Energy transfer in the nanostar: the role of coulombic coupling and dynamics

We present a theoretical investigation of energy transfer in the phenylene ethynelene dendrimer known as the nanostar. Data from extensive molecular dynamics simulations are used to model the dynamical effects caused by torsional motion of the phenyl groups. We compare rate constants for energy transfer between the two-ring chromophore and the three-ring chromophore obtained via the F?rster model, the ideal dipole approximation (IDA), and the transition density cube (TDC) method, which has as its limit an exact representation of the Coulombic coupling. We find that the rate constants obtained with the TDC are extremely sensitive to the phenyl group rotation, whereas the constants computed with the F?rster model and the IDA are not. The implications of these results for the interpretation of recent pump-probe experiments on the nanostar are discussed in detail. Finally, we predict the temperature dependence of the rate constant for energy transfer.     

Novel oxa-di-pi-methane and Norrish type I reactions in the S2 (pi,pi*) excited state of a series of beta,gamma-unsaturated ketones

[reaction: see text] Beta,gamma-unsaturated methyl ketones with electron-withdrawing groups at the gamma-position of the ene moiety undergo ODPM rearrangements and Norrish type I reactions on direct irradiation at 254 nm. The results are consistent with the involvement of alkene S(2) (pi,pi*) as reactive excited states in these processes.     

Ground and excited states of NH4: Electron propagator and quantum defect analysis

Vertical excitation energies of the Rydberg radical NH4 are inferred from ab initio electron propagator calculations on the electron affinities of NH4+. The adiabatic ionization energy of NH4 is evaluated with coupled-cluster calculations. These predictions provide optimal parameters for the molecular-adapted quantum defect orbital method, which is used to determine Einstein emission coefficients and radiative lifetimes. Comparisons with spectroscopic data and previous calculations are discussed.     

Phosphate diester hydrolysis and DNA damage promoted by new cis-aqua/hydroxy copper(II) complexes containing tridentate imidazole-rich ligands

The tridentate Schiff base [(2-(imidazol-4-yl)ethyl)(1-methylimidazol-2-yl)methyl)imine (HISMIMI) and its reduced form HISMIMA were synthesized and characterized, as well their mononuclear cis-dihalo copper(II) complexes 1 and 2, respectively. In addition, the dinuclear [CuII(mu-OH)2CuII](2+) complexes (3) and (4) obtained from complexes 1 and 2, respectively, were also isolated and characterized by several physicochemical techniques, including magnetochemistry, electrochemistry, and EPR and UV-vis spectroscopies. The crystal structures of 1 and 2 were determined by X-ray crystallography and revealed two neutral complexes with their tridentate chelate ligands meridionally coordinated. Completing the coordination spheres of the square-pyramidal structures, a chloride ion occupies the apical position and another is bonded in the basal plane. In addition, complexes 1 and 2 were investigated by infrared, electronic, and EPR spectroscopies, cyclic voltammetry, and potentiometric equilibrium studies. The hydrolytic activity on phosphate diester cleavage of 1 and 2 was investigated utilizing 2,4-BDNPP as substrate. These experiments were carried out at 50 degrees C, and the data treatment was based on the Michaelis-Menten approach, giving the following kinetic parameters (complex 1/complex 2): vmax (mol L(-1) s(-1))=16.4x10(-9)/7.02x10(-9); KM (mol L(-1))=17.3x10(-3)/3.03x10(-3); kcat (s(-1))=3.28x10(-4)/1.40x10(-4). Complex 1 effectively promoted the hydrolytic cleavage of double-strand plasmid DNA under anaerobic and aerobic conditions, with a rate constant of 0.28 h(-1) for the decrease of form I, which represents about a 10(7) rate increase compared with the estimated uncatalyzed rate of hydrolysis.     

Carbohydrate-derived spiroketals: stereoselective synthesis of di-D-fructose dianhydrides via intramolecular aglycon delivery

[reaction: see text] A one-pot synthesis of C(2)-symmetric di-d-fructose dianhydrides having the 1,6,9,13-tetraoxadispiro [4.2.4.2]tetradecane skeleton has been accomplished via intramolecular aglycon delivery from (6 --> 6) xylylene-tethered fructofuranose precursors. The stereochemical outcome of the glycosylation-spiroketalization process is governed by the geometrical constraints imposed by the rigid tetracyclic structure of the final compound.     

Novel microscopic mechanism of intermixing during growth on soft metallic substrates

Generic computer simulations using empiric interatomic potentials suggest a new, collective mechanism that could be responsible for mixing at heteroepitaxial interfaces. Even if single adsorbate atoms diffuse by hopping on the substrate surface and do not mix at the terraces, two-dimensional islands formed by nucleation may become unstable above a certain critical size and explode upwards forming clusters of several atomic layers. This process is accompanied by strong distortions of the underlying atomic layers, and on soft materials it can result in surface etching and incorporation of substrate atoms into the islands.