This work considers how the properties of hydrogen bonded complexes, X–H⋯Y, are modified by the quantum motion of the shared proton. Using a simple two-diabatic state model Hamiltonian, the analysis of the symmetric case, where the donor (X) and acceptor (Y) have the same proton affinity, is carried out. For quantitative comparisons, a parametrization specific to the O–H⋯O complexes is used. The vibrational energy levels of the one-dimensional ground state adiabatic potential of the model are used to make quantitative comparisons with a vast body of condensed phase data, spanning a donor-acceptor separation (R) range of about 2.4 − 3.0 Å, i.e., from strong to weak hydrogen bonds. The position of the proton (which determines the X–H bond length) and its longitudinal vibrational frequency, along with the isotope effects in both are described quantitatively. An analysis of the secondary geometric isotope effect, using a simple extension of the two-state model, yields an improved agreement of the predicted variation with R of frequency isotope effects. The role of bending modes is also considered: their quantum effects compete with those of the stretching mode for weak to moderate H-bond strengths. In spite of the economy in the parametrization of the model used, it offers key insights into the defining features of H-bonds, and semi-quantitatively captures several trends.
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7 May 2014
Research Article|
May 07 2014
Effect of quantum nuclear motion on hydrogen bonding
Ross H. McKenzie;
Ross H. McKenzie
a)
1School of Mathematics and Physics,
University of Queensland
, Brisbane 4072, Australia
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Christiaan Bekker;
Christiaan Bekker
1School of Mathematics and Physics,
University of Queensland
, Brisbane 4072, Australia
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Bijyalaxmi Athokpam;
Bijyalaxmi Athokpam
2Department of Inorganic and Physical Chemistry,
Indian Institute of Science
, Bangalore 560 012, India
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Sai G. Ramesh
Sai G. Ramesh
2Department of Inorganic and Physical Chemistry,
Indian Institute of Science
, Bangalore 560 012, India
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a)
E-mail: r.mckenzie@uq.edu.au. URL: condensedconcepts.blogspot.com.
J. Chem. Phys. 140, 174508 (2014)
Article history
Received:
January 30 2014
Accepted:
April 15 2014
Citation
Ross H. McKenzie, Christiaan Bekker, Bijyalaxmi Athokpam, Sai G. Ramesh; Effect of quantum nuclear motion on hydrogen bonding. J. Chem. Phys. 7 May 2014; 140 (17): 174508. https://doi.org/10.1063/1.4873352
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