Ring structure of the NO dimer radical cation: A possible new assignment of the mysterious IR absorption at 1424 cm−1

Abstract

The complete assignment of IR absorptions for the nitric oxide dimer radical cation has been a difficult task for some time. Although the 1619 cm−1 band was recently assigned to the antisymmetric N–O stretch mode for the trans and cis ONNO+ structures, the 1424 cm−1 band has remained a mystery. The ring or rectangular structure of the (NO)2+ cation was examined in this research with density functional theory (DFT) and high-level ab initio methods in the prospect that it might be an energetically low-lying isomer. In conjunction with the above methods, two sets of basis functions were utilized. One is double-ζ plus polarization, and another is triple-ζ plus double polarization with f functions. The ground state of the ring (NO)2+ cation is of Bu2 symmetry. The antisymmetric N–O stretch vibrational frequency is predicted to be nonphysically large with the self-consistent field method due to the symmetry breaking problem. This fundamental is predicted in the ∼1800 cm−1 region based upon DFT methods, but the result is also doubtful because (NO)2+ exhibits the inverse symmetry breaking problem. Since these problems may impair the theoretical vibrational frequencies, higher theoretical levels, namely Brueckner coupled-cluster methods, were ultimately applied, and the harmonic vibrational frequency of this challenging mode was eventually predicted to be about 1400 cm−1. This ring structure lies only ∼5 kcal/mol above the global minimum, so that it might be observable in the laboratory. Moreover, the ring structure is predicted to lie ∼10 kcal/mol below separated NO+NO+. Since no other low-lying isomers were found, it is plausible to assign the 1424 cm−1 band to the ring structure.