Anisotropies magnetic susceptibility

Certain amphiphatic molecules arrange themselves in discoidal bilayered structures known as bicelles. The bicelles have highly anisotropy magnetic susceptibility and thus the property to align themselves in the magnetic field with the plane normal perpendicular to the field [434]. This results in a uniaxial medium suitable for... 

The aromatic character of thiazole has been deduced from the magnetic susceptibility anisotropy of the molecule (319). 

A dramatic decrease in the magnitude of the magnetic susceptibility anisotropy is observed on going from thiirane to the open-chain analog, dimethyl sulfide, and has been attributed to non-local or ring-current effects (70JCP(52)5291). The decrease also is observed to a somewhat lesser extent in oxirane relative to dimethyl ether. 

In the light of these results, it is not surprising that a very large anisotropy has been calculated [24] to exist in the magnetic susceptibility of nanotubes. The susceptibility with the field parallel to the tube axis is predicted to be as much as 3 orders of magnitude smaller than that with the field perpendicular. Again,... 

With respect to the physical properties mentioned, band-structure calculations have attracted considerable interest, e.g., for SbSBr, SbSI, and SbSel (234). For the compounds having reference 22i in column 4 of Table XXIX, a temperature-independent diamagnetism has been found, with values of about 10 cm" g between 77 and 340 K. A small temperature-dependence is exhibited by BiTel, a narrow-gap semiconductor (41). The anisotropy of the magnetic susceptibility has been studied for SbSI, BiSel, and BiTel (41, 420). 

The anisotropy of the magnetic susceptibility of a cyclic conjugated system, attributable to induced ring currents in its rc-electron network, is one of the important quantities indicative of 7t-electron delocalization. The method used for the calculation of the magnetic susceptibilities of nonalternant hydrocarbons is the London-Hoarau method taken together with the Wheland-Mann SCF technique . The resonance integral is assumed again to be of exponential form but... 

On the basis of comparison of the AFM images between the SWNT/MEHPPV composites and the SWNT in the absence and presence of NaHC03, the magnetic orientation of SWNT/MEHPPV composites can most likely be ascribed to the anisotropy in susceptibilities of the SWNTs. 

Breslow etal.133 investigated the microwave spectrum of cyclopropenone and determined data for bond lengths, bond angles, dipole moment (4.39 D from the molecular Stark effect), and magnetic susceptibility anisotropy (Ax) as seen in Table 5 in comparison with cyclopropene5 3 ... 

The isotropic shift. The isotropic shift is the sum of two contributions the contact and the dipolar contributions. The former is due to the presence of unpaired electron density on the resonating nucleus. The latter arises from the anisotropy of the magnetic susceptibility tensor, modulated by the distance between the unpaired electron and the resonating nucleus, and is also dependent on the orientation of the metal nucleus vector with respect to the principal axes of the magnetic susceptibility tensor. Some problems arise when the spin delocalization is taken into account in calculating the dipolar coupling, but we will not address this problem except when strictly necessary. 

Ring currents cannot be directly determined by experimental methods. However, comparison of experimental values of magnetic susceptibilities and their exaltations and anisotropies as well as of H-NMR chemical shifts with the respective data calculated from the ring current model points to the adequacy of this model for the interpretation of experimental results. The magnetic susceptibility associated with the ring current / (83BCJ1853), known as the London susceptibility, is given by... 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

Since the magnetic susceptibility anisotropy Ax is a characteristic attribute of aromatic molecules (66MI1 75MI2), its value could play the role of an aromaticity index... 

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). 

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