13C NMR studies

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

Characterization439 Inherent viscosity before and after solid-sate polymerization is 0.46 and 3.20 dL/g, respectively (0.5 g/dL in pentafluorophenol at 25°C). DSC Tg = 135°C, Tm = 317°C. A copolyester of similar composition440 exhibited a liquid crystalline behavior with crystal-nematic and nematic-isotropic transition temperatures at 307 and 410°C, respectively (measured by DSC and hot-stage polarizing microscopy). The high-resolution solid-state 13C NMR study of a copolyester with a composition corresponding to z2/zi = 1-35 has been reported.441... 

The rationale for the cyclopent[Z>]indole design discussed above was that the quinone methide would build up in solution and intercalate/alkylate DNA. Enriched 13C-NMR studies indicate that the quinone methide builds up in solution and persists for hours, even under aerobic conditions (Fig. 7.21). In contrast, the quinone methide species formed by known antitumor agents (mitomycin C) are short lived and highly reactive. The spectrum shown in Fig. 7.21 also shows the N to O acyl transfer product that we isolated and identified. However, we could not determine if the quinone methide structure actually has the acetyl group on the N or O centers. 

Ouyang, A. Skibo, E. B. The iminium ion chemistry of mitosene DNA alkylating agents, enriched 13C-NMR studies. Biochemistry 2000, 39, 5817-5830. 

Gelsevirine (2) was first isolated in 1953 from G. sempervirens as a minor component (3). Its structure was later elucidated on the basis of mass spectrometry as well as H-NMR and 13C-NMR studies (4). Gelsevirine has been found to be the predominant alkaloid in G. rankinii (24), and it was claimed that some of the previously reported 1 H-NMR and 13C-NMR data should be revised. Thus the previous assignments of H-16, H-15, H-14a, H-14e, and H-6 for gelsevirine should be changed to H-15, H-14a, H-16, H-6, and H-14e, respectively, from the evidence of the more accurate homonu-clear 2D COSY experiments. Similarly, from the heteronuclear 2D correlation spectrum, the assignments for C-16, C-15, C-6, and 1V-CH3 should be revised to C-15, C-16,1V-CH3, and C-6, respectively. 

A number of 13C NMR studies on zinc amine complexes have been reported.101-104... 

Zinc complex formation with 1,3-diketones in aqueous solution has been investigated with pentane-2,4-dione, l,l,l-trifluoropentane-2,4-dione, and 4,4,4-trifluoro-l-(2-thienyl)butane-l, 3-dione. The buffer dimethylarsinic acid was shown to have a catalytic effect on complex formation with pentane-2,4-dione and the proton transfer reactions were affected.471,472 High-resolution solid state 13C NMR studies of bis(2,4-pentanedionato) zinc complexes have been carried out.473... 

The zinc nitrate complexes of pyridyl functionalized 12[ane]N3 ligands l-(2-pyridylmethyl) -1,5,9-triazacyclododecane and l-(2-pyridyl-2 -ethyl)-l,5,9-triazacyclododecane were formed. 13C NMR studies were used to determine conformation in solution suggesting the former gave a trigonal bipyramidal isomer in solution with a water bound and the latter gave a 2 1 mixture of tetrahedral and asymmetric trigonal bipyramidal isomers. The crystal structure of the zinc complex of l-(2-pyridyl-2 -ethyl)-1,5,9-triazacyclododecane reveals a tetrahedral geometry in the solid state.679... 

C NMR studies suggest that 1,4,7,10-tetra-azacyclododecane-l,7-diacetic acid binds to zinc in a cis octahedral geometry—the two carboxylate oxygens are cis and the remaining four donors from the cyclen macrocycle. The formation constant was determined for the complex.733... 

Jezowska, I., A. Wolak, W.I. Gruszecki, and K. Strzalka. 1994. Effect of beta-carotene on structural and dynamic properties of model phosphatidylcholine membranes. II. A 31P-NMR and 13C-NMR study. Biochim. Biophys. Acta 1194 143-148. 

The gas chromatographic analysis of the unreacted monomers in the experiments from Table II discloses a constant C5/C8 ratio comparing the starting comonomer composition to the final composition. This means that monomer conversion is the same for 1,5-cyclooctadiene and cyclopentene in the copolymerization so that copolymer compositions are equal to the charge ratios. This result is consistent with the product analysis by 13C NMR spectroscopy where the copolymer composition is nearly identical to the starting comonomer composition. 13C NMR is used to determine the composition of the cyclopentene/1,5-cyclooctadiene copolymers as part of a detailed study of their microstructure (52). The areas of peaks at 29-30 ppm (the pp carbon from cyclopentene units) and at 27.5 ppm (the four ap carbons from the 1,5-cyclooctadiene) are used to obtain the mole fractions of the two comonomers (53, 54, 55). 13C NMR studies and copolymer composition determinations are described by Ivin (51, 56, 57) for various systems. 

P. putida grown with hexanoic acid contained approximately 75, 11, and 10 mol% of 3HHx,3HO, and 3HD units and also small amounts of four unsaturated repeating units. The mechanism for the formation of 3HO unit was investigated by 13C NMR study, which showed that the most of 3HO units found in this PHA were formed by the reaction of hexanoic acid with acetyl-CoA [53]. These results confirmed that P. putida produces 3HA units by fatty acid synthesis pathway, through a -oxidation and chain elongation process. 

The importance of the platinum-tin linkage in hydroformylation chemistry has promoted the publication of several articles dealing with this subject. 31P, 119Sn, 195Pt, and 13C NMR studies have been helpful in this aspect of the reaction.93-95... 

Solid-state 9Be and 13C NMR studies have been used to characterize the structure and dynamics of the methyl-substituted beryllocenes. The isotropic 9Be chemical shift has been found to vary with the coordination of the Cp rings, from —19.8ppm in 11 (rf/rf) to — 24.4ppm in 10 (rf/rf). The Cp rings of 11 display comparatively little motion from —100 to 80°C, whereas the rate of reorientation of the CsMes rings decreases substantially at low temperatures in 10.50... 

The interactions of (212) and related species with monovalent and divalent metal cations have been studied by nmr spectroscopy (Lehn Simon, 1977). The study indicated that sequential formation of mono-and di-cation complexes occurs (see [4.7]). These studies, and especially a 13C nmr study of the 1 1 complex of (212) with barium nitrate, suggest that the 1 1 species are unsymmetrical with the metal ion being contained in one of the azacrown cavities. Nevertheless, the nmr data also indicate that these 1 1 species undergo internal cation exchange between the respective azacrown sites. This intramolecular dynamic behaviour serves... 

For the 1 2 complexes, a full H- and 13C-NMR study has been carried out and has been interpreted on the basis of a similar process initially involving the movement of the H(a) atom between the two metal edges and finally the equilibration of the H(a)/H(/3) systems (90). The... 

The number of 13C NMR studies on metal carbonyls has greately increased over the last few years as shown by the presently available data on tetranuclear carbonyls (Table 11) compared with that available in 1974 when the last review232 on this subject appeared and contained only two references to tetranuclear clusters. 

An IR study of the relative proportions of the various Fe(CO)5 JC(PF3)x species as a f (temperature) used to provide data for a corresponding 13C NMR study... 

The 13C NMR study utilized cross polarization-magic angle spinning (CP-MAS) with spin counting. The elemental and functional group analyses provided input for a series of analytical constraints calculations that yield an absolute upper limit for the amount of aromatic carbon and most probable estimates of both aromatic and non-carboxyl aliphatic carbon in each sample. Spin counting experiments demonstrate that less than 50% of the... 

G ) (89CRV1225). According to X-ray data on substituted azepines and oxepines (81AG713 86CC970), these molecules have nonplanar boatlike structures. The results of low-temperature 13C-NMR studies on 2-... 

High-resolution 13C NMR studies have been conducted on intact cuticles from limes, suberized cell walls from potatoes, and insoluble residues that remain after chemical depolymerization treatments of these materials. Identification and quantitation of the major functional moieties in cutin and suberin have been accomplished with cross-polarization magic-angle spinning as well as direct polarization methods. Evidence for polyester crosslinks and details of the interactions among polyester, wax, and cell-wall components have come from a variety of spin-relaxation measurements. Structural models for these protective plant biopolymers have been evaluated in light of the NMR results. 

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