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Substituent chemical shifts in NMR spectroscopy. Part 11. Does C—C bond anisotropy contribute to proton chemical shifts?

✍ Scribed by Raymond J. Abraham; Mark A. Warne; Lee Griffiths

Publisher
John Wiley and Sons
Year
1998
Tongue
English
Weight
231 KB
Volume
36
Category
Article
ISSN
0749-1581

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✦ Synopsis

A previous model for the calculation of proton chemical shifts in substituted alkanes based upon partial atomic charges and steric interactions has been modiÐed by the replacement of the CÈC bond anisotropy term with an orientation-dependent c e †ect (i.e. C É C É C É H). The new scheme (CHARGE5) predicts the proton chemical shifts of a variety of acyclic, cyclic and polycyclic hydrocarbons over 188 data points spanning 2 ppm to within \0.1 ppm, an improvement over the previous model with three fewer variable parameters. Systems considered include cyclobutanes, cyclopentanes, cyclohexanes, norbornanes, cis-and trans-decalin, perhydrophenalene, anthracene, adamantane, androstane, methylbutanes and tert-butylmethanes. The signiÐcance of these results is discussed with respect to the development of a comprehensive theory of proton chemical shifts and it is concluded that CÈC bond anisotropy does not in general contribute signiÐcantly to proton chemical shifts, although a possible speciÐc shielding e †ect in planar eclipsed C É C É C É C fragments is noted.

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