Gas phase acidities of aliphatic carboxylic acids, based on measurements of proton transfer equilibria

Abstract

Measurements of the gas phase proton transfer equilibrium [1]: AH + B − = A − + BH with a pulsed electron high pressure mass spectrometer (PHPMS) lead to [Formula: see text] and [Formula: see text] data. These relative acidities are converted to the absolute acidities [2]: AH = A − + H + bycalibration of the relative [Formula: see text] and [Formula: see text] scale to the known values for [Formula: see text] and [Formula: see text] of a reference compound(HCl). Earlier determinations that included 16 aliphatic AH are extended in the present work and provide data for 47 aliphatic carboxylic acids. These include halogen, OH, CH 3 O, C 2 H 5 O, PhO, and NH 2 substituted acetic acids, and halogen substituted, unsaturated, and cyclic RCO 2 H of higher carbon number. The effects of the various substituents on the gas phase acidity are briefly examined. Keywords: acidities in gas phase, proton transfer equilibria, ion-molecule equilibria, mass spectrometry.