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

We have investigated the model system H2S∴-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 Å 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 (ΔE = ΔE2c-3e + ΔEelst) is nearly 60% provided by the three-electron bond (ΔE2c-3e) between the unpaired sulfur 3px electron on H2S+• and the sulfur 3px lone pair on H2S; electrostatic attraction (ΔEelst) is important, too, with a contribution of somewhat more than 40%. We show furthermore that the three-electron bond (ΔE2c-3e = Δ2c-1e + ΔEpauli) can be conceived as and quantitatively analyzed in terms of a one-electron bond (AE2c-1e), arising from the β-electron of the H2S lone pair interacting with the corresponding empty β-spin orbital of H2S+•, opposed by the Pauli repulsion (ΔEPauli) between the α-electrons of the H2S lone pair and H2S+• SOMO