Ring current model

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... 

The question to be answered in the first place is whether the ring current model criterion is compatible with the chief energy criterion of aromaticity and antiaromaticity. The answer will be positive if a relationship is revealed between ring currents and resonance energies. 

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). 

We have used density functional calculations to calibrate intensity factors for the DNA and RNA bases (43). While these are not necessarily optimal values, they are used here to provide an initial idea of the performance of ring-current models in explaining DNA shift patterns. A more extensive test of alternative values is in progress. 

It is concluded that one isomer has the methyl group below the basal plane of the boron framework (endo-) with the CF3 group nearly in the plane of the base in an axial (exo-) position and in the other isomer the conformation is the opposite, but no assignment was made. Marynick and Onak 180> have suggested, on the basis of their ring current model for the correlation of chemical shifts in pyramidal boron compounds, that the proton chemical shifts reported for the methyl groups of these two isomers 174> favor assignment of the endo-methyl conformation to isomer A. 

A ring current model has been shown to correlate proton and boron-11 chemical shifts in a number of pyramidal boron compounds including pentaborane(9) and its derivatives 180>. This model is analogous to the model used to explain proton chemical shifts in benzene and similar aromatic compounds. 

Torsional influences on ring current may be deduced from a comparison of cis- and trans-OEC (12) (69TL1145). A ring current model for chlorophyll derivatives has been described (82JA4332). 

The ring current model may also be used to interpret data on such molecules as porphyrins, whose large ring currents lead to substantial shifts to higher frequency for protons outside the ring and a very large shift to lower frequency for the NH protons inside the electron ring. 

Tabulated compilations of the chemical shifts of the protons in oxiranes are frequently found in the literature, " but structural conclusions are rarely drawn from the chemical shifts as the coupling constant is a more exact means of establishing the configuration. Configurations and conformations of oxiranes derived from prostaglandin intermediates have been determined from H shifts with the aid of the ring current model. ... 

Different kinds of basket handle porphyrins have been reported recently (Fig. 17) [169, 170, 182, 186], The distortion in the porphyrin ring is clearly reflected in the up or down field shifts of various protons and magnitude of these shifts directly exposes the amount of distortion created by the linking chains. Momenteau and coworkers [186] used the ring current model of Abraham and... 

A ring-current model, used to calculate the magnetic anisotropy of a cyclopropane ring, permits estimates of the shielding contribution of a cyclopropane ring to the chemical shifts of neighbouring protons. Illustrations include 3a,5a-, 3/3,5 -, and 5j8,7)5-cyclosteroids. 

Arguably the most important observable magnetic property in molecular chemistry is the chemical shift in NMR. Consequently, ring current models have been tested according to their performance in predicting NMR chemical shifts. Pople, in 1956, was the first to apply ring current approach to the calculation of NMR shifts [41]. 

Figure 13.5 Semi-classical ring current models to predict the proton chemical shift of benzene, (a) Simplified magnetic dipole model (infinitesimally small dipole in the center of the ring), (b) current loop model with two loops (blue) above and underneath the ring, (c) partial cancellation of...

In a recent development of a ring current model for the porphyrin ring system (see later) we had occasion to compare the equivalent dipole and current loop models. ... 

A Ring Current Model of the Porphyrin and Related Rings. 

We wished to obtain a porphyrin ring current model for use In metalloporphyrln complexatlon studies (see later). Insertion of a diamagnetic metal (In our case zinc) Into the porphyrin does not affect the proton chemical shifts, thus the ring current Is unchanged. However, the application to metalloporphyrlns allows the use of the high field shifts of protons on apical ligands (e.g. pyridine) attached to the metal to be used as additional tests of the ring current 3K>del. 

The dilution shifts of figure 6 can now be used with the ring current model to deduce the structure of the ZP... 

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