Structures and absorption spectra of complexes with charge transfer between the N-methylquinolinium cation and the barbituric acid anions

Charge transfer (CT) bands were observed in the absorption spectra of the complexes of N-methylquinolinium with 1,3-dimethylbarbiturate anions between 19,000–24,000 cm−1. The frequency of the bands decreases with the length of the alkyl substituent. According to quantum chemical calculations, the ionization potentials of the anions increase from 3.18 to 3.29 eV in going from the methyl substituent to the bytyl one. The CT energies obtained in the calculations of the absorption spectra of the ion complexes are significantly lower than the corresponding experimental values (13,000–14,000 cm−1). Only taking into account the association of the counterions with the ion complexes provides a satisfactory description of the observed spectra. The calculated energy of quadrupole formation is about 2 eV. In chloroform, as well as in more polar solvents (acetonitrile, methanol), ion reactions occur stimulated by a decrease in the excited levels of the complexes to thermal values.