McConnell equation double bond

Figure 7. GIAO-calculated shielding increment (Ao) of "ghost atoms" (triangles) and the McConnell equation-calculated shielding increment (circles) 2.0 A above ethene vs. lateral distance from the center of the carbon-carbon double bond.

The anisotropy of double bonds is more difficult to assess, because they have three nonequivalent axes. Thus, the McConnell equation, with only two axes, does not apply. Protons situated over double bonds are, in general, more shielded than those in the plane (Figure 3-8), both for alkenes (Figure 3-8a) and for carbonyl groups (Figure 3-8b). The position of the methylene protons in norbornene (3-8) may be explained in this fashion. 

ApSimon and co-workers have modihed the McConnell equation to allow the calculation of the screening effects of C—C and C—H bonds at nuclei relatively close to them new values of the anisotropy constants for these bonds are given. A new treatment of the C=C bond by these workers takes account of the complete anisotropy of this bond (i.e., three magnetic susceptibilities must be used). The idea usually adopted for shielding cones of C=C bonds is rejected instead it is pointed out that deshielding of nuclei occurs only within a restricted region at the ends of the bonds, elsewhere nuclei either in or out of the plane of the double bond being shielded. Calculations based on these considerations are applied in the diterpene series. 

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