Application of the kinetic method to bifunctional bases: ESI tandem quadrupole experiments

The applicability of the kinetic method to the determination of the basicity of bidentate molecules has been assessed by considering several molecules M previously studied by equilibrium method (M=acetone (1), 1,3-propanediol (2), glycerol (3), 1,4-butanediol (4), 1,3-propanolamine (5), 1,4-butanolamine (6), 1,3-propanediamine (7) and 1,4-butanediamine (8)). Protonated adducts [MHB_i]⁺ (where B_i is a reference base) were produced by electrospray ionisation and analysed by tandem quadrupole mass spectrometry. Application of the classical correlation of the natural logarithm of the ratio of peak intensities, ln([MH]⁺/[B_iH]⁺), with either the proton affinities (PA) or the gas phase basicities (GB) of the reference bases B_i to deduce PA(M) or GB(M) has been examined. It is confirmed that only the use of several experiments at different collision energies ("extended" or "isothermal point" methods) may lead to meaningful results. Good agreement is observed between the PA(M) values tabulated or obtained from the isothermal point for molecule M=1 and 5-8. However, the present PA values appear to be different from that obtained by equilibrium method in the case of diols 2-4. It is generally observed that the measured "apparent" protonation entropies of all the bifunctional molecules examined are significantly less than that obtained by equilibrium methods.