Shift reagents relaxation times

If the amount of the sample is sufficient, then the carbon skeleton is best traced out from the two-dimensional INADEQUATE experiment. If the absolute configuration of particular C atoms is needed, the empirical applications of diastereotopism and chiral shift reagents are useful (Section 2.4). Anisotropic and ring current effects supply information about conformation and aromaticity (Section 2.5), and pH effects can indicate the site of protonation (problem 24). Temperature-dependent NMR spectra and C spin-lattice relaxation times (Section 2.6) provide insight into molecular dynamics (problems 13 and 14). 

Most biochemically relevant high-spin systems have such short 7j-relaxation times that their EPR is broadened beyond detection at ambient temperatures. An exception is the class of S = 5/2 Mn" systems with D hx. Also, S = 7/2 Gd"1-based MRI shift reagents exhibit readily detectable room-temperature EPR spectra. Otherwise, aqueous-solution transition ion bioEPR is limited to complexes of S = 1/2 metals, in particular Cu", and to a lesser extent VIV02+, NiIn, Ni1, Mov, and Wv. Cupric is the stable oxidation state of biological copper under aerobic conditions, however, the other metals are stable as Vv, Ni", MoVI, and WVI, and, therefore, the other oxidation states associated with S = 1/2 paramagnetism may exhibit oxidative or reductive reactivity and may thus require specific experimental precautions such as strict anaerobicity over the course of the EPR experiment. 

In 2-substituted adamantanes 25 both types of 8-positioned carbon atoms (8syn and 8 ) exist within one molecule (Scheme 37). Early measurements with limited spectral resolution (176) did not differentiate between their signals. Later (124,244), differences of up to 0.7 ppm were detected, and application of various independent methods, including addition of lanthanide shift reagents (245), determination of longitudinal relaxation times T, (246), and evaluation of deuterium... 

The study of nucleic acid bases by NMR has been reported in a number of monographs (/), but very little data is available on the, 3C and, 5N NMR chemical shift tensors in these compounds. The low sensitivity of NMR spectroscopy and the long relaxation times exhibited by many of these compounds have posed the main impediments for these studies. The use of sample doping with free radical relaxation reagents, to reduce the relaxation times facilitating 2D multiple pulse experiment (2, 3), enables one to measure and analyze the principal values of the chemical shift tensors in natural abundance samples. In previous papers from this laboratory we have presented, 5N NMR chemical shift principal values for adenine, guanine, cytosine, thymine and uracil (4, 5). 

In a study combining 2H relaxation-time measurements and use of paramagnetic lanthanide shift reagents, the ratio of the relaxation times of axial and equatorial deuterons in 2,2,6,6-tetradeuterio-4-t-butylcyclohexanone [20]... 

As we pointed out in Section 4.11, lanthanide shift reagents owe their utility partly to the fact that the electron spin-lattice relaxation time for the lanthanides is very short, so that NMR lines are not exceptionally broad. On the other hand, there are shiftless paramagnetic reagents that shorten both Tx and T2 to a moderate degree without causing contact or pseudocontact shifts. 

Metal acetylacetonates can be used as NMR shift reagents. Thus, Cr(acac)3 and Fe-(acac)3 can be used as paramagnetic relaxation agents to decrease the spin-lattice relaxation times of all C signals in poly(Me methacrylate), poly(Bu methacrylate) and poly(hydroxyethyl methacrylate) . ... 

Such n.m.r. shift reagents as Ni ", Co, Fe + low-spin Fe, and lanthan-ide(III) ions (other than Gd, a relaxation ret ent) are characterized by having very fast electron-spin relaxation times 10 s), which pre-... 

The decision as to which metal to choose for a particular experiment is also dictated by the fact that, in addition to shift changes, all of the lanthanides produce some change in the nuclear relaxation-times of the substrate, and thereby increase the line-widths of the n.m.r. transitions. The ratio of shift change to line broadening is optimal for such metals as europium (III) and praseodymium (III), which are, therefore, the most suitable for use as shift reagents. In contrast, gadolinium (III) is totally unsuited for this particular purpose, as it induces substantial... 

Nuclear Magnetic Resonance Spectroscopy.—As noted above, conformational analysis of bicyclo[3.3.1]nonanes is still a topic of considerable interest. A variable-temperature n.m.r. analysis now provides the first case in which the boat-chair-chair-boat equilibrium is directly observed in the amines (17) and (18). In a related case, re-examination of the acetal (19) suggests that the preferred conformation involves a chair carbocyclic ring and a boat heterocyclic ring. This conclusion was made by n.m.r. analysis, using lanthanide shift reagents, by a study of nuclear Overhauser effects, and by measurement of relaxation times of protons. Details have been reported for other 3-azabicyclo[3.3.1]nonanes, and the non-additivity of substituent effects on chemical shifts in 9-thiabicyclo[3.3.1]non-2-enes has been analysed. Both and n.m.r. data have been reported for a series of 9-borabicyclo[3.3.1]non-anes and their pyridine complexes. 

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