2011 Vol. 24, No. 5

Content
Content
2011, 24(5): 0-0. doi: 10.1088/1674-0068/24/5/0-0
Special Issue
To advance hierarchical equations of motion as a standard theory for quantum dissipative dynamics, we put forward a mixed Heisenberg-Schr?dinger scheme with block-matrix imple-mentation on efficient evaluation of nonlinear optical response function. The new approach is also integrated with optimized hierarchical theory and numerical filtering algorithm. Dif-ferent configurations of coherent two-dimensional spectroscopy of model excitonic dimer systems are investigated, with focusing on the effects of intermolecular transfer coupling and bi-exciton interaction.
The dynamics of F+HD→HF+D reaction has been studied at ten collision energies rang-ing from 5.43 kJ/mol to 18.73 kJ/mol using high-resolution H/D atom Rydberg tagging time-of-flight method. Product vibrational and rotational state-resolved differential cross sections have been determined. The intensity of the HF(v′=2) forward products decreases as the collision energy increases, suggesting that the resonance contribution is reduced as the collision energy increases. The forward peak of HF(v′=3) product has also been observed above the threshold of this product channel. Product energy disposals in different degrees offreedom have been analyzed. The collision energy dependence of the HF vibrational product branching was also determined. This work presents a comprehensive dynamic picture of this resonance mediated reaction in a wide collision energy regime, providing a good test ground for theoretical understandings of this interesting reaction at higher collision energies.
The interaction of mineral oxides (α-Al2O3, MgO, Fe2O3, and SiO2) with hydrogen peroxide was investigated using the Knudsen cell reactor. The initial reactive uptake coefficients for the commercially available powders are measured as (1.00±0.11)×10-4 for α-Al2O3, (1.66±0.23)×10-4 for MgO, (9.70±1.95)×10-5 for Fe2O3, and (5.22±0.9)×10-5 for SiO2. These metal oxide powders exhibit some catalytic behavior toward the decomposition of hydrogen peroxide excluding SiO2. H2O2 can be destroyed on Fe2O3 surface and O2 is formed. The experimental results suggest that the heterogeneous loss on mineral surface can represent an important sink of hydrogen peroxide.
The crossed beams scattering dynamics of the F+HD→DF+H reaction have been studied at collision energies ranging from 8.19 kJ/mol to 18.98 kJ/mol using the high resolution H-atom Rydberg tagging time-of-flight method. Product rotational state-resolved differen-tial cross sections have been measured. Most of the DF products are backward scattered at low collision energies and then gradually shift to the sideway as the collision energy in-creases. In addition to the backward and sideway scatterings, we have also observed the DF(v′=4) product in the forward direction for the first time for this reaction. The forward scattering DF(v′=4) product also increases with the collision energy. Angular and collision energy dependence of the product energy disposals in different degrees of freedom have been determined. Collision energy dependence of the vibrational branching ratios has also been examined. Possible dynamical origins of the forward scattering DF(v′=4) products were discussed.
In our recent work [Phys. Chem. Chem. Phys. 11, 9149 (2009)], a molecular-mechanics force field-based amide-I vibration frequency map (MM-map) for peptides and proteins was constructed. In this work, the temperature dependence of the MM-map is examined based on high-temperature molecular dynamics simulations and infrared (IR) experiments. It is shown that the 298-K map works for up to 500-K molecular dynamics trajectories, which reasonably reproduces the 88 oC experimental IR results. Linear IR spectra are also simulated for two tripeptides containing natural and unnatural amino acid residues, and the results are inreasonable agreement with experiment. The results suggest the MM-map can be used to obtain the temperature-dependent amide-I local mode frequencies and their distributions for peptide oligomers, which is useful in particular for understanding the IR signatures of the thermally unfolded species.
Triplet-triplet energy transfer in fluorene dimer is investigated by combining rate theories with electronic structure calculations. The two key parameters for the control of energy transfer, electronic coupling and reorganization energy, are calculated based on the diabatic states constructed by the constrained density functional theory. The fluctuation of the electronic coupling is further revealed by molecular dynamics simulation. Succeedingly, the diagonal and off-diagonal fluctuations of the Hamiltonian are mapped from the correlation functions of those parameters, and the rate is then estimated both from the perturbationtheory and wavepacket diffusion method. The results manifest that both the static and dynamic fluctuations enhance the rate significantly, but the rate from the dynamic fluctuation is smaller than that from the static fluctuation.
We present nonadiabatic quantum dynamical calculations on the two coupled potential en-ergy surfaces (12A′ and 22A′) [J. Theor. Comput. Chem. 8, 849 (2009)] for the reaction. Initial state-resolved reaction probabilities and cross sections for the N+ND→N2+D reaction and N′+ND→N′D+N reaction for collision energies of 5 meV to 1.0 eV are determined, re-spectively. It is found that the N+ND→N2+D reaction is dominated in the N+ND reaction.In addition, we obtained the rate constants for the N+ND→N2+D reaction which demand further experimental investigations.
The ultrafast dynamics through conical intersections in 2,6-dimethylpyridine has been stud-ied by femtosecond time-resolved photoelectron imaging coupled with time-resolved mass spectroscopy. Upon absorption of 266 nm pump laser, 2,6-dimethylpyridine is excited to the S2 state with a ππ* character from S0state. The time evolution of the parent ion sig-nals consists of two exponential decays. One is a fast component on a timescale of 635 fs and the other is a slow component with a timescale of 4.37 ps. Time-dependent photo-electron angular distributions and energy-resolved photoelectron spectroscopy are extracted from time-resolved photoelectron imaging and provide the evolutive information of S2 state. In brief, the ultrafast component is a population transfer from S2 to S1 through the S2/S1 conical intersections, the slow component is attributed to simultaneous IC from the S2 state and the higher vibrational levels of S1 state to S0 state, which involves the coupling of S2/S0 and S1/S0 conical intersections. Additionally, the observed ultrafast S2→S1 transition occurs only with an 18% branching ratio.
The AgOCH3- and Ag-(CH3OH)x (x=1, 2) anions are studied by photoelectron imaging as well as ab initio calculations. The adiabatic and vertical detachment energies (ADE and VDE) of AgOCH3- are determined as 1.29(2) and 1.34(2) eV, respectively, from the vibrational resolved photoelectron spectrum. The Ag-(CH3OH)1,2 anionic complexes are characterized as metal atomic anion solvated by the CH3OH molecules with the electron mainly localized on the metal. The photoelectron spectra of Ag-(CH3OH)x (x=0, 1, 2) show a gradual increase in VDE with increasing x, due to the solvent stabilization. Evidence for the methanol-methanol hydrogen bonding interactions appears when the Ag- is solvated by two methanol molecules.
The structure-property characteristics of a series of newly synthesized intramolecular charge-transfer (ICT) compounds, single-branch monomer with triphenylmethane as electron donor and 2,1,3-benzothiadiazole as acceptor, the corresponding two-branch dimer and three-branch trimer, have been investigated by means of steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) techniques in different polar solvents. The TD-DFT calculations are further performed to explain the observed ICT properties. The interpretation of the experimental results is based on the comparative stud-ies of the series of compounds which have increased amount of identical branch moiety. The similarity of the absorption and fluorescence spectra as well as strong solvent-dependence of the spectral properties for the three compounds reveal that the excited state of the dimer and trimer are nearly the same with that of the monomer, which may localize on one branch. It is found that polar excited state emerged through multidimensional intramolecular charge transfer from the donating moiety to the acceptor upon excitation, and quickly relaxed to one branch before emission. Even so, the red-shift in the absorption and emission spectra and decreased fluorescence radiative lifetime with respect to their monomer counterpart still suggest some extent delocalization of excited state in the dimer and trimer upon excitation. The similar behavior of their excited ICT state is demonstrated by FS TR-SEP FD mea-surements, and shows that the trimer has the largest charge-separate extent in all studied three samples. Finally, steady-state excitation anisotropy measurements has further been carried out to estimate the nature of the optical excitation and the mechanism of energy redistribution among the branches, where no plateau through the ICT band suggests the intramolecular excitation transfer process between the branches in dimer and trimer.
Oxygen-poor vanadium oxide clusters, V2On+ (n=1, 2), V3On+ (n=1, 2, 3), and V4O3+, were produced by laser vaporization and were mass-selected and photodissociated with 532 and 266 nm photons. The geometric structures and possible dissociation channels of these clusters were determined based on the comparison of density functional calculations and pho-todissociation experiments. The experiments show that the dissociation of V2O+, V2O2+, and V3O3+ mainly occurs by loss of VO, while the dissociation of V3O+ and V4O3+ mainly occurs by loss of V atom. For the dissociation of V3O2+, the VO loss channel is slightly dominant compared to the V loss channel. The combination of experimental results and theoretical calculations suggests that the V loss channels of V3O+ and V4O3+ are single photon processes at both 532 and 266 nm. The VO loss channels of V2O2+ and V3O3+ are multiple-photon processes at both 532 and 266 nm.
Pentachlorophenol, a widespread environmental pollutant that is possibly carcinogenic to humans, is metabolically oxidized to tetrachloroquinone (TCBQ) which can result in DNA damage. We have investigated the photochemical reaction dynamics of TCBQ with two pyrimidine type nucleobases (thymine and uracil) upon UVA (355 nm) excitation using the technique of nanosecond time-resolved laser flash photolysis. It has been found that 355 nm excitation populates TCBQ molecules to their triplet state 3TCBQ*, which are highly reactive towards thymine or uracil and undergo two parallel reactions, the hydrogen abstraction and electron transfer, leading to the observed photoproducts of TCBQH· and TCBQ·- in transient absorption spectra. The concomitantly produced nucleobase radicals and radical cations are expected to induce a series of oxidative or strand cleavage damage to DNA afterwards. By characterizing the photochemical hydrogen abstraction and electron transfer reactions, our results provide potentially important molecular reaction mechanisms for understanding the carcinogenic effects of pentachlorophenol and its metabolites TCBQ.
Vanadium oxide clusters VxOyq (x≤8, q=0,±1) are classified according to the oxidation index (△=2y+q-5x) of each cluster. Density functional calculations indicate that clusters with the same oxidation index tend to have similar bonding characters, electronic structures, and reactivities. This general rule leads to the findings of new possible ground state struc-tures for V2O6 and V3O6+ clusters. This successful application of the classification method on vanadium oxide clusters proves that this method is very effective in studying the bonding properties of early transition metal oxide clusters.
A two-dimensional generalized Langevin equation is proposed to describe the protein con-formational change, compatible to the electron transfer process governed by atomic packing density model. We assume a fractional Gaussian noise and a white noise through bond and through space coordinates respectively, and introduce the coupling effect coming from both fluctuations and equilibrium variances. The general expressions for autocorrelation functions of distance fluctuation and fluorescence lifetime variation are derived, based on which the exact conformational change dynamics can be evaluated with the aid of numerical Laplace inversion technique. We explicitly elaborate the short time and long time approximations. The relationship between the two-dimensional description and the one-dimensional theory is also discussed.
The photoelectron imagings of LaO-, CeO-, PrO-, and NdO- at 1064 nm are reported. The well resolved photoelectron spectra allow the electron a±nities to be determined as 0.99(1) eV for LaO, 1.00(1) eV for CeO, 1.00(1) eV for PrO, and 1.01(1) eV for NdO, respectively. Density functional calculations and natural atomic orbital analyses show that the 4f electrons tend to be localized and suffer little from the charge states of the molecules. The photodetached electron mainly originates from the 6s orbital of the metals. The ligand field theory with the δ=2 assumption is still an effective method to analyze the ground states of the neutral and anionic lanthanide monoxides.
High-resolution ro-vibrational spectroscopy of 15N216O in 1650-3450 cm-1 region is studied using highly enriched isotopologue sample. The positions of more than 7300 lines of 15N216O isotopologue were measured with a typical accuracy of 5.0×10-4 cm-1. The transitions were rovibrationally assigned on the basis of the global effective Hamiltonian model. The band by band analysis allowed for the determination of the rovibrational parameters of a total of 73 bands. 29 of them are newly reported and more rotational transitions have been observed for the others. The maximum deviation of the preidictions of the effective Hamiltonian model is up to 0.70 cm-1 for the 15N216O species.
The minimum-energy configurations and energetic properties of the ArN-CO2 (N=1-19) van der Waals clusters were investigated by a simulated annealing algorithm. A newly de-veloped Ar-CO2 potential energy surface together with the Aziz Ar-Ar interaction potential was employed to construct the high dimensional potential functions by pairwise additive ap-proximation. The global minimal conformations were optimized by sampling the glassy phase space with a circumspectively formulated annealing schedule. Unlike the lighter RgN-CO2 clusters, the size-dependent structural and energetic characteristics of ArN-CO2 exhibit a different behavior. The dramatically variations with number of solvent were found for small clusters. After the completion of the first solvation shell at N=17, the clusters were evolved more smoothly.
Using density functional theory and polarizable continuum models, we study the Raman spectra of aqueous peroxynitric acid. The calculated results indicate that the solvent effect has significant influence on the electric dipole transition moments between the ground and excited electronic state and Raman polarizabilities. The theoretical Raman spectra agree well with the experimental results. From the experimental depolarization ratio, we can conclude that peroxynitric acid is not a plane molecule. We also find that the hydrogen bond can enhance IR intensity of hydroxyl group by several times.
We present a first velocity map imaging study on the 234 nm photodissociation dynamics of two carbon-chain branched alkyl bromides, neopentyl bromide (denoted as NPB) and tert-pentyl bromide (denoted as TPB). Unlike the 234 nm photodissociation of the unbranched n-C5H11Br molecule where only a direct fission of the C-Br bond is involved, the branched NPB and TPB molecules exhibit one and two more independent dissociation pathways with much energy being decayed via an extensive excitation of the bending modes of the parent molecules prior to the C-Br bond fission. This observation strongly suggests that the disso-ciation coordinate for the two carbon-chain branched molecules is no longer solely ascribed to the C-Br stretching mode but rather a combination of the bending-stretching modes.