ON THE SINGLET DIRADICAL CHARACTER OF S2N2

The results of some 6-31G(d) CISD MO calculations for the ground state of cyclic S2N2 are reported. They give sulfur 3p pi and nitrogen 2p p i 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 nit rogen singlet diradical structures are in accord with this conclusion. However STO-6G CISD calculations give sulfur 3p pi and nitrogen 2p pi NAO occupancies of 1.664 and 1.335, respectively, thereby implying th at the STO-6G calculations overestimate the extent of nitrogen diradic al character. Consideration is given to the use of two equivalent incr eased-valence structures to provide a valence bond representation of t he electronic structure of S2N2. Regardless of the degree of nitrogen or sulfur singlet diradical character, resonance between these structu res, which is equivalent to resonance between the two singlet diradica l and four zwitterionic Lewis structures, must provide a lower energy VB represention than does that which is obtained by use of either sing let diradical structure alone.