0000000000335924
AUTHOR
M. T. Picher
showing 9 related works from this author
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…
Flavones, sesquiterpene lactones and glycosides isolated from Centaurea aspera var. Stenophylla
1984
Abstract From the alcoholic extract of Centaurea aspera var. stenophylla benzoic acid, p-hydroxybenzoic acid, apigenin, 6-methoxyluteolin, 11,13-dehydromelitensin, melitensin, stenophyliolide, ethyl-7-O-apigenin-glucuronate and the glucosides of sitosterol and stigmasterol were isolated and characterized. Stenophyllolide was shown to be 9,15-dihydroxygermacra-1(10),4,11-trien-12,6-olide.
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.
A DFT study of the polar Diels–Alder reaction between 4-aza-6-nitrobenzofuroxan and cyclopentadiene
2005
Abstract The polar Diels–Alder reaction between 4-aza-6-nitrobenzofuroxan (ANBF) and cyclopentadiene has been studied using DFT procedures at the B3LYP/6-31G* level. Only one highly asynchronous transition state structure associated to the formation of the [4+2] adduct 13 is found. A further [3,3] sigmatropic shift on the [4+2] cycloadduct 13 allows its conversion into the thermodynamically more stable [2+4] cycloadduct 14 . The analysis of the global and local electrophilicities of the reagents correctly explain the behaviour of ANBF as a strong electrophile in polar cycloadditions.
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 on the Molecular Mechanism of the Domino Cycloadditions between Dimethyl Acetylenedicarboxylate and Naphthaleno- and Anthracenofura…
2001
AM1, B3LYP/6-31G//AM1, and B3LYP/6-31G computational studies were performed to select the reaction pathway controlling the reactions between dimethyl acetylenedicarboxylate (DMAD) and two furanophanes, naphthalenofuranophane and anthracenofuranophane. For these domino reactions, several pathways have been characterized on the potential energy surface corresponding to two consecutive cycloadditions. The first step corresponds to a [4 + 2] intermolecular cycloaddition of DMAD with the furan ring or with the naphthalene or anthracene ring of both furanophane systems to yield an oxabicyclo[2.2.1]heptadiene or a bicyclo[2.2.2]octadiene intermediate, respectively. The second step corresponds to […
Toward an understanding of the selectivity in domino reactions. A DFT study of the reaction between acetylenedicarboxylic acid and 1, 3-Bis(2-furyl)p…
2000
The mechanism of the domino reaction between acetylenedicarboxylic acid and 1,3-bis(2-furyl)propane has been theoretically studied in the framework of density functional theory. This domino process comprises two consecutive cycloaddition reactions: the first one is initialized by the nucleophilic attack of the C5 position of the furan ring to a conjugate position of acetylenedicarboxylic acid to give a zwitterionic intermediate, which by a subsequent ring-closure process affords an oxanorbornadiene intermediate. The second reaction is an intramolecular concerted cycloaddition of this intermediate to give the final dioxapentacyclic adduct. For the second cycloaddition, which corresponds to t…
Toward an Understanding of the Molecular Mechanism of the Reaction between 1-Methylpyrrole and Dimethyl Acetylenedicarboxylate. An ab Initio Study
1998
The molecular mechanism for the reaction between 1-methylpyrrole and dimethyl acetylenedicarboxylate (DMAD) has been studied using ab initio methods. Two alternative reaction pathways have been considered, both of which correspond to stepwise processes with initial, rate-determining formation of a common zwitterionic intermediate. This intermediate is formed by nucleophilic attack of the pyrrole ring to the carbon−carbon triple bond of DMAD. Closure of this intermediate (pathway A) affords a [4 + 2] cycloadduct, whereas intramolecular proton transfer (pathway B) affords a Michael adduct. The much larger potential energy barrier of the second step in pathway B relative to pathway A is respon…