ANALYSIS OF THE SENSITIVITY OF THE GAS ELECTRON DIFFRACTION METHOD TO THE DETERMINATION OF THE CONFORMATIONAL COMPOSITION OF PHENYL AND THIOPHENYL SUBSTITUTED AZA-BODIPY: THEORETICAL STUDY

Abstract

Conformational manifold of 1,3,5,7-tetraphenyl-aza-BODIPY and 1,3,5,7-tetra(2-thiophenyl)-aza-BODIPY was studied by B3LYP calculations. The addition of four cyclic groups to the aza-BODIPY core defines the non-planarity of the molecule and movement of the substituents from the heterocycle plane. The rotation angles of the phenyl and thiophenyl groups in the molecules are about 30° and 17°, respectively. For phenyl-substituted aza-BODIPY four conformers were found using quantum chemical calculations (B3LYP/6-31G* and B3LYP/cc-pVTZ), for thiophenyl-substituted aza-BODIPY – 30 conformers. Relative energies of some conformers are quite low, therefore they should be taken into account in the treatment of the gas electron diffraction (GED) experimental data. In current work the sensitivity of the GED method to the structural changes induced by different positions of phenyl and thiophenyl substituents relative to the aza-BODIPY core and each other was examined. Model radial distribution curves f(r) for all conformers were compared. It demonstrates that the bond distances can be reliably determined from experimental data, while the refinement of mutual orientations of neighboring groups relating to each other is at the limit of the possibilities of the GED method. Based on experimental data, it is possible to distinguish 1,3,5,7-tetra(2-thiophenyl)-aza-BODIPY conformers, which differ in the mutual arrangement of sulfur atoms. The use of the results of various quantum chemical calculations leads to the same conclusions about the possibility of the gas electron diffraction method to determine the conformational composition.