Carbon multiple resonance

Anthracene has the formula Cl4Hln. It is similar to benzene but has 3 six-membered rings that share common C—C bonds, as shown below. Complete the structure by drawing in multiple bonds to satisfy the octet rule at each carbon atom. Resonance structures are possible. Draw as many as you can find. 

ADDITION TO CARBON-CARBON MULTIPLE BONDS unstabilized by resonance ... 

The proton decoupled carbon 13 NMR spectra for three poly( cyclohexylmethyl-co-isopropylmethyl) copolymers are shown in Figure 4. The backbone methyl group is observed as occurring between -4 and -1 ppm and consists of multiple resonances which are due to polymer microstructure. Multiple resonances are also observed for the methyl and tertiary carbon of the isopropyl group and for the methine carbon of the cyclohexyl group. Microstruc-tural assignments for these resonances remain to be made. It has also been found that increasing the bulky character of the substituent yielded broader resonance peaks in the carbon-13 NMR spectra. 

Recording of the broad-band decoupled 13C NMR spectrum and DEPT experiments allows to obtain the number of carbon resonances (usually, even a non-quantitative spectrum gives good agreement between line intensities and number of carbon atoms) and carbon multiplicities. 

In general, the resonant frequencies can be used to determine molecular structures. H resonances are fairly specific for the types of carbon they are attached to, and to a lesser extent to the adjacent carbons. These resonances may be split into multiples, as hydrogen nuclei can couple to other nearby hydrogen nuclei. The magnitude of the splittings, and the multiplicity, can be used to better determine the chemical structure in the vicinity of a given hydrogen. When all of the... 

The multiple resonances observed for the carbon of the carboxylic acid group in N-benzoyl-L-phenylalanine were shown to be related to different types of hydrogen bonding. These results are in good agreement with earlier studies using... 

As in the case of carbene complexes, 13C NMR spectroscopy is particularly useful in that the carbyne carbon typically resonates to low field (240 and 360 ppm), with heteroatom substituents shifting this to higher field. As noted above for carbene complexes, X-ray crystallography reveals that carbyne complexes have very short metal-carbon bonds, typically the shortest of any metal-carbon multiple bond, but lengthened if heteroatom substituents are present. 

All compounds with more than one chiral centre yield diastereomeric mixtures. All heteroalkylated species have a chiral centre at the sulphur. In the compounds 6 and 2 the carbon 5 is chiral, too. After phosphorylation there is one more centre at the phosphorus. So the appearance of two diastereomeres b 1 and 6 and four b 2 respectively is expected. This expectation is verified by NMR-results. In l3C-NMR as well as 31P-NMR spectra one can see multiple resonances corresponding to almost all nuclei of the compounds. The 31P-spectrum of 1 and the l3C-spectrum of 6 are the most significant for tbs phenomenon. 

HSQC) or heteronuclear multiple quantum correlation (HMQC). The combined experiments such as 2D HSQC(HMQC)-TOCSY experiments are powerful tools for the assignment of the 13C and 11 resonances belonging to the same sugar residue providing enhanced dispersion of TOCSY correlations in the carbon dimension. More recendy, different carbon multiplicity editing methods, for example, DEPT (distortionless enhanced polarization transfer)-HMQC and E-HSQC, have been developed to reduce the complexity of proton-carbon correlation spectra and to enhance the resolution by narrowing the applied spectral window.11... 

Figure 6.24. Editing of a proton spectrum according to carbon multiplicities. In (b) only those resonances arising from methylene groups have been selected from the conventional spectrum (a). Clean suppression of all other resonances is achieved with pulsed field gradients, although some phase errors remain on the selected signals.

Regions "E" and "F" involve multiple resonances but from only one type of carbon in each case. 

The first applications of the new technique to cellulose " demonstrated the resolution of multiple resonances for some of the chemically equivalent carbons in the anhydroglucose units. It became clear that the rationalization of the spectra that were observed would provide valuable additional information concerning the structure of the celluloses investigated. The first step in such a rationalization was the assignment of the resonances that appear in the spectra. 

Going from a phosphaalkene to a phosphaalkyne, we increase the. r-contribution in the carbon phosphorus multiple bond, and would therefore expect a further down-field shift of the phosphorus resonance. However, a glance at the situation in carbon carbon multiple bond systems in particular, alkenes and alkynes, tells us that C-NMR spectra of these molecules show the carbon resonance of alkynes upheld from that of alkenes. This is usually explained by anisotropic effects associated with the linear rod-shaped structure of alkynes versus the bend structure of alkenes. As the geometries of phosphaalkenes andphosphaalkynes are analogous to alkenes and alkynes, respectively, we can assume that the explanation given for the appearance of the carbon resonance in alkynes upheld from that for alkenes in C-NMR spectra is also applicable for the respective unsaturated phosphoms compounds. 

---