Chemical Physics Letters (v.377, #3-4)

Open-shell fullerene C74: phenalenyl-radical substructures by V.I Kovalenko; A.R Khamatgalimov (263-268).
The π-bond distribution and biradical structure of empty fullerene C74 molecule was shown for the first time by the local symmetry analysis followed by DFT calculation. There are two phenalenyl-radical substructures, which are symmetrically disposed on molecular C3 axis of symmetry. An open-shell electronic structure of fullerene C74 is the main reason of its instability as empty species. A pair of added electrons stabilizes C74 as, for example, in its endohedral metallofullerenes. The probable sites of metal atom(s) inside fullerene C74 are discussed. Possible ways of C74 stabilization are outlined.

Modelling pattern formation in CO + O2 on Pt{1 0 0} by I.M. Irurzun; R.B. Hoyle; M.R.E. Proctor; D.A. King (269-278).
We extend a detailed kinetic model for CO + O2 on Pt{1 0 0} to describe pattern formation. The model includes: (i) a non-linear power law to describe the phase transition, (ii) trapping and untrapping processes explicitly considered, and (iii) experimentally determined coverage-dependent sticking probabilities and rate constants. This model is extended to include diffusion and gas global coupling. Diffusion is included through a mass-balance equation which couples the migration of CO with the phase transition. Gas global coupling is introduced considering realistic values of the pumping flow, the reactor volume and the size of the crystal.

The angular and velocity distributions of desorbing CO2 and LEED spot intensities were examined in steady-state CO oxidation on Rh(1 1 0) and Pd(1 1 0). On Rh(1 1 0), in the limited CO pressure range, the angular distribution splits into bi-directional lobes collimating at ±24° off normal along the [0 0 1] direction and the meta-stable (1×2) structure stabilized by oxygen is maximized. On the other hand, neither inclined CO2 desorption nor meta-stable (1×2) was found on Pd(1 1 0).

Temperature and pressure effects were investigated on spectral holes burned in the inhomogeneous S1 absorption band of octaethylporphine in poly(methyl methacrylate) glass. The temperature-induced hole shift and broadening were recorded between 7 and 30 K. The dynamic, phonon-induced shift depends strongly on burning position. According to theory, negative quadratic electron–phonon coupling (QEPC) constant corresponds to a decrease of phonon frequencies in the excited state. This mode-softening behaviour prevails on the short-wavelength part of absorption, where the thermal shift is large and bathochromic. The absence of shift on the red edge cannot be explained with negligible absolute values of QEPC constants, since thermal hole broadening (that depends on the square of QEPC constant) is large and essentially independent on burning position.

Synthesis of multi-walled carbon nanotubes and nano-fibres using the aerosol method with metal-ions as the catalyst precursors by Marianne Glerup; Henning Kanzow; Robert Almairac; Marie Castignolles; Patrick Bernier (293-298).
We report on MWNTs synthesised using the aerosol method. The catalyst particles are formed by dissolving metal salts in organic solvents. This solution is sprayed as aerosols into a furnace. The active catalyst is formed in-situ, using H2 as carrier gas and as reducing agent of the metal ions. This synthesis route gives possibilities for using a uniquely wide range of metal catalyst precursors and carbon sources. Using this approach, the size of the active catalyst particles can be calculated. MWNTs, prepared using Co(NO3)2  · 6H2O as precursor and tetrahydrofuran, serving as solvent and carbon source demonstrate the reliability of these predictions.

Emission spectra and decay times of fac-Ir(ppy)3 dissolved in THF were registered for 1.2  K⩽T⩽300  K and magnetic fields (at T=1.5 K) up to B=10 T. Three emissive triplet substates I, II, and III could be identified and classified as substates of a 3MLCT term of 3(Ir5d-ppyπ*) character. The energy separations and decay times are ΔE II,I=13.5 cm−1, ΔE III,I=83 cm−1 and τ I =145  μ s, τ II =11  μ s, τ III=750 ns. All three substates are involved in the emission process at 300 K resulting in an average decay time of 2.1 μs. Magnetic field application at T=1.5 K alters the dominant radiative deactivation process from a forbidden to a significantly allowed process. The magnetic field-induced change of the emission decay rate can be described by a first-order perturbation approach.

Head-to-tail organization of terthiophene–vinylbenzoate in Langmuir–Blodgett films by Stefano Cattaneo; Camila Rouhento; Elina Vuorimaa; Aleksandre Efimov; Helge Lemmetyinen; Martti Kauranen (306-310).
The Langmuir–Blodgett films of terthiophene–vinylbenzoate (TSe) embedded in an octadecyl amine (ODA) inactive matrix were found to maintain an ordered head-to-tail (Z-type) structure up to at least 20 molecular layers. UV–Vis polarized absorption and polarized second-harmonic generation indicate that TSe is oriented in the films with the molecular axis aligned about the film normal. The degree of orientation increases for thick films suggesting that the ordering of the first few layers is compromised by substrate roughness. Improved orientation is obtained also by depositing pure ODA layers before the active layers.

Orientational and phase-coexistence behaviour of hard rod–sphere mixtures by Dmytro Antypov; Douglas J. Cleaver (311-316).
Results are presented from Monte Carlo simulations of bulk mixtures of hard Gaussian overlap particles with an aspect ratio of 3:1 and hard spheres with diameters equal to the breadths of the rods. For sphere number–concentrations of 50% and lower, compression of the isotropic fluid results in formation of a homogeneous (i.e., compositionally mixed) nematic phase. The volume fraction of this isotropic–nematic transition is found to increase approximately linearly with sphere concentration. On compression to higher volume fractions, however, this homogeneous nematic phase separates out into coexisting nematic and isotropic phases.

Crystalline WO3 nanowires synthesized by templating method by Kake Zhu; Heyong He; Songhai Xie; Xuan Zhang; Wuzong Zhou; Songlin Jin; Bin Yue (317-321).
A new method is developed to obtain crystalline nanowires of WO3 using mesoporous silica SBA-15 as template. The method includes aminosilylation of the surface silanols within SBA-15 channels, anchoring of the heteropoly acid (HPA) to the grafted amine groups, thermal decomposition of the HPA, and removal of the silica framework with HF. The formation of the crystalline nanowires is monitored by the in situ XRD technique. TEM images intuitively confirm that the nanowires are uniform in diameter and HRTEM images further indicate that each nanowire belongs to single crystal although the growth orientations of these nanowires are different.

We present a simple two-dimensional (2D) solid-state exchange NMR method to suppress significant undesirable diagonal signals. Two delays, τ, incorporated in standard 2D exchange experiments modulate the 2D spectra with sin{(Ω1−Ω2)τ}, where Ω 1 and Ω 2 are precession frequencies of a given site before and after the mixing time, respectively, yielding the spectra free of diagonal signals. The suppression of diagonal signals is demonstrated for dimethyl sulfone. This technique is combined with magic angle spinning and is applied to polyethylene. Not only the exchange signals between the crystalline and noncrystalline components but also those among the noncrystalline components in different states are clearly observed.

Ultraviolet lasing of ZnO whiskers prepared by catalyst-free thermal evaporation by Y.G. Wang; Clement Yuen; S.P. Lau; S.F. Yu; B.K. Tay (329-332).
ZnO whiskers were synthesized by vapor transportation via thermal evaporation of ZnO powders in open air. Preferred aligned hexagonal ZnO whiskers with length of 30–40 μm were obtained. Room temperature photoluminescence spectrum of the whiskers exhibits intense ultraviolet excitonic emission and weak defect related visible emission. Optical pumped lasing actions have been observed at 393 nm at room temperature when the excitation intensity exceeds 150 kW/cm2. Laser cavities are formed in the whiskers between the smooth interfaces of the two ends, which acted as reflectors.

High-resolution NMR of anisotropic samples with spinning away from the magic angle by Dimitris Sakellariou; Carlos A Meriles; Rachel W Martin; Alexander Pines (333-339).
High-resolution NMR of anisotropic samples is typically performed by spinning the sample around an axis at the ‘magic’ angle of 54.7° with the static magnetic field. Geometric and engineering constraints often prevent spinning at this specific angle. Implementations of magic angle field rotation are extremely demanding due to power requirements or an inaccessible geometry. We present a methodology for taking the magic out of MAS while still obtaining both isotropic and anisotropic spectral information during sample spinning or field rotation at arbitrary angles. Using projected-MAS, we obtained resolved scaled isotropic chemical shifts in inhomogeneously broadened spinning samples.

Effects of an external electrical field on the polarization of growing organic crystals: a theoretical study by Jürg Hulliger; Martin Losada; Claire Gervais; Thomas Wüst; Felix Budde (340-346).
In certain non-ferroelectric molecular crystals a static external electrical field applied during the growth in the high vacuum may induce effects of permanent poling or depoling through interaction with dipolar molecules getting attached to crystal faces. In centrosymmetric crystal structures a field may effect a substantial deviation from a 50%:50% fractional site occupation with respect to the molecular dipole orientation. For channel-type inclusion compounds the electric field may affect growth sectors as such as to annihilate polarity in one sector and to enhance polarity in symmetry related sectors. Numerical simulations are given to show the change in the spatial distribution of polarity, including its temperature dependence.

The absorption and emission properties and film structure of co-evaporated mixed thin solid films of vanadyl-phthalocyanine (VOPc) and N,N -bis(neopentyl)-3,4,9,10-perylenebis(dicarboximide) (BNPTCD) are reported. Exciplex emission from mixed films is clearly identified and the intensity of new near-infrared light source depends on the degree of mixing and molecular orientation in the film that control the exciplex formation. A clear correlation between molecular organization and emission properties is established. The IR spectroscopic data and AFM topographical analysis show that thermal annealing of the mixed films induces phase separation enhancing energy transfer as main deactivation pathway of the absorbing BNPTCD molecules.

Hexagonal zigzag network of boron nitride (BN) nanotubes was investigated by high-resolution electron microscopy (HREM) and image processing. From the HREM image, lattice planes of {0 0 2} and hexagonal rings of BN nanotube were confirmed by comparing image simulation and nanostructure models. The present work indicated that arrangement of hexagonal rings of BN nanotubes could be confirmed by combination of HREM image analysis and simulation.

The ultra-violet photodissociation of ozone revisited by Zheng-Wang Qu; Hui Zhu; Reinhard Schinke (359-366).
We have performed new electronic structure calculations for the five lowest 1A states of ozone using the multi-reference configuration interaction method with an augmented triple zeta valence atomic basis set. Several avoided crossings, which are important for interpreting the Huggins–Hartley band system, are identified and two-dimensional diabatic potential energy surfaces are constructed. It is argued that the Huggins and the Hartley band systems are due to excitation of the same electronic state.

Single-crystal ZnSe nanowires have been synthesized via a thermochemical method in a N2 atmosphere with CO and H2 gases. The as-prepared ZnSe nanowires have a mean diameter of 40 nm and a length of 1 μm. The nanowires are single crystals with a hexagonal structure growing along the [0 0 1] direction. A self-catalyzed vapor–liquid–solid process is proposed for the formation of such nanowires. The as-prepared nanowires show two emission bands at ca. 447 and 617 nm.

Fluorescence of the peripheral LH2 antenna complex from the photosynthetic bacterium Rhodobacter sphaeroides has been studied at 5 K upon selective excitation into the B870 exciton absorption band. Heterogeneous nature of the LH2 emission band has been confirmed by a double spectral selection technique simultaneously utilizing hole burning and fluorescence line narrowing spectroscopy. The phonon structure of the spectra related to the B870 band has been analyzed resulting in a genuine single-exciton state absorption/emission profile. The total electron–phonon coupling strength S=0.8±0.2, larger than previously assumed, has been determined.

Silicon nanowires (SiNW) have been prepared at different temperatures by chemical vapor deposition of silane over a titania-supported Au catalyst. It was found that the SiNW produced at 500 °C have a well-crystallized silicon core with a very thin amorphous silicon dioxide outer layer. At temperatures lower or higher than 500 °C, both yield and quality greatly decrease. Different controlling rate-limiting steps are proposed to explain the difference in quantity and quality of the products obtained as a function of temperature.

The infrared spectrum of acetylene–HF in helium nanodroplets by G.E. Douberly; K. Nauta; R.E. Miller (384-390).
Rotationally resolved infrared spectra are reported for the T-shaped acetylene–HF complex in helium nanodroplets, corresponding to excitation of both the H–F and asymmetric C–H stretches. In comparison to the isolated complex, rotation about the A-axis is relatively unhindered by the helium, while overall rotation is slower by a factor of approximately two. An ab initio potential surface is also reported for this system. Although the bent complex, with acetylene acting as a hydrogen donor, is determined to be a local minimum, only the T-shaped isomer is experimentally observed, indicating that the barrier is too small to prevent the metastable bent structure from rearranging to the global minimum.

The design of a molecular logic circuit based on sequential forward S2–S2 energy transfer and back S1–S1 energy transfer (cyclic energy transfer) for a system comprising of an azulene and a zinc porphyrin is discussed. Using a computational dynamics simulation, the effect of varying the orientation factor via the viscosity of the medium on the cyclic energy transfer dynamics is investigated and is incorporated into the logic circuit.

First-principle path integral study of DNA under hydrodynamic flows by Shilong Yang; James B Witkoskie; Jianshu Cao (399-405).
We use the worm-like chain as a first-principles model to study single molecule experiments of double stranded DNA subject to constant plug, elongational, and shear flows. The steady-state configurations of the polymer correspond to a locally defined potential and result in a path integral description of the canonical partition function. The parameters of this model are consistent with previous theory and experimental measurements. The time averaged mean extension reproduces experimental results and compares well with computationally more expensive Brownian dynamics simulations of reduced models.

Recently new experimental findings on vibrational predissociation dynamics of I2(B)–Ne have been reported. VSCF–DWB–IOS approximate method for vibrational predissociation is applied to understand the predissociation dynamics. Using the vibrational self-consistent field (VSCF) method with a modified potential function, we determined the vibrational structure of I2(B)–Ne that agrees with experiments very well. Using the distorted-wave Born (DWB) and the infinite-order sudden (IOS) approximations for dissociation process, we have calculated the predissociation lifetimes that are also in good agreement with other theories or experiments. The advantage and limit of the proposed method and the modified potential function are discussed.

Search for most stable structure of Si8H8 cluster by Mingsheng Tang; Wencai Lu; C.Z. Wang; K.M. Ho (413-418).
We have performed a global structure optimization for Si8H8 cluster using a genetic algorithm (GA) coupled with an efficient and accurate tight-binding (TB) potential to describe the energies of the clusters. Structure models selected from the GA/TB optimization are further verified by ab initio calculations. Our study shows that a new structure with C2 symmetry is energetically more stable than the octasilacubane structure previously proposed for Si8H8. This is in contrast to Ge8H8 where the octasilacubane structure is energetically more stable than the new C2 structure. Vibration and electronic properties of the new Si8H8 structure have also been studied.

Multiple ionization effects on the yields of HO2 /O2 •− and H2O2 produced in the radiolysis of liquid water with high-LET 12C6+ ions: a Monte-Carlo simulation study by Jintana Meesungnoen; Abdelali Filali-Mouhim; Nitaya Snitwongse Na Ayudhya; Samlee Mankhetkorn; Jean-Paul Jay-Gerin (419-425).
Monte-Carlo simulations are used to investigate the effects of multiple ionization on the formation of HO2 /O2 •− and H2O2 in the radiolysis of water by 12C6+ ions up to ∼430 keV/μm, at neutral pH and 25 °C. Taking into account the double and triple ionizations of water molecules, the primary yields G HO2 /O2 •− and G H2O2 are calculated as a function of linear energy transfer (LET). Our results quantitatively reproduce the large increase observed in G HO2 /O2 •− at high LET. They also simultaneously predict a slight maximum in G H2O2 at ∼180–200 keV/μm, in excellent agreement with experiment. Under the conditions of this study, triple ionization is found to contribute only negligibly to G HO2 /O2 •− .

Determination of nitrogen chemical shift anisotropy from the second-order cross-term in 14N MAS NMR spectroscopy by Tania Giavani; Henrik Bildsøe; Jørgen Skibsted; Hans J. Jakobsen (426-432).
A new method for determination of nitrogen chemical shift anisotropies (CSAs) employing 14N MAS NMR spectroscopy is presented. The method utilizes the effect of the second-order cross-term between the quadrupolar coupling and chemical shift interaction in the average Hamiltonian on the lineshape for the individual resonances within the manifold of 14N spinning sidebands, as illustrated by determination of the 14N CSA for Pb(NO3)2. The second-order functionality for this cross-term ensures that the CSA cannot be averaged by MAS. This is illustrated by determination of the 14N CSA for Pb(NO3)2 spinning (12 kHz) at twice the magnitude of the 14N CSA.

After illumination by single-beam polarized pulse laser (532 nm) with energies below the ablation threshold energy, periodic microstructure was prepared on an azobenzene-containing poly(urethane-imide) (PUI) film surface. The reflective polarized IR results proved that the azobenzene group tended to be oriented perpendicular to the surface periodic microstructure. The illuminated PUI surface was used as the alignment layer of liquid crystal (LC) molecules. The experimental results indicated that the LC molecules tended to align closely along the surface microgrooves of the surface periodic microstructure. But the anchoring energy of LC molecules on the PUI film surface was smaller than that calculated based on the Berreman theory, which indicated that LC alignment on the laser-illuminated PUI film surface was controlled by the surface microgrooves and anisotropic orientation of the azobenzene group.

Diode laser cavity ring-down spectroscopy (CRDS) has been used to measure rotational line intensities and air pressure broadening coefficients for the 3ν 3 (N–N stretching) vibrational overtone band of N2O. The line intensity measurements are in quantitative agreement with previously reported Fourier transform spectroscopy results [J. Mol. Spectrosc. 197 (1999) 158]. The air pressure broadening coefficients increase from 0.086 ± 0.015 cm−1 atm−1 for the P(34) line to 0.139 ± 0.010 cm−1 atm−1 for the P(3) line, and are greater than values reported for other ro-vibrational bands of N2O observed at lower frequencies. The effects of pressure broadening on line-centre absorption cross-sections, and thus for the sensitivity of CRDS detection of N2O using narrow bandwidth near-IR lasers are quantified.

Exfoliating KTiNbO5 particles into nanosheets by G.H. Du; Y. Yu; Q. Chen; R.H. Wang; W. Zhou; L.-M. Peng (445-448).
KTiNbO5 nanosheets have been prepared by a simple intercalation and exfoliation method. The structures of these nanosheets were analyzed, the formation mechanism was discussed and bandgap was measured to be about 3.2 eV.

The electronic structure of the first members of the ladderane family has been theoretically studied. The compounds are predicted to be highly symmetric and stable. Their geometries, heat of formation, vibrational frequencies and NMR spectra have been calculated and analyzed. The reported theoretical results can be useful for the experimental detection of this interesting family of molecules.

Mean-field approach to extended Su–Schrieffer–Heeger models by Matthias Rateitzak; Thorsten Koslowski (455-461).
In this work, we present a new approach to the numerical solution of the Su–Schrieffer–Heeger model. It is based on a polaron transformation that decouples nuclear and electronic degrees of freedom and a mean-field approach to the resulting electronic Hamiltonian. The emerging self consistent field procedure allows the simultaneous computation of the microscopic and electronic structure of Su–Schrieffer–Heeger models that are extended by explicitly taking electron–electron interactions into account. As an example, we apply the scheme to models of carbon nanotubes.

The geometries and the vibrational frequencies for several SiSO+ and GeSO+ species at doublet state have been predicted at density functional theory level with a 6-311+G* basis set. The detailed bonding character is discussed, and the state–state energy separations of various stable states relative to the ground state are calculated. The ground states are linear Si–OS+ and cyclic GeSO+ for two systems, respectively. Result analysis indicates that the 2 A cyclic state should be classified as the thiosuperoxide, the bent structure and the linear M–OS+ structure have some thiosuperoxide characters, but the linear S–M–O+ may be classified as thio-oxide.

Sugar, water and free volume networks in concentrated sucrose solutions by Valeria Molinero; Tahir Çaǧın; William A. Goddard III (469-474).
We used molecular dynamics simulations to determine the sucrose hydrogen bond network (HBN) in amorphous sucrose with 0–50% w/w water. We find that the onset of a percolated sugar HBN network (between 33% and 40%) coincides with the experimental observation of the decoupling of viscosity and probe diffusion for these mixtures. The analysis of the free volume (FV) in these mixtures shows a non-monotonic behavior with water content which is consistent with experimental observations of hydrogen bond compaction and negative apparent partial volume of dilution, but in disagreement with the FV theory model of plasticization by water.

Nanoscale domain switching at crystal surfaces of lithium niobate by Dongfeng Xue; Sixin Wu; Yingchun Zhu; Kazuya Terabe; Kenji Kitamura; Jiyang Wang (475-480).
The domain switching at the nanoscale is studied at both −Z and +Z surfaces of a lithium niobate crystal. A remarkable difference between both surfaces is experimentally observed by scanning probe microscopy, and is theoretically analyzed by the structural property and the chemical bonding structure of the lithium niobate crystal. The domain switching at the −Z surface is much easier and more stable than that at the +Z surface, which makes lithium niobate crystals have potential applications in the future domain engineering, especially at the nanoscale level.

Based on recently reported experimental results from various groups, the barrier height (or transition state energy) for the T1 dissociation of acetaldehyde, CH3CHO → CH3  + HCO, is determined to lie between 12.3 and 12.9 kcal mol−1. This result is compared with predictions from recent ab initio calculations.