Evaluation of thermodynamic parameters for highly correlated chemical systems: a spectrophotometric study of the 1 : 1 and 2 : 1 equilibria between I2 and 1,1′-methylenebis(3-methyl-4-imidazoline-2-thione)(mbit) and 1,1′-ethylenebis(3-methyl-4-imidazoline-2-thione)(ebit). Crystal and molecular structures of mbit·2I2 and ebit·2I2

Abstract

The reactions of I₂ with 1,1′-methylene- and 1,1′-ethylene-bis(3-methyl-4-imidazoline-2-thione)(mbit and ebit) have been investigated in CHCl₃ solution at different temperatures by spectrophotometry. The experimental data have been processed using two different procedures of computer analysis: SUPERSPEC based on the conventional Gauss–Newton–Marquardt least-squares method; and POWELSPEC, based on Powell's direct search method for the refinement of ΔH° and ΔS°. The ability of the two methods to identify the involved equilibria correctly was compared. Evidence for the stepwise formation of the 1 : 1 and 1 : 2 adducts has been obtained by the two methods for the mbit case, while POWELSPEC only was able to solve the ebit case satisfactorily. The crystal structures of the adducts mbit·2I₂ and ebit·2I₂ have been determined. They show that the thionic sulfur atoms co-ordinate two diiodine molecules, with S–I 2.683(2)(mbit·2I₂) and 2.642(3)(ebit·2I₂)Å and with I–I 2.897(1) and 2.903(2)Å respectively, the two S–I–I groups beings related by a two-fold axis. The structural features of the S–I–I linear group are in accordance with Fourier-transform IR and Raman spectral data and can be taken into account with a three-centre two-electron axial orbitally deficient bonding scheme.