C Chemical shifts

The C chemical shift ranges for organic compounds in Table 2.2 show that many carbon-containing functional groups can be identified by the characteristic shift values in the C NMR spectra. 

Other functional groups that are easily differentiated are cyanide (5c =110-120) from isocyanide (5c = 135- 150), thiocyanate (5c =110-120) from isothiocyanate (5c = 125 - 140), cyanate (5c = 105- 120) from isocyanate (5c = 120- 135) and aliphatic C atoms which are bonded to different heteroatoms or substituents (Table 2.2). Thus ether-methoxy generally appears between 5c = 55 and 62, ester-methoxy at 5c = 52 N-methyl generally lies between 5c = 30 and 45 and. S-methyl at about 5c = 25. However, methyl signals at 5c = 20 may also arise from methyl groups attached to C=X or C=C double bonds, e.g. as in acetyl, C//j-CO-. 

If an H atom in an alkane R-// is replaced by a substituent X, the C chemical shift 8c in the a-position increases proportionally to the electronegativity of X (-/ effect). In the (1-position, Sc generally also increases, whereas it decreases at the C atom y to the substituent (y-effect, see Section 2.3.4). More remote carbon atoms remain almost uninfluenced (dSc 0). 

In contrast to H shifts, C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the C and the H NMR spectra. By analogy with their proton resonances, the C chemical shifts of k electron-deficient heteroaromatics (pyridine type) are larger than those of k electron-rieh heteroaromatic rings (pyrrole type). 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OCH3, SCH3, N(C//i)2] attached to a C=C double bond shields the 3-C atom and the -proton +M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

Low-temperature NMR spectra in CD2CI2 at -98 °C revealed two types of NH resonances and that the T-IH form is dominant. This is reversed already at -60 °C. Temperature is therefore also important and so is concentration. In a less polar solvent as toluene- fg at 300 °C the T-2H dominates to the extent that it exists in this form upto 97% [9]. 

Carbon Calculated nuclear shieldings Predicted chemical shifts Observed chemical shifts 

1-pentene with butanal. Replacing the methyl group in pentane by the more electronegative oxygen deshields the carbon in 1-butanol. Likewise, replacing C-1 in 1-pentene by oxygen deshields the carbonyl carbon in butanal. 

PROBLEM 13.13 Consider carbons x, y, and z in p-methyianisoie. One has a chemicai shift of 8 20 ppm, another has 8 55 ppm, and the third 8 157 ppm. Match the chemicai shifts with the appropriate carbons. 

Two features that are fundamental to NMR spectroscopy—integrated areas and splitting patterns—are not very important in NMR. 

PROBLEM 13.12 How many signals would you expect to see in the NMR spectrum of each of the following compounds  

Just as chemical shifts in NMR are measured relative to the protons of tetramethyl-silane, chemical shifts in NMR are measured relative to the carbons of tetra-methylsilane as the zero point of the chemical-shift scale. Table 13.3 lists typical chemical-shift ranges for some representative types of carbon atoms. 

In general, the factors that most affect chemical shifts are  

Both can be illustrated by comparing the chemical shifts of the designated carbon in the compounds shown. (The numbers are the chemical shift of the indicated carbon in parts per million.) 

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C=C stretch 80 C-13 chemical shifts 22, 53 C60 31,32 C60O isomers 54 carbon dioxide 120, 182 carbon monoxide 175,191 carbonyl series 84 carbonyl stretch 84, 220 in solution 244 Carmichael 136 Carpenter 152, 196 Cartesian coordinates 52, 286, 287 CASSCF keyword 228 CASSCF method 228,229,230,231, 232,233, 234,235 state-averaged 233... 

Figure 1. Flat of C-13 chemical shift of C-/3 of tyrosine in enkephalin as a function of pH meter reading in D O, 30° C...

Figure 2. C-13 chemical shifts of the glycyl residues in [2-[2-C-13]glycine]me-thionine enkephalin and [3-[2-C-13]glycine]methionine enkephalin in the presence of 75.0 and 78.5 mg of PS, respectively, as a function of pH, 30°C. Shifts observed for enkephalin in the absence of PS (see Figure 3), (-------...

Figure 3. C-13 chemical shifts of the glycyl-2 and glycyl-3 residues of methionine enkephalin as a function of pH, 30°C...

The spectra of the reacted 1,2-EB, DPMS and Al(acac)3 is shown in Fig. 6. The C-13 chemical shifts are shown in Table 6. There is a signal at 128.3 ppm that does not appear in the unreacted silanol material. This peak is due to benzene 51). This indicates phenyl cleavage of the silanol under the reactive conditions. The 1,2-EB and DPMS without Al(acac)3 did not show any peak at 128.3 ppm which indicates that the aluminium species is responsible for phenyl cleavage and benzene formation. 

Chemical shifts for aromatic azoles are recorded in Tables 17-20. Fast tautomerism renders two of the C-13 chemical shifts equivalent for the NH derivatives just as in the proton spectra (Table 17). However, data for the A-methyl derivatives (Table 18) clearly indicate that the carbon adjacent to a pyridine-like nitrogen shows a chemical shift at lower field than that adjacent to a pyrrole-like N-methyl group (in contrast to the H chemical shift behavior). In azoles containing oxygen (Table 19) and sulfur (Table 20), the chemical shifts are generally at lower field than those for the wholly nitrogenous analogues, but the precise positions vary. 

Coordination-Induced C-13 Chemical Shifts in Vanadium(V) Dipeptide Complexes... 

Polyethylene Backbone and Side-Chain C-13 Chemical Shifts in ppm from TMS (+0.1) as a Function of Branch Length (y Carbon Chemical Shifts, which occur near 30.4 ppm, are not given because they are often obscured by the major 30 ppm resonance for the "n" equivalent, recurring methylene carbons). Sol vent 1,2,4-trichlorobenzene. Temperature 125°C. 

Drumond M, Aoyama M, Chen C-L, Robert D (1989) Substituent effects on C-13 chemical shifts to aromatic carbons in biphenyl type lignin model compounds J Wood Chem Technol 9 421-441... 

Harper, J. K. and Grant, D. M. (2000). Solid state C-13 chemical shift tensors in terpenes. 3. Structural characterization of polymorphous verbenol. J. Am. Chem. Soc., 122, 3708-14. 

Inamoto, N. and Masuda, S. (1977). Substituent Effects on C-13 Chemical Shifts in Aliphatic and Aromatic Series. Proposal of New Inductive Substituent Parameter (i iota) and the Application. Tetrahedron Lett., 18, 3287-3290. 

Table 5. C-13 chemical shifts of 2,4,5-trichlorophenyl acrylate (AOTcp), polyAOTcp and equimolar copoly(AOTcp-styrene)...

Table 7 C-13 chemical shifts of clonidine HCl. Garbon assignment Chemical shift 6 (ppm)...

In general, there are several kinds of non-bonded interactions, which may influence the C-13 chemical shifts. Among them, the electrical environmental effects are expected to be the major contribution to the a-CD complexation-induced C-13 displacements of head and tail carbons of the guest compounds, as these shifts are induced by moving the guest molecule from the free state, surrounded by polar water molecules, to the relatively non-polar a-CD cavity. 

To calculate C-13 chemical shifts, we used the Karplus and Pople s average excitation energy method using CNDO/2 parameters[8], since this method was found to give good linearity between the calculated and observed C-13 shifts, as shown in Fig.2. 

As the model of solvent effect on C-13 chemical shifts, we used one developed by Ando et al. [6] based on a Klopman s "Solvaton" theoiry[9]. This model has been successfully applied to interpret the dielectric solvent effect on C-13 chemical shifts of many orgeinic compounds. According to this model, the interaction of solute with solvent molecules is incorporated into semi-empirical MO calculations by an assumption of a virtual particle called a solvaton. 

To examine the applicability aind the limits of the CNDO/2-solvaton combined method, the C-13 chemical shifts were calculated for PHBA ais a function of dielectric constant ( ), aind the results were compaired with those obseirved in several solvents of a wide range of values as listed in Table I. In the calculations, effective van der Waals radii... 

In the present case we have the NMR spectra of several demulsifiers where the spectra are described by the peak intensities at 46 specific peak locations. Of these variables, 17 C-13 chemical shifts are from the alkane region, 11 are from the ethylene/propylene oxide region, and 18 are from the aromatic region. These data are represented by a matrix of 198 rows (the demulsifiers) and 46 columns (the peak positions). 

H. Saito, R. Tabeta, T. Asakura, Y. Iwanaga, A. Shoji, T. Ozaki, I. Ando, High-resolution C-13 NMR-study of silk fibroin in the solid-state by the cross-polarization magic angle spinning method—conformational characterization of silk-I and silk-II type forms of Bombyx mori fibroin by the conformation-dependent C-13 chemical-shifts. Macromolecules 17 (1984) 1405-1412. 

The temperature-dependent NMR spectrum can be analyzed to show that there is a barrier (8.4 kcal/mol) for ring flip which interchanges the hydrogens of the methylene group. The C-13 chemical shifts are also compatible with the homoaromatic structure. 

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