The puzzling infrared spectra of the nitric oxide dimer radical cation: a systematic application of Brueckner methods

Since symmetry breaking occurs in the ONNO+ cation with the self-consistent field (SCF) method and inverse symmetry breaking occurs when density functional theory (DFT) methods are used, Brueckner double excitation coupled cluster methods (BD) and BD plus perturbative triple-excitation contributions [BD(T)]have been used to study the geometries and vibrational frequencies for the trans and cis structures of the ONNO+ cation. Double-zeta plus polarization (DZP) basis sets and triple-zeta plus double polarization with f functions (TZ2Pf) basis sets were utilized. The ground state of the trans-ONNOcation is of 2Ag symmetry, which has a slightly (0.36 kcalmol-1) lower energy than the cis conformer (2A1). The controversial vibrational frequency corresponding to the asymmetric N-O stretching mode for both trans and cis structures is predicted as about 1600cm-1. This value is discussed in the context of the many (sometimes variant) experimental assignments.