Proton affinity determinations using the kinetic method in an ion trap mass spectrometer

Abstract

Proton affinities for various compounds have been estimated using a quadrupole ion trap by generating and mass-selecting proton-bound dimers and measuring their dissociation kinetics (A–H + −B → AH + + B and/or BH + + A). From the relative abundances of the fragment ions ([BH +] and [AH +]), which are related to their relative proton affinities by ln ([AH +]/[BH +]) = ΔPA/ RT, it is shown that the proton affinities of the alicyclic carboxylic acids decrease in the order: cyclohexane- > cyclopropane- > cyclopentane- > cyclobutanecarboxylic acid. Proton affinity values for these species, measured from their proton-bound dimers with specific ketones, esters and carboxylic acids of known PA, are determined to be 198.3 ± 0.2 kcal mol −1, 198.0 ± 0.2 kcal mol −1, 197.8 ± 0.2 kcal mol −1 and 197.0 ± 0.2 kcal mol −1, respectively. The major contribution to the estimated uncertainties in these values results from the uncertainties in literature proton affinity values for the reference compounds. Proton affinity differences of ≤ 0.1 kcal mol −1 are measurable and differences as small as 0.04 ± 0.02 kcal mol −1 are estimated to be detectable. Similarly, the proton affinities for o-, m- and p-methylbenzoic acid are determined to be 201.3 ± 0.2, 200.0 ± 0.2, 201.4 ± 0.2 kcal mol −1, respectively. The ability to measure such small PA differences is believed to be a consequence of the low internal energy of the activated proton-bound dimers. The kinetic method has also been used to examine the effect of deuterium labeling on proton affinity. Secondary isotope effects on proton affinity are examined for both meta and para deuterated benzoic acid proton-bound to benzoic acid ( k H/ k D = 1.0 ± 0.1 and 0.9 ± 0.1, respectively), for acetophenone proton-bound to deuterated-acetophenone (C 6H 5C(O)CD 3) ( k H/ k D = 0.7 ± 0.1) and for 2-pentanone proton-bound to deuterated 2-pentanone (CH 3CH 2CH 2C(O)CH 3) ( k H/ k D = 2.1 ± 0.2). These results, as well as those for the carboxylic acids and benzoic acids given above, are accounted for in terms of stabilizing electronic effects in the protonated molecules.