Vibrationally resolved dynamics of the reaction of Cl atoms with 2,3-dimethylbut-2-ene in chlorinated solvents

Broadband transient infra-red absorption measurements are reported that contrast the dynamics of reaction of Cl atoms with 2,3-dimethylbut-2-ene and n-pentane in chlorinated solvents. In both cases, H-atom transfer produces HCl and a hydrocarbon radical, but the energy release in the former reaction is greater because of the formation of a resonance-stabilized allylic radical. The reaction of Cl atoms with n-pentane in solution in CH2Cl2 forms HCl exclusively in its lowest vibrational level (v = 0), and our measured rate coefficient agrees with a prior report by Sheps et al., J. Phys. Chem. A, 2006, 110, 3087. The time-dependence of the growth of intensity in the HCl fundamental absorption band shows two domains of reaction: a prompt rise is associated with reaction of the photolytically generated Cl atoms with n-pentane molecules lying within the first solvent shell, whereas a slower further rise is attributed to reaction following diffusion through the solution. For the reaction of Cl atoms with 2,3-dimethylbut-2-ene, these two domains of reaction are also observed, and fitting to a kinetic model incorporating these components gives bimolecular rate coefficients for formation of HCl of (1.7 +/- 1.4) x 10(10) M-1 s(-1) in CDCl3 and (3.4 +/- 1.2) x 10(10) M-1 s(-1) in CCl4. However, an additional transient absorption is observed similar to 115 cm(-1) lower in wavenumber than the fundamental HCl absorption band and is assigned to the v = 2