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 (5500 Torr) in nitrogen buffer gas. At T g 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 e T e 694 K: k1a ) 5.44 × 10-11 exp(-1250/T) cm3 molecule-1 s-1. At T e 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, k1 ) k1a + k1b ) 3.0 × 10-11 cm3 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: ∆H°298 ) - 53.6 ( 3.4 kJ mol-1, ∆H°0 ) - 52.2 ( 3.5 kJ mol-1, and ∆S°298 ) - 88 ( 11 J mol-1 K-1. In conjunction with the well-known heats of formation of Cl and CH3I, the above ∆H values lead to the following heats of formation for CH3ICl at 298 and 0 K: ∆H°f,298 ) 82.3 ( 3.5 kJ mol-1 and ∆Hf,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-Cl bond angle is 85.2° 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.