Kinetic and mechanistic study of the reaction of atomic chlorine with methyl iodide over the temperature range 218-694 K

A laser flash photolysis-resonance fluorescence technique has been employed to study the kinetics of the reaction of chlorine atoms with methyl iodide as a function of temperature (218-694 K) and pressure (5-500 Torr) in nitrogen buffer gas, At T greater than or equal to 364 K, measured rate coefficients are pressure independent and a significant H/D kinetic isotope effect is observed, suggesting that hydrogen transfer is the dominant reaction pathway; the following Arrhenius expression adequately describes all kinetic data at 364 K less than or equal to T less than or equal to 694 K: k(1a) = 5.44 x 10(-11) exp(-1250/T) cm(3) molecule(-1) s(-1), At T less than or equal to 250 K, measured rate coefficients are pressure dependent and much faster than computed from the above Arrhenius expression for the H-transfer pathway, suggesting that the dominant reaction pathway at low temperature is formation of a stable adduct; at T = 218 K and P = 500 Torr, For example, k(1) = k(1a) + k(1b) = 3.0 x 10(-11) cm(3) molecule(-1) s(-1), with 99.4% of the reactivity being attributable to the addition channel 1b, At temperatures in the range 263-309 K, reversible addition is observed, thus allowing equilibrium constants for CH3ICl formation and dissociation to be determined, Second-and third-law analyses of the equilibrium data lead to the following thermochemical parameters for the association reaction 1b: Delta H(298)degrees = -53.6 +/- 3.4 kJ mol(-1), Delta H-0 degrees = -52.2 +/- 3.5 kJ mol(-1), and Delta S(298)degrees = -88 +/- 11 J mol(-1) K-1. In conjunction with the well-known heats of formation of Q and CH3I, the above Delta H values lend to the following heats of formation for CH3ICl at 298 and 0 K: Delta H(f,298)degrees = 82.3 +/- 3.5 kJ mol(-1) and Delta H-f,H-0 = 91.6 +/- 3.6 kJ mol(-1). Ab initio calculations using density functional theory (DFT) and G2 theory reproduce the experimental bond strength reasonably well. The DFT calculations predict a structure (used in the third-law analysis) where the C-I-CI bond angle is 85.2 degrees and the methyl group adopts a staggered orientation with a pronounced tilt toward chlorine. Bonding in CH3ICl is discussed as are the implications of the new kinetic data for atmospheric chemistry.