Methylene resonance

Randall used C-NMR to study the methylene spectrum of polystyrene. In 1,2,4-trichlorobenzene at 120°C, nine resonances were observed. These were assumed to arise from a combination of tetrads and hexads. Using m and r notation, extend Table 7.8 to include all 20 possible hexads. Criticize or defend the following proposition Assuming that none of the resonances are obscured by overlap, there is only one way that nine methylene resonances can be produced, namely, by one of the tetrads to be split into hexads while the remaining tetrads remain unsplit. 

Isomer ratios of mixtures of 5//-dibenz[c,e]azepines have been determined by integration of the C7 methylene resonance signal in their HNMR spectra.48... 

It is hence easy to detect by this technique different polymorphic forms having different chain conformations. For instance, the a or p forms of s-PS (tram-planar chain conformations) present only a single methylene resonance at 48.1 ppm (vs.TMS), while the y form (helical conformation) presents two methylene resonances at 37.3 and 47.3 ppm (Fig. 20) [114]. 

Differences in the. solid-state NMR signals of crystalline forms having identical conformations have been also observed. For instance, well-crystallized a form samples of i-PP show splittings for the methyl (22.6, 22.1 ppm) and methylene resonances (45.2, 44.2 ppm) into two lines with relative intensities 2 1 [117,118]. These splittings have been interpreted in terms of the known crystalline packing of the a form, which is characterized by pairs of 3/1 helices of opposite handedness at closer distances (Fig. 10). This generates inequivalence between the carbons indicated as A and those indicated as B in Fig. 10 [117,118]. 

ACCORD-ADEQUATE spectrum using a 500-MHz spectrometer equipped with a 5-mm cryoprobe. The data were acquired as 180 hypercomplex points in the second frequency domain using 256 transients/fi increment. The broad 14-vinyl methylene resonance was located in the structure based on correlations in the ACCORD-ADEQUATE spectrum from H14 to C13 and from H12 to both Cll and C13. The C18 aromatic methine resonance afforded ADEQUATE correlations to the flanking C17 and C19 non-protonated carbons and, finally, the C23 methine provided a correlation to the C22 non-protonated carbon. 

The 5 Hz optimized l,n-ADEQUATE spectrum of strychnine (1) is shown in Figure 9.70 The high resolution 600 MHz proton spectrum is shown in Figure 10A and compared to slices extracted at the Fi frequency of the C15 methylene resonance in the 5 Hz optimized l,n-ADEQUATE spectrum (B) and the 60 Hz optimized 1,1-ADEQUATE spectrum (C). The adjacent carbons (via Vcc) show correlations in the 5 Hz optimized INADEQUATE spectrum that are unsuppressed but all possible three-bond... 

B) Slice taken through the 5 Hz optimized INADEQUATE spectrum of 1 at the F1 shift of the C15 methylene resonance. (C) Comparison slice taken through the 60 Hz optimized 1,1-ADEQUATE spectrum of 1 at the Fn shift of the C15 methylene resonance. 

Figure 21 Expansion of the aliphatic region of the HSQC-1,1-ADEQUATE spectrum of the CDK-2 inhibitor dinaciclib (48). The connectivity network is traced out for the 2-(P-hydroxyethyljpiperidine moiety contained in the structure. Methylene resonances are inverted and plotted in grey methine and methyl resonances have positive intensity and are plotted in black.

Fig. 10.2. Expansion of a portion of the proton NMR spectrum of strychnine (1 inset structure). The full proton spectrum is shown in Fig. 10.1. The resonances for the H22 vinyl proton and the H12 and H23 oxygen-bearing methine and methylene resonances, respectively, are shown. The inset expansion of the H23 methylene protons shows a splitting diagram for this resonance. The larger of the two couplings is the geminal coupling to the other H23 resonance and the smaller coupling is the vicinal coupling to the H22 vinyl proton.

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

Fig. 10.16. (A) GHSQC spectrum of strychnine (1) using the pulse sequence shown in Fig. 10.15 without multiplicity editing. (B) Multiplicity-edited GHSQC spectrum of strychinine showing methylene resonances (red contours) inverted with methine resonances (black contours) with positive phase. (Strychnine has no methyl resonances.) Multiplicity-editing does have some cost in sensitivity, estimated to be 20% by the authors. For this reason, when severely sample limited, it is preferable to record an HSQC spectrum without multiplicity editing. Likewise, there is a sensitivity cost associated with the use of gradient based pulse sequences. For extremely small quantities of sample, non-gradient experiments are preferable.

Figure 34 shows spectra of TA and SOL from a 24-year-old male volunteer recorded at 1.5 T (a) and 3.0 T (b). Besides the clearly improved SNR, which is elevated by a factor of 1.7 to 1.8 at 3.0 T, distinct differences can be observed for the two magnetic field strengths In both muscles, IMCL and EMCL are clearly better separated at 3.0 T as the methylene resonance of IMCL shows smaller natural linewidths (in ppm). However, EMCL signals remain with a broad lineshape in TA as well as in SOL, since the lineshape is dominated by susceptibility induced static field inhomogeneities. Crs and TMA signals are... 

G. Babbitt, Allied Chemical Corp., N.J. How do you know that a methylene resonance was under the methyl signals in the spectrum of hydrochlorinated 1,4-polydimethylbutadiene ... 

The C-13 spectrum of polystyrene, shown In Figures 2 and 3, contains two regions where stereochemical Information can be extracted. There are nine methylene resonances and at least 20-22 aromatic quaternary carbon resonances. No other carbons In polystyrene e lblt a configurational sensitivity. Tentative assignments have been made for the methylene carbons based on an assumed Bernoulllan behavior (IJ). 

Exceptions do exist as shown in the spectrum of polyacrylonitrile in Figure 6. The methine resonances show a distinct triplet with very little splitting exhibited by the methylene resonances. The greatest sensitivity toward configuration occurs for the nitrile resonances where an almost ideal Bemoulllan distribution is observed. ... 

I was wondering more about the physical problem of obtaining areas, for example, cutting and weighing, when some of the methylene resonances may be truncated. 

Fig. 20. Plot of increase in methylene resonance against the decrease in the epoxide group75)...

Primary alkyl isocyanides are known not to comply simply to homopolymerization. It was recently reported that polyisocyanides prepared with nickel(II) compounds varied in color from yellow to black (22,23). The latter occurred at preparative temperatures in excess of 25° C, or when polymer non-solvents or acids were used. Further, the addition of acids to solutions or suspensions of the yellow polymers also led to black compounds. The NMR spectrum of a soluble polymer sample (i.e. MVPO = 1100) of black poly(ethyl isocyanide) shows methylene resonance shifts to values as occur in nitriles, which were interpreted as giving evidence of rearrangement to a polycyanide... 

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