Methyl carbon resonance

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

CPMAS spectrum typical of the regular twofold helical conformation (Figure 2.45b).147 191 Samples of sPP which present X-ray powder diffraction patterns typical of the isochiral form II (Figure 2.45c) show a different solid-state 13C NMR spectrum with additional signals in the region of the methylene and methyl carbon resonances (Figure 2A5d). These additional resonances have... 

A number of cis/trans 4,6-dialkyl-2,2-dimethyl-l,3-dioxanes were studied by C NMR spectroscopy (93JOC5251). The C NMR shifts of C -Me groups (Scheme 8) were found to be very sensitive to the 1,3-dioxane conformation [chair form Me(ax) ca. 19 ppm and Me(eq) ca. 30 ppm— pure 30.89 ppm in the twist-boat form both methyl carbons resonate at ca. 25 ppm (pure 24.70 ppm)]. With these values, AG° of the chair to twist-boat equilibrium was calculated (Table IV). For 13a (nitrile), 13b (alkyne), and 13e (methyl ester) (Scheme 8) in CH2CI2, the temperature dependence of the AG° values was determined. Depending on the substituent, small negative or positive entropy terms were found generally the enthalpy term dominates the -AG° value. In the tram isomers 13, the cyano and alkyne substituents favor the chair conformation, but CHO, ester, alkene, and alkyl substituents, respectively, clearly favor the twist-boat conforma-... 

Diastereomeric 1,3-diols 5 could be easily identified by the equivalence and nonequivalence of the geminal methyl carbon resonances in the meso- and <7,/-form, respectively415. 

If the radical were restricted to resonance between the KekulA structures A and B, with the free valence on the methyl carbon, resonance would stabilize the radicals to just the same extent as the undissociated molecules, which would then have only the same tendency to dissociate as a hexaalkylethane. But actually the five structures A, B, C, D and E (each with three double bonds) contribute about equally to the structure of the radical, which thus resonates among five structures instead of two and is correspondingly stabilized by the additional resonance energy. 

Inversion of ring A causes significant shifts of the carbon signals of rings A and B in steroids. In 5/5-cholestane, the C-19 methyl carbon resonates about 12 ppm towards... 

The COSY spectrum for caryophyllene oxide can be understood more clearly when interpreted in conjunction with the information from an HMQC spectrum (Figure 5.17). From the DEPT spectrum (see Figure 5.15), we already know that caryophyllene oxide has three methyl carbon resonances (16.4, 22.6, and 29.3 ppm), six methylene carbon resonances (26.6,29.2, 29.5, 38.4, 39.1, and 112.0 ppm), three methine carbon resonances (48.1, 50.1, and 63.0 ppm) and three quaternary carbon resonances (33.3,59.1, and 151.0 ppm)... 

The 13C spectra of several sydnones and sydnonimines have been determined (74JCS(P2)875). In 3-methylsydnone the IV-methyl carbon resonance is at 39.8 p.p.m. and the ring carbons 4 and 5 resonate at 96.8 and 169.2 p.p.m., respectively. These data and those for other sydnones have been correlated with ring structure. 

Interrupted-Proton-Decoupling Experiments. Interrupted-pro-ton-decoupling experiments were carried out with a 50- xs interruption. This technique selects quaternary carbons (those lacking an attached proton) and rapidly moving methyl carbons. Resonances from methylene and methine carbons are suppressed unless, for some reason, their motion within the solid occurs at a rate similar to that of methyl groups. In essence, the interrupted-proton-decoupling experiment, which has not been previously reported for amber samples, provides an alternative fingerprint for the samples. 

In Fig. 11.1(a), which shows the a-form, both methylene and methyl carbon resonances are splitted by approximately Ippm [2, 3j. The ratio of intensities of the high to the low frequeney eomponent is 2 1 for both earbon... 

Table 21.2. Chemical shifts and peak intensities of methyl carbon resonances in DD/MAS NMR spectra... 

The experimental lineshapes of the carboxyl and methyl carbon resonances of fully enriched L-Alanine have been studied in detail at different MAS frequencies and decoupling field strengths. Complex lineshapes at intermediate spinning speeds were explained by the joint effect of off rotational resonance and coherent CSA-dipolar cross-correlation. It was found that coherent CS A-dipolar cross-correlation introduces either a differential intensity and/or a differential broadening of the lines of the J-multiplet. The conditions that lead to such effects were explained and experimentally verified. Additional simulations showed that these effects can be expected over a wide range of static magnetic fields and are not restricted to L-Alanine. 

Of the nuclei, H and which both possess nuclear spin and are common to synthetic polymers, C is by far the more sensitive spin probe for polymer NMR studies. NMR spectra suffer neither from a narrow dispersion of chemical shifts (see Figs. 20.8 and 20.9) nor firom extensive homonuclear spin-spin (scalar) coupling, both of which complicate the analyses of H NMR spectra. (See below how two-dimensional observations increase the sensitivity of H NMR to molecular microstructures.) It is the sensitivity of resonance frequencies or chemical shifts, to the microstructures of polymers which makes NMR so useful as a structural probe. We noted in Fig. 20.9 that the methyl carbon resonances observed in the 25 MHz C... 

NMR spectrum of atactic PP were sensitive to pentad stereo-sequences. At 90.5 MHz (see Fig. 20.10), the methyl carbon resonances show sensitivity to heptad stereosequences (rmnmmmm, rrrrrr, mrmmrr, etc.) [13]. The NMR spectra of PPs are sensitive to stereosequences extending over 4 (pentads) and 6 (heptads) bonds in both directions along the PP backbone. This long-range sensitivity to microstructural detail makes NMR a valuable tool in the determination of polymer structures. 

The NMR parameters for the Pt-methyl group provide an interesting contrast when compared to those of the formyl group. The methyl carbon resonance is found at 0.48 ppm, with /(Pt, C) expected to be in the range 620-710 Hz. 

Comparison of the methyl resonances in P-VC and PP reveals a decreased sensitivity to stereosequence for the P-VC copolymer. The methyl carbon resonances in P-VC are sensitive to pentad stereosequences, whereas in PP heptad sensitivity is observed. In Table 2.6 the C chemical shifts calculated for the methyl carbons in several heptad stereosequences of P-VC and PP are compared. As observed, the methyl carbon chemical shifts calculated for P-VC are sensitive to pentads, but PP methyl carbons show significant heptad sensitivity. This difference in stereosequence sensitivity between the methyl carbons in P-VC and PP is directly attributable to differences in their conformational behavior as embodied in their RIS models. Local bond conformations reflect pentad sensitivity in P-VC and heptad dependence in PP. In addition, note that the overall spreads in methyl carbon chemical shifts observed in P-VC and PP are 2.7 and 2.0 ppm, respectively, with the P-VC methyl carbons resonating about 1 ppm upfield from those in PP. These observations are also reproduced by the calculated chemical shifts, which employ the same y-effect (ycHj.cH = Ppm)... 

Since all of the methods discussed above have been applied to analysis of the methyl carbon resonances in polypropylene, the assignment of these resonances will be covered in detail to illustrate the methods. Nine methyl carbon resonances are observed (Figure 7), the lowest field one of which has the same chemical shift as the methyl carbons in isotactic polypropylene and... 

In another approach to assign the methyl carbon resonances of polypropylene, ZanJDelli and coworkers [58]... 

The CP-MAS C-NMR spectra of the different forms of iPP are shown in Fig. 9.4 (Tabel 9.1) [28-30]. Both the methylene and methyl carbon resonances in the a form are split by approximately 1 ppm, but the methine resonance shows only a shoulder. 

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