Singlet–triplet competition in the low-lying energy states of C4O4−n S n (n = 1–3) molecules

Abstract

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, C4O4−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.