Nature of the Three-Electron Bond in H 2 S∴SH 2 +

We have investigated the model system H 2 S∴SH 2 +, i.e., the sulfur−sulfur bound dimer radical cation of H 2 S, 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 (Δ E = Δ E 2c-3e + Δ E elst ) is nearly 60% provided by the three-electron bond (Δ E 2c-3e ) between the unpaired sulfur 3p x electron on H 2 S +• and the sulfur 3p x lone pair on H 2 S; electrostatic attraction (Δ E elst ) is important, too, with a contribution of somewhat more than 40%. We show furthermore that the three-electron bond (Δ E 2c-3e = Δ E 2c- 1e + Δ E Pauli ) can be conceived as and quantitatively analyzed in terms of a one-electron bond (Δ E 2c-1e ), arising from the β-electron of the H 2 S lone pair interacting with the corresponding empty β-spin orbital of H 2 S +•, opposed by the Pauli repulsion (Δ E Pauli ) between the α-electrons of the H 2 S lone pair and H 2 S +• SOMO.