Positional isomers of indolecarboxylic acid and their derivatives are used in various fields due to their diverse properties and effects, especially as intermediates in pharmacy, electrochemistry, agrochemistry, materials science, etc. However, despite importance and frequency of their use, the crystal structures of most of these compounds have not yet been described in the literature.
In this master’s thesis, we therefore analyzed commercial compounds of positional isomers of indolecarboxylic acid using single-crystal X-ray diffraction and electron diffraction, examined their packing, and also performed Hirshfeld surface analysis using the CrystalExplorer software. We also analyzed the positional isomers using several different characterization methods, namely X-ray powder diffraction, nuclear magnetic resonance, thermogravimetric analysis and differential scanning calorimetry.
Due to the suitable size and quality of the crystals, we determined structures of I2CA and I7CA using single-crystal X-ray diffraction, while I4CA and I6CA were measured by electron diffraction. We found that the I4CA and I2CA isomers each form two polymorphic forms. Due to the impurities in isomer I5CA, its more detailed analysis has been omitted in this master’s thesis.
All positional isomers of ICA stack into offset parallel layers; all except I4CA P21/c also contain a second, identical layer that lies at a specific angle to the first. They are linked to one another by π…π, C–H…π, and C–H…H–C interactions of varying strengths. The two polymorphic forms of I2CA are the only ones that in the same plane assemble into a R_2^2 (9) motif, in a zigzag pattern, while all others form centrosymmetric dimers in a R_2^2 (8) motif. With the exception of I7CA, these isomers form an additional N–H…O hydrogen bond.
Based on acquired data, we determined that a change in the position of the carboxylic group on the indole ring has a visible effect on the arrangement of molecules in a crystal, on the presence and extent of intermolecular interactions, and consequently also on the other properties of the positional isomers.
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