Critical analysis of the calculated frequency shifts of hydrogen-bonded complexes

The frequency shift of the proton donor in hydrogen bonded complexes is an important quantity which enables to discuss the nature of the hydrogen bond . Calculations of frequency shifts by quantum chemical methods are usually performed within the harmonic approximation and therefore the comparison wi th experimental data is biased. We have investigated the importance of anha rmonic corrections in the case of twelve complexes in which either FH or Cl H is the proton donor. Hartree-Fock, Moller-Plesset second and third order (MP2, MP3), density functional theory (DFT), and hybrid Hartree-Fock/DFT me thods have been used for the calculations. It is shown that the anharmonic contribution to the frequency shift is rather method dependent. Its magnitu de is usually 10%-20% of the total shift though it can be as large of simil ar to 50% for (ClH)(2). Once anharmonicity is taken into account, most meth ods tend to noticeably overestimate the frequency shifts. In the case of DF T related approaches this trend is interpreted in terms of a poor descripti on of the exchange in the intermolecular region. (C) 1999 American Institut e of Physics. [S0021-9606(99)31038-2].