Influência do empacotamento cristalino nas conformações de N,N -bis(piridina-n-ilmetil) etanodiamidas e tioamidas análogas (n= 2, 3, 4)

Abstract

The rational development of crystal structures requires consistent and directional formation of its synthons to control the supramolecular arrangement. Hydrogen bonds between atoms with high electronegativity (OH O and N-H...O) have greater strength and directionality compared to other interactions (C-H...N, C-H..._, _..._ ), so they are reliable for the design of these structures. However, some studies show that these interactions are not enough to predict a specific supramolecular arrangement, for example, synthons of carboxylic acids have centrosymmetric dimers {..H-O-C=O}2 in only one third of the structures with the potential to achieve these interactions. In this work several conformers of N, N'-bis (pyridin-n-ylmethyl)ethanediamide and their thioamides analogous were studied theoretically (B3LYP/631-G**) and their stabilities were compared to those of the conformation adopted in the crystal. The origin of the rotational barrier in the C-N bonds of amides and thioamides were analyzed through the changes in geometric parameters, partial charges and bond orders. It was observed that (thio)amide bonds in the gas phase have properties that direct the crystal packing to the ZZ conformation. As to the orientations of the pyridyl rings (Planar, U shaped and S shaped), the electronic structures of individual molecules do not present stabilizing forces that can be carried to the solid state without the influence of intermolecular interactions, thus, the choice of the crystalline conformation related to the pyridyl rings is dictated by intermolecular forces. It was also proved that the origin of the rotational barrier can be described by the classical resonance model.