Oligomers, spectra

The c.d. spectra of a number of other derivatives have been measured as models for the linkages found in oligomers. Spectra of the methyl a-and )8-D-ketopyranosides of N-acetylneuraminic acid are given in Fig. 29. They do not differ substantially from the c.d. spectra for the fundamental sugars, except that the a-ketopyranoside has a positive c.d. band of low intensity at 250 nm. These types of bands occur from time to time for still obscure reasons, as discussed for carboxyl derivatives. 

Shifts between the film photoluminescencc spectra of the various oligomers follow the same trend as found for the solution spectra, but the film spectrum is strongly red-shifted relative to the solution spectrum in each case (Tables 16-2 and 16-5). The phoiolumincscence spectra are strongly influenced by intermolecu-... 

Figure 4-2. INDO/SCI-siuiulaled linear absorption spectrum of the eleven-ring PPV oligomer. The vertical lines represent the oscillator strength of the transitions that are described the site labeling used tor the wavcluiiclion analyses is shown on lop.

In contrast to PEG-1000, the ESR-spectra obtained for labelled grafted PEO-oligomers (N = 1-9) were reported to differ strongly from that for N = 23 and exhibited only slowly moving labels. As the fine width of the fast domain spectrum depends on the mobility of the label, it can be concluded that the tails for N = 23 must consist of at least 4-9 repeated units. It appears that some tails are rather long. 

The oligomers as produced from saponified MHR were isolated using Sephadex G50 and preparative HPAEC. ID and 2D NMR experiments (COSY and ROESY) were used to determine the structure of the smallest oligosaccharide, eluting at 23 min upon HPAEC. The chemical shifts of the assigned peaks in the H NMR spectrum are summarised in Table II. 

Effect of dimer formation on deactivation. Another possible mode of deactivation is formation of inactive Co dimers or oligomers. To test for these species, we examined the ESI-mass spectram of fresh and deactivated Co-salen catalysts in dichloromethane solvent (22). The major peak in the mass spectram occurred at m/z of 603.5 for both Jacobsen s Co(II) and Co(III)-OAc salen catalysts, whereas much smaller peaks were observed in the m/z range of 1207 to 1251. The major feature at 603.5 corresponds to the parent peak of Jacobsen s Co(II) salen catalyst (formula weight = 603.76) and the minor peaks (1207 to 1251) are attributed to dimers in the solution or formed in the ESI-MS. The ESI-MS spectrum of the deactivated Co-salen catalyst, which was recovered after 12 h HKR reaction with epichlorohydrin, was similar to that of Co(II) and Co(III)-OAc salen. Evidently, only a small amount of dimer species was formed during the HKR reaction. However, the mass spectram of a fresh Co(III)-OAc salen catalyst diluted in dichloromethane for 24 h showed substantial formation of dimer. The activity and selectivity of HKR of epichlorohydrin with the dimerized catalyst recovered after 24 h exposure to dichloromethane were similar to those observed with a fresh Co-OAc salen catalyst. Therefore, we concluded that catalyst dimerization cannot account for the observed deactivation. 

Figure 5. Infrared Spectrum of Uncured IP-600 Isoimide Oligomer.

Figure 62 The unreacted (100), partially reacted (101), and completely reacted (102) diacetylene group-containing oligomers in the product from the reaction between 95 and Cp2Mo2(CO)6 (top). The X-ray diffraction spectrum (middle, left), TEM micrographs (middle, right), and conductivity plots (bottom) of the product from the reaction between 95 and Cp2Mo2(CO)6 after pyrolysis to 1000°C. (Adapted from ref. 130.)...

Figure 25 MALDI-TOF mass spectrum from di-hydroxyl end-capped PPG, indicating the presence of hydroxyl and ally end-capped oligomers (see text for description of peak annotation).

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