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Spin-Spin Relaxation time T2 (transverse)

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 and T1 are equal (small/medium molecules and fast tumbling rate). 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. Spin-Spin relaxation mechanisms

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 (dynamic NMR and scalar coupling with quadrupolar nuclei)

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:

The scalar coupling J can be time dependent due to chemical exchange If nuclei S is jumping in and out of a site in which it is coupled to spin I, splitting will collapse at high rate. At intermediate rate, the line will broaden due to partial scalar coupling (this can be observed on exchangeable protons like OH or NH) the spin S can be time dependent due to rapid relaxation of spin S (like in quadrupolar nuclei)

Relaxation induced by Quadrupolar nuclei

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.