CH3 resonance

DSC 31 The signals of the carbons within the crystalline phase have the same chemical shifts as at 303 K. The signals of the carbons of the amorphous phase changed as can be seen most obviously at the CH3 resonances. Figure 17 shows the CP-MAS 13C-NMR spectra of the threodiisotactic poly(l,2-dimethyltetramethylene)... 

Fig. 10. 220 MHz proton NMR spectra of solutions of polypropylene, (a) Isotactic polypropylene broad bands between 8.62 and 8.8 r, others hidden by CH3 resonances in the region of 9.0 and 9.25 r (85). (b) Syndiotactic polypropylene a single set of peaks between 8.8 and 9.0 r (85). (c) Soluble polypropylene obtained by polymerization with Zr(benzyl)4 (38).

Methylcarborane is an air-stable, white crystalline solid which is soluble in common organic solvents. The infrared spectrum (Nujol mull) contains major absorption bands at 3.90 (s), 8.83 (w), 9.12 (w), 9.69 (w), 9.83 (w), 10.03 (w), and 13.85 (s) p. The proton nuclear magnetic resonance spectrum of a carbon tetrachloride solution of methylcarborane contains a broad —CH resonance of intensity 1 at 6.52 r and a —CH3 resonance of intensity 3 at 8.02 r. 

If the substrate is enantioselective, as for example meta-methylstyrene, the satellites split into (at least) two separate resonances, respectively, for the CH2- and the CH3-resonances of the resulting product meta-methyl-ethylbenzene (Fig. 12.20). 

Fig. 12.22 CH2- and CH3-resonances observed in the -PHIP-NMR spectrum of the intermediate attached to an achiral catalyst during the hydrogenation of styrene.

Of the multitude of ID 13C NMR experiments that can be performed, the two most common experiments are a simple broadband proton-decoupled 13C reference spectrum, and a distortionless enhancement polarization transfer (DEPT) sequence of experiments [29]. The latter, through addition and subtraction of data subsets, allows the presentation of the data as a series of edited experiments containing only methine, methylene and methyl resonances as separate subspectra. Quaternary carbons are excluded in the DEPT experiment and can only be observed in the 13C reference spectrum or by using another editing sequence such as APT [30]. The individual DEPT subspectra for CH, CH2 and CH3 resonances of santonin (4) are presented in Fig. 10.9. 

For acetamido resonemces, a similar upward shift of about 0.1 ppm can be expected when going from CH—NHAc to CfCHs)—NHAc >. That this in fact is observed, is demonstrated by the data summarized in the table. The C—CH3 resonances of the compounds listed vary considerably within a rather broad range from 8.22—8.68 t, thus excluding any stereochemical deductions from their chemical shift differences. However, the methyl protons of the acetamido groups at the bremching point in compounds No. 1—11 appear within the very narrow range of 0.05 ppm (8.15—... 

All this data has prompted various authors to present a discussion of P-C-H couplings as a function of the molecular geometry and more especially of the dihedral angle H-C-P or H-C-P=X. (1967,77 1968,30) A more complete analysis has been presented by Albrand et al. (1968,62 1969,76) (Fig. 4) to improve the Karplus-like description of 27(P-C-H) values for trivalent phosphorus. As is seen zero coupling is possible with a dihedral angle of about 45° the CH and CH3 resonances appear as singlets in 21(1966,42) and 22(1967,78 1969 77) respectively. ... 

Figure 9-33 Schematic proton nmr spectra X and Y are nonmagnetic nuclei. For 2-propane derivatives, as at the top, the CH3 resonances are double because of the splitting produced by the single proton on C2. For the ethane derivatives, the right set of lines is always a triplet when observable because of the two protons of the X—CH2— group. We assume here that the chemical shifts of the CH Y3 protons are independent of the number of Y substituents.

Lyerla et al. measured Tic over a wide temperature range from room temperature down to 105 K [94], and concluded that Tic s of not only CH3 but also CH resonances depend on CH3 rotational motion, and that the broadening of the CH3 resonance below -100 °C is also due to modulation of CH3 rotational motion at the frequency of proton nutation in the presence of the decoupling field. Gomez et al. have also reported solid-state high-resolution 13C NMR spectra of isotactic polypropylenes [95]. They used samples characterized by X-ray crystallography and reconfirmed the results obtained by Bunn et al. 

Fig. 5. DNMR spectrum of the N-CH3 resonances of lFe(CH3, Bzdtc)313F4 in CD, Cl, solution. From Ref.36 ...

The use of aqueous chiral lanthanide complexes in the determination of the enantiomeric purity of chiral a-hydroxy acids has also been assessed by H NMR [21], Large lanthanide induced shifts, chemical shift non-equivalence and an apparent absence of kinetic resolution in complex formation is observed upon addition of racemic lactate to [Yb.3a]3+ (Figure 1). The lactate CH3 resonances are clearly resolved for the... 

R and S diastereomers (AA5 = 10 ppm) and experience a large lanthanide induced shift with the CH3 resonating at +21 and +31 ppm respectively for (It)- and (5)-lactate respectively (c.f. +1.3 ppm in the free form). The large shifts exhibited by this unique chiral derivatising agent may be compared to the much smaller AA8 values reported for shift reagents which are generally of the order 1 ppm, and often <0.1 ppm. 

Fig. 13. Left-, (a) ID CP spectrum (b) SS-APT spectrum with r = 4.5 ms and (c) SS-APT spectrum with r = 6 ms of cholesteryl acetate. Right Expansion of the spectra shown on the left between 12 and 45 ppm. The assignments are indicated above the peaks CH and CH3 groups can be distinguished by the fact that the CH resonances give intense negative peaks for r = 4.5 ms, that diminish in intensity for r = 6 ms, whereas the opposite effect is observed for CH3 resonances the CH2 groups give peaks that are weakly positive (like those around 27 ppm) or even null (like those around 30 ppm), whereas the quarternary carbons give intense positive resonances. (Taken from Lesage et al.201 with permission.)...

Steric complications arise when the acceptor together with C2 forms a nine-membered ring. Hafner and coworkers88 89 have studied 10,10-bis(dimethylamino)methylene cyclo-nonatetraene (40) and found that the compound participates in a solvent-dependent equilibrium between a twisted form with low polarity (40a, Amax = 330 nm), favored by nonpolar solvents, and a nearly planar dipolar form (40b, Amax = 403 nm), favored by polar solvents and low temperature. The N—CH3 resonance splits into a doublet... 

FIGURE 6.5 H NMR spectrum of acetaldehyde. Right CH3 resonance, with the two spin states of the aldehyde proton indicated. Left, CHO resonance, with the four spin orientations of the methyl protons indicated. 

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