Electric field effects on the shielding of protons in CH bonds

✍ Scribed by Martin Grayson; William Thomas Raynes

• Publisher: John Wiley and Sons
• Year: 1995
• Tongue: English
• Weight: 609 KB
• Volume: 33
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1260330210

✦ Synopsis

Abstract

The coefficients which determine the electric field dependence at first and second order of the shielding of protons in CH bonds were calculated by a finite‐field SCF method for the molecules, CH~4~, C~2~H~6~, CH~3~F, CH~3~Cl, CH~3~CN, C~2~H~4~, H~2~CO, HCN and C~2~H~2~. Values are given for individual tensor components and for the rotationally averaged coefficients. Diamagnetic and paramagnetic parts of the coefficients are generally of the same order of magnitude. This differs from carbon, nitrogen, oxygen and fluorine shielding studied previously where the paramagnetic parts are often very much larger than their diamagnetic counterparts. The linear electric field coefficients A~z~ (electric field along the CH bond from C to H) for the above molecules (except H~2~CO and HCN) are very similar in value, with a mean of 70.0 ppm a.u. This is very close to an empirical value of 72.2 ppm a.u. obtained by Zürcher in a classic study of steroids. It further appears that the value may be transferable among all C(sp^3^)H bonds. A representative value for the mean quadratic coefficient B is 68.0 ppm a.u. Empirical values of this vary widely.