Quadrupole coupling tensor

Oxirane is an important Lewis base in the present context. The O atom carries two equivalent n-pairs of electrons, as it does in H20, but oxirane has the advantage over water in that it is possible to determine both angles 0 and 9 for its complexes with HC1 and ClF because the non-zero off-diagonal element Xab(Cl) of the Cl nuclear quadrupole coupling tensor is available. The corresponding Lewis base in which an S atom carries two equivalent n-pairs is thiirane. Each of the pair of complexes (CL S- -HC1 and (CL S- -ClF has Cs symmetry and here it is the off-diagonal element Xac(Cl) that is non-zero... 

No effort to examine this possibility was undertaken until Byberg observed a similarity between the quadrupole coupling tensors in chloryl and perchlorate radicals. The species CIO4 could be interpreted as an exchange coupled complex of CIO2 and O2 [7]. 

Nuclear Quadrupole Coupling Tensors in Some Aluminosilicates... 

Conventional single-crystal studies of aluminosilicates date back to the early 1950 s, and principal components of the quadrupole-coupling tensors together with asymmetry parameters have been determined for a number of minerals (see Table XI). Wide-line-NMR measurements of the dependence of the 27A1 and 23Na quadrupole-coupling constants in analcime and zeolites Na-X and Na-A on the water content were carried out by Gabuda et al. (137). 

Water and ice polymorphs. Water is a very interesting compound as the oxygen quadrupole coupling tensor of the free molecule has been accurately measured by beam-maser spectroscopy (27) and various ice polymorphs have been studied by NMR or NQR (28). This gives us an opportunity to show that the PCM formalism is able to switch easily between isolated molecules and fully reticulated networks. Table II gives our results obtained using the following parametrization for... 

Lowenstein and Igner86 have determined the quadrupole coupling tensor of 33S in dimethyl sulphone, both in liquid crystalline solvents and in CHCI3 solutions. In solution, an approximate value of xc has been measured from 2H y and Tj. 33S y values are as follows 1.63MHz for rj — 0 and 1.41 MHz for q — 1. In liquid crystals, x is about 1.8 MHz. These values seem to be in agreement within... 

Naito A, McDowell CH (1984) Determination of the UN quadrupole coupling tensors and the 13C chemical shielding tensors in a single crystal of L-aspaigine monohydrate. J Chem Phys 81 4795-4803... 

Such order can be described in terms of the preferential alignment of the director, a unit vector that describes the orientation of molecules in a nematic phase. Because the molecules are still subject to random fluctuations, only an average orientation can be described, usually by an ordering matrix S, which can be expressed in terms of any Cartesian coordinate system fixed in the molecule. S is symmetric and traceless and hence has five independent elements, but a suitable choice of the molecular axes may reduce the number. In principle, it is always possible to diagonalize S, and in such a principal axis coordinate system there are only two nonzero elements (as there would be, for example, in a quadrupole coupling tensor). In the absence of symmetry in the molecule, there is no way of specifying the orientation of the principal axes of S, but considerable simplification is obtained for symmetric molecules. If a molecule has a threefold or higher axis of symmetry, its selection as one of the axes of the Cartesian coordinate system leaves only one independent order parameter, with the now familiar form ... 

In 2008, Alonso and coworkers reported a combined experimental and computational study of the conformations of cysteine. They located 11 low lying conformers at MP4/6-311-i i-G(d,p)//MP2/6-311-l-l-G(d,p), and the five lowest energy structures are shown in Table 3.15. Using laser ablation molecular beam Fourier transform microwave spectroscopy, they identified six conformers present in the gas phase. Comparing the computed rotational constants and nuclear quadrupole coupling tensor components with the experiment, they were able to decidedly match up all six experimental conformers with computed structures. Of the five low energy conformers listed in Table 3.15, four of them were identified in the experiment. 

Dreizler and co-workers used quantum-chemical calculations at the MP4(SDQ) level to compute the nuclear quadrupole coupling tensor of in a series of molecules to compare them to values obtained from rotational spectra <1997ZNA297>. Improved values for thiirane are caa =—329425(78), ebb =—16.402(14), and ccc = 49.345(14) MHz, respectively. 

In rigid aromatic and aliphatic compounds, the quadrupole splitting of (Fig. 3.1.1 (a)) is of the order of 130 kHz. The interaction is quadratic with respect to the spin vector I. It is described by the quadrupole coupling tensor Q, which is proportional to the tensor of the electric field gradient. The average of Q is determined by its trace Spur (in German). Following the Laplace equation, the trace is zero,... 

The validity of (3.2.9) is restricted to the symmetries mentioned above, that is to cylindrical molecules, macroscopically uniaxial samples, and r] = 0. For many samples, these conditions are fulfilled when using NMR, because the quadrupole coupling tensor of aliphatic deuterons is often found to be axially symmetric. In wideline NMR, the anisotropy of the magnetic shielding is used. Here the angular resolution is lower, and the calculation has to be extended to include p > 0 [Henl]. In combination with MAS (cf. Section 3.3), the Legendre subspectral analysis has been used successfully for the determination of molecular order in partially ordered polymers [Harl]. 

The approach of Pickard and Mauri allows the calculation of both chemical shift tensors and quadrupole coupling tensors and is available through the CASTEP code.11... 

Fig. 24. Calculated 2H quadrupole echo and MAS NMR spectra for a two-site reorientation35 such that the 2H quadrupole coupling tensor unique principal axis moves through 106°, i.e. the motion appropriate for the two-site motion of the methyl groups in deuterated DMS. The simulations assume an inhomogeneous symmetric log-Gaussian distribution of correlation times with a mean correlation time of 5 x 10 5s and a standard deviation ranging from 0 to 3 decades, (a) Quadrupole echo spectra with echo delay time t = 30 /is. (b) MAS spectra.

---