Association of Cl with C ₂ H ₂ by unified variable-reaction-coordinate and reaction-path variational transition-state theory

Abstract

Barrierless unimolecular association reactions are prominent in atmospheric and combustion mechanisms but are challenging for both experiment and kinetics theory. A key datum for understanding the pressure dependence of association and dissociation reactions is the high-pressure limit, but this is often available experimentally only by extrapolation. Here we calculate the high-pressure limit for the addition of a chlorine atom to acetylene molecule (Cl + C2H2→C2H2Cl). This reaction has outer and inner transition states in series; the outer transition state is barrierless, and it is necessary to use different theoretical frameworks to treat the two kinds of transition state. Here we study the reaction in the high-pressure limit using multifaceted variable-reaction-coordinate variational transition-state theory (VRC-VTST) at the outer transition state and reaction-path variational transition state theory (RP-VTST) at the inner turning point; then we combine the results with the canonical unified statistical (CUS) theory. The calculations are based on a density functional validated against the W3X-L method, which is based on coupled cluster theory with single, double, and triple excitations and a quasiperturbative treatment of connected quadruple excitations [CCSDT(Q)], and the computed rate constants are in good agreement with some of the experimental results. The chlorovinyl (C2H2Cl) adduct has two isomers that are equilibrium structures of a double-well C≡C–H bending potential. Two procedures are used to calculate the vibrational partition function of chlorovinyl; one treats the two isomers separately and the other solves the anharmonic energy levels of the double well. We use these results to calculate the standard-state free energy and equilibrium constant of the reaction.

Atmospheric Kinetics: Bimolecular Reactions of Carbonyl Oxide by a Triple-Level Strategy

“…Since there are two bottlenecks in sequence, we calculated the overall rate constant k m by the canonical unified statistical theory − (CUS), but with the assumption that the flux through the van der Waals well is much larger than the flux through either transition state. This yields Since there is tunneling in k m ″, we label this CUS/SCT.…”

Temperature-dependent kinetics of the atmospheric reaction between CH2OO and acetone

“…Our final estimate of the high-pressure limit rate constants was calculated by the extension of the canonical unified statistical theory [81][82][83][84] to the high-pressure limit. For the present application, we ignore the flux through the CA and CB intermediates and the flux through the through the variational transition state between reactants and CA based on the assumption that they are much larger than the fluxes through the TSA and TSB transition states.…”

Structure and Vibrational study of Entrance Channel of Monomer C2H2+ Cl via ab initio calculation

“…Therefore, it is worth to study spectroscopy of C 2 H 2 attached with halogen atoms particularly Cl atom. Recently, investigations have been reported for interaction Cl atom with C 2 H 2 [28]. The results explored reaction coordinates of the transition states of Cl+C 2 H 2 [28], however the work did not predict IR vibrational frequencies and equilibrium structures.…”