Chain-stretching mode

The remaining bands must arise, therefore, from vibrations of the infinite chains. In the 500-600-cm region, the observed infrared (600 and 500 cm" ) and Raman bands (518 and 526 cm" ) are attributed to chain stretching modes. Chain-square-plane deformational (381 cm" ) and torsional-rotational modes (184, 124 cm" ) are also seen in the Raman spectra. 

The presence of methylenic bands shifted at higher frequency in the very early stages of the polymerization reaction has also been reported by Nishimura and Thomas [114]. A few years later, Spoto et al. [30,77] reported an ethylene polymerization study on a Cr/silicalite, the aluminum-free ZSM-5 molecular sieve. This system is characterized by localized nests of hydroxyls [26,27,115], which can act as grafting centers for chromium ions, thus showing a definite propensity for the formation of mononuclear chromium species. In these samples two types of chromium are present those located in the internal nests and those located on the external surface. Besides the doublet at 2920-2850 cm two additional broad bands at 2931 and 2860 cm are observed. Even in this favorable case no evidence of CH3 groups was obtained [30,77]. The first doublet is assigned to the CH2 stretching mode of the chains formed on the external surface of the zeolite. The bands at 2931 and... 

One major interest in vibrational surface spectroscopy is the ability to directly probe lipid layers. Similarly to the previous case, the structure of the alkyl chains of phospholipids is readily determined from the ratio of the magnitude of the CH2 and CH3 symmetrical stretching modes [136,137]. At the D2O-CCI4 interface, a layer of... 

Intramolecular chain transfer, 20 220 Intramolecular cycloacylations, 72 177 Intramolecular self assembly, 20 482 Intramolecular stretching modes, 74 236 Intraoperative auto transfusion, 3 719 Intraparticle mass transfer, 75 729-730 Intrapellet Damkohler number, 25 294,... 

The Kieffer model correctly predicts the systematic change of the reduced partition functions of various minerals with structure, as indicated by Taylor and Epstein (1962). For anhydrous sihcates, the decrease in the sequence framework-chain-orthosilicate reflects the decreasing frequency of antisymmetric Si-O stretching modes. The internal frequencies of the carbonate ion give a high reduced partition function at all T. The value for rutile is low because of the low frequencies of the Ti-0 modes (Kieffer, 1982). 

It has been suggested [Tadokoro, Seki, and Nitta (218) Tadokoro (215b)] that this band arises from a symmetric C—C stretching mode, which would be consistent with its polarization parallel to the zig-zag chain, but this assignment presents difficulties too. In particular, it places such a skeletal mode at what seems to be much too high a frequency, and the skeletal vibration involved, viz., i>+(0), is already adequately assigned to the band at 915 cm-1. 

Fig. 15.4. Averaged Raman spectra from within the SC (top) and lipid (bottom) regions as noted in Fig. 15.3b and c as factors 1 and 2, respectively, a The 590-780cm-1 region with bands at 605 and 700cm-1 assigned to cholesterol as marked, b The 800-1380cm-1 region displays three major bands (1058, 1130, and 1296 cm-1) characteristic for ordered (all-bans) lipid chains, c The C-H stretching (2800-3000 cm-1) region with symmetric (2850 cm-1) and asymmetric (2880 cm-1) methylene stretching modes noted. The frequencies of the bands are consistent con-formationally ordered chains...

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