Spin-lattice relaxation time compounds

It appears that purification of commercially available solvents is sometimes required for the complete elimination of impurity resonances. Occasionally, these impurities may be turned into advantage, as in the case of C2D2CI4 where the (known) C2DHCI4 content may be used as an internal standard for quantitation. Thus, removal of every impurity peak is not always essential for identification and quantitative analysis of stabilisers in PE. Determination of the concentration of additives in a polymer sample can also be accomplished by incorporation of an internal NMR standard to the dissolution prepared for analysis. The internal standard (preferably aromatic) should be stable at the temperature of the NMR experiment, and could be any high-boiling compound which does not generate conflicting NMR resonances, and for which the proton spin-lattice relaxation times are known. 1,3,5-Trichlorobenzene meets the requirements for an internal NMR standard [48]. The concentration should be comparable to that of the analytes to be determined. 

A function closely related to QCC is the Li quadrupolar splitting constant (QSC), defined as QSC = (1 -h/7 /3) x( Li), where r is the asymmetry parameter. The Li QSC values can be estimated from the Li and C(para) spin-lattice relaxation times. The QSC values are correlated with the effects of structure, solvent and temperature on association in solution for aryllithium compounds (155, 171, 172). Conclusions can be drawn about the structure of the associated species in cases where no supporting XRD evidence is available. ... 

Within rigid molecules the correlation time rc is equal for all carbon atoms. Moreover, the C —H bond lengths in nearly all organic compounds are approximately 0.109 nm, except for alkynes (rCH 0.106 nm). Under these conditions, the spin-lattice relaxation time 7, of a 13C nucleus relaxing by the DD mechanism is shown by eq. (3.23) to depend solely upon the number N of directly bonded H atoms ... 

Finally, a coupled and decoupled 13C NMR spectrum of 2,2 -bipyrrole (Fig. 4.14(a,b) and an inversion-recovery series of 2,2 -bi pyridine (Fig. 4.15 [73 i]) illustrate signal assignments of heteroaromatic compounds by means of carbon-proton couplings and spin-lattice relaxation times. These spectra also exemplify the characteristic shift differences between n-excessive (2,2 -bipyrrole) and n-deficient heteroaromatic compounds (2,2 -bipyridine). 

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

Si NMR studies of solutions are difficult because of the long spin-lattice relaxation times of the nucleus and its negative nuclear Overhauser enhancement. The 29Si-1H dipole-dipole relaxation is inefficient because in most compounds the intemuclear distance is large. Fortunately, the problem of relaxation can often be overcome by resorting to cross-polarization (see Section II,E). 

One of the mayor drawbacks is that only volatile and temperature-resistant compounds can be investigated. Gases are magnetized faster than liquids, because they have shorter spin-lattice relaxation times (T ), due to an effective spin rotation mechanism. Therefore, pulse repetition times in flow experiments can be in the range of 1 s and some dozen transients can be accumulated per separated peak. Nevertheless, the sample amounts used nowadays in capillary GC are far from the detection limit of NMR spectroscopy, and therefore the sensitivity is low or insufficient, due to the small number of gas molecules per volume at atmospheric pressure in the NMR flow cell. In addition, high-boiling components (> 100 °C) are not easy to handle in NMR flow probes and can condense on colder parts of the apparatus, thus reducing their sensitivity in NMR spectroscopy. 

The big disadvantage of 13C NMR spectroscopy is its low sensitivity. Due to the natural abundance of 1.1% of the 13C isotope and due to long spin-lattice relaxation times (Th) of the order of seconds, the acquisition of a routine 13C NMR spectrum of a 0.1 M solution of an organic compound takes at least one minute. 

Pulse FT NMR has been used to study the spin-lattice relaxation times, Tv of U9Sn in a number of organic (37, 38, 40, 42, 43) and inorganic (44) tin compounds. The most important relaxation mechanism for this nucleus in a series of tetraorganotins appears to be spin-rotation (SR) (38, 43) although for larger molecules, such as hexabutylditin, dipole-dipole (DD) relaxation is important, even at room temperature. (37)... 

Spin-lattice relaxation times have been used to assist in the assignment of the NMR spectrum of the naturally occurring organometallic compound, 5 -deoxyadenosylcobalamin (65). In the context of assignment of resonances and the determination of unknown structures, spin-lattice times offer information on the number of protons attached to carbons in fused-ring structures and can be used to detect groups that have internal motion. 

Silicon only has one naturally occurring isotope ( Si) with a nonzero nuclear spin and Si NMR spectroscopy has become one of the most widely used techniques for the identification of silicon compounds. Unfortunately, the natural abundance of Si is only 4.7%, which combined with its long spin-lattice relaxation times, means that relatively long acquisition times may be needed to obtain high quality spectra. Si NMR spectroscopy has been the subject of a number of reviews and tables of chemical shift data and coupling constants are available. " ... 

Interestingly, the spin-lattice relaxation time according to the direct process involving the triplet substates II and I remains unchanged within fimits of experimental error. For Pt(2-thpy-hg)2 and for Pt(2-thpy-dg)2 the sir times at T = 1.3 K are r jj. (720 10) ns and (710 10) ns, respectively (see Sect. 4.2.7.2 and Ref. [23]). Obviously, perdeuteration of the chromophore does not strongly influence the sir at low temperatures. Moreover, it is indicated that the matrix cages of the two compounds in n-octane are similar. Otherwise one would expect to observe distinctly different sir times as has been shown for Pt(phpy)2 (compare Fig. 1) in n-octane [64]. 

Fig. 26. Energy level diagram for the triplet sublevels of (a) perprotonated, (b) partially deu-terated, and (c) perdeuterated Pt(2-thpy)2 dissolved in an n-octane matrix (Shpol skii matrix). Emission decay times and spin-lattice relaxation times are given for T= 1.3 K. Several vibrational satellites are specified, HT = Herzberg-Teller active vibration, FC = Franck-Condon active vibration. The emission spectrum of the partially deuterated compound (b) exhibits vibrational satellites of the two different ligands. (Compare Fig. 27b.) The data given for Pt(2-thpy-hg)(2-thpy-d6) (b) refer to the lower lying site A. (Compare Ref. [23])...

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