bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic extracule moments of atoms in position and momentum spaces
Authors: Koga Toshikatsu , Matsuyama Hisashi
Published: July 2002

The electronic extracule density represents the probability density function for the center-of-mass motion of any pair of electrons. Two methods are presented for the calculation of electronic extracule moments 〈Rn〉 in position space and 〈Pn〉 in momentum space of atomic systems within the Hartree–Fock framework, where R and P are respectively the position and momentum distances between the center-of-mass of any pair of electrons and the coordinate origin. The methods are applied to the neutral atoms from He to Xe in their ground state, and the moments 〈Rn〉 with n=−2 to +4 and 〈Pn〉 with n=−2 to +3 are reported based on the numerical Hartree–Fock calculations. These extracule moments are pointed out to have intimate relations with their partner intracule moments. The present results, together with the previous ones for the electronic intracule moments [J. Chem. Phys., 107, 8510 (1997)], compile accurate Hartree–Fock electron-pair moments for all the 53 atoms He through Xe in both position and momentum spaces.
Torsional splitting of the intermolecular vibrations of phenol (H2O)1 and its deuterated isotopomers
Authors: Schmitt M. , Jacoby Ch. , Kleinermanns K.
Published: July 2002

The intermolecular vibrations of phenol–water and their tunneling (torsional) splittings have been assigned in the S1 state by mass resolved spectral hole burning. The abundance of transitions in the low frequency region of the spectra can be traced back to torsional tunneling of the water moiety, which splits all vibronic levels. Especially the in plane wag vibration β2 exhibits a large splitting which points to a strong coupling with the H2O torsion τ and a substantial lowering of the effective torsional barrier after β2 excitation. Based on the discrimination of different isotopomers and their isomers and of the torsional sublevels a reassignment of some intermolecular transitions could be given.
Laser induced desorption of thin molecular films investigated with high time resolution
Authors: Handschuh Martin , Nettesheim Stefan , Zenobi Renato
Published: July 2002

The temporal evolution of the surface coverage of aniline submonolayers desorbing from graphite under nanosecond laser induced thermal desorption (LITD) conditions was investigated in a novel two laser experiment. The experimental findings reveal that nanosecond LITD of submonolayers can be described by thermal equilibrium kinetics.
Infrared diode laser spectroscopy of the CCO radical: The 2ν1−ν1 and ν1+ν3−ν3 difference bands
Authors: Moazzen-Ahmadi N. , Boeré R. T.
Published: July 2002

The 2ν1−ν1 and ν1+ν3−ν3 difference bands of the CCO radical in the gas phase have been studied using a tunable infrared diode laser spectrometer. The CCO radical was produced using a flowing mixture of carbon suboxide and helium subjected to a hollow-cathode discharge. The spectral region between 1920–1960 cm−1 was probed. Ninety and seventy-five rovibrational transitions were measured in the 2ν1−ν1 and ν1+ν3−ν3 bands, respectively. The analyses of these bands yielded spectroscopic constants for the (001), (200), and (101) vibrational states. The band origins for 2ν1−ν1 and ν1+ν3−ν3 were determined to be 1941.85761(54) cm−1 and 1936.79402(56) cm−1, respectively.
Direct observation of the ionization threshold of triplet methylene by photoionization mass spectrometry
Authors: Litorja Maritoni , Ruscic Branko
Published: July 2002

The photoionization spectrum of the ionization threshold region of methylene has been recorded for the first time. The CH2 radical was produced in situ by successive hydrogen abstractions from methane precursor. The observed steplike onset corresponds to the vibrationless transition CH2+X̃ 2A1←CH2X̃ 3B1 and leads to the adiabatic ionization energy of CH2 of 10.393±0.011 eV. This value is slightly higher than the nominal midrise of the threshold step structure, which is depressed by rotational autoionization effects. In a separate set of experiments, the threshold region of the CH2+ fragment from CH3 was recorded at room temperature. The fragment appearance energy was accurately determined by fitting to be 15.120±0.006 eV at 0 K. The combination of these two measurements provides the best current experimental value for the bond dissociation energy of the methyl radical, D0(H–CH2)=4.727±0.012 eV=109.0±0.3 kcal/mol (corresponding to 110.4±0.3 kcal/mol at 298 K), and yields ΔHf0⊖(CH2,X̃ 3B1)=93.2±0.3 kcal/mol (93.3±0.3 kcal/mol at 298 K) and ΔHf0⊖(CH2,ã  1A2)=102.2±0.3 kcal/mol (102.3±0.3 kcal/mol at 298 K). The latter makes the reaction CH2 (ã  1A2)+H2O→CH3+OH essentially thermoneutral, ΔHr0⊖=0.0±0.3 kcal/mol.
Molecular mechanisms for disparate miscibilities of poly(propylene) and head-to-head poly(propylene) with other polyolefins
Authors: Freed Karl F. , Dudowicz Jacek , Foreman K. W.
Published: July 2002

Recent experiments find large differences in binary blend miscibilities between poly(propylene) in its ordinary and head-to-head forms with poly(ethylene propylene). This miscibility variation cannot be understood from simple Flory–Huggins (FH) theory but can readily be explained with the lattice cluster theory (LCT), a generalization of FH theory that accounts for nonrandom mixing, monomer structure, and chain semiflexibility in a systematic manner. Calculations are also presented of miscibility trends for poly(propylene)/poly(ethylene butylene) blends, one of which has yet to be studied. Experiments for this system should provide a nontrivial test of the LCT predictions.
A,
C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory
Authors: Shafizadeh N. , Bréchignac Ph. , Dyndgaard M. , Fillion J. H. , Gauyacq D. , Levy B. , Miller J. C. , Pino T. , Raoult M.
Published: July 2002

The A–X transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the C and D states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the 3sσ,A, 3pπ,C, and 3pσ,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A–X, C–X, and D–X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar A′ and A″ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the observed and calculated spectra. In contrast to the ground state, the Renner–Teller splitting of the 3pπ,C state into two A′ and A″ components is very large, of the order of 4000 cm−1. This effect is complicated by further mixing between the states of A′ symmetry induced by the argon atom. The A state is anisotropic and weakly bound with a small potential well at the linear configuration (the argon atom being on the side of the oxygen). The C(A″) and the bound electronic component of the strongly mixed C+D(A′) states exhibit a vibrational structure close to that of the ion and, consequently, present some Rydberg character even if the Coulomb field central symmetry (s-p) is broken by the perturbing argon atom.
Coexistence curve of perfluoromethylcyclohexane+carbon tetrachloride near the critical point in composition-pressure and composition-temperature space
Authors: Dobashi Toshiaki , Koizumi Jun-ichi , Kita Rio , Nakata Mitsuo
Published: July 2002

Coexistence curves in volume fraction-temperature (φ–T) space and in volume fraction-pressure (φ–p) space have been measured for the binary system perfluoromethylcyclohexane+carbon tetrachloride in the temperature range Tc–T<0.8 K and in the pressure range p–pc<3.2 MPa near the critical point (Tc,pc,φc). The observed coexistence curves were expressed by the simple scaling relation φ+–φ−=BT(Tc−T)βT in φ–T space and by φ+–φ−=Bp(p–pc)βp in φ–p space. The exponent was determined as βT=0.326±0.001 and βp=0.330±0.006, which agree with each other and with the theoretical value β=0.327 for the Ising model. The observed coefficients give (Bp/BT)1/β=0.42±0.02 K/MPa, which may be compared with the slope (dT/dp)c=0.38 K/MPa of the p–T critical line in light of a thermodynamic argument. This means that the pressure range is converted into the temperature range with the slope (dT/dp)c as a scale factor.
Vibrational energy localization in the stretching vibrational (1000A1/F2), (2000A1/F2), and (3000A1/F2) band systems of SnD4120
Authors: Halonen M. , Halonen L. , Bürger H. , Jerzembeck W.
Published: July 2002

High resolution Fourier transform infrared spectra of the stretching fundamental (1000A1/F2) as well as the first (2000A1/F2) and second (3000A1/F2) stretching vibrational overtones of monoisotopic deuterated stannane, SnD4120, were measured using a Bruker 120 HR interferometer. The symmetric top K structure of a prolate symmetric rotor was observed in the second stretching vibrational overtone, which indicates that localization of vibration has occurred and the dynamic symmetry of the molecule has changed. Rotational analyses of the spectra were performed, and the local mode relations obtained confirm that vibrational energy localization takes place in the second stretching vibrational overtone of deuterated stannane. The results indicate a surprisingly similar behavior of deuterated stannane and normal stannane.
Quantum rate constants for the H2+OH reaction with the centrifugal sudden approximation
Authors: Zhang Dong H. , Light John C. , Lee Soo-Y.
Published: July 2002

The cumulative reaction probability (CRP) has been calculated for the H2+OH↔H2O+H in its full dimensionality by using the centrifugal sudden (CS) approximation for J>0. The Boltzmann average of the CRP provides the most accurate thermal rate constant to date for the title reaction on the Walch, Dunning, Schatz, Elgersma (WDSE) potential energy surface (PES). It is found that the theoretical rate is larger than the experimental value in the low temperature region (a factor of ∼1.8 at 300 K), and smaller than the experimental value for temperatures higher than 500 K, indicating that a more accurate PES is needed to provide a quantitative description of the title reaction. We also demonstrate that the “J-shifting” approximation in which we calculate N(J>K,K) from N(J=K,K) by an energy shift works very well for this reaction. However, the “J- and K-shifting” approximation [calculating N(J,K) from N(J=0,K=0)] overestimates the rate for this reaction by about 60% for all the temperatures investigated. It is also found that the CS rate constant is substantially lower than the rate constant for the ground rovibrational state of the reagents calculated on the same PES, indicating that initial rotational excitation is important to the thermal rate constant for this reaction (it causes a decrease).
Spectral dispersion of second molecular hyperpolarizability of diacetylene derivatives: Correlation with electronic and chemical structure
Authors: Nampoothiri A. V. V. , Puntambekar P. N. , Singh Bhanu P. , Sachdeva R. , Sarkar A. , Saha Dipti , Suresh A. N. , Talwar S. S.
Published: July 2002

Dispersion of the second hyperpolarizability γ(−ω;ω,ω,−ω) of a series of conjugated diacetylene derivatives measured by single beam Z-scan technique, is presented. The spectrum of γ is explained by a three-level model involving a one-photon excited state, a two-photon excited state and the ground state. The location of one and two-photon states and corresponding transition dipole moments have been estimated. The influence of various electronic states on the nonlinear optical properties of these derivatives have been discussed. Broad correlations of the nonlinearity with the structure have been drawn.
A new class of stochastic simulation models for polymer stress calculation
Authors: Feigl Kathleen , Öttinger Hans Christian
Published: July 2002

We introduce a new class of stochastic models for polymer stresses which offers a blending of continuum mechanics, network theory and reptation theory. The stochastic dynamics of the model involve two independent Gaussian stochastic processes, Q1 and Q2. Associated with each random vector, Qi, is a random variable, Si, that describes the vector’s survival time during which it evolves according to a deterministic equation of motion. The expression for the stress tensor is an ensemble average of f1(Q12,Q22)Q1Q1+f2(Q12,Q22)Q2Q2, where the fi are scalar functions of Q12=Q1⋅Q1 and Q22=Q2⋅Q2. The relationship between this new class of models and the class of factorized Rivlin-Sawyers integral models is indicated, and simulation models from this new class are used to predict rheological behavior of three low-density-polyethylene melts. We find that the steady-state shear data of all three melts, and the time-dependent elongational viscosity of one of the melts, can be predicted well by models with the same fi, but different probability densities for Si which are obtained from the different relaxation spectra.
Electronic spectroscopy and ab initio quantum chemical study of the Ã(1A″)−X̃(1A′) transition of CFBr
Authors: Knepp Pamela T. , Scalley Christopher K. , Bacskay George B. , Kable Scott H.
Published: July 2002

The Ã(1A″)−X̃(1A′) electronic transition of jet-cooled CFBr has been investigated by spectroscopic and ab initio theoretical methods. Laser induced fluorescence (LIF) excitation spectroscopy was used to explore the rovibronic levels in the Ã state, and dispersed fluorescence spectroscopy was used to study ground-state vibrations. Analysis of these spectra yielded gas-phase vibrational frequencies and anharmonicity constants in both electronic states. The computed ab initio vibrational frequencies in both X̃ and Ã states are in good accord with the experimental values. The Ã-state fluorescence lifetimes varied between 100 ns and 3 μs as a function of excited vibronic state. The highest lying levels displayed a shortened fluorescence lifetime, and some vibronic states that involved ν1 (the CF stretch) exhibited shortened lifetimes (300–500 ns) irrespective of the vibrational energy. Vibronic structure in the LIF spectrum disappeared for vibrational energy in excess of 2957 cm−1. Calculations of the Ã-state potential-energy surface show that it has a small barrier to dissociation to CF+Br with a barrier height in good accord with observed termination of fluorescence. The predicted photochemical pathway to production of CF+Br fragments was proven experimentally by detection of CF fragments. The photofragment excitation spectrum showed strong, increasingly broad vibronic structure at higher energies than the LIF spectrum. At lower energy, sharp but weaker vibronic structure was still evident, overlapping the LIF spectrum. There appears to be two photochemical mechanisms to produce CF+Br, one direct and one indirect. We estimate the height of the barrier to direct dissociation to lie 3250±150 cm−1 above the zero-point level of the Ã state. The asymptotic thermochemical dissociation limit is estimated to lie ⩾1100 cm−1 lower. The thermochemical bond dissociation energy for the C–Br bond in CFBr was thereby estimated to be Ediss⩽23 180 cm−1, which led to an estimate of the heat of formation for CFBr, ΔfH2980⩾86 kJ mol−1.
A new scheme to calculate dipole-allowed singlets in conjugated polymers
Authors: Zhang G. P. , George Thomas F. , Pandey Lakshmi N.
Published: July 2002

Properly combined with the Lanczos iteration algorithm, the density matrix renormalization group enables us to directly calculate the dipole-allowed singlet states and the transition moment elements between these states and the ground state for longer chains. Our new scheme does not resort to reflection symmetry and electron-hole symmetry, nor to the construction of different superblocks for the ground state and these singlets separately. As an example, we calculate the ground state and first-excited state in the extended Hubbard model. Our calculations show that for a large U, the transition matrix elements decrease with the chain length; for V around U/2, the transition matrix elements can increase with the chain length, which is consistent with recent experimental results.
Critical viscosity of the ionic mixture triethyl n-hexyl ammonium triethyl n-hexyl borate in diphenyl ether
Authors: Wiegand Simone , Berg Robert F. , Levelt Sengers Johanna M. H.
Published: July 2002

We report measurements of the viscosity near the consolute point of triethyl n-hexyl ammonium triethyl n-hexyl borate in diphenyl ether. Until recently, this ionic mixture was the leading candidate for a “mean-field” ionic fluid composed of small molecules. The measurements of the coexistence curve of Singh and Pitzer and the measurements of turbidity of Zhang et al. had indicated mean-field static behavior. In contrast, the present measurements show a critical viscosity enhancement similar to that seen in Ising fluids. Such an enhancement is not expected in either a mean-field fluid or a fluid with sufficiently long-ranged forces. The measurements were made in two very different viscometers. Both viscometers achieved low shear rates by use of a flow impedance larger than in a conventional capillary viscometer. The first viscometer’s impedance was a glass frit consisting of about 105 pores of 5.5 μm diam each. The second viscometer’s impedance was a single 1 m long, 203 μm diam capillary. In both viscometers, the sample was sealed entirely in glass, in order to inhibit decomposition of the sample.
Polar orientational phase transition and differential dielectric constant of smectic monolayers on a water surface
Authors: Wu Chen-Xu , Ou-Yang Zhong-can , Iwamoto Mitsumasa
Published: July 2002

We build up a self-consistent equation and derive a general equation of orientational phase transition between a normal-director phase and a tilted-director phase for smectic monolayers on a water surface under monolayer compression with a consideration of molecular configuration. A differential dielectric constant for monolayer films is discovered and discussed. The change of the first orientational order parameter and the dielectric constant at the polar orientational phase transition point is also examined. It is found that the influence of the polar orientational phase transition on the first orientational order parameter in monolayer films is clear compared with that on the dielectric constant. This reveals that Maxwell-displacement-current measuring technique would be a useful way to measure such a phase transition.
Time-resolved photoelectron spectroscopy of the allyl radical: The lifetimes of the ultraviolet bands
Authors: Schultz Thomas , Fischer Ingo
Published: July 2002

We report [1+1′] picosecond time-resolved pump–probe photoelectron spectra of the UV bands of the allyl radical. The experiments are performed in a molecular beam of allyl radicals, generated by supersonic jet flash pyrolysis. Photoelectron spectroscopy in a magnetic bottle is shown to be a suitable method for investigating the photophysics of organic radicals. Lifetimes were obtained for all vibronic bands between 250 and 238 nm previously assigned by MPI spectroscopy to the electronically excited B 2A1, C 2B1, and D 2B2 states, with values ranging from 20 ps to 9 ps. The nonradiative decay is due to internal conversion to the electronic ground state. Information on the structure of the allyl cation is deduced from the photoelectron spectrum.
Anisotropy of pump-probe absorption of the hydrated electron: A statistical model
Authors: Bratos S. , Leicknam J-Cl.
Published: July 2002

A statistical model is presented to interpret the pump-probe absorption anisotropy of the hydrated electron. Its basic ingredient is the assumption of cascade-type deactivation processes between its excited p-states. The theory uses the correlation function formalism of nonlinear statistical mechanics. A number of technical problems are discussed: construction of a relaxation operator describing the cascade, solution of the corresponding Heisenberg equation of motion, calculation of various tensorial quantities, etc. According to this model, the presence of a cascade reduces the pump-probe dichroism, particularly if the p-state splitting is small. Very short cascade relaxation times of the order of 10 fs are required for that purpose. As the upper state of an s↔p transition is quasi-degenerate, the magic angle theories do not apply here. The similarity between the deactivation processes in excited electronic states of polyatomic molecules and of the hydrated electron is emphasized.
One dimensional carrier trapping in the crystalline phase of a columnar liquid crystal
Authors: Donovan K. J. , Kreouzis T. , Boden N. , Clements J.
Published: July 2002

A study of charge transport in the crystalline phase of the columnar liquid crystal, hexakis(n-hexyloxy)triphenylene, HAT6, is reported. A simple model is introduced and fitted to observations of photocurrent transient decays in the crystalline phase of HAT6. From these a quasi one-dimensional trapping process is inferred with a field dependent trapping rate determined by carrier drift at high fields and controlled by carrier diffusion in the limit of zero field. The detrapping rate is found to be independent of field indicative of a neutral barrier. The trap concentration is found from fits of the transients to the model. The mobility of holes in the crystalline phase is found and compared to the time-of-flight mobility previously found for HAT6 in the discotic mesophase. Also found is the field dependence of the generation rate in the presence of an absorbing electrode adjacent to the photocreation site.
Conformational dynamics of a low temperature protein: Free base cytochrome-c
Authors: Schlichter J. , Fritsch K.-D. , Friedrich J. , Vanderkooi J. M.
Published: July 2002

Spectral diffusion waiting time experiments at 100 mK combined with aging time experiments are used to shed light on the features of the energy landscape of a cytochrome c-type protein and the respective conformational dynamics. The energy landscape shows features of a hierarchical organization. The time law which governs the dynamics in conformational phase space is a power law. The respective processes seem to be related to generalized diffusive-like motions.
Density functional calculation of structure and vibrational spectra of polyenes
Authors: Schettino Vincenzo , Luigi Gervasio Francesco , Cardini Gianni , Remigio Salvi Pier
Published: July 2002

The vibrational frequencies of a number of all trans polyenes H–(CH=CH)n–H ranging from n=1 to n=12 have been calculated within the density functional approach using a generalized gradient corrected exchange-correlation functional and a gaussian basis set. A remarkably good agreement with available experimental data is obtained. A correlation of the normal frequencies along the polyene series is presented describing the motions according to a nodal analysis. The results obtained are also correlated with the vibrations of an infinite polyenic chain.
Extended symmetry-adapted discrete variable representation and accelerated evaluation of Ĥψ
Authors: Chen Rongqing , Guo Hua
Published: July 2002

It has long been realized that symmetry-adapted discrete variable representation for molecular systems with nontrivial symmetry leads to significant reduction of the grid size, and thus computational costs. In this work, we show that even for molecules with no symmetry it is still possible to achieve significant computational savings by symmetry adaptation. The extended symmetry-adapted discrete variable representation takes advantage of the higher symmetry in a part of the total Hamiltonian denoted as the reference Hamiltonian, e.g., a kinetic energy operator. When the evaluation of its action constitutes the more expensive part of the calculation such as in discrete variable representation (DVR) based methods, significant savings may ensue. Applications to several commonly used kinetic energy operators are discussed. Numerical tests in simple as well as realistic systems are carried out to demonstrate the efficiency of the method.
Formation, stability, and structures of antimony oxide cluster ions
Authors: Kaiser Bernhard , Bernhardt Thorsten M. , Kinne Markus , Rademann Klaus , Heidenreich Andreas
Published: July 2002

The formation of positively charged antimony oxide clusters has been investigated as a function of oxygen partial pressure using time of flight mass spectrometry. With increasing oxygen partial pressure magic number patterns are observed, which can be attributed to the clusters of (Sb2O3)n+ and (Sb2O3)n(SbO)+ with 2⩽n<7 and 1⩽n<7, respectively. Oxygen rich clusters, i.e., clusters formed by the addition of one or more oxygen atoms to the above observed series, gain in intensity with increasing cluster size and increasing oxygen partial pressure. To obtain information about structures and general building principles of these clusters we have performed ab initio quantum chemistry calculations for the series (Sb2O3)n, (Sb2O3)n+, (Sb2O3)n(SbO)+, (Sb2O3)n(SbO2)+, and (Sb2O3)n(SbO3)+. Except from a defect center in the cationic series, antimony atoms are trivalent and oxygen atoms divalent. For the cationic series (Sb2O3)n(SbO)+, an open valence at one antimony atom is avoided by the formation of an additional Lewis-donor Lewis-acceptor Sb–O bond with a trivalent oxygen atom. Open structures with terminal oxygen atoms or with Sb–Sb bonds are very high in energy, expressing a principle of the formation of a maximum number of Sb–O bonds. In the series (Sb2O3)n(SbO2)+, an excess oxygen valence leads to structures with a central Sb+VO4 unit (i.e., without terminal oxygen). For n⩾3, such structures have a very pronounced energetic stability compared to isomers with a terminal oxygen atom or with an O–O bond. Characteristic building blocks in the neutral and in the cationic series are eight-membered rings, which are also found in the bulk antimony(III)oxide modification senarmonite, as well as Sb4O5 units bridged by oxygen atoms.
Ab initio calculation of atomic contributions to the magnetic susceptibility by continuous transformation of the origin of the current density in HF, H2O, NH3, and CH4 molecules
Authors: Ferraro M. B. , Caputo M. C.
Published: July 2002

The conventional random phase approximation (RPA) of the polarization propagator theory and a computational method based on continuous transformation of origin for the current density (CTOCD) induced within the electron cloud by an external homogeneous, static magnetic field has been employed to calculate atomic contributions to magnetic susceptibilities. The diamagnetic part of the magnetic susceptibility is written in terms of the polarization propagator. Since the paramagnetic term may also be obtained from the propagator it is thus possible to compute both contributions at the same level of approximation. The evaluated average susceptibility is independent of the origin of the vector potential, but depends on the origin of the reference frame. The atomic contributions to the diamagnetic and paramagnetic parts of the magnetic susceptibility are derived by applying off-diagonal hypervirial relations which are exactly fulfilled if the state functions are exact eigenfunctions of a model Hamiltonian. The rationalization of the magnetic susceptibilities into atomic contributions is applied to some small molecules: HF, H2O, NH3 and CH4, and the sum of these contributions is compared to the corresponding calculated total values and the experimental data for the molecular magnetic susceptibility for the same compounds. Computations are performed using basis sets of increasing quality. A series of sum rules for gauge independence of the computed results and charge-current conservation have been tested to document the accuracy of the calculation of magnetic properties.
Anomalous isotope effect in Ar–H2S versus the normal effect in Ne–H2S
Authors: de Oliveira Glenisson , Dykstra Clifford E.
Published: July 2002

Substitution of deuterium for hydrogen in the weakly bound cluster Ar–H2S is known to produce an anomalous effect in the ground vibrational state’s average rotational constant 〈B+C〉/2 in contrast to deuteration of Ar–H2O. The anomalous effect develops from the vibrational dynamics, and we show in this report that the effect is not likely to be found for deuteration of the Ne–H2S cluster. Ab initio calculations were used to generate a potential-energy surface for Ne–H2S. That surface and a surface derived from it were used in rigid body diffusion quantum Monte Carlo calculations to obtain the rotational constants of the ground vibrational states and thereby determine whether deuterium substitution increases or decreases 〈B+C〉/2. From these results, we show that the exceptional vibrational dynamics of Ar–H2S are a consequence of a surface feature, the extensiveness of a low-energy trough. Though a similar trough is found for Ne–H2S, it is not as extensive. In carrying out these calculations, we achieve an assessment of the role certain surface features and their representations play in vibrational averaging. The calculations also provide a prediction that 〈B+C〉/2 for Ne–H2S, for which there is no spectroscopic value available yet, is within the range of 2344–2493 MHz.
Thermodynamical stability of CH3ONO and CH3ONO−: A coupled-cluster and Hartree–Fock-density-functional-theory study
Authors: Gutsev Gennady L. , Jena Puru , Bartlett Rodney J.
Published: July 2002

The structure and thermodynamic stability of methylnitrite and its anion are studied by the infinite-order coupled-cluster method with all singles and doubles and noniterative inclusion of triple excitations [CCSD(T)] and Hartree–Fock-density-functional theory (HFDFT). We have optimized the geometries and computed the harmonic vibrational frequencies of major fragments, H2, CH, NH, OH, CN, N2, CO, NO, O2, CH2, NH2, H2O, HCN, HNC, HCO, HNO, O2H, CO2, NO2, CH3, NH3, CNH2, HCO2, HNO2, CH3N, CH3O, CH3NO, CH3ON, CH2NO2, and their anions, when the latter exist. Fragmentation energies obtained at both levels of theory are rather close to each other, except for channels involving CN as a product. The CH3ONO− and CH3NO2− anions are shown to possess lower fragmentation energies than their neutral parents. This implies that the attachment of an extra electron to CH3NO2 or CH3ONO may have a crucial role in initiating the decomposition of these compounds. Also, the attachment of an extra electron to CH3NO2 or CH3ONO leads to the appearance of new exothermic decay channels of the anions.
Application of contracted distributed approximating functions to solving vibrational eigenvalue problems
Authors: Szalay Viktor , Smith Sean C.
Published: July 2002

It has been shown that an approximately band-limited function can be reconstructed by using the function’s values taken at appropriate equidistant grid points and a generalized Hermite-contracted-continuous-distributed-approximating-function (Hermite-CCDAF) as the reconstruction function. A sampling theorem prescribing the possible choices of grid spacing and DAF parameters has been derived and discussed, and discretized-Hermite-contracted DAFs have been introduced. At certain values of its parameters the generalized Hermite-CCDAF is identical to the Shannon–Gabor-wavelet-DAF (SGWDAF). Simple expressions for constructing the matrix of a vibrational Hamiltonian in the discretized-Hermite-contracted DAF approximation have been given. As a special case the matrix elements corresponding to sinc-DVR (discrete variational representation) are recovered. The usefulness and properties of sinc-DVR and discretized-Hermite-contracted-DAF (or SGWDAF) in bound state calculations have been compared by solving the eigenvalue problem of a number of one- and two-dimensional Hamiltonians. It has been found that if one requires that the same number of energy levels be computed with an error less than or equal to a given value, the SGWDAF method with thresholding is faster than the standard sinc-DVR method. The results obtained with the Barbanis Hamiltonian are described and discussed in detail.
Fluctuation effects on quadratic autocatalysis fronts
Authors: Velikanov Mikhail V. , Kapral Raymond
Published: July 2002

A Markov chain model for spatially distributed autocatalytic systems with a quadratic reaction rate is considered. An approximate solution for the local probability distribution is obtained in the form of a perturbation expansion for the regimes where diffusion is relatively fast. Using this approximate distribution, properties of the chemical wave fronts found in these autocatalytic systems are studied, and deviations of the minimum propagation velocity and the concentration profile from deterministic predictions are analyzed. A comparison with numerical results from lattice–gas automaton simulations is also provided.
Photoionization efficiency spectrum and ionization energy of S2O2
Authors: Cheng Bing-Ming , Hung Wen-Ching
Published: July 2002

The spectrum of photoionization efficiency (PIE) of S2O2 was measured in the spectral range 105–130 nm by means of a discharge flow and a photoionization mass spectrometer coupled to a synchrotron as a radiation source. S2O2 was generated from self reaction of SO radicals, SO+SO→S2O2, in a gaseous flow system. SO radicals were produced on reacting O atoms with CS2 and OCS. The ionization energy (IE) of S2O2 was determined to be (9.93±0.02) eV.
Prediction of quantum dot-like behavior in polymer nanoparticles: Quantum drops
Authors: Runge Keith , Sumpter Bobby G. , Noid Donald W. , Barnes Michael D.
Published: July 2002

Recent technological advances in the production of polymer particles suggests the feasibility of generating polymer nanoparticles from solution with discrete electronic structure in close analogy with quantum dots. We call these new particles quantum drops, which have controllable physical properties including radius, composition, chemical potential, and magnetic properties. We consider the electronic properties of electrons confined on the surface of these spherical polymer nanoparticles using a new Hamiltonian and semiclassical quantization.
Total electron scattering cross section for Cl2
Authors: Cooper G. D. , Sanabia J. E. , Moore J. H. , Olthoff J. K. , Christophorou L. G.
Published: July 2002

Absolute measurements of the total electron scattering cross section, σsc,t(ε), for chlorine, Cl2, are reported for electron energies, ε, ranging from 0.3 to 23 eV. The present data are in reasonable agreement with previous measurements of the cross sections for total electron scattering and total rotational excitation, and indicate significant indirect vibrational excitation due to negative ion resonances.
A test of quantum transition state theory for a system with two degrees of freedom
Authors: Liao Jie-Lou , Pollak Eli
Published: July 2002

A recently formulated quantum transition state theory is applied to scattering over an Eckart barrier coupled bilinearly to a harmonic mode. Results are compared with the numerically exact and the centroid density method computations of McRae et al. [J. Chem. Phys. 97, 7392 (1992)]. We find that quantum transition state theory is of comparable accuracy to the centroid approximation for all parameter ranges studied.
Fluid structure near heterogeneous surfaces
Authors: Delattre C. , Dong W.
Published: July 2002

Surface heterogeneity, geometric or energetic, can arise in various circumstances, e.g., surface defects, porous adsorbents, surface of amorphous materials, etc. Much effort, both in theory and in simulations, has been devoted to study fluid absorption on perfect plane surfaces up to now. In contrast, many fundamental questions remain open concerning the effect of surface disorder on the structure and thermodynamic properties of adsorbates near heterogeneous surfaces. In this work, we propose a simple model for heterogeneous surfaces which allows us to address some of these questions by carrying out Monte Carlo simulations. An interesting feature of this model is that the counterpart with a structureless plane surface can be defined precisely for the model heterogeneous surface. Thus, meaningful comparison between the two cases can be made which have allowed us to clearly reveal the influence of surface heterogeneity on the fluid structure near rough surfaces. We also propose a simple approach which relates the fluid density profile near a heterogeneous surface to that near a plane surface through a distribution function characterizing the surface corrugation. Results in very good agreement with simulations have been obtained from this approach.
On the efficient generation of Arn–SH free-radical clusters via ultraviolet photolysis of closed-shell Arn–H2S complexes
Authors: Mackenzie S. R. , Votava O. , Fair J. R. , Nesbitt D. J.
Published: July 2002

Detailed investigations are described for the generation of open-shell radical complexes formed by unimolecular photolysis of closed-shell van der Waals precursors. As a specific test case, ultraviolet photolysis of slit-jet cooled Arn–H2S (n⩽2) complexes at both 248 and 193 nm are shown to yield Ar–SH and Ar2–SH radical cluster species with surprisingly high efficiencies. Analysis of the laser induced fluorescence (LIF) spectra indicates that the radical complexes are produced with extensive van der Waals stretch/bend and overall rotational excitation, which is consistent with a simple ballistic model of the dissociation dynamics. The LIF spectra obtained as a function of expansion distance downstream provide clear evidence for remarkably efficient cooling of the newly formed radical cluster species by low-energy collisions with jet-cooled inert gas atoms at ≈10 K. Spectrally resolved Ar–SH and Ar2–SH LIF signals have been investigated as a function of Ar composition, which yields information on relative branching ratios for fragmentary (e.g., Ar2–H2S→Ar–SH+H+Ar) and nonfragmentary (e.g., Ar2–H2S→Ar2–SH+H) photolysis events.
Adiabatic diffusion Monte Carlo approaches for studies of ground and excited state properties of van der Waals complexes
Authors: Lee Hee-Seung , Herbert John M. , McCoy Anne B.
Published: July 2002

Several adiabatic extensions to the diffusion Monte Carlo approach are presented. In the first, an adiabatic form of the finite field method is developed for the systematic evaluation of expectation values. In addition, an adiabatic flexible node method for calculating excited states is described. The above methods are applied to NeSH and Ar2HCl where comparisons to results of variational calculations can be made.
A field-oriented chain of dipolar particles in elongational flow
Authors: Miguel M.-Carmen , Rubı́ J. M.
Published: July 2002

We study the behavior of an isolated field-oriented chain of dipolar particles in elongational fluid flow. Our main goal is to emphasize the effect of dipolar interactions on the chain’s contribution to the pressure tensor and to the viscosities of a dilute suspension of these linear aggregates. In our model, despite the overall rigid appearance of the chain at rest, the constituent beads may move slightly relative to one another, conferring a certain degree of flexibility to the chain. This flexibility is quantified in terms of a dimensionless parameter, λ−1, comparing thermal and dipolar energies. We perform an expansion in λ−1, and obtain the first correction to the rigid chain contribution to the Kramers’ pressure tensor for different flow geometries. The interplay of the elongational flow field and the field-induced chain orientation gives rise to a rich variety of scenarios. We compute the elongational, shear, and rotational viscosities in some representative situations.
Pressure–volume properties of endlinked hard-chain polymer networks
Authors: Kenkare N. R. , Hall C. K. , Khan S. A.
Published: July 2002

Equilibrium molecular dynamics simulations are used to obtain the pressure and configurational chain properties of near-perfect, off-lattice, trifunctional hard-chain networks of chain lengths 20, 35, 50, and 100, and of tetrafunctional hard-chain networks of chain lengths 20, 35, and 50 over a range of packing fractions. Our simulation results show that the variation of network pressure with density is similar to that of uncrosslinked chain systems of the same chain length, except at low densities where the network pressure shows a negative region, as first observed by Escobedo and de Pablo. We present a theoretical treatment leading to an analytical expression for the network pressure as the sum of liquid-like and elastic contributions. The liquid-like contribution is obtained by extending the generalized Flory-dimer theory to networks, and the elastic contribution is obtained by treating the network as a set of interpenetrated dendrimers and using an ideal chain-spring analogy to calculate the free energy. The theoretical predictions for network pressure are in good agreement with simulation data.
Ethanol force fields: A molecular dynamics study of polarization effects on different phases
Authors: González M. A. , Enciso E. , Bermejo F. J. , Bée M.
Published: July 2002

We have studied two simple approaches to consider thermodynamic state dependent electrostatic interactions for molecular modeling of different phases of ethanol: the addition of an isotropic point polarizability located in the oxygen, and a self-consistent calculation of the effective dipole moment at each thermodynamic state. We have performed molecular dynamics simulations in order to investigate the thermodynamic properties, structure and dynamics of the liquid phase at three experimental densities, as well as in the monoclinic crystal and at critical conditions. In order to rationalize the effects of changing the dipole moment of the molecules, simulations with a nonpolarizable model for ethanol were also performed. The results show that a nonpolarizable model with an effective dipole moment is able to reproduce most of the static and dynamic properties of the condensed phases of ethanol, while the need to take into account the real dipole moment of the isolated molecule by using a polarizable model is more evident in the low-density states.
Study on microstructural lattice deformation of polypyrrole by positron annihilation lifetime spectroscopy
Authors: Chakrabarti Swapan
Published: July 2002

Microstructural defects (polarons, bipolarons), responsible for conduction in polypyrrole (PPy) has been studied using positron annihilation lifetime (PAL) spectroscopy, conductivity and electron spin resonance (ESR) measurements. Pyrrole, oxidized at different oxidizing strengths [0.13(N)–2.0(N)] by ammonium ferric sulphate has been used for the preparation of PPy. It is observed from the PAL measurement that of the two lifetime components the longer one originates from annihilation of positron in the defect (polaron and bipolaron) sites and the corresponding intensity is a measure of the defect concentration. Further it is evident from the ESR spectra that the dominant defects are polarons at lower oxidizing strengths [⩽ 0.75(N)] and bipolarons at strengths ⩾ 1.0(N). Conductivity data corroborate the findings.
Chain formation in homogeneous gas–liquid nucleation of polar fluids
Authors: Rein ten Wolde Pieter , Oxtoby David W. , Frenkel Daan
Published: July 2002

We report a computer-simulation study of homogeneous gas–liquid nucleation in a model for strongly polar fluids. We find that the nucleation process is initiated by chain-like clusters. As the cluster size is increased, the chains become longer. However, beyond a certain size, the nuclei collapse to form compact, spherical clusters. Nevertheless, in the interface of the collapsed nuclei a high degree of chain formation is preserved. We compare the interface of the collapsed nuclei with the planar interface and find that the interface of the globule-like nuclei differs markedly from the flat interface. Classical nucleation theory underestimates both the size of the critical nucleus and the height of the nucleation barrier.
Ab initio study of the n-π* electronic transition in acetone: Symmetry-forbidden vibronic spectra
Authors: Liao D. W. , Mebel A. M. , Hayashi M. , Shiu Y. J. , Chen Y. T. , Lin S. H.
Published: July 2002

Ab initio calculations of geometry and vibrational frequencies of the first singlet excited A21(1A″) state of acetone corresponding to the n-π* electronic transition have been carried out at the CASSCF/6-311G** level. The major geometry changes in this state as compared to the ground state involve CO out-of-plane wagging, CO stretch and torsion of the methyl groups, and the molecular symmetry changes from C2v to Cs. The most pronounced frequency changes in the A″1 state are the decrease of the CO stretch frequency v3 by almost 500 cm−1 and the increase of the CH3 torsion frequency v12 from 22 to 170 cm−1. The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for calculations of Franck–Condon factors. Transition matrix elements over the one-electron electric field operator at various atomic centers calculated at the state-average CASSCF/6-311+G** level are used to compute vibronic couplings between the ground A11, A21, and Rydberg B21(n-3s), 2 A11(n-3py), 2 A21(n-3px), 2 B21(n-3pz), and B11(n-3dxy) electronic states, and the Herzberg–Teller expansion of the electronic wave function is applied to derive the transition dipole moment for A11→A21 as a function of normal coordinates. The results show that the intensity for this transition is mostly borrowed from the allowed A11-B21(n-3s) transition due to vibronic coupling between A21 and B21 through normal modes Q20, Q22, and Q23 and, to some extent, from the A11-B11 transition due to Q19 (CO in-plane bend) which couples A21 with B11(n-3dxy). The calculated total oscillator strength for the n-π* transition through the intensity-borrowing mechanism, 3.62×10−4, is in close agreement with the experimental value of 4.14×10−4. Ninety-four percent of the oscillator strength comes from the perpendicular component (b1 inducing modes) and 6% from the parallel component (b2 modes). Calculated spectral origin, 30 115 cm−1 at the MRCI/6-311G** level, underestimates the experimental value by ∼300 cm−1. Calculated positions of the most intense peaks in the spectra also reasonably agree with the experimental band maximum. The presence of numerous weak vibronic peaks densely covering a broad energy range (∼12 000 cm−1) explains the diffuse character of the experimental n-π* band. Most of the bands observed in fluorescence excitation spectra [Baba and Hanazaki, Chem. Phys. Lett. 103, 93 (1983); Baba, Hanazaki, and Nagashima, J. Chem. Phys. 82, 3938 (1985)] can be assigned based on the computed spectrum.
Freezing in polyampholytes globules: Influence of the long-range nature of the interaction
Authors: Angerman Hindrik Jan , Shakhnovich Eugene
Published: July 2002

In random heteropolymer globules with short-range interactions between the monomers, freezing takes place at the microscopic length scale only, and can be described by a one-step replica symmetry breaking. The fact that the long-range Coulomb interaction has no intrinsic length scale suggests that freezing in random polyampholyte globules might take place at all length scales, corresponding to an overlap parameter q(x) that increases continuously from zero to its maximum value. Study of the polyampholyte globule within the independent interaction approximation seems to confirm this scenario. However, the independent interaction model has an important deficiency: it cannot account for self-screening, and we show that the model is only reliable at length scales shorter than the self-screening length. Using the more realistic sequence model we prove that in the general case of a random heteropolymer globule containing two types of monomers such that unlike monomers attract each other, freezing at arbitrarily large length scales is not possible. For polyampholyte globules this implies that beyond the self-screening length, the freezing behavior is qualitatively the same as in the case of short-range interactions. We find that if the polyampholyte globule is not maximally compact, the degree of frustration is insufficient to obtain freezing.