Electronic states of HNO+ and HON+ Original Research Article

The three-dimensional CASSCF-MRCI potential energy functions have been calculated for the X 2A′ and A 2A″ electronic states of the HNO+ and HON+ isomers. In both cases, these two states form a bent-linear Renner–Teller pair in 2Π symmetry. The barriers to linearity of the X state are calculated to be 6846 cm−1 for HNO+ and 8050 cm−1 for HON+ (RCCSD(T)). Both isomers possess a conical intersection arising from the crossing between the first excited 2Σ+ state and the X 2Π state along the collinear (NO+(X 1Σ+)+H(2S)) dissociation path. The barrier for isomerisation has been calculated to be 19457 cm−1 (MRCI) and the HNO+ isomer is more stable than HON+ by 5819 cm−1. The rovibronic levels in the HNO+ Renner–Teller system have been calculated by a variational method taking into account the coupling of the rovibrational, electronic and electron spin angular momenta. Since for HON+ the barrier to dissociation of the X 2A′ ground state is located below the barrier to linearity, only low lying vibrational levels have been calculated for this isomer.