Singlet-triplet competition in the low-lying energy states of C₄O_4-n S _n (n = 1-3) molecules

Cyclobutanetetrone, C₄O₄, has a triplet ground state, although the ground state of C₄S₄ is singlet. This computational study focuses on the mono-, di-, and trithiosquarate, C₄O_4-n S _n (n = 1-3), molecules as transition stages between the two ending points (C₄O₄ and C₄S₄), and investigates the trends for the changes in the energies, geometry, partial atomic charges, partial spin densities, as well as orbital energies of the four low-lying electronic states. As the number of the sulfur atoms is increasing, the singlet spin state becomes energetically more and more preferred. For C₄O₃S molecule, where only one oxygen atom is substituted by sulfur, the CCSD(T) calculations predict a triplet ground state, but the error of the calculations is most likely higher than the calculated 0.5 kcal/mol singlet-triplet energy gap.