Proton Spin-Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

Area of Science:

• Solid-state chemistry
• Materials science
• Physical chemistry

Background:

• Understanding molecular dynamics and solid-state structures is crucial for predicting material properties.
• Van der Waals molecular solids exhibit complex behaviors influenced by their nanoscale structure.
• Polymorphism can significantly alter the physical characteristics of organic compounds.

Purpose of the Study:

• To investigate the solid-state properties of 2-ethylanthracene (EA) and 2-ethylanthraquinone (EAQ) using various physical characterization techniques.
• To explore the relationship between sample preparation, molecular structure, and relaxation dynamics.
• To elucidate the origins of anomalous relaxation behavior observed in EAQ samples.

Main Methods:

• Solid-state ¹H spin-lattice relaxation measurements at variable temperatures.
• Single-crystal and powder X-ray diffraction (XRD) analyses.
• Field emission scanning electron microscopy (FESEM) for morphological characterization.
• Differential scanning calorimetry (DSC) for thermal analysis.

Main Results:

• ¹H spin-lattice relaxation in both EA and EAQ was non-exponential at high temperatures, consistent with methyl group rotation.
• ¹H spin-lattice relaxation experiments proved sensitive to the cluster structure of these van der Waals solids.
• Unusual low-temperature non-exponential relaxation in EAQ, fitted by stretched or double exponentials, correlated with powder XRD findings.

Conclusions:

• The observed anomalous relaxation in EAQ at low temperatures is attributed to the presence of multiple crystalline polymorphs (concomitant polymorphism).
• Physical purification methods can influence the polymorphic composition and solid-state properties of EAQ.
• Solid-state NMR relaxation is a powerful tool for detecting subtle structural variations, including polymorphism, in molecular solids.