Vitamin B12 catalyzes the reductive dechlorination of several ubiquitous pollutants including the conversion
of trichloroethylene (TCE) to ∼95% cis-1,2-dichloroethylene (DCE) and small amounts of trans-DCE and
1,1-DCE. The origins of this unexpected selectivity were investigated using density functional and coupled-cluster theory. At all levels of theory considered, the initially formed trichloroethylene radical anion is an
unstable species. Breakage of one of the three C−Cl bonds during the dissociative process gives the most
stable ion complex when the two remaining chlorines occupy a cis geometry. Once formed, the cis-1,2-dichloroethen-1-yl radical is about 6 kJ/mol more stable than the corresponding trans radical and 21 kJ/mol
more stable than the 1,1-dichloroethen-2-yl radical. The calculated relative energies can be rationalized by
delocalization of the unpaired electron over the nonbonding orbitals of the α-chlorine. The computed geometries
of the radicals suggest significant interactions between the orbital occupied by the unpaired electron and the
σ* orbital of the β C−Cl bond trans to the radical. The barrier for interconversion of the two 1,2-dichlorinated
vinyl radicals lies between ∼30−40 kJ/mol depending on the level of theory. The reactivities of the three
radicals with respect to hydrogen atom abstraction from methanol (C−H or O−H) as well as chlorine
elimination were investigated. All three radicals show a strong preference for abstraction of the α-hydrogen
atom of methanol (17−25 kJ/mol), with a significant positive reaction energy for chlorine elimination (60−80 kJ/mol). These results are discussed further in relation to the experimentally observed product distribution.
We present a self-consistent field algorithm for the restricted open-shell Kohn-Sham method which can be used to calculate excited states that have the same spatial symmetry as the corresponding ground states. The method is applied to -* transitions in polyenes, cyanines, and protonated imines. Excitation energies obtained with gradient corrected functionals are found to be significantly redshifted; the shift is constant within a homologous series. Planar excited state geometries have been optimized for all systems.
We present first‐principles molecular dynamics simulations of azobenzene and a sterically hindered derivative in the first excited state. The restricted open‐shell Kohn–Sham (ROKS) approach is employed to describe the motion in the lowest excited state. The rotational pathway is observed in the molecular dynamics simulations for both azobenzene and its azacrown ether capped derivative.
The bipropellant monomethylhydrazine/nitrogen tetroxide is used as a rocket fuel. Molecular dynamics simulations provide an explanation for the "cold prereaction", which yields methyldiazene and nitrous acid, and for the subsequent explosive reaction via dimethyltetrazane. I. Frank and co-workers give details in the following Communication. The bipropellant monomethylhydrazine (MMH)/nitrogen tetroxide (NTO) has been employed in astronautics for a long time, [1] for example, as fuel for the upper stage engine of the European launch vehicle Ariane 5 and in various U.S. spacecraft engines of the RS series (e.g. RS-21, RS-25, RS-28, RS-41, and RS-42). However, the mixture is unfortunately problematic in its handling and application. In 2001, flight 510 of the Ariane 5 project encountered serious problems due to complications within the upper stage engine.[2]On contact the components of the mixture react violently, explosively emitting heat and gas, and therefore the mixture is known as hypergolic. However, the mixture does not heat up during the reaction that precedes ignition ("cold prereaction"). Such a fast process is not easy to examine experimentally in full detail, as it is especially difficult to create a well-defined mixture of the reactants. Here theory provides an attractive alternative in terms of molecular dynamics simulations [5][6][7][8] within the Car-Parrinello approach, [3,4] where the extreme reactivity of the system is even advantageous. Car-Parrinello Molecular Dynamics (CPMD) allows the simulation of different types of reactions in a system through the description of the electronic structure with the density functional theory approach. [9,10] It is not necessary to predefine a reaction coordinate; rather, all degrees of freedom of the system can be considered. This study does not focus on highly precise computations of properties of isolated molecules. It aims rather at determining the relevant reaction schemes under varying reaction conditions like the ratio of starting materials, temperature, and impurities.As the first step, an MMH/NTO mixture with a ratio of 1:1 and a density of 0.9 g cm À3 was simulated (Scheme 1 and Figure 1). At temperatures above 300 K we initially observe electron transfer: an electron is transferred from an MMH molecule to an NTO molecule while the latter dissociates. The resulting ions are neutralized by a consecutive proton transfer. Afterwards a second electron is transferred, and again proton transfer yields neutral products (Scheme 1; details on the energetics are given in the Supporting Information). The oxidation state of the MMH molecule and its products during the molecular dynamics run can easily be determined from Scheme 1. Redox reaction leading from MMH to methyldiazene as observed in the CPMD simulations.
We present first principles molecular dynamics simulations of the photochemistry of butadiene and cyclohexadiene. The excited state is described with restricted open-shell Kohn–Sham theory. We observe cis–trans isomerizations for 1,3-butadiene and conrotatory ring opening of cyclohexadiene. The excitation of a sample of several butadiene molecules leads to the formation of an excimer.
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