Quantitative evaluation of hyperconjugation in the cyclopropylcarbinyl cation and in cyclopropylborane
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An orbital deletion procedure (ODP) at HF/6-311G** have been used to evaluate the hyperconjugation effects in the cyclopropylcarbinyl cation (1) and in cyclopropylborane (2), as well as the conjugation effects in the allyl cation (3) and in vinylborane (4). The hyperconjugation (or conjugation) energies have been quantified by ODP in which the critical "vacant" carbocation (or boron) p orbital is "deactivated". Comparisons between the bisected conformations of 1 with 3, and 2 with 4 demonstrate that cyclopropane can be just as effective as a pi-electron donor as a C=C double bond. (C) 1997 Elsevier Science B.V.