Low temperature kinetics, crossed beam dynamics and theoretical studies of the reaction S(1D) + CH4 and low temperature kinetics of S(1D) + C2H2

Abstract

The reaction between sulfur atoms in the first electronically excited state, S(¹D), and methane (CH₄), has been investigated in a complementary fashion in (a) crossed-beam dynamics experiments with mass spectrometric detection and time-of-flight (TOF) analysis at two collision energies (30.4 and 33.6 kJ mol⁻¹), (b) low temperature kinetics experiments ranging from 298 K down to 23 K, and (c) electronic structure calculations of stationary points and product energetics on the CH₄S singlet potential energy surface. The rate coefficients for total loss of S(¹D) are found to be very large (ca. 2 × 10⁻¹⁰ cm³molec⁻¹ s⁻¹) down to very low temperatures indicating that the overall reaction is barrier-less. Similar measurements are also performed for S(¹D) + C₂H₂, and also for this system the rate coefficients are found to be very large (ca. 3 × 10⁻¹⁰ cm³molec⁻¹ s⁻¹) down to very low temperatures. From laboratory angular and TOF distributions at different product masses for the reaction S(¹D) + CH₄, it is found that the only open reaction channel at the investigated collision energies is that leading to SH + CH₃. The product angular, T(θ), and translational energy, P(E′_T), distributions in the center-of-mass frame are derived. The reaction dynamics are discussed in terms of two different micromechanisms: a dominant long-lived complex mechanism at small and intermediate impact parameters with a strongly polarized T(θ), and a direct pickup-type (stripping) mechanism occurring at large impact parameters with a strongly forward peaked T(θ). Interpretation of the experimental results on the S(¹D) + CH₄ reaction kinetics and dynamics is assisted by high-level theoretical calculations on the CH₄S singlet potential energy surface. The dynamics of the SH + CH₃ forming channel are compared with those of the corresponding channel (leading to OH + CH₃) in the related O(¹D) + CH₄ reaction, previously investigated in crossed-beams in other laboratories at comparable collision energies. The possible astrophysical relevance of S(¹D) reactions with hydrocarbons, especially in the chemistry of cometary comae, is discussed.