Phase transitions, hydrogen bond and crystal dynamics of p-methylbenzyl alcohol as studied by single crystal X-ray diffraction and 2H NMR

The title compound (pMBA) was found to undergo a first-order phase transition at 211 K (T-c1). Another transition with subtle enthalpy change appeared at 172 K (T-c2). Crystal structure determinations at various temperatures revealed that the transition at T-c1 was accompanied by remarkable changes in the molecular conformations around the CH2-C and O-CH2 bonds and a reversal of the direction of the O-(HO)-O-... hydrogen bond. Experiments of H-2 NMR were carried out on pMBA-d where the hydroxyl hydrogen of pMBA was selectively deuterated. Analyses of the H-2 NMR spectra and the temperature dependence of T-1 of the H-2 NMR indicated occurrence of jumping motions of H-2 between asymmetric potential wells at temperatures lower than T-c1.