Chemical Physics Letters (v.373, #3-4)

Molecular beams generated from the vapors above the surfaces of ternary mixtures, water–methanol–ethanol, water–ethanol–1-propanol and water–methanol–1-propanol have been examined by mass spectrometry. The propensity for surface enrichment of the alcohols is obtained in terms of the vapor mole fractions of the alcohols, which in turn were estimated from the cluster populations in the molecular beam. The enriching propensities in the ternary mixtures are compared with those in the binary mixtures. The net surface enrichment in ternary mixtures is generally lowered in comparison to that in the binary mixtures, except in the case of water–methanol–ethanol, where it is similar. While the surface enriching ability of methanol is nearly unaffected, that of ethanol is enhanced. The enriching ability of the longer chain propanol, however decreases significantly.

Electron impact ionisation and partial ionisation cross-sections of iso-octane by K Bouamra; J.R Vacher; F Jorand; N Simiand; S Pasquiers (237-244).
Electron impact ionisation of iso-octane is studied using mass spectrometry. Cross-sections for the formation of fragment ions are measured between 17 and 79 eV. C4H9 + is the most abundant ion, C8H18 + is completely absent. A total ionisation cross-section of 2×10−15 cm2 is found at 79 eV. Ab initio calculations show that the principal ionic species observed, C4H9 +, C4H8 +, C7H15 +, C3H6 + and C3H11 +, may be issued directly from iso-octane by simple C–C bond split processes or with H atom rearrangements. Ionisation of the two major neutral species issued from bond split, C4H9 and C4H10, can lead to the principal ions observed.

Ab initio design on new push–pull sila- and germastilbene by Yoshiaki Amatatsu; Yuhki Ohara (245-250).
Ab initio complete active space self-consistent field and second order Möller–Plesset perturbation calculations have been performed to examine the capabilities of the second order non-linear optics (NLO) of 4-dimethylamino,4-cyano,i-sila(or germa)stilbenes (i-DCSS, or i-DCSG) where the carbon at the position i of the parent 4-dimethylamino,4-cyanostilbene (DCS) is replaced by more electro-positive silicon or germanium atom. 1-DCSS and 1-DCSG, which have not been synthesized yet, were found to have better NLO properties than DCS.

Recently we proposed a new model describing rotationally inelastic collisions: the angular momentum and energy corrected sudden approximation (AECS) based on an effective angular momentum parameter l c=2ℏ limiting the possible amount of angular momentum transfer due to the restricted interaction time. In this work rotational energy transfer in N2 induced by collisions with rare gases (He, Ne, Ar and Kr) is investigated with time-resolved CARS. We find it necessary to complement the AECS model with an energy gap term that can be formulated without additional model parameters.

In this Letter we present two conical intersection lines (CLs) between adiabatic potential energy surfaces (PESs) of oxonium correlated to the asymptotic systems H+H 2 O +( X ), H ++H 2 O( X ) and H+H 2 O +( A ̃ ). Such PESs features are related to the dynamics of the charge transfer process H++H2O→H+H2O+ studied by Toennies and coworkers. A new three steps method is also presented whereby to determine the CLs. The generalized Heitler–London approximation is used first to obtain the analytical expression of a qualitative CL in whose neighborhood the accurate CL is finally ab initio computed after an intermediate searching step. Using this strategy the 3D configurational subspace where to search for crossing points is drastically reduced allowing for a faster determination of the CLs.

Macroscopic oriented web of single-wall carbon nanotubes by Xinluo Zhao; Sakae Inoue; Makoto Jinno; Tomoko Suzuki; Yoshinori Ando (266-271).
A macroscopic oriented web (30 cm length) of single-wall carbon nanotubes (SWNTs) with purity higher than 70 at.% has been produced by dc arc discharge evaporation of a carbon electrode including 1 at.% Fe catalyst in H2–Ar mixture gas. The SWNTs possess high crystallinity and a ‘clean’ surface. Moreover, the coexisting Fe catalyst nanoparticles can be completely eliminated by a two-step purification process (heating in air at 693 K and mild hydrochloric acid treatment). This study provides a simple, inexpensive and efficient method for producing high-quality SWNTs on a large scale, and enables us to investigate the physical properties of SWNTs in bulk.

Naturally produced carbon nanotubes by C. Velasco-Santos; A.L. Martı́nez-Hernández; A. Consultchi; R. Rodrı́guez; V.M. Castaño (272-276).
Carbon nanotubes represent an impressive kind of materials with diverse unexpected properties, and different methods to artificially produce them have been developed. Recently, they have also been synthesized at low temperatures, demonstrating that these materials might exist in fluids or carbon rocks of the Earth’s crust. A new type of natural encapsulated carbon nanotubes found in a coal–petroleum mix is presented. These findings show that all allotropic carbon forms known up to date can be produced in Nature, where pressure, catalysts particles, shear stress and parameters other than exclusively very high temperature, seem to play an important role for producing nanotubes.

In situ functionalized multi-walled carbon nanotube (MWNT)/Phenoxy composites were prepared by melt mixing Phenoxy with MWNT and 1-(aminopropyl)imidazole (NIm). Composites with 4.8 wt% or more of in situ functionalized MWNT show higher storage modulus than Phenoxy, provided a sufficient amount of NIm was used to aid in the dispersion of the hydrophobic MWNTs in the hydrophilic Phenoxy matrix.

Why is the thermalization of excited electrons in semiconductor nanoparticles so rapid? Studies on CdSe nanoparticles by Qusai Darugar; Christy Landes; Stephan Link; Alexander Schill; M.A. El-Sayed (284-291).
Quantum confinement of electronic motion in semiconductor nanoparticles leads to quantization of its band continua of the bulk. The relaxation between the resulting quantized levels by electron phonon coupling was expected, but not found, to be slow due to the small phonon frequencies (phonon bottleneck). Studying the electronic relaxation from the band gap and a higher excited state in CdSe dots and rods under different perturbations suggest the importance of coupling the excited electron to the surface. The surface species act as an efficient heat bath or as electron trapping sites in the linear or nonlinear (Auger) relaxation processes.

Resonance enhanced multiphoton dissociation of polycyclic aromatic hydrocarbons cations in an RF ion trap by Denis Rolland; August A. Specht; Mike W. Blades; John W. Hepburn (292-298).
We present a novel method for recording the absorption spectra of cations of non-volatile molecules using resonance enhanced multiphoton dissociation of trapped ions. Ions are produced through a 2-laser desorption/ionization process in an RF ion trap mass-spectrometer. A third tunable visible laser induces fragmentation of the trapped ions with a yield dependent on the absorption cross-sections of the ions. Using this method we have recorded the visible absorption spectrum of two PAH isomers, phenanthrene and anthracene.

A theory of one-color two-photon excitation fluorescence depolarization in organized molecular assemblies and solutions, is outlined and discussed. The discussed theory is based on the formalism of spherical irreducible tensorial sets and the group theory methods. The obtained general formulas are symmetry simplified and discussed for the case of macroscopically ordered planar membranes, Langmuir–Blodgett (LB) films, stretched polymer films, membrane vesicles, labeled macromolecules and solutions.

Quantum chemistry study on the open end of single-walled carbon nanotubes by Shimin Hou; Ziyong Shen; Xingyu Zhao; Zengquan Xue (308-313).
Geometrical and electronic structures of open-ended single-walled carbon nanotubes (SWCNTs) are calculated using density functional theory (DFT) with hybrid functional (B3LYP) approximation. Due to different distances between carbon atoms along the edge, reconstruction occurs at the open end of the (4,4) armchair SWCNT, i.e., triple bonds are formed in the carbon atom pairs at the mouth; however, for the (6,0) zigzag SWCNT, electrons in dangling bonds still remain at ‘no-bonding’ states. The ionization potential (IP) of both (4,4) and (6,0) SWCNTs is increased by their negative intrinsic dipole moments, and localized electronic states existed at both of their open ends.

We investigate a new type of structural instability of regular 1-D molecular stacks consisting of homonuclear disc-like molecules, in particular of polycyclic aromatic hydrocarbons. If the repeating units are separated by mirror planes of symmetry, it is shown that the interaction energy of such systems is non-bonding, i.e., the ground state is unstable with regard to a distorsion which eliminates the symmetry plane.

Chirped pulse ionization: bondlength dynamics and interference effects by T. Lohmüller; M. Erdmann; V. Engel (319-327).
Ionization with chirped pulses is studied using the Na2 molecule as a numerical example. Bondlength expectation-values are determined for different linear chirp parameters. It is shown that under certain conditions the photoelectron spectra can exhibit a pronounced interference structure which has an analogy in oscillatory patterns and the rainbow structure found in the scattering amplitude for atom–atom collisions.

The distance d(u,v|G) between the vertices u and v of a molecular graph G is the length of a shortest u,v-path. We consider a class of Wiener-type topological indices W λ (G), defined as the sum of the terms d(u,v|G) λ over all pairs of vertices of G. Several special cases of W λ (G), namely for λ=+1 (the original Wiener number) as well as for λ=−2,−1, +1/2, +2 and +3, were previously studied in the chemical literature, and found applications as molecular structure-descriptors. We establish a relation between W λ+1 and W λ , applicable for benzenoid molecules, phenylenes, chemical trees, and other types of molecular graphs.

Surface modification of TiO2 film by iron doping using reactive magnetron sputtering by Wenjie Zhang; Ying Li; Shenglong Zhu; Fuhui Wang (333-337).
Fe-doped TiO2 films are prepared by pulse dc reactive magnetron sputtering. Iron concentration is controlled by varying the surface area of Fe pieces fixed on the pure titanium target. TiO2 films are in anatase phase when iron concentration is less than 10 at.%. Only Ti(IV) is found in the pure TiO2 film while Fe-doped TiO2 films show mixed titanium oxidation states. The absorption edges of Fe-doped TiO2 films show red shift. The films with low iron concentrations perform better photocatalytic activity than the pure TiO2 film and the best doped iron concentration is 0.58 at.%.

Photochromic materials are now anticipated as promising candidates for erasable memory media of the next generation because organic materials are easy to tailor. Density Functional calculations at the B3LYP/6-311+G(d,p)//HF/3-21G and partly at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) levels were carried out for thermal irreversibility study of the titled compounds, a kind of photochromic materials which undergo cyclization and cycloreversion reactions. Eighteen configurations were characterized for both the cis and the trans forms of three diarylethene-type compounds. Among them, six were transition states. The results from the relative energy barriers and from the normal coordinate analyses of the imaginary frequencies show that Woodward–Hoffman principle about disrotatory and conrotatory mechanisms is valid for the thermal irreversibility study on the compounds. The thermal irreversibility of changing phenyl to thiophenyl groups in stilbene, of introducing methyl groups on central carbons that were responsible for the electrocyclic reaction was discussed. The applicability of Irie’s rule was investigated.

The internal rotation of the methanol molecule is studied in full dimensionality i.e., the 12 coordinates are treated explicitly using an adiabatic separation of the 1D-torsional and the 11D-inactive wave functions. The potential energy surface is calculated with the help of quantum chemistry codes and is expanded in a Taylor series up to the second order along the torsional path. Our main results show that the torsional energy levels of this 1+11D-adiabatic model are noticeably different from those of the 1D-models, whether the zero point energy correction along the path is included or not.

Theoretical enthalpy of formation of the acetonyl radical by Joaquı́n Espinosa-Garcı́a; Antonio Márquez; Sándor Dóbé (350-356).
The standard enthalpy of formation of the acetonyl radical (CH2COCH3) was theoretically estimated using several working chemical reactions, with four variants of theoretical approaches (levels) and four extended basis sets. Our best theoretical enthalpy of formation is Δ f H0 298(CH 2 COCH 3)=−32±4  kJ mol −1 . This computed value corresponds to the bond dissociation energy of DH0 298(H–CH 2 COCH 3)=403±4  kJ mol −1 , and to the resonance stabilization energy (RE0) and the intrinsic stabilization energy (SE0) of 16.5 and 14.2  kJ mol −1 , respectively. These energies indicate a greater stabilization of the acetonyl radical than previously thought.

By substituting the standard mass-weighted normal coordinates with either Morse-like or Gauss-like coordinates, it is demonstrated that significant improvements can be made to the vibrational spectra of polyatomic molecules calculated variationally. Quartic force fields in the form of Taylor expansions are generated by density functional theory for water, formaldehyde and methane, and their vibrational spectra calculated by the perturbation normal coordinate code SPECTRO. These are then compared with three sets of spectra arising from the variational code MULTIMODE. Initial spectra are obtained using the identical Taylor expansion force fields. A subsequent set of spectra are then obtained for which the symmetric normal coordinates of the force fields are replaced by Morse-like coordinates and a final set of spectra are obtained for which the asymmetric normal coordinates of the force field are replaced by Gauss-like coordinates. The restriction is imposed that the complete set of derivatives to quartic are preserved under these coordinate transformations.

The role of nonadiabatic pathways and molecular rotations in the oxygen abstraction reaction on the Al(1 1 1) surface by Marcello Binetti; Olaf Weiße; Eckart Hasselbrink; Gil Katz; Ronnie Kosloff; Yehuda Zeiri (366-371).
The adsorption dynamics of O2 on Al(1 1 1) has been studied experimentally using molecular beams and laser spectroscopy as well as theoretically using a multi-dimensional multi-electronic surfaces model. Experiments find that: (i) abstractive chemisorption is operative at all translational energies; (ii) it increases markedly with translational energy; and (iii) rotational excitation of the molecules suppresses the abstraction process. The model calculations semi-quantitatively reproduce the experimental findings and demonstrate that: (i) a nonadiabatic description is necessary to reproduce the experimental results and that (ii) the rotational hindrance is due to a restricted cone of acceptance in direction of the surface normal.

Fluorescence efficiency of four infrared polymethine dyes by M. Casalboni; F. De Matteis; P. Prosposito; A. Quatela; F. Sarcinelli (372-378).
We report on the fluorescence efficiency of four infrared (IR) polymethine dyes. These figures have been determined using an indirect method based on a fluorescence standard (Rhodamine B in ethanol). The fluorescence decay times have been measured too and their correlation with the quantum efficiency data has been shown. The optical behavior of the IR dyes in three solvents with different polarities has been compared and discussed.

The dehydrogenation and cracking reactions of isobutane over the ZSM-5 zeolite by Ivan Milas; Marco Antonio Chaer Nascimento (379-384).
The dehydrogenation and cracking reactions of isobutane over zeolite HZMS-5 were studied at the DFT/B3LYP level of calculation. The zeolite was represented by the‘double-ring’ 20T cluster. The activation energies for the reactions were 9–12 kcal/mol lower than those obtained with the linear 5T cluster. In both cases the attack of the acid site proton was directly on a carbon atom of the substrate, and not on the C–H and C–C bonds, evidencing carbonium-ion-type transition states. The results suggest that the reactions should be competitive, although the more hindered acid sites should favor the dehydrogenation over the cracking reaction.

The iron(II) spin crossover dinuclear compound [Fe(dpa)(NCS)2]2bpym where dpa = 2,2-dipyridylamine and bpym = 2,2-bipyrimidine has been synthesized and characterized. Variable-temperature magnetic susceptibility and 57 Fe Mössbauer spectroscopy data provide evidence for a rather complete and continuous S=2 (HS)↔S=0 (LS) spin-crossover behavior taking place in the temperature range 400–50 K (T1/2=245  K) without the presence of a plateau at 50% of conversion. The absence of such plateau, which is characteristic of all dinuclear compounds so far studied, is interpreted in terms of synergetic effect between intramolecular and intermolecular interactions.

We calculate the mobility and conductivity of the proton transfer in hydrogen-bonded systems by quantum mechanics. We first give the equation of change of velocity of the kink–antikink pair resulting from proton transfer, from the Hamiltonian in our model. Utilizing the values of the physical parameters we finally find out the mobility of kink–antikink pairs and the electrical conductivity due to the proton transfer in an externally applied electric-field in the systems. These are about (6.5–6.9)×10−6   m 2/V s and (7.6–8.1)×10−3/(Ω   m) for ice, respectively, which are basically consistent with experimental data.

Limiting negative zero-field splitting in tetrakis(imidazole)bis(acetato) nickel(II) complex by Roman Boča; Lubor Dlháň; Wolfgang Haase; Radovan Herchel; Anna Mašlejová; Blažena Papánková (402-410).
Magnetic properties of bis(acetato-O)tetrakis(imidazole-N3)nickel(II) have been investigated by low-temperature magnetic susceptibility as well as high-field magnetisation and subjected to theoretical analysis. A simultaneous fitting of susceptibility and magnetisation gave the following set of magnetic parameters: g z =2.181, g x =2.039, and D/hc=−22.3 cm−1. Thanks to the magnetisation measurements the sign of the axial zero-field splitting parameter is fixed unambiguously. The negative value of the D-parameter, unexpected for the geometry of an elongated tetragonal bipyramid, is understandable by introducing different orbital reduction factors for axial (acetate) and equatorial (imidazole) ligands.

The absolute pK as of eight carboxylic acids have been computed using a Born–Haber cycle, the most recent experimental value for the proton solvation energy and completely ab initio structures, energies, and harmonic frequencies of acids and conjugated bases both in vacuo and in aqueous solution. The proton affinities computed by the PBE0 hybrid functional are in very good agreement with experimental values and with the results of the most sophisticate (and expensive) quantum mechanical models (G2, G3, and CBS). The Gibbs energies in aqueous solution have been computed by our last version of the polarizable continuum model, which takes into the proper account escaped charge effects in a very effective computational implementation. The encouraging results obtained for this training set and the linear scaling both of the electronic and solvation models pave the route for the evaluation of reliable pK values for the large systems of interest in biological or material science frameworks.

Optical–optical double resonance (OODR) spectra in the vibrationless S1←S0 absorption transition of jet-cooled pentacene are reported. Spectral holes as well as resolved and individually assigned single rovibronic OODR-lines corresponding to different selected ground states are observed. The experimental OODR-spectra are in good agreement with calculated spectra using rotational constants which were obtained from a band contour analysis. The accuracy of the rotational constants for the S0 and S1 state was improved in comparison to Heinecke et al. [J. Chem. Phys. 109 (1998) 906].

We have studied the bonding of N2 to a (9, 0) carbon nanotube using the MP2 and ONIOM methods with extended basis sets. We find a weak dispersion type bond with the N2 ∼3.3 Å away from the surface. Our best estimate of the binding energy, including the effect of expanding the high accuracy piece from ethylene to cluster A, is 1.88 kcal/mol for bonding in a twofold site and 1.97 kcal/mol for bonding in a sixfold site. We find N2 is bound more strongly to a graphitic surface by ∼0.3 kcal/mol.

We report on emission studies of excited phototautomers of 1- hydroxy-2-acetonaphthone in aqueous solutions of α-cyclodextrin (α-CD). Caging by α-CD provokes a ∼40 nm blue shift of the emission, and increases the emission lifetime from 90 ps to 1 ns. The rotational time of the caged phototautomers varies from 50 to 180 ps, while that of global rotational time of 1:2 complex (HAN:(α-CD)2) is ∼950 ps. These results are explained on the basis of confinement effects on the twisting motion of the formed phototautomers within the cavities provided by one or two CD.