Carbocations calculating charge distribution

Effects of nitrogen substitution can be predicted by quantum chemical calculations (42,43), or more qualitatively by examining the charge distribution in the relevant carbocations. When a carbocation is generated at C-l, the positive charge can be delocalized as shown below. 

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. 

Table 1 Charge Distribution on /3-H Atoms and sp2-Hybridized C + Atoms Calculated by RHF/6-31G for Carbocations Derived from Various Alkenes...

The relative stability, charge distribution, and bond orders obtained by the calculation identified three main factors in carbocation stabilization as Tr-donation, (3-hyperconjugation, and the 01-polar effects. 

Even more important is the fact that the formation of the triol carbocations (PAHTC) has not been correctly calculated. Any treatment based on a simple Hiickel-MO or PMO calculations for odd AH ions neglect the effect of the differently charged carbon atoms and hence, must be in error. The ionic charge distributed over the aromatic system affects the electronegativity of carbon atoms in specific ways and this has a profound effect on the 7i-energy. Breakdowns of both the PMO and HMO approximations with ionic reaction intermediates are documented in the work of Dewar and Thompson [36,70], Streitwieser et al [35,71] and Szentpaly [39]. The reactivity patterns with radical and ionic reaction intermediates of PAH are different [34-39]. It has been pointed out by Dewar [36] that the PMO method works better for radical than ions, and adequate modifications of the PMO method have been developed for ionic intermediates [16,38,39]. 

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