Original scientific paper

Proton and Heavy Atom Motions during the Stepwise Proton Tautomerism of Various Oxalamidines. A Semiempirical PM3-MNDO Study

Gerd Scherer ; Institut für Organische Chemie der Freien Universitat Berlin, Takustr. 3, D-14195 Berlin, Federal Republic of Germany
Hans-Heinrich Limbach ; Institut für Organische Chemie der Freien Universitat Berlin, Takustr. 3, D-14195 Berlin, Federal Republic of Germany

Abstract

The tautomerism of oxalamidine (OA) and the substituted analogs tetraphenyloxalamidine (TPOA), 2,2’-bis-(4,5-dihydro-1,3-diazole) (OA5), 2,2'-bis-(3)4,5,6-tetrahydro-l,3-diazixine) (OA6) and 2,2'-bis-(4,5,6,7-tetra- hydro-1,3-diazepine) (OA7) has been studied theoretically using the semiempirical PM3-MNDO method. According to a previous experimental study, this process consists of an intramolecular degenerate double proton transfer in weak intramolecular N-H- N hydrogen bonds, where the two protons are transferred stepwise involving a zwitterionic intermediate. In addition, evidence was obtained for a substantial heavy atom reorganisation which is strongly dependent on the chemical structure. In the present study, this interpretation is confirmed theoretically by calculation of the energies and geometries of the initial states, transition states and intermediate states of the tautomerism. For A06 and A07, sin- and anti-forms are obtained which differ in the confirmation of methylene bridges. The geometry of anti-OKI agrees remarkably well with the crystal geometry of OA7. Therefore, although the calculated barrier heights of the tautomerism highly exceed the experimental values, the calculated molecular geometries seem to be reliable. Especially interesting is the nature of reorganisation of the molecular skeleton during proton transfer in various oxalamidines.
This reorganisation mainly involves a decrease of the nitrogen-nitrogen distances of the hydrogen bond in which the proton transfer takes place, thus lowering the barrier for the tautomerism. The reorganisation energy strongly depends on the chemical structure. In the case of OA and TPOA, compression of the proton transferring hydrogen bond is possible without major changes in the geometry of the other hydrogen bond. By contrast in bicyclic oxalamidines, compression of one hydrogen bond involves elongation of the other bond or a ring deformation, requiring additional energy. Ring deformation requires less energy in OA7 as compared to OA5 and OA6, in accordance with experimental findings.

Keywords

Hrčak ID:

136765

URI

https://hrcak.srce.hr/136765

Publication date:

1.12.1994.

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