ELIMINATION OF METHANE FROM PROTONATED ACETALDEHYDE - THE AB-INITIO TRANSITION-STATE

The transition state (TS) for loss of CH4 from protonated acetaldehyde has been located at the second-order Moller-Plesset (MP2)/6-31G(d, p) level of theory. The activation energy is predicted to be 263.9 kJ/mo l starting from the more stable form (methyl and hydrogen E) and 261.6 kJ/mol starting from the less stable form (methyl and hydrogen Z) tha t is required for reaction. The products (methane and the formyl ion) are predicted to lie 136.6 kJ/mol below the TS for their formation. MP 2 methods underestimate the heats of formation of both the TS and the reaction products by about 40 kJ/mol when compared with experiment. Re stricted Hartree-Fock (RHF) calculations give much more accurate relat ive energies. The MP2 TS leads directly to fragmentation and is descri bed as a protonation of the methyl group by the acidic proton on oxyge n. Under RHF theory the reaction is stepwise. An RHF TS similar to the MP2 TS leads to a nonclassical intermediate (which is stable at this level of theory) that has one of the C-H bonds protonated. This mechan ism (protonation of an alkyl group) appears to be a general one for hi gh energy 1,2 eliminations from organic cations.