Tentative study of strong hydrogen bond dynamics. Part II. Vibrational frequency considerations

Experimental data on the O⋯O equilibrium distances Re (2.40 ≤ Re ≤ 2.69 Å) and the proton and deuteron stretching vibrational frequencies (vH and vD) for compounds with short hydrogen bonds OHO and ODO are tabulated. Data is included for compounds in which the proton (deuteron) moves in a potential having a symmetry centre. For compounds with 2.60 {less-than or approximate} Re ≤ 2.69 Å both symmetric and asymmetric hydrogen bonds are included. The earlier suggested (but slightly modified) mathematical model of the OHO (ODO) fragment potential-energy surface with a new set of fitting parameters was used for calculations of the proton and deuteron adiabatic terms EHυ and EDυ, as well as the Franck-Condon transition frequencies vH and vD as functions of Re. Simultaneously, the isotopic frequency ratio γ=vH/vD was calculated depending on vH. These dependencies are compared with respective distributions of the ca. 100 experimental points in order to assign the observed frequencies to respective transitions. The non-monotonic γ dependence on vH, as well as the low ({less-than or approximate}1) and high ( > 1.37) γ values are also interpreted. The same model was used to compute the following quantities as functions of Re: (a) the O⋯O equilibrium distance change on isotopic substitution ΔRRDe - RHe; (b) the δH distance between two points of maximum proton density in the double-well OHO potential: and (c) the isotopic difference ΔδδD-δH. A qualitative and in some cases semi-quantitative agreement with experiment was obtained. © 1990.