13C NMR resonances

The carbonyl-group carbon atoms of aldehydes and ketones have characteristic 13C NMR resonances in the range 190 to 215 8. Since no other kinds of carbons absorb in this range, the presence of an NMR absorption near 200 8 is clear evidence for a carbonyl group. Saturated aldehyde or ketone carbons usually absorb in the region from 200 to 215 8, while aromatic and a,p-unsaturated carbonyl carbons absorb in the 190 to 200 5 region. 

Complex 12 was found to be a good reactant in the double-silylation reaction. Thus, thermolysis of a toluene solution of 12 and diphenylacetylene at 120°C for 12 h afforded 5,6-carboranylene-1,1,4,4-tetramethyl-2,3-diphenyl-1,4-disilacyclohex-2-ene 13. When 1-hexyne was employed in the reaction with 12 under the same reaction conditions, the five-membered disila ring compound 18 was isolated. A key feature in the h NMR spectrum of 18 includes a singlet at 6.24 ppm assigned to the vinyl proton. A characteristic high-frequency 13C NMR resonance at 138.50ppm provides evidence for a tethered sp2 carbon atom between the two silicon atoms. 

The disilanickela compound 21 is not effective in the nickel-catalyzed double silylation reaction with styrene. However, the stoichiometric reaction of 21 with styrene afforded 4,5-carboranylene-l,l,3,3-tetramethyl-2-phenylmethylene-1,3-disilacyclopentane 29. A key feature in the 111 NMR spectmm of 29 includes a singlet at 7.71 ppm assigned to the vinyl proton. A characteristic low-frequency 13C NMR resonance at 139.75 ppm provides evidence for a tethered carbon atom of the two silicon moieties. Unambiguous confirmation was provided by X-ray crystallographic analysis of 29. 

Three or more equivalents of the nucleophilic Wanzlick-type carbene00 l,3-dimethylimidazolin-2-ylidene react with BeCl2 to cleave its bridging Be-Cl-Be bonds, forming the stable pale yellow ionic coordination compound [(l,3-Me2-C3N2H2)3BeCl]+Cl- 19.67 There is a large upfield shift in the 13C NMR resonances of the carbene atom... 

C NMR spectroscopy. 13C NMR resonances for both carbonyls of the TV-acyloxy-V-alkoxyamides and that of the parent hydroxamic ester are given for most compounds in Table 2. 

Methyl end groups resulting from main-chain scission in ethylene-propylene copolymers have observed by their characteristic 13C NMR resonance and determined quantitatively to give values of G(scission). 

Further confirmation of the structure of the AHLs often follows from the determination of their proton and carbon-13 NMR spectra. A large number of AT-acyl, AT-(3-oxoacyl) and AT-(3-hydroxyacyl)-L-HSL derivatives have been prepared and their NMR data reported [14-16]. Also a detailed study by Lao et al. [49] on the complete assignments of the 13C NMR resonances of AT-acyl and N-(3-oxoacyl)-L-HSL derivatives has been published. These assignments were made by comparison with values for AT-butanoyl-L-HSL (C4-HSL), whose structure was comprehensively established by a combination of 1-D and 13C spectra and 2-D COSY,NOESY, HSQC and HMBC experiments. The assignments... 

TABLE 2.8. CHARACTERISTIC 13C NMR RESONANCES FOR iV-SILYLOXY NITROSO ACETALS... 

The carbide atom in 1 is located in the center of the square face such that it is partially exposed whereas the carbide atom in 2 is completely encapsulated by the six ruthenium atoms. From a spectroscopic viewpoint, carbide atoms are very distinctive and the earlier reviews have dealt with these aspects in detail.7 8 The IR spectrum of 1 contains peaks at 701 (s) and 670(m) cm 1, and 2 contains peaks at 717(sh), 703(s), 680(m), and 669(m) cm-1.22 I3C-NMR spectra of 1 and 2 do not appear to have been reported. This is probably due to the low yields in which these compounds were initially obtained at a time when, 3C-NMR was still not in widespread use in cluster chemistry. In general, the 13C-NMR resonance of carbide atoms ranges from 8 250 to 500. The high frequency resonances exhibited in 13C-NMR spectra reflect the different diamagnetic and paramagnetic effects experienced by a nucleus in such an unusual chemical environment.23... 

The key feature of the NMR spectra of 91 is its simplicity. Thus the 13C NMR spectrum consists of only two resonances at 4.9 and 17.6 ppm, indicating either a symmetrical trishomocyclopropenium cation, 93, or rapid equilibration between three equivalent structures (Scheme 37). The positions of the 13C NMR resonances of the cation strongly suggested the formulation of its structure as the trishomocyclopropenium ion, 93210. This conclusion was reinforced by the preparation of the deuterated cation and examination of the isotopic perturbation of its 13C chemical shifts208 211, and measurement of the 13C H coupling constants209. 

Andersen, 0.M., Aksnes, D.W., Nerdal,W., and Johansen, O.P. 1991a. Structure elucidation of cy-anidin-3-sambubioside and assignments of the H and 13C NMR resonances through two-dimensional shift-correlated NMR techniques. Phytochem. Anal. 2 175-183. 

It should be noted that the kinetics for the human B isoenzyme are more complicated in that the pH dependence indicates that additional groups influence the rate. Studies with the isoenzyme carboxymethylated at His-200 prepared from 13C-labelled bromoacetate show that the pH dependence of the 13C NMR signal can be fitted to a curve with two pKa values of 6.0 and 9.2, but not to a curve with a single pKa. The second group could be the imidazole side-chain of His-200. Paramagnetically shifted 13C NMR resonances in the modified Co11 human carbonic anhydrase-B have been located by a novel method 498 This should allow the confirmation of an earlier postulate that the carboxymethyl carboxylate is a ligand for zinc in the modified enzyme. 

More highly stabilized l,8-bis(diarylmethyl)naphthalene dications have been prepared, including the p -methoxyphenyl derivative 53.20 This dication is generated from ionization of the diol in HBF4 and (CF3CO)2O.20a Dication 53 has been characterized by experimental studies (single crystal X-ray analysis and NMR) and theoretical calculations. The carbenium ion centers are found to be separated by just 3.076 A (X-ray and ab initio results) and show 13C NMR resonances at A3C 191.8. Two electron reduction is also shown to give the acenaphthene derivative 54. 

Copper, silver and gold - The paramagnetism of Cu11 has limited the use of NMR for the direct study of copper complexes, but information is readily obtained from studies of ligand nuclei. For example, the broadening of the NMR signal by copper in copper-zinc and copper-cobalt superoxide dismutases (SODs) has been used to determine the distance between the copper and the proton on bound formate (Sette et al., 1992). Also, broadening of the formate 13C NMR resonance reveals information about the orientation of the formate. 

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