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Evolution of Quantum Similarity Measures: How They Perform in Modeling Hammett Constants

Sadlej-Sosnowska N.

Polish Journal of Chemistry

2009

Vol. 83, nr 12

2215-2224

This study was aimed at comparing a few measures of similarity of carboxylic acids’ molecules, based on the electronic descriptors derived from molecular wave functions. Five series of molecules were investigated: benzoic acids, thiophene-2-carboxylic acids, furane-2-carboxylic acids, trans-cinnamic acids, and 3-phenylpropionic acids. It has been previously found that in stead of comparing densities or other electronic properties of the entire molecules, it is sufficient to use the properties within a molecular fragment, active towards a given reaction. In this vein, three measures have been compared: the one pertaining to the properties of bond critical points, the one derived from atomic potentials, and that based on Fermi-hole densities, confined to the COOH group. Never the less, it appeared that other ways of defining a similarity measure may also be practical, namely that based on charges of the COOH group and that based on the properties of another molecular fragment, i.e. the substituent and the ring carbon atom connected to the so-called substituent active region. The calculations were carried out at the B3LYP/cc-pVDZ level.

Correlations of Charges of Substituent Active Region in C-substituted Tetrazoles with Their Gas-phase Acidity, Hammett Constants, and Charge Distribution in Tetrazolic Rings

Sadlej-Sosnowska N., Murłowska K.

Polish Journal of Chemistry

2008

Vol. 82, nr 4

935-945

Electron distribution was investigated in molecules of 5C-substituted tetrazoles by means of Natural Population Analysis. The analysis of the impact of substituents on the excess of p electrons in the ring and on the uniformity of their distribution produced similar results as were recently reported for derivatives of p-benzoic acid. The significant correlation coefficients of the two properties with the Hammett constants have been found. The more uniform the p electron distribution, measured by means of standard deviation of p electrons amounts localized on individual atoms in the ring, the more acidic is the molecule. On the other hand, an increase of the p electron number in the ring brings about an opposite effect. No correlation was found between the substituent charges and the Hammett constants. However, a new variable, namely charge of the substituent active region, composed of charge of a substituent added to charge of the ring Cipso atom, was highly correlated with the Hammett constants, as well as with charge of acidic hydrogen of the >N-H group. The results prove that the set of charges of the substituent active region may be perceived as a theoretical equivalent of the experimental set of Hammett constants and that it is closely related to properties of this group of compounds.