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Experimental determination of NMR tensors
Liquid crystal NMR spectroscopy (LCNMR) is a very applicable method for the determination of physical properties of solute molecules. The orientation distribution of molecules in a liquid-crystalline environment is anisotropic resulting in non-zero dipolar couplings, Dexp, which consist of information on the molecular structure, orientation and the indirect spin-spin coupling tensors. Furthermore, the experimental chemical shifts can be used for studies of nuclear shielding tensors.
Spin-spin coupling tensors have experimentally been determined, for example, for the following molecules (recent studies include also quantum chemical calculations, see Theory and computation of NMR parameters): acetonitrile, benzene, ethane, ethylene, ethyne, methylsilane, methylisocyanide, fluoromethanes, p-difluorobenzene, dimethylmercury, etc. A review article (J. Vaara, J. Jokisaari, R.E. Wasylishen and D. Bryce, Spin-Spin Coupling Tensors as Determined by Experiment and Computational Chemistry) will appear soon in Progress in Nuclear magnetic Resonance Spectroscopy.
The investigation of spin-spin coupling tensors is motivated also by the fact that if one wants to utilize experimental D, for example DCC, DCN, and DCF, couplings in structure and conformation studies of liquid crystal molecules or molecules dissolved in liquid crystals (including proteins in the so-called dilute liquid-crystalline solutions) one has to know the possible contribution of the corresponding indirect spin-spin coupling tensor to D.
The nuclear shielding tensor elements may be derived by changing the orientational order of the molecule under investigation and dissolved in a LC or by changing the orientation of the LC director. There exist several possibilities which are described, for example, in J. Lounila and J. Jokisaari, Anisotropies in Spin-Spin Coupling Constants and Chemical Shifts as Determined from the NMR spectra of Molecules Oriented by Liquid Crystal Solvents, Progress in Nuclear magnetic Resonance Spectroscopy, 15, 249 (1982), and in J. Jokisaari, Anisotropy of Shielding & Coupling in Liquid Crystalline Solutions, Encyclopedia of Nuclear Magnetic Resonance (eds. D.M. Grant and R.K. Harris), 2, 839 (1996). The method has been applied to a number of molecules and to nuclei from 1H to 199Hg. A recent study dealt with the 199Hg shielding tensor anisotropy in methylmercury halides combined with ZORA DFT calculations.
Solid state MAS NMR is used to the investigation of 31P and 199Hg shielding tensors in a few mercurous phosphates. Details will be given later once the spectral analyses are completed.