Comparative theoretical study of transition structures, barrier heights, and reaction energies for the intramolecular tautomerization in acetaldehyde…

1998

The transition structures associated with the possible intramolecular tautomerization for acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems as models of keto/enol and imine/enamine interconversion processes, respectively, were characterized. The relative stabilities of the tautomers and the associated barrier heights were calculated. Ab initio analytical gradients and second derivatives at the HF level of theory and 3-21G, 6-31G, 6-31G**, 6-31++G**, and 6-311++G** basis-set, DFT (BP86/6-311++G** and BLYP/6-311++G**), and semiempirical (AM1 and PM3) procedures were used to identify the stationary points. Correlation effects were estimated using the perturbational approach at MP2…

On Transition Structures for Hydride Transfer Step: A Theoretical Study of the Reaction Catalyzed by Dihydrofolate Reductase Enzyme

1996

Abstract A theoretical study is presented of the catalytic mechanism of dihydrofolate reductase (DHFR) enzyme based upon the characterization of the transition structure (TS) for the hydride transfer step. Analytical gradients at AM1 and PM3 semiempirical levels have been used to characterize the saddle point of index one (SPi-1) on global energy hypersurface for the hydride transfer in the active site of DHFR enzyme. The geometry, stereochemistry, electronic structure, and transition vector (TV) components associated to SPi-1 are qualitatively computational level independent. The TV amplitudes show primary and secondary isotope effects to be strongly coupled. The geometrical arrangement of…

ChemInform Abstract: Evidence for the Formation of 1,3- and 1,4-Dehydrobenzenes in the Thermal Decomposition of Diaryliodonium-carboxylates.

1987

Abstract Abstract: Generation of m- and p-benzynes in decomposition of diaryliodonium- 3- and 4-carboxylates is demonstrated by three-phase method.

Transition structures for hydride transfer reactions in vacuo and their role in enzyme catalysis

1996

A general discussion as to the role of in vacuo transition structure in enzyme catalysis is presented. Quantum mechanical aspects are emphasized. The transition structures defined as saddle points ...

Potential energy surface for the decomposition of mandelic acid

1997

Abstract The decomposition of mandelic acid has been studied by ab initio calculations at the MP2/6-31G ∗∗ level. Three competitive reaction pathways have been characterized, two are stepwise processes with the formation of an α-lactone intermediate, achieved by the nucleophilic attack of either the carbonylic oxygen atom (mechanism A) or the hydroxylic oxygen atom (mechanism B) of the carboxyl group, followed by a ring opening; the third pathway (mechanism C) is a one-step process. The calculated rate coefficient agrees with experimental data. The decomposition is energetically favourable along mechanism A.

Theoretical Study of the Elimination Kinetics of Carboxylic Acid Derivatives in the Gas Phase. Decomposition of 2-Chloropropionic Acid

1997

The reaction mechanism for the decomposition of 2-chloropropionic acid in the gas phase to form hydrogen chloride, carbon monoxide, and acetaldehyde has been theoretically characterized. Analytical gradients have been used by means of AM1 and PM3 semiempirical procedures and ab initio methods at HF and DFT (BLYP) levels with the 6-31G** basis set. The correlation effects were also included by using the perturbational approach at the MP2 level with the 6-31G** and 6-31++G** basis sets and the variational approach at the CISD/6-31G** level and by means of MCSCF wave functions with a (6,6) complete active space and the 6-31G** basis set. The global potential energy surface has been studied, an…

A Theoretical Study of the Favorskii Rearrangement. Calculation of Gas-Phase Reaction Paths and Solvation Effects on the Molecular Mechanism for the …

1997

The molecular mechanism of the α-chlorocyclobutanone transposition to yield cyclopropanecarboxylic acid, as a model of the Favorskii rearrangement, has been theoretically characterized in vacuo by means of ab initio molecular orbital procedures at the Hartree−Fock (HF) level of theory with the 6-31G* and 6-31+G* basis sets. The electron correlation has been estimated at the MP2/6-31G* level and calculations based on density functional theory, BLYP/6-31G*. The solvent effects are included at HF/6-31G* level by means of a polarizable continuum model. The questions related to the two accepted molecular mechanisms, the semibenzilic acid and the cyclopropanone transpositions, as well as the comp…

The tandem Diels-Alder reaction between acetylenedicarboxyaldehyde and N,N'-dipyrrolylmethane. An ab initio study of the molecular mechanisms

1998

Abstract An extensive exploration at RHF/3-21G and RHF/6-31G ∗ levels of the potential energy surface for the tandem cycloaddition of acetylenedicarboxyaldehyde to N,N'-dipyrrolylmethane allows us to characterize the reaction pathways and the associated stationary points. The formation of the pincer and/or domino adducts can be described as a stepwise mechanism. The first step, associated with an intermolecular [4 + 2] cycloaddition, is the rate determining step and an azanorbornadiene intermediate is obtained. The second step is an intramolecular [4 + 2] cycloaddition. The formation of the pincer adduct is the step which kinetically controls the global process, due to the low barrier heigh…

On Transition Structures for Hydride Transfer Step in Enzyme Catalysis. A Comparative Study on Models of Glutathione Reductase Derived from Semiempir…

1996

As a model of the chemical reactions that take place in the active site of gluthatione reductase, the nature of the molecular mechanism for the hydride transfer step has been characterized by means of accurate quantum chemical characterizations of transition structures. The calculations have been carried out with analytical gradients at AM1 and PM3 semiempirical procedures, ab initio at HF level with 3-21G, 4-31G, 6-31G, and 6-31G basis sets and BP86 and BLYP as density functional methods. The results of this study suggest that the endo relative orientation on the substrate imposed by the active site is optimal in polarizing the C4-Ht bond and situating the system in the neighborhood of the…

Theoretical study of the molecular mechanism of the domino pathways for squarate ester sequential reactions

1999

Ab Initio Study of Stereo- and Regioselectivity in the Diels−Alder Reaction between 2-Phenylcyclopentadiene and α-(Methylthio)acrylonitrile

1997

Ab initio molecular orbital calculations have been used to investigate the Diels−Alder reaction between 2-phenylcyclopentadiene and α-(methylthio)acrylonitrile. Geometry optimizations were performed with the RHF/3-21G, RHF/3-21G*, and RHF/6-31G* calculations while the correlation energy has been calculated at MP2 and MP3 levels. Four asynchronous transition structures corresponding to the formation of different stereoisomers and regioisomers associated with the four reaction channels have been located. The observed endo/exo stereoselectivity and para/meta regioselectivity are correctly described at all levels of calculation, and the calculated activation energies reflect accurately the rela…

Theoretical Study of the Gas Phase Decomposition of Glycolic, Lactic, and 2-Hydroxyisobutyric Acids

1997

The reaction mechanism associated with the decomposition of three α-hydroxycarboxylic acids (glycolic, lactic, and 2-hydroxyisobutyric) in the gas phase to form carbon monoxide, water, and the corresponding carbonyl compounds has been theoretically characterized by using ab initio analytical gradients at the MP2 level of theory with the 6-31G** and 6-31++G** basis sets. A detailed characterization of the potential energy surface points out the existence of three competitive reaction pathways for the decomposition process. The first pathway describes a two-step mechanism, with water elimination and formation of an α-lactone intermediate, achieved by the nucleophilic attack of the carbonylic …

A theoretical study of the addition of CH3MgCl to chiral α-alkoxy carbonyl compounds

1998

Abstract A theoretical characterisation of the addition of CH 3 MgCl to the carbonyl group of 2-hydroxypropanal and 3,4-di- O -methyl-1- O -trimethylsilyl- l -erythrulose has been carried out. The PM3 semiempirical procedure and the ab initio method at HF level of theory with the 3–21G and 6–31G ∗ basis sets have been applied to identify the stationary points on the potential energy surface. The correlation effects have been included by using the MP2/6–31G ∗ method, as well as by means of MP3/6–31G ∗ // MP2/6–31G ∗ and MP4/6–31G ∗ //MP2/6–31G ∗ single point calculations. The geometry, harmonic vibrational frequencies, transition vector and electronic structure of the transition structures h…

A theoretical study on the decomposition mechanism of β-propiolactone and β-butyrolactone

1998

Abstract The reaction mechanism associated with the decomposition of β-propiolactone and β-butyrolactone in the gas phase to form carbon dioxide and ethylene or propene, respectively, has been theoretically characterized by using analytical gradients at the B3LYP/6-31G**, B3LYP/6-31++G**, MP2/6-31G** and MP2/6-31++G** calculation levels, as well as by single point calculations at QCISD(T)/6-31++G**//MP2/6-31++G**. A detailed analysis of the results points out that the reaction pathway can be described as an asynchronous concerted process. The geometrical parameters and the components of the transition vector are weakly dependent on the computational method. By applying transition state theo…

Evidence for the formation of 1,3- and 1,4-dehydrobenzenes in the thermal decomposition of diaryliodonium-carboxylates

1986

Abstract Abstract: Generation of m- and p-benzynes in decomposition of diaryliodonium- 3- and 4-carboxylates is demonstrated by three-phase method.