METHYL CATION AFFINITIES OF N, O, AND C LONE-PAIR DONORS

Methyl cation transfer equilibria were measured by use of pulsed high-pressure mass spectrometry. A ladder of ΔG°600 values gives relative methyl cation affinities (MCAs) for several cyanides, ethers, and iodides, spanning a range of 15 kcal/mol. The ladder is anchored to an estimated MCA((CH3)2O) = 93 kcal/mol, giving the following MCA values: CH3CN, 98; HCCCN, 95; i-C3H7CN, 100; 1,4-dioxane, 96; c-C5H10O, 98; c-C4H8O, 98; CH3I, 86; C2H5I, 90 kcal/mol. The MCAs of (CH3)2O, CH3CN, HCCCN, and several model compounds were calculated ab initio with basis sets ranging from MPn/6-31G* to MPn/6-311++G(2d,2p). The MP2/6-31G*(6-31G**) results agree best with the experimental results. MCAs show a good correlation with the amount of electron density transferred from the bases to the CH3+ group, suggesting that the bonding is largely covalent. The bonding between a methyl cation and a base arises primary from the interaction between the σ-HOMO of the base and the σ-LUMO of the methyl cation; π-effects are much less important. © 1990 American Chemical Society.