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 -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 i nvestigated by spectrophotometry at different temperatures. Evidence f or 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 e nthalpy and entropy changes have been calculated. The 1:2 complex has been characterized in the solid state by an X ray structural study: th e 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)degre es, Z = 4, R = 0.0579. The molecular structure shows that the two thio nic 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 previo us investigations on Et(4)todit and Ph(4)todit, where the absorbance d ata were explained assuming the formation of the 1:1 complex only. A n ew calculation procedure for the least-squares minimization of spectro photometric data, based on Powell's direct search method, has been dev eloped. This procedure allows for the refinement of the standard entha lpy and entropy changes from a set of absorbance data at different tem peratures. 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).