Stereoelectronic interaction effects on the conformational properties of hydrogen peroxide and its analogues containing S and Se atoms: an ab initio, hybrid-DFT study and NBO analysis.

Ab initio molecular orbital (MP2/6-311+G**//MP2/6-31G+G**) and hybrid-density functional theory (B3LYP/6-311+G**//MP2/6-311+G**) methods and NBO analysis were used to study the stereoelectronic interaction effects on the conformational properties of hydrogen peroxide (1), hydrogen disulfide (2) and hydrogen diselenide (3). The results showed that the Gibbs free energy difference (G(T)-G(S)) values at 298.15K and 1atm between the skew (S) and trans (T) conformations (DeltaG(T-S)) increase from compound 1 to compound 2 but decrease from compound 2 to compound 3. The C conformations of compounds 1-3 are less stable than their S and T conformations. Based on these results, the racemization processes of the axial symmetrical (C(2) symmetry) conformations of compounds 1-3 take place via their T conformations. Based on the optimized ground state geometries using the MP2/6-311+G** level of theory, the NBO analysis of donor-acceptor (bond-antibond) interactions revealed that the stabilization (resonance) energy associated with LP(2)M2-->sigma*(M3-H4) electronic delocalization for the S conformations of compounds 1-3 are 1.35, 5.94 and 4.68 kcal mol(-1), respectively. There is excellent agreement between the variations of the calculated DeltaG(T-S) and stabilization (resonance) energies associated with LP(2)M2-->sigma*(M3-H4) electronic delocalization for the S conformations of compounds 1-3. The correlations between resonance energies, orbital integrals, dipole moments, bond orders, structural parameters and conformational behaviors of compounds 1-3 have been investigated. Test were made of complete basis set methods (CBS-QB3, CBS-4 and CBS-Q), the first two gave results essentially indistinguishable from those we used, but the CBS-Q results were in disagreement with experimental and other theoretical results.