Isotope effects in hydrogen-bonded complexes. Calculation of geometrical and vibrational characteristics of asymmetric isotopologues of [F(HF)2]-

The geometrical and vibrational characteristics of isolated H-bonded anionic complexes FHFDF](-), FHFTF](-), and FDFTF](-) are calculated quantum-mechanically. The four-dimensional anharmonic vibrational problems are solved by the variational method using the potential energy and dipole moment surfaces calculated in the MP2/6-311++G(3df,3pd) approximation with the basis set superposition error taken into account. Changes in the bond lengths of molecular fragments LF (L = H, D, T) and in the distances between the F(-) anion and the centers of mass of LF are used as the vibrational coordinates. For each isotopologue, the vibrational energy levels, the transition frequencies and absolute intensities for the H-bond and L-F stretching vibrations are determined. To study the isotope effects on the geometrical parameters, the values of internuclear separations and the asymmetry parameter of the F(-)···L-F bridges, averaged over the ground state and several excited vibrational states, are calculated, as well as their standard deviations. The calculations revealed an extremely strong influence of anharmonic coupling between different vibrations on the absorption intensities and a significant mass-dependence of spectroscopic and structural parameters. The geometry and harmonic frequencies of KH(2)F(3), KD(2)F(3), and KHDF(3) are also calculated at a lower ab initio level. The results obtained for FHFDF](-), FHFTF](-), and FDFTF](-) are compared with the available experimental data and the results of earlier calculations of the symmetric complexes F(HF)(2)](-), F(DF)(2)](-), and F(TF)(2)](-) and complexes containing a positive K-meson.