Measuring coupling constants quadrupole moment

One method of determining nuclear quadrupole moment Q is by measuring the quadrupole coupling constant, given by eqQ/h, where e is the charge of the electron and q the electric field gradient due to the electrons at the atomic nucleus. The extraction of Q depends on an accurately calculated q. As a test of our finite-field relativistic coupled cluster approach, preliminary results for Cl, Br, and I are presented. 

NMR shieldings at the oxygen can also be obtained for mineral systems and are calculated to show the same angular trends as for silicon (Tossell and Lazzeretti, 1988a). However, O is a quadrupolar nuclide (i.e., with a nonzero quadrupole moment), and its nuclear quadrupole coupling constant is thus an easier to measure quantity and, probably, one of more interest. When few data on the O NMR of silicates existed, Janes and Oldfield (1986) noted that different bonding models for silicates predicted different dependence of q° upon Si-O-Si angle. In particular. 

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... 

The frequencies of the observed hyperfine components were also used to estimate the quadrupole coupling constant for HCN. The splitting of the three components is related to the value of the quadrupole parameter, eQq, which is associated with the electric field gradient of the electrons across the nucleus with the quadrupole moment (3). The students measured the line frequencies, and calculated eQq using the following equation,... 

The 2 2-component of the EFG operator q is now to be determined by electronic structure theory either from relativistic or nonrelativis-tic wave functions. The expression e Qqzz in eq- (17) is the nuclear quadrupole coupling constant (NQCC) and can be obtained by experiment leading to the sought quadrupole moment eQ. Due to its dependence the EFG operator especially stresses the core region of the electronic wave function and relativity can be expected to play a major role. For the adequate treatment of heavy atoms and molecules containing heavy elements relativity is therefore indispensable and we will mention nonrelativistic results only for comparative purposes. Before we treat the subject of relativistic qzz calculations in detail a few common experimental techniques for accurate NQCC measurements are briefly discussed and the underlying physical principles mentioned. 

---