Electric field gradients, point-charge model

Terms up to order 1/c are normally sufficient for explaining experimental data. There is one exception, however, namely the interaction of the nuclear quadrupole moment with the electric field gradient, which is of order 1/c. Although nuclei often are modelled as point charges in quantum chemistry, they do in fact have a finite size. The internal structure of the nucleus leads to a quadrupole moment for nuclei with spin larger than 1/2 (the dipole and octopole moments vanish by symmetry). As discussed in section 10.1.1, this leads to an interaction term which is the product of the quadrupole moment with the field gradient (F = VF) created by the electron distribution. 

In the case of compounds where the ligand contribution determines the the point-charge model can be applied to calculate the electric field gradient as the sum of independent contributions of each ligand. Based on the point-charge model, the calculation of for trans... 

M. Pempointner, M. Seth, P. Schwerdt-feger. A point-charge model for the nuclear quadrupole moment Coupled-cluster, Dirac-Fock, Douglas-KroU, and nonrelativistic Hartree-Eock calculations for the Cu and F electric field gradients in CuF. /. Chem. Phys., 108(16) (1998) 6722-6738. 

Nuclear quadrupoje resonance (NQR) has been employed by Carlson and co-workers [Carlson 5969)] to measure the nuclear quadrupole interaction of C1 in the trichlorides of La, Ce, Pr, Nd, Sm, Eu, and Gd in the paramagnetic state at 4.2 K. The point-charge model for the electric field gradient was found not to predict satisfactorily the trend of the observed resonance frequencies. 

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