Spectroscopy carbon-13 NMR

The 13C NMR assignments of a series of bisquinolizidine compounds related to sparteine (compounds 24-27) are summarized in Table 3 2003JST275 . Detailed 13C NMR assignments for other sparteine derivatives are also available in the literature 2003T5531, 2005JST75 . 

The combination of H NMR, 13C NMR data and H- H and H- C correlations has been widely employed for the structural assignment of quinolizidine natural products. One example is the alkaloid senepodine A 30, 

Bicyclic 6-6 Systems with One Bridgehead (Ring Junction) Nitrogen Atom No Extra Heteroatom 

How many signals would you expect to obtain in the H NMR spectrum of undecadeu-teriocyclohexane at room temperature  

1 IB One Peak for Each Magnetically Distinct Carbon Atom 

The interpretation of NMR spectra is greatly simplified by the following facts  

Whereas carbon-carbon signal splitting does not occur in NMR spectra, hydrogen atoms attached to carbon can split NMR signals into multiple peaks. However, it is 

Most NMR spectra are obtained in the simplified broadband decoupled mode first and then in modes that provide information from the H couplings (Sections 9.1 ID and 9.1 IE). 

Where does a carbonyl group absorb in the NMR Where does an internal alkyne absorb In the proton NMR, both of these groups are invisible. Sometimes we can infer their presence If the carbonyl group has a proton attached (an aldehyde proton), the peak between 89 and 10 alerts us to its presence. If the adjacent carbon atom has hydrogens, their signals between 82.1 and 2.5 are suggestive, but we still can t see the carbonyl group. An internal alkyne is even more difficult, because there are no distinctive absorptions in the proton NMR and usually none in the IR either. 

The development of Fourier transform NMR spectroscopy made carbon NMR ( C NMR or CMR) possible, and high-field superconducting spectrometers allowed it to become nearly as convenient as proton NMR ( H NMR). Carbon NMR determines the magnetic environments of the carbon atoms themselves. Carbonyl carbon atoms, alkyne carbon atoms, and aromatic carbon atoms all have characteristic chemical shifts in the C NMR spectrum. 

Carbon NMR took longer than proton NMR to become a routine technique because carbon NMR signals are much weaker than )roton signals. About 99% of the carbon atoms in a natural sample are the isotope C. This isotope has an even number of 

Because C NMR is less sensitive than H NMR, special techniques are needed to obtain a spectrum. If we simply operate the spectrometer in a normal (called continuous wave or CW) manner, the desired signals are very weak and become lost in the noise. When many spectra are averaged, however, the random noise tends to cancel while the desired signals are reinforced. If several spectra are taken and stored in a computer, they can be averaged and the accumulated spectrum plotted by the computer. Since the NMR technique is much less sensitive than the H NMR technique, hundreds of spectra are commonly averaged to produce a usable result. Several minutes are required to scan each CW spectrum, and this averaging procedure is long and tedious. Fortunately, there is a better way. 

A Fourier transform is the mathematical technique used to compute the spectrum from the free induction decay, and this technique of using pulses and collecting transients is called Fourier transform spectroscopy. A Fourier transform spectrometer is usually more expensive than a continuous wave spectrometer, since it must have fairly sophisticated electronics capable of generating precise pulses and accurately receiving the complicated transients. A good C NMR instrument usually has the capability to do H NMR spectra as well. When used with proton spectroscopy, the Fourier transform technique produces good spectra with very small amounts (less than a milligram) of sample. 

Carbon-13 C C) has a nuclear spin of 16, and it can be detected by C-NMR experiment. NMR allows us to detect the structural environment of carbon atoms. This is often an advantage, especially for carbon atoms that are not bonded to hydrogen atoms, and thus cannot be detected by hydrogen NMR. 

NMR spectra can be easily obtained for many isotopes with half integer spins, such as and P, because their natural abundance is 100%. The detection of the isotope is more difficult because it has an abundance of only 1%. However, C NMR spectra are easily obtained. Lets consider the location ofin a compound such as 2-butanol. Most of the carbon atoms are C, which has no nuclear spin. The probability is equal for the location of at any of the positions in a molecule. The probabihty of finding a at C-1 of a molecule is 1%. The probability of finding a C at C-2 is also 1% and so on. The probabihty of finding two or more C in the same molecule and simultaneously bonded to each other is very low. For example, the probability of finding in the same molecule at both C-1 and C-2 is only 0.01%. As a result, a C NMR spectrum shows a sum of the signals generated by individual atoms at all of the possible sites in a collection of isotopically substituted molecules. 

The 13C NMR spectra of some parent A,B-diheteropentalenes and T(C,H) coupling constants are summarized in Tables 4 and 5. The sensitivity of C-3 to substituent effects is the same in both thieno[2,3-/ ]thiophene (33) and thieno[3,2-6]thiophene (12) 76ACS(B)417 . The substituted carbon atoms also show comparable sensitivity. Differences were observed for the substituent transmittance effects to similar positions in these two systems. For example the resonance effect is much more efficiently transmitted over two sulfur atoms in derivatives of compound (12). On the other hand, for the 6a position the system (33) shows a great similarity with 2-substituted thiophenes. 

Carbon-13 NMR parameters for 4//-thieno[3,2-6]pyrrole (9) and some other furo-, thieno-, and selenolo[3,2-6]pyrroles have been reported 76ACS(B)39i . Carbon-13 chemical shifts and T(C,H) coupling constants of the 2-, 4-, and 6-substituted furo[3,2-6]pyrroles have been determined (Tables 6 and 7) 90MRC830 . The largest effect of a substituent attached at C-2 was observed on the C-2, C-3, and C-6a carbons. Additional 13C NMR data on furo[3,2-6]pyrrole derivatives have been reported 93CCC2139, 94MI 701-01). 

A comparison of the spectra of thieno[2,3-6]thiophene (33) 76ACS(B)417 and selenolo[2,3-6] selenophene (35) 76CS159 reveals that the resonances of all carbons except for C-6a occur at lower field for compound (35). 

The carbon signals of selenolo[2,3-c]thiophene (19) were assigned from the undecoupled spectrum and the values of chemical shifts and coupling constants T(C,H) were compared with C-4 and C-6 substituted derivatives 81IZV1285 . 

The l3C NMR spectrum of compound (10) (84TL5669) shows for C-3 and C-6 a deviation of 16.7 ppm from the chemical shift of pyrrole at the -position this suggests an electron density increase due to the electron-donating character of the adjacent nitrogen atom. An analogous upheld shift of C-3 and C-6 was observed in the furo[3,2-6]pyrrole (8). 

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Breitmaier E and Voelter W 1986 Carbon-13 NMR Spectroscopy High Resolution Methods and Applications in Organic Chemistry (New York VCH)... 

Proton and carbon-13 nmr spectroscopy provides detailed information on all types of hydrogen and carbon atoms, thus enabling identification of functional groups and types of linkages ia the lignin stmcture. Detailed a ssignments of signals ia proton and carbon-13 nmr spectra have been pubHshed... 

J. B. Stothers, Carbon-13 NMR Spectroscopy, Academic Press, Inc., New York, 1972. 

H.-O. Kalinowski, S. Berger and S. Braun, Carbon-13 NMR Spectroscopy, Wiley, Chichester, 1988. 

Using proton NMR of solutions, the composition of polymers can be analyzed.47 Carbon-13 NMR spectroscopy is a useful tool for studying the sequence length of segments in copolymers and thereby determining the blockiness of the copolymer. With solid-state NMR, the mobility of chain segments can be studied and the crystallinity determined. 

Carbodiimides, 81 Carbodiimidization, 226-227 Carbon-13 NMR spectroscopy. See 13C NMR spectroscopy Carbon-carbon structure, 4 Carbonyl-containing polyester polyols,... 

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

Beckmann, N. Carbon-13 NMR Spectroscopy of Biological Systems Academic Press New York, 1995. 

Kahnowski H- O, Berger S, Braun S (1988) Carbon-13 NMR spectroscopy. Wiley, Chichester... 

T. A. Gerken and N. Jentoft, Structure and dynamics of porcine submaxillary mucin as determined by natural abundance carbon-13 NMR spectroscopy, Biochemistry, 26 (1987) 4689-4699. 

G.C. Levy and G.L. Nelson, Carbon-13 Nuclear Magnetic Resonance for Organic Chemists, Wiley, New York, 1972 J.B. Stothers, Carbon-13 NMR Spectroscopy, Academic Press, New York, 1972. 

The carbon-skeleton has been viewed directly with the help of Carbon-13 NMR spectroscopy on a particle basis since early 1970 s whereas -NMR spectrometry started in late 1950 s. The valuable contribution made by various researchers, between 1976 and 1980, has virtually placed 13C-NMR to a strategically much advanced stage where it gives a clear edge over 1H-NMR in terms of not only its versatility but also its wide application in analysis. 

Wehrli, FW, and T. Wirthin., Interpretation of Carbon-13 NMR spectra , London, Heyden, 1976 Abraham, RJ, and P. Loftus., Proton and Carbon-13 NMR spectroscopy , London, Heyden, 1978 Levy, GC, RL Lichter, and GL Nelson, Carbon-13 Nuclear Magnetic Resonance, 2nd, ed., New York, Wiley-Interscience, 1980. 

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