Proton Affinities of Simple Amines; Entropies and Enthalpies of Activation and Their Effect on the Kinetic Method for Evaluating Proton Affinities

The competing unimolecular dissociations of a variety of proton-bound pairs of amino compounds of formula R 1 NH 2, R 1 R 2 NH, where R 1 and R 2 are chiefly alkyl groups C 1 to C 9, have been investigated by tandem mass spectrometry. Metastable and collision-induced dissociations were studied. The relative product yields, [B 1 H + ]/[B 2 H + ], from B 1 H + B 2 ions (B n = amine) have been related to the proton affinities (PA) of B 1 and B 2 by the kinetic method. In its simplest form, the method assumes no entropy effects and a zero reverse energy barrier for the competing dissociations. The general effects expected from nonzero entropies of activation are described in terms of how they influence log(rate constant) vs internal energy plots for such competing dissociations. For these homologous series such effects were observed to be minimal and the kinetic method well reproduces most of the reference PA values. In view of the very close agreement between many reference PA values obtained from equilibrium studies, it is possible that small reverse energy barriers (ca. 5 kJ mol -1 ) may be identifiable by the kinetic method, in particular for t -C 4 H 9 NH 2, the homologous di- n -alkylamines, and possibly larger barriers for di- sec -alkylamines. The method is quite sensitive to small discrepancies in PA, and new values have been proposed for benzylamine, 924 ± 4 kJ mol -1, (CH 3 )( n -C 4 H 9 )NH, 951 ± 4 kJ mol -1, and ( i -C 4 H 9 ) 2 NH, 969 ± 4 kJ mol -1 . Results have also been obtained for some bidentate species, the 1,2-, 1,3-, and 1,4-diaminoethane, propane, and butane, respectively. Particular attention was given to the effects of activation entropies, Δ S ⧧, and the possible presence of reverse energy barriers, E rev, both of which are here a significant problem for the kinetic method. It was found that for 1,2-diaminoethane there was a Δ S ⧧ effect, particularly for collisionally activated ions. The derived PA of 949 ± 4 kJ mol -1 is quite close to the reference value for this compound, 952 ± 4 kJ mol -1 . It was concluded that for 1,3-diaminopropane (reference PA 987 ± 4 kJ mol -1 ), the method failed due both to an activation entropy effect and the presence of a reverse energy barrier. The latter was estimated to be ca. 20 kJ mol -1, making ca. 967 kJ mol -1 the apparent PA resulting from the kinetic method. For 1,4-diaminobutane (reference PA = 1006 ± 4 kJ mol -1 ), here too the kinetic method failed and similar difficulties arose with an estimated reverse energy barrier of ca. 32 kJ mol -1 . The overall results for these three compounds were compared with a recent report in this journal on low-energy collision-induced fragmentations. These results for species having a bidentate structure indicate that similar difficulties may apply to biomolecules and that PA values obtained by the kinetic method may be flawed.

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