Paramagnetic relaxation reagent

The study of the fiillerenes by C NMR spectroscopy is not straightforward. C has a low natural isotopic abundance (1.11%) and a low relative receptivity. In addition, the carbon atoms in fiillerenes have relatively long relaxation times and spectra for the higher, less symmetric fiillerenes can only be obtained within a reasonable time by using paramagnetic relaxation reagents, typically Cr(acac)3. 

Representative and spectra are presented in Figures 5, 6, 7, and 8 for the Amax and Monterey samples. The quantitative reference (peak at 0.0 ppm) in each spectrum is hexamethyldisiloxane ((CH3)3-Si-0-Si(0113)3). A paramagnetic relaxation reagent (the paramagnetic relaxation reagents used were either tris(acetylacetonato) iron (III), Fe(acac)3, or tris(acetylacetonato) chromium (III), Cr(acac)3 at concentrations of 2-6 X lO M). was added to decrease spin lattice relaxation times (Ti s) and suppress nuclear Overhauser effects (5). In addition,... 

NMR Spectrometry. Liquid phase and NMR spectra were recorded on a Varian XL300 NMR spectrometer at carbon and nitrogen resonant fi equencies of 75.4 and 30.4 MHz, respectively, using a 10 mm broadband probe. Quantitative NMR spectra of the unreacted fulvic and humic acid samples were recorded in DMSO-d6, 99.9 atom % as previously described (23). INEPT (24) and ACOUSTIC (25) N NMR spectra were recorded on the aniline-reacted fulvic and humic acids. Refocussed INEPT (proton decoupled) spectra were recorded as previously described (9). ACOUSTIC spectra, with the exception of the bimessite catalyzed sample, were recorded with the use of paramagnetic relaxation reagent (100-200 mg chromium (III) acetylacetonate). Acquisition parameters included an 18,656.7 Hz spectral window (613.7 ppm), 0.5-s acquisition time, 45° pulse angle, 2.0-s pulse delay, and t delay of 0.1 ms. Neat formamide in a 5 mm NMR tube, assumed to be 112.4 ppm, was used as an external reference standard for all spectra. N NMR chemical shifts are reported in ppm downfield of ammonia, taken as 0.0 ppm. 

Paramagnetic relaxation reagents (pa.r.r.), such as Gd, Mn ", Cu ", and Cr ", and other paramagnetic metal ions with isotropic charge-distribution, affect the nuclear relaxation rates, T, and T2, of species under investigation by n.m.r. spectroscopy through electron-nuclear spin-spin coupling. There are two classes of pa.r.r.s which are characterized by the predominant mechanism of relaxation enhancement. 

For paramagnetic relaxation-reagents, the electron-spin relaxation-rates are long (r,e 2e > 10 ° s), SO that, for small molecules, = x, and x = x. Because this is the same correlation-time as in all other terms of the nuclear-spin relaxation-time r, where... 

Fig. 5.—The Effect of Gd + on the C Resonances of the Proton-decoupled, Natural-abundance, K -N.m.r. Spectrum of 6. [The concentration of compound 6 was 168 toM in HjO, pH 7. The vertical gam of the spectra of solutions containing large proportions of paramagnetic, relaxation reagent was increased slightly, so that broadening effects could be clearly observed. (A) Sample contained no Gd , and requited 27,000 accumulations. A linebroadening factor of 2.3 Hz was used during the data processing. (B) Sample contained...

Of course, loss of part of the NOE means many repetitions to build up the signal intensity. The time required can be shortened by addition of a paramagnetic relaxation reagent, such as the metal-organic complex Cr(acac)3 to reduce all of the 7, and 72 relaxations. 

Paramagnetic Relaxation Reagents. Alternatives or Compliments to Lanthanide Shift Reagents in NMR Spectral Analysis, G. C. Levy and R. A. Ko-moroski, J. Amer. Chem. Soc., 96,678 (1974). 

Paramagnetic relaxation reagents capable of specifically interacting with a basic site of the investigated substrate can also be... 

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