Phenyl rotation barrier in Z‐azobenzene: A physical organic problem in honor of J.D.R.

Abstract

AbstractThe Z isomer of azobenzene exhibits nonbonded interactions between ortho substituents on adjacent phenyl groups. For o,o′‐difluoroazobenzene derivatives, through‐space 19F‐19F spin‐spin coupling is observed. The coupling constant decreases as the nuclear magnetic resonance probe temperature is decreased culminating in collapse to a single peak. We have used density functional theory and X‐ray crystallography to argue that phenyl group rotation is responsible for this loss of through‐space coupling. We experimentally measured phenyl rotation barriers in the range of 37 to 44 kJ/mol for o,o′‐difluoroazobenzene derivatives. These rotation barriers are qualitatively similar to literature experimental values for fluxional systems but higher than calculated values for Z‐azobenzene.This is the only project in 35 years that I discussed with J.D.R. This mechanistic interpretation of an unusual nuclear magnetic resonance phenomenon is a classic physical organic investigation inspired by my training with Roberts. I tell this scientific story interwoven with a few observations about working with Roberts.