The results of STO-6G valence bond calculations for S2N2, with sulfur 3d orbitals omitted, support a conclusion obtained from a previous INDO study, namely that the primary Lewis valence bond structure is a spin-paired diradical (or "long-bond") structure. If this structure is excluded from the set of 5 = 0 Lewis structures, the ground state is calculated to be an S = 1 spin state. "Increased-valence" structures are used to provide qualitative valence bond mechanisms for the thermal polymerization of S2N2 to (SN), and electron conduction in (SN)r.When sulfur 3d orbitals are omitted from the bonding scheme for S2N2, it is usually assumed that the zwitterionic Lewis structures 1-4 are the primary structures required to provide a qualitative VB description for this molecule. No consideration is given to the possibility that there may be an appreciable contribution from the spin-paired diradical or "long-bond" structure1 (5) to the ground-state resonance.
Figure 1. Potential energy diagramme for CH3N2+ isomers. Relative energies (referred to the energy of H2NCH2+ (J)) are given in kcal/mol.The reaction co-ordinate is a complex co-ordinate. transition state of singlet diradicaloid character. As the UHF method (which has been used to localize this saddle point) is known for its poor description of singlet diradicals, the energy of TS14 is likely to be too high. But, ion 6 also seems to be prevented by significant barriers from spontaneous dissociation. In Figure 1 the various isomerization/dissociation processes are given in a simplified two-dimensional potential energy surface.Heats of formation AH0, of 5 and 6 can be estimated from combining the AH0, of reactions 1-5 with the experimentally determined heats of formation of the various mono-cations;14 by doing this, for H2NCH2+ (5) an averaged value of AH°= 644 kcal/mol and for H3NC2+ (6) of AH°= 700 kcal/mol is obtained.Both numbers indicate the extreme thermodynamic instability of the two dications, and it is highly unlikely to ever generate a stable CH3N2+ species in solution. Proton transfer to the solvent shell or capture of negatively charged species from it will occur with avidity.Acknowledgment. The financial support of our work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. We are particularly indebted to the Computer Centres of TU Berlin and the Konrad-Zuse-Zentrum Berlin for generously providing computer time.
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