AN INVESTIGATION INTO INTRAMOLECULAR HYDROGEN-BONDING - IMPACT OF BASIS-SET AND ELECTRON CORRELATION ON THE AB-INITIO CONFORMATIONAL-ANALYSIS OF 1,2-ETHANEDIOL AND 1,2,3-PROPANETRIOL

Electron correlation effects are especially important in systems with strong nonbonded interactions. Despite this, very few ab initio studie s of polyfunctional alcohols have included correlation effects in thei r geometry optimizations. In order to better understand intramolecular hydrogen bonding and to develop more reliable energy and geometric pa rameters for future molecular modeling, we optimized the geometries of 10 conformers of 1,2-ethanediol (ethylene glycol) and 11 conformers o f 1,2,3-propanetriol (glycerol) at the HF/4-21G, HF/6-311G(*) and MP2 /6-311G(*) levels of theory. All three computational methods are able to predict differences between internal coordinates optimized in diff erent regions of conformational space, to within typical experimental accuracies. However, the inclusion of electron correlation has a major impact on the absolute values of these internal coordinates and on th e depths of the associated energy minima. Compared with our HF/6-311G( *) results, the MP2-optimized structures have longer C-O bonds by up to 0.020 Angstrom, O-H bonds that are up to 0.023 Angstrom longer, O-C -C angles that are often 1 degrees smaller, H-O-C angles in excess of 4 degrees smaller, and torsional angles that may deviate from ideal tr ans or gauche values by an additional 5 degrees for heavy-atom torsion s and 8 degrees for H-O-C-C torsions. The net effect of all these conf ormational rearrangements is to greatly enhance intramolecular hydroge n bonding. Nonbonded O ... H distances invariably decrease, by up to 0 .19 Angstrom, and the alignments of hydrogens with hypothetical lone-p air orbitals on oxygen accepters improve. Electron correlation selecti vely stabilizes those conformers with more intramolecular hydrogen bon ds, but decreases the energy differences among conformers with the sam e number of hydrogen bonds. The errors associated with single-point MP 2 energy calculations at HF-optimized geometries appear to increase wi th the size of the system, as do differences between MP2- and HF-optim ized O-C-C-O torsional angles. Thus, molecular mechanics parameters de rived from MP2/6-311G(*) optimizations of prototypical small molecule s such as ethylene glycol and glycerol are expected to result in signi ficantly different macromolecular energies and structures than those b ased on HF/6-311G(*) optimizations.