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.
Highlights
When the size of the molecular reactive system increases, in atom number and/or electron number, the construction of the full-dimensional potential energy surface (PES), representing nuclear motion, becomes increasingly complicated, and with the direct application of ab initio calculations it is practically prohibitive
Hopefully recent quasi-classical trajectory (QCT) calculations[50,51] on the similar HBr + OH - Br + H2O reaction which presents a stabilized HBr–OH complex in the entrance channel, presents negative temperature dependence, in this case below 160 K, reproducing the experimental evidence which is available for this reaction
The present paper presents two different parts, first the development of an analytical full-dimensional potential energy surface for the title reaction and its isotopic variants, and second, the calculation of thermal rate constants and kinetic isotope effects, and their comparison with the only experimental values at 298 K
Summary
When the size of the molecular reactive system increases, in atom number and/or electron number, the construction of the full-dimensional potential energy surface (PES), representing nuclear motion, becomes increasingly complicated, and with the direct application of ab initio calculations it is practically prohibitive. In these cases, the development of PESs based on functional forms (based on a reduced number of ab initio calculations) acquires greater importance.
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