Wyniki wyszukiwania - BazTech

1

Proton or Hydrogen Transfer? Charge Distribution Analysis

Waluk J.

Polish Journal of Chemistry

|

2008

|

Vol. 82, nr 4

947-962

EN

The meaning of terms "proton transfer" (PT) and "hydrogen transfer" (HT), commonly encountered in the literature, is not always unambiguous, especially in the case of intramolecular tautomerization processes. Depending on the used criterion, the same reaction may be classified as either PT or HT. Two criteria for distinguishing between PT and HT are discussed: (a) formal translocation of charge; (b) the reaction mechanism, in particular the coupling between the motion of the proton and the electron density redistribution. The analysis is illustrated using the values of ground and excited state partial charges calculated for three well-known examples of molecules that undergo photo-induced and/or ground state tautomerization: -hydroxyacetophenone, 2,2'-bi pyridyl- 3,3'-diol, and porphycene.

2

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

EN

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.

3

On the Way to Physical Interpretation of Hammett Constants: How Substituent Active Space Impacts on Acidity and Electron Distribution in p-Substituted Benzoic Acid Molecules

Sadlej-Sosnowska

Polish Journal of Chemistry

|

2007

|

Vol. 81, nr 5-6

1123-1134

EN

Electron distribution was investigated in molecules of p-substituted benzoic acids by means of Natural Population Analysis. The analysis offers a valuable insight into the impact of a given substituent on the individual atoms’ charges in the molecules, on the excess of pi electrons in the ring and on the uniformity of their distribution. The more uniform the distribution, measured by means of standard deviation of pi electrons amounts localized on individual atoms in the ring, themore acidic is the molecule. On the other hand, an increase of the pi electron number in the ring brings about an opposite effect. No correlation was found between the total substituent charges and the Hammett constants. However, a new variable, composed of charges of substituents, added to charges of the ring Cipso atoms, was highly correlated with charges of acidic hydrogens, with the pi-electron populations on the individual ring atoms, and eventually, with the Hammett constants. On that basis a notion of the active substituent space was introduced, which comprises the substituent itself and the ring ipso carbon atom. It has been found that it is the charge of the active space, but not the charge of the substituent itself, which is well correlated with the acidity and other properties of p-substituted benzoic acids.