Prototypal 1,2,3,5-Dithia- and 1,2,3,5-Diselenadiazolyl [HCN₂E₂]^• (E = S, Se): Molecular and Electronic Structures of the Radicals and Their Dimers, by Theory and Experiment

The reactions of N,N,N′-tris(trimethylsilyl)formamidine with ECl₂ (E = S, Se) afford 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolium chloride, which can be reduced with triphenylantimony to the corresponding dithia- and diselenadiazolyl radicals [HCN₂E₂]^•. The solid state structure of the cofacial dimer [HCN₂S₂]₂ has been determined by X-ray diffraction; crystals of [HCN₂S₂]₂ belong to the monoclinic space group P2₁/n, with a = 6.833(6), b = 16.463(4), c = 19.161(4) Å, β = 93.57(4)°, FW = 210.30, Z = 12. The crystal structure consists of stacked dimers, with three dimers in the asymmetric unit. Along the stacking axis the mean intradimer S- - -S contact is 3.11 Å; the mean interdimer contact is 3.76 Å. Ab initio calculations with split-valence and larger basis sets have been employed to evaluate the structures and energies of both the gas phase radicals and their dimers. Minor changes in geometry are predicted upon association of the monomers; this relative structural invariance is consistent with low binding energies, the best predictions (including zero point vibrational energy corrections) being ca. 4 kcal/mol (for E = S) and ca. 10 kcal/mol (for E = Se). Theory also provides spin distributions to support the interpretation of the observed ESR parameters (for E = S), confirms the a₂ symmetry of the SOMO, yields Koopmans' theorem and ΔSCF ionization potentials for analysis of the PE spectra of the radicals, and furnishes vibrational frequencies and infrared intensities of the sulfur radical. The latter facilitate assignment of the gas-phase IR spectrum.