Chemical Physics Letters (v.336, #3-4)

Novel types of spatio-temporal patterns in catalytic CO oxidation on a facetted Pt(1 1 0) surface by F. Meißen; A.J. Patchett; R. Imbihl; A.M. Bradshaw (181-186).
Spatio-temporal patterns in catalytic CO oxidation have been studied on a facetted Pt(1 1 0) surface in the 10−5   mbar range using low energy electron microscopy (LEEM). Prior to the LEEM experiments the surface had been facetted by exposing the sample to reaction conditions in the 10−4   mbar range. Irregularly shaped spiral waves, interacting oxygen fronts, travelling CO islands with slowly decaying trails and travelling wave fragments exhibiting a superimposed slow rotation have been found. The experimental results are discussed in terms of reversible facetting changing the anisotropy of the surface and adding a slow time scale to the dynamics of the reaction.

Nodal surface approximations to the P,  G,  D and I-WP triply periodic minimal surfaces by Paul J.F. Gandy; Sonny Bardhan; Alan L. Mackay; Jacek Klinowski (187-195).
The cubic P,  G,  D and I-WP triply periodic minimal surfaces (TPMS) may be closely approximated using periodic nodal surfaces (PNS) with few Fourier terms, thus enabling easy generation of TPMS for use in various chemical and physical applications. The accuracy of such approximations is quantitatively discussed and represented visually using a colour coding.

In the purification of single-wall carbon nanotubes (SWNTs), it has been difficult to remove graphitic particles. We were able to increase the percentage of SWNTs and decrease impurities in the form of graphite particles and amorphous carbon by using a new target. The target material was made of crude SWNTs, which contains SWNTs (20–30%), particles of graphite (about 10–20%), amorphous carbon (50–60%) and a little of Ni and Co (about 0.6%). The high yield of SWNTs obtained was attributed to the composition of the target and its porous structure.

New method of carbon onion growth by radio-frequency plasma-enhanced chemical vapor deposition by X.H Chen; F.M Deng; J.X Wang; H.S Yang; G.T Wu; X.B Zhang; J.C Peng; W.Z Li (201-204).
Large quantities of carbon onions with high purity were synthesized by radio-frequency plasma-enhanced chemical vapor deposition. The produced carbon onions are solid, clean and can be separated easily from the catalytic particles. The formation of onions is based on the formation of many cages in successive stages from the core to the surface. Around the edge of a carbon onion, discontinuous curved lines are shown in the high-resolution TEM image, reflecting the wavy behavior of carbon onions.

Gas adsorption in the inside and outside of single-walled carbon nanotubes by Akihiko Fujiwara; Kenji Ishii; Hiroyoshi Suematsu; Hiromichi Kataura; Yutaka Maniwa; Shinzou Suzuki; Yohji Achiba (205-211).
Adsorption properties of nitrogen and oxygen gases in single-walled carbon nanotube (SWNT) bundles were investigated by the isotherm and X-ray diffraction (XRD) studies. In the as-grown (AG) nanotubes with close-ended caps, both the gases are adsorbed only in the interstitial channels between triangular packed nanotubes. In the heat-treated (HT) nanotubes with open ends, the gases are adsorbed first in the inside of tubes, and next in the interstitial channels. In each site, gases can be adsorbed with the stoichiometory of C20N2 or C20O2 as a monolayer. These results indicate that the inside of nanotube has strong affinity for gas adsorption than the interstitial channels of bundles.

Hydrogen bond breaking in aqueous solutions near the critical point by Robert A. Mayanovic; Alan J. Anderson; William A. Bassett; I-Ming Chou (212-218).
The nature of water–anion bonding is examined using X-ray absorption fine structure spectroscopy on a 1m    ZnBr 2/6m NaBr aqueous solution, to near critical conditions. Analyses show that upon heating the solution from 25°C to 500°C, a 63% reduction of waters occurs in the solvation shell of ZnBr4 2−, which is the predominant complex at all pressure–temperature conditions investigated. A similar reduction in the hydration shell of waters in the Br aqua ion was found. Our results indicate that the water–anion and water–water bond breaking mechanisms occurring at high temperatures are essentially the same. This is consistent with the hydration waters being weakly hydrogen bonded to halide anions in electrolyte solutions.

The collisional quenching of vibrationally excited methyl-substituted pyrazine series with an initial energy of 38 000–39 000 cm−1 by He, Ar, and Kr has been investigated using time-resolved infrared fluorescence of C–H stretching vibrations. The methyl group facilitates the quenching process, but its enhancement effect decreases as the number of methyl groups increases. The quenching efficiency of dimethylpyrazines depends on the position of methyl groups, which becomes significant when going from He to Kr. The steric hindrance of methyl groups plays an important role in the quenching of dimethylpyrazines and trimethylpyrazine by inert gas.

Mn–Te bond in the rocksalt Sn1−x Mn x Te alloys and octahedral radius of Mn: X-Ray absorption- and diffraction study by R.J. Iwanowski; W. Paszkowicz; K. Lawniczak-Jabłońska; M.H. Heinonen; B. Witkowska; J. Feldhaus (226-233).
The studies of local atomic structure in Sn1−x Mn x Te (x<0.07) alloys, with an attention to the nature of Mn–Te bond, were performed for the first time with use of extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) techniques, supplemented by photoelectron measurements. XRD unanimously indicated a single phase NaCl-type structure of these compounds. Analysis of Mn K-edge EXAFS data provided us with the bond length and Debye–Waller (DW) factor for Mn–Te bond in octahedral coordination of rocksalt Sn1−x Mn x Te. The magnitude of DW factor suggested a moderate ionicity of Mn–Te bond (in the host SnTe matrix) indicative of its mixed covalent–ionic character. The determined value of Mn–Te bond length in Sn1−x Mn x Te enabled us to estimate the octahedral covalent radius of Mn.

The solid-state photoluminescent quantum yield of triboluminescent materials by G. Bourhill; L.O. Pålsson; I.D.W. Samuel; I.C. Sage; I.D.H. Oswald; J.P. Duignan (234-241).
Ground rules defining design of triboluminescent (TL) materials are currently missing, in part since the triboluminescent efficiency is the product of the efficiency of excited-state formation and the efficiency of radiative relaxation from these excited-states. In order to de-couple these two processes, we have measured, for the first time, the solid-state photoluminescent quantum yield of various triboluminescent materials. The measurements highlighted: (i) some triboluminescent materials possess near-optimised radiative yields; (ii) structurally similar triboluminescent compounds can exhibit widely varying yields and (iii) some triboluminescent materials, which appear to the eye as reasonably efficient, possess low radiative yields, allowing possible triboluminescent enhancement.

Reaction rates of O(3 P) atom with fluoroethanes at 1000–1400 K by Hiroumi Shiina; Kentaro Tsuchiya; Masaaki Oya; Akira Miyoshi; Hiroyuki Matsui (242-247).
The overall rate constants for reactions of O(3 P) with three fluoroethanes, O(3 P)+CH 3 CH 2 F→OH+CH 2 CH 2 F/CH 3 CHF (1), O(3 P)+CH 2 FCH 2 F→OH+CHFCH 2 F (2), and O(3 P)+CH 3 CHF 2→OH+CH 2 CHF 2/CH 3 CF 2 (3), were measured by laser flash photolysis in shock heated sample gases at 1000–1400 K. The rate constants were obtained in the simple Arrhenius form, k=Aexp(−E a/RT), where A=2.19×10−9, 1.24×10−9 and 7.43×10−9   cm 3   molecule −1   s −1 and E a=58.0, 57.0 and 82.7 kJ mol−1, and uncertainty factors for k at 2σ level are F=1.36, 1.27 and 1.19, respectively. The effect of substitution of the fluorine atom on the overall rate constant is discussed in comparison with the corresponding rates for a series of alkanes and fluoromethanes.

Generalization of topological indices by Ernesto Estrada (248-252).
A generalized (molecular) graph-theoretical matrix is introduced in such a way that the adjacency and distance matrices of (molecular) graphs are particular cases of it. By using this matrix, two new molecular graph-theoretical vectors and a vector–matrix–vector multiplication procedure, a generalized invariant is introduced. This invariant permits a generalization of some of the classical topological indices (TIs). Thus, Wiener W index, Zagreb M 1 and M 2 indices, Balaban J index, Harary H number, Randic χ index and valence connectivity index χ v resulted as particular cases of an infinite set of molecular descriptors that can be derived from the same invariant.

We present a computer simulation study of the effect of molecular reorientation on the Förster-type energy transfer (ET) process in a nematic and isotropic phase. Monte Carlo (MC) equilibrium configurations and a stepwise diffusion algorithm are employed to model the ET process and the molecular rotational dynamics in the case of arbitrary time scales. We find faster fluorescence depolarization at higher rotational dynamic rates, but the transfer efficiency and directionality observed for fixed molecules is also maintained in the presence of molecular reorientation.

Classical molecular dynamics (MD) is used to study vibrational relaxation and predissociation processes in I2 embedded in a rare-gas environment. It is shown that the simultaneous effect of the two mechanisms determines the deviation of experimental pump–probe signals from a single-exponential decay [Lienau and Zewail, J. Phys. Chem. 100 (1996) 18629].

Influence of the transverse velocity on TC-RFWM spectra of jet-cooled CH by A.A. Villaeys; K.K. Liang; S.H. Lin (268-277).
A saturation dip has been observed recently by A. Kumar et al. [Chem. Phys. Lett. 297 (1998) 300] in degenerate four-wave mixing (DFWM) and two-color resonant four-wave mixing (TC-RFWM) spectra of jet-cooled CH. This observation suggests that expressions of the signal intensity established for double-resonance molecular spectroscopy in the weak field limit are no longer valid. While the line splitting has been explained, this is not the case for the asymmetry observed on this spectrum. Here, we clearly establish the origin of this asymmetry as resulting from the transverse molecular velocity which acts even for a small velocity component. The model conveniently reproduces line splitting and asymmetry.

Ab initio investigations on neutral clusters of ammonia: (NH3) n (n=2–6) by Sudhir A Kulkarni; Rajeev K Pathak (278-283).
Hydrogen-bonded neutral clusters of ammonia, (NH3) n (n=2–6), have been theoretically investigated employing the 6–31++G(d,p) (and wherever necessary, higher) basis sets at the Hartree–Fock (HF) level as well as with second-order Møller–Plesset (MP) perturbation theory. While the ammonia trimer and tetramer exhibit perfect molecular symmetries and are non-polar, the pentamer and the hexamer both optimize with slight deviations from perfect symmetries and are seen to possess marginal, but nonzero dipole moments, in contradiction to a conjecture made in 1979. The (NH3) n `linear' clusters are seen to be chemically softer than the corresponding `cyclic' ones.

We have sketched the broad outlines of a new theory of elementary electron transfer in condensed matter in the light of which the nature of the well-known narrow optical J-band is due to an unusual resonance between the electron and environmental nuclear reorganization motion, not to averaging an environmental statistical disorder by the quickly moving Frenkel exciton as is generally believed.

Polarizable and nonpolarizable potentials for K+ cation in water by Oleg Borodin; Richard L. Bell; Yi Li; Dmitry Bedrov; Grant D. Smith (292-302).
Quantum chemistry based potentials for classical molecular dynamic (MD) simulations have been derived for K+ cation in water. Water was represented using the SPC/E, TIP4P, RPOL potentials. The K+–water potentials were parameterized to reproduce the quantum chemistry binding energy of a K+/water complex. The gas-phase binding energies and enthalpies, the excess enthalpy and free energy of K+ hydration, K+ hydration structure and K+ mobility were determined from molecular mechanics and MD simulations, and compared with experimental data. All potentials were able to accurately predict K+ hydration structure and K+ self-diffusion coefficient at infinite dilution limit.

The spin–orbit induced predissociation dynamics of the A 2Σ+ (v;N) rovibronic states of HCl+ has been investigated by numerical solution of four coupled time-dependent Schrödinger equations based on ab initio data. The rotational dependence of the predissociation lifetime exhibits pronounced peaks with lifetimes up to one order of magnitude larger than that of neighboring levels. The origin and possible applications of these `rotational islands of stability' are discussed.

A computationally simple theoretical method, i.e., self-consistent field (SCF) method to determine the vibrational structure of HeBr2, distorted-wave Born (DWB) approximation for dissociating process, and the infinite-order sudden (IOS) approximation for the continuum dissociating product of Br2 (SCF–DWB–IOS), has been applied to the vibrational predissociation of the HeBr2 (B state) van der Waals (vdW) complex. The lifetimes of transient excited vibrational states, dissociation linewidths, and the rotational distributions of the product, Br2 have been computed. A comparison is made with other accurate theoretical and experimental results and reasonable agreement is found for the vibrational predissociaiton from the low vibrational states of HeBr2. The advantage and disadvantage of the methodology are also discussed.

Evaluation of rate constants for conformational transitions using single-molecule fluorescence spectroscopy by Alexander M Berezhkovskii; Marián Boguñá; George H Weiss (321-324).
We develop formulae for translating single-molecule fluorescence spectroscopic data into estimates of the rate constants for slow conformational transitions between two states with different lifetimes of the fluorescent probe. These rate constants cannot be determined separately from the bulk experiment.

Theoretical study on the mechanism of the reaction: HCCCH 2  ++C 2 H 2→c-C 3 H 3  ++C 2 H 2 by Zheng-wang Qu; Hui Zhu; Ze-sheng Li; Qi-yuan Zhang (325-334).
The gas phase ion–molecule reaction of propargylium (HCCCH 2  +) with acetylene (C2H2) to produce cyclopropenylium (c-C 3 H 3  +) with C2H2 has been investigated theoretically at the B3LYP/6–31G(d) and single-point QCISD/6–311G(d,p) levels. The detailed mechanism for the observed isotope exchange between HCCCH 2  + and C2D2 has also discussed. Three intermediates 1 CH2CCH2CCH+, 2 H 2 C 2·CHCCH 2  + and 3 c-C 4 H 3 –CH 2  + are shown to play important roles in the product formation and isotope exchange processes, rather than the low-lying isomers 4 c-C 3 H 2 –CH 2  + , 7 c-(CH)5  + and 8 pyramidal C 5 H 5  + . Our calculated results agree well with the available experimental data and may be helpful for understanding the mechanism for combustion processes.

Ab initio anharmonic intermolecular potential of the C2H2–HCl hydrogen bonded complex by P Çarçabal; V Brenner; N Halberstadt; P Millié (335-342).
A grid calculation of the intermolecular potential of the hydrogen bonded C2H2–HCl complex has been performed at the CCSD(T)/cc-pVTZ level of theory. The effects of anharmonicity, of coupling between intramolecular and intermolecular motions, and of the basis set superposition error (BSSE) are independently considered. This calculation yields the center of mass separation of the complex, vibrationaly averaged and corrected of BSSE. It also provides the binding energy D e and the anharmonic vibrational frequency shift of the ν HCL band of the complex correlated with the HCl stretching mode. These properties are compared with experimental results and found to be in very good agreement.

The structures and energetics of nine valence isomers of phosphinine, (CH)5P, have been investigated by ab initio (HF, MP2 and CCSD(T)) and hybrid density functional (B3LYP) methods. The relative stability ordering of the (CH)5P isomers is similar to those of (CH)6. Strain energies are evaluated for all the non-planar isomers based on the sum of standard bond strengths, taking the planar resonance stabilized isomer, phosphinine as the reference. Lower magnitudes of the frequencies corresponding to the first few normal modes compared to their benzene isomers account for smoother isomerization reactions among them.

The variation of the magnetic properties of 2,2,5,5,tetrametyhl-4,methyl-imidazoline nitroxide with the pH of the embedding medium is investigated by means of quantum mechanical calculations. The hyperfine coupling constants of the nitroxide are determined by mixed QCISD/PBE0 calculations, while the geometry of the neutral and protonated nitroxide and the relative dissociation constants are calculated at the PBE0 level. Solvent effects are modelled both by including a few explicit solvent molecules (strongly bound to the solute) and by using the CPCM method. The agreement between the calculated and the experimental titration curve is quite good, confirming the reliability of our computational strategy.

Calculation of refractive indices and local electric field tensors in α-sexithiophene crystal by R.W Munn; M Andrzejak; P Petelenz; A Degli Esposti; C Taliani (357-363).
Polarizabilities calculated for sexithiophene using Hartree–Fock and density-functional approaches with a 6-31G basis set are used to calculate refractive indices and local electric field tensors in the α-sexithiophene crystal. The calculations under-estimate the (incomplete) experimental refractive indices but polarizabilities are devised that fit them. These predict that the unknown refractive index perpendicular to the crystal bc plane is larger than those in the plane. The tilted molecular long axis produces sizeable off-diagonal local-field components in the crystal ac plane. This complicates the analysis of electro-absorption spectroscopy.

The search for resonance signatures in H+D2 reaction dynamics by Sheng Der Chao; Rex T Skodje (364-370).
To investigate the recent report of the observation of a reactive resonance in the title reaction, we have calculated the ro-vibrationally state-resolved integral cross-sections (ICS) and differential cross-sections (DCS), as well as the vibrational branching ratios for the H+D2(v=j=0)→HD(v ,j )+D reaction. Oscillatory structure occurring in the state-to-state reaction probabilities does survive partial wave averaging to appear in the integral cross-sections for low j -values, although not significantly for the j =7 state recently reported. The resonance picture interpretation of the oscillations is shown to be problematic.