Nuclear Overhauser enhancement relaxation times

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>. 

The 50.31 MHz 13C NMR spectra of the chlorinated alkanes were recorded on a Varian XL-200 NMR spectrometer. The temperature for all measurements was 50 ° C. It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the NMR data, carbon-13 Tj data were recorded for all materials using the standard 1800 - r -90 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu)3SnH, (n-Bu)3SnCl, DCP, TCH, pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of DCP and TCH in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4-chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the NMR analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 Tj. The only exception to this is heptane where the shortest T[ is 12.3 s (delay = 2.5 ). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the TCH reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical... 

T3C n.m.r. spectra were recorded for the oils produced at 400°, 450°, 550° and 600°C. As the temperature increased the aromatic carbon bands became much more intense compared to the aliphatic carbon bands (see Figure 8). Quantitative estimation of the peak areas was not attempted due to the effect of variations in spin-lattice relaxation times and nuclear Overhauser enhancement with different carbon atoms. Superimposed on the aliphatic carbon bands were sharp lines at 14, 23, 32, 29, and 29.5 ppm, which are due to the a, 8, y, 6, and e-carbons of long aliphatic chains (15). As the temperature increases, these lines... 

C (or 15N) spin-lattice relaxation times (T C), spin-spin relaxation times (T2c) and nuclear Overhauser enhancement (NOE rj) are generally given... 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

C Spin-Lattice Relaxation-Times, Line Widths, and Nuclear Overhauser Enhancements (n.O.e.) of PS 13140... 

Spin-lattice relaxation times were measured by the fast inversion-recovery method (24) with subsequent data analysis by a non-linear three parameter least squares fitting routine. (25) Nuclear Overhauser enhancement factors were measured using a gated decoupling technique with the period between the end of the data acquisition and the next 90° pulse equal to eibout four times the value. Most of the data used a delay of eibout ten times the Ti value. (26)... 

The results that have been obtained indicate that the major influence of the crystalline regions on segmental motions, and hence to the structure of the non-crystalline regions, is in the linewidth and T2. The different morphologies are reflected in different values of T2- The segmental motions in long chain molecules which exert major influence on the spin-lattice relaxation times and the nuclear Overhauser enhancements are not in general the same motions which determine the resonant linewidth. 

R. Mathur-De Vre, C. Maerschalk and C. Delporte, Spin-lattice relaxation times and nuclear Overhauser enhancement effect for P metabolites in model solutions at two... 

Nuclear Overhauser enhancements and spin-lattice relaxation times are individual for each carbon. As a result, signal intensities cannot be evaluated from PFT 13C NMR spectra obtained with continuous proton broadband decoupling. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

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). 

A complementary article by Dais (Iraklion, Crete) addresses the theoretical principles underlying the phenomenon of carbon-13 nuclear magnetic relaxation, encompassing spin-lattice (Tt) and spin-spin (T2) relaxation times, the nuclear Overhauser enhancement, and their relation to the motional behavior of carbohydrates in solution. With examples broadly selected from simple sugar derivatives, oligosaccharides, and polysaccharides, the author shows how qualitative treatments have provided useful interpretations of the gross mobility of molecules in solution, but demonstrates how a quantitative approach may be of greater ultimate value. 

T2, and the nuclear Overhauser enhancement, NOE, comprise a set of parameters which characterize molecular motions. In the case of simple isotropic motion, the dependence is in terms of a single correlation time characterizing the exponential decay of the autocorrelation function. However, in many instances, the assumption of isotropic motion is not valid. For rigid systems, the relaxation behavior can then often be predicted by assuming simple anisotropic motion (1 ). Often, superposition of two or more independent motions must be used to satisfactorily interpret observed relaxation behavior. Recently, however, the wide-... 

Table XIV. Calculated Methylene Spin-Lattice Relaxation Times and Nuclear Overhauser Enhancement Factors for the C-2 Carbon...

NMR spectrum depends on the type of starch (amylose-to-amylopectin ratio) and is associated with the numbers of carbon atoms in the branching points and thermal glucose units. Tables X and XI present 13C spin-lattice relaxation times (Tus) and nuclear Overhauser enhancement (n.O.e) for l3C nuclei of starches of various origins. Figure 21 shows H NMR spectra of amylose and a high-amylopectin waxy sorghum starch. 

FIG. I. Spin-lattice relaxation time of Li in 3-9 M LiClin HiOasafunctionofinverse absolute temperature. Circles experimental relaxation time. 7 ," crosses dipolar contribution, obtained from T,"" and the nuclear Overhauser enhancement factor t). The linear least-squaresfit for the latter yields an activation energy of 3-6kcalmol (16)... 

FIG. 11. Be spin-lattice relaxation time in 1 m aqueous Be(N03)j as a function of temperature. Open circles experimental relaxation time squares dipolar relaxation time. T°°. obtained from 7 " and the nuclear Overhauser enhancement factor, t) filled circles nondipolar relaxation time, (1/T -p- l/ri°°)- -. (102)... 

These aspects are discussed in an excellent recent review [24]. It must also be noted that NMR experiments are indispensable tools in stmcture elucidation and furnish specific information regarding chemical shift, (5), spin-spin coupling constants [25] (J), spin-lattice relaxation times (T1), spin-spin relaxation times (T2) as well as nuclear Overhauser enhancement (nOE). These... 

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