Infrared spectroscopy of ClONO2 and BrONO2 investigated by means of anharmonic force fields

The structures of ClONO2 and BrONO2 are optimized at the B3LYP/6-311++G(3df) level. Nine-dimensional models for the two molecules are constructed with mode couplings described through cubic and quartic anharmonic force fields, which are calculated by employing the finite difference procedure. The infrared spectra are computed using a multi-configuration time-dependent Hartree representation of the wave function. Further, the interaction with ultrashort infrared laser pulses was investigated for the purpose of assigning certain spectral bands and studying intramolecular couplings. The changes of spectral properties and mode interactions that arise upon substitution of the chlorine by the bromine atom are addressed.