An NMR crystallographic characterisation of solid (+)-usnic acid

Area of Science:

• Solid-state chemistry
• Spectroscopy
• Computational chemistry

Background:

• (+)-Usnic acid is a natural product found in lichens.
• Accurate structural and signal assignment is crucial for understanding its properties.

Purpose of the Study:

• To achieve complete signal assignment for solid (+)-usnic acid using NMR and DFT.
• To investigate tautomeric forms and compare pure usnic acid with lichen samples.

Main Methods:

• One- and two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy.
• Density functional theory (DFT) calculations for signal assignment.
• Variable-temperature NMR measurements.

Main Results:

• Full assignment of ¹H and ¹³C signals for solid (+)-usnic acid was achieved.
• Absolute assignment of signals to specific crystallographic molecules was possible.
• Tautomeric variations influenced ¹³C chemical shifts with temperature.
• The crystalline form of usnic acid in *Usnea dasopoga* lichen matched the pure sample.

Conclusions:

• Combined NMR and DFT provides robust signal assignment for complex molecules.
• (+)-Usnic acid in its natural lichen source is directly observable and structurally consistent with pure samples.