Tautomerism and isomerism in some antitrichinellosis active benzimidazoles: Morphological study in polarized light, quantum chemical computations

Abstract The morphology of the crystal structure of some antitrichinellosis active benzimidazole derivatives including (1 H -benzimidazol-2-ylthio)acetic acids, [1,3]thiazolo[3,2- a ]benzimidazol-3(2 H )-ones, 1 H -benzimidazol-2-ylthioacetylpiperazines and starting 2-mercapto benzimidazoles, was studied by the use of Polarized Light Microscopy (PLM). Characterization of the crystal phase was complimented by Differential scanning calorimetry analysis (DSC) and spectroscopic data. DFT computations were performed in order to investigate the prototropic tautomerism and the geometry of the molecule of the synthesized compounds. One distinct type of crystal structure for each one of 5 or 6-methyl-(1H-benzimidazol-2-ylthio)acetic acid 6 was observed by PLM – dendritic and needle-shaped formations. Compound 14 , containing a methyl substituent in the benzimidazole ring crystallized also into two phases; while for the unsubstituted compound 13 a separation of phases does not take place. The influence of the both solvents - chloroform and ethanol on the phase separation and the formation of the crystalline structure of compound 14 was investigated. The morphological study showed that the cyclization of 6 in the presence of acetic anhydride in pyridine medium led to a mixture of 6-methyl-[1,3]tiazolo[3,2- a ]benzimidazol-3(2 H )-one ( 10a ) and 7-methyl-[1,3]thiazolo[3,2- a ]-benzimidazole-3(2 H )-one ( 10b ), which crystallized in the form of fibrils and spherulites respectively. It was found that a difference in the crystal structures of substituted and unsubstituted benzimidazol-2-thiones, respectively benzimidazol-2-thiol derivatives exists, which may be due not only to the thiol-thione tautomerism but to the prototropic properties of the hydrogen atom in first position of the ring. The calculation results indicated that the thione form is more stable than the thiol tautomer by 51–55 kJ mol −1 . But at the same time ΔG for the two thiol tautomers is below 0.5 kJ mol −1 . In solid phase the 5(6)-substituted-1 H -benzimidazol-2-thiols crystallized in two different crystal structures while the unsubstituted 1 H -benzimidazol-2-thiol possess one type of crystal structure.