Chemical Physics Letters (v.356, #5-6)

Chemical potential of quadrupolar two-centre Lennard-Jones fluids by gradual insertion by Jadran Vrabec; Matthias Kettler; Hans Hasse (431-436).
The gradual insertion method for direct calculation of the chemical potential by molecular simulation is applied in the NpT ensemble to different quadrupolar two-centre Lennard-Jones fluids at high density state points. The results agree well with Widom's test particle insertion but show at very high densities significantly smaller statistical uncertainties. The gradual insertion method, which is coupled here with preferential sampling, extends the density range where reliable information on the chemical potential can be obtained. Application details are reported.

A transferable interatomic potential for MgO from ab initio molecular dynamics by Andrés Aguado; Leonardo Bernasconi; Paul A. Madden (437-444).
A refinement of the parameters appearing in the Aspherical Ion Model (AIM) potential is presented. The strategy involves fitting to the ab initio forces and stresses as obtained from the Density Functional Theory code CASTEP at ion configurations obtained from molecular dynamics runs at T=1500 K and P=0 GPa on a small system. The resulting potential is then shown to reproduce the experimental phonon dispersion curves at 300 K with a quality comparable to that of ab initio methods. The potential is also shown to predict thermal expansivities and isothermal compressibilities of MgO in excellent agreement with experimental results over the experimentally characterised range, demonstrating that the potential is transferable to (T,P) values far from those employed in the fitting.

Ab initio calculations of NQR 35Cl frequency in organo-germanium chlorides and its dependency on Ge–O distance by Nasser L. Hadipour; Marjan A. Rafiee; Masoumeh Javaheri; Morteza K. Mousavie (445-450).

Semiempirical calculations of aniline oligomers hyperpolarisabilities by A.E.de A. Machado; D.V. Petrov; E.H.L. Falcão; A.A.S. da Gama; W.M. de Azevêdo (451-456).
Theoretical study of the hyperpolarisabilities β and γ of aniline oligomers is presented. The results obtained for β at the AM1/TDHF level are promising for the emeraldine form, indicating a potential use as a second-order nonlinear optical material. The calculations of static cubic hyperpolarisability of trimers agree with the experimentally observed trends for polyaniline.

The proton intra-pair dipolar order relaxation time (T 1D) was measured for nematic methyl-deuterated para-azoxyanisole (PAAd6) as a function of both the temperature and concentration in perdeuterated PAA (PAAd14), at 27 MHz. Since the results coincide for all measured concentrations in all the nematic temperature range, we conclude that the intermolecular contribution to the relaxation of the dipolar energy in this compound is negligible, at the studied frequency. The observed temperature dependence of T 1D is typical of the order fluctuations of the director (ODF), which clearly indicates that the ODF is the relevant mechanism producing dipolar order relaxation, in accordance with previous field cycling experiments.

Density analysis of imaginary part of γ related to two-photon absorption by Masayoshi Nakano; Harunori Fujita; Masahiro Takahata; Kizashi Yamaguchi (462-468).
We have developed a method for elucidating the spatial contributions of electrons to dynamic second hyperpolarizability, γ(−ω;ω,ω,−ω), using an analytical expression of the γ density. The merit of this method is that the dynamic γ density, whose imaginary part is related to the two-photon absorption (TPA) cross-section, can be calculated easily for each virtual excitation process. This method can also be applied to other dynamic response properties. We present, as an example, an interpretation of the spatial contributions of π-electrons to Im  γ for trans-stilbene and its donor-disubstituted derivative, i.e., 4,4-diamino-trans-stilbene.

Structural minima of neutral and negatively charged CuO6 clusters have been investigated within density functional theory and a plane-wave approach. Among the lower energy structures, three isomers made of CuO2 complexes and two isomers featuring a double ozonide Cu(O3) unit lie within a few hundredths of eV. Isomers containing CuOOO chains are higher in energy. Formation of Cu(O3) units is accompanied by hybridization between O2p- and Cu3d-like states. We highlight analogies and differences between our results and the structural identification proposed in the framework of photoelectron spectroscopy experiments.

Direct observation of OH photofragment from triplet hydroxyacetone by Pradyot K. Chowdhury; Hari P. Upadhyaya; Prakash D. Naik; Jai P. Mittal (476-482).
In contrast to the photoexcitation of hydroxyacetone (HA) at 193 nm resulting in an instantaneous dissociation of the Rydberg 1(n,3s) state, on photoexcitation at 248 nm the singlet 1(n,π *) excited HA molecule first undergoes intersystem crossing (ISC) to the triplet state, followed by a minor dissociation channel to CH3COCH2 and OH radicals. The real time formation of OH, which is probed by laser-induced fluorescence (LIF), shows a rate constant to be ⩾10 8   s −1 . The initial rotational state distribution of OH  (X 2Π) is found to be Boltzmann-like, characterized by a single rotational temperature T rot of 450±40 K. The average relative translational energy of the photofragments is determined by Doppler spectroscopy to be 8.7±2.0    kcal mol −1 . The observation of OH with a modest rotational energy, no vibrational energy, and a large amount of translational energy suggests significant exit energy barrier with the dissociating surface.

The nature of the Au–Rg bond in the [AuRg4]2+ (Rg=Ar, Kr and Xe) molecules by Sławomir Berski; Zdzisław Latajka; Juan Andrés (483-489).
A topological analysis of the electron localisation function of the [AuRg4]2+ (Rg = Ar, Kr and Xe) molecules reveals the `closed-shell' nature of the Au–Rg binding, which is dominated by electrostatic interactions. A similar finding is obtained by means of the AIM method where the Laplacian of the electron density computed for the bond critical point of the Au–Rg linkage is positive and decreases from 0.163 [AuAr4]2+ to 0.087 [AuXe4]2+. A charge separation according to the Au+1.30[Ar+0.18]4, Au+1.19[Kr+0.20]4 and Au+0.29[Xe+0.43]4 formulas shows an increase of the positive charge on the rare-gas atom and accordingly decrease on the gold atom. The unpaired electron density is localised mainly on Au and the integrated spin density decreases from 0.28 e for [AuAr4]2+ to 0.16 e for [AuXe4]2+.

Gas-phase reaction of dimethyl sulfoxide (DMSO) with OH radical is investigated using ab initio calculations. The CH3S(O)OH+CH3 product channel has an overall negative reaction activation energy, and it could proceed by formation of an addition complex (CH3)2S(O)·OH and subsequent dissociation into CH3S(O)OH+CH3 via an energy barrier at −29.8 kJ/mol below DMSO+OH. The other two product channels, CH3S(O)CH2+H2O and CH3SO+CH3OH, have energy barriers of 12.4 and 78.4 kJ/mol above DMSO+OH, respectively. The CH3S(O)OH+CH3 product channel is likely the dominant pathway in the DMSO+OH reaction.

Unified representation of MQMAS and STMAS NMR of half-integer quadrupolar nuclei by Jean-Paul Amoureux; Clarisse Huguenard; Frank Engelke; Francis Taulelle (497-504).
High resolution solid state NMR has been achieved for nuclei that exhibit quadrupolar interaction to the second order by MQMAS and more recently by STMAS. Both methods extend to several variants, 3QMAS, 5QMAS, … for MQMAS as well as ST1MAS, ST2MAS,… for STMAS. These experiments contain the same information content i.e. anisotropic second-order powder pattern and isotropic terms. A unified representation allows us to display the 2D spectra independently of the method used. Direct comparison of any such spectra is therefore straightforward and does not need any further renormalization.

Nascent products of C2H3+O2 reaction have been studied by time-resolved FTIR emission spectroscopy. Vibrational excitation of reaction products H2CO, HCO, CO, and CO2 was observed, providing the evidence of existing two exothermic reaction channels: HCO+H2CO and CH3+CO2 for the reaction. The latter channel was firstly observed in the experiment. Simulation of the spectrum of the products reveals that the highest excitation of CO and CO2(ν 3) at 5 μs is v=4 and 6, respectively; the branching ratio of the channel HCO+H2CO to the channel CH3+CO2 is estimated to be 17.

Synthesis of silicon carbide nanowires in a catalyst-assisted process by S.Z. Deng; Z.S. Wu; Jun Zhou; N.S. Xu; Jian Chen; Jun Chen (511-514).
At elevated temperatures, silicon carbide nanowires were synthesized in a catalyst-assisted process using aluminum as a catalyst. Transmission electron microscopy shows that the nanowires are around 20 nm in diameter and around 2 μm in length. High resolution transmission electron microscopy shows that the nanowires are crystalline β-SiC. Raman spectra show the typical features of nano-SiC. A model based on vapor–liquid–solid process is proposed to explain our finding.

Six-dimensional quantum and classical dynamics study of H2(ν=0,J=0) scattering from Pd(1 1 1) by H.F. Busnengo; E. Pijper; M.F. Somers; G.J. Kroes; A. Salin; R.A. Olsen; D. Lemoine; W. Dong (515-522).
We present results of a six-dimensional quantum dynamics study of H2(ν=0,J=0) scattering from Pd(1 1 1) using a potential energy surface obtained from density functional theory calculations. Quantum dissociative adsorption probabilities agree with experimental data and provide new evidence on the importance of dynamic trapping (induced by translation to rotation energy exchange) at low energies. We analyse the role of the vibrational zero point energy in classical calculations and the suitability of the so-called classical and quasi-classical approaches to describe dissociative adsorption dynamics for a non-activated system.

Theoretical study of C 1Π–X 1Σ+ transition of InCl by Wenli Zou; Meirong Lin; Xinzheng Yang; Baozheng Zhang (523-526).
Using coupled-cluster with single and double substitutions (CCSD) and based on the density functional theory (DFT), geometry optimization calculations have been performed for the ground state of InCl, and the spectroscopic constants are obtained. The potential energy curve, the spectroscopic constants and the radiative lifetimes for the vibrational levels of the C 1Π state are also computed by the use of CCSD, the results being in accordance with the experimental values, and showing that the C 1Π system is a quasi-bound state with a shallow well. Considering the effects of avoided crossing and barrier penetration, the maximal vibrational quantum number of the C 1Π state should not be more than 4.

Transition between `base' and `tip' carbon nanofiber growth modes by Anatoli V Melechko; Vladimir I Merkulov; Douglas H Lowndes; Michael A Guillorn; Michael L Simpson (527-533).
Carbon nanofibers (CNFs) have been synthesized by catalytically controlled dc glow discharge plasma-enhanced chemical vapor deposition (PECVD). Both base-type and tip-type nanofibers have been produced on identical substrates. We have observed a sharp transition between these two growth modes by controlling the kinetics of the growth process without changing the substrate and catalyst materials. This transition is brought about by changing the parameters used in the deposition process such as the flow ratio of the carbonaceous and etchant gasses and others. This study of the initial growth stages as a function of time for both regimes provides a basis for a model of the growth mode transition.

Hyper-Rayleigh scattering studies of silver, copper, and platinum nanoparticle suspensions by Robert C. Johnson; Jiangtian Li; Joseph T. Hupp; George C. Schatz (534-540).
The optical frequency doubling properties of silver, copper, and platinum nanoparticles in aqueous solution are studied via hyper-Rayleigh scattering (HRS) spectroscopy. Substantial HRS responses are observed for silver and copper particles. The response is attributed to enhancement due to resonance of the nonlinearly scattered light with the particles' surface plasmon absorption band. Platinum particles, which lack visible-region plasmon absorption, do not display detectable HRS signals. The largest signals from silver particles are observed under conditions of two-photon resonance, qualitatively consistent with the predictions of available theory; smaller yet still impressive signals are observed under pre- and post-resonant conditions.

Excited singlet state relaxation yields of pentacene in Shpol'skii matrices by M. Banasiewicz; I. Deperasińska; D. Fabjanowicz; B. Kozankiewicz (541-546).
We studied absorption and fluorescence spectra and decays of pentacene (Pc) in Shpol'skii matrices of n-heptane (C7), n-nonane (C9), n-decane (C10), n-dodecane (C12), n-tetradecane (C14) and n-hexadecane (C16) in the temperature range 1.7–200 K. The fluorescence quantum yields of Pc in C12, C14 and C16 at 1.7 K were determined to be 20±6%, 26±6% and 22±6%, respectively. The kinetic analysis provided values of the radiative, internal conversion and intersystem crossing rate constants for the relaxation channels of the S1 state of Pc in different matrices. Calculated fully saturated emission rates of single molecule were in very good relation with previous observation of single Pc molecules in Shpol'skii matrices.

The first detection of the 3Ag state in carotenoids using resonance-Raman excitation profiles by Kentaro Furuichi; Tokutake Sashima; Yasushi Koyama (547-555).
The singlet 3Ag state that had been theoretically predicted in shorter polyenes [P. Tavan and K. Schulten J. Chem. Phys. 85 (1986) 6602; Phys. Rev. B 36 (1987) 4337] was first identified in bacterial carotenoids by measurements of resonance-Raman excitation profiles. It is almost overlapped with the 1Bu + state in spheroidene (the number of conjugated double bonds, n=10), and located in-between the 1Bu + and 1Bu states in lycopene, anhydrorhodovibrin and spirilloxanthin (n=11–13). The slopes when the 2Ag -, 1Bu - and 3Ag -state energies were expressed as linear functions of 1/(2n+1) exhibited the ratio of 2:3.1:3.8 in excellent agreement with that theoretically predicted, 2:3.1:3.7.

A one-electron model is developed which is able to describe the electronic structure and potential-energy (PE) surface of the Rydberg molecule H3O, in particular the Rydberg-to-valence transition of the singly occupied orbital along the H3O→H2O+H reaction path. An analytic PE function for the unpaired electron is obtained by numerical inversion of the one-particle Schrödinger equation and a least-squares fitting using a Thomas–Fermi-type model. Application of this procedure to H5O2 indicates the transferability of the model parameters from H 3 O to H 3 O–water clusters in a good approximation. The long-term goal of this work is the development of an accurate pseudopotential which can be employed in computer simulations of excess electrons and excess protons in liquid water.

Preparation of monodispersed multi-walled carbon nanotubes in chemical vapor deposition by D.S. Tang; S.S. Xie; Z.W. Pan; L.F. Sun; Z.Q. Liu; X.P. Zou; Y.B. Li; L.J. Ci; W. Liu; B.S. Zou; W.Y. Zhou (563-566).
Monodispersed multi-walled carbon nanotubes were prepared in a chemical vapor deposition (CVD) method by using monodispersed nanoparticles of Fe3O4 (1 nm) as the precursor of catalyst particles. High-resolution electron microscopy (HREM) revealed that over 50% of these carbon nanotubes synthesized at 600 °C and 180 Torr were quadruple walled nanotubes with mean outer diameter of 5.6 nm. At the same time, in the close tips of these carbon nanotubes, no catalyst particles were detected by HRTEM, which means that the catalyst particles were too small to be detected by HRTEM, even though they exit in the tips of carbon nanotubes in our experiments.

Purification and alignment of arc-synthesis single-walled carbon nanotube bundles by Houjin Huang; Hisashi Kajiura; Atsuo Yamada; Masafumi Ata (567-572).
We report here a scalable method for purification and alignment of single-walled carbon nanotubes (SWNT) in an aqueous solution. Arc-synthesis soot containing SWNTs is first treated with a concentrated nitric acid. After removal of most of the impurities and water, macroscopic and well-aligned SWNT bundles up to several centimeters long are formed in a rotary evaporator. Alignment of the SWNT bundles is ascribed to the liquid flow induced by rotary evaporation and van der Waals interactions among the bundles. The aligned SWNT bundles are further purified by ultrasonic Soxhlet extraction and annealing.

Two-photon-pumped frequency-upconverted yellow lasing in a novel dye solution by Xin-Jing Tang; Li-Zhu Wu; Li-Ping Zhang; Chen-Ho Tung (573-576).
Two-photon induced frequency upconversion emission and two-photon-pumped (TPP) lasing properties of a novel dye, trans-2-[p-(N-ethyl-N-(hydroxyethyl)amino)styryl]-N-methylthiazolinium iodide (1) were investigated. This dye exhibits excellent TPP lasing at ∼577 nm when it is pumped with a 1064 nm pulsed laser beam. The net conversion efficiency from the absorbed 1064 nm pump energy to the ∼577 nm upconverted lasing energy was measured to be ∼2.2%.

Remarkably high electrochemical charge uptake for modified electrodes of polyacetylene molecular wires encapsulated within zeolites and mesoporous MCM-41 aluminosilicate by Mercedes Alvaro; Belén Ferrer; Hermenegildo Garcı́a; Alexander Lay; Francisco Trinidad; Jesús Valenciano (577-584).
Heavy loaded samples of polyacetylene and polypropyne incorporated within zeolites or MCM-41 were obtained by in situ polymerisation of the corresponding alkynes on Ni2+-exchanged aluminosilicates. Charge uptakes were obtained as high as 250  C g −1 (the theoretical amount for complete polyacetylene oxidation), the highest ever reported for a zeolite modified electrode. Voltammetric peaks were however irreversible and extensive degradation occurs.

MRCI calculations are reported for O 4(3 A) using two distinct CASSCF reference wave functions, and the results compared with previous ab initio calculations. By combining the force field of the best ab initio CASSCF surface at the saddle point with the recommended estimate for the barrier height, a four-body analytic term is obtained which added to the popular double many-body expansion potential energy surface for the title system makes it fit well such properties.

The production of high-purity multi-wall carbon nanotubes (MWNTs) without metal catalysts was produced using radio-frequency plasma. The MWNTs had diameters of about 5 nm, with the innermost tubes having a diameter of 0.4 nm, the smallest value possible for a single-wall carbon nanotube. Two types of aggregates were obtained, depending on the conditions of formation: moss-like aggregates (diameter: about 1  μm ) and long bundles (length: 10  μm , diameter: 100 nm). The MWNTs in the moss-like aggregate, referred to as acute MWNTs, had tips with a cone angle of 19.2°.

A density function theory study on the properties and decomposition of CF3OF by Hong-Ming Yin; Ju-Long Sun; Ya-Min Li; Guo-Zhong He; Ke-Li Han (601-606).
The density function theory was used to calculate the potential energy surface for the decomposition of CF3OF. The geometries, vibrational frequencies and energies of all stationary points were obtained. The calculated harmonic frequencies agreed well with the experimental ones. Three decomposition channels of CF3OF were studied. The calculated reaction enthalpy (29.85 kcal/mol) of the elimination reaction CF3OF→CF2O+F2 was in good agreement with the experimental value (27.7 kcal/mol). The O–F bond of CF3OF is broken easily by comparing the energies, while the decomposition channel to yield the CF3O and F radicals is the main reaction path.

Third-order optical non-linearities in titanium bis-phthalocyanine/toluene solutions by Félix Fernández-Alonso; Paolo Marovino; Anna Maria Paoletti; Marcofabio Righini; Gentilina Rossi (607-613).
We report Z-scan experiments performed on titanium bis-phthalocyanine (TiPc2)/toluene solutions as a function of the molar concentration. The TiPc2 cubic molecular hyperpolarizability, γ hyper, has been found to be (−70±30)×10−32 esu at 800 nm, approximately 105 times larger in magnitude than the third-order susceptibility of the standard reference molecule CS2. Given the non-proximity of the laser wavelength to the main electronic bands of this molecule and its unusual molecular structure, we compare our results with previous measurements in other phthalocyanine systems. We also discuss the most probable factors influencing the non-linear optical response of our molecular system. The results indicate that the large non-linear optical response observed for TiPc2 cannot be explained using optical-pumping, resonance-enhancement or electronic delocalization arguments.

Author index (614-624).