Measuring coupling constants relaxation delay

As in similar cases, where different nuclides of the same element are discussed, the chemical shifts of Li and Li (in ppm) are identical because primary isotope effects can safely be neglected, scalar spin-spin coupling constants are related by the factor y( Li)/y( Li) = 2.64, and relaxation mechanisms as well as NMR linewidths differ. Nuclear properties which are important for Li NMR experiments are collected in Table 2, where data for the widely used nuclei H and as well as N and P, which are of interest in the present context, are included for comparison. Both Li and Li possess a quadrupole moment, Q, but that of Li is the smallest one known for any nucleus. Li ium-6 NMR is, therefore, not dominated by the quadrupole moment and Li has been termed an honorary spin-1/2 nucleus [11]. Long relaxation times may sometimes cause difficulties for Li NMR measurements and this has to be taken into account by choosing a sufficiently long relaxation delay between individual... 

Since the short relaxation times associated with a quadrupolar nucleus drastically reduce the time delay to be applied in an NMR experiment between two pulses, measuring times are short or, in other words, distinct NMR signals can often be detected with a limited time spent down to micromolar concentrations. Along with this apparent advantage, quadrupolar nuclei provide information in addition to the classical parameters chemical shift (or shielding) and nuclear spin-spin coupling constants. Variations in linewidths for quadrupolar nuclei are another sensitive quantity allowing for the evaluation of the electronic and the steric situation in the first coordination sphere of a vanadium compound, its periphery, its (local) symmetry and its interaction with the matrix, i.e. counter-ions, solvent molecules and other constituents present in solution. 

Actually, the two terms of equation (8.7) have to be modified in order to account for a relaxation time distribution on both polarisation and conduction phenomena. As x is the time constant describing the electric field delay to 0, only a molecular relaxation process having Tj dielectric measurements using the electric modulus, the lower frequency relaxation will always be associated with the relaxation of long-range transport phenomena. Nevertheless, due to the similarity of equation (8.3) and (8.7), the first sight of a relaxation curve doesn t allow us to distinguish a relaxation caused by a conduction mechanism coupled with a polarisation mechanism from a relaxation of the conduction mechanism characterised by a broad distribution of relaxation times. A more precise study on several samples has then to be done. 

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