O(3P) + CO2 collisions at hyperthermal energies: dynamics of nonreactive scattering, oxygen isotope exchange, and oxygen-atom abstraction

The dynamics of O((3)P) + CO(2) collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at = 98.8 kcal mol(-1) were performed with isotopically labeled (12)C(18)O(2) to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering ((16)O((3)P) + (12)C(18)O(2)), isotope exchange ((18)O + (16)O(12)C(18)O), and oxygen-atom abstraction ((18)O(16)O + (12)C(18)O). Stationary points on the two lowest triplet potential energy surfaces of the O((3)P) + CO(2) system were characterized at the CCSD(T)/aug-cc-pVTZ level of theory and by means of W4 theory, which represents an approximation to the relativistic basis set limit, full-configuration-interaction (FCI) energy. The calculations predict a planar CO(3)(C(2v), (3)A') intermediate that lies 16.3 kcal mol(-1) (W4 FCI excluding zero point energy) above reactants and is approached by a C(2v) transition state with energy 24.08 kcal mol(-1). Quasi-classical trajectory (QCT) calculations with collision energies in the range 23-150 kcal mol(-1) were performed at the B3LYP/6-311G(d) and BMK/6-311G(d) levels. Both reactive channels observed in the experiment were predicted by these calculations. In the isotope exchange reaction, the experimental center-of-mass (c.m.) angular distribution, T(θ(c.m.)), of the (16)O(12)C(18)O products peaked along the initial CO(2) direction (backward relative to the direction of the reagent O atoms), with a smaller isotropic component. The product translational energy distribution, P(E(T)), had a relatively low average of = 35 kcal mol(-1), indicating that the (16)O(12)C(18)O products were formed with substantial internal energy. The QCT calculations give c.m. P(E(T)) and T(θ(c.m.)) distributions and a relative product yield that agree qualitatively with the experimental results, and the trajectories indicate that exchange occurs through a short-lived CO(3)* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak (16)O(18)O product signal from an abstraction reaction was observed, which could only be detected in the forward direction. A small number of QCT trajectories leading to abstraction were observed to occur primarily via a transient CO(3) intermediate, albeit only at high collision energies (149 kcal mol(-1)). The oxygen isotope exchange mechanism for CO(2) in collisions with ground state O atoms is a newly discovered pathway through which oxygen isotopes may be cycled in the upper atmosphere, where O((3)P) atoms with hyperthermal translational energies can be generated by photodissociation of O(3) and O(2).     

A modified greedy algorithm for dispersively weighted 3-set cover

The set cover problem is that of computing a minimum weight subfamily F^′, given a family F of weighted subsets of a base set U, such that every element of U is covered by some subset in F^′. The k-set cover problem is a variant in which every subset is of size at most k. It has been long known that the problem can be approximated within a factor of by the greedy heuristic, but no better bound has been shown except for the case of unweighted subsets. In this paper we consider approximation of a restricted version of the weighted 3-set cover problem, as a first step towards better approximation of general k-set cover problem, where any two distinct subset costs differ by a multiplicative factor of at least 2. It will be shown, via LP duality, that an improved approximation bound of H(3)-1/6 can be attained, when the greedy heuristic is suitably modified for this case. A key to our algorithm design and analysis is the Gallai-Edmonds structure theorem for maximum matchings.     

Consistent scheme for computing standard hydrogen electrode and redox potentials

The standard hydrogen electrode (SHE) potential in aqueous solution was evaluated with new computational procedure that provides the Gibbs energy of a proton in aqueous solution from the experimental pK_a value and the Gibbs energy change by deprotonation reactions of several neutral alcohol molecules. With our computational scheme, the CCSD(T)/aug‐cc‐pVDZ method provides the SHE potential of 4.52 V, which is almost the same as the experimental SHE potential. This scheme also reproduces well the redox potentials of several typical reactions within almost 0.1 V. B3LYP also gives excellent redox potentials of the same reactions with almost the same accuracy with our new computational scheme. © 2012 Wiley Periodicals, Inc.     

Nanostructures formed by combination of nanotube and polymer chain

We have investigated polymeric supramolecular systems of inclusion complexes between molecular nanotube and polymer chains theoretically and experimentally. This system indicates inclusion-dissociation behavior much sharper than the inclusion complex between cyclic molecule and small compounds. And we propose some functional supramolecules utilizing the combination of the nanotube or nanoring and polymer chains such as insulated molecular wire and topological gel.     

Syntheses of 2‐alkylthio‐1,3‐thiazine derivatives from S‐alkyldithiocarbamates and α,β‐unsaturated ketones

Various 2‐alkylthio‐1,3‐thiazine derivatives were synthesized by the reactions of S‐alkylthiocarbamates with α,β‐unsaturated ketones in the presence of BF₃ · Et₂O. The thiazine was converted into two isomeric dehydrated products in the presence of a Lewis acid. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:377–379, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10055     

Complex Formation Between Poly(dimethylsilane) and Cyclodextrins

Poly(dimethylsilane)s form inclusion complexes with cyclodextrins in high selectivity to give crystalline compounds. β‐Cyclodextrin forms complexes with poly(dimethylsilane)s of low molecular weight only, γ‐cyclodextrin with poly(dimethylsilane)s of high molecular weight in high yield, and α‐cyclodextrin does not form complexes with poly(dimethylsilane) at all. Complexes were isolated and characterized by spectroscopic methods and X‐ray diffraction.