Theoretical study of the reaction of hydrogen with nitric acid: ab initio MO and TST/RRKM calculations

Abstract

The kinetics and mechanism of the H + HNO 3 reaction have been elucidated with ab initio molecular orbital and statistical theory calculations. Our room temperature reaction rate results accord well with available experimental data. The reaction is dominated by an indirect metathetical process taking place via vibrationally excited dihydroxyl nitroxide, ON(OH) 2, producing OH + cis-HONO. The excited ON(OH) 2 also undergoes molecular elimination, yielding H 2O + NO 2 as a minor competing reaction. The direct H abstraction reaction forming H 2 + NO 3 was found to be the least important one. At atmospheric pressure, we recommend the following expressions for the three rate constants, in units of cm 3/molecule s, from the 300–3000 K temperature range for H + HNO 3 collision yielding the products H 2 + NO 3 by direct mechanism k a = (9.24 × 10 −16) T 1.53e −8253/ T based on CTST calculations, OH + cis-HONO by indirect mechanism k b = (6.35 × 10 −19) T 2.30e −1.53/ T ), and H 2O + NO 2 by indirect mechanism k c = (1.01 × 10 −22) T 3.29e −3126/ T , the latter two are based on Arrhenius fits to the solution of the master equation which includes RRKM microscopic rate constants and tunneling corrections.