224 research outputs found
R-matrix calculation of differential cross sections for low-energy electron collisions with ground and electronically excited state O2 molecules
Differential cross sections for electron collisions with the O molecule
in its ground state, as well as excited
and states are calculated. As previously, the fixed-bond
R-matrix method based on state-averaged complete active space SCF orbitals is
employed. In additions to elastic scattering of electron with the O
, and states, electron
impact excitation from the state to the
and states as well as '6 eV states' of
, and states is
studied. Differential cross sections for excitation to the '6 eV states' have
not been calculated previously. Electron impact excitation to the
state from the metastable state is also
studied. For electron impact excitation from the O
state to the state, our results agree better with the
experimental measurements than previous theoretical calculations. Our cross
sections show angular behaviour similar to the experimental ones for
transitions from the state to the '6 eV states', although
the calculated cross sections are up to a factor two larger at large scattering
angles. For the excitation from the state to the
state, our results marginally agree with the experimental
data except for the forward scattering direction
R-matrix calculation of electron collisions with electronically excited O2 molecules
Low-energy electron collisions with O molecules are studied using the
fixed-bond R-matrix method. In addition to the O ground
state, integrated cross sections are calculated for elecron collisions with the
and excited states of O molecules. 13
target electronic states of O are included in the model within a valence
configuration interaction representations of the target states. Elastic cross
sections for the and excited states are
similar to the cross sections for the ground state. As in
case of excitation from the state, the O
resonance makes the dominant contribution to excitation cross sections from the
and states. The magnitude of excitation
cross sections from the state to the
state is about 10 time larger than the corresponding cross sections from the
to the state. For this
transition, our cross section at
4.5 eV agrees well with the available experimental value. These results should
be important for models of plasma discharge chemistry which often requires
cross sections between the excited electronic states of O.Comment: 26 pages, 10 figure
Palladium-catalyzed regioselective and stereo-invertive ring-opening borylation of 2-arylaziridines with bis(pinacolato)diboron: Experimental and computational studies
A palladium catalyzed regioselective borylative ring opening reaction of 2-arylaziridines to give β-amino-β-arylethylborates was developed. The reaction reported herein represents the first example of ring-opening borylation of non-vinylic aziridines and direct borylative C(sp3)-N bond cleavage of neutral organic substrates. NMR studies and density functional theory (DFT) calculations suggested that the active intermediate for the reaction is a PdL2 complex [L = P(t-Bu)2Me]. The multi-component artificial force-induced reaction method (MC-AFIR) located the transition states for the regioselectivity-determining aziridine ring opening that proceeds in an SN2 fashion, and explained the selectivity of the reaction. The full catalytic cycle consists of a selectivity-determining aziridine ring opening (oxidative addition), a proton transfer, phosphine ligand dissociation from the catalyst, boron-boron bond cleavage, and reductive elimination. Water is important to the drive the transmetalation step. The calculated overall mechanism and selectivity are consistent with the experimental results
Ab Initio Study of the Molecular and Electronic Structure of CoCH2+ and of the Reaction Mechanism of CoCH2+ + H2
Both CASSCF and MR-SDCI-CASSCF methods have been used with two different effective core potentials to investigate the molecular and electronic structures of CoCH2+, as well as the mechanism for the reaction CoCH2+ + H2. Four electronic states of CoCH2+ are very low lying: the ground state is a nearly degenerate pair (3A2 and 3A1), and the 3B1 and 3B2 states are only 4-8 kcal/mol higher in energy. The binding energy of C O C H ~ + ( ~ Are~la)t,iv e to that of C ~ + ( ~ F , s l d+~ C)H 2(3B1), is estimated to be 70-80 kcal/mol. A similar hydrogenolysis reaction mechanism holds for the 3A2 and 3A1 states of the CoCH2+ + H2 reactants: In the first step, the reactants yield an ion-molecule complex, (H2)CoCH2+, stabilized by 8-9 kcal/mol. Subsequently, the H-H bond is activated, leading to a four-center transition state with an energy barrier of about 31-34 kcal/mol. An intermediate complex, HCoCH3+, is predicted to be a minimum at the CASSCF level, but MR-SDCI-CASSCF single-point calculations suggest that this minimum disappears at the higher level of theory. Following H-H bond cleavage, a CoCH4+ ion-molecule complex is formed, with a stabilization energy of 19-22 kcal/mol. The CoCH2+ hydrogenolysis reaction is predicted to be exothermic by 20-30 kcal/mol. The channels leading to formation of CoH+ + CH3 and CoCH3+ + H are endothermic by about 5-1 2 kcal/mol. The reverse reaction Co+ + CH4 may give only one product, the ion-molecule complex CoCH4+ at moderate temperatures. An increase in the available kinetic energy would make it possible to form dissociation products: CoH+ + CH3 and CoCH3+ + H. Although the channel leading to CoCH2+ + H2 is thermodynamically more favorable, a large barrier prevents it from taking place. Hay-Wadt and Stevens-Krauss-Basch-Jasien pseudopotentials give qualitatively the same results
An unexpected Ireland–Claisen rearrangement cascade during the synthesis of the tricyclic core of Curcusone C: Mechanistic elucidation by trial-and-error and automatic artificial force-induced reaction (AFIR) computations
In the course of a total synthesis effort directed toward the natural product curcusone C, the Stoltz group discovered an unexpected thermal rearrangement of a divinylcyclopropane to the product of a formal Cope/1,3-sigmatropic shift sequence. Since the involvement of a thermally forbidden 1,3-shift seemed unlikely, theoretical studies involving two approaches, the “trial-and-error” testing of various conceivable mechanisms (Houk group) and an “automatic” approach using the Maeda–Morokuma AFIR method (Morokuma group) were applied to explore the mechanism. Eventually, both approaches converged on a cascade mechanism shown to have some partial literature precedent: Cope rearrangement/1,5-sigmatropic silyl shift/Claisen rearrangement/retro-Claisen rearrangement/1,5-sigmatropic silyl shift, comprising a quintet of five sequential thermally allowed pericyclic rearrangements
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Mapping behavioral specifications to model parameters in synthetic biology
With recent improvements of protocols for the assembly of transcriptional parts, synthetic biological devices can now more reliably be assembled according to a given design. The standardization of parts open up the way for in silico design tools that improve the construct and optimize devices with respect to given formal design specifications. The simplest such optimization is the selection of kinetic parameters and protein abundances such that the specified design constraints are robustly satisfied. In this work we address the problem of determining parameter values that fulfill specifications expressed in terms of a functional on the trajectories of a dynamical model. We solve this inverse problem by linearizing the forward operator that maps parameter sets to specifications, and then inverting it locally. This approach has two advantages over brute-force random sampling. First, the linearization approach allows us to map back intervals instead of points and second, every obtained value in the parameter region is satisfying the specifications by construction. The method is general and can hence be incorporated in a pipeline for the rational forward design of arbitrary devices in synthetic biology
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Chemical Fate of Contaminants in the Environment: Chlorinated Hydrocarbons in the Groundwater
Chlorinated hydrocarbons (CHCs) are the most common contaminant found at hazardous waste sites and are the most prevalent contaminants on U.S. Department of Energy (DOE) weapons production sites. Many of the CHCs are either known or suspected carcinogens and thus pose health risks to the public and/or site workers. Unlike simple hydrocarbons, CHCs are resistant to biodegradation, but can degrade by abiotic processes such as hydrolysis, nucleophilic substitution, and dehydrochlorination. Unfortunately, few studies of the reactions of chlorinated hydrocarbons have been reported in literature, and disagreement still exists about the mechanisms and rates of many of the key reactions. In this work, we modeled the reactions involved in the degradation of CHCs in the groundwater. The goals of the research proposed are: • development of a computational approach that will allow reaction pathways and rate constants to be accurately calculated • development of more approximate approaches, evaluated against the more accurate approach, which will lay the groundwork for exploratory studies of more complex CHCs • application of these approaches to study the degradation pathways of CHCs in aqueous liquids • application of the more approximate approaches to study the mechanism of forming complex CHC polychlorinated benzene compounds and dioxins. We examined elementary reactions involved in the aqueous-phase chemistry of chlorinated methanes and ethylenes in an attempt to obtain a detailed understanding of the abiotic processes involved in the degradation of this important class of contaminants. We began by studying the reactions of CHnCl(4-n) and C2HnCl(4-n) with OH¯, as these are thought to be the dominant processes involved in the degradation of these chlorinated species. We used state-of-the-art theoretical techniques to model the elementary reactions of CHCs important in the groundwater. We employed high-accuracy electronic structure methods (e.g., perturbation theory and coupled cluster methods with correlation-consistent basis sets) to determine the energies of the various stable species, intermediates, and transition states involved in the elementary reactions of CHCs. Effects of solvation on the reaction energetics were studied by including small numbers of solvent molecules (microsolvation). Our own N-layered molecular orbital + molecular mechanics (ONIOM) method was used because it allows the number of solvent molecules to be increased, and hybrid quantum mechanical/molecular mechanics (QM/MM) methods and continuum solvation models were used to estimate the effects of bulk solvation. Rate constants for the gas-phase, microsolvated, and bulk-phase reactions were computed using variational transition state theory (VTST)
НАПРАВЛЕНИЯ СОВЕРШЕНСТВОВАНИЯ ОБРУДОВАНИЯ ДЛЯ ФЛОТАЦИОННОГО ОБОГАЩЕНИЯ ТОНКОДИСПЕРСНыХ МАТЕРИАЛОВ
Проблема и ее связь с научными и практическими задачами. В связи с тем, что в поступающем на обогатительные фабрики сырье содержится до 30% ма-териала крупностью менее 1 мм, роль процесса флотации существенно возрас-тает. Этому способствует и возможность создания достаточно простых замкну-тых водно-шламовых схем, включающих флотацию в качестве основного эле-мента очистки оборотных вод. Многими исследованиями, которые проводились ранее и продолжают выполняться и в настоящее время, установлены направле-ния совершенствования этого достаточно сложного физико-химического про-цесс
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