Vibrational spectroscopy crystallinity

More recently, Raman spectroscopy has been used to investigate the vibrational spectroscopy of polymer Hquid crystals (46) (see Liquid crystalline materials), the kinetics of polymerization (47) (see Kinetic measurements), synthetic polymers and mbbers (48), and stress and strain in fibers and composites (49) (see Composite materials). The relationship between Raman spectra and the stmcture of conjugated and conducting polymers has been reviewed (50,51). In addition, a general review of ft-Raman studies of polymers has been pubUshed (52). 

There has been considerable discussion about the extent of hydration of the proton and the hydroxide ion in aqueous solution. There is little doubt that this is variable (as for many other ions) and the hydration number derived depends both on the precise definition adopted for this quantity and on the experimental method used to determine it. H30" has definitely been detected by vibration spectroscopy, and by O nmr spectroscopy on a solution of HF/SbFs/Ha O in SO2 a quartet was observed at —15° which collapsed to a singlet on proton decoupling, 7( 0- H) 106 Hz. In crystalline hydrates there are a growing number of well-characterized hydrates of the series H3O+, H5O2+, H7O3+, H9O4+ and H13O6+, i.e. [H(0H2) ]+ n = 1-4, Thus... 

This last reaction is typical of many in which F3CIO can act as a Lewis base by fluoride ion donation to acceptors such as MF5 (M = P, As, Sb, Bi, V, Nb, Ta, Pt, U), M0F4O, Sip4, BF3, etc. These products are all white, stable, crystalline solids (except the canary yellow PtFe ) and contain the [F2CIO] cation (see Fig. 17.26h) which is isostructural with the isoelectronic F2SO. Chlorine trifluoride oxide can also act as a Lewis acid (fluoride ion acceptor) and is therefore to be considered as amphoteric (p. 225). For example KF, RbF and CsF yield M [F4C10] as white solids whose stabilities increase with increasing size of M+. Vibration spectroscopy establishes the C4 structure of the anion (Fig. 17.29g). 

Since the vibrational spectra of sulfur allotropes are characteristic for their molecular and crystalline structure, vibrational spectroscopy has become a valuable tool in structural studies besides X-ray diffraction techniques. In particular, Raman spectroscopy on sulfur samples at high pressures is much easier to perform than IR spectroscopical studies due to technical demands (e.g., throughput of the IR beam, spectral range in the far-infrared). On the other hand, application of laser radiation for exciting the Raman spectrum may cause photo-induced structural changes. High-pressure phase transitions and structures of elemental sulfur at high pressures were already discussed in [1]. 

Both vibrational spectroscopies are valuable tools in the characterization of crystalline polymers. The degree of crystallinity is calculated from the ratio of isolated vibrational modes, specific to the crystalline regions, and a mode whose intensity is not influenced by degree of crystallinity and serves as internal standard. A significant number of studies have used both types of spectroscopy for quantitative crystallinity determination in the polyethylenes [38,74-82] and other semi-crystalline polymers such as polyfethylene terephthalate) [83-85], isotactic poly(propylene) [86,87], polyfaryl ether ether ketone) [88], polyftetra-fluoroethylene) [89,90] and bisphenol A polycarbonate [91]. 

The frequencies of absorption bands present gives diagnostic information on the nature of functional groups in materials as well as information from any observed frequency shifts on aspects such as hydrogen bonding and crystallinity. In many cases, spectra can be recorded non-destructively using either reflection or transmission procedures. IR spectra of small samples can also be obtained through microscopes (IR microspectrometry). Chalmers and Dent [8] discuss the theory and practice of IR spectroscopy in their book on industrial analysis with vibrational spectroscopy. Standard spectra of additives for polymeric materials include the major collection by Hummel and Scholl [9]. 

The period under review has seen a small, but apparently real, decrease in the annual number of publications in the field of the vibrational spectroscopy of transition metal carbonyls. Perhaps more important, and not unrelated, has been the change in perspective of the subject over the last few years. Although it continues to be widely used, the emphasis has moved from the simple method of v(CO) vibrational analysis first proposed by Cotton and Kraihanzel2 which itself is derived from an earlier model4 to more accurate analyses. One of the attractions of the Cotton-Kraihanzel model is its economy of parameters, making it appropriate if under-determination is to be avoided. Two developments have changed this situation. Firstly, the widespread availability of Raman facilities has made observable frequencies which previously were either only indirectly or uncertainly available. Not unfrequently, however, these additional Raman data have been obtained from studies on crystalline samples, a procedure which, in view of the additional spectral features which can occur with crystalline solids (vide infra), must be regarded as questionable. The second source of new information has been studies on isotopically-labelled species. 

Although X-ray crystallography, NMR, and circular dichroism are extremely valuable techniques for determining the structure of crystalline proteins or proteins in solution, they cannot be used to study proteins adsorbed on surfaces. Vibrational spectroscopy (infrared and Raman) appears to be the best approach available for bridging the gap between adsorbed proteins and proteins in solution. 

The presence of monomeric H3BO3 and B(OH)7 in aqueous solutions has been confirmed by spectroscopic techniques. Infrared (126, 415-417) and Raman (176,247) spectra of boric acid solutions show similar absorptions to crystalline H3BO3 (176), for which a planar BO3 arrangement has been found (446). The monoborate ion BtOHlJ has similarly been identified by vibrational spectroscopy (119, 161, 176) its expected tetrahedral structure has been confirmed by comparison of its spectra with that of teepleite NaB(OH)4 NaCl (213, 342) and bandy-lite Cu[B(OH)J2 CuCl (342), which are known to contain monomeric tetrahedral B04 units. 

Vibrational spectroscopy has proved to be the most useful technique yet for identifying which boron species are present in solution. Close agreement between solution spectra and those of crystalline borates of known structure have confirmed the presence of hitherto postulated polyborate ions. Details of the IR spectra of the NajO-BaOs-H O system at 26°C with absorptions assigned to polyborate species are shown in Table III (416, 417). In a more recent study (126), the major ions in 0.5 M boron solution were identified as BsOefOH), B405(0H)4 , and B(OH)4-. 

Photo-oxidation leads to the formation of carbonylic products and this is classically monitored by vibrational spectroscopy. To investigate the relation between the accumulation of the oxygenated photoproducts and the change in the crystallinity of polycyclooctene, the decrease of the heat of crystallization was compared with the rise of the concentration of carbonyl function (1721 cm 1 band) as displayed in Figure 10.9. The enthalpy of crystallization falls at early stages of irradiation before significant accumulation of the carbonyl. Assuming that the decrease of the... 

Vibrational spectroscopy shows that inclusion of cholesterol in phospholipid bilayers tends to decrease the fluidity of the hydrophobic region above the main transition point Tm and to increase it below Tm. The presence of cholesterol in DPPC or DMPC muti-layered vesicles does not affect the transition point but simply broadens the transition by decreasing the CH2-stretching wavenumber in the liquid crystalline phase and by increasing it in the gel-like phase (Lippert and Peticolas, 1971 Spiker and Levin, 1976 Casal and Mantsch, 1984). There is also evidence that lipid-cholesterol interaction increases the amount of bound water in the headgroups (Levin et al., 1985). 

The effect of toxins such as melittin (from the honey-bee venom), myotoxin a, and cardiotoxin (from the snake venom) was investigated by vibrational spectroscopy (Pezolet et al., 1982 Faucon et ah, 1983 Liddle and Tu, 1985 Lafleur et ah, 1987). Monitoring the Raman intensity ratio I(1060)/I(1080) indicated that the lipid/melittin assemblies in DPPC are characterized by a high conformational order, little intermolecular chain-chain interaction, and a low cooperativity of the gel-like liquid crystalline phase transition. The effect of ricin, a toxic lectin, on DPPC and DPPC-cerebroside mixtures was studied by Raman and IR spectroscopy. It was suggested that ricin mainly interacts with the interfacial domains of the bilayers (Picquart et ah, 1989). 

Commonly, the vibrational spectroscopy covers a wavenumber range from 200 to 4000 cm-1. We should know that crystalline solids also generate lattice vibrations in addition to molecular vibrations. The lattice vibrations refer to the vibrations of all the atoms in crystal lattice in a synchronized way. Such vibrations exhibit lower frequencies compared with those of common molecular vibrations and have a wavenumber range of about 20-300 cm-1. Coupling between lattice and molecular vibrations can occur if the molecular vibrations lie in such a low wavenumber range. Molecular vibrations can be distinguished from the lattice vibrations because they are not as sensitive to temperature change as lattice vibrations. 

To address this gap in understanding, the surface specific nonlinear vibrational spectroscopy, sum frequency generation (SFG) has been applied to study the hexagonal ice surface, Ih. The Ih crystalline form is chosen as the focus of this work because it is the stable form of ice for ambient conditions on Earth. Further, the most abundant exposed face is the hexagonal or basal face. The second most abundant face consists of the cylinder sides, also called the prism face. This work examines both of these common faces. 

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