Chemical Physics Letters (v.346, #1-2)

Mechanism of metal cationization in organic SIMS by I. Wojciechowski; A. Delcorte; X. Gonze; P. Bertrand (1-8).
A mechanism for metal cationization of phenyl group containing hydrocarbons is discussed. Intact molecules and their fragments are emitted from a thin organic layer covering a metal surface bombarded by fast ions. It is shown that the process of associative ionization of a neutral hydrocarbon molecule and a neutral excited metal atom, occurring above the surface, may contribute to the yield of cationized molecules. To demonstrate this we have calculated the potential energy curves for the model system C6H6+Me (Me=Ag, Cu, Au) making use of the density functional theory. The initial states of the metal atoms approaching the benzene ring along the C6 symmetry axis were set as the ground, ionic, and excited in (n−1)d9 ns2 electronic configuration.

The absorbance spectra of individual methylene blue aggregates adsorbed by spin coating on a fused silica surface are resolved. Using singular value decomposition and a model incorporating Beer's Law and elemental conservation, a varied concentration family of UV–Vis spectra are fit. Results indicate that H-dimer formation occurs beginning at a surface concentration of 3×1013   molecules/cm 2 , and further aggregate formation, including J-type dimers, is significant above 2×1014   molecules/cm 2 . Based on our results, past assumptions that a spin-coated surface (using a millimolar coating solution) results in a monolayer of methylene blue monomers should be reexamined.

Kinetics of the reactions of Cl (2 P 1/2) and Cl(2 P 3/2) atoms with C3H8,C3D8, n-C4H10, and i-C4H10 at 298 K by Kanami Hitsuda; Kenshi Takahashi; Yutaka Matsumi; Timothy J. Wallington (16-22).
The title reactions were studied using laser flash photolysis in conjunction with vacuum ultraviolet laser-induced fluorescence techniques. Separate monitoring of the two spin orbit states, Cl (2 P 1/2) and Cl(2 P 3/2), was used to measure the kinetics of the chemical reactions of Cl(2 P 3/2) and Cl (2 P 1/2), and physical quenching of Cl (2 P 1/2). The rate constants for chemical reactions of Cl (2 P 1/2) atoms with C3H8, n-C4H10, and i-C4H10 are approximately 30% of those for Cl(2 P 3/2) atoms. There was no observable chemical reaction of Cl (2 P 1/2) atoms with C3D8 and C2H6 (<17% of Cl(2 P 3/2) reaction rate).

Ethylene flame synthesis of well-aligned multi-walled carbon nanotubes by Liming Yuan; Kozo Saito; Wenchong Hu; Zhi Chen (23-28).
A stainless steel grid baked by a propane–air premixed flame had iron, chromium and nickel oxide deposits on the grid surface. With this grid, entangled and curved shape multi-walled carbon nanotubes (MWNTs) were harvested from an ethylene–air diffusion flame with yield rate of 3 mg/min. Nitrogen addition to the flame was found to straighten the entangled tubes probably by lowering the flame temperature. A cobalt-electrodeposited stainless steel grid was finally applied to the nitrogen-diluted ethylene diffusion flame; well-aligned and well-graphitized carbon nanotubes consisting of 20 nm diameter and 10 μm long element tubes were obtained.

We used a field emission high-resolution 300 kV electron microscope (Omega filter) to study the nanostructures appearing during consecutive stages of chemical reaction between multi-walled chemical-vapor-deposited (CVD) C nanotubes (NTs) and boron oxide in flowing nitrogen at >1573 K. We found that C→BN conversion in the tubular layers occurs through inhomogeneous crystallization of B/N domains onto and within undulating defective graphitic C shells opened by oxidation. The crystallization preferentially takes place at open edges, internal and external tube surfaces. Enrichment of shells with B and N at the expense of outflowing C gradually occurs. In the course of the transformation unique longitudinal nanotubular heterojunctions composed of C-rich and BN-rich domains form.

C―N bond formation in the reaction of nitrogen ions N+ with benzene molecules by D. Ascenzi; P. Franceschi; T.G.M. Freegarde; P. Tosi; D. Bassi (35-40).
We have investigated the ring-opening reaction of N+(3P) ions with benzene in a guided ion beam tandem mass spectrometer under single collision conditions. Our attention is focussed on the mechanisms by which C―N bonds are formed. We have measured the kinetic energy dependence of the integral cross-section for the reaction channels leading to the formation of H2CN+, H2C2N+, H2C3N+, H3C4N+, HC5N+ and H2C5N+. The present work indicates that the title ion molecule reaction represents a potential route to form cyano molecules from aromatic hydrocarbons, and thus may be relevant for the production of molecules of biological interest in the interstellar space.

Relaxation dynamics of polydiacetylene was investigated with femtosecond time-resolved absorption spectroscopy between 1120 and 1280 cm−1. A new photo-induced IR band was found at 1225  cm −1 just after photo-excitation, and it disappeared with a decay time constant of 4.1±0.7  ps . This IR band is assigned to the vibrational mode which induces self-trapping of the 1B1u exciton.

Ultrafast polarisation spectroscopy is employed to investigate excited state relaxation in the green fluorescent protein (GFP) chromophore in anion and neutral forms. The dominant relaxation channel is internal conversion (IC). The rate of IC is larger in the neutral than the anion. In both the relaxation time is a few picoseconds and independent of viscosity. This rules out IC induced by large scale intramolecular reorganisation. Alternative mechanisms are discussed. How the protein so effectively suppress this radiationless channel remains unclear. A second slower component in the ground state recovery indicates the existence of a bottleneck. This component arises through diffusive reorientation.

The ejection intensities of dopants incorporated within a C6H5CH3 matrix are examined as a function of successive laser pulses in UV ablation (λ=248 nm). The dopants include the strongly bound to the matrix C10H22 and the weakly bound (CH3)2O. For C10H22, the results are fully consistent with the volume expansion character of ablation. In contrast, for (CH3)2O, a high contribution through diffusion from underlying (i.e., nonejected) layers is demonstrated. This high diffusivity is consistent with melting of these layers during irradiation. Consequently, overheating of the upper/ejected layers well above the melting point is strongly suggested, in very good agreement with the `phase explosion' model for ablation.

A supercooled glycerol–water mixture: evidence for the large-scale heterogeneity? by A.V Pastukhov; D.V Khudyakov; V.R Vogel; A.I Kotelnikov (61-68).
The results of an experiment on the time-resolved spectral dynamics of the eosin phosphorescence in a low-temperature glycerol–water mixture are explained supposing a cluster structure of the supercooled liquid and the glass. An estimation of the size and the restructurization dynamics time of the clusters at about 237 K are performed. A modification concerning the procedure for the calculation of the relaxation function of a time-resolved spectral shift obtained in a supercooled liquid, possessing a cluster structure, is suggested. A conclusion on the existence of a new amorphous phase is inferred from the change in the spectral dynamics of the eosin phosphorescence observed in the low-temperature mixture.

Nascent OH(X 2Π) product state distributions from the reaction of O(1D) with ethylene. by Miguel González; Marı́a P. Puyuelo; Jordi Hernando; Rodrigo Martı́nez; R. Sayós; Pedro A. Enrı́quez (69-80).
The full characterization of the OH( X 2Π, v″=0–3, N″, J″, Λ″) product state distributions for the O(1 D)+C 2 H 4→OH+C 2 H 3 reaction was experimentally performed using the laser-induced fluorescence (LIF) technique. Statistical spin–orbit distributions were obtained, while some preference for the formation of the Π(A) Λ-doublet level was observed. The rovibrational populations obtained suggest that the reaction preferentially evolves via insertion, yielding rovibrationally cold OH through slow decomposition of an alcohol-type collision complex and rovibrationally excited OH by fast decomposition. Moreover, some evidences were found about the implication of an abstraction mechanism, which would produce rotationally cold and highly vibrationally excited OH.

Electronic states of NH 4(NH 3)n  (n=0–4) cluster radicals by Kota Daigoku; Nobuaki Miura; Kenro Hashimoto (81-88).
We have investigated geometries, ionization potentials (IPs), and vertical transition energies (VTEs) of NH 4(NH 3)n  (n=0–4) cluster radicals by ab initio MO method at the correlated level. The structures in which NH4 donates as many NH bonds as possible to the hydrogen bonding with surrounding NH3 molecules are the most stable for each n. The calculated IPs agree well with experiment. The spatial expansion of the unpaired electron occurs with stepwise solvation. The growing one-center Rydberg-like nature of the cluster radicals results in the successive decrease in the transition energies to the low-lying excited states, which is responsible for the red shifts of the electronic absorption bands.

Photodissociation spectroscopy and dynamics of Si4 by Alexandra A. Hoops; Ryan T. Bise; Hyeon Choi; Daniel M. Neumark (89-96).
The photodissociation of Si4 has been investigated using fast beam photofragment translational spectroscopy. The photofragment yield (PFY) spectrum shows features between 21 370 and 22 220 cm−1 corresponding to the 1 B 1u ← X ̃ 1 A g transition that are attributed to multi-photon dissociation. Single-photon dissociation was examined at higher excitation energies ranging from 5.17 to 6.42 eV. The dominant product channel was found to be Si3+Si. Experimental photofragment translational energy distributions were modeled by phase space theory (PST), indicating statistical photodissociation with no exit barrier along the dissociation coordinate. PST modeling yields a dissociation energy of 4.60±0.15  eV and ΔHf,0 0(Si 4)=6.75±0.24  eV .

Alkaline 2-deoxyguanosine undergoes one- and two-photon laser photoionization at 266 nm. Similar to tryptophan and indole, the one-photon quantum yield increases five-fold between 296 and 362 K. Earlier Arrhenius analyses of tryptophan and indole photoionization rate constants, yielded unreasonably large pre-exponential factors (1013–1018   s −1 ). An analysis employing pre-exponential factors for diffusion controlled reactions (1010–1011   s −1 ) provides evidence of activation barriers that decrease with increasing temperature over a range where water structure undergoes significant changes. In the context of Marcus theory, this analysis suggests that curvatures of the electron-transfer free energy surfaces decrease as temperature increases.

Ground-state rotational spectra of four isotopomers Ar⋯79 Br 35 Cl , Ar⋯81 Br 35 Cl , Ar⋯79 Br 37 Cl and Ar⋯81 Br 37 Cl of a complex involving argon and bromine monochloride were observed. Spectroscopic constants were interpreted on the basis of a linear species Ar⋯BrCl, with atoms in the indicated order and r(Ar⋯Br)=3.466  A ̊ . When intermolecular electron transfer was assumed negligible, the Br and Cl nuclear quadrupole coupling constants were interpreted to show that a fraction δ(Br→Cl)=3.5(5)×10−3 of an electron is transferred from Br to Cl on complex formation. The values kσ=2.79(1)  N m −1 and kββ=4.7(2)×10−20   J rad −2 were established for the intermolecular stretching and bending quadratic force constants, respectively.

Inelastic neutron scattering (INS) studies on low frequency vibrations of 1,4-benzoquinone by A. Pawlukojć; I. Natkaniec; I. Majerz; L. Sobczyk; E. Grech (112-116).
Inelastic neutron scattering (INS) infrared (IR) and Raman (R) spectra in the region below 1700  cm −1 of solid 1,4-benzoquinone (p-BQ) are compared with calculated frequencies and intensities for the gas phase. In the case of INS spectra a very good agreement is found when the frequency scaling factor markedly higher than unity (1.05) is assumed. This is most probably due to the fact that in the solid state all hydrogen and oxygen atoms are involved in the C–H⋯O hydrogen bond formation. The internal dynamics of p-BQ molecules reflected in the low frequency region is to a marked extent affected by these interactions. The environment effect is best reflected in the INS spectra recorded at 22 K. The complementarity of INS, IR and R spectra, appearance of overtones and summation modes and overlapping of closely located bands are discussed.

We present a method for the determination of dissociation energies of polyatomic systems that undergo sequential fragmentation with energy-dependent decay pathway branching. It allows to experimentally determine the dissociation energy of any polyatomic system that shows such fragmentation behaviour without the need for a specific modelling of the system or of its fragmentation process, thus eliminating several systematic errors of traditional methods. The new method has been applied to the sequential fragmentation of Au+ 14 and Au+ 16. The resulting dissociation energies are highly accurate and in good agreement with model-free values based on rates of sequential decays.

Influence of time delayed global feedback on pattern formation in oscillatory CO oxidation on Pt(1 1 0) by Michael Pollmann; Matthias Bertram; Harm Hinrich Rotermund (123-128).
Spatiotemporal pattern formation in the catalytic CO oxidation on Pt(1 1 0) is controlled by means of global delayed feedback applied through the gas phase. Using photoemission electron microscopy we have investigated the dynamical response of the system to a change of feedback intensity and time delay. Well defined synchronization and desynchronization regimes alternate when the delay is varied. In the case of synchronization the surface shows spatial homogeneous oscillations whereas in the case of desynchronization non-uniform patterns are observed. Furthermore two-phase clusters with period doubled local oscillations are studied. Phase balance between single cluster areas and breathing cluster modes are observed.

Rotationally resolved infrared spectra are reported for the linear and T-shaped isomers of acetylene–hydrogen cyanide formed in helium nanodroplets. The results for the latter show that the overall rotation ((B+C)/2) is slowed by the helium, while rotation about the A-axis is only weakly affected. A comparison of the relative abundances of the two isomers formed in helium and in a gas phase free jet expansion provides insights into the nature of the growth in helium.

The solvent effects on the relative free energies of binding of K+ and Na+ ion to 18-crown-6 and ΔlogK s (the difference of stability constant of binding) have been investigated by a Monte Carlo simulation of statistical perturbation theory (SPT) in several solvents. Comparing the relative free energies of binding of K+ and Na+ ion to 18-crown-6, in H2O (TIP3P) and CH3OH of this study with molecular dynamic simulation and experimental works, there is a good agreement among the studies.

High-resolution 1H NMR in the solid state using symmetry-based pulse sequences by P.K Madhu; Xin Zhao; Malcolm H Levitt (142-148).
We demonstrate new rotor synchronised pulse sequences for obtaining high-resolution 1H NMR spectra in the presence of fast magic angle spinning (MAS). The new sequences exploit selection rules generated by appropriate synchronisation of the radio-frequency (rf) field modulations and the sample rotation. We show preliminary results demonstrating the feasibility of high-resolution proton NMR spectroscopy in the presence of fast MAS, and also demonstrate the resolution of 1H–13C J-couplings in the solid state.

Non-orthogonal basis sets for hyperspherical coordinate calculations on chemical reactions by Sergei K. Pogrebnya; Andrew J. Richardson; David C. Clary (149-154).
In quantum reactive scattering calculations using hyperspherical coordinates a problem can arise in calculating Hamiltonian matrix elements between non-orthogonal basis functions from different reaction arrangement channels. The calculation of these matrix elements is considered here. It is shown that the use of particle-in-a-box functions defined within a limited interval requires special care in treating the kinetic energy operator.

Nuclear quadrupole moments of Kr and Xe from molecular data by Vladimir Kellö; Pekka Pyykkö; Andrzej J. Sadlej (155-159).
Nuclear quadrupole coupling constants for KrH+, XeH+ and XeD+ are combined with Douglas–Kroll (DK) CCSD(T) calculations to obtain the nuclear quadrupole moments of +259(1) and −114(1) millibarn (1 mb=10−31 m2 for 83Kr and 131Xe, respectively. For the radioactive isotopes 81Kr and 85Kr, the available atomic isotopic ratios give a Q of +644(4) and +443(3) mb, respectively. For the 83Kr 9.4 keV I=7/2 Mössbauer state, Q(83Kr ) of 507(3) mb is derived from our data. For the 129Xe 40 keV I=3/2 Mössbauer state, Q(129Xe ) is −393(10) mb.

Electric field gradients are highly pair-additive by Markus G Müller; Barbara Kirchner; Patrick S Vogt; Hanspeter Huber; Debra J Searles (160-162).
The electric field gradients at the central deuterons in one hundred clusters consisting of heavy water molecules, previously calculated in a supermolecular approach including many-body effects, is calculated assuming pair-additivity. Excellent pair-additivity of the electric field gradient is found for the water clusters. This result is confirmed for one hundred clusters extracted from a water–DMSO mixture. The use of pair-additivity results in substantial computer time savings in the quantum chemical calculation of the electric field gradients for nuclei in liquid systems using the so-called cluster approach, and hence their quadrupole coupling constants. It also permits the simulation of quadrupolar relaxation times from electric field gradient hypersurfaces obtained within the pair-additive approximation.

DFT studies of interaction between O2 and Au clusters. The role of anionic surface Au atoms on Au clusters for catalyzed oxygenation by Mitsutaka Okumura; Yasutaka Kitagawa; Masatake Haruta; Kizashi Yamaguchi (163-168).
Density functional and ab initio molecular orbital (MO) calculations have been carried out for the AuO, AuO2, AuO2 , and the Au13O2 clusters. The results show that the surface atoms of the icosahedral Au13 cluster are negatively charged and the anionic Au atoms in the cluster have a stronger interaction with O2 than a neutral Au atom. These findings suggest that the negatively charged surface Au atoms on the surface of the Au clusters are the active sites for oxygenation on the supported and unsupported Au catalysts.

We demonstrate that quantities such as classical paths, action integrals, stability matrix, caustics, and so on, which are all required in semiclassical chemical dynamics, can be integrated very efficiently by means of a locally analytic integrator (LAI). Hulme's collocation method is improved to carry out these integrations systematically. LAI solves ordinary differential equations (ODEs) by recasting the set of ODEs into a set of nonlinear equations. An individual solution in each dimension is represented in terms of an analytic function of time for a short interval. We explicitly show that the local analyticity brings about distinct advantages.

We present a new local multi-reference singles and doubles configuration interaction (MRSDCI) algorithm. The method presented here eliminates configurations if they involve simultaneous excitations out of widely separated internal orbitals and is therefore based on the weak pairs approximation of Saebø and Pulay. Although the resulting truncated CI expansions have only about 50% as many CSFs as the non-local MRSDCI, we show that they can recover over 99% of the correlation energy. Additionally, we show for the first time that they can accurately describe bond dissociation.