Chemical Physics Letters (v.330, #1-2)

We have employed scanning tunnelling microscopy (STM) in ultra-high vacuum (UHV) to explore the assembly and electron transport properties of an ordered layer of passivated gold clusters adsorbed onto an alkanethiol passivated Au(1 1 1) surface. The passivation of the Au(1 1 1) surface stabilises the cluster layer. STM images show ordered hexagonal arrays of the nanoparticles extending over distances >100 nm with a mean nearest neighbour spacing of 6.5 nm. I–V measurements show a strong non-linear I–V relationship, as well as equidistant steps of width ∼0.15 V above a bias voltage of ∼2 V, attributed to a Coulomb staircase, i.e., single electron charging, in this structure.

Substrate-induced demixing in a confined liquid crystal film by Richard Latham; Douglas J Cleaver (7-14).
We present results from molecular dynamics simulations of a confined equimolar binary liquid crystal mixture of generalised Gay–Berne particles with axial ratios of 3.0:1 and 3.5:1. Particle–substrate interactions are based on a non-separable anisotropic 9-3 form augmented by an azimuth-dependent prefactor. At high and moderate temperatures, the longer particles are preferentially adsorbed at the substrates. Unlike the equivalent 3-d bulk system, however, a significant degree of temperature-driven demixing is observed on cooling, the longer particles migrating into the centre of the cell at low temperatures. This leads to a marked decrease in the surface-region order parameter.

Local-mode oscillations at a liquid interface by Leon F Phillips (15-20).
The small-scale motions of a liquid surface due to capillary waves are found to be dominated by over-damped, local modes of oscillation. For values of the wave vector k in the under-damped regime, where viscosity is relatively unimportant, the long-range part of a local mode can excite the normal modes that are the usual focus of capillary-wave theory. The calculated surface roughness at high k values is larger than is given by normal mode treatments, so is in better agreement with the results of recent studies of the scattering of synchrotron radiation from liquid interfaces.

CH4 and CO adsorption isotherms on microporous ice surfaces have been performed simultaneously by pressure measurements and infrared spectrometry in the 50–80 K temperature range in order to compare the two techniques: volumetric isotherms allow evaluation of the surface area of the ice and the net heat of adsorption by the BET model, while infrared spectrometry gives complementary information on the nature of the chemical or physical bonding interaction. Our results show that the two methods are well correlated and that their comparison provides valuable information on the different adsorption sites.

Both the photophysics and the fluorescence quenching behavior of an anionic conjugated polymer towards various small molecule quenchers can be modulated effectively by complexing the polymer with a simple countercharged detergent. For example upon adding odecyltrimethylammonium bromide (DTA) to the polymer, cationic quenchers such as methyl viologen become less effective while the quenching by neutral reagents – most notably nitroaromatics or cyanoaromatics – is enhanced. Thus, the polymer–detergent complex provides a new platform for sensing chemical agents via fluorescence quenching. Thin films formed from the complex exhibit high sensitivity to quenching by nitroaromatic vapor and reasonable reversibility.

Electron and positron scattering from CF3I molecules below 600 eV: a comparison with CF3H by Michihito K Kawada; Osamu Sueoka; Mineo Kimura (34-40).
The total cross-sections (TCSs) for electron and positron scattering from CF3I molecules have been studied experimentally. A theoretical analysis based on the continuum multiple-scattering (CMS) method has been performed to understand the origin of resonances and the elastic cross-sections. The present TCS for electron scattering is found to be larger by about 20% than that of T. Underwood-Lemons, D.C. Winkler, J.A. Tossel, J.H. Moore [J. Chem. Phys. 100 (1994) 9117] although the general shape agrees well in the entire energy studied. The difference in the cross-sections for CF3I and CF3H is explained by the sizes and the dipole moments of these molecules.

Catalytic growth of carbon nanoballs with and without cobalt encapsulation by Ziyi Zhong; Huayi Chen; Songbei Tang; Jun Ding; Jianyi Lin; Kuang Lee Tan (41-47).
Carbon nanoballs encapsulated with cobalt were prepared by decomposition of methane. After acid treatment, the encapsulated cobalt can be removed to produce hollow carbon nanoballs. The content of hollow carbon nanoballs as high as 90 vol% in acid-treated carbon product can be obtained by increasing the content of cobalt from 50 to 75 mol% in the catalysts. However, under the same catalytic conditions, the main carbon products are multi walled carbon tubes (MWNTs) over Co/Al2O3, Co/La2O3, Co/CeO2 and Ni/MgO catalysts by decomposition of CH4, or by decomposition of CO over Co/MgO catalysts. These carbon nanoballs encapsulated with cobalt are novel magnetic materials.

Deposition of carbon nanotubes on Si nanowires by chemical vapor deposition by Y.F Zhang; Y.H Tang; Y Zhang; C.S Lee; I Bello; S.T Lee (48-52).
By using a hot filament chemical vapor deposition (HFCVD) method, deposition of carbon on Si nanowires (Si NWs) has been studied. Multi-walled carbon nanotubes (CNTs) were found to form on the surfaces of Si NWs at 900°C with a good surface coverage and adherence. However, as the temperature of deposition increased to 1000°C, Si cores tended to transform into β-SiC cores and the carbon layers grown on β-SiC cores were distorted. When the temperature of deposition was as high as 1100°C, the carbon layers bucked openly to form many feather-like carbon sheets sprouting from the surface of the nanowires. A mixture of large carbon sheets and β-SiC nanowires was formed when the temperature was over 1300°C.

Novel decay channels of carbon cluster ions, C40 z+ and C41 z+ (z=3,4) by R Parajuli; P Scheier; V Grill; S Matt; O Echt; T.D Märk (53-60).
We have analyzed spontaneous decay reactions of triply and quadruply charged C40 z+ and C41 z+ cluster ions which are formed from C60 fullerenes by electron-impact ionization. A new but very weak decay reaction for the even-sized carbon cluster ions is observed, namely loss of C3. The odd-sized cluster ions preferentially decay by loss of carbon atoms and, to a lesser degree, trimers. A weak signal due to C2 loss is observed for C41 3+. We discuss these decay channels in terms of the geometric structure of these metastable, relatively cold cluster ions.

Electrochemical impedance spectroscopy (EIS) was used to study the kinetics and insertion mechanism of alkali metals in single wall carbon nanotubes (SWNT). Three distinct processes with very different time constants were identified: charge transfer across the macroscopic electrolyte/electrode interface, diffusion through the mesoscale porosity of the SWNT electrode, and nanoscale diffusion in individual SWNT ropes. Electrode resistance and capacitance from EIS compare well with direct dc measurements. We propose that alkalis decorate the internal and external surfaces of the SWNT ropes.

Vibrational frequencies of gaseous Br2O and OBrO were observed using the Fourier transform infrared spectrometer. For the first time, bands at 629.0 cm−1 (ν 3) and 532.9 cm−1 (ν 1) were recorded for both Br–O asymmetric and symmetric stretching vibrations of gaseous Br2O. Two fundamental vibrations were observed at 798.7  cm −1  (ν1) and 846.3  cm −1  (ν3) for the O 18 BrO radical. In addition, two new peaks at 2333 cm−1 and 668 cm−1 were observed in a HOBr spectrum. They are tentatively assigned to the H–Br and Br–O stretching vibrations of a HOBr isomer on the basis of ab initio computational results.

Controlling the folding/unfolding transition of the DNA–histone H1 complex by direct optical manipulation by Yuko Yoshikawa; Shin-ichirou M. Nomura; Toshio Kanbe; Kenichi Yoshikawa (77-82).
Single giant duplex DNA (T4DNA, 166 kbp) complexed with a polycationic protein, histone H1, is trapped by a continuous-wave infrared laser, without chemical modification of the DNA such as attachment to a micrometer-sized bead. When the DNA–H1 complex is transported from a low-salt solution (0.2 M NaCl) to a high-salt solution (2 M NaCl), its conformation changes from a compact to an elongated structure. With the reverse procedure, i.e., with the transportation from the high-salt to the low-salt environment, the elongated chain shrinks into a compact state. These results afford the direct evidence that histone H1 induces a salt-dependent transition in the higher-order structure of giant DNA molecules.

Femtosecond time-resolved fluorescence intensities of 1,8-dihydroxyanthraquinone (chrysazin) in hexane have been measured at room temperature for a wide visible wavelength region (470–670 nm) by using the up-conversion method. Time-resolved fluorescence spectra were reconstructed after deconvolution taking account of the finite instrumental response. It was found that both the `normal-form type' fluorescence and the `tautomeric-form type' fluorescence appear almost instantaneously, which indicates that a barrierless excited-state proton transfer occurs within 50 fs reflecting delocalization of the excited-state wave function. A fluorescence spectral change was also observed in a few picosecond time scale, which was assigned to an additional proton translocation induced by the intramolecular vibrational relaxation.

Nuclear motion in the O 1s−1u core-excited states of CO2 probed by sub-natural-width resonant Auger emission spectroscopy by Y Muramatsu; Y Shimizu; H Yoshida; K Okada; N Saito; I Koyano; H Tanaka; K Ueda (91-96).
A promotion of the O 1s electron in the CO2 molecule to the lowest unoccupied molecular orbital 2πu brings about the stretching and bending motions in the core-excited states. These nuclear motions are investigated by use of the sub-natural-width resonant Auger emission spectroscopy.

Transient absorption measurement of radical anions of 1,1-diarylmethanols bearing electron-withdrawing groups in dimethyl-formamide at room temperature has revealed that symmetric compounds form intramolecular dimer radical anions as an intermediate of their reductive C–O bond dissociation giving diarylmethyl radicals. The dimer radical anions showed an absorption band in the near infrared region (NIR) (1000–1600 nm) characteristic to their charge resonance (CR) properties. On the contrary to 1,1-diarylmethanols, corresponding radical anion of 1,3-diarylpropan-1-ol showed no CR band. It is suggested that formation of dimer radical anions is energetically favorable for diaryl compounds with one-atom spacer but unfavorable for them with three-atom spacer being different from the n=3 rule for the favorable formation of intramolecular excimers.

The technique of velocity map imaging, an enhanced resolution variant of ion imaging, is shown to be a useful method for determining the dissociation energy of van der Waals complexes. The method is demonstrated by measuring the dissociation energy of p-difluorobenzene–Ar in the S1 state. From the spectroscopic shift of the S1←S0 transition and the change in the ionisation energy between the complex and free p-difluorobenzene, the dissociation energies in the ground state of the neutral and cationic complexes are determined. The values so determined are 339±4 and 369±4  cm −1 for the S0 and S1 states of the neutral complex, respectively, and 576±4  cm −1 for the cation ground (D0) state.

Zero-field muon spin rotation study on genuine organic ferromagnets, 4-arylmethyleneamino-2,2,6,6-tetramethylpiperidin-1-yloxyls (aryl=4-biphenylyl and phenyl) by Takayuki Ishida; Seiko Ohira; Tomoaki Ise; Kensuke Nakayama; Isao Watanabe; Takashi Nogami; Kanetada Nagamine (110-117).
Long-range magnetic orderings are unequivocally demonstrated by zero-field muon spin rotation/relaxation measurements on the genuine organic free radical crystals of 4-(p-phenylbenzylideneamino)- and 4-(benzylideneamino)-TEMPOs (TEMPO=2,2,6,6-tetramethylpiperidin-1-yloxyl). The T C's are 0.22 and 0.17 K and the critical exponents (β's) 0.356 and 0.362, respectively. Dominant two-dimensional ferromagnetic interaction is confirmed and inter-sheet dipolar interaction is proposed for bulk ferromagnetism.

Basic ideas for the correction of semiempirical methods describing H-bonded systems by M.I. Bernal-Uruchurtu; M.F. Ruiz-López (118-124).
In this Letter, we show how semiempirical methods may be improved to describe hydrogen-bonded systems and proton transfer reactions. The approach consists in a redefinition of the core–core interaction terms that, as previously shown, are at the origin of spurious artifacts in standard methods. The parameterization of the new core–core functions is done using ab initio data of the intermolecular potential energy surfaces (PESs), which permits reaching the correct behavior at short and long interatomic distances. Here we report the parameters for O–O, O–H and H–H interactions. Extension to other atom pairs seems feasible, so the development of a semiempirical method adapted to the study of intermolecular interactions might be envisaged.

We employ a reference interaction-site model (RISM) theory to estimate the relaxation dynamics of the average energy of solute–solvent systems as well as the time-dependent radial distribution functions of solvents viewed from the solute. The theoretical results indicate that the rearrangement of the second solvent shell is about an order of magnitude slower compared with that of the first shell. We suggest that the slower relaxation of further solvent shells can play a significant role in the dynamic relaxation of the inhomogeneous spectral width, which has been observed in our previous experiments of the time-resolved hole-burning and fluorescence spectroscopy.

Based on density functional theory (DFT) and atom–bond electronegativity equalization method (ABEEM), the condensed linear response function related to the chemical bond region is defined, determined and calculated for some molecules and the results are in good agreement with the usual chemical knowledge. The strengths among all the couplings between the electron density and the potential are in the order of atom–atom, atom–bond and bond–bond. The effect of a change in the potential of every H atom on the electron density of any chemical bond is almost negligible, and the electron density of all the C–H bonds rarely varies with a shift in the potential of any atoms.

Apparent tunneling in chemical reactions by Niels E. Henriksen; Flemming Y. Hansen; Gert D. Billing (139-145).
A necessary condition for tunneling in a chemical reaction is that the probability of crossing a barrier is non-zero, when the energy of the reactants is below the potential energy of the barrier. Due to the non-classical nature (i.e, momentum uncertainty) of vibrational states this is, however, not a sufficient condition in order to establish genuine tunneling as a result of quantum dynamics. This proposition is illustrated for a two-dimensional model potential describing dissociative sticking of N2 on Ru(s). It is suggested that the remarkable heavy atom tunneling, found in this system, is related to the high momentum tail of the initial vibrational state of nitrogen, allowing for amplitude to cross over the barrier.

Full-CI calculations of frequency-dependent dipole polarizabilities of ground state LiH have been performed in the imaginary frequency range 0–20 a.u. using an extended set of 109 Gaussian type orbitals (GTOs). A 32-points Gauss–Legendre quadrature of the Casimir–Polder formula over imaginary frequencies allows calculation of the dipole dispersion constants for the LiH–LiH homodimer, from which isotropic C6 and anisotropy γ6 dispersion coefficients are derived for the first time.

Time-dependent density functional theory (TDDFT) is applied to calculate vertical excitation energies of s-tetrazine, both in the gas-phase and in aqueous solution. The model density functional (PBE0) is obtained by combining the Perdew–Burke–Erzenrhof (PBE) generalized gradient functional with a predetermined amount of exact exchange, while the polarizable continuum model (PCM) is used to mime solvent effects on electronic transitions. Our results in the gas-phase show that the PBE0 functional provides accurate excitations both to valence and to low-lying Rydberg states. At the same time, the experimental solvent shifts in aqueous solution are well reproduced when the solute and the first solvation shell are embedded by a continuum solvent. These results show the potentialities of the combined TDDFT/PCM approach for the study of UV spectra of aromatic compounds.

Distribution of effectively unpaired electrons by Viktor N. Staroverov; Ernest R. Davidson (161-168).
The density of effectively unpaired electrons employed recently by the authors [J. Am. Chem. Soc. 122 (2000) 186] is identical with the distribution D( r ) of `odd' electrons proposed much earlier by Takatsuka, Fueno, and Yamaguchi [Theor. Chim. Acta 48 (1978) 175]. The function D( r | r ) is also implicitly present in several definitions of the free valence index. Basic properties of D( r | r ) are reviewed and a number of new results are reported. The distribution D( r ) is advocated as a rigorous criterion of the radical character in molecules of any spin. Numerical examples deal with the dissociation of N2 and O2 molecules.

We develop a feasible transcorrelated method for accelerating the convergence of reproducing the dynamic correlation effects with the size of one-electron basis. The effective Hamiltonian is parameterized in such a way that the Coulomb repulsion is compensated at short inter-electronic distances in terms of a frozen Gaussian geminal. The geminal is chosen to be independent of the position and orientation of pair-electrons. The extra part of the transcorrelated Hamiltonian is also short-ranged, size-consistent, and universal to the states of interest. We preliminarily applied the method to the single-reference many body perturbation theory with some pilot calculations.

Formulas are developed for evaluating the three-electron integrals, which appear in the transcorrelated method using a frozen Gaussian geminal. First, we derive expressions with intermediates, in which the angular momentum indices of the inter-electronic operators are temporarily increased. Following the inspection on the factorization property, we further derive more straightforward recurrence relations corresponding to the Obara–Saika one for electron repulsion integrals. It is shown that the relations can be applied outside the contraction loops over geminal primitives by introducing three-indexed auxiliary integrals.

A density functional theory study of the anthracene anion by Caroline E.H. Dessent (180-187).
The optimized geometries, harmonic vibrational frequencies, and absolute energies of anthracene and anthracene were determined using density functional methods. Adiabatic electron affinities (EAs) of 0.360, 0.274, 0.590 and 0.530 eV were calculated using B3LYP/cc-pVDZ, BLYP/cc-pVDZ, BP86/cc-pVDZ and B3LYP/6-31+G*, respectively. The EAs are in good agreement with experiment, with the B3LYP/6-31+G* value matching the experimental value and the BP86/cc-pVDZ value displaying an absolute error of only 0.060 eV. Accurate predictions of experimental parameters were obtained using all three functionals. Single point calculations using the aug-cc-pVDZ, cc-pVTZ and aug-cc-pVTZ basis sets are included to investigate the effect of basis set expansion and diffuse functions on the computed EAs. The excellent performance of density functional theory (DFT) for this system contrasts with poor results generated using Hartree–Fock theory.

Quantum mechanical cross-sections for the collinear SN2 reactions Cl +CH 3 Cl  →  ClCH 3+Cl ′− and Cl +CH 3 Br  →  ClCH 3+Br have been calculated within the time-independent hyperspherical coordinate approach employing two-dimensional CCSD(T) potential energy surfaces. Compared to the results obtained from the potentials of Hase and co-workers, the recent cross-sections show several different features. However, the cross-sections calculated using the CCSD(T) potential energy surface for Cl+CH3Br do not explain the experimentally observed independence of the rate constant on initial CH3Br temperature.

An ab initio study of the low-lying electronic excited states of CH3OBr by Sujata Guha; Yumin Li; Joseph S Francisco (195-198).
Using high level ab initio molecular orbital methods, theoretical studies were performed on the low-lying excited states of CH3OBr. The complete active space self-consistent field (CASSCF) and multi-reference configuration interaction (MRCI) methods, with the cc-pVTZ basis set, were used to calculate the vertical excitation energies for the lowest three 1 A , two 1 A″, two 3 A , and two 3 A″ states. The CASSCF method was used to obtain the potential energy curves for the lowest three 1 A , two 1 A″, two 3 A , and two 3 A″ states of CH3OBr along the O–Br coordinate. All of the eight excited states were found to be strongly repulsive along the O–Br coordinate.

On the photoelectron spectrum of Ga2P2 by Edet F Archibong; Alain St-Amant (199-206).
The experimental photoelectron spectrum of Ga2P2 reported by Taylor, Asmis, Xu and Neumark [Chem. Phys. Lett. 297 (1998) 133] is interpreted using quantum chemical calculations. The anion has a C2v distorted tetrahedron structure and the neutral has a D2h rhombic structure. Vertical electron detachment energies (VEDE) from the 2 B 1 ground electronic state of Ga2P2 to various electronic states of Ga2P2 are presented, discussed and correlated with the experimental results. Harmonic vibrational frequencies and adiabatic electron detachment energies (AEDE) are also reported to aid future assignment of the vibrationally resolved spectrum of the Ga2P2 /Ga2P2 system.

The Letter deals with modelling of the rate coefficients for the dissociation reaction N2(i)+N=3N. The dependence of dissociation rate coefficients on the vibrational quantum level has been obtained by using trajectory calculations and the phenomenological Treanor–Marrone model. A comparison of the results obtained on the basis of different models is discussed and practical recommendations concerning the parameters of the phenomenological model are given.

A comparison of the ab initio quantum chemical methods: Hartree–Fock (HF) and hybrid density functional theory (DFT)/B3LYP for the treatment of tautomeric equilibria both in the gas phase and in the solution is made. The solvent effects were investigated in terms of the self-consistent reaction field (SCRF). Ionization potentials (IP), calculated by DFT/B3LYP, are also compared with those calculated previously within the HF frame.