CPMAS NMR
Definition and meaning of CPMAS NMR in chemistry.
CPMAS NMR (cross-polarization magic-angle-spinning nuclear magnetic resonance) is a solid-state NMR technique that transfers magnetization from abundant, high-sensitivity nuclei (usually ¹H) to dilute, low-sensitivity nuclei (such as ¹³C or ¹⁵N) while spinning the sample rapidly at 54.74° to the magnetic field, producing narrow, high-resolution spectra from otherwise broad solid-state signals.
In more detail
In rigid solids, strong dipolar coupling and chemical shift anisotropy broaden NMR lines far beyond what is seen in solution. Magic angle spinning averages these orientation-dependent interactions toward zero because they scale with (3cos²θ − 1), which vanishes at the "magic angle" of 54.74°. Cross polarization separately boosts sensitivity by transferring magnetization from plentiful ¹H nuclei to sparse, weakly polarized nuclei through dipolar coupling during a set contact time, while also letting experiments recycle faster since they exploit ¹H's typically short spin-lattice relaxation time. Together, CP and MAS make routine solid-state ¹³C and ¹⁵N NMR practical.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Magic angle | 54.74° (arccos(1/√3)) relative to the magnetic field |
| Common target nuclei | 13C, 15N, 29Si, 31P |
| Key components | Cross polarization (CP) + magic angle spinning (MAS) |
Recording a ¹³C CPMAS NMR spectrum of a crystalline drug powder to distinguish polymorphic forms by their distinct isotropic chemical shifts, without dissolving the sample.
Frequently asked questions
Why is magic angle spinning required?
Because dipolar coupling and chemical shift anisotropy in solids depend on (3cos²θ − 1); spinning the sample rapidly about an axis at 54.74° to the magnetic field drives this term toward zero, narrowing peaks to near-isotropic linewidths.
Why use cross polarization rather than direct excitation?
Cross polarization transfers magnetization from abundant ¹H nuclei to dilute nuclei like 13C, increasing signal intensity and allowing shorter recycle delays because it relies on ¹H's often much faster spin-lattice relaxation.