THE ABILITY OF THE SEMIEMPIRICAL PM3 METHOD TO MODEL PROTON-TRANSFER REACTIONS IN SYMMETRICAL HYDROGEN-BONDED SYSTEMS

Geometries and energies of small proton-bound symmetric dimers and min imum energy pathways for the proton transfer from one subunit to the o ther were calculated both at the semiempirical MNDO/PM3 and at ab init io HF and post HF levels. Except for systems, in which carbon atoms ar e involved or oxygen atoms are part of acceptor molecules, PM3 can be used for a good quantitative description of hydrogen bonds near the eq uilibrium geometry. If the subunits are separated by more than 2.5 Ang strom, attractive forces will be lost quickly. Proton transfer reactio ns are modeled in a qualitative acceptable manner. If proton transfer occurs between nitrogen acid/base pairs, PM3 barriers of activation ar e about 5-10 kcal/mol higher than ab initio results.