Single-Crystal NMR Characterization of Halogen Bonds

Area of Science:

• Solid-state chemistry
• Supramolecular chemistry
• Nuclear Magnetic Resonance (NMR) spectroscopy

Background:

• Halogen bonds are crucial non-covalent interactions in supramolecular chemistry.
• Understanding halogen bond geometry and strength is vital for designing new materials.
• Nuclear Magnetic Resonance (NMR) spectroscopy is a powerful tool for characterizing molecular structure and interactions in solids.

Purpose of the Study:

• To characterize halogen-bonded cocrystals using oxygen-17-enriched triphenylphosphine oxide.
• To investigate the relationship between NMR interaction tensors and halogen bond geometry.
• To establish single-crystal NMR spectroscopy as a method for probing halogen bonds in solids.

Main Methods:

• Synthesis and characterization of oxygen-17-enriched triphenylphosphine oxide and its halogen-bonded cocrystals.
• Single-crystal ³¹P and ¹⁷O NMR spectroscopy to measure NMR interaction tensors (chemical shift, quadrupolar coupling, spin-spin coupling).
• Gauge-including projector-augmented wave (GIPAW) computations to corroborate experimental findings.

Main Results:

• ³¹P and ¹⁷O chemical shift tensors, ¹⁷O quadrupolar coupling tensors, and ³¹P-¹⁷O spin-spin coupling tensors were reported for P═O···I halogen bonds.
• Angular deviations of NMR tensor components correlate with deviations in the linearity of the P═O···I halogen bond.
• Formation of halogen bonds led to decreased anisotropy and increased asymmetry in the ³¹P-¹⁷O coupling tensors.

Conclusions:

• Single-crystal NMR spectroscopy is a novel and effective probe for studying halogen bonds in the solid state.
• NMR interaction tensor magnitudes and orientations provide insights into the local geometry of halogen bonds.
• The observed changes in NMR tensors are consistent with the weak nature of the investigated iodine-oxygen halogen bonds.