An experimental and theoretical study on some thiocarbonyl-I2 molecular complexes

Intermolecular charge-transfer (CT) spectra of several complexes between thiocarbonyl compounds and molecular iodine were studied in the UV-visible region. Equilibrium constants and Gibbs energy changes of 1:1 charge-transfer complexes were determined in solution. Two different kinds of complexes were detected, those which present the CT band in the 300 nm region and those which absorb around 350 nm. Ab initio calculations at HF/LANL2DZ* and MP2(full)/LANL2DZ*//LANL2DZ* were carried out to clarify their structure. Complexes with the CT band around 300 nm correspond to chose where the molecule of iodine lies in the same plane of the C=S group, while in those absorbing in the 350 nm region the It moiety is almost perpendicular to that plane. These perpendicular complexes are formed when the substituents around the thiocarbonyl group are voluminous, due to steric hindrance and to the different nature of the HOMO. In bath kinds of complexes, the thiocarbonyl-iodine interaction is essentially electrostatic. The substituent effects were analyzed by Taft-Topsom's model. Experimental data in solution and theoretical estimates were found to follow a good linear relationship. The gas-phase basicity of the set of thiocarbonyl compounds investigated toward proton is linearly correlated with their basicity toward molecular iodine in solution. This finding strongly supports previous conclusions regarding the relationship between gas-phase and solution reactivity data.