Carbon 13 chemical shifts methyl complexes

Todd et al. (23) have studied the NMR spectra of complexes of the type (CO)sMCRR, M = Cr, W R = CHs, Ph, m- andp-Ph R = NH2, OR (Tables XXXVI and XXXVII). They have also found (cf Ref. 123) that substitution of a methyl for a phenyl group (R) causes an 8 to 11 ppm downfield shift. A unique solvent effect was observed for this system the carbene carbon chemical shift was 6 to 7 ppm upfield in THF from the value in CHCI3. This effect was attributed to the formation of a solvent-solute complex in THF solution. 

Analysis of literature NMR data for hundreds of 1,3-diol acetonides have proven this method reliable. Only a few types of substituents (Ri and/or R2) are problematic. The chemical shift correlations shown in Figure 6.27 only become unreliable when the substituents in the 4 and/or 6 position of the dioxolane ring are an yp-hybridized carbon (alkyne or nitrile). Use of the acetal carbon chemical shift correlation is not quite as reliable, but of the hundreds of acetonides examined, fewer than 10% of yyn-1,3-diol acetonides and 5% of anti-l,3-diol acetonides would be misassigned based on the chemical shift of the acetal carbon alone—and practially none will be misassigned if the acetal chemical shift is considered in conjunction with the acetonide methyl chemical shifts. The only drawbacks to this method is that the acetonide derivatives must be prepared from the diol substrates, but this is easily accomplished with a mixture of acetone, 2,2-dimethoxypropane, and pyridinium/ p-toluenesulfonate (PPTS). When only a small amount of sample is available, C-enriched acetone can be used to prepare the acetonides. The [ Cjacetonide method is also readily applied to complex natural products containing several different 1,3-diols. 

Fig. 31. A comparison of the 1H chemical shifts for the E11 valine 72-CH3 protons of the various isonitrile complexes of isolated a chains (o) and p chains (A) of Hb A, the o ( ) and p ( ) chains as identified within an intact Hb A molecule, and sperm whale myoglobin ( ). Ligands CO, carbon monoxide M, methyl isocyanide E, ethyl isocyanide P, n-propyl isocyanide B, n-butyl isocyanide iP, isopropyl isocyanide and tB, tert-butyl isocyanide. [From Mims et al. (1983a)].

Nonbonded complexes. The equilibrium constants, enthalpies, and entropies for the weak complexation of pyrazine with dichloromethane, chloroform, or carbon tetrachloride have been determined from changes in the n — 77 absorptions of solutions at various concentrations (in cyclohexane) and temperatures 568 similar data for pyrazine-aromatic hydrocarbon complexes were obtained from variations in the H NMR chemical shift values.1037 The spectral effects of complexation with borane have been studied in the pyrazine diborane adduct and its methyl derivatives.254... 

The complexes Cp (PP)Fe ( i-C=CC=C) Fe(CO)2Cp (PP = dppe 100, dippe 102) are protonated (HBF4Et20) or methylated (MeOTf) to give [ Cp (PP)Fe i-C=C=C=CR[Fe(CO)2Cp ] ]BF4 (190a-d) (R = H, Me), which can be isolated as moderately stable powders (Scheme 43). The UV-vis spectra of these deep purple butatrienylidene complexes have characteristic absorptions between 527 and 546 nm (s ca. 15,000).259 The absence of v(CC) bands between 1500 and 1650 cm-1 excludes the ethynylvinylidene formulation, and bands at 1952, 1776 (dppe/H), 1945 (dippe/H), 1942, 1882, 1830 (dppe/Me), and 1946 cm-1 (dippe/Me) were tentatively assigned to v(CC) modes. In 13C NMR spectra, resonances beween 250 and 265 ppm [2/(CP) ca. 35 Hz] were assigned to the carbon adjacent the Fe(PP)Cp moiety. The other carbon resonances were assigned on the basis of /(CH) and the assumption that the chemical shifts will move upfield along the chain. The Mossbauer spectra contained two quadrupole doublets, with 5 values similar to those shown by Fp Me or allenylidene... 

The nmR chemical shifts of the methyl groups in boron-capped iron(II) diacetylmonooxime hydrazonates (14.0 and 17.7 ppm) are lower than that in the [Fe(thz)](BF4)2 complex (18.6 ppm). A second signal was assigned to the methyl substituent in a hydrazonate fragment. A signal from the azomethine carbon atoms at 168 ppm is also attributed to this fragment, whereas a line at 154 ppm corresponds to an oxime fragment. The NMR... 

The increasing interest in C-NMR spectroscopy has resulted in a number of such studies in the imidazole field. Carbon-13 chemical shifts in a variety of solvents have been reported for imidazoles, 4-methyl-imidazole and its cation, phenylimidazoles, 1-acetylimidazole, and Fe(III) porphyrin-imidazole complexes.The influence of Mn(II) and Cu(II) on nuclear relaxation rates in imidazole has been studied C-2 appears at lowest field (136.2), while C-4 and C-5 are at 122.3 Hz. While C-NMR studies have been of some value in conformational studies,the shifts are of limited value only in ascertaining positions of tautomeric equilibrium for rapidly interconverting tautomers. ... 

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