Kinetic and Mechanistic Study of the Reaction of Atomic Chlorine with Methyl...

Ayhens, Y. V., J. M. Nicovich, M. L. McKee, and P. Wine (1997), Kinetic and Mechanistic Study of the Reaction of Atomic Chlorine with Methyl Iodide over the Temperature Range 218-694 K, J. Phys. Chem. A, 101, 9382-9390.

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.

Research Program: 
Upper Atmosphere Research Program (UARP)