Effect of quantum nuclear motion on hydrogen bonding

• Published in: The Journal of chemical physics 2014-05, Vol.140 (17), p.174508-174508
• Main Authors: McKenzie, Ross H, Bekker, Christiaan, Athokpam, Bijyalaxmi, Ramesh, Sai G
• Format: Article
• Language: English
• Subjects: BOND LENGTHS; Bonding strength; Energy levels; GROUND STATES; HAMILTONIANS; HYDROGEN; Hydrogen bonding; Hydrogen bonds; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; Isotope effect; ISOTOPE EFFECTS; Parameterization; PROTONS
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.4873352

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.