Bond anisotropy

Direct application of Ax for the quantitative evaluation of aromaticity is, however, not practicable since its magnitude is not determined by ring currents only. Quite substantial effects may be played by a local contribution by the 7r-bond anisotropy and the anisotropy of CC and CH (r-bond magnetic susceptibilities as well as by the anisotropy due to local paramagnetic currents (for more detail, see, e.g., 66MI1). 

Anharmonic Thermal Motion and the Bonding Anisotropy of Transition Metal Complexes... 

In noncubic materials, A must be replaced by the 3x3 exchange-stiffness tensor A v, and the energy is X J A v dM/dxy dM/dxv dV. Here the indices ju and v denote the spatial coordinates x, y, and z of the bonds. The energy is anisotropic with respect to the nabla operator = d/d (bond anisotropy) but isotropic with respect to the magnetization M. By contrast, the relativistic anisotropic exchange Xa i Ha(i VMa VMP dV is isotropic with respect to V but anisotropic with respect to M. 

Jeffrey GA, Ruble JR, Yates JH (1984) r-Bond anisotropy in the molecular structure of thioacetamide. J Am Chem Soc 106 1571-1576... 

H9 by 0.11 ppm with respect to values in 5. These cyclopropane ring-induced shifts were accounted for in a reasonable manner using bond anisotropy values indeed the data were less well rationalized when a ring current in the cyclopropane ring was invoked. 

Figure 11-12 shows the seven normal modes of vibration of square-planar XY4 molecules. Vibrations P3, nd p are infrared active, whereas Pi, P2 and P4 are Raman active. Table II-6j lists the vibrational frequencies of some ions belonging to this group XeF, (Sec. I-10) is an unusual example of a neutral molecule which takes a square-planar structure. Bos worth and Clark measured the relative intensities of Raman-active fundamentals of some of these ions and calculated their bond polarizability derivatives and bond anisotropies. They also measured the resonance Raman spectra of several [AuBr4] salts in the solid state, and observed progressions such as np,... 

In summary, the phenomenon of chemical shift enables the chemist to obtain some fundamental information about electronegativities, bond anisotropies, and so on. Above all, the ability to observe the chemical shift causes hydrogen atoms (and to some extent other nuclei) to become functional groups that can be qualitatively and quantitatively estimated. 

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