Electron reorganization induced by intramolecular hydrogen bonding in phenol derivatives: relationship between proton chemical shifts and the hydroxylic electron populations

Abstract

Intramolecular hydrogen-bonding effects in ortho-substituted phenols have been studied by ¹H n.m.r. and i.r. spectroscopy and CNDO/2 calculations. Full geometry optimization using the CNDO/2 method indicates definite variations of the OH bond length in intramolecular hydrogen-bonded phenols whereas these lengths remain nearly constant in para- or meta-substituted derivatives. There is a good linear relationship between the OH bond length and the i.r. ν_OH frequency shift, considered as reflecting the intramolecular association strength. Comparison of ortho- and para-substituted phenols shows that intramolecular hydrogen-bond formation induces important δ electron transfer from the substituent X to the hydroxylic oxygen via the bonded hydrogen atom whereas the electron population of this hydrogen atom decreases. This transfer results in π-electron reorganization of the X–C(1)–C(2)–OH fragment, and thereby increases the electron-releasing character of the OH group and the electron-withdrawing power of acceptor X groups, but the remaining atoms of the ring are little affected. With such an unusual electron reorganization, the chemical shifts of the phenolic hydrogen in ortho-, meta-, and para-substituted phenols can be simultaneously correlated with the electron population only if we consider both the proton investigated and the oxygen atom bearing it. The correlation Δδ=–31.6Δq_HM+ 16.1ΔQ^σ_O+ 0.08 satisfactorily reflects the proton shifts for the 22 compounds investigated.