DEFINITIVE INVESTIGATION OF THE GAS-PHASE 2-CENTER 3-ELECTRON BOND IN [H2S-THEREFORE-SH2](+), [ME(2)S-THEREFORE-SME(2)](+), AND [ET(2)S-THEREFORE-SET(2)](+) - THEORY AND EXPERIMENT

The association products of reactions 1 and 2, examples of two-center three-electron (2c-3e) S therefore S bonds, were studied in the gas phase. Me(2)S(+) + SMe(2) + M reversible arrow [Me(2)S therefore SMe(2)](+) + M (1) and Et(2)S(+) + SEt(2) + M reversible arrow [Et(2)S therefore SEt(2)](+) + M (2). The binding enthalpies were determined by measuring the temperature dependence of the equilibrium constants for eqs 1 and 2. The experimental bond enthalpy and entropy of association were determined for reaction 1 at 576 K (Delta H degrees(bond,576) = 111 +/- 2 kJ/mol, Delta S degrees(rxn,576) = -112 +/- 3 J/mol K) and for reaction 2 at 520 K (Delta H degrees(bond,520) = 116 +/- 3 kJ/mol, Delta S degrees(rxn,520) = -132 +/- 5 J/mol K). The calculated bond enthalpies with zero-point and heat capacity corrections are 122 kJ/mol at 576 K at the [PMP4/6-31+G(2df,p)]//MP2/6-31G(d) level and 112 kJ/mol for [Et(2)S therefore SEt(2)](+) at 520 K at the PMP2/6-31G//(d)//HF/6-31G(d) level. Three conformations of Et(2)S and Et(2)S(+) are predicted to be within 4 kJ/mol of each other. The preferred [Et(2)S therefore SEt(2)](+) 2c-3e complex (C-2 symmetry) is formed between C-1 conformers of Et(2)S and Et(2)S(+). The calculated bond energy for [H2S therefore SHt(2)](+) is 119.6 kJ/mol using G2 theory. All three 2c-3e complexes were further studied by calculating the lowest optical transition and the hydrogen hyperfine coupling constants.