Structure of thiocyanate dimer radical anion: An ab initio study

The various possible structures of thiocyanate dimer radical anion, (SCN)2·−, have been optimized using Hartree-Fock (HF), density functional theory with B3LYP functional (DFT) and second order Moller-Plesset perturbation (MP2) methods. The most stable structure has been found to be the one where the two SCN subsystems are mutually out-of-plane with respect to each other involving a weak bonding between sulfur-sulfur atoms. The sulfur-sulfur bond length and dihedral angle have been calculated to be larger as compared to that of thiocyanogen, (SCN)2. Binding energy for the global minimum of (SCN)2·− has been obtained as 53.7, 129.3 and 139.0 kJ/mol with the aid of HF, DFT and MP2 methods, respectively, suggesting a strong electron correlation effect. Similar to other dimer radical ion systems, the agreement in binding energy between DFT and MP2 results is quite satisfactory for (SCN)2·−. However, the DFT calculated S⋯S bond distance is highly overestimated as compared to the corresponding MP2 value. Energy and geometry of these optimized structures have been discussed and compared with similar systems.