Chemical Physics Letters (v.392, #4-6)

Photo-induced reactions in the gaseous metal ion-nucleobase complex Mg+–cytosine by Ju-Long Sun; Haichuan Liu; Hong-Ming Wang; Ke-Li Han; Shihe Yang (285-290).
Complexes of magnesium cation with cytosine were produced by laser ablation of a magnesia and cytosine mixture. Photo-induced reactions in the complexes of Mg+–cytosine have been studied at 230–440 nm. The evaporation product, Mg+, as well as other four and five reactive products were observed in the long and short wavelength regions, respectively. The ground state structures of the complexes composed of magnesium cation and the low-lying tautomers of cytosine were fully optimized at the B3LYP/6-31+G** level. The bond dissociation energies (BDEs) of the complexes for the channels we observed were calculated at the same theoretical level.

Molecular zippers – designing a supramolecular system by Shahar Keinan; Mark A. Ratner; Tobin J. Marks (291-296).
We report here the first phase in the rational design of a versatile hydrogen-bonded supramolecular `zipper', by creating a molecular `tool box'. These are novel structures that are predicted to undergo self-assembled and to form stable `zippers', which should be amendable to molecular manipulations that change the physical properties of the supramolecule, while preserving the three-dimensional structure. We emphasize design rationale, including examples, and also demonstrate the importance of using highly accurate computational methods in molecular design.

Anharmonicity of the core-excited state potential of an organic molecule from NEXAFS vibronic fine structure by A Schöll; D Hübner; Th Schmidt; S.G Urquhart; R Fink; E Umbach (297-302).
We present a high-resolution C K- and O K-NEXAFS study for the large aromatic molecule acenaphthenequinone (ANQ) in the condensed phase. This is the largest molecule for which a distinct vibronic fine structure at the O K-edge has been resolved so far. The most prominent results are the appearance of only one predominant vibrational mode, a C–H stretching mode, in the O 1s NEXAFS spectra and the analysis of the anharmonicity of the excited state potential.

Total photoabsorption cross-sections of CF3SF5 in the C, F and S K-shell regions by T Ibuki; Y Shimada; S Nagaoka; A Fujii; M Hino; T Kakiuchi; K Okada; K Tabayashi; T Matsudo; Y Yamana; I.H Suzuki; Y Tamenori (303-308).
Total photoabsorption cross-sections of trifluoromethyl sulfur pentafluoride, CF3SF5, were measured in the C, F and S K-shell regions by using a double ionization chamber and synchrotron radiation at the SPring-8 facility. The maximum cross sections were found to be 6.3, 5.0 and 1.1 Mb for the C, F and S K-edges, respectively. The spectral features in the F K-shell region were characterized by the photoabsorption spectra of CF4 and SF6 and those in the S K-shell region resemble the peaks of SF6. The observed peaks for CF3SF5 were tentatively assigned on the basis of a comparison with those for CF4 and SF6 measured simultaneously for references.

Single-wall carbon nanotubes (SWNTs) were synthesized directly on cobalt fine particles by chemical vapor deposition, where nanometer-sized catalyst particles were supplied by pulsed laser vaporization in an ethanol-vapor-flowing quartz tube. Transmission electron microscope observation revealed that the bundle of SWNTs with less amorphous carbon material was obtained using this technique. Dependences of the amount and the diameter distribution of the obtained SWNTs on the furnace temperature were investigated by Raman spectroscopy.

Recently, we have shown that suspended metallic nanowires support Rydberg-like electron image states. Here, we investigate the possibility of tuning such `tubular image states', formed around pairs of parallel nanowires, by electric and magnetic fields. In the presence of a magnetic field, directed along the nanowires, we observe the formation of `Landau-like' image states, with simple elliptic-like orbits, that are highly detached from the surfaces of both nanowires. An additional electric field, induced by opposite charging of the two nanowires, spatially shifts these `molecular' states to one of the wires, while strongly chaotic nodal patterns emerge.

Gating effect of suspended multiwalled carbon nanotube with all-shell rooted from electrodes: parallel growth from ferromagnetic catalytic contact by Yun-Hi Lee; Chang-Woo Lee; Dong-Ho Kim; Yoon-Teak Jang; Chang-Hoon Choi; Kyung-Sik Shin; Byeong-Kwon Ju (319-323).
We describe the gate coupling of a rooted grown suspended carbon nanotube (CNT) between two microsized catalytic contact electrodes using a direct parallel growth technique. All-shell Multiwalled carbon nanotubes (MWNTs) were bonded to contact electrodes because MWNTs were grown from the electrodes. The response characteristics of the devices with the gate voltage show typical p-type field effect transistor function at a high-temperature regime and asymmetric Coulomb blockage behavior in low temperatures. Results of the observation suggest that our unique structure, i.e., catalytic ferromagnetic electrode/all-shell bonded MWNT/ferromagnetic electrode, made by direct parallel growth may be a promising key element for nanoelectronics and nanoelectro-mechanical-systems (NEMS).

Ultrafast spectrally resolved stimulated vibrational echo experiments measure the dephasing of the CO stretching mode of hemoglobin–CO (Hb–CO) inside living human erythrocytes (red blood cells). A method is presented to overcome the adverse impact on the vibrational echo signal from the strong light scattering caused by the cells. The results are compared to experiments on Hb–CO aqueous solutions. It is demonstrated that the dynamics of the protein as sensed by the CO ligand are the same inside the erythrocytes and in aqueous solution, but differences in the absorption spectra show that the cell affects the protein's potential energy surface.

Internal water molecules of light-driven chloride pump proteins by Mikihiro Shibata; Norikazu Muneda; Kunio Ihara; Takanori Sasaki; Makoto Demura; Hideki Kandori (330-333).
Halorhodopsin and bacteriorhodopsin convert light into energy in archaea through light-driven chloride and proton pumps, respectively. Three water molecules are present in their active centers, which presumably stabilize the quadrupole structures and play crucial roles in pumps. The present low-temperature Fourier-transform infrared (FTIR) study revealed that hydration of the negative charges by the internal water molecules is much weaker in halorhodopsin than in bacteriorhodopsin, suggesting that chloride ion is stabilized by weak hydrogen bonds of waters in halorhodopsin.

Reactivity of molecular oxygen: conversion of methanol to formate at low temperatures on Pt(1 1 1) by Takeshi Sawada; Zhengxin Liu; Noriaki Takagi; Kazuya Watanabe; Yoshiyasu Matsumoto (334-339).
The oxidation of methanol by molecular oxygen on a Pt(1 1 1) surface has been investigated by infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy. Formate is produced when the surface co-adsorbed with molecular oxygen and methanol is annealed to 70 K; the temperature is far much lower than the dissociation temperature of molecular oxygen on a clean Pt(1 1 1) surface. The attractive interaction between the co-adsorbates is postulated to lower a dissociation barrier of molecular oxygen. When a methanol-precovered Pt(1 1 1) surface is exposed to O2, the sticking probability of O2 decreases with increase of surface temperature and the major product of methanol oxidation is changed from formate to CO.

Dynamics of solvation and rotational relaxation in neutral Brij 35 and Brij 58 micelles by Debdeep Chakrabarty; Partha Hazra; Anjan Chakraborty; Nilmoni Sarkar (340-347).
Solvation dynamics of Coumarin 480 (C-480) and Coumarin 490 (C-490) in neutral polyoxyethylene (23) lauryl ether (Brij 35) and polyoxyethylene (20) cetyl ether (Brij 58) micelles is severely retarded compared to pure water. The solvation dynamics is biexponential in all these micelles. The fast component of the solvation dynamics arises due to the water molecules present in the palisade layer of the micelles. The slow nanosecond component in solvation dynamics may arise due to the hydrogen bonding of water molecules with the polar part of micelles. The rotational relaxations in all these micelles are bimodal and comprising of a picosecond and a nanosecond component.

Terahertz spectroscopy of naphthalene, α-naphthol, β-naphthol, biphenyl and anthracene by Jiaguang Han; Hui Xu; Zhiyuan Zhu; Xiaohan Yu; Wenxin Li (348-351).
The far-infrared THz spectra of naphthalene, α-naphthol, β-naphthol, biphenyl and anthracene have been measured using THz time-domain spectroscopy. The low-energy vibrational modes of measured molecules are discussed based on their characteristic and common features. The complex dielectric functions are obtained. Meanwhile, it is shown that the experimental results can be well fitted by a standard Lorentz model.

The intricate dynamical processes in photochemical reactions are not fully accessible to either experiment or conventional theory. Here, we outline a technique for simulations in photochemistry, which employs classical trajectories for the nuclei moving in a mean field, with the electrons coupled to a laser pulse by the time-dependent Peierls substitution. We demonstrate that it provides an illuminating description of photoisomerization. One observes a nontrivial sequence of events which include multiple electronic excitations, conversion of double bonds to single bonds (and vice versa), nonadiabatic depopulation of excited levels at avoided crossings, vibrational energy redistribution, and an elegant interdependence of the various electronic and vibrational degrees of freedom.

Vibrational relaxation following ultrafast internal conversion: comparing IR and Raman probing by T Schrader; A Sieg; F Koller; W Schreier; Q An; W Zinth; P Gilch (358-364).
Femtosecond IR and Raman techniques are compared with respect to their abilities to monitor vibrational relaxation following ultrafast internal conversion. The study was performed on para-nitroaniline which undergoes sub-picosecond internal conversion to the ground state populating a variety of vibrational modes. The NO2 stretch vibration at 1315 cm−1 responds to the excitation by a strong red shift, visible in transient IR and Raman experiments. It is due to the off-diagonal anharmonic coupling with other modes. The population of the NO2 stretch mode itself can only be observed in the (anti-Stokes) Raman spectrum, whereas IR spectroscopy is superior in detecting the coupling induced shift.

Coriolis couplings have been added to perturbative and variational implementations of anharmonic computations of vibrational spectra. The results for several medium size molecules show that Coriolis couplings cannot be neglected for quantitative studies, since they can account in some cases for more than 20% of the overall anharmonic correction. The effect is particularly important for fundamental vibrations involving out of plane motions and for combination bands involving stretchings and out-of-plane bendings. In all cases perturbative inclusion of these terms leads to results very close to their variational counterparts. This is particularly significant for an effective perturbative-variational approach to the anharmonic spectra of large molecules.

Laser induced conversion of methane into methanol over WO3, TiO2 (rutile) and NiO semiconductor photocatalysts, at room temperature, under the irradiation of a strong UV laser beam at 355 nm has been studied for the first time. In addition to monitoring the yield of the targeted product methanol, the yield of hydrogen, produced as by-product, was also monitored. The comparison of the amount of hydrogen produced in the presence and absence of methane was used to assess the mechanism of degradation of methanol immediately after its formation in the aqueous suspension.

The solvolysis rate constants (k obs) of tert-butyl chloride are measured in 20%(v/v) 2-PrOH–H2O mixture at 15 temperatures ranging from 0 to 39 °C. Examination of the temperature dependence of the rate constants by the weighted least squares fitting to two to four terms equations has led to the three-term form, lnk obs=a 1+a 2 T −1+a 3lnT, as the best expression. The activation parameters, ΔH and ΔS , calculated by using three constants a 1, a 2 and a 3 revealed the steady decrease of ≈1 kJ mol−1 per degree and 3.5 J K−1  mol−1 per degree, respectively, as the temperature rises. The sign change of ΔS at ≈20.0 °C and the large negative heat capacity of activation, ΔC p =−1020 J K−1  mol−1, derived are interpreted to indicate an SN1 mechanism and a net change from water structure breaking to electrostrictive solvation due to the partially ionic transition state. Confidence intervals estimated by the Monte Carlo method are far more precise than those by the conventional method.

Electron–phonon interaction and relaxation time in graphite by J. Jiang; R. Saito; A. Grüneis; G. Dresselhaus; M.S. Dresselhaus (383-389).
The electron–phonon (e–ph) interaction and the relaxation time of photo-excited electrons are calculated in graphite within the tight-binding scheme. The e–ph matrix element thus obtained, shows anisotropy in k-space. The calculated relaxation time is of the same order of magnitude as the experimental one. Moreover, below an energy close to the Fermi energy, the absorption rate exceeds the emission rate, which indicates that graphite can emit far infra-red electro-magnetic waves by absorbing heat. We also compare this result with Planck's formula for black-body radiation.

Direct observation of vibrational dynamics in tin phthalocyanine by Masakatsu Hirasawa; Yuzo Sakazaki; Hiroki Hane; Takayoshi Kobayashi (390-395).
Charge-transfer (CT)-excited state in phthalocyanine (Pc) tin dichloride is observed by real-time vibrational spectroscopy with a 6-fs pulse laser. Transient negative and positive absorbance change were observed for the CT- and Q-bands, respectively. The observed signal for the CT excitation dominantly couples with intermolecular vibrational modes in the Pc columns, while that for the Q-band couples only the in-plane deformation modes of the Pc macro-cycle. The Q-band, which is the main absorption in the visible and near-infrared region, has no obvious CT character in the primary stage of the photocarrier generation, which suggests the two-step carrier-generation model for Q-band excitation.

Spectro-electrochemical studies of single wall carbon nanotubes films by P Corio; A Jorio; N Demir; M.S Dresselhaus (396-402).
The dependence of the resonance Raman spectra of single wall carbon nanotubes (SWNTs) on an externally applied potential (V ap) was studied in different media. The spectro-electrochemical data indicate that the reaching of van Hove singularities (vHSs) of SWNTs occurs at a different V ap for each electrolyte, and that there is a dependence of the charge transfer and intercalation processes on the chemical nature of the media. We present evidence to suggest that the position of the vHSs of SWNTs is perturbed by V ap. Asymmetry between empty and filled vHSs is revealed by the spectroscopic response upon electrochemical p- and n-doping.

This Letter presents a proton fast field cycling Nuclear Magnetic Resonance study of the molecular dynamics in the region above the smectic A-isotropic phase transition in thermotropic liquid crystals. The proton spin–lattice relaxation time was measured as a function of temperature at different Larmor frequencies and the experimental results were interpreted in term of the Landau–De Gennes theory. Mechanisms provided by both individual reorientational fluctuations and fluctuations of the local order parameter can be distinguished at the different Larmor frequency scales.

Infrared spectroscopy of water adsorption on vanadium cluster cations (V x +; x=3–18) by T.D Jaeger; A Fielicke; G von Helden; G Meijer; M.A Duncan (409-414).
Vanadium cluster cations with one, two or three adsorbed water molecules are investigated with infrared photodissociation spectroscopy in the region of the bending mode of water. In all of these complexes, the bending mode of adsorbed water is measured near the corresponding frequency of the isolated molecule. Dissociation processes are efficient, indicating that these resonances are characteristic of a substantial fraction of all complexes present. This indicates that water is adsorbed without significant dissociation on these clusters. Clusters with two or three water molecules have similar resonances near the bending mode of free water, indicating independent absorption without hydrogen bonding.

Thermal melting of solid materials induced by ultrafast laser pulse irradiation as explosively homogeneous nucleation by Jingsong Wei; Zengrong Sun; Feng Zhang; Wendong Xu; Yang Wang; Fei Zhou; Fuxi Gan (415-418).
By using the classical nucleation and growth dynamics to describe the melting process of superheated solid materials induced by ultrafast laser pulse irradiation, we found that the melting process of highly superheated solid materials is governed by explosively homogeneous nucleation. For the superheating degree of 1.6–1.9, the melting time for aluminum is several tens of picoseconds. In this case the lattice is heated within a few picoseconds, thus the total melting time of the solid materials is governed by the explosively homogeneous nucleation. However, for the very short laser pulse irradiation, such as femtosecond laser pulses with high fluences, the nonthermal mechanism plays a critical role in the melting process, and the classical homogeneous nucleation theory and growth dynamics is not useful. That is to say, the classical homogeneous nucleation theory and growth dynamics is useful for describing the rapid thermal melting process, however, is not suitable for explaining the nonthermal melting process.

The sharing of electrons in the donor–acceptor complexes BH3NH3, BCl3NH3, AlH3NH3, AlCl3NH3, and GaCl3NH3 is investigated by the one-electron delocalization indices. Three prevailing aspects of the analysis are: (1) the nature of the compounds based upon the average number of electrons in atomic basins, (2) the quantitative extent of electron delocalization based upon the basin–basin indices, and (3) the sharing of electrons between the groups constituting the molecules. Because the analysis is rooted in the first-order density matrix, no reference to orbitals has been invoked. This study provides new insight into the electronic basis of the DA interactions.

The electronic charge distributions in boron–nitride nanocones with 240° disclination are investigated through first-principles calculations based on the density-functional theory. The charge distributions in BN cones are analyzed and compared with a planar BN sheet and BN nanotubes. The cones are submitted to different external electric fields applied along the axis, ranging from 0 up to 1.7 V/Å, and the charge rearrangements are studied. The tip charge concentrations and the electronic behaviors of these cones show different characteristics depending on the terminating atoms. The cone's particular structure and the response to the electric field make the BN cones promising candidates to be used as probes in electronic microscopy as well as electron field emitters.

Radial surface segregation in free heterogeneous argon/krypton clusters by Marcus Lundwall; Maxim Tchaplyguine; Gunnar Öhrwall; Raimund Feifel; Andreas Lindblad; Andreas Lindgren; Stacey Sörensen; Svante Svensson; Oile Björneholm (433-438).
Free clusters produced by a co-expansion of argon and krypton have been studied by means of core level photoelectron spectroscopy. In this study we have determined: (i) the produced clusters are heterogeneous; (ii) the radial distribution of the two atomic species in the cluster differs. The proposed structure model places the majority of the krypton atoms in the bulk, leaving the argon atoms to be found more frequently on the cluster surface. The suggested structure is qualitatively discussed using the cohesive energies of the two-components.

Nitric oxide is synthesized from l-Arg by nitric oxide synthases (NOSs). DFT calculations carried out in the present study demonstrate that there is direct coupling between the heme bound oxygen and the tetrahydrobiopterin (H4B) cofactor in the activated state of NOS. Results indicate that radicalization of H4B causes the coupled reduction of heme bound oxygen. In our model system H3B radical formation is prompted by proton dissociation from the N5 site of the cofactor; spin density is transferred to the heme bound oxygen, which we found in an orientation preconditioned for H abstraction from the substrate.

The third-order resonant and static nonlinear optical polarizabilities of the donor–donor and donor–acceptor substituted π conjugated molecules are calculated using the third-order response formalism in combination with time-dependent Hartree–Fock (TD-HF) and density functional theory (TD-DFT) methods. Performance of different levels of theory for excited state structure and nonlinear optical responses has been analyzed. Since the exact computations are fairly expensive, and only a few components of the cubic polarizability (corresponding to the Liouville space paths) are important, numerically efficient approximations are suggested.

The transition from the planar to three-dimensional structures in Au clusters occurs at the cluster consisting of 15 Au atoms, which is predicted based on the first-principles density functional theory (DFT) calculations including the spin–orbital coupling. The results show that the spin–orbit coupling does not alter the relative stability of Au clusters but increases the binding energy of the cluster by about 0.08 eV/atom for all the clusters studied here. The energy gap between the highest occupied and the lowest unoccupied orbitals, on the other hand, decreases when the spin–orbit coupling is included.

The hydrophobic effect has been explained by the peculiar properties of liquid water. However, recent works by Marmur proposed a novel explanation: the hydrophobic effect is caused by the self-assembly of hydrophobic solutes driven by the theoretical upper bound of excess free energy. Here I show that this argument is erroneous, based upon a rigorous statistical mechanical analysis.

Structure of an ionic liquid, 1-n-butyl-3-methylimidazolium iodide, studied by wide-angle X-ray scattering and Raman spectroscopy by Hideki Katayanagi; Satoshi Hayashi; Hiro-o Hamaguchi; Keiko Nishikawa (460-464).
The structure of a room temperature ionic liquid (IL), 1-n-butyl-3-methylimidazolium iodide ([bmim]I), is studied by wide-angle X-ray scattering (WAXS) and Raman spectroscopy. The radial distribution function obtained by WAXS shows prominent peaks ascribable to a structure constructed by iodide anions; this means that the structure of the anions has a long-range correlation. The Raman spectrum of liquid [bmim]I agrees with those of [bmim]Cl and [bmim]Br in the molten phase and in crystals. The results show that the cation structures in the liquids of three [bmim] halides are similar to those in the crystals. However, the arrangement of the iodide anions in liquid [bmim]I differs slightly from those in crystalline [bmim]Cl and [bmim]Br, although it partly retains the hydrophilic channel structure characteristic of the crystals.

Fragmentation of propanoic acid by subexcitation electrons by Andrzej Pelc; Wolfgang Sailer; Paul Scheier; Tilmann D Märk; Eugen Illenberger (465-469).
Low energy electron attachment to propanoic acid (CH3CH2COOH) leads to a variety of negative fragment ions formed by dissociative electron attachment. The most intense reactions are operative at subexcitation energies (<3 eV) at cross-sections on the 10−20 m2 scale. They are due to dehydrogenation from the O site (direct cleavage of the O–H bond), but also more complicated reactions associated with hydrogen transfer and substantial geometric and electronic rearrangement. The present results demonstrate the potential of slow electrons to efficiently decompose organic acids.

Femtosecond studies of orientational anisotropy decay of benzopyranthione in the excited S2 state in hydrocarbons by G Burdzinski; G Buntinx; O Poizat; P Toele; H Zhang; M Glasbeek (470-475).
Reorientational dynamics of 4H-1-benzopyrane-4-thione (BPT) in the S2 state, in hydrocarbon solution, is studied from the anisotropy decay of fluorescence upconversion and pump–probe absorption transients with femtosecond time resolution. The anisotropy decay is found to be monoexponential in n-hexane, n-heptane, n-octane, n-decane, n-dodecane and cyclohexane. The dependence of the rotational time constant, τ or, on the solvent viscosity is approximated by a linear function with a slope of 2.6 ± 1 ps/cP indicating slip conditions. The decay of orientational anisotropy is observed for the first time for a molecule in the S2 state.

High-pressure UV spectroscopy on oxygen up to 1.5 GPa by Yuichi Akahama; Haruki Kawamura (476-479).
UV absorption spectra of molecular oxygen were measured at pressure up to 1.5 GPa and temperature between 17 and 297 K. In the supercritical phase, an absorption band was observed in a UV region between 4.5 and 7 eV and showed an appreciable enhancement in the intensity with pressure. The absorption cross-section, σ eff, of the UV band at 1.5 GPa, estimated from the molar volume obtained by present X-ray diffraction experiments, reached three orders of magnitude of previous one measured at 1.0 MPa. The observed UV absorption of high-density oxygen was attributed to the two-molecule process by an antiferromagnetic O2 pair.

Photo-excited transient species of 2,3-dicyanonaphthalene produced during UV irradiation was studied by low-temperature matrix-isolation Fourier transform infrared (IR) spectroscopy. To identify the transient species, the density-functional-theory method was used for optimization of the geometrical structures and for estimation of the IR spectral patterns of the T1 and T2 states. By a comparison of the observed IR spectrum with the predicted spectral patterns, the observed photo-excited transient species was identified as the T1 state of 2,3-dicyanonaphthalene.

Quantum monodromy for diatomic molecules in combined electrostatic and pulsed nonresonant laser fields by Carlos A Arango; William W Kennerly; Gregory S Ezra (486-492).
We investigate the classical and quantum mechanics of diatomic molecules in combined electrostatic and pulsed laser fields. The integrable case of collinear static and linearly polarized laser fields exhibits both classical and quantum monodromy, and energy–momentum diagrams are presented for several physically relevant field combinations. Although the tilted field problem is nonintegrable, a quantum 〈 H ̂ 〉: 〈m2〉 lattice for eigenstates retains much of the structure of the integrable limit and is organized by classical periodic orbits. The regular structure of the quantum lattice in the integrable limit is disrupted in an energy range associated with the onset of chaotic classical motion.

Studies on photodissociation of alkyl bromides at 234 and 267 nm by Ying Tang; Lei Ji; Bifeng Tang; Rongshu Zhu; Song Zhang; Bing Zhang (493-497).
We studied the photodissociation dynamics of alkyl bromides (CHBr3, CH2Br2, C2H5Br, C2H4Br2) near 234 and 267 nm using resonance-enhanced multiphoton ionization (REMPI) with time-of-flight mass spectrometer. After photodissociation of alkyl bromides, bromine fragments of Br ( 2 P 1/2 0 ) (denoted Br*) and Br ( 2 P 3/2 0 ) (denoted Br) released. Branching ratios of N(Br*)/N(Br) were determined, and a possible photodissociation mechanism of alkyl bromides was proposed. Additionally, the REMPI spectrum of bromine atoms has been recorded between 231 and 268 nm, 42 transition lines were found and assigned, and most of them coincided with the previous results except 12 atomic transitions that were new.

Cluster surface interactions: small Fe clusters driven nonmagnetic on graphite by K Fauth; S Gold; M Heßler; N Schneider; G Schütz (498-502).
We present a study of the changes in the magnetic and electronic properties of small, deposited Fe clusters upon exposure to the graphite surface. The clusters exhibit strong X-ray magnetic circular dichroism (XMCD) at the L3 edge while matrix isolated in a thin Ar film. XMCD and photoemission experiments show that the clusters are driven into a nonmagnetic state by the interaction to graphite. Our results support earlier calculations for adatoms and dimers and extend their validity to larger cluster sizes. They also provide a basis for an understanding of the magnetic properties of carbon encapsulated transition metal particles.

Neutron diffraction with isotopic titanium substitution allows the nature of the cation sites to be determined beyond nearest-neighbor distances for the first time in a sol–gel derived titania–silica glass. Reverse Monte Carlo (RMC) modelling of the results shows unequivocally that titanium substitutes for silicon within the homogeneous glass network, existing mostly as TiO4, with a minority TiO6 component. The fundamental scientific interest, and wide-ranging technologically significant properties of titania additions to silica, demand an understanding of the precise nature of the titanium site.

Following a recent experimental report on the synthesis and thermal stability of imidazopyridopyrimidine-containing structures with the ability to form four hydrogen bonds, the geometries and relative stabilities of the pairs of model bases have been studied using the HF, MP2 and B3LYP approaches. The 1H NMR chemical shifts of the base pairs have been estimated by the SOS-DFPT-IGLO method and employed in the analysis of the experimental proton spectra.

Properties of an atom–bond additive representation of the interaction for benzene–argon clusters by M Albertı́; A Castro; A Laganà; F Pirani; M Porrini; D Cappelletti (514-520).
In this work the properties of a new potential energy surface for the benzene–argon clusters are investigated. The proposed functional form is expressed as a sum of atom–bond interaction pairs and the relevant parameters for Ar–CC and Ar–CH cases are given. The main static and dynamical features of the potential energy surface in argon–benzene clusters are computed and compared with those available from the literature.

Molecular hosts for triplet emission in light emitting diodes: A quantum-chemical study by P Marsal; I Avilov; D.A da Silva Filho; J.L Brédas; D Beljonne (521-528).
Correlated semiempirical and ab initio quantum-chemical methods are applied to the description of the lowest-lying triplet excited state, T1, in conjugated molecules used as hosts in phosphorescent light emitting diodes. Density functional theory is found to lead to the best agreement between measured and calculated excitation energies in a set of reference molecules. The trade-off between the barrier for charge injection and the singlet–triplet S0  → T1 energy spacing is discussed in the context of the design of molecular hosts for blue triplet guest emitters.

Purification and characterization of zeolite-supported single-walled carbon nanotubes catalytically synthesized from ethanol by Hideyuki Igarashi; Hiroto Murakami; Yoichi Murakami; Shigeo Maruyama; Naotoshi Nakashima (529-532).
Nearly complete removal of zeolite particles and Fe/Co catalysts from raw single-walled carbon nanotubes (SWNTs) produced by chemical vapor deposition was achieved by oxidation of raw material at 240 °C or 340 °C and treatment with 1% aqueous solution of hydrofluoric acid. The yields of SWNTs were estimated to be ≈95% and the purities of SWNTs thus obtained were more than 95%, as characterized by thermo-gravimetric analysis. Raman spectra of the purified SWNTs were essentially identical with those of the corresponding raw samples. The raw and purified SWNTs were also characterized by EDX spectroscopy and transmission electron microscopy.

A multiconfiguration theory for electronic dynamics of molecules in an intense laser field is developed based on the Dirac–Frenkel time-dependent variational principle. The equations of motion for spin–orbitals and configuration-interaction coefficients are explicitly given. Numerical calculations of electronic dynamics of a hydrogen molecule in an intense electronic field are performed as a practical application of the theory.

The disilenyl, H2SiSiH(X2A), and the d3-isotopomer were detected for the first time via infrared spectroscopy in low temperature silane matrices upon an irradiation of the sample matrices with energetic electrons. The ν 5 fundamental was observed at 651 and 493 cm−1, respectively. In the d4-silane matrix, the ν 4 at 683 cm−1 was noticed, too. Our investigations suggest that this radical is formed via radiolysis of silylsilylene, H3SiSiH(X1A), and disilene, H2SiSiH2(X1Ag). The new absorption of the H2SiSiH(X2A) radical may be employed in future spectroscopic monitoring of chemical vapor deposition processes and in astronomical searches of silicon-bearing molecules toward the carbon star IRC + 10216.

`Optical' fatigue in a solid state diarylethene polymer by Andrea Lucotti; Chiara Bertarelli; Giuseppe Zerbi (549-554).
The issue dealt with in this Letter is the study of the optical fatigue of poly-1,2-bis(2-methylthien-3-yl)perfluorocyclopentene (I) as prototype of backbone photochromic polymers recently proposed as materials for the development of new optical devices in photonics and optical imaging. An instrument has been built and measurements have been carried out, thus showing that a film cast from a chloroform solution of the photochromic compound (I) + polymethylmethacrylate reaches the limits set by technology as acceptable (20% decrease of the absorbance with the respect to the absorbance of the stationary state) at 3400 cycles.

Electronic properties of Mo-doped cylindrical and scroll-like divanadium pentoxide nanotubes by A.N Enyashin; V.V Ivanovskaya; Yu.N Makurin; V.L Volkov; A.L Ivanovskii (555-560).
Atomic models of molybdenum-doped divanadium pentoxide (V1.9Mo0.1O5) nanotubes with cylindrical as well as with scroll-like morphology are presented and their electronic properties are studied by means of tight-binding band structure calculations. It was found that owing to Mo → V substitutions all V2O5 curved nanostructures (which are uniformly semi-conducting) transform into a metallic state in addition to an increase of the Fermi energy into the conduction band. A qualitative picture of decreasing stability of V2 −  x Mo x O5 tubes with an increase in the content of Mo is presented. A possible effect of impurity Mo atom distribution (`randomly' doped tubes vs Mo atoms `clustering') on the electronic properties of V1.9Mo0.1O5 tubes is also discussed.

We show that the expectation value 〈cosθ 12〉 and its subshell-pair components 〈cosθ 12 nl,n l are bounded from below and above in the Hartree–Fock theory of atoms, where θ 12 is the interelectronic angle subtended by the position vectors r 1 and r 2 of a pair of electrons, and n and l are the principal and azimuthal quantum numbers. The lower bounds to 〈cosθ 12 nl,n l are −l/[2(2l−1)(2l+1)] for ll pairs with l=l +1, while they are 0 for the other ll pairs, independently of n and n . A weighted sum of these subshell-pair bounds gives a lower bound to 〈cosθ 12〉 . The upper bounds are 0 in all the cases.

Author index (568-578).