Deuteron NQR spectra of several model systems involving alcoholic of phenolic-OD groups are discussed. The spectra of alpha hydroquinone and its two isomers resorcinol and catechol show complex structure due to the presence of inequivalent O–D⋅⋅⋅O hydrogen bonds. In the case of hydroquinone, this structure collapses to that characteristic of a single type of hydrogen bond in the beta- or clathrate-forming phase. An attempt is made to place the data in theoretical perspective by calculations of the deuterium field gradient in hydroxide ion, hydroxyl radical, methanol, and methanol dimer and by comparison with precise Hartree–Fock computations from the literature.
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.
The purpose of this article is to examine experimental data for deuterium quadrupole coupling in a number of hydrogen bonded systems in the context of approximate, but moderately accurate, ab initio calculations of the field gradient, and to suggest a quantitative interpretation of experimental data in terms of the modification of the local electronic structure on hydrogen bonding. Systems discussed include HF, dimers of HF in the solid configuration and in the geometry of the gas phase complex, HCl, dimers and clusters of HCl in the geometry of the orthorhombic solid, formic acid, maleic acid, and several other carboxylic acids, and salts of pyridinium ion. For the case of DF, for which a plethora of calculations is available, computations of the deuterium field gradient at the STO-3G, −4/31G, −6/31G*, and −6/31G** are presented in the context of previous work. Hydrogen bonding has, according to the calculations, a substantial effect upon the magnitude of the field gradient and a much smaller effect upon its principal axis directions. According to these calculations, the deuterium coupling constant in the gas phase dimer in the interior position should be depressed by ∼ 5% from the monomer value, while the depression of the coupling constant at the exterior position should be less than 1%. In a model dimer at the solid geometry, the former is depressed by ∼ 10%. In solid DCl, these effects are somewhat smaller, of the order of 5%, but observable. For the other species examined in this investigation, effects of hydrogen bond formation on coupling constant are also substantial and always in the direction of decreasing field gradient. It is found that although the absolute values of the field gradient are strongly basis dependent, differences are remarkably constant. Calculations at STO-4/31G seem to predict the changes observed on hydrogen bonding to good precision. The partition of these changes into variation with bond length and with charge distribution is discussed. Cross-sectional charge density difference maps are presented for a variety of cases and discussed in terms of the changes which occur at constant distance. A normal feature which results from hydrogen bonding is depletion of charge at the hydrogen, accompanied by a strong accretion in the bond region. A very small buildup of charge is also found in the hydrogen bond region and other manifestations of polarization are evident in the density difference maps as well.
Das NQR‐Spektrum von Deuterium am Phenol‐Sauerstoff wurde für die chlorierten Phenole 2‐, 3‐, 4‐Chlor‐phenol, 2,3‐, 2,4‐, 2,5‐, 2,6‐, 3,4‐, 3,5‐Dichlor‐phenol, 2,4,6‐Trichlor‐pheno1 sowie für 2,4‐Dimethyl‐ phenol aufgenommen.
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