Conformational Phase Transitions Associated with Reversal of Hydrogen Bond Direction in 4-Chloro- and 4-Bromobenzyl Alcohols. An X-Ray Study

Notable changes in the molecular conformations of 4-chlorobenzyl alcohol around the C(H2)-Car and O-C(H2) bonds were found to take place at the first-order phase transition point (Tc1 = 236 K). Quite similar phenomena were observed at Tc1 (217 K) of the isomorphic 4-bromo compound. The two kinds of conformational changes are considered to couple and cause the reversal of the direction of the linear O-H. . . O hydrogen bond as the compound undergoes the first-order transition. The CH2OH moiety participates in several short intermolecular contacts, and a C-H. . .π interaction exists between the CH2 and the neighboring phenyl group. These intermolecular factors seem to contribute to the regulation of the molecular conformation in the crystal. The C-O bond length of each compound shortens considerably at T > Tc1.