Theoretical Study of the Structure of 2,6-Dibromopyridinium Halide Salts

Abstract

Theoretically, the role played by the halogen…halide interactions (C-Br…X^–) and hydrogen bonding interaction (N-H…X^–) in the crystal structure of 2,6-dibromopyridinium halide (2,6-DBP)X (X = Cl^–, Br^–) has been studied. The computational results are compared with previous experimental results for analogous compounds. There is a clear presence of (C-Br…X^–) interactions and (N-H..X^–) interactions in (2,6-DBP)X with almost linear (C-Br…X^–) interactions angles (178.55º) and (N-H…X^–) interactions angles (ca. 179.92º) and the results are then compared to experimental results for analogous compounds. This study shows that the (C-Br…X^–) interactions have comparable strength with the corresponding strong NH…X^– hydrogen bonds in (2,6-DBP)X. This is different from a previous result in which the (C-Br…X^–) interactions are stronger than hydrogen bonding interactions in (2,5-DBP)X (X=Cl, Br and I ). The study has proven the existence of N(p)…..X^– (X = Cl, Br and I) interaction. The existence of nitrogen (p) … halide ion interaction [N(p)….X^–] is investigated. Calculations has been performed for the following compounds: (2,6DBP)Br, (2,6-DBP)Cl, (2,5-DBP)Br and (2,5-DBP)Cl. Calculations of energy changes that occur when the halide ion site changes above the nitrogen atom in the pyridinium bromide salt have been conducted. The position of the bromine anion was varied in three dimension forming an imaginary cubic shape above a nitrogen atom. The studied ranges of the coordinates are as follows: x= -2 to 2 Å, y = -2 to 2 Å and z = 1.5-4.5 Å. In the pyridinium bromide salt, the minimum energy for the N(p)…Br^– interaction is located at a distance of 3.5-3.7 Å from the nitrogen atom to the halide anion and at an angle of ca. 25º between the N…Br^– line and the normal to the plane of the pyridinium cation. These values are in good agreement to previously studied systems of similar nature.