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

Abstract

Cyclobutanetetrone, C 4 O 4 , has a triplet ground state, although the ground state of C 4 S 4 is singlet. This computational study focuses on the mono-, di-, and trithiosquarate, C 4 O 4-n S n (n = 1-3), molecules as transition stages between the two ending points (C 4 O 4 and C 4 S 4 ), 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 C 4 O 3 S 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.