Torsional barriers in para-substituted phenols from ab initio molecular orbital theory and far infrared spectroscopy

The effects of para substituents (OH, F, Me, CHO, CN, and NO2) on the barrier to rotation about the C–O bond in phenol as determined by ab initio molecular orbital calculations and far i.r. spectroscopic measurements are generally in close agreement; substituents which are π-electron donors lower the observed barrier while π-electron acceptors raise the barrier.