Structure elucidation with infrared spectra

This simple fact is the major advantage of the database approach — which is also the basis for the modeling approach — against conventional spectra catalog searches. This approach is designed for use with an expert system and user-supplied databases. It generally provides a fast prediction within 1 to 20 seconds and a higher success rate than any other automated method of structure elucidation with infrared spectra. 

In problems of structure elucidation an NMR spectrum may provide useful, even vital data, but it is seldon the sole piece of information available. A knowledge of the source of the compound or its method of synthesis is frequently the single most important fact. In addition, the interpretation of the NMR spectrum is carried out with concurrent knowledge of other physical properties, such as elemental analysis from combustion or mass spectral studies, the molecular weight, and the presence or absence of structural features, as indicated by infrared or ultraviolet spectra or by chemical tests. Obviously, the procedure used for analyzing the NMR spectrum is highly dependent on such ancillary knowledge. 

Structure elucidation with an infrared spectrum cannot be performed simply by interpreting the individual peaks one after another. As a result, we are in need of some mechanism that is able to find patterns in infrared spectra or to use the entire spectrum as pattern, like it is done in the conventional infrared software to identify a compound. If we are not able to obtain a structure from the details, we can either try to use patterns or even the entire data set for comparison. This is how infrared spectra are typically used as pattern for structure elucidation. 

Interhalogen Compounds. Several of these compounds have been investigated by the infrared method and their structures elucidated. The compound IF5 illustrates how Raman and infrared spectra can together aid in structure determination. The Raman spectrum of the liquid and the infrared spectrum of the gas [ ] were measured. Lord et al. compared their observations with the predicted number of frequencies for seven possible symmetries for the molecule (Table I). 

The similarity of the ultraviolet spectrum of 4,5-diphenyloxazol-2-one (91) with those of both alternative methyl derivatives preclude application of the spectral comparison method to the elucidation of their structures, but the fluorescence spectra of these compounds indicate that 91 exists in the oxo form. ° Infrared data for a number of substituted oxazol-2-ones support this conclusion. ... 

Mass spectrometry is an analytical technique to measure molecular masses and to elucidate the structure of molecules by recording the products of their ionization. The mass spectrum is a unique characteristic of a compound. In general it contains information on the molecular mass of an analyte and the masses of its structural fragments. An ion with the heaviest mass in the spectrum is called a molecular ion and represents the molecular mass of the analyte. Because atomic and molecular masses are simple and well-known parameters, a mass spectrum is much easier to understand and interpret than nuclear magnetic resonance (NMR), infrared (IR), ultraviolet (UV), or other types of spectra obtained with various physicochemical methods. Mass spectra are represented in graphic or table format (Fig. 5.1). 

Infrared and Raman spectroscopy are in current use fo r elucidating the molecular structures of nucleic acids. The application of infrared spectroscopy to studies of the structure of nucleic acids has been reviewed,135 as well as of Raman spectroscopy.136 It was noted that the assignments are generally based on isotopic substitution, or on comparison of the spectrum of simple molecules that are considered to form a part of the polynucleotide chain to that of the nucleic acid. The vibrational spectra are generally believed to be a good complementary technique in the study of chemical reactions, as in the study76 of carbohydrate complexation with boric acid. In this study, the i.r. data demonstrated that only ribose forms a solid complex with undissociated H3B03, and that the complexes are polymeric. 

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