C. MacLean

Anisotropy of the diamagnetic susceptibility of benzene. Determination by high-field deuterium NMR

Abstract A doublet due to quadrupole splitting cannot be detected in the deuterium NMR spectrum of perdeuterobenzene at 141 kG, whether neat, in dilute solution, or in the gas phase. However, the proton-decoupled spectrum of monodeuterobenzene at 296 K consists of a doublet with spacing of 0.50 Hz at 1 mol% concentration in acetone- d 6 , decreasing to 0.42 Hz at 80 mol% concentration. The observed splitting provides a value for the anisotropy of the diamagnetic susceptibility of -1.11 :K 10 −28 emu/molecule at I% concentration and -0.93 × 10 −28 emu/molecule at 80% concentration, where transient dimers may be formed. The difficulty with perdeuterobenzene is traced to the effect of the scalar spin-spin coupling between deuterons.

N.M.R. study of the dipolar alignment in fluid mixtures induced by strong electric fields

This paper deals with N.M.R. electric field experiments on mixtures of polar and non-polar liquids. Alignment of the molecular dipoles by application of a strong electric field is manifested in the N.M.R. spectrum of the polar component. 2H and some 14N N.M.R. electric field experiments are reported for perdeuteronitrobenzene, dissolved in benzene and carbon tetrachloride. The data display the dependence of the local field on the composition of the mixture in a direct manner. The Lorentz and Onsager local field models are shown to give no adequate description in concentrated solutions of perdeuteronitrobenzene in benzene and carbon tetrachloride. Molecular interactions in the theoretical expression of the alignment of the polar component are described in terms of correlation functions for the mixture. A detailed formula for the dipolar alignment, which should be valid for dilute solutions, is compared with experiment.

Alignment of non-polar molecules—electric field effects in the 2H N.M.R. spectrum of perdeuterobenzene in solutions with nitrobenzene

Electric field experiments on the 2H N.M.R. spectrum of perdeuterobenzene, dissolved in nitrobenzene, are reported. The alignment of the polar molecules, induced by a strong electric field, is transferred to the non-polar ones and as a result their N.M.R. spectrum is modified. Local field models, e.g. those of Lorentz and Onsager, treat the environment of a molecule as a continuum and are unable to explain the alignment of the non-polar molecules. Intermolecular interactions are essential. In an expression for the alignment these interactions are described in terms of correlation functions for the mixture. N.M.R. and Kerr data of nitrobenzene-benzene solutions have been compared. The Kerr constant of a binary mixture is shown to depend on the alignments of both the polar and non-polar component. N.M.R. electric field experiments may be useful to separate these two contributions.

N.M.R. of some nitriles in electric fields

14N N.M.R. experiments are reported for pure liquid nitriles, partly aligned by strong electric fields. The average value of the quadrupole coupling does not vanish, and results in a splitting of the N.M.R. lines. From the N.M.R. splittings it follows that the internal field in these liquids, excepting chloroacetonitrile, is well described by Onsagers model. This result is interpreted to mean that intermolecular short-range interactions are of limited importance.

Chemical shielding anisotropy of 13C in CH3 CN determined by NMR spectroscopy of the dielectrically oriented molecule

A successful determination of a shielding anisotropy with electric field NMR is reported for the first time. The nucleus studied is the nitrile 13C in liquid acetonitrile. From 13C frequency changes and 14N quadrupolar splittings upon dielectrical alignment, the chemical shift anisotropy of 13C is found to be 378 ± 20 ppm. The 14N quadrupole coupling constant is obtained from 1H and 14N spectra of the orientated molecule, giving a value of 3·47 ± 0·16 MHz.

NMR of molecules partially oriented in the gas phase

Abstract The vapor phase 2 H NMR spectra of monodeuterobenzene and pentadeuterofluorobenzene were recorded at 14.1 T. Reduction of the collision time, and thus of the linewidth, was accomplished by compressing the gas to 10.8 atm with ethane. The molecules are aligned by the field of the spectrometer and, as a consequence, quadrupolar couplings are recorded. Differences from the liquid-phase spectra arise in some of the quadrupolar splittings as well as in the scalar D-F couplings. It is rationalized that the deviating quadrupolar interaction cannot be caused by interactions resulting from incomplete quenching of the rotation, but originates from environmental effects on the quadrupolar coupling in the liquid. The results agree with recent microwave experiments.

Studies of quadrupolar and dipolar electric field effects in the NMR spectra of binary mixtures of liquids

Abstract Molecular alignment of binary mixtures, induced by an external electric field, has been studied as a function of mixture composition. The alignment factors were calculated from the quadrupolar splittings in the 2 H-NMR spectra. The investigated mixtures were: nitrobenzene/ethylenecarbonate, nitrobenzene/acetonitrile and acetonitrile/benzene. Intermolecular interactions strongly affect alignment. The results are discussed in terms of geometry changes and/or changes in the asymmetry parameters of 2 H quadrupole coupling constants.

Anisotropy and asymmetry of diamagnetic susceptibility, quadrupole coupling constants and molecular motion of fluorene and the fluorenyl carbanion

Fluorene and the fluorenyl carbanion, both perdeuterated, have been studied by 2H N.M.R. at 11·7 Tesla. From deuteron splittings and spin-spin relaxation times, information is obtained about the anisotropy and asymmetry of the diamagnetic susceptibility, the quadrupole coupling constants of the different deuterium nuclei and the rotational diffusion constants of the two species.

The determination and assignment of a complete 14N quadrupole coupling tensor in liquid solution

Abstract 14 N line splittings in the spectrum of nitrobenzene (neat liquid) and metadinitrobenzene (dissolved in benzene), induced by an external electric field, have been used to determine the complete 14 N quadrupole coupling tensor of these substances. Assuming that both molecules are rigid and planar, and that the quadrupole coupling tensors at the 14 N nuclei are identical, the principal components in a local reference frame ( x ′, y ′, z ′) are ( eQ/h ) V x ′ x ′ = ±0.34 MHz ( eQ/h ) V y ′ y ′= ±1.18 Hz and ( eQ/h ) V z ′ z ′ = ±1.52. The z′-direction is parallel to the CN bond and the y−direction is perpendicular to the plane of the nitrogroup. With these data the asymmetry parameter η = 0.55.

The quadrupole coupling tensor of 95Mo in mesitylene molybdenum tricarbonyl

The quadrupole coupling tensor of 95Mo in mesitylene-d1 molybdenum tricarbonyl is determined in benzene solution by NMR of the dielectrically aligned molecule. The largest principal component e 2 qQ/h, which lies along the dipole moment of the complex, is ±(954 ± 50) kHz. The asymmetry parameter η is zero because of symmetry considerations. A value of 10·9 ± 0·6 MHz for the quadrupole coupling constant of 97Mo is obtained by evaluating the quadrupole-moment ratio of 97Mo and 95Mo. Rotational diffusion of the complex in solution is investigated by relaxation measurements. The dipole moment of the complex is estimated to be (2·1 ± 0·1) × 10-29 Cm (6·2 ± 0·4 D).