Nuclear Overhauser Enhancement NOE

FIGURE 4.4 (a) Inversion recovery pulse sequence with inverse gated proton decoupling for 7, measurement. 

The actual enhancement for the 13C— H system can be anywhere from 0 to 1.98 depending on the mechanism of relaxation for each individual nucleus. In practice, for carbons with no attached protons, the enhancement is essentially zero since there is practically no l3C— H dipolar relaxation. For small to medium-sized organic 

Spin-coupling J values—at least as an initial consideration—are less important in l3C NMR than in H NMR. Since routine 13C spectra are usually decoupled, 13C— H coupling values are discarded in the interest of obtaining a spectrum in a short time or on small samples—a spectrum, furthermore, free of complex, overlapping absorptions. 

FIGURE 4.5 Gated proton decoupling pulse sequence. Rd is relaxation delay, 6 is a variable pulse angle, t2 is the acquisition time. 

The 3/ch values are roughly comparable to 2JCh values for sp3 carbon atoms. In aromatic rings, however, the 3/ch values are characteristically larger than 2JCH values. In benzene itself, 3/CH = 7.6 Hz and 2./CH = 1.0 (see Table 4.2). 

When we obtain a proton-decoupled spectrum, the intensities of many of the carbon resonances 

For a proton-decoupled spectrum, we would calculate, using the values in Table 3.2, 

Signal enhancement due to NOE is an example of cross-polarization, in which a polarization of the spin states in one type of nucleus causes a polarization of the spin states in another nucleus. Cross-polarization will be explained in Section 4.6. In the current example (proton-decoupled spectra), when the hydrogens in the molecule are irradiated, they become saturated and attain a distribution of spins very different from their equilibrium (Boltzmann) state. There are more spins than normal in the excited state. Due to the interaction of spin dipoles, the spins of the carbon nuclei sense the spin imbalance of the hydrogen nuclei and begin to adjust themselves to a new equilibrium state that has more spins in the lower state. This increase of population in the lower spin state of carbon increases the intensity of the NMR signal. 

In a proton-decoupled spectrum, the total NOE for a given carbon increases as the number of nearby hydrogens increases. Thus, we usually find that the intensities of the signals in a spectrum (assuming a single carbon of each type) assume the order 

nuclei must be rather close together in the molecule in order to exhibit the NOE effect. The effect is greatest for hydrogens that are directly attached to carbon. 

The three-peak pattern centered at 5 = 77 ppm is due to the solvent CDCI3. This pattern results from the coupling of a deuterium ( H) nucleus to the nucleus (see Section 6.13). Often the CDCI3 pattern is used as an internal reference, in place of TMS. 

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186 Nuclear Magnetic Resonance Spectroscopy Part Two Carbon-13 Spectra 

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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. 

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. 

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. 

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... 

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

Now if S is strongly irradiated, then it is saturated and S is no longer at its Boltzmann equilibrium. Therefore it cannot maintain the Boltzmann equilibrium of spins 7, and the intensity of the 7 signal is changed. Equalizing S populations produces a proportional change in 7 populations such that equation 3.41 can be written in which r IS is called the nuclear Overhauser enhancement (NOE) factor. 

D. Cross-relaxation rates - nuclear Overhauser enhancement (nOe) factors... 

D. Cross-relaxation Rates - Nuclear Overhauser Enhancement (nOe) Factors... 

Although crosslinked polymers and polymer gels are not soluble, the spectra of swollen, low crosslink density networks exhibit reasonably narrow C-13 NMR line widths, sufficiently resolved to reveal details of microstructure 13S). Thus, recording the spectra under scalar low power decoupling yields characterization information and some dynamic measurements, concerning T, T2 (line widths) and nuclear Overhauser enhancement (NOE) for lightly crosslinked polymers. 

NMR analyses were done on an IBM Instruments NR-300 spectrometer and an Oxford 7 Tesla superconducting narrow-bore magnet. Silicon-29 (Si-29) NMR spectra were recorded at 59.6 MHz and hydrogen (also commonly called proton or H-l) NMR spectra at 300.13 MHz. Spectra were recorded using conventional single-pulse techniques with proton decoupling for Si-29 acquisitions. Si-29 experiments were structured so as to suppress nuclear Overhauser enhancement (NOE). For Si-29 acquisitions, spectral widths were 50 kHz and Fourier transform (FT) sizes were 4K points. For protons, spectral widths were 7.5 kHz and FT sizes were 16K points. Si-29 rf pulse widths were approximately 12 fits and proton rf pulse widths were 8 jj.s. 

Within a molecule, a nucleus is characterized by its magnetic properties and electronic environment and by the following consequent parameters associated with the corresponding NMR signals chemical shift, coupling constants, relaxation rates,15 and nuclear Overhauser enhancement (NOE).1617 All these values may be used to extract qualitative or quantitative information about the structure, the conformation, and the behavior of molecules in solution. 

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-... 

Conformational analyses of JOM-13 and [L-Ala3]DPDPE have proven to be critical for the determination of the bioactive conformation of enkephalin-like peptides at the delta receptor. H-NMR studies of JOM-13 in aqueous solution revealed that this tetrapeptide exists in two distinct conformations on the NMR time scale as evidenced by two sets of resonances [63]. Large differences in the observed chemical shifts and coupling constants for the D-Cys2 residue in the two conformers suggested that the major differences between the two NMR conformers reside in the disulfide portion of the molecule however, a paucity of conformationally informative nuclear Overhauser enhancement (NOE) interactions precluded the development of a detailed structural model from the NMR studies. In order to develop such a model a thorough conformational analysis of JOM-13 was undertaken, in which the NMR data were complemented by x-ray diffraction results and by molecular mechanics calculations [64]. The results indicate that the 11-... 

The nuclear Overhauser enhancement (NOE, see Section 4.2.4) varies among 13C nuclei, and the signal intensities vary accordingly. 

The structure determination of organic molecules in solution — constitution, configuration and conformation — is probably the most important application of high-resolution NMR spectroscopy. Sophisticated methods based on classical NMR parameters like chemical shifts, /-couplings and nuclear Overhauser enhancement (NOE) have been developed for deriving structural models.1 2 The approach has been successfully applied to a vast number of molecules including thousands of biomacromolecules and uncountable natural and synthetic products. 

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