Experimental and theoretical study of the reaction of HO− with NO

Abstract

Hydroxide ion (HO−) reacts with nitric oxide by slow reactive electron detachment with a rate coefficient ∼4×10−12 cm3 s−1 at 298 K. The detachment process is presumably associative detachment forming nitrous acid and an electron. Observations, data analysis, and alternative explanations for these observations are discussed. The associative detachment reaction was also investigated theoretically through calculations of the geometries, relative energies, and normal-mode vibrational frequencies of the relevant species HO−, HO, NO, cis- and trans-HONO, and cis- and trans-HONO−. These calculations indicate that in the ion HONO−, the cis conformer is more stable, while in the neutral HONO, the trans conformer is more stable. The HO–NO bond in HONO, which is formed in this reaction, is much stronger than the HO−–NO bond in HONO− with an energy of 198.7±1.8 kJ mol−1 for cis-HONO [J. Phys. Chem. Ref. Data 14, 1 (1985)] and 52.2±5 kJ mol−1 for cis-HONO− at 0 K. HONO− is bound with respect to HONO. The adiabatic electron detachment energy resulting from detachment from cis-HONO− forming the same conformer of the neutral molecule cis-HONO is 0.29±0.05 eV. The HO–NO equilibrium bond distance in HONO− is considerably longer than that in HONO, with values of 1.750 and 1.640 Å for trans- and cis-HONO−, respectively, and 1.429 and 1.392 Å for trans- and cis-HONO, respectively. These geometric and energetic characteristics of HONO− and HONO are combined with calculations of relative energies of these species at nonequilibrium/distorted HO–NO bond lengths to give a qualitative picture of the potential energy curves for these species along the reaction coordinate. While no significant energy barrier to autodetachment of HONO− is present, the Franck–Condon wave function overlap for autodetachment is small and is likely the reason for the observed inefficiency. The maximum calculated rate constant for associative detachment is 4×10−12 cm3 s−1, in good agreement with the observed value.