Internal rotation of para-substituted phenols. A low temperature dielectric relaxation study

The dielectric relaxation rates of 18 phenols and 3 thiophenols have been determined in paraffin solutions at 4.2 or 77 K and used to obtain relative values of the tunnel splitting of the ground torsional state. Where comparison is possible the results are compatible with vapour phase microwave spectroscopic data. The barriers to hydroxyl rotation are calculated and are fairly consistent with barriers derived from far infrared data. Para-substituents F, CH₃, Cl, C(CH₃)₃, Br and I in order of decreasing effectiveness, lower the barrier. COOH and CHO raise it by 350–400 cm^?1.