The Nuclear Quadrupole Coupling Constant

For chemists, the important factor in the nuclear quadrupole coupling con-e2qQ.  

Similar relations can be used in comparing two isotopes of the same element. By observation of the same transition frequency of the two isotopic nuclei in one compound, one can assume a constant electric field gradient (that is, no measurable influence of the nuclear mass and the nuclear charge distribution on the core electrons). 

The situation is more complicated in cases where one wishes to compare the electric field gradients at the sites of different nuclei A and B (belonging to different chemical elements). Here the only reliable way to discuss and compare electric field gradients measured in different substances at the sites of different nuclei supposes an independent knowledge of QAIQn from theory or experiment. 

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Other key properties of these complexes relevant for comparison with experiment are the nuclear quadrupole coupling constants. Theoretical NQCC values, as calculated via electric field gradients, are discussed exhaustively in several recent theoretical investigations [26,28,30,33,34] and in experimental work [4-11]. BH HLYP/aug-cc-pVTZ calculated NQCCs [30,34], as evaluated for XY- -NH3 complexes, are listed in Table 10. 

We assume that, on formation of B- XY, a fraction 5j (i = intermolecular) of an electronic charge is transferred from the electron donor atom of Z of the Lewis base B to the npz orbital of X and that similarly a fraction 5p (p = polarisation) of an electronic charge is transferred from npz of X to n pz of Y, where z is the XY internuclear axis and n and n are the valence-shell principal quantum numbers of X and Y. Within the approximations of the Townes-Dailey model [187], the nuclear quadrupole coupling constants at X and Y in the hypothetical equilibrium state of B- -XY can be shown [178] to be given by ... 

Preliminary Cu-ENDOR results on Cu(salen) show that the nuclear quadrupole coupling constant of the copper ions lies in the range between those for square planar 04 and S202 coordinated copper complexes177. ... 

The nuclear quadrupole coupling constant, e Qq, of a singly-bonded chlorine atom, R—Cl, is related to the halogen atom valence p and p orbital populations, a and a , by the equation (7,2)... 

The unpaired electron in these two molecules is now in exactly the right orbital for the present purposes, since d —Px bonding is usually invoked in order to stabilise such antibonding or non-bonding orbitals. As we have seen both the nuclear quadrupole coupling constant and the anisotropic electron-nuclear hyperfine coupling constant are dominated... 

The nuclear quadrupole coupling constant for low-spin Co11 in a square planar environment was determined114 by ESR of Co-doped (Et4N)2Ni(mnt)2. A qualitative discussion was given to explain the finding that QD and QE were unexpectedly small. 

It is important to note that for 0 the nuclear quadrupole coupling constant is also an important NMR parameter, which can be used to characterize both the identity of the tetrahedrally coordinated atoms, T, in a T-O-T linkage, and the value of the T-O-T angle. Earlier calculations (24) showed a decrease in the O NQCC as the Si-O-Si angle decreases, a trend verified by experiment (34, 35). Later calculations(36) indicating a progessive decrease in the O NQCC for Si-O-Si, Si-O-Al and Al-O-Al angles were also verified. 

The subscript to q is omitted when no ambiguity can arise. The quantity e2Qq/h is the nuclear quadrupole coupling constant, usually quoted in MHz. 

The study of the nuclear quadrupole coupling constants, and their temperature dependence, of a series of similar compounds can thus provide a great deal of useful information about the bonding in such compounds. Unfortunately, in many instances such extensive series as discussed above are not available and much less information is obtainable. 

When a nucleus with a quadrupole moment is situated in an inhomogeneous electric field, there is a quadrupole splitting of nuclear energy states directly proportional to the quantity e Qq, the nuclear quadrupole coupling constant. In an axially symmetric field (t) = 0), the energy arising from the quadrupole moment (eQ) in an electric-field gradient (q) is ... 

Ab initio calculations of the nuclear quadrupole coupling constant suggest that the cyanate nitrogen atom is more ionic than that of the thiocyanate ion (118), but measurements show the latter to be more ionic than that in the selenocyanate ion (397). 

X e ctQlh is the nuclear quadrupole coupling constant, which can be obtained from microwave, NQR, or NMR spectroscopy. 

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