Carbon-13 nuclear magnetic resonance complexation

Characterization Tools for Pyrolysis Oils. It wasn t too many years ago that the only tools available to the scientist interested in pyrolysis oil composition were gas chromatography and thermogravi-metric analysis. The complexity of the pyrolysis oils demands high performance equipment, and a list of such equipment mentioned during the symposium would include proton and carbon nuclear magnetic resonance spectroscopy, free-jet molecular beam/mass spectrometry (16.25), diffuse reflectEuice Fourier transform infrared spectrometry ( ), photoelectron spectroscopy ( ), as well as procedures such as computerized multivariate analysis methods (32) - truly a display of the some of the most sophisticated analytical tools known to man, and a reflection of the difficulty of the oil composition problem. 

Generally, the most powerful method for stmctural elucidation of steroids is nuclear magnetic resonance (nmr) spectroscopy. There are several classical reviews on the one-dimensional (1-D) proton H-nmr spectroscopy of steroids (267). C-nmr, a technique used to observe individual carbons, is used for stmcture elucidation of steroids. In addition, C-nmr is used for biosynthesis experiments with C-enriched precursors (268). The availability of higher magnetic field instmments coupled with the arrival of 1-D and two-dimensional (2-D) techniques such as DEPT, COSY, NOESY, 2-D J-resolved, HOHAHA, etc, have provided powerful new tools for the stmctural elucidation of complex natural products including steroids (269). 

Fig. 22. Carbon-13 nuclear magnetic resonance of carbonyl carbon chemical shifts on complexation of K.+.

We saw in Chapter 12 that mass spectrometry gives a molecule s formula and infrared spectroscopy identifies a molecule s functional groups. Nuclear magnetic resonance spectroscopy does not replace either of these techniques rather, it complements them by "mapping" a molecule s carbon-hydrogen framework. Taken together, mass spectrometry, JR, and NMR make it possible to determine the structures of even very complex molecules. 

Halpin RA, GD Hegeman, GL Kenyon (1981) Carbon-13 nuclear magnetic resonance studies of mandelate metabolism in whole bacterial cells and in isolated, in vivo cross-linked enzyme complexes. Biochemistry 20 1525-1533. 

Theng BKG, Newman RH, Whitton JS (1998) Characterization of an alkylammonium-montmorillonite-phenanthrene intercalation complex by carbon-13 nuclear magnetic resonance spectroscopy. Clay Miner 33 221-229... 

Laughlin et al. [122] analysed chloroform extracts of tributyltin dissolved in seawater using nuclear magnetic resonance spectroscopy. It was shown that an equilibrium mixture occurs which contains tributyltin chloride, tributyl tin hydroxide, the aquo complex, and a tributyltin carbonate species. Fluorometry has been used to determine triphenyltin compounds in seawater [123]. Triph-enyltin compounds in water at concentrations of 0.004-2 pmg/1 are readily extracted into toluene and can be determined by spectrofluorometric measurements of the triphenyltin-3-hydroxyflavone complex. 

B. Akermark, B. Krakenberger, S. Hansson, Ligand Effects and Nucleophilic Addition to (T)3-Allyl)palladium Complexes. A Carbon-13 Nuclear Magnetic Resonance Study, Organometallics, 1987, 6, 620-628. 

Infrared and nuclear magnetic resonance data for the U(C5H5)3R complexes are consistent with a metal-carbon a bond, and this fact has been confirmed by the structural analysis of tricyclopentadienylphenyethynyluranium(IV) (72). The molecular geometry (Fig. 13) about the uranium is a distorted tetrahedron with... 

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. 

The dependence of the principal components of the nuclear magnetic resonance (NMR) chemical shift tensor of non-hydrogen nuclei in model dipeptides is investigated. It is observed that the principal axis system of the chemical shift tensors of the carbonyl carbon and the amide nitrogen are intimately linked to the amide plane. On the other hand, there is no clear relationship between the alpha carbon chemical shift tensor and the molecular framework. However, the projection of this tensor on the C-H vector reveals interesting trends that one may use in peptide secondary structure determination. Effects of hydrogen bonding on the chemical shift tensor will also be discussed. The dependence of the chemical shift on ionic distance has also been studied in Rb halides and mixed halides. Lastly, the presence of motion can have dramatic effects on the observed NMR chemical shift tensor as illustrated by a nitrosyl meso-tetraphenyl porphinato cobalt (III) complex. 

Evidence for the interaction between tributylamine and BN nanotubes has been obtained by nuclear magnetic resonance spectroscopy. We have studied the H and 13C NMR spectra of tributylamine-functionalized BN nanotubes in comparison with the spectra of tributylamine. We observe a small increase in the H chemical shift by 0.02 ppm in the amine-BN adduct. In the case of nC NMR spectra, we observe a significant increase in the chemical shifts of the y and S carbons by 0.4 ppm and a decrease in the chemical shift of the P carbon atom by 0.3 ppm. The chemical shift of the acarbon is also higher in the amine-BN adduct by 0.1 ppm. The changes in the H and L1C spectra of tributylamine found on interaction with BN are comparable to those reported in the literature for similar complexes.17,18... 

Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (equations 1 and 2). Resonance form (2), considered as the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this carbon-heteroatom bond. This effect is responsible for the restricted rotation often observed for this bond in nuclear magnetic resonance (NMR) studies. For example cis and trans isomers (6) and (7) of methoxymethyl carbenes rapidly interconvert at room temperature, but can be frozen out in the proton NMR at -40 °C. By contrast, the M-C bond is close to single and often rotates freely. 

Critical phenomena are also inherent to other types of the complexes. Thus, for iso-and syndio-PMAA complex the miiiimum length of stereosequence, which is necessary for the complex formation, has been determined by means of high resolution nuclear magnetic resonance For syndio-poliads it is equal to 8-10 linkages in aromatic solvents and to 3 linkages in acetonitrile and carbon tetrachloride. 

In Order to prove that stable alkyl cations, and not exchanging donoracceptor complexes were obtained, we also investigated the l3C nuclear magnetic resonance of the potentially electropositive carbenium carbon atom in alkyl cations31-32 ... 

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