Triad carbon resonances

The Triad Chemical Shift Sequence with Respect to an Increasing Field Strength for Some Representative Vinyl Polymer Backbone Methlne Carbon Resonances... 

More recent work with nitrido clusters within the iron triad revealed chemical shifts in the expected region. For instance, [Ru6N(CO),6] appears at 538 ppm (NH4 ) 91) which can be compared to the carbon resonance of [Ru6C(CO),6] which is at 461.2 ppm (CH4) 112). Although the value is not twice as large as it is at least larger than the carbon resonance. Of the approximately 15 compounds measured, a range from —450 to —620 ppm is observed. Because many of the compounds are closely related some useful trends have appeared. 

Triad, pentad and heptad assignments were developed for the aromatic C-l carbon resonance patterns and triad assignments previously proposed for the methine carbon resonances of polystyrene were verified. Based on the assignments developed, the 13C-NMR spectrum of polystyrene indicates it to be almost atactic, P(m)=0.45. 

The of-CHs resonances in the carbon NMR spectra in PMMA [124] were resolved into three peaks of different chemical shift, one corresponding to each of the triad sequetKes. The fiaction of each triad sequence present in a polymer is th directly proportional to the relative areas of the three peaks. Also other carbon resonances (carbonyl, quaternary carbon and methylene carbon) in PMMA... 

C-13 NMR of copolymers provides information regarding composition of a copolymer, monomer sequence distribution and end chain propagation mechanism (22-24). In the present studies, TCA-ST copolymers were studied at different mole compositions in order to see the effect of mole composition on the splitting pattern. An alkyl ester group of a copolymer displayed certain influence on the chemical shift of the carbonyl and quaternary carbon resonances. Quarternary carbons in ST-ST-ST triad always resonate at higher field than that of TC-TC-TC triad. 

HSQC) and Total Correlated spectroscopy (TOCSY) experiments. The methylene, methine and methyl (A/M and A/G) carbon resonances show both stereochemical (triad level) and compositional (dyad, triad, tetrad, pentad and hexad level) sensitivity. 2D Double Quantum Filtered Correlated Spectroscopy (DQFCOSY) experiment was used in A/B copolymers to assign the complex NMR spectrum to different compositional sequences. 

The next step is to assign carbon-13 spectra using the results of proton assignment. This can be done by CHCOSY. Tliis method coirelates carbon resonance with proton which is directly attached to the carbon. Figure 5 shows the CHCOSY spectrum of PVA. Since we have assigned the proton spectrum, we can easily assign carbon-13 spectra at the triad-tetra level. 

In the carbon-13 NMR spectrum of this system, each of the triads gives rise to a well-resolved signal as shown in Figure 2.4 (A-centred sequence abundances were determined from the nitrile resonances S-centred sequence abundances were obtained from the styrene quaternary aromatic carbon resonances). From the observed triad distributions for each sample, various number-fractions of styrene and acrylonitrile sequences were calculated. For example, which represents the number-fraction of acrylonitrile sequences... 

The low field region shows three methine carbon resonances, peaks 1-3. Referring to Terao [2], and concentrating for the moment only at PV(, peaks 1-3 in the solid state cannot simply be assigned to the tacticity induced splitting of the mm, mr and rr triads respectively the relative area intensities of the three peaks in the solid state are not consistent with the triad tacticity observed in solution [2]. Moreover a significant downfield shift of the resonances 1 and 2 occurs compared with the observed chemical shifts for mm and mr triad resonances in solution, which results in much larger mutual chemical shift differences between the methine carbon resonances in the solid. 

In the assignment of the roethine carbon resonances in the E-VOH copolymers we have to take in account both tacticity effects, as in PVOH, and sequence distribution effects. For the calculation of the chemical shifts in the sequence triads of solid E-VOH copolymer, the chemical shift additivity rules for aliphatic alcohols, as calculated by Ovenall [5] from solution NMR measurements, have been used ... 

Hz), is very close to that (190 Hz) determined for the CF carbon resonances of polydtFS, Since the CF carbon resonances of poly-aFS occur at 6 =92,1 and 6 =101 6 ppm and are not split appreciably by stereosequence effects, it seems that the CF carbon resonance patterns of the copolymers may be useful for sequence distribution measurements CF-carbon resonances of SSS and MSM triads should be observed at (6 =92.4 and 101.9 ppm) and (6 =94.3 and 103.2 ppm), respectively those associated with (MSS + SSM) triads should be observed in the vicinity of 6 =93 and 102,5 ppm. As can be seen in Figure 10, this seems to be the case for the copolymer containing 48 mole percent aFS the observed CF-carbon resonance pattern seems to be consistent with aFS triad fractions calculated for this sample, which are (SSM4MSS) > SSS" ... 

Fig. 10. High field CF-carbon resonance pattern of an a-fluoro-styrene-methyl acrylate copolymer containing 48.1 mole percent a-fluorostyrene, showing possible contributions due to a-fluorostyrene-centered triads.

Table V. Comparison of Methine Carbon Resonance Areas to Triad Fractions Calculated for the Copolymers.

Mole % aFS in Copolymer Relative Methine Carbon Resonance Areas MA-Centered Triad Fractions ... 

These workers discuss, under separate headings, the following types of triads BB, ABA, ABB, BBA, MBM, MBB and BBM under olefinic carbon resonances and BBB, ABA, ABB triads under nitrile carbon resonances. 

The methine proton resonance pattern of poly(vinyl thiophene) is the same as that observed for polystyrene. The lowest field resonance component amounts to 25 percent of the total methine proton resonance and is consistent with poly(vinyl thiophene) being atactic if this lower field component can be assigned to mm triads. Finally, the C-1 carbon resonance spectrum of poly-(vinyl thiophene) (Figure 11) seems to consist of triad resonances, with the lower field area being further subdivided into pentad resonances. This spectrum also suggests that poly(vinyl thiophene) is atactic. If we can conclude on the basis of the results presented here that poly(vinyl thiophene) is indeed atactic, then it seems reasonable to expect the same thing to be true for polystyrene, since the benzene and thiophene rings have very similar steric requirements and similar chemical behavior. 

Several procedures are available for preparing polybutadienes that contain equal amounts of 1,4-cis and 1,2- units [104,105]. The structures of these equibinary" polymers have been investigated extensively by cmr spectroscopy [21,22,106,107]. The olefinic carbons of the 1,4-cis units are particularly useful for determining cis unit centered triad distributions, and some of the aliphatic carbon resonances have provided measures of dyad distributions. The... 

In addition to the copolymer composition, the NMR spectrum can provide information about the triad fractions of both the acrylonitrile and the styrene units in the copolymer. The acrylonitrile triad fractions can be obtained from the splittings in the cyano carbon resonances, and the styrene triad fractions can be obtained from the splittings in the resonance of the quaternary carbon in the styrene side chain, as shown in Figure 2. The experimentally determined triad fractions are given in Figure 4 for all triads over the range of monomer feed compositions, for bulk copolymerization of the monomers at 60 C. 

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