Kinetic and Mechanistic study of the Reactions of O(1D2) with HCN and CH3CN

Abstract

A laser flash photolysis-resonance fluorescence (LFP-RF) technique is employed to investigate the kinetics and mechanism of the reactions of O((1)D(2)) with HCN [reaction (1)] and CH(3)CN [reaction (2)] as a function of temperature over the range 193-430 K. The experiments involve time-resolved RF detection of O((3)P(J)) or H((2)S(1/2)) following LFP of O(3)/X/He mixtures (X=HCN or CH(3)CN), some of which also contain N(2), H(2), and/or NO(2). Measured rate coefficients for total removal of O((1)D(2)) by HCN and CH(3)CN are well-described by the following Arrhenius expressions (units are 10(-10) cm(3) molecule(-1) s(-1)): k(1)(T)=1.08exp(+105/T) and k(2)(T)=2.54exp(-24/T). Temperature-dependent product yields of O((3)P(J)), k(1a)/k(1) and k(2a)/k(2) are well-described by the following Arrhenius-type expressions: k(1a)/k(1)=0.150exp(+200/T) and k(2a)/k(2)=0.0269 exp(+137/T). The H((2)S(1/2)) yield from reaction (2) is found to be 0.16±0.03 independent of temperature (200-423 K). Large 298 K yields of H((2)S(1/2)), 0.68±0.12 produced per O((1)D(2)) destroyed by HCN, are observed for reaction (1). However, observed kinetics suggest that only about half of detected H((2)S(1/2)) is generated as a primary product of the O((1)D(2))+HCN reaction, with the remainder generated via a fast secondary reaction. The implications of the reported kinetic and mechanistic results for understanding the atmospheric chemistry of HCN and CH(3)CN are discussed.