Theoretical and matrix isolation studies of infrared spectra of the H2CO∙∙∙HF hydrogen-bonded complex

Abstract

The equilibrium geometry, the binding energy, and harmonic and anharmonic spectral parameters of H2CO and HF monomers and the H2CO∙∙∙HF complex are calculated in the MP2/6-311++G(3df,3pd) approximation with the basis set superposition error taken into account. Anharmonic calculations are carried out using the second-order vibrational perturbation theory. The changes in transition frequencies and intensities of monomeric IR bands upon formation of the 1:1 complex are analyzed. A high intensity value (53 km mol–1) is predicted for the overtone band of the out-of-plane HF libration. The variational method was used to solve anharmonic vibrational problems in 1D–4D subspaces involving the H–F stretch. The influence of anharmonicity on spectral and structural parameters was studied. The variational calculation confirmed the high intensity value for the overtone of the HF librational mode from the perturbative calculation. Matrix-isolation experiments were performed in N2 at T = 8 K to record the absorption spectra of pure H2CO and HF and H2CO/HF mixtures in the IR region. The spectral features related to the H–F, C=O, and C–H stretches and the in-plane HF librational motion were reliably identified. The predictions of our perturbative calculations for isolated compounds, especially for the band shifts upon complexation, are in satisfactory agreement with the experimental matrix-isolation findings. The only exception is the H–F stretching mode in the complex because of the high anharmonicity of HF and strong interaction of free HF molecules with a matrix environment. Of particular interest is the observation of a rather strong band at 1069 cm–1, which is close to 1072 cm–1 predicted for the overtone of HF out-of-plane librational mode.