The topological similarity index rAB introduced recently is generalized by using reduced 2nd orde... more The topological similarity index rAB introduced recently is generalized by using reduced 2nd order density matrices. The comparison with the results of our previous study demonstrates enhanced ability to discriminate between allowed and forbidden reactions.
A topological similarity index is introduced, allowing to characterize quantitatively the extent ... more A topological similarity index is introduced, allowing to characterize quantitatively the extent of reorganization of electron density in the course of chemical reactions. The possible application of this new index for the description and the classification of chemical reactions is discussed on the basis of certain analogy of similarity criteria with the so-called least-motion principle.
The basis of this method is a simple idea that chemical reaction is nothing but a specific transf... more The basis of this method is a simple idea that chemical reaction is nothing but a specific transformation converting the reactant, characterized by the set of bonds µR into the product characterized by the set of bonds µP In classifying the character of the reaction, the key role is played by the mutual relation of corresponding sets. A similar situation is also encountered with the technique of correlation diagrams, where the question of the mutual relation between these sets is solved by looking for the mutual assignment between the molecular orbitals as the elements of individual sets. The overlap determinant method solves this problem globally by determining the value of the overlap integral of the approximate wave functions which, in a given reaction, characterize the reactant and the product. This approach, first proposed and practically applied by Trindle [33], arises from the simple intuitive idea that any changes in the structure of molecules, as far as allowed processes are concerned, should not change the nodal structure of the molecular orbitals so that the corresponding overlap integral should be nonzero. On the other hand, for forbidden reactions, which are intuitively connected with deeper changes of the nodal structure, the above criterion requires the zero value of the overlap integral. Although the correctness of this idea has already been proven in the original study by Trindle [33], the substantial numerical complexity of his method has so far prevented its practical exploitation. In the overlap determinant method this important conceptual shortcoming has been successfully eliminated. The result is a simple universal formalism, the great advantage of which is that its philosophy is close to the thinking of organic chemists. This circumstance finds its reflection in that the discrimination between allowed and forbidden processes is solved only on the basis of the knowledge of classical structural formulae of the reactant and the product. In order to demonstrate the principles on which the overlap determinant method is based, it is best to illustrate its practical use by concrete examples, taken first from the field of thermal pericyclic reactions.
<jats:p>Bersuker's concept of redox charge transfer in catalysis has been adopted to st... more <jats:p>Bersuker's concept of redox charge transfer in catalysis has been adopted to study initiating activity of metal ions in radical addition reactions of model organic addends with alkenes. It was found that the most important factor determining the activity of the metal ions was the magnitude of the redox charge transfer δ<jats:italic>q</jats:italic>. Additionally, the spin-allowedness or spin-forbiddenness of the redox step was essential. Two-valent metal ions with configuration d<jats:sup>5</jats:sup>-d<jats:sup>9</jats:sup> do not exhibit, regardless of the spin-allowedness of the redox step, a significant oxidation activity owing to low values of δ<jats:italic>q</jats:italic>. Three-valent metal ions with configurations d<jats:sup>1</jats:sup>-d<jats:sup>3</jats:sup> are also inactive because of relatively lowest values of δ<jats:italic>q</jats:italic> and spin-forbiddenness of the redox step. A transient position is occupied by the Mn<jats:sup>3+</jats:sup> (d<jats:sup>4</jats:sup>) ion for which the redox step is still spin-forbidden, but the value of δ<jats:italic>q</jats:italic> is high enough to evoke satisfactory oxidation activity. Three-valent ions Fe<jats:sup>3+</jats:sup>, Ni<jats:sup>3+</jats:sup> with configurations d<jats:sup>5</jats:sup>-d<jats:sup>7</jats:sup> as well as four-valent ions Pb<jats:sup>4+</jats:sup> and Mn<jats:sup>4+</jats:sup> show, according to the magnitude of δ<jats:italic>q</jats:italic>, the highest oxidation activity in chain initiation.</jats:p>
On the basis of minimization of the properly defined objective function, a general method for sep... more On the basis of minimization of the properly defined objective function, a general method for separation of inductive and resonance effect has been proposed. Comparison of the so calculated results with characteristics obtained independently by perturbation theory of substituent effect made it possible to discuss mechanism of the transmission of inductive effect in aromatic and aliphatic series.
The second order similarity index gRP, which has been proposed recently as a new means for a qual... more The second order similarity index gRP, which has been proposed recently as a new means for a qualitative characterization of correlation effects in chemical reactivity, was generalized by incorporation into a topological model of the overlap determinant method. The resulting approach, which provides information about the variation of electron correlation during chemical reactions, was applied to the investigation of several selected pericyclic reactions. Consistent with what can be expected, the role of electron correlation was found to be the most critical for transition states of other transient species near the top of the energy barrier. The systematic differences in the extent of electron correlation between allowed and forbidden reactions are also discussed.
On the basis of CNDO/2 calculations,the geometrical changes accompanying the activation of the et... more On the basis of CNDO/2 calculations,the geometrical changes accompanying the activation of the ethanol molecule in the acid catalysed dehydration have been analysed. The activation of the ethanol molecule via protonation of its hydroxyl group manifests itself above all in the lengthening of the C-O bond. Further elongation of this bond leads to the formation of nonclassical structure, in extreme case to the nonclassical ion which is more stable then the classical one. The above reaction mechanism is confronted with the known facts and provides acceptable unifiying explanation of e.g. the stereoselectivity anomalies and the isotope effect of β-deuterated alcohols as well as other experimental and theoretical data.
Recently proposed overlap determinant method is applied to the description of reactions induced b... more Recently proposed overlap determinant method is applied to the description of reactions induced by the presence of the transition metal catalysts. The use of this technique is demonstrated on a detailed analysis of a number of examples of thermally forbidden pericyclic reactions in which the interaction with the catalyst removes the symmetry restrictions imposed by Woodward-Hoffmann rules.
ABSTRACT On the bsis of an EHT model study of PtII and PdII complexes, it has been shown that the... more ABSTRACT On the bsis of an EHT model study of PtII and PdII complexes, it has been shown that the coordination of ethene to the MCl3− species results in a lowering of the CC torsional barrier by 9% in PtII complexes and 31% in PdII complexes, in comparison with free C2H4 molecule. The larger decrease of this barrier for palladium suggests that PdII acts as a more efficient “CC bond softening agent” than PtII. This conclusion is consistent with the observed cis-trans isomerisation activity of Li2PdCl4 in acidified solutions of cis- or trans-2-butene.
A simple model was proposed allowing to estimate the Brønsted exponents in acid-base catalysis on... more A simple model was proposed allowing to estimate the Brønsted exponents in acid-base catalysis on the basis of the pK values of the species participating in the proton transfer process. The approach was tested using the experimental data on the basically catalyzed halogenation of carbonyl compounds and on the proton removal from nitroalkanes. It has been shown that the model is able to reproduce the Brønsted exponents not only in the case of…
The topological similarity index rAB introduced recently is generalized by using reduced 2nd orde... more The topological similarity index rAB introduced recently is generalized by using reduced 2nd order density matrices. The comparison with the results of our previous study demonstrates enhanced ability to discriminate between allowed and forbidden reactions.
A topological similarity index is introduced, allowing to characterize quantitatively the extent ... more A topological similarity index is introduced, allowing to characterize quantitatively the extent of reorganization of electron density in the course of chemical reactions. The possible application of this new index for the description and the classification of chemical reactions is discussed on the basis of certain analogy of similarity criteria with the so-called least-motion principle.
The basis of this method is a simple idea that chemical reaction is nothing but a specific transf... more The basis of this method is a simple idea that chemical reaction is nothing but a specific transformation converting the reactant, characterized by the set of bonds µR into the product characterized by the set of bonds µP In classifying the character of the reaction, the key role is played by the mutual relation of corresponding sets. A similar situation is also encountered with the technique of correlation diagrams, where the question of the mutual relation between these sets is solved by looking for the mutual assignment between the molecular orbitals as the elements of individual sets. The overlap determinant method solves this problem globally by determining the value of the overlap integral of the approximate wave functions which, in a given reaction, characterize the reactant and the product. This approach, first proposed and practically applied by Trindle [33], arises from the simple intuitive idea that any changes in the structure of molecules, as far as allowed processes are concerned, should not change the nodal structure of the molecular orbitals so that the corresponding overlap integral should be nonzero. On the other hand, for forbidden reactions, which are intuitively connected with deeper changes of the nodal structure, the above criterion requires the zero value of the overlap integral. Although the correctness of this idea has already been proven in the original study by Trindle [33], the substantial numerical complexity of his method has so far prevented its practical exploitation. In the overlap determinant method this important conceptual shortcoming has been successfully eliminated. The result is a simple universal formalism, the great advantage of which is that its philosophy is close to the thinking of organic chemists. This circumstance finds its reflection in that the discrimination between allowed and forbidden processes is solved only on the basis of the knowledge of classical structural formulae of the reactant and the product. In order to demonstrate the principles on which the overlap determinant method is based, it is best to illustrate its practical use by concrete examples, taken first from the field of thermal pericyclic reactions.
<jats:p>Bersuker's concept of redox charge transfer in catalysis has been adopted to st... more <jats:p>Bersuker's concept of redox charge transfer in catalysis has been adopted to study initiating activity of metal ions in radical addition reactions of model organic addends with alkenes. It was found that the most important factor determining the activity of the metal ions was the magnitude of the redox charge transfer δ<jats:italic>q</jats:italic>. Additionally, the spin-allowedness or spin-forbiddenness of the redox step was essential. Two-valent metal ions with configuration d<jats:sup>5</jats:sup>-d<jats:sup>9</jats:sup> do not exhibit, regardless of the spin-allowedness of the redox step, a significant oxidation activity owing to low values of δ<jats:italic>q</jats:italic>. Three-valent metal ions with configurations d<jats:sup>1</jats:sup>-d<jats:sup>3</jats:sup> are also inactive because of relatively lowest values of δ<jats:italic>q</jats:italic> and spin-forbiddenness of the redox step. A transient position is occupied by the Mn<jats:sup>3+</jats:sup> (d<jats:sup>4</jats:sup>) ion for which the redox step is still spin-forbidden, but the value of δ<jats:italic>q</jats:italic> is high enough to evoke satisfactory oxidation activity. Three-valent ions Fe<jats:sup>3+</jats:sup>, Ni<jats:sup>3+</jats:sup> with configurations d<jats:sup>5</jats:sup>-d<jats:sup>7</jats:sup> as well as four-valent ions Pb<jats:sup>4+</jats:sup> and Mn<jats:sup>4+</jats:sup> show, according to the magnitude of δ<jats:italic>q</jats:italic>, the highest oxidation activity in chain initiation.</jats:p>
On the basis of minimization of the properly defined objective function, a general method for sep... more On the basis of minimization of the properly defined objective function, a general method for separation of inductive and resonance effect has been proposed. Comparison of the so calculated results with characteristics obtained independently by perturbation theory of substituent effect made it possible to discuss mechanism of the transmission of inductive effect in aromatic and aliphatic series.
The second order similarity index gRP, which has been proposed recently as a new means for a qual... more The second order similarity index gRP, which has been proposed recently as a new means for a qualitative characterization of correlation effects in chemical reactivity, was generalized by incorporation into a topological model of the overlap determinant method. The resulting approach, which provides information about the variation of electron correlation during chemical reactions, was applied to the investigation of several selected pericyclic reactions. Consistent with what can be expected, the role of electron correlation was found to be the most critical for transition states of other transient species near the top of the energy barrier. The systematic differences in the extent of electron correlation between allowed and forbidden reactions are also discussed.
On the basis of CNDO/2 calculations,the geometrical changes accompanying the activation of the et... more On the basis of CNDO/2 calculations,the geometrical changes accompanying the activation of the ethanol molecule in the acid catalysed dehydration have been analysed. The activation of the ethanol molecule via protonation of its hydroxyl group manifests itself above all in the lengthening of the C-O bond. Further elongation of this bond leads to the formation of nonclassical structure, in extreme case to the nonclassical ion which is more stable then the classical one. The above reaction mechanism is confronted with the known facts and provides acceptable unifiying explanation of e.g. the stereoselectivity anomalies and the isotope effect of β-deuterated alcohols as well as other experimental and theoretical data.
Recently proposed overlap determinant method is applied to the description of reactions induced b... more Recently proposed overlap determinant method is applied to the description of reactions induced by the presence of the transition metal catalysts. The use of this technique is demonstrated on a detailed analysis of a number of examples of thermally forbidden pericyclic reactions in which the interaction with the catalyst removes the symmetry restrictions imposed by Woodward-Hoffmann rules.
ABSTRACT On the bsis of an EHT model study of PtII and PdII complexes, it has been shown that the... more ABSTRACT On the bsis of an EHT model study of PtII and PdII complexes, it has been shown that the coordination of ethene to the MCl3− species results in a lowering of the CC torsional barrier by 9% in PtII complexes and 31% in PdII complexes, in comparison with free C2H4 molecule. The larger decrease of this barrier for palladium suggests that PdII acts as a more efficient “CC bond softening agent” than PtII. This conclusion is consistent with the observed cis-trans isomerisation activity of Li2PdCl4 in acidified solutions of cis- or trans-2-butene.
A simple model was proposed allowing to estimate the Brønsted exponents in acid-base catalysis on... more A simple model was proposed allowing to estimate the Brønsted exponents in acid-base catalysis on the basis of the pK values of the species participating in the proton transfer process. The approach was tested using the experimental data on the basically catalyzed halogenation of carbonyl compounds and on the proton removal from nitroalkanes. It has been shown that the model is able to reproduce the Brønsted exponents not only in the case of…
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