MODELING OF THE INTERCONVERSION BETWEEN Z-ISOMERIC AND E-ISOMERIC FORMS OF PYRIDINE-2-FORMYL THIOSEMICARBAZONE

The isomeric interconversion between Z and E forms of a thiosemicarbaz one derivative, namely, pyridine-2-formyl thiosemicarbazone (PATS-2) h as been modeled in this work with the semi-empirical program GEOMOP. P ATS-2 is an electrode surface modifier, relevant in the context of met alloproteins electrochemistry. The strategy of modeling included the d efinition and geometry optimization of eight starting conformations fo r each of the two PATS-2 isomers (Z and E), Rotations over PATS-2 CN d ouble bond, a supposedly prohibited conversion, were imposed to the mo lecule in order to monitor the Z-E transition, since that is the most likely route for the experimentally observed interconversion, Results indicated that PATS-2 CN bond order has a significantly lower value th an that expected for a conventional double bond of the same kind, sugg esting that a rotation at that point in the molecule, a condition for the observed isomerization, is feasible. The greater stability of the E isomer, experimentally observed, could be accounted for by energetic and entropic considerations based on results from the modeling work, We found that the Z <-> E trajectories proceed through small shifts in energy and imply intermediate conformational rearrangements during in terconversion. Most stable transient conformers along a 180 degrees ro tation, including the most likely transition state geometry, were iden tified. (C) 1998 Published by Elsevier Science B,V. All rights reserve d.