Methyl-Substituted Allyl Cations. A Comparison of Experimental Stability, Rotational Barrier, and Solvolysis Data with ab Initio 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 distri- bution. 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 I1 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 4-1 2 kcal/mol higher than the activation free enthalpies deter- mined in superacid solution; the difference provides further evidence for differential solvation effects-the more highly delo- calized 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 f 2 kcal/mol in superacid solution.