Molecular structure elucidation

A major part of work in a chemical laboratory, both in research and industry, deals with analytical challenges. Important problems are separation of mixtures into pure compounds, and elucidation of their structures. Often, determination of a compound s constitution is the foremost aim, though a true identification of a compound should also include determination of its stereochemistry, i.e. its configuration and possibly conformation. We group this together under molecular structure elucidation. 

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In contrast to the well-established methods for identifying and quantifying naturally occurring chlorophylls, very few reports concern quantitative analysis of chlorophyllin copper complexes in color additives and in foodstuffs. Analytical methods proposed are based on spectral properties, elemental analysis, chromatographic separation, and molecular structure elucidation or a combination of these procedures. 

M.E. Elyashberg, Y. Karasev, E.R. Martirosian, H. Thiele and H. Somberg, Expert systems as a tool for the molecular structure elucidation by spectral methods, strategies of solution to the problem. Anal. Chim. Acta, 348 (1997) 443 64. 

Elyashberg ME, Martirosian ER, Karasev YuZ, Thiele H, Somberg H (1997) X-Pert a user-friendly expert system for molecular structure elucidation by spectral methods. Anal Chim Acta 337 265... 

Molecular structure elucidation, principally by single crystal X-ray diffraction, has become almost routine and is now available for the majority of the metal amides presently discussed. In the 1980 book, however, such data were provided for just 112 compounds, 54 of which were for d- and/-block metals and 41 for the Group 13 metal amides. The contrast with the developing situation is illustrated by reference to Group 1 metal amides from four X-ray data sets in 1980 there were more than 200 by the end of 2007. 

Several compounds of interest have had their crystal and molecular structures elucidated using x-ray analysis. 7-Methylbenzothieno[3,2-c][l,2,5]oxadiazole 1-oxide (3) was found to align itself in the crystal lattice so that the benzene ring of one molecule faces the 1,2,5-oxadiazole oxide moiety of another, thus superposing the least number of atoms <75CSC13>. 

Computer Methods of Molecular Structure Elucidation from Unknown Mass Spectra... 

Almost every NMR book begins with a discussion of basic theory. Short, elementary accounts of theory are given in such books as Nuclear Magnetic Resonance by P. J. Hore28 and Introduction to NMR Spectroscopy by R. J. Abraham, J. Fisher, and P. Loftus,29 as well as in books mentioned later that are directed toward biological applications or molecular structure elucidation. 

Elyashberg, M.E., et al., X-Pert A User-Friendly Expert System for the Molecular Structure Elucidation by Spectral Methods, Anal. Chim. Acta, 337, 265, 1997. 

Besides the most abundant isotope (98.89%) there are also the isotope C (1.11%) and the radioactive C (traces). The molar weight of carbon (12.011 gmol ) results from this isotopic composition. " C is employed to determine the age of archeological objects (radiocarbon dating). The stable isotope is a valuable tool for molecular structure elucidation by NMR spectroscopy because its nuclear-spin quantum number is I =... 

Biotechnology is one of the most influential sciences at the end of this century and the analytical control of biotechnological processes in laboratories and pilot and industrial plants Is therefore essential. Automation is a boon to the five analytical techniques most frequently employed In this field, namely electrophoresis, Immunoassay, chromatography, protein and DNA sequencing, and molecular structure elucidation. 

The interpretation that is hest (i.e., correct) is often the simplest. This simplicity is perhaps the most important guiding precept for molecular structure elucidation. The hest course is to initially explore all possibilities and then reduce the manifold options to the simplest explanation for all evidence. This admonition is a restatement of Occam s razor, which is... 

Some researchers went one step farther towards molecular structure elucidation by including C into the coupling network. Szalma and Pelczer proposed the... 

Acta, 141 (1995). Molgen-F, a Generator of Connectivity Isomers and Stereoisomers for Molecular Structure Elucidation. 

Expert systems have been extensively applied in many branches of analytical science, and in a number of noteworthy cases (generally involving molecular structure elucidation from spectroscopic data) such applications have led to the development of the technology. In addition to organic, molecular spectroscopy automated spectral interpretation systems have also been developed for X-ray diffraction. X-ray fluorescence, and, as advisors for instrument optimization, for atomic absorption spectrometry. 

X-ray diffractometry, which requires only a single crystal of the analyte for molecular structure elucidation, is also used for the solution of the most difficult problems in the investigation of steroids. This is illustrated by atlases containing over 400 steroid structures. The potential of this technique in the investigation of receptor binding of steroid hormones merits special mention. 

Molecular structure elucidation. Computer-aided structure elucidation (CASE) uses algorithms that construct all mathematically possible structural formulas for a given molecular formula and optional structural restrictions (often obtained from a spectrum). This has to be performed efficiently and without redundance (i.e. no duplicates allowed). Virtual spectra can be calculated for generated structures and compared with the experimental spectrum to rank the generated structure candidates. The corresponding algorithms that we need for such a formula-based structure generation will be described. 

In Chapter 6 we describe the basic principles of supervised statistical learning and show how it can be used in computer chemistry when a causal connection between structure and property is not known, or can only be calculated with extremely high effort. Such problems occm quite often in combinatorial chemistry as well as in molecular structure elucidation. 

We used the steuidard valences Vx for RC. These numbers were also used in [194] and [237] in order to derive an expression for the plausibility of a molecular formula in molecular structure elucidation. 

Inverse QSPR/QSAR is the search for compounds exhibiting prescribed prop-erty/activity values. Molecular structure elucidation aims at identifying an unknown compound, i.e. deducing its molecular structure from measured physicochemical properties, most often spectra. 

Example In molecular structure elucidation the observations are pairs of spectra and compounds. Predictors used are spectral predictors, functions that map spectra onto real numbers. The target variable is, for example, a binary molecular descriptor of a structural property SP, equal to 1 if a compound has property SP, and equal to 0 otherwise. The search is for a function able to predict whether or not the corresponding compound has property SP for a given spectrum. We will calculate such predicting functions in Section 8.5. 

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