Electronic Spectra of SCnH Radicals (n = 2−4): An ab Initio Study

Abstract

The MCSCF and MRCI methods have been used to study the electronic spectra of SCnH radicals for n = 2−4. Vertical excitation energies and oscillator strengths have been computed for a number of states. The lowest-lying excited doublet states are due to an excitation from the highest inner-valence π orbital to the HOMO, which is of π type. The corresponding vertical excitation energies are compared to the experimental results. We encountered very intense transitions to high-lying 2Π terms [42Π ← X2Π (n = 2), (6,7)2Π ← X2Π (n = 3), and (7,8,9)2Π ← X2Π (n = 4)], as well as moderately intense transitions to lower-lying terms [12Π ← X2Π, 12Σ+ ← X2Π (n = 2), (2,3,4)2Π ← X2Π (n = 3), and 12Π ← X2Π (n = 4)]. These intense transitions are frequent in cumulenic carbon chains but have not been registered so far in the present systems. The X2Π−14Π vertical energy gaps are (in electronvolts) 4.94 (n = 2), 2.87 (n = 3), and 3.71 (n = 4); the adiabatic electronic energy gaps should be about 1.95, 0.42, and 0.93 eV smaller, respectively.