Susceptibility anisotropy

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. 

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 ... 

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... 

Magnetic susceptibility anisotropy has been used for the estimation of relative aromaticity of some azines in comparison with benzene (77JCS(P2)897). If the extent of ir-electron delocalization for benzene is taken as 1.0, the corresponding values for azines are pyridine 0.7, pyridazine 0.7, pyrimidine 0.5, and 1,3,5-triazine 0.3. 

The molecular magnetic susceptibility anisotropies for the pyranones have been determined by microwave techniques (71JA5591, 73JA2766). Values for the parameter A, which represents the out-of-plane minus the average in-plane molecular magnetic susceptibilities, were obtained for benzene, furan, 2- and 4-pyranone, and tropone. A may be separated into local and non-local contributions with the aid of known local group contributions for non-aromatic molecules. The results are presented in Table 12. 

The magnetic susceptibility anisotropy of pyran-4-one was determined by microwave molecular Zeeman effect studies. Data for 2- and 4-pyranone are given in Table 12. Since pyran-4-one exhibits a negligible non-local effect it was suggested that this molecule is best regarded as being non-aromatic (73JA2766). 

From the lJ residual dipolar coupling the molecular magnetic anisotropy tensor is obtained, which differs from the metal contribution by an extent which depends on the magnetic anisotropy of the diamagnetic part. For example, in cytochrome b the diamagnetic, the paramagnetic and the total susceptibility anisotropy values are A Xax = —0.8, 2.8, 2.20 x 10 32 m3, respectively, and Axrh = 0.1, —1.1, — 1.34 x 10-32 m3, respectively [60]. The corresponding tensors sum up as expected. 

Note that (/. ) is slightly misaligned with respect to the direction of Bq, k it would be coincident with the k direction only if xn = Xx> i.e. in the absence of magnetic susceptibility anisotropy. 

Since most liquid crystalline phases used today are lyotropic mesophases with a relatively small magnetic susceptibility anisotropy, it should be noted that the orientation of the director is also influenced by the inertial torque when... 

Magnetic susceptibility anisotropy has been used to estimate relative aromaticities of some azines <1977JOC897>. If the extent of -electron delocalization for benzene is taken as 1.0, the corresponding values for azines are pyridine 0.7, pyridazine 0.7, pyrimidine 0.5, and 1,3,5-triazine 0.3. Another quantitative magnetic index is the exaltation of the total magnetic susceptibility (A). All aromatic systems reveal large A values, whereas for nonaromatic compounds A is close to zero and it is assumed that aromaticity increases with A. For six-membered monocycles the following values of A have been reported (in units of cm3 mol-1 x —106) benzene (17.9), pyridine (18.3), pyridazine (8.7), pyrimidine (18.2), pyrazine (12.7), l-ethyl-2-pyridone (13.0), and 1,3,5-triazine (19.0). 

Data for 0-(BEDT-TTF)2I3 [20,98] are also included in Fig. 19. Ru is positive and falls sharply below 20 K. Fortune et al. [99] attribute this fall to the effect of a phase transition at 23 K (possibly a SDW) and suggest that the latter causes a reduction of about 50% in the electronic density of states. This is the reason put forward for the lower superconducting transition temperature (Tsc = 1.5 K) of the ambient pressure 0L phase compared with the pH phase, which is stable above 0.5 kbar and has Tsc = 8 K. However, in contrast to this, Bulaevskii [3] attributed the suppression of to the larger disorder, corresponding to a residual resistivity of about 170 pilcm, which is larger than for other (BEDT-TTF)2X superconductors. So more experiments are needed to verify the above hypothesis. Susceptibility anisotropy measurements may be useful in this respect, because they are more sensitive to the formation of a SDW than the static susceptibility of randomly oriented single crystals (Fig. 4, Section IV). 

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