Ab Initio Investigation of the Abstraction Reactions by H and D from Tetramethylsilane and Its Deuterated Substitutions

Abstract

Thermal rate constants for chemical reactions using the corrections of zero curvature tunneling (ZCT) and of small curvature tunneling (SCT) methods are reported. The general procedure is implemented and used with high-quality ab initio computations and semiclassical reaction probabilities along the minimum energy path (MEP). The approach is based on a vibrational adiabatic reaction path and is applied to the H + Si(CH3)4 → H2 + Si(CH3)3CH2 reaction and its isotopically substituted variants. All of the degrees of freedom are optimized, and harmonic vibrational frequencies and zero-point energies are calculated at the MP2(full) level with the cc-pVTZ basis set. Single-point energies are calculated at a higher level of theory with the same basis set, namely, CCSD(T,full). The influence of the basis set superposition error (BSSE) on the energetics is tested. The method is further exploited to predict primary and secondary kinetic isotope effects (KIEs and SKIEs, respectively). Rate constants computed with the ZCT and SCT methods over a wide temperature range (180-2000 K) show important quantum tunneling effects at low temperatures when compared to rates obtained from the purely classical transition-state theory (TST) and from the canonical variational transition state theory (CVT). For the H + Si(CH3)4 reaction, they are given by the following expressions: k(TST/ZCT) = 9.47 × 10(-19) × T(2.65) exp(-2455.7/T) and k(CVT/SCT) = 7.81 × 10(-19) × T(2.61) exp[(2704.2/T) (in cm(3) molecule(-1) s(-1)). These calculated rates are in very good agreement with those from available experiments.

Quantum scattering study of the abstraction reaction of H atoms from tetramethylsilane

“…Several steps are needed to obtain the effective potential energy adapted to further dynamics calculations. In a first step, adopting the same methods as described in the previous semiclassical study [11], an adiabatic approach is followed to obtain the potential energy surface which treats explicitly the breaking (r 1 ) and the forming (r 2 ) bonds. For fixed values of r 1 and r 2 , spectator degrees of freedom are fully optimized at the secondorder Moller-Plesset perturbation theory level(MP2) [30,31] with the correlation-consistent polarized valence basis set of Dunning [32,33], namely cc-pVTZ.…”

“…[11], the CCSD(T) method insures a good description of the reaction energetics and kinetics for the H abstraction from TMS. The calculations lead to a vibrational adiabatic barrier height ( V a ) of 10.48 kcal/mol and a reaction exothermicity of −3.08 kcal/mol.…”

“…For the geometries (optimized at the MP2 level) of the Rvdw, Pvdw and transition state (TS) involved in TMS + H reaction, we refer the reader to Table 3 in Ref. [11].…”

“…In a recent work [11], we have studied, using semi classical methods, the abstraction reaction of hydrogen atoms from tetramethylsilane (TMS):…”

“…The semiclassical study [11] showed that the reaction mechanism is a direct hydrogen abstraction one with a collinear saddle point. Quantum tunneling as well as isotopic effects were found to be important at low temperatures.…”