Intermolecular Hydrogen Bonds between a Radical and a Diamagnetic Matrix: CW-ESR Investigations of 2,6-Di-tert-butyl-4-hydroxymethylphenol

Abstract

We propose a novel formation model of an intermolecular hydrogen bond between radicals and a diamagnetic matrix in the condensed phase. W-band and multi-frequency ESR studies revealed that the polycrystalline radical sample generated from PbO 2 oxidation of 2,6-di-tert-butyl-4-hydroxymethylphenol in toluene solution in vacuum at room temperature has an anomalously large 1 H-hyperfine interaction. It was concluded that this interaction is attributed to hydrogen bonding between the O• which is a paramagnetic center in the molecular structure of the radicals and the para-OH or the phenolic OH of the primary phenol. Although the phenol shows interesting radical chemical reactions unlike other 2,6-di-tert-butylphenol derivatives, the generated radical species were successfully defined by the solution ESR investigations. This may be the first ESR observation of an anomalously large hyperfine doublet structure coming from a proton on an intermolecular hydrogen bond between a radical and a diamagnetic phenol.

The Phenoxyl Radical–Water Complex—A Matrix Isolation and Computational Study

“…Radicals are in principle also capable of accepting hydrogen bonds, although to a lesser extent. − Previous theoretical studies indicate that indeed radical centers can act as weak hydrogen bond acceptors. ,− ,, There is a principal difference between hydrogen bonding to closed shell molecules and to radicals: whereas a “normal” hydrogen bond can be looked upon as an intermediate along the reaction coordinate resulting in proton transfer, hydrogen-bonded radicals are intermediates toward hydrogen atom transfer, although proton transfer can also be an option. There are only few experimental studies of radicals as hydrogen bond acceptors. − ,,,− For the phenyl radical interacting with water, an OH···π complex was found under the conditions of matrix isolation. , This complex is thermally and photochemically highly labile, and excitation with visible light results in the formation of a π-complex between the hydroxyl radical and the benzene. Thus, a hydrogen atom is transferred from water to the phenyl radical.…”