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The kinetic method is a now well-established technique for determining thermochemical properties such as acidities and proton affinities. We present here a study of the application of the kinetic method to the proton affinities (PA) of a series of homologous primary alkanols, namely ethanol through n-octanol. Both metastable and collisionally activated dissociations of proton-bound alkanol pairs were studied, the latter as a function of the target gas and its pressure. Plots of ln([R1OH2 +]/[R2OH2 +]) vs PA for both experiments were obtained to determine new PA values and investigate the significance of the “effective temperature” term. When the experiments are considered in detail, it is apparent that the kinetic method is essentially a semiempirical relationship, without a sound physicochemical basis.

This study examines critically the present state and utility of the Benson additivity schemes for estimating the enthalpy of formation of organic compounds. Old and new group additivity values (GAV) for a wide variety of compounds containing C, H and O are described and are revised where appropriate. When new terms are proposed, or old ones significantly altered, the rationale for so doing is provided. Corrections for such items as cis-isomer effects, gauche interactions, ring strain energies, double-bond position, conjugation effects, steric hindrance in aromatic molecules, etc. are included and discussed. Also provided are the thermochemical consequences of functional group replacements, in which one group in a molecule is substituted by another, thus providing quick short cuts to estimating new Δ(f)H(0) values. Results derived from the new additivity terms are consistent with those produced by computational chemistry methods in general use.

The competing unimolecular dissociations of a variety of proton-bound pairs of amino compounds of formula R1NH2, R1R2NH, where R1 and R2 are chiefly alkyl groups C1 to C9, have been investigated by tandem mass spectrometry. Metastable and collision-induced dissociations were studied. The relative product yields, [B1H+]/[B2H+], from B1H+B2 ions (B n = amine) have been related to the proton affinities (PA) of B1 and B2 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-C4H9NH2, 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, (CH3)(n-C4H9)NH, 951 ± 4 kJ mol-1, and (i-C4H9)2NH, 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, ΔS ⧧, 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 ΔS ⧧ 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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