Title

Determination of the Gas-Phase Acidity of Methylthioacetic Acid Using the Cooks’ Kinetic Method

Authors

Jianhua Ren, University of the PacificFollow
Chirag G. Patel, University of the Pacific

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 (ΔH_acid) 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 (ΔG_acid) of 333.0±1.7 kcal/mol. The entropy of the acid dissociation, ΔS_acid, 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 ΔH_acid 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