DISCHARGE FLOW-PHOTOIONIZATION MASS-SPECTROMETRIC STUDY OF HNO - PHOTOIONIZATION EFFICIENCY SPECTRUM AND IONIZATION-ENERGY AND PROTON AFFINITY OF NO

Photoionization efficiency (PIE) spectra of HNO were measured over the wavelength range lambda = 110-125 nm and in the ionization threshold region, lambda = 118-124 nm, using a discharge flow-photoionization ma ss spectometer (DF-PIMS) apparatus coupled to a synchrotron radiation source. HNO was generated in situ by the following reaction sequence: N + NO --> N-2 + O; O + C2H4 --> CH3 + HCO; HCO + NO --> HNO + CO. The PIE spectrum displays steplike behavior near threshold and an HN-O st retching frequency in the cation of 1972 +/- 67 cm(-1). A value of 10. 18(4) +/- 0.01(2) eV for the adiabatic ionization energy (IE) of HNO w as obtained from photoionization thresholds, which correspond to the H NO+(X(2)A') <-- HNO(X(1)A') transition. This result is the first PIMS determination of IE(HNO). Also, an ab initio molecular orbital calcula tion (QCISD(T)/aug-cc-pVQZ) was performed that yields a value for IE(H NO) of 10.186 +/- 0.050 eV, There is good agreement between the experi mental and the theoretical values for IE(HNO) reported here and that f rom a recent photoelectron spectroscopy study. The present experimenta l value for IE(HNO) was employed along with other, known thermodynamic quantities to obtain values for the heat of formation of the HNO cati on and the absolute proton affinity of NO: Delta(f)H degrees(298)(HNO) = 1089.73 +/- 1.18 kJ mol(-1) (Delta(f)H degrees(0)(HNO+) = 1092.65 +/- 1.18 kJ mol(-1)); PA(298)(NO) = 531.55 +/- 1.26 kJ mol(-1) (PA(0)( NO) = 526.12 +/- 1.26 kJ mol(-1)).