Like (CO) 4 , Do (CS) 4 and (CSe) 4 Have a Triplet Ground State?

Cyclobutane‐1,2,3,4‐tetraone, (CO) 4 , was computationally predicted and, subsequently, experimentally confirmed to have a triplet ground state, in which a b 2g σ MO and an a 2u π MO were each singly occupied. In contrast, the (U)CCSD(T) calculations reported herein found that cyclobutane‐1,2,3,4‐tetrathione, (CS) 4 , and cyclobutane‐1,2,3,4‐tetraselenone, (CSe) 4 , both had singlet ground states, in which the b 2g σ MO was doubly occupied and the a 2u π MO was empty. Our calculations showed that both the longer CX distances and smaller coefficients on the carbon atoms in the b 2g and a 2u  MOs of (CS) 4 and (CSe) 4 contributed to the difference between the ground states of these two molecules and the ground state of (CO) 4 . An experimental test of the prediction of a singlet ground state for (CS) 4 is proposed.