A high pressure mass spectrometric study of proton transfer between tri-, tetra-, penta-, and hexamethylbenzene

Abstract

The proton affinities (PA) of some methylaromatic compounds and the rates of proton transfer reactions have been measured using high pressure mass spectrometry. The equilibria studied were of the form [Formula: see text]. Van't Hoff plots yielded the following PA values (kcal mol−1) relative to PA(ethylacetate) = 200.7 as standard: mesitylene 201.0, 1,2,3,5-tetramethylbenzene 203.2, pentamethylbenzene 204.4, hexamethylbenzene 206.6. ΔS0 values for the proton transfer equilibria are not fully determined by changes in rotational symmetry numbers and it is suggested that vibrational and torsional changes must be considered. Proton transfer is slow in both the forward (exothermic) and reverse (endothermic) directions and, in addition, for all except the most endothermic reaction studied (1,2,3,5-tetramethylbenzene + protonated hexamethylbenzene) the rate constants for proton transfer increase with decreasing temperature. Such behaviour can be associated with the potential energy profile along the reaction coordinate for the reaction. A limited number of charge transfer experiments involving molecular ions [Formula: see text] showed that reaction occurs at every collision in the forward (exothermic) direction and the reaction efficiency in the reverse (endothermic) direction is small but increases with increasing temperature.