Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Gradient enhanced NMR spectroscopy

Gradient enhanced NMR is a method for obtaining high resolution nuclear magnetic resonance spectra without the need for phase cycling. Gradient methodology is used extensively for two purposes, either rephasing (selection) or dephasing (elimination) of a particular magnetization transfer pathway. It includes the application of magnetic field gradient pulses to select specific coherences. By using actively shielded gradients, a gradient pulse is applied during the evolution period of the selected coherence to dephase the transverse magnetization and another gradient pulse refocuses the desired coherences remaining during the acquisition period.

Advantages

• Significant reduction in measuring time
• Reduced T1 artifacts
• Elimination of phase cycling and difference methods
• Possibility for three and four-quantum editing
• The ability to detect resonances at the same chemical shift as a strong solvent resonance

Drawbacks

• A need for field-frequency-lock blanking during long runs.

Examples

• Selection of transverse magnetization (I_x, S_x, I_y etc.):

(+)gradient      180°(x)      (+)gradient 

• Suppression of transverse magnetization (I_x, S_x, I_y etc.):

(+)gradient      180°(x)      (-)gradient
 

References

• Ralph E. Hurd, Gradient-Enhanced Spectroscopy, Journal of magnetic resonance. 87, 422-428 (1990)