Aromatic carbon chemical shift

In an organic solid representative broadenings are 150 ppm for aromatic carbon chemical shift anisotropy and 25 kHz (full width at half-height) for a rather strong carbon-proton dipolar interaction. At a carbon Larmor frequency of 15 MHz, the shift anisotropy corresponds to 2.25 kHz. In high magnetic fields the forms of the respective Hamiltonians are... 

Finally, the aromatic carbon chemical shift anisotropy (R j spin-lattice relaxation rates are determined from Eq. (4.4-10). Equation (4.4-11) is then used to calculate the chemical shift anisotropy (Au) for an axially symmetric chemical-shift tensor. 

From the spectrum there are 7 carbon environments 4 carbons are in the typical aromatic/olefinic chemical shift range, 2 carbons in the aliphatic chemical shift range and 1 carbon at low field (167 ppm) characteristic of a carbonyl carbon. The molecular formula is given as C9HJJNO2 so there must be an element (or elements) of symmetry to account for the 2 carbons not apparent in the spectrum. 

From the off-resonance decoupled spectrum there are 2 CH resonances in the aromatic/olefinic chemical shift range, one CH2 and one CH3 carbon in the aliphatic chemical shift range. 

As is the case for substituent effects on proton and carbon, but not phosphorous NMR chemical shifts, nitrogen chemical shifts exhibit deshielding when in the vicinity of substituents that are more electronegative than carbon (Scheme 2.11). Presumably, although there are little data to support this assertion, the effect diminishes rapidly with distance of the substituent from the nitrogen, as is the case for proton and carbon chemical shifts. However, in aromatic heterocycles, such as pyridine, the effect of the substituent is transmitted through the n system, as indicated by the significant influence of para substituents of pyridines (Scheme 2.12).10... 

Representative examples of ring proton and carbon chemical shifts of all known l,4-(oxa/thia)-2-azoles were reported in CHEC-II(1996). A special notice should be given for H and 13C nuclear magnetic resonance (NMR) spectra of both dithiazolium 5 (X = Y = S) and oxathiazolium salts 6 (X = 0 Y = S) and 7 (X = S Y = 0) <1996CHEC-II(4)489>. A S downfield shift for both 3-H and 5-H as well as C-2 and C-5 is correlated with a potential 7r-electron delocalization and thus the aromaticity of these ring systems <1996CHEC-II(4)498>. 

The carbon chemical shifts of the azolium salts can be found at the downheld end of the aromatic range at 5 = 140-160 ppm and the carbenes themselves about A5 = 100 ppm downheld of the imidazolium salts. Coordinahon to transihon metals brings the carbon chemical shift upheld from the value of the free carbene. Whereas the resonance in [Cp Ru(NHC)Cl] complexes are typically around 8 = 200 ppm [116], the same signal in [Ag(NHC)Cl] complexes can be found at 8 = 170-190 ppm [50] (see Figure 1.23). 

The XH NMR spectrum shows bands in the region 4.0-6.0 ppm (See Table 6). Following the discussion of priority of paths of delocalization of aceheptylene dianion 232 and acenaphthylene dianion 82 also 332 and 342 show that specific paths of delocalization are favoured. While in the neutral structure 33 and 34 the competition is between aromatic and nonaromatic structures, in the respective dianions the competition is between nonaromatic and antiaromatic structures (Fig. 9). From the spectroscopic parameters, i.e., chemical shifts and coupling constants of the bridge protons it can be concluded that the neutral systems are best represented by structures with an aromatic skeleton connected to a virtually isolated double bond. In the charged systems, viz. 332 and 342 it seems that a nonaromatic path of conjugation is preferred to an antiaromatic path (Fig. 9). These considerations are also reflected in the carbon chemical shifts and in their HOMO-LUMO gap (AE) (vide infra) 122). It can be concluded from all these observations that there is a tendency of aromatic systems to remain so and to avoid as much as possible paratropic antiaromatic contributions. 

The resulting spectrum displays crosspeaks correlating carbon chemical shifts in f2 and proton shifts in f] which are further spread by homonuclear proton couplings in fi. Fig. 6.32 displays a part of the carbon-proton shift correlation spectrum of the palladium complex 6.11. Despite the extensive crowding in the aromatic region, the carbon shifts are sufficiently dispersed to resolve all correlations (note some resonances are broadened by restricted dynamic processes within the molecule and some are split by coupling to phosphorus). 

A similar range of values is noted for the ipso carbon chemical shifts of aromatic - S02- compounds as shown in the following tables. 

Figure 1. Aromatic part of the phase-sensitive 2D resolved dipolar spectrum of the liquid crystal 4 -methoxybenzylidene-4-n-butylaniline (MBBA) in the nematic phase. The spread in the ardirection is governed by the proton-carbon dipolar couplings C+ carbon chemical shifts) while the spread along the horizontal axis is determined by the carbon chemical shifts, exclusively. The corresponding proton-decoupled ID spectrum is sketched at the top of the 2D spectrum for clari ation. Projection of the 2D spectrum onto the

The chemical shift anisotropies for the carbonyl and aromatic carbons of Hytrel were reconstructed from a Herzfeld-Beiger analysis (24) of the intensities of the sidebands from NMR magic angle spinning experiments. The results in Table III indicate that the carbonyl carbon chemical shift anisotropy is axially symmetric for each terephthalate ester. We attribute this axial symmetry to a general property of terephthalate esters, rather than as a consequence of molecular motion, as the highly crystalline dimethyl terephthalate also has an axially symmetric carbonyl carbon chemical shift tensor. 

Figure 3. Diagrams showing the chemical shift changes observed in the peptide-DNA hairpin samples relative to the isolated DNA hairpin sample. (A) (upper panel) Phosphorus-31 chemical shift changes of the 5 - P for each residue of the 1 1 peptide-hairpin sample and (lower panel) carbon-13 chemical shift changes of the aromatic C6/C8 (open bar) and the deoxyribose Cl (thatched bar) signals in the 1 1 peptide-hairpin sample. (B) Aromatic proton chemical shift changes for the 1 1 (upper panel) and the 2 1 (lower panel) peptide-hairpin samples. The changes for the H6 8 protons (open bar), the A H2 and the C H5 protons (thatched bar) are plotted against the hairpin sequence.

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