Electronic excitation spectra of radical anions of cyanoethylenes and cyanobenzenes: symmetry adapted cluster-configuration interaction study

Electronic excitation spectra of the radical anions of cyanoethylenes (trans-dicyanoethylene and tetracyanoethylene) and cyanobenzenes (1,2-dicyanobenzene: o-DCNB, 1,3-dicyanobenzene: m-DCNB, and 1,4-dicyanobenzene: p-DCNB) were studied by the symmetry adapted cluster-configuration interaction (SAC-CI) method. Theoretical calculations predicted positive electron affinities for all the molecules in good agreement with the experimental observations. Electronic excitation spectra of open-shell radicals is a topic that has not been studied as much as such spectra of closed-shell molecules, but this can be easily addressed using SAC-CI theory. The present paper systematically describes the calculation procedures for radical anions by investigating several basis sets, including anion diffuse and Rydberg functions. The calculated excitation energies were in good agreement with the experimental UV∕NIR (near infrared region) spectra, which had been observed by one of the present authors in 2-methyltetrahydrofuran matrix frozen to transparent glassy solids at 77 K. For p-DCNB, the SAC-CI theoretical spectrum agreed particularly well with the experimental spectrum. An extremely weak π*(SOMO) - π* excitation at 1.41 eV predicted in the present work, but had been overlooked in the previous experimental spectrum published in 1988, was confirmed to be real by a careful re-examination of the old spectrum.