Relaxation times, nuclear

Meiboom S and Gill D 1958 Modified spin-echo method for measuring nuclear relaxation times Rev. Sci. Instrum. 29 688-91... 

The addition of paramagnetic species, such as the metal ions Cu ", Mn, or CF", can have dramatic effects on both the observed spectmm and the relaxation behavior of a molecule. The added ion reduces nuclear relaxation times, and permitting more rapid data collection. In addition, faster relaxation rates minimize NOE effects in the spectra, which can be useful in obtaining quantitative intensity data. The most widely used reagent for this purpose is chromium acetylacetonate [13681 -82-8] known as Cr(acac)2. Practically speaking, the use of such reagents requires care, because at... 

Turning from chemical exchange to nuclear relaxation time measurements, the field of NMR offers many good examples of chemical information from T, measurements. Recall from Fig. 4-7 that Ti is reciprocally related to Tc, the correlation time, for high-frequency relaxation modes. For small- to medium-size molecules in the liquid phase, T, lies to the left side of the minimum in Fig. 4-7. A larger value of T, is, therefore, associated with a smaller Tc, hence, with a more rapid rate of molecular motion. It is possible to measure Ti for individual carbon atoms in a molecule, and such results provide detailed information on the local motion of atoms or groups of atoms. Levy and Nelson " have reviewed these observations. A few examples are shown here. T, values (in seconds) are noted for individual carbon atoms. 

Overhauser limit (Gloss and Gloss, 1969 Gloss, 1969 Gloss and Trifunac, 1969) (v) polarization was obsei ved in radical systems where the radical lifetimes were longer than the nuclear relaxation times in the individual radicals involved (Ward and Lawler, 1967 Lawler, 1967 Fischer, 1969). 

Failure to observe polarization in a particular reaction is significant only to the extent that any negative evidence is significant. If other evidence points to a radical pathway for the reaction, it may well be worth checking that the nuclear relaxation times for nuclei in the product are not unexpectedly short and also that polarization is not observable in a different spectral region from that expected for the final product owing to the formation of a metastable intermediate. 

In the photolysis of. -dibromo-diphenyl-diazomethane in toluene the geminate pair effect is observed 87). However, it is accompanied by the enhanced emission by the escape product 7. This means that the carriers of the original polarization were the free radicals, whose lifetime is obviously shorter than the nuclear relaxation time. 

T jo = nuclear relaxation times of solvent molecules outside the first coordination shell. 

As the TieS lengthen and l/Tie (s ) approaches the size of the electron—nuclear interaction, considerable NMR line broadening can occur, and it may not be possible to acquire high-resolution NMR spectra under these conditions. The effect of Tie on the nuclear relaxation times is discussed in more detail in ref 22. Large hyperfine constants are observed (of many MHz) when the nuclear and electronic spins are on the same atom. For example, a hyperfine constant A/h of —324 MHz was measured by electron... 

From nuclear relaxation time measurements, Alexandre and Rigny (3) were able to determine the chemical shift difference between the equatorial and the 2 axial fluorine atoms as 50 2 ppm. They also obtained a value of 195 Hz for the mean 01—F coupling constant and values for the exchange time between the fluorine atoms. 

Another important nuclear characteristic is the nuclear quadrupole moment which, possessed by nuclei for which 7 1, has given rise to the important field of nuclear quadrupole resonance spectroscopy. A major importance of the quadrupole moment with respect to NMR absorption resides in the effects of quadrupole coupling constants on nuclear relaxation times and, therefore, on the line widths and saturation characteristics of NMR absorption (9). In addition, in favorable situations, quadrupole coupling constants can be derived from the characteristics of nuclear resonance of quadrupolar nuclei 127). Some examples of these effects will be described in Sections III, IV and VI of this chapter. 

When the CM complex is fully formed, [M] = 0 and [CM] = Cm, therefore, from Eq. (4.40), e = So- w thus defined as the enhancement factor measured in a solution where all the metal is complexed. Since usually q < p,eo should be smaller than unity if the intrinsic nuclear relaxation times are the same in the metal complex and in the aquaion. However, as often T cm < T m owing to a longer correlation time xc in the complex (Chapter 3), q can be larger than unity. This is particularly true when C is a macromolecule (e.g. a protein) and M is a metal ion with long electronic relaxation times. 

The hyperfine shifts of groups bound to the donor atom are largely dominated by the contact interaction, even if pseudocontact shift contributions are sizable and any quantitative use of the shifts should rely on the separated contributions. Longitudinal nuclear relaxation times can be used, and have been used in the case of cobalt substitute stellacyanin, to determine metal-proton distances [101]. The contribution of Curie relaxation, estimated from the field dependence of the linewidths, can be used both for assignment and to determine structural constrains [101]. 

D experiments are devised in the assumption that the various times involved in the cycle of Fig. 8.1 (with the exception of when present) are small with respect to the nuclear relaxation times. When the latter are short for any reason, e.g. in the case of paramagnetic molecules because of the presence of unpaired electrons, the system of spins may have reached the equilibrium, or almost reached the equilibrium, before the detection pulse. Under these circumstances no memory is left for the state of the spins during the preceding steps. As a consequence, cross peaks may be decreased in intensity until below detectability. It is necessary, therefore, to match all the time intervals with the nuclear relaxation times, in order to detect the maximum possible cross peak intensities. The ideal case is that t ... 

Selective T values are generally measured with the 180 r 90 pulse sequence using a soft 180° pulse. When the nuclear relaxation times are short, it may become impossible to invert the magnetization and at the same time maintain the required selectivity with the soft pulse. When such difficulties arise, a good compromise is to use a soft pulse that can at least saturate the signal. Then, the sequence becomes equivalent to a 90 r 90 pulse sequence. 

B crystal-field parameters of rank k Ti longitudinal nuclear relaxation time... 

Let us finally also mention here the results of proton nuclear relaxation time 7 measurements on TEA(TCNQ)2 [53,54], From the frequency dependence of 7, it is deduced that the spin motion is a nearly one-dimensional diffusion. Moreover, the temperature dependence of the on-chain spin diffusion rate shows a quite remarkable feature while it is thermally activated below 220 K, it suddenly becomes temperature independent above 220 K. 

Here Tq g and Tq q are the nuclear spin lattice relaxation times of the scavenged and in-cage products, respectively is the nuclear relaxation time in the escaped radical and ks is the rate constant of the formation of the scavenged products. Thus, unless ks is greater than or comparable to [T ] l, CIDNP of the scavenged products would not be observed. 

From CIDNP patterns it is possible to derive magnetic properties of free radicals as magnitudes and signs of hyperfine couphng constants, g factors and nuclear relaxation times. 

Meiboom S, Gill D. Modified spin-echo method for measuring nuclear relaxation times. Rev. Scient. Inst. 1958 29 688-691. Millet O, et al. The static magnetic field dependence of chemical exchange linebroadening defines the NMR chemical shift time scale. J. Am. Chem. Soc. 2000 122 2867-2877. 

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