Molecules, vibrational spectroscopy molecular crystal

Raman spectroscopy of the solid state differs from that of gases or liquids. A fluid is usually considered to be an assembly of noninteracting, randomly oriented molecules. In contrast, in a solid, i.e., a molecular crystal, the molecules have essentially fixed orientation with respect to the crystal axis. Thus, the molecules lose the rotational and translational degrees of freedom found in the free molecule. In the molecular crystal, these degrees of freedom are replaced by so-called external vibrations torsional motions of the molecule about its axis at the lattice site (librations) and restricted translational excursions within the lattice site. The external vibrations give rise to many new features in the low-frequency region of the spectrum. In addition, the vibrational bands seen for the free molecule (the internal vibrations) can be perturbed in the crystal. These vibrational bands may be split, for example, due to the symmetry of the crystal site, interactions with other molecules in the unit cell, or interaction between the vibrations of the free molecule and the external vibrations in the crystal. 

Since the oxides do not have to be isolated, the sulfur solution after addition of the peroxyacid solution is simply kept in the refrigerator until S,(, has formed which is then isolated by cooling and recrystallization When both Sg and S g are dissolved in CS and the solution is cooled, then, under special concentration conditions, a new sulfur allotrope crystallizes out as orange-yellow opaque crystals of m.p. 92 °C. This compound has been shown by vibrational spectroscopy and X-ray structural analysis to consist of equal amounts of Sg and molecules in their usual conformations. In solution the mean molecular weight of 258 corresponding to 8 atoms per molecule indicates complete dissociation This is the first example of an allotrope of a chemical element consisting of molecules of different sizes. 

These analytical dilemmas interfere with the methods of alkaloid analysis. Each group of alkaloids has its own methods of extraction, isolation and crystallization, as well as detection in structure, molecule and dynamicity. Not all these stages are still possible in the majority of alkaloids. In recent years, many techniques have been used in alkaloid detection. There are atomic and molecular electronic spectroscopy, vibration spectroscopy and electron and nuclear spin orientation in magnetic fields, mass spectroscopy, chromatography, radioisotope and electrochemical techniques. Although important developments in methodology and... 

Vibrational Spectroscopy [Infrared (mid-IR, NIR), Raman]. In contrast to X-ray powder diffraction, which probes the orderly arrangement of molecules in the crystal lattice, vibration spectroscopy probes differences in the influence of the solid state on the molecular spectroscopy. As a result, there is often a severe overlap of the majority of the spectra for different forms of the pharmaceutical. Sometimes complete resolution of the vibrational modes of a particular functional group suffices to differentiate the solid-state form and allows direct quantification. In other instances, particularly with near-infrared (NIR) spectroscopy, the overlap of spectral features results in the need to rely on more sophisticated approaches for quantification. Of the spectroscopic methods which have been shown to be useful for quantitative analysis, vibrational (mid-IR absorption, Raman scattering, and NIR) spectroscopy is perhaps the most amenable to routine, on-line, off-line, and quality-control quantitation. 

Such considerations are of considerable importance in the study of oriented molecules, as in a crystal, but are less important for liquids and gases, in which the molecular orientation is random. However, in the case of Raman spectroscopy, not all information about the components of a is lost by orientational averaging. In particular one finds that it is possible to distinguish totally symmetric from other molecular vibrations from a measurement of the depolarization ratio,... 

In the weakly anharmonic molecular crystal the natural modes of vibration are collective, with each internal vibrational state of the molecules forming a band of elementary excitations called vibrons, in order to distinguish them from low-frequency lattice vibrations known as phonons. Unlike isolated impurities in matrices, vibrons may be studied by Raman spectroscopy, which has lead to the establishment of a large body of data. We will briefly attempt to summarize some of the salient experimental and theoretical results as an introduction to some new developments in this field, which have mainly been incited by picosecond coherent techniques. 

In the following sections of this chapter, we first treat intramoleular vibrations then, in much more detail, phonons using typical examples and finally, very briefly, stochastic rotational motions ( reorientations ) and translational diffusion of molecules. Although the experimental methods for the characterisation of dynamics in molecular crystals are in principle no different from those used to investigate inorganic crystals, we shall briefly describe inelastic neutron diffraction, Raman scattering, infrared and far-infrared spectroscopy, as well as NMR spectroscopy to the extent necessary or useful for the specific understanding of molecular and lattice... 

Molecular conformations are determined by a wide variety of experimental techniques. Conformations of molecules in crystals may be determined by X-ray diffraction, and molecules in the gas phase may be investigated by electron diffraction and microwave spectroscopy. NMR, vibrational spectroscopy, UV spectroscopy, and circular dichroism studies are also useful. Other techniques include calorimetry and the determination of physical properties such as pK values and dipole moments. " In addition, conformation may be inferred from kinetics of reactions of functional groups that may be in different conformational environments. ... 

Polarization effects are another feature of Raman spectroscopy that improves the assignment of bands and enables the determination of molecular orientation. Analysis of the polarized and non-polarized bands of isotropic phases enables determination of the symmetry of the respective vibrations. For aligned molecules in crystals or at surfaces it is possible to measure the dependence of up to six independent Raman spectra on the polarization and direction of propagation of incident and scattered light relative to the molecular or crystal axes. 

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