Solid state separated-local-field NMR spectroscopy on half-integer quadrupolar nuclei: principles and applications to...

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Materials Science
• Physical Chemistry

Background:

• Quadrupolar nuclei (spin >/= 3/2) present unique challenges in solid-state NMR due to complex interactions.
• Existing NMR techniques often require stringent conditions like homonuclear proton decoupling, limiting their applicability.
• Separated-local-field magic-angle spinning (SLF MAS) has been successful for spin-1/2 nuclei but not readily extended to quadrupolar nuclei.

Purpose of the Study:

• To introduce and exemplify novel multidimensional NMR methods for solid-state quadrupolar nuclei.
• To extend SLF MAS NMR techniques to study half-integer quadrupolar spins (S >/= 3/2).
• To develop NMR experiments that correlate quadrupolar and heteronuclear dipolar interactions without homonuclear decoupling.

Main Methods:

• Development of multidimensional NMR sequences based on SLF MAS principles.
• Utilizing moderately fast magic-angle spinning (MAS) rates (6-15 kHz).
• Employing rotor-synchronized constant-time pulse sequences to amplify dipolar coupling strengths.

Main Results:

• A suite of simple 2D NMR experiments was developed, providing valuable information from line shapes.
• These methods were successfully applied to boron-containing samples, yielding new insights into their structure and dynamics.
• The techniques were extended to 3D NMR experiments incorporating multiple-quantum MAS for enhanced resolution.

Conclusions:

• The new NMR methods effectively correlate quadrupolar and heteronuclear dipolar interactions in the solid state.
• These techniques offer a powerful tool for analyzing the structure and dynamics of solids containing quadrupolar and proton nuclei.
• The developed 3D NMR experiments facilitate the assignment of inequivalent sites in complex solid materials.