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Chemical Sciences Division
Zhang, Q.; Zhang, D.; Wang, S.; Gu, Y.|
Ab Initio and Kinetic Calculations for the Reactions of H with (CH3)(4-n)GeHn (n=1,2,3,4)
J. Phys. Chem. A
122 - 129
GeH4 + H· → H2 + GeH3
300 - 2000
4.79x10-12 [cm3/molecule s] (T/298 K)2.16 e-2444 [J/mole]/RT
Ab initio transition states for the abstraction reaction were calculated using MP2/6-31G(d) optimized geometries and frequencies and using energies from G2MP2 calculations. It was found that calculated barriers and heats of reactions were relatively insensitive to basis set, while the energetics were sensitive to the degree of electron correlation. MP2 energies were found to be inadequate, while value calculated using QCISD(T) energies were in good agreement with the experimental heat of reaction for GeH4 + H = GeH3 + H2. The transition states were used with Truhlars Polyrate program and including a CVT/SCT (canonical variational transition state theory / small curvature tunneling) correction. The transition states for the Ge-H+H abstraction reactions from GeH4 through GeH(CH3)3 were essentially collinear (Ge-H-H bond angle 177-180o).
Calculated barriers for the abstractions were relatively insensitive to methyl substitution, with the computed barrier heights being 2-3 kcal/mol. This compares to computed barriers of 10-13 kcal/mol for abstraction of C-H from the methyl groups (thus the latter being a minor channel).
Energies relatively independent of basis set 6-311G(d,p) to 6-311++G(3df,3pd)but MP2 give barriers about 3 kcal/mol higher (6 vs. 3 kcal/mol) and poor agreement between calculated and experimental heat of reaction.
Calculated barrier is 3.0 kcal/mol. Calculated rate constant is about 2-2.5 times lower than experimental value.
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Rate constant values calculated from the Arrhenius expression:
|T (K)||k(T) [cm3/molecule s]|