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Obtaining reliable excitation energies of the isoelectronic series, UO22+, NUO+, and UN2, is a challenge in quantum chemistry calculations due to importance of electron correlation and spin-orbit coupling (SOC). Vertical spin-free and spin-orbit coupled excitation energies of these molecules are calculated in this work using equation-of-motion coupled-cluster approach at the CC singles and doubles level (EOM-CCSD). SOC is included in post-SCF calculations and this treatment has been shown to provide SOC effects with high accuracy for heavy elements. Excitation energies for UO22+ with the present approach are in good agreement with previous results using linear response CCSD, multiconfiguration perturbation theory (CASPT2), and multireference configuration interaction (MRCI). As for NUO+ and UN2, excitation energies with CASPT2 from two previous calculations differ significantly and the present results usually lie between these two sets of CASPT2 results. On the other hand, excitation energies with intermediate Hamiltonian Fock space CC method (IHFSCC) are generally too small compared with our results. This work provides new estimates on excitation energies of these molecules and it could be helpful in investigating spectroscopic and luminescence properties of larger uranium complexes. [ABSTRACT FROM AUTHOR]

A Brønsted acid catalyzed tandem Diels-Alder/aromatiza- tion reaction of 2-vinylindoles has been developed. The reaction provides a highly efficient and concise approach to 3-indolyl-substituted tetrahydrocarbazoles with various substituents in high yields under mild conditions. [ABSTRACT FROM AUTHOR]

Yb3+/Al3+/P5+-co-doped silica glasses with different P/Al ratios were prepared using the sol-gel method combined with high-temperature sintering. The evolution of composition-dependent color centers caused by X-ray irradiation in these glasses was correlated with their structural changes, which are controlled by the P/Al ratio. Nuclear magnetic resonance (NMR) and Raman spectra have been used to characterize the glass network structure, and advanced pulse electron paramagnetic resonance (EPR) has been employed to study the local coordination atomic structures of Yb3+ ions in pristine glasses as a function of the P/Al ratio. Si- (Si-E'), Al- (Al-E', Al-ODC, AlOHC), P- (P1, P2, POHC), and Yb-related (Yb2+) color centers in irradiated glasses have been observed and explained by optical absorption and continuous wave-EPR spectroscopies. The formation mechanisms of these centers, the structural models of glasses, and the relationship between them were proposed. Direct evidence confirms that the formation of Yb2+ ions induced by radiation is highly dependent on the coordination environment of Yb3+ ions in glasses. In addition, the glass network structure significantly affects the generation of oxygen hole color centers (AlOHCs/POHCs) caused by radiation. These results are useful in understanding the microstructural origin and the suppression mechanism of the radiodarkening effect by phosphorus co-doping in Yb3+-doped silica fibers. [ABSTRACT FROM AUTHOR]

Bond lengths and force constants of a set of closed-shell sixth-row and superheavy element monohydrides and monofluorides are calculated in this work. Kramers restricted coupled-cluster approaches (KR-CC) with spin-orbit coupling (SOC) included at the self-consistent field (SCF) level as well as CC approaches with SOC included in post-SCF treatment (SOC-CC) are employed in calculations. Recently published relativistic effective core potentials are employed, and highly accurate results for superheavy element molecules are achieved with KR-CCSD(T). SOC effects on bond lengths and force constants of these molecules are investigated. Effects of electron correlation are shown to be affected by SOC to a large extent for some superheavy element molecules. Bond lengths and force constants with SOC-CC agree very well with those of KR-CC for most of the sixth-row element molecules. As for superheavy element molecules, SOC-CCSD is able to afford results that are in good agreement with those of KR-CCSD except for 111F, while the error of SOC-CCSD(T) is more pronounced. Large error would be encountered with SOC-CC approaches for molecules when both SOC and electron correlation effects are sizable. [ABSTRACT FROM AUTHOR]

On the basis of our quantum mechanical calculation, we propose that homogeneous single-stranded adenine bases (Ade-DNA) form helical H aggregates, and the photoexcited states can be described as Frenkel excitons. The calculated excitonic coupling between adjacent transition dipoles is in good agreement with the measured absorption spectrum of 20-base homogeneous adenine stacks that exhibits a blue shift of 2.6 nm relative to that of the monomeric species. [ABSTRACT FROM AUTHOR]

Achieving both bond dissociation energies (BDEs) and their trends for the R-X bonds with R = Me, Et, i-Pr, and t-Bu reliably is nontrivial. Density functional theory (DFT) methods with traditional exchange-correlation functionals usually have large error on both the BDEs and their trends. The M06-2X functional gives rise to reliable BDEs, but the relative BDEs are determined not as accurately. More demanding approaches such as some double-hybrid functionals, for example, G4 and CCSD(T), are generally required to achieve the BDEs and their trends reliably. The fixed-node diffusion quantum Monte Carlo method (FN-DMC) is employed to calculated BDEs of these R-X bonds with X = H, CH3, OCH3, OH, and F in this work. The single Slater-Jastrow wave function is adopted as trial wave function, and pseudopotentials (PPs) developed for quantum Monte Carlo calculations are chosen. Error of these PPs is modest in wave function methods, while it is more pronounced in DFT calculations. Our results show that accuracy of BDEs with FN-DMC is similar to that of M06-2X and G4, and trends in BDEs are calculated more reliably than M06-2X. Both BDEs and trends in BDEs of these bonds are reproduced reasonably with FN-DMC. FN-DMC using PPs can thus be applied to BDEs and their trends of similar chemical bonds in larger molecules reliably and provide valuable information on properties of these molecules. [ABSTRACT FROM AUTHOR]

Hydrogen abstraction from toluene by OH, H, O, CH3, and HO2 radicals are important reactions in oxidation process of toluene. Geometries and corresponding harmonic frequencies of the reactants, transition states as well as products involved in these reactions are determined at the B3LYP/6-31G(2df,p) level. To achieve highly accurate thermochemical data for these stationary points on the potential energy surfaces, the Gaussian-4(G4) composite method was employed. Torsional motions are treated either as free rotors or hindered rotors in calculating partion functions to determine thermodynamic properties. The obtained standard enthalpies of formation for reactants and some prodcuts are shown to be in excellent agreement with experimental data with the largest error of 0.5 kcal mol-1. The conventional transition state theory (TST) with tunneling effects was adopted to determine rate constants of these hydrogen abstraction reactions based on results from quantum chemistry calculations. To faciliate its application in kinetic modeling, the obtained rate constants are given in Arrhenius expression: k (T) = ATn exp(-EaR/T). The obtained reaction rate constants also agree reasonably well with available expermiental data and previous theoretical values. Branching ratios of these reactions have been determined. The present reaction rates for these reactions have been used in a toluene combustion mechanism, and their effects on some combustion properties are demonstrated. [ABSTRACT FROM AUTHOR]

To provide insight on the reaction mechanism of the methyperoxy (CH3O2∙) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH3O2∙) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH3OOCH3 + Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. can be produced directly from 2(CH3O2∙), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH3O∙ + O2 and CH3OH + CH2O + O2 as determined before, the channels leading to CH3OOOH + CH2O and CH3O∙ + CH2O + HO2∙ are also energetically favorable in the self-reaction of CH3O2∙ especially at low temperature according to our results. [ABSTRACT FROM AUTHOR]

A new class of organocatalysts has been designed by rational combination of proline with cinchona alkaloids. The chiral amine 3a, prepared from L-proline and cinchonidine, has been found to be an efficient catalyst for the direct aldol reactions of acetone or 2-butanone with a wide range of aldehydes (up to 98% ee). The cinchonidine backbone is essential to the reaction efficiency and enantioselectivity. [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory (TDDFT) coupled to the relativistic two-component zeroth-order regular approximation, both available in the last version of the ADF package, have been successfully used to simulate X-ray absorption spectra of TiCl4, Ti(5-C5H5)Cl3, and Ti(5-C5H5)2Cl2in terms of their oscillator strength distributions. Besides allowing a first principle assignment of Ti 1s, Cl 1s, and Ti 2p (L2,3edges) core excitation spectra, theoretical outcomes provide a rationale for deviations from the expected L3/L2branching ratio. [ABSTRACT FROM AUTHOR]