On the Singlet Diradical Character of S2N2

The results of some 6-31G(d) CISD MO calculations for the ground state of cyclic S 2 N 2 are reported. They give sulfur 3pπ and nitrogen 2pπ NAO occupancies of 1.501 and 1.443, respectively. These results are interpreted to indicate that the nitrogen singlet diradical character is larger than the sulfur singlet diradical character. The results of STO-6G valence bond calculations of the energies of the sulfur and nitrogen singlet diradical structures are in accord with this conclusion. However STO-6G CISD calculations give sulfur 3pπ and nitrogen 2pπ NAO occupancies of 1.664 and 1.335, respectively, thereby implying that the STO-6G calculations overestimate the extent of nitrogen diradical character. Consideration is given to the use of two equivalent increased-valence structures to provide a valence bond representation of the electronic structure of S 2 N 2 . Regardless of the degree of nitrogen or sulfur singlet diradical character, resonance between these structures, which is equivalent to resonance between the two singlet diradical and four zwitterionic Lewis structures, must provide a lower energy VB represention than does that which is obtained by use of either singlet diradical structure alone.