Search results for "COMPUTATION"
showing 10 items of 7362 documents
Theoretical Studies on the Mechanism of the Formation of Cyclopentadienes and Dihydropyridazines
2021
A theoretical study of the molecular mechanism of the reaction between N,N-dimethylmethyleneammonium cation and cyclopentadiene.
2001
WHY DIELS-ALDER REACTIONS ARE NON-CONCERTED PROCESSES
2014
ABSTRACT Two exhaustive topological analyses using the Electron Localisation Function (ELF) along the non-polar Diels-Alder reactions of 1,3-butadiene [J. Phys. Chem. A 107 , 6014 (2003)] and cyclopentadiene [Org. Biomol. Chem. 8 , 5495, (2010)] with ethylene established that the bonding changes along these reactions are non-concerted. Herein, the ELF bonding changes along the intramolecular Diels-Alder reaction of 1-( hex-5-enyl)cyclohexa-1,3-diene is analysed. The geometrical restrictions imposed by the tether break the synchronicity in the single bond formation in this non-polar reaction. The present ELF topological analysis along this intramolecular Diels-Alder reaction supports the ear…
Molecular modeling of intercalation complexes of antitumor active 9-aminoacridine and a [d, e]-anellated isoquinoline derivative with base paired deo…
1996
Intercalators are molecules capable of sliding between DNA base pairs without breaking up the hydrogen bonds between the DNA bases. On the basis of molecular mechanics calculations structural, models of B-DNA tetranucleotide intercalation complexes of some cytostatic active 9-aminoacridines and of a [d, e]-anellated isoquinoline derivative are presented. The drug complexes are stabilized by energetically favouredvan der Waals interactions and by selective hydrogen bonds between the side chains of the drugs and the DNA bases. Semiempirical quantum chemistry calculations revealed that the chromophoric system of the intercalators is able to form π,π-charge-transfer interactions with the purine…
Relation between resonance energy and substituent resonance effect inP-phenols
2005
Molecular geometries of phenol and its 17 p-substituted derivatives were optimized at the B3LYP/6–311 + G** level of theory. Three homodesmotic and three isodesmotic reaction schemes were used to estimate aromatic stabilization energies (ASE) and the substituent effect stabilization energy (SESE). Other descriptors of π-electron delocalization (HOMA and NICS, NICS(1) and NICS(1)zz) were also estimated. The SESE and ASE values correlated well with one another as well as with substituent constants. Much worse correlations with substituent constants were found for other aromaticity indices. The NICS(1)zz values are the most negative for unsubstituted phenol, indicating its highest aromaticity;…
Ab initio molecular orbital study of the substituent effect on phosphine–borane complexes
1998
Abstract Ab initio molecular orbital calculations have been used to study the substituent effect on H 3 BPH n Me 3− n and Me 3− n H n BPH 3 ( n =0–3) phosphine–borane complexes. The ab initio results show that successive methyl substitution on the phosphine favours complex formation, contrary to successive methyl substitution on the borane. The natural bond orbitals partitioning scheme suggests that, in general, there is no correlation between the charge transfer and the complexation energies. It also shows the shortening of the P–H and P–C bond lengths, upon complexation, is due to the increasing `s' character of these bonds.
An Understanding of the Electrophilic/Nucleophilic Behavior of Electro-Deficient 2,3-Disubstituted 1,3-Butadienes in Polar Diels−Alder Reactions. A D…
2008
The electrophilic/nucleophilic behavior of dimethyl 2,3-dimethylenesuccinate 1, an electron-deficient 2,3-disubstituted 1,3-butadiene, in polar Diels-Alder reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The electronic nature of bonding of the transition structures involved in the cycloaddition reactions of the diene 1 toward the nucleophilically activated dienophile 6 and the strong electrophilically activated dienophile 7 has been carefully examined within the natural bond orbital (NBO) and the topological analysis of the electron localization function (ELF) frameworks. Additionally, a study of the global electrophilicity pattern of the reagents at the …
An AM1 study on π-facial selectivity in Diels-Alder reactions of 2-aza-1, 3-dienes with azodienophiles
1994
Abstract A theoretical study of the π-facial selectivity in Diels-Alder reactions of chloro derivatives of 2-aza-1,3-dienes with azodienophiles has been carried out using the semiempirical AM1 method. Three transition structures for the model reaction of the diene 3 with the dienophile 4 were located. The Diels-Alder reaction of the non-substituted 2-aza-1,3-diene 6 with the trans -diimide was studied. All transition structures located show an important asynchronicity and the calculations reveal the presence of stereoelectronic effects. The predicted stereoselectivity is in agreement with the experimental evidence.
Understanding the reactivity of captodative ethylenes in polar cycloaddition reactions. A theoretical study
2008
The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for…