Protonation of Chlorine Nitrate and Nitric Acid: Identification of Isomers by Vibrational Spectroscopy

Abstract

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 (HXNO_3^+, X = H, D, ^(35)Cl, ^(37)Cl). Two protonated isomers of both ClONO_2 and HNO_3 were identified from the vibrational spectra. The lowest energy isomer was the ion−molecule complex NO_2^+(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 NO_2^+ + HO_X 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.