Carbon NMR studies

Carbon NMR in solution is the method of choice for many types of material characterisation. While the sensitivity is much lower than for protons (Table 3.1), synthetic polymers are often soluble at concentrations as high as 5-20 wt%, so C-NMR spectra with a high signal-to-noise ratio can be acquired in a few hours. One major advantage of carbon NMR is that the chemical shifts are dispersed over 200 ppm rather than the 10 ppm typically observed for protons [1]. In addition, C-NMR is frequently used to study polymer molecular dynamics because the relaxation is predominantly due to carbon-hydrogen dipolar interactions with directly bonded protons. The C-H distances are fixed by the bond lengths, so the relaxation times can be used to measure the rotational correlation times directly (eqn (3.4)). 

The carbon chemical shifts depend not only on the stereochemistry and the chain defects, but also on chain conformation through the y-gauche effect (7). The y-gauche effect is an empirical correlation between the chemical shift of a particular carbon and the orientation of its y neighbouring carbons. The y-gauche effect can shift resonances by as much as 5 ppm, depending on the chain conformation and the microstructure. In many cases the chemical shifts can be assigned by comparison of the calculated and observed line intensities [7]. 

Carbon NMR relaxation is one of the most important methods for the study of polymer dynamics, since the relaxation times are directly related to the rotational correlation times [1]. The correlation times for many polymers have 

---

Hunkapillar, M. W., Smallcombe, S. H., Whitaker, D. R., and Richards,J. H. (1973). Carbon NMR studies of the histidine residue in alpha-lytic protease. Implication for the mechanism of serine proteases. Biochemistry 12,4732-4743. 

Extensive discussions have focused on the conformation of the alkyl chains in the interior ". It has been has demonstrated that the alkyl chains of micellised surfactant are not fully extended. Starting from the headgroup, the first two or three carbon-carbon bonds are usually trans, whereas gauche conformations are likely to be encountered near the centre of tlie chain ". As a result, the methyl termini of the surfactant molecules can be located near the surface of the micelle, and have even been suggested to be able to protrude into the aqueous phase "". They are definitely not all gathered in the centre of tire micelle as is often suggested in pictorial representations. NMR studies have indicated that the hydrocarbon chains in a micelle are highly mobile, comparable to the mobility of a liquid alkane ... 

The poly(vinyl alcohol) made for commercial acetalization processes is atactic and a mixture of cis- and /n j -l,3-dioxane stereoisomers is formed during acetalization. The precise cis/trans ratio depends strongly on process kinetics (16,17) and small quantities of other system components (23). During formylation of poly(vinyl alcohol), for example, i j -acetalization is more rapid than /ra/ j -acetalization (24). In addition, the rate of hydrolysis of the trans-2iQ. -A is faster than for the <7 -acetal (25). Because hydrolysis competes with acetalization during acetal synthesis, a high cis/trans ratio is favored. The stereochemistry of PVF and PVB resins has been studied by proton and carbon nmr spectroscopy (26—29). 

Syn- and anti-orientations are possible and there is evidence that the anti-orientation does not favor orbital overlap such an orientation is favored with larger branched-chain substituents. A C-nmr study found that the TT-electron density on the vinyl P-carbon is lower as the reactivity of the monomer increases (20). Methyl vinyl ether exists almost entirely ia the syn-stmcture, a favorable orbital overlap situation, and MVE for this reason is less reactive to both polymerization and hydrolysis (21). 

Katritzky and co-workers studied the mechanism of this reaction in detail. His work involved a NMR study of 16 reactions of methyl-, phenyl-, 1,2-dimethyl-, and l-methyl-2-phenylhydrazine with /3-keto esters. In many cases starting materials, intermediates, and products were detected simultaneously. Most reactions proceed by nucleophilic addition of the less hindered hydrazine nitrogen atom to the keto carbon of the keto ester. For example, the pathway given in Scheme 3 for the reaction of methyl 3-oxobutanoate 9 with methyl- or phenyUiydrazine 2 (R = Me or Ph) was found to be dominant. The initially formed addition product 10 dehydrates to hydrazone 11, which then isomerizes to hydrazone 12. Intermediate 12 then cyclizes to pyrazol-3-one 13, which tautomerizes to the kinetically more stable pyrazol-3-otie 14 [87JCS(P2)969]. 

A recent nuclear magnetic resonance (NMR) study of the structure of 2,3,4,5-tetramethyl, 2,3,4- and 2,3,5-trimethyl, and 2,4-dimethyI-pyrrolium ions in concentrated HCl has shown that they are all protonated on the -carbon and are thus of type (13)... 

Besha, K., Key, C, Glimcher, M., Schimizu, M. and Griffin, R.G. 1990 Solid state carbon-13 and proton NMR studies of carbonate-containing calcium phosphates and enamel. Journal of Solid State Chemistry 84 71-81. 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

Jurkiewicz A, Maciel GE. 1995. Solid state C NMR studies of the interaction of acetone, carbon tetrachloride and trichloroethylene with soil components. Sci Total Environ 164 195-202. 

This technique is the most widely used and the most useful for the characterization of molecular species in solution. Nowadays, it is also one of the most powerful techniques for solids characterizations. Solid state NMR techniques have been used for the characterization of platinum particles and CO coordination to palladium. Bradley extended it to solution C NMR studies on nanoparticles covered with C-enriched carbon monoxide [47]. In the case of ruthenium (a metal giving rise to a very small Knight shift) and for very small particles, the presence of terminal and bridging CO could be ascertained [47]. In the case of platinum and palladium colloids, indirect evidence for CO coordination was obtained by spin saturation transfer experiments [47]. 

Shaw, C.F. Ill, Eldridge, J. and Cancro, M.O. (1981) Carbon-13 NMR studies of aurothioglucose ligand exchange and redox disproportionation reactions. Journal of Inorganic Biochemistry, 14, 267-274. 

Isab, A.A. (1992) The carbon-13 NMR study of the binding of gold(I) thiomalate with ergothionine in aqueous solution. Journal of Inorganic Biochemistry, 45, 261-267. 

H/D exchange of H and Hg protons of sulfone 86 and estimated the difference in the free energies of activation for 79a and 79b to be < 1.2 kcal mol , based on the kjk value of 3 0.5. In the base-catalyzed H/D exchange of 87, kjk = 1.6, where k and k are the rate constants of H/D exchange of H, and H, respectively. Based on the small kjk value. Brown and colleagues suggested that if the carbanion is pyramidal, the steric stabilities of 79a and 79b are almost identical. Meanwhile, based on their C-NMR study Chassaing and Marquet proposed that the hybridization of the carbon atom of the sulfonyl carbanion, PhSOjCHj , would be between sp and sp . 

The structure of 65 was determined via a detailed mono- and bidimen-sional NMR study. In C NMR a signal at 20.3 ppm corresponds to the methyl groups and the carbon nucleus = C2 - H carbon appeared at 142.3 ppm. The gem-dimethylcyclopropene structure of 65 was consistent with an analysis of the HMBC spectrum. 

---