L3C-NMR spectrum

Figure 13.3 shows both the H and the l3C NMR spectra of methyl acetate, CH3CO2CH3. The horizontal axis shows the effective field strength felt by the nuclei, and the vertical axis indicates the intensity of absorption of rf energy. Each peak in the NMR spectrum corresponds to a chemically distinct 1H or 13C nucleus in the molecule. (Note that NMR spectra are formatted with the zero absorption line at the bottom, whereas IR spectra are formatted with the zero absorption line at the top Section 12.5.) Note also that 1H and 13C spectra can t be observed simultaneously on the same spectrometer because different amounts of energy are required to spin-flip the different kinds of nuclei. The two spectra must be recorded separately. 

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. 

A second interesting point about both spectra in Figure 13.8 is that the peaks aren t uniform in size. Some peaks are larger than others even though they are one-carbon resonances (except for the two 2-carbon peaks of j ara-bromoaceto-phenone). This difference in peak size is a general feature of l3C NMR spectra. 

Techniques developed in recent years make it possible to obtain large amounts of information from l3C NMR spectra. For example, DEPT-NMR, for distortionless enhancement by polarization transfer, allows us to determine the number of hydrogens attached to each carbon in a molecule. 

AH C-C bonds are equivalent one resonance line in both and l3C NMR spectra. 

Selective labeling of the initiator with 13C allow s substantial enhancement of the signals of the initiator residues relative to signals due to the backbone in l3C NMR spectra. Initiators labeled with or containing NMR active nuclei such as 19F or J P can also be applied. These methods are described in Section 3.5.4.2,... 

Fig. 6. CP-MAS l3C-NMR spectra of polydimethylsiloxane at 75.47 MHz above and below the melting transition. Chemical shifts refer to TMS = 0 ppm and correspond to the scale at the bottom (Ref.10))...

Fig. 8. Slow exchange-fast exchange transition for the conformational interconversion of crystalline cyclotetraeicosane in CP-MAS l3C-NMR spectra at 75.47 MHz. Chemical shifts refering to TMS = 0 ppm and temperatures in K are indicated at the spectra. (Ref.7 )...

The second choice is a simpler solution. According to Sarko and Muggli,66 all 39 observed reflections in the Valonia X-ray pattern are indexable by a two-chain triclinic unit cell with a = 9.41, b =8.15 and c = 10.34 A, a = 90°, 3 = 57.5°, and y = 96.2°. Ramie cellulose, on the other hand, is completely consistent with the two-chain monoclinic unit cell. Also, there are significant differences between their high-resolution solid-state l3C NMR spectra, indicating that Valonia and ramie celluloses, the two most crystalline forms, reflect two distinct families of biosynthesis. On this basis, the Valonia triclinic and the ramie monoclinic forms are classified69 as Ia and Ip, respectively. It has been shown from a systematic analysis of the NMR spectra by these authors, and from electron-dif-... 

Per-O-acetyl dihexulose dianhydrides l3C NMR spectra, 247 H NMR spectra, 250-251 optical rotations and melting points, 244 Per-O-acetyl fructose glucose, H-NMR spectra, 252... 

The diacetyl compound 33 shows MeCO signals corresponding to one Z and one ZZ form with respect to the Ac—C bonds (Scheme 2) at - 122°C, and both H and l3C NMR spectra are in agreement with a system in which 33a and 33b (the EZ forms) are in fast equilibrium by C=C rotation, although all other rotations are slow at this temperature. 

The IR, H NMR, and l3C NMR spectra of this material are identical with those for distilled geranyl chloride (bp 49-51cC at 0.2 mm) Distillation on a small scale significantly reduces the yield, and there is no improvement in the yield of the phosphorylation reaction using distilled material. A synthesis of geranyl chloride was reported earlier in this series. We find, however, that the procedure of Corey, Kim, and Takeda4 is more convenient. ... 

Licania heteromorpha var. heteromorpha Bentham is a tree up to 30 m high native to the Amazonian forest. Phytochemical study of its aerial parts yielded both triterpenes and flavonoids triterpenes were obtained from the chloroform extract by silica gel column followed by RP-HPLC and were characterised as betulinic acid (11), alphitolic acid (48), 3(3-0-trans-p-coumaroyl alphitolic acid (49), 3(3-0-cw-p-coumaroyl alphitolic acid (50), 3 -0-trans-p-coumaroyl maslinic acid (51), 3fi-O-cis-p-coumaroyl maslinic acid (52), 3(3-0-tnms,-/ -coumaroyl-2a-hydroxy-urs-12-en-28-oic acid (53), 3 3-0-m-p-coumaroyl-2a-hydroxy-urs-12-en-28-oic acid (54) [see Fig. (2) and (6)] [15], Compounds 11 and 48-54 were identified comparing their H and 13C NMR data with those previously described. Triterpenoids 48-54 were found for the first time in Licania, while betulinic acid had been isolated previously from L. carii [9]. On the other hand, flavonoids were isolated from the chloroform-methanol and methanol residues by Sephadex LH-20 and HPLC they were identified as myricetin 3-0- 3-D-galactopyranoside (17), myricetin 3-0-a-L-rhamnopyranoside (32), myricetin 4 -methylether-3-0- 3-D-glucopyranoside (55), myricetin 4 -methylether-3-0-a-L-rhamnopyranoside (45), myricetin 3,4 -di-0-a-L-rhamnopyranoside (56), myricetin 7-methylether 3,4 -di-0-a-L-rhamnopyranoside (57), and myricetin 4 -methylether-3-0- 3-D-galactopyranoside (58) [see Fig. (2), (4), and (6)]. The last three myricetin derivatives were new natural compounds [16]. Known compounds were identified by comparison of their H and l3C NMR spectra with those reported in the literature [15]. 

The reaction of pyrazine with phenyllithium in THF or TMEDA at -45°C leads quantitatively to 102, as shown by the H-NMR spectrum. The assignment of the structure is made possible by comparison with the amino anaIog31. Both H- and l3C-NMR spectra of 102 are rather poorly diagnostic because they consist of broad signals. 

The response of an atom to the strength of the external magnetic field is different for different elements, and for different isotopes of the same element. The resonance frequencies of most nuclei are sufficiently different that an NMR experiment is sensitive only to a particular isotope of a single element. The frequency for H is 200 MHz at 4.7 T, but that of l3C is 50.4 MHz. Thus, when recording the NMR spectrum of an organic compound, we see signals only for H or 13C, but not both H and l3C NMR spectra are recorded in separate experiments with different instrument settings. 

Section 13.15 13C signals are more widely separated from one another than proton signals, and l3C NMR spectra are relatively easy to interpret. Table 13.3 gives chemical shift values for carbon in various environments. 

Section 20.21 Acyl chlorides, anhydrides, esters, and amides all show a strong band for C=0 stretching in the infrared. The range extends from about 1820 cm-1 (acyl chlorides) to 1690 cm-1 (amides). Their l3C NMR spectra are characterized by a peak near 8 180 for the carbonyl carbon. H NMR spectroscopy is useful for distinguishing between the groups R and R in esters (RC02R ). The protons on the carbon bonded to O in R appear at lower field (less shielded) than those on the carbon bonded to C=0. 

Fig. 32. GHPD (left) and cross-polarization (right) l3C NMR spectra of natural rubuer cured with 10% sulfur at 150 °C for 15, 30 and 60 minutes (adapted from Ref.196))...

The configuration at C-13 of the diterpenes has been a problem for many years. NMR spectroscopy using chiral shift reagents has been suggested as a method to differentiate manool from 13-epi-manool [131]. Most of the diterpenes with a saturated side chain were present as mixtures of C-13 epimers. Small differences in chemical shifts in the H and l3C - NMR spectra did not allow assignement of the stereochemistry at C-13 [132]. 

Fig.4.17. Proton broadband-decoupled l3C NMR spectra of polypropylene ((a-c) 25 MHz 200 mg/mL 1,2,4-trichlorobenzene at 140 JC (d-e) 90.52 MHz 200 mg/mL heptane at 67 X) (a) isotactic (b) syndiotactic (c) atactic sample (d) methyl carbon spectrum, simulated for calculated carbon shifts and Lorentzian signals of < 0.1 Hz width at half-height (e) experimental spectrum [521]. Numbers in (d) refer to the 36 possible heptads ...

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