Search results for "COMPUTATION"

Differentiation of isomeric cyclic diamides by electron impact mass spectra

1976

The behaviour under electron impact of two series of isomeric cyclopentane- and cyclohexane-1, 2- and -1, 3-dicarboxylic acid dipiperidides was studied. Diagnostic fragmentation pathways were found to differentiate the isomeric diamides. Additional evidence was obtained from the metastable transitions.

Quantum Chemistry of Excited States in Polyhedral Boranes

2015

In this Chapter we describe the electronic structure of ground states and excited states of the two isomers of octadecaborane (22), anti- and syn-B18H22, and the new derivative of anti-B18H22, the polyhedral substituted borane 4,4′-(HS)2-anti-B18H20. A theoretical interpretation is given on the fluorescence of the anti-B18H22 isomer, and the non-radiative decay of the syn-B18H22 isomer, an unsolved problem since 1962. For the new derivative of anti-B18H22, substitution of hydrogen atoms in positions 4 and 4′ by SH groups allows the tuning of the photophysical properties in 4,4′-(HS)2-anti-B18H20, facilitating intersystem crossing from the excited singlet state to the triplet state.

Better understanding of the ring-cleavage process of cyanocyclopropyl anionic derivatives. A theoretical study based on the electron localization fun…

2005

[reaction: see text] Theoretical calculations at the B3LYP/6-31+G(d), MP2/6-31+G(d), and G3(MP2) levels have been carried out to understand the alternative reaction pathways (the cyclopropyl ring cleavage (RC) and the retrocycloaddition reaction (rCA)) of a constrained tricyanocyclopropyl anionic derivative. The more energetically favorable path is found to be the RC process, a formally "forbidden" rearrangement (Leiviers, M.; Tam, I.; Groves, K.; Leung, D.; Xie, Y.; Breslow, R. Org. Lett. 2003, 5, 19, 3407) yielding an allylic anion system via a concerted transition structure, in agreement with experimental outcomes. rCA is more energetically favorable along a two-stage mechanism, via an i…

Synthesis and Properties of Some Simple Organotitanium Compounds

1986

Before embarking on a detailed discussion of the titanation of carbanions, synthetic and physical organic aspects of several typical organotitanium (IV) compounds shall be surveyed, including thermal stability, aggregation state, X-ray structural data and bond energies. Some of this information is useful in understanding reactivity and selectivity in reactions with organic substrates. Low valent Ti (II) and Ti (III) shall be mentioned only on passing; the interested reader is referred to reviews [1].

An Accurate Quartic Force Field and Fundamental Frequencies for the Ozonide Anion: A Rare Positive Anharmonicity for the Antisymmetric Stretch

2003

The CCSD(T) method has been used to compute a highly accurate quartic force field and fundamental frequencies for all 16O and 18O isotopomers of the ozonide anion. The CCSD and CASPT2 methods have also been used to verify the reliability of the CCSD(T) fundamental frequencies. The computed fundamental frequencies are in agreement with gas-phase experiments, but disagree with matrix isolation experiments for the antisymmetric stretch, ν3. CASPT2 calculations show that the antisymmetric part of the O3- potential surface is sensitive to the external environment. It is concluded that the antisymmetric stretch exhibits a significant matrix shift in the matrix isolation experiments and that the m…

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.

Study of the pentalene and heptalene molecules according to the SCF theory (Roothaan method)

1965

Activation of Mitochondria-Driven Pathways by Artemisinin and Its Derivatives

2014

Mitochondria have recently emerged as promising agents for cancer therapy. Of particular interest and potential clinical relevance are agents that target these organelles, promoting cell death. There are literally thousands of compounds that act on mitochondria and destabilise them. Of these, naturally occurring compounds are particularly interesting, since they are often more ‘biocompatible’; besides, natural compounds can be lead drugs for the design of novel and more efficient anti-cancer agents. In this paper, we focus on the natural product artemisin and its semisynthetic derivatives, and document the molecular mechanism of their activity and their potential use as clinically relevant …

Why do Electron-Deficient Dienes React Rapidly in Diels?Alder Reactions with Electron-Deficient Ethylenes? A Density Functional Theory Analysis

2004

The Diels−Alder reaction of the electron-deficient (ED) dimethyl 2,3-dimethylenesuccinate with two electron-rich (ER) and two ED ethylenes has been studied at the B3LYP/6-31G* level of theory. The analysis of the geometry and electronic structure of the transition state of the reaction with the ED dimethyl 2-methylenemalonate along with the analysis of the global and local electrophilicity indices of the reagents provide an explanation of the participation of this ED diene as nucleophile against powerful electrophiles in polar Diels−Alder reactions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Density Functional Theory Study of the Cycloaddition Reaction of Furan Derivatives with Masked o-Benzoquinones. Does the Furan Act as a Dienophile in…

2002

The molecular mechanism for the cycloaddition reaction between 2-methylfuran and a masked o-benzoquinone has been characterized using quantum mechanical calculations at the B3LYP/6-31G theory level. An analysis of the results on the reaction pathway shows that the reaction takes place along a polar stepwise mechanism. The first and rate-determining step corresponds to the nucleophilic attack of the furan ring on the doubly conjugated position of the 2,4-dienone system present at the masked o-benzoquinone to give a zwitterionic intermediate. Closure of this intermediate affords the formally [2 + 4] cycloadduct. For the second step two reactive channels have been characterized corresponding t…