During the study of the HOOH–HOO complex, we noted that the positions of some of its bands were sensitive to the concentration of dopants in the matrix, shifting several wavenumbers between different experiments.

Bands are assigned to the hydrogen peroxide peroxy radical complex, if they appear only when hydrogen peroxide and peroxy radicals are simultaneously present in the matrix, if they have constant intensity ratios directly after deposition, independent of the concentrations of hydrogen peroxide and peroxy radicals, and if they decrease at the same rate when the matrix is irradiated.

It should be noted that the strongest bands which we assign to a hydrogen peroxide peroxy radical complex are too strong to be due to 1∶2 (2∶1) complex.

When hydrogen peroxide is added to the matrix, they grow strongly as expected if they are due to a hydrogen peroxide peroxy radical complex (Fig. 1).

It therefore seems clear that the 3206 cm⁻¹ band is due to the OH stretch of HOO complexed to hydrogen peroxide.

The complex shift, −207 cm⁻¹, is slightly larger than the corresponding shift of the peroxy radical water complex, −177 cm⁻¹.

Experiments with partially deuterated hydrogen peroxide show that the 3432 cm⁻¹ band is due to HOOH.

Also for the HOOH (DOOD) band the complex shift is relatively large, −156 cm⁻¹ (−110.0 cm⁻¹), giving clear evidence for a hydrogen bond from hydrogen peroxide to the peroxy radical.

Deuteration experiments show that the 1428 cm⁻¹ band is due to HOOH and the 1487 cm⁻¹, 1497 cm⁻¹ doublet is due to HOO.

Both bands are strongly blue shifted from the HOO bends of hydrogen peroxide and the peroxy radical as expected for a cyclic, hydrogen bonded complex.

The OO stretch of the complexed peroxy radical is observed at 1121 cm⁻¹, very close to the corresponding band of the peroxy radical water complex (1120 cm⁻¹, ^{ref. 2}).

A weak band at 868 cm⁻¹ is assigned to the OO stretch of complexed HOOH.

The corresponding bands of HOOH–DOO have been observed in experiments where D₂ was passed through the microwave discharge instead of H₂.

Experiments with partially deuterated hydrogen peroxide have made it possible to observe some of the intramolecular fundamentals of the D₂O₂ and HDO₂ complexes with HOO (Table 1) and DOO (Table 2).

In addition to the intramolecular fundamental bands, we observe a number of relatively weak bands which fulfill our criteria for assignment to the peroxy radical hydrogen peroxide complex.

The assignment of the mid-infrared spectrum is collected in Tables 1, 2 and 3.

We have observed a number of intermolecular bands of the hydrogen peroxide complex (Fig. 2).

The two bands with highest frequencies are observed in the region where the bolometer spectra and the MCT spectra overlap.

The results in this region are in very good agreement.

Compared to mid-infrared bands of matrix isolated molecules, bands in the far infrared tend to be weaker and broader.

One is closer to the phonon band in the far infrared, this makes relaxation a low order process and therefore decreases lifetimes of excited states.

Both hydrogen peroxide monomer and dimer have clear and strong far infrared bands, which partially obscure the bands of the peroxy radical hydrogen peroxide complex.

The assignment is significantly aided by the fact that the hydrogen peroxide peroxy radical complex decomposes very rapidly when irradiated with 266 nm radiation, significantly faster than the hydrogen peroxide dimer.

The hydrogen peroxide monomer is hardly touched during the time it takes to eliminate the peroxy radical complex.

The irradiation has the additional advantage that the base line is almost unchanged by the irradiation.

The assignment of the far infrared spectrum is collected in Table 4.

This conclusion is supported by the observation of two strongly blue shifted HOO bends, one from each component.

For the HOO bends of HOOH, the shifts are larger in the HOOH–DOOD dimer than in the peroxy radical complex.

For both bends, the complex shifts are smaller for the peroxy radical complex than for the mixed dimer.

The two HOO bends are strongly coupled, in HOOD the HOO bend is found at 1343 cm⁻¹, approximately midway between the two HOO bends of HOOH (1385 cm⁻¹ and 1271 cm⁻¹) and the DOO bend is found at 981 cm⁻¹ close to the average of the two DOOD bends.

The lowest band, at 227 cm⁻¹, is close to a band at 219 cm⁻¹ of the hydrogen peroxide dimer, which we believe is due to the libration of HOOH in the dimer.¹⁴

We suggest that the 227 cm⁻¹ band of HOOH–HOO should be assigned to the libration of HOOH around its OO axis.

The precise position depends on the isotopic composition of the complex partner.

We therefore assign the 557 cm⁻¹ band to the libration of complexed HOO around its OO bond.

This assignment implies that the 277 cm⁻¹ band and the 477 cm⁻¹ band are due to the HOOH torsion.

The bands at 485 cm⁻¹ and 272 cm⁻¹ are assigned to the torsion of HOOH and the 220 cm⁻¹ band to the libration of HOOH, the libration of DOO is observed at 415 cm⁻¹.

In these experiments we observe a number of far infrared bands whose photolysis rates clearly shows that they are due to hydrogen peroxide peroxy radical complexes.

We found it difficult to control the H/D ratio sufficiently well to make it possible to avoid the presence of significant concentrations of HOOD in these experiments.

This makes the assignment very difficult, since the mid-infrared spectra indicate that there are two different isomers of the HOOD peroxy radical complex.

The assignment is collected in Table 4.

The B3LYP calculations give very large shifts for the OH stretches of hydrogen peroxide and the peroxy radical for the six-membered ring structure, 1.4 and 1.8 times larger than the observed shifts respectively.

For the five membered ring structure the OH shift of the hydrogen peroxide is 0.50 of the observed shift and the OH shift of the peroxy radical is 1.5 times the observed shift.

The calculated OO shifts of the peroxy radical are approximately 10 times larger than the observed shift for both structures.

The complex shifts of the QCISD calculations are in reasonable agreement with the observations, the OH shifts of the OH bonds involved in the hydrogen bonds are somewhat smaller the the observed shifts but the general pattern of the shifts agrees well with the observations.

It is more difficult to choose between the two structures.

The largest difference in the calculated shifts is obtained for the HOOH torsion, which is calculated to be blue shifted in the five membered ring complex and red shifted in the six-membered ring complex.

Unfortunately it is impossible to compare the calculated torsions with the experimental torsion since the low trans barrier of the complex allows rapid tunneling in the torsionally excited state.

The calculated bound OH shifts of the six-membered ring complex are both significantly closer to the observed shifts.

Also most other calculated shifts of the six-membered ring are closer to the experimentally observed shifts that those of the five membered ring (Table 5).

It is difficult to compare the calculated intermolecular vibrations with the experimentally observed far infrared bands, since no description of the calculated bands is given in the paper and the calculations are done only for the HOOH–HOO isotopologue.

Both calculations and experiment agree that this is an intense band.

We have difficulties in understanding the nature of the second largest calculated intermolecular fundamental at 544.6 cm⁻¹.

The hydrogen bond from the peroxy radical to hydrogen peroxide is calculated to be stronger than the hydrogen bond from hydrogen peroxide to the peroxy radical.

The hydrogen peroxide libration is therefore expected to be less than 0.7 of the peroxy radical libration, the calculated frequency is 100 cm⁻¹ larger than one expects from this argument.

The hydrogen peroxide peroxy radical complex is clearly more strongly bound than the hydrogen peroxide dimer.

The complex shift of the OH stretch of the peroxy radical is −207 cm⁻¹ suggesting that the hydrogen bond from the peroxy radical to hydrogen peroxide is stronger than the hydrogen bond from hydrogen peroxide to the radical.

The shifts of several wavenumbers of some complex bands between different experiments is very surprising (Fig. 4).

Even more surprising is that the hydrogen peroxide dimer bands, which have very stable positions in experiments where only hydrogen peroxide is present in the matrix⁷ show shifts in these experiments, these shifts correlate with the shifts of the hydrogen peroxide peroxy radical complex bands.

However, the C₂ symmetric isomer, which has the two non hydrogen bonded hydrogens on the same side of the plane of the heavy atoms has a relatively large dipole moment and is expected from ab initio SCF calculations to be approximately 50 cm⁻¹ higher in energy than the C_i isomer.