Chemical Physics Letters (v.363, #5-6)

A large density of multiwall carbon nanotubes (MWCNTs) has been grown on the fibers of a carbon paper using a sulfonated silane intermediary to uniformly disperse (1:1) Ni–Co catalytic sites used in the decomposition of ethylene. The nanotubes were obtained at 800 °C by Ohmically heating the carbon paper support. The MWCNTs are typically up to 20  μm in length and between 30 and 50 nm in diameter. These tubes are easily opened at room temperature, while remaining firmly attached to the carbon paper.

A master equation approach including weak exciton–phonon coupling is applied to the exciton dynamics of a dendritic molecular aggregate modeled after a phenylacetylene dendrimer, D25, which exhibits an efficient light-harvesting property. The mechanism of exciton migration from the periphery to the core is studied by analyzing relaxation terms among the exciton states originating in weak exciton–phonon coupling. Partial overlaps of exciton distributions between neighboring exciton states are found to be important for realizing the unique migration behavior by stepwise transfer from the periphery to the core via multi-step exciton states.

An ab initio study of ion-pair formation from the third 1 A state of ArHCl by Tadashi Sato; Katsuyuki Nobusada; Kiyoshi Tanaka (429-434).
Photoexcited molecules in the vacuum ultraviolet region electronically relax via Rydberg and intra-valence excited states. As an example of such relaxation processes, ion-pair formation of ArHCl+→ArH++Cl from the third 1 A electronic state of ArHCl is studied theoretically. The potential energy surfaces are calculated at the level of multi-reference single and double excitation configuration interaction theory. Detailed energetics of the ion-pair formation is discussed. It is found that the present molecular system has a potentiality to enhance the ion-pair formation owing to the large electronic polarizability and high ionization threshold of Ar.

A novel theory of vibronic intensity borrowing by large-amplitude vibrations was developed in a form to modify the conventional Herzberg–Teller theory. The ingredients are: (1) Crude Born–Oppenheimer wavefuntions (CBO Wf’s) at more-than-one (2×2×3=12) conformations were used as building blocks of the adiabatic BO Wf. (2) The latter was made to belong to the molecular symmetry group (G 12) by Fourier-series expansion terminated at the low (ninth) order. (3) The non-Born–Oppenheimer effect was shown to be less significant. The experimental observation of the subtitle was successfully interpreted with this formalism, endorsing the previous assertion by Walker et al. [J. Chem. Phys. 102 (1995) 8718].

A first principles description of electronic excitation in Li–He colliding pairs is developed introducing l-dependent pseudopotentials and including two- and three-body polarization terms. The treatment combines an eikonal approximation and time-dependent molecular orbitals to provide interatomic potentials, their non-adiabatic couplings, and state populations during interactions. We discuss the effects of the basis set size on the calculations, and compare our results with experiment and other calculations. Our integral cross-sections obtained with a large basis set are in excellent agreement with experiment.

Anion radicals of o- and p-bis(trimethylsilyl)benzenes were found by ESR spectroscopy to form the corresponding triplet dimers in 2-methyltetrahydrofuran (MeTHF) glass matrices at low temperatures. The temperature dependence of the signal intensity of a triplet dimer has shown that the dimer has the triplet ground state. The spin–spin distances estimated with the D values increased with the increasing size of alkali metal countercations, indicating that the triplet dimer has a π–M+–π type sandwich structure. The stability of the triplet dimer depended remarkably on the coordination ability of the solvent; no triplet dimer was observed in tetrahydrofuran (THF).

Density Functional Theory has been applied to investigate isomerization processes of aluminum tris(8-hydroxyquinolinate) (Alq3). Both mer–fac isomerization and mer-Alq3 chirality inversion have been reported. This represents an attempt to describe phenomena which could take place during Alq3 amorphous-films deposition or during solid-state changes of phase. Furthermore, it represents the opportunity to apply modern theoretical methodologies to shed light on the general problem of isomerizations in tris–chelate complexes which were widely studied experimentally and by approximate theoretical methods in the seventies and in part of the eighties.

An ab initio direct molecular dynamics study of the fragmentation of the unstable, neutral F(H2O) complex has been performed. A photodetachment experiment of the stable F(H2O) anion complex produces F(H2O) in the vicinity of the transition-state region of the hydrogen abstraction reaction, F+H2O→FH+OH. Ab initio energy and gradient evaluations in the direct molecular dynamics simulation were done at the QCISD/6-311+G** level of theory. The product vibrational and rotational states were computed and the mechanism of the reaction examined. The primary products of reaction are vibrationally excited HF and rotationally warm but vibrationally cold (v″=0) OH. The reaction proceeds through three channels. In order of the frequency of occurrence, these are: (1) direct abstraction of a hydrogen atom by F; (2) dissociation into F+H2O; and (3) direct abstraction of a hydrogen atom by F with subsequent formation of a weakly bound FH–OH complex.

Hyper-Rayleigh scattering is used to compare the effect of the addition of n-butylamine to large (3.2 nm) and small (1.6 nm) CdSe nanoparticles (NPs). It is found that although the adsorption of n-butylamine on large NPs enhances their nonlinear optical response, it has the opposite effect when adsorbed on the small nanoparticles for which the amine adsorption was found previously to induce structural changes. This observation is consistent with an increase in the symmetry of the nanocrystal structure such as would be observed in a phase change from a 4-coordinate wurtzite crystal structure to that of a 6-coordinate zinc-blend form.

A chemical kinetic theory on muscle contraction and spontaneous oscillation by Wei-Sheng Guo; Liao-Fu Luo; Qian-Zhong Li (471-478).
From a set of chemical kinetic equations describing the actin-activated myosin ATPase cycle, we show that, in active muscle, the fraction of myosin heads in any given biochemical state is independent of both [ADP] and [Pi]. Combining muscle mechanics data of Pate and Cooke, we deduce the muscle state equation in which muscle force is a state variable of the muscle system. The theoretical results are consistent with Baker's experimental data but somewhat different from conventional muscle theory. Based on the muscle state equation with the knowledge of special structure of muscle, we present a physical mechanism which can lead to both contraction and oscillation of sarcomeres. It explains the muscle spontaneous oscillatory contraction in a natural way and agrees well with experimental data. The model will be helpful in studying the oscillatory behavior of cilia and flagella.

Coupled-oscillator model for nonlinear optical activity by M.A. Belkin; Y.R. Shen; C. Flytzanis (479-485).
The coupled-oscillator model, previously used to describe linear optical activity, is extended to the study of nonlinear optical activity of dimer-like chiral molecules. The result shows good agreement between theory and experiment for optically active sum-frequency generation from 1,1-bi-2-naphthol solution. It is seen that linear and nonlinear optical activities generally probe different aspects of the chiral structure of the molecules.

Spatial symmetry breaking is examined numerically for the linear C n + (n=3, 5, 7, 9) radical cations with hybrid B3LYP and local BP86 functionals. B3LYP clearly breaks spatial symmetry for 2Σ u +   D ∞h structures for C3 + and C5 +. BP86 does not break spatial symmetry but generates lower-symmetry lower-energy 2ΣC ∞v solutions which are not connected with the 2Σ u +   D ∞h structures starting from C5 +. The 2Π g and 2Π u electronic states do not suffer from spatial symmetry breaking. The bent C3 +, kinked C5 +, and quasi-linear C2v and C2h structures of C7 + and C9 + are the lowest energy chains.

Nonlinear optical response of a polycarbazolyldiacetylene film through femtosecond two-photon spectroscopy by Daniela Grando; GianPiero Banfi; Davide Comoretto; Giovanna Dellepiane (492-497).
We report the two-photon absorption (TPA) spectrum of a polycarbazolyldiacetylene film in the interval 900–1550 nm. In this range, the TPA coefficient exhibits a broad peak with a maximum of 33 cm/GW around 1125 nm, which indicates a two-photon state positioned at 2.2 eV, slightly below the one-photon state at 2.36 eV. The magnitude of the nonlinear refraction in the infrared is predicted on the base of a Three Level Model, which accounts fairly well for the nonlinear absorption in the explored range.

Collision induced absorption (CIA) bands in molecular oxygen, at 630 and 577 nm, were measured at low temperatures, down to 132 K. Besides providing confirmation of previous room temperature data, and new information about the temperature dependence of CIA bands, these measurements demonstrate the performance of cavity ring down spectroscopy in a rather unusual configuration. The cavity ring down spectroscopy (CRDS) setup includes a broadly tunable commercial pulsed optically pumped oscillator (OPO) laser coupled with a ring down cell, a section of which can be cooled down to liquid nitrogen temperature. In addition, a spectrograph is used as a wavelength-selective element at the cavity output, before measuring the ring down signal. This was needed given the broad OPO lineshape and the coexistence of other wavelengths generated by the OPO system.

Theoretical ab initio study of Xenon pentafluoride anion. Mechanism of Xenon pseudorotation by Paul Fleurat-Lessard; Olivier Durupthy; François Volatron (505-508).
Ab initio calculations have been performed on XeF5 anion at the MP2 and CCSD(T) levels with a large basis set. Four extrema have been optimized and characterized by frequencies analysis. We find the absolute minimum to be of D5h symmetry in accordance with the experimental data; the theoretical vibrational spectrum of this minimum is in good agreement with the experimental one. Three other extrema are found to be higher in energy depending on the angular separation of the Xenon lone pairs as predicted by the VSEPR theory. Finally the characterized transition state has been found to belong to the Xenon pseudorotation pathway.

Anatase titanium dioxide nanocrystalline films with (0 0 1) preferred orientation were successfully grown on the industrial indium tin oxide conducting glass through induction of Langmuir–Blodgett monolayer of surfactant Triton X-100 at room temperature. The (0 0 1) oriented titanium dioxide nanocrystalline film with 1000 nm in thickness consists of 98% anatase phase and 2% rutile phase and gives an actual surface area of 50. Compared with that based on titanium dioxide polycrystalline with the same actual surface area and without the preferred orientation, a remarkable preference in energy conversion was observed for dye-sensitized solar cell based on (0 0 1) oriented titanium dioxide nanocrystalline. This is attributed to their differences in crystallinity and crystal orientation.

We report herein the interaction of 2-phenyl-5-(4-diphenylyl)1,3,4-oxadiazole (PBD) with α-, β-, and γ-cyclodextrins (CDs). It was found that the reaction patterns between PBD and CDs are remarkably different. α-CD can form a simple 1:2 (guest:host) inclusion complex with PBD, while β-CD and γ-CD can form nanotube with PBD at higher concentration of both PBD and CDs. It was also found that the interaction pattern of butyl-2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (butyl-PBD) with CDs is similar to that of PBD with CDs. On the basis of the above results, possible structures of these cyclodextrin nanotubes were discussed.

A full-dimensional quantum dynamical study of vibrational relaxation in H2+H2 collisions using a recently developed potential energy surface of Boothroyd et al. is reported for both ortho- and para-H2. A comparison of theoretical and experimental rate coefficients is made. The existence of a minimum in rate coefficients as a function of temperature is predicted.

We report calculations on the CH 3 D+O(3 P)→CH 3+OD reaction using a reduced dimensionality model with four degrees of freedom. The model has already been used to evaluate the effect of the symmetric and asymmetric stretching vibrations of methane on the CH 4+O(3 P) and CH4+H reactions. Unfortunately, it is not easy to establish a direct comparison between these previous calculations and experimental results. As the difficulties disappear when CH4 is replaced with CH3D we decided to try our model on the title reaction. The results of these new calculations are presented in this Letter.

Macroscopically structured polymer formation governed by spatial patterns in the Belousov–Zhabotinsky reaction by Yevhen Yu. Kalishyn; Vyacheslav O. Khavrus; Peter E. Strizhak; Michael Seipel; Arno F. Münster (534-539).
We report the formation of macroscopically structured cross-linked polyacrylamide hydrogel in the Belousov–Zhabotinsky (BZ) system (oxidation of malonic acid by bromate catalyzed by ferroin). Here, acrylamide, the cross-linker bis-acrylamide, and polymerization initiator are added into the BZ system. We show that the formation of waves and ripples in the polymer is governed by spatial structures emerging in the BZ system. Without any spatial structures in the BZ system only the formation of a spatially uniform polymer is observed. Without cross-linker, a spatially uniform polymer was observed as well. Structured polymer formation is caused by the interaction of chemical reactions in the BZ system and the polymerization process including gelation and cross-linking of the monomer units.

Full-CI calculations of frequency-dependent dipole and dipole–quadrupole polarizabilities of ground state LiH have been performed in the imaginary frequency range 0–56 a.u. using a set of 58 Gaussian type orbitals (GTOs) giving a Full-CI dimension of about 700.000 determinants in each symmetry-adapted subspace. A 16-point Gauss–Legendre quadrature of the Casimir–Polder formula over imaginary frequencies allows calculation of the dipole–quadrupole dispersion constants for the LiH–LiH homodimer, from which C7 dispersion coefficients are derived for the first time.

Ordinary LCAO-MO theory allows for the ab-initio definition of α and β parameters occurring in the Hückel theory including overlap of simple σ homonuclear systems. One- and two-electron terms not included in Hückel theory are explicitly considered. When such correction terms are neglected in the very spirit of Hückel theory, simple calculation in the hydrogenic approximation for the ground states of H2 +, H2, He2 +, He2 at the corresponding bond lengths shows semiquantitative agreement with experimental or accurate theoretical results for the bond energies, providing a simple and clear physical picture of the nature of the chemical bond.

Theoretical study of the molecular mechanism of the Li(2 S 1/2)+N 2 O(X 1Σ+) reaction by Oksana Tishchenko; Eugene S Kryachko; Christian Vinckier; Minh Tho Nguyen (550-558).
The present work aims to elucidate the mechanism of the oxidation reaction of lithium atoms with nitrous oxide based on the complete-active-space-self-consistent-field plus second-order perturbation theory (CASSCF–MP2(11e/12o)) ab initio calculations. The title reaction is found to occur via two lower-lying channels with the energy barriers of 4.5 and 6.0 kcal/mol. Both barriers originate as a result of an avoided crossing of the two lowest 2 A potential energy surfaces (PES), corresponding to the neutral and ionic reactant states. Due to a large energy separation between surfaces in the transition regions, the reaction likely occurs on the lowest adiabatic PES. Earlier photoelectron spectroscopic experiments related to the ionization of LiO are also discussed within the present model.

The interference effects induced by two-color excitation in the photodissociation of IBr by Hideki Ohmura; Taisuke Nakanaga; Hironori Arakawa; M. Tachiya (559-566).
We have investigated the interference effects in the two-color dissociation of IBr using fundamental (560 nm) and its second-harmonic (280 nm) light. We have performed one-dimensional photofragment translational spectroscopy in the presence of two-color excitation. In the strong-excitation regime for the fundamental light (∼1.3×1010   W cm −2 ), the yield of the spin–orbit excited iodine atoms dissociated from IBr molecules shows an oscillating behavior that is dependent on the relative phase between the fundamental and the second-harmonic light. This phase-dependent behavior can be explained by the interference between a one-photon transition induced by the second-harmonic light and a dipole-forbidden two-photon transition induced by the strong fundamental light.

Reduced diameter distribution of single-wall carbon nanotubes by selective oxidation by E Borowiak-Palen; T Pichler; X Liu; M Knupfer; A Graff; O Jost; W Pompe; R.J Kalenczuk; J Fink (567-572).
We report an easy way to narrow the diameter distribution of single-walled carbon nanotubes (SWNT) by oxidization treatments. Both a chemical treatment in 2 M HNO3 as well as oxidation in a reduced O2 atmosphere lead to a selective burning of the narrower SWNT in bulk samples and to a diameter distribution which is smaller by a factor of two. This is a first important step towards a selective production of SWNT with a defined diameter on a bulk scale.

Competition between collective and stimulated effects in 130 Te 2 superfluorescence by J. Meziane; S. Oullemine; K. Amezian; E. Boursey (573-578).
The unique properties of superfluorescence need a clear theoretical understanding. Our goal is to present a comparison of our experimental data on Te2 superfluorescence with simulations based on semi-classical theory and mean field quantum theory. Quantitative agreement can be found with quantum theory results provided time dependance of the field is taken in account.

Synthesis of La1−x Ca x MnO3 nanowires by a sol–gel process by Xiangyang Ma; Hui Zhang; Jin Xu; Junjie Niu; Qin Yang; Jian Sha; Deren Yang (579-582).
La1−x Ca x MnO3 (LCMO) nanowires were prepared using a simple sol–gel process and nano-channel alumina (NCA). X-ray diffraction (XRD) and selected-area electron diffraction (SAED) indicate that the nanowires are of monoclinic perovskite structure. Transmission electron microscopy (TEM) shows that the diameters of nanowires are around 30 nm. Furthermore, high-resolution TEM reveals a clear lattice image of {0 2 1} planes in a LCMO nanowire. It can be expected that the method presented in this Letter is also appropriate for the preparation of nanowires of other mono-component or multi-component oxides.

Opening and thinning of multiwall carbon nanotubes in supercritical water by Jia-Yaw Chang; Anil Ghule; Jia-Jiu Chang; Shin-Hwa Tzing; Yong-Chien Ling (583-590).
Supercritical water (SCW) in the presence and absence of oxygen is used for the first time for the opening and thinning of multiwall carbon nanotubes (MWNTs). The influence of variation of pressure, temperature, and time on the opening and thinning of MWNTs is examined. In SCW, opening and thinning of MWNTs is observed both in the presence and absence of oxygen. In addition, the presence of oxygen (∼2 mmol) shows improved thinning of MWNTs with the collapsed outer graphene layers tending towards the inner layers. The morphology of MWNTs are critically analyzed using transmission electron micrographs (TEM) and Raman spectra.

Photoreaction mechanisms of 2-bromo, 2-bromo-4-chloro and 2,4-dibromophenols in low-temperature argon matrices have been investigated by Fourier transform infrared spectroscopy. Density-functional-theory calculations are performed to assign the observed infrared bands of photoproducts, where a 6-311+G(d,p) basis set is used to optimize geometrical structures. It is concluded that 4-bromo-2,5-cyclohexadienone is mainly produced from 2-bromophenol, while formation of cyclopentadienylidenemethanone⋯HBr complex is a minor pathway, in contrast to the case of 2-chlorophenols. The branching ratio between the two reaction pathways is estimated from the absorbance changes of the infrared bands to be 5.4±0.3.

For wave functions of atoms expressed by linear combinations of Slater determinants, a general formalism is presented to evaluate the angular expectation values 〈cosθ 12〉 and 〈cos θ ̄ 12〉 where θ 12 and θ ̄ 12 are the angles subtended, respectively, by the position and momentum vectors of two electrons. The results are applied to the 102 atoms He through Lr in their ground states within the Hartree–Fock framework. It is found that both 〈cosθ 12〉 and 〈cos θ ̄ 12〉 are non-positive for all the atoms, implying that the interelectronic angles θ 12 and θ ̄ 12 are never smaller than 90° in an average sense.

The energy expressions for the Coulomb and dipole–dipole interactions in systems with one-dimensional periodicity are derived. In the calculations we used the integral representation of the gamma function, the one-dimensional Poisson summation formula, and two-dimensional Fourier transforms of the functions depending on position components perpendicular to the periodicity direction. In this method the reciprocal-space summation can be expressed as a sum of structure factors, containing charge or dipole positions, multiplied by coefficients depending on the reciprocal-space vector, however, in a general case two parameters must be introduced.

Optical limiting properties of soluble poly(thienyleneethynylene)s by Guijiang Zhou; Shuang Zhang; Peiji Wu; Cheng Ye (610-614).
In solution, the soluble poly(thienyleneethynylene)s (PTEs) all show obvious optical limiting effect. The thin composite plate of PTE-3 in polystyrene (PS) shows stronger optical limiting response than its solution sample and C60 solution at the same linear transmittance 76%. Even with 86% linear transmittance, the thin plate still shows pretty good optical limiting effect which is better than that of C60. All the interesting results may be attributed to strap-like structure of PTE-3 formed in PS matrix. So it can be anticipated that the PTEs may be one of the most potential materials to develop practical application for optical limiting.

Author Index (615-624).