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Gaseous-phase proton affinity of anilines: A quantum chemical evaluation and discussion in view of aqueous basicity

Pankratov Alexei N. (N.G. Chernyshevskii Saratov State University, Department of Chemistry, Astrakhanskaya Street, Saratov, Russia)
Uchaeva Inna M. (N.G. Chernyshevskii Saratov State University, Department of Chemistry, Astrakhanskaya Street, Saratov, Russia)
Doronin Sergei Yu. (N.G. Chernyshevskii Saratov State University, Department of Chemistry, Astrakhanskaya Street, Saratov, Russia)
Chernova Rimma K. (N.G. Chernyshevskii Saratov State University, Department of Chemistry, Astrakhanskaya Street, Saratov, Russia)

Using the PM3 method, enthalpies and free energies of the gaseous-phase proton affinity (PA) have been computed for aniline and 62 of its derivatives with different kinds of electron-donor and electron-acceptor substitution in the aromatic ring and at the nitrogen atom. Linear correlations of the type pKa vs. PA have been found. Deviations of the data for ortho substituted anilines from the above relationships was discussed in view of possible hydrophobic hydration of the molecular fragments ajacent to the protonation centre. Linear dependecesPexper = bPtheor (wherePis standard entropy, heat or Gibbs energy of formation, first ionization potential, molecular dipole moment) were found.

Keywords: anilines, proton affinity, basicity, quantum chemical computation, theory-experiment correlation, hydrophobic hydration