High-level ab initio calculations of torsional potential of phenol, anisole, and o-hydroxyanisole: Effects of intramolecular hydrogen bond

The internal rotational barrier heights of phenol and anisole were calculat ed using several basis sets up to cc-pVQZ with MP2-level electron correlati on correction to evaluate the basis set effects. The calculations showed th at the effects of the further improvement of the basis set beyond the cc-pV TZ were very small. Although the electron correlation substantially increas ed the barrier heights of the two molecules. the effects of the electron co rrelation beyond the MP2 method were not large. The barrier heights calcula ted with the CCSD(T) method were close to those with the MP2 method. The in ternal rotational potentials of methoxy and hydroxyl groups of o-hydroxyani sole were calculated at the MP2/cc-pVTZ//HF/G-311G** level. The calculated potentials were compared with those of phenol and anisole, o-Hydroxyanisole preferred planar structure in which the hydroxyl group had an intramolecul ar hydrogen bond with the oxygen atom of the methoxy group. The calculated torsional potential of the methoxy group had the maximum (7.30 kcal/mol) wh en the methoxy group rotated 180 degrees from the minimum energy structure, in which the hydroxyl group did not have the hydrogen bond. The barrier he ight of the methoxy group of o-hydroxyanisole was considerably larger than that of anisole (2.99 kcal/mol). The large internal rotational barrier heig ht of o-hydroxyanisole showed that the intramolecular hydrogen bond greatly stabilized the energy minimum structure and that the hydrogen bond strictl y restricted the conformational flexibility of the methoxy group.