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Back | Show only these items: | Microsolvation of the indole cation (In+) in a nonpolar environment: IR spectra of In+–Ln complexes (L = Ar and N2, n ≤ 8)

1
Microsolvation of the indole cation (In⁺) in a nonpolar environment: IR spectra of In⁺–L_n complexes (L = Ar and N₂, n ≤ 8)

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Infrared photodissociation (IRPD) spectra of complexes composed of the indole cation (In⁺ = C₈H₇N⁺) and several neutral ligands (L = Ar and N₂) were recorded in the vicinity of the N–H stretch vibration (ν₁) of bare In⁺ in its ²A″ electronic ground state.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

The analysis of systematic size-dependent ν₁ band shifts and photofragmentation branching ratios in the spectra of In⁺–Ar_n (n ≤ 5) and In⁺–(N₂)_n (n ≤ 8) provides information about the stepwise microsolvation of In⁺ in a nonpolar hydrophobic environment, including the existence of structural isomers and the determination of ligand binding energies.

Type: Object | Advantage: Yes | Novelty: New | ConceptID: Obj2

The IR spectra of the In⁺–L dimers reveal two transitions, which are attributed to ν₁ fundamentals of the H-bound and π-bound isomers on the basis of their complexation-induced ν₁ frequency shifts, Δν₁.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

In both cases, the H-bound isomer is found to be more stable than possible π-bound isomers.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

The Δν₁ shifts are used to derive the first experimental estimate for the proton affinity of the indolyl radical (∼920 ± 30 kJ mol⁻¹).

Type: Method | Advantage: None | Novelty: New | ConceptID: Met1

The IR spectra of In⁺–Ar_n (n ≤ 5) suggest that the preferred microsolvation path for this cluster system begins with the formation of the H-bound In⁺–Ar dimer core, which is further solvated by (n − 1) π-bound ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

In contrast, the spectra of In⁺–(N₂)_n with n ≤ 8 suggest that this cluster grows by the formation of an In⁺–(N₂)₂ trimer core with two H-bound N₂ ligands, to which (n − 2) π-bound N₂ molecules are attached.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

The In⁺–L_n complexes were generated in an electron impact (EI) ion source, which predominantly produces the most stable isomer of each cluster ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

For several In⁺–L_n complexes, the geometry of the most stable isomer produced in this ion source differs drastically from the structures previously observed by resonant photoionization of the corresponding neutral precursors, demonstrating the severe restriction of photoionization techniques (given by the Franck–Condon principle) for the spectroscopic characterization of cluster ions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Most of the In⁺–L_n complexes investigated exhibit a distinct ionization-induced change in the preferred substrate–ligand recognition pattern.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

Introduction

Many biophysical and biochemical phenomena, including molecular recognition, protein folding, and biological activity, strongly depend on the environment.^1,2

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

Solvation effects are particularly important for charged species, because of the larger strength and longer range of ion–solvent interactions compared to corresponding forces between neutral molecules.^3–7

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Biological molecules are often (locally) charged due to either (de-)protonation or charge separation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

Isolated complexes of the type X^±q–L_n are simple and attractive model systems to investigate the effects of stepwise solvation of an ion with charge ±q (X^±q) by a well-defined number (n) of ligands (L) with both experimental and theoretical techniques.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The present work reports infrared (IR) spectra and quantum chemical calculations of complexes of the indole cation (In⁺ = C₈H₇N⁺) solvated by several inert ligands (L = Ar and N₂) to characterize the interaction of In⁺ with a hydrophobic environment.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

Indole is the aromatic chromophore of the amino acid trypthophan, which plays an important role in solvation-dependent photophysical processes of proteins.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

Hence, size-selected In⁺–L_n clusters serve as model systems to investigate the interaction between tryptophan-containing biomolecules and surrounding solvent molecules under controlled solvation conditions.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

In general, aromatic molecules (X = A) with acidic functional groups (YH_k) offer two principal recognition sites for a solvent molecule.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

The ligand L can either interact with the aromatic π-electron system (π-bond) or form a hydrogen bond to one of the protons of the YH_k group (H-bond).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

Other binding sites are usually less stable.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

The preferred recognition site depends strongly on the electronic excitation and charge of A, the acidity of the YH_k group, and the type of ligand (e.g., nonpolar or polar).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

For example, complexes of neutral aromatic molecules with the spherical rare gas atoms (e.g., A–Ar) generally feature π-bound equilibrium structures in the singlet electronic ground state (S₀).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

Illustrative examples inlude Ar complexes of phenol (Ph–Ar, YH_k = OH)^8,9 and aniline (An–Ar, YH_k = NH₂).¹⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

The dominant contribution to the attraction in neutral A–Ar complexes is the dispersion interaction between Ar and the aromatic π-electron system of A, which is most favorable for π-bonding.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

So far, no H-bound isomer has been detected for any aromatic A–Ar complex in the S₀ state.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

The situation changes for complexes with molecular nitrogen, A–N₂.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

The quadrupole moment of N₂ leads to additional electrostatic interactions with the polar YH_k group of A. Consequently, the H-bound A–N₂ isomer often competes energetically with the π-bound one.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

For example, the intermolecular potential of Ph–N₂ has a H-bound global minimum,⁸ whereas the less acidic An molecule prefers π-bonding with N₂.¹¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

The interaction in A⁺–L_n cation clusters is usually rather different from that in the corresponding neutral A–L_n complexes, because of the substantial additional electrostatic and inductive attraction arising from the positive charge distribution in A⁺.^3,4,6

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

Hence, A⁺–L_n and A–L_n complexes often have rather different equilibrium structures and binding energies.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

For example, the Ph⁺–Ar^12–15 and An⁺–Ar¹⁶ cation dimers have H-bound global minima in the ground electronic state (D₀), whereas the π-bound structures, which are global minima in S₀, are only local minima on the intermolecular potential in D₀.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

A similar situation is observed for Ph⁺–N₂^8,12,13,17 and An⁺–N₂ dimers.¹⁸

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

In general, the interaction strength in A⁺–N₂ is larger than that in the corresponding A⁺–Ar dimer, mainly because of the additional charge–quadrupole interaction in the former complexes.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

The substantial difference in the topolopgy of the interaction potentials in neutral A–L_n and ionic A⁺–L_n clusters, with respect to both their equilibrium structure and binding energy, leads to important consequences for spectroscopic studies of the cation clusters prepared by photoionization of the corresponding neutral precursor.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Frequently, A⁺–L_n complexes are generated by the formation of neutral A–L_n complexes in a supersonic molecular beam expansion, followed by resonance enhanced multiphoton ionization (REMPI).

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

For the spectroscopic characterization of A⁺–L_n, the REMPI process is usually combined with photoionization efficiency measurements (PIE), mass analysed threshold ionization (MATI), zero kinetic energy photoelectron (ZEKE) spectroscopy, or photodissociation (PD) spectroscopy.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

However, all these ionization techniques suffer from the severe restrictions imposed by the Franck–Condon principle, which prevents the efficient population of the most stable isomer of A⁺–L_n in cases where the global minimum structures of the neutral and the cation complex are rather different.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met2

This scenario is quite common for complexes of (substituted) aromatic molecules with polar and nonpolar molecules.¹⁹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

To overcome the limitations of the REMPI technique for cluster ion generation (“REMPI ion source”), the present work employs electron impact (EI) ionization within a supersonic expansion for the production of cold A⁺–L_n complexes.⁷

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

This EI cluster ion source was shown to produce predominantly the most stable isomer of a given A⁺–L_n cluster ion, because the reaction sequence begins with EI ionization of A (to yield A⁺), which is followed by three-body cluster aggregation reactions.^{12–16,18,20,21}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

Hence, the EI source forms predominantly the most stable A⁺–L_n isomer, independent of the most stable geometry of the corresponding neutral A–L_n species.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met3

This is in contrast to the REMPI source, which often generates only local minima of A⁺–L_n.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met2

For example, EI-IR spectra of Ph⁺–Ar_n^12–15 and An⁺–Ar_n¹⁶ were interpreted with global minimum structures of these complexes, which escaped detection in the corresponding REMPI-IR spectra as well as other photoionization spectra (PIE, ZEKE, MATI).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

Similar to most aromatic molecules with an acidic YH_k group, the planar indole molecule offers at least two principal binding sites for neutral ligands, namely H-bonding to the N–H group or π-bonding to the aromatic π-electron system.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

This recognition pattern yields two major isomers for dimers, namely the H-bound and π-bound In⁽⁺⁾–L structures shown in Fig. 1(a) and (b), respectively.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

(Although there are possibly more than one stable π-bound isomer on the potential of In⁽⁺⁾–L, they are not distinguished in the present work.) Accordingly, three combinations are possible for the In⁽⁺⁾–L₂ trimer, one with two H-bonds (Fig. 1(e), 2H isomer), one with two π-bonds (Fig. 1(d), 2π), and one with one H-bond and one π-bond (Fig. 1(c), H/π).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

In the present work, the notation xH/yπ is used to describe an In⁽⁺⁾–L_n isomer with x H-bound and y π-bound ligands (x + y = n).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Spectroscopic information about the structure of neutral In–Ar_n clusters (with n ≤ 2) in the two lowest singlet electronic states, S₀ and S₁, is available from laser-induced fluorescence excitation (LIF),^22,23 dispersed LIF,²⁴ and REMPI spectroscopy.^24,25

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

All available spectral information is consistent with the observation of a π-bound In–Ar dimer (Fig. 1(b)) and a 2π isomer of In–Ar₂ (Fig. 1(d)) in both electronic states.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

Such structures were also predicted by simple Lennard-Jones pairwise additive atom–atom potentials.²⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

No evidence for an H-bound In–Ar isomer has been reported.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot3

The analysis of the rotationally resolved spectrum of the S₁ ← S₀ transition of π-bound In–Ar yields a structure, in which the Ar ligand is approximately located R ≈ 3.4 Å above or below the indole plane.²³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

The attraction between Ar and the lone pair of the N atom results in some preference for Ar toward the five-membered ring of In.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

The interaction in the S₁ state (R = 3.40 Å) is slightly stronger than in S₀ (R = 3.43 Å),²³ with experimental dissociation energies of D₀ = 477 ± 10 and 451 ± 10 cm⁻¹, respectively.²⁶

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

The larger interaction in the excited state arises from an increase in the polarisability of the aromatic π-electron system upon electronic excitation, which enhances the dispersion attraction.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

The S₁ ← S₀ transitions of In–Ar_n with n = 0–2 display nearly additive red shifts upon Ar complexation, ΔS₁ = −26 cm⁻¹ (n = 1) and −53 cm⁻¹ (n = 2),²² indicating that both Ar ligands in the 2π isomer of In–Ar₂ form almost equivalent π-bonds on opposite sides of the molecular plane, with little interaction between both Ar ligands.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

Spectroscopic information about the interaction of In⁺–Ar_n (with n ≤ 2) in the cation ground state (D₀) is provided by PIE,²⁷ MATI,²⁶ and ZEKE²⁵ spectroscopic studies.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

Similar to the corresponding neutral complexes, all spectral features in the cation spectra could be rationalized by a π-bound In⁺–Ar isomer and a 2π isomer of In⁺–Ar₂.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

No evidence for the formation of an intermolecular H-bond in In⁺–Ar_n clusters has been reported.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot4

This observation is not surprising because all cation spectra recorded so far were obtained by REMPI of the neutral clusters featuring only intermolecular π-bonds.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met2

In general, the interaction in the cation ground state of π-bound In⁺–Ar (D₀ = 537 ± 10 cm⁻¹)²⁶ is significantly stronger than in the neutral electronic states because of the additional charge–induced dipole attraction.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac22

In addition, the additive red shifts in the ionization potentials of In–Ar_n with n = 0–2 indicate that both π-bonds in the 2π isomer of In⁺–Ar₂ are nearly equivalent.²⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

Little activity in the intermolecular bending (ν_b) and stretching vibrations (ν_s) observed in the dispersed LIF, LIF excitation, ZEKE, and MATI spectra of π-bound In–Ar_n (n = 1, 2) suggests only small changes in the geometry of the intermolecular π-bond(s) along both the bending and stretching coordinates upon electronic excitation and subsequent ionization.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

In contrast to In–Ar_n, neither spectroscopic nor theoretical information is available for the neutral and ionic states of In⁽⁺⁾–(N₂)_n complexes.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot5

The major goal of the present work is the characterization of the intermolecular interaction in In⁺–(N₂)_n and In⁺–Ar_n complexes in their cation ground states by EI-IR spectroscopy in the vicinity of the N–H stretch fundamental of bare In⁺ (ν₁ = 3454 cm⁻¹).²⁸

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

Of particular interest is the competition between H-bonding and π-bonding in the In⁺–L dimers and its consequences on the microsolvation process (e.g., the geometry of the most stable isomer of larger In⁺–L_n clusters).

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

Comparison of the EI-IR spectra obtained for In⁺–Ar_1,2 with previous photoionization data demonstrates the deficiencies of REMPI techniques for the production of the most stable isomers of this type of cluster ions.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

In addition to the EI-IR spectra, the intermolecular interaction in the H-bound isomers of In⁺–(N₂)_n (n ≤ 2) and In⁺–Ar is characterized by density functional calculations, as no previous theoretical studies of the cation PES of these clusters are available.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

Comparison of In⁺–(N₂)_n and In⁺–Ar_n with corresponding complexes of Ph⁺ and An⁺ reveals the effect of the acidic YH_k group on the recognition pattern of aromatic ions with hydrophobic solvent molecules, including the preferred sequence for cluster growth and the determination of ligand binding energies.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

Experimental

IRPD spectra of mass-selected In⁺–L_n complexes were recorded in a tandem quadrupole mass spectrometer coupled to an electron impact cluster ion source and an octopole ion trap.⁷

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Briefly, In⁺–L_n cluster ions were produced in an ion source which combines a pulsed supersonic expansion with electron impact ionization close to the nozzle orifice.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The expanding gas mixture was generated by seeding indole vapor (heated up to T ≈ 340 K) in Ar and/or N₂ carrier gas at backing pressures ranging from 5 to 10 bar.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

As outlined in detail in ^{ref. 13}, the dominant production mechanism of In⁺–L_n begins with electron ionization of In, which is followed by three-body association reactions: In + e → In⁺ + 2eIn⁺–L_n−1 + L + M → In⁺–L_n + M (M = L, In) This reaction sequence produces predominantly the most stable isomers of In⁺–L_n clusters and to smaller extent less stable isomers.¹³

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

A typical mass spectrum of the ion source is shown in Fig. 2 for the generation of In⁺–N₂.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The spectrum is dominated by N_n⁺ and In⁺.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Only little fragmentation of In⁺ is observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

Major cluster series include [X − (N₂)_n]⁺ with X = N₂, N₃, In, Ar, and the impurity H₂O. The intensity ratio of In⁺ and In⁺–N₂ is of the order of 200∶1, indicating the formation of a weakly-bound noncovalent complex.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

The central part of the supersonic plasma expansion was extracted into an initial quadrupole mass spectrometer, which was tuned to the mass of In⁺–L_n.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

The mass selected In⁺–L_n beam was then deflected by 90° and injected into an octopole ion guide, where it interacted with a tunable IR laser pulse.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

Resonant vibrational excitation of In⁺–L_n induced the evaporation of weakly bound ligands: In⁺–L_n + hν → (In⁺–L_n)* → In⁺–L_m + (n − m)L Only the rupture of the weak intermolecular bonds was observed upon IR excitation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

The In⁺–L_m fragment ions were mass selected with a second quadrupole mass filter and detected with a Daly ion detector.²⁹

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

The IRPD spectrum of In⁺–L_n was obtained by monitoring the In⁺–L_m fragment current as a function of the laser frequency.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

For In⁺–L_n clusters with n > 1, several fragment channels m were possible, and in these cases IRPD spectra were recorded simultaneously in the two dominant channels.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

As an example, Fig. 3 shows the mass spectrum obtained for resonant excitation of In⁺–(N₂)₆ at ν₁ = 3413 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

For this cluster, the major fragment channels were m = 1 (30%) and m = 2 (65%).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

In general, the IRPD spectra recorded in different fragment channels were rather similar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

Consequently, only the spectra obtained in the dominant channel are shown in the figures.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

Tunable IR laser radiation with a bandwidth of 0.02 cm⁻¹ was generated by a single-mode optical parametric oscillator laser system pumped by a seeded Nd:YAG laser.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

The IRPD spectra were calibrated to an accuracy of better than 1 cm⁻¹ by simultaneously recording optoacoustic spectra of ammonia and analysing water absorptions along the IR laser path.^30,31

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

All IRPD spectra were normalized for laser intensity variations recorded with an InSb detector.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met8

Quantum chemical calculations

Density functional calculations were carried out for In⁺, In⁺–Ar, In⁺–N₂, and In⁺–(N₂)₂ at the unrestricted B3LYP/6-31G* level of theory³² to investigate the intermolecular interaction between In⁺ and H-bound ligands as well as the effects of the intermolecular H-bond(s) on the properties of In⁺.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met9

Previous calculations for the related complexes involving An⁺ and Ph⁺ revealed that this theoretical level is sufficient to describe the properties of the H-bonds of these ions to Ar and N₂ with reasonable accuracy (in particular the frequency shifts of the proton donor stretch vibrations) and to facilitate vibrational assignments.^12,15,16,18

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met9

In general, all coordinates were relaxed during the search for stationary points.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met10

Intermolecular interaction energies were counterpoise corrected for basis set superposition error.³³

Type: Method | Advantage: None | Novelty: New | ConceptID: Met11

Harmonic frequencies were scaled by a factor of 0.9589 to bring the calculated frequency of the N–H stretch vibration of In⁺ into agreement with the corresponding experimental value, ν₁ = 3454 cm⁻¹.²⁸

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

Table 1 summarizes the results of the calculations of In⁺–L_n relevant for the present work.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The table includes structural and energetic properties of the intermolecular H–L bond, such as length (R_H–L), angle (θ_N–H–L), and dissociation energy (D_e), as well as fundamental attributes of the intramolecular N–H bond, namely length (r_N–H) and stretching frequency (ν₁).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The proton-bound In⁺–L minima have nearly linear H-bonds of L to the acidic N–H proton donor group of In⁺, leading to planar equilibrium geometries with C_s symmetry (Fig. 1(a)).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

100

The calculated intermolecular bond angles and separations are 170° and 2.64 Å for Ar and 177° and 2.16 Å for N₂, respectively.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

101

As expected, the interaction in In⁺–Ar is significantly weaker than in In⁺–N₂ (D_e = 291 and 1201 cm⁻¹).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

102

The effects of H-bonding on the intramolecular N–H bond of In⁺ are an elongation (Δr_N–H), a reduction in the stretching frequency (Δν₁), and an enhancement in the IR oscillator strength (ΔI₁).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac25

103

As expected, the magnitude of these effects is correlated with the strength of the H-bond (Δr_N–H = 0.0011 and 0.0045 Å, Δν₁ = −19 and −68 cm⁻¹, and ΔI₁ = 100 and 290% for Ar and N₂).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

104

The experimental IR spectra discussed in section IV suggest that In⁺–(N₂)₂ has a structure, in which both N₂ ligands are H-bonded to the N–H group of In⁺.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

105

All efforts in searching for such a 2H isomer of In⁺–(N₂)₂ resulted in a planar equilibrium geometry with two nonequivalent N₂ ligands (Fig. 1(e)).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

106

The H-bond to the first N₂ ligand, which is closer to the six-membered ring of In⁺, is characterized by R_H–N = 2.29 Å, θ_N–H–N = 155°, and D_e = 805 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

107

The interaction with the second ligand is considerably weaker (R_H–N = 2.70 Å, θ_N–H–N = 122°, D_e = 549 cm⁻¹).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

108

The effects of both ligands on the N–H bond of In⁺ (Δr_N–H = 0.0033 Å, Δν₁ = − 39 cm⁻¹, ΔI₁ = 180%) are smaller than those of the single ligand in H-bound In⁺–N₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

109

Thus, because of noncooperative interactions, the addition of the second H-bound ligand destabilizes the intermolecular bond to the first ligand, which in turn leads to a stabilization of the intramolecular N–H bond.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

110

Complexes of In⁺ with π-bound ligands were not investigated theoretically, because density functional calulations do not properly describe interactions, which are dominated by dispersion forces.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

111

However, as π-bound ligands in In⁺–L_n have only little effects on the properties of the N–H bond, the changes of the ν₁ frequency and IR intensity of In⁺ upon formation of π-bonds are assumed to be small^12,15,18.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

Experimental results and discussion

112

The IRPD spectra of In⁺–Ar_n (n ≤ 5) and In⁺–(N₂)_n (n ≤ 8) are shown and analysed in Figs. 4–8, and the band maxima and widths of the transitions observed are listed in Table 2, along with their assignments.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

In⁺–L dimers

113

Fig. 4 compares the IRPD spectra of In⁺–L (L = Ar, N₂) in the vicinity of the ν₁ vibration of bare In⁺ (ν₁ = 3454 cm⁻¹).²⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

114

Two transitions are observed in each spectrum in the frequency range investigated (3250–3600 cm⁻¹), and they are attributed to the ν₁ fundamentals of (at least) two different isomers, denoted H and π.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

115

The more intense bands at 3425 (Ar) and 3379 (N₂) cm⁻¹ are assigned to the H-bound isomers (Fig. 1(a)).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

116

They feature significant red shifts of Δν₁ = −29 and −75 cm⁻¹ upon complexation, which compare favorably with the calculated values of −19 and −68 cm⁻¹, respectively.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

117

The shift is larger for H-bound In⁺–N₂ than for In⁺–Ar, consistent with the stronger interaction in the former dimer.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

118

The weaker bands at 3451 (Ar) and 3450 (N₂) cm⁻¹ are close to the In⁺ transition (3454 cm⁻¹)²⁸ and display only modest red shifts of Δν₁ = −3 and −4 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

119

This observation suggests an assignment of these bands to the ν₁ fundamentals of the π-bound isomers of In⁺–L (Fig. 1(b)), because in these structures the ligands have only little influence on the N–H bond of In⁺ with respect to both frequency and IR intensity.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

120

There are actually other binding sites which also have little impact on the N–H bond, such as H-bonding to the aromatic C–H groups.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

121

Although at present these alternative binding sites cannot be completely ruled out both from the experimental and theoretical point of view, π-bound isomers are presently the favored assignment for the transitions near 3450 cm⁻¹ and this interpretation is used as working hypothesis for the rest of this paper (see also section V).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

122

In addition to the band shifts, the observed ν₁ band profiles support the isomer assignment in Fig. 4 and Table 2.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

123

The ν₁ bands of the H-bound dimers are significantly shaded to the blue, with sharp heads in the P branch and slowly decreasing signal to higher frequency of the band maximum.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

124

Such band profiles are typical for the excitation of proton donor stretch vibrations in H-bound dimers,^{4,12,13,34–37} because the intermolecular bond becomes stronger and shorter in the vibrationally excited state leading to larger rotational constants.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

125

In the limit of adiabatic separation of inter- and intramolecular degrees of freedom, the red shifts Δν₁ correspond directly to the increase of the interaction energy upon ν₁ excitation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

126

In contrast to the H-bound dimers, the π-bound isomers feature rather symmetric ν₁ band profiles, because ν₁ excitation has nearly no influence on the strength of the intermolecular π-bond, consistent with the small complexation shifts Δν₁.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

127

The ratios of the integrated ν₁ band intensities observed in the IRPD spectra of the In⁺–L dimers can be used to estimate the relative abundance of the H-bound and π-bound isomers in the expansion, assuming the calculated relative ν₁ IR oscillator strengths for the H-bound In⁺–L dimers listed in Table 1.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met12

128

The π-bound In⁺–L isomers are assumed to have the same ν₁ IR oscillator strength as In⁺.^12,16,18

Type: Method | Advantage: None | Novelty: New | ConceptID: Met12

129

This procedure results for In⁺–Ar (In⁺–N₂) in a relative abundance of n_H∶n_π ≈ 2∶1 (7∶1), on the basis of the experimental intensity ratio of 4∶1 (28∶1) and the theoretical ratio of the oscillator strength of 2∶1 (4∶1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

130

The larger abundance of the H-bound In⁺–L dimers suggests that they are more stable than the corresponding π-bound isomers, because the electron impact ion source produces predominantly the most stable isomer of a given cluster.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

131

Moreover, the energy difference between both isomers appears to be larger for In⁺–N₂ than for In⁺–Ar leading to a more efficient production of the π-bound isomer in the latter case.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

132

Similar to the previous studies of Ph⁺–Ar,^12,14 the relative intensity ratio of the ν₁ bands of H-bound and π-bound In⁺–Ar depended considerably upon the expansion conditions, confirming that both transitions arise from different isomers.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

133

The spectrum in Fig. 4 was obtained when the conditions were optimized for the production of π-bound In⁺–Ar.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

134

Moreover, the relative concentration of the π-bound isomer decreased for lower effective temperatures of the ion source.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

135

This observation is taken as clear evidence that the H-bound isomer of In⁺–Ar corresponds to the global minimum of the potential, whereas the π-bound structure is a less stable local minimium.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

136

Similar conclusions apply also to the In⁺–N₂ dimer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

137

The dissociation energy of π-bound In⁺–Ar in the ground vibrational state was determined as D₀ = 537 ± 10 cm⁻¹.²⁶

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

138

Hence, the shift of Δν₁ = −3 cm⁻¹ implies a binding energy of D₀ = 540 ± 10 cm⁻¹ (+0.6%) in the ν₁ excited state.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

139

As the interaction in H-bound In⁺–Ar should be somewhat stronger than in π-bound In⁺–Ar (because of the larger abundance in the EI ion source), the dissociation energy calculated at the B3LYP/6-31G* level, D_e = 291 cm⁻¹, substantially underestimates the true interaction energy.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

140

Comparison with the related Ph⁺–Ar dimer yields an estimated binding energy of D₀ ≈ 670 ± 140 cm⁻¹ in the ground state (see section IV.B).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

141

On the basis of this value, the interaction increases by +4.5% upon ν₁ excitation (Δν₁ = −29 cm⁻¹), resulting in D₀ ≈ 700 ± 140 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

142

No experimental information is available for the binding energy of H-bound and π-bound In⁺–N₂.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac29

143

For H-bound Ph⁺–N₂, the dissociation energy calculated at the B3LYP/6-31G* level, D_e = 1787 cm⁻¹,¹² is close to the experimental binding energy, D₀ = 1640 ± 10 cm⁻¹.¹⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

144

Assuming similar accuracy for the corresponding H-bound In⁺–N₂ dimer, the theoretical value, D_e = 1201 cm⁻¹ (Table 1), leads to an estimated dissociation energy of D₀ ≈ 1100 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res29

145

On the basis of this value, the observed red shift of Δν₁ = −75 cm⁻¹ corresponds to an increase of the interaction strength of ≈7% upon ν₁ excitation (D₀ ≈ 1175 cm⁻¹).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

146

The dissociation energy of π-bound In⁺–N₂ may be estimated as D₀ ≈ 700 cm⁻¹ from those of the corresponding π-bound isomers of Ph⁺–N₂ (D₀ = 750 ± 150 cm⁻¹)¹³ and An⁺–N₂ (D₀ = 700 ± 200 cm⁻¹).^18,38

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

147

This value is also in line with the analysis of the photofragmentation data of larger complexes outlined in section IV.B.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

Larger In⁺–L_n clusters

148

Fig. 5 shows the IRPD spectra of In⁺–Ar_n (n ≤ 5) recorded between 3380 and 3480 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

149

All spectra were obtained in the In⁺ fragment channel, indicated as n → 0.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

150

The spectrum of In⁺–Ar displays two bands at 3425 and 3451 cm⁻¹, which are assigned to the ν₁ fundamentals of the H-bound and π-bound isomers (section IV.A).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res30

151

Similar to In⁺–Ar, the spectrum of the In⁺–Ar₂ trimer reveals also two transitions (at 3424 and 3450 cm⁻¹), which are barely shifted upon addition of the second Ar ligand.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res31

152

Consequently, they are assigned to ν₁ bands of In⁺–Ar₂ isomers, which are obtained by adding a π-bound ligand to the H-bound and π-bound In⁺–Ar dimer isomers, respectively.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

153

The intense band at 3424 cm⁻¹ corresponds to the H/π isomer (Fig. 1(c)), whereas the weak band at 3450 cm⁻¹ arises from the 2π isomer (Fig. 1(d)).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res32

154

The relative intensity ratio of the ν₁ bands of the 2π and H/π isomers is of the order of 1∶20 and is significantly lower than the corresponding ratio of the π-bound and H-bound dimers (1∶4).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res33

155

This observation confirms that the intermolecular H-bond between Ar and In⁺ is more stable than the π-bond.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

156

The spectra of In⁺–Ar_n with n = 3–5 are dominated by a single transition, which is assigned to ν₁ of an isomer with one H-bound and (n − 1) π-bound ligands, denoted H/(n − 1)π.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res34

157

No evidence for an isomer with only π-bound ligands is observed for n ≥ 3, again demonstrating that the H-bond is more stable than the π-bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res35

158

Hence, the preferred solvation sequence in In⁺–Ar_n complexes begins with the formation of a H-bound In⁺–Ar dimer core, which is further solvated by (n − 1) π-bound ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

159

In general, the widths of the ν₁ bands of In⁺–Ar_n decreases with increasing cluster size from 14 (n = 1) to 5 cm⁻¹ (n = 5).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

160

This effect is attributed to (i) lower internal energy, (ii) smaller rotational constants, and (iii) smaller effects of ν₁ excitation on the intermolecular modes of the larger clusters (i.e. sequence bands of ν₁ involving intermolecular modes better overlap with the ν₁ fundamental).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res36

161

Fig. 6 shows the dependence of the ν₁ frequencies of In⁺–Ar_n as a function of the cluster size n.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

162

The plot mirrors the growth of the solvation (sub)shells of the different cluster isomers, nπ and H/(n − 1)π.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res37

163

In the case of the nπ isomers of In⁺–Ar_n (observed up to n = 2), the incremental red shifts are of the order of −2 ± 1 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

164

In the more stable H/(n − 1)π cluster series (observed up to n = 5), the large ν₁ red shift of the H-bound ligand (−29 cm⁻¹, n = 1) is followed by a small red shift for the first π-bound ligand (−1 cm⁻¹, n = 2) and a small blue shift for the second π-bound ligand (+2 cm⁻¹, n = 3).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res38

165

No change in the ν₁ frequency is observed for clusters in the size range n = 3–5 within experimental resolution.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

166

In general, the incremental complexation-induced shifts of π-bound ligands (|Δν₁| ≤ 3 cm⁻¹) are significantly smaller than the widths of the observed transitions (fwhm = 5–14 cm⁻¹), making it difficult to extract more definitive information on the structure of the complexes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res39

167

Fig. 7 compares the IRPD spectra of In⁺–(N₂)_n with n ≤ 8 recorded in the dominant In⁺–(N₂)_m fragment channel, indicated as n → m.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res40

168

Similar to In⁺–Ar, the spectrum of In⁺–N₂ displays two bands at 3379 and 3450 cm⁻¹, which are assigned to the ν₁ fundamentals of the H-bound and π-bound isomers, respectively (section IV.A).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res41

169

In contrast to In⁺–Ar₂ and In⁺–N₂, however, the spectrum of the In⁺–(N₂)₂ trimer reveals only a single transition at 3404 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res42

170

As this transition is significantly red shifted from ν₁ of π-bound In⁺–N₂ (Δν₁ = −46 cm⁻¹), it is not attributed to the 2π isomer of In⁺–(N₂)₂ (Fig. 1(d)).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res43

171

On the other hand, the band also displays a large blue shift from ν₁ of H-bound In⁺–N₂ (Δν₁ = 25 cm⁻¹), indicating that the second N₂ ligand has a significant stabilizing effect on the N–H bond.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res44

172

Consequently, the second ligand can not be π-bonded to H-bound In⁺–N₂, excluding an assignment of the 3404 cm⁻¹ transition to the H/π isomer of In⁺–(N₂)₂ (Fig. 1(c)).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

173

This observation initiated our theoretical search for In⁺–(N₂)₂ trimer structures, in which both N₂ ligands interact with the acidic proton of In⁺.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot6

174

All efforts in locating such minimum structures, using the B3LYP calculations described in section III, converged toward the planar In⁺–(N₂)₂ geometry with two nonequivalent H-bonds shown in Fig. 1(e) (2H isomer, Table 1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res45

175

Interestingly, the ν₁ frequency of this In⁺–(N₂)₂ isomer is predicted as 3414 cm⁻¹, in nice agreement with the observed value of 3404 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res46

176

In addition, the calculated shifts from ν₁ of H-bound In⁺–N₂ and bare In⁺, Δν₁ = +28 and −40 cm⁻¹, are compatible with the corresponding experimental values, Δν₁ = +25 and −50 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res47

177

Hence, the 3404 cm⁻¹ band is attributed to the 2H isomer of In⁺–(N₂)₂.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

178

The absence of any pronounced absorptions of In⁺–(N₂)₂ near the ν₁ bands of H-bound and π-bound In⁺–N₂ suggests that the 2π and H/π isomers of In⁺–(N₂)₂ are significantly less stable than the 2H isomer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res48

179

The low concentration of the 2π isomer in the expansion confirms that H-bound In⁺–N₂ is more stable than π-bound In⁺–N₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res49

180

The ν₁ blue shift of In⁺–(N₂)₂ with respect to H-bound In⁺–N₂ indicates that ν₁ excitation reduces the binding energy of the second ligand by 25 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res50

181

Moreover, addition of the second ligand strengthens the N–H bond and weakens the first H-bond in H-bound In⁺–(N₂)₂, leading to the observed increase in the ν₁ frequency.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res51

182

This trend is in line with the results of the B3LYP calculations (Table 1).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con18

183

The fact that the ν₁ band of the 2H isomer is shaded to the red indicates that the rotational constants in the ν₁ state of In⁺–(N₂)₂ are smaller than in the ground state.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res52

184

The spectra of In⁺–(N₂)_n with n = 3–8 are dominated by a single band, which is close to the ν₁ band of the 2H isomer of In⁺–(N₂)₂.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

185

Consequently, this transition is assigned to the ν₁ fundamental of In⁺–(N₂)_n structures, in which the 2H isomer of In⁺–(N₂)₂ is further solvated by (n − 2) π-bound ligands, denoted 2H/(n − 2)π.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res53

186

Similar to In⁺–Ar_n, the widths of the ν₁ transitions tend to decrease as the size of the cluster increases.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res54

187

Fig. 8 shows the dependence of the ν₁ frequencies of In⁺–(N₂)_n as a function of the cluster size n.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res54

188

Similar to In⁺–Ar_n, this plot clearly reflects the structure of the subshells of the first solvation shell and the sequence of In⁺ solvation.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con19

189

The less stable nπ isomer is only observed up to n = 1, with a red shift (−4 cm⁻¹) that is larger than the one for In⁺–Ar_n.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res55

190

The preferred solvation sequence in the more stable 2H/(n − 2)π cluster series of In⁺–(N₂)_n begins with the formation of a strongly H-bound In⁺–(N₂)₂ trimer core, in which the first H-bond (−75 cm⁻¹, n = 1) is much stronger than the second H-bond (+25 cm⁻¹, n = 2).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res56

191

The 2H In⁺–(N₂)₂ trimer core is then further solvated by π-bound ligands.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res57

192

These induce small incremental blue shifts in the ν₁ frequency, which decrease almost monotonically from 3 (n = 3) to ≈1 cm⁻¹ (n = 8).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res58

193

Thus, the N–H bond of the H-bound In⁺–(N₂)₂ core is slightly stabilized by further sequential addition of π-bound ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con20

194

This observation is in line with a destabilization of the two H-bonds of the In⁺–(N₂)₂ core by filling the π-bound solvation (sub)shell.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con21

195

Table 3 summarizes the photofragmentation branching ratios measured for resonant ν₁ excitation of the most stable isomers of In⁺–Ar_n and In⁺–(N₂)_n.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res59

196

In agreement with previous studies on related systems,^{13,18,37–42} the range of fragment channels (m) for a given parent cluster size (n) is rather narrow.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con22

197

This information may be used to estimate ligand binding energies within the framework of a simple model.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con23

198

This model assumes that the absorbed photon energy (ν₁) is available for subsequent ligand evaporation.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

199

Moreover, the ligands are classified into H-bound and π-bound ligands, with dissociation energies of D₀(H) and D₀(π), respectively.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

200

Equivalent ligands are assumed to have the same binding energy.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

201

In⁺–Ar_n clusters up to n ≤ 5 evaporate all n ligands upon ν₁ excitation, implying that the sum of the dissociation energies of one H-bound and four π-bound ligands is below 3425 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res60

202

The dissociation energy of π-bound In⁺–Ar has been determined as D₀(π) = 537 ± 10 cm⁻¹,²⁶ yielding an upper limit for the dissociation energy of the H-bound ligand, D₀(H) < 1320 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res61

203

This limit can be further reduced because several observations suggest that the dissociation energy of H-bound In⁺–Ar should be significantly lower than the one of H-bound Ph⁺–Ar, D₀(H) = 670 ± 140 cm⁻¹.¹³

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con24

204

First, the relative ν₁ red shift of H-bound In⁺–Ar is much smaller than the one of H-bound Ph⁺–Ar (0.8 vs.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res62

205

2.0%).¹⁴

Type: Result | Advantage: None | Novelty: None | ConceptID: Res62

206

Second, the abundance ratio of H-bound and π-bound In⁺–Ar observed in the EI-IR spectra is much lower than the one observed for Ph⁺–Ar under comparable experimental conditions.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res63

207

For example, when the conditions are optimized for the production of the π-bound isomers, this ratio is n_H∶n_π ≈ 2∶1 for In⁺–Ar and ≈7∶1 for Ph⁺–Ar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res63

208

This observation indicates that the difference in binding energies between the H-bound and π-bound dimers, ΔD₀(H–π) = D₀(H) − D₀(π), is larger for Ph⁺–Ar than for In⁺–Ar.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con25

209

Moreover, the 2π isomer of In⁺–Ar₂ could clearly be detected (in addition to the more stable H/π isomer).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

210

In contrast, for Ph⁺–Ar₂ only the H/π isomer was observed, whereas the 2π isomer was below the detection limit.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac31

211

This observation indicates again that ΔD₀(H–π) of In⁺–Ar is smaller than for Ph⁺–Ar.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con25

212

As D₀(π) of Ph⁺–Ar (535 ± 3 cm⁻¹)⁹ and In⁺–Ar (537 ± 10 cm⁻¹)²⁶ are comparable, D₀(H) of Ph⁺–Ar (670 ± 140 cm⁻¹)¹³ provides an upper limit to D₀(H) of In⁺–Ar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res64

213

In conclusion, D₀(H) of In⁺–Ar can be bracketed by D₀(π) of In⁺–Ar (537 ± 10 cm⁻¹) and D₀(H) of Ph⁺–Ar (670 ± 140 cm⁻¹), resulting in D₀(H) ≈ 670 ± 140 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res65

214

For the analysis of the photofragmentation branching ratios of In⁺–(N₂)_n, the ligands are classified into three categories with three distinct binding energies, D₀(H₁) > D₀(H₂) > D₀(π).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

215

For this scenario, the photofragmentation data yield D₀(π) = 600 ± 100 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

216

Information about the values of D₀(H₁) and D₀(H₂) are less accurate, with 600 cm⁻¹ ≤ D₀(H₂) ≤ 1450 cm⁻¹ and 650 cm⁻¹ ≤ D₀(H₁) ≤ 2300 cm⁻¹, and the additional restriction 1300 cm⁻¹ ≤ D₀(H₁) + D₀(H₂) ≤ 2900 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

217

These values are compatible with the theoretical predictions of D_e(H₁) = 805 cm⁻¹ and D_e(H₂) = 549 cm⁻¹ for In⁺–(N₂)₂ (Table 1).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res66

Further discussion

218

Because of the additional charge, the intermolecular PES of the In⁺–Ar cation differs drastically from that of the corresponding neutral dimer, leading to very different equilibrium structures.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res67

219

Electronic spectroscopy at the level of rotational resolution suggests that the π-bound structure corresponds to the most stable isomer of neutral In–Ar in the S₀ state.²³

Type: Result | Advantage: None | Novelty: None | ConceptID: Res68

220

No evidence for a less stable H-bound In–Ar isomer has been reported so far.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con26

221

The π-bound structure is favored by the dispersion forces, which dominate the attractive part of the potential in S₀.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res69

222

In contrast, the EI-IR spectrum of In⁺–Ar demonstrates that the intermolecular H-bond is the preferred recognition site in the cation ground state (D₀), whereas the π-bond is less favorable: D₀(H) ≈ 670 ± 140 cm⁻¹ and D₀(π) = 537 ± 10 cm⁻¹.²⁶

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con27

223

Comparison with the related Ph⁺–Ar and An⁺–Ar dimers suggests that the isomerization barrier from π-bound to H-bound In⁺–Ar is of the order of V_b ≈ 100–200 cm⁻¹.^14,16

Type: Result | Advantage: None | Novelty: None | ConceptID: Res70

224

The preference for H-bonding in the In⁺–Ar cation arises from the additional induction forces between the charge distribution of In⁺ and the polarisability of Ar.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res71

225

The AIM (atoms-in-molecules) population analysis at the UMP2/6-311G(2pd,2df) level yields a large positive partial charge of q_H = 0.50 e on the acidic N–H proton of In⁺, which leads to substantial additional stabilization of the H-bound isomer on ionization.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res72

226

In contrast to the acidic N–H proton, the aromatic C–H protons carry only little positive charges (q_H = 0.11–0.15 e), resulting in the formation of relatively weak C–H–Ar hydrogen bonds.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

227

Similar to C₆H₇⁺–Ar,³⁸ this result supports the interpretation that the C–H–Ar bonds in In⁺–Ar are less favorable than π-bonding.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con28

228

The drastic ionization-induced change in the interaction potential and the equilibrium structure of In–Ar explains why the most stable H-bound In⁺–Ar geometry has completely escaped detection in all previous spectroscopic studies of this cation dimer.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con29

229

These include PIE,²⁷ MATI,²⁶ and ZEKE²⁵ measurements, which are all based on resonant photoionization of the neutral π-bound In–Ar dimer via the S₁ state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con29

230

For this REMPI process, the Franck–Condon factors for populating the H-bound global minimum of In⁺–Ar are very small.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac33

231

As a consequence, the ionization potentials determined by these photoionization methods should not be considered as the adiabatic ionization potential, as claimed by the authors in .^{refs. 25–27}

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con30

232

The ionization-induced change in the preferred recognition site in A⁽⁺⁾–Ar and also A⁽⁺⁾–CH₄ dimers from π-bonding to H-bonding has now been observed for a variety of aromatic molecules A⁽⁺⁾ featuring acidic functional YH_k groups (Y = O, N).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

233

It appears to be a quite general phenomenon and has so far been demonstrated for phenol,^12–14 halogenated phenols,¹⁵ aniline,¹⁶ indole, and recently also for naphthol.⁴³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

234

H-bonds to Ar have also been inferred as most stable binding motif for protonated aromatic molecules with acidic functional groups, such as protonated phenol,³⁴ protonated halogenated phenols,⁴⁴ and protonated naphthol.⁴³

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

235

In contrast, (protonated) aromatic cations without functional groups, such as (protonated) benzene, apparently prefer π-bonding with Ar and CH₄ in the cation ground state, because the C–H bonds are much less acidic and carry only small positive partial charges, so that dispersion (favoring π-bonds) overrides the induction forces (favoring H-bonds).^38,45,46

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

236

As the optimal In⁺–Ar interaction is for any orientation stronger than the Ar–Ar attraction (≈100 cm⁻¹), the cluster growth in In⁺–Ar_n is mainly governed by the In⁺–Ar dimer potential.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

237

Hence, the preferred solvation sequence in In⁺–Ar_n complexes begins with the formation of a H-bound In⁺–Ar dimer core, which is further solvated by (n − 1) π-bound ligands.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

238

This process is very similar to the cluster growth observed in Ph⁺–Ar_n and An⁺–Ar_n, which also starts with the solvation of the acidic protons of the OH and NH₂ groups by H-bound ligands, before further π-bound ligands are attached to the aromatic ring.^13,16

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con31

239

Interestingly, the less stable 2π isomer of In⁺–Ar₂ could be weakly observed, whereas the concentrations of the corresponding isomers of Ph⁺–Ar₂ and An⁺–Ar₂ were below the detection limit.^13,16

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

240

This observation may indicate that the difference in the binding energies of the H-bound and π-bound dimers, ΔD₀(H–π) = D₀(H) − D₀(π), decreases along the series Ph⁺–Ar > An⁺–Ar > In⁺–Ar.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con32

241

Similar to Ph⁺–N₂ and An⁺–N₂, the H-bound isomer corresponds to the global minimum on the In⁺–N₂ dimer potential.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res73

242

The anisotropy of the long-range charge–quadrupole and charge–induced dipole interactions aligns the N₂ ligand in such a way that the molecular axis points toward the positive charge, resulting in a nearly linear N–H⋯N–N configuration in H-bound In⁺–N₂.^{4,12,18,35,38,42,47}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac37

243

The EI-IR spectra clearly demonstrate that the π-bound isomer of In⁺–N₂ is substantially less stable than the H-bound one.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con33

244

Interestingly, the π-bound isomers of An⁺–N₂ and Ph⁺–N₂ were not detected in the corresponding EI-IR spectra, suggesting that π-bonding becomes more competitive in In⁺–N₂.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res74

245

Actually, N₂ complexes of (protonated) aromatic ions without acidic functional groups, such as C₆H₆⁺ or C₆H₇⁺, appear to prefer π-bonding with N₂ in the cation ground state.^38,45,46

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac38

246

According to the current interpretation of the EI-IR spectra, the preferred structure of the first solvation subshell of In⁺–(N₂)_n differs qualitatively from those of In⁺–Ar_n, Ph⁺–(N₂)_n¹³ and An⁺–(N₂)_n.¹⁸

Type: Result | Advantage: None | Novelty: None | ConceptID: Res75

247

In the latter three cluster systems, each acidic proton of the YH_k group is solvated by a single ligand and subsequently π-bound ligands are attached.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac39

248

In contrast, in In⁺–(N₂)_n the acidic N–H proton appears to be solvated by two H-bound ligands before π-bonding sets in.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res76

249

The reason for this different behaviour is unclear at the present stage.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con34

250

Reliable high-level quantum chemical calculations and/or higher-resolution spectroscopic data are required to definitively determine the structures of the first solvation shell of these aromatic ion–ligand complexes.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con35

251

Previous spectroscopic studies revealed that the magnitude of the complexation-induced red shift in the proton donor stretch vibration, Δν_X−H, of H-bound X–H⁺–L dimers is correlated with the difference in the proton affinities (PA) of the involved bases X and L.^{4,13,35,48–50}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac40

252

In general, the smaller PA(X) − PA(L), the stronger the intermolecular H–L bond and the larger Δν_X−H.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac40

253

Table 4 and Fig. 9 summarize the relative red shifts |Δν_X−H|/ν_X−H for a series of H-bound XH⁺–N₂ and XH⁺–Ar dimers, including XH⁺ = SiOH⁺, Ph⁺, In⁺ and An⁺.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

254

In general, the shifts are larger for N₂ compared to Ar, because PA(N₂) > PA(Ar) (494 > 369 kJ mol⁻¹).⁵¹

Type: Result | Advantage: None | Novelty: None | ConceptID: Res77

255

Similarly, for a fixed ligand L the shifts decrease in the order XH⁺ = SiOH⁺ > Ph⁺ > In⁺ > An⁺ because of increasing PA(X).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res78

256

To the best of our knowledge, no experimental value has been reported for the PA of the indolyl radical.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot7

257

Linear extrapolation from the known PA values of the anilino and phenoxy radicals (PA = 950 and 873 kJ mol⁻¹, respectively) and the measured frequency shifts yields an estimated value of PA ≈ 920 ± 30 kJ mol⁻¹ for indolyl.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res79

258

This example demonstrates that cluster ion IR spectroscopy may be used to probe thermochemical properties of transient radicals.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con36

259

In general, the attraction in weakly H-bound X–H⁺–L dimers is based on electrostatic and inductive forces.^4,38,52

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac41

260

For a series of X–H⁺–L dimers with L = Ar and N₂, the strength of the H-bond was shown to correlate with the partial charge located on the intermediate proton, q_H,³⁸ and the In⁺–L dimers confirm this trend.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res80

261

Thus, the dissociation energies of H-bound In⁺–L dimers are systematically lower than those of the corresponding Ph⁺–L dimers, because q_H in In⁺ (0.50 e) is smaller than in Ph⁺ (0.66 e).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con37

262

For example, D₀(H) = 6520 ± 50 and 4790 ± 10 cm⁻¹ for Ph⁺–H₂O and In⁺–H₂O, respectively.²⁶

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con37

263

This trend is also compatible with PA(indolyl) > PA(phenoxy).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con38

Concluding remarks

264

IR photodissociation spectra of In⁺–Ar_n (n ≤ 5) and In⁺–(N₂)_n (n ≤ 8) have been analysed in the vicinity of the N–H stretch vibration (ν₁) to characterize the preferred recognition sites of In⁺ interacting with hydrophobic ligands.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa4

265

Analysis of the size-dependent complexation-induced frequency shifts (Δν₁) and photofragmentation branching ratios provide valuable information about the microsolvation of In⁺ in a nonpolar environment.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con39

266

The IR spectra of all In⁺–L_n clusters investigated are dominated by the ν₁ fundamental of the most stable isomer of each cluster ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res81

267

The In⁺–Ar and In⁺–N₂ dimers have H-bound equilibrium structures (global minima), whereas the π-bound isomers are identified as less stable local minima.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res82

268

In general, the interaction strength in In⁺–N₂ is larger than that in the corresponding In⁺–Ar dimers, mainly because of the additional charge–quadrupole interaction in the former complexes.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res83

269

The Δν₁ shifts yield a first experimental estimate for the proton affinity of the indolyl radical as ∼920 ± 30 kJ mol⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res84

270

This example demonstrates that cluster ion IR spectroscopy may be used to probe thermochemical properties of bio-active transient radicals.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con40

271

The IR spectra of In⁺–Ar_n (n ≤ 5) indicate that the preferred microsolvation path for this cluster system begins with the formation of the H-bound In⁺–Ar dimer core, which is further solvated by (n − 1) π-bound ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

272

In contrast, the spectra of In⁺–(N₂)_n with n ≤ 8 are interpreted with cluster structures, in which a In⁺–(N₂)₂ trimer core with two H-bound N₂ ligands is generated and subsequently solvated by (n − 2) π-bound N₂ molecules.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

273

Unfortunately, neither Ar nor N₂ matrix isolation studies are available for In⁺,⁵³ preventing a detailed comparison of the In⁺–L_n cluster band shifts with the bulk limit (n → ∞).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con41

274

The In⁺–L_n clusters have been generated in an EI cluster ion source, which predominantly produces the most stable isomer of a given cluster ion.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res85

275

In the case of In⁺–Ar_n, the structures found differ qualitatively from those deduced from (REMPI) photoionization techniques, demonstrating that the EI ion source is more generally applicable than REMPI for the spectroscopic characterization of global minima of cation clusters.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con42

276

In general, the structures of the most stable In⁺–Ar_n clusters differ qualitatively from those of the corresponding neutral species, emphasizing the large impact of ionization on the intermolecular potential and the preferred recognition pattern of the interaction between aromatic molecules and neutral ligands.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con43

277

The ionization-induced change in the preferred recognition site in A⁽⁺⁾–Ar dimers from π-bonding to H-bonding has now been demonstrated for a variety of aromatic molecules A⁽⁺⁾ featuring acidic functional YH_k groups (Y = O, N) and appears to be a quite general phenomenon.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con44

278

This ionization-induced variation in the recognition pattern is of importance for biological aromatic molecules with acidic functional groups surrounded by a hydrophobic environment.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con45

Introduction

Experimental

Quantum chemical calculations

Experimental results and discussion

In+–L dimers

Larger In+–Ln clusters

Further discussion

Concluding remarks

In⁺–L dimers

Larger In⁺–L_n clusters