Chemical Physics Letters (v.371, #1-2)

Study on the reduction of Eu3+→Eu2+ in Sr4Al14O25: Eu prepared in air atmosphere by Mingying Peng; Zhiwu Pei; Guangyan Hong; Qiang Su (1-6).
Compounds of Sr4Al14O25: Eu were prepared in air atmosphere by high temperature solid state reaction. The reduction of Eu3+→Eu2+ was firstly observed in the aluminate phosphor of Sr4Al14O25: Eu synthesized in air condition. This made aluminate a new family and Sr4Al14O25 a new member of compounds in which Eu3+ ion could be reduced to Eu2+ form when fired in air atmosphere. The reduction of Eu3+→Eu2+ in Sr4Al14O25: Eu was explained by means of a charge compensation model. Experiments based on the model were designed and carried out, and the results supported this model.

Ab initio electron correlation calculations based on quantum chemical methods are successfully applied to a metallic system. As a test system we select one-dimensional Li n rings up to n=62. The correlation energy is determined within an incremental scheme, where the individual energy increments are calculated with local orbitals. The local orbitals are generated by projecting the atomic 2s orbitals onto the occupied Hartree–Fock space. Multi-reference methods are applied to deal with the low-lying excitations in a metal.

Thermoluminescence and electroluminescence of annealed polyfluorene layers by Dessislava Sainova; Dieter Neher; Ewa Dobruchowska; Beata Luszczynska; Ireneusz Glowacki; Jacek Ulanski; Heinz-Georg Nothofer; Ulli Scherf (15-22).
As-prepared and annealed layers of polyfluorene were studied by thermoluminescence (TL), photoluminescence (PL) and electroluminescence (EL). Upon annealing at the transition to the liquid-crystalline phase, the green emission in PL and EL becomes more pronounced. At the same time the TL spectrum broadens towards higher temperatures, indicating the introduction of additional trapping sites. In contrast, photooxidation does not broaden the TL spectrum, even so it significantly affects the emission spectra. We conclude that annealing causes the formation of non-/weakly emitting aggregates which compete with the undisturbed chains with respect to charge carrier capture and exciton recombination.

Selective solvation caused by size effects by M. Sakurai; A. Yoshimori (23-28).
A simple binary-solvent model is theoretically employed to study the size-effect on selective solvation. We consider multi-component hard-sphere molecules with solute–solvent attraction. We employ the PY approximation to calculate coordination numbers. The calculated coordination number shows that the solute attracts more large solvent molecules than small solvent molecules at a weak attraction. The solute, however, attracts small solvent molecules at strong attraction. To study the selectivity, we divide the size-effect into the exclude effect and the steric hindrance. Attractive strength increases the steric hindrance of large solvent molecules more than that of the small solvent molecules.

The reaction of formaldehyde with chlorine atom by Feng Dong; Zhengwang Qu; Qiyuan Zhang; Fanao Kong (29-34).
The elementary reaction of formaldehyde (H2CO) with chlorine atom has been experimentally investigated by the time-resolved infrared emission spectroscopy. The nascent products HCO and HCl (v⩽3) are observed. The major product channel has been identified as Cl+H2CO→HCl+HCO. Theoretical calculations have revealed that the mechanism is a nearly barrierless peripheral hydrogen abstraction reaction.

Potential energy surface of AlW and W2 clusters has been studied by density functional theory. The results indicate that at a given spin multiplicity, usually more than one local minimum are found on the potential energy surface depending on the initial geometry. For AlW neutral cluster, global minimum is found at spin multiplicity 6, while for cation and anion species, spin multiplicities at 7 and 3 gives the global minimum, respectively. For W2 cluster, global minimum is found at the lowest spin state, i.e., spin multiplicities 1, 2 and 2 for neutral, cation and anion species, respectively.

Picosecond and femtosecond laser photolysis studies of a photochromic diarylethene derivative: multiphoton gated reaction by Hiroshi Miyasaka; Masataka Murakami; Tadashi Okada; Yasushi Nagata; Akira Itaya; Seiya Kobatake; Masahiro Irie (40-48).
Cycloreversion (ring-opening) process of one of the photochromic diarylethene derivatives, 1,2-bis(2-methyl-3-benzothienyl)perfluorocyclopentene (BPFC), was investigated by means of picosecond and femtosecond laser photolysis methods. The drastic enhancement of the reaction yield was observed by the picosecond laser exposure. On the other hand, the cycloreversion reaction yield under femtosecond laser exposure was consistent with the steady-state light irradiation. The excitation intensity effect of the reaction profiles revealed that the successive multiphoton absorption process leading to higher excited states opened the efficient cycloreversion process.

Geometry optimization within a localized CAS-SCF approach by Thierry Leininger; Celestino Angeli; Stefano Evangelisti; Renzo Cimiraglia; Daniel Maynau (49-55).
A CAS-SCF algorithm based on molecular orbitals that conserve their physical nature during the iterative process has been recently proposed. The algorithm is based on the iterative partial diagonalization of the one-body density matrix obtained from a SuperCI-like procedure. If localized guess orbitals are used, the locality property is conserved by the final orbitals, and the algorithm is particularly suitable for the study of local processes (e.g., bond breaking) in large molecules. In this work, it is shown that the localized orbitals obtained in such a way can be supplied to a standard CAS-SCF geometry-optimization package, in order to find the optimal geometry relaxation with a given local active space. The procedure is illustrated in the case of the rotation of the CH2 group around the CC double bond in the acroleine molecule.

Nuclear quadrupole coupling constants (χ) of 27Al, 35Cl and 81Br in AlX3 monomers as well as Al2X6 (X=Cl, Br) dimers are calculated at the RHF/6-311G* and B3LYP/6-311G* levels, using Gaussian 98 package. Correlations are made between χ and dihedral angles θ, of Al2X6. These θs are produced through the ring puckering motions about the hinge line which joins the two bridge halogens (Xb). Nuclear quadrupole coupling constants of 35Cl, 81Br and 27Al are used as probes for monitoring the departure of the symmetry of Al2X6 from a high symmetry point group D2h to a lower one. The χs of chlorine nuclei of AlCl3 differ significantly from those of Al2Cl6. These differences appear negligible for AlBr3 in comparison to Al2Br6. This work demonstrates the considerable sensitivity of nuclear quadrupole resonance in distinguishing between Al2X6 conformers. This is in comparison to the usage of energy differences which is customarily employed.

Coupled-cluster connected-quadruples corrections to atomization energies by Torgeir A Ruden; Trygve Helgaker; Poul Jørgensen; Jeppe Olsen (62-67).
Using the CCSDTQ method, we have examined the contributions from the connected quadruple excitations to the atomization energies of CH2, H2O, HF, N2, F2, and CO. For N2 and F2, the quadruples contribution is larger than 3 kJ/mol. On average, the quadruples corrections to the CCSDT atomization energies reduce the mean and maximum errors from 2.3 and 5.0 kJ/mol to 0.5 and 1.1 kJ/mol, respectively, for these six molecules. Moreover, the largest error occurs for N2, where the quintuple contribution is shown to account for a large part of this deviation. The calculations presented here establish unequivocally that the small errors in the CCSD(T) atomization energies in part arise from a cancellation of the errors resulting from the approximate treatment of the connected triples and from the neglect of connected quadruples.

Sensitized reactions by benzophenones in the higher triplet excited state by Xichen Cai; Michihiro Hara; Kiyohiko Kawai; Sachiko Tojo; Tetsuro Majima (68-73).
Transient behaviors of benzophenone (BP) and several substituted benzophenones in the higher triplet excited state (T n ,n>2) were investigated by two-color two-laser excitation technique. Energy transfer from BP(T n ) to quenchers (Q) having higher E T1 than that of BP and the second energy transfer from Q(T1) to BP(S0) were observed. The lifetime of BP(T n ) is estimated to be 450 ps which is long enough for a bimolecular reaction to occur. The decay of Q(T1) depended on the concentration and structure of the Q. BP(T n )-sensitized cleavages of C–Cl bond of CCl4 and C–Br bond of bromobenzene occurred.

Computations are presented for the alanine–(H2O) n (n=1–3) and its zwitterion–(H2O) n clusters. We find that at least two water molecules need to bind to alanine to give stable alanine zwitterion–water cluster. Structures of the conformers are predicted, and their relative energies are compared. Detailed analysis is presented on the dynamic (proton transfer) pathways between the neutral alanine–(H2O) n and the zwitterionic alanine–(H2O) n (n=2, 3) clusters, including the structures of the transition states. A variety of proton transfer pathways are predicted between alanine–(H2O) n and the zwitterion–(H2O) n clusters, depending on their structures: direct proton transfer, concerted double and triple proton transfer mechanism.

XRD and XPS analysis of the degradation of the polymer electrolyte in H2–O2 fuel cell by Chengde Huang; Kim Seng Tan; Jianyi Lin; Kuang Lee Tan (80-85).
Nafion ® is frequently used as electrolyte membrane in polymer electrolyte fuel cells (PEFC). In this Letter the degradation of the Nafion ® polymer electrolyte was investigated using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). XRD showed that the electrode potential and low gas humidification temperature could decrease the crystallinity of Nafion ® . XPS analysis indicated that the Nafion ® was decomposed in the hydrogen potential region of the fuel cell, through the interaction of the hydrophobic (CF2) n groups of the membrane with H or/and C atoms.

We have selected a new class of material which shows presence of large conductance switching. The acceptor groups present in the molecules result in low OFF-state current in the devices and hence large ON/OFF ratio of 100,000. The ON-state in the molecules has been due to conjugation restoration via electroreduction. The ON/OFF ratio decreased to 4 in devices based on molecules without any acceptor groups. The devices have undergone ‘write-read-erase-read’ sequence for hours, showing their applicability in random-access-memory elements.

New insights into the bonding arrangements of l- and d-glutamates from solid state 17O NMR by V. Lemaitre; K.J. Pike; A. Watts; T. Anupold; A. Samoson; M.E. Smith; R. Dupree (91-97).
Magic angle spinning (MAS) from l- and d-glutamic acid–HCl at 14.1 T produces highly structured and very similar NMR spectra. Lines from all 4 oxygen sites are readily distinguished and assigned. These 17 O NMR spectra are very different from the previously reported 17 O spectrum of the d,l-form presumably because that was a racemic crystal. 17 O NMR from l-monosodium glutamate–HCl is very different again requiring the application of double angle rotation and 3 quantum MAS NMR to provide resolution of 5 different sites. Hence high resolution 17 O solid state NMR techniques offer possible new insight into biochemical bonding processes.

Characterization of aza-fullerene C58N2 isomers by X-ray spectroscopy by Stepan Kashtanov; Oscar Rubio-Pons; Yi Luo; Hans Ågren; Sven Stafström; Stefan Csillag (98-104).
X-ray photoelectron and near-edge X-ray absorption fine-structure spectra (NEXAFS) of four isomers of C58N2 are predicted by means of density-functional theory calculations. A strong isomer dependence for both types of spectra are found. The spectroscopic findings are discussed in conjunction with results from electronic and geometric structure optimizations.

Nitrogen K-shell photoabsorption spectra of KCN, KAu(CN)2 and KAg(CN)2 were recorded using the synchrotron based X-ray absorption fine structure (XANES) spectroscopy. All types of transitions commonly appearing in the near edge region, i.e., K→π* transitions, Rydberg transitions, double excitations and shape resonances, are observed in the nitrogen XANES spectra of KCN, KAu(CN)2 and KAg(CN)2. The results show that there is a blue shift of the lowest energy transition (K→π* resonance) after CN forms [Au(CN)2 ] n and [Ag(CN)2 ] n complexes.

Infrared spectra and structures of (CH3NH2) n H+ (n=1–4). Binding features of an excess proton by Takayuki Michi; Kazuhiko Ohashi; Yoshiya Inokuchi; Nobuyuki Nishi; Hiroshi Sekiya (111-117).
Infrared photodissociation spectra of CH3NH3 +–Ar and (CH3NH2) n H+ with n=2–4 are measured in the 2600–3500 cm−1 region and analyzed with the aid of ab initio calculations. The intensities of the CH-stretching transitions relative to the NH-stretching transitions increase with increasing n, suggesting the change of the binding features of an excess proton in the clusters. Two CH3NH2 molecules in (CH3NH2)2H+ equally share the proton. On the other hand, the proton is localized on the central molecule in (CH3NH2)4H+, forming the CH3NH3 + core solvated by three CH3NH2 molecules.

Two complementary solid-state NMR techniques – variable contact time cross-polarization magic angle spinning (VCT CPMAS) and 2D wide-line separation (2D WISE) spectroscopy have been used to probe the dynamics of methylene chains in an intercalated surfactant bilayer, formed within the galleries of layered cadmium thiophosphate by the insertion of cetyl trimethyl ammonium ions. A gradient of conformational mobilities is observed in the intercalated bilayer; the cationic heads are immobile while the terminal methylenes of the surfactant tail undergo rapid motion. In the crystalline salt, the gradient is reversed, the tails are stationary and the heads mobile.

Lifetime-broadening removed X-ray absorption near edge structure by resonant inelastic X-ray scattering spectroscopy by Hisashi Hayashi; Yasuo Udagawa; Wolfgang A. Caliebe; Chi-Chang Kao (125-130).
A novel method to measure lifetime-broadening removed (LBR) X-ray absorption near edge structure (XANES) spectra is presented. It makes use of monochromatic incident X-rays near resonant condition as well as a high-resolution spectrometer. Fine structures observed in the resonant inelastic X-ray scattering spectra are compared to high-resolution fluorescent excitation XANES spectra for several copper compounds, demonstrating that the present method can be employed to obtain LBR-XANES.

Selective area growth of aligned carbon nanotubes by ion beam surface modification by A. Gohel; K.C. Chin; K.Y. Lim; S.T. Tay; R. Liu; G.S. Chen; A.T.S. Wee (131-135).
O2 + ion beams are used to modify the surface of 50 nm thick Fe catalyst films deposited on Si(100) substrates to demonstrate selective area growth of aligned multiwall carbon nanotubes (MWNT). Aligned MWNTs were grown on the modified Fe/Si substrate using the hot filament plasma enhanced chemical vapor deposition (HF-PECVD) method. A higher growth rate and density of MWNTs was observed on the ion-modified areas, facilitating selective area growth of aligned MWNTs at temperatures as low as 560 °C. Deposition of graphitic sheets at the initial growth process on the unmodified areas inhibits the deposition of MWNTs.

Solvent geometry and exciplex stability by Sharmistha Dutta Choudhury; Samita Basu (136-140).
Exciplex formation between several unlinked donor–acceptor systems has been studied in benzene, toluene and p-xylene solvents. It has been found that for small planar molecules the exciplex stability increases with increase in the solvent bulk. It has been concluded that an optimum distance between the donor and acceptor is of paramount importance for exciplex stability and this distance can be modulated by the intervening solvent medium.

The ion pairs C7H7 +I and [(η7-C7H7)Mo0(CO)3]+I in CH2Cl2 show I→C7H7 + CT absorptions at λ max=570 and 640 nm, respectively. This outer-sphere CT appears also as an I→C7H7 + ligand-to-ligand CT band (λ max=735 nm) in the absorption spectrum of the complex (η7-C7H7)Mo0(CO)2I.

Reaction of OH* radicals with H2 in sub-critical water by Timothy W. Marin; Charles D. Jonah; David M. Bartels (144-149).
The rate constant for the reaction of hydroxyl radicals (OH) with hydrogen in aqueous solutions has been measured from 200 to 350 °C by competition kinetics using nitrobenzene as a competing OH* scavenger. Measurements below 250 °C agree with previous results in other laboratories. At higher temperatures, the rate constant undershoots an extrapolation of the Arrhenius plot and actually decreases in value above 275 °C. At 350 °C, the measured rate constant is more than a factor of 5 below the Arrhenius extrapolation. We propose an explanation based largely on the hydrophobic solvation properties of the H2 molecule.

A new phase of lithiated titania predicted from first principles by Marina V. Koudriachova; Simon W. de Leeuw; Nicholas M. Harrison (150-156).
A new phase of Li-intercalated rutile-structured titania, Li x TiO2 for x=0.75, is predicted on the basis of first principles calculations. The existence of this phase has been noted in X-ray diffraction measurements but its structure has not previously been determined. The new phase has important consequences for the performance of Li x TiO2 as a battery cathode material. On the basis of the computed energetics of phases at x=0.75, 0.5 and 0.25 phases, and a careful comparison of the computed and measured diffraction data, a two phase model of intercalation for 0.25<x<0.75 is proposed. It is suggested that nucleation of the new x=0.75 phases results from kinetic restrictions under operating conditions. The structural features of the new phase and their manifestation in X-ray diffraction are discussed. The transformation of the new phase to an hexagonal phase at x=0.8 is analysed and its role in cathode degradation discussed.

Electronic states in 2-aminopurine revealed by ultrafast transient absorption and target analysis by Olaf F.A. Larsen; Ivo H.M. van Stokkum; Marie-Louise Groot; John T.M. Kennis; Rienk van Grondelle; Herbert van Amerongen (157-163).
Subpicosecond polarized transient-absorption measurements on the fluorescent adenine analogue 2-aminopurine have been performed in the wavelength region from 320 to 690 nm. Global target analysis of the data reveals structural heterogeneity of the chromophore in the excited state. Two distinct states with remarkably different spectroscopic properties were resolved. One state does show stimulated emission, whereas the other state does not and exposes a very long excited-state lifetime.

Torsional potentials of 9-phenylanthracene both in the ground and excited states were determined by using ab initio molecular orbital calculations. Molecular structure in the ground state was optimized at the restricted Hartree–Fock level under the C 2v symmetry. Then, the torsional potentials between the anthracene ring and the phenyl group in the S0 and S1 states were calculated by means of multi-reference perturbation theory based on the complete active space (CAS) SCF wave function. The present findings were consistent with the jet-laser experimental results by Barbara et al.

The chemistry of HN3 on Ge(1 0 0)-2×1: a theoretical study by Xin Lu; Zheng Su; Xin Xu; Nanqin Wang; Qianer Zhang (172-177).
The chemistry of HN3 on the Ge(1 0 0)-2×1 surface has been investigated by means of first-principle density functional cluster model calculations. The calculations revealed three possible reaction pathways for HN3 reaction with the surface dimer. Based on the predicted energetics, a conclusion can be made that at low temperatures HN3 can either undergo barrierless 1,3-dioplar cycloaddition onto the surface dimer, resulting in a 5-member-ring metallocyclic surface species, or adsorb molecularly via its HN-end, forming a dative N→Ge bond with the buckled-down Ge atom of the surface dimer.

Ce3+ and/or Tb3+-doped LaPO4 nanocrystalline thin films and their patterning were fabricated by a sol–gel process combined with soft lithography on silicon and quartz glass substrates. The results of XRD indicated that the films began to crystallize at 700 °C. The 1000 °C annealed single layer films are transparent by eyes, uniform and crack-free with a thickness of about 200 nm and an average grain size of 100 nm. Patterned thin film with different band widths (5–50 μm) were obtained by micro-molding in capillaries technique. The luminescence and energy transfer properties of Ce3+ and Tb3+ were studied in LaPO4 films.

Naphthenic hydrocarbon oils transmissible for transverse waves by Mikio Fukuhara; Toshiyuki Tsubouchi (184-188).
The acoustic shear transmissibility and rigidity of saturated naphthenic hydrocarbon oils (NHO) with bridged carbon rings were determined by ultrasonic transmission analysis. In contrast to an established theory, we found that the NHO has superior transmissibility for transverse waves. It arises from appearance of quasi-elastic shear response under ultrasonic vibration, based on interlocking of the sterically hindered molecules with bridged rings. The shear and bulk moduli are of the order of 0.8–1.3 and 2.3–5.7 GPa at room temperature, respectively, indicating shear rigidity like solid polymers.

Aluminum cluster anions were reacted with methyl iodide vapor in a fast-flow tube apparatus, yielding primarily I and clusters of the type Al n I. Based on thermodynamic considerations, the presence of I is taken to indicate a neutralization reaction pathway whereby clusters of the type Al n CH3 are formed. Comparison of relative reaction rates to previously measured electron affinities suggests that the Al n –CH3 interaction is covalent, rather than ionic, in nature. It is suggested that the neutral cluster complexes reported here may hold potential as high energy fuel dopants.

A new formulation of the base rate and the adiabatic energy correction used in the energy corrected sudden approximation is proposed. Explicitly, the common energy gap laws are replaced with an angular momentum gap law. This law is based upon the introduction of an effective angular momentum parameter (ℓc=2ℏ) limiting the collision-induced intra-molecular rotational energy transfer (RET) due to the finite interaction time. The experimental results of collision induced RET for the N2–N2, CO–CO and N2–He systems obtained by the application of femtosecond coherent anti-Stokes Raman spectroscopy (fs-CARS) are in excellent agreement with the proposed model.

Analysis of similarity/dissimilarity of DNA sequences based on novel 2-D graphical representation by Milan Randić; Marjan Vračko; Nella Lerš; Dejan Plavšić (202-207).
The recently proposed 2-D graphical representation of DNA based on four horizontal lines involves an arbitrary assignment of the four types of bases to the lines. While each such assignment is legitimate, it is desirable to have a scheme free of such arbitrary choices among non-equivalent geometrical representations. We outline one such approach, which is based on the construction of a 12-component vector whose components are the leading eigenvalues of the L/L matrices associated with DNA. The examination of similarities/dissimilarities among the coding sequences of the first exon of β-globin gene of different species illustrates the utility of the approach.

Isotope effect of the photoinduced H(D)-transfer reaction in indole–ammonia clusters by H. Lippert; V. Stert; L. Hesse; C.P. Schulz; I.V. Hertel; W. Radloff (208-216).
The intracluster reaction dynamics initiated by femtosecond laser pulses at 263 nm has been studied in nondeuterated and deuterated indole–ammonia clusters. No isotope effect is observed on the sub-ps timescale. On the long-timescale (of few 100 ps), however, the structural reorientation dynamics of the parent as well as the product clusters is prolongated (up to about 3 times) for the deuterated complexes. This effect is interpreted as due to the difference of the torsion modes and due to low energetic barriers between different isomers of both isotopic complexes.

The diffusion mechanism of noble gases in the α form crystal of syndiotactic polystyrene was investigated using a molecular dynamics simulation. The diffusion process was analyzed in connection with the cavity structure. A capillary-like cavity structure was clearly found in the α form. The efficiency of the gas diffusion in the capillary was much higher. The diffusion coefficients were more than one order higher than those in amorphous polystyrene. The anisotropy of diffusion becomes more significant for the larger size gases, whose diffusion in the α form can be effectively described by a one-dimensional transport.

Isotopic effects in the product vibrational distribution of the OH(OD) + HCl reaction by Aurelio Rodriguez; Ernesto Garcia; M. Luz Hernandez; Antonio Laganà (223-228).
Measured product vibrational distributions of the OH+HCl→H2O+Cl and OD+HCl→HOD+Cl reactions show a marked isotopic effect since the HOD product is vibrationally excited while the H2O one is not. Quasiclassical trajectories calculated on a new largest angle generalization of the rotating bond order (LAGROBO) potential energy surface offer a rationale for such an effect in addition to reproducing the measured temperature dependence of the thermal rate coefficient.