EVALUATION OF THERMODYNAMIC PARAMETERS ON HIGHLY CORRELATED CHEMICAL-SYSTEMS - A SPECTROPHOTOMETRIC STUDY OF THE 1/1-EQUILIBRIA AND 1/2-EQUILIBRIA BETWEEN I-2 AND R(4)TODIT=4,5,6,7-TETRATHIOCINO[1,2-B-3,4-B']DIIMIDAZOLYL-1,3,8,10-TETRAALKYL-2,9-DITHIONE, (R=BU, ME (NEW DATA), ET, PH (REINVESTIGATION)) - CRYSTAL-STRUCTURE AND MOLECULAR-STRUCTURE OF THE CHARGE-TRANSFER COMPLEX BU(4)TODIT-CENTER-DOT-2I2

The reactions between Bu(4)todit and I-2 in CHCl3 solution have been investigated by spectrophotometry at different temperatures. Evidence for the stepwise formation of the 1:1 and 1:2 charge-transfer complexes has been obtained by computer analysis of the experimental data. The stability constants of both complexes and the corresponding standard enthalpy and entropy changes have been calculated. The 1:2 complex has been characterized in the solid state by an X ray structural study: the crystals of Bu(4)todit . 2I(2) are monoclinic, space group C2/c, a = 16.352(6), b = 9.376(5), c = 23.470(7) Angstrom, beta = 99.78(2)degrees, Z = 4, R = 0.0579. The molecular structure shows that the two thionic sulfur atoms coordinate two diiodine molecules, the bond distances and angles being similar to those observed previously in Et(4)todit . 2I(2). On the contrary, the solution studies disagree with the previous investigations on Et(4)todit and Ph(4)todit, where the absorbance data were explained assuming the formation of the 1:1 complex only. A new calculation procedure for the least-squares minimization of spectrophotometric data, based on Powell's direct search method, has been developed. This procedure allows for the refinement of the standard enthalpy and entropy changes from a set of absorbance data at different temperatures. Convergence has been attained for the reaction of diiodine with Bu,todit and Me(4)todit (new data) and also for Et(4)todit and Ph(4)todit (old data).