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
by Teppen, B. J.; Cao, M.; Frey, R. F.; Vanalsenoy, C.; Miller, D. M.; Schafer, L.
Electron correlation effects are especially important in systems with strong nonbonded interactions. Despite this, very few ab initio studies of polyfunctional alcohols have included correlation effects in their geometry optimizations. In order to better understand intramolecular hydrogen bonding and to develop more reliable energy and geometric parameters for future molecular modeling, we optimized the geometries of 10 conformers of 1,2-ethanediol (ethylene glycol) and 11 conformers of 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 different 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 the 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 trans or gauche values by an additional 5 degrees for heavy-atom torsions and 8 degrees for H-O-C-C torsions. The net effect of all these conformational rearrangements is to greatly enhance intramolecular hydrogen bonding. Nonbonded O ... H distances invariably decrease, by up to 0.19 Angstrom, and the alignments of hydrogens with hypothetical lone-pair orbitals on oxygen accepters improve. Electron correlation selectively stabilizes those conformers with more intramolecular hydrogen bonds, but decreases the energy differences among conformers with the same number of hydrogen bonds. The errors associated with single-point MP2 energy calculations at HF-optimized geometries appear to increase with the size of the system, as do differences between MP2- and HF-optimized O-C-C-O torsional angles. Thus, molecular mechanics parameters derived from MP2/6-311G(**) optimizations of prototypical small molecules such as ethylene glycol and glycerol are expected to result in significantly different macromolecular energies and structures than those based on HF/6-311G(**) optimizations.