Relative vibrational overtone intensity of cis–cis and trans–perp peroxynitrous acid

The vibrational overtone spectrum of HOONO is examined in the region of the 2 nu(OH) and 3 nu(OH) bands using action spectroscopy in conjunction with ab initio intensity calculations. The present measurements indicate that the oscillator strength associated with the higher energy trans-perp conformer of HOONO is stronger relative to the lower energy cis-cis conformer for both these vibrational overtone levels. Ab initio intensity calculations carried out at the QCISD level of theory suggest that this disparity in oscillator strength apparently arises from differences in the second derivative of the transition dipole moment function of the two isomers. The calculations indicate that the oscillator strength for the trans-perp isomer is approximately 5.4 times larger than that of the cis-cis isomer for the 2 nu(OH) band and approximately 2 times larger for 3 nu(OH) band. The band positions and intensities predicted by the calculations are used to aid in the assignment of features in the experimental action spectra associated with the OH stretching overtones of HOONO. The observed relative intensities in the experimental action spectra when normalized to the calculated oscillator strengths appears to suggest that the concentration of the higher energy trans-perp isomer is comparable to the concentration of the cis-cis isomer in these room temperature experiments.