Conformational Preferences and Pathways for Enantiomerization and Diastereomerization of Benzyl Alcohol. Data Mining and ab Initio Quantum-Mechanical Study
datasetposted on 07.01.2000, 00:00 by Rainer Glaser, G. Richard Nichols
The potential energy surface of benzyl alcohol has been explored at the RHF/6-31G* and MP2(full)/6-31G* levels of ab initio theory. The exo and endo minimum structures and all of the transition-state structures for their enantiomerization and diastereomerization were located. The thermochemical functions were computed, and relative enthalpies and free enthalpies were determined. The computed relative free enthalpy is in excellent agreement with known NMR data. Benzyl alcohol is a highly flexible molecule, which allows for essentially unhindered enantiomerization and diastereomerization of the minima; all pertinent relative energies and free enthalpies are less than 3 kcal/mol, and the entire conformational space about the CC and CO bonds is easily accessible. An examination of the crystallographic record shows a much greater variety of conformations to occur in the solid state as a result of the optimization of intermolecular interactions in the crystals. Data mining shows that the conformations about the CC and CO bonds of all of the CC-staggered and CO-gauche structures are correlated, and consequently, the conformational preferences about the C−O bond greatly deviate from the expected 3-fold barrier. The theoretical study shows the correlation to be instrinsic and offers an explanation. Natural population analysis provides a rational for the relative stabilities of the benzyl alcohol conformations on the basis of the conformation dependence of the effects of electron−electron repulsion between the O-lone pairs and the benzene π-cloud, the charge−charge attraction between the hydroxyl H-atom and the benzene π-density, and the dipole−dipole interaction associated with the CH2−Cipso and OH bonds. Our analysis emphasizes the importance of the dipole alignment of the CH2−Cipso and OH bond dipole moments.