Direct-dynamics VTST study of hydrogen or deuterium abstraction and C–C bond formation or dissociation in the reactions of CH3 + CH4, CH3 + CD4, CH3D + CD3, CH3CH3 + H, and CH3CD3 + D

Abstract

Direct-dynamics variational transition-state theory calculations are studied at the MPWB1K∕6-311++G(d,p) level for the four parts of reactions. The first part is hydrogen or deuterium abstraction in the reactions of CH3 + CH4, CH3 + CD4, and CH3D + CH3. The second part involves C-C bond formation in these reactions. The third one is the reactions of CH3CH3 + H and CH3CD3 + D to form of H2, HD, and D2. The last one is the dissociation of C-C bonds in the last group of reactions. The ground-state vibrational adiabatic potential is plotted for all channels. We have carried out direct-dynamics calculations of the rate constants, including multidimensional tunneling in the temperature range T = 200-2200 K. The results of CVT∕μOMT rate constants were in good agreement with the experimental data which were available for some reactions. Small-curvature tunneling and Large-curvature tunneling with the LCG4 version were used to include the quantum effects in calculation of the rate constants. To try to find the region of formation and dissociation of bounds we have also reported the variations of harmonic vibrational frequencies along the reaction path. The thermally averaged transmission probability (P(E)exp (-ΔE∕RT)) and representative tunneling energy at 298 K are reported for the reactions in which tunneling is important. We have calculated kinetic isotope effect which shows tunneling and vibrational contributions are noticeable to determine the rate constant. Nonlinear least-squares fitting is used to calculate rate constant expressions in the temperature range 200-2200 K. These expressions revealed that pre-exponential factor includes two parts; the first part is a constant number which is important at low temperatures while the second part is temperature dependent which is significant at high temperatures.

Dynamic and Kinetic Parameters and Energy Exchanges of Particles in Reaction of NH + OH and Deuterated Analogues on an Interpolated Potential Energy Surface

“…Previously, one of the authors of this paper investigated the primary kinetic isotope effect (KIE) for the reactions of methyl radical with methane and trichloromethyl radical …”

Accelerating quantum instanton calculations of the kinetic isotope effects

“…(Note that although B k(γ) th also depends on the force, it depends only on the force acting on a single bead, and hence, its cost is negligible for large P.) It should be emphasized, however, that gradient-and Hessian-dependent parts of virial estimators can be calculated by finite difference, without the need to evaluate the gradient or Hessian explicitly. For example, ⟨w, ∇V ⟩ 0 and ⟨w, ∂ 2 V/∂r 2 , w⟩ 00 are the first and second derivatives of V in the direction of w and therefore can be evaluated by finite difference using just one and two additional evaluations of V , respectively. As a result, the effective cost is only one extra potential evaluation per bead for F r,cv , one per unconstrained bead for F ‡ cv , two per unconstrained bead for (G v + F It should be emphasized that it is not necessary to evaluate these estimators after each MC step due to finite correlation lengths inherent to MC simulations.…”

“…It should be emphasized, however, that gradient-and Hessiandependent parts of virial estimators can be calculated by finite difference, without the need to evaluate the gradient or Hessian explicitly. For example, w, ∇V 0 and w, ∂ 2 V /∂r 2 , w 00 are first and second derivatives of V in the direction of w, and therefore can be evaluated by finite difference using just one and two additional evaluations of V , respectively. As a result, the effective cost is only one extra potential evaluation per bead for F r,cv , one per unconstrained bead for F ‡ cv , two per unconstrained bead for (G v + F 2 v )/2, and three for f v,v .…”

“…1 Several approaches are currently used for calculating rate constants and KIE's in situations where quantum effects are not negligible. One approach consists in adding a tunneling correction to transition state theory, 2 others use an approximation for the propagator by treating it semiclassically 3 or by treating only one or two degrees of freedom quantum mechanically. 4 Another promising method is the ring polymer molecular dynamics 5 (RPMD), which can partially capture both quantum effects and classical recrossing.…”

Ab initio calculations and kinetic modeling of thermal conversion of methyl chloride: implications for gasification of biomass

“…6 Substantial corrosion of gas turbine blades may also occur at low chlorine concentration 5 and a tolerance of 0.5 ppm is reported for gas turbines. 6 Chlorine species are furthermore known to poison catalysts used for conversion of syngas 57 and levels below 1 ppm are desirable. 6 In addition to this, chlorine species may result in deposition and enhanced corrosion in the downstream processes.…”