Chemical Physics Letters (v.341, #3-4)

We report the observation of intramolecular features on isolated silicon nanoscale clusters. Unpassivated silicon clusters are fabricated under ultrahigh vacuum (UHV) conditions, deposited onto a graphite substrate, and imaged in situ using scanning tunneling microscope (STM). We were able to resolve certain structural features of the clusters with atomic resolution. A sequence of images indicates atomic scale instability, either from diffusion or tip induced modification. The inert surface of the graphite substrate appears to allow significant mobility for atoms and small clusters, and does not strongly influence the lateral arrangement of material on the surface.

The optical properties of NNBis (2,6-xylyl)perylene-3,4:9,10-bis(dicarboximide) (DPP-PTCDI) thin films of various thicknesses have been studied. While the absorption spectra are redshifted compared to the monomer absorption spectra in solution, the photoluminescence spectra show an additional inhomogeneously broadened unstructured band, which increases in intensity with increasing film thickness. Photoluminescence measurements reveal that this broadband emission originates from defect states that exist in the ground state. The bulky functional group of the DPP-PTCDI molecule inhibits excimer formation, otherwise typical for perylene derivatives. Time-resolved photoluminescence (TRPL) measurements show that the molecular decay is directly related to the film thickness due to an increasing density of defects with increasing film thickness.

Completely polarized photoluminescence emission from a microcavity containing an aligned conjugated polymer by T. Virgili; D.G. Lidzey; M. Grell; S. Walker; A. Asimakis; D.D.C. Bradley (219-224).
We present a study of 1-D optical microcavities based on a dielectric mirror, a layer of an aligned fluorescent conjugated polymer (poly(9,9-dioctylfluorene) (PFO)) and a metallic mirror. The aligned polymer film is birefringent, with a refractive index difference calculated to be 0.36 between directions parallel and perpendicular to the alignment direction. Because of the large birefringence, two cavity modes are observed within the microcavity stop band. They are split by 32 nm and are polarized parallel and perpendicular to the alignment direction. Photoluminescence from the PFO film correspondingly shows two emission bands with mutually perpendicular polarizations. The polarization ratio of the most intense mode exceeds 300:1. We discuss how this structure might be used to construct highly polarized light-sources.

Site-switching for CO2 formation and structural transformation of (1×2)⇄(1×1) in steady-state CO oxidation on Pt(1 1 0) by Md. Golam Moula; Asht B.P. Mishra; Izabela Rzeźnicka; Mikhail U. Kislyuk; Suwen Liu; Yuichi Ohno; Tatsuo Matsushima (225-231).
The angular and velocity distributions of desorbing product CO2 were studied in steady-state CO oxidation on Pt(1 1 0) by means of cross-correlation time-of-flight techniques. In the active region where the reaction was first order in CO, the angular distribution in the (0 0 1) plane was split into two-directional lobes collimated at ±25° from the surface-normal direction, indicating the CO2 formation on inclined (1 1 1) terraces. The translational temperature was maximized at these collimation angles, reaching about 1820 K. On the other hand, in a limited range of the inhibited region, where CO retarded the reaction, CO2 desorption sharply collimated along the surface-normal direction, suggesting CO2 formation on flat (1×1) domains.

The effects of thickness-extensional mode resonance oscillation (TERO) on the catalytic activity for CO oxidation of thin TiO2 and NiO films deposited on a z-cut LiNbO3 (z-LN) single crystal were investigated. TERO at 3 W caused 4 – 7-fold increases in the catalytic activity of TiO2/z-LN and NiO/z-LN and remarkable decreases in the activation energy of the reaction. Vertical lattice displacement by TERO was decreased remarkably by the presence of the metal oxides. The catalyst activation is due to the interactions of TERO with the carriers in the semiconductor metal oxides.

Interaction between colloidal particles of C60 hydrosol and cationic dyes by N.O. Mchedlov-Petrossyan; V.K. Klochkov; G.V. Andrievsky; A.A. Ishchenko (237-244).
Interaction of the dispersed phase of the fullerene C60 hydrosol with cationic dyes, Indopolycarbocyanines, Methylene Blue and others, results in the adsorption of the dyes on the surface of colloidal particles. The phenomenon is accompanied by expressed changes in the absorption spectra of all the dyes, as well as in fluorescence quenching in the case of Indotricarbocyanine, and leads to the coagulation of the sol. The structural forces, caused by hydrophobization of the fullerene–water interface, play a decisive role in the coagulation phenomena.

Diameter-controlled growth of carbon nanotubes using thermal chemical vapor deposition by Cheol Jin Lee; Seung Chul Lyu; Young Rae Cho; Jin Ho Lee; Kyoung Ik Cho (245-249).
The diameter and the growth rate of vertically aligned carbon nanotubes (CNTs) are controlled by modulating the size of catalytic particles using thermal chemical vapor deposition (CVD). The size of iron catalytic particles deposited on silicon oxide substrate is varied in a controlled manner by adjusting the condition of ammonia pretreatment. We found an inverse relation between the diameter and growth rate of carbon nanotubes. As the diameter increases, the compartment layers of bamboo-shaped carbon nanotubes appear more frequently, which is suitably explained by the base growth mechanism.

We present molecular dynamics results on the hydrogen bond structure of water at supercritical conditions adsorbed inside carbon nanotubes (CN). Water was described by means of a flexible version of the simple point charge potential and the water–tube interaction was of the Lennard-Jones type. We found significative reductions in the average number of hydrogen bonds for all considered tubes with respect to bulk water. This effect is much more important than for confined water at ambient conditions.

A new method for analyzing the cybotactic zone in a ternary solvent system by means of electronic absorption spectroscopy has been presented. The position of the intramolecular charge transfer absorption band of 2,6-diphenyl-4(2,4,6-triphenyl-1-pyridino) phenolate has been monitored in four ternary solvent mixtures in which one of the pairs are partially miscible. Investigations have been done in the completely miscible region of the ternary mixtures including the binodal curve. Results have been explained by extending the existing models of solvation in binary solvent mixtures to ternary systems.

Fluorescence polarization experiments on N,N-dimethylaminobenzonitrile (DMABN) and derivatives bearing stronger acceptor substituents, and of amino-bridged model compounds have been conducted at low temperature. They allow the classification into two groups differing in the nature of the locally excited (LE) state. From the comparison with these and further observations, the suitability of different mechanistic models can be assessed. The twisted intramolecular charge transfer (TICT) model (twisted amino group) presents no conflict with the experiments. The RICT model (cyano bending) fails partially. The PICT model (planar charge transfer (CT) structure) fails to account for all four key observations considered.

Fluorescence excitation spectra of jet-cooled 4-(diisopropylamino)benzonitrile and related compounds by Rainer Daum; Sergey Druzhinin; Dietmar Ernst; Lutz Rupp; Jörg Schroeder; Klaas A. Zachariasse (272-278).
Fluorescence excitation spectra of 4-(diisopropylamino)benzonitrile (DIABN) and 4-(dimethylamino)benzonitrile (DMABN) in thermal vapour and seeded jet expansions are compared. The spectrum of jet-cooled DIABN shows an intense 0–0-transition at 31751.8  cm −1 . The spectrum collapses at excess excitation energies above 800  cm −1 , indicating the presence of an efficient non-radiative decay channel. In the gas phase, fluorescence emission of DIABN occurs from the intramolecular charge transfer (ICT) state. The non-radiative decay channel, therefore, is attributed to rapid ICT in the isolated molecule. The related compounds 4-(methylamino)-3,5-dimethylbenzonitrile (MHD), 4-(azetidinyl)-3,5-dimethylbenzonitrile (M4D), and 4-(dimethylamino)-3,5-dimethylbenzonitrile (MMD) in the jet show extremely weak and structureless emission.

Optical and magnetic properties induced by structural confinement of ternary chalcogenide in AlMCM-41 nanotube by Weon-Sik Chae; In-Wook Hwang; Jin-Seung Jung; Yong-Rok Kim (279-284).
For ternary chalcogenide, Ni3(SbTe3)2, with the potential applications of magnetic and opto-magnetic storage media, we investigated the properties induced by the incorporation of ternary chalcogenide into long range ordered mesoporous AlMCM-41. Structural confinement in the mesoporous channel induces the enhanced crystallinity of the ternary chalcogenide and is possibly responsible for the observed weak ferromagnetic transition (∼70 K). Such confinement also causes the suppression of phonon coupling to photoexcited carriers in the confined ternary chalcogenide, resulting narrower photoluminescence (PL) band and slower PL lifetime than those of the bulk ternary chalcogenide.

Effect of temperature in a closed unstirred Belousov–Zhabotinsky system by Marco Masia; Nadia Marchettini; Vincenzo Zambrano; Mauro Rustici (285-291).
Complex periodic and aperiodic behaviours are reported in an unstirred Belousov–Zhabotinsky oscillatory reaction performed at temperatures varying between 0°C and 8°C. A route to chaos following a Ruelle–Takens–Newhouse (RTN) scenario is identified. Thus, temperature effects on the coupling between chemical kinetics, diffusion and convection, seem to be responsible for the observed RTN scenario. In this Letter we demonstrate that the temperature is a bifurcation parameter for the sequence period-1 → quasiperiodicity → chaos.

Picosecond time-resolved resonance Raman observation of Iso-CH2Br–I following A-band photodissociation of CH2BrI in the solution phase by Wai Ming Kwok; Chensheng Ma; David Phillips; Anthony W Parker; Michael Towrie; Pavel Matousek; David Lee Phillips (292-298).
We present picosecond time-resolved resonance Raman spectra that demonstrate formation of the iso-CH2Br–I species following A-band excitation of bromoiodomethane in cyclohexane solvent at room temperature. The iso-CH2Br–I species is produced within several ps and appears to decay with a half-life on the 2–3 ns time scale. We compare these results to previous studies on solution phase photodissociation of dihalomethanes.

Light scattering spectra of liquid and plastic phases of succinonitrile have been measured around the phase transition temperature for a wide frequency range using a Sandercock-type tandem Fabry–Perot interferometer and a double monochromator. Although the molecular structures in these two phases have been found similar, the spectral difference becomes prominent with decreasing frequency. Particularly, the onset of the difference lies in the so-called low-frequency modes (<200 cm−1), implying that the cooperativity among various modes is essential for the manifestation of macroscopic nature.

Current–voltage characteristics of PTSA/I2 doped poly(4 vinyl pyridine) by R Vijayalakshmi Rao; M.H Shridhar; S Ganesh; K.C Prashanth (306-312).
Current–voltage characteristics of high molecular weight poly(4 vinyl pyridine) (P4VP) doped with P-toluene sulphonic acid (PTSA) and I2 is carried out in the voltage range 0–100 V and temperature range 300–440 K. It is found that for pure P4VP and PTSA doped P4VP, the current shows a linear variation with voltage following omhic characteristics with slope 0.77∼1 in the initial 0–45 V and then with slope 1.02∼1.72 leading to a partial SCLC type conduction. When the P4VP+PTSA complex is further doped with I2 the current increases drastically due to a large number of low energy carriers released by the charge transfer complex (CTC) formed. The transition voltage also shifted to the higher voltage side (75–80 V) indicating omhic conduction for a wide range of applied field. The variation is found to be nonomhic after 75 V and the SCLC mechanism along with thermally activated omhic conduction dominates. The suitability of different mechanisms are justified by considering Poole–Frenkel (PF)/Richardson–Schottky (RS) and SCLC approaches.

The electronic structure of ZnO and ZnF determined by anion photoelectron spectroscopy by Vicki D. Moravec; Stephen A. Klopcic; Bappaditya Chatterjee; Caroline Chick Jarrold (313-318).
The 3.49 and 4.66 eV photoelectron spectra of ZnO and ZnF are reported. The PES of ZnO shows five electronic transitions between two states of the anion and four states of the neutral. The EA of ZnO is 2.087(8) eV. The vibrational frequencies of all but the excited anion state are determined, in addition to relative bond lengths. The photoelectron spectrum of ZnF exhibits one electronic transition between the ground states of the anion and the neutral. The electron affinity of ZnF is 1.974(8) eV. Vibrational frequencies and relative bond lengths are determined for these two states.

Assessment of a new local exchange functional OPTX by Wee-Meng Hoe; Aron J. Cohen; Nicholas C. Handy (319-328).
We evaluate the performance of a new local exchange functional OPTX [N.C. Handy and A.J. Cohen, Mol. Phys. 99 (2001) 403] in predicting thermochemistry and molecular structures. OPTX was developed taking into account that exchange and left–right correlation are non-separable. Used with the correlation functional LYP, OLYP (=OPTX+LYP) significantly improves upon the well-established BLYP. This is attributed to the accurate description by OPTX of the Kohn–Sham exchange (KSX) energy, parameterised against atomic exchange energies. The superiority of OPTX is most evident based on molecular energetic predictions at stretched geometries, where hybrid functionals severely deteriorate. OLYP is recommended in place of BLYP for computational chemistry.

We developed a theoretical method that can systematically treat the phase-matching condition of nonlinear infrared or Raman measurements. This method might be a rational tool for the analysis of observed signals from various directions based on the response function especially under non-impulsive excitation. Model calculations of the third-order nonlinear IR spectroscopy for a Brownian harmonic oscillator system are performed. We discuss the variation of signals that depend on the observation direction and the temporal width of excitation pulse in comparison with the behavior within the impulsive limit approximation.

We regard dissociation processes induced by femtosecond excitation of iron pentacarbonyl. In common time-resolved ionization experiments, the total ion yield is detected as a function of the delay time between pump- and probe-pulse. If several ionic fragments are produced, these measurements cannot distinguish between charged molecules produced via ionization of neutral molecules or fragmentation of larger molecular ions. We show that time-resolved photoelectron spectroscopy does not suffer from this deficiency and thus provides a unique tool to investigate neutral fragmentation processes.

Numerical simulations of the oxygen production in the oscillating Bray–Liebhafsky reaction by Katarı́na Kissimonová; Ivan Valent; L'ubica Adamčı́ková; Peter Ševčı́k (345-350).
Numerical simulations of the oxygen production in the batch Bray–Liebhafsky (BL) oscillating reaction were performed. The dynamic behaviour of the BL reaction was modelled by a scheme suggested by Schmitz and Kolar-Anič, complemented by the physical processes of iodine and oxygen interphase transport. The calculated number and periods of oscillations of the rate of the gaseous oxygen production are in satisfactory agreement with experiment. Oscillations in iodate concentration have been found if some escape of iodine from the batch reactor was allowed.

Rate constants and isotope effects for the CH 3+H 2  →  CH 4+H reaction by an approximate semiclassical initial-value representation method by Antonio Fernández-Ramos; Emilio Martı́nez-Núñez; Zorka Smedarchina; Saulo A. Vázquez (351-357).
Rate constants and kinetic isotope effects are calculated for the CH 3+H 2  →  CH 4+H reaction by two theoretical methods: variational transition state theory with semiclassical corrections for tunneling and an approximate (linearized) semiclassical initial-value representation method, recently proposed by H. Wang, X. Sun, W.H. Miller [J. Chem. Phys. 108 (1998) 9726]. The theoretical results agree well with each other and with the experimental data in the temperature range 500–1500 K. For high temperatures, the differences between the two theoretical rate constants arise from the more accurate treatment of dividing surface recrossings by Miller's method.

Predicted rovibronic spectra of CH2 + and CD2 + by P.R. Bunker; M.C. Chan; W.P. Kraemer; P. Jensen (358-362).
We present simulations of the A 2 B 1← X 2 A 1 electronic band systems of CH2 + and CD2 + in absorption at 200 K. For each isotopomer we calculate the spectrum over the range from 5000 to 18 000  cm −1 for the purpose of assisting the experimental search of the spectrum in a cooled hollow-cathode discharge. We make use of our previously determined ab initio potential energy surfaces, dipole moment and transition moment surfaces in a calculation that includes the Renner–Teller effect and spin–orbit coupling. To complete the picture we also present simulations of the rotation and rotation–vibration spectra of CD2 +.

On the influence of ionic association on the capacitance of an electrical double layer by Myroslav Holovko; Vitalyj Kapko; Douglas Henderson; Dezsö Boda (363-368).
The concept of ionic association is applied to give an explanation of the anomalous temperature dependence of the capacitance of the electrical double layer. It is shown that, except in the intermediate region where the temperature derivative of the capacitance changes sign, a Bjerrum-like correction of the mean spherical approximation, with Ebeling's expression for the association constant, satisfactorily reproduces the computer simulation data at high and low temperatures. Including polarization effects that arise from ionic pairs improves the description in this region.

Density-dependent 17O magnetic shielding in the gas phase by Włodzimierz Makulski; Karol Jackowski (369-372).
Density-dependent 17O magnetic shielding has been measured for the first time. For pure CO, CO2, OCS and N2O gases at 300 K an increase in density linearly diminishes the oxygen shielding constants. It permits to separate quantitatively the shielding contributions due to intermolecular interactions (σ 1) and the shielding parameters of isolated molecules (σ 0). The new experimental results may be used for the better verification of ab initio calculations of oxygen shielding.

High vibrational excitation and bond breaking by generalized Raman ladder climbing by Bo Y. Chang; Ignacio R. Solá; Jesús Santamarı́a (373-381).
By coherent stimulated non-resonant Raman techniques using chirped pulses, it is possible to invert the population between adjacent vibrational levels in a sequence that follows a ladder climbing pattern. Using a realistic model of Na2 we numerically demonstrate the efficiency of the method applied to (1) selective excitation of very highly excited vibrational levels and (2) photodissociation. We explore the different conditions for the optimal performance of the previous goals in the linear chirp regime and we propose and test novel Raman ladder climbing patterns via two or three quantum steps.

Highly accurate values of the adiabatic electron affinity (EA) of the ground state (X 2Σ+) of the BO molecule, and the dissociation energy (D 0) of the anion BO (X 1Σ+), have been determined using the CCSD(T) approach in conjunction with a series of doubly augmented correlation consistent basis sets, d-aug-cc-pVnZ, n=3–6. In addition, the full potential energy curve of BO (X 1Σ+) has been constructed at the multireference configuration interaction, n=5 level. Our final values are, EA(BO)=2.50  eV and D0(BO )=215.7  kcal/mol , in excellent agreement with experimental results.

Application of an integrated MOZYME+DFT method to pKa calculations for proteins by K Ohno; N Kamiya; N Asakawa; Y Inoue; M Sakurai (387-392).
We propose an integrated MOZYME + DFT method for calculation of the pKa values of ionizable residues in a protein. The method is applied to two protein systems, bacteriorhodopsin (bR) and ribonuclease T1. It is shown that the present method reproduces the experimental pKa values of several targeted residues better than other conventional methods. Thus, it is available for pKa calculation for large molecular systems.

B3LYP and CCSD(T) calculations with various basis sets have been performed to investigate the potential energy surface for the ScO+H2→Sc+H2O reaction, which is shown to proceed by formation of the HScOH molecule. HScOH can isomerize to the molecular Sc–OH2 complex and dissociate to Sc atoms and water with endothermicity of 51–59 kcal/mol. The HScOH molecule is thermodynamically stable and the barrier required to release H2, ∼28 kcal/mol, is much lower than the energy needed for the Sc+H2O dissociation. The Sc–OH2 molecular complex has a binding energy of 7–11 kcal/mol and is stabilized kinetically with respect to isomerization to HScOH by a significant barrier.