Determination of the gas-phase acidity of methylthioacetic acid using the Cooks' kinetic method

We determined the gas-phase acidity of methylthioacetic acid (MTA) in a triple-quadrupole mass spectrometer using the Cooks' kinetic method with the consideration of entropy effects. The negatively charged proton-bound dimers were generated by electrospray ionization. Collision-induced dissociation was applied to the dimer ions and the product ion ratios were measured at four different collision energies. The gas-phase acidity (ΔHacid) of MTA was determined to be 340.0 ± 1.7 kcal/mol using the extended kinetic method and 339.8 ± 1.7 kcal/mol using the standard kinetic method. The entropy term is insignificant in this case and can be ignored. The standard kinetic method yielded a free energy of deprotonation of MTA (ΔGacid) of 333.0 ± 1.7 kcal/mol. The entropy of the acid dissociation, ΔSacid, was estimated to be 22.8 cal/mol K. Theoretical prediction at the B3LYP/6-31 + G* level of theory gives a similar value for ΔHacid of 338.9 kcal/mol. In the gas-phase, MTA is a stronger acid than methoxyacetic acid, although in solution, MTA is a weaker one.