Nuclear Overhauser enhancement,

If the relaxation of spin I is significantly affected by the motions of a different spin, S, then in general any deviation from Boltzmann equilibrium of the S spins will also render the I spin populations non-Boltzmann. The resulting change in the intensity of resonance I is called a nuclear Overhauser enhancement (NOE) when spins I and S both belong to nuclei. The nuclear Overhauser enhancement factor is conventionally ri, so that 1 -l- tjjs is defined as the intensity of the I resonance when the S spin populations are equalized, divided by the intensity of the I resonance when the 

However, if nf is made equal to for example by irradiation of the S resonance before and possibly during observation of the I resonance, then the second exponential becomes equal to unity. Hence PF /PFL rises to exp — (yi + ys)fiBo/kT, and so, using the relation e K +x when x is small, we find that the fractional population difference — now equals yi- -y )hBQlkT, where beforehand it was just 

Nuclear Overhauser enhancements are not just an experimental curiosity. They are independent relaxation parameters which can be used to investigate molecular motions in detail. Also, the technique of measuring specific H- H Overhauser enhancements by NOE difference spectroscopy detects spatial rather than chemical proximity and hence adds a powerful new tool to conformational and structural analysis. 

We may now borrow the general two-spin expression for Tj in the presence of S irradiation, equation (10), and extend it to include the small differences between absorptive and emissive transition probabilities.If 

Now c =l j to a very good approximation, so that the term in parentheses becomes 

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Nuclear Overhauser enhancement (NOE) spectroscopy has been used to measure the through-space interaction between protons at and the protons associated with the substituents at N (20). The method is also useful for distinguishing between isomers with different groups at and C. Reference 21 contains the chemical shifts and coupling constants of a considerable number of pyrazoles with substituents at N and C. NOE difference spectroscopy ( H) has been employed to differentiate between the two regioisomers [153076 5-0] (14) and [153076 6-1] (15) (22). N-nmr spectroscopy also has some utility in the field of pyrazoles and derivatives. 

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 main contribution to the spin-lattice relaxation of C nuclei which are connected to hydrogen is provided by the dipole-dipole interaction (DD mechanism, dipolar relaxation). For such C nuclei a nuclear Overhauser enhancement of almost 2 will be observed during H broadband decoupling according to ... 

The difference between 2-CH2 and 6-CH2 is shown by the nuclear Overhauser enhancement (NOE) on the proton at Su = 6.67, if the methylene protons at 5 = 2.87 are irradiated. The assignment of the methylene C atoms can be read from the CH COSY segment. The C atoms which are in close proximity to one another at 5c = 113.3 and 113.8 belong to C-5 and C-7. Carbon atom C-5 is distinguished from C-7 by the pseudo-quartet splitting CJqh = 3.4 Hz to 1-H and A-H2) that involves the methylene group in the ortho position. 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

The most convenient technique used to study organotin(IV) derivatives in solution and in solid state is Sn NMR spectroscopy. The Sn nucleus has a spin of 1 /2 and a natural abundance of 8.7% looking only at the isotopic abundance, it is about 25.5 times more sensitive than The isotope Sn is slightly less sensitive (natural abundance 7.7%) but it has not been used as much. Both nuclei have negative gyromagnetic ratios, and, as a consequence, the nuclear Overhauser enhancements are negative. Some examples of the applications of this method are mentioned later, in different sections. 

More recent studies on the folded toxin structure by Norton and colleagues have utilized h- and C-NMR techniques (19,20). By using 2D-FT-NMR, it was possible to localize a four stranded, antiparallel )5-pleated sheet "backbone structure in As II, Ax I, and Sh I (21,22), In addition, Wemmer et al. (23) have observed an identical )5-pleated structure in Hp II. No a-helix was observed in these four variants. In the near future, calculated solution conformations of these toxins, utilizing distance measurements from extracted Nuclear Overhauser Enhancement (NOE) effects should greatly stimulate structure-activity investigations. 

One disadvantage of the APT experiment is that it does not readily allow us to disdnguish between carbon signals with the same phases, i.e., between CH3 and CH carbons or between CH2 and quaternary carbons, although the chemical shifts may provide some discriminatory information. The signal strengths also provide some useful information, since CH3 carbons tend to be more intense than CH carbons, and the CH2 carbons are usually more intense than quaternary carbons due to the greater nuclear Overhauser enhancements on account of the attached protons. 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

Figure 4.2 Energy levels and populations for an IS system in which nuclei I and S are not directly coupled with each other. This forms the basis of the nuclear Overhauser enhancement effect. Nucleus S is subjected to irradiation, and nucleus I is observed, (a) Population at thermal equilibrium (Boltzmann population).

Why do we need to involve zero-quantum (VI()) or double-quantum Wi) processes to explain the origin of the nuclear Overhauser enhancement ... 

NOESY/ROESY nuclear Overhauser enhancement spectroscopy... 

Table 1. Nuclear Overhauser Enhancement Difference Value for...

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... 

NOE Nuclear Overhauser effect/nuclear Overhauser enhancement. Enhancement of the intensity of a signal via augmented relaxation of the nucleus to other nearby nuclei that are undergoing saturation. See also ... 

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... 

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