Substituted N,N-Dialkylanilines: Relative Ionization Energies and Proton Affinities through Determination of Ion-Molecule Reaction Equilibrium Constants

Article abstract of DOI:10.1021/jo00203a009

The relative ionization energies and proton affinities of N,N-dimethyl-, N,N-diethyl-, and N,N-di-n-propylaniline, and meta- and para-methyl-substituted analogues (as well as N,N,3,5-tetramethylaniline and 4-chloro-N,N-diethylaniline) have been determined in the gas phase through measurements of the equilibrium constants of charge-transfer and proton-transfer reactions in an ion cyclotron resonance spectrometer.Absolute values are assigned to the ionization energies and proton affinities generated in these experiments.Comparison atandards were the ionization potential (7.12 eV) and proton affinity (223.4 kcal/mol) for N,N-dimethylaniline taken from the literature.The heats of formation of the parent radical cations, M⁺, and the corresponding protonated molecules, MH⁺, vary in the same way, differing from one another by 21 +/- 2 kcal/mol over the entire set; that is, the radical cations of these compounds display a constant hydrogen affinity of 74 +/- 2 kcal/mol.This is interpreted to mean that all the compounds protonate at the nitrogen atom; previous work had suggested that meta-substituted isomers protonate on the ring.Further, it is demonstrated that variations in both the ionization energy and the proton affinity values upon changes in ring substitution can be predicted from the appropriate Hammett ? values, but not from the corresponding ?⁺ values; changes brought about by differing N-substituents correlate with ?^* values.