Lecture 4: Spin Relaxation

Question 1

What is the definition of NOW enhancement factor? Know the equations for transient NOE enhancement. Know the relationship between NOE enhancement and the distance of nuclei. What factors influence the observation of transient NOE?

Answer 1

-NOE enhancement factor, n, is defined as:
~ n = ?enhanced - reference)/ (reference)
-See equation for Transient NOE

Question 2

What does pulse sequence mean? List three components of a pulse sequence.

Answer 2

-pulse sequence: describes a set of RF pulses applied to a sample to produce a specific form of NMR signal.
~ Pulse: excite, invert, or decouple the magnetization; they can have various shapes, lengths, and phases.
~Delay: time intervals between pulses, allows spin magnetization to evolve.
~ Data acquisition: time got receiver to record FID signal; receiver can be phase shifted between different scans..

Question 3

Know the definition of Bo field and B1 field.

Answer 3

• Bo: static magnetic field.

- B1: The magnetic field generated by an RF pulse.

Question 4

What does a 90 degree pulse mean? What does a 180 degree pulse mean?

Answer 4

90 pulse = A burst of radiation that cause M to tip 90 degrees.
180 pulse = A burst of radiation that causes M to tip 180 degrees.

Question 5

What is NMR relaxation?

Answer 5

-After the pulse:
~Because of random fluctuations of local magnetic fields, the individual (magnetic moment) vectors will quickly get out of phase and the net magnitude of M will decrease.
~ The system will then relax toward equilibrium; that is, M will slowly rise back to its equilibrium value along the Z axis.

Question 6

What are the definitions of NMR T1 and T2 relaxations? What is the major mechanism for NMR T1 and T2 relaxation?

Answer 6

T1 = spin lattice relaxation, excited spins return to ground state.
T2 = spin spin relaxation, loss of spin coherence in the XY plane.
~oscillations of the local magnetic field also causes the magnetic moments of the nuclei to rotate with different speeds in the XY plane, thereby leads to T2 relaxation.

Question 7

Know the dependence of T1 and T2 relaxations over molecular tumbling time tc, what kind of molecular information can be derived from T1 and T2 relaxation?

Answer 7

-Rate of T1 depends on:
~Strength and frequency of fluctuating magnetic field which depend on:
▪︎ magnetic moment of nuclei
▪︎ distance between nuclei involved.
▪︎the molecular tumbling time (tc)
~ T1 relaxation is most efficient when rate of tumbling motion is near to NMR frequency.

-T2 relaxation increases as the tumbling gets slower (NMR peaks become broader).

Question 8

Know the correlation between line broadening and T2 relaxation time:

Answer 8

-T2 relaxation increases as the tumbling gets slower (NMR peak become broader).

Question 9

Able to describe/draw the pulse sequence for spin echo experiment. Can explain how the magnetization of different nuclei (with different chemical shifts) become refocused in the end.

Answer 9

• An initial excitation pulse decays with time due to both spin relaxation and inhomogeneous effects which cause different spins in the sample to precess at different rates.
  ~The first set leads to an irreversible loss of magnetization.
  ~ Inhomogeneous effects can be removed by applied a 180 inversion pulse that inverts the magnetization vectors.
  *Inhomogeneous effects include magnetization field gradient and difference of chemical shifts.