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| - Conformational Preferences and Pathways for Enantiomerizationand Diastereomerization of Benzyl Alcohol. Data Mining and abInitio Quantum-Mechanical Study
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| - 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 thetransition-state structures for their enantiomerization and diastereomerization were located. Thethermochemical functions were computed, and relative enthalpies and free enthalpies weredetermined. 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 enthalpiesare less than 3 kcal/mol, and the entire conformational space about the CC and CO bonds is easilyaccessible. An examination of the crystallographic record shows a much greater variety ofconformations to occur in the solid state as a result of the optimization of intermolecular interactionsin the crystals. Data mining shows that the conformations about the CC and CO bonds of all of theCC-staggered and CO-gauche structures are correlated, and consequently, the conformationalpreferences about the C−O bond greatly deviate from the expected 3-fold barrier. The theoreticalstudy shows the correlation to be instrinsic and offers an explanation. Natural population analysisprovides a rational for the relative stabilities of the benzyl alcohol conformations on the basis ofthe conformation dependence of the effects of electron−electron repulsion between the O-lone pairsand the benzene π-cloud, the charge−charge attraction between the hydroxyl H-atom and thebenzene 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 bonddipole moments.
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