On the Singlet Diradical Character of S₂N₂

Abstract

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.

Electronic Structures and Molecular Properties of Chalcogen Nitrides Se2N2 and SeSN2

“…Despite the non-physical nature of the symmetry-broken spin densities, they usually provide valuable information about the origin of the diradical character. 41 A good illustration of this is the symmetry-broken spin density of H2 at elongated bond lengths, which correctly describes the homolytic dissociation of the bond by localizing one Together with the previous results 16,[20][21][22][23][24] the spin density analysis clearly settles the discussion about the nature of the singlet diradical character in S2N2 in favor of the nitrogen-localized VB structure 2. It should, however, be noted that our recent analysis of the [22,12]-CAS wave function of S2N2 in terms of different Lewis-type VB structures 16 showed that the VB structure 2 has only 34% weight in the CAS wave function.…”

“…In CAS optimizations, the active space consisted of full valence space and 7 included all possible configurations that arise from the distribution of 22 valence electrons in 11 highest occupied and 5 lowest virtual orbitals i.e. [22,16] Vibrational frequencies were calculated utilizing all aforementioned methods within the harmonic approximation. For methods for which analytical second derivatives were not implemented, frequencies were calculated using finite differences of energy.…”

“…In CAS and CASPT2 calculations the active space size was [22,16]. First, a state averaged CAS calculation was performed (with equal weights for all states) for the ground state and the lowest eight excited singlet (triplet)…”

Notes on Valence Bond Structures for S2N2 and Related Systems

“…Therefore to include all Lewis structures, resonance between structures III-VI is needed. The ten Lewis structures can also be included by replacing the a, b, c and d AOs in Equations (8) and (9) for structures I' and II' by two-centred orbitals.…”

“…When the one-electron bond polarity parameters k = l are both set equal to unity, an average SÀN bond character of 1.375 [8] is associated with resonance between increased-valence structures I and II. This bond character reflects better than does resonance between the zwitterionic Lewis structures 3-6 (with an average SÀN bond character of 1.25) that the SÀN bond lengths of 1.645 are nearly halfway [2] between those of an SÀN single bond (1.74 ) and an S=N double bond (1.54 ).…”