Chemical Physics Letters (v.325, #5-6)

Role of nickel particles in selected growth of boron carbonitride tubular structures by X.D. Bai; Jie Yu; Shuang Liu; E.G. Wang (485-489).
Using Ni as catalyst, boron carbonitride (BCN) nanotubes/nanofibres were synthesized from a mixture of N2, H2, CH4, and B2H6 by means of plasma-assisted hot filament chemical vapor deposition. The activated Ni catalysts exhibit some preferred crystal orientations at different inlet B2H6 concentrations, such as [011] at 1.0%, [1̄11] at 1.5%, and [1̄13] at 2.0%, respectively. Our results imply that Ni catalytic particles play an important role in the formation of the stable phases of BCN nanotubes/nanofibres with different chemical compositions.

Methane hydrate, amoeba or a sponge made of water molecules by Hisako Hirai; Masashi Hasegawa; Takehiko Yagi; Yoshitaka Yamamoto; Kazushige Nagashima; Mami Sakashita; Katsutosi Aoki; Takumi Kikegawa (490-498).
A new technique fabricating single crystals and polycrystalline aggregates of methane hydrate at room temperature under high pressure was established using a diamond anvil cell. In-situ observations by optical microscopy and X-ray diffractometry revealed high-pressure behavior up to 5.5 GPa, including growth, a compression process associated with changes in cage occupancy, and decomposition into high-pressure ice and solid methane. Interesting features such as an amoeba-like motion at crystallization and sponge-like behavior with pressure changes were observed. Cage occupancy, the so-called hydration number, was estimated from the relative intensity of the X-ray diffraction pattern, and changes in cage occupancies dependent on pressure were clearly observed.

Hydrogenated diamond-like carbon (DLC) films were deposited on conductive glass from acetonitrile by liquid deposition using a pulse-modulated source at room temperature. The structure of the films was characterized by XPS, Raman and IR spectroscopy. The deposits were hydrogenated amorphous DLC films. The films have high transmission ratio (>70%) in the range of 330∼2000 nm with VIS/UV spectroscopy. The resistivity of the films was of the order of 1010 Ω·cm.

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the structure of behenic acid monolayers on the surface of aqueous Cd2+ sub-phase in a Langmuir trough at the room temperature of 293 K. The PM-IRRAS spectra were recorded at representative points throughout 0.401∼0.100 nm2 and the surface-pressure area isotherms were recorded during compression to these points. It is found that compressed cadmium behenate (CdB) monolayers possess an orthorhombic unit cell. On the other hand, single monolayers of CdB deposited on CaF2 substrates at 20 mN m−1 possessed a triclinic parallel packing.

Formation of covalent Si–N linkages on pyrrole functionalized Si(100)-(2×1) by Ming Hua Qiao; Yong Cao; Jing Fa Deng; Guo Qin Xu (508-512).
The covalent attachment of a pyrrolyl monolayer has been efficiently achieved by directly exposing pyrrole molecules to the clean Si(100)-(2×1) surface. The high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS) studies show that the robust pyrrolyl monolayer is directly bonded to the Si(100) surface via the Si–N linkage with the preservation of the pyrrolyl ring structure. The as-prepared pyrrole-modified silicon surfaces can serve as templates for further electrochemical polymerization of polypyrrole thin films onto silicon substrates, providing new opportunity for fabrication of new polypyrrole/silicon heterojunctions with improved performance.

Hyper-Rayleigh light scattering measurements of molecular motion in several liquids find that the orientations of the molecules are correlated over macroscopic distances. Such collective molecular rotation occurs in addition to the usually observed, highly localized, diffusive mode of molecular reorientation. Long range correlation of molecular orientation may have significant unrecognized effects on the structure and properties of liquids.

Second-order Stark effect is analysed in terms of the third-order non-linear susceptibility χ (3)(ω;ω,0,0) within a centrosymmetric one-dimensional three-level model. The Stark effect manifests itself in the electroabsorption spectrum mainly as the first or the second derivative of the unperturbed absorption spectrum that corresponds to either Stark shift or Stark broadening, respectively. Domination of the first or the second derivative in the electroabsorption lineshape depends on the energy gap between the first dipole-allowed state and a nearby excited dipole-forbidden state. The model takes into account inhomogeneous broadening and vibronic sidebands of both excited states.

Electric dipole moments of the cyclic trimers (H2O)2HCl and (H2O)2HBr from Stark effects in their rotational spectra by Z. Kisiel; J. Kosarzewski; B.A. Pietrewicz; L. Pszczółkowski (523-530).
The electric dipole moments of two weakly bound cyclic trimers, (H2O)2HCl and (H2O)2HBr, have been determined from Stark effect measurements in rotational spectra recorded at conditions of supersonic expansion. The experimental results are compared with ab initio and induction calculations and the dipole moments of all three monomers are found to be subject to appreciable enhancement on complexation. The effect on the total dipole moment of each trimer is, however, less marked owing to partial cancellation of the various component moments. A novel electrode system for Stark effect measurements in a cavity Fourier transform microwave spectrometer is also described.

Hydrated electron in subcritical and supercritical water: a pulse radiolysis study by G. Wu; Y. Katsumura; Y. Muroya; X. Li; Y. Terada (531-536).
Temperature dependences of the eaq spectrum and Gε max were investigated by the pulse radiolysis method over a temperature range of 25–400°C including the supercritical condition and the eaq formation in supercritical water (D2O) was confirmed. With increasing temperature, the absorption peak (λ max) of eaq shifts significantly to longer wavelength. The value of Gε max in supercritical water is considerably smaller than in liquid water at room temperature. The behavior of eaq revealed here is helpful for an extensive understanding of water radiolysis and for the study of eaq involving radical reactions in supercritical water.

A new energy transfer chemical laser at 1.315 μm by Thomas L. Henshaw; Gerald C. Manke II; Timothy J. Madden; Michael R. Berman; Gordon D. Hager (537-544).
CW laser action has been demonstrated on the electronic I ( 2 P1/2)  →  I( 2 P3/2) transition of atomic iodine at 1.315 μm from the NCl (a   1 Δ )+I( 2 P 3/2) energy transfer reaction. The stimulated emission was generated in a transverse subsonic flow device when hydrogen azide, HN3, was injected into a flow of iodine and chlorine atoms. The measured laser output power was 180 mW.

The complete expression for the macroscopic second-order susceptibility (SOS) tensor χ (2) for a monolayer with C symmetry at the air–liquid interface is derived as functions of the molecular SOS tensor α (2) and the orientational order parameters. The chiral (non-chiral) terms of χ (2) are distinguished by their association with even (odd) order parameters, by which a second-harmonic generation (SHG) circular-dichroism effect can be better understood. An SHG experiment of a 4-cyano-4′-5-alkyl-biphenyl monolayer on an air–water interface by monolayer compression is also discussed.

Tuning the red emission of a soluble poly(p-phenylene vinylene) upon grafting of porphyrin side groups by J. Morgado; F. Cacialli; R.H. Friend; R. Iqbal; G. Yahioglu; L.R. Milgrom; S.C. Moratti; A.B. Holmes (552-558).
We investigate the energy transfer in donor–acceptor systems with reduced spectral overlap to assess the scope for emission colour tunability and the influence of spectral overlap on the luminescence efficiency. We use copolymers derived from poly [2-methoxy, 5- (2′-ethylhexyloxy)-1,4-phenylene vinylene], randomly attaching tetraphenylporphyrin side-chain units. We find that both photoluminescence and electroluminescence are widely tunable via control of the porphyrin concentration, with relatively efficient energy transfer in spite of limited spectral overlap. We also find that luminescence efficiency is controlled by concentration quenching of the tetraphenylporphyrin, with only a minor effect of spectral overlap. This has implications for electrophosphorescent devices.

Threshold photoelectron spectroscopy of iodine monochloride by Andrew J. Yencha; M.Cristina A. Lopes; George C. King (559-567).
A high-resolution (1–11 meV) threshold photoelectron spectroscopic study of ICl has been performed using synchrotron radiation and a penetrating-field electron spectrometer over the valence ionization region of the molecule. Accurate vibrational constants have been obtained for both spin–orbit components of the ICl+ (X   2 Π i ) state through the observation of extended vibrational bands in the Franck–Condon gap regions of the molecular ion. The appearance of these vibrational bands is attributed to resonant autoionization of Rydberg states residing in this energy region of the ionic state. The adiabatic (v +=0) ionization potentials for formation of the ground state of the ICl+ ion are found to be 10.076±0.002 eV for (X   2 Π 3/2 ) and 10.655±0.002 eV for (X   2 Π 1/2 ) yielding a spin–orbit splitting in this state of 0.579±0.002 eV. The adiabatic (v +=0) ionization potential for formation of the (A   2 Π i ) state of ICl+ is estimated to be at ∼12.5 eV. Additional autoionization effects are noted in the formation of the (A   2 Π i ) and (B   2 Σ + ) band systems in ICl+.

Measurements are reported of infrared-optical double-resonance spectra (3100–3800 cm−1) for jet-cooled perylene/(H2O) n clusters ranging up to n=8. Unusual patterns of OH-stretch resonances, which are particularly evident for the n=3–6 clusters, imply that some significant structural perturbations are taking place in the presence of the large aromatic molecule.

Wavepacket dynamics and predissociation of the D  1Πu state of Rb2 by Bo Zhang; Lars-Erik Berg; Tony Hansson (577-583).
We present the first experimental investigation of the real-time dynamics of predissociation in a diatomic molecule involving more than two electronic states – the predissociation of the D  1Πu state of Rb2. Our results show that the state is strongly predissociated, τ≈5 ps, above a sharp energy threshold. We propose that mainly the (1)   3 Δu state causes the fast predissociation and that the fine-structure components of the products are mixed by coupling among molecular states at large internuclear distances. Furthermore, an outward–inward asymmetry of the wavepacket signal is attributed to autoionisation of the wavepacket evolving in the probe (Rydberg) state.

The Fourier transform infrared spectrum of the ν 12 band of cis-d2 -ethylene (cis-C2H2D2) has been recorded with an unapodized resolution of 0.0024 cm−1 in the frequency range of 1280–1400 cm−1. This band was found to be mutually coupled by Coriolis interaction with the unobserved 2ν 10 band situated approximately 10 cm−1 below ν 12. By fitting a total of 771 infrared transitions of ν 12 with a standard deviation of 0.00075 cm−1 using the Watson's Hamiltonian with the inclusion of a c-type Coriolis resonance term, a set of accurate rovibrational constants for V12=1 state was derived. The ν 12 band is A type with a band centre at 1341.1512±0.0001 cm−1. Accurate rovibrational constants for the V10=2 state were also derived.

Radiative electron transfer in planar donor–acceptor quinoxaline derivatives by Rafał Czerwieniec; Jerzy Herbich; Andrzej Kapturkiewicz; Jacek Nowacki (589-598).
Synthetic and spectral studies have been performed for a family of electron donor–acceptor (D-A) quinoxaline derivatives possessing an aromatic amine as an electron donor. A photophysical behaviour of the compounds with an internal degree of freedom for internal D-A rotation and their rigid analogues with a fixed planar conformation appears to be very similar. Electronic transition dipole moments related to the charge-transfer (CT) absorption and fluorescence are determined by both the direct interactions between the 1 CT and ground states and by the contributions from the locally excited configurations. The radiative properties of the D-A systems under study can be explained in terms of the simple model which assumes that the electronic coupling elements are mainly determined by the interactions between the atoms forming the A-D bond.

Coupling between intramolecular and lattice vibrations in solid para-diiodobenzene by Raffaele Guido Della Valle; Aldo Brillante; Elisabetta Venuti; Luca Palazzi (599-604).
Optical spectra, phonon density of states and constant volume specific heat of the α-phase of para-diiodobenzene are computed by the harmonic lattice dynamics method. The results provide a detailed assignment of the spectra. The role of intramolecular vibrations is found to be essential to reach agreement with the experimental data.

Fluorescence and absorption spectra of 1-dimethylaminonaphthalene (DMAN) in aqueous-organic binary solvents were recorded as a function of the content of organic components (methanol, ethanol, n- and i-propanol, and acetonitrile) for the purpose of understanding the photophysics of DMAN. Whereas the absorption spectra underwent minor change with increasing organic component, the fluorescence spectra experienced a continuous blue-shift in the band position and the fluorescence quantum yield increased reaching a maximum before continuous decrease. It was found that, in a series of aqueous-organic binary solvents, the variations of the quantum yield versus solvent polarity turned at nearly the same solvent polarity equivalent to an E T(30) value of ca. 57 kcal mol−1. It is hence suggested that while the emissive state is intramolecular charge transfer in nature, the radiationless decay channel is solvent-polarity dependent.

Theoretical and experimental research on excimer like (N2)2 dimer: potential energy curves and spectra by Ajmal H Hamdani; Zuochun Shen; Yunhua Dong; Huide Gao; Zuguang Ma (610-618).
Theoretical ab initio calculations for a number of symmetry groups of (N2)2 dimer are performed with an emphasis on the intermolecular distance 1.2⩽R⩽2.5 Å. Except for the D2h group, the ground and excited state potential energy curves of most of the symmetry groups are repulsive in this region. The theoretical results indicated that there exists an excimer like electric dipole transition between the excited singlet states a1B2g and a1B3u of the D2h symmetry group. The oscillator strength and theoretical spectra are calculated. Experimentally observed emission spectra for the excimer-like (N2)2 dimer is found to be in good agreement with our theoretical calculations.

Temperature dependence of solute vibrational relaxation in supercritical fluids: experiment and theory by D.J. Myers; Motoyuki Shigeiwa; C. Stromberg; M.D. Fayer; Binny J. Cherayil (619-626).
Vibrational lifetime (T 1) data for the asymmetric CO stretching mode of W(CO)6 in supercritical ethane and carbon dioxide as a function of temperature at various fixed densities are compared to a recent extended hydrodynamic theory . In ethane at the critical density, as the temperature is raised, T 1 initially becomes longer, reaching a maximum ∼70 K above the critical temperature. T 1 decreases with further increase in temperature. The theory is able to reproduce this behavior nearly quantitatively without free parameters. At high density in ethane and in CO2, the inverted temperature dependence is not observed, in agreement with theoretical calculations.

The Raman spectrum intensity of CCl4 is measured in the temperature range from 0°C to 65.0°C with the optical-fiber A maximum value exists when it alters with a variety of temperatures. Several main factors influencing the Raman spectrum intensity in an optical fiber are calculated. Theorical and experimental results are in agreement.

Metropolis Monte Carlo NVT computer simulations of 50:50 mixtures of L/D=5 hard spherocylinders (N HSC=510) and L/D=0.12 hard cut spheres (N CS=510) have been performed. At the start of the simulation, the system is taken as a totally demixed state of pure rods and pure plates. For a packing fraction of 44% the system is found to be a fully mixed isotropic state, while at 48% two ordered phases (one rich in rods and one rich in plates) are found to coexist. In this Letter we examine the stable free interface between the two liquid crystalline phases paying special attention to the orientational order through the interface.

Theoretical study on reforming of CO2 catalyzed with Be by Der-Yan Hwang; Alexander M. Mebel (639-644).
Ab initio G2(MP2) calculations of the potential energy surface for Be+CO 2  →  BeO+CO show that the reaction proceeds by formation of the cyclic BeOCO intermediate, 6.9 kcal/mol below the reactants, with a barrier of 22.8 kcal/mol. From the cyclic BeOCO structure the reaction continues to produce the linear OBeOC complex, 4.6 kcal/mol above the reactants, with a barrier of 13.0 kcal/mol relative to the reactants. The OBeOC complex decomposes to BeO+CO without an exit barrier. The calculated endothermicity of the Be+CO 2  →  BeO+CO reaction is ca. 26 kcal/mol. with 66 kcal/mol for Mg+CO 2  →  MgO+CO and 125 kcal/mol for unimolecular decomposition of CO2, making beryllium atoms more efficient than magnesium atoms in the reforming of carbon dioxide into carbon monoxide.

For a specific force constant, the spatial form of the ground-state wavefunction Ψ(r 1,r 2) is known analytically for the Hookean atom in which two electrons repel coulombically. Here, it is first shown that the wavefunction Ψ(r,r) for the antiparallel spin electrons at the same position r is sufficient to construct the ground-state density in this model example. An off-diagonal generalization is then effected, which relates the one-particle density matrix to the electron pair correlation function, again for this same model.

Theoretical photoabsorption spectra of Ar n + clusters by Nikos L. Doltsinis; Peter J. Knowles (648-654).
The photoabsorption spectra of selected Ar n + clusters (n=7, 8, 17, 19, 23) have been investigated theoretically using an extended Diatomics-in-Molecules approach including induced dipole – induced dipole and spin-orbit coupling interaction effects. Our calculations at 0 K confirm the experimentally observed spectral red-shift of the visible photoabsorption peak in the region 15<n<20 [Levinger et al., J. Chem. Phys. 89 (1988) 5654]. Furthermore, we have been able to reproduce the additional red-shift measured for 7⩽n⩽9 [Haberland et al., Phys. Rev. Lett. 67 (1991) 3290] by carrying out finite temperature Monte Carlo simulations.

Anomalies in the double layer properties of electrolytes with strong ionic interactions have been observed in recent simulations. As yet these anomalies have not been predicted by any theory. The anomalous temperature dependence of the capacitance has been observed experimentally. In this note the generalized mean spherical approximation (GMSA) is applied to the electrochemical interface. In contrast to other theories, the GMSA predicts, in agreement with simulations, partial drying and an adsorption isotherm that is negative and large in magnitude. The anomalous behavior of the capacitance remains unexplained.

The influence of transiently bound collision on the diffusion coefficient of free radicals was examined using molecular dynamics simulations and the Green–Kubo formula. It was found that transiently bound collisions significantly increase the diffusion coefficients of free radicals at temperatures relevant to combustion. The present study suggests that a molecular theory beyond the Chapman–Enskog equation is needed to evaluate the diffusion coefficients of free radicals in laminar flame and other high-temperature reacting flow simulations.

NMR chemical shifts in solution: a RISM-SCF approach by Takeshi Yamazaki; Hirofumi Sato; Fumio Hirata (668-674).
The NMR chemical shift induced by solvation is formulated based on the ab initio electronic structure theory coupled with the integral equation method of molecular liquids. In order to examine the validity of the theory, the chemical shift of the atoms in a water molecule in water is calculated. The preliminary result with respect to hydrogen gives a reasonable account for the solvation shift, and for its temperature and density dependence.

A corrected 3D Ewald calculation of the low effective temperature properties of the electrochemical interface by Paul S. Crozier; Richard L. Rowley; Douglas Henderson; Dezsö Boda (675-677).
The corrected 3D Ewald method is used to verify charged sheets method results that show increasing double-layer capacitance with increasing temperature in the low effective temperature region. The restricted primitive model is used where ions are represented as charged hard spheres and the solvent is represented by a uniform dielectric constant. It is shown that the capacitance temperature plot for the test system exhibits increasing capacitance with increasing temperature in the low effective temperature region, which contradicts common theories of the electrochemical interface. For this system, the corrected 3D Ewald method results coincide well with the charged sheets method results.

The spin–orbit splittings A SO  (R) of the A  2Π state of LiRg (Rg=Ar, Kr and Xe) complexes have been obtained from the configuration interaction calculations using relativistic effective core potentials and two-component spinors. The results confirm that the mixing between the p orbitals of the lithium atom and the valence np orbitals (and not the Rydberg orbitals) of the rare gas (Rg) atom produces a large A SO  (R) for the A  2Π states at short internuclear distances. The spectroscopic constants for the ground and A states of LiRg complexes are also reported.

Structures and isomerization reactions of the CH2CHCOOH isomers in S0, T1 and S1 states have been studied with ab initio molecular orbital methods. The obtained results support a mechanistic model for the photochemical rearrangement of acrylic acid based on a non-radiative decay route. The CH2CHCOOH molecules are populated in the S1 state by photo-excitation at 248 nm. Then the T1 intermediate is formed via the S1/T1 intersystem crossing. A significant fraction of the CH2CHCOOH molecules in T1 returns to the ground state by a decay through the T1/S0 crossing point. Vibrationally hot ground-state isomers are generated, which can undergo unimolecular decompositions.

The role of bridged structures in the mechanism of the reaction between chlorine atom and ethylene by P. Braña; B. Menéndez; T. Fernández; J.A. Sordo (693-697).
The potential energy surface for the reaction between a chlorine atom and ethylene was extensively explored by using ab initio methodologies. Two different routes for the 1,2 migration of the chlorine atom were identified. One of them involves a C2v ( 2 B2) transition structure (TSsb) that directly connects two equivalent structures (P and P′) of the 2-chloroethyl radical with the chlorine atom attached either to C1 (P) or to C2 (P′) carbon atoms in ethylene (shuttling motion). In the second pathway, the 2-chloroethyl radical (P) coverts into a C2v ( 2 A1) intermediate (Iadd) through a Cs ( 2 A′) transition structure (TSadd ). Then Iadd leads to the 2-chloroethyl radical (P′) through a transition structure equivalent to TSadd (TSadd′ ). The `indirect shuttling motion' described along this latter route is notably lower in energy and allows one to rationalize some mechanistic aspects experimentally observed in reactions involving haloethyl radicals.

It has been suggested by Dunsyuryun, Karpov and Morozov that two different terms should be used to describe the solvation of halide ions in aqueous solutions. The term co-ordination number gives the primary `solvation number' (ca. 6), whilst the term hydration number gives the number of water molecules that stay co-ordinated to the anions as they move through the liquid (ca. 2). Here it is suggested that since these two terms are widely used to mean the same thing, it is better not to change one of them. It is also suggested that the number of water molecules that move with ions is variable and ill defined and that it is not appropriate to specify a precise number for this.

We do not think that we have misled readers by our use of the `hydration number' N(h) and the `coordination number' N(c). Our definitions coincide with the conventional ones. We agree with Symons that the number of water molecules in the ion cluster, moving in solution, fluctuates.

Index (703-714).