Pursuing structure in microcrystalline solids with independent molecules in the unit cell using 1H-13C correlation...

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Crystallography
• Materials Science

Background:

• Characterizing solids with multiple independent molecules in the unit cell (asymmetric units) is challenging for many analytical methods.
• Accurate assignment of NMR spectral shifts to specific molecular positions is crucial for understanding solid-state structures and dynamics.

Purpose of the Study:

• To demonstrate the utility of the (1)H-(13)C solid-state NMR heteronuclear correlation (HETCOR) experiment for assigning spectral shifts in solids with multiple molecules per asymmetric unit.
• To enable conformational characterization of individual molecules within the asymmetric unit using only solid-state NMR data.
• To compare the sensitivity and efficiency of (1)H-(13)C HETCOR with previous (13)C-(13)C correlation methods.

Main Methods:

• Application of the (1)H-(13)C solid-state NMR heteronuclear correlation (HETCOR) experiment.
• Verification of the technique using materials with known assignments from INADEQUATE data (santonin and Ca(OAc)(2) phase I).
• Extension of the method to analyze unknown solids: (+)-catechin and Ca(OAc)(2) phase II.

Main Results:

• HETCOR successfully assigns spectral shifts to specific molecular positions and associates resonances with individual molecules in the asymmetric unit.
• The experiment achieved sufficient sensitivity and resolution to assign over 54% of sites to specific molecules in the asymmetric unit for the tested unknown solids.
• The (1)H-(13)C HETCOR method demonstrated higher sensitivity compared to (13)C-(13)C correlation techniques, leading to shorter analysis times for natural abundance materials.

Conclusions:

• The (1)H-(13)C solid-state NMR HETCOR experiment is a powerful tool for detailed structural and conformational analysis of complex solids.
• This method overcomes limitations of other techniques in characterizing solids with multiple molecules per asymmetric unit.
• HETCOR provides a more sensitive and efficient approach for resonance assignment in solid-state NMR studies.