Nature of the three-electron bond in H2S∴SH2+

We have investigated the model system H2S therefore SH2+, i.e., the sulfur-sulfur bound dimer radical cation of H2S, using both density functional theory (LDA, BP86, PW91) and traditional ab initio theory (up to CCSD(T)). Our purpose is to better understand the nature of the three-electron bond. The S-S bond length is 2.886 Angstrom and the bond enthalpy (for 298.15 K) amounts to -40.7 kcal/mol at the BP86/TZ2P level. The best ab initio estimates for the S-S bond strength (our CCSD(T)/6-311++G(2df,2pd)//MP2(full) and literature values) are some 10 kcal/mol weaker than those from nonlocal DFT. It is shown, using an energy decomposition scheme for open-shell systems, that the sulfur-sulfur bond (Delta E = Delta E2c-3e + Delta E-elst) is nearly 60% provided by the three-electron bond (Delta E2c-3e) between the unpaired sulfur 3p(x) electron on H2S+. and the sulfur 3p(x) lone pair on H2S; electrostatic attraction (Delta E-elst) is important, too, with a contribution of somewhat more than 40%. We show furthermore that the three-electron bond (Delta E2c-3e = Delta E2c-1e + Delta E-Pauli) can be conceived as and quantitatively analyzed in terms of a one-electron bond (Delta E2c-1e), arising from the beta-electron of the H2S lone pair interacting with the corresponding empty beta-spin orbital of H2S+., opposed by the Pauli repulsion (Delta E-Pauli) between the alpha-electrons of the H2S lone pair and H2S+. SOMO.