Intramolecular Dynamics of RS-SR′ Systems (R, R′ = H, F, Cl, CH3, C2 H5): Torsional Potentials, Energy Levels, Partition Functions

The equilibrium and cis and trans geometric structures of a series of disulfides RS-SR' (HSSH, HSSF, HSSCl, HSSCH3, HSSC2H5, FSSF, CISSCl, ClSSF, CH3SSCH3, C2H5SSC2H5) have been studied using both density functional theory (B3LYP/6-311++G(3df, 3pd)) and second-order Moller-Plesset perturbation theory (MP2(FU)/aug-cc-pVTZ). The effects of internal rotation on the structural parameters have been analyzed and the torsional potentials around the S-S bond have been compared with experimental and theoretical information, when available, as well as with the analogues of the peroxydic family. Torsional levels were calculated, and their distribution as a function of temperature was determined, obtaining partition functions. This information is of interest for statistical approaches to equilibrium properties and to rates of processes in which torsional anharmonicity is relevant. It is also required for recent atmospheric modeling studies and also for prototypical chiral separation experiments, in view of a possible dynamic mechanism for chirality exchange by molecular collisions. In general, barriers for such processes are higher than for the corresponding peroxides, and accordingly, rates for chirality change are estimated to be consistently smaller.