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A study was conducted to explore the interaction of molecular hydrogen with the small molecules HF, [H.sub.2]O, N[H.sub.3], and LiOH. The interaction energy for the simultaneous interaction of [H.sub.2] with two LiOH molecules has not exceeded the value obtained for the interaction with a single LiOH molecule.

The correlated coupled-cluster theory with singles and doubles (CCSD) approach is employed for the calculation of the reaction barrier height for hydrogen abstraction by the hydroperoxyl radical from methane. The reactions of the [C.sub.n][H.sub.2n+2], where n = 2-4 alkanes, are also studied.

The article describes the benchmark, near basis set limit fc-CCSD(T) equilibrium harmonic frequencies for a set of 12 closed-shell and five open-shell molecules, which is computed numerically by using the CCSD(T)-R12 method. The corrections due to core-valance correlation are used for assessing the accuracy of the benchmark data, along with corrections from scalar relativistic effects.

The effect of basis set on second order Moller-Plesset perturbation (MP2) of several hydrogen-bonded complexes is studied. The possibilities of utilizing basis set extrapolations for geometries and dissociation energies are discussed along with the results of R12 methods and density functional and local correlation methods.

The larger aromatic systems should be favored over single benzene rings in order to increase the interaction of dihydrogen with aromatic systems. The investigations of different ways to store hydrogen are described.

We present a study of the structural evolution of tantalum cluster anions Ta n - , 6 ≤ n ≤ 13 using a combination of trapped ion electron diffraction (TIED) experiments with a variety of electronic structure methods. A genetic algorithm has been employed to establish a set of likely structures for each cluster, their geometries and energetics have been studied by density functional theory (DFT), random phase approximation, and two-component (2C) DFT methods, which include spin-orbit coupling. We find octahedral structures for Ta 6 - and Ta 8 - as well as structures based on the pentagonal bipyramid (Ta 7 - and Ta 9 - ). Ta 10 - -Ta 12 - are defective icosahedral structures and Ta 13 - is a distorted icosahedron. For most clusters, we find a good agreement between the theoretically predicted ground-state structures, especially those determined by the 2C method and the TIED results.

Dynamical effects on the mechanochemistry of linear alkane chains, mimicking polyethylene, are studied by means of molecular dynamics simulations. Butane and octane are studied using density-functional theory (DFT), whereas higher homologues are studied using a simple one-dimensional model in which the molecules are represented by a linear chain of Morse potentials (LCM). The application of a fixed external force to a thermodynamically pre-equilibrated molecule leads to a preference for cleavage of the terminal C-C bonds, whereas a sudden application of the force favors bond breaking in the central part of the chain. In all cases, transition-state theory predicts higher bond-breaking rates than found from the more realistic molecular dynamics simulations. The event of bond dissociation is related to dynamic states involving symmetric vibrational modes. Such modes do in general have lower frequencies of vibration than antisymmetric modes, which explains the deviation between the statistical theory and the dynamics simulations. The good qualitative agreement between the DFT and LCM models makes the latter a useful tool to investigate the mechanochemistry of long polymer chains.

The photophysics of chromophores is strongly influenced by their environment. Solvation, charge state, and adduct formation significantly affect ground and excited state energetics and dynamics. The present study reports on fluorescence emission of rhodamine B cations (RhBH + ) and derivatives in the gas phase. Substitution of the acidic proton of RhBH + by alkali metal cations, M + , ranging from lithium to cesium leads to significant and systematic blue shifts of the emission. The gas-phase structures and singlet transition energies of RhBH + and RhBM + , M = Li, Na, K, Rb, and Cs, were investigated using Hartree-Fock theory, density functional methods, second-order Møller-Plesset perturbation theory, and the second-order approximate coupled-cluster model CC2. Comparison of experimental and theoretical results highlights the need for improved quantum chemical methods, while the hypsochromic shift observed upon substitution appears best explained by the Stark effect due to the inhomogeneous electric field generated by the alkali ions.

We present gas-phase dispersed photoluminescence spectra of europium(III) 9-hydroxyphenalen-1-one (HPLN) complexes forming adducts with alkali metal ions ([Eu(PLN)3M](+) with M = Li, Na, K, Rb, and Cs) confined in a quadrupole ion trap for study. The mass selected alkali metal cation adducts display a split hypersensitive (5)D0 → (7)F2 Eu(3+) emission band. One of the two emission components shows a linear dependence on the radius of the alkali metal cation whereas the other component displays a quadratic dependence thereon. In addition, the relative intensities of both components invert in the same order. The experimental results are interpreted with the support of density functional calculations and Judd-Ofelt theory, yielding also structural information on the isolated [Eu(PLN)3M](+) chromophores.

The CH4 + HO2(*) reaction is studied by using explicitly correlated coupled-cluster theory with singles and doubles (CCSD-R12) in a large 19s14p8d6f4g3h basis (9s6p4d3f for H) to approach the basis-set limit at the coupled-cluster singles-doubles level. A correction for connected triple excitations is obtained from the conventional CCSD(T) coupled-cluster approach in the correlation-consistent quintuple-zeta basis (cc-pV5Z). The highly accurate results for the methane reaction are used to calibrate the calculations of the hydroperoxyl-radical hydrogen abstraction from other alkanes. For the alkanes C(n)H(2n+2) with n = 2 --> 4, the reactions are investigated at the CCSD(T) level in the correlation-consistent triple-zeta (cc-pVTZ) basis. The results are adjusted to the benchmark methane reaction and compared with those from other approaches that are commonly used in the field such as CBS-QB3, CBS-APNO, and density functional theory. Rate constants are computed in the framework of transition state theory, and the results are compared with previous values available.