Barriers to rotation in intramolecularly hydrogen-bonded 2-phenylazoresorcinols

Abstract

Separate peaks at δ 13.0 and 7.5 are observed for the intramolecularly hydrogen-bonded and nonhydrogen-bonded hydroxy groups in the ¹H n.m.r. spectrum of 2-phenylazoresorcinol in [²H₈]toluene at temperatures below ca. 300 K. At 357 K, equilibration of the two conformers in which the phenylazo group is hydrogen-bonded to each hydroxy group is rapid (k 5.4 × 10⁴ s^–1) and an averaged hydroxy peak at δca. 10 is observed. Energy barriers for substituted 2-phenylazoresorcinols in [²H₈]toluene and CD₂Cl₂ have been obtained from linewidth measurements of the hydroxy peaks and from observation of coalescence temperatures of aromatic ¹H and ¹³C signals.