The Electronic Structure and Vibrational Spectrum of trans-HNOO^†

This paper reports the theoretical results of a thorough, state-of-the-art, coupled-cluster, renormalized coupled-cluster, and vibrational study on the molecule imine peroxide, HNOO, in its trans conformation. This molecule is isoelectronic with ozone and presents many of the same difficulties for theory as ozone. We report both the theoretical geometry and the vibrational frequencies, including anharmonic corrections to the computed harmonic vibrational frequencies obtained by calculating the quartic force field at the high levels of coupled cluster theory, including CCSD(T) and its renormalized and completely renormalized extensions and methods including the combined effect of triply and quadruply excited clusters [CCSD(TQ_f) and CCSDT-3(Q_f)]. The motivation behind our study was the disagreement between two previous reports that appeared in the literature on HNOO, both reporting theoretical (harmonic) and experimental (matrix isolation) vibrational spectra of HNOO. Our new theoretical results and our analysis of the previous two papers strongly suggest that the correct assignment of vibrational spectra is that of Laursen, Grace, DeKock, and Spronk (J. Am. Chem. Soc. 1998, 120, 12583−12594). We also compare the electronic structure of HNOO with the isoelectronic molecules HONO and O₃. The NO and OO bond lengths are practically identical in HNOO, in agreement with the identical OO bond lengths (by symmetry) in ozone. Correspondingly, the NO and OO stretching frequencies of trans-HNOO are in close proximity to each other, as are the symmetric and antisymmetric OO stretching frequencies in O₃. This is in contrast to the electronic structure of HONO, which has a large difference between the two NO bond lengths, and a correspondingly large difference between the two NO vibrational frequencies. These results are readily understood in terms of simple Lewis electron dot structures.