Dynamics of the hydrogen bond: two-dimensional model and isotope effects

Abstract

In the adiabatic approximation the AHB linear triatomic model of a hydrogen bond is suggested to have two degrees of freedom: x (an AH anharmonic stretching vibration of a reduced mass μ 1 - fast subsystem) and X (the A…B stretching vibration - slow subsystem). An asymmetric hydrogen bond of the type A-H…B) as well as a single-well symmetric one (A…H…A) are discussed. The coupling of the subsystems induces a change in the equilibrium interatomic distance R ) (A…B) by X 0. Allowance for the latter permits one to establish the relations between the frequency ν(AH), the equilibrium bond lengths r′ o(AH) and R o of asymmetric hydrogen bonds, and explaining some relevant experimental regularities. The μ 1 dependence of X 0 and that of a ν(AH) anharmonicity permits one to account for the isotope effects for R o, r′ o and ν for both asymmetric and symmetric hydrogen bonds. The relation between the isotopic frequency ratio, ν(AH)/ν(AD), and ν 0(AH)/ν(AH) for asymmetric bonds is derived and seems to be confirmed by all published experimental data.