Determination of the Gas-Phase Acidity of Methylthioacetic Acid Using the Cooks’ Kinetic Method
Document Type
Article
Publication Title
Journal of the American Society for Mass Spectrometry
Department
Chemistry
ISSN
1044-0305
Volume
16
Issue
4
DOI
10.1016/j.jasms.2004.12.012
First Page
535
Last Page
541
Publication Date
4-1-2005
Abstract
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.
Recommended Citation
Ren, J.,
&
Patel, C. G.
(2005).
Determination of the Gas-Phase Acidity of Methylthioacetic Acid Using the Cooks’ Kinetic Method.
Journal of the American Society for Mass Spectrometry, 16(4), 535–541.
DOI: 10.1016/j.jasms.2004.12.012
https://scholarlycommons.pacific.edu/cop-facarticles/682
