bluuebunny/crossref_metadata_2025 huggingface dataset:
On the spectrum and structure of the isolated O4− anion in solid argon
Authors: Chertihin G. V. , Andrews L.
Published: July 2002

Laser ablation of metal targets in front of condensing Ar/O2 mixtures produced the strong antisymmetric fundamental of isolated O4− at 953.5±0.3 cm−1 in solid argon from the capture of ablated electrons by O2 followed by the addition of another O2 molecule. The sharp sextet mixed isotopic spectrum suggests a structure with two equivalent O2 subunits and equivalent atomic positions in each subunit. DFT/UB3LYP calculations reveal doublet trans and rectangular structures with the latter more stable by 8.8 kcal/mol. Of more importance, the mixed oxygen isotopic spectrum calculated for the rectangular structure matches the observed spectrum much better than values calculated for the trans structure; thus, it is concluded that isolated O4− probably has an approximately rectangular structure. It is further suggested that the earlier observed alkali metal M+O4− species have rectangular pyramidal structures and that the sharp 954±1 cm−1 bands attributed to a different structural isomer are in fact due to the isolated anion owing to photoionization of the alkali metal atoms by radiation from the Nernst glower source of the grating spectrophotometer.
Off resonance second order optical activity of isotropic layers of chiral molecules: Observation of electric and magnetic contributions
Authors: Schanne-Klein M. C. , Hache F. , Roy A. , Flytzanis C. , Payrastre C.
Published: July 2002

We perform nonresonant surface second harmonic reflection from an isotropic layer of a chiral pentamethinium salt. The combination of gated integration and photon counting techniques allows for a very high sensitivity of our experimental setup. We measure a second harmonic optical rotation angle of 5°, as well as linear and circular dichroisms of 5% to 140%. We quantitatively compare these three experimental techniques and obtain a good agreement. The nonresonant configuration of our experiments enables us to distinguish between electric and magnetic contributions to the second order optical activity. The nonvanishing circular dichroism, as well as the nonvanishing s-polarized second harmonic signal with s-polarized fundamental beam, indicates that electric and magnetic effects are of the same order of magnitude.
Electric multipolar induction in the far-infrared spectra of CO in liquid Ar: Translational/rotational contributions and static cancellation effects
Authors: Medina A. , Roco J. M. M. , Hernández A. Calvo , Velasco S.
Published: July 2002

We have analyzed by means of molecular dynamics simulations the temperature and density dependence of the different pure induced components of the far-infrared integrated absorption coefficient of CO in liquid Ar (an interesting example for which experimental far-infrared spectra show the existence of high order electric multipolar induction). The main contribution to these components (for all the thermodynamic conditions we elected) comes from the rotation of the diatomic except for dipole induced-dipole absorption, where quantitatively rotation and translation are similar. By studying static cancellation effects separately in rotation and translation, we can assert an interesting result: All those contributions to the integrated absorption coefficient with radial decay equal or faster than the first repulsive decay of the solute–solvent interaction present enhancement effects and all those decaying slower present cancellation effects.
Assessment of the validity of intermolecular potential models used in molecular dynamics simulations by extended x-ray absorption fine structure spectroscopy: A case study of Sr2+ in methanol solution
Authors: Roccatano D. , Berendsen H. J. C. , D’Angelo P.
Published: July 2002

Molecular dynamics simulations have been carried out for Sr2+ in methanol using different Sr2+ Lennard-Jones parameters and methanol models. X-ray absorption fine structure (EXAFS) spectroscopy has been employed to assess the reliability of the ion-ion and ion-methanol potential functions used in the simulations. Radial distribution functions of Sr2+ in methanol have been calculated for each simulation and compared with the EXAFS experimental data. This procedure has allowed the determinations of reliable Sr2+-methanol models which have been used in longer simulations providing an accurate description of the dynamic and structural properties of this system.
Ω-type doubling reversal in the B 3Π1 state of HgAr200 as a probe of the long-range potential of the A 3Π+ state
Authors: Hishikawa Akiyoshi , Sato Hirokazu , Yamanouchi Kaoru
Published: July 2002

The mass-resolved excitation spectrum of the B 3Π1–X 1Σ+ transition of HgAr was measured by the two-color resonantly enhanced two-photon ionization method using a high-resolution reflectron time-of-flight mass spectrometer. The rotational structures were recorded with high spectral resolution (∼0.05 cm−1) for the (vB,0) bands (vB=0–9) of a single isotopomer HgAr200. From the rotational analysis, the presence of an Ω-type doubling was identified for the first time, which is caused by the rotational perturbation from the nearby highly excited vibrational levels of the A 3Π0+ state. The spacing between the Ω-type doublets was found to exhibit a characteristic reversal as a function of vB. From the deperturbation of the Ω-type doubling, the vibrational level energies of the A state in the vibrationally highly excited region were derived, from which its long-range interaction potential was determined.
Linear viscoelasticity of colloidal mixtures
Authors: Nägele G. , Bergenholtz J.
Published: July 2002

In this work we develop a unifying and general method for calculating linear viscoelastic properties of multicomponent colloidal mixtures of spherical particles. Using linear response theory based on the many-body Smoluchowski diffusion equation, we derive an exact expression for the zero shear rate shear relaxation function, together with a Green-Kubo formula for the static shear viscosity. From these results, we obtain an exact expression for the high frequency elastic shear modulus of colloidal mixtures. We present, in addition, the first derivation of a self-consistent mode coupling scheme for the linear viscoelasticity of concentrated colloidal mixtures. This scheme offers the opportunity for a unified description of linear viscoelasticity and diffusion mechanisms. It accounts further for polydispersity and mixing effects, and leads naturally to a diverging shear viscosity at a glass transition point. Various limiting cases are considered to assess the accuracy of the approach. It is shown to be a valuable method for evaluating the rheological properties of concentrated colloidal mixtures.
Photodissociation dynamics of OClO at 157 nm
Authors: Lin Jim J. , Hwang Dennis W. , Lee Yuan T. , Yang Xueming
Published: July 2002

Photodissociation of OClO at 157.6 nm excitation has been investigated using the photofragment translational spectroscopic technique. Two distinctive chemical dissociation channels have been observed; one is the binary dissociation process, OClO+hν→ClO+O; the other one is the triple dissociation process, OClO+hν→Cl+O+O. The branching ratio of the binary dissociation channel to the triple dissociation channel is determined to be 0.59:0.41. Bimodal vibrational distribution of the ClO product has been observed for the OClO→ClO(X 2Π)+O(3P,1D) channel, implying that two distinctive dissociation routes possibly exist in the binary dissociation process. The bimodal distribution is likely caused by the two dissociation pathways from two excited electronic states: the D(2A1) and E(2B1) states of OClO. These arguments are further supported by the results of the anisotropy parameter measurements for the binary dissociation channels. Experimental results also show that the OClO+hν→ClO(X 2Π)+O(1S) and OClO+hν→ClO(A 2Π)+O(3P) channels might also exist in addition to the ClO(X 2Π)+O(3P,1D) channel. In the triple dissociation process, experimental results show that the main product channel is the OClO+hν→Cl(2P)+O(1D)+O(3P) channel, while the OClO+hν→Cl(2P)+O(3P)+O(3P) channel is the minor one. The branching ratio of these two channels is determined to be 0.89:0.11. From the modeling of the time of flight spectra of the O atom product, it is believed that the triple dissociation process of OClO is a simultaneous process within the time scale of one rotation period. Two-photon dissociative ionization process OClO+hν→Cl++O2+e− has also been observed.
Classical trajectory and statistical adiabatic channel study of the dynamics of capture and unimolecular bond fission. V. Valence interactions between two linear rotors
Authors: Maergoiz A. I. , Nikitin E. E. , Troe J. , Ushakov V. G.
Published: July 2002

The combination of two linear rotors forming linear or nonlinear adducts is treated using standardized valence potentials. Classical trajectory (CT) and statistical adiabatic channel (SACM) calculations are used for the calculation of thermal capture rate constants. At very low temperatures, only SACM applies. At intermediate temperatures SACM and CT approach each other; however, Landau–Zener-type multiple crossings of adiabatic channel potentials introduce local nonadiabaticity which has to be accounted for. The high-temperature transition from globally adiabatic to nonadiabatic (sudden) dynamics is studied by CT. Thermal rigidity factors, accounting for the influence of the anisotropy of the potential on the capture rate constant, are expressed in simple analytical form which facilitates practical applications. The present work complements similar studies on the addition of atoms to linear molecules in standardized valence potentials (part IV of this series).
Rotational motion compensates the energy defect in near-resonant vibration–vibration energy transfer: A state-to-state study of NO(v)+N2O
Authors: Drabbels Marcel , Wodtke Alec M.
Published: July 2002

Our lack of understanding of the factors that compensate energy defects in near resonant V–V energy transfer constrains our ability to accurately predict resonance widths and, thus, the overall importance of such processes. We have carried out one of the first truly state-to-state measurements of near resonant V–V energy transfer under single collision conditions, employing the crossed molecular beams, stimulated emission pumping technique. We have varied the energy defect ΔE for the process: NO X 2Π(v)+N2O(0,0,0)→NO X 2Π(v−1)+N2O(0,0,1), by changing the prepared vibrational state from v=22 (ΔE=+14 cm−1) to v=21 (ΔE=−18 cm−1) to v=20 (ΔE=−49 cm−1). Changes in the energy transfer efficiencies and rotational distributions of vibrationally inelastically scattered NO with energy defect both strongly suggest that molecular rotation (both of NO and N2O) is responsible for compensating the energy defect. Furthermore it appears that relative translation is ineffective in compensating the energy defect. A ΔJNO∼ΔJN20 approximation also appears valid.
Transition pathways in a many-body system: Application to hydrogen-bond breaking in water
Authors: Csajka Félix S. , Chandler David
Published: July 2002

We apply a stochastic method introduced by Dellago et al. [J. Chem. Phys. 108, 1964 (1998)] to sample transition paths in high-dimensional systems. The method connects two endpoint regions (for example a reactant and a product region) by a set of space-time paths. This approach is an importance sampling for rare events that does not require prior knowledge of the location of dynamical bottlenecks. Transition paths are generated with a weight corresponding to a chain of Metropolis Monte Carlo steps. We derive Monte Carlo algorithms and apply the technique to the dynamics of hydrogen-bond breaking in liquid water. We obtain averages in a transition path ensemble for the structure and energy along the trajectory. While characterized by a rate constant, hydrogen-bond breaking in water occurs frequently enough to be studied by standard methods. The process therefore provides a useful test of path sampling methods. The comparison between path sampling and standard Monte Carlo demonstrate the feasibility of transition path sampling for a many-body system with a rough potential energy surface.
Hydration of ions at various temperatures: The role of electrostriction
Authors: Danielewicz-Ferchmin I. , Ferchmin A. R.
Published: July 2002

A new statistical approach to the electric properties of water at elevated temperatures is proposed. The local permittivity and electrostriction pressure in hydration shells of Sr2+ and Ni2+ ions at ambient conditions, and of Ni2+ ion at elevated temperatures are calculated. The insensitivity of Ni2+ hydration characteristics to the applied pressure is explained by the role played by the much higher local electrostriction pressure. A correlation between the temperatures at which destroyed hydration structures have been observed in recent neutron scattering experiments and the calculated local electrostriction pressures on the one hand, and the melting line of H2O on the other hand, is pointed out. Temperatures at which analogous changes in hydration structure may be expected are predicted for several ions.
Solvation and conformational dynamics of dicarboxylic suberic acid
Authors: Nilsson J. Arvid , Laaksonen A. , Eriksson Leif A.
Published: July 2002

Solvation structure and conformational preferences of the dicarboxylic suberic acid, HOOC(CH2)6COOH, both neutral and protolyzed in water and neutral in methanol have been studied using molecular-dynamics (MO) computer simulations. According to results from MD simulations in water solution, the backbone hydrocarbon chain shows a very clear tendency to curl up into a helical structure, forming either a tgggt or a tg′g′g′t conformation. The carboxylic head groups are strongly hydrated in the water solution, while the hydrophobic hydrocarbon skeleton is surrounded by water molecules in a packed structure. In the helical conformation, the surface area of the nonpolar part of the fatty acid is minimized against water phase. Transitions between the right and the left-handed helices are observed in neutral and mono-anionic forms of suberic acid in water solution. Suberic acid dissolved in methanol does not show any conformational preferences. Along the hydrocarbon chain, g, g′, and t conformers are equally populated. The head group torsional angles, however, strongly prefer trans conformation due to dipolar interactions between the carboxyl groups and the solvent hydroxyl groups. In addition to MD simulations, corresponding water and methanol solutions are prepared and C13 NMR (nuclear magnetic resonance) chemical shifts are measured in both solutions. Using a time-averaged geometry for suberic acid from MD simulations in water, chemical shielding constants are calculated quantum chemically. Agreement between the theoretical and the experimental chemical shifts is good, and gives indirect support to the simulated conformations of suberic acid in the investigated solutions. The simulation results are also consistent with recent Raman investigations of suberic acid in both water and methanol solutions.
Effects of initial conditions on the development of spinodal decomposition
Authors: Chikina I.
Published: July 2002

Self-consistent theory of the early spinodal decomposition stage is developed. Calculations show the existence of two maxima in the wave number dependence of the structure factor S(q,t). The main maximum grows with time and has a position between qsaddle (14) and qm (1) at a finite wave number q+. The second one is localized near q≃0, its height being practically independent of time. Characteristics of the main maximum are sensitive to the initial conditions. Experiments confirm these predictions.
Molecular self assembly of rodlike particles: An application to collagen
Authors: Evans G. T.
Published: July 2002

A simplified form of the Wertheim theory of chemical association has been used to address how ellipsoidal particles with three highly localized interaction sites (head, tail, side) polymerize to form aggregates. Aggregation is allowed for heads with tails subject to an energy parameter, εa, and for heads or tails with the side spot subject to a different energy parameter, εc. No side-by-side interactions are allowed. As εc/εa→0, the aggregate becomes rodlike, whereas as εc/εa→∞, the object becomes a freely rotating chain or a herringbone pattern. When the present theory is applied to self assembly of type I collagen, it is found that each εc and εa correspond to the anchoring caused by two H bonds.
Infrared spectroscopy and time-resolved dynamics of the ortho-H2–OH entrance channel complex
Authors: Anderson David T. , Schwartz Rebecca L. , Todd Michael W. , Lester Marsha I.
Published: July 2002

The rotationally resolved infrared spectrum of the prereactive o-H2–OH complex in its ground electronic state is obtained in the OH overtone region at ∼1.4 μm using an IR-UV double resonance fluorescence enhancement technique. The pure OH overtone band of o-H2–OH is observed as well as approximately 20 additional rovibrational transitions extending out to the OH (X 2Π,v=2)+o-H2(X 1Σg+) dissociation limit. These transitions are assigned as combination bands involving the simultaneous excitation of the OH vibrational overtone and intermolecular bending (internal rotor) states. The assignment of the experimental spectrum is aided by a detailed comparison with the bound states computed for the ab initio potential of Clary, Werner, and co-workers [Mol. Phys. 83, 405 (1994)]. The infrared spectroscopy results also verify the topology of this ab initio potential in the entrance channel to the OH+H2 hydrogen abstraction reaction. Direct time-resolved experiments indicate that the lifetime of the vibrationally activated o-H2–OH complex in the ground intermolecular state is 115(26) ns. The initial excitation is found to stay localized in the OH intramolecular stretching mode for a long period of time prior to vibrational predissociation or chemical reaction.
Analytical evaluation of energy derivatives in extended systems. I. Formalism
Authors: Sun Jun-Qiang , Bartlett Rodney J.
Published: July 2002

A method is developed to analytically evaluate energy derivatives for extended systems. Linear dependence among basis functions, which almost always occurs in extended systems and brings instability to the coupled-perturbed equations, is automatically eliminated in this method. The remaining independent basis functions are transformed into semiorthogonal orbitals. The derivatives of the orbitals and the overlap matrix over them are obtained via a set of coupled-perturbed equations, similar to those of the coupled-perturbed Hartree-Fock (CPHF) equations which are used to calculate the derivatives of the Hartree-Fock (HF) orbitals and the orbital energies. By introducing symmetrized coordinates, these coupled-perturbed equations can be easily solved. Explicit expressions for calculating gradients and Hessians of the HF energy for extended systems are given. With this method, we can calculate energy derivatives with respect to displacements of the nuclei, including those which break the translational symmetry. Therefore, the method not only provides an efficient and accurate approach to calculate energy derivatives of any order, but also enables us to determine the force constants for individual nuclei, the interatomic force constants, and phonon dispersion curves in the whole Brillouin zone. With this method, the computational cost to calculate phonon spectrum with k≠0 in the Brillouin zone is the same as that needed for the spectrum at k=0.
Real-time control of electronic motion: Application to NaI
Authors: Gro/nager Michael , Henriksen Niels E.
Published: July 2002

We study theoretically the electronic and nuclear dynamics in NaI. After a femtosecond pulse has prepared a wave packet in the first excited state, we consider the adiabatic and the nonadiabatic electronic dynamics and demonstrate explicitly that a nonstationary electron is created in NaI corresponding to electron transfer between Na and I. The electronic motion is introduced via nuclear motion, more specifically, through nonadiabatic curve crossing and the electronic motion is here on the same time scale as the nuclear motion. We show that the branching ratio between the channels Na+I and Na++I− depends on the electron distribution (i.e., where the electron “sits”) prior to the time where the bond is broken by a subpicosecond half-cycle unipolar electromagnetic pulse. Thus we control, in real time, which nucleus one of the valence electrons will follow after the bond is broken.
A stochastic simulation scheme for studying pore volume trapping in a structure of growing particles
Authors: Sotirchos Stratis V. , Tomadakis Manolis
Published: July 2002

A stochastic computational scheme is developed for investigating formation of trapped (isolated) pore volume in a structure of growing particles, with particle growth occurring only on surface elements exposed to the connected (accessible) part of the pore space. The simulation procedure combines a gradual increase of the particle size with a random walk scheme, the latter used to determine whether a randomly chosen point in the unit cell of the two-phase structure lies in the particle phase or in the connected or isolated part of the matrix phase. The formulated algorithm is applied to structures of freely overlapping, unidirectional cylinders, and results are obtained both for the volume fractions and the specific surface areas of the accessible and inaccessible parts of the pore phase. The trapped volume results are compared with those obtained for ordinary continuum percolation, that is, for uniformly growing particles.
The isotropic–nematic phase transition in a fluid of square well spherocylinders
Authors: Williamson Dave C. , del Rio Fernando
Published: July 2002

The isotropic–nematic phase transition in a fluid of hard spherocylinders with a spherocylindrical square-well attraction is examined using Monte Carlo simulations and two theoretical approaches. The first theory is a first-order perturbation theory which incorporates the Parsons decoupling approximation for the pair distribution function. The second theory is a simple resummation of the virial coefficients in the nematic phase which maps the thermodynamics of the nematic phase to those of the isotropic phase. In general both the theoretical approaches and the simulation results show a destabilization of the nematic phase with respect to the isotropic phase as the temperature is decreased. However, close comparison between the simulation results and the theories reveals that the Parsons approach is quantitatively deficient. On the other hand, the results for the resummation procedure are in good agreement with the simulation results over the full isotropic range and for the isotropic–nematic phase transition. The comparison of the nematic phase close to the phase transition shows reasonable agreement between theory and simulation, however, the theoretical results become much poorer deep in the nematic phase. The reason for this is attributed to the crude manner in which the orientational dependence is included into the attractive contribution to the free energy.
A new chemical concept: Shape chemical potentials
Authors: Chan Garnet Kin-Lic , Handy Nicholas C.
Published: July 2002

Within the density functional formalism, we introduce the shape chemical potential μin for subsystems, which in the limiting case of point subsystems, is a local chemical potential μn(r). It describes the electron withdrawing/donating ability of specified density fragments. The shape chemical potential does not equalize between subsystems, and provides a powerful new method to identify and describe local features of molecular systems. We explore the formal properties of μin especially with respect to discontinuities, and reconcile our results with Sanderson’s principle. We also perform preliminary calculations on model systems of atoms in molecules, and atomic shell structure, demonstrating how μin and μn(r), identify and characterize chemical features as regions of different shape chemical potential. We present arguments that shell structure, and other chemical features, are not ever obtainable within Thomas–Fermi-type theories.
Resonant ion-dip infrared spectroscopy of the S4 and D2d water octamers in benzene-(water)8 and benzene2-(water)8
Authors: Gruenloh Christopher J. , Carney Joel R. , Hagemeister Fredrick C. , Arrington Caleb A. , Zwier Timothy S. , Fredericks Sharon Y. , Wood John T. , Jordan Kenneth D.
Published: July 2002

The techniques of resonant two-photon ionization (R2PI), UV–UV (ultraviolet) hole-burning, and resonant ion-dip infrared (RIDIR) spectroscopies have been employed along with density functional theory (DFT) calculations to assign and characterize the hydrogen-bonding topologies of two isomers each of the benzene-(water)8 and (benzene)2(water)8 gas-phase clusters. The BW8 isomers (B=benzene, W=water) have R2PI spectra which are nearly identical to one another, but shifted by about 5 cm−1 from one another. This difference is sufficient to enable interference-free RIDIR spectra to be recorded. As with smaller BWn clusters, the BW8 clusters fragment following photoionization by loss of either one or two water molecules. The OH stretch IR spectra of the two BW8 isomers bear a close resemblance to one another, but differ most noticeably in the double-donor OH stretch transitions near 3550 cm−1. Comparison to DFT calculated minimum energy structures, vibrational frequencies, and infrared intensities leads to an assignment of the H-bonding topology of the BW8 isomers as nominally cubic water octamers of S4 and D2d symmetry surface attached to benzene through a π H-bond. A series of arguments based on the R2PI and hole-burning spectra leads to an assignment of additional features in the R2PI spectra to two isomers of B2W8. The OH stretch RIDIR spectra of these isomers show them to be the corresponding S4 and D2d analogs of B2W8 in which the benzene molecules each form a π H-bond with a different dangling OH group on the W8 sub-cluster.
Theoretical study of the OH−(H2O)2 system: Nature and importance of three-body interactions
Authors: Turki Naima , Milet Anne , Rahmouni Ali , Ouamerali Ourida , Moszynski Robert , Kochanski Elise , Wormer Paul E. S.
Published: July 2002

The nature and importance of nonadditive three-body interactions in the ionic OH−(H2O)2 cluster have been studied by supermolecule Mo/ller–Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Mo/ller–Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters OH−(H2O)n, n=2,3,4,10, with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree–Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler–London and exchange-induction/deformation terms. This implies that the induction energy alone is not a reliable approximation to the Hartree–Fock nonadditive energy. At the correlated level, the most important contributions come from the induction-dispersion and the MP2 exchange energies. The exchange-dispersion and dispersion nonadditivities are much smaller, and for some geometries even negligible. This suggests that it will be difficult to approximate the three-body potential for OH−(H2O)2 by a simple analytical expression. The three-body energy represents only 4%–7% of the pair CCSD(T) intermolecular energy for the OH−(H2O)2 cluster, but can reach as much as 18% for OH−(H2O)4. Particular attention has been paid to the effect of the relaxation of the geometry of the subsystems.
Electronic spectra of jet-cooled 1-phenylpyrrole: Large-amplitude torsional motion and twisted intramolecular charge-transfer phenomenon
Authors: Okuyama Katsuhiko , Numata Yasushi , Odawara Shino , Suzuka Isamu
Published: July 2002

The fluorescence excitation spectrum and the single vibronic level dispersed fluorescence spectra in the region of the S0⇆S1 transition were measured for jet-cooled 1-phenylpyrrole. The 0–0 band was observed at 35 493 cm−1. Long and low-frequency progressions with somewhat irregular intensity distributions appeared on both spectra, and were assigned to torsional motion. The torsional energy levels in the S0 and S1 states were obtained up to 25 and 16 quanta, respectively. The torsional potentials in both states could be determined from the sufficient number of energy levels observed. In the S0 state the most stable conformation was determined to be a twisted form with a dihedral angle of 38.7°, where the planar barrier height was calculated to be 457 cm−1, and the perpendicular to be 748 cm−1. On the other hand, it was discovered that 1-phenylpyrrole in the S1 state also had a twisted form with a somewhat smaller dihedral angle of 19.8°, and that the barrier to planarity was 105 cm−1 and to perpendicularity, 1526 cm−1. These facts indicated that the electronic excitation caused 1-phenylpyrrole to be rigid to twist. 1-Phenylpyrrole and its derivatives have been reported as a group of twisted intramolecular charge-transfer (TICT) molecules. No indication of TICT appeared on the shape of the S1-state torsional potential determined. The relation between torsional potential and TICT is discussed based on the results of this study.
On the influence of reactant’s induced surface transformations in the behavior of a heterogeneously catalyzed dimer–monomer reaction model
Authors: Albano Ezequiel V.
Published: July 2002

A surface reaction model for the catalyzed reaction A+(12)B2→AB is studied by means of Monte Carlo simulations. Motivated by experiments on the catalytic oxidation of CO on certain Pt surfaces; i.e., A≡CO, B2≡O2, and AB≡CO2; it is assumed that the catalyst surface undergoes reactant’s induced reversible phase transitions (RIRPT’s) between two different structures, i.e., a stable (reconstructed) phase in the low (high) A-coverage regime. Also the sticking coefficient of B2-species depends on the surface structure, being negligible in the stable phase. Two cases are studied in detail depending on whether the RIRPT’s involve the whole surface and are due to the average coverage of the sample or, on the other hand, they are restricted to small patches of the surface and are driven by the local coverage. For the case of global RIRPT’s, the reactive regime of the system exhibits self-sustained oscillatory behavior. However, considering local RIRPT’s oscillations are observed in finite samples but it is shown that they will vanish in the thermodynamic limit.
Resonant two-photon ionization spectroscopy of jet-cooled RuC
Authors: Langenberg Jon D. , DaBell Ryan S. , Shao Lian , Dreessen Dawn , Morse Michael D.
Published: July 2002

A resonant two-photon ionization study of the jet-cooled RuC molecule has identified the ground state as a Σ+1 state arising from the 10σ211σ25π42δ4 configuration. The Δi3 state arising from the 10σ211σ25π42δ312σ1 configuration lies very low in energy, with the Δ33 and Δ23 components lying only 76 and 850 cm−1 above the ground state, respectively. Transitions from the X 1Σ+, Δ33, and Δ23 states to the Π23, Π13, Φ33, Φ43, Φ31, and Π11 states arising from the 10σ211σ25π42δ36π1 configuration have been observed in the 12 700–18 100 cm−1 range, allowing all of these states to be placed on a common energy scale. The bond length increases as the molecule is electronically excited, from r0=1.608 Å in the 2δ4, X 1Σ+ state, to 1.635 Å in the 2δ312σ1, Δ3 state, to 1.66 Å in the 2δ36π1, Π3 and Φ3 states, to 1.667 Å in the 2δ36π1, Φ1 and 1.678 Å in the 2δ36π1, Π1 state. A related decrease in vibrational frequency with electronic excitation is also observed. Hyperfine splitting is observed in the 2δ312σ1, Δ33 state for the Ru99(I=5/2)12C and Ru101(I=5/2)12C isotopic combinations. This is analyzed using known atomic hyperfine parameters to show that the 12σ orbital is roughly 83% 5sRu in character, a result in good agreement with previous work on the related RhC and CoC molecules.
An analytical model for collisional effects on spectral line shape from the Doppler to the collision regime
Authors: Lance B. , Robert D.
Published: July 2002

A spectral line shape model accounting for both the collisional confinement narrowing of the Doppler distribution and the inhomogeneous effects due to the radiator speed dependence of the collisional broadening and shifting parameters is proposed. The velocity changes are assumed to be induced as well by hard as soft collisions. Doppler-collision correlations and speed-class exchanges are taken into account. A comparison with the previous models used for the Doppler regime and for the collisional one is done. The present model also applies for the intermediate regime where the two above mechanisms are simultaneously efficient. The spectral line shape characteristics are exemplified through the H2–Ar and C2H2–Xe prototype molecular systems.
On the singlet excited states of C70 and C60
Authors: Dauw X. L. R. , Bronsveld M. V. , Krüger A. , Warntjes J. B. M. , Witjes M. R. , Groenen E. J. J.
Published: July 2002

The fullerenes C70 and C60 have been studied by excitation and luminescence spectroscopy. Highly resolved phosphorescence-excitation spectra of C70 in n-pentane at 1.5 K are reported. Besides the origin and e1′ vibronic bands of the lowest-energy A2′←S0 excitation, the origin, and an a1′ vibronic band of the E1′←S0 excitation are assigned. For both C70 and C60 the fluorescence spectrum changes with temperature, which is explained in terms of thermally induced population of excited states above S1. For C70, this concerns the E1′ state and the corresponding fluorescent state is found to be 130 cm−1 above the A2′ state in decalin/cyclohexane. For C60 in the same matrix, it concerns the state of predominant Gg character at 85 cm−1 above the T1g state.
Molecular dissolution processes in lipid bilayers: A molecular dynamics simulation
Authors: Xiang Tian-xiang , Anderson Bradley D.
Published: July 2002

A thorough understanding of the effects of chain ordering on solute partitioning and transport across biomembranes requires a detailed account of various dissolution processes in lipid bilayers. In this study, the dissolution properties and related molecular processes for noble gases in the alkyl chain region of lipid bilayers were obtained by means of molecular dynamics simulation. The excess chemical potential exhibits a plateau value in the ordered peripheral region followed by a steep decline near the center of the bilayer. The strong entropic effects as manifested by the larger Barclay–Butler constants than commonly encountered in hydrocarbon solvents indicate that solute partitioning into membranes is driven primarily by changes of lipid chain conformation or/and an extra confinement of solute in the bilayer interior. Solute partitioning into lipid bilayers is analyzed in terms of two contributions: (1) the free energy for cavity creation to accommodate a solute, which is analyzed by scaled particle theory; and (2) the interaction energy between the inserted solute and surrounding molecules in the bilayer. The unfavorable free energy for cavity creation is found to be primarily responsible for the substantial decrease of solubility into the membranes from that into a hydrocarbon solvent (dodecane) when the solute size is increased. The observed linear decrease of the excess chemical potential with solute surface area arises from linear but opposite dependencies of the reversible work for cavity creation and the intermolecular interaction energy on solute surface area and may be described by an anisotropic surface-tension model.
Towards a statistical theory of associating telechelics: Equilibrium molecular structural distribution and one-cluster static scattering
Authors: Ermoshkin A. V. , Erukhimovich I.
Published: July 2002

A rigorous statistical treatment of the system of associating telechelics with due regard for formation of complicated many-chains clusters is presented for the case when the active telechelic ends associate only in two (A2) and threefold (A3) aggregates. A diagram technique to describe the structure of these clusters and find the corresponding equilibrium molecular structural distribution is introduced and a regular procedure to calculate the average structural and scattering properties of the system is elaborated. A competition between forming of the tree- and looplike fragments and its influence on the critical concentration cg of gelation and structural characteristics of the system are analyzed in detail. The dependences of cg as well as the number- and weight-averaged degrees of associations both for the tree- and looplike fragments and the clusters as a whole are plotted as functions of the properly reduced number density of telechelics and the reduced equilibrium constant of the chemical reaction 3A2↔2A3.
Fitting classical microcanonical unimolecular rate constants to a modified RRK expression: Anharmonic and variational effects
Authors: Song Kihyung , Hase William L.
Published: July 2002

Classical RRKM (i.e., microcanonical) rate constants are often calculated for analytic potential energy surfaces (PESs) developed to represent unimolecular decomposition reactions. The values for these rate constants reflect the complete anharmonicities of the PESs. A common procedure is to fit these rate constants with the expression of RRK theory, which is the harmonic limit of classical RRKM theory. These RRK fits often give values for s significantly less than 3n−6(5), which are not meaningful, since anharmonic rate constants are represented with a harmonic model. In this work different schemes are proposed for introducing anharmonic and also variational transition state effects into the RRK expression. These modified RRK schemes allow one to fit classical anharmonic RRKM rate constants with s=3n−6(5). The fits give information about the importance of anharmonicity in the unimolecular reactant’s density of states and the transition state’s sum of states.
Resonant two-color photoionization threshold measurements of the Zn+(4s)⋅Ar bond strength: Model-potential analysis of M+(ns)⋅Ar interactions
Authors: Leung Allen W. K. , Bellert D. , Julian Ryan R. , Breckenridge W. H.
Published: July 2002

The bond energy of the Zn(4s)+⋅Ar ground-state ion has been determined to be De=2085±100 cm−1 by measuring the threshold for two-color resonant photoionization of a ZnAr metastable state. This bond energy is larger (and the estimated bond length is smaller) than the values for other similar M(ns)+⋅Ar states, where M=Mg, Ca, V, Co, Ba, Cd, and Hg, consistent with our earlier postulate that the Zn(4s)+ “core” is smaller than the other M(ns)+ cores in the analogous electronically excited MAr van der Waals complexes for M=Mg, Ca, Zn, Cd, and Hg. We analyze and compare the M+(ns)⋅Ar potential curves using a model potential consisting of calculated 1/R4 (dipole induction) and 1/R6 (quadrupole induction plus dispersion) attractive terms, and an empirically derived Ae−bR repulsion term. The repulsion term for Zn+Ar is consistent with our postulate, in that at smaller R it is the least repulsive of the M+(ns)⋅Ar repulsive potentials when M=Zn, Hg, V, Co, Cd, Mg, Ca, and Ba.
Transient dielectric and birefringence relaxation of a system of rigid polar particles in strong dc electric fields
Authors: Kalmykov Yu. P. , Déjardin J. L.
Published: July 2002

The problem of the nonlinear transient response of a system of rigid dipolar particles (macromolecules) diluted in a nonpolar solvent to a sudden change of an external dc field both in magnitude and direction is considered. On solving the noninertial Fokker–Planck equation for the rotational Brownian motion of a dipolar particle, the relaxation functions and relaxation times are calculated for the transient dynamic Kerr effect and nonlinear dielectric relaxation. This is accomplished by using the matrix-continued fraction method, which allows one to find the solution of the infinite hierarchy of differential-recurrence relations for the first- (electric polarization) and second- (electric birefringence) order transient responses of ensemble averages of the spherical harmonics (relaxation functions). The results are compared with solutions previously obtained and illustrated by plots for the relaxation times and spectra of the relaxation functions.
Variational transition state theory of vapor phase nucleation
Authors: Schenter Gregory K. , Kathmann Shawn M. , Garrett Bruce C.
Published: July 2002

An expression for the rate of vapor phase nucleation is developed that is based on variational transition state theory. The method depends on a definition of a dividing surface in phase space that separates reactants from products. For this surface we choose a spherical shell in coordinate space that is centered about the center of mass of a cluster of i molecules having an interior volume v. In a manner that is consistent with variational transition state theory, we vary v to minimize the reactive flux through our chosen dividing surface. The resulting expression for the rate constant involves a definition of a physical cluster that is consistent with previous developments in nucleation theory. In formulating the rate in this manner we obtain a new expression for the evaporation rate constant that is proportional to the derivative with respect to v of the Helmholtz free energy for cluster formation. In addition, we have a fundamentally justified procedure for selecting a unique volume v for each i cluster. Application of the method to the nucleation of water clusters will be presented.
Picosecond mobilities of neat diamino poly(propylene imine) dendrimer DAB(CN)64 from a Raman study of its CN stretching fundamental
Authors: Perrot Michel , Rothschild Walter G. , Cavagnat Raymond M.
Published: July 2002

The paper is based on mid-frequency Raman scattering data of neat diamino poly(propylene imine) dendrimer molecule DAB(CN)64; time correlation functions were obtained from a band profile analysis of its fundamental CN mode near 2246 cm−1 at four temperatures between 300 and 123 K; lattice relaxation processes were found to be governed by a stretched exponential modulation function with dispersion parameter α=0.90±0.05 (300 K) and 0.85 (123 K) and corresponding time constant τ0=0.78 and 1.15 ps, respectively. The results suggest that the fast fluctuations of the molecular environment of the CN end groups of the dendrimer system regain equilibrium by independent, parallel pathways with Poisson relaxation times τ that show a broader distribution at lower system temperatures. The results imply that the shape and width of the CN band contour are generated by a vibrational dephasing process in the intermediate modulation regime, under inseparable contributions of intra- and intermolecular oscillator–lattice coupling.
Rotation-tunneling spectrum of the deuterated ammonia dimer
Authors: Karyakin E. N. , Fraser G. T. , Loeser J. G. , Saykally R. J.
Published: July 2002

The millimeter and submillimeter-wave molecular-beam spectrum of the perdeuterated ammonia dimer (ND3)2 has been measured between approximately 50 and 400 GHz using an electric-resonance optothermal spectrometer (EROS). As in the case of the (NH3)2, the spectrum is complicated by the threefold internal rotation of the ND3 subunits, the interchange tunneling of the two subunits, and the inversion of the subunits through their respective centers of masses. These tunneling motions split the rigid-molecule energy levels into 22 components, which all have nonzero statistical weights in the case of the deuterated dimer. Transitions have been assigned for rotation-tunneling states correlating to A–A (ortho–ortho) combinations of the ND3 monomer states, where A designates the rovibronic symmetries of the ND3 subunits. One K=1←1, one K=1←0, one K=0←1, and two K=0←0 progressions have been assigned. The data have been fit to 0.28 MHz using linear molecule-type energy-level expressions to determine rotational constants, band origins, l/K-type double constants, and centrifugal distortion constants. The two K=0←0 subbands, with origins near 264 GHz, are split by 64 MHz due to monomer inversion, as observed previously in the NH3 dimer. The 264 GHz, K=0 splitting arises predominantly from monomer interchange tunneling and is nearly a factor of 2 less than the 483 GHz value for the NH3 dimer. The separation is also approximately 25% smaller than predicted by Olthof et al. [E. H. T. Olthof, A. van der Avoird, and P. E. S. Wormer, J. Chem. Phys. 101, 8430 (1994)] from dynamical calculations on a model potential energy surface adjusted to fit the observed far-infrared rotation-tunneling spectrum of the NH3 dimer.
Surface-induced fragmentation of higher fullerenes and endohedral metallofullerenes
Authors: Kimura Takumi , Sugai Toshiki , Shinohara Hisanori
Published: July 2002

We report the first results of surface collisions of pure hollow fullerenes (C60, C70 and C78) and endohedral metallofullerenes (Y@C82, Ca@C82 and Ca@C84), isolated by liquid chromatography, against solid (silicon and gold) surfaces and self-assembled monolayer (SAM) films. The experiments have been performed by a reflectron type time-of-flight mass spectrometer modified for measuring surface-induced dissociation (SID) spectra. No surface-induced fragment is observed for the surface collisions with the solid surfaces and the alkanethiolate SAM film. In contrast, sequential C2-loss fragments have been observed for the surface collisions of hollow fullerenes and Ca@C84 with the fluorinated SAM film.
A comparative study of existing and new design techniques for protein models
Authors: Micheletti Cristian , Maritan Amos , Banavar Jayanth R.
Published: July 2002

We present a detailed study of the performance and reliability of design procedures based on energy minimization. The analysis is carried out for model proteins where exact results can be obtained through exhaustive enumeration. The efficiency of design techniques is assessed as a function of protein length and the number of classes into which amino acids are coarse grained. It turns out that, while energy minimization strategies can identify correct solutions in most circumstances, it may be impossible for numerical implementations of design algorithms to meet the efficiency required to yield correct solutions in realistic contexts. Alternative design strategies based on an approximate treatment of the free energy are shown to be much more efficient than energy-based methods while requiring nearly the same CPU time. Finally, we present a novel iterative design strategy that incorporates negative design with the use of selected decoy structures that compete significantly with the target native state in housing the designed sequences. This procedure allows one to identify systematically all sequences that fold on a given target structure.