Rotational isomerism—22 : An nmr study of oh..π bonding and the conformations of benzyl alcohol and derivatives

Abstract

The conformations of benzyl alcohol, the ortho and para nitro and methoxy derivatives and benzyl methyl ether have been investigated by NMR in CCL 4 and DMSO solutions. The 3J(CH.OH) and 2J(H.C.H) couplings (the latter via the 2J(H.C.D)coupling)and the OH chemical shift (in DMSO and ∞ dil XXX as conformational probes. The δ ∞ (OH) for ROH (R = Me, Et, iPr) is also given. The results provide no support for the existence of an intramolecular H-bond in benzyl akohol The endo conformation of the OH proton ( anti to a CH proton) is favoured by ca. 1 kcal mole −1 over the exo conformation (H anti to phenyl) and these conformers are responsible for the separate OH frequencies observed in the IR spectrum. The results do not support an extreme conformation of the phenyl ring (C.C.C.O dihedrals of 0 or 90°) but are consistent with either an 6̃0° conformation of the phenyl ring or a freely rotating model. In ortho nitrobenzyl alcohol intramolecular H-bonding is present, but in ortho methoxy benzyl alcohol little or no bonding to the substituent occurs.