Nuclear Overhauser enhancement relaxation-rate measurements

N-protonation the absolute magnitude of the Ad values is larger than for Af-methylation <770MR(9)53>. Nuclear relaxation rates of and have been measured as a function of temperature for neat liquid pyridazine, and nuclear Overhauser enhancement has been used to separate the dipolar and spin rotational contributions to relaxation. Dipolar relaxation rates have been combined with quadrupole relaxation rates to determine rotational correlation times for motion about each principal molecular axis (78MI21200). NMR analysis has been used to determine the structure of phenyllithium-pyridazine adducts and of the corresponding dihydropyridazines obtained by hydrolysis of the adducts <78RTC116>. 

Nuclear Overhauser enhancements (NOE) were used to measure the nuclear relaxation rates of C as a function of temperatures for neat thiazole <84MP141i>. A study of the spin-spin coupling between C nuclei in five-membered heteroaromatic ring systems showed the high aromaticity of thiazole <94MRC62>. 

IT was calculated for different temperatures. The nuclear Overhauser enhancement factors T were set as independent of temperature, which was confirmed by our results. Table 2 lists measured and calculated relaxation times and rates for the silane mixture mentioned above. 

Rates of nuclear relaxations can be altered by the deliberate addition to the solutions used for NMR spectroscopy of species containing paramagnetic nuclei. The usual relaxation agent employed with substrates dissolved in organic solvents is the chelate chromium tris(acetylacetonate) (Cr(acac)3), [Me C6 CH C(6)Me]3Cr, i.e. chromium(III)-2,4-pentane-dionate. It acts to reduce values of via dipole-dipole interactions, leaving chemical shift values unaltered. Cr(acac)3, when used at concentrations of about 0.05 M, substantially reduces values of T, particularly those for quaternary nuclei simultaneously it removes the nuclear Overhauser enhancement from proton-decoupled and other resonances. Use of this reagent could therefore considerably shorten the times required for quantitative NMR measurements. 

Dynamic NMR parameters such as spin-lattice relaxation rates and the nuclear Overhauser enhancement (NOE) have also been used as sources of information. Cardin et al. (15) have measured... 

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