Carbon 13C

The Scheme depicts our most comprehensive understanding of the processes operative during the WGSR catalyzed by group 6b metal carbonyls at temperature < 100°C. As noted in the Scheme when the reaction is carried out in the presence of 13C0 both Cr(C0)3 and Cr(C0)sH are enriched in 13C-carbon monoxide, the latter species to a greater extent (see Figure 4). 

The results as shown in Figure 4.19 suggest that there is no source relationship between MW-2 and MW-4. The enrichment of heavy 13C carbon isotope in MW-4 may be attributed to the presence of heavy refined product, originating from an isotopically heavy rich (rich in 13C) crude oil feedstock mixture. 

Infrared and Raman spectroscopic results were identical to those in the literature, as were H proton and 13C carbon NMR spectra. 

DNP in liquids has also been reported at higher fields of 5 T (140 GHz electron frequency) by Loening et al46 In this report, the chemical systems were specifically selected to optimize scalar coupling, as the decrease in the coupling factor with increasing field is less dramatic for scalar-coupled species.4 Enhancements of E = 181 for 31P triphenylphospine, E = 41 for 13C carbon tetrachloride, E = — 35 for 15N aniline and E = 9.4... 

Each of carbon s isotopes contains 6 protons. 6 protons define carbon., 2C (carbon-12) contains 6 neutrons, 13C (carbon-13) contains 7 neutrons, and HC (carbon-14) contains 8 neutrons. 

Fig. 11. The LG-CP experiment of Fig. 9(a) applied to - C dipolar couplings in polycarbonate.15 (a) The full two-dimensional spectrum, (b) The LG-CP dipolar lineshape for the quaternary 13C (carbon 4), which as expected shows only very small - C dipolar couplings (as evidenced by the lack of observable splitting of the lineshape). (c) The LG-CP dipolar lineshape for carbon 3. (d) The best-fit simulation to the lineshape in (c). The motion used in this simulation (fast limit) assumes that the phenyl ring containing the carbon reorients about the local twofold axis of the ring through an angle of 120°, with the reorientation axis wobbling with an amplitude of 20°.

A further point about mass spectrometry, noticeable in the spectrum of propane (Figure 12.2), is that the peak for the molecular ion is not at the highest m/z value. There is also a small peak at M + l because of the presence of different isotopes in the molecules. Although 12C is the most abundant carbon isotope, a small amount (1.10% natural abundance) of 13C is also present. Thus, a certain... 

At its simplest, 13C NMR makes it possible to count the number of different carbon atoms in a molecule. Look at the l3C NMR spectra of methyl acetate and 1-pentanol shown previously in Figures 13.3b and 13.6b. In each case, a single sharp resonance line is observed for each different carbon atom. 

Most 13C resonances are between 0 and 220 ppm downfield from theTMS reference line, with the exact chemical shift of each 13C resonance dependent on that carbon s electronic environment within the molecule. Figure 13.7 shows the correlation of chemical shift with environment. 

Predict the number of carbon resonance lines you would expect in the 13C NMR spectra of the following compounds ... 

The information derived from 13C NMR spectroscopy is extraordinarily useful foT structure determination. Not only can we count the number of nonequivalent carbon atoms in a molecule, we can also get information about the electronic environment of each carbon and can even find how many protons each is attached to. As a result, we can answer many structural questions that go unanswered by TR spectroscopy or mass spectrometry. 

Problem 13.12 We saw in Section 8.3 that addition of HBr to a terminal alkyne leads to the Markovnikov addition product, with the Br bonding to the more highly substituted carbon. How could you use 13C NMR to identify the product of the addition of 1 equivalent of HBr to 1-hexyne ... 

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

Carbon atoms of an aromatic ring absorb in the range 110 to 140 8 in the 13C NMR spectrum, as indicated by the examples in Figure 15.16. These resonances are easily distinguished from those of alkane carbons but occur in the same range as alkene carbons. Thus, the presence of l3C absorptions at 110 to 140 8 does not in itself establish the presence of an aromatic ring. Confirming evidence from infrared, ultraviolet, or 1H NMR is needed. 

Ether carbon atoms also exhibit a downfield shift in the 13C NMR spectrum, where they usually absorb in the 50 to 80 5 range. For example, the carbon atoms next to oxygen in methyl propyl ether absorb at 58.5 and 74.8 8. Similarly, the methyl carbon in anisole absorbs at 54.8 8. 

The carbonyl-group carbon atoms of aldehydes and ketones have characteristic 13C NMR resonances in the range 190 to 215 8. Since no other kinds of carbons absorb in this range, the presence of an NMR absorption near 200 8 is clear evidence for a carbonyl group. Saturated aldehyde or ketone carbons usually absorb in the region from 200 to 215 8, while aromatic and a,p-unsaturated carbonyl carbons absorb in the 190 to 200 5 region. 

Table 2.1 may be useful for calculating the number of carbon, bromine, chlorine, and sulfur atoms in the molecular formula. This table shows that for every 100 12C atoms there are 1.1 13C atoms. Also, for every 100 32S atoms, there are 0.8 33S atoms and 4.4 34S atoms. The following examples... 

For example, the 13C isotope will contribute approximately 2% to the M + 2 in compounds containing 20 carbon atoms. 

The 13C-NMR spectra of 4-7, 9-11 show a close similarity to the spectral data of analogous carbene complexes. The shift differences between the metal carbonyls of the silylene complexes and the related carbon compounds are only small. These results underline the close analogy between the silicon compounds 4-7, 9-11 and Fischer carbene complexes. This view is also supported by the IR spectral data. On the basis of an analysis of the force constants of the vco stretching vibration,... 

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. 

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