Benzene at 1GHz. Magnetic field-induced fine structure

The deuterium NMR spectrum of benzene-d6 in a high field spectrometer (1 GHz protons) exhibits a magnetic field-induced deuterium quadrupolar splitting ??. The magnitude of ?? observed for the central resonance is smaller than that observed for the 13C satellite doublets ???. This difference, ?(??) = ??? ? ??, is due to unresolved fine structure contributions to the respective resonances. We determine the origins of and simulate this difference, and report pulse sequences that exploit the connectivity of the peaks in the 13C and 2H spectra to determine the relative signs of the indirect coupling, JCD, and ??. The positive sign found for ?? is consonant with the magnetic field biasing of an isolated benzene molecule—the magnetic energy of the aromatic ring is lowest for configurations where the C6 axis is normal to the field. In the neat liquid the magnitude of ?? is decreased by the pair correlations in this prototypical molecular liquid

Benzene at 1GHz. Magnetic field-induced fine structure

International audienceThe deuterium NMR spectrum of benzene-d(6) in a high field spectrometer (1 GHz protons) exhibits a magnetic field-induced deuterium quadrupolar splitting Delta v. The magnitude of Delta v observed for the central resonance is smaller than that observed for the C-13 satellite doublets Delta v'. This difference, Delta(Delta v) Delta v' - Delta v, is due to unresolved fine structure contributions to the respective resonances. We determine the origins of and simulate this difference, and report pulse sequences that exploit the connectivity of the peaks in the C-13 and H-2 spectra to determine the relative signs of the indirect coupling, J(CD), and Delta v. The positive sign found for Delta v is consonant with the magnetic field biasing of an isolated benzene molecule-the magnetic energy of the aromatic ring is lowest for configurations where the C-6 axis is normal to the field. In the neat liquid the magnitude of Delta v is decreased by the pair correlations in this prototypical molecular liquid

Vibrational averaging of residual dipolar couplings: dependence on orientational order

Observed residual, inter-nuclear dipolar couplings are averages over vibrational motion. Calculations of this effect are made on the assumption that the vibrational modes can be approximated as harmonic in order to demonstrate the dependence on the orientational order of solute molecules dissolved in anisotropic solvents

Tetraethyl stannane: structure, conformations, and orientational order when dissolved in a nematic liquid crystalline solvent

Quantum chemical calculations are used to investigate the structure and the distribution of conformers of tetraethyl stannane, and these are compared with those of tetraethyl methane. The calculations predict that the most abundant conformers in tetraethyl stannane are those with symmetry C-1, in contrast to tetraethyl methane, in which the D-2d and S-4 conformers are dominant. The structural and conformational information on tetraethyl stannane are then used, together with the additive potential model, to explain the finite quadrupolar splittings observed in the deuterium NMR spectrum of tetraethyl stannane dissolved in a nematic liquid crystalline solvent. This analysis emphasizes that to understand the orientational order of a flexible molecule in a liquid crystalline phase it is essential to consider the symmetry of individual conformers rather than that of an average structure

Forty years of Progress in Nuclear Magnetic Resonance Spectroscopy

This issue completes the 50th volume of Progress in NMR Spectroscopy, edited since its initiation 40 years ago by Jim Emsley, Jim Feeney and Les Sutcliffe (Fig. 1). The journal was founded in 1966 shortly after the publication of their comprehensive (at the time) NMR text-book [1]. This was written when the authors were at Liverpool University during a period when NMR was expanding at an astonishing rate. After its publication it was realised that it would be virtually impossible for such a comprehensive text to be kept up-to-date by simply publishing further editions. For this reason Les Sutcliffe approached our publisher with the proposal to set up a review series based on invited articles from carefully chosen NMR experts to provide updated coverage of selected areas across a broad front of the subject. His proposal was readily accepted and Progress in NMR Spectroscopy was born. The journal has continued to grow and flourish initially with Pergamon Press and latterly (since 1991) with Elsevier. We encourage our selected authors to write thorough, detailed and authoritative review articles that will be seen by the NMR community as being the primary source for learning about a topic. The extent to which this has been successful can be judged by an examination of the contents of the first 50 volumes given in Appendix A. The electronic versions of any of these articles can be accessed via Science Direct. From the diversity of the review titles in Appendix A it can be seen that our aim to cover all aspects of NMR and its wide application in chemistry, biology and medicine is being met

The pressure and temperature dependence of the orientational order in the nematic phase of 4-n-pentyl-d11-4'-cyanobiphenyl. A deuterium NMR study

Deuterium NMR spectroscopy has been used to measure the temperature and pressure dependence of quadrupolar splittings in alkyl chain deuteriated 4-n-pentyl-4'-cyanobiphenyl. The quadrupolar splittings are used to obtain order parameters SiCD for each position in the chain, and these are found to be independent of pressure at the nematic-isotropic transition temperatures. The data are used together with an equation of state for the nematic phase to derive the variation of Si CD with temperature at constant volumes of 0.243 dm3 mol -1 and 0.248 dm3 mol-1. These two sets of SiCD are compared with the predictions of a mean field theory of flexible molecules. The density dependences are obtained of the averaged interaction coefficients which appear in the theory, and the results suggest that relatively short range forces dominate the potential of mean torque.On utilise la résonance magnétique du deutérium pour mesurer la dépendance en température et en pression des dédoublements quadrupolaires de 4-n-pentyl-4'-cyanobiphényl à chaînes alkyl deutérées. Ces dédoublements donnent les paramètres d'ordre SiCD pour chaque position sur la chaîne, qu'on trouve indépendants de la pression sur la ligne de transition nématique-isotrope. On utilise les résultats pour obtenir, par une équation d'état de la phase nématique, la variation des SiCD avec la température à volumes constants de 0,243 dm3/mole-1 et 0,248 dm 3/mole-1. On compare ces deux groupes de Si CD avec les prédictions d'une théorie de champ moyen pour des molécules flexibles. On obtient ainsi la dépendance en densité des coefficients d'interaction de la théorie ; les résultats suggèrent que le potentiel de couple moyen est dominé par des forces à courte portée

The conformational distribution in diphenylmethane determined by nuclear magnetic resonance spectroscopy of a sample dissolved in a nematic liquid crystalline solvent

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d(3) dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, D-ij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles tau(1) and tau(2). Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(tau(1),tau(2)), which is then used to define a probability density distribution, P(tau(1),tau(2)). It is shown here that when attempting to obtain P(tau(1),tau(2)) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(tau(1,)tau(2)). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on tau(1)=tau(2)=56.5+/-0.5degrees, which is close to the values calculated for an isolated molecule of 57.0degrees, and significantly different from the asymmetric structure found in the crystalline state

The effect of a chiral nematic solvent on the orientational order and conformational distribution of a flexible prochiral solute

Deuterium NMR spectroscopy is used to detect the reduction in symmetry which occurs when a flexible, prochiral molecule, 4-pentyl-4'-cyanobiphenyl, is dissolved in a chiral nematic solvent produced by dissolving poly-gamma-benzyl-L-glutamate in organic solvents. The quadrupolar splittings obtained are used to investigate whether this phenomenon is the result of a reduction in symmetry of the anisotropic, solute - solvent interaction potential or whether the conformational distribution is also affected. It is demonstrated that the first assumption explains the enantiotopic discrimination in chiral liquid crystals and it is possible in this chiral medium to determine the relative configuration of the deuteriums in the methylene groups