Chemical shift ring current, effect

The effects of ring currents on chemical shifts have been used more recently to study the presence of nonrandom coil sequences in fragments of large peptide hormones, such as glucagon (Boesch et al, 1978) and parathyroid hormone (Bundi et a/., 1978). 

An alternative characterization of the local features of conjugated polycyclic hydrocarbons in terms of ring currents involves computation of the effect of ring currents on chemical shifts. It was suggested by Schleyer and collaborators that the absolute mag-... 

Some available data on H NMR spectra of non-aromatic azoles containing two ring-double bonds are given in Table 10. Here there is no ring current effect and the chemical shifts are consequently more upheld. 

FIG. 13 Chemical shift change of the EPC H NMR signals induced by the ring current effect of BPA. Negative value means upheld shift. (From Ref. 47. Copyright 1999 American Chemical Society.)... 

In conjunction with the evaluation of ring current effects and NICS values (Nuclei Independent Chemical Shift), calculations of 3H and 13C NMR chemical shifts for a series of fluorenylidene dications were performed.104... 

In fact, it may even be difficult to ascertain its existence as was shown by several authors (cf. Scheme 2). The C(ll) and C(12) signals in benzo-[g/u ]fluoranthene (1) are 6.6 ppm apart. Since this value is much larger than the 0.3-ppm difference of the corresponding signals in acenaphthylene (2), it was ascribed to ring-current effects (67). The 7-ppm difference of the two carbonyl chemical shifts in 3 (8 = 206.1) and 4 (8 = 213.0) has been attributed the same origin (68). 

Various techniques are suitable for the study of binding locations of organic solubi-lizates in micelles and vesicles. Typically, these methods have included changes in NMR chemical shifts as a result of aromatic ring current effects, paramagnetic relaxation... 

Studies of the stereochemical dependence of 1H chemical shifts in cyclobutanes, benzocyclobute-nes and /J-lactams have been collected463. In a number of cases the protons or proton groups are shielded if they are cis to vicinal halogen, hydroxy or phenyl substituents, due to a ring-current effect. Carbonyl groups, however, can give rise to deshielding effects. 

The relation between UV- and PES-spectra is quite similar as found for planar aromatic compounds. All chemical shifts in NMR-spectra can well be explained by normal ring-current effects and Van der Waals interactions. Polarographic data do not deviate from those for planar compounds. 

The H NMR spectra of a number of 2-, 5- and 6-monosubstituted quinoxalines have also been analyzed and their chemical shifts and coupling constants reported. In these compounds J2i is 1.7-1.9 Hz J67 5.0-8.3 Hz J1S 8.4-10.3 Hz 1.4-2.7 Hz J6B 0.7-2.9 Hz and Jss 0.3-0.8 Hz.256 The very small value for Jn is noteworthy. The chemical shifts of the ring hydrogens in 6-substituted quinoxalines have been correlated with -electron charge density, after correction for N-anisotropic and ring current effects.257... 

Trifluoromethyl-substituted517 and perfluorophenyl-substitutedcarbocations518,519 have also been prepared and studied. Because of the relatively large fluorine chemical shifts, anisotropy and ring current effects play a relatively much smaller role than they... 

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