bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical comparison of single-reference and multireference coupled-cluster methods: Geometry, harmonic frequencies, and excitation energies of N2O2
Authors: Tobita Motoi , Perera S. Ajith , Musial Monika , Bartlett Rodney J. , Nooijen Marcel , Lee Jae Shin
Published: December 2003

Calculated vertical excitation energies, optimized geometries, and vibrational frequencies of the nitric oxide dimer are reported. The “multireference” (MR) nature of the problem and weak bond between the monomers make a proper description of the system difficult, and standard methods are not as applicable to this system. In this study, recently developed methods such as the double-electron-affinity similarity-transformed equation-of-motion coupled cluster method (DEA-STEOM-CCSD), MR Brillouin–Wigner CCSD (MR-BWCCSD), MR average quadratic CCSD (MR-AQCCSD), and others are used along with a series of basis sets of increasing accuracy. The calculated excitation energies are consistent and convergent with respect to the basis set in DEA-STEOM-CCSD, MR-BWCCSD, and MR-AQCCSD methods. The geometries are highly sensitive to the basis set size and the challenge to obtain the right answers in the basis set limit remains. Nevertheless, we obtain qualitative agreement with the experimental geometry and harmonic vibrational frequencies. The results from the above multireference methods show dramatic improvement over the coupled cluster with singles and doubles and perturbative triples excitation [CCSD(T)] results. Like O3, (NO)2 offers an extremely challenging example in its ground and excited states for single-reference and multireference theory. It deserves to be a standard test molecule as new methods are developed.
Dispersion interaction of high-spin open-shell complexes in the random phase approximation
Authors: Żuchowski Piotr S. , Bussery-Honvault Béatrice , Moszynski Robert , Jeziorski Bogumil
Published: December 2003

Equations of the random-phase approximation (RPA) for high-spin open-shell molecules are derived using the time-dependent variational principle and a unitary exponential representation of the high-spin determinantal wave function parametrized with a nonredundant set of variational parameters. The restricted open-shell RPA theory is applied to derive expressions for the polarization propagator, and for the dispersion energy of high-spin open-shell complexes. It is also used to define the intramonomer correlation expansions of the RPA dispersion energy using various Møller–Plesset-type partitionings of the Hamiltonian. A close relation between the present treatment and the multipole expansion approach of Hettema and Wormer [H. Hettema and P. E. S. Wormer, J. Chem. Phys. 93, 3389 (1990)] is established and discussed. Numerical results for various high-spin open-shell–closed-shell and open-shell–open-shell complexes are presented. Comparison of the dispersion energy computed in the random phase approximation with highly correlated results from the full configuration interaction or coupled-cluster singles, doubles, and approximate triples calculations shows that the random phase approximation accounts for the major part of the intramonomer correlation effects in the dispersion energy. For open-shell–closed-shell complexes the convergence of the intramonomer correlation expansion through the second order is very good, while for the more difficult case of open-shell–open-shell complexes is much less satisfactory, and full RPA calculations are necessary to get reliable results.
Restricted density-functional linear response theory calculations of electronic g-tensors
Authors: Rinkevicius Zilvinas , Telyatnyk Lyudmyla , Sałek Paweł , Vahtras Olav , Ågren Hans
Published: December 2003

A method for calculations of electronic g-tensors based on a spin-restricted open-shell Kohn–Sham formalism and linear response theory is described. Test calculations for main group organic radicals and transition metal compounds have been carried out using two different spin–orbit approximations: Scaled spin–orbit and atomic mean-field spin–orbit operators. The results indicate slightly better performance of the proposed spin-restricted approach compared to previous methods based on the unrestricted Kohn–Sham formalism. An exception to this general improvement are the anions, as they show considerable spin-polarization. The results also show the superiority of the atomic mean field spin–orbit approximation of the spin–orbit operator with respect to the scaled approximation. For main group radicals, quantitative agreement with high level ab initio as well as experimental data are achieved, whereas for transition metal compounds the results systematically underestimate experimental values, showing also a considerable dependency on the employed exchange correlation functional, similarly to previous reports on calculations using unrestricted density functional theory. From this work we conclude that these problems are not primarily associated with the spin-contamination problem, and that they must be referred to the current time-dependent density functional theories as such.
The aminoboranylidene–iminoborane isomerization
Authors: Rosas-Garcia Victor M. , Crawford T. Daniel
Published: December 2003

We have determined the barrier and exothermicity of the aminoboranylidene (H2NB) to iminoborane (HNBH) isomerization reaction using coupled cluster energies extrapolated to the complete basis set limit, including core-correlation corrections and zero-point vibrational energies based on computed fundamental frequencies. Our best estimates of the reaction energy and reaction barrier are −41.35 kcal/mol and 27.40 kcal/mol, respectively. In addition, coupled cluster structural properties and fundamental vibrational frequencies, including isotopic shifts, are compared against experimental data.
Reversible work of the formation of a layer of a new phase on a spherical charged conductor within a uniform multicomponent macroscopic mother phase
Authors: Noppel M. , Vehkamäki H. , Kulmala M.
Published: December 2003

A thermodynamically consistent formalism is applied to calculate the reversible work needed to form a small layer of a new phase (embryo) around a charged insoluble conducting sphere within a uniform macroscopic mother phase. To treat the embryos of arbitrary size which are not the critical nuclei (i.e., which are not fragments that are in unstable equilibrium with the mother phase), a constraint in the form of an external field is introduced. When a critical nucleus is of primary interest, the specification of this constraint is not required. The results that are obtained for a critical cluster do not depend on which path approaches a critical state. However, the properties of noncritical clusters depend on the external constraining field and this effect is discussed in detail. The obtained new expression for the work of embryo formation differs from the one commonly used in the nucleation literature. The conditions of its extrema yield the correct conditions of equilibrium between the critical nucleus and the mother phase. Expressions are derived that are suitable for practical calculations of the size and composition of a critical nucleus.
Evidence of charge-enhanced C–H–O interactions in aqueous protonated imidazole probed by high pressure infrared spectroscopy
Authors: Su Chih-Chia , Chang Hai-Chou , Jiang Jyh-Chiang , Wei Pei-Ying , Lu Li-Chuan , Lin Sheng Hsien
Published: December 2003

Charge-enhanced C–H–O interactions are investigated for imidazole hydrochloride/D2O mixtures. Based on the concentration-dependence results, the infrared spectra reveal a strong C–H band and a weak shoulder at ∼3065 cm−1. The pressure-dependence measurements also exhibit considerable spectral changes as the mixtures transform to high-pressure ices. The absorption intensity of the weak shoulder, located in the region of 3060–3090 cm−1, drastically increases as the pressure is elevated. Observation of this low frequency band provides the experimental evidence of C2–H–O hydrogen bonds in the aqueous imidazolium solutions. Ab initio calculation results, predicting the frequency shift of the C–H stretching vibrations as C–H–O is interacting via hydrogen bonding, are discussed. Structural identification of the hydrophobic isomers in infrared spectra seems to be complicated by the presence of more than one stable isomeric form.
Zwitterion formation in hydrated amino acid, dipole bound anions: How many water molecules are required?
Authors: Xu Shoujun , Nilles J. Michael , Bowen Kit H.
Published: December 2003

While the naturally occurring amino acids are not zwitterions in the vapor phase, they are in aqueous solutions, implying that water plays an important role in inducing zwitterion formation. Together, these observations inspire the question, “How many water molecules are required to induce zwitterion formation in a given amino acid molecule?” In this paper, we address this question in the context of mass spectrometric and size-selected photoelectron spectroscopic studies of hydrated amino acid anions. We utilize the facts that zwitterions possess very large dipole moments, and that excess electrons can bind to strong dipole fields to form dipole bound anions, which in turn display distinctive and recognizible photoelectron spectral signatures. The appearance of dipole-bound photoelectron spectra of hydrated amino acid anions, beginning at a given hydration number, thus signals the onset of greatly enhanced dipole moments there and, by implication, of zwitterion formation. We find that five water molecules are needed to transform glycine into its zwitterion, while four each are required for phenylalanine and tryptophan. Since the excess electron may also make a contribution to zwitterion stabilization, these numbers are lower limits for how many water molecules are needed to induce zwitterion formation in these amino acids when no extra (net) charges are involved.
Impact of interatomic electronic decay processes on Xe 4d hole decay in the xenon fluorides
Authors: Buth Christian , Santra Robin , Cederbaum Lorenz S.
Published: December 2003

A hole in a 4d orbital of atomic xenon relaxes through Auger decay after a lifetime of 3 fs. Adding electronegative fluorine ligands to form xenon fluoride molecules, results in withdrawal of valence-electron density from Xe. Thus, within the one-center picture of Auger decay, a lowered Xe 4d Auger width would be expected, in contradiction, however, with experiment. Employing extensive ab initio calculations within the framework of many-body Green’s functions, we determine all available decay channels in XeFn and characterize these channels by means of a two-hole population analysis. We derive a relation between two-hole population numbers and partial Auger widths. On this basis, interatomic electronic decay processes are demonstrated to be so strong in the xenon fluorides that they overcompensate the reduction in intra-atomic Auger width and lead to the experimentally observed trend. The nature of the relevant processes is discussed. These processes presumably underlie Auger decay in a variety of systems.
Size dependence of CH stretching features on diamond nanocrystal surfaces: Infrared spectroscopy and density functional theory calculations
Authors: Chen Y.-R. , Chang H.-C. , Cheng C.-L. , Wang C.-C. , Jiang J. C.
Published: December 2003

Surface CH stretching features on diamond nanocrystals of different origins and sizes have been characterized by Fourier transform infrared spectroscopy and density functional calculations. Independent of the origins of the samples, a single sharp transition, corresponding to the CH stretch on C(111)-1×1, is observed at 2834 cm−1 for particles of sizes (d) larger than 100 nm. The feature, however, gradually diminishes as the particle size decreases and disappears nearly completely at d=5 nm. We interpreted the observation in terms of a domain size effect, namely, the domains of the {111}-type facets on the 5 nm diamonds are too small to yield the characteristic CH stretching feature [Chen et al., J. Chem. Phys. 116, 1211 (2002)]. The interpretation is supported by the present density functional calculations performed at the B3LYP/6-31G* level for three H-truncated diamond model clusters, C22H28, C38H42, and C58H58. Both “isolated” and “coupled” frequencies of the C–H stretching features on the {111}-type facets of these clusters are calculated and the results agree satisfactorily with the values obtained from the mixed isotope measurements conducted on diamond C(111)-1×1 single crystal surfaces [Cheng et al., J. Chem. Phys. 106, 7411 (1997)].
The formic acid dimer (HCOOH)2 probed by time-resolved structure selective spectroscopy
Authors: Matylitsky V. V. , Riehn C. , Gelin M. F. , Brutschy B.
Published: December 2003

Rotationally resolved spectra of the formic acid dimer (HCOOH)2 have been recorded by femtosecond degenerate four-wave mixing (fs DFWM) in the gas cell at room temperature (≈10 mbar, 298 K) and under supersonic jet conditions (≈60 K). Rotational recurrences of both, the monomeric and the dimeric species have been identified and assigned by fitted simulations of the fs DFWM spectra. For the first time the study of molecular clusters by fs DFWM is reported. From the fitted simulation based on a general nonrigid asymmetric rotor model the rotational and centrifugal distortion constants, and parametrized polarizability were derived for the dimeric structure of type O–H⋯O/O⋯H–O as well as for the formic acid monomer. With the assumption of unperturbed monomers a center-of-mass distance of R=2.990±0.001 Å for the monomers within the dimer is calculated from the spectroscopic results. The structure of the formic acid dimer and a possible isomer of the type C–H⋯O/O⋯H–O is discussed and compared to recent ab initio calculations.
Close coupling calculations for rotational relaxation of CO in argon: Accuracy of energy corrected sudden scaling procedures and comparison with experimental data
Authors: Martinez R. Z. , Domenech J. L. , Bermejo D. , Thibault F. , Bouanich J.-P. , Boulet C.
Published: December 2003

Fully quantal scattering calculations are carried out for CO in argon using both the close coupling (CC) and coupled states (CS) methods. CC and CS cross sections σ(j→j′) generally agree to within 15% or less with the exception of those corresponding to low Δj=|j′−j| values at low j where differences may reach 50%. The discrepancy arises mostly from efficient collisions with large orbital angular momentum, where the rotation of the quantization axis can no longer be neglected. Then, the CC calculations were used to test a scaling procedure based on the energy corrected sudden (ECS) approximation: given a set of basic cross-section σ(j→0), is it possible to predict the entire σo(j→j′) relaxation matrix? The ECS procedure yields reasonable agreement, on average, at the 13% level. However it fails at reproducing the Δj=1 cross sections in cases where the concept of a mean adiabaticity factor losses its physical meaning since the duration of the efficient collisions varies too much with the orbital angular momentum. On that basis, we have examined another question: the validity of an inversion procedure, based on the ECS scheme. Is it possible to determine the basic cross sections σ(j→0) starting from the knowledge of the easily measurable diagonal elements σo(j→j)? The ECS inverted basic rates agree with the CC ones to within about 20% up to j=15 and strongly diverge for higher j while the inversion leads to an overestimation of the mean duration of the efficient collisions. Then, using a high resolution Raman spectrometer, we recorded the Q-branch head of the fundamental band of CO in mixture with Ar at three temperatures, 87, 195, and 300 K, and total pressures up to 1.25 bar. Line-mixing effects in experimental Raman profiles are compared with CC theoretical predictions. Finally the close coupling results are also used to predict rotational relaxation times measured in free jets.
2,5-dichlorothiophenol on Cu(111): Initial adsorption site and scanning tunnel microscope-based abstraction of hydrogen at high intramolecular selectivity
Authors: Rao Bommisetty V. , Kwon Ki-Young , Liu Anwei , Bartels Ludwig
Published: December 2003

We investigated the adsorption of 2,5-di-chloro-thio-phenol (DCTP) on Cu(111) at 15 K and the formation of the thiolate upon electronic and thermal excitation. Initially, the sulfur atom of DCTP adsorbs at an on-top site and the molecule is able to rotate through six almost identical surface orientations. Attachment or removal of electrons from anywhere within the molecule at several hundred mV bias leads to the abstraction of the hydrogen atom from the thiol group in a nonthermal one-electron process with perfect selectivity. The resultant thiolate is locked into position on the surface.
Multilayer growth and chemisorbate reactivity of methanol on Pd{110}
Authors: Pratt Stephanie J. , Escott Daniel K. , King David A.
Published: December 2003

The development of a methanol multilayer on Pd{110} at 124 K has been examined with reflection–absorption infrared spectroscopy (RAIRS). The spectra indicate that a crystalline multilayer grows in which hydrogen-bonded chains of molecules are parallel to the substrate. A “sandwich” phase is identified and consists of several physisorbed layers in between the chemisorbed layer and the crystalline phase. Methanol desorption and decomposition on heating have been followed with simultaneous temperature-programmed RAIRS and TPD measurements. Desorption of chemisorbed methanol is observed at 214 K, whereas the decomposition reaction yielding adsorbed CO and H proceeds at ∼295 K on heating at 2 K s−1. Spectral bands observed on adsorption at 180 K to low exposure, which precede the appearance of the CO signal, are tentatively attributed to chemisorbed methanol. No conclusive evidence for a methoxy intermediate in the decomposition reaction is found.
Are both symmetric and buckled dimers on Si(100) minima? Density functional and multireference perturbation theory calculations
Authors: Jung Yousung , Shao Yihan , Gordon Mark S. , Doren Douglas J. , Head-Gordon Martin
Published: December 2003

We report a spin-unrestricted density functional theory (DFT) solution at the symmetric dimer structure for cluster models of Si(100). With this solution, it is shown that the symmetric structure is a minimum on the DFT potential energy surface, although higher in energy than the buckled structure. In restricted DFT calculations the symmetric structure is a saddle point connecting the two buckled minima. To further assess the effects of electron correlation on the relative energies of symmetric versus buckled dimers on Si(100), multireference second order perturbation theory (MRMP2) calculations are performed on these DFT optimized minima. The symmetric structure is predicted to be lower in energy than the buckled structure via MRMP2, while the reverse order is found by DFT. The implications for recent experimental interpretations are discussed.
Analytical implementation and critical tests of fluid thermodynamic perturbation theory
Authors: Ben-Amotz Dor , Stell George
Published: December 2003

We appreciably broaden the scope of existing thermodynamic perturbation theory for pure fluids in several ways. Analytical approximation strategies are combined to express the thermodynamic properties of fluids in analytical and/or algebraic form and applied to a generalized class of pair potentials. The results are illustrated by comparing previously reported simulation measurements with our predictions obtained using a new class of generalized Lennard-Jones potential functions with independently adjustable repulsive and attractive length scales. Results are also obtained for various purely repulsive fluids, including inverse-power potentials with exponents ranging from 12 to 72 and a Lennard-Jones repulsive reference fluid. Applications to argon (Lennard-Jones) and C60 (Girifalco potential) are used to probe the limits of applicability of first-order perturbation theory to systems with very short range attractive potentials. Furthermore, we discuss the role of these new results in a number of additional applications.
The cooperative dynamics of the H-bond system in 2-propanol/water mixtures: Steric hindrance effects of nonpolar head group
Authors: Sato Takaaki , Buchner Richard
Published: December 2003

We have determined the complex dielectric spectra of 2-propanol/water mixtures employing time domain reflectometry for the entire mixing range in 0.1⩽ν/GHz⩽25 at 20 °C, 25 °C, and 30 °C. At the specific concentrations, a frequency range was expanded up to 89 GHz with the help of waveguide interferometry. The excess activation free energy, ΔGE, enthalpy, ΔHE, and entropy, ΔSE, and their partial molar quantities, ΔGiE, ΔHiE, and ΔSiE [i=2-propanol (2PA) and water (W)] were derived from the relaxation time of the cooperative process, τ1. In the water-rich region, ΔHE and ΔSE exhibit conspicuous behavior with large positive values, which suggests an increase of the “time averaged” number and the strength of H-bonds. τ1 becomes considerably larger for 2-propanol/water than that for 1-propanol/water, resulting in ∼10% larger maxima of ΔH and ΔS. This indicates that the steric hindrance caused by the nonpolar group is more efficient for the branched isopropyl unit than that for the linear n-propyl moiety in preventing “disturbing fifth neighbor water molecules” from acting as new H-bond partners for a hydroxyl group of alcohols. The two pertinent maxima in ΔH2PAE and ΔS2PAE at X(molar fraction of 2-propanol)∼0.03 and ∼0.07 support the view that around X∼0.05, water molecules are pushed out of the first coordination shell of the nonpolar group into the bulk due to solute–solute association, commonly called hydrophobic interaction. In X⩾0.15, ΔH2PAE and ΔS2PAE become nearly zero, suggesting the generation of zigzag H-bonded alcohol chains similar to the pure alcohol. The analysis of the effective correlation factor, geff, revealed a strong structural perturbation effect of water in 0.5⩽X⩽1.0.
On different possibilities of a thermodynamically consistent determination of the work of critical cluster formation in nucleation theory
Authors: Schmelzer Jürn W. P. , Baidakov Vladimir G.
Published: December 2003

In a recent paper [J. Chem. Phys. 118, 1837 (2003)], Kashchiev claimed to have developed a general thermodynamically consistent formula allowing one to describe correctly the work of critical cluster formation in one-component systems for a variety of cases of formation of gaseous, liquid, and solid phases. It is shown that most of the consequences drawn lack any substantiation.
Low-frequency Raman study of water–ethylene glycol oligomer binary mixtures
Authors: Amo Yuko , Inadachi Yuriko , Tominaga Yasunori
Published: December 2003

Low-frequency Raman scattering of water–ethylene glycol oligomer (EGO) binary mixtures have been carried out. Reduced spectra of mixtures were decomposed into a linear combination of pure water and neat EGO spectra. The superposed spectra systematically deviate from the measured spectra of mixtures below 80 cm−1. Integrated intensity of the difference between obtained spectrum and superposed spectrum, Δχ″, depends on both the concentration and the unit numbers of EGO. Small Δχ″ and nonlinear concentration dependence of the coefficients of linear combination are related to the cooperative motions of EGO and water molecules.
Studies of metal hydrosulfides. III: The millimeter/submillimeter spectrum of BaSH (X̃ 2A′)
Authors: Janczyk A. , Ziurys L. M.
Published: December 2003

The pure rotational spectrum of BaSH (X̃ 2A′) and that of its deuterium isotopomer were measured using millimeter direct absorption techniques in the range 305–365 GHz. This work is the first time barium hydrosulfide has been observed by any spectroscopic technique. These radicals were synthesized in a dc discharge by the reaction of barium vapor, produced in a Broida-type oven, and either H2S or D2S. Thirteen rotational transitions of BaSH were recorded, as well as five transitions of BaSD; for each transition, asymmetry components for Ka=0 through Ka=6 or 7 were typically measured. Fine structure splittings, which generally ranged from 52 to 62 MHz in magnitude, were also observed in every transition. These data unambiguously demonstrate that BaSH is a bent molecule with Cs symmetry, following the trend established in the lighter alkaline earth hydrosulfides. Perturbations were found in the pure rotational spectra, however, likely resulting from accidental degeneracies and strong vibration-rotation coupling. From these measurements, rotational and fine structure parameters were established for BaSH and BaSD. An r0 analysis indicates that the Ba–S–H angle is 88°, while rBa–S=2.807 Å and rS–H=1.360 Å. A comparison of spin-rotation parameters with other hydrosulfides suggests that this interaction is dominated by second-order spin–orbit contributions in BaSH, generated by rotational mixing of nearby excited electronic states. Calculation of the g-tensor values for the unpaired electron in this radical suggests an elongated distribution about the metal atom primarily along the ĉ axis, in contrast to MgSH and CaSH, where the major asymmetry lies along the b̂ axis.
Infrared spectra of trans-HOCO, HCOOH+, and HCO2− trapped in solid neon
Authors: Forney Daniel , Jacox Marilyn E. , Thompson Warren E.
Published: December 2003

When a Ne:HCOOH sample is codeposited at ca. 5 K with neon atoms that have been passed through a microwave discharge, new absorptions appear in the infrared spectrum of the resulting solid that can be assigned to trans-HOCO, trans-HCOOH+, and HCO2−. The absorptions of trans-HOCO are readily identified by a comparison with those previously reported for that molecule trapped in solid argon. Preliminary assignments of infrared absorptions of HOCO+, confirmed in studies using another experimental system, are also suggested. The identifications of trans-HCOOH+ and of HCO2− are aided by study of the photodestruction characteristics of these products when the deposit is exposed to various wavelengths of visible and ultraviolet radiation, by an analysis of the spectra obtained from isotopically substituted samples, and by a comparison with the results of ab initio and density functional calculations. Three previously unidentified vibrational fundamentals of trans-HCOOH+ have been assigned, as have been four vibrational fundamentals of HCO2−, in the inert, nonionic environment of solid neon. The CH-stretching fundamental of HCO2− appears at an exceptionally low frequency. The results of density functional calculations of the structures and vibrational fundamentals of trans-HCOOH+, HCO2−, the W(C2v) structure of C(OH)2+, and cis- and trans-HCOOH− are given.
Infrared spectra of HOCO+ and of the complex of H2 with CO2− trapped in solid neon
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: December 2003

When a Ne:H2:CO2=800:10:1 mixture is codeposited at 4.3 K with neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit includes not only absorptions characteristic of experiments from which H2 is absent, but also numerous product absorptions that are common to similar experiments on Ne:HCOOH samples. The carriers of these latter absorptions include HCOOH itself, as well as trans-HOCO, HCOOH+, and HCO2−. Detailed isotopic substitution studies confirm the infrared identifications of HCOOH+ and HCO2− and permit the identification of two previously unassigned vibrational fundamentals each of HOCO+ and DOCO+. Density functional calculations for the latter species support the proposed assignments. Evidence is presented for the stabilization of the H2⋯CO2− anion complex, in which the H2 moiety becomes infrared-active. The processes that result in the stabilization of these products are considered.
A quantum dynamical study of the He++2He→He2++He reaction
Authors: Xie Junkai , Poirier Bill , Gellene Gregory I.
Published: December 2003

The temperature dependent rate of the He++2He→He2++He three-body association reaction is studied using two complementary quantum dynamical models. Model I presumes a two-step, reverse Lindemann mechanism, where the intermediate energized complex, He2+*, is interpreted as the rotational resonance states of He2+. The energy and width of these resonances are determined via “exact” quantum calculation using highly accurate potential-energy curves. Model II uses an alternate quantum rate expression as the thermal average of the cumulative recombination probability, N(E). This microcanonical quantity is computed approximately, over the He2+ space only, with the third-body interaction modeled using a special type of absorbing potential. Because Model II implicitly incorporates both the two-step reverse Lindemann mechanism, and a one-step, reverse collision induced dissociation mechanism, the relative importance of the two formation mechanisms can be estimated by a comparison of the Model I and Model II results. For T<300 K, the reaction is found to be dominated by the two-step mechanism, and a formation rate in good agreement with the available experimental results is obtained with essentially no adjustable parameters in the theory. Interestingly, a nonmonotonic He2+ formation rate is observed, with a maximum identified near 25 K. This maximum is associated with just two reaction intermediate resonance states, the lowest energy states that can contribute significantly to the formation kinetics.
A ground level interpretation of the dielectric behavior of diluted alcohol-in-carbon tetrachloride mixtures
Authors: Pieruccini M. , Saija F. , Ponterio R. , Vasi C.
Published: December 2003

The dielectric behavior at room temperature of alcohol/carbon tetrachloride binary mixtures over the whole composition range is analyzed in the low frequency domain (100 kHz). Methanol, ethanol, 1-, 2-propanol, and 1-pentanol are considered. The results are compared with the ideal case as described in the framework of Onsager theory [J. Am. Chem. Soc. 58, 1486 (1936)]. We find that the observed negative deviation from the ideal dielectric constant at high alcohol dilution is almost independent of the alcohol species. This behavior is interpreted as an effect of the presence of closed chainlike alcohol clusters. Good quantitative agreement is found with a simple lattice model describing the equilibrium distribution of alcohol clusters in the framework of the semigrand canonical ensemble. The observed dielectric behavior is then compared with the results of the calorimetric investigations carried out by Otterstedt and Missen [Trans. Faraday Soc. 57, 879 (1962)]. It is concluded that the dielectric and the calorimetric behaviors of these mixtures at high dilutions can be both interpreted consistently as effects of homogeneous association of the alcohol.
Zero-variance zero-bias principle for observables in quantum Monte Carlo: Application to forces
Authors: Assaraf Roland , Caffarel Michel
Published: December 2003

A simple and stable method for computing accurate expectation values of observables with variational Monte Carlo (VMC) or diffusion Monte Carlo (DMC) algorithms is presented. The basic idea consists in replacing the usual “bare” estimator associated with the observable by an improved or “renormalized” estimator. Using this estimator more accurate averages are obtained: Not only the statistical fluctuations are reduced but also the systematic error (bias) associated with the approximate VMC or (fixed-node) DMC probability densities. It is shown that improved estimators obey a zero-variance zero-bias property similar to the usual zero-variance zero-bias property of the energy with the local energy as improved estimator. Using this property improved estimators can be optimized and the resulting accuracy on expectation values may reach the remarkable accuracy obtained for total energies. As an important example, we present the application of our formalism to the computation of forces in molecular systems. Calculations of the entire force curve of the H2,LiH, and Li2 molecules are presented. Spectroscopic constants Re (equilibrium distance) and ωe (harmonic frequency) are also computed. The equilibrium distances are obtained with a relative error smaller than 1%, while the harmonic frequencies are computed with an error of about 10%.
Relativistic coupled cluster calculations of the energies for rubidium and cesium atoms
Authors: Chaudhuri Rajat K. , Sahoo Bijaya Kumar , Das Bhanu Pratap , Merlitz Holger , Mahapatra Uttam Sinha , Mukherjee Debashis
Published: December 2003

Ionization potentials and excitation energies of rubidium and cesium atoms are computed using the relativistic coupled cluster (CC) method. The effect of electron correlations on the ground and excited state properties is investigated using different levels of CC approximations and truncation schemes. The present work demonstrates that the even-parity channel truncation scheme produces results almost as accurate as obtained from the all-parity channel approximation scheme at a reduced computational cost. The present study also indicates that for a given basis the linearized CC method tends to overestimate the ground and excited state properties compared to the full CC method.
Computational study of the rate constants and kinetic isotope effects for the CH3+HBr→CH4+Br reaction
Authors: Sheng Li , Li Ze-Sheng , Liu Jing-Yao , Sun Chia-Chung
Published: December 2003

Ab initio direct dynamics methods have been used to study the title reaction. The electronic structure information including geometries, gradients and force constants (Hessians) is calculated at the QCISD/6-31+G(d) level. With the aid of intrinsic reaction coordinate theory, the minimum energy path (MEP) is obtained at the same level, and the energies along the MEP are further refined by performing the single-point calculations at the PMP4/6-311++G(3df,3pd) level. For this reaction, the theoretical rate constants by using improved canonical variational transition state theory incorporating small-curvature tunneling correction are in excellent agreement with the more recent experimental values. Our calculated results show that for the CH3+HBr reaction, the rate constants have a strong nonlinear Arrhenius behavior, i.e., the CH3+HBr reaction has negative temperature dependence at T<536 K, but clearly shows positive temperature dependence at T>536 K. The current work predicts that the kinetic isotope effect for the title reaction is normal in the temperature range 200–482 K, i.e., kHBr/kDBr>1. This result is in good agreement with the experimental one.
On one-dimensional discrete variable representations with general basis functions
Authors: Szalay Viktor , Czakó Gábor , Nagy Ádám , Furtenbacher Tibor , Császár Attila G.
Published: December 2003

The method of discrete variable representation (DVR) is based on standard orthogonal polynomial bases and the associated Gaussian quadratures. General basis functions correspond either to nonpolynomial expressions or to nonstandard orthogonal polynomials. Although one cannot directly relate any Gaussian quadrature to general basis functions, the DVR-like representation derivable with such basis sets via the transformation (diagonalization) method is, as proved here, of Gaussian quadrature accuracy. The optimal generalized DVR (GDVR) is an alternative to and entirely different from this DVR-like representation. Yet, when built from the same general basis functions and the corresponding quadrature points obtained by the diagonalization method, the two representations are found to give almost identical numerical results. The intricate relationship between the optimal GDVR and the transformation method is discussed.
Experimental and theoretical study of distribution of O2 molecules over vibrational levels in O2(a 1Δg)–I mixture
Authors: Antonov Ivan O. , Azyazov Valery N. , Ufimtsev Nikolay I.
Published: December 2003

The vibrationally excited oxygen in O2(a 1Δg)–I mixture was detected by emission spectroscopy. The analysis of a luminescence spectra of oxygen molecules on O2(b 1Σg+,v′)→O2(X 3Σg−,v″) transitions has shown that vibrationally excited O2(b 1Σg+) molecules up to v=5 are generated in the active medium of chemical oxygen–iodine laser (COIL). The highest values of relative O2(b 1Σg+,v=1) population of 22±2% and O2(b 1Σg+,v=2) of 10±3.5% are reached for I2 content in an oxygen flow ≈1%. It is shown theoretically that the relative populations of O2(X 3Σg−), O2(a 1Δg), and O2(b 1Σg+) molecules at the first and the second vibrational levels are approximately equal because of fast EE energy exchange between oxygen molecules. Up to 20% of oxygen molecules in COIL active medium are vibrationally excited. Comparison of intensities of O2(b 1Σg+,v′)→O2(X 3Σg−,v″) bands has demonstrated that a few percent of O2(b 1Σg+,v) molecules are vibrationally excited with v=3, 4, or 5. It is suggested that the following pooling reaction forms them: O2(a 1Δg,v=i)+I(2P1/2)→O2(b 1Σg+,v′=i+2)+I(2P3/2), where i=1, 2, or 3.
Detailed dynamics of a complex photochemical reaction: Cis–trans photoisomerization of stilbene
Authors: Dou Yusheng , Allen Roland E.
Published: December 2003

Detailed simulations are reported for the dynamics of electrons and nuclei during the cis to trans photoisomerization of stilbene. Our method, which employs a semiclassical description of both the nuclear motion and the radiation field, is described in the text. After excitation of electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) by a femtosecond-scale laser pulse, two principal avoided crossings are observed between the HOMO and LUMO levels, each of which leads to substantial depopulation of the LUMO. Based on our results and those of other groups, we propose that the first such HOMO–LUMO coupling can lead to the formation of 4a,4b-dihydrophenanthrene (DHP). The second coupling, on the other hand, leads to the formation of trans-stilbene. It is found that pyramidalization of the two carbon atoms of the vinyl group is involved significantly in both couplings, and that rotation of the two phenyl rings, together with their interaction, plays an important role in the first coupling. The occurrence along the same trajectory of two couplings, one leading to DHP and the other leading to isomerization, is a significant observation, and one which indicates that further theoretical and experimental investigations would be of considerable interest.
Studies of Ar:HBr using fast scan submillimeter-wave and microwave coaxial pulsed jet spectrometers with sub-kHz precision
Authors: McElmurry B. A. , Lucchese R. R. , Bevan J. W. , Leonov I. I. , Belov S. P. , Legon A. C.
Published: December 2003

Coaxial pulsed jet submillimeter spectra are reported for Ar:H79Br and Ar:H81Br transition frequencies measured with sub-kHz precision. This is confirmed by comparing combination frequency differences associated with ΔF=+/−1 hyperfine components in the rovibrational transitions of the low frequency Σ bending mode with corresponding sum frequencies of the rotational transitions in the ground state precisely measured with a pulsed-nozzle FT microwave spectrometer (1.5 kHz resolution, 200 Hz accuracy). Ground state molecular parameters evaluated using the submillimeter-wave and microwave techniques are demonstrated to have comparable accuracy. Furthermore, excited Σ state molecular constants for the isotopic band origins of Ar:H79Br and Ar:H81Br at 329 611.4284(10) and 329 225.6778(10) MHz are shown to be determined with equivalent accuracy.
Comparison of experimental and calculated infrared spectra of aminocarbynes on the Pt(111) surface
Authors: Chatterjee Bappaditya , Kang Dae-Hyuk , Herceg Eldad , Trenary Michael
Published: December 2003

Infrared spectra of the three aminocarbynes CNH2, CNHCH3, and CN(CH3)2 attached to Pt, Pt7H6, Pt2, Pt9H14, and Pt4 clusters are calculated from density functional theory and compared with the corresponding experimental spectra measured on the Pt(111) surface. The different clusters provide models for adsorption at on-top, twofold bridge, and threefold bridge sites. For the Pt2 and Pt9H14 models of the twofold bridge site, the calculations successfully reproduce the observed frequency shifts for C13, N15, and D substitutions and yield fair agreement with the observed relative intensities. In the case of CNHCH3, the calculations for one of two possible conformers give better agreement with the experimental spectra. No improvement was achieved with the larger Pt9H14 cluster compared with the Pt2 model, indicating that the differences between the experimental and calculated spectra for the latter cluster were not due to finite cluster size effects. The calculated spectra for the different adsorption site models indicate that the experimental spectra are most consistent with the twofold bridge site.
Manipulating bond lengths adiabatically with light
Authors: Sola Ignacio R. , Chang Bo Y. , Rabitz Herschel
Published: December 2003

In this paper we propose a new method to manipulate bond lengths in molecules. The scheme uses strong fields that prepare an artificial potential with the desired equilibrium bond length, by controlling the mixing of electronic configurations, and finds an adiabatic path that connects the initial state with the laser created potential. As an illustration, we show that it is possible to control the bond length of a diatomic molecule by directly coupling the ground state and a dissociative potential, with the aid of a second excited bound potential.
Molecular dynamics simulation of nucleation in the presence of a carrier gas
Authors: Toxvaerd S.
Published: December 2003

Nucleation in the presence of an inert carrier gas is investigated by molecular dynamics simulation of an immiscible binary mixture. The distribution of clusters in the quenched and quasiequilibrium state is used to localize the critical nucleation size, which is 100 particles for a nucleation time of approximately 50 ns. The density profiles in clusters before and at onset of nucleation are determined from an ensemble of 25 independent nucleations. The density profiles show that the inert carrier gas particles, which have a particle fraction of 0.75 in the quenched state, are squeezed out of the clusters before onset of nucleation. This is in agreement with the classical nucleation theory.
Effect of protein–protein interactions on protein aggregation kinetics
Authors: Zhang Jing , Liu Xiang Y.
Published: December 2003

The mutual diffusion coefficient Dm of dilute lysozyme solutions is measured by the dynamic light scattering technique. The relation between the osmotic second virial coefficient B22 and the protein mutual diffusion coefficient Dm of dilute solutions is examined experimentally and theoretically. The role of B22 in relation with Dm in protein aggregation kinetics was analyzed based on the effect of the average protein pairwise interactions on the protein aggregation kinetics. The empirical “crystallization window” proposed by George and Wilson [Acta Crystallogr., Sect. D: Biol. Crystallogr. 50, 361 (1994)] can be interpreted from the point view of crystallization kink kinetics.
A direct classical trajectory study of the acetone photodissociation on the triplet surface
Authors: Martı́nez-Núñez E. , Fernández-Ramos A. , Cordeiro M. N. D. S. , Vázquez S. A. , Aoiz F. J. , Bañares L.
Published: December 2003

Product energy distributions (PEDs) for the photodissociation of acetone at 266, 248, and 193 nm were evaluated by direct classical trajectory calculations on the lowest triplet potential energy surface. CASSCF(8,7) and MRCI+Q calculations were first performed to obtain a set of high-level ab initio data with which the semiempirical parameters were refined. The trajectories were initiated at the barrier, using two different microcanonical sampling methods. The results obtained for the excess energies corresponding to excitation at 266 and 248 nm are in good agreement with the experimental product energy partitioning, supporting a dissociation event taking place on the T1 surface after intersystem crossing from the initially exited S1 state. At 193 nm, the results obtained with the two sampling methods show significant discrepancies. The PEDs calculated with the anharmonic sampling procedure appear to be consistent with the experimental data.
Infrared spectroscopy of V+(H2O) and V+(D2O) complexes: Solvent deformation and an incipient reaction
Authors: Walker N. R. , Walters R. S. , Pillai E. D. , Duncan M. A.
Published: December 2003

V + ( H 2 O ) Ar n and V+(D2O)Arn complexes are studied with mass-selected infrared photodissociation spectroscopy in the O–H (O–D) stretch region. Two vibrational bands are measured 50–80 cm−1 to the red from the symmetric and asymmetric stretches in free water. Rotational analysis for V+(H2O)Ar indicates a C2v geometry with argon opposite water and significant expansion of the H–O–H angle. The v=1 level in the asymmetric stretch undergoes distortion consistent with selective excitation into the coordinate of an insertion reaction.
Monitoring chemical reactions at a liquid–solid interface: Water on CuIn(S,Se)2 thin film solar cell absorbers
Authors: Heske C. , Groh U. , Fuchs O. , Weinhardt L. , Umbach E. , Schedel-Niedrig Th. , Fischer Ch.-H. , Lux-Steiner M. Ch. , Zweigart S. , Niesen T. P. , Karg F. , Denlinger J. D. , Rude B. , Andrus C. , Powell F.
Published: December 2003

The chemical and electronic structure of the interface between liquid water and a CuIn(S,Se)2 thin film surface was studied with synchrotron-based, high energy-resolution soft x-ray emission spectroscopy (XES). By probing the local environment around the sulfur atoms, an x-ray-induced sulfate formation at the CuIn(S,Se)2 surface can be monitored, correlated with a substantial enhancement of sodium impurity atoms from the CuIn(S,Se)2 film and its glass substrate. The results demonstrate that, with XES, an experimental probe is available to in situ study chemical reactions at liquid–solid interfaces or at surfaces in a high-pressure gas environment in a chemically sensitive and atom-specific way.
In phase and antiphase synchronization of coupled homoclinic chaotic oscillators
Authors: Leyva I. , Allaria E. , Boccaletti S. , Arecchi F. T.
Published: December 2003

We numerically investigate the dynamics of a closed chain of unidirectionally coupled oscillators in a regime of homoclinic chaos. The emerging synchronization regimes show analogies with the experimental behavior of a single chaotic laser subjected to a delayed feedback.
Unsteady solution of viscous fluid flow in an infinite cylinder with a variable body force
Authors: Fang Tiegang
Published: December 2003

In this Brief Communication, the unsteady viscous flow with a variable body force in an infinite cylinder is solved exactly. The solution describes the transient velocity profiles and pressure gradients, which can be used in the application of sedimentation of spherical particles in viscous fluid. The current solution is compared with the previously solved problem between two infinite parallel plates. It is found that, for a similar body force, the central-line velocity of the cylinder will be greater than that of the two-infinite-plate case. But the slip velocity at the wall of cylinder will be smaller than that of the flow between two infinite plates. The transient pressure gradients are also different, resulting in a smaller value for the cylindrical case with a similar body force.