METHYL-SUBSTITUTED ALLYL CATIONS - COMPARISON OF EXPERIMENTAL STABILITY, ROTATIONAL BARRIER, AND SOLVOLYSIS DATA WITH ABINITIO CALCULATIONS

Three sources of quantitative data for methyl-substituted allyl cations are available experimentally: Gas phase heats of formation, rotational barriers in solution, and rates of solvolysis. NMR chemical shifts also provide an index of charge distribution. This paper draws all these lines together in comparison with results of STO-3G ab initio molecular orbital calculations performed on planar and perpendicular methyl-substituted allyl cations. Sequential substitution of the terminal positions by one, two, three, and four methyl groups stabilizes allyl cations electronically by 17, 15, 13, and 11 kcal/mol, respectively; a methyl group on the central carbon has a much smaller stabilizing effect (~5 kcal/mol). The steric strain for the first endo-methyl group was determined to be 3 kcal/mol, whereas a second endo-methyl experiences a somewhat greater strain of 5 kcal/mol. Allyl solvolysis rates correlate well with gas phase allyl cation stability data, but there is a reduction in magnitude due to solvation. The calculated rotational barriers also are 4-12 kcal/mol higher than the activation free enthalpies determined in superacid solution; the difference provides further evidence for differential solvation effects-the more highly delocalized planar forms are solvated to a lesser extent than their rotational transition states. The rotational barrier of the parent allyl cation is predicted to be 34 kcal/mol in the gas phase but to decrease to 23.7 ± 2 kcal/mol in superacid solution. © 1979, American Chemical Society. All rights reserved.