Carbon connectivity

The one-bond C— coupling potentially contains a wealth of structural information, as it indicates carbon-carbon linkage. Unfortunately, only 1 in about 10,000 pairs of carbon atoms contains two atoms and hence displays a coupling in the spectrum. 

These resonances can be detected as very low intensity satellites on either side of the center band that is derived from molecules containing only isolated atoms. For bonded pairs of y is about 30-50 Hz, and the satellites are separated from the center band by half that amount. Coupling also may be present over two or three bonds ( 7, J) in the range of about 0-15 Hz. Not only are these satellites low in intensity and possibly obscured by the center band, but also, spinning sidebands, impurities, and other resonances may get in the way. 

The pulse sequence INADEQUATE (Incredible Natural Abundance DoublE QVAntum Transfer Experiment) was developed by Freeman to suppress the usual (single-quantum) resonances and exhibit only the satellite (double-quantum) resonances. The pulse sequence is 90°-T-180y-T-90°-A-90. The homonuclear 180° pulse refocuses field inhomogeneities, but allows the vectors from different coupling arrangements to contin- 

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Of the sp hybridized carbons C 1 is the most shielded because it is bonded to only one other carbon The least shielded carbon is the ring carbon to which the side chain is attached It is the only sp hybridized carbon connected to three others... 

Conjugated diene (Section 10 5) System of the type C=C—C=C in which two pairs of doubly bonded carbons are joined by a single bond The tt electrons are delocalized over the unit of four consecutive sp hybridized carbons Connectivity (Section 1 6) Order in which a molecule s atoms are connected Synonymous with constitution Constitution (Section 1 6) Order of atomic connections that defines a molecule... 

Fig. 1. The two principal elements of secondary stmcture in proteins, (a) The a-helix stabilized by hydrogen bonds between the backbone of residue i and i + 4. There are 3.6 residues per turn of helix and an axial translation of 150 pm per residue. represents the carbon connected to the amino acid side chain, R. (b) The P sheet showing the hydrogen bonding pattern between neighboring extended -strands. Successive residues along the chain point...

Carbon atoms and protons are assigned by means of the proton-carbon connectivities as identified in the HC HSQC and HMBC experiment (b and c). The latter also permits the derivation of the connection of the ethyl groups to the porphyrin ring. The cross signals in the relevant part a of the HH COSY plot (a) are used to connect the methyl and methylene subunits to the ethyl groups. 

Figure 134 is a sketch of some of the details around the anode. The carbon piece is 35 cm long and 3.5 cm in diameter. The metal hanger to the carbon connection shown was made with a nickel split sleeve with commercial galvanized steel banding clamps. (Nickel and Monel alloy banding clamps did not work well they stretched.) This mode of connec-... 

Triarylmethyl cations (14) are propeller shaped, though the central carbon and the three ring carbons connected to it are in a plane The three benzene rings cannot all be in the same plane because of steric hindrance, though increased resonance energy would be gained if they could. 

Both drawings above are commonly used, and you should train your eyes to see triple bonds either way. Don t let triple bonds confuse you. The two carbon atoms of the triple bond and the two carbons connected to them are drawn in a straight fine. All other bonds are drawn as a zigzag ... 

One possibility would be to put the Cl on the less substituted carbon (carbon connected to two other carbon atoms), and the other possibihty would be to put the Cl on the more substituted carbon (carbon connected to three other carbon atoms). If we put the Cl on the more substituted carbon, we call this a Markovnikov addition. If we put the Cl on the less substimted carbon, we call it an anti-Markovnikov addition. How do we know whether we get Markovnikov addition or anti-Markovnikov addition This is an issue of regiochemistry. 

How can we determine the carbon-carbon connectivities in a molecule through a combination of homo- and heteronculear shift[Pg.259]

The 2D INADEQUATE spectrum of a-picoline (Fig. 5.57) provides carbon—carbon connectivity information. The pairs of coupled carbons... 

A line is then drawn vertically upward from F to satellite peak F. The mirror image partner of F, appearing on the same horizontal axis, is A, which establishes the connectivity of the C-2 quaternary carbon with the attached methyl carbon (8 24.3). The carbon-carbon connectivity assignments based on the 2D INADEQUATE experiment are presented around the structure. 

Carbon-Carbon Connectivity Assignment by 2D INADEQUATE Experiment... 

The 2D INADEQUATE spectrum contains satellite-peaks representing direct coupling interactions between adjacent C nuclei. The 2D INADEQUATE spectrum and C-NMR data of methyl tetrahydrofuran are shown. Assign the carbon-carbon connectivities using the 2D INADEQUATE plot. 

COSY and HETCOR experiments are extremely useful in the structure elucidation of complex organic molecules. The geminal and vicinal protons and their one-bond C-H connectivities are first identified from the HETCOR spectrum, and then the geminal couplings are eliminated from the COSYspectrum, leaving vicinal connectivities. By careful interpretation of the COSY and the one-bond HETCOR spectra, it is then possible to obtain information about the carbon-carbon connectivities of the protonated carbons ( pseudo-INADEQUATE information). In this way the carbon-carbon connectivity information of protonated carbons is obtainable through a combination of COSY and HETCOR experiments. 

The 2D INADEQUATE spectrum provides carbon-carbon connectivity information and allows the entire carbon framework to be built up. The best strategy for the interpretation of a complex INADEQUATE spectrum is to start with the most downfield satellite carbon resonance and to trace the subsequent connectivities. Using this strategy,... 

Once acquired, processed, and plotted, carbon-carbon connectivity information can be extracted from a 1,1-ADEQUATE spectrum. There are multiple ways of going about this task and whichever method is used is probably best determined by the preferences of the individual investigator. 

Carboranes and other aspects of the boron/carbon connection... 

As a second illustrative aspect of the boron/carbon connection, we consider the dicarbollide ion, C2B9H112-, which can be pictured as derived from Bi2Hi22- by removing one BH(t) vertex and replacing two adjacent borons of the exposed pentagonal face by carbon atoms, as shown in Fig. 3.116. This ion exhibits remarkable electronic mimicry of the cyclopentadienyl ion (C5H5-, Section 3.3.2), forming similar coordination and sandwich complexes with many metallic species.162... 

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