Hyperconjugation orientation-dependent

The dependence of the j3-deuterium effect on the spatial orientation of the isotopic bond with respect to the developing -orbital, on the a-carbon atom was elegantly demonstrated by Shiner and Humphrey (1963). This work will not be discussed in detail here suffice it to say the suggestion is made that j8-deuterium effects are better correlated by the postulate of hyperconjugation and its angular dependence than by the simple steric model (Shiner and Humphrey, 1963). 

Stronger effects than those of hyperconjugation can be expected for substituents with stronger interacting power. The effect of aryl substituents will depend on their orientation relative to the plane of the silicon backbone.63... 

This stabilization has been shown to be primarily a delocalization effect by demonstrating a strong dependence on the dihedral angle between the Si—C bond and the empty p orbital of the carbocation. Kinetic rate enhancements of about 1012, 0, and 104 were found for compounds with angles of 180°, 90°, and 0°, respectively. This is consistent with a hyperconjugation mechanism and with analyses of orbital overlap that show considerably stronger overlap for the anti (180°) than syn (0°) orientations. ... 

The hyperconjugative origin of /3-isotope effects received strong confirmation when Streitwieser and his co-workers (54,156) and Shiner (157-159) showed them to depend on the spatial orientation of the jS-hydrogen atoms. 

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