Skip to content
DraconiticResearch
Search Sources API
203,234 studies31 sourcesUpdated 18 min ago
PreprintCC BYChemRxiv

An analytical potential energy surface and variational transition-state theory kinetics of the NH3 + Br → NH2 + HBr hydrogen-abstraction reaction

  • Cipriano Rangel1,2
  1. 1Departamento de Quimica Organica e Inorganica (Area de Quimica Organica
  2. 2Universidad de Extremadura
Open at sourcePDFDOICite

Abstract

We report the first full-dimensional analytical potential energy surface (PES) and a variational transitionstate theory (VTST) study of the endothermic gas-phase reaction NH3 + Br −−→ NH2 + HBr, completing the halogen series (NH3 + F, Cl, Br) of the group’s analytical surfaces. Stationary points and energetics were benchmarked at the CCSD(T)/CBS level with the bromine 3d shell frozen; a −3.51 kcal/mol spinorbit correction was added to the NH3 + Br asymptote for the Br(2P) splitting. The benchmark locates a genuine, product-like (Hammond) saddle 17.6 kcal/mol above reactants but submerged 2.1 kcal/mol below the NH2+HBr asymptote (zero-point-and spin–orbit-corrected). A valence-bond/molecular-mechanics PES was calibrated to reproduce these stationary points to better than 0.25 kcal/mol; it retains an exact analytical gradient and dedicated van der Waals wells for the entrance (NH3···Br) and exit (NH2···HBr) complexes. Canonical variational theory with multidimensional tunneling (µOMT) yields forward rate constants over 200 K to 2000 K. Because the adiabatic barrier lies below the product asymptote, the tunneling coefficient is unity and the reverse reaction NH2 +HBr −−→ NH3 +Br exhibits a negative apparent activation energy, as measured for the carbon analogue CH3 +HBr. Quasi-classical trajectories on the same surface, once product zero-point leakage is accounted for, reproduce the variational forward rate as an independent check of the barrier. In the absence of experimental data, the forward rate constants are presented as predictions anchored to the high-level energetics.

Related

  1. 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
    Cipriano Rangel
    ChemRxivPreprintCC BYFull textAtomic, molecular & optical physicsChemRxiv
  2. Learning multistate kinetics with a variational multistate committor network
    Chenyu Tang, Cheng Giuseppe Chen, Benoît Roux +1
    arXivPreprintCC BY-NC-SA 4.0Full textPhysical chemistryComputational physicsarXiv
  3. Extended Onsager's variational principle for nonlinear chemical reaction kinetics in active matter
    Xinpeng Xu, Changhao Li, Zhongqiang Xiong +3
    arXivPreprintCC BY 4.0Full textPhysical chemistryBiological & medical physicsarXiv
  4. A Variational Framework for Guiding-Center Kinetics, Anisotropic Equilibria, and Quasisymmetry in Stellarators
    Lanke Fu, Amitava Bhattacharjee
    arXivPreprintCC BY 4.0Full textPlasma physicsarXiv
  5. First-Principles Study of I2 and CH3I Adsorption on Transition Metal Decorated 2D-Material substrates : Insights from Electronic Structure and Reaction Kinetics
    M. Meena, P. Anees
    arXivPreprintarXiv licenceFull textCondensed matterarXiv

Cite

BibTeXRIS
BibTeX
@misc{rangel2026analytical,
  title = {{An analytical potential energy surface and variational transition-state theory kinetics of the NH3 + Br → NH2 + HBr hydrogen-abstraction reaction}},
  author = {Cipriano Rangel},
  howpublished = {ChemRxiv},
  year = {2026},
  month = sep,
  doi = {10.26434/chemrxiv.15007326/v2},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15007326/v2},
  note = {CC BY},
}
RIS
TY  - UNPB
TI  - An analytical potential energy surface and variational transition-state theory kinetics of the NH3 + Br → NH2 + HBr hydrogen-abstraction reaction
AU  - Cipriano Rangel
PY  - 2026
DA  - 2026/09/23
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15007326/v2
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15007326/v2
AB  - We report the first full-dimensional analytical potential energy surface (PES) and a variational transitionstate theory (VTST) study of the endothermic gas-phase reaction NH3 + Br −−→ NH2 + HBr, completing the halogen series (NH3 + F, Cl, Br) of the group’s analytical surfaces. Stationary points and energetics were benchmarked at the CCSD(T)/CBS level with the bromine 3d shell frozen; a −3.51 kcal/mol spinorbit correction was added to the NH3 + Br asymptote for the Br(2P) splitting. The benchmark locates a genuine, product-like (Hammond) saddle 17.6 kcal/mol above reactants but submerged 2.1 kcal/mol below the NH2+HBr asymptote (zero-point-and spin–orbit-corrected). A valence-bond/molecular-mechanics PES was calibrated to reproduce these stationary points to better than 0.25 kcal/mol; it retains an exact analytical gradient and dedicated van der Waals wells for the entrance (NH3···Br) and exit (NH2···HBr) complexes. Canonical variational theory with multidimensional tunneling (µOMT) yields forward rate constants over 200 K to 2000 K. Because the adiabatic barrier lies below the product asymptote, the tunneling coefficient is unity and the reverse reaction NH2 +HBr −−→ NH3 +Br exhibits a negative apparent activation energy, as measured for the carbon analogue CH3 +HBr. Quasi-classical trajectories on the same surface, once product zero-point leakage is accounted for, reproduce the variational forward rate as an independent check of the barrier. In the absence of experimental data, the forward rate constants are presented as predictions anchored to the high-level energetics.
ER  -