Spin-Spin relaxation or transverse relaxation - T2
Spin-Spin relaxation mechanisms
Measurement of T2 relaxation process | Hahn echo | Carr-Purcell-Meiboom-Gill sequence| Diffusion
The existence of relaxation implies that an NMR line
must have a width. The smallest width can be estimated from the uncertainty
principle. Since the average lifetime of the upper state cannot exceed T1,
this energy level must be
broadened to the extent: h/T1. This means that line width at half-height of the NMR line must be at least: 1/T1.
In Solution NMR, very often T2
are equal (small/medium molecules and fast tumbling
For solids, T1 is usually much larger than T2. The very fast spin-spin relaxation time provide very broad signals.
There are other processes that can increase the line width substantially over the expected value extracted from T1 analysis. T2 represents the lifetime of the signal in the transverse plane (XY plane) and it is this relaxation time that is responsible for the line width. The "true" line width on an NMR signal depends on the relaxation time T2 (line width at half-height=1/T2).
In fact, by measuring the experimental line width, one can in principle determine the T2 relaxation time. Unfortunately, the experimental line width depends also on the inhomogeneity from the magnetic field. The experimental relaxation rate, extracted from the line width, is called 1/T2* (1/T2*=1/T2 + 1/T2(inhomogeneity)). The inhomogeneity factor is more critical for nuclei with higher frequency (more critical for proton than for Carbon or Phosphorus). The reason behind this statement can be explained as follows: imagine that identical nuclei (identical chemical shift) in a NMR tube are submitted to slightly different field (inhomogeneity from the magnet). The resonance frequency for each individual nuclei is described by the equation: . If the field vary through the NMR tube, so do the observed frequency, broadening the NMR line. This broadening would affect more the proton (higher ) than lower frequency nuclei. The broadening is in fact directly proportional to the frequency.
The same mechanism that are active in Spin-Lattice are active for spin-spin relaxation. You can consult them on the T1 description. We will describe here the mechanisms that can bring extra broadening to the peak.
Scalar relaxation occurs when two spins interact through bond (electron mediated) - J coupling.
Spin I can feel fluctuating in the field from spin S in two ways:
If the relaxation time of the quadrupolar nuclei is rapid
(1/T1(S) >> 2*pi*J),
nuclei I will not couple to the quadrupolar nuclei.
e.g. 1H next to 14N or 11B (T1 => 10-100 msec) => produce broadened lines
e.g. 1H next to 35Cl (T1 => 1 usec) => insignificant broadening
Scalar coupling with rapidly relaxing quadrupolar nucleus can be determined based on T1 and T2 analysis.
In non-viscous liquids, usually T2 = T1. But some process like scalar coupling with quadrupolar nuclei, chemical exchange, interaction with a paramagnetic center, can accelerate the T2 relaxation such that T2 becomes shorter than T1.
In principle T2 can be obtained by measuring the signal width at half-height (line-width = (pi * T2)-1
However the line width for non-viscous liquids is most often dominated by field inhomogeneity. Fortunately, the dephasing of spins isochromats resulting from field inhomogeneity is a reversible process: it can be refocused by using a 180 degree pulse inserted in the center of an evolution time.
The Carr-Purcell-Meiboom-Gill (CPMG) sequence shown above is derived from the Hahn spin-echo sequence. This sequence is equipped with a "built-in" procedure to self-correct pulse accuracy error. For a description of the first half of the sequence, look above in the Hahn echo section. In the picture above, only the first shift is shown but with field inhomogeneity. The letters f and s means that those nuclei affected by the inhomogeneity of the magnet precess faster and slower than the chemical shift respectively.
The following animation.presents those two echo techniques discussing the effect of scalar coupling.
This experiment can also be used to measure diffusion in the NMR tube by using
To have more information about the gradient version of that experiment
To learn more about this topic, you can consult the following animation
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