Anisotropy of magnetic susceptibility and

B. Criteria for Aromaticity Involvino Anisotropy of Magnetic Susceptibility and Magn ic Exaltation... 

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). 

Table 70 Calculated magnetic parameters of thiophenes magnetic susceptibility Xisol anisotropy of magnetic susceptibility Xanisoi the susceptibility component perpendicular to the ring plane, XzzI exaltation of the three parameters, A, Axamso. and Axzz, and nuclear-independent magnetic shift calculated at ring centers (NICS) and 1 A above the ring centers (NICS(1)) in ppm...

For an earlier discussion of the relation of magnetic susceptibility and aromaticity on the basis of calculated diamagnetic susceptibilities of 28 carbo- and heterocyclic unsaturated compounds including 1, see <1974TL253>. Magnetic susceptibility Xiao, its anisotropy Xaj,iso, its component perpendicular to the ring plane, the exaltations A,... 

The different factors contributing to the chemical shifts of arenium ion protons are important enough to be studied more closely. As noted above C. MacLean and E. Mackor believed the anisotropy of magnetic susceptibility for the protonated benzene ring to be comparable to that of usual unsaturated systems. An attempt was made to verify experimentally this assumption by the protonation of 2,4,6,2, ... 

A similar phenomenon occurs with magnetic fields and is related to the anisotropy of magnetic susceptibility. 

Table 13.2 Magnetic susceptibilities anisotropies of magnetic susceptibilities A/ (10 mol ), magnetic susceptibilities calculated from local increments and magnetic...

Parameter S2 can be found from the anisotropy of magnetic susceptibility, optical dichroism and birefringence, NMR, etc. The determination of higher order parameters requires for more sophisticated techniques. For instance, S4 can be found from Raman light scattering [15], luminescence or other two-wave interaction optical experiments. Data on 5e, Sg are not available at present. In some cases, the X-ray scattering can even provide/(1 ) as a whole but with limited accuracy. 

Basu, C., Ghosh, U.S. J. Phys. Chem. Solids 32 (1971) 2259. Estimation of bonding parameters and overlap integrals on the basis of magnetic susceptibility and anisotropy data. 

The calculated magnetic functions for nearly-octahedral Mo(IV) complexes are presented in Fig. 36. The anisotropy of the susceptibility components is well seen, and it escapes at ca. 1000 K. The overall magnetic productivity is somewhat suppressed in comparison with V(III) owing to the decreased g-factors (Fig. 37). The TIP value cannot be extrapolated safely, neither at high temperatures, since all nine members of the multiplets arising from the 3Tig-term become thermally populated. 

The calculations confirm a visible anisotropy of the susceptibility components at low temperatures, and the overall magnetic productivity is decreased as a consequence of the decreased g-factors (compare the effective magnetic moments in Figs. 50 and 41). 

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