Ab initio self-consistent field study of the vibrational spectra for NO₃ geometric isomers

Ab initio self-consistent field results at the UHF/6-31G* and UHF-DZP levels for harmonic vibrational frequencies of symmetric NO₃ with C_2v, C_s, and D_3h symmetry; cis and trans forms of OONO are reported. At both levels of calculations (6-31G* and DZP), the theoretical vibrational frequencies for C_2v sym-NO₃ are in agreement with the recent experimental results. Our calculations for cis and trans isomers of OONO show large deviations from the only observed vibrational frequency (1838 cm^-1) for trans OONO. The trans isomer of OONO is predicted to be more stable than the cis isomer by approximately 2 kcal/mol and in each case, the ground state for OONO is predicted to be ²A″.