Molecular modelling Raman spectroscopy

This comprehensive review of theoretical models and techniques will be invaluable to theorists and experimentalists in the fields of infrared and Raman spectroscopy, nuclear magnetic resonance, electron spin resonance and flame thermometry. It will also be useful to graduate students of molecular dynamics and spectroscopy. 

For crystals with molecule-like constituents, like the BO, " and BO4 " groups in some borates, semi-quantitative models of the molecular component as a gas-phase entity have been proposed (Oi et al. 1989). This is conceptually similar to the approximation made for species in solution, although in practice most studies of crystals consider additional frequencies that reflect inter-molecular vibrations. The spectroscopic data on these vibrations (which typically have lower frequencies than the intra-molecular vibrations) are often available, at least approximately, from infrared and Raman spectroscopy and elastic properties. This type of hybrid molecule-in-crystal model has been applied to many minerals in theoretical studies of carbon and oxygen isotope fractionation, the most noteworthy being studies of calcite (Bottinga 1968 Chacko et al. 1991) and sihcates (Kieffer 1982). Because specfroscopic dafa are always incomplete (especially for subsfances substifufed wifh rare isolopes), some amounl of vibralional modeling is necessary. 

Raman spectroscopy s sensitivity to the local molecular enviromnent means that it can be correlated to other material properties besides concentration, such as polymorph form, particle size, or polymer crystallinity. This is a powerful advantage, but it can complicate the development and interpretation of calibration models. For example, if a model is built to predict composition, it can appear to fail if the sample particle size distribution does not match what was used in the calibration set. Some models that appear to fail in the field may actually reflect a change in some aspect of the sample that was not sufficiently varied or represented in the calibration set. It is important to identify any differences between laboratory and plant conditions and perform a series of experiments to test the impact of those factors on the spectra and thus the field robustness of any models. This applies not only to physical parameters like flow rate, turbulence, particulates, temperature, crystal size and shape, and pressure, but also to the presence and concentration of minor constituents and expected contaminants. The significance of some of these parameters may be related to the volume of material probed, so factors that are significant in a microspectroscopy mode may not be when using a WAl probe or transmission mode. Regardless, the large calibration data sets required to address these variables can be burdensome. 

In 1961 a molecular model of a (7/2) helix [Figure 6(a)] was proposed by the author and his coworkers (13.) based on the information from x-ray, infrared, and Raman spectroscopy, but the crystal structure could not be determined at that time. After ten years, owing to the development of methods and apparatus, especially the constrained least-squares method and a vacuum cylindrical camera with a radius of 10 cm, the crystal structure has been determined as shown in Figure 6(c) (22.). The internal rotation angles are considerably distorted from the uniform helix, although the molecular conformation is essentially the (7/2) helix and close to the TTG sequences. 

By combining all of the possible TEM modes, we are able to explain the behavior of carbonaceous materials primarily in terms of the local molecular orientations established in the final stages of liquid-phase pyrolysis. The models established from these observations are supported by the results of other techniques, such as infrared analyses (33), optical microscopy (27), X-ray diffraction (24), and Raman spectroscopy (22). 

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