Bruker BioSpin Solid State NMR, User Manual

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User Manual Version 002 BRUKER BIOSPIN 201 (327)
16
Relaxation
Measurements
16
In NMR experiments, one is generally concerned with measuring resonance fre-
quencies, and relating these to the local molecular environment. To do this the
state of the system of spins in the sample must be changed from equilibrium. At
equilibrium, the net magnetization due to the spins is aligned along the magnetic
field axis. By applying a radio frequency pulse the net magnetization is tilted away
from the field axis, and the resulting precessing magnetization generates the ob-
served signal. The pulse has disturbed the system from equilibrium, and over time
the system will return to its equilibrium state. This process is called relaxation.
This chapter describes experiments used for measuring relaxation rates in solid-
state NMR. A basic description of relaxation is provided in order to define terms
and introduce the techniques involved, but discussion of the significance and use
of relaxation data is outside the scope of this manual. Many textbooks provide
more detail on the theory of relaxation: the classic is Abragam:
A. Abragam, Principles of nuclear magnetism , Oxford: Clarendon Press,
(1961)
but simpler descriptions can be found in the books of Slichter and Levitt:
C.P. Slichter, Principles of magnetic resonance, Springer (1996, 3
rd
ed.)
M.H. Levitt, Spin dynamics: Basics of nuclear magnetic resonance, Wiley
(2001)
Some discussion of T
1r
relaxation, including effects of dipolar coupling to proton
spins, can be found in:
D.L. VanderHart and A.N. Garroway,
13
C NMR rotating frame relaxation in
a solid with strongly coupled protons: polyethylene, J. Chem. Phys. ,
71 :2773-2787, 1979
Details of the X T
1
experiment with CP are in:
D.A. Torchia, The measurement of proton-enhanced
13
C T
1
values by a
method which suppresses artifacts, J. Magn. Reson. , 30 :613-616, 1978
The TOPSPIN software includes a tool for processing the data obtained in relax-
ation measurements, and this will be demonstrated for the different types of relax-
ation experiment.
Describing Relaxation 16.1
Relaxation of the net magnetization can be described in terms of two processes.
After a pulse, the state of the system differs from the equilibrium in two ways: the
z-magnetization is not equal to the equilibrium value, and the net magnetization in
the transverse plane is non-zero. The return of the z-magnetization to equilibrium
is termed longitudinal relaxation, or spin-lattice relaxation, and the return of trans-