The Quadrupole Splitting Constants

The quadrupole splitting constant, QSC, of Li has been used by Jackman et al. [151] and Johnels [152] as an empirical parameter to obtain structural information on lithium enolates in solution. Tlie QSC is given by equation (11). 

If Tc can be determined from the dipolar longitudinal relaxation of a nucleus in the same molecule, which has its intemuclear distance vector 

An important result of the relaxation studies on Li carried out by Wehrli [9,40] was the finding that an appreciable H, Li nuclear Overhauser effect (NOE) exists which amounts to ti = 2.61 and 1.19 for Li in aqueous LiCl and n-butyllithium in n-hexane, respectively. The theoretical value is Ti=y( H)/ 2y( Li) = 3.40 (Table 1). Due to the inefficient quadrupolar relaxation, the Li nucleus shows considerable dipolar interactions with neighbouring protons which can yield valuable structural information. Similar effects for Li are uncommon because of the stronger dominance of quadrupolar relaxation for this nucleus, but appreciable H, Li NOEs have been found for systems like 7 [37,38]. 

The great potential of the H, Li NOE for structural research was recognized in 1986 independently by two groups. Avent et al. [154] reported ID NOE difference experiments for hydrido[tris(trimethylsilyl)methyl]-metalates, while Bauer et al. [155] introduced the H, Li HOESY experiment. The pulse sequence for these types of experiment, (ix). 

The dynamic nature of most organolithium compounds in solution forms the basis for broad applications of dynamic NMR (DNMR) in this held. Lineshape changes are observed for all relevant nuclei ( H, Li, P) 

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These parameters are close in the order of magnitude to the quadrupole splitting constants of Li in LiNhOs (K=46 kHz), in lithium nitrate LiNOs (K=39 kHz) [35]. Therefore, it can he assumed that the structure of the first coordination sphere of... 

Table 1. The 72-atom model examined by different theoretical methods. The energy differences (AE in kcal/mol) are calculated with respect to the lowest SCF energy. q(Fe) stands for Mulliken population charges on the Fe atoms q(S) and SS(b.i.) are the Mulliken population charges and the bond index for the bridging S atoms, respectively AEq is the calculated Mossbauer quadrupole splitting constant [mm/sec]. The PUHF spin states are those projected from the UHF wavefunction with 5 = 5,. 

PUHF, yield the lowest energies as well as reasonable Mossbauer quadrupole splitting constants, A Jq. 

The experimentally observed quadrupole splitting AEq for Fe in inorganic compounds, metals, and solids reaches from 0 to more than 6 mm s [30, 32]. The range of AEq for other Mossbauer isotopes may be completely different because of the different nuclear quadrupole moment Q of the respective Mossbauer nucleus, and also because the EFG values may be intrinsically different due to markedly different radial distributions of the atomic orbitals (vide infra). As Q is constant for a given isotope, variations in the quadrupole coupling constants eQV can only arise from... 

Fig. 4.14 Magnetically perturbed quadrupole spectra simulated for powder distributions of the EFG (Vzz > 0) with an applied field B = 4T which is fixed in the laboratory system perpendicular to the y-beam/ The value of the quadrupole splitting is kept constant at AEq = +4 mm s For negative quadrupole splitting (V z < 0), the spectra would be inverted on the velocity scale. Note the difference in relative intensities for the spectrum for ry = 0 and the single-crystal type spectrum given in Fig. 4.13. Similar patterns are obtained for B y...

Chemical information from hafnium Mossbauer spectroscopy can primarily be deduced from the quadrupole-splitting parameter. In Table 7.4, we have listed the quadrupole coupling constants eQV for some hafnium compounds. Schafer et al. 

Finally, we mention that Li T measurements have been used to study the rate of interconversion between tight and loose ion pairs of lithium fluorenide [44]. The quadrupolar relaxation contribution was derived from Li and Li T data, and a change of the Li quadrupole splitting constant, QSC, obtained via Li quadrupolar and dipolar relaxation rates (see Section 4) with ion pair structure, was indicated. 

The quadrupole splitting, Ai q of 117.8 kHz observed for the oriented block of [2,2- H2]Gly labeled silk fibroin fiber is the same as the value in the spectrum of an unaligned sample as shown in Fig. 23.9(B). Therefore, if the latter quadrupole splitting is used to provide a value of the proportionality constant, (3/4)e qQ/h, which describes the relationship between the quadrupole splitting and the angle CD (the angle of the C H bond vector of Gly relative to the fiber axis) > CD is calculated as 90°. By taking into account an... 

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,... 

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