Rotational effects in the dissociative adsorption of H2 on the Pt211 stepped surface

Rotational effects in the dissociative adsorption of H2 on the Pt211 stepped surface have been studied using classical trajectory calculations on a six-dimensional, density-functional theory potential-energy surface. Reaction of rotating molecules via an indirect trapping mechanism exhibits an unexpected nonmonotonic dependence on the initial rotational quantum number J. Indirect reaction is first quenched with increasing J but is enhanced again for high J initial states. The quenching is attributed to rotational-to-translational energy transfer, which facilitates escape from the chemisorption wells responsible for molecular trapping. For high J, rotational and translational motions decouple, and the energy transfer is no longer possible, which leads again to trapping. Degeneracy-resolved calculations show that for high initial J, molecules rotating in a "cartwheel" fashion (mJ=0) are more likely to become trapped and react indirectly than "helicoptering" molecules (mJ=J). Experimental confirmation of this finding would lend strong support to the existence of the chemisorption wells that trap molecules prior to reaction.     

Asymptotic correction of the exchange-correlation kernel of time-dependent density functional theory for long-range charge-transfer excitations

Time-dependent density functional theory (TDDFT) calculations of charge-transfer excitation energies omegaCT are significantly in error when the adiabatic local density approximation (ALDA) is employed for the exchange-correlation kernel fxc. We relate the error to the physical meaning of the orbital energy of the Kohn-Sham lowest unoccupied molecular orbital (LUMO). The LUMO orbital energy in Kohn-Sham DFT--in contrast to the Hartree-Fock model--approximates an excited electron, which is correct for excitations in compact molecules. In CT transitions the energy of the LUMO of the acceptor molecule should instead describe an added electron, i.e., approximate the electron affinity. To obtain a contribution that compensates for the difference, a specific divergence of fxc is required in rigorous TDDFT, and a suitable asymptotically correct form of the kernel fxc(asymp) is proposed. The importance of the asymptotic correction of fxc is demonstrated with the calculation of omegaCT(R) for the prototype diatomic system HeBe at various separations R(He-Be). The TDDFT-ALDA curve omegaCT(R) roughly resembles the benchmark ab initio curve omegaCT CISD(R) of a configuration interaction calculation with single and double excitations in the region R=1-1.5 A, where a sizable He-Be interaction exists, but exhibits the wrong behavior omegaCT(R)相似文献   

EM Heating-Stimulated Water Flooding for Medium–Heavy Oil Recovery

We report a study of heavy oil recovery by combined water flooding and electromagnetic (EM) heating at a frequency of 2.45 GHz used in domestic microwave ovens. A mathematical model describing this process was developed. Model equations were solved, and the solution is presented in an integral form for the one-dimensional case. Experiments consisting of water injection into Bentheimer sandstone cores, either fully water saturated or containing a model heavy oil, were also conducted, with and without EM heating. Model prediction was found to be in rather good agreement with experiments. EM energy was efficiently absorbed by water and, under dynamic conditions, was transported deep into the porous medium. The amount of EM energy absorbed increases with water saturation. Oil recovery by water flooding combined with EM heating was up to \(37.0\%\) larger than for cold water flooding. These observations indicate that EM heating induces an overall improvement in the mobility ratio between the displacing water and the displaced heavy oil.     

Interaction of microbubbles with high intensity pulsed ultrasound

High intensity pulsed ultrasound, interacting with microbubble contrast agents, is potentially useful for drug delivery, cancer treatment, and tissue ablation, among other applications. To establish the fundamental understanding on the interaction of a microbubble (in an infinite volume of water) with an ultrasound pressure field, a numerical study is performed using the boundary element method. The response of the bubble, in terms of its shape at different times, the maximum bubble radius obtained, the oscillation time, the jet velocity, and its translational movement, is studied. The effect of ultrasound intensity and initial bubble size is examined as well. One important outcome is the determination of the conditions under which a clear jet will be formed in a microbubble in its interaction with a specific sound wave. The high speed jet is crucial for the aforementioned intended applications.     

Electronic structure,chemical bond,and optical spectra of metal bis(porphyrin) complexes: a DFT/TDDFT study of the bis(porphyrin)M(IV) (M = Zr,Ce, Th) series

The electronic absorption spectra of the bis(porphyrin) sandwich complexes of the metals Zr, Ce, and Th are studied with time-dependent density functional theory (TDDFT). A ground-state electronic structure analysis reveals that the highest occupied one-electron levels are, as expected, composed of the porphyrin a(1u) and a(2u) highest occupied orbitals (the Gouterman orbitals), but the level pattern is not simply a pair of low-lying nearly degenerate in-phase combinations and a pair of high-lying approximately degenerate antibonding combinations. Instead, the a(1u) split strongly and the a(2u) do not. Since the calculated spectrum agrees very well with experiment, the assignment leaves little doubt that although the experimental spectrum has porphyrin-like features, such as the well-known Q and B bands, the actual composition of the states is rather different from that in porphyrin. In particular the strong mixing of a(1u) --> e(g) and a(2u) --> e(g) is absent, there is mixing with excitations of non-Gouterman type, and, in Ce, ring to metal charge-transfer transitions play an important role. The composition of the states as calculated in this work does not lead to a classification of the excitations as purely "excitonic" or "charge-resonance".     

Mapping of a discontinuous and highly conformational binding site on follicle stimulating hormone subunit-β (FSH-β) using domain Scan™ and Matrix Scan™ technology

This paper describes the application of two novel screening technologies, i.e. Domain Scan (24- and 30-mer peptides) and Matrix Scan (24-mer peptides) technology, in the mapping of a discontinuous epitope on FSH-beta for a series of 20 monoclonal antibodies. 11 out of 20 mAb's, mapping of which was not successful by conventional Pepscan technology (12-mer peptides), showed selective binding to peptide-constructs corresponding to the beta3-loop of FSH in the Domain and/or Matrix Scan. Systematic replacement analysis studies with peptide-construct 57VYETVRVPGCAC-SAc-ADSLYTYPVATQ81 revealed that for most mAb's the amino acids R62, A70, D71, and L73 form the core of the epitope. A Domain Scan performed in the C-O format showed highly selective binding for mAb's 1 and 2 with only three beta1-beta3 peptide-constructs covering the residues 60TVRVPGCAHHADSLY74 in combination with 10IAIEKEECRFAI21, while for mAb 10 binding was observed with peptide-constructs containing the C-terminal residues 97RGLGPSYCSFGEMKE114 in combination with the residues 10IAIEKEECRFAI21. A Matrix Scan of mAb 17 showed that peptides from four different regions on FSH (1st strand beta3-loop, alpha 1-loop, long alpha2-loop, det. loop) showed enhanced binding in combination with several 70ADSL73-containing peptides. BIACORE measurements with mAb's 1, 2, 13, and 17 using a set of 21 different peptide(-construct)s partially confirmed the Domain and Matrix Scan screening results. Only 24- and 33-mer peptides covering both the 1st and 2nd strand of the beta3-loop showed measurable binding. Cyclic beta3-loop peptide mimics were found to bind significantly stronger (Kd approximately 5 microM) than the lineair analogues, in agreement with the fact that the discontinuous epitope is part of a loop structure. Coupling of the lineair beta1-peptide 1oIAIEKEECRFAI21 to the linear beta3-peptide *52TFKELVYETVRVPGCAHHADSLYTYPVATQAH83# via disulfide bond formation showed a 2-3 fold increase in Kd, thus conforming participation of the beta 1-loop in antibody binding for these mAb's.     

Solvation of p-coumaric acid in water

The photoactive yellow protein (PYP) is an important model protein for many (photoactive) signaling proteins. Key steps in the PYP photocycle are the isomerization and protonation of its chromophore, p-coumaric acid (pCA). In the ground state of the protein, this chromophore is in the trans configuration with its phenolic oxygen deprotonated. For this paper, we studied four different configurations of pCA solvated in water with ab initio molecular dynamics simulations as implemented in CP2K/Quickstep. We researched the influence of the protonation and isomerization state of pCA on its hydrogen-bonding properties and on the Mulliken charges of the atoms in the simulation. The chromophore isomerization state influenced the hydrogen-bonding less than its protonation state. In general, more charge yielded a higher hydrogen-bond coordination number. Where deprotonation increases both the coordination number and the residence time of the water molecules around the chromophore, protonation showed a somewhat lower coordination number on two of the three pCA oxygens but much higher residence times on all of them. This could be explained by the increased polarization of the OH groups of the molecule. The presence of the chromophore also influenced the charge and polarization of the water molecules around it. This effect was different in the four systems studied and mainly localized in the first solvation shell. We also performed a proton-transfer reaction from hydronium through various other water molecules to the chromophore. In this small charge-separated system, the protonation occurred within 6.5 ps. We identified the transition state for the final step in this protonation series.     

Helmholtz equation and non-singular boundary elements applied to multi-disciplinary physical problems

The famous scientist Hermann von Helmholtz was born 200 years ago. Many complex physical wave phenomena in engineering can effectively be described using one or a set of equations named after him: the Helmholtz equation. Although this has been known for a long time, from a theoretical point of view, the actual numerical implementation has often been hindered by divergence-free and/or curl-free constraints. There is further a need for a numerical method that is accurate, reliable and takes into account radiation conditions at infinity. The classical boundary element method satisfies the last condition, yet one has to deal with singularities in the implementation. We review here how a recently developed singularity-free three-dimensional boundary element framework with superior accuracy can be used to tackle such problems only using one or a few Helmholtz equations with higher order (quadratic) elements which can tackle complex curved shapes. Examples are given for acoustics (a Helmholtz resonator among others) and electromagnetic scattering.