Infrared data, phase structure

The recent advent of infrared microspectroscopy (10) suggested the possibility that infrared absorbance data could serve as the basis for analysis. It seemed unlikely that the absorbance of vater in a liquid crystal phase vould be anisotropic to the same degree as is refractive index. Further, infrared data could possibly provide qualitative information as to phase structure - a kind of information not contained in refractive index data (11). 

In addition to providing data on the compositions of coexisting phases, infrared data is also of potential value in providing structural information about phases. The mid-IR spectral region, particularly the CH-stretching and -bending regions, has been extensively used for this purpose (17-18). The near-IR bands ve observe between 4500 and 4000 cur"1" are the CH bend-stretch combination bands. It seemed likely that their band positions and intensities could also be sensitive to phase structure. 

The molecular structural data are from the compilation of Sutton (1 ). Infrared data (1, j, for vapor phase MoOgCl ... 

Stanners et al. [29] used VSFS to study the structure of the alcohols at the liquid I air interface. For low-molecular-mass alcohols, namely, methanol to butanol, the alcohol molecule is oriented with the hydroxy group in the liquid and the alkyl group pointing into the gas phase. This orientation is attributed to the effects of hydrogen bonding involving the hydroxy group and other alcohol molecules in the bulk. The infrared data involved both the -OH and -CH stretch-... 

In practice, one usually defines a training set of molecules and associated experimental properties and fits the relevant data with an assumed force field.The next step is to test the results on molecules and data outside the training set. Experimental data that depend on the energy surface and that may be used to determine the parameters of the intramolecular interactions consist mainly of gas-phase structural data derived from microwave spectra or electron diffraction patterns, crystal structures derived from X-ray and inelastic neutron scattering measurements, and vibrational frequencies obtained from infrared and Raman spectra. Torsional barriers are derived from NMR band shapes and relaxation times, whereas conformational energies are determined from spectroscopic and thermochemical data. Nonbonded parameters are determined mainly from... 

Infrared spectra are now widely used in the examination of pharmaceuticals. The sixteenth revision of The Pharmacopoeia of the United States (U.S.P.) and the eleventh edition of the National Formulary (N.F.) have presented identification tests which used infrared spectroscopy, whereas no infrared tests were used in U.S.P. XV or N.F. X. Infrared spectra have attained acceptance in legal considerations and are now given in patent applications as characteristics of antibiotics of unknown structure. In the pharmaceutical industry there are many applications for quantitative infrared analyses in research and development work, pharmacy research, and in various phases of pharmaceutical production. For example, infrared data are used to characterize reaction conditions and yields, to assay the purity of intermediate products, to examine such problems as the stability of a drug in the material in which it is suspended, and to maintain quality control in the chemical production of bulk drugs. A recent review (Papendick et al, 1969) has given many references to fractionation and isolation methods for pharmaceutical analysis, such as the various types of chromatography, electrophoresis, countercurrent distribution, and extraction. The authors presented many references to infrared analyses for a wide variety of compounds (Table 16.1). 

In the different calcium phosphates, the hydrogen-bonded O—O distance, obtained from infrared data, decreases from phase to phase as a function of acidity. This is corroborated by X-ray diffraction data, which indicate that the phosphate groups in a more acid calcium phosphate occupy a higher percentage of the structural volume, resulting in shorter hydrogen-bonded O—O distances. Table 19.11 presents... 

In 1994, Villegas and Weaver [114] reported an elegant combined STM, and in situ ETIR study of CO adsorption at the Pt(lll) electrode surface this is a key paper in the area of in situ infrared spectroscopy. The work essentially showed that, in CO-saturated aqueous HCIO4, the spatial structure of the compressed CO adlayer undergoes a phase transition from a close-packed (2 x 2)-3CO adlayer, (0co = 0.75), at potentials <0 V versus SCE, to a ( 19 x yi9)R23.4°-13CO unit cell structure, (0co = 13/19), at potentials between 0 V and the onset of CO oxidation at ca. 0.25 V. The STM data were supported by the infrared data in terms of the changing distribution of linearly adsorbed CO, (COl), and CO adsorbed at doubly bridging, (COb), and threefold hollow sites, (COh). 

In order to derive structural information from infrared frequencies, input is required from quantum chemical calculations at computational levels which match the experimental resolution. Experimentally, gas-phase conditions imply extremely low sample densities, requiring special techniques in order to acquire infrared data. Some of those techniques involve double resonance approaches which provide unique opportunities for isomer selective IR spectroscopy. This facet is among the advantages of gas-phase experiments, making it possible to follow certain properties, such as excited state dynamics, as a function of molecular structure. At the same time, the availability of gas-phase data provides opportunities to calibrate computational methods, force fields, and functionals. 

The transient infrared absorption spectra of the co-ordinatively unsaturated Cr(CO) species [x = 5, 4, 3 and 2), generated via excimer laser photolysis of gas phase Cr(C0)6, have been described. The results indicate that the gas-phase structures of these species are similar to those observed in condensed-phase experiments. Reactions, including some kinetic data, have been reported on several of these species. " Several photolyses experiments have been carried out on Fe(C0)5, " and rate constants for the reaction of Fe(C0)p (n = 4, 3 or 2) with CO determined. The generation and some reac-... 

Fourier transform reflection-absorption infrared (RA-IR) spectroscopy is used to probe the structure and properties of sodium dodecyl sulfonate (C12S) monolayers that are self-assembled from dilute solution at an air-water interface. Recent optical models for the interpretation of signal intensity measurements are briefly reviewed. The methylene stretching peaks of C12S monolayers in the RA-IR spectra are used to determine die chain orientation, the surface concmtration and the conformational state of the alkyl chains. Conqiarisons are drawn between monolayers and C12S crystals. A phase transition is found as the concentration of C12S in the subphase below the monolayer is reduced. The effect of salt on the monolayers is presented. The infrared data is interpreted in terms of the surface tension behavior. 

Spectra of carbenes are very useful sources of information on the structure of the free carbenes, e,g. the R—C—R angle, or the multiplicity of their lowest state. However, these data were mostly obtained under conditions different from those in solution, where chemical reactions normally occur. The spectra are usually recorded either in matrices at low temperatures, say at 4 or 77 °K, or in the gas phase. Only very few investigations of that type have been carried out in solution. The most important spectroscopic technique used in the investigations of carbenes is ESR. Other spectroscopic methods, such as flash photolysis which produces electronic spectra of carbenes, and infrared and lately CIDNP spectroscopy have been successfully employed. 

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