Theoretical study of intramolecular hydrogen bonding and molecular geometry of 2-trifluoromethylphenol

The conformational behavior of 2-trifluoromethylphenol was investigated by means of theoretical calculations. Four characteristic structures have been found on the potential energy hypersurface of the compound: anti form (local minimum), in which the hydroxy hydrogen points away from the trifluoromethyl group; and three syn forms (the hydrogen points towards the trifluoromethyl group), with different trifluoromethyl torsions (global minimum, one low and another one high lying saddle-point). The geometry of these conformers were optimized by ab initio calculations using 6-31G** basis set. The effects of electron correlation were investigated by MP2 and various DFT methods. To investigate the intramolecular interaction in the syn forms, the electron density distribution was calculated at the MP2 level of theory. In the structure corresponding to the global minimum at the MP2/6-31G** level a bond critical point was found in Bader's sense between the hydroxy hydrogen and a fluorine of the trifluoromethyl group indicating hydrogen bonding interaction. The length of the hydrogen bond, 1.98 Å, corresponds to medium strength interaction. The O(SINGLE BOND)H bond is slightly twisted and the C(SINGLE BOND)F bond, interacting with it, is considerably twisted out of the plane of the benzene ring to the same side of the ring. The most pronounced geometrical consequence of the hydrogen bond is the 0.02-Å lengthening of the C(SINGLE BOND)F bond participating in its formation. All the other geometrical changes in 2-trifluoromethylphenol, as compared with trifluoromethylbenzene and phenol, are also consistent with the phenomenon of resonance-assisted hydrogen bonding. © 1996 by John Wiley & Sons, Inc.