bluuebunny/crossref_metadata_2025 huggingface dataset:
Mixed-mode oscillations in an electrochemical system. I. A Farey sequence which does not occur on a torus
Authors: Albahadily F. N. , Ringland John , Schell Mark
Published: July 2002

We analyze measurements of an oscillatory current in an electrochemical process in which copper dissolves into phosphoric acid from a rotating-disk electrode. The focus is on a set of states in which each member consists of a different combination of large and small oscillations (mixed-mode oscillations). This set of mixed-mode oscillations is shown to constitute a Farey sequence, i.e., a periodic sequence for which a one-to-one correspondence exists with an ordered sequence of rational numbers. Plots of a measured quantity known as the ‘‘firing number’’ are presented which reveal a structure that is similar to a ‘‘devil’s staircase.’’ The states surrounding the mixed-mode oscillations are analyzed by examining one-dimensional maps, surfaces of section, and phase portraits constructed from experimental data. This analysis shows that the Farey sequence of these mixed-mode oscillations is of a different nature than the Farey sequences associated with phase locking on a torus.
Radial distribution of carbon atoms in crystalline and molten polyethylene from x-ray diffraction
Authors: Narten A. H.
Published: July 2002

X-ray diffraction data for crystalline and molten polyethylene are analyzed to yield radial distribution functions. Use of scattering factors for CH2 groups in the data reduction yields unambiguous, model-independent information on the relative positions of carbon atoms. Direct comparison of the atom pair distribution functions rules out extended chain structures in the melt.
Comment on ‘‘Study on the liquid–vapor interface of water. I. Simulation results of thermodynamic properties and orientational structure’’
Authors: Wilson Michael A. , Pohorille Andrew , Pratt Lawrence R.
Published: July 2002

This comment on the article by Matsumoto and Kataoka [J. Chem. Phys. 88, 3233 (1988)] identifies a discrepancy between recent calculations of the surface potential of the water liquid–vapor interface. The cited work assumes that the field of a water molecule in the interfacial region is strictly the field of a point molecular dipole, whereas other works have made more detailed assumptions about the molecular charge distributions. The difference between the values for the surface potential obtained from these different assumptions is large compared to the surface potential value in question. It is shown that the numerical difference is associated with the densities of molecular quadrupole moment in the coexisting bulk phases. The correction can be evaluated analytically and applied to the molecular dynamics results after the fact.
Chemical association in simple models of molecular and ionic fluids
Authors: Stell George , Zhou Yaoqi
Published: July 2002

A simple approximation scheme is given for the association constant of a family of models of associating particles introduced and considered by Cummings and Stell, Rasaiah et al., and Stell. The result correctly reduces to the law of mass action at low concentrations. For the ionic-species version, the theory requires accurate assessment of the cavity function of charged hard spheres, for which we develop a recipe. We show the close similarity (and some differences) between the above models and those considered by Wertheim, through a simple extensions of his models. We also note the equivalence of our approximation and Wertheim’s first-order thermodynamic perturbation theory.
The vibrationally mediated photodissociation dynamics of nitric acid
Authors: Sinha Amitabha , Vander Wal Randall L. , Crim F. Fleming
Published: July 2002

Vibrationally mediated photodissociation, in which one photon prepares a highly vibrationally excited molecule by vibrational overtone excitation and a second photon dissociates the vibrationally excited molecule, is a means of studying the spectroscopy and photodissociation dynamics of highly vibrationally excited states. Applying this dissociation scheme to nitric acid (HONO2) excited in the region of the third overtone of the O–H stretching vibration (4νOH) and detecting the OH fragment by laser induced fluorescence determines the energy partitioning and identifies the influence of vibrational excitation prior to dissociation. Vibrationally mediated photodissociation using 755 and 355 nm photons deposits more energy in relative translation than the isoenergetic single photon dissociation with 241 nm light. The former process also produces three times more vibrationally excited OH fragments, and both processes form electronically excited NO2, which receives over three-quarters of the available energy. In these experiments, vibrational overtone excitation enhances the cross section for the electronic transition by about three orders of magnitude. The observed differences are consistent with the motion of the vibrationally excited molecule on the ground electronic state surface strongly influencing the dissociation dynamics by allowing access to different electronic states in the photolysis step.
Photodissociation dynamics of acetone at 193 nm: Photofragment internal and translational energy distributions
Authors: Trentelman Karen A. , Kable Scott H. , Moss David B. , Houston Paul L.
Published: July 2002

The photofragment internal and translational energy distributions resulting from the 193 nm photolysis of acetone have been measured. Vacuum-ultraviolet laser-induced fluorescence was used to probe the CO fragment, and multiphoton ionization time-of-flight mass spectrometry was used to probe the CH3. A Boltzmann distribution was observed to fit each degree of freedom with the following characteristic temperatures: CO: Tvib =2700 K, Trot =3000 K, Ttrans =3000 K; CH3: Tvib =800 K, Trot =500 K, Ttrans =3500 K. No evidence was found for two distinct CH3 populations, as might be characteristic of a stepwise reaction. Energy partitioning between the fragments was fit well by a simple impulsive model in which the available energy is divided equally between the two dissociating C–C bonds, the two bonds cleaving in rapid succession on a time scale short enough to allow little redistribution of energy into the methyl degrees of freedom.
Heisenberg spin exchange and molecular diffusion in liquid crystals
Authors: Nayeem Akbar , Rananavare S. B. , Sastry V. S. S. , Freed Jack H.
Published: July 2002

Heisenberg spin exchange (HE) studies of translational diffusion of the nitroxide radicals PD-tempone and P probe in two liquid crystalline solvents 6OCB–8OCB and 4O,6 are described. It is shown that while PD-tempone undergoes strong exchange in the two solvents, the more anisotropic P-probe exhibits a tendency toward weak exchange which becomes more prominent in the low temperature mesophases. The molecular diffusion rates measured from our HE studies are compared with rates measured over larger distances using electron-spin resonance (ESR) imaging methods; we find that, in similar thermotropic liquid crystals, the former are somewhat faster. Also, while diffusion rates for PD-tempone (using HE) in the ordered phases of 6OCB–8OCB are consistent with a single activation energy, those in 4O,6 show variations; suggesting that probe expulsion from core to chain regions in the former most likely occurs prior to SA formation, whereas in the latter it occurs in the SA phase. The absence of discontinuities in our diffusion data at the N–SA–RN transitions supports the belief that these transitions are subtle, with nothing dramatic occurring as the reentrant nematic (RN) phase is formed. The effect of including a potential of mean force U(r) between colliding radicals due to the liquid-crystal structure, is also considered. Our analyses indicate that the potential is of a repulsive nature [i.e., U(d)>0] suggesting the possibility of solvent molecules inhibiting collisions of radicals at distances shorter than the sum of their solvated radii. The influence of orientational ordering on HE involving nonspherical radicals is considered, but changes from strong to weak exchange in the ordered phases appear to depend on how τ1 the lifetime of the interacting radical pair is influenced by U(r). A careful effort is made to separate the HE effects from the intermolecular electron–electron dipolar (EED) interactions. It is suggested that anomalies in D obtained from HE vs EED in this and earlier studies may also be rationalized in terms of the effects of U(r).
Raman scattering from freely suspended liquid crystal films
Authors: Van Winkle David H. , Dierker S. B. , Clark N. A.
Published: July 2002

Using Raman scattering we have studied the molecular vibrations in freely suspended liquid crystal films as thin as two molecular layers. The effect of the free surfaces on several aspects of molecular order were examined by comparing the Raman spectra obtained on both thin and thick films. The liquid crystal materials 8CB, 8SI, and 65OBC were studied in the crystal, hexatic, and fluid smectic phases. Changes were observed in both intramolecular and intermolecular vibrations as a function of thickness.
Ã 1B2–X̃ 1A1 26v transitions of 18O-enriched tropolone
Authors: Redington Richard L. , Redington Theresa E. , Hunter Martin A. , Field Robert W.
Published: July 2002

Laser excitation spectra with v=0, 2, 4, and 6 in the Ã 1B2–X̃ 1A1 26v0 progression of jet-cooled 18O/16O isotopomers of tropolone are reported. The isotope shift for ν26, an out-of-plane deformation mode at 39 cm−1 in the Ã state, is 2% for tropolone-18O18O. This large 18O isotope effect indicates that Q26 for tropolone resembles the analogous normal mode of tropone, which is a ring deformation towards the boat conformation of 2, 4, 6-cycloheptatriene accompanied by a large O atom displacement. Tunneling by tropolone in the Ã state is quenched by exciting the 26v overtone states and a mechanism for this quenching is proposed in terms of the indicated normal coordinate. Tunneling splittings are <0.3 cm−1 for the zero point levels of the X̃ state of the symmetrical isotopomers. In contrast, vibrational isotope effects dominate the tunneling interactions to split the corresponding levels of tropolone-16O18O by 1.7 cm−1. In the Ã state of this isotopomer the tunneling interactions are dominant. Because they are determined by the overlap between localized and delocalized wave functions, the Franck–Condon factors of tropolone-16O18O are smaller than those of the symmetrical isotopomers.
Potential-energy surfaces for Pt2+H and Pt+H interactions
Authors: Balasubramanian K. , Feng P. Y.
Published: July 2002

Potential-energy surfaces for ten electronic states of Pt2H as a function of Pt2–H distance are constructed using the complete active space multiconfiguration self-consistent field followed by multireference singles plus doubles configuration interactions which included up to 407 000 configurations. In addition, the spin-orbit effects were included through the relativistic configuration-interaction method. The potential-energy curves of low-lying electronic states of PtH and spectroscopic constants of 11 states of PtH are also computed using the same method. The experimental A′–X2 spectroscopic bands of PtH are reassigned to the 2Σ+(II)–1/2(I) transition. A tentative reassignment of the observed B′–X1 system is also suggested. The ground state of Pt2H is found to be of E(I) symmetry when spin-orbit effects are incorporated with a bridged structure [Re(Pt–Pt)=2.46 Å, Re(Pt–H)=1.684 Å]. A very low-lying excited state E(II) (Te=0.14 eV) was found for Pt2H. The De(Pt2H)=59 kcal/mol compared to the De(PtH)=72 kcal/mol, indicating lower stability of the Pt2H bridge compared to PtH diatomic. Some of the low-lying electronic states of PtH, especially 3/2 states, exhibit avoided crossings.
The vibrational spectra of molecular ions isolated in solid neon. III. N+4
Authors: Thompson Warren E. , Jacox Marilyn E.
Published: July 2002

When a Ne:N2=100 or 200 mixture is codeposited at 5 K with a beam of neon atoms excited by a microwave discharge, a weak to moderately intense infrared absorption appears at 2237.6 cm−1 which is assigned to the N+4 molecular ion. The analysis of the infrared spectra of the nitrogen-15 substituted species of N+4 supports the conclusion from earlier ab initio calculations and electron spin resonance observations that N+4 has a linear, centrosymmetric ground-state structure. For the N+4 species with noncentrosymmetric isotopic substitution, the in-phase end-atom stretching fundamental becomes infrared active and has also been observed. Although the anion responsible for overall charge neutrality of the deposit has not been definitively identified, secondary photolysis studies provide some information regarding its properties.
Overtone intensities and dipole moment surfaces for the isolated CH chromophore in CHD3 and CHF3: Experiment and a
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Authors: Ha Tae-Kyu , Lewerenz Marius , Marquardt Roberto , Quack Martin
Published: July 2002

The band strengths of fundamentals (N=1) and overtones (up to N=6) of the strongly coupled CH stretching and bending vibrations in CHD3 and CHF3 are calculated using high level ab initio (SCF-CI) dipole moment functions and potential surfaces in one and two (three) dimensions. The calculations are performed in approximate normal coordinate and internal coordinate subspaces, the former giving generally superior results. The overall prediction of relative and absolute intensities ranging over many orders of magnitude is often accurate to within a factor of 2, but not to within experimental accuracy. Different dipole model functions and potential surfaces are investigated and an empirical adjustment of the dipole function to experiment is proposed for CHF3. The comparison of experimental and ab initio overtone intensities for the Fermi resonance system is discussed in some detail, as well as the importance of the results for IR spectroscopy and IR multiphoton excitation.
Diffusion of penetrants in amorphous polymers: A molecular dynamics study
Authors: Müller-Plathe Florian
Published: July 2002

Molecular dynamics calculations are performed that simulate the diffusion of penetrant molecules through an amorphous polymer. For the polymer and the penetrant molecules, model potentials are chosen that approximate polyethylene and methane, respectively. The simulations extend over several hundred picoseconds. The results indicate that permeation proceeds by a hopping mechanism: The penetrants dwell in voids in the polymer matrix for a considerable time and perform fast jumps between neighboring voids. A potential scaling technique based on the fact that individual jump events are activated processes is presented by which the calculation of penetrant self-diffusion coefficients is speeded up.
Properties of a macroscopic system as a thermal bath
Authors: Suárez Alberto , Silbey Robert
Published: July 2002

A macroscopic system acting as a heat reservoir affords a simplified description in terms of a collection of independent modes. Although in most cases this model is unable to account for the behavior of the macroscopic system itself, it provides a consistent desciption of the macroscopic degrees of freedom acting as a dissipation mechanism for a microscopic system. This is found to be a consequence of the statistical properties of bath quantities relevant to the dynamics of the microscopic system. Furthermore, these properties ensure that the unphysical features of the independent bath model do not affect its performance as a heat reservoir.
Toward a molecular theory of vapor-phase nucleation. I. Identification of the average embryo
Authors: Ellerby H. Michael , Weakliem Cheryl Li , Reiss Howard
Published: July 2002

A statistical-mechanical theory of vapor-phase nucleation is developed which improves on that introduced by Reiss, Katz, and Cohen (RKC), and revised by Reiss, Tabazadeh, and Talbot (RTT) by identifying a physical embryo appropriate to the nucleation process. Put in the simplest manner, the goals of this paper are (1) to define embryos in such a manner that their thermodynamic properties can be easily calculated by means of computer simulation; (2) to define embryos in such a manner that (ideally) they can be used as entities in terms of which the systematic exact evaluation of the classical phase integral can be performed, and the equilibrium distribution of embryos therefore determined. This model of a pre-nucleating embryo prevents redundant counting of configurations in the partition function needed to derive the metastable equilibrium distribution of embryos. The embryo definition does not artificially constrain the molecules within the embryo, but remains only a procedural device for organizing the counting. The present treatment also resolves a problem not noticed by RTT involving an inconsistency concerning dimensions in their theory. Using the embryo definition the metastable equilibrium distribution of embryos is derived. Finally, a brief discussion of how the distribution can be used in a theory of nucleation rate is provided. Work is presently under way on the rate theory.
Calculated intensity in the local mode overtone spectra of hydrogen peroxide
Authors: Kjaergaard Henrik G. , Goddard John D. , Henry Bryan R.
Published: July 2002

Calculations of OH stretching vibrational intensities for hydrogen peroxide are performed with a harmonically coupled anharmonic local mode model for the OH stretching vibrational wave functions and ab initio calculations to obtain the dipole moment function. The ab initio calculations are carried out at three different levels: Self-consistent field (SCF), second-order Mo/ller–Plesset perturbation theory (MP2), and configuration interaction including all single and double excitations (CISD), all with a 6–31G* basis set, to yield the dipole moment function. From these results, total oscillator strengths are calculated for the three different types of rotational bands in a given OH stretching transition. The calculated overtone intensities for ΔvOH=2–5 are not sensitive to the particular dipole moment function. However, a significant decrease in intensity occurs for the fundamental transition in going from the molecular orbital to the correlated level calculations. Our calculated results are compared to near infrared, photoacoustic, and molecular beam experiments by other workers. The calculations can explain many of the experimental results, although the interpretation is not always straightforward. Our calculations suggest that transitions to the symmetric OH stretching mode, should be included in the analysis and assignment of the OH stretching overtone regions.
Isomerization and inherent structures in liquids. A molecular dynamics study of liquid cyclohexane
Authors: Harris Jonathan G. , Stillinger Frank H.
Published: July 2002

The effect of conformational isomerization on liquid structure has been examined, using a realistic model of cyclohexane as an example. Molecular dynamics simulations at 300 K and 1 atm have been carried out for two liquid samples: one consisted of an all-chair medium, the other was prepared with about half of the molecules trapped in the twisted-boat form. During each molecular dynamics run, sets of liquid phase configurations were mapped onto representative potential energy minima (inherent structures) by a steepest-descent procedure. Comparison of distribution functions before and after this configurational mapping shows that thermal motions strongly obscure the effects of chair, twisted-boat isomerization on local liquid order, though such effects are clearly present in the inherent structures. The latter contain resolved characteristic intramolecular and intermolecular distance distributions that should be observable in low temperature amorphous deposits of cyclohexane.
Population transfer by stimulated Raman scattering with delayed pulses: Analytical results for multilevel systems
Authors: Coulston George W. , Bergmann Klaas
Published: July 2002

Stimulated Raman scattering involving adiabatic passage (STIRAP) is a proven technique for population transfer in three-level systems with strong oscillator strengths. We show that the STIRAP process should also yield high transfer efficiencies when the densities of states near the intermediate and final levels are high, provided certain criteria for the experimentally controllable parameters are met. We also show that high transfer efficiencies may even be possible when the pump and Stokes lasers can access levels outside of the three-level system. The criteria are derived with an approach that emphasizes the adiabatic nature of the transfer process in an eigenvector of the interaction Hamiltonian that resembles a trapped state. The results are compared to density matrix calculations.
Isomerization of C60 fullerenes
Authors: Yi Jae-Yel , Bernholc J.
Published: July 2002

Car–Parrinello calculations show that the transformation between C60 isomers involves barriers in excess of 5 eV, although the energetic difference between the buckyball and the next lowest isomer is only 1.6 eV. C60 isomerization thus requires high temperatures and long annealing times.
Morphology dependent anomalous frequency shifts of infrared absorption bands of polymer crystals: Interpretation in terms of transition dipole–dipole coupling theory
Authors: Kobayashi Masamichi , Sakashita Mami
Published: July 2002

For such crystalline linear polymers as trigonal poly(oxymethylene) (t-POM), orthorhombic poly(oxymethylene) (o-POM), and poly(ethylene oxide) (PEO), it has been found that the infrared bands polarized parallel to the chain axis exhibit anomalously large high-frequency shifts as the crystal morphology of the sample goes from the extended-chain crystal (ECC) to the folded-chain crystal (FCC). The common features of the remarkable spectral change were investigated systematically and summarized as follows: (1) the anomalous bandshifts were observed only for the infrared-active parallel bands, (2) the magnitude of the shift was proportional to the oscillator strength of the band, (3) the overtones corresponding to the parallel fundamental bands did not show such a remarkable shift, and (4) the appearance of the high-frequency bands in FCC was ascribed not to the folded molecular structure, but to the lamellar-type crystal morphology. The morphology-dependent bandshifts were interpreted quantitatively in terms of the transition dipole–dipole coupling theory. By integrating the interacting energy over cylindrical crystal having various radius/height ratios, it was demonstrated that the energy due to the dipole interaction vanished for the idealized ECC and became maximum for the infinitely thin lamellar crystal. The magnitudes of the bandshifts between the two extreme cases were calculated on the basis of the absolute values of the oscillator strengths of the bands measured on highly crystalline samples of t-POM, o-POM and PEO and their crystal structures. The results agreed well with the observed band gaps between the ECC and FCC samples. The change in the absorption profiles of the parallel bands with change in crystal morphology was simulated. The result reproduced well the observed spectral change.
Random sequential adsorption of anisotropic particles. I. Jamming limit and asymptotic behavior
Authors: Viot P. , Tarjus G. , Ricci S. M. , Talbot J.
Published: July 2002

We study the random sequential adsorption (RSA) of unoriented anisotropic objects onto a flat uniform surface, for various shapes (spherocylinders, ellipses, rectangles, and needles) and elongations. The asymptotic approach to the jamming limit is shown to follow the expected algebraic behavior, θ(∞)−θ(t)∼t−1/3, where θ is the surface coverage; this result is valid for all shapes and elongations, provided the objects have a nonzero proper area. In the limit of very small elongations, the long-time behavior consists of two successive critical regimes: The first is characterized by Feder’s law, t−1/2, and the second by the t−1/3 law; the crossover occurs at a time that scales as ε−1/2 when ε→0, where ε is a parameter of anisotropy. The influence of shape and elongation on the saturation coverage θ(∞) is also discussed. Finally, for very elongated objects, we derive from scaling arguments that when the aspect ratio α of the objects becomes infinite, θ(∞) goes to zero according to a power law α−p, where p=1/(1+2√2). The fractal dimension of the system of adsorbed needles is also discussed.
Degeneracy breaking in the Hilbert-space coupled cluster method
Authors: Berkovic Sigalit , Kaldor Uzi
Published: July 2002

The Hilbert space multireference coupled cluster method with single and double excitations (HS–MRCCSD) is applied to excited states of the N2 molecule. Particular attention is given to the breaking of degeneracy, where sublevels which should be degenerate by symmetry come out separated by as much as 0.3 eV. This feature is explained by the fact that a truncated coupled cluster expansion starting from an open-shell determinant does not preserve spin or spatial symmetry.
Structure and dynamics of model metal clusters
Authors: Uppenbrink Julia , Wales David J.
Published: July 2002

The structure and dynamics of model metal clusters containing 13–55 atoms are investigated by molecular dynamics with regular quenching along the trajectory. The atoms interact via Sutton–Chen many-body potentials suitable for describing surface and bulk properties of gold and silver. The features of the potential energy surfaces of the clusters are elucidated by structural analysis of the quenched configurations and by eigenvector-following to find connecting transition states, and these features are related to the dynamical behavior. The observation of time-scale coexistence is shown to be strongly dependent upon cluster size as well as potential parameters. The Sutton–Chen silver and gold clusters have very different structural properties, with the silver potential stabilizing the icosahedral geometry, and the gold potential favoring glasslike structures. The results are compared with the available experimental observations.
Intensities of CH- and CD-stretching overtones in 1,3-butadiene and 1,3-butadiene-d6
Authors: Kjaergaard Henrik G. , Turnbull David M. , Henry Bryan R.
Published: July 2002

Gas phase vibrational overtone spectra of 1,3-butadiene are recorded in the ΔvCH=2–6 regions by conventional near infrared–visible spectroscopy, and in the ΔvCH=4–7 regions by intracavity dye/titanium:sapphire, laser photoacoustic spectroscopy (ICL-PAS). Gas phase vibrational overtone spectra of 1,3-butadiene-d6 are recorded in the ΔvCD=2–5 regions with conventional spectroscopy and in the ΔvCD=5–8 regions by ICL-PAS. Oscillator strengths are calculated from wave functions that are obtained from a harmonically coupled anharmonic oscillator (HCAO) local mode model and from a dipole moment function that is obtained from ab initio calculations. The experimental oscillator strengths are compared to the values that are calculated for both the CH- and CD-stretching components of the spectrum. Our simple calculations, which contain no adjustable parameters, are in very good agreement with the relative intensities of the peaks corresponding to the three different CH oscillators in 1,3-butadiene. As expected, the local mode description is not as good for the CD oscillators in 1,3-butadiene-d6. Nonetheless, the calculations can provide a reasonable explanation of the CD-stretching intensity distribution in the higher overtone spectra of 1,3-butadiene-d6. Small hydrogen impurities in the fully deuterated sample give rise to isolated CH-stretching overtones. The relative intensities of the CD peaks and the CH impurity peaks in the 1,3-butadiene-d6 sample spectra are predicted by the calculations. A comparison of the 1,3-butadiene-d6 sample spectra in the CH-stretching region with the CH-stretching overtone spectra in 1,3-butadiene dramatically illustrates the effects of vibrational coupling between CH oscillators.
A distributed Gaussian approach to the vibrational dynamics of Ar–benzene
Authors: Faeder J.
Published: July 2002

A method for calculating the vibrational eigenstates of van der Waals clusters is presented and applied to argon–benzene. The method employs the linear variational principle with a nonorthogonal basis set of Gaussian functions in both the stretching and bending coordinates. These localized functions allow greater flexibility than the standard spherical harmonics or Wigner D functions and should be more efficient when the motion is confined to specific regions of the potential energy surface. Calculations are performed on several potential surfaces including two recent fits to a previously published ab initio calculation. Accurate results with rapid convergence are obtained here for the states of zero total angular momentum (J=0). The results agree with calculations recently performed on the same potential surfaces by a different method [J. Chem. Phys. 98, 5327 (1993)] and suggest a reassignment of the experimentally observed bands. An extension of the basis set to nonzero J is presented in the Appendix.
Molecular dynamics simulation study of the adsorption of chain alkanes from solution onto graphite
Authors: Hentschke Reinhard , Winkler Roland G.
Published: July 2002

We use the molecular dynamics simulation method to study the adsorption of an n alkane from a benzene solution onto the basal plane of graphite. We show that it is possible to simulate the entire adsorption process, i.e., the diffusion of the alkanes to the surface, the penetration of the highly structured solvent layers near the surface, and the physisorption of the chains on the graphite. Specifically, we consider a solution of C14H30 in benzene confined between two graphite surfaces. The alkane chains are initially dispersed near the center of the slit where the solvent is bulklike. During the simulation, the chains are attracted by the surfaces where they adsorb replacing benzene molecules in the structured liquid near the surfaces. The alkane chains adsorb parallel to the graphite plane assuming predominately stretched conformations without tails extending into the solution. The simulation also shows evidence for the onset of self-organization due to strong lateral interactions between the adsorbed molecules.
Slit-jet near-infrared diode laser spectroscopy of (DCl)2: ν1, ν2 DCl stretching fundamentals, tunneling dynamics, and the influence of large amplitude ‘‘geared’’ intermolecular rotation
Authors: Schuder Michael D. , Nelson David D. , Nesbitt David J.
Published: July 2002

The first high resolution spectra of (DCl)2 are reported using direct IR laser absorption spectroscopy in a slit supersonic expansion. The spectral data are analyzed to obtain vibrational frequencies, rotational constants, and tunneling (interconversion) level splittings for isotopically symmetric (D35Cl)2 and (D37Cl)2, and mixed D35Cl–D37Cl dimers. Six dimer absorption bands are observed and analyzed for both (D35Cl)2 and D35Cl–D37Cl. These include two perpendicular Ka=1←0, v1=1←0 (i.e., ‘‘free’’ DCl stretch) bands, one each originating from the lower (+) and the upper (−) tunneling sublevels in the ground vibrational state. Four parallel v2=1←0 (i.e., ‘‘bound’’ DCl stretch) bands are also observed, one for each of the Ka=0←0 and Ka=1←1 subbands originating from both the lower (+) and upper (−) tunneling components.

In addition, two bands are observed only for the isotopically mixed dimer (i.e., complexes from D35Cl and D37Cl), which acquire oscillator strength by virtue of the breaking of inversion symmetry. This complete set of bands provides the necessary data to determine interconversion splittings for the mixed dimer in the ground [5.9595(6) cm−1] and the two DCl vibrationally excited states [3.2286(6) cm−1 for v1=1 and 2.9935(6) cm−1 for v2=1], as well as to make accurate predictions for the symmetric (D35Cl)2 dimer. These experimental splittings for the ground state are compared to results from (i) a 1D quantum calculation for adiabatic motion over a minimum energy tunneling path; and (ii) a 3D variational calculation in a basis set of free DCl rotors which treats all three internal bend and torsion angles (Θ1, Θ2, and φ1–φ2). These calculations, performed on an approximate dipole and quadrupole model of the electrostatic potential surface, reproduce the ground state tunneling splittings to within 15%. The corresponding eigenfunctions provide direct evidence for highly correlated, ‘‘geared’’ internal rotation of the two DCl subunits over a low barrier. The fivefold decrease in tunneling splitting for the symmetric (DCl)2 upon v1=1 or v2=1 excitation is qualitatively consistent with previous models of vibrationally diminished tunneling rates due to intramolecular V→V energy transfer at the C2h transition state. However, this decrease is nearly identical to the 4.8-fold decrease observed in (HCl)2, which is quantitatively inconsistent with a simple dipole–dipole vibrational energy transfer mechanism. Measured linewidths in these dimer spectra are all at the resolution limit of the diode laser apparatus, which translates into vibrational predissociation lifetimes in excess of 3 ns.
A quantum chemical determination of diabatic states
Authors: Ruedenberg Klaus , Atchity Gregory J.
Published: July 2002

A simple electronic-structure-based construction of diabatic states from adiabatic states is formulated. It is accomplished by maximizing the configurational uniformity of the diabatic states with respect to the dominant configurations throughout the entire coordinate space. No configurational constraints are introduced. No matrix elements of additional, derivative, or nonderivative operators have to be calculated. The matrix elements between the diabatic states are simply expressible in terms of the adiabatic energies and wave function coefficients.
266 nm CH3I photodissociation: CH3 spectra and population distributions by coherent Raman spectroscopy
Authors: Zahedi Mansour , Harrison James A. , Nibler Joseph W.
Published: July 2002

High resolution coherent anti-Stokes Raman spectroscopy has been used to study the symmetric CH stretching mode of methyl radical formed by UV laser photolysis of CH3I cooled in a free jet expansion. Spectra obtained under near-nascent conditions (∼3 to ∼6 collisions) show that most of the CH3 product is formed in the ground vibrational state, with little excitation seen in the ν2 out-of-plane bending coordinate [v2=1/v2=0, population ratio 0.27(10)]. This is in accord with recent theoretical calculations favoring slow, adiabatic CH3 relaxation from a pyramidal to planar structure as the C–I bond breaks. Extensive N,K rotational structure is resolved and the distributions obtained lend support to those deduced for nascent CH3 by Chandler and co-workers from modeling of unresolved resonance enhanced multiphoton ionization spectra. The results are consistent with conservation during dissociation of CH3 ortho-para nuclear spin forms and of K spinning angular momentum about the symmetry axis. CH3 product tumbling motion is slightly greater (1–2 units of angular momentum) than predicted theoretically. Rapid collisional excitation of higher rotational levels is seen and the rich spectrum observed after ∼50 collisions has been analyzed to give improved or new vibrational-rotational parameters for 1000 and 1100 states. The 1100 band origin is determined to be 2996.21(4) cm−1, yielding −8.23(5) cm−1 for the x12 anharmonicity constant.
High resolution near infrared spectroscopy of HCl–DCl and DCl–HCl: Relative binding energies, isomer interconversion rates, and mode specific vibrational predissociation
Authors: Schuder Michael D. , Nesbitt David J.
Published: July 2002

Both D- and H-bonded isomers of the mixed dimers formed between HCl and DCl are investigated via high resolution infrared difference frequency and diode laser spectroscopy in the 2885 and 2064 cm−1 regions. From an analysis of the relative integrated absorption intensities, the D-bonded complex (i.e., HCl–DCl) is determined to be more stable by 16±4 cm−1 than the H-bonded (i.e., DCl–HCl) species. All four chlorine isotopic combinations of the lower energy (HCl–DCl) complex are probed via excitation of both HCl (vHClacc=1←0) and DCl (vDCldon=1←0) stretches. Additionally, two chlorine isotopomers of the higher energy (DCl–HCl) complex are investigated through HCl excitation. Compared to the facile tunneling observed in both (HCl)2 or (DCl)2 complexes, these mixed dimers exhibit more rigid behavior characteristic of two distinct isomeric species. However, the relatively small energy difference (16±4 cm−1) between the two isomers still allows the wave functions for both species to sample both the HCl–DCl and DCl–HCl local minima on the potential surface. This intermediate level of angular localization of the wave function is modeled via 3D quantum mechanical calculations including all three internal rotor angular degrees of freedom. Additionally, a 1D treatment along the minimum energy tunneling path is investigated, which quantifies the asymmetry in the tunneling coordinate due to isotopic dependence of the H- and D-bonded zero point bending and torsion energies.

Vibrational predissociation lifetimes in excess of the slit jet instrument line shape are determined from homogeneous broadening of the spectral line widths. The HCl stretch excited lifetime of H-bonded DCl–HCl [ΔνHCldon=44(6) MHz, τHCldon=3.6(5) ns] is threefold shorter than the corresponding lifetime of D-bonded HCl–DCl [ΔνHClacc=16(3) MHz, τHClacc=9.6(16) ns]. This ratio is quite comparable to the results obtained in investigations of (HCl)2 and consistent with a stronger, mode specific coupling to the dissociation coordinate for excitation of the bonded-HX vs free-HX moiety. However, the absolute lifetimes of both vHClacc=1 HCl–DCl and vHCldon=1 DCl–HCl complexes are tenfold shorter than the corresponding excited vibrational state lifetimes in (HCl)2. This suggests a near resonant channel for predissociation into HCl(v=0)+DCl(v=1) which minimizes the energy deposited into rotation and relative translation of the diatomic fragments.
Intermolecular dynamics of benzene–rare gas complexes as derived from microwave spectra
Authors: Brupbacher Th. , Makarewicz J. , Bauder A.
Published: July 2002

The rotational spectra of three benzene–X complexes, where X=20Ne, 129Xe, or 132Xe, and of the benzene-1,3,5-d3–Ar complex have been observed using pulsed nozzle Fourier transform microwave (FTMW) spectroscopy. Rotational transitions assigned in the 8–18 GHz range have been found to match symmetric top spectra. Rotational constants B and centrifugal constants DJ and DJK were determined from the measured frequencies. Intermolecular motions between benzene and the rare gas atom have been modeled with a rovibrational Hamiltonian. The three-dimensional interaction potential has been assumed of a simple form with three adjustable parameters. These parameters, one of which represents the equilibrium distance of the rare gas atom from the plane of benzene, have been adjusted for all benzene–rare gas complexes in a least-squares fit by direct inversion of the observed rotational transition frequencies. From the potential, the force constants and frequencies of the van der Waals vibrations and the binding energies have been deduced for all benzene–rare gas complexes.
Temperature variation of the structural parameters in actinide tetrafluorides
Authors: Kern S. , Hayward J. , Roberts S. , Richardson J. W. , Rotella F. J. , Soderholm L. , Cort B. , Tinkle M. , West M. , Hoisington D. , Lander G. H.
Published: July 2002

We have used neutron diffraction on polycrystalline samples of AnF4 (An=Th, U, and Np) to examine the structures. All are isostructural with monoclinic space group C2/c. The structure of UF4 has been examined in some detail as a function of temperature. The volume of the unit cell has a minimum not at lowest temperature (15 K) examined, but rather at ∼350 K. ThF4 also has a larger volume at low rather than room temperature; suggesting that this effect is caused by structural rather than electronic (as ThF4 has no 5f electrons) interactions. An examination of the coordination around the two independent An sites in UF4 shows that the fluorine configuration changes appreciably around An2 as a function of temperature. At low temperatures the distorted cubic antiprism structure is considerably more ‘‘puckered’’ than at higher temperatures. This puckering requires more volume for the molecule, thus causing the volume to expand as the temperature is lowered. The temperature factors and the interatomic distances vary smoothly with temperature and show no unusual behavior.
Organic hydrogen-bonded dielectrics: Quantum paraelectricity based on tautomerization of 9-hydroxyphenalenone derivatives
Authors: Mochida Tomoyuki , Izuoka Akira , Sugawara Tadashi , Moritomo Yutaka , Tokura Yoshinori
Published: July 2002

Dielectric properties based on tautomerization in 9-hydroxyphenalenone derivatives were examined. While the 5-methyl derivative 2 exhibits an antiferroelectric transition at 41 K, such a phase transition was not observed in 5-bromo derivative 3 down to 4.2 K. Suppression of the phase transition in the latter may be interpreted in terms of quantum paraelectricity based on proton-tunneling along the hydrogen bond. The reason for the appearance of antiferroelectric ordering coupled with the phase transition in 2 may be ascribed to reduction of the tunneling frequency due to symmetry breaking caused by the methyl group. The present system can be regarded as zero-dimensional hydrogen-bonded system and is interesting in reference to inorganic hydrogen-bonded dielectrics.
Relative role of surface interactions and topological effects in nuclear magnetic resonance of confined liquids
Authors: Korb J.-P. , Delville A. , Xu Shu , Demeulenaere G. , Costa P. , Jonas J.
Published: July 2002

The relative roles of surface and topological effects on the nuclear relaxation rates T−11, T−12, and T−11ρ of polar or nonpolar liquids in porous sol-gel silica glasses are identified via their very different pore size and frequency dependences. On the basis of theory, experimental relaxation rates, and molecular dynamics simulations for the modeled porous systems, the 1/Ti’s are interpreted in terms of a linear combination of bulk, confinement, and surface effects: 1/Ti = 1/Tibulk + ai/R2+ bi/R, where R is the average pore size and ai and bi are given in terms of the usual relaxation parameters of the studied molecular species. This simple expression which allows the determination of the relative roles of surface and topological effects has been used to fit the observed 1H NMR relaxation rates as a function of pore size and frequency for methylcyclohexane, nitrobenzene, pyridine, and toluene both for nonmodified and surface modified porous silica glasses. Using this method, the surface (∝1/R) and pure geometrical (∝1/R2) relaxation contributions are evaluated and the surface and translational correlation times are calculated. More generally, the experimental data allow us to explain the following seemingly paradoxical results obtained for confined liquids: (i) The pure confinement effect is independent of the polarities of the liquid molecules in pores and is very sensitive to the frequency. (ii) The finding of the frequency variation of T−11 and T−11ρ both for polar or nonpolar liquids confined to small pores, shows that the geometrical confinement effects dominate over the surface interaction effects at low frequency and for small pores.
The structure of small nickel clusters. II. Ni16–Ni28
Authors: Parks E. K. , Zhu L. , Ho J. , Riley S. J.
Published: July 2002

The molecular adsorption of nitrogen on nickel clusters is used to probe the clusters’ geometrical structures. The application of nitrogen binding rules derived from earlier studies of both larger and smaller nickel clusters allows a determination of structure from nitrogen uptake patterns. In the 16- and 28-atom size region cluster structure is dominated by local pentagonal symmetry, a consequence of a preference for close packing of atoms on clusters with curved surfaces. In most cases, the structures that result can be derived from the 13-atom icosahedron, the polyicosahedral 19-, 23-, and 26-atom clusters, and the 55-atom icosahedron, by adding or removing atoms. Icosahedral and polyicosahedral clusters often have substantial surface strain, which in some cases is relieved by deviations from the ideal geometry. Structures are proposed for all clusters in the Ni16 to Ni28 size range, with the exception of Ni27. Generally, there is no evidence for structural changes as a consequence of nitrogen binding, so that the proposed structures are those of the bare as well as the nitrogenated clusters. Where possible, comparison with existing theoretical calculations of nickel cluster structure is made.
The correlation functions of hard-sphere chain fluids: Comparison of the Wertheim integral equation theory with the Monte Carlo simulation
Authors: Chang Jaeeon , Sandler Stanley I.
Published: July 2002

The correlation functions of homonuclear hard-sphere chain fluids are studied using the Wertheim integral equation theory for associating fluids and the Monte Carlo simulation method. The molecular model used in the simulations is the freely jointed hard-sphere chain with spheres that are tangentially connected. In the Wertheim theory, such a chain molecule is described by sticky hard spheres with two independent attraction sites on the surface of each sphere. The OZ-like equation for this associating fluid is analytically solved using the polymer-PY closure and by imposing a single bonding condition. By equating the mean chain length of this associating hard sphere fluid to the fixed length of the hard-sphere chains used in simulation, we find that the correlation functions for the chain fluids are accurately predicted. From the Wertheim theory we also obtain predictions for the overall correlation functions that include intramolecular correlations. In addition, the results for the average intermolecular correlation functions from the Wertheim theory and from the Chiew theory are compared with simulation results, and the differences between these theories are discussed.
The Wertheim integral equation theory with the ideal chain approximation and a dimer equation of state: Generalization to mixtures of hard-sphere chain fluids
Authors: Chang Jaeeon , Sandler Stanley I.
Published: July 2002

We have extended the Wertheim integral equation theory to mixtures of hard spheres with two attraction sites in order to model homonuclear hard-sphere chain fluids, and then solved these equations with the polymer-Percus–Yevick closure and the ideal chain approximation to obtain the average intermolecular and overall radial distribution functions. We obtain explicit expressions for the contact values of these distribution functions and a set of one-dimensional integral equations from which the distribution functions can be calculated without iteration or numerical Fourier transformation. We compare the resulting predictions for the distribution functions with Monte Carlo simulation results we report here for five selected binary mixtures. It is found that the accuracy of the prediction of the structure is the best for dimer mixtures and declines with increasing chain length and chain-length asymmetry. For the equation of state, we have extended the dimer version of the thermodynamic perturbation theory to the hard-sphere chain mixture by introducing the dimer mixture as an intermediate reference system. The Helmholtz free energy of chain fluids is then expressed in terms of the free energy of the hard-sphere mixture and the contact values of the correlation functions of monomer and dimer mixtures. We compared with the simulation results, the resulting equation of state is found to be the most accurate among existing theories with a relative average error of 1.79% for 4-mer/8-mer mixtures, which is the worst case studied in this work.