The pulsed laser photolysis-pulsed laser induced fluorescence technique has been employed to determine absolute rate coefficients for the reaction OH + CH3CN (1) and its isotopic variants, OH + CD3CN (2), OD + CH3CN (3), and OD + CD3CN (4). Reactions 1 and 2 were studied as a function of pressure and temperature in N2, N2/O2, and He buffer gases. In the absence of O2 all four reactions displayed well-behaved kinetics with exponential OH decays and pseudo-first-order rate constants which were proportional to substrate concentration. Data obtained in N2 over the range 50-700 Torr at 298 K are consistent with k1 showing a small pressure dependence. The Arrhenius expression obtained by averaging data at all pressures is k1(T) = (1.1-0.3+0.5) × 10-12 exp[(-1130 ± 90)/T] cm3 molecule-1 s-1. The kinetics of reaction 2 are found to be pressure dependent with k2 (298 K) increasing from (1.21 ± 0.12) × 10-14 to (2.16 ± 0.11) × 10-14 cm3 molecule-1 s-1 over the pressure range 50-700 Torr of N2 at 298 K. Data at pressures >600 Torr give k2(T) = (9.4-5.0+13.4) × 10-13 exp[(-1180 ± 250)/T] cm3 molecule-1 s-1. The rates of reactions 3 and 4 are found to be independent of pressure over the range 50-700 Torr of N2 with 298 K rate coefficients given by k3 = (3.18 ± 0.40) × 10-14 cm3 molecule-1 s-1 and k4 = (2.25 ± 0.28) × 10-14 cm3 molecule-1 s-1. In the presence of O2 each reaction shows complex (non-pseudo-first-order) kinetic behavior and/or an apparent decrease in the observed rate constant with increasing [O2], indicating the presence of significant OH or OD regeneration. Observation of regeneration of OH in (2) and OD in (3) is indicative of a reaction channel which proceeds via addition followed by reaction of the adduct, or one of its decomposition products, with O2. The observed OH and OD decay profiles have been modeled by using a simple mechanistic scheme to extract kinetic information about the adduct reactions with O2 and branching ratios for OH regeneration. A plausible mechanism for OH regeneration in (2) involves OH addition to the nitrogen atom followed by O2 addition to the cyano carbon atom, isomerization, and decomposition to D2CO + DOCN + OH. Our results suggest that the OH + CH3CN reaction occurs via a complex mechanism involving both bimolecular and termolecular pathways, analogous to the mechanisms for the important atmospheric reactions of OH with CO and HNO3.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry