Isotope effects in the spectra of hydrogen-bonded complexes. Calculation of the structure and vibrational absorption spectra of the H$_{2}$O $\dots$ HF, H$_{2}$O $\dots$ DF, D$_{2}$O $\dots$ HF, and D$_{2}$O $\dots$ DF complexes

The frequencies and intensities of IR absorption bands of the H$_{2}$O $\dots$ HF, H$_{2}$O $\dots$ DF, D$_{2}$O $\dots$ HF, and D$_{2}$O $\dots$ DF hydrogen-bonded complexes are calculated using the second-order vibrational perturbation theory. The MP2/aug-cc-pVTZ method with the basis set superposition error taken into account is used to calculate the electronic wave functions in determining the equilibrium configuration, the potential energy and dipole moment surfaces of these complexes, as well as in calculating spectral parameters. It is shown that upon complexation the frequencies and intensities of the stretching vibration of HF (DF) molecules and the intensities of stretching vibrations of H$_2$O (D$_2$O) molecules change most significantly. A variational calculation of librational motion of the water molecule in the two-well potential explained the reason for the higher value of the fundamental transition frequency of the $\nu_1$ (H–F) mode as compared to the frequency of this mode upon hot transition from the first excited librational state. The dependence of the intermode anharmonic interaction on isotope substitution was analyzed