Compound Representation

Figure 9.13—Effect of resonance on carbonyl-containing compounds. Representation of the delocalisation of valence electrons in mesomeric forms of organic compounds. In1 C NMR, the signal corresponding to a carbonyl in an ester is at 165 ppm, whereas it is at 205 ppm for a ketone.

The dimensionality of chemical structure space exceeds that of known biological functional space. The dimensionality of biological functional space has increased in recent years due to the discovery of a multitude of genes, largely from the Human Genome Project. This chapter, however, will focus on chemical diversity rather than functional diversity. Quantification of chemical diversity involves two areas first, the predefmition of a chemical space, accomplished by selection of a diversity metric and a compound representation (i.e., molecular descriptors) and second, a rational subset selection, or classification, method dependent on efficient dimensionality reduction. Here, we describe these methods, prerequisites for a definition... 

There is a wide variety of descriptors available to describe molecules the molecular representation they encode is key to the measurement of diversity. Descriptors directly influence the metrics and algorithms used in the design or analysis, the nature of the chemical space in question, and the location of molecules within the chemical space. Therefore, it is important to select descriptors most appropriate to the problem at hand. Such a selection of the appropriate descriptors is nontrivial, the requirement of compound representation is that it contains enough information to incorporate structure and, in some investigations, function. 

Triazoles with alkyl, aryl or acyl substituents on N-1 or N-4 can be quaternized. Because of the mesomeric distribution of the positive charge on triazolium compounds, representations such as (63) are convenient but the equivalent formula (64) may be used to denote the site at which quaternization has taken place (the mesomeric nature of (64) must be kept in mind of course). Trialkyloxonium tetrafluoroborates are powerful quaternizing reagents (70JOC2246). 

Cotarnine reacts with methyl iodide as though it were a secondary base, there being formed a mixture of cotarnine hydriodide, more properly, cotarninium iodide, and methylcotarnine methiodide. Roser (14) therefore suggested that cotarnine is X and that its yellow salts are quaternary ring compounds representable by XIV, thus accounting for the loss of water in salt formation. Decker (17) preferred the carbinolamine structure XV, since he argued that the secondary amine and the aldehyde functions... 

The dissociation constant for a model compound representation of propagating vinyl ether carbenium ions has been estimated as 10 moldm at — 6°C. For polymerizations in which the initiator concentration is of this magnitude, the propagating ions could be expected to be largely unpaired. Kinetic studies have provided estimates of the rate constants of propagation for such ions. 

Cambridge Structural Database Chemical Abstracts Service Information System Factual Information Databases Inorganic Compound Representation Inorganic Three-dimensional Structure uiatabases Online Databases in Chemistry. 

Electronegativity and the Periodic Table Experimental Data Evaluation and Quality Control Factual Information Databases Inorganic Chemistry Databases Inorganic Compound Representation Internet-based Computationai Chemistry Tools Lanthanides and Actinides Materiais Properties Online Databases in Chemistry Structural Chemistry Application of Mathematics Symmetry in Chemistry X-Ray Crystallographic Analysis and Semiempirical Computations Zeolites Applications of Computational Methods. 

Chemometrics Multivariate View on Chemical Problems Combinatorial Chemistry Factual Information Databases Fuzzy Methods in Chemistry Infrared Data Correlations with Chemical Structure Infrared Spectra Interpretation by the Characteristic Frequency Approach Inorganic Chemistry Databases Inorganic Compound Representation NMR Chemical Shift Computation Ab Initio NMR Chemical Shift Computation Structural Applications NMR Data Correlation with Chemical Structure Online Databases in Chemistry Spectroscopy Computational Methods Standard Exchange Formats for Spectral Data Structure and Substructure Searching Structure Determination by Computer-based Spectrum Interpretation Structure Generators Synthesis Design. 

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