Intermolecular crystalline splitting

For polypropylene (PP), on the other hand, it has not been possible to observe any IR splittings resulting from intermolecular or crystalline packing. In fact, there are many features in the PP spectrum that look like pairs of bands, and indeed they are. However, the band pairs arise from intramolecular helical splitting, not from intermolecular crystalline splitting. Actually, intermolecular packing influences the IR spectrum of PP very little. In Fig. 4.42, the X-ray patterns of the monoclinic a phase and the smectic S phase are shown. The monoclinic a phase consists of well-ordered crystalline 31 helices, while the smectic S phase consists of 31 helices that are out of register with each other. The smectic phase will convert with time to the... 

The infrared absorption spectra of the same polymer in the crystalline and amorphous states may differ because of the following two reasons (i) Specific intermolecular interactions may exist in the crystalline polymer which lead to sharpening or splitting of certain bands and (ii) Some specific conformations may exist in one but not the other phase, which may lead to bands characteristic exclusively of either crystalline or amorphous material. For example in polyethylene terepthalate), the 0CH2CH20 portion of each repeat unit is restricted to the all trans-conformation in the crystal, but... 

Chandross and Ferguson64 find that the absorption spectra of dimers, produced65 by photolytic cleavage of photodimers of anthracene and mono-derivatives in a rigid methylcyclohexane glass at 77°K, are consistent with a symmetrical sandwich configuration these dimers also emit the characteristic excimer fluorescence. On the other hand, it is necessary to assume a 60° rotation of one component about the intermolecular axis of the 9,10-di-chloroanthracene dimer (as in the crystalline compound) to account for the observed resonance splittings of both absorption bands.64... 

Not only can molecules constituting the oriented phase be thus studied. Solutes present in this mesophase will also be oriented through operation of anisotropic intermolecular forces. An example is that of deuterium-labeled D gramicidin. When dissolved in a nematic mesophase, it displays a series of deuterium doublets. Their residual splittings 6v are almost temperature-independent. This points to a rigid structure for the peptide, an helix that reorients about the director of the liquid crystalline phase (18). 

Berkowitz and Wahl9 have reviewed the experimental and theoretical estimates of the dissociation energy of molecular fluorine. The Raman and far-i.r. spectra of crystalline F2 show that in this state the element resembles 02 more closely than it does the other halogens.10 The intermolecular forces, in particular, are extremely weak, as exemplified by the small shifts of the internal frequencies from their gas-phase values, the absence of observable factor-group splitting of the fundamental and overtones, and the low value of the external (lattice) vibrations. 

MHz) VT/ MAS solid state spectra DP 1700, 7600 and 15,500 (Kuraray Co.) phase structure of single crystals from triethylene glycol TMS standard CH resonance splits into four peaks at 77.5 (two intra H-bonds) 71.5 (one intra h.bond) 65.0 (no intra H-bond) and 62.4 (intermolecular H-bond) frachon of OH groups with intra H-bond is 0.35 for crystalline domains decreases from 0.66 (DP 1700) to 0.44 (DP 15,500) in noncrystalline regions (60)... 

Infrared bands should only be designated as crystalline bands if they disappear on melting, are predicted from group theory, and X-ray or other data proves the polymer to be crystalline. Only in a few rare cases have true crystalline bands been observed for example, the 720-730 cm CH2 rocking doublet of crystalline PE is a reflection of a genuine crystallinity effect, as the splitting results from intermolecular interaction of segments in the orthorhombic unit cell (37). More frequently, reported crystalline bands are associated with a preferred crystalline conformation that may also be present (in lower concentration) in the noncrystalline phase. 

Under the conditions of elastic deformation a specimen of a simple crystalline solid suffers a definite deformation when a definite load is applied the deformation does not depend on the duration of application of the load and disappears completely when the applied forces are removed. When the strain is small the stress and strain are linearly related. Elastic behaviour can be readily explained in terms of the intermolecular forces existing within a crystal, as discussed in section 2.10, based on the assumption that the force between neighbouring planes of molecules in a simple crystal varies with the separation of the planes in qualitatively the same way as the force between a pair of molecules, shown schematically in Fig. 1.2(a). This figure also indicates that if F reaches the value —the two molecular planes would fly apart and the crystal would separate into two parts. The applied force equal to —Fj, would be the tensile force needed to produce rupture between neighbouring molecular planes. In macroscopic terms, the solid would deform elastically in tension and then suddenly split into two parts, a process known as brittle fracture. A number of materials, e.g. rock salt and bismuth, behave approximately in this manner at room temperature. 

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