An intermediate neglect of differential overlap technique for spectroscopy: Pyrrole and the azines

Abstract

An LCAO-MO-SCF-CI model along the lines introduced by Del Bene and Jaffé is developed that is capable of reproducing the better identified observed spectra of nitrogen heterocycles with a rms error of ∼ 1000 cm⁻¹. The model is applied to the spectra of pyrrole, benzene, pyridine, the diazines, symmetric triazine and symmetric tetrazine. The benzene and pyridine spectra are reproduced nearly exactly. The band observed in pyrrole at ∼ 6.5 eV is calculated as two bands at ∼ 6.5 eV, but they are assigned π→σ ^* and not π→π. No evidence is found for the low lying ¹ B _2g in pyrazine, reported at ∼ 30400 cm⁻¹ in pure crystals. The lowest excited singlet of sym. triazine is calculated as ¹ E″ (n→π ^*), not ¹ A″2 (n→π), in agreement with a recent interpretation of Fischer and Small. Several bands are reassigned, and the electronic nature of the transitions discussed. Naphthalene and quinoxaline are examined to insure that no large drift of results are met with molecules of other sizes. Comparison of eigenvalues with molecular ionization potentials is made. Here the numerical agreement appears satisfactory for the first few ionization potentials only.