Proton-Decoupled 13C Spectra

Exercise 9-42 With reference to the data summarized in Figure 9-47 and the discussion in this section, sketch qualitatively the proton-decoupled 13C spectra you would expect for... 

Figure 29-9 Proton-decoupled 13C spectra of different polypropene samples taken in CHCI2CHCI2 solution at 150° at 15.9 MHz. The upper spectrum is of a highly isotactic polypropene, which shows only the faintest indication of lack of stereoregularity. The middle spectrum is of atactic polypropene, which shows a variety of chemical shifts for the CH3 groups as expected from the different steric interactions generated by random configurations of the methyl groups. The lower spectrum is of a sample of so-called stereoblock polymer, which is very largely isotactic. The 13C spectrum of syndiotactic polypropene looks exactly like that of the isotactic polymer, except that the CH3— peak is about 1 ppm upfield of the position of the isotactic CH3 peak and the CH2 peak is about 1 ppm downfield of the isotactic CH2 peak.

Figure 5 75.4 Mhz proton -decoupled 13c spectra of 30% menthol in deuteriochloroform, (A)b before and (B) after zero- and first-order phase correction.

C NMR of linear cross-linked PS. The proton decoupled 13C NMR spectra of linear and 1% cross-linked PS at 75 MHz in chloroform are illustrated in Figure 3. These spectra are similar to those for linear and cross-linked chloromethylated PS previously reported at lower field (14), although we have been able to resolve more structure in tHe" aliphatic and aromatic regions here. The quarternary and methylene carbon resonances at about 146 ppm and between 40 and 50 ppm respectively, are the most strongly affected by stereochemistry (20). The ortho and meta resonances at 128.4 ppm show partially resolved structure in the linear PS, as does the para carbon at 126.1 ppm. The methine resonance at... 

Proton-decoupled 13C-NMR spectra were recorded on a Varian XL-300 operating at 75.4 MHz. Approximately 250 mg of the sample was dissolved in 3 ml of deuterated chloroform. 13C chemical shifts were referenced internally to CDCL (77 ppm). A delay of 200s was used to ensure relaxation of all the carbon nuclei and 1000 transients were collected to assure a good signal-to-noise ratio. 

Heteronuclei such as 13C (this magnetically active nucleus has 1.1% natural abundant) and 15N (0.3% natural abundance) are routinely measured with modem NMR spectrometers. Proton decoupled 13C NMR spectra in natural abundance exhibit singlets for each specific carbon atom, which are easier to count than overlapping multiplet lines in H NMR. ID 13C NMR can be used to investigate whether a peptide exhibits a single set of lines or a double (or more) set, which indicate conformational or configurational isomers (see Section 7.5.3). However, ID 13C NMR is rather insensitive and if there is not enough material or the solubility is low, more sensitive techniques have to be applied. 

Figure 9.19—Proton decoupled 13C NMR spectrum of ethylbenzene. Each of the carbon atoms gives a signal consisting of a singlet. These large band decoupled spectra are simpler but contain less information.

For illustration, the proton decoupled 13C NMR spectra of quinoline, obtained from one CW scan and one pulse, respectively, are compared in Fig. 2.18. A further signal noise enhancement of up to 40% arises from application of the QD technique outlined in Section 2.5.4.1 (Fig. 2.18(c)). 

C— lH) into singlets. NMDR spectra are often symbolized by putting the nuclei to be decoupled between brackets besides the nuclei to be observed A X. Proton decoupled 13C NMR experiments are thus referred to as 13C 1H NMR spectra. 

Measurement of Proton-Decoupled 13C NMR spectra with Suppressed NOE... 

The stacked series of selectively proton-decoupled 13C NMR spectra with varying decoupling frequency is conveniently recorded fully computer-controlled, as can be seen for nicotine in Fig. 2.25 (b). The series clearly shows that 13C signals, e.g. those of C-2, C-6, C-4, C-5 and C-4, are unequivocally assigned if the protons attached to these carbons do not overlap with others in the H NMR spectrum. For overlapping proton resonances (e.g. the pairs 2 -H, 5 -H and 3 -H, CH3) more than one carbon is affected by decoupling, of course, and other assignment aids such as two-dimensional CH correlation (Section 2.10) have to be taken into account (e.g. for the pairs C-2, C-5 and C-3, CH3 of nicotine in Fig. 2.25 (b)). 

Fig. 4.18. Proton-decoupled 13C NMR spectra (22.63 MHz 100-150 mg/ mL deuteriochloroform) of guttapercha (trans-) (a) and natural rubber (m-polyisoprene) (b) [73 i].

Comparison of the spectrum of uridine with data of 4-thiouridine, 2,4-dithiouridine, thymidine, 4-thiothymidine, cytidine and 2-thiocytidine allows the assignment of all carbon resonances of these pyrimidine moieties. Further confirmation of these assignments is obtained from the proton off-resonance decoupled 13C-spectra of these nucleosides, as has been demonstrated for thymidine The pyrimidine CH3 and CH resonances of the proton broadband-decoupled spectrum are split into a quartet and a doublet, respectively, in the proton off-resonance spectrum, and can thus be easily assigned. 

A second difficulty of fully decoupled 13C NMR spectra is that die connectivity in the molecule is difficult to establish (except by chemical shift correlation) because coupling patterns are absent. This dilemma is partially resolved by die use of a technique called off-resonance decoupling. In off-resonance decoupled 13C spectra, the carbons are coupled only to diose protons directly attached to diem and die coupling is first order. Thus quaternary carbons are singlets, methine carbons are doublets, methylene carbons are triplets, and methyl carbons are quartets. It is possible to use diis information to establish proton-carbon connectivity,... 

Sketch the proton decoupled 13C NMR spectrum and DEPT spectra for each of the compounds in question 4.1. 

APT is a technique for -decoupled 13C spectra, which uses the phase (normal or upside down) of the 13C peaks as a way to encode information about the number of protons attached to a carbon Cq (quaternary carbon, no protons), CH (methine, one proton), CH2 (methylene, two), or CH3 (methyl, three). These spectra are called edited because the phase (positive absorptive or negative absorptive) is modified relative to a normal 13C spectrum in order to encode additional information. APT gives all of the information of a normal carbon spectrum with somewhat reduced sensitivity, and it tells you whether the number of attached protons is odd (CH3 or CH) or even (CH2 or quaternary). 

The most important, practical application of 13C-n.m.r. spectroscopy is probably the simple characterization and identification of organic compounds. Because of the simplicity of proton-decoupled carbon spectra, and the sensitivity of carbon-13 chemical-shifts towards structural changes, carbon spectra are extremely well suited for this purpose (see, for example, Ref. 84), and it is for this reason that the emphasis of the present article has been placed on presenting chemical-shift data of monosaccharides and their derivatives. Such data are also important for structural studies of oligo- and poly-saccharides,3 and for the investigation of such mixtures as those arising from mutarotation85-87 (see Section 11,4) or from other reactions.34... 

In general, broad-band proton-decoupled 13C-NMR spectra of unenriched samples consist of single resonances for each type of carbon... 

Figure 1. The fully proton-decoupled 13C NMR spectra of amber samples from Simojovel and Totolapa, Chiapas, Mexico, taken on the solid samples with magic angle spinning and cross polarization. The top sample was orange-red and the middle sample yellow.

On the other hand, 13C NMR is extremely effective in characterizing these types of polymers. These spectra not only display well-separated lines for the various structural isomers that may be present, but also provide sequence distribution information and readily allow quantitative analysis. For example, the proton decoupled 13C NMR spectrum of polybutadiene shown in Figure 7-32 has bands that can be assigned to specific toad sequences. We have marked with arrows those that have a central cis unit, like the sequence shown at the top of the figure. 

An important development was the use of broadband decoupling (i.e, irradiation) of protons. Because of the large J values for 13C—H ( 110-320 Hz) and appreciable values for 13C—C—H and 13C—C—C—H, proton-coupled 13C spectra usually show complex overlapping multiplets that are difficult to interpret but... 

Unlike sugar phenylosotriazoles which possess a flexible side chain,130 some cyclitol phenylosotriazoles possess symmetrical structures, as indicated by the simplicity of their proton-decoupled 13C NMR spectra,392,532 Thus the 1H NMR spectra of inositol phenylosotriazoles, 292a, and their esters revealed the presence of a simple, twofold axis of symmetry and the ring protons were symmetrical about a midpoint (see 294), making them examples of four-nucleus AA BB systems. 

Proton decoupled 13C FTnmr spectra obtained on a Brukcr 11FX-90 multi-nuclei spectrometer at 22-63 MHz on 0-2f> M sample, ft Proton decoupled 13C FTnmr spectra obtained on a Brukcr IIFX-90 multi-nuclei spectrometer at 22-628 MHz relative to external TMS. 

Proton decoupled 13C FTnmr spectra obtained on a Bruker HFX-90 spectrometer at 22-63 MHz on 0-1 M samples. 

Figure 5. Plot of peak frequencies in the 1H off-resonance selectively decoupled 13C spectra of NAD+ as a function of position of irradiation in the 1H spectrum, expressed in ppm from internal dioxan. The positions of the peaks in the 1H noise decoupled 13C spectrum are shown by lines on the ordinate and the position of the proton peaks by lines on the abscissa. The arrows t indicate the point of collapse of the 1 3C doublet and the connection between a given 1 3C peak and proton peak. The errors in the position of measurements are indicated by the size of the points except near the cross-over positions where the errors are larger ( 0-15 ppm). (Taken from Birdsall et al., 1972a.)...

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