Critical evaluation of anharmonicity and configurational averaging in QM/MM modelling of equilibrium isotope effects
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INVESTIGACIONMetadata
Title
Critical evaluation of anharmonicity and configurational averaging in QM/MM modelling of equilibrium isotope effectsDate
2020Publisher
Royal Society of ChemistryISSN
1463-9076; 1463-9084Bibliographic citation
ROCA, Maite; UPFOLD, Catherine M.; WILLIAMS, Ian H. Critical evaluation of anharmonicity and configurational averaging in QM/MM modelling of equilibrium isotope effects. Physical Chemistry Chemical Physics, 2020, vol. 22, no 28, p. 16267-16276.Type
info:eu-repo/semantics/articleVersion
info:eu-repo/semantics/publishedVersionAbstract
Anharmonic effects upon vibrational frequencies and isotopic partition function ratios are modelled
computationally by means of quantum mechanics/molecular mechanics (QM/MM) methods for two systems.
First, the methyl ... [+]
Anharmonic effects upon vibrational frequencies and isotopic partition function ratios are modelled
computationally by means of quantum mechanics/molecular mechanics (QM/MM) methods for two systems.
First, the methyl cation in explicit water is considered using a B3LYP/6-31+G(d)/TIP3P method in order
to check the previous prediction of an inverse equilibrium isotope effect (EIE) KH3/KD3 for transfer from
vacuum to water at 298 K. A full QM/MM treatment including Lennard-Jones interactions predicts
significantly inverse contributions from both internal (0.843 0.001) and external (0.894 0.001)
modes of the solute. This treatment yields a much larger harmonic EIE (0.753 0.002, averaged over
928 independent solvent configurations) than is obtained either by projecting out the translational and
rotational contributions (0.853) or by treating the solvent by a point-charge representation (0.9360
0.0006, harmonic; 0.9366 0.0006, anharmonic). The contribution of anharmonicity to the EIE affects
the value only in the 3rd significant figure. Second, anharmonicity is investigated by means of QM/MM
potential-energy scans along 12 normal modes for internal and external vibrations of methyl cation in
water and for three modes (one stretching and two bending) for the Ha atom at the carbenium-ion
centre in cyclopentyl, cyclohexyl, tetrahydrofuranyl and tetrahydropyranyl cations in explicit water and
cyclohexane solvents, as obtained by means of atomic Hessian analysis [-]
Is part of
Physical Chemistry Chemical Physics, 2020, vol. 22, no 28.Investigation project
RYC-2014-16592, UJI-B2016-28 and UJI-B2019-43Rights
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