Width of NMR lines

For the evaluation of NMR measurements, there are a few very important parameters second moment M2 and width of NMR line. They can offer some... 

The width of NMR lines is related to the phenomenon of relaxation. This concept has been defined. There are two main relaxation processes in NMR, namely, spin-lattice or longitudinal, relaxation and spin-spin , or transverse, relaxation. Spin-lattice relaxation is the process by which the nuclei in the upper spin state lose energy externally to the crystal lattice (in a crystal) or, less effectively, to the surrounding liquid (in a solution). The speed at which spin-lattice relaxation occurs is characterized by the longitudinal relaxation time Ti. 

If the radiofrequency spectmm is due to emission of radiation between pairs of states - for example nuclear spin states in NMR spectroscopy - the width of a line is a consequence of the lifetime, t, of the upper, emitting state. The lifetime and the energy spread, AE, of the upper state are related through the uncertainty principle (see Equation 1.16) by... 

As for the width of NMR resonance lines, it is inversely proportional to mobility of resonating nuclei. The width calculated from the experimental NMR lines of the three samples is of the highest value for the sample CT ODA 5 and there is only a small difference between the linewidths of CT and CT ODA 2. 

The lines in the spectrum of Fig. 1 are, even under optimum experimental conditions, quite broad. This is due to small unresolved hyperfine interaction with the eighteen equivalent hydrogen atoms. Either the width of the lines, or nmr measurements (La Mar et al., 1973) can reveal the magnitude of this y-hydrogen splitting (ca. 0.15 G—dependent on temperature and solvent). 

If you look at the nmr spectra of many different kinds of organic compounds, you will notice that some resonances are sharp and others are broad. In a few spectra, all of the peaks may be broad as the result of poor spectrometer performance, but this is not true for the spectra of Figures 9-29 (p. 312) and 24-2 (p. 1173) where, within a given spectrum, some resonances will be seen to be sharp and others broad. We can understand these differences by consideration of the lifetimes of the magnetic states between which the nmr transitions occur.1 The lifetimes of the states can be related to the width of the lines by the Heisenberg uncertainty principle. 

Doskocilova D, Tao DD, Schneider B, Effects of macroscopic spinning upon line width of NMR signals of liquid in magnetically inhomogeneous systems, Czech. J. Phys. B, 25 202-209, 1975. 

An interesting variation of this experiment involves using the same technique of NMR line-width measurements to study the cis-trans exchange rate in N,N-dimethylacetamide. By determining this rate as a function of temperature over the range 300 to 500 K, one can evaluate the rotational barrier for this cis-trans isomerization. ... 

Generally the sweep width of broad-line NMR is about 10 to 10 Hz, while that of high-resolution NMR is about 10 Hz at 36.4 MHz for a proton. But the H NMR spectra of the mesophase for Kureha pitch (Figure 8) and ethylene tar pitch (Figure 9) were swept over 2.5 x 10" Hz. Thus the sweep width of the H NMR spectrum for the carbonaceous mesophase is intermediate between those of broad-line H NMR... 

In the NMR speetroscopy of liquids, the spin lattice relaxation time falls in the range 10 to 10 s. This influences the width of the NMR adsorption line. Estimate the corresponding minimum width of the line using the relationship... 

Here, the experimental error for each 0 value is introduced and shown as the width of the lines. The black area indicates the most accurate region which satisfies all of the observed bond orientations within experimental error. By addition of the angular constraint 0cr> to those obtained from and solid-state NMR, a further narrow (more... 

At temperatures above 23 K the NMR line corresponds to a typical NMR quadrupolar Pake pattern with an asymmetry oitj = 0.2. The satellite pattern has completely disappeared. The width of the line decreases slowly with increasing temperature, which is an indication of a weakening of the T/ -bond between the metal and dihydrogen. 

Theoretically, it should be possible to study any element present in a soil by NMR. However, there are a number of limitations. Some nuclei are intrinsically insensitive and if present in only trace concentrations, cannot be detected. This is particularly troublesome for elements for which rare nuclides have to be studied. A further problem is that the width of the lines in the NMR spectra can be greater than the differences between individual resonance frequencies so that whatever the structure, only one broad line is obtained. As already noted, to date only a few nuclei ( H, Al, Si, N, P) have been... 

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