Study of the electronic structure and the spectra of methyl-substituted carbanions of nitrogen heterocycles by the selfconsistent field MO LCAO method

The -electron density distribution in the ground state and the energies of the spectral transitions were calculated by a semi-empirical version of the Pople-Pariser-Parr method for a number of carbanions derived from the removal of a proton from the methyl groups of toluene, isomeric methylnaphthalenes, methylpyridines, and benzoquinones. The results obtained are compared with the results obtained in a Hückel calculation of the same anions and with experimental data from the electronic spectra of methyl-substituted nitrogen heterocycles in a solution of potassium amide in liquid ammonia.The calculations reflect satisfactorily the characteristic features of the observed spectra. The effect of the choice of parameters for the self-consistent calculation is discussed and the contribution of singly or doubly excited configurations is explained.