Discharge Flow-Photoionization Mass Spectrometric Study of HNO: Photoionization Efficiency Spectrum and Ionization Energy and Proton Affinity of NO

Abstract

Photoionization efficiency (PIE) spectra of HNO were measured over the wavelength range λ = 110−125 nm and in the ionization threshold region, λ = 118−124 nm, using a discharge flow-photoionization mass spectrometer (DF-PIMS) apparatus coupled to a synchrotron radiation source. HNO was generated in situ by the following reaction sequence: N + NO → N2 + O; O + C2H4 → CH3 + HCO; HCO + NO → HNO + CO. The PIE spectrum displays steplike behavior near threshold and an HN−O stretching frequency in the cation of 1972 ± 67 cm-1. A value of 10.184 ± 0.012 eV for the adiabatic ionization energy (IE) of HNO was obtained from photoionization thresholds, which correspond to the HNO+(X2A‘) ← HNO(X1A‘) transition. This result is the first PIMS determination of IE(HNO). Also, an ab initio molecular orbital calculation (QCISD(T)/aug-cc-pVQZ) was performed that yields a value for IE(HNO) of 10.186 ± 0.050 eV. There is good agreement between the experimental and the theoretical values for IE(HNO) reported here and that from a recent photoelectron spectroscopy study. The present experimental value for IE(HNO) was employed along with other, known thermodynamic quantities to obtain values for the heat of formation of the HNO cation and the absolute proton affinity of NO: ΔfH°298(HNO+) = 1089.73 ± 1.18 kJ mol-1 (ΔfH°0(HNO+) = 1092.65 ± 1.18 kJ mol-1); PA298(NO) = 531.55 ± 1.26 kJ mol-1 (PA0(NO) = 526.12 ± 1.26 kJ mol-1).