NMR, 13C

The strong interactions between the water molecules also become obvious from NMR measurements by Tsujii et al..57) 13C-NMR experiments were used for determining the microviscosity of water in reversed micelles of dodecylammonium-propionate with 13C glycine cosolubilized. It was found that the apparent viscosity of the water-pool corresponds to the viscosity of a 78 % aqueous glycerol solution, obviously as a consequence of the extended network formation by strong hydrogen bonding. 

Active Figure 13.3 la) The H NMR spectrum and (b) the 13C NMR spectrum of methyl acetate, CH3C02CH3. The small peak labeled "TMS" at the far right of each spectrum is a calibration peak, as explained in Section 13.3. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Interactive to learn to utilize 13C NMR spectroscopy to deduce molecular structures. 

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. 

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

Assign the resonances in the 13C NMR spectrum of methyl propanoate, CH3CPJ2C02CH3 (Figure 13.9). 

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. 

Figure 13.11 The 13C NMR spectrum of 1-methylcyclohexene, the E2 reaction product from treatment of 1-chloro-1-methylcyclohexane with base.

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 ... 

How many absorptions would you expect the following compound to have in its lH and 13C NMR spectra ... 

Sketch what you might expect the 1H and 13C NMR spectra of the following compound to look like (yellow-green = Cl) ... 

Thomson 1 Click Organic Interactive to learn to use 13C NMR, 1H NMR, infrared, and mass spectrometry together to deduce molecular structures. 

Why do you suppose accidental overlap of signals is much more common in H NMR than in 13C NMR ... 

How many 13C NMR absorptions would you expect for c/s-1,3-dimethyl-cyclohexane For fra/is-l,3-dimethylcyclohexane Explain. 

How could you use FI and 13C NMR to help you distinguish among the following isomeric compounds of formula C4H8 ... 

The and 13C NMR spectra of compound A, C8H9Br, are shown. Propose a structure for A, and assign peaks in the spectra to your structure. 

Compound D is isomeric with compound C (Problem 13.6 L) and has the following 13C NMR spectral data. Propose a structure. 

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. 

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