Trajectory surface hopping study of the O(3P) + ethylene reaction dynamics

Spin-orbit coupling (SOC) induced intersystem crossing (ISC) has long been believed to play a crucial role in determining the product distributions in the O(3P) + C2H4 reaction. In this paper, we present the first nonadiabatic dynamics study of the title reaction at two center-of-mass collision energies: 0.56 eV, which is barely above the H-atom abstraction barrier on the triplet surface, and 3.0 eV, which is in the hyperthermal regime. The calculations were performed using a quasiclassical trajectory surface hopping (TSH) method with the potential energy surface generated on the fly at the unrestricted B3LYP/6-31G(d,p) level of theory. To simplify our calculations, nonadiabatic transitions were only considered when the singlet surface intersects the triplet surface. At the crossing points, Landau-Zener transition probabilities were computed assuming a fixed spin-orbit coupling parameter, which was taken to be 70 cm-1 in most calculations. Comparison with a recent crossed molecular beam experiment at 0.56 eV collision energy shows qualitative agreement as to the primary product branching ratios, with the CH3 + CHO and H + CH2CHO channels accounting for over 70% of total product formation. However, our direct dynamics TSH calculations overestimate ISC so that the total triplet/singlet ratio is 25:75, compared to the observed 43:57. Smaller values of SOC reduce ISC, resulting in better agreement with the experimental product relative yields; we demonstrate that these smaller SOC values are close to being consistent with estimates based on CASSCF calculations. As the collision energy increases, ISC becomes much less important and at 3.0 eV, the triplet to singlet branching ratio is 71:29. As a result, the triplet products CH2 + CH2O, H + CH2CHO and OH + C2H3 dominate over the singlet products CH3 + CHO, H2 + CH2CO, etc.     

Theoretical investigation of hyperthermal reactions at the gas-liquid interface: O (3P) and squalane

Hyperthermal collisions (5 eV) of ground-state atomic oxygen [O ((3)P)] with a liquid-saturated hydrocarbon, squalane (C(30)H(62)), have been studied using QM/MM hybrid "on-the-fly" direct dynamics. The surface structure of the liquid squalane is obtained from a classical molecular dynamics simulation using the OPLS-AA force field. The MSINDO semiempirical Hamiltonian is combined with OPLS-AA for the QM/MM calculations. In order to achieve a more consistent and efficient simulation of the collisions, we implemented a dynamic partitioning of the QM and MM atoms in which atoms are assigned to QM or MM regions based on their proximity to "seed" (open-shell) atoms that determine where bond making/breaking can occur. In addition, the number of seed atoms is allowed to increase or decrease as time evolves so that multiple reactive events can be described. The results show that H abstraction is the most important process for all incident angles, with H elimination, double H abstraction, and C-C bond cleavage also being important. A number of properties of these reactive channels, as well as inelastic nonreactive scattering, are investigated, including angular and translational energy distributions, the effect of incident collision angle, variation with depth of the reactive event within the liquid, with the reaction site on the hydrocarbon, and the effect of dynamics before and after reaction (direct reaction versus trapping reaction-desorption).     

Significant nonadiabatic effects in the S(1D) + HD reaction

A nonadiabatic quantum dynamics calculation involving four coupled potential energy surfaces (two degenerate 3A' ', one 3A', and one 1A') and the spin-orbit coupling matrix for these states is reported for the title reaction. The results show that the important discrepancy between theoretically calculated and experimentally measured intramolecular isotope effects can at least in part be attributed to significant nonadiabatic effects.     

Sharp melting of polymer-DNA hybrids: an associative phase separation approach

An associative equilibrium theory describing the sharp melting behavior of polymer-DNA hybrids is developed. The theory considers linear polymers with attached DNAs on each polymer that serve as "stickers" and with a two-state model governing the DNA melting equilibrium. For three or more oligonucleotides on each polymer, solutions of polymer-DNA hybrids are found to undergo phase separation at sufficiently low temperatures. The dense phase dissolves as temperature increases, which leads to a sharp increase in the fraction of non-hybridized DNA near the phase transition temperature, in agreement with experimental absorbance profiles at 260 nm. The melting temperature is predicted to have the same dependence on salt concentration as a solution of unattached DNAs and be weakly sensitive to the concentration of DNA in solution. The melting temperature is predicted to be higher than that of unattached DNA in solution, with the magnitude of the increase sensitive to the DNA hybridization cooperativity. The theoretical predictions are generally in good quantitative agreement with new experimental data (also presented here), which show the effect of the polymer-DNA hybrid length and salt concentration on the melting profiles.     

Calix[4]arenes as Molecular Platforms for Magnetic Resonance Imaging (MRI) Contrast Agents

The novel amphipilic conjugate of a calix[4]arene with four Gd–1,4,7,10‐ tetra(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane (DOTA) chelates has potential as a magnetic resonance imaging contrast agent, both in its monomeric and in its micellar form. The system, illustrated here with its nuclear magnetic relaxation profile, shows good relaxivities, thanks to its high rigidity.

     

Polysaccharide‐block‐polypeptide Copolymer Vesicles: Towards Synthetic Viral Capsids

Natural inspiration : Amphiphilic polysaccharide‐block‐polypeptide copolymers were synthesized by click chemistry from dextran end‐functionalized with an alkyne group and poly(γ‐benzyl L ‐glutamate) end‐functionalized with an azide group. The ability of these copolymers to self‐assemble into small vesicles (see picture) suggests the possibility of a new generation of drug‐ and gene‐delivery systems whose structure mimics that of viruses.

     

Enhancing the melting properties of small molecule-DNA hybrids through designed hydrophobic interactions: an experimental-computational study

Detailed experimental and computational studies revealed the important role that hydrophobic interactions play in the aqueous assembly of rigid small molecule-DNA hybrid (rSMDH) building blocks into nanoscale cage and face-to-face (ff) dimeric structures. In aqueous environments, the hydrophobic surfaces of the organic cores in these nanostructures are minimized by interactions with the core in another rSMDHs, with the bases in the attached DNA strands, and/or with the base pairs in the final assembled structures. In the case that the hydrophobic surfaces of the cores could not be properly isolated in the assembly process, an ill-defined network results instead of dimers, even at low concentration of DNA. In contrast, if ff dimers can be formed with good minimization of the exposed hydrophobic surfaces of the cores, they are highly stable structures with enhanced melting temperatures and cooperative melting behavior.     

CO2 hydrogenation to formic acid on Ni(110)

Hydrogen (H) in the subsurface of transition-metal surfaces exhibits unique reactivity for heterogeneously catalyzed hydrogenation reactions. Here, we explore the potential of subsurface H for hydrogenating carbon dioxide (CO₂) on Ni(110). The energetics of surface and subsurface H reacting with surface CO₂ to form formate, carboxyl, and formic acid on Ni(110) is systematically studied using self-consistent, spin-polarized, periodic density functional theory (DFT-GGA-PW91) calculations. We show that on Ni(110), CO₂ can be hydrogenated to formate by surface H. However, further hydrogenation of formate to formic acid by surface H is hindered by a larger activation energy barrier. The relative energetics of hydrogenation barriers is reversed for the carboxyl-mediated route to formic acid. We suggest that the energetics of subsurface H emerging to the surface is suitable for providing the extra energy needed to overcome the barrier to formate hydrogenation. CO₂ hydrogenation to formic acid could take place on Ni(110) when subsurface H is available to react with CO₂. Additional electronic-structure based dynamic calculations would be needed to elucidate the detailed reaction paths for these transformations.     

Isochronous Mass Measurements of Hot Exotic Nuclei

A novel method for mass measurements of short-lived exotic nuclides is presented. Exotic nuclides were produced and separated in flight at relativistic energies with the fragment separator (FRS) and were injected into the experimental storage ring (ESR). Operating the ESR in the isochronous mode we performed mass measurements of neutron deficient fragments of ⁸⁴Kr with half-lives larger than 50 ms. However, this experimental technique is applicable in a half-life range down to a few μs. A mass resolving power of 110000 (FWHM) has been achieved. Results are presented for the masses of ⁶⁸As, ^70,71Se and ⁷³Br. This revised version was published online in July 2006 with corrections to the Cover Date.