Carbon nucleus

Figure 1.9. NMR spectra of a mixture of ethanol and hexadeuterioethanol [27 75 v/v, 25 °C, 20 MHz], (a) H broadband decoupled (b) without decoupling. The deuterium isotope effect Sch - d on chemical shifts is 1.1 and 0.85 ppm for methyl and methylene carbon nuclei, respectively...

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

The C NMR spectrum of the metabolite shows 16 signals instead of 8 as expected from the elemental composition determined by high-resolution mass spectrometry. Moreover, aromaticity of the 2,6-xylenol is obviously lost after metabolism because two ketonic carbonyl carbon atoms (5c = 203.1 and 214.4) and four instead of twelve carbon signals are observed in the shift range of trigonal carbon nuclei (5c = 133.1, 135.4, 135.6 and 139.4) in the C NMR spectra. To conclude, metabolism involves oxidation of the benzenoid ring. 

CH COLOC Correlation via long-range CH coupling, detects CH connectivities through two or three (more in a few cases) bonds in the CH COSY format, permits localisation of carbon nuclei two or three bonds apart from an individual proton... 

We start by assuming for a conjugated molecule a fully-symmetrical arrangement of carbon nuclei as an unperturbed system. Electronic wavefunctions and the corresponding energies... 

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... 

In the carbon-13 experiments so far discussed, only a single radio-frequency pulse has been used to irradiate the spin system. This gave us information on the chemical shifts of the carbon nuclei in the molecule. The coupled spectrum obtained using gated decoupling (1.2.2) told us how many protons are bound to any one carbon atom however, this experiment requires a lot of time. There are however other experiments which give us this information... 

There is also the rather famous experiment known as 2D INADEQUATE (Incredible Natural Abundance DoublE QUAntum Transfer Experiment) which allows us to correlate carbon-13 with carbon-13. Potentially this experiment is very useful, since it allows us to see directly which carbon atoms are directly bonded. However, you will remember that the natural abundance of carbon-13 is only 1.1%, so a carbon-13/carbon-13 correlation requires us to detect only about 0.01% of the carbon nuclei present. Thus the experiment is very insensitive and requires large amounts of both sample and measuring... 

We mentioned above that it is possible to carry out carbon-carbon correlation experiments using the 2D INADEQUATE procedure. There, as you may remember from the discussion of one-dimensional INADEQUATE, we have a very difficult problem to solve carbon-13 represents only 1.1% of the total carbon nuclei present in a sample. And in the INADEQUATE experiment we need to detect only those molecules containing two carbon nuclei which couple with one another, i.e. about 10"4 of the nuclei present at the same time we have to get rid of the signal coming from those molecules containing only one carbon-13 (the great majority) ... 

First of all we have three problems where the structure is known. Here you are asked to calculate coupling constants between phosphorus and carbon or hydrogen (Problem 36) and relaxation times T for carbon nuclei (Problem 37) and phosphorus nuclei (Problems 38a and 38b). Note that the equation you will require for Tj calculations can be found in Fig. 10 on p. 19... 

Ti determination of the carbon nuclei Determine Ti for each of the four carbon-13 nuclei using the equation given in Fig. 10... 

The pair of electrons in the n orbital are more diffuse and less firmly held by the carbon nuclei, and so more readily polarisable, than those of the a bond, leading to the characteristic reactivity of such unsaturated compounds. As the it electrons are the most readily accessible feature of the carbon-carbon double bond, we should expect them to shield the molecule from attack by nucleophilic reagents and this is indeed found to be the case (cf. p. 198, however). The most characteristic reactions of the system are, hardly surprisingly, found to be initiated by electron-deficient species such as X and X (radicals can be considered electron-deficient species as they are seeking a further electron with which to form a bond), cations inducing heterolytic, and... 

Proton NMR has the advantage of relative experimental ease due mainly to its intrinsic high sensitivity, though the relaxation properties of the proton resonances are generally more difficult to interpret in terms of dynamics than are those from protonated carbon nuclei. The widths of the proton resonances can conveniently be used as a qualitative measure of some of the motional properties. 

The similarity of results for peptide resins in both solvents fits nicely with the estimate that this resin sample with about 50 weight % peptide should have similar swelling characteristics in the two solvents (3). In these samples we have measured the integrals of the aromatic region relative to an unenriched peptide peak, and the results are consistent with the observation of all the carbon nuclei, and hence the results are typical of all portions of the cross-linked matrix. 

The net effect is that the carbon nuclei are polarized by the proton magnetization and the S/N is increased both directly by the ratio Yh/Yc (4 in the case of 13C) and indirectly because the experiment now depends only on the recovery of the proton magnetization which will usually be much faster than the 13C relaxation. 

The general pattern of the spectra of aminals is similar to that of the corresponding ketals, and the measurement of enantiomer composition can be done on the same carbon nuclei. In addition, the signals are clearly distinguishable in the aminals, giving more accurate results.38... 

Proton-decoupled 13C-NMR spectra were recorded on a Varian XL-300 operating at 75.4 MHz. Approximately 250 mg of the sample was dissolved in 3 ml of deuterated chloroform. 13C chemical shifts were referenced internally to CDCL (77 ppm). A delay of 200s was used to ensure relaxation of all the carbon nuclei and 1000 transients were collected to assure a good signal-to-noise ratio. 

This lower sensitivity can be pulsed using Fourier-transform NMR, wherein a high-power microsecond pulse of radio frequency energy sets all the carbon nuclei into resonance at once, thus... 

Where the circle represents the two carbon and sticks the bonds. Now, if we allow the stick joining the two carbon nuclei to rotate freely in the holes, then an infinite number of different atomic arrangements becomes possible. 

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