13C NMR techniques

In an NMR analysis of the effects of /-irradiation induced degradation on a specific polyurethane (PU) elastomer system, Maxwell and co-workers [87] used a combination of both H and 13C NMR techniques, and correlated these with mechanical properties derived from dynamic mechanical analysis (DMA). 1H NMR was used to determine spin-echo decay curves for three samples, which consisted of a control and two samples exposed to different levels of /-irradiation in air. These results were deconvoluted into three T2 components that represented T2 values which could be attributed to an interfacial domain between hard and soft segments of the PU, the PU soft segment, and the sol... 

A subsequent study of A. cavernosa from Thailand by California workers revealed two additional F-type kalihinols in addition to kalihinol-X (109) and -Y (110) [45]. The structures of kalihinol-I (121) and -J (122) were secured by low resolution chemical ionization mass spectral and NMR data. Both 1H-1H COSY and 13C NMR techniques were used extensively. Furthermore, hydrolysis of compound 109 to kalihinol-J (122) confirmed the assignments. 

Invention of Fourier transform instruments and consequent development of 13C-NMR techniques have supplied the chemist with excellent tools in characterizing complex molecules and making fine stereochemical distinctions. During the last two decades following the fundamental work of Wenkert (130, 228, 308), substantial amounts of l3C-NMR data of corynantheine and yohimbine alkaloids have been published. Some numerical values of interest are cited in Tables VIII, IX, X, and XI. 

Nuclear magnetic resonance (NMR) has proved to be a very useful tool for structural elucidation of natural products. Recent progress in the development of two-dimensional 1H- and 13C-NMR techniques has contributed to the unambiguously assignment of proton and carbon chemical shifts, in particular in complex molecules. The more used techniques include direct correlations through homonuclear (COSY, TOCSY, ROESY, NOESY) [62-65] and heteronuclear (HMQC, HMBC) [66. 67] couplings. 

Detection of the adducts in the NH2"-NH3 system has involved both H-NMR and 13C-NMR techniques. The reference spectra can be obtained in CDCIj and other nonprotic solvents because changes in chemical shifts with respect to liquid NH3 are generally small.8 ... 

By means of the 13C NMR technique Zambelli et al. proved311 that in polymerization of propylene catalyzed by TiCl3 + (I3CH3)2A1I the stereochemistry of the very first insertion is the same as that of subsequent units despite the lack of asymmetry in the original alkyl group of the activator. 

The interaction of metal ions with vitamin Bi (thiamine hydrochloride, 135) continues to be of interest, and a combination of IR, XH and 13C NMR techniques have been applied to the... 

H and 13C NMR techniques have widely been used to determine the configuration of new dioxepins and dithiepins and to elucidate the constitution and conformation of new naturally occurring substances. For example, the configuration of oximes 14 was determined by H and 13C correlated spectra, correlation spectroscopy (COSY), nuclear Overhauser enhancement spectroscopy (NOESY), heteronuclear correlation (HETCOR) spectroscopy, and hetero-nuclear multiple bond correlation (E1MBC) spectroscopy <1998CCA557>. 

Many studies have been performed, mostly in the 1980s, on BPA-PC with the various solid-state lHy 2H and 13C NMR techniques. 

The fractional aromaticity observations were performed by the solid state 13C NMR technique by Prof. R. Pugmire and co-workers at the University of Utah. 

Sequence distribution studies on several types of rubber by 13C-NMR technique have been reported. Some of the more recent reports include silicone rubbers [28-30], SBR [31], acrylonitrile-butadiene rubber (NBR) [32,33], polyurethane [34,35], polyepichlorohydrin [36], ethylene-norbonene [37] and ethylene-propylene rubber [4, 16, 25, 38-44]. The NMR studies on EPDM have been carried out extensively, because it is one of the important parameters, which control the physical properties of the elastomer. For example, ethylene sequence can influence the crystallisation kinetic and melting behaviour of the rubber [38]. 

Recently, a novel method for determining the microstructure of crosslinked polybutadiene in latex using solution 13C-NMR technique was reported [133]. The surfactant and polymer concentrations in the latex were adjusted to give a good signal resolution of the latex sample, as indicated by half-width of the resonance peak at 32.7 ppm. Under these conditions, the S/N ratio was almost identical to that of sample in solution, as shown in Figure 11.31. The microstructure of sol and gel fractions in a radical initiated polybutadiene, determined by this technique, was similar to that of solution measurements. 

Principal solid state 13C NMR techniques, 337 Pristine, 337-8 Processability, 800 Processing... 

The biosynthesis of 27 2 Hy has been studied in vivo with both radiotracer and 13C-NMR techniques. Labeled acetate was incorporated in vivo about equally into the saturated and diene hydrocarbons, whereas [1- 4C]1inoleic acid was preferentially incorporated into the diene fraction (18). Furthermore, [9,10-3H]oleate was incorporated at low levels but almost exclusively into the 27 2 Hy, with less than... 

In the 13C-NMR spectra, PEG gives a singlet signal at 8 71.5 ppm due to the inner chain carbons 228). In the conformational studies of PEG-bound sequences of ala-methiein, this signal has been eliminated from the spectrum by making use of a double resonance experiment by selective saturation of the methylene carbons 209). Distinct differences between random-coil and a-helical conformations were observed from these high-resolution 13C-NMR techniques. 

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