Rate constant calculations of the GeH₄ + OH/OD → GeH₃ + H₂O/HOD reactions using an ab initio based full-dimensional potential energy surface

Abstract

We report an analytical full-dimensional potential energy surface for the GeH4 + OH → GeH3 + H2O reaction based on ab initio calculations. It is a practically barrierless reaction with very high exothermicity and the presence of intermediate complexes in the entrance and exit channels, reproducing the experimental evidence. Using this surface, thermal rate constants for the GeH4 + OH/OD isotopic reactions were calculated using two approaches: variational transition state theory (VTST) and quasi-classical trajectory (QCT) calculations, in the temperature range 200-1000 K, and results were compared with the only experimental data at 298 K. Both methods showed similar values over the whole temperature range, with differences less than 30%; and the experimental data was reproduced at 298 K, with negative temperature dependence below 300 K, which is associated with the presence of an intermediate complex in the entrance channel. However, while the QCT approach reproduced the experimental kinetic isotope effect, the VTST approach underestimated it. We suggest that this difference is associated with the harmonic approximation used in the treatment of vibrational frequencies.

Kinetics study of the OH + SiH4 hydrogen abstraction reaction: A theoretical analysis

“…The section finishes with a comparison of the kinetics behavior of the OH free radical with IV group hydrides, OH + XH 4 (X≡C, Si, Ge), which has been investigated in our lab 51,52 . As noted in Section 1, the corresponding broken X–H dissociation energies decrease in the series.…”

Chemical Kinetics Approves the Occurrence of C (3Pj) Reaction with H2O

“…The use of scale factor has been usually considered to be adequate for anharmonicity correction, for example, see refs and . In the case of 3 INTb, which possesses a torsional degree of freedom, torsional anharmonicity was included by treating the lowest vibrational frequency as a hindered rotor, while the other frequencies were treated as separable harmonic oscillators modified by a frequency scale factor . No multistructural anharmonicity was concerned because all reactants, products, TSs, and intermediates could be described by a single structure .…”

“…However, a contradiction is evident in Figure ; MESMER and the study of Hickson et al show negative temperature dependency at lower temperatures and positive temperature dependency at higher temperatures, while RRKM–SSA flags positive temperature dependency over the entire range of temperature. Negative temperature dependency is probable for the reactions of free radicals and atomic or ionic species that form a stabilized reactant complex in their entrance channels . On the other hand, the presence of positive apparent activation energy enforces positive temperature dependency .…”

“…In the case of 3 INTb, which possesses a torsional degree of freedom, torsional anharmonicity was included by treating the lowest vibrational frequency as a hindered rotor, while the other frequencies were treated as separable harmonic oscillators modified by a frequency scale factor. 103 No multistructural anharmonicity was concerned because all reactants, products, TSs, and intermediates could be described by a single structure. 102 The applied anharmonicity corrections are expected to influence kinetics of the reaction in two ways: 104 (i) declining the effect of dividing surface position on the rate coefficients by decreasing the effect of transitional modes and approaching the maximal Gibbs free energy change of the corresponding reaction coordinate to the saddle point and (ii) lowering the rate coefficients depending on the studied reaction and temperature.…”

“…Negative temperature dependency is probable for the reactions of free radicals and atomic or ionic species that form a stabilized reactant complex in their entrance channels. 103 On the other hand, the presence of positive apparent activation energy enforces positive temperature dependency. 121 Furthermore, the presence of barrier heights at the entrance channel might invoke tunneling at low temperatures and lead to negative temperature dependency.…”