Molecular Orbital Calculations on Some Nitrogen Derivatives of Conjugated Hydrocarbons: Base Strength of Benzacridines and Their Amino Derivatives

Abstract

The relations which exist between the electronic structure and base strength of benzacridines and their amino derivatives were investigated using semiempirical molecular orbital calculations. The calculations are complicated by a number of factors which affect equilibria in solution. The results indicate that the energy to protonate a nitrogen derivative of a conjugated hydrocarbon may be divided into the terms ∆Eσ and ∆Eπ for changes in localized and delocalized electron energies, respectively, an energy term for solvation, and a term for steric hindrance to protonation. When the term ∆Eπ alone is used to determine the pKa values, it yields a linear relationship within each family of derivatives. The term ∆Esolv appears to depend primarily on the size of the molecule and may be calculated by the use of a modified Born equation. The combination of ∆Eπ and ∆Esolv, as a representation of ∆E, yields a single relationship when plotted against pKa for a number of benzacridines and their amino derivatives as well as the derivatives of pyridine, isoquinoline, quinoline, and acridine, which were reported previously. The deviation from this relationship for some of the compounds appears to be due to structural factors which depend on the ∆Ester and ∆Eσ terms.