Spectrum carbon

Example 1, Poly(vinyl alcohol). The first example is given for the carbon-13 spectrum of Poly(vinyl alcohol). Figure 2 shows a plot of the carbon spectrum and a peak listing with assignments from the user s database. The assignments constitute a difficult part of the analysis... 

So basically there is no point in integrating a broad-band decoupled carbon spectrum. This is not so much of a drawback as it sounds, because the signals are distributed over a range of more than 200 ppm, so that line overlap is very unusual. 

By now we are used to the appearance of such spectra, and again the central rectangle contains the actual 2D spectrum, while the carbon spectrum (decoupled) is shown on the left and the proton spectrum at the top. 

The carbon spectrum, both in the broad-band decoupled form and as an APT spectrum. 

The proton-carbon correlation spectrum, which tells you directly which signals in the proton spectrum correspond with which signals in the broad-band decoupled carbon spectrum. This information, together with the integration values and the multiplicities obtained from APT (or DEPT), is invaluable in putting together the molecular fragments. 

The carbon spectrum of this compound (Fig. 5.8) is typical of such spectra, with the CF3 group and the carbonyl group having chemical shifts of 8 115.7 and 192.1, respectively, and F—C couplings of 292 and 34 Hz, respectively. In this case, no three-bond F—C coupling is able to be discerned at the CH2 carbon, which appears as a broad singlet at 30.1 ppm. 

The carbon spectrum is wonderfully discernable, with all three carbons appearing as quartets in the same general region, with the highly split (270 Hz) trifluoromethyl carbon at 122.8 ppm, the C2 carbon... 

The carbon spectrum of 3,3,4,4,4-pentafluoro-2-butanone provides a good example of the carbon signals of an isolated C2F5 group (Fig. 6.11b). 

As we have already pointed out in the section dealing with heteronuclear coupling that it is not always necessary to confirm the presence of a particular hetero atom by acquiring the NMR spectrum of that nucleus. More often than not, the hetero atom will have a clear signature in the proton or carbon spectrum. Fluorine and phosphorus are both examples of nuclei that couple to protons over two, three, four and even more bonds. 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

The initial observation is that PMMA is essentially completely degraded to monomer by heating to 375°C in a sealed tube while heating a mixture of red phosphorus and PMMA under identical conditions yields a solid, non-deqraded, product as well as a lower yield of monomer. One may observe, by 3C NMR spectroscopy, that the methoxy resonance is greatly decreased in intensity and methyl, methoxy phosphonium ions are observed by 31P NMR. Additional carbonyl resonances are also seen in the carbon spectrum, this correlates with a new carbonyl vibration near 1800 cm 1 in the infrared spectrum and may be assigned to the formation of anhydride. The formation of anhydride was also confirmed by assignment of mass spectra obtained by laser desorption Fourier transform mass spectroscopy, LD-FT-MS. 

A, which is shown in the second spectrum from the bottom in Fig. 10.24. Comparing the carbon spectrum of Form A to that of a different crystalline form, Form B, shown in the topmost spectrum of Fig. 10.24, reveals several notable differences in both chemical shifts and line shapes. This, in fact, is often the case for different polymorphs of the same chemical entity, and it enables a clear distinction between the various crystalline forms when present in a mixture. A 13C CPMAS spectrum of a standard sample of the excipients, in the appropriate proportions for the formulation, was also acquired for reference. This is shown as the second spectrum from the top in Fig. 10.24. 

To continue the investigation, carbon detected proton T relaxation data were also collected and were used to calculate proton T relaxation times. Similarly, 19F T measurements were also made. The calculated relaxation values are shown above each peak of interest in Fig. 10.25. A substantial difference is evident in the proton T relaxation times across the API peaks in both carbon spectra. Due to spin diffusion, the protons can exchange their signals with each other even when separated by as much as tens of nanometers. Since a potential API-excipient interaction would act on the molecular scale, spin diffusion occurs between the API and excipient molecules, and the protons therefore show a single, uniform relaxation time regardless of whether they are on the API or the excipients. On the other hand, in the case of a physical mixture, the molecules of API and excipients are well separated spatially, and so no bulk spin diffusion can occur. Two unique proton relaxation rates are then expected, one for the API and another for the excipients. This is evident in the carbon spectrum of the physical mixture shown on the bottom of Fig. 10.25. Comparing this reference to the relaxation data for the formulation, it is readily apparent that the formulation exhibits essentially one proton T1 relaxation time across the carbon spectrum. This therefore demonstrates that there is indeed an interaction between the drug substance and the excipients in the formulation. 

An important first step in interpreting the C-13 spectra is to distinguish a-carbons from 3-carbons, i.e. methine from methylene. Observation of multiplicity when the proton decoupler is off is one way, but this is not always easy if the lines are broadened by chemical shift multiplicity. Measurement of has been used for this purpose since the 3-carbon with two bonded protons relaxes about twice as fast as the a-carbon with only one. A very positive way is by deuterium labelling. In Fig. 3 is shown the main-chain 25 MHz carbon spectrum of two styrene-S02 copolymers containing 58 mol% styrene, or a ratio of styrene to SO2 of 1.38 (7 ). In the bottom one, 3,3-d2-styrene has been used, cind all the 3-carbon resonances are distinguishable from the a-carbon resonances since the presence of deuterium has eliminated their nuclear Overhauser effect because of this eind the deuterium J coupling ( 20 Hz), they are markedly smaller eind broader than the a-carbon resonances. 

The answer to the cis-trans question is to be found in the methylene carbon spectrum of Fig. 7. If we look at the (61 ppm) and C4 (53 ppm) peaiks for the -78° polymers, —which we recall has an almost exclusively alternating structure—, we see that they are clearly split, but by less than 1 ppm. We might at first think this represents cis and trans structures. However, ejqierience with diene polymer spectra shows that when methylene carbons are involved in a cis structure they shield each other by 8 to 10 ppm. This is due to the operation of the Y steric effect, particularly strong when the carbon bonds actually eclipse each other rather than being merely gauche. In chloroprene units one ejqiects the C4 carbon to shift little between a cis and trans structure because it always sees a bulky substituent across the... 

Similar spectra can be obtained more rapidly and with less sample if the data are acquired through the proton signals, which are much more intense. Basically, the H NMR data are acquired and the H- C coupling constant used as the delay in a pulse sequence, which enables us to obtain the carbon spectrum. This method of obtaining the data is called inverse-mode , since the carbon atoms are detected through their attached hydrogen atoms rather than by direct detection, with obvious benefits in the sensitivity and the time taken to obtain a spectrum. HMQC and HMBC are both examples of inverse-mode spectra and this method is so much quicker than CH COSY that an entire HMQC spectrum can be obtained in much less time than it takes to obtain the proton-decoupled C... 

With this dual display still on the screen select in the file manager window the ID carbon spectrum D NMRDATA GLUCOSE 1D C GC 001999.1R and use the drag and drop method to move it directly and most conveniently into the 1D WIN-NMR application window. 

Check the New Instance function and load the proton spectrum of glucose D NMRDATA GLUCOSE 1D H GH 001999.1R in the first and the carbon spectrum of glucose D NMRDATA GLUCOSE 1D C GC 001999.1R in the second instance of 1D WIN-NMR. 

The most valuable information one should be able to obtain from the carbon spectrum of a coal derivative is the ratio of aromatic to nonaromatic carbon atoms. We know of no other direct method by which this value can be obtained. Since the literature pertinent to C13 NMR is sparse, we wish to... 

It is appropriate now to re-emphasize just what reasonably quantitative data one can derive from the proton and C13 spectra. In this early work we assume that atoms other than carbon and hydrogen are present in only negligible amounts. The atomic hydrogen to carbon ratios can be evaluated from the elemental analyses of the materials and can be used to normalize the hydrogen and carbon NMR measurements so that they add to unity. The proton spectrum gives, after this minor modification, three items of information hnr, the fraction of total atoms in the material which is present as hydrogen atoms directly bonded to aromatic carbons hay the fraction bonded to carbons situated a to aromatic rings and hp, the fraction bonded to other nonaromatic carbons. The carbon spectrum in conjunction with the aromaticity calibration curve yields c.r, the fraction of total atoms in the material... 

Fig.4.17. Proton broadband-decoupled l3C NMR spectra of polypropylene ((a-c) 25 MHz 200 mg/mL 1,2,4-trichlorobenzene at 140 JC (d-e) 90.52 MHz 200 mg/mL heptane at 67 X) (a) isotactic (b) syndiotactic (c) atactic sample (d) methyl carbon spectrum, simulated for calculated carbon shifts and Lorentzian signals of < 0.1 Hz width at half-height (e) experimental spectrum [521]. Numbers in (d) refer to the 36 possible heptads ...

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