Direct Experimental Determination of the Energy Barriers for Methyl Cation Transfer in the Reactions of Methanol with Protonated Methanol, Protonated Acetonitrile, and Protonated Acetaldehyde: A Low Pressure FTICR Study

Methyl cation transfer reactions between methanol and protonated methanol, protonated acetonitrile, and protonated acetaldehyde have been investigated experimentally by low-pressure FT-ICR mass spectrometry. The temperature dependencies of the rate constants for these reactions were determined in an Arrhenius-type analysis to obtain activation energies, enthalpies, and entropies of activation. The enthalpies of activation were determined to be -16.9 ( 0.6, -16.5 ( 0.6, and -18.4 ( 0.7 kJ mol -1 for the methanol/protonated methanol, methanol/protonated acetonitrile, and methanol/protonated acetaldehyde reactions, respectively. These values agree quite well with ab initio-calculated values. The entropies of activation were found to be quite similar for all three reactions within experimental uncertainty, which is expected due to the similar transitionstate structures for all reactions. Ab initio potential energy surfaces calculated at the MP2/6-311G** level and basis set are reported for the three reactions. For the methanol/protonated acetonitrile and methanol/ protonated acetaldehyde reactions, isomerization of the initially produced proton-bound dimer to a methylbound complex is suggested prior to methyl cation transfer. The barrier for the first isomerization is predicted to be significantly lower than the barrier for methyl cation transfer such that it does not interfere with the experimental determination of the latter.