Quadrupole coupling constant principles

For nuclei with /> 1, the quadrupole coupling constant is defined as the product of the nuclear quadruple moment eQ and the maximum principle value of electric field gradient tensor q. 

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. 

Slichter, C. P. Principles of Magnetic Resonance, 2nd ed. New York Springer (1978) Lucken, E. A. C. Nuclear Quadrupole Coupling Constants, New York Academic Press (1969)... 

Two of the three constants eqQaa, eqQbb id eqC c were determined from microwave optical double resonance (MODR) spectra of NH2 [1 to 4], microwave absorption spectra of ND2 [5], and by ab initio (UHF) calculations [6]. The third constant follows from eqQaa+ eqQbb + QQcc = 0 [3]- The nuclear quadrupole coupling constants listed in the following table are given in terms of the principle axis system (b C2, c 1 molecular plane). 

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