Spectral Properties and Structure Identification

Once the wavefunction is in hand, all observable properties can, at least in principle, be computed. This can include all varieties of spectral properties, which are particularly valuable in ascertaining molecular structure. The full theoretical and computational means for computing spectral properties are mathematically involved and beyond the scope of this chapter. Instead, we will focus on the use of quantum chemical computations to help identify chemical structure. The purpose of this chapter is to inspire the routine use of computed spectra to aid in structural identification. 

As discussed in Chapter 1, the full three-dimensional structure of a compound can be optimized with almost any of the quantum computational techniques. Since most quantum chemical computations are still performed on a single molecule in the gas phase, these computed structures can be most readily compared to gas-phase experimental structures. In the following chapters, we will present a number of case studies where computed and experimental geometries are compared. To get a sense of the quality of computed geometries, a few selected cases are discussed next. 

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COMPUTED SPECTRAL PROPERTIES AND STRUCTURE IDENTIFICATION TABLE 2.13 Boltzmann-Weighted Values of [a]n for 28 and 29... 

Thus, a more complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR, Raman, UV and esr spectroscopic methods are mutually complementary. Since IR spectroscopy is the most informative method of identification of matrix-isolated molecules, this review is mainly devoted to studies which have been performed using this technique. 

The results described in this review show that matrix stabilization of reactive organic intermediates at extremely low temperatures and their subsequent spectroscopic detection are convenient ways of structural investigation of these species. IR spectroscopy is the most useful technique for the identification of matrix-isolated molecules. Nevertheless, the complete study of the spectral properties and the structure of intermediates frozen in inert matrices is achieved when the IR spectroscopy is combined with UV and esr spectroscopic methods. At present theoretical calculations render considerable assistance for the explanation of the experimental spectra. Thus, along with the development of the experimental technique, matrix studies are becoming more and more complex. This fact allows one to expect further progress in the matrix spectroscopy of many more organic intermediates. 

Beside the main contaminants chlorobenzo(b)thiophene (one isomer) was also tentatively identified. Since reference material is commercial not available the identification based mainly on the gas chromatographic and mass spectral properties. First structural evidence for this compound was... 

During the GC/MS screening analyses several still unknown contaminants were detected. Consequently, structure elucidating analyses were performed on two different groups of compounds both appearing in the third extracts. Thus, polar and acidic properties have to be stated for these substances. The identification of the individual compounds was based mainly on the interpretation of their mass spectral properties and subsequent verification of the proposed molecular structures by synthesized reference material. 

A number of physical tests emphasizing stress-strain behavior will be covered in Chapter 14. Here, we will concentrate on other areas of testing, emphasizing thermal and electrical properties and on the characterization of polymers by spectral means. Spectroscopic characterization generally concentrates on the structural identification of materials. Most of these techniques, and those given in Chapter 14, can also be directly applied to nonpolymeric materials such as small organic molecules, inorganic compounds, and metals. 

The spectral properties of ethylene oxides are among the most important, not only for the information derivable from them concerning tbe intimate structure of the three-merabered oxide ring, but also in connexion with the detection and identification of this function in complex molecules of unknown constitution, e.g. natural product. The present review is concerned with the following three types of gpectroBoopy A) infrared spectroscopy, ( ) ultraviolet spectroscopy, and ( 7) nuclear magnetic resonance spectroscopy,... 

RDF descriptors exhibit a series of unique properties that correlate well with the similarity of structure models. Thus, it would be possible to retrieve a similar molecular model from a descriptor database by selecting the most similar descriptor. It sounds strange to use again a database retrieval method to elucidate the structure, and the question lies at hand Why not directly use an infrared spectra database The answer is simple. Spectral library identification is extremely limited with respect to about 28 million chemical compounds reported in the literature and only about 150,000 spectra available in the largest commercial database. However, in most cases scientists work in a well-defined area of structural chemistry. Structure identification can then be restricted to special databases that already exist. The advantage of the prediction of a descriptor and a subsequent search in a descriptor database is that we can enhance the descriptor database easily with any arbitrary compound, whether or not a corresponding spectrum exists. Thus, the structure space can be enhanced arbitrarily, or extrapolated, whereas the spectrum space is limited. 

However, they represent a group of still unnoticed riverine contaminants, interestingly, substituted with bromine atoms. Thus, also structure elucidating analyses were performed on this substances. These compounds were identified by comparison of their analytical data with those of reference material in case of the mono-substituted contaminants. The identification of the dibrominated compounds is based on their mass spectral properties as well as on their gas chromatographic elution in comparison with the monobrominated compounds. In the following the mass spectral, gas chromatographic and IR spectroscopical properties are illustrated (Fig. 6 to 9) and discussed shortly. 

An additional 14-membered ring peptide alkaloid, scutianine-B (58 R = = CH2Ph), has been isolated from Scutia buxifoliaP Its structure rests on the typical mass and other spectral data and on the identification of the usual amino-acid fragments obtained by acidic hydrolysis. Amphibine-A, an alkaloid isolated from Ziziphus amphibia, appears to be identical with discarine A [58 R = /i-indolyl-CH2, R = CH(Me)Et] on the basis of comparison with the reported physical, spectral, and chemical properties. ... 

In these oxides, the 6s and 5d and, to a lesser extent, the 4f electrons of the rare earth atom are mainly responsible for electrical transport and structural properties, whereas the localized 4f electrons govern the magnetic properties. X-ray absorption spectroscopy (XAS) offers the important advantage of simultaneously probing the 4f and the ds conduction states in these oxides. In XAS, the dipole selection rules are strictly obeyed and this facilitates the identification of the spectral features. Generally, the 3d—>4f (Mjv-v) or 4d—>4f (Niv-v) absorption transitions are studied. In these absorption processes the excited 4f electron participates directly in the transition. The resulting multiplet structure is observed to provide a finger-print of the 4f population of the rare earth atom. The modification in the valence band electron distribution introduced by the delocalization of a 4f electron is probed by the transition of a 2p (Ln-m) electron in the vacant sd conduction states. In this case the 4f electron does not participate direct in the transition. 

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