Investigating tautomeric polymorphism in crystalline anthranilic acid using terahertz spectroscopy and solid-state...

Area of Science:

• Solid-state chemistry
• Molecular spectroscopy
• Crystallography

Background:

• Terahertz (THz) spectroscopy is a powerful technique for probing intermolecular interactions in solids.
• Polymorphism, the ability of a solid material to exist in multiple crystalline forms, significantly impacts material properties.
• Tautomeric polymorphism, arising from different tautomeric forms within the crystal lattice, presents unique challenges in solid-state characterization.

Purpose of the Study:

• To investigate tautomeric polymorphism in anthranilic acid using terahertz spectroscopy.
• To analyze the vibrational modes and relative stabilities of different anthranilic acid polymorphs.
• To elucidate the molecular interactions governing the stability of these polymorphs.

Main Methods:

• Terahertz spectroscopy was employed to study three polymorphs (Forms I, II, and III) of anthranilic acid.
• Solid-state density functional theory (SS-DFT) calculations, including London dispersion corrections, were used for vibrational analysis and stability assessment.
• Analysis focused on identifying spectral signatures associated with neutral and zwitterionic forms within the crystal structures.

Main Results:

• Terahertz spectroscopy successfully differentiated between the three polymorphs of anthranilic acid.
• Form I was identified as the most stable polymorph, containing both neutral and zwitterionic anthranilic acid molecules.
• Forms II and III, consisting solely of neutral molecules, were found to be less stable than Form I.
• Computational analysis indicated that a balance of steric and electrostatic forces dictates the stability of the mixed neutral/zwitterionic form.

Conclusions:

• Terahertz spectroscopy is a valuable tool for investigating tautomeric polymorphism.
• The stability of anthranilic acid polymorphs is influenced by the coexistence of neutral and zwitterionic species.
• Understanding these intermolecular forces is crucial for controlling crystal forms and their properties.