# Discovery of a New Polymorph of 5-Methoxy-1H-Indole-2-Carboxylic Acid: Characterization by X-ray Diffraction, Infrared Spectroscopy, and DFT Calculations

## TL;DR

Scientists discovered a new crystal form of a compound called MI2CA using X-ray and infrared methods, which could be important for drug development.

## Contribution

The discovery of a new MI2CA polymorph and its structural characterization using X-ray, IR, and DFT calculations.

## Key findings

- The new MI2CA polymorph crystallizes in the monoclinic system with space group P21/c.
- Cyclic dimers formed via O−H⋯O hydrogen bonds and C–H⋯O interactions define the crystal structure.
- DFT calculations and IR spectroscopy confirm the structural and spectroscopic features of the new polymorph.

## Abstract

This study presents a new 5-methoxy-1H-indole-2-carboxylic acid (MI2CA) polymorph investigated by single-crystal X-ray diffraction, infrared spectroscopy, and density functional theory (ωB97X-D) calculations employing two basis sets (6-31++G(d,p) and aug-cc-pVTZ). The compound crystallizes in the monoclinic system, space group P21/c (a = 4.0305(2) Å, b = 13.0346(6) Å, c = 17.2042(9) Å, β = 91.871(5)°, Z = 4). In the crystalline structure, the formation of cyclic dimers via double hydrogen bonds O−H⋯O between MI2CA molecules was observed. Interactions between the NH groups of the indole rings and the adjacent methoxy groups, as well as C–H⋯O contacts, significantly influence the spatial arrangement of molecules. The results from DFT calculations, including dimeric and trimeric structures, agree well with the experimental structural and spectroscopic data. Analysis of the infrared spectra confirms the conclusions drawn from X-ray diffraction studies and reveals differences between the IR spectra of the newly obtained polymorph and that reported earlier in the literature. This comprehensive study sheds some light on the MI2CA polymorphism and is important for a potential pharmacological applications of this compound.