Carbon polarization spectra

The carbon and DEPT (distortionless enhanced polarization transfer) spectra are shown in Figure 10. The HETCOR (heteronuclear two-dimensional proton-carbon correlation) spectrum is shown in Figure 11. The carbon assignments are listed in Table 5. Long-range HETCOR experiments were used to make the assignments for the thiophene carbons. 

The insensitivity of the DEPT sequence to different JCH coupling constants, as illustrated in Fig. 2.46, makes it useful for editing 13C NMR spectra. To edit a carbon-13 spectrum, three DEPT experiments for the polarization transfer angles... 

Fig. 6. Simulated spectral contributions of different methyl and methylene groups to the expected ( H)- C cross-polarization spectrum of cetyl pamitate. The integral of each single spectrum corresponds to the number of carbon atoms being represented by the particular segment. All simulations are based on the data listed in Table 1.

Figure 14. (a) Absorption polarization spectrum of pyrene-2-carbonic methyl ester (PCME). Solvent 1.85 1 by weight mixture of cholesteryl chloride and cholesteryl laurate 30°C). Concentrations lO M and 3.3. 10 M, respectively. The spectrum PIIE was taken with the linear polarizers P oriented parallel to the electric field E (or parallel to the optix axis L). 

Certain alkyl halides and tosylates undergo E2 eliminations faster when treated with such weak bases as Cl in polar aprotic solvents or PhS than with the usual E2 strong bases such as RO in ROH. In order to explain these results Parker et al. proposed that there is a spectrum of E2 transition states in which the base can interact in the transition state with the a carbon as well as with the p hydrogen. At one end of this spectrum is a mechanism (called E2C) in which, in the transition... 

The APT pulse sequence provides limited information about the number of hydrogens bonded to the carbons in a molecule, since it does not readily allow us to distinguish between the CH, and CH carbons or between CH and quaternary carbons. The INEPT spectrum not only can yield information about the multiplicity of all the carbons, but also affords sensitivity-enhanced C signals due to polarization transfer. 

This broad band at 1500 cm was ascribed by Kaufman. Metin, and Saper-stein [10], to an IR observation of the amorphous carbon Raman D and G bands. This is forbidden by the selection rules, and has been attributed to the symmetry breaking introduced by the presence of CN bonds in the amorphous network. As carbon and nitrogen have different electronegativities, the formation of CN bonds gives the necessary charge polarity to allow the IR observation of the collective C=C vibrations in the IR spectrum. This conclusion was stated by the comparison of spectra taken from films deposited from N2 and N2. In the N2-film spectrum, no shift was observed for the 1500-cm band, whereas all other bands shifted as expected from the mass difference of the isotopes. Figure 25 compares... 

Fig. 47 Carbon-13 solid state NMR spectrum of camphor (50.309 MHz, cross-polarization contact time 5 ms, spin rate 1650 Hz)...

The first of these tools is the distortionless enhancement by polarization transfer (DEPT) pulse sequence. There are a number of versions of this experiment which can be very useful for distinguishing the different types of carbons within a molecule. Of these, we have found the DEPT 135 sequence to be the most useful. In this experiment, the quaternary carbons are edited out of the spectrum altogether. 

A very recent addition to the already powerful spectrum of microwave Heck chemistry has been the development of a general procedure for carrying out oxidative Heck couplings, that is, the palladium)11)-catalyzed carbon-carbon coupling of arylboronic acids with alkenes using copper(II) acetate as a reoxidant [25], In a 2003 publication (Scheme 6.6), Larhed and coworkers utilized lithium acetate as a base and the polar and aprotic N,N-dimethylformamide as solvent. The coupling... 

In addition, the technique of cross polarization introduced and developed by Pines, Gibby and Waugh (9) is used to increase the signal-to-noise ratio of the spectrum. The proton magnetization is spin-locked along the y axis with a spin-locking field % and the carbons subjected to an RF pulse chosen such that the two fields fulfill the Hartmann-Hahn condition (10), equation [3] (Figure 2). 

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