Carbocations bond lengths

Larger changes in bond lengths, as expected, are observed for more localized carbocations. Most of the structures available are for stabilized systems, such as protonated carbonyl compounds [e.g. the protonated cyclopropyl ketones referred to on page 110 (Childs et al., 1990), and dioxacarbocations (Paulsen and Dammeyer, 1973, 1976 Paulsen and Schuttpelz, 1979 Childs et al., 1986, 1991). It is normal to see one of the atoms of the counterion (in most cases MXJ or MX ) packing in the position expected for addition to the activated C=OH(R)+ system, apparently just within the sum of the van der Waals radii for the neutral centres (Childs et al., 1986). This can happen without significant pyramida-lization, however (Childs et al., 1991), and on both sides of the planar carbon centre it tells us little new about reactivity. 

Ab initio MO calculations have been carried out for two carbocation-generating reactions the 6 nI reaction of protonated 1-phenylethanol (H2O leaving group) and the acid-catalysed hydration of styrene. Optimizations were done at the MP2/6-31G level. The 6 nI transition state lies half way between the reactant and the product with respect to the bond lengths, charge distribution, and secondary deuterium isotope effects. 

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