An Analytical Full-Dimensional Potential Energy Surface for the H + C3H8 Hydrogen Abstraction Reaction: Non-Equivalence of the Primary Hydrogens and Variational Transition State Theory Kinetics
- 1Departamento de Química Orgánica e Inorgánica (Área de Química Orgánica
- 2Universidad de Extremadura
Abstract
A new analytical full-dimensional potential energy surface, PES-2026, is reported for the gas-phase hydrogen abstraction reaction H + C3H8 → H2 + C3H7. It is built within the valence bond–molecular mechanics, VB-MM, framework and depends on 72 parameters, 65 of which were optimized against a focal-point CBS benchmark and against UMP2/cc-pVTZ geometries, harmonic frequencies and reaction paths. The surface describes the two chemically distinct abstraction channels, primary and secondary, and resolves them into three kinetically distinct channels: the six primary hydrogens are not equivalent, two of them being abstracted through an anti and four through a gauche saddle point whose adiabatic barriers differ by 0.26 kcalmol−1. Classical barrier heights of 12.05, 11.68 and 9.44 kcalmol−1 were obtained for the anti-primary, gauche-primary and secondary channels, reproducing the benchmark to within 0.04 kcalmol−1, and a relaxed two-dimensional torsional scan that did not enter the fit is reproduced with a root-mean-square deviation of 0.11 kcalmol−1. Thermal rate constants were computed between 200 and 1000 K with variational transition state theory and multidimensional tunnelling, CVT/µOMT, treating the three channels separately and summing them. At 300 K the total rate constant, 9.4×10−16 cm3 molecule−1 s−1, is 0.85 times the Tsang evaluation, and the calculated primary contribution, 5.6%, compares with the 4.3–4.7% derived from experiment and improves on the 9.5% of the most recent direct-dynamics study. Kinetic isotope effects are predicted for substrate and for attacking-atom substitution; the latter is inverse below ∼800 K, which is traced to the lowering of the adiabatic barrier upon deuteration of the incoming atom. The muonium analogue is also examined and is found to lie beyond the reach of a harmonic treatment.
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BibTeX
@misc{rangel2026analytical,
title = {{An Analytical Full-Dimensional Potential Energy Surface for the H + C3H8 Hydrogen Abstraction Reaction: Non-Equivalence of the Primary Hydrogens and Variational Transition State Theory Kinetics}},
author = {Cipriano Rangel},
howpublished = {ChemRxiv},
year = {2026},
month = oct,
doi = {10.26434/chemrxiv.15010442/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010442/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - An Analytical Full-Dimensional Potential Energy Surface for the H + C3H8 Hydrogen Abstraction Reaction: Non-Equivalence of the Primary Hydrogens and Variational Transition State Theory Kinetics
AU - Cipriano Rangel
PY - 2026
DA - 2026/10/14
PB - ChemRxiv
DO - 10.26434/chemrxiv.15010442/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010442/v1
AB - A new analytical full-dimensional potential energy surface, PES-2026, is reported for the gas-phase hydrogen abstraction reaction H + C3H8 → H2 + C3H7. It is built within the valence bond–molecular mechanics, VB-MM, framework and depends on 72 parameters, 65 of which were optimized against a focal-point CBS benchmark and against UMP2/cc-pVTZ geometries, harmonic frequencies and reaction paths. The surface describes the two chemically distinct abstraction channels, primary and secondary, and resolves them into three kinetically distinct channels: the six primary hydrogens are not equivalent, two of them being abstracted through an anti and four through a gauche saddle point whose adiabatic barriers differ by 0.26 kcalmol−1. Classical barrier heights of 12.05, 11.68 and 9.44 kcalmol−1 were obtained for the anti-primary, gauche-primary and secondary channels, reproducing the benchmark to within 0.04 kcalmol−1, and a relaxed two-dimensional torsional scan that did not enter the fit is reproduced with a root-mean-square deviation of 0.11 kcalmol−1. Thermal rate constants were computed between 200 and 1000 K with variational transition state theory and multidimensional tunnelling, CVT/µOMT, treating the three channels separately and summing them. At 300 K the total rate constant, 9.4×10−16 cm3 molecule−1 s−1, is 0.85 times the Tsang evaluation, and the calculated primary contribution, 5.6%, compares with the 4.3–4.7% derived from experiment and improves on the 9.5% of the most recent direct-dynamics study. Kinetic isotope effects are predicted for substrate and for attacking-atom substitution; the latter is inverse below ∼800 K, which is traced to the lowering of the adiabatic barrier upon deuteration of the incoming atom. The muonium analogue is also examined and is found to lie beyond the reach of a harmonic treatment.
ER -