Deuteron quadrupole coupling in hydrogen bonded systems. IV. Deuteron quadrupole coupling in substituted phenols

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Abstract
The pure nuclear quadrupole spectrum of deuterium at oxygen in chlorinated phenols has been observed by level crossing double resonance with the ring protons. Observed coupling constants, ranging from 200 to 250 kHz, exhibit the effects of strong hydrogen bonding through their correlation with the intermolecular O–H⋅⋅⋅O bond lengths in those cases where structure data are available. This is interpreted below to indicate that a primary effect of chlorine substitution is the influence it exerts on intermolecular hydrogen bonding through modification of the barrier to hydroxyl group rotation. From an extrapolation of the observed distance dependence, one may obtain a coupling constant of ∼283 kHz for deuterium at the oxygen position in non-hydrogen-bonded phenol. An interpretation of these data is made using model calculations on phenol, vinyl alcohol, and chlorovinyl alcohol. Barriers to internal rotation, as well as the dependence of the deuterium quadrupole coupling constant on the orientation of the –OD bond with respect to the ring plane are reported using a standard STO-4/31G basis. A value of 299 kHz is obtained for the deuterium coupling constant in the gas phase by interpolation from STO-4/31G calculations and gas phase data for water, methanol, and formic acid. Calculation demonstrates that the coupling constant of deuterium is essentially independent of –OD twist angle in phenol, and that vinyl alcohol is an excellent model fragment for phenol insofar as deuterium field gradients are concerned. Chlorovinyl alcohol exhibits a substantially larger barrier to internal –OD rotation than phenol, and also shows a weak dependence of field gradient on twist angle. Since solid phenol and substituted phenols show substantial deviations of the aromatic ring and –OD group from coplanarity, one may rationalize trends in the coupling constants on the basis of a modification of the barrier to internal hydroxyl group rotation by chlorine substitution. Attempts were made to obtain spectra in methyl- and bromo-substituted phenols, but with the exception of 2,4-dimethylphenol, low field relaxation times were too short to permit their observation.

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