Protonation of chlorine nitrate and nitric acid: Identification of isomers by vibrational spectroscopy

Predissociation spectra in the 2.6-3.3 μm region were observed for protonated chlorine nitrate and protonated nitric acid as well as some of their isotopomers (HXNO₃⁺, X = H, D, ³⁵Cl, ³⁷Cl). Two protonated isomers of both ClONO₂ and HNO₃ were identified from the vibrational spectra. The lowest energy isomer was the ion-molecule complex NO₂⁺(HOX) formed by protonation of the XO group. The second isomer was the metastable species (HO)(XO)NO⁺ formed by protonation of a terminal oxygen; this isomer was generated only under hotter ionizing conditions. The vibrational band centers of these isomers agreed well with ab initio predictions. Vibrational excitation of the HXNO₃⁺ species studied here led solely to NO₂⁺ + HOX products. Predissociation of the covalently bound metastable isomers (HO)(XO)NO⁺ to these products required an IR-induced rearrangement involving simultaneous 1,3 hydrogen shift and charge transfer. The results presented here were consistent with predictions of ab initio calculations and previous mass spectrometric and kinetic studies.