Model compound analysis, nuclear

The 50.31 MHz 13C NMR spectra of the chlorinated alkanes were recorded on a Varian XL-200 NMR spectrometer. The temperature for all measurements was 50 ° C. It was necessary to record 10 scans at each sampling point as the reduction proceeded. A delay of 30 s was employed between each scan. In order to verify the quantitative nature of the NMR data, carbon-13 Tj data were recorded for all materials using the standard 1800 - r -90 ° inversion-recovery sequence. Relaxation data were obtained on (n-Bu)3SnH, (n-Bu)3SnCl, DCP, TCH, pentane, and heptane under the same solvent and temperature conditions used in the reduction experiments. In addition, relaxation measurements were carried out on partially reduced (70%) samples of DCP and TCH in order to obtain T data on 2-chloropentane, 2,4-dichloroheptane, 2,6-dichloroheptane, 4-chloroheptane, and 2-chloroheptane. The results of these measurements are presented in Table II. In the NMR analysis of the chloroalkane reductions, we measured the intensity of carbon nuclei with T values such that a delay time of 30 s represents at least 3 Tj. The only exception to this is heptane where the shortest T[ is 12.3 s (delay = 2.5 ). However, the error generated would be less than 10%, and, in addition, heptane concentration can also be obtained by product difference measurements in the TCH reduction. Measurements of the nuclear Overhauser enhancement (NOE) for carbon nuclei in the model compounds indicate uniform and full enhancements for those nuclei used in the quantitative measurements. Table II also contains the chemical... 

With the development of polymer structural characterizations using spectroscopy, there has been a considerable effort directed to measurements of tacticity, sequence distributions and number average sequence lengths (59 65). Two methods have been traditionally used for microstructure analysis from polymer solutions. Vibrational spectroscopy (infrared) and Nuclear Magnetic Resonance (NMR). Neither of these techniques is absolute. The assignment of absorption bands requires the use of model compounds or standards of known structure. 

As mentioned above, the determination of atomic level structure, i.e., the backbone torsion angles for an oriented protein fiber, is possible by using both solid-state NMR method described here and specifically isotope labeling. This is basically to obtain the angle information. Another structural parameter is distance between the nuclei for atomic coordinate determination. The observation of Nuclear Overhauser Enhancements (NOEs) between hydrogen atoms is a well known technique to determine the atomic coordinates of proteins in solution [14]. In the field of solid-state NMR, REDOR (rotational echo double resonance) for detection of weak heteronuclear dipole interactions, such as those due to C and N nuclei [15, 16] or R (rotational resonance) for detection of the distance between homonuclei, are typical methods for internuclear distance determination [17,18]. The REDOR technique has been applied to structure determination of a silk fibroin model compound [19]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required which frequently becomes a problem. A review of these approaches has appeared elsewhere [16]. 

While nuclear magnetic resonance spectroscopy has become an invaluable tool in lignin analysis, relatively little effort has been directed toward the calculation of such spectra for lignin model compounds. Liptaj et al. [68] compared conformational data for cinnamaldehyde models derived from NMR to calculation, and similar work on quinone methides at both semiempirical and ab initio levels has been reported by Konschin et al. [69]. 

The elimination reaction of this chloride was carried out with potassium 2-methyl-2-butoxide to give the desired monomer. Monomer 2 was prepared in the same way using 4-methyl catechol. The structures were confirmed by infrared (IR), proton magnetic resonance (PMR), 13-C nuclear magnetic resonance (CMR) and elemental analysis. Model compounds 3 - 6 were synthesized as aids in establishing the structure of polymers derived from 1. 

Comparison of the nuclear magnetic-resonance spectrum of methyl 4-0-acetylmycaroside with the spectra of some model compounds, and the failure to obtain an isopropylidene derivative of methyl a-L-mycaroside, led Foster and coworkers to propose for mycarose the ij-xylo configuration. However, the more detailed analysis made of the nuclear magnetic-resonance spectrum of di-O-acetylmycarose by Hofheinz and coworkers as well as the stereospecific syntheses of mycarose by Korte and coworkers and by Woodward and coworkers, leave little doubt that mycarose is 2,6-dideoxy-S-C-methyl-L-nho-hexose (61) and that cladinose is its 3-methyl ether (62). 

CP measurements on static samples use of spectral simulation based on line shapes obtained from model compounds Optimization of sensitivity for analysis of coals, with judicious choice of pulse flip angles and use of a special pulse sequence to correct baseline problems Use of advanced NMR methods (nuclear Overhauser enhancement, DNP, 2D correlation and other 2D methods, spectral editing, etc.)... 

Different nuclear models and contributions of the Breit interaction between valence, inner and outer core shells of uranium, plutonium and superheavy elements El 12, E113, and El 14 are considered in the framework of allelectron four-component and (G)RECP methods. It is concluded on the basis of the performed calculations and theoretical analysis that the Breit contributions with inner core shells must be taken into account in calculations of actinide and SHE compounds with chemical accuracy whereas those between valence and outer core shells can be omitted. 

The fundamental vibrations have been assigned for the M-H-M backbone of HM COho, M = Cr, Mo, and W. When it is observable, the asymmetric M-H-M stretch occurs around 1700 cm-1 in low temperature ir spectra. One or possibly two deformation modes occur around 850 cm l in conjunction with overtones that are enhanced in intensity by Fermi resonance. The symmetric stretch, which involves predominantly metal motion, is expected below 150 cm l. For the molybdenum and tungsten compounds, this band is obscured by other low frequency features. Vibrational spectroscopic evidence is presented for a bent Cr-H-Cr array in [PPN][(OC)5Cr-H-Cr(CO)5], This structural inference is a good example of the way in which vibrational data can supplement diffraction data in the structural analysis of disordered systems. Implications of the bent Cr-H-Cr array are discussed in terms of a simple bonding model which involves a balance between nuclear repulsion, M-M overlap, and M-H overlap. The literature on M-H -M frequencies is summarized. 

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