Topic 2: linking NMR theory and practice

Question 1

• Nuclei have … and respond to the presence of a … …
• They precess at their Larmor frequency, following the equation v = γB_o/2π (…) where γ is the … … that defines a nucleus
• A pulse of certain … will excite nuclei from low to high energy states of … spin

Answer 1

• Nuclei have spin and respond to the presence of a magnetic field.
• They precess at their Larmor frequency, following the equation v = γB_o/2π (hz) where γ is the Gyromagnetic ratio that defines a nucleus
• A pulse of certain frequency will excite nuclei from low to high energy states of opposite spin.

Question 2

How does magnetisation along an applied field form? How can this magnetisation be transferred

Answer 2

• Transverse excess of low and high Energy state cancels, resulting in a vector M_z aligned with the applied field.
• This can be transferred to the xy plane through an RF pulse.

Question 3

Outline the components of a 1D pulse sequence

Answer 3

• Pulse pushes magnetisation into xy-plane where it can be measured
• t_rd – how long after collecting FID until we can remeasure. Time for magnetisation to return to +z axis (applied field), returning to equilibrium state.
• Acquisition (Acq) – contains information of the frequencies of nuclei in the sample.
• Each species gets its own line

Question 4

• Outline the differences in 1,2 &3D NM

Answer 4

Preparation phase is to get magnetisation into transverse (xy) plane through pulse (black bar)

Question 5

Why do we have a mixing period in 2/3D NMR.

Answer 5

• In multi-dimensional NMR we want to know how nuclei interact with one another
• Can be through scalar (bond) or dipolar (space) coupling
• Mixing allows coherence between signals to transfer to find degree of each coupling.

Question 6

How are solvent peaks eliminated from FID acquisition?

Answer 6

• Continuous wave decoupling destroys the coherence of large ¹H peaks due to water
• A high power 90^O acquisition pulse if done quickly before water has a chance to relax back in to spectrum.

Question 7

• A good 1D spectrum has good suppression of large solvent peaks. It can also tell us about the chemical shift dispersion in the spectrum. What is this and what else can a spectrum tell us?

Answer 7

• Chemical shift dispersion describes the general spread of the chemical shifts for a given nucleus in the NMR spectrum. If this is very clustered, it would not be wise to proceed with further 2D/3D NMR, due to high overlap.
• A good spectrum will also tell us the concentration of our sample of interest as well as its purity.

Question 8

• Outline the details of a ¹H-¹H 2D NOESY experiment

Answer 8

• 2^nd 90^O pulse puts magnetisation from xy plane into Z direction
• Allows NOE information transferred between coupled nuclei that are dipolar coupled.
• If mixing time is short, only strongest dipolar coupled nuclei show in signal.

Question 9

What is the evolution delay, t₁, in 2D NOESY?

Answer 9

• Evolution delay creates a second dimension via Fourier transform
• Incrimented in time to capture different chemical shift information in each plane.
• Collect # of planes with a different value for given incremented t₁ delay.