bluuebunny/crossref_metadata_2025 huggingface dataset:
Interactions and kinetic arrest in an adhesive hard-sphere colloidal system
Authors: Pontoni D. , Finet S. , Narayanan T. , Rennie A. R.
Published: September 2003

The evolution of microstructure and dynamics of a colloidal suspension transforming from hard-sphere to sticky hard-sphere system is investigated by small-angle x-ray scattering techniques. The colloidal system comprised of sterically stabilized silica particles suspended in a marginal solvent. The repulsive to attractive transition was realized by varying the temperature. While the particle form factor showed few changes, the structure factor of interparticle interactions exhibited liquidlike features in the attractive phase. The measured structure factors up to a gelation transition can be adequately described by the square-well model of short-ranged attractive fluids. The particle dynamics showed a continuous change from single to stretched exponential decay as the system transformed from repulsive to attractive behavior. A complete jamming of the particle dynamics was observed when the depth of attractive well attained several kBT. Although, static and dynamic behavior are reversible with respect to temperature, considerable hysteresis was noticed. The observed behavior is consistent with the existence of a gelation/attractive glass transition induced by the strong short-ranged attractive interaction in the colloidal gas–liquid coexistence region.
Structural and dynamical properties of Co(III) in aqueous solution: Ab initio quantum mechanical/molecular mechanical molecular dynamics simulation
Authors: Kritayakornupong Chinapong , Plankensteiner Kristof , Rode Bernd M.
Published: September 2003

A combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation has been performed to evaluate the structural and dynamical properties of Co(III) in water. The average Co(III)–O distance resulting from the simulation is 1.97 Å for the first hydration shell, and the coordination numbers are 6/15 for first/second hydration shell. Ligand exchanges between the second hydration shell and the bulk occur with a mean ligand residence time of ∼55 ps, estimated from three events observed during the simulation. The hydration enthalpy of −1144±8 kcal/mol, determined by the QM/MM molecular dynamics simulation, is in good agreement with experimental estimations.
A time correlation function theory for the fifth order Raman response function with applications to liquid CS2
Authors: DeVane Russell , Ridley Christina , Space Brian , Keyes T.
Published: September 2003

A new theory for the fifth order Raman response function, R(5)(t1,t2), is presented. Using this result, R(5)(t1,t2) is shown to have a classical limit given by a combination of time derivatives of the real and imaginary parts of a two time correlation function (TCF) of the polarizability. In contrast with one time correlation functions, no exact analytic relationship exists between the real and imaginary parts of the quantum mechanical TCF that would allow the classical limit to be written in terms of classical TCF’s. Writing the nonlinear response function in terms of classical TCF’s would allow R(5)(t1,t2) to be calculated with minimal computational effort, in contrast to existing (exact) classical formulations. However, a simple approximate relationship is shown to exist between the real and imaginary parts of the two time TCF for a harmonic system with nonlinear polarizability. In the spirit of quantum correction, this relationship is used to write the exact classical response function in terms of classical TCF’s. The resulting TCF expression is then calculated from (fully anharmonic) molecular dynamics calculations supplemented by a suitable spectroscopic (polarizability) model. The approximate expression is demonstrated to have correct limiting behaviors and leads to a two-dimensional spectrum for ambient carbon disulfide in excellent agreement with existing experimental and theoretical work. The proposed approach makes the calculation of fifth order response functions practical for a wide variety of chemically interesting systems.
Density fluctuations in the intermediate glass-former glycerol: A Brillouin light scattering study
Authors: Comez Lucia , Fioretto Daniele , Scarponi Filippo , Monaco Giulio
Published: September 2003

Brillouin scattering has been used to measure the dynamic structure factor of glycerol as a function of temperature from the high temperature liquid to the glassy state. Our investigation aims at understanding the number and the nature of the relaxation processes active in this prototype glass forming system in the high frequency region. The associated character of glycerol is reflected by a rather simple relaxation pattern, while the contributions coming from intramolecular channels are negligible in the GHz frequency region. The temperature behavior of the characteristic frequency and lifetime of the longitudinal acoustic modes is analyzed, suggesting that a phenomenological model which only includes the structural (α) process and the unrelaxed viscosity is able to catch the leading contributions to the dynamics of the density fluctuations. This ansatz is also supported by a combined analysis of light and inelastic x-ray scattering spectra. The temperature dependence of the characteristic time of the α-process, τα, obtained by a full-spectrum analysis conforms to the α-scale universality, i.e., the values τα revealed by different experimental techniques are proportional to the others. The nonerogodicity parameter smoothly decreases on increasing the temperature, and no signature of the cusplike behavior predicted by the idealized mode coupling theory and observed in other glass-formers is found in glycerol.
Effect of polarizability of halide anions on the ionic solvation in water clusters
Authors: Yoo S. , Lei Y. A. , Zeng X. C.
Published: September 2003

Molecular dynamics simulation has been performed to study the effect of the polarizabilities of model anions on the ionic solvation in water clusters. The primary focus is given to the surface versus interior solvation behavior of the anions. To this end, various combinations of polarizable/nonpolarizable water and anion models were considered. Using the nonpolarizable TIP4P water with polarizable Cl− and Br− models, the Cl− is fully solvated inside the (H2O)60 cluster, whereas the Br− is partially solvated at the surface of the cluster. However, when the polarizability of the Br− is turned off, the “Br−” anion is fully solvated. Using the polarizable Dang–Chang water, both Cl− and Br− reside at the surface of (H2O)60 as well as (H2O)500 clusters, consistent with the finding of Stuart and Berne [J. Phys. Chem. 100, 11934 (1996)] based on the polarizable TIP4P-FQ water with the polarizable Drude halide model. When the polarizabilities of the halide anions are turned off, the smaller size “Cl−” anion is fully solvated in the interior of the Dang–Chang water cluster, whereas the larger “Br−” anion is still partially solvated at the surface of the cluster, indicating the importance of the anion-size effect. We have also calculated the free energy change for the Cl− moving from the center of a lamella water slab to the surface. The free-energy change is on the order of 1 kcal/mol, indicating that the Cl− can easily access the surface region of the Dang–Chang water slab.
Combinatorial fabrication and study of doped-layer-thickness-dependent color evolution in bright small molecular organic light-emitting devices
Authors: Cheon K. O. , Shinar J.
Published: September 2003

The behavior of [N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD)]/[5±0.6 wt % [(2-methyl-6-{2-[2,3,6,7-tetrahydro-1H,5H-benzo(i,j)quinolizin-9-yl] ethenyl}-4H-pyran-4-ylidene)propane-dinitrile (DCM2)-doped α-NPD]/[4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl] organic light-emitting devices is described. The color of the devices shifts from blue to red as the thickness of the doped layer increases from 0 to 35 Å. The 2-Å-thick doped layer device exhibited the highest brightness L∼120 Cd/m2 and external quantum efficiency ηext∼4.4% at a current density of 1 mA/cm2.
Study of the interaction potential between He and a self-assembled monolayer of decanethiol
Authors: Bracco Gianangelo , Scoles Giacinto
Published: September 2003

Self-assembled monolayers of decanethiol on Au(111) have been investigated by means of low energy He beam scattering. In particular the specular reflectivity has been measured and features associated with resonances due to bound states of He atom interacting with the CH3 groups of the standing-up organic molecules have been observed. The analysis of the data has allowed the determination of four bound state levels and the He–surface interaction potential averaged over the unit cell has been estimated.
Interaction of H, O2, and H2O with 3C-SiC surfaces
Authors: Amy Fabrice , Chabal Yves J.
Published: September 2003

Infrared absorption spectroscopy studies of Si-dimer-terminated 3C-SiC(100) 3×2 and c4×2 surfaces reveal marked differences between their atomic scale reactivity with H, O2, and H2O and that of Si(100) surfaces. While atomic hydrogen is well known to passivate dangling bonds on all Si surfaces, H exposure on both 3C-SiC(100) 3×2 and c(4×2) surfaces induces a metallic state instead. Furthermore, hydrogenated 3C-SiC(100) 3×2 surfaces exposed to O2 clearly show that oxygen atoms are inserted below the top surface without any loss in the H coverage at room temperature. Finally, while H2O decomposes into H and OH on both Si(100) 2×1 and 3C-SiC(100) 3×2, subsequent atomic H exposure induces a metallic state on the latter only, creating a thin oxide strip on the top surface separated by a metallic trough.
The catalytic role of water in CO oxidation
Authors: Gong Xue-Qing , Hu P. , Raval R.
Published: September 2003

Water, one of the most popular species in our planet, can play a catalytic role in many reactions, including reactions in heterogeneous catalysis. In a recent experimental work, Bergeld, Kasemo, and Chakarov demonstrated that water is able to promote CO oxidation under low temperatures (∼200 K). In this study, we choose CO oxidation on Pt(111) in the presence of water as a model system to address the catalytic role of water for surface reactions in general using density functional theory. Many elementary steps possibly involved in the CO oxidation on Pt(111) at low temperatures have been investigated. We find the following. First, in the presence of water, the CO oxidation barrier is reduced to 0.33 eV (without water the barrier is 0.80 eV). This barrier reduction is mainly due to the H-bonding between the H in the H2O and the O at the transition state (TS), which stabilizes the TS. Second, CO can readily react with OH with a barrier of 0.44 eV, while COOH dissociation to produce CO2 is not easy (the barrier is 1.02 eV). Third, in the H2O+OH mixed phase, CO can be easily converted into CO2. It occurs through two steps: CO reacts with OH, forming COOH; and COOH transfers the H to a nearby H2O and, at the same time, an H in the H2O transfers to a OH, leading to CO2 formation. The reaction barrier of this process is 0.60 eV under CO coverage of 1/6 ML and 0.33 eV under CO coverage of 1/3 ML. The mechanism of CO oxidation at low temperatures is discussed. On the basis of our calculations, we propose that the water promotion effect can in general be divided into two classes: (i) By H-bonding between the H of H2O and an electron negative species such as the O in the reaction of CO+O+H2O→CO2+H2O, H2O can stabilize the TS of the reaction and hence reduce the barrier. (ii) H2O first dissociates into H and OH and then OH or H participates directly in the reaction to induce new reaction mechanism with more favorable routes, in which OH or H can act as an intermediate.
Insight into association reactions on metal surfaces: Density-functional theory studies of hydrogenation reactions on Rh(111)
Authors: Liu Zhi-Pan , Hu P. , Lee Ming-Hsien
Published: September 2003

Hydrogenation reaction, as one of the simplest association reactions on surfaces, is of great importance both scientifically and technologically. They are essential steps in many industrial processes in heterogeneous catalysis, such as ammonia synthesis (N2+3H2→2NH3). Many issues in hydrogenation reactions remain largely elusive. In this work, the NHx (x=0,1,2) hydrogenation reactions (N+H→NH, NH+H→NH2 and NH2+H→NH3) on Rh(111) are used as a model system to study the hydrogenation reactions on metal surfaces in general using density-functional theory. In addition, C and O hydrogenation (C+H→CH and O+H→OH) and several oxygenation reactions, i.e., C+O, N+O, O+O reactions, are also calculated in order to provide a further understanding of the barrier of association reactions. The reaction pathways and the barriers of all these reactions are determined and reported. For the C, N, NH, and O hydrogenation reactions, it is found that there is a linear relationship between the barrier and the valency of R (R=C, N, NH, and O). Detailed analyses are carried out to rationalize the barriers of the reactions, which shows that: (i) The interaction energy between two reactants in the transition state plays an important role in determining the trend in the barriers; (ii) there are two major components in the interaction energy: The bonding competition and the direct Pauli repulsion; and (iii) the Pauli repulsion effect is responsible for the linear valency-barrier trend in the C, N, NH, and O hydrogenation reactions. For the NH2+H reaction, which is different from other hydrogenation reactions studied, the energy cost of the NH2 activation from the IS to the TS is the main part of the barrier. The potential energy surface of the NH2 on metal surfaces is thus crucial to the barrier of NH2+H reaction. Three important factors that can affect the barrier of association reactions are generalized: (i) The bonding competition effect; (ii) the local charge densities of the reactants along the reaction direction; and (iii) the potential energy surface of the reactants on the surface. The lowest energy pathway for a surface association reaction should correspond to the one with the best compromise of these three factors.
Hydrogen bonding between water and methanol studied by temperature-programmed time-of-flight secondary ion mass spectrometry
Authors: Souda R. , Kawanowa H. , Kondo M. , Gotoh Y.
Published: September 2003

The interactions between condensed molecules at cryogenic temperatures (15–200 K) have been investigated on the basis of secondary ion mass spectrometry. It is demonstrated that the protonated molecular ions, emitted via the proton transfer reactions, provide us unique information about the reorganization of hydrogen-bonded molecules. From the CH3OH molecules adsorbed on the D2O–ice surface, the D+(CH3OH) ions are sputtered predominantly in the temperature range between 100 and 150 K since most of the CH3OH molecules are bound to the D2O layer via hydrogen bonds. A rapid and almost complete H/D exchange, yielding the D+(CH3OD) species, occurs above 150 K due to the enhanced mobility of the surface D2O molecules. Up to the desorption temperature of 180 K, a considerable amount of methanol exists on the surface without mixing with the heavy-water layer due to hydrophobicity of the methyl group. On the methanol–ice surface, the adsorbed D2O molecules form hydrogen bonds preferentially with the CH3OH molecules and tend to be incorporated in the thin-layer bulk of methanol above 120 K.
Strong optical anisotropies of F16CuPc thin films studied by spectroscopic ellipsometry
Authors: Alonso M. I. , Garriga M. , Ossó J. O. , Schreiber F. , Barrena E. , Dosch H.
Published: September 2003

We have used spectroscopic ellipsometry to measure the optical functions of F16CuPc (copper-hexadecafluoro-phthalocyanine, C32F16N8Cu) films grown by organic molecular beam deposition simultaneously onto MgO(001), SiO2/Si, and A-plane (112̄0) sapphire substrates. The latter allows one to produce, under suitable conditions, highly ordered films with interesting anisotropic optical properties. These films are shown to be oriented with two principal axes of the dielectric tensor on the film surface, with one of them almost aligned with the c axis of the substrate. The main spectral features, corresponding to the strong Q- and B-band absorptions of phthalocyanines, are polarized along the other in-plane axis which is perpendicular to c. The Q band contains a particularly sharp excitonic peak near 1.55 eV whose intensity variations point to different molecular stackings in the films.
Internal segregation and side chain ordering in hairy-rod polypeptide monolayers at the gas/water interface: An x-ray scattering study
Authors: Fukuto Masafumi , Heilmann Ralf K. , Pershan Peter S. , Yu Seungju M. , Soto Carissa M. , Tirrell David A.
Published: September 2003

We report studies of the structure and packing of Langmuir monolayers (LMs) of polypeptide poly(γ-4-(n-hexadecyloxy)benzyl α,L-glutamate) (C16–O–PBLG) on the surface of water. The molecule is a “hairy rod” and consists of side attachments of hexadecyloxy chains (–O–C16) to the rigid rod-like core made up of α-helical poly(γ-benzyl L-glutamate) (PBLG). Measurements include surface pressure (Π) versus area/monomer (A) isotherms, x-ray specular reflectivity (XR), and grazing incidence diffraction (GID). In contrast to the LM of bare PBLG on water, which undergoes a monolayer/bilayer transition with increasing Π, monolayers of C16–O–PBLG remain stable up to the highest densities. On the basis of XR and GID results, the structure of the C16–O–PBLG monolayer is characterized by the following main features. First, hydrophobicity causes the –O–C16 chains to segregate towards the film/gas interface and away from water and the PBLG cores, which sit parallel to and near the water/film interface. Since the attachment position of some of the side chains is at the core/water interface, the segregation forces these chains into the space between neighboring core rods. Compression associated with increasing Π thickens the film but the internally segregated structure is maintained for all Π (i.e., >∼30 dyne/cm). Second, the C16–O–PBLG rods form domains in which the rods are aligned parallel to each other and to the interface. The correlation length for the interhelix positional order of the rods is short and typically comparable to or less than the length of the rods. With increasing Π the spacing d between nearest-neighbor rods decreases linearly with A at high Π, indicating a direct correspondence between the macroscopic compressibility and the microscopic interhelix compressibility. Third, as Π increases past ∼5 dyne/cm, the local packing of tethered –O–C16 chains displays the same herringbone (HB) order that is common for high-density bulk and monolayer phases of alkyl chains. Various features of the observed GID peaks also imply that the HB order of –O–C16 chains is oriented with respect to the helical axes of aligned PBLG cores. We propose that the HB order is established initially by one-dimensionally confined chains between aligned rods at low Π and grows laterally with compression.
The reduction of NO on Pt(100) by H2 and CO studied with synchrotron x-ray photoelectron spectroscopy
Authors: Rienks E. D. L. , Bakker J. W. , Baraldi A. , Carabineiro S. A. C. , Lizzit S. , Weststrate C. J. , Nieuwenhuys B. E.
Published: September 2003

Using synchrotron radiation x-ray photoelectron spectroscopy we have determined the surface composition during the reduction reactions of nitric oxide with hydrogen and carbon monoxide on Pt(100). It is found that NO dissociates readily on this surface and that the dissociation product oxygen is only partially removed by reducing agents H2 and CO. Two oxygen species have been identified after NO dissociation, and they appear to react with H2 and CO at different rates. With increasing total pressures in the NO–H2 reaction, new N-containing surface species are observed. Some preliminary results are presented concerning the reduction of NO in the presence of both CO and H2. For this system partially hydrogenated CO molecules are found in addition to partially hydrogenated N-species.
Response of the bicontinuous cubic D phase in amphiphilic systems to compression or expansion
Authors: Babin V. , Ciach A.
Published: September 2003

Effects of confinement on the cubic bicontinuous phase with double-diamond symmetry (D phase) are studied in the lattice, Ciach–Hoye–Stell model of oil–water–surfactant mixtures within mean-field approximation. We focus on the system confined between two parallel hydrophilic walls, which are perpendicular to the diagonal of the cubic cell. The solvation force decays very slowly, and oscillates with the period λ/∛, where λ is the edge length of the conventional cubic unit cell. The mechanical properties depend on the spectrum of the bulk metastable phases, on the difference between surfactant concentrations in the coexisting D and lamellar phases, Δρs, and on the thermodynamic state. The smaller Δρs and the closer to the transition to the lamellar phase, the stronger the deviations from the elastic response to compression or expansion. The inelastic behavior is associated with formation of lamellar wetting films at the surfaces, and the stress is accommodated in the near-surface regions, whereas the D phase in the central part of the slit is only weakly deformed. The analog of capillary condensation of the lamellar phase does not obey the Kelvin equation, due to periodic nature and long range of the solvation force in both phases. When the wall separation L is decreased for a fixed thermodynamic state, a sequence of transitions between the D and lamellar phases occurs for L spanning decades of λ.
A quasiclassical trajectory study of the reaction OH+CO→H+CO2
Authors: Lakin Matthew J. , Troya Diego , Schatz George C. , Harding Lawrence B.
Published: September 2003

A new, full-dimensional potential energy surface has been constructed to describe the OH+CO↔H+CO2 reactive system. The new surface modifies the existing many body expansion potential of Bradley and Schatz based on recent ab initio calculations and incorporates an entirely new hybrid surface to accurately describe the OH+CO entrance channel and two possible van der Waals complexes, OH–CO and OH–OC. Quasiclassical trajectory calculations have been performed for the reaction OH+CO→H+CO2 using the new surface in order to examine the impact of the changes in the surface, to evaluate the accuracy of the surface by comparing to experimental results, and to investigate the reaction dynamics of this interesting complex-forming system. It is shown that the improvement in the description of the entrance channel has a rather large effect on overall reactivity and response to reagent rotational and vibrational excitation, but has little effect on various product properties such as angular and translational energy distributions, which still compare well to experiment. Also, although the reaction forms the intermediate complex HOCO, it is shown that energy is not completely equilibrated among all internal modes and, as a result, there is a strong correlation between reagent and product excitation.
Raman spectra of P4 at low temperatures
Authors: Östmark Henric , Wallin Sara , Hore Nathan , Launila Olli
Published: September 2003

Raman spectra of solid P4 have been recorded from 12 K up to room temperature using Fourier transform-Raman spectroscopy. The three Raman-active modes corresponding to Td symmetry have been resolved, and line shifts depending on temperature and matrix environment have been measured. Two phase transitions have been observed at T≈80 K (irreversible) and T≈195 K (reversible) corresponding to the γ→β and β↔α transitions, respectively. In the β phase, the ν2 mode is split into two lines, νsplit≈7.7 cm−1. The splitting of the ν2 mode is an indication of a breaking of Td symmetry for the β phase. A matrix shift of ≈3 cm−1 for P4 in a N2 matrix (1:5) was measured. Comparing experimental transitions of pure P4 with literature values on matrix isolated P4 in N2 (1:1000) we can conclude that there is a matrix shift in nitrogen of between 6 and 9 cm−1 depending on vibrational mode. The line positions found for pure P4 in the γ phase at 12 K are ν1=599.8 cm−1, ν2=361.6 cm−1, and ν3=459.0 cm−1. The corresponding values in the α phase at 220 K are ν1=598.8 cm−1, ν2=360.6 cm−1, and ν3=458.0 cm−1. The present work also adds valuable insight into the behavior of other potential molecules with similar Td structure such as N4(Td).
He
*
(2
3
S)
Penning ionization of H2S. I. Theoretical Franck–Condon factors for the H2S(X̃ 1A1,v′=0)→H2S+(X̃ 2B1,Ã 2A1) ionization and H2S+(Ã–X̃) transition
Authors: Tokue Ikuo , Yamasaki Katsuyoshi , Nanbu Shinkoh
Published: September 2003

In order to elucidate the ionization dynamics, in particular the vibrational distribution, of H2S+(Ã) produced by the Penning ionization of H2S with He*(2 3S) atoms, the Franck–Condon factors (FCFs) were presented for the H2S(X̃)→H2S+(X̃,Ã) ionization and the H2S+(Ã–X̃) transition, and Einstein’s A coefficients were presented for the latter transition. The FCFs were obtained by quantum vibrational calculations using the global potential energy surfaces (PESs) of H2S(X̃ 1A1) and H2S+(X̃ 2B1,Ã 2A1,B̃ 2B2) electronic states. The global PESs were determined by the multireference configuration interaction calculations with the Davidson correction and the interpolant moving least squares method combined with the Shepard interpolation. The obtained FCFs exhibit that the H2S+(X̃) state primarily populates the vibrational ground state since its equilibrium geometry is almost equal to that of H2S(X̃), while the bending mode (ν2) is strongly enhanced for the H2S+(Ã) state; the maximum in the population is around ν2=6–7. In the same manner, the bending progressions are expected to consist of the great part of the H2S+(Ã–X̃) emission. A detailed comparison with the experimental study for this system is reported in the accompanying paper, Paper II.
He
*
(2
3
S)
penning ionization of H2S. II. Formation of the SH+(A 3Π) and H2S+(Ã 2A1) ions
Authors: Tokue Ikuo , Yamasaki Katsuyoshi , Nanbu Shinkoh
Published: September 2003

Emissions in the 200–750 nm region produced by the collision of He*(2 3S) with H2S were studied under single-collision conditions. The hydrogen Balmer lines and the SH+(A 3Π–X 3Σ−) and H2S+(Ã 2A1–X̃ 2B1) bands were assigned. The total emission cross section (σem) was evaluated to be (1.7±0.3)×10−20 m2 at a collision energy of 150 meV. The σems of the SH+(A–X) and H2S+(Ã–X̃) bands decreased with increase in the collision energy in the 115–200 meV range, indicating that attractive forces are effective for the incident channels with regard to the formation of these species. The rotational distribution of SH+(A 3Π,ν′=0) is represented by a Boltzmann temperature of 870±80 K. The H2S+(Ã 2A1–X̃ 2B1) emission, which was assigned for the first time in the Penning ionization of H2S, primarily consists of the bending progressions. The internal populations of H2S+(Ã) were analyzed using the vibrational energies and Einstein’s A coefficients calculated in this study. The details of the calculation and derived spectroscopic constants are reported in the accompanying paper, Paper I. The populations obtained for the bending vibration (ν2′) of H2S+(Ã) show an inverted distribution with a peak at ν2′=3. This distribution is shifted lower compared that with a peak at ν2′=4–5 observed by He*(2 3S) Penning ionization electron spectroscopy and that with a peak at ν2′=6–7 predicted by the theoretical Franck–Condon factors for the H2S(X̃)–H2S+(Ã) ionization. The origin of the difference is discussed concerning the formation mechanism of H2S+(Ã 2A1).
Exact quantum mechanical calculations of rovibrational energy levels of hydrogen peroxide (HOOH)
Authors: Lin Shi Ying , Guo Hua
Published: September 2003

Full-dimensional quantum calculations of rovibrational energy levels of the HOOH molecule have been carried out using the Lanczos algorithm and a potential energy surface based on high quality ab initio calculations. The calculated results for J⩽4 show excellent agreement with available experimental data. The molecule behaves like a rigid asymmetric top for low-lying vibrational states with n4⩽3. However, rotational energy levels of higher torsional states deviate significantly from the rigid-rotor model, indicating strong coupling between the torsional and rotational degrees of freedom. Such rotational level patterns should be observable and will provide valuable insight into intramolecular hindered rotation.
Kinetics of boiling in binary liquid–gas solutions: Comparison of different approaches
Authors: Schmelzer Jürn W. P. , Baidakov Vladimir G. , Boltachev Grey Sh.
Published: September 2003

A comparative analysis of the results of determination of the work of critical cluster formation in nucleation theory for three different methods of evaluation—Gibbs’ method (employing the capillarity approximation), the van der Waals–Cahn and Hilliard and a newly developed modified Gibbs’ approach—is given in application to phase formation in multicomponent systems. As an example, processes of boiling in binary liquid–gas solutions, in particular, in nitrogen–helium mixtures are investigated. In addition to the work of critical cluster formation, the bulk properties of the critical bubbles, their characteristic sizes and the values of the surface tension are determined in dependence on the initial supersaturation in the system or, equivalently, on the size of the critical clusters. It is shown that latter two mentioned methods (the van der Waals–Cahn and Hilliard and the modified Gibbs’ approach) lead, in the determination of the work of critical cluster formation, to qualitatively and widely even quantitatively equivalent results. As one of the more general consequences from the present analysis, it has been proven that the modified Gibbs’ approach represents a highly effective tool for the determination of the work of formation of clusters or bubbles of critical sizes not only for one-component and quasibinary systems, discussed earlier, but for phase formation in multicomponent systems of, in general, arbitrary numbers of components as well. It is shown that the modified Gibbs’ approach is preferable as compared with Gibbs’ original treatment not only due to its advantages with respect to an appropriate determination of the properties of clusters of critical sizes, but also from general theoretical considerations. In the limit of large sizes of the critical clusters, both approaches—Gibbs’ original treatment and the modified or generalized Gibbs’ approach—lead to equivalent results.
Structure and properties of the weakly bound cyclic trimer (H2O)2HBr observed by rotational spectroscopy
Authors: Kisiel Z. , Pietrewicz B. A. , Desyatnyk O. , Pszczółkowski L. , Struniewicz I. , Sadlej J.
Published: September 2003

The weakly bound cyclic trimer (H2O)2HBr was observed in supersonic expansion and its rotational spectrum was measured in the region 3.6–17.7 GHz by cavity Fourier-transform microwave spectroscopy. Rotational, centrifugal distortion constants, and inertial and principal hyperfine splitting constants were determined for seven isotopomers of (H2O)2HBr. In addition to the large bromine hyperfine splitting each rotational transition exhibits a fine vibrational splitting into four components, at relative intensities consistent with expectations from the G8 group classification of the vibration–rotation–tunneling motions in the trimer. The associated four low-lying states are either very close together or well below the inversion barriers, since the differences between their rotational constants are all below 0.02%. The experimental moments of inertia were used to determine rs, r0, rz, and rm(1L) geometries, in all of which the heavy atom distances are considerably shorter than similar distances in H2O–HBr and (H2O)2. An improved analysis of the measured electric dipole moment of (H2O)2HBr is reported, and all experimental results are confronted with predictions from ab initio calculations.
Hydrogen fluoride phase behavior and molecular structure: Ab initio derived potential models
Authors: Wierzchowski Scott J. , Kofke David A.
Published: September 2003

Several variations of ab initio based molecular models for hydrogen fluoride (HF) are examined by Monte Carlo molecular simulation to determine their bulk-phase properties. The models are taken from the literature, and represent fits of functional forms to the potential energy surface of the HF dimer as given by ab initio computational chemistry calculations. For one of these models, we examine three variations for bulk-phase modeling. In particular, we consider first the effect of including versus neglecting an Ewald sum for the long-range dipole–dipole interactions; second, we examine a modification of the form for the short range repulsive region of the potential; and third, we add three-body contributions to the energy via an available 12-dimensional potential for the trimer, again representing a fit to ab initio energy calculations. The simulations examine the density (via isothermal–isobaric simulation) and radial distribution function (via canonical–ensemble simulations) each at two state points where corresponding experimental data are available. We also examine vapor–liquid coexistence properties, considering the saturation densities, heat of vaporization, and vapor pressure from 225 K to states approaching (but not closely) each model’s critical point. Inclusion of the three-body energy is the only variation that has any beneficial effect on the radial distribution function as compared to experiment, and this variation also gives good results for the vapor pressure, and significantly raises the critical point toward the experimental value. However this model also grossly overestimates the liquid-phase coexistence density. In almost all regards none of the models or variations can be considered to give a satisfactory representation of the bulk-phase behavior. Improvements to the models require more careful attention to the balance between repulsive and attractive pair interactions at short range.
Simulation study of the order formation dynamics in the melt crystallization of flexible chain molecules induced by rigid molecular nuclei
Authors: Miura Toshiaki , Kishi Ryoichi , Mikami Masuhiro
Published: September 2003

Order formations in the crystallization processes of flexible chain molecules with regard to the nucleation effect of rigid crystalline molecules have been studied by means of molecular dynamics simulations. When the short polymer melts consist of complete flexible chains, no orientation order was observed under cooling. However, additives of rigid molecules induce the crystallization in the entire system. The time evolutions of the local and global orientation orders were examined at different concentrations of the rigid molecular nuclei. The local orientation order of flexible chains emerges sharply at a certain concentration threshold and it becomes nearly constant in the high concentration region. In the meantime, the global orientation order takes maximum values near this concentration threshold and it gradually decreases with the increase of concentration of rigid molecular nuclei. The comparison of the scaled time evolution curves reveals that the ordering dynamics during the crystallization are strongly influenced by the concentration of rigid molecular nucleating agents.
Structure and dynamics of hexafluoroisopropanol-water mixtures by x-ray diffraction, small-angle neutron scattering, NMR spectroscopy, and mass spectrometry
Authors: Yoshida Koji , Yamaguchi Toshio , Adachi Tomohiro , Otomo Toshiya , Matsuo Daisuke , Takamuku Toshiyuki , Nishi Nobuyuki
Published: September 2003

The structure and dynamic properties of aqueous mixtures of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) have been investigated over the whole range of HFIP mole fraction (xHFIP) by large-angle x-ray scattering (LAXS), small-angle reutron scattering (SANS), F19-, C13-, and O-NMR17 chemical shifts, O-NMR17 relaxation, and mass spectrometry. The LAXS data have shown that structural transition of solvent clusters takes place at xHFIP∼0.1 from the tetrahedral-like hydrogen bonded network of water at xHFIP⩽∼0.1 to the structure of neat HFIP gradually formed with increasing HFIP concentration in the range of xHFIP⩾0.15. The Ornstein–Zernike plots of the SANS data have revealed a mesoscopic structural feature that the concentration fluctuations become largest at xHFIP∼0.06 with a correlation length of ∼9 Å, i.e., maximum in clustering and microhetrogeneities. The F19 and C13 chemical shifts of both CF3 and CH groups of HFIP against xHFIP have shown an inflection point at xHFIP∼0.08, implying that the environment of HFIP molecules changes due to the structural transition of HFIP clusters. The O17 relaxation data of water have shown that the rotational motion of water molecules is retarded rapidly upon addition of HFIP into water up to xHFIP∼0.1, moderately in the range of ∼0.1<xHFIP≲0.3, and almost constant at xHFIP≳0.3, reflecting the structural change in the solvent clusters at xHFIP∼0.1. The mass spectra of cluster fragments generated in vacuum from HFIP-water mixtures have shown that the predominant clusters are A1Wn (n<12, A=HFIP, W=water) and water clusters Wn (n=5–8) at xHFIP=0.09 and 0.20 and only HFIP oligomers in a water-rich region of xHFIP=0.005∼0.01. From all the information obtained in the present study, the models are proposed for the aggregation of HFIP and water molecules in HFIP-water mixtures.
How approximate is the experimental evaluation of quadrupole coupling constants in liquids? A novel computational study
Authors: Hardy Edme H. , Müller Markus G. , Vogt Patrick S. , Bratschi Christoph , Kirchner Barbara , Huber Hanspeter , Searles Debra J.
Published: September 2003

Model calculations to investigate the deuteron quadrupolar relaxation in liquid water are performed. Techniques not amenable to experiment, such as switching on and off the intermolecular or intramolecular electric field gradients and simulating rigid liquid water, give insight into the microscopic effects leading to relaxation. In experimental studies it is usually assumed that the deuteron quadrupolar relaxation is governed largely by the reorientational motion of an average electric field gradient, and the error in this assumption is readily extracted from the model calculations. As expected, this error is significant for deuterons in hydrogen bonds. These model calculations should provide a guide to better understanding of quadrupolar relaxation and experimental evaluation of relaxation.
Vibrational modeling of the thermodynamic properties of magnetite (Fe3O4) at high pressure from Raman spectroscopic study
Authors: Shebanova Olga N. , Lazor Peter
Published: September 2003

A Raman study of natural magnetite has been performed at high pressure up to 20 GPa. Upon compression, the vibrational spectra do not exhibit any changes, except for the continuous shift to higher frequencies. Vibrational modeling of the specific heat and entropy based on the modified Kieffer’s model allowed the estimation of the magnetic contribution to the thermodynamic properties of magnetite by comparison with published calorimetric measurements. High-pressure data provided information on the volume dependence of phonon modes of magnetite expressed by the mode Grüneisen parameters γiT. The volume dependence of vibrational modes of magnetite is described by the averaged value γ̄=1.33, weighted by the contribution of each mode to the total lattice specific heat. This value was employed for the calculation of the pressure-temperature dependence of heat capacity and entropy. The effect of the uniaxial stress on the compressional behavior and related thermodynamic parameters of magnetite was inferred from experiments conducted in hydrostatic and quasi-hydrostatic environments.
Effects of supercritical clustering and selective confinement on reaction equilibrium: A molecular simulation study of the esterification reaction
Authors: Turner C. Heath , Gubbins Keith E.
Published: September 2003

We explore the effects of different molecular surroundings on the equilibrium conversion of ethanol and acetic acid to ethyl acetate and water. While the equilibrium conversion of this reaction is limited from 66% to 68% in the liquid phase, experimental measurements have found that the reaction yield can be enhanced within a supercritical carbon dioxide solvent and within microporous carbons. The reactive Monte Carlo simulations presented mirror these experimental findings, and provide the molecular-level information needed to explain the experimental observations. The simulations demonstrate that selectivity is the driving force for the enhanced conversion in carbon slit-pores and in the supercritical carbon dioxide solvent. Ethyl acetate tends to be selectively adsorbed within carbon slit-pores in the range of 1.0 to 2.0 nm in width, and is selectively solubilized within carbon dioxide clusters near the critical point of the fluid.
Quantum chemical study of the lanthanide bond length contraction on Ln3+-doped Cs2NaYCl6 crystals (Ln=Ce to Lu)
Authors: Ordejón Belén , Seijo Luis , Barandiarán Zoila
Published: September 2003

The lanthanide–chlorine bond length, Re, and the frequency of the symmetric stretching mode, ν̄a1g, of the (LnCl6)3− octahedral defect clusters embedded in Cs2NaYCl6 have been calculated for all 14 Ce3+ to Lu3+ impurities in their ground 4fn electronic state using wave-function-based ab initio methods of solid state quantum chemistry which include relativistic effects and electron correlation within the (LnCl6)3− defect clusters and quantum mechanical interactions between the (LnCl6)3− electronic group and the Cs2NaYCl6 embedding host ions. The bond distance values obtained provide useful data to improve the Judd–Morrison model of the 4f→5d energy separation by explicitly including the local distortions the Ln3+ ions produce in the Cs2NaYCl6 host. The values of the structural parameters Re and ν̄a1g, and their variation across the series have also been studied using simpler models of the embedding host (in vacuo and Madelung embeddings), which has revealed that host effects, particularly those associated with quantum mechanical interactions, strengthen the Ln–Cl bond (decreasing the bond distances and increasing the frequencies of the symmetric bond stretch) and are smaller towards the right end of the series. Electron correlation within the (LnCl6)3− clusters also reduces the bond distance values, but this reduction increases going right from Ce to Lu; its effects on the frequencies of the symmetric stretching mode are negligible. The comparison of the theoretical results with the few available experiments is very satisfactory.
Calculations of the electronic structure of 3d transition metal dimers with projector augmented plane wave method
Authors: Valiev M. , Bylaska E. J. , Weare J. H.
Published: September 2003

The projected augmented plane wave method provides an all-electron solution to the Kohn–Sham local density approximation to the electronic Schrödinger equation. By projecting the fast-varying parts of the single-particle orbitals onto a local basis it allows accurate calculations for molecules and solids throughout the periodic table using a plane wave basis without the introduction of pseudopotentials. The method, thereby, preserves the efficiency required for applications to first-principles molecular dynamics simulations, while still allowing an unambiguous solution for systems containing strong scattering atoms such as transition metals. In this work, we made a comprehensive comparison of the structural and energetic properties of homonuclear 3d transition metal diatomic molecules as predicted by projector augmented plane wave and by various local basis methods. These molecules are particularly difficult to treat with plane-wave pseudopotential methods. The bond energies, bond lengths, and vibrational frequencies for the lowest-lying multiplet states are calculated. Our results demonstrate that projector augmented plane wave calculations deliver the same level of accuracy as local basis methods.
Impact dynamics of molecular clusters on surfaces: Fragmentation patterns and anisotropic effects
Authors: Tomsic A. , Schröder H. , Kompa K.-L. , Gebhardt C. R.
Published: September 2003

The fragmentation dynamics of (H2O)1032 clusters colliding with a repulsive surface at incident velocities of 1753 m/s and 2909 m/s, corresponding to kinetic energies of 0.5 and 1.5 times the cluster binding energy, has been examined in a classical molecular dynamics simulations study. The results show a large anisotropy in the energy redistribution inside the cluster upon impact, which leads to asymmetric fragmentation, starting in the leading part of the cluster. The low-mass region of the fragment size distribution can be described by a power law with an exponent close to −1.6, and the range of this region increases with increasing incident velocity. The formed fragments have rather uniform internal temperatures close to the standard boiling point of water, but the translational energy of the monomers formed upon collision is much larger, pointing at the asymmetric energy distribution inside the cluster. The angular distributions of fragment mass and fragment kinetic energy peak at grazing exit angles. For the investigated conditions, the dynamics is insensitive to the details of the initial structure of the cluster.
Dynamics of a doped polymer at temperatures where the two-level system model of glasses fails: Study by single-molecule spectroscopy
Authors: Naumov A. V. , Vainer Yu. G. , Bauer M. , Kador L.
Published: September 2003

We investigated the spectra of a large number of single tetra-tert-butylterrylene molecules embedded in an amorphous polyisobutylene matrix and analyzed the distributions of their linewidths (widths of single spectral peaks). The measurements were performed at 2, 4.5, and 7 K. This is a temperature region, where the standard two-level system (TLS) model of low-temperature glasses begins to fail. At T=2 K the temporal behavior (history of frequency jumps) of most of the measured spectra and their linewidth distributions were found to be consistent with the TLS model. At higher temperatures the main features of individual spectra (number of spectral peaks, temperature variation of peak widths, ratio of intensities of different peaks, etc.) still appear consistent with the predictions of this model. An increase of temperature leads mainly to the broadening of spectral peaks. A detailed analysis of the linewidth distributions reveals deviations from a standard TLS model at T=4.5 and 7 K. This difference is attributed to the influence of quasi-local low-frequency modes (LFM) of the amorphous matrix. By comparing the measured linewidth distributions with computer simulations, we quantitatively determined the LFM contribution to the single-molecule spectra in our dye-matrix system at different temperatures.
Strong Jahn–Teller effect in the excited state: Anomalous temperature dependence of the zero-phonon line
Authors: Hizhnyakov V. , Boltrushko V. , Kaasik H. , Sildos I.
Published: September 2003

We are studying the optical spectra of impurity centers in the case when the optical transition takes place between nondegenerate and two-fold degenerate electronic E-states characterized by a strong vibronic coupling. In the latter state the adiabatic surface has a shape of a deformed Mexican hat with three flat minima on the trough of the “hat.” We have found that in this case the usual T7-dependence of the homogeneous width of the zero-phonon line at low temperatures is replaced by a dependence close to the T3-one. Besides, the usual T4-dependent redshift of the line is replaced by the T2-dependent shift. Depending on the flatness of the adiabatic surface, the latter shift may be to the red or to the blue. The theory is applied to the zero-phonon line 637 nm of the nitrogen-containing N-V center in a diamond. In this system the temperature dependence of the ZPL results from the common action of the Jahn–Teller effect and the pseudo-Jahn–Teller effect in the excited E-state: the pseudo-Jahn–Teller effect “helps” to get the system close to the dynamical instability due to the enlargement of the Jahn–Teller distortion.