Chemical Physics Letters (v.331, #2-4)

Phonon confinement in ultrathin nickel films by A. Melikyan; H. Minassian (115-118).
We present an interpretation of the recently obtained data related to the measurements of the sound velocity in ultrathin nickel films. The interpretation is based on the phonon frequency spectrum size quantization, which in turn leads to a decreasing of the phonon group velocity in the confinement direction as the film thickness decreases. A good agreement with the experimental data is obtained for all film thicknesses for which the sound velocity has been measured.

Recent scanning tunneling microscope (STM) studies emphasize the important role of the tip/surface interaction even at typical imaging conditions. The tip-induced adatom diffusion on anisotropic Ag(1 1 0) has been investigated upon scanning parallel to the close-packed rows, where diffusion is preferred. In this scanning direction Ag atoms, which were removed from kink sites, follow the tip into an originally empty surface region, until it passes onto the adjacent close packed row. Due to attractive interaction diffusion is drastically suppressed by the tip.

Nano-mechanical cutting and opening of single wall carbon nanotubes by I Stepanek; G Maurin; P Bernier; J Gavillet; A Loiseau; R Edwards; O Jaschinski (125-131).
We pioneered a simple and soft mechanical cutting process for single wall carbon nanotubes involving diamond particles as abrasive material. High resolution transmission electron microscopy observation reveals shortened nanotubes always organised in bundles. Micrograph analysis allows us to determine the breaking mechanism of the bundles. The average nanotube lengths, estimated using AFM images, are ranging from 300 to 700 nm. Adsorption measurements show a topological modification of the material with the presence of new microporosities ranging in the average nanotube diameter.

Three-center semi-empirical LEPS potential energy surfaces were constructed for the F+HX  (X=OH) and F+OH reactions. Trajectory calculations on these surfaces reproduced quite well the available experimental results with regard to rate constants and vibrational state distributions of the products. The results are presented and discussed.

Tumbling atoms and evidence for charge transfer in La2@C80@SWNT by Brian W Smith; David E Luzzi; Yohji Achiba (137-142).
Using transmission electron microscopy, we demonstrate the insertion of the metallofullerene La2@C80 into 1.4 nm diameter single wall carbon nanotubes. Micrographs indicate that the spheroidal D2h isomer having an IhC80 cage ∼0.8 nm in diameter is preferred. For the first time, endohedral atoms inside a metallofullerene are directly imaged. The La atoms are seen to undergo a discontinuous tumbling motion inside their C80 cage. The measured La–La separation is slightly larger than is predicted by theory. These data suggest that interactions between the nanotube and La2@C80 have changed the molecules' behaviors. Our results prove that extrinsic molecular species can be introduced into carbon nanotubes.

Photoelectric response of pyrrolidinofullerene and bis-pyrrolidinofullerene LB films by Sheng Zhang; Liangbing Gan; Chunhui Huang (143-148).
Two new pyrrolidinofullerene derivatives and their corresponding dimers are prepared. All these compounds form stable Langmuir–Blodgett (LB) films, which can be readily transferred onto an indium-tin-oxide (ITO) electrode. The photoelectric conversion ability of the bis-fullerenes is more than twice that of the mono-fullerenes.

Brillouin scattering from C70 and C60 films: a comparative study of elastic properties by P Murugavel; Chandrabhas Narayana; A Govindaraj; Ajay K Sood; C.N.R Rao (149-153).
Brillouin scattering measurements carried out on a thin film of C70 deposited on a Si(1 0 0) substrate show modes at 4.8 and 25 GHz due to the surface Rayleigh wave and the bulk longitudinal acoustic mode, respectively, the corresponding modes for C60 being found at 4.3 and 30 GHz. Based on the Brillouin data, the values of the bulk, shear and Young's moduli of C70 are found to be 11.3, 3.2 and 8.7 GPa, respectively (the corresponding values for C60 are 14.9, 4.7 and 12.7 GPa). Poisson's ratios for C70 and C60 are comparable, the values being 0.37 and 0.36, respectively.

Sub-picosecond, time-resolved absorption spectra of all-trans-neurosporene, both free in n-hexane solution and bound to the LH2 complex from Rhodobacter sphaeroides G1C, were recorded and analyzed by singular-value decomposition followed by global fitting using a sequential model. The former experiment identified the 1Bu +→1Bu →2Ag internal conversion, whereas the latter experiment identified a transformation of 1Bu +→2Ag →T1(13Bu +). Excitation to the 1Bu +(0) or 1Bu +(1) vibronic level resulted in enhancement of stimulated emission from the particular level, showing that vibrational relaxation in the 1Bu + state has a time constant comparable to, or larger than, that of electronic relaxation.

Transition metal-chlorine anions and cations: monomers, clusters, and periodic trends by Chuan-Fan Ding; Yongzhi Yu; Roy H Jensen; Walter J Balfour; Charles X.W Qian (163-169).
Laser ablation and time-of-flight mass spectrometry have been used to generate and characterize aluminum-chlorine and transition metal-chlorine ionic species. Many new complexes have been observed. The structures of the monometal-chlorine species are discussed.

Rate coefficients and cross-sections for the reactions of C(3 P J ) atoms with methylacetylene and allene by Delphine Chastaing; Sébastien D. Le Picard; Ian R. Sims; Ian W.M. Smith; Wolf D. Geppert; Christian Naulin; Michel Costes (170-176).
The reactivity of C(3 P J ) atoms towards methylacetylene and allene has been investigated using complementary experimental methods. Kinetic measurements performed over the range T=15–295  K have yielded thermal rate coefficients: (2.7±0.6)×10−10(T/298  K)−(0.11±0.07) and (3.5±0.8)×10−10(T/298  K)−(0.01±0.12)   cm 3   molecule −1   s −1 , respectively. Crossed beam experiments conducted in the range of relative translational energies ε tr =4.9–282  meV have yielded integral cross-sections for H(2 S 1/2) production that are proportional to (ε tr)−0.56±0.02 and (ε tr)−0.55±0.01, respectively, in excellent agreement with the temperature dependencies of the rate coefficients.

We show that vibrational energy can be used to tune the chemisorption state of an adsorbed molecule. When dosing C2H4 with a supersonic molecular beam on Ag(0 0 1) we demonstrate by vibrational spectroscopy that, with increasing nozzle temperature, T N, firstly no stable adsorption occurs, then a π-bonded configuration is populated, switching finally to a more strongly, undissociated, di-σ-bonded state. Direct dissociation is observed at still higher T N with the formation of dehydrogenated radicals.

Rotational spectrum of the Kr–HCO+ ionic complex by Kouji Seki; Yoshihiro Sumiyoshi; Yasuki Endo (184-188).
The Kr–HCO+ ionic complex and its isotopomers have been detected for the first time by Fourier transform microwave spectroscopy, where pure rotational transitions have been observed for six isotopomers under a jet-cooled condition. The rotational and the centrifugal distortion constants of the six isotopomers have been determined by least squares fittings. The complex has been determined to have a linear proton-bound form, with a Kr–H distance to be 2.222 Å and an estimated intermolecular stretching frequency of 129 cm−1. Results of Kr–HCO+ are compared with those of Ar–HCO+ and neutral complexes containing Kr.

Ionic conductivities of hydrated zeolite Li-, Na- and K-A at room temperature and at pressures up to 4.8 GPa have been measured. Anomalous increases in conductivity with pressure up to 1.7–1.9 GPa were observed for all zeolites. In this pressure range, activation volumes of conduction are −1.4, −3.9 and −6.8 cm3/mol for Li-, Na- and K-A zeolites, respectively. The conductivity goes through a maximum, where pressure-induced amorphization occurs. Following the maximum, there is a rapid decrease in conductivity due to the collapse of long-range ordering. On decompression, conductivity increases to values that are higher than the conductivity values at equal pressures on compression.

The Δv=0 sequence of the NF(b 1Σ+v→X 3Σv″) spin forbidden emission is studied from the intensity point of view. The NF b 1Σ+ emitting state is produced by NF3 windowless photolysis in a microwave flowing afterglow apparatus. Although the observed NF b 1Σ+ vibrational population is at a Boltzmann equilibrium, v =1 through v =3 emitting levels appear to be depleted. This behaviour is attributed to interactions of the emitting b 1Σ+ state with the ground state continuum. A value of 60.7±1.5 kcal/mol for the NF X 3Σ dissociation limit is thus estimated.

The kinetics of the two-photon ionization (TPI) of N,N,N ,N -tetramethylbenzidine (TMB) in CH2Cl2-doped solid argon using 313 nm Hg lamp radiation is investigated by means of ultraviolet/visible (UV/VIS) absorption spectroscopy. The experimental findings can be quantitatively understood on the basis of a rate equation model. The ionization cross-section of TMB in the lowest triplet state and the rate constant of cation–electron recombination are obtained by adapting calculated to measured irradiation time-dependent concentrations of the cation.

The recombination of H3 + ions with electrons: dependence on partial pressure of H2 by J. Glosı́k; R. Plašil; V. Poterya; P. Kudrna; M. Tichý (209-214).
We observed an increase in the effective recombination coefficient (α eff) of the recombination of H3 + ions with electrons with a number density of H 2, n(H 2). With increasing n(H2) from 1×1011 to 2×1012   cm −3 the α eff increased from 1.3×10−8 to 1.5×10−7   cm 3   s −1 . The dependence of α eff on n(H2) indicates that the recombination of H3 + ions in an afterglow plasma is a complex process in which collisions with H2 take place. It is stressed that at conditions corresponding to plasma in interstellar space the dissociative recombination of H3 + ions is very slow with a rate coefficient α⩽1.3×10−8   cm 3   s −1 .

Excited state molecular configuration of biphenyl by P. Sett; S. Chattopadhyay; P.K. Mallick (215-223).
Contributions of different electronic states to Raman scattering have been studied by critical analyses of Raman excitation profiles of several normal modes of vibration of biphenyl. In this context, the possible structure and other interesting properties of the molecule in the excited electronic states have been discussed. It is found that the first allowed transition to the excited state occurs at 248 nm whose major geometry change involves CC stretching vibrations, whereas, for the second allowed transition (corresponding to 205 nm) inter (i.e., substituent-sensitive) and intra (i.e., ring-breathing) ring CC stretching and in plane CCH angle bending modes are important.

Deuteron transfer in heavy water is investigated by means of mixed quantum/classical molecular dynamics simulation and compared to proton transport in water, which was treated in a previous study [D. Zahn, J. Brickmann, Isr. J. Chem. 39 (1999) 483]. Therein the migration of a positive charge is treated as a two-step process; (i) the displacement of a H3O+/D3O+ ion, followed by (ii) proton/deuteron transfer to an adjacent (heavy) water molecule. The water molecules as well as the H3O+/D3O+ ions are treated classically. The proton/deuteron transfer between two neighbor molecules is treated on the basis of a quantum degree of freedom. It is shown that our model allows a numerically very effective simulation of proton/deuteron transfer processes in aqueous solution.

Some uranium (U(VI) and U(V)) triatomic molecules and cations with the general formula XUY (X,Y= C,N,O) and a charge varying from 0 to +2 have been studied using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) and Density Functional Theory (DFT). Geometry optimizations and infrared harmonic frequency calculations have been carried out and a comparison with available experimental data has been performed. This study shows that CASSCF/CASPT2 is capable of handling actinide systems, where multiconfiguratioanl and relativistic effects are essential.

Absolute chloroform/water partition coefficients for organic solutes were calculated by using a linear response method for determining the free energy of solvation and Monte Carlo simulations. In the used linear equations, in addition to the Coulombic and van der Waals components of the solute–solvent interaction energy, the cavity terms such as molecular surface area, molecular volume and ovality have also been used. For most solutes, the calculated log  P values and predicted free energy of solvation in both water and chloroform are in good agreement with available experimental values and other theoretical methods. However, including ovality produces better results. The trend of ΔG solvation in water and chloroform media were matched with their polarities.

Dipolar spectroscopy and spin alignment in electron paramagnetic resonance by G. Jeschke; M. Pannier; A. Godt; H.W. Spiess (243-252).
Two single-frequency techniques for refocusing (SIFTER) dipolar couplings between electron spins are introduced. The experiments are based on the solid-echo and Jeener–Broekaert sequences, well established in dipolar NMR spectroscopy of solids, and open up new routes to high-resolution two-dimensional EPR spectroscopy with only moderate requirements on the spectrometer. For distances between paramagnetic centres larger than 3 nm, SIFTER provides better resolution than double electron–electron resonance (DEER). Good agreement between distances from SIFTER measurements and force-field computations is found for shape-persistent biradicals with distances up to 5.1 nm corresponding to a dipolar frequency of 390 kHz.

Ab initio calculations on indole–water, 1-methylindole–water and indole–(water)2 by Tanja van Mourik; Sarah L. Price; David C. Clary (253-261).
The structures of indole–water are optimized with second-order Møller–Plesset (MP2) perturbation theory and augmented correlation consistent basis sets. The binding energies and hydrogen bond distances are corrected for BSSE. By extrapolating the correlation interaction energy to estimate the complete basis-set limit, the equilibrium binding energies D e of the two indole–water minima are 24.27±0.5 and 21.23±0.5 kJ/mol. We calculated binding energies D 0 of 18.13 and 17.9 kJ/mol for the global minimum of indole–water and methylindole–water, which are, unlike previous theoretical estimates, in quite good agreement with experiment (19.52–20.25 and 17.15 kJ/mol, respectively).

The electronic structure calculations locate the local minima for bicyclic N10 and the fullerene analog N60, both as high-energy density species. The bridging NN bond in N10 is quite remarkably strong (ΔE [N 10→2N 5]=93 kcal/mol), yet flexible to allow a facile rotation of one ring with respect to the other. This property could permit N10 to serve as a building block for specific clustering into the nitrogen buckminsterfullerene structure.

The CH(2Π)+N 2 reaction over the ground state potential energy surface has been investigated at the G2M level of theory. This reaction is directly relevant to hydrocarbon combustion chemistry, in particular, to `prompt NO' formation. A detailed mechanism via stepwise and concerted pathways to form HNCN, involving chain and cyclic intermediates, is presented. The proposed mechanism for NO formation is more favorable than the commonly assumed spin-forbidden path producing HCN+N(4 S). The theoretically predicted heats of formation for NCN and HNCN are in excellent agreement with the recently reported experimental values.

Should one adjust the maximum step size in a Metropolis Monte Carlo simulation? by Mark A. Miller; Lynn M. Amon; William P. Reinhardt (278-284).
It is common practice to adjust the maximum step size during Monte Carlo simulations to achieve a target fraction of accepted trial moves. Although simulation experts are well aware that this procedure is technically incorrect, the continuing publication of studies that employ it indicates that the pitfalls are not more widely known, or at least are assumed to be negligible. In this pedagogical Letter, it is shown that dynamic step size adjustment may lead to unwanted correlations and statistical undersampling of flat regions of the potential energy surface, thereby violating balance and producing systematic errors. A variety of examples highlight the conditions where the problem is likely to be worst.

On the interaction of two conical intersections: the H6 system by A.J.C. Varandas; A.I. Voronin; I. Borges (285-289).
We report a study on the interaction of two H3 systems, which are known to possess a conical intersection when infinitely separated from each other. The topology of the crossing seam is analysed for different geometrical arrangements of the two interacting partners.

Structural and electronic properties of neutral and anionic CuO2 molecules are investigated within density functional theory. The lowest energy structures are the bent CuOO and the linear OCuO. Consideration of temperature effects via first-principles molecular dynamics simulations allows to conclude that two CuO2 isomers (bent CuOO and CuO2 side-on) coexist at very close energies in the measured photoelectron spectrum, for different spin states. Among the isomers of CuO2, bonding is the most covalent in the linear OCuO molecule.

Semi-empirical (INDO/s) calculations have been conducted on molecular fragments with zero to three phenylenevinylene (PV) units attached to 4 and 4 positions of a 2,2-bipyridine (bpy) group, with and without chelated metal ions, mimicking metal-free and metal-chelated photoconducting polymers 1 and 2 [Chen et al., J. Phys. Chem. B 104 (2000) 1950]. The calculations suggest that: (1) a global lowering of the molecular orbital energy levels due to metal-chelation is responsible for the observed red-shift in the lowest energy transitions; and (2) metal chelation attenuates π-electron delocalization. The relevance of these effects to photoluminescence of metal-chelated polymers is also discussed.

MP2 and CCSD(T) calculations are used to analyse the structures and vibrational spectra of HRgF molecules, where the rare gas atom is He, Ne, Ar, Kr, Xe or Rn. We extend the analysis of the vibrational spectra of these molecules to include anharmonic corrections for the most likely candidates for experimental detection, i.e., HArF, HKrF, HXeF, and their deuterated isotopomers. The anharmonic correlation-corrected vibrational self-consistent-field (CC-VSCF) calculations are used for this, and fundamental, overtone and combination frequencies and their absorption intensities are computed.

Ab initio dipole moment function of H2S by Thibaud Cours; Pavel Rosmus; Vladimir G. Tyuterev (317-322).
We report new ab initio dipole moment surfaces (DMS) of the electronic ground state of the H2S molecule. The two components of the dipole moment function have been evaluated using the coupled-cluster single and double excitations augmented by a perturbative contribution from connected tripled excitation (CCSD[T]) ab initio method. The rovibrational line intensities calculated with these ab initio DMS are in good agreement with the experimental data for fundamental ν1, ν2 , and ν 3 bands. Our calculations reproduce the observed anomalies in the intensity distributions of rovibrational bands of H2S unlike the previous ab initio calculations. Since the DMS are very flat near the equilibrium geometry, the fundamental ν 1 and ν 3 bands are abnormally weak which makes the first principle intensity calculation a challenging problem.

The state He 2 4 I g (1σ g 2  1σ u  1δ g  1φ u ) of Adamowicz and Pluta [Chem. Phys. Lett. 179 (1991) 517] is shown to be (1σ g 2  1σ u  2δ g  1φ u ). A variational calculation using one electron diatomic molecular orbitals, as well as a complete active space self-consistent field calculation, predict it to be unbound with respect to its neutral parent states. It is argued that He 2 4 I g (1σ g 2  1σ u  1δ g  1φ u ) cannot exist.

We have calculated cross-sections and rate constants for the title reaction by using the quasiclassical trajectory method and a recently reported two-valued energy-switching potential energy surface for the water molecule. By varying the amplitude and rate of decay of a local Gaussian term which controls the appearance of a barrier along the C 2v minimum energy profile, an attempt has been made to answer the title issue. A comparison of the calculated rate constants with the available experimental data suggests that the barrier, if existing, lies below the energy of the reactants, and separates the small van der Waals well from the deep chemical one at short distances.

Dynamical and thermal properties of polyethylene by ab initio simulation by S. Serra; S. Iarlori; E. Tosatti; S. Scandolo; G. Santoro (339-345).
The structural, dynamical, and thermal properties of crystalline polyethylene are addressed with first-principles methods based on the density functional theory. The relatively low accuracy of the local density approximation for molecular crystals is corrected with the generalized gradient approximation, supplemented with empirical van der Waals' corrections, so as to optimize the description of the static (structural and elastic) properties of crystalline polyethylene. Based on this description, we perform first-principles finite temperature molecular dynamics simulations of warm, solid polyethylene. We analyze in particular thermal disorder effects, revealing the spontaneous appearance of trans-gauche defects close to the melting temperature (430 K).