Hydrogen Bonding Systems Containing Hydrogen Fluoride. III. Infrared Frequency Shifts and CNDO/2 Calculations on HF-Carbonyl Compound Complexes

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Abstract
CNDO/2 calculations were carried out for the hydrogen bonded complexes between hydrogen fluoride (HF) and a series of organic compounds having a carbonyl group as a proton acceptor. Infrared spectral measurements were also made to obtain the frequency shifts of H–F stretching vibration, ΔνHF, for the same complexes in dilute carbon tetrachloride solution. Optimum geometry was obtained on the assumption of planar complex. For all the carbonyl compounds used, the interatomic angles, ∠C–O···H, were 120°–123° for the most stable conformation. The changes in the distance between H atom and F atom, ΔRHF, were in good correlation with ΔνHF obtained from IR spectra. Calculated hydrogen bond energies were also in good correlation with ΔνHF, though their absolute values were somewhat larger than those derived from experimental observations. This indicates that the CNDO/2 calculations give relatively correct enthalpy values for the hydrogen bond complexes of a given donor with a series of molecules having a common acceptor group. Other correlations of ΔνHF with properties expected to change with the hydrogen bond formation were also confirmed. These include the energies of each bond obtained by the partitioning of the total energy and the force constant of H–F bond. The electron population on the oxygen atom and other atoms participating in the hydrogen bond formation, the charge transfer, ionization potentials and dipole moment enhancements were also calculated and discussed in terms of the measure of hydrogen bond strength.