L3C-NMR

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. 

At what approximate positions would you expect H2C=CHC02CH2CH3, to show l3C NMR absorptions ... 

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. 

Assign as many of the resonances as you can to specific carbon atoms in the l3C NMR spectrum of ethyl benzoate. 

Carboxylic acid groups can be detected by both and A3C NMR spectroscopy. Carboxyl carbon atoms absorb in the range 165 to 185 8 in the l3C NMR spectrum, with aromatic and unsaturated acids near the upheld end of the range (—165 8) and saturated aliphatic acids near the downfield end (—185 8). Nitrile carbons absorb in the range 115 to 130 8. 

Problem 20,16 How could you distinguish between the isomers cyclopentanecarboxylie acid and 4-hydroxycyclohexanone by and l3C NMR spectroscopy (See Problem 20.14.)... 

Although l3C NMR is useful for determining the presence or absence of a carbonyl group in a molecule, the identity of the carbonyl group is difficult to determine. Aldehydes and ketones absorb near 200 8, while the carbonyl carbon atoms of various acid derivatives absorb in the range 160 to 180 8 (Table 21.4). 

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

Methyl-l-propanol, l3C NMR spectrum of. 453 2-Methylpropene, heat of hydrogenation of. 187 Mevalonate, decarboxylation of, 1075 isopentenyl diphosphate from, 1072-1075... 

Naphtho analogues, naphtho[2,l-e]tetrazolo[l,5-6][l,2,4]triazine, naph-tho[l,2-e]tetrazolo[l,5-b][l,2,4]triazine, and naphtho[2,3-e]tetrazolo[l,5-Zj][1, 2,4]triazine, were prepared (82JOC3168 84JOC3199) by cyclization of the respective hydrazine with sodium nitrite in acetic acid or by azide displacement of a leaving group. Elucidation of the site of annulation of the tetrazole ring was accomplished by X-ray analysis and l3C-NMR spectroscopy (Scheme 189). 

Several 5,6-disubstituted-3-azido[l,2,4]triazine-l-oxides 1021 were prepared (77JHC1221) by treatment of 1020 with nitrous acid. 3-Azido[l, 2,4]-triazine-2-oxide 1023 was prepared by reacting the corresponding 3-bromo derivative 1022 with either tetramethylguanidinium azide in chloroform or sodium azide in aqueous acetone. These azido derivatives were proven to exist in the open-chain form by H- and l3C-NMR and 1R spectra (77JHC1221) (Scheme 191). 

H NMR spectroscopy frequently has been used in kinetic studies, for example, in the isomerization of 2,4,6-triphenyl-4//-thiopyran 56 (R= Ph) to its 2//-isomer 60 (R = H, 81JHC1517). l25Te NMR spectra were also measured for 4//-teluropyran 77 and related compounds (88MI1). Oxo-enol tautomerism of 4-hydroxy-2//-thiopyrans llOa-c in the solid state as well as in CDC13 solution was successfully studied by l3C NMR [86JCS(P2) 1887]. 

The ratio is determined by capillary GLC. c The reagent is prepared from CH3Li and CeCI,. d The reagent is prepared from CH3MgBr and Cul. e The reagent is prepared from R"Li and Cul. f The ratio is determined by l3C NMR. 

Conjugate Addition. To a solution of 1.5 mmol of lithium dialkylcuprate at — 25 CC is added 1 mmol of methyl ( )-3-[(25,45,55)-3-benzyloxycarbonyl-4-methyl-5-phenyl-2-oxazolidinyl]-propenoate dissolved in 1 mL of dry diethyl ether. After 30 ntin at — 25 C, the mixture is treated with an aq NH3/NH4C1 pH 8 buffer solution and then stirred at r.t. for 15 min. After diethyl ether extraction, the organic layers are dried over Na,S()4 and filtered and the solvent is evaporated under reduced pressure. The crude products are checked by H- and l3C-NMR analyses in order to determine the diastereomer ratios (g 95 5) and then purified by flash chromatography (hexane/ethyl acetate 80 20) yield 70-72%. 

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,... 

The mechanism of B polymerization is summarized in Scheme 4,9. 1,2-, and cis- and trews-1,4-butadiene units may be discriminated by IR, Raman, or H or nC MMR speclroseopy.1 92 94 PB comprises predominantly 1,4-rra//.v-units. A typical composition formed by radical polymerization is 57.3 23.7 19.0 for trans-1,4- c7a -1,4- 1,2-. While the ratio of 1,2- to 1,4-units shows only a small temperature dependence, the effect on the cis-trans ratio appears substantial. Sato et al9J have determined dyad sequences by solution, 3C NMR and found that the distribution of isomeric structures and tacticity is adequately described by Bernoullian statistics. Kawahara et al.94 determined the microslructure (ratio // measurements directly on PB latexes and obtained similar data to that obtained by solution I3C NMR. They94 also characterized crosslinked PB. 

The mechanism of chloroprene polymerization is summarized in Scheme 4.11. Coleman et ai9iM have applied l3C NMR in a detailed investigation of the microstructure of poly(chloroprene) also known as neoprene. They report a substantial dependence of the microstructure on temperature and perhaps on reaction conditions (Table 4.3). The polymer prepared at -150 °C essentially has a homogeneous 1,4-tra/rv-niicrostructure. The polymerization is less specific at higher temperatures. Note that different polymerization conditions were employed as well as different temperatures and the influence of these has not been considered separately. 

The extent of short-chain branching in PE may be quantitatively determined by a variety of techniques including IR. 1 pyrolysis Ct., 7" and y-radiolysis."84 The most definitive information comes from l3C NMR studies.2S5"2 0 The typical... 

Stames el al.I7 have provided support for the above mechanism (Scheme 6.29) by determining the unsaturated chain ends (112) in low conversion PVAc by l3C NMR. They were able to distinguish (112) from chain ends that might have been formed if transfer involved abstraction of a vinylic hydrogen. The number of unsaturated chain ends (112) was found to equate with the number of -CH OAc ends suggesting that most chains arc formed by transfer to monomer. Stames et a . 13 also found an isotope effect k kD of 2.0 for the abstraction reaction with CTTpCHOiCCD as monomer. This result is consistent with the mechanism shown in Scheme 6.28 but is contrary to an earlier finding.174... 

H and nC NMR studies on PVAc or PVA also provide information on the nature of branches.30 1,204 508,209 Dunn and Naravane"0 and Bugada and Rudin204 proposed that the difference in intensity of the methylene and methine regions of the l3C NMR spectrum could be used as a quantitative measure of the non-hydrolyzable branches (short chain f long chain) in PVA. However, this approach has been questioned by Vercauteren and Dormers" 4 because of the relatively large errors inherent hi the method. 

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 )...

Similar experiments suggested that 4-hydroxy-L-threonine (43) was an intermediate in synthesis of the three-carbon unit, C-6, C-5, C-5 (after decarboxylation). This was rigorously proved by a chemical synthesis of 4-hydroxy-L-(2,3-13C2)threonine. Incubation of E. coli mutant WG2 with this substrate produced a sample of pyridoxol that was examined by l3C NMR. The presence of doublets in the signals originating from C-5 and C-6 of pyridoxol exclusively, showed that the C-2-C-3 bond of the substrate had been incorporated intact into the predicted site (Scheme 18).42... 

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-... 

---