C and 13C

Previous studies using l C and 13c/12c measurements in connection with other source apportionment techniques have demonstrated their value in assessing the importance of various sources (3). In winter studies in Portland, OR, at a site in which the impact from wood-burning is large, the fraction of contemporary carbon was near 100% urban plume values of l C for Denver samples were substantially lower (10-5A% contemporary) (8). Previous values reported for Long Island, NY, and Barrow, AK, sites were in the range of 30% contemporary carbon (5), with 13c/12c ratios consistent with a combination of fossil fuel and C-3 plant biogenic sources ( 13c ---26). 

Junicke et al,134 employed 3 mm probe capabilities in a study of the carbohydrate complexes of 13C-labeled platinum(IV) through the use of 1H-1H, H- C, and 13C-13C spin-spin coupling constants. The structure of the complex (64) is shown. 

Another example is dimethyl 3,6-dichloro-l-tosyl-l//-azepine-4,5-dicarboxylate which, on the basis of ]H and 13C NMR spectral studies, is a 9 1 equilibrium mixture of azepine and benzene imine tautomers at —70 C(AG = 50.16 kJ mol-1) 22 however. X-ray analysis indicates that the mixture crystallizes as the 1//-azepine isomer. 

Structure of Oxy-F Compound F is extremely unstable and is difficult to obtain at a level of purity suitable for NMR studies. However, an oxidation product, Oxy-F, formed when F is left standing at — 20° C, is considerably more stable than F and can be purified to a sufficiently high level of purity. Oxy-F is nonfluorescent and shows absorption maxima at 237 nm and 275 nm (shoulder). The high-resolution FAB mass spectrum indicated the molecular formula of Oxy-F to be C33H3809N4Na2 [m/z 703.2363 (M + Na)+ and 681.2483 (M + H)"1"]. The H and 13C NMR data allowed the assignment of structure 7 to oxy-F (Fig. 3.2.6 Nakamura et al., 1988). 

Although the o-xylylene complex is thermally unstable, it was characterized at — 50 °C by its 1H- and 13C-NMR spectra showing the exocyclic methylene at 5 = 5.04,4.42 ppm (JH) and 5 = 144.8 ppm (13C) using C6D5CD3 as the solvent. Its reaction with benzoyl chloride on the exocyclic carbon leaves a very acidic methylene group which transfers a proton onto the adjacent methylene unit. The double bond is benzoylated again in in situ and a di-cation of the [bis(arene)Fe]2+ type is obtained [47] Scheme VIII. 

Variable-temperature H and 13C NMR spectroscopic study of this seemingly crowded molecule 38 revealed that the ring rotation about the iron ring axis occurs freely down to — 95 °C, although at lower temperatures the rotation of the... 

Fig. 17. CP-MAS 13C-NMR spectra of threodiisotactic poly(l, 2-dimethyltetrame-thylene) below, c), and above the glass transition, b), in comparison to the spectrum in CDC13 solution at 303 K, a). The chemical shifts given at the signals refer to TMS = 0 ppm. (Ref. 32))...

When file polymerization is earned out in the same experimental conditions but without addition of p-TSA (S11CI2 2H20 = 0.4% mass, 180°C and 20 h), the resulting polymer is amorphous with Mw = 40,000 and a degree of racemization equal to 75% (determined from 13C NMR measurements). 

The activation energy for the reaction, a, was determined for the above Pt-porous nanoparticles from the first cycle of CV measurement in the temperature range between 30 and 60 °C, Figure 13c. The activation energy was obtained from the slope, —EJR, of the Arrhenius relationship and equal to SOklmoP. This value was similar to some of those obtained for the electro-oxidation of methanol on electrodes of Pt particles dispersed in Nation [50, 51]. 

Two types of adducts, C-C (35) and C-O (36) adducts, were identified and characterized by H and 13C NMR [10]. A pair of diastereomeric C-C adducts 35 were the predominant products formed. These adducts were converted to the desired double-bond-containing product with heating. This reaction most likely involves a pericyclic elimination (38) which is favored by formation of the aromatic hydro-quinone 37, as highlighted in Scheme 3.14. 

C-N.m.r. Chemical-shift Data for the Anomeric-Carbon Atoms and C, and Coupling Constants ( /ch) Measured for Selected Gland /S-D-Glycosyl-L-threonine Model Compounds19- 1,82-84... 

Table XVIII contains the 13C-n.m.r.-spectral data for the model compounds a,/ -Araf — Hyp and a,/ -Ara/— Hyp. The assignments of Qfi and Cs of Hyp were readily obtained19 by comparison with 13C data for L-Hyp in the free amino acid form and for L-Hyp residues of peptides.109 The chemical shift of C 5 was, as expected, 7-8 p.p.m. downfield from the values reported for nonglycosylated L-Hyp. The chemical shifts of C and Cy appeared in the same region as carbohydrate resonances and were therefore identified through their chemical shift-pH dependence.19 The chemical shift of the / -anomeric carbon atom in compound 60 is rather up-field, in agreement with a syn orientation for the C-1 - O-1 and...

The H and 13C NMR data for the three metal derivatives were similar. An X-ray crystallographic structure for the gallium derivative (Fig. 6) indicated a shortened Ga—C (carbene) distance of 1.935(6) A, whereas the Ga—C(Me) distances had normal single bond values, 1.994(8) and 1.988(8) A. Thus, the structure may be visualized as being composed of the two canonical forms... 

The H- and 13C-NMR spectra of gelsemine (1) have been reinvestigated with 2D homonuclear NOESY and heteronuclear COSY techniques (31). As a result some of the original assignments (4,32) have been revised. Thus the <5H of values of H-14a, H-14e, H-15, and H-16 are revised from 2.37, 2.0,2.83, and 2.30 to 2.83, 2.01, 2.30, and 2.43 ppm, respectively, while the most significant corrections of <5C values involve that of C-6 from 40.2 to 50.47 ppm and 1V-CH3 from 50.4 to 40.40 ppm. These adjustments will be helpful in future studies of alkaloids in this series. 

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. 

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