engleski

Anionic vs. Neutral Protonation - An Ab Initio Analysis with a Triadic Formula

A simple formula which enables delineation of the initial and final state effects in the proton affinity calculations of neutral bases (Z. B. Maksić, R. Vianello, J. Phys. Chem. A 2002, 106, 419) is extended to encompass acidity of Br?nsted AH acids. The latter is estimated by computing the proton affinities of conjugate bases PA(A–). It is shown that the PA(A–)s are predominantly determined by the Koopmans' ionization potentials of A–, or in other words, acidity is decisively affected by the final state effects. In contrast, proton affinity and the closely related basicity of alcohols and carboxylic acids are dominated by the initial state effects as a rule. A simple rationalization is provided, which explains a well known fact that the deprotonation energies of acids are much higher than the protonation energies of neutral bases. It is expected that the triadic formula could prove useful in bridging the gap between the proton affinities of superbases PA(B) and the deprotonation energies PA(A–) of superacids enabling an overlapping between these two scales, which are completely separated at present.

acidity; basicity; ionization potential; proton affinity; bond dissociation energy

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