Chemical Physics Letters (v.328, #4-6)

Molecular knife throwing: aiming for dissociation at specific surface sites through state-selection by Drew A. McCormack; Geert-Jan Kroes; Roar A. Olsen; Jeroen A. Groeneveld; Joost N.P. van Stralen; Evert Jan Baerends; Richard C. Mowrey (317-324).
We perform a quantum study of the dissociative adsorption of H2 molecules on the Cu(1 0 0) surface. By calculating the reaction probability at specific surface sites, we find that, even at low collision energies, reaction is not necessarily dominated by the site with the lowest potential barrier to reaction. By controlling the vibration and collision energy of the molecules one could, in principle, selectively induce reaction at different specific surface sites, probing them individually. We also suggest an experiment that could provide additional evidence for our predictions.

We have used first-principles calculations to understand water adsorption on the SnO 2(1 1 0) surface. Particular attention has been paid to the roles played by surface geometry and inter-adsorbate interactions. For this purpose a full comparison between adsorption on SnO2 and TiO2 surfaces is made. There are several points of contrast between adsorption mechanisms for the two materials. The most significant differences are that molecular water is only stable on the SnO2 surface when bound to a neighbouring OH group, and that the results favour dissociative adsorption on SnO2 at monolayer coverage. The `mixed adsorption state' first suggested for TiO2 is metastable on SnO2(1 1 0). For both materials there are small energy differences between alternative adsorption states at monolayer coverage.

Oxygen molecules on Ag(0 0 1): superstructure, binding site and molecular orientation by Stéphane Messerli; Silvia Schintke; Karina Morgenstern; Jouko Nieminen; Wolf-Dieter Schneider (330-336).
Oxygen molecules on Ag(0 0 1), adsorbed at about 60 K, are found to form two-dimensional c(2×4) compact islands. We determine binding site and orientation of the molecules within the superstructure by comparing experimental and calculated scanning tunneling images in combination with molecular dynamics simulations. The molecule adsorbs in the thermodynamically stable fourfold hollow site with its molecular axis in the direction of short periodicity of the superstructure. Rehybridization of 1π and 2π orbitals on adsorption is at the origin of the observed image contrast.

The multi-photon ionization process of the hydrogen-bond cluster of pyridine–methanol has been investigated using a conventional and reflectron time-of-flight mass spectrometer (RTOF-MS) at 355 and 266 nm laser wavelengths, respectively. The sequences of the protonated cluster ions (CH3OH) n H+ and (C5H5N n )(CH3OH) m H+ (n=1,2) were observed at both laser wavelengths, while the sequence of the cluster ions (CH3)OH n (H2O)H+ was observed only at 355 nm laser wavelength. The difference between the relative signal intensities of the protonated methanol cluster ions at different laser wavelengths is attributed to different photoionization mechanisms. Some nascent cluster ions in metastable states dissociated during free flight to the detector. The dissociation kinetics is also discussed.

A simple route to annihilate defects in silicon nanowires by Y.H. Tang; Y.F. Zheng; C.S. Lee; S.T. Lee (346-349).
Defects inside silicon nanowires (SiNW) could be significantly reduced by annealing the nanowires at 1100° C for 6 h. High-resolution transmission electron microscopy (HRTEM) showed that stacking faults and twins were annihilated upon annealing. In particular, the tips of the nanowires demonstrated perfect lattices free of defects after annealing. Raman spectra also confirmed that the bulk specimen was almost defect-free. By using thermal annealing, defect-free silicon nanowires can be prepared in a simple and practical way, which holds promise for nanoelectronic applications.

Mizoguti et al. (Chem. Phys. Lett. 321 (2000) 297) reported that amorphous carbon (a-C) contained in as-grown single-wall carbon nanotubes could be burned preferentially by using ultrafine gold particles and cationic surfactant, benzalkonium chloride (BKC). We confirmed this result and found additionally that the optimum concentration of the ultrafine gold particles and BKC were, respectively, 0.6 atom% and 7 g/l. We studied the roles of ultrafine gold particles and BKC in this phenomenon; the ultrafine gold particles catalyzed the oxidation of carbonaceous materials leading to the decrease of the burning temperatures. BKC had the function of homogenizing the a-C aggregation states, which resulted in the burning of a-C in a narrow temperature range.

The energetic characteristics of two-shell carbon nanoparticles (onions) with different shapes of second shell are calculated. The barriers of relative rotation of shells are found to be surprisingly small; therefore, free relative rotation of shells can take place at room temperature. The intershell orientational melting of the nanoparticle is studied by molecular dynamics. The parameters of the Arrhenius formula for jumprotational intershell diffusion are calculated. The rotation of shells can be observed beginning from a temperature of 70 K.

In situ Raman and Vis-NIR spectroelectrochemistry at single-walled carbon nanotubes by Ladislav Kavan; Peter Rapta; Lothar Dunsch (363-368).
The population of valence-band electronic states of single-walled carbon nanotubes (SWCNT) was tuned electrochemically in aqueous electrolyte solution. Depending on the applied potential, reversible changes of intensity and frequency of the Raman-active tangential mode and radial breathing mode were observed. The maximum intensity occurs at ca. −0.2 V vs. Ag/AgCl independent of the electrode material used as a support (Pt, Au, Hg). SWCNT are sensitive to photoanodic breakdown upon prolonged exposure to anodic bias and laser light. Reversible bleaching of the first band-gap transition in semiconducting tubes (at 0.7 eV) occurs after anodic polarization.

A simple method for the production of large arrays of aligned carbon nanotubes by Frank Rohmund; Lena K.L Falk; Eleanor E.B Campbell (369-373).
Aligned carbon nanotubes are a technologically relevant member of the family of novel carbon materials, which find applications, e.g., as field emitters in flat panel displays. Different strategies of various complexities for their production have been demonstrated. Here we present an efficient and versatile but simple method for the production of large arrays of aligned carbon nanotubes based on thermal chemical vapor deposition from common precursor molecules. Iron catalyst particles are obtained from thermal decomposition of Fe(CO)5, while C2H2 serves as carbon feedstock. Growth of aligned nanotubes is achieved under both co-deposition and deposition in separate steps of the carbonyl and acetylene.

Effect of oxidation on single-wall carbon nanotubes by Shinichi Nagasawa; Masako Yudasaka; Kaori Hirahara; Toshinari Ichihashi; Sumio Iijima (374-380).
Two common methods of oxidation, gas-phase oxidation by heat treatment in oxygen gas and liquid-phase oxidation using nitric acid, were applied to single-wall carbon nanotubes (SWNT). The heat treatment in oxygen showed that thinner SWNTs burn more quickly. The nitric acid treatment showed that SWNTs are relatively inert to oxidation using acids. When the nitric acid treated samples were further oxidized by heat treatment in oxygen, selective oxidation of thinner SWNTs occurred.

Adsorption and catalytic properties of single-wall carbon nanohorns by J Adelene Nisha; M Yudasaka; S Bandow; F Kokai; K Takahashi; S Iijima (381-386).
The recently discovered single-wall carbon nanohorns (SWNH) are found to adsorb liquid ethanol effectively. The amount of liquid ethanol adsorbed by SWNHs is about 3.5 times larger than that of a super high surface area carbon. It is also found that SWNHs act as a catalyst for the oxidation of ethanol into acetaldehyde. Their thermal stability in oxygen is also discussed.

Optimized geometries and 13 C NMR chemical shifts of fullerene C80 have been calculated by density functional theory using B3LYP/6-31G* for all isolated-pentagon-rule isomers with non-zero HOMO–LOMO gap (isomers 1, 2, 3, 4 and 5). D2 distorted isomer 7 is predicted as first-order saddle-point by B3LYP/STO-3G. The calculated NMR spectrum of isomer 2 agrees well with experiment of Hennrich, confirming the assignment unequivocally. The predicted spectra of other isomers either show unusually large spectral span or have unusual chemical shifts for some sites, indicating unfavorable electron distribution. Both energetic and NMR properties indicate these isomers are less stable than isomer 2.

The reactions of polycyclic aromatic hydrocarbons with OH by Alessandra Ricca; Charles W Bauschlicher Jr (396-402).
The OH radical adds to naphthalene and naphthalene cation without a barrier. For the neutrals, the most favorable path for this intermediate is the loss of the OH, and the next most favorable option is the loss of an H atom to form the alcohol. For the cation, the most favorable path appears to be a hydrogen migration followed by the loss of a hydrogen to form the alcohol. The OH at carbon atom 1 is energetically most favorable for both the inital complex and final product. This is true for both the neutrals and cations.

Photoinduced intermolecular electron transfer between oligo(p-phenylene vinylene)s and N-methylfulleropyrrolidine in a polar solvent by Paul A. van Hal; Edwin H.A. Beckers; Emiel Peeters; Joke J. Apperloo; René A.J. Janssen (403-408).
Photoinduced intermolecular electron transfer in mixtures of oligo(p-phenylene vinylene)s (OPVns, with n=2–7, the number of phenyl rings) and N-methylfulleropyrrolidine (MP-C60) is studied in o-dichlorobenzene. Photoinduced absorption (PIA) spectroscopy is used to show that photoexcitation of MP-C60 generates the corresponding triplet state, which is quenched in a one-electron reduction reaction by OPVn for n>2. The electronic transitions of the photogenerated OPVn +• radical cations evolve with conjugation length in a near linear relation with 1/n. Charge transfer in these donor–acceptor systems for n>2 is in full agreement with the calculated change in free energy for charge separation.

Growth of hydrogenated silicon cluster ions using an ion trap by Hidefumi Hiura; Toshihiko Kanayama (409-414).
Medium-sized hydrogenated silicon cluster ions, Si n H x + (10<n<50), have been produced using an ion trap filled with silane (SiH4) at a pressure of ∼10−6 Torr. Time-resolved time-of-flight mass spectroscopic measurements revealed that there are two kinds of fast, efficient, cluster growth occurring via repetitive increments of eight Si atoms. One cluster growth continued beyond Si54H x +, which has a considerably larger n value than has been previously reported (n⩽10). Experimental evidence indicates that neutral Si n H x molecules, where n=6–8, formed from SiH4 and accumulated on the chamber wall, serve as the source material for both successive reactions.

Could uranium(XII)hexoxide, UO6 (O h ) exist? by Pekka Pyykkö; Nino Runeberg; Michal Straka; Kenneth G Dyall (415-419).
The formally dodecavalent, octahedral UXIIO6 is found to be a local energy minimum at several theoretical levels, including quasirelativistic single-reference (HF, B3LYP, MP2 and CCSD(T)) and multireference CI approaches. The fully relativistic, single-configuration Dirac–Fock (DF) approach gives similar U–O bond lengths but has one imaginary frequency. In this hypothetical compound the largest formal oxidation state of uranium would be increased from +VI to +XII.

Brownian dynamics simulations of the distribution of reaction times for Forster energy migration between the two ends of a polymer chain in solution have been carried out. The distribution is non-exponential and broad in almost all cases except when the Forster radius (R F) is close to the mean radius (R 0) of the polymer chain. Similar results are also obtained for the Heaviside sink function.

Doppler effect for bound nuclear motion and its manifestation in resonant photoemission of oriented systems by Pawel Sałek; Faris Gel'mukhanov; Timofei Privalov; Hans Ågren (425-430).
In contrast to the common notion that Doppler effects have relevance only for atoms or molecules in free motion, we show that such effects should clearly be observable – and accounted for – in resonant scattering also when the nuclei are in bound state quantized motion. This untrivial effect does not influence the position of single resonances but only the center of gravity of the spectra. It is found to strongly depend on the duration time of the scattering.

A new diagnostic for open-shell coupled-cluster theory by Matthew L. Leininger; Ida M.B. Nielsen; T.Daniel Crawford; Curtis L. Janssen (431-436).
We present a new diagnostic for open-shell coupled-cluster theory, readily computed from the single substitution amplitudes in the CCSD wavefunction. The new diagnostic, D 1(ROCCSD), is designed to be comparable to the previously proposed D 1(CCSD) diagnostic. Unlike other approaches, the D 1 diagnostics are independent of system size and have the same invariance properties as the energy with respect to orbital rotations. Calibration of the D 1(ROCCSD) diagnostic on 34 molecular systems indicates that for values of D 1(ROCCSD) of 0.025 or below the quality of the CCSD results are, in general, excellent, whereas values larger than 0.025 signal inadequacies in the CCSD approach.

The excited-state intermolecular proton transfer from 1-naphthol to urea has been studied in methanol. In the absence of urea, the excited 1-naphthol (ROH*) decays exponentially in methanol. However, upon the addition of urea, the decay becomes non-exponential. It is attributed to a geminate recombination of an ion-pair in the solvent cage. The quenching constant of ROH* by urea and some rate constants are evaluated by using both the steady-state fluorescence and picosecond fluorescence decay measurements.

We have studied the behaviour of static and dynamic polarisabilities for a set of small molecules and compared it with earlier experimental and computational work. The polarisabilities have been calculated using non-local and hybrid functionals under density functional theory (DFT). Calculations at several low frequencies enable the Cauchy moment S(−4) to be found for both the isotropic and anisotropic polarisabilities. The DFT values obtained with the hybrid functional are in good agreement with experimental values. The PBE0 functional gives slightly more accurate results than the traditional CCSDLR method.

We have calculated bond lengths, harmonic vibrational frequencies, and dissociation energies for (1 1 1)H using relativistic effective core potentials (RECP) including one-electron spin–orbit operators at the Hartree–Fock and coupled-cluster levels of theory. The spectroscopic constants calculated using shape-consistent RECPs compare favorably with available four-component results, but energy-consistent RECPs are found to underestimate the spin–orbit effects. The best computed(estimated) spectroscopic constants of (1 1 1)H are 1.512(1.524)  A ̊ , 2668(2647)  cm −1 , and 2.87(2.77) eV for R e, ω e, and D e, respectively. The calculated spin–orbit effects (+0.009  A ̊ ,−113  cm −1 , and −0.64 eV) are modest, although the molecule has a closed-shell electronic structure.

General-order equation-of-motion coupled-cluster methods for ionization potentials and electron affinities (IP-EOM-CC and EA-EOM-CC) are developed by employing a determinantal algorithm. With these, principal ionization potentials or electron affinities of diatomic molecules and the excitation energies of their ionized or electron-attached counterparts are computed across different approximations of the cluster operator and the ionization (electron-attachment) operator. IP-EOM-CC(2,2h-1p)=IP-EOM-CCSD and EA-EOM-CC(2,1h-2p)=EA-EOM-CCSD or EA-EOM-CC(2,2h-3p) prove to be well-balanced models for principal ionization potentials and electron affinities, whereas for the quantitative descriptions of non-Koopmans ionization or electron-attachment processes IP-EOM-CC(3,3h-2p)=IP-EOM-CCSDT and EA-EOM-CC(2,2h-3p) appear to be the minimal levels.

A modified Cashion–Herschbach potential for the H3 potential energy surface by T.I. Sachse; K.T. Tang; J.P. Toennies (469-472).
The H3 potential energy surface has been calculated using the Cashion–Hershbach (CH) potential which contains all orders of two-body Coulomb, exchange, and overlap terms. For collinear configurations, the dominant three-body contribution comes from the Axilrod–Teller–Muto triple–dipole dispersion energy. By adding only this term, properly damped, the saddle point region potential agrees with recent ab initio calculations to within 9×10−5 a.u. (0.06 kcal/mole).

Chemically reliable uncontracted Gaussian-type basis sets for atoms H to Lr by Toshikatsu Koga; Hiroshi Tatewaki; Tsuyoshi Shimazaki (473-482).
Highly accurate Gaussian-type function basis sets are developed for 103 atoms from H (atomic number Z=1) to Lr (Z=103). Choosing the last atoms of the seven periods and referring to the numerical Hartree–Fock total energies, the sizes of the present sets are so determined that the total energy errors are less than 1 millihartree. The average total energy error over the 103 atoms is only 0.79 millihartrees and the maximum error is 1.47 millihartrees for the Zn atom, indicating that the new sets are well-balanced throughout the atoms in the periodic table. The basis sets are tested for the GdF molecule.

MCSCF and DFT calculations of EPR parameters of sulfur centered radicals by Maria Engström; Olav Vahtras; Hans Ågren (483-491).
The EPR parameters of sulfur centered radicals are different depending on the radical structure, charge and solvent. That is, the g- and A-tensor components provide significant patterns which may distinguish sulfur radical structures from each other. In the present work, these EPR parameters were calculated for monosulfide radicals (RS), disulfide radicals (RSS), radical cations (RSSR+) and anions (RSSR), with R=CH3, using the MCSCF linear response and DFT/B3LYP methods. Results were in agreement with experimental data for the cases when well-resolved EPR spectra are available. Especially, the assignment of the disulfide anion in ribonucleotide reductase was confirmed. The results indicate that investigations with the present computational methods on refined structures and solvent modeling may provide interpretations of experimental data on unassigned radical species.

The facile hydrolysis of chlorine nitrate in aqueous sulfate aerosols by Jonathan P McNamara; Ian H Hillier (492-499).
High level electronic structure calculations show that chlorine nitrate is readily hydrolysed in aqueous sulfate aerosols. For the reaction catalysed by H2SO4 and three water molecules, the reaction products involve H3O+NO3 /H2SO4/HOCl. Catalysis by HSO4 leads to an essentially spontaneous reaction, where the reaction products involve NO3 /H2SO4/HOCl or HONO2/HSO4 /HOCl depending on the size of the water cluster.

Dynamics of the Cl+D2 reaction: a comparison of crossed molecular beam experiments with quasi-classical trajectory calculations on a new ab initio potential energy surface by Nadia Balucani; Laura Cartechini; Piergiorgio Casavecchia; Gian Gualberto Volpi; F.Javier Aoiz; Luis Bañares; Marta Menéndez; Wenshen Bian; Hans-Joachim Werner (500-508).
The dynamics of the Cl+D2 reaction has been studied experimentally at the collision energies 4.9 and 6.3 kcal mol−1 by the crossed molecular beam technique. The experimental results have been compared with theoretical predictions based on quasi-classical trajectory (QCT) calculations on the new BW potential energy surface (PES), and a good general agreement has been obtained. The QCT results obtained on the BW PES have been compared with those obtained on the previous semiempirical G3 PES, and striking differences regarding the effect of reagent rotation j on the reactivity have been found.

Ab initio and density functional theory methods are applied to investigate the molecular structures, intramolecular orbital interactions, and 19 F and 77 Se NMR chemical shifts of o-selenobenzyl fluoride derivatives, ArSeX (Ar=C 6 H 4 CH 2 F;  X=CN,Cl,Me), at both RHF and B3LYP levels with the basis sets 6-311G ∗∗ and 6-311+G ∗∗ . There are two stable rotational conformers for ArSeX. The energy differences between both conformers for each compound are small (within 2 kcal/mol) at various levels.

Semi-classical versus exact eigenvalues of He–benzene using cross-correlation filter-diagonalization by Sybil M. Anderson; Jaejin Ka; Peter M. Felker; Daniel Neuhuaser (516-521).
We determine semi-classically the eigenvalues of a He–benzene model where the benzene is held fixed in space (and non-rotating). The results are analyzed with an efficient approach toward extracting the eigenvalues from a short-time (set) of correlation functions: cross-correlation filter-diagonalization. The use of this analysis tool allows quite accurate determination of the eigenvalues, when compared to exact eigenvalues. One of the reasons that the eigenvalues are so accurate is that the cross-correlation filter-diagonalization approach allows the use of very short times, for which semi-classical is very accurate.

Index (522-529).