Chemical Physics Letters (v.416, #4-6)

Theoretical study of the reaction of ketenyl and nitrogen dioxide radicals (HCCO + NO2) by Mai Thu Hien; Thanh Lam Nguyen; Shaun A. Carl; Minh Tho Nguyen (199-205).
Portions of potential energy surface containing low-energy pathways of HCCO + NO2 reaction were constructed using the G3B3 method. These show that only two separate reaction pathways are possible leading to HCNO + CO2 and HCO + NO + CO. Selectivity between product channels (CO2 vs. CO) occurs very early on the attractive entrance channels. The absence of an entrance barrier is consistent with available rate constants showing their negative dependence on temperature. The product distribution estimated in this Letter is in agreement with recent experimental results of Meyer and Hershberger [J.P. Meyer, J.F. Hershberger, J. Phys. Chem. A, 109 (2005) 4772].

We report the anharmonic spectra calculated for formaldehyde in acetonitrile solution using quartic force fields obtained at different levels of theory in connection with the SCI-PCM continuum solvent model. The fair agreement observed with the experimental data and with the observed shifts relative to the gas phase shows that the CCSD(T)/cc-pVQZ/SCIPCM and the hybrid CCSD(T)/cc-pVTZ//B3LYP/6-31+G(d,p)/SCIPCM approaches can be recommended to calculate reliable vibrational spectra in solution for medium size systems.

Prior to any thermodynamics studies of water clusters, one has to know all possible patterns of their hydrogen bondings. Recently, Miyake and Aida have developed directed graph model which generates all possible topological distinct hydrogen-bonded clusters. Here, a new method for generating such clusters is developed which is much faster then the method of Miyake and Aida. The results are given for all clusters with up to 12 water molecules.

Organic solvent dependence of plasma resonance of gold nanorods: A simple relationship by Jian Yang; Jen-Chia Wu; Yi-Chou Wu; Juen-Kai Wang; Chia-Chun Chen (215-219).
We report a theoretical and experimental study to reveal the linear relationship between the square of the peak position of the longitudinal plasmon mode of gold nanorods and the square of medium refractive indices. It is predicted theoretically and confirmed experimentally. The linear relationship indicates that gold nanorods can potentially be exploited as a new optical platform to develop novel, specific, sensitive and miniaturized biological sensors after suitable surface modification.

High pressure photoinduced polymerization of the orthorhombic polymeric phase of C60 by K.P. Meletov; V.A. Davydov; A.V. Rakhmanina; V. Agafonov; G.A. Kourouklis (220-224).
The stability of the linear orthorhombic polymer of C60 as a function of pressure has been studied by Raman scattering and X-ray measurements. The in situ Raman study shows an irreversible transition to a new phase occurring at pressures as low as ∼0.3 GPa. The specimens treated at pressure up to 3 GPa without laser irradiation do not show any structural changes after pressure release. The Raman spectrum of the new phase differs from those of the known 2D polymerized phases of C60. These data indicate that the simultaneous application of pressure and laser irradiation results in pressure photoinduced polymerization of the pristine polymeric chains of C60 thus transforming it to a new polymeric phase of C60.

Crystal structure and optical properties of (Ga1−x Zn x )(N1−x O x ) oxynitride photocatalyst (x  = 0.13) by Masatomo Yashima; Kazuhiko Maeda; Kentaro Teramura; Tsuyoshi Takata; Kazunari Domen (225-228).
The crystal structure and optical properties of (Ga0.87Zn0.13)(N0.83O0.16) were investigated. This material was demonstrated to absorb visible light at wavelengths of up to ca. 500 nm, and to exhibit photoluminescence at around 650 nm (1.9 eV) even at room temperature. The Rietveld analyses of neutron powder diffraction data of (Ga0.87Zn0.13)(N0.83O0.16) (P63 mc) confirmed that oxygen substitutes for nitrogen in the crystal structure, and may be responsible for the desirable optical properties of (Ga0.87Zn0.13)(N0.83O0.16) as a photocatalyst for visible light-driven overall water splitting. The nuclear density mapping revealed the structure to be free of interstitial sites and large disorder.

Generation of carotenoid radical cation in the vicinity of a chlorophyll derivative bound to titanium oxide, upon excitation of the chlorophyll derivative to the Q y state, as identified by time-resolved absorption spectroscopy by Xiao-Feng Wang; Yoshinori Kakitani; Junfeng Xiang; Yasushi Koyama; Ferdy S. Rondonuwu; Hiroyoshi Nagae; Shin-ichi Sasaki; Hitoshi Tamiaki (229-233).
Electron injection from a chlorophyll derivative (methyl 3-carboxy-3-devinylpyropheophorobide a, abbreviated as PPB a) to TiO2 nanoparticle took place in ≈30 fs following the decay of an excimer that was generated immediately after excitation to the Q y state (681 nm). Then, electron transfer from carotenoids (Cars) to PPB a •+ took place in ≈200–240 ps. The latter observation supports the electron-transfer mechanism proposed in a previous investigation, in which Cars were added as redox spacers to the PPB a-sensitized TiO2 solar cells to enhance their performance (X.-F. Wang, J. Xiang, P. Wang, Y. Koyama, S. Yanagida, Y. Wada, K. Hamada, S. Sasaki, H. Tamiaki, Chem. Phys. Lett. 408 (2005) 409).

High efficiency and long lifetime OLED based on a metal-doped electron transport layer by Jiun-Haw Lee; Meng-Hsiu Wu; Chun-Chieh Chao; Hung-Lin Chen; Man-Kit Leung (234-237).
The OLED performance of cesium (Cs) doped 4,4′-bis(5-phenyl-[1,3,4]oxadiazol-2-yl)-2,2′-dinaphthylbiphenyl (bis-OXD), a metal-doped electron transport layer, is reported. Device lifetime increases because: (1) Cs is heavy and difficult to diffuse in an organic matrix, and (2) The host material, bis-OXD, exhibits a high glass-transition temperature (T g) of 147 °C. The average roughness of the thin film is small hence the leakage current of the corresponding OLED devices is low. By using a silver cathode, an OLED with a 2.59 V reduction in driving voltage, a 47.3% increase in current efficiency, and a 3.14 times enhancement in operation lifetime was demonstrated.

Ordering of biaxial solutes in a smectic solvent by Giorgio Cinacchi (238-245).
The positional–orientational probability distribution function of a number of hard-body biaxial solute particles, treated in the Zwanzig approximation and dissolved in the smectic A phase of parallel hard spherocylinders, has been calculated using a second-virial theory. It is found that the tendency of a solute to be partitioned between intra- and inter-layer regions, each one associated with a different degree of solute orientation, exhibits a non-monotonic behavior with the geometrical parameters defining its shape and size. These results, compared with the available experimental and numerical data, could be of help in qualitatively interpreting ordering phenomena of interest in both material and biological sciences.

Cation-exchange reaction between Ag+ and Cd2+ was employed to transform single-crystal Ag2Se nanowires into single-crystal CdSe nanowires as a case study of transformation from one chalcogenide to another one. The CdSe nanowires of ∼30 nm in diameter had a wurtzite crystal structure and showed preservation of both the morphology and single crystallinity of the Ag2Se nanowires. This synthetic route is useful in producing large quantities of CdSe nanowires with uniform diameters and exceptionally long lengths (>50 μm).

Pore characterization of multi-walled carbon nanotubes modified by KOH by Sang Moon Lee; Soon Chang Lee; Jong Hwa Jung; Hae Jin Kim (251-255).
Multi-walled carbon nanotubes (MWNTs) were modified by KOH in order to obtain high surface area with micropores. Pore characterization of modified MWNTs was performed with the observation of adsorption–desorption isotherms of N2. According to the modification temperature of MWNTs increases, the specific surface area of MWNTs increases from 407 to 650 m2  g−1, on the other hand, the average pore width decreases from 2.77 to 2.13 nm. The isosteric heat value (q st,  ϕ  = 1/e ) of modified MWNTs increased with the rise of modification temperature. The q st,  ϕ  = 1/e of MWNTs increased in inverse proportion to the average pore width (w) and depended on the pore width.

Colored non-Gaussian noise induced resonant activation by Pradip Majee; Gurupada Goswami; Bidhan Chandra Bag (256-260).
We have investigated the mean first passage time (MFPT) problem in the presence of a fluctuating barrier. The fluctuation is carried out through colored multiplicative noise. Both Gaussian and non-Gaussian colored noises are considered. Our study shows that the resonant activation (RA) can appear at slower rate of increase of external noise strength with noise correlation time (τ) for non-Gaussian noise compared to Gaussian noise. When RA appears for both the noises then minimum in plot of MFPT vs. τ occurs at greater value of τ for Gaussian noise than that for non-Gaussian noise. Our calculation also shows that rate of decrease of the MFPT with increasing strength of additive white noise, is faster for Gaussian colored multiplicative noise compared to corresponding for non-Gaussian case.

Three conformational isomers of o-fluorobenzoic acid dimer have been identified in a supersonic jet expansion by use of laser-induced fluorescence excitation and dispersed fluorescence spectroscopy. Using a mixed dimer of o-fluorobenzoic acid with benzoic acid, we have provided further evidence that the three isomers in the previous case originate due to two distinct internal rotational isomeric forms of each of the dimer moiety. Relative stability and geometries of all the dimers are computed by use of DFT theoretical method. The resolved fluorescence spectra of the three isomeric homodimers have been tentatively assigned by correlating the observed frequencies with the normal mode frequencies of the isomers predicted by DFT calculation.

Energy resonance in electron transfer from nitric oxide to atomic metal cations at room temperature by Michael J.Y. Jarvis; Voislav Blagojevic; Gregory K. Koyanagi; Diethard K. Bohme (268-271).
Rate coefficients have been measured for the spin-allowed transfer of an electron from NO(X2Π) to Au+(1S), Zn+(2S), Se+(4S), As+(3P) and Hg+(2S) at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The observed range of a factor of 3 in the corresponding reaction efficiencies can be accounted for qualitatively by invoking energy resonance and Franck–Condon factors in the formation of NO+(X1Σ+, v). The electron transfer to Se+(4S) may leave the neutral Se product atom in an excited Se*(4p4, 3P) state.

It has long been a challenge to calculate electronic structures and properties of molecular systems containing more than 10 000 electrons using first-principles approaches. We have developed an elongation method that allows to treat 59 nm long single-walled carbon nanotubes (SWCNTs) containing 4770 carbon atoms at the hybrid density functional theory level. An unexpected oscillating behaviour of the energy gap with respect to the length of the nanotube is observed. The calculated current–voltage characteristics of sub-20 nm SWCNTs are in excellent agreement with the experimental results, revealing the underlying mechanism of electron transport in nano-sized SWCNTs for the first time.

Ab initio prediction of extremely large first hyperpolarizability of polyphosphaacetylene and polyphosphasilyne by Daisy Y. Zhang; Claude Pouchan; Eric A. Perpète; Denis Jacquemin (277-281).
Calculations at MP2/6-31G(d) level predict a very large first hyperpolarizability (β) for two phosphorus-containing oligomers, namely, polyphosphaacetylene, – ( P = CH ) n – and polyphosphasilyne, – ( P = SiH ) n – , i.e., 212 777 and 90 811 a.u., respectively, at n  = 16. These values are 5.3 and 2.3 times of that for the nitrogen analogue of polyphosphaacetylene, that is, polymethineimine. In striking contrast, polyborophosphene, – ( PH = BH ) n – , is predicted to have extremely small β values, with a β/n value approaching zero at large n. The causes for the relative β values in these four series are discussed, in light of the bonding structure, the π bond strength, the size of the nuclei, and the degree of delocalization.

CCSD(T) and DFT-PBE calculations with the Pol and HyPol basis sets have been performed in the series formaldehyde, formamide and urea to obtain accurate gas and solution phase molecular structures, rotational barriers, vibrational and electronic (hyper)polarizabilities in the static and dynamic (ω  = 0.04282 a.u.) regime. Comparison with available experimental data shows good agreement for polarizabilities and second hyperpolarizability of urea, while β(SHG) of urea is strongly underestimated because of overerestimation of solvent effects.

Rectification in molecular assemblies of donor–acceptor monolayers by Biswanath Mukherjee; Amlan J. Pal (289-292).
We report molecular rectification based on donor/acceptor (D/A) assemblies. A monolayer of a donor and a monolayer of an acceptor have been bound electrostatically to fabricate a D/A assembly. While neither of the donor or the acceptor monolayer showed any rectification, the D/A assembly showed asymmetric current–voltage characteristics. Molecular rectification in a D/A assembly has been discussed in terms of a band diagram. The advantages of a D/A assembly over unimolecular D–σ–A, where complex chemical synthesis is involved, have been discussed. In this Letter, we have used a monolayer of a phthalocyanine and Rose Bengal as donor and acceptor, respectively.

High-resolution FTIR analysis of the 2ν 6 overtone of cis-CHF=CHF and Fermi resonance study with the ν 2 fundamental by R. Visinoni; P. Stoppa; A. Pietropolli Charmet; S. Giorgianni; G.D. Nivellini (293-299).
The 2ν 6 overtone band (1659.43 cm-1) of cis-CHF=CHF has been recorded with an unapodized resolution of 0.004 cm-1, using a multipass cell and a Fourier transform infrared spectrometer. More than 2300 transitions have been assigned and fitted to the Watson’s A-reduced Hamiltonian in the I r representation yielding accurate spectroscopic parameters up to three sextic coefficients. The vibrational resonance of 2ν 6 with the ν 2 fundamental was also considered and, as a result, the Fermi coupling term, the unperturbed vibrational levels and molecular parameters have been calculated. Additionally, information on the infrared inactive ν 6 vibration have been derived.

Based on the Kawski–Gryczyński method the values of angle between absorption and fluorescence transition moments of carbocyanines are given. This method is applied to the linear molecules: 3,3′-diethylthiacyanine iodide (DTTHCI), diethyloxacarbocyanine iodide (DOCI), 3,3′-diethyl-9-methylthiacarbocyanine iodide (MDTCI), diethylthiacarbocyanine iodide (DTCI) and 3,3′-diethyloxadicarbocyanine iodide (DODCI).Similarly located transition moments polarized approximately along the long axis of DTTHCI, DOCI, MDTCI, DTCI and DODCI are responsible for absorption and fluorescence (β  ⩽ 10°), when exciting in the long wavelength absorption band. The results are compared with relevant data obtained from linear dichroism measurements, energy migration data in partly ordered films and general Perrin formula.

Keto–enol tautomerism of two structurally related Schiff bases: Direct and indirect way of creation of the excited keto tautomer by P. Fita; E. Luzina; T. Dziembowska; D. Kopeć; P. Piątkowski; Cz. Radzewicz; A. Grabowska (305-310).
Femtosecond time-resolved absorption spectra of two structurally related, internally H-bonded Schiff bases are reported. The 2-hydroxynaphthylidene-1′-naphthylamine (HNAN) stable as an enol tautomer undergoes an ultrafast excited state intramolecular proton transfer, while the 2-hydroxynaphthylidene-(8′-aminoquinoline) (HNAQ), stable as a keto structure, reveals unusual relaxation routes after electronic excitation. In particular, the rise of the bleaching band with the characteristic time of ∼700 fs was found and attributed to a gradual population of the S1 fluorescent state from a ‘hot’ excited state. The results accompanied by TDDFT calculations are used to construct the diagram of relaxation routes of an excited HNAQ molecule.

Optical two-dimensional Fourier transform spectroscopy of semiconductors by Camelia N. Borca; Tianhao Zhang; Xiaoqin Li; Steven T. Cundiff (311-315).
Two-dimensional Fourier transform spectra of optical excitations in semiconductor quantum wells are obtained. Coupling between heavy-hole excitons, light-hole excitons and unbound electron–hole-pairs results in off-diagonal features. A qualitative analysis of these features reveals the dominance of many-body interactions.

A model for combined electron and proton transfer in electrochemical systems by J. Grimminger; S. Bartenschlager; W. Schmickler (316-320).
We propose a model Hamiltonian for electrochemical electron and proton transfer; it contains terms for the two transfer steps themselves and for their coupling to solvent and vibrational modes. As first applications, we present potential-energy surfaces for the case where the two transitions couple only to classical modes, and show some examples of the proton dynamics near the saddle point region. Finally we discuss, under what circumstances a concerted transfer of both particles may be more favorable than two subsequent transitions.

Indium nanocrystals of different shapes and morphologies, including several micrometers long nanowires and two-dimensional nanoplatelets, were directly grown on the surface of mother Indium Phosphide nanorods during weak beam electron irradiation inside a transmission electron microscope. As-grown nanocrystals revealed pure single-crystalline tetragonal In phase. Low melting point of In crystals led to the continuous transitions of crystals from a solid to liquid state during the in situ growth, which were observed in a real-time and video-recorded. The structure and chemistry of the nanocrystals were thoroughly analyzed using high-resolution TEM and STEM imaging, energy dispersion X-ray spectroscopy, electron diffraction and elemental mapping.

Role of dielectric properties in organic photovoltaic devices by Basudev Pradhan; Amlan J. Pal (327-330).
We have fabricated organic photovoltaic devices via layer-by-layer electrostatic self-assembly technique with varied number of donor/acceptor interfaces. Upon illumination, the short-circuit current of the devices depended on the number of such interfaces. We have studied impedance spectroscopy of the photovoltaic cells under dark and illumination conditions. We compared the changes in bulk resistance of the device and relative dielectric constant of the active material with short-circuit current of the devices due to illumination. The results show that the photovoltaic devices should be designed in such a way that the dielectric constant of the active material exhibits large decrease under illumination.

Solvation of 2,2′-bithiophene: Influence of the first solvation shell in the properties of π-conjugated systems by Francisco Rodríguez-Ropero; Jordi Casanovas; Carlos Alemán (331-335).
The internal rotation of 2,2′-bithiophene has been investigated in aqueous and acetonitrile solutions using three different solvation models: the discrete, the continuum self-consistent reaction-field and the combined discrete/self-consistent reaction-field, the polarizable continuum model being chosen for continuum calculations. Results indicate that the polarizable continuum model provides a satisfactory description of the solvent effects in acetonitrile solution. However, combined discrete/self-consistent reaction-field calculations are more appropriated in aqueous solution, where both the first-solvation shell effects and the polarization induced by the bulk solvent affect the rotational profile of 2,2′-bithiophene. These results indicate that combined calculations are needed to model water soluble thiophene-derivatives.

Molecular dynamics simulations have been employed to investigate the volatilization of the enkephalin embedded in a rapidly heated H2O film. Simulations are performed for three different energy inputs, 103%, 89% and 69%, to vaporize the entire film. The cluster formed for each system undergoes a fast decomposition. The decomposition leads to a rapid temperature drop and proceeds differently for the different energy inputs. For 103% energy input, the explosive evaporation is the dominant process while, for 69% energy input, the normal evaporation plays a significant role in the cluster decomposition.

Synthesis and state of art characterization of BN bamboo-like nanotubes: Evidence of a root growth mechanism catalyzed by Fe by J.J. Velázquez-Salazar; E. Muñoz-Sandoval; J.M. Romo-Herrera; F. Lupo; M. Rühle; H. Terrones; M. Terrones (342-348).
We report the synthesis of bamboo-like BN nanotubes by annealing amorphous BN powders at 1100 °C in an Ar atmosphere. The amorphous powders were obtained after ball-milling h-BN for times longer than 60 h. The materials were characterized using high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS), elemental mapping, energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Based on our observations, we propose a novel root growth mechanism catalyzed by Fe-based alloy nanoparticles that arise from the ball-milling container.

NC4NC2: A new isomer of dicyanodiacetylene isolated in a cryogenic matrix by Anne Coupeaud; Nathalie Piétri; Isabelle Couturier-Tamburelli; Jean-Pierre Aycard (349-353).
The photoreactivity of the dicyanoacetylene:acetylene complex, trapped in an argon matrix at 10 K, has been studied at λ  > 120 nm using FTIR spectroscopy and complemented by DFT calculations at the B3LYP/6-31G** level. Two products have been formed during the experiment. In addition to the spectral bands assigned to dicyanodiacetylene, NC6N, a second set of bands has been observed: its carrier is attributed to NC4NC2 ([3-cyanoprop-2-ynylidyne)ammonio]ethynide) based on DFT calculations and isotopic experiments. On the basis of this assignement, this is the first time that this C6N2 isomer has been identified.

Electrostatic control on endo/exo selectivity in ionic cycloaddition by V. Tamilmani; C.A. Daul; P. Venuvanalingam (354-357).
DFT method has been used in combination with various basis sets to model the ionic cycloaddition of cationic heteroaromatic diene, 2,3-dimethylisoquinoliniuim ion with cyclopentadiene with a view to understand the factors that influence the stereochemical outcome of the reaction. Calculations show that this reaction is an inverse electron demand type reaction and it passes through highly asynchronous transition states and mainly electrostatic repulsion govern the endo/exo selectivity of the reaction. Endo TS is more destabilized than exo TS due to repulsion between positive charges delocalized over the two reacting partners in the transition state and hence, exo adduct is more preferred than endo adduct in total agreement with experiment.

Effects of strong paramagnetic interactions on solid-state deuterium NMR spectra by Motohiro Mizuno; Naohisa Itakura; Kazunaka Endo (358-363).
The method for analyzing the 2H NMR spectrum affected by strong paramagnetic interaction is discussed. The 2H NMR spectral simulation including the effects of paramagnetic shift and paramagnetic spin–spin relaxation was performed for several molecular motions. The contribution of distant paramagnetic ions to the paramagnetic shift and the effect of anisotropic spin–spin relaxation on the lineshape of the 2H NMR spectrum were investigated by the spectral simulation. The temperature variation of 2H NMR spectrum of [Mn(H2O)6][SiF6] observed by the Exorcycled quadrupole-echo sequence was well reproduced using the proposed method.

Dynamics of rigidly rotating spirals under periodic modulation of excitability by Supichai Kantrasiri; Pramote Jirakanjana; On-Uma Kheowan (364-369).
The dynamics of rigidly rotating spiral waves under periodic modulation of excitability is studied experimentally in the photosensitive Belousov–Zhabotinsky reaction. The experiments of sinusoidal and pulsatory modulation reveal that the motion of the spiral tips can be forced to describe a wide range of cycloidal trajectories, depending strongly on the modulation period. The results are complemented with numerical simulations and discussed in the framework of the kinematical theory. We found that the critical parameter for a linear resonance drift under the periodic modulation is not only ‘the modulation period’, as reported earlier, but also the amplitude and duration of modulation.

Potential curves of alkaline-earth dimers, Be2, Mg2 and Ca2, bound by London dispersion forces, are determined with the recently developed range separated hybrid method with perturbational long-range correlation corrections (RSH + MP2), and are compared to wave function approaches (MP2), to conventional density functional (LDA, PBE, BLYP, BPW91) and hybrid functional (PBE0, B3LYP) results. In contrast to rare gas dimers, functionals involving Becke’s exchange are not repulsive, but yield a relatively profound potential well. The RSH + MP2 approach outperforms the MP2 method considerably and reduces the basis set superposition error. It represents a promising alternative for the study of larger alkaline-earth clusters.

The quantitative use of momentum-space descriptors by Jabir H. Al-Fahemi; David L. Cooper; Neil L. Allan (376-380).
We explore the possible use of various momentum-space quantities as molecular descriptors in QSAR and QSPR studies. It is found that three- or four-descriptor models, that include molecular weight, provide useful correlations for a range of property and activity data, such as gas-chromatography retention times, gas-hexadecane partition coefficients and tadpole narcosis concentrations. The underlying reason for this success suggests the utility of hybrid models that take account also of the atomic character, albeit corrected for the number of bonds.

A systematic in situ transmission electron microscopy study was carried out on the formation and evolution of germanium (Ge) nanoclusters and nanoparticles in a structure consisted of a Ge plus silicon oxide or Ge plus germanium oxide layer sandwich between two SiO2 layers. Nanoclusters were observed in the Ge plus silicon oxide and the Ge plus germanium oxide samples when irradiated with an electron beam current with unheated substrate. Nanoparticles were observed in the Ge plus silicon oxide samples under electron beam irradiation with the substrate heated to 250 °C. The growth mechanism of the nanoparticles was well described by the classical model.

Author Index (387-394).