DENSITY-FUNCTIONAL STUDY OF HYDROGEN-BONDED SYSTEMS - ENERGETIC AND VIBRATIONAL FEATURES OF SOME GAS-PHASE ADDUCTS OF HYDROGEN-FLUORIDE

A series of detailed density functional calculations on hydrogen-bonde d gas-phase adducts of the FH molecule for which both experiments and calculations based on traditional approaches were available have been carried out. The adducts were: (FH)(2), FH/CO and FH/NH3. The adopted methods were: SCF, MP2, S-VWN, B-LYP, B-P86 and B3-LYP. Basis sets wer e: 6-31G(d,p), 6-31+G(d,p), 6-311++G(d,p), 6-311+G(2d,2p), 6-311+G(3df ,2pd), DZ(d,p), cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ. Comput ed properties were: geometries,harmonic vibrations, binding energies a nd intensity of the F-H stretching mode of the donor. A detailed analy sis of the anharmonicity associated with the F-H stretching mode was a lso carried out by numerically solving the nuclear Schrodinger equatio n associated with the F-H mode for both the FH/NH3 and FH/CO cases. Co mparison with previous studies as well as with available experimental data shows that heats of formation computed with the gradient correcte d DF methods are as accurate as MP2 or better, when a basis set of at least triple-zeta valence quality with a double set of polarization fu nctions is chosen. All functionals are able to give the correct order of stability for the FH/CO adduct, i.e. the carbon-bound structure as the more stable one, in agreement with MP2 studies. The Pople basis se t 6-311+G(2d,2p) appears as good as the correlation-consistent basis s et proposed by Dunning, namely aug-cc-pVDZ. Some care should be taken when adopting the 6-31G(d,p) basis set, as for instance in the case of (FH)(2) where spurious interactions stabilize cyclic structures with respect to open ones. All DF methods overestimate the shifts in the F- H stretching mode due to the hydrogen-bond interaction, calculated at the harmonic level, and no improvement is brought about by anharmonici ty. The increase in the infrared intensity of the FH donor molecule is correctly accounted for. DF methods based on local electron density o verestimate both the binding energies and the shifts in the F-H stretc hing mode showing their unrealiability for studying hydrogen-bonding i nteractions. (C) 1997 Elsevier Science B.V.