bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational spectrum of a dark state in 2-fluoroethanol using microwave/radio-frequency-infrared multiple resonance
Authors: Miller C. C. , Philips L. A. , Andrews A. M. , Fraser G. T. , Pate B. H. , Suenram R. D.
Published: July 2002

Microwave/radio-frequency-infrared multiple resonance has been used with an electric-resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the ν14 C–O stretching vibration of 2-fluoroethanol. The infrared spectrum of 2-fluoroethanol was recorded at a resolution of ∼2 MHz using a tunable microwave-sideband CO2 laser. The spectrum is fit by an asymmetric-rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 413 upper state which are split into doublets by an interaction between the 413 level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio-frequency-infrared double and triple resonance reveals that the 413 level of the C–O stretching vibration is interacting with the 431 level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the ν18+4ν21 combination vibration of the lowest energy conformer, where ν18 is the CCO bending vibration and ν21 is the C–C torsional vibration. From the weak ΔKa=2 matrix element between ν14 and ν18+4ν21 it is possible to derive a J=0 anharmonic interaction between these states of ∼3.5 GHz.
An intermediate neglect of differential overlap technique for actinide compounds
Authors: Cory Marshall G. , Köstlmeier Sibylle , Kotzian Manfred , Rösch Notker , Zerner Michael C.
Published: July 2002

An intermediate neglect of differential overlap method for examining the electronic structure of actinide complexes is developed. It is characterized by a basis set obtained from relativistic Dirac–Fock atomic calculations, the inclusion of all one-center two-electron integrals, and a parameter set based on molecular geometry and ionization spectra. The model is successful in reproducing the geometries of many small test molecules, especially the hexahalides and tetrahalides of the early actinides. We also investigate the bonding in actinocenes and the photoelectron spectra of pentavalent uranium amide/imide complexes as two diverse examples in which this model can be used to help in understanding and prediction.
The effect of reagent translational excitation on the dynamics of the reaction H+Cl2→HCl(v′,J′)+Cl
Authors: Berg P. A. , Sloan J. J.
Published: July 2002

We use fast time-resolved Fourier transform spectroscopy and low pressure infrared chemiluminescence techniques to determine the product energy distribution in the title reaction. We create the reagent hydrogen atoms with 2.3 eV of translational excitation by photofragmentation of H2S at 193 nm, and observe the time evolution of the infrared chemiluminescence from the product HCl(v′,J′) under single gas kinetic collision conditions. The initial vibrational distribution, determined from the first observation after creation of the H atoms is P(v′=1:2:3:4:5:6:7:8)=1.0:0.84:0.74:0.59:0.34:0.24:0.17:0.13. The initial HCl rotational distribution in each vibrational level is broad, showing no identifiable maximum. The fraction of the total available energy entering HCl vibration and rotation, respectively, are 0.19 and 0.10. The time evolution of the observed vibrational and rotational distributions gives information about the changes in the reaction dynamics consequent on reduction of the reagent translational energy.
The identification of InOH in the gas phase and determination of its geometric structure
Authors: Lakin Nicholas M. , Brown John M. , Beattie Ian R. , Jones Peter J.
Published: July 2002

The first gas phase observation of the species InOH is reported through the detection of its electronic spectrum in the near ultraviolet region, between 345 and 377 nm. The molecule was generated by the high temperature reaction between H2O and In metal or between H2 and In2O3, and cooled in a free jet expansion. Two separate electronic transitions have been identified and are tentatively assigned as α1A′←X̃1A′ and β1A″←X̃1A′. Values for the vibrational wavenumbers ν2 (bending vibration) and ν3 (In–O stretching vibration) have been determined for InOH and InOD in all three electronic states involved. There is evidence that the molecule is quasilinear in its ground electronic state which somewhat complicates the values determined for ν2 in this state. Rotational structure was easily resolved at the lowest temperature achieved in this work (Trot≊12 K). Analysis of this structure shows that the molecule is bent in all of the electronic states studied, with a bond angle of about 132° in the X̃ state and about 105° in the α and β states.
Frequency spectra for fluids beyond the mean spherical approximation
Authors: Ho/ye Johan S. , Lomba Enrique
Published: July 2002

In this paper, we investigate the frequency spectra of fluids (or fluid-like disordered materials) composed of particles with embedded harmonic oscillators. The general treatment is particularized at the low density limit, and a working scheme to add corrections to the mean spherical approximation for arbitrary density is introduced. A detailed comparison with an exactly solvable model in one dimension is also presented.
A Monte Carlo wave function study of the effect of collisions on the coherent multiphoton excitation of SF6
Authors: Mitra S. S. , Bhattacharyya S. S.
Published: July 2002

Collisional interruption of coherent excitation of SF6 in the lower discrete region of its laser absorbing mode (ν3) have been studied using the recently developed quantum Monte Carlo wave function (QMCWF) method. The usual pure pump mode description up to 3ν3 and complete mixing of all modes (QC) above it have been assumed. The rotational and anharmonic splitting of the vibrational states upto 3ν3 are taken into account but splitting due to tensor interaction terms are neglected. Excitation to QC is represented by irreversible leakage from the coherent ladder modeled by an imaginary term in the Hamiltonian of the coherently excited vibrational rotational levels. QMCWF study has been carried out at the laser frequency 942.8 cm−1 for which the local time average populations in the intermediate excited vibrational states are found to be negligible. Large leakage occurs only from narrow band of ground rotational states due to 3 photon resonances but their number increases with increasing intensity. Two different collisional energy transfer models, one obeying symmetry imposed restrictions and propensity rule, and the other free from such restrictions except the fact that collisions restore the thermal distribution, have been used. Results show different pressure effects at different temperature and for different intensities. However, the two different models used for collisional rotational transition probabilities give similar enhancement of leakage at high intensities.
Manifestations of wave packet fractional revivals in a Morse-like anharmonic system
Authors: Eryomin V. V. , Vetchinkin S. I. , Umanskii I. M.
Published: July 2002

The general properties of the time-dependent characteristics of one-dimensional vibrational wave packets in an anharmonic system with the quadratic spectrum are discussed. The local behavior of the autocorrelation function and its square modulus (the survival function) is studied near the moments of fractional revival t=(p/q)Trev, where Trev is the period of the complete revival, p/q is an irreducible fraction, and q is the order of the revival. It is shown that all the wave packets have the same values of the survival function at the moments of fractional revival provided that the number of states in the packet exceeds the order of the revival q. It is also found that for the given packet, all the fractional revivals are identical to each other in the sense that their lifetimes do not depend on the order of the revival. The lifetimes of all fractional revivals are expressed in terms of the parameters of the initial packet (the effective number of states and the period of classical motion of the packet center) and the anharmonicity of the molecular system.
Al3H stable and transition state structures
Authors: Gonzales Nick , Simons Jack
Published: July 2002

In this study we predict, based on our multiconfigurational and higher level correlated ab initio electronic structure calculations, the geometries and relative energies of the Al3H and Al3 molecules. We found three minima on the Al3H potential energy surface, two of which are nearly energetically degenerate: a C2v σ-bonded structure and a C3v π-bonded structure. Two Al3H transition states were also found: one that connects the C3v and C2v minima and another that connects one C3v minimum to another C3v minimum. We also predict the lowest Al3 electronic state to be the 2A1′ state which has the valence orbital occupation of (a1′)2(e′)4(a2″)2(a1′)1.
Comparison of μ2-scaled Hückel theory and Hartree–Fock theory of boranes and carboranes
Authors: Rousseau Roger , Lee Stephen
Published: July 2002

The μ2-scaled Hückel method is used to calculate the electronic energy surfaces of the four boranes BnH2−n (n=8–11) and the carborane C2B8H2−10. These electronic energy surfaces and their minimum energy geometries are directly compared to both the single crystal x-ray determined structures and to Hartree–Fock optimized geometries. Bond distances differ on the average by 0.04 Å between alternate methods. It is shown that μ2-scaled Hückel results may be directly interpreted by analysis of the highest occupied and lowest unoccupied molecular orbitals. Also studied by the μ2-scaled Hückel and Hartree–Fock methods are the isomerization pathways of B8H2−8, B11H2−11, and C2B8H2−10. Reaction barriers and transition state geometries found by the two different calculational methods are in fair agreement with each other and known literature values. Using the μ2-scaled Hückel method one can readily deduce that the B8H2−8 and B11H2−11 isomerizations are Woodward–Hoffmann allowed reactions. In the case of B8H2−8 this allowed mechanism is contrasted to an alternate Woodward–Hoffmann forbidden pathway. Hartree–Fock calculations on the C2B8H2−10 confirm earlier μ2-scaled Hückel based findings, that a second less stable isomer of C2B8H2−10 exists which, in contradiction to Wade’s rules of electron deficient clusters, has a pair of open square faces in the cluster.
Rotational and vibrational interactions of singlet gerade Rydberg states of H2 near the ionization limit
Authors: Glab W. L. , Qin K. , Bistransin M.
Published: July 2002

We have experimentally and theoretically studied the gerade Rydberg states of molecular hydrogen near the ionization limit, detecting ionization and dissociation separately. The spectra in this region show a rich variety of effects due to interactions between Rydberg series with different rotational and vibrational quantum numbers. The states which we have observed are all well described by Hund’s case (d) coupling. We discuss the spectroscopic and dynamic properties of the states as revealed by our experiment, and apply theoretical models to the simulation of several regions of the spectra. These models allowed us to deduce interseries couplings and relative dipole matrix elements, as well as the decay rates for some states.
Far-infrared spectra and two-dimensional potential energy surface for the ring-bending and ring-twisting vibrations of 5,6-dihydro-4H-thiopyran
Authors: Choo Jaebum , Meinander N. T. , Villarreal John R. , Laane Jaan
Published: July 2002

5,6-Dihydro-4H-thiopyran has been synthesized and its far-infrared spectrum has been recorded. Eleven ring-bending bands originating at 120.7 cm−1 and four ring-twisting bands originating at 274.5 cm−1 were observed. Twelve sum and difference bands in the 383–397 and 148–166 cm−1 regions were also observed and these facilitated the construction of a detailed energy map including numerous excited vibrational states of the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined to be V=9.48 ×104x4−4.13×104x2+1.37×104τ4−1.82×104τ2+1.10 ×105x2τ2, where x and τ are the bending and twisting coordinates, respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1. Both of these values have large uncertainties since the vibrational data only extend to 800 cm−1 above the potential surface minimum. The relatively low bending energy and high barrier to planarity can both be explained by the low force constant for the C–S–C angle bending.
Electrostatic interactions between particles with an ion-penetrable charged membrane
Authors: Hsu Jyh-Ping , Kuo Yung-Chih
Published: July 2002

The electrostatic interactions between spherical particles suspended in an electrolyte solution is analyzed. The surfaces of these particles are coated with an ion-penetrable membrane, which bears nonuniformly distributed fixed charges. We consider two classes of fixed charge distributions; these distributions all yield the same amount of total fixed charges in the membrane phase. The exact solution to the governing Poisson–Boltzmann equation is derived for the case the potential of the system under consideration is low. Based on this solution, the stability ratio and the critical coagulation concentration of a dispersed system are estimated.
Dynamic zone structure model for the surface scattering of large van der Waals clusters at thermal kinetic energies
Authors: Vach H. , Benslimane M. , Châtelet M. , De Martino A. , Pradère F.
Published: July 2002

A semiempirical dynamic zone structure model is presented to explain the behavior of the grazing exit angle component previously observed in experiment and simulation during the scattering of large van der Waals clusters from surfaces. The proposed model that is based on a simple energy balance and the Leidenfrost phenomenon does not only qualitatively reproduce the measured importance of the grazing exit angle component as a function of incident cluster size, incident velocity, angle of incidence, and surface temperature, but it also gives an order-of-magnitude estimate for the size of the large fragments scattered in this component and for the involved picosecond interaction times. Recent results obtained from both trajectory calculations and experiments show very good agreement with the predictions concluded from the proposed model.
Identification of the 4pσu 1Πu Rydberg state of O2
Authors: England J. P. , Lewis B. R. , Ginter M. L.
Published: July 2002

New high-resolution measurements of the O2 photoabsorption cross section in the 1147–1155 Å region are presented. The observation of three previously-unreported narrow lines in a band near 1148 Å has enabled unambiguous assignment of the band to a 1Πu←X3Σg− transition which gains strength through a spin-orbit interaction with the upper state of a 3Πu←X3Σg− transition appearing at 1153 Å. Other lines in the 1148 Å band are broadened by predissociation through a heterogeneous interaction with a nearly energetically coincident 1Δu level. The states, labelled h1Πu, F3Πu and i1Δu, are predominantly of 4pλ Rydberg molecular orbital configurations although there are indications that the Πu states interact strongly with valence states of the same symmetry.
Density functional theory of nucleation: A semiempirical approach
Authors: Nyquist Rebecca M. , Talanquer Vicente , Oxtoby David W.
Published: July 2002

We present a semiempirical approach to the density functional theory of gas–liquid nucleation, in which the same experimental properties used in classical nucleation theory (equilibrium vapor pressure, liquid density, and surface tension) are used to fit three adjustable parameters in the intermolecular potential. This approach allows direct comparison of nucleation rates with experimental data. Agreement with results on nonane from three different experimental groups is reasonable, although the comparison clearly reveals the scatter in those results and suggests that further experimental measurements by different groups on the same systems would be very valuable. For water and the n-alcohols, this version of density functional theory gives results quite close to classical nucleation theory, implying that it is not a good approximation to describe polar fluids by effective spherically symmetric potentials.
Comment on the accuracy of Wertheim’s theory of associating fluids
Authors: Müller Erich A. , Gubbins Keith E. , Tsangaris Dimitrios M. , de Pablo Juan J.
Published: July 2002

The results of recently developed Monte Carlo methods for simulation of strongly associating fluids show an apparent disagreement with Wertheim’s theory of associating fluids. In this comment we show that the disagreement observed arises largely from approximations used for the reference-fluid pair distribution function required by the theory.
Excited state enol-keto tautomerization in salicylic acid: A supersonic free jet study
Authors: Bisht Prem B. , Petek Hrvoje , Yoshihara Keitaro , Nagashima Umpei
Published: July 2002

Excited state enol-keto isomerization in salicylic acid (SA) monomer and dimer has been studied in a supersonic free jet expansion. Two carboxylic group rotamers of SA with significantly different photophysical properties are found in the expansion. Rotamer I, the major form of SA in the expansion, has an intramolecular hydrogen bond and can undergo excited state tautomerization reaction. Its S1 origin is at 335.34 nm. Single vibronic level emission spectra are dominated by progressions in OH stretching (3230 cm−1), and in-plane bending of the carboxylic group (240 cm−1). The spectra appear to consist of two components, normal (UV) and tautomer (BLUE) emissions, even at the origin. The intensity of the BLUE relative to the UV emission depends on the vibronic state, rather than the excess vibrational energy between the origin and 1100 cm−1. The fluorescence decay time profiles for both the emission components of rotamer I are identical within ∼1 ns experimental time resolution. A nonradiative decay process with an activation energy of ∼1100 cm−1 is deduced from an abrupt decrease in fluorescence lifetimes above this energy. The rotamer II cannot undergo excited state tautomerization. Its electronic origin is at 311.52 nm and emits only UV fluorescence. Upon increasing the concentration of the SA sample, a new spectrum is observed. Due to a nonlinear concentration dependence of the intensity and the propensity of SA to form dimers in solution, it is assigned to the SA dimer. This spectrum shows possible evidence of double proton transfer in the S1 state.
Ab initio potential curves, dipole moments, and transition probabilities for the low-lying states of arsenic oxide
Authors: Alekseyev Aleksey B. , Sannigrahi Abani B. , Liebermann Heinz-Peter , Buenker Robert J. , Hirsch Gerhard
Published: July 2002

Relativistic effective core potentials (RECPs) are employed in the framework of spin-orbit configuration interaction method to compute potentials curves and one-electron properties for a large number of electronic states of the arsenic oxide molecule. Good agreement is noted between calculated and experimental data for the spectroscopic constants of states with Te values at or below 40 000 cm−1. The calculations predict that the lowest excited Λ–S state is the π→π* a 4Π and it is argued that some experimental results of Kushawaha et al. originally thought to correspond to the A″ 2Σ+–X 2Π transition should be reassigned as a 4Π–X 2Π. There is general agreement that the corresponding π→π* 2Π is the upper state in the G1,G2→X 2Π band systems, with computed Te values only 600 cm−1 smaller than observed, and discrepancies in re and ωe values of 0.01 Å and 16–20 cm−1, respectively. The b 4Σ− and I 2Φ Ω components are found to be the next lowest-energy states, but it is pointed out that the experimental L–F splitting is too large to be attributed to the b1 4Σ−1/2–b2 4Σ−3/2 energy difference.

Strong perpendicular transitions are computed for the A 2Σ+–X 2Π band system, and the upper state is found to undergo homogeneous perturbations by a number of neighboring states which should have important effects on the A–X vibrational intensity distribution. The B 2Σ+ state has a large amount of Rydberg character and is the only low-lying AsO state with As−O+ polarity. The minimum in its potential curve appears to be almost coincident with a maximum in the A 2Σ+ potential, leading to an onset of a break-up in the otherwise strong B–X emission intensity pattern at v′=0 and N′=21. On the basis of the present calculations an estimate for the D00 value of the AsO ground state of 4.22 eV can be made, which is ∼0.7 eV smaller than the upper limit for this value given in the literature. Numerous comparisons with analogous calculated results for the heavier Group V oxides, SbO and BiO are made, allowing for a systematic evaluation of the changing role of relativistic effects with increasing atomic number of the heavy atom in this class of molecules.
Time-dependent reactive scattering in hyperspherical coordinates: A close coupled wave packet approach
Authors: Echave Julián
Published: July 2002

We present a close coupled wave packet method to treat reactive collisions in hyperspherical coordinates. Within this approach, the information needed for the time-dependent propagation is the same as that required for a time-independent close coupling treatment. The method is tested on the simple collinear D+H2 reaction. We compare the time-dependent technique with the more established time-independent close coupling treatment. Finally, we show that time-dependent properties are useful to gain insight into the detailed mechanism of the reaction.
Diffusion-influenced radical recombination in the presence of a scavenger
Authors: Kim Joohyun , Jung Younjoon , Jeon Jonggu , Shin Kook Joe , Lee Sangyoub
Published: July 2002

The recombination reaction of radicals photolytically produced in solution proceeds via two distinguishable kinetic phases. At short times recombination between geminate radicals predominates, while at long times recombination between nongeminate radicals that have diffused away from their geminate counterparts to the bulk solution takes place. In most previous theories dealing with the effect of added scavenger molecules on the radical recombination reaction, only the geminate recombination phase has been considered. However, if the concentration of radical pairs generated is high, the interplay of bulk recombination in the dynamics needs to be considered. In the present paper, we formulate a theory that deals with the scavenging effect in both the geminate and bulk recombination phases within a unified framework. We show that the scavenger concentration CS and the scavenging rate coefficient kS have independent effects on the reaction, in general. In contrast, conventional theories involve these two quantities only as a product kSCS, usually in the form of a Laplace transformation variable.
Vibrational overtone activation of methylcyclopropene
Authors: Snavely D. L. , Grinevich O. , Hassoon S. , Snavely G.
Published: July 2002

Laser vibrational overtone activation has been used to investigate the reaction channel competition in the isomerization of 1-methylcyclopropene (MCPene). The vibrational overtone activation of three types of CH stretches (methyl, methylenic, and olefinic) in the 5νCH and 6νCH transitions initiated the isomerization and all three products (2-butyne, 1,3-butadiene, and 1,2-butadiene) were detected by gas chromatography. Stern–Volmer plots were constructed for the appearance of each individual product and the derived experimental specific rate coefficients were compared to those of the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The rate coefficients for the 6νCH transitions were in good agreement with the predicted values but those for the 5νCH transition were as much as a factor of 5 too large. Product ratios of 1,3-butadiene to 2-butyne and 1,2-butadiene to 2-butyne were independent of pressure. In general, these ratios were lower than the RRKM predicted ratios due to collisional deactivation. No evidence of mode specific behavior was observed in these product yield ratios.
Rheology of self-assembled fluids
Authors: Pätzold Gerald , Dawson Kenneth
Published: July 2002

We investigate the rheological properties of a Landau–Ginzburg model that has competing interaction terms. These interactions have earlier been shown to produce mesoscopic ordering and such models have been helpful in explaining microemulsion behavior. Our present study is based on time-dependent Landau–Ginzburg equations for the order parameter and velocity field. The possible influence of hydrodynamic fluctuations, though discussed, is neglected in our treatment. General expressions for the excess viscosity and the first normal stress coefficient are derived in terms of the quasistatic structure factor. For steady shear flows and in the mean field approximation, explicit relations are given in two space dimensions for a nonconserved order parameter and in three space dimensions for a conserved order parameter. The former case is the easiest one to study numerically in computer simulations. Our numerical results show that mean field theory for the excess viscosity is satisfactory at some distance from the ‘‘transition’’ curve to the lamellar phase. The normal stress coefficient turns out to be very small. It only becomes appreciable close to the phase boundary. Here the nonlinear dependencies of excess viscosity and stress coefficient on the shear rate become important. To explain the general behavior we have considered terms up to fourth order in the shear rate. Computer simulations as well as mean field theory indicate that the quadratic corrections to both coefficients are negative in the microemulsion region. With increasing shear rate one therefore first enters a regime of shear-thinning. The quartic corrections are found to be positive, so further increase of the shear rate will lead to shear-thickening.
Elastic scattering from diblock copolymer chains in dilute solution
Authors: Grigera Tomás S. , Alessandrini José Luis
Published: July 2002

The elastic scattering structure factor of a single diblock copolymer chain is calculated up to first order in ε(=4−d, with d the dimensionality of the space) with the conformational space renormalization group method. The influence of several thermodynamic conditions on the scattering intensity is analyzed. Our results are compared with small-angle neutron scattering experiments under variable contrast conditions and numerical simulations of the zero average contrast case.
Constant pressure molecular dynamics simulations of the dodecamers: d(GCGCGCGCGCGC)2 and r(GCGCGCGCGCGC)2
Authors: Norberg Jan , Nilsson Lennart
Published: July 2002

Here we report constant pressure molecular dynamics simulations of the B-DNA dodecamer d(GCGCGCGCGCGC)2 and the A-RNA dodecamer r(GCGCGCGCGCGC)2 in aqueous solution. The molecular dynamics simulations were performed at reference pressures of 1 and 6000 atm. We obtained stable trajectories, especially for the A-RNA dodecamer, using the standard atom based truncation cutoff method. For the B-DNA dodecamer, a tendency to transform to the A-DNA form was obtained. The atomic mean square fluctuations were lower for the A-RNA dodecamer than for the B-DNA, and were further reduced by increased pressure for the A-RNA dodecamer. The backbone and glycosidic torsion angles were well preserved for the A-RNA dodecamer and only small changes were observed for the B-DNA dodecamer. In both dodecamers the Watson–Crick hydrogen bonds were preserved without any constraints. The differences in the exposed surface area between the dodecamers were mainly due to the 2′OH group and to the cytosine base being more exposed in the B-DNA.
Orientation and electronic structure of methylene blue on mica: A near edge x-ray absorption fine structure spectroscopy study
Authors: Hähner G. , Marti A. , Spencer N. D. , Caseri W. R.
Published: July 2002

Methylene blue adsorption from aqueous solution is widely used for the determination of the specific surface area of mica. In this procedure, knowledge of the space requirement for each ion-exchange-reacted methylene blue (MB) molecule is a prerequisite for a precise determination of the specific surface area. There is, however, some disagreement about the correct value due to the unknown orientation of the adsorbed MB molecules. In the present study the electronic structure and orientation of MB on muscovite mica is investigated with x-ray photoelectron spectroscopy and the synchrotron-based technique of near edge x-ray absorption fine structure spectroscopy. It was found that the MB molecules are tilted, with their largest face inclined at 65–70° with respect to the surface. This value disagrees with common a priori assumptions that MB lies flat on the surface, i.e., the specific surface area of mica is overestimated in many cases by a significant factor. The results, however, agree with a recent study proposing an apparent surface area of 66 Å2 for MB molecules on mica.
Ab initio study on the stability and properties of XYCO⋅ ⋅ ⋅HZ complexes. III. A comparative study of basis set and electron correlation effects for H2CO⋅ ⋅ ⋅HCl
Authors: Nowek Andrzej , Leszczyński Jerzy
Published: July 2002

Ten basis sets of double and triple-zeta quality augmented by polarization and diffusion function in conjunction with density functional theory (DFT, with the Becke–Lee–Yang–Parr exchange-correlation potential), Mo/ller–Plesset MP2 to MP4 (SDTQ), coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitation [CCSD(T)] levels of theory were applied in studies of the molecular geometry and stability of the H2CO...HCl complex. Interaction energy (corrected for the basis set superposition error and zero-point vibrational energy contributions) predicted at three highest levels used; the MP4(SDTQ)/6-311++G(2df,2pd)//MP2/6-311++G(2df,2pd), CCSD(T)/6-311G(2d,2p)//CCSD/6-311G(2d,2p), and MP2/aug-cc-pVTZ (augmented correlation consistent polarized valence triple-zeta)//MP2/6-311+G(2df,2pd) amounts to −2.65, −2.61, and −3.88 kcal/mol, respectively, while the DFT/6-311++G(2df,2pd) level gives −2.86 kcal/mol. It appears that within a given computational method (e.g., MP2, DFT) interaction energy slightly depends on the chosen reference geometry (e.g., optimized at HF, MP2, CCSD, or DFT levels). A significant influence of the BSSE on the interaction energy and molecular geometry of the complex is analyzed. Reliable MP2/6-311++G(2df,2pd) and CCSD/6-311G(2d,2p) levels of harmonic frequencies, infrared intensities and potential energy distribution analysis are presented for the complex and its components and compared to the available experimental data.
An ab initio study of solvent shifts in vibrational spectra
Authors: Stefanovich Eugene V. , Truong Thanh N.
Published: July 2002

Using analytical second derivatives of the generalized conductorlike screening model (GCOSMO) we calculate vibrational frequency shifts for several molecules (acetone, methylamine, formic acid, acetic acid, and trans-NMA) solvated in water. In these calculations, results from dielectric continuum approach with and without several explicit water molecules are compared with traditional supermolecule approach. The simple GCOSMO model, where all solvent molecules are treated as a continuum medium, reproduces quite accurately solvent shifts in solutes having moderate hydrogen bondings with water, such as acetone and methylamine. To represent strong solvent effects in formic acid and acetic acid, one should add at least one explicit water molecule in GCOSMO calculations. Solvent effects on solute structure correlate well with frequency shifts. Geometry optimizations and frequency calculations in the GCOSMO-supermolecule approach require only 10%–20% more computational effort than similar calculations in the gas phase. Therefore, this method provides a promising and effective tool for studying reactivity, structural, and spectroscopic properties of realistic solutes.
Calculation of nuclear magnetic shieldings. XI. Vibrational motion effects
Authors: Fukui H. , Baba T. , Narumi J. , Inomata H. , Miura K. , Matsuda H.
Published: July 2002

Nuclear magnetic shieldings in first- and second-row hydrides were calculated with electron correlation taken into account through third order. The calculation was performed using London’s gauge-invariant atomic orbitals (GIAOs) and finite-field Mo/ller–Plesset perturbation theory (FF-MPPT). Furthermore, the vibrational motion corrections to the magnetic shieldings were evaluated. It was shown that the calculated isotropic shielding constants at the experimental geometries are higher than the experimental values, but that vibrational corrections are generally negative and improve the calculated shielding constants.
The unimolecular dissociation of HCO. II. Comparison of calculated resonance energies and widths with high-resolution spectroscopic data
Authors: Keller Hans-Martin , Floethmann Heiner , Dobbyn Abigail J. , Schinke Reinhard , Werner Hans-Joachim , Bauer Cornelia , Rosmus Pavel
Published: July 2002

We present a theoretical study of the unimolecular dissociation resonances of HCO in the electronic ground state, X̃1A′, using a new ab initio potential energy surface and a modification of the log-derivative version of the Kohn variational principle for the dynamics calculations. Altogether we have analyzed about 120 resonances up to an energy of ≊2 eV above the H+CO threshold, corresponding to the eleventh overtone in the CO stretching mode (v2=11). The agreement of the resonance energies and widths with recent stimulated emission pumping measurements of Tobiason et al. [J. Chem. Phys. 103, 1448 (1995)] is pleasing. The root-mean-square deviation from the experimental energies is only 17 cm−1 over a range of about 20 000 cm−1 and all trends of the resonance widths observed in the experiment are satisfactorily reproduced by the calculations. The assignment of the states is discussed in terms of the resonance wave functions. In addition, we compare the quantum mechanical state-resolved dissociation rates with the results of classical trajectory calculations and with the predictions of the statistical model.
Fluctuations in the number of particles adsorbed under the influence of diffusion and flow
Authors: Adamczyk Zbigniew , Siwek Barbara , Szyk Lilianna , Zembala Maria
Published: July 2002

Fluctuations in the number of colloid particles adsorbed irreversibly under diffusion and flow were determined. The experimental measurements were carried out in the impinging-jet cells using as model colloids the monodisperse polystyrene latex particles of micrometer size range adsorbing at mica sheets. The surface concentration of adsorbed particles was determined quantitatively using the direct microscope observation method coupled with an image analyzing system. Two series of experiments were performed (i) for diffusion controlled adsorption when the random sequential adsorption (RSA) mechanism was valid and (ii) for flow controlled adsorption. It was found that in the case of RSA the reduced variance of the distributions decreased markedly for increasing surface concentration θ in accordance with theoretical predictions based on the mean-field approximation. The experimental results were in a good agreement with the numerical simulations performed according to the RSA algorithm. It was also determined that the magnitude of fluctuations in our irreversible system was very similar to reversible systems described by the scaled-particle theory. A significantly different behavior was observed for flow affected adsorption when the reduced variance (at the same surface concentration) was much smaller than for the RSA model, therefore deviating considerably from an equilibrium system. The decrease in the variance indicated that the surface exclusion effects (described by the available surface function) were more important under flow due to the hydrodynamic scattering effect.
van der Waals clusters in the ultraquantum limit: A Monte Carlo study
Authors: Meierovich M. , Mushinski A. , Nightingale M. P.
Published: July 2002

Bosonic van der Waals clusters of sizes three, four, and five are studied by diffusion quantum Monte-Carlo techniques. In particular we study the unbinding transition, the ultraquantum limit where the ground state ceases to exist as a bound state. We discuss the quality of trial wave functions used in the calculations, the critical behavior in the vicinity of the unbinding transition, and simple improvements of the diffusion Monte Carlo algorithm.
Embedding procedure for cluster calculations of ionic crystals
Authors: Bredow Thomas , Geudtner Gerald , Jug Karl
Published: July 2002

An embedding procedure suitable for Hartree–Fock studies of ionic systems is presented and implemented on the semiempirical level. In this approach model clusters are embedded in finite fields of pseudoatoms with average self-consistent orbital occupation numbers or fixed orbital occupation numbers for best charge equilibration of cluster atoms. These pseudoatoms are of the same kind and described with the same set of parameters as the real cluster atoms. The implementation in the semiempirical self-consistent field molecular orbital method SINDO1 is described. Calculations are performed for the systems TiO2 and MgO and the influence of the embedding on geometry and electronic structure is discussed.
Sum-over-states density functional perturbation theory: Prediction of reliable 13C, 15N, and 17O nuclear magnetic resonance chemical shifts
Authors: Olsson Lars , Cremer Dieter
Published: July 2002

Sum-over-states density functional perturbation theory (SOS-DFPT) has been used to calculate 13C, 15N, and 17O NMR chemical shifts of 20 molecules, for which accurate experimental gas-phase values are available. Compared to Hartree–Fock (HF), SOS-DFPT leads to improved chemical shift values and approaches the degree of accuracy obtained with second order Mo/ller–Plesset perturbation theory (MP2). This is particularly true in the case of 15N chemical shifts where SOS-DFPT performs even better than MP2. Additional improvements of SOS-DFPT chemical shifts can be obtained by empirically correcting diamagnetic and paramagnetic contributions to compensate for deficiencies which are typical of DFT.
Quantum energy gap law of outer-sphere electron transfer reactions: A molecular dynamics study on aqueous solution
Authors: Ando Koji
Published: July 2002

The quantum energy gap law for electron transfer (ET) reactions in water is examined. Molecular dynamics (MD) simulation analysis is carried out to obtain the solvent reorganization energies, time correlation functions (TCF), spectral density functions, and quantum rate constants. Their dependence on the reaction free energy and on the donor–acceptor distance is explored along with the solvent isotope effects. Properties of the imaginary-time saddle-point for the TCF expression of the ET rate formula are also examined. The high-frequency intramolecular vibrational modes of the solvent water are found to present marked quantum effects on the ET rate, while their contribution to the static reorganization energy is small (less than 6%). The energy gap dependence of the quantum activation free energy is shown to become nearly independent of the donor–acceptor distance when renormalized by the reorganization energy. Approximations to compute quantum rate constants from MD simulation data are briefly discussed in light of the present results.
New measurements of appearance potentials and optical absorption of Na2F and Na3O
Authors: Vituccio D. T. , Herrmann R. F. W. , Golonzka O. , Ernst W. E.
Published: July 2002

Oxides and fluorides of sodium clusters were produced by reacting a beam of metal clusters with N2O or SF6 in the region between nozzle and skimmer. The threshold for single photon ionization was measured by using a pulsed, frequency doubled visible dye laser or a UV dye laser and detecting ions with a quadrupole mass spectrometer. Resonant two-photon ionization was employed to record optical spectra. Comparisons with previous measurements and theoretical predictions are discussed.
A general mixture theory. II. Mixtures of nonspherical molecules
Authors: Hamad Esam Z.
Published: July 2002

A recently suggested mixture theory for the prediction of mixture properties is formulated for nonspherical molecules. The theory is capable of handling equations of state that are not conformal (do not obey the corresponding states principle). The exact composition dependence of the density expansion of the mixture equations, plus other features of the original theory, are retained. Two methods for evaluating the cross molecular parameters for nonspherical molecules are suggested. They become identical for spherical molecules, and give similar predictions for mixture properties of nonspherical molecules at high densities. The two methods predict the properties of mixtures of convex bodies and fused hard spheres within simulation uncertainty. Properties of diatomic Lennard-Jones mixtures, and ethane–carbon dioxide are predicted fairly accurately.
A matrix isolation FT–IR and quasirelativistic density functional theory investigation of the reaction products of laser-ablated uranium atoms with NO, NO2 and N2O
Authors: Kushto Gary P. , Souter Philip F. , Andrews Lester , Neurock Matthew
Published: July 2002

The reaction products of laser-ablated uranium atoms with nitrogen oxide reagent gases were trapped in excess argon at 6–7 K. Infrared spectra of the resulting matrices revealed the presence of several new product species. Major bands at 983.6 and 818.9 cm−1 in the infrared spectra of the reaction products of U atoms with 14N16O are due to the U–N and U–O stretching vibrations, respectively, of the linear NUO molecule. Weaker absorptions at 845.9 cm−1 and 718.3 cm−1 in similar experiments are indicative of a “T” shaped NUO2 species. A weaker 1017.0 cm−1 band provides evidence for the cation/anion pair [NUO]+[NO]−. Several possible products of the reaction of uranium atoms with nitrogen oxides have been further characterized by quasirelativistic density functional calculations.
Perturbation theory for coupled anharmonic oscillators
Authors: Suvernev Andrei A. , Goodson David Z.
Published: July 2002

Perturbation theory is applied to a pair of coupled oscillators with cubic anharmonicity. Large-order perturbation theory is shown to be more efficient computationally than numerical diagonalization of the Hamiltonian. Quadratic Padé summation of the energy expansions yields convergent results for the real and the imaginary parts of resonance eigenvalues.
Total cross sections for low energy collisions of H3+ with molecular hydrogen and rare gases
Authors: Peko B. L. , Champion R. L.
Published: July 2002

Measurements of total cross sections for collision induced dissociation (CID) and proton abstraction have been made for the reactants H3++H2, Ar, and He. The laboratory collision energies range from a few up to 400 eV and D3+ has been substituted to investigate possible isotope effects. The CID cross sections of H3+→H++H2 or H2++H maximize at a value of a few Å2. Proton abstraction is the dominant process for relative collision energies below 5 eV and is observed to lead to a highly excited H3+ product which often autodissociates. Dissociative charge transfer at higher collision energies is responsible for producing H2+ product ions. The role of internal energy contained within the H3+ primaries in previous experiments and the effects of internal energy on the cross section measurements presented here are also discussed.
On the molecular basis of hydrophobicity: A Monte Carlo study of propionic acid hydration
Authors: SanRomán-Zimbrón M. L. , Ortega-Blake I.
Published: July 2002

In this work we present a Monte-Carlo simulation of propionic acid in aqueous solution and look into the structural and thermodynamic behavior of an amphiphile at temperatures of 285, 298 and 330 K. All Monte-Carlo simulations were done with one solute and 338 water molecules, a NVT ensemble, a polarizable ab initio potential, Metropolis sampling, and periodic boundary conditions. On the technical side we show that very large statistical samples (8×108) are needed to attain reliable results and that accounting for microscopic detail is needed to find patterns that are hidden by averaging. Related to the molecular basis of hydrophobicity we found that solute–water interactions, even around the hydrophobic tail, lead to a favorable enthalpic effect, in agreement with other simulations. We also found that the tail enhances water–water interactions as compared to pure water, in agreement with the experiment. An important finding is that the anomalous solubility seems to come from an induced structure of waters neighboring the hydrophobic tail. On the contrary, at low temperatures the polar head loses its hydration shell as the water network becomes important. From a detailed analysis we propose an interplay of several factors as the reason for hydrophobicity.
An ab initio molecular orbital study on structures and energetics of a C3H2− anion
Authors: Ikuta Shigeru
Published: July 2002

The geometrical structures of a C3H2− anion are surveyed at the coupled-cluster doubles and coupled-cluster singles, doubles (triples) [CCSD(T)] levels of theory with the aug-cc-pVDZ basis set. The cyclopropenylidene, propadienylidene, and C2, Cs, and C1 propargylene conformers are considered. The final energies are calculated at the CCSD(T) and multireference configuration-interaction levels of theory with the aug-cc-pVTZ basis set. The most stable C3H2− anion is propadienylidene, 2B1. The vertical electron detachment energy of the propadienylidene anion is 1.797 eV, in excellent agreement with the experiment (1.794 eV) by Oakes and Ellison. The most stable cyclopropenylidene anion is 2A1, which is higher in energy than the corresponding neutral. Thus the cyclopropenylidene neutral has no electron affinity. The π electrons on the C3 ring in the cyclopropenylidene anion are equal to 2.0, but the electrostatic repulsion overcomes the aromatic stability based on the 4n+2π-electron rule.
Existence diagram for chemical wave patterns in the NO+H2 reaction on Rh(110)
Authors: Schaak A. , Imbihl R.
Published: July 2002

Employing photoemission electron microscopy as spatially resolving method the existence range for pattern formation in the NO+H2 reaction on Rh(110) has been determined.The boundaries between bistable and excitable regions and between different types of chemical wave patterns have been mapped out in pH2, T-parameter space with fixed pNO=1.6×10−6 mbar.
Photochemistry of hydrogen peroxide in solid argon
Authors: Khriachtchev L. , Pettersson M. , Tuominen S. , Räsänen M.
Published: July 2002

UV-induced (from 193 to 380 nm) photochemical processes on H2O2 in solid argon (18 K) are studied by using Fourier transform infrared and luminescence spectroscopy. The major photolysis products were identified as (i) a complex of monomeric water with atomic oxygen and (ii) free OH radicals. A three-component photokinetic scheme H2O⋯O↔H2O2→2 OH describes well all the observed features. The wavelength dependence of the kinetics was measured, which gives the rate constants as a function of excitation wavelength. A novel photoinduced recovery of H2O2 from a water molecule complexed with an oxygen atom is experimentally demonstrated. For the origin of this reaction of importance in various areas, we propose a photoinduced charge transfer between H2O and O with concomitant recombination into H2O2.