The NMR experiment L2

Question 1

Describe the NMR experiment

Answer 1

Sample is held in a static homogeneous strong magnetic field . Radio frequency (RF) pulse is applied perpendicular to magnetic field direction. When the RF pulse is switched of the nuclei relax. The energy emitted on relaxation is detected

Question 2

What does applying radio frequency pulse do the the sample

Answer 2

Induces resonance in nuclei of appropriate magnetogyric ratios. The pulse induces transitions between spin states.

Question 3

Describe the pulse induces transitions between the NMR experiment

Answer 3

Very short powerful pulse applied, the short pulse contains the spread of frequencies which excites all nuclei simultaneously

Question 4

Why is it important that the nuclei relax

Answer 4

The nuclei need to relax to re establish Boltzmann distribution of spin states

Question 5

What does precession of nuclear spin states cause

Answer 5

Oscillating magnetic field. This is then approximated by summation of sign and co-sign functions for each chemical environment of nuclei under study

Question 6

What is the NMR signal detected as

Answer 6

NMR signal detected as free induction decay FID

Question 7

Describe FiD

Answer 7

Fid is a time domain signal that decays exponentially due to relaxation of nuclear spins

Question 8

How is FID processed

Answer 8

Processed by a computer into a frequency domain NMR spectrum. The FID undergoes Fourier transformation

Question 9

Why do you need to wait between pulses in the nmr experiment

Answer 9

Pulse delay so nuclei can relax back to Boltzmann distribution of spin states