title: Effect of quantum nuclear motion on hydrogen bonding.
abstract: 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.
authors.affiliation: School of Mathematics and Physics, University of Queensland, Brisbane 4072, Australia.
authors.forename: Ross H
authors.initials: RH
authors.lastname: McKenzie
authors.affiliation: School of Mathematics and Physics, University of Queensland, Brisbane 4072, Australia.
authors.forename: Christiaan
authors.initials: C
authors.lastname: Bekker
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India.
authors.forename: Bijyalaxmi
authors.initials: B
authors.lastname: Athokpam
authors.affiliation: Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India.
authors.forename: Sai G
authors.initials: SG
authors.lastname: Ramesh