1,721,079 research outputs found
Lilith: From childhood to adolescence
No full textComputer assisted organic synthesis is the best to date effort for the understanding and the optimization of the synthesis planning activity which represents one of the most exciting and funny tasks for a chemist. Several approaches have been developed in past years,1 each one contributing ideas and theoretical achievements which resulted in several beautiful works. Our contribution to the field, named Lilith, is still one of the youngest, but nevertheless it has come time for her to grow a little more, leaving childhood for adolescence. In her young age Lilith acquired some important attributes which qualified her behavior (a strategy, a reactivity analysis, a first level of evaluation capability); now she is going to complete her power improving her possibility of controlling operations and results. In this perspective a completely new and powerful nucleus of the program is being studied; it mainly consists of a true expert system whose tasks are (1) the control of the work flow, (2) the analysis and the evaluation of intermediate and final results, and (3) the optimization of the application of the IAIA (initial approximation increasing accuracy) principle.2 Here its first version is reported with particular attention to the analysis of the up-to-data achievements in the field of the characterization of its theoretical skeleton
Reaction classification by similarity: the influence of steric congestion
No full textRecently, we introduced a new methodology that can classify reactions using similarity measures. The classification has the following properties: is hierarchical, is property based, uses similarity. We are now considering the addition of steric demand: a fast methodology for calculating steric hindrance is presented, the obtained descriptor is used in the calculation of a steric similarity index between reactions, and it is used inside the existing classification scheme to group together either reaction with similar steric demand, or reactions with both similar electronic and steric properties
Classification of Polar and Nonpolar Aquatic Pollutants Using Simple Descriptors. Differences between Polarity Prediction and Narcosis Classification
No full textSimilarity Measures: Is It Possible To Compare Dissimilar Structures?
No full textThe determination of the similarity between dissimilar compounds is a problem only partially solved. The necessity of defining unconnected description spaces to evaluate the diversity of chemical structures is an important drawback of many existing methodologies. Our objective is thus the definition of a method capable of making acceptable comparisons between any kind of structures. The definition of a virtual similarity index, indirectly calculated from standard similarity analyses, and the selection of a proper representation space allow the calculation of two new similarity indexes: the global and the local similarity indexes. The performance of the method is discussed by its application to three sets of compounds at different complexity levels
A new definition of functional groups and a general procedure for their identification in organic structures
No full textFunctional groups (FGs) have always been used to describe structures and reactivities of organic molecules, therefore their univocal identification is important. Current approaches are often subject to limits and errors, using descriptions that are either too approximated or too rigid. A new definition, and the corresponding search procedure, based on the calculation of a molecular descriptor is introduced and applied to some molecules of different complexity. Results show a consistent set of FGs for each structure. They are divided into two classes - first level FGs and second level FGs - that describe interactions either among atoms (first level) or among groups of atoms (second level)
Empirical atomic charges: a 3D approach
No full textA novel empirical method of calculating atomic charges in 3D structures is presented as an extension of our previous topological approach. It is based on the use of atomic electronegativity and covalent radius in order to control the electronic distribution of the molecule. Some examples are discussed in detail and the results compared with those obtained by quantum mechanical approaches. Merits and inadequacies are described, focusing on the sensitivity of the method to the distribution of atoms in 3D space and on its capability of calculating feasible charges
Question of data format in organic chemistry
No full textOrganic chemistry of necessity has developed mechanized information retrieval. The problem of characterizing the format of the compounds' structures and reactivity is examined. Current and future trends are given with comments and critical observations. Information about both informatic and chemical aspects of data collection is treated from both user and producer viewpoints. Techniques and tricks in structuring and maintaining data are given
Reaction prediction: the suggestions of the Beppe program
No full textThe Beppe project represents a modeling experiment that aims at contributing a better management of the prediction of reaction products. From this perspective, a completely theoretical system has been initially studied, independent of the result control given by experimental data. The connected program realized until now can evaluate the different aspects of the molecular reactivity; consequently, it can generate some of the reaction products starting from a molecular ensemble and give a set of numerical rules to estimate both the result reliability and the possibility to get different results using different experimental procedures
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