Reaction dynamics of Cl+CH3SH:: Rotational and vibrational distributions of HCl probed with time-resolved Fourier-transform spectroscopy

Rotationally resolved infrared emission spectra of HCl(v=1-3) in the reaction of Cl+CH3SH, initiated with radiation from a laser at 308 nm, are detected with a step-scan Fourier-transform spectrometer. Observed rotational temperature of HCl(v=1-3) decreases with duration of reaction due to collisional quenching; a short extrapolation to time zero based on data in the range 0.25-4.25 mus yields a nascent rotational temperature of 1150+/-80 K. The rotational energy averaged for HCl(v=1-3) is 8.2+/-0.9 kJ mol-1, yielding a fraction of available energy going into rotation of HCl, =0.10+/-0.01, nearly identical to that of the reaction Cl+H2S. Observed temporal profiles of the vibrational population of HCl(v=1-3) are fitted with a kinetic model of formation and quenching of HCl(v=1-3) to yield a branching ratio (68+/-5):(25+/-4):(7+/-1) for formation of HCl(v=1):(v=2):(v=3) from the title reaction and its thermal rate coefficient k(2a)=(2.9+/-0.7)x10(-10) cm(3) molecule(-1) s(-1). Considering possible estimates of the vibrational population of HCl(v=0) based on various surprisal analyses, we report an average vibrational energy 36+/-6 kJ mol-1 for HCl. The fraction of available energy going into vibration of HCl is =0.45+/-0.08, significantly greater than a value =0.33+/-0.06 determined previously for Cl+H2S. Reaction dynamics of Cl+H2S and Cl+CH3SH are compared; the adduct CH3S(Cl)H is likely more transitory than the adduct H2SCl. (C) 2004 American Institute of Physics.