Direct ab initio dynamics study on the rate constants and kinetic isotope effect for the reactions of H atoms with GeD_n(CH₃)_4-n (n = 1-4)

Direct ab initio dynamic calculations are performed on the reactions of atomic hydrogen with GeD_n(CH₃)_4-n (n = 1-4) over the temperature range 200-2000 K at the PMP4SDTQ/6-311+G(3df,2p)//MP2/6-31 + G(d) (for n = 2-4) and G2//MP2/6-31 + G(d) (for n = 1) levels. The corresponding k_H/k_D ratios are then calculated in order to determine the kinetic isotope effect for the four reactions. For the simplest GeD₄ + H reaction, the only one that has available experimental data, the calculated canonical variational transition state theory incorporates small-curvature tunneling correction (CVT/SCT) thermal rate constants, and the k_H/k_D values are in good agreement with the experimental values within the experimental temperature range 293-550 K. For the four GeD_n(CH₃)_4-4 (n = 1-4) reactions, the variational effect is small over the whole temperature range, whereas the small-curvature effect is important in the lower temperature range. Finally, the overall rate constants are fitted to the three-parameter expression over the whole temperature range 200-2000 K as 5.8 × 10⁸T^1.68exp(-929/T), 1.7 × 10⁸T^1.80exp(-691/T), 2.58 × 10⁸T^1.71exp(-706/T), and 1.0 × 10⁷T^2.08exp(-544/T) cm³ mol^-1 s^-1 for the n = 4, 3, 2, and 1 reactions. Our work may represent the first theoretical study of the kinetic isotope effect for the H-attack on the Ge-H bonding.