bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational predissociation of triatomic van der Waals molecules
Authors: Beswick J. A. , Jortner Joshua
Published: February 2003

In this paper we advance a quantum mechanical colinear model for vibrational predissociation on a single electronic potential surface of a linear triatomic van der Waals molecule X⋅⋅⋅BC, where BC is a conventional diatomic, while X represents a rare-gas atom. The zero-order states of the system are represented as products of an eigenfunction of the vibrating BC bond and a function describing the (bound or unbound) motion of X relative to the center of mass of BC bond which is frozen at its equilibrium configuration. The residual interaction representing the deviation between the interaction potential of X with the vibrating BC molecule, and the interaction of X with the frozen diatomic, induces discrete–continuum and continuum–continuum coupling. On the basis of the analysis of these coupling terms we assert that the zero-order basis provides a reasonable description of the initial and final states. We have also demonstrated that the zero-order resonance widths are small relative to their spacings and, furthermore, we have shown that continuum–continuum couplings prevail essentially only between adjacent continua. The dynamics of vibrational predissociation were reduced to the problem of the decay of a single resonance into a manifold of adjacently coupled continua. Closed analytical expressions for the rate of vibrational predissociation and for the vibrational distribution of the products were derived incorporating the effects of discrete–continuum and continuum–continuum coupling. We have explored the dependence of the rate of vibrational predissociation on the frequency of the BC molecule establishing a new energy gap law for this process. We have also investigated the dependence of the rate on the potential parameter of van der Waals bond and on the mass of the rare-gas atom. Finally, a study of the nature of the final vibrational distribution of the diatomic fragment resulting from the vibrational predissociation process was provided.
Renormalized configuration interaction method for electron correlation in the excited states of polyenes
Authors: Ohmine I. , Karplus M. , Schulten Klaus
Published: February 2003

Extensive configuration interaction (CI) is needed to achieve satisfactory descriptions of the optical spectra and photochemical properties of the π-electron systems of polyenes. Although a basis of single and double excitations with respect to the SCF ground state yields a qualitatively correct energy level scheme, such a treatment introduces an imbalance in the ground state correlation (well described) relative to that of the excited states (poorly described). The result is a divergence in the excitation energies with increasing size of the π-electron system. A renormalized configuration interaction method is developed to account correctly for the excited state correlation. The method is based on the finding that the main contribution to the correlation energy in the excited states is from the electrons not directly involved in the excitation, so that the correlation correction closely resembles that in the ground state. A detailed analysis of the excited state energy permits one to isolate the dominant ground-state correlation term and to determine the smaller, but not negligible, rearrangement correction. The former does not contribute to the excitation energy. The fact that the latter is approximately constant, independent of chain length, provides an explanation for the success achieved by appropriately parametrized single excitation calculations in the assignment of the optically allowed states of polyenes. To implement the renormalized CI method a localized SCF orbital set is employed and the basis functions used for the CI expansion are expressed in terms of single and double excitations with respect to the correlated ground state. It is demonstrated that for the 1B+u, 3Bu+, and 3Ag+ states, which can be characterized in terms of ’’elementary’’ single excitations, this approach gives excellent agreement with the results of more extended CI calculations. Further, it is shown that the correlation energy of the excited state can be estimated using the results of a single-excitation calculation and the ground-state double excitation coefficients.
Conformation of n-alkane molecules in the melt and in cyclohexane solution studied by small-angle neutron scattering
Authors: Dettenmaier M.
Published: February 2003

Small-angle neutron scattering experiments on hexadecane and hexatriacontane molecules in various states have been performed. The molecules in the melt and in cyclohexane solution show the same scattering behavior. The measured radii of gyration are in good agreement with those calculated by Flory on the basis of the rotational isomeric state theory. There is no evidence for any intermolecular interaction that favors elongated chain conformations. One must rather come to the conclusion that n-alkane molecules assume their unperturbed dimensions in the molten state.
Two-parameter statistical model for atoms
Authors: Gázquez José L. , Parr Robert G.
Published: February 2003

A simple two-parameter charge density of the form ρ (r) =C/(1+βr)n with β=α/n, where C is determined by normalization and α and n are determined by minimization of the total energy, is examined in connection with an energy density functional that contains kinetic energy terms up to fourth order in the gradient expansion and the classical Dirac exchange term. This charge density is finite at the nucleus, is monotonically decreasing, becomes pure exponential when n→∞, is extremely accurate for determining two-electron Hartree–Fock densities, and gives good energy values for first-row atoms. For higher atomic number, the energy values are much superior to the traditional Thomas–Fermi–Dirac values. Also presented, following Fermi [Z. Phys. 48, 73 (1928)] is a prediction with this charge density of the number of electrons with a given quantum number l in an atom with atomic number Z. The agreement with the known values is excellent: whereas Fermi found that the d shell begins to fill at Z=19.33 and the f shell at Z =53.02, the new model has the d shell beginning to fill at Z =20.96, and the f shell at Z=57.51; actual values are 20 to 21 and 57 to 58, respectively.
Preparation and photoionization potentials of molecules of sodium, potassium, and mixed atoms
Authors: Herrmann A. , Schumacher E. , Wöste L.
Published: February 2003

In supersonic nozzle beams of sodium, potassium, and Na/K molecular aggregates Mx have been investigated by several techniques based on mass spectrometry. By electron impact ionization with energy of 20 eV clusters with x≳4 are not observable. Photoionization near the threshold allows identification of Naxx?16, Kxx?12, and NaxKyx+y?6. Single photon ionization thresholds for Naxx?14 are given, this being the hitherto longest contiguous series of any property as a function of cluster size. For Kxx?8 and for 8 Na/K mixed clusters PI thresholds are similarly determined. Very accurate two photon photoionization thresholds for Na2 and K2 have been obtained. Theoretical models of metal clusters from recent publications are not able to adequately interpret the electronic properties of these particles as reflected by the ionization potentials.
Theoretical study on a reaction pathway of Ziegler–Natta-type catalysis
Authors: Novaro O. , Blaisten-Barojas E. , Clementi E. , Giunchi G. , Ruiz-Vizcaya M. E.
Published: February 2003

A b initio SCF–LCAO–MO all-electron calculations have been performed for the catalyzed reaction, related to the Ziegler–Natta polymerization process TiCl4⋅Al(CH3)3+C2H4 →TiCl4⋅Al(CH3)2⋅C3H7. The energy of the Ti–Al–ethylene complex during the reaction pathway is associated to the following processes: First, the ethylene coordinates to the catalyst complex and subsequently it reacts with the methyl moiety producing an activation energy barrier mainly due to the breaking of the ethylene double bond. The computed barrier height (15 kcal/mole) is in reasonable agreement with experimental activation energies. The identification of the energy barrier is based on the analysis of the electron population, bond energies, and molecular orbitals of the system as they evolve during the reaction. A comparison with the qualitative picture resulting from the classical model of Cossee and with semiempirical calculations is presented. From these calculations a model is derived which leads to a tentative suggestion for the enhancement of the catalytic activity. We consider this suggestion (regardless of its practical value), as well as the model, as examples of the type of theoretical results that can be furnished by quantum mechanics.
Theoretical study of reactive processes in the FH+2 system by a
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o MCSCF–CI and diatomics-in-molecules calculations
Authors: Kendrick J. , Kuntz P. J. , Hillier I. H.
Published: February 2003

A b initio MCSCF–CI calculations of the diatomic potential curves of HF+ arising from H++F(2P) and F+(3P)+H(2S) atoms are used in the diatomics-in-molecules (DIM) formalism to obtain potential energy surfaces for six 3A′ and six 3A″ states of FH+2. Energy splittings between the two lowest 3A′ surfaces in the region of an ’’avoided crossing’’ seam in the entrance channel are made to agree with ab initio calculations on the triatomic FH+2. The resulting DIM surfaces are used to find energetically accessible pathways for the reaction F+(3P)+H2(X 1Σ+g) →HF+(X 2Π)+H and related reactions which have been observed experimentally. Empirical approximations to the interaction between F+(1D) and H atoms are used in constructing model diabatic potential surfaces for the 1A′ states of FH+2. Intersections of these surfaces illustrate what products might be expected from the reaction of F+(1D) metastable ions with H2 molecules; our results support recent experimental evidence that the dynamics of F+(1D) ions reacting with H2 are quite different from those of F+(3P) ions.
Relativistic effects in a
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o effective core potentials for molecular calculations. Applications to the uranium atom
Authors: Kahn Luis R. , Hay P. Jeffrey , Cowan Robert D.
Published: February 2003

The procedure of deriving ab initio effective core potentials (ECP) to incorporate the Coulomb and exchange effects as well as orthogonality constraints from the inner core electrons is extended to include the dominant relativistic effects on the valence orbitals. An ab initio approach is then described which enables the valence electrons in heavy atoms to be treated in a standard nonrelativistic manner by including the effect of the relativistic core–valence interactions directly into the ECP. The starting point for this procedure is the Pauli Hartree–Fock relativistic treatment of Cowan and Griffin. The pseudo-orbital transformation and derivation of the l-dependent effective core potentials are analogous to the nonrelativistic case with certain modifications. Analytic forms for the pseudo-orbitals and ECP’s are derived for the U atom, and results of valence electron calculations are presented.
Observation of infrared–optical double resonance in NO2
Authors: Herman I. P. , Javan A. , Field R. W.
Published: February 2003

Several experiments are described in which infrared–optical double resonance is observed for the first time in NO2. A cw argon-ion laser (4545–5145 Å) is used to excite molecules to the 2B2 and 2B1 states and a chopped cw CO2 laser (9.2–10.8 μ) is employed to induce an excited vibronic state transition; detection is through infrared laser-induced amplitude modulation of the visible fluorescence. Double resonance is, in fact, observed at each visible–infrared wavelength pair employed. This effect is studied as a function of laser wavelengths, lateral separation of the two laser beams, and NO2 pressure, and the modulated fluorescence is spectrally analyzed. These results suggest that isoenergetic collisional transfer may contribute to the observations. In addition, evidence is obtained that the optically excited levels participating in the double resonance have very short zero-pressure lifetimes, ∼0.6±0.2 μsec.
Quasiclassical trajectory studies of the chlorine–hydrogen system. II. Cl+D2→DCl+D
Authors: Persky Avigdor
Published: February 2003

Results of three-dimensional quasiclassical trajectory calculations for the reaction Cl+D2(O,J) →DCl+D on a LEPS potential energy surface are reported. Calculations were performed for the initial vibrational state v=0, rotational states J=0 to 5, and collision energies E between threshold and 15.0 kcal/mole. Reaction probabilities, reaction cross sections, product energy partitioning, product angular distributions, and thermal rate constants for different initial conditions are presented. The results of the present study are compared with results for the isotopic reaction Cl+H2→HCl+H, which were reported in an earlier publication. From the calculations for the two isotopic reactions kinetic isotope effects kCl+H2/kCl+D2 are derived and compared with results of transition state theory calculations, using the same potential energy surface, and with experimental data. The agreement between the trajectory results and the experimental kinetic isotope effects is found to be very good.
Solvent, isotope, and magnetic field effects in the geminate recombination of radical ion pairs
Authors: Werner Hans-Joachim , Staerk Hubert , Weller Albert
Published: February 2003

The magnetic field dependence of the geminate recombination triplet yield of radical ion pairs generated via photoinduced electron transfer in polar solvents is investigated for the systems pyrene/N,N-dimethylaniline (Py/DMA), pyrene/3,5-dimethoxy-N,N-dimethylaniline (Py/DMDMA), and the perdeuterated system Py-d10/DMA-d11. The magnetic field dependence characterized through its B1/2 value is found to be dependent on the sum of the hyperfine coupling constants in the radical pair in agreement with previous theoretical predictions. A drastic reduction of the B1/2 value is observed with the perdeuterated system. By means of measurements of the radical ion and triplet absorption signals with nanosecond time resolution, the influence of the solvent on the geminate singlet and triplet recombination yields is investigated. Complementary measurements of exciplex lifetimes and quantum yields are carried out in a series of solvents with different polarities in order to determine the rate constants of fluorescence emission and intersystem crossing in the exciplexes.
Reorientational motion of ammonium ions in (NH4)2SO4 and [(NH4)0.16K0.84]2SO4
Authors: Goyal P. S. , Dasannacharya B. A.
Published: February 2003

We report measurements of quasielastic incoherent neutron scattering from paraelectric (NH4)2SO4 and [(NH4)0.16K0.84]2 SO4 in the temperature range of 227 to 420°K and in the wave vector region of 0.8 to 2.38 Å−1. (NH4)2SO4 has two inequivalent NH+4 ions in a unit cell, labeled I and II, whereas the mixed salt has only one type of NH+4 ion, that is, type II. It is shown that it is possible to get information about the two NH+4 ions separately by combining the data from the two salts. The form factors for the two NH+4 ions and the temperature dependence of the characteristic reorientation time are derived. From the form factors it is shown that for NH+4 (II) ion there is a distribution of characteristic relaxation times for reorientation about the four N–H bonds. On the other hand, for NH+4 (I) ion the reorientation rate about all the four N–H bonds is nearly same. The average characteristic times τI and τII for the two NH+4 ions at room temperature are found to be 12.0±2.0 and 15.8±1.1 psec, respectively. The temperature dependence of τ is very similar for the two salts and can be described by an average description of the type τ=τ0expV/kBT with V=1.27±0.17 kcal/mole and τ0=1.65±0.7 psec. The present data are not accurate enough at all temperatures to give separate temperature dependences for τI and τII. The values of τ0 and V are compared with those derived by NMR.
Molecular negative surface ionization of UF6
Authors: Dittner P. F. , Datz S.
Published: February 2003

Investigations have been carried out to elucidate the process of formation of UF6− molecular ions on heated surfaces. On a carbon coated Pt surface the absolute efficiency of conversion of UF6 to UF6− was found to be 99+1−5% over a wide temperature range in accordance with anticipations based solely on measured thermionic work functions and the molecular electron affinity of UF6. The heat of adsorption of UF6− determined from residence time measurements is 33.6±1.1 kcal/mol on this surface. On clean Pt and other pure metal surfaces the ionization efficiency is orders of magnitude lower than predictions based on the Saha–Langmuir equation. The reasons for this behavior are discussed in terms of alternative dissociation processes on the surface.
An accurate three-dimensional potential energy surface for H3
Authors: Siegbahn P. , Liu B.
Published: February 2003

An accurate three-dimensional potential energy surface for H3 has been obtained by the configuration interaction (CI) method. The calculated energies, for 156 nuclear configurations, with the energy of the saddle point taken to be zero, are believed to lie within 0.1 kcal/mole of the exact clamped-nuclei limit. The CI calculations used an extended one-particle basis set of 4 s-type, 3 p-type, and 1 d-type contracted Gaussian functions, and a nearly complete n-particle basis set. In order to solve the large secular problem, the direct CI method was adapted to the problem of complete CI for three valence electrons. The properties of the accurate H3 potential surface were used to evaluate ab initio and semiempirical methods for potential surface calculations, with emphasis on their applications to other exchange reactions.
Functional representation of Liu and Siegbahn’s accurate a
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o potential energy calculations for H+H2
Authors: Truhlar Donald G. , Horowitz Charles J.
Published: February 2003

Liu and Siegbahn’s recent calculations on the potential energy surface start for H+H2 provide us with the most accurately known potential energy surface for any chemical reaction. We have made an accurate least-squares fit to this surface which satisfies several criteria for use in scattering calculations, including essentially exact agreement with all saddle point properties and being reasonably compact. With eight nonlinear parameters and 15 linear parameters we fit all 267 ab initio points with a root-mean-square error of 0.17 kcal/mol and a maximum absolute deviation of 0.55 kcal/mol. The spherical average of the interaction potential is in good agreement with the recent experimental estimate of Gengenbach, Hahn, and Toennies.
Low temperature heat capacity and magnetic susceptibility of the DPPH–benzene complex
Authors: Duffy William , Strandburg Donald L. , Deck Joseph F.
Published: February 2003

An experimental study of the heat capacity in fields of 0 and 50 kOe from 0.55 to 20 K and the powder magnetic susceptibility from 0.4 K to room temperature of the DPPH–benzene complex is reported. Magnetic and lattice contributions to the heat capacity are well resolved. The susceptibility and magnetic heat capacity are compared with pair, chain, and quadratic net Heisenberg model predictions, none of which give satisfactory agreement with experiment. Calculations based on McConnell’s spin density Hamiltonian indicate the likelihood that dominant intermolecular exchange coupling is of comparable magnitude to six near neighbors, in layers separated from adjacent layers by benzene solvent molecules.
Electronic to vibrational energy transfer, from Br(42P1/2) to H2O
Authors: Hariri A. , Wittig C.
Published: February 2003

A pulsed dye laser has been used to photolytically produce excited bromine atoms in the 4 2P1/2 state in a flowing gas mixture containing Br2, H2O, and Ar. By monitoring the time resolved molecular fluorescence from the stretching vibrations of H2O, it was possible to determine (1) the rate coefficient for the quenching of Br* by H2O, (2) the fraction of the quenching collisions which result in excitation of the (001) and/or (100) state, and (3) various V–V, R, T energy transfer rate coefficients for H2O. Our results indicate that energy transfer from Br* to H2O is a fast and mode specific process. A comparison to other Br*–hydride systems is made.
The potential energy curve for the X1Σg+ state of Mg2 calculated with many-body perturbation theory
Authors: Purvis George D. , Bartlett Rodney J.
Published: February 2003

The ground state potential curve for the van der waals molecule, Mg2, is calculated by adding to the Hartree–Fock potential curve those many-body perturbation theory (MBPT) correlation corrections which arise from double excitation type diagrams through fourth order (DE–MBPT). The fourth-order binding energy is shown to be unaffected by higher order double excitation diagrams. The DE–MBPT potential curve is compared to the fourth-order Rayleigh–Schrödinger perturbation theory (RSPT) approximation of the double excitation configuration interaction (DECI) potential curve. The DE–MBPT curve is found to be in much better agreement with experiment. Since the only difference between the MBPT equation and the RSPT equation is the size-inconsistent E2Δ renormalization term contained in the double CI and its fourth-order RSPT approximation, the importance of having a size-consistent model for molecular binding is demonstrated. The inclusion of additional correlation effects, due to the fourth-order EPV rearrangement diagrams, are found to further improve the computed binding energy. Unlike the terms in the DE–MBPT method, these EPV terms are not invariant to a unitary transformation of degenerate orbitals, and this lack of invariance causes an incorrect dissociation limit. A breakdown of the correlation energy into pair contributions is made and discussed with particular emphasis on the origin of the molecular binding energy.
Generalized differential oscillator strengths for the electron impact ionization of helium determined for large and intermediate momentum transfers at 300 to 500 eV incident energies.
Authors: Dillon Michael A.
Published: February 2003

Differential oscillator strengths for the electron impact ionization (ejected electron energy=1 a.u.) of helium have been determined to ±6% accuracy for incident electron energies of 300 to 500 eV and
Liquid-state diffusion of Na2O–CaO–SiO2 system
Authors: Wakabayashi Hajimu , Oishi Y.
Published: February 2003

The concentration distributions after liquid-state diffusion at 1200°C were measured by using an electron probe microanalyzer for two diffusion couples with an average composition of 16Na2O⋅12CaO⋅72SiO2 (wt%). The diffusion profiles agreed satisfactorily with the theoretical profiles calculated by the five-component diffusion equations derived from the Nernst–Planck equation with assumptions of two oxygen ionic species with differing diffusivities and the oxygen density conservation.
Quantum corrections to classical photodissociation models
Authors: Heller Eric J.
Published: February 2003

An unconventional time dependent formula for total photodissociation cross sections shows the importance of short time dynamics in direct photofragmentation. This is exploited to provide a systematic expansion in powers of h/ for the cross section. The lowest order term is a classical cross section which is shown to be an improvement upon the venerable reflection approximation. Terms to higher order in h/ lead to even greater improvements in accuracy as shown by simple numerical examples. Our formulas are directly applicable to polyatomic photofragmentation, and as a spinoff we derive the polyatomic generalization of the (diatomic) reflection method.
Vibrational anharmonicity in CF4
Authors: Jeannotte A. C. , Marcott Curtis , Overend John
Published: February 2003

A simple anharmonic force field, consisting of a general quadratic force field in curvilinear coordinates, augmented with principal cubic and quartic valence-bond stretching force constants based on a Morse function, has been determined by least-squares adjustment to the vibrational energies of CF4. Since this model did not give a value of the cubic normal-coordinate force constant k344 in agreement with experiment, we considered a second force field identical to the first one except for the introduction of cubic stretch–bend–bend interaction force constants. These force fields have been used to calculate the tetrahedral anharmonic splittings of overtone and combination levels of CF4 and the calculated splittings have been compared with observed values determined by re-examining the previously published infrared spectra of CF4 in liquid–argon solution.
On the Stokes problem for a suspension of spheres at finite concentrations
Authors: Freed Karl F. , Muthukumar M.
Published: February 2003

We consider the problem of the evaluation of the ensemble averaged translational friction coefficient of, and the velocity field produced by, a single nonrotating sphere, moving with uniform velocity through a stationary suspension of similar spheres at finite concentrations, where the average is performed over the spatial distribution of the remaining spheres. The coupled equations for the forces exerted by the spheres on the fluid are iterated to provide a multiple scattering representation of the friction coefficient and velocity field. This expansion can be viewed as an expansion in powers of the concentration, and all terms beyond the trivial leading one are shown to involve divergent integrals because of the long range nature of hydrodynamic disturbances. However, the integrands are shown to form simple geometric series which produce absolutely convergent integrals after the summation. The result displays the phenomenon of hydrodynamic screening (providing a derivation of Darcy’s law) that has been utilized to explain the concentration dependence of the hydrodynamics of polymer solutions. An effective medium approach is presented which incorporates the hydrodynamic screening in lowest order, leading to a theory which is free of divergent integrals.
Direct determination of the imperfect gas contribution to the refractive index of ethylene and a discussion of the quadrupole moment
Authors: St-Arnaud J. M. , Bose T. K.
Published: February 2003

The experimental value of the second refractivity virial coefficient of ethylene obtained directly is higher than the classically calculated theoretical value. The quadrupole moment of ethylene determined by combining the results of the second dielectric virial coefficient and the second refractivity virial coefficient compares very favorably with the one derived from the microwave absorption as well as the value obtained directly by the method of induced birefringence.
Monte Carlo study of a hard-sphere fluid near a hard wall
Authors: Snook Ian K. , Henderson Douglas
Published: February 2003

Monte Carlo value of the density profile and pair distribution function of a hard-sphere fluid near a hard wall are reported for four densities. The density profile is found to be extremely high near the wall, in agreement with earlier simulation and integral equation studies. However, the number of molecules near the wall is no higher than in a comparable volume of the bulk fluid. The high density near the wall is caused by the fact that the molecules near the wall are generally constrained to remain near the wall and undergo a quasi-two-dimensional random walk. A layering is also observed in the pair distribution function.
An analysis of Gaussian basis sets in terms of molecular one-electron properties
Authors: Goddard John D. , Csizmadia Imre G.
Published: February 2003

A large number of nitrogen sp and hydrogen s Gaussian basis sets were obtained from the literature and employed in LCAO–MO–SCF calculations on ammonia at a fixed geometry. The resultant wavefunctions were analyzed in terms of a large number of one-electron properties. Minimal, split valence, double zeta, and extended (sp/s) basis sets were considered. An assessment is made of the size and type of basis set necessary to yield values for various one-electron properties that approach the best calculated or experimental results.
Energy transfer in one-dimensional substitutionally disordered systems. I. The effects of coherence, tunneling, and thermal promotion
Authors: Zwemer D. A. , Harris C. B.
Published: February 2003

A theory for energy transfer in substitutionally disordered solids is presented that includes the effects of coherent and incoherent migration of the wave packet, the statistical distribution of fluctuations in the potential energy surface the wave packet encounters, and tunneling. Explicit expressions for the ’’diffusion’’ coefficient in these limits and the energy-partitioning ratios in binary systems are derived. It is also shown that an activation energy dependent upon the composition of the binary system can emerge from such a treatment and is an indication that energy transfer between localized states is facilitated by concurrent tunneling and thermal promotion.
Potential interactions between boron ions and rare gases
Authors: Ding A. , Karlau J. , Weise J. , Kendrick J. , Kuntz P. J. , Hillier I. H. , Guest M. F.
Published: February 2003

Theoretical and experimental results on the elastic scattering of boron ions, B+, on rare gas atoms X (He, Ne, Ar, Kr, Xe) are described. The elastic scattering observed in these systems depends very strongly on the particular rare gas employed. In particular, the scattering of B+ on He and Ne shows no oscillatory structure, B+ on Ar and Kr shows two rainbow structures, and B+ on Xe shows only one rainbow structure. These results are interpreted in terms of the potential energy curves arising from different states of the BX+ molecules, some of which correlate with the first excited state of B+(3P). The potential energy curves for the ground and first excited state of NeB+ and ArB+ have been calculated by ab initio methods and allow an interpretation of the experimental results to be made.
Symmetries of the static electric field induced optical effects in rarefied fluids
Authors: Chemla D. S. , Bonneville R.
Published: February 2003

The expression for the energy of a molecule in a static electric field is investigated in terms of the irreducible spherical polarizability tensors. The symmetries of its dependence upon the molecular orientation is discussed in detail. The case of the various molecular point groups and the usual approximations are analyzed. The generalized Langevin functions, which describe the thermodynamical averaging over the molecular orientations in a fluid at equilibrium, are calculated exactly and their properties are discussed for the case where molecular correlations can be neglected (i.e., for rarefied fluids). The use of the formalism is illustrated by its application to two well known electro-optical effects; the Kerr effect and the static field induced second harmonic generation.
Statistical mechanics of isomerization dynamics in liquids and the transition state approximation
Authors: Chandler David
Published: February 2003

In this article, time correlation function methods are used to discuss classical isomerization reactions of small nonrigid molecules in liquid solvents. Molecular expressions are derived for a macroscopic phenomenological rate constant. The form of several of these equations depend upon what ensemble is used when performing averages over initial conditions. All of these formulas, however, reduce to one final physical expression whose value is manifestly independent of ensemble. The validity of the physical expression hinges on a separation of time scales and the plateau value problem. The approximations needed to obtain transition state theory are described and the errors involved are estimated. The coupling of the reaction coordinate to the liquid medium provides the dissipation necessary for the existence of a plateau value for the rate constant, but it also leads to failures of Wigner’s fundamental assumption for transition state theory. We predict that for many isomerization reactions, the transmission coefficient will differ significantly from unity and that the difference will be a strong function of the thermodynamic state of the liquid solvent.
Surface tension of a hard sphere fluid near a wall. Expansion in powers of the density
Authors: Navascués G. , Tarazona P.
Published: February 2003

The Percus–Yevick and superposition approximations are used to expand the surface tension of a hard sphere fluid near a wall in powers of the bulk density. Two approaches have been followed. A macroscopic one (using the thermodynamic Gibbs adsorption equation) and a statistical mechanical one (using a recently obtained equation for the surface tension of an interface). The expansions obtained are compared with each other. An inconsistence has been found in the superposition approximation.
Molecular orbital theory of the Compton profile of F2
Authors: Snyder Lawrence C. , Weber Thomas A.
Published: February 2003

Theoretical directional and spherical average Compton profiles have been computed for the fluorine molecule from an SCF–MO wavefunction. The Compton profile of F2 is found to be highly anisotropic due to an atomic contribution and a prominent overlap or diffraction effect from which the bond length can be deduced with about 5% error.
The photodissociation of HNC and DCN
Authors: Stein I. , Gedanken A.
Published: February 2003

Photodissociation study of HCN and DCN is reported using the keypton resonance lives. (AIP)
Singlet–triplet energy separation for silaethylene
Authors: Hood Diane M. , Schaefer Henry F.
Published: February 2003

Using the minimum basis SCF structure of Csizmadia and co-workers4, the singlet triplet equilibrium geometries of silaethelene have been calculated. (AIP)
Atomic iron chemiluminescence from Ar* (3P) collisions with Fe(CO)5
Authors: Hartman Dennis C. , Winn John S.
Published: February 2003

First observation of chemiluminescence resulting from collisional electronic energy transfer to a metal carbonyl are described. The process Ar* (3P)+Fe(CO)5→Ar+Fe*+5CO has been observed. (AIP)
Dissociation of NF3 in shock waves
Authors: Breshears W. D. , Bird P. F.
Published: February 2003

The thermal dissociation rate of NF3 in mixtures of 5% and 10% NF3 in Ar has been measured behind incident shock waves over the temperature range 1330–2000 K. Dissociation rates were determined from postshock density gradients measured by laser beam deflection. The second order rate coefficient determined for NF3–Ar collisions is kd=2.31×1015 exp(−20500/T) cm3 mole sec.
Comments on the interpretation of hypersonic sound velocity data in supercooled water
Authors: Trinh Eugene , Apfel Robert E. , Marston Philip L.
Published: February 2003

Hypersonic sound velocity measurements in supercooled and normal liquid water have provided the basis for the suggestion that the thermodynamic sound velocity might vanish at a thermodynamic singularity at −45 °C. We would like to point out that present available evidence may be interpreted to predict a different temperature dependence.
3-quantum transition probabilities in the ν3 manifold of UF6 and the assignment of 3ν3 in the observed infrared spectrum
Authors: Marcott Curtis , Golden W. G. , Overend John
Published: February 2003

Although there are two F1u components in 3ν3 of UF6 and transitions are allowed from the vibrational ground state to both, only a single band has been observed in the ir spectrum. These two states may be identified in a Cartesian representation of the normal coordinates as 3ν3x and 2ν3x+ν3y; on the basis of calculated intensities, the observed transition has been assigned to 3ν3x. The calculation of the dipole strengths required us to derive the octahedral splitting parameters G33 and T33 and these have been used to calculate the anharmonic splittings in the ν3 vibrational manifolds of 238UF6 and 235UF6.
Low energy ionizing collisions between N2 and CO beam molecules and CO, N2, NO, CH4, and CO2 target molecules
Authors: Utterback Nyle G. , Van Zyl Bert
Published: February 2003

Absolute total negative charge production cross sections for N2+CO, CO+N2, CO+CO, N2+NO, N2+ CH4, and N2+CO2 collisions have been measured to within about 1 eV of their respective thresholds. Mass spectrometric identification of the product ions scattered in the forward direction has been used to investigate specific reaction channels and explain characteristic structures in the total charge production cross sections in the near-threshold regions. Such structures, the most dramatic of which results from the N2+CO →NO++CN− ion pair formation reaction, tend to dominate the cross section shapes at low energies. Dissociative ionization is also shown to be important in such collisions. In fact, ’’simple’’ ionization, where one of the initial molecules simply gives up an electron, is not even an important process for some reactants in this low energy regime. The results of the cross section measurements and the mass spectrometric investigations are presented and a simple model of the collision to aid in understanding the results is discussed. Also presented are charge transfer cross sections for CO++CO, CO++CH4, and N+2+CH4 and secondary electron emission coefficients for CO molecules on a gold surface.
Mobilities and interaction potentials for Cs+–Ar, Cs+–Kr, and Cs+–Xe
Authors: Gatland I. R. , Thackston M. G. , Pope W. M. , Eisele F. L. , Ellis H. W. , McDaniel E. W.
Published: February 2003

The mobilities of Cs+ ions in Ar, Kr, and Xe gases at 300 °K have been measured in a drift tube mass spectrometer for a wide range of values of the energy parameter E/N (the ratio of the electric field strength to the gas number density). Ion neutral interaction potentials have also been derived for each case (Cs+–Ar, Cs+–Kr, and Cs+–Xe). A kinetic theory appropriate to the nonthermalized ion motion is used to derive the mobilities from the potentials, and an iterative technique is used to modify the potentials to fit the experimental data. Various tests of the accuracy and uniqueness of the method indicate that this determination of the potentials gives errors in the potential parameters, e.g., well depth, well position, etc., which are less than about 10%.
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o Hartree–Fock crystal orbital studies. II. Energy bands of an infinite carbon chain
Authors: Kertesz M. , Koller J. , A?man A.
Published: February 2003

An ab initio restricted Hartree–Fock study of an infinite carbon chain is undertaken. In the equidistant model there appears a charge-density wave solution with lower total energy indicating that the alternating structure is energetically more favorable. Indeed, the alternating model is found to be energetically much more stable. The most stable geometry is predicted with alternating bonds with an average of 1.275±0.005 Å and an alternation of 0.26±0.02 Å. The nature of the CDW is remarkable: two out-of-phase waves with the same amplitude appear in the perpendicular π-electron systems.
Dipole moment derivatives and infrared intensities. IV. Prediction of absolute infrared intensities of the fundamental vibrations of CF2 and CF3 radicals
Authors: Newton James H. , Person Willis B.
Published: February 2003

Absolute infrared intensities have been predicted for the fundamental absorption bands of CF2 and CF3 radicals, using an F atomic polar tensor transferred from CH3F. The predicted relative intensities have been compared with the experimental spectra of the matrix-isolated radicals with satisfying results. The effect on the predictions resulting from modification of the transferred polar tensor is examined by using F atom polar tensors from CF2H2 and from CNDO/2 and INDO calculations. The scatter of the predicted values suggests that the predictions of the absolute integrated molar absorption coefficients may be expected to be accurate within a range of about ±30%, and so would be of potential use in semiquantitative analytical applications.
Dissociation energies of CaI, SrI, and BaI from high temperature mass spectrometry
Authors: Kleinschmidt P. D. , Hildenbrand D. L.
Published: February 2003

Equilibrium effusion beams containing the gaseous molecules CaI, SrI, and BaI were generated by the reaction of HI with the corresponding metal oxides at 1500 to 1900 K. Beam composition data obtained by mass spectrometry were used to determine the equilibrium constants of gaseous reactions involving the monoiodides and certain reference molecules. Reaction enthalpies then were evaluated by second and third law methods, from which the dissociation energies D0° (Cal) =62.1±2.5, D0° (SrI) =63.6±1.4, and D0° (BaI) =71.4±1.0 kcal/mole were derived. The results are consistent with data obtained earlier for the diatomic metal fluorides, chlorides, and bromides, and they are compatible with the predictions of the Rittner polarizable-ion model. Various features of the chemical bonding are discussed. A correlation of the thermochemical properties across the entire halide series allows one to estimate reliable dissociation energies for BeBr, BeI, MgBr, and MgI.
The effect of vibrational excitation on the reaction of O(3P) with H2 and the distribution of vibrational energy in the product OH
Authors: Light Glenn C.
Published: February 2003

The overall rate constant and the product branching rate constants have been measured for the first time for the reaction where k2=k2a+k2b. The measurements were made in a flow tube apparatus at T=302±2 °K at a total pressure of 3 Torr. O(3P) was produced by the titration of active nitrogen with NO. H2(v=1) was produced by a heated tungsten filament and its concentration was determined from vuv absorption measurements in the Lyman band system of H2. Absolute concentrations of OH(v=0) and OH(v=1) were deduced from laser induced fluorescence in OH in a calibrated fluorescence system. The rates and uncertainties are k2= (1.0+0.9−0.6) ×10−14 cc/molecule-sec, k2a= (1.0+0.4−0.6) ×10−14 cc/molecule-sec, k2b?4.7×10−15 cc/molecule-sec.

The reported value for k2 represents an increase owing to vibrational excitation in hydrogen by a factor of ?2.6×103 over the fully ground state reaction rate constant. Measurements of the fully ground state reaction rate constant and recent theoretical treatments of the O+H2 reaction are discussed in relation to these results.
Variation of chemical shielding with intermolecular interactions and rovibrational motion. III. 31P nucleus in PH3
Authors: Jameson Cynthia J. , Jameson A. Keith , Parker Harriet
Published: February 2003

The functions σ1(T) and σ0(T) have been determined for the 31P nucleus in PH3. The former is a measure of the intermolecular effects on chemical shielding whereas the latter is a property of the isolated molecule and is due to centrifugal distortion and anharmonic vibration. The PH3 molecule is of special interest since it is a good test case for the contributions to σ0(T) from the thermal average of bond angle deformation. It is also of interest that the observed temperature dependence of σ0 is in a direction opposite to that observed for other isolated molecules.
Ionic radii and diffraction patterns of molten alkali halides
Authors: Abramo M. C. , Caccamo C. , Pizzimenti G. , Parrinello M. , Tosi M. P.
Published: February 2003

The mean spherical approximation for a fluid of charged hard spheres of different radii is applied to the evaluation of diffraction patterns for the whole family of molten alkali halides. The ionic radii entering the model are determined from the liquid-state compressibility. The liquid-state radii are shown to be characteristic temperature-dependent lengths for each ion through the family of salts, and a good one-parameter fit of the whole set of compressibilities at each temperature is obtained through the assumption that the radius ratios are those for basic radii yielded by the Born–Mayer theory of alkali halide crystals. The analysis also provides new evidence against the older sets of crystal ionic radii. Calculations of partial structure factors and of x-ray diffraction patterns are then reported for all the salts and compared with the results of x-ray and neutron diffraction experiments wherever possible. The model emerges as having semiquantitative value in the prediction of x-ray diffraction from simple ionic fluids and in its detailed interpretation.
The hydrated proton H+(H2O)n. VII. A neutron diffraction study of the isolated diaquooxonium ion H3O+⋅2H2O in 2,5-dichlorobenzenesulphonic acid trihydrate
Authors: Roziere Jacques , Williams Jack M.
Published: February 2003

A neutron diffraction study of 2,5-dichlorobenzenesulphonic acid trihydrate has been undertaken in order to investigate the precise hydrogen atom arrangement within the ’’H7O+3’’ unit. The cationic species can be unambiguously formulated as H3O+⋅2H2O, the diaquooxonium ion, although the hydrogen bond distances are short and comparable to those found in diaquohydrogen ions, (H2O⋅H⋅OH2)+. Considerable asymmetry is observed in these hydrogen bonds. The oxonium ion geometry is highly distorted from trigonal symmetry, with dO–H of 1.08(2), 0.98(1), and 0.94(2) Å, and the ion is quite flat. The derived H3O+ ion geometry, and the high thermal motion of the oxygen atom, may indicate some sort of disorder involving oxonium ion inversion. The structure was determined using 1906 independent three dimensional data measured with a neutron wavelength of 1.142(1) Å. The crystals are monoclinic, space group P21/c with unit cell dimension (x-ray) a=7.350(2) Å, b=23.516(3) Å, c=6.862(1) Å, and β=100.68(4) °. The structure was refined by full matrix least-squares techniques on F2o, with anisotropic thermal parameters for all atoms, to R=0.089.
The ground electronic state of B2
Authors: Dupuis M. , Liu B.
Published: February 2003

An accurate ab initio configuration interaction calculation has established that the ground electronic state of B2 is of 3Σ−g symmetry, and that the Douglas–Herzberg emission system near 3200 Å is due to transitions from the second 3Σ−u state to the X 3Σ−g state. The lowest 5Σ−u state, suggested by previous calculations as the ground state, is found to lie about 1300 cm−1 above the X 3Σ−g state. The assignment of the Douglas–Herzberg transition is based on the close agreement between calculated and observed molecular constants which, with observed values in parentheses, are Re(3Σ−g) =1.622(1.590) Å, ωe(3Σ−g) =988.5(1035.2) cm−1, Re(2 3Σ−u) =1.660(1.625) Å, ωe(2 3Σ−u) =884.5(929.3) cm−1, and Te(2 3Σ−u) =31 438(30 546.1) cm−1. Further support for this assignment is seen in the close agreement between calculated and observed isotope shifts and intensity distributions. An explanation is given for why the 1 3Σ−u −X 3Σ−g transition has not been observed.
Vibrational modes of H-bonded chains in liquid and solid HF
Authors: Ring J. W.
Published: February 2003

Analysis of the inelastic scattering of neutrons from HF in terms of the vibrational modes of extended hydrogen-bonded chains shows that a dominant feature of the time-of-flight spectra can be explained both as to its dependence on angle of scattering and its dependence on temperature by scattering from two out of the plane modes of the hydrogen atoms. Contributions of other chain modes are consistent with the neutron scattering data but there are parts of the spectra which can not be explained by these vibrational modes and hence must be due to other causes.
Bivariational calculations of bounds on complex-frequency polarizabilities
Authors: Weinhold F. , Robinson P. D.
Published: February 2003

Recent bivariational principles for upper and lower bounds on generalized complex-frequency polarizabilities are cast into a computationally convenient form which gives automatic optimization with respect to the linear parameters of the two trial functions. The relative accuracy of the ’’explicit’’ and ’’implicit’’ versions of these principles is explored by a trial application to the generalized dipole polarizability of the helium ground state. The numerical results exhibit the significant advantages of the bivariational techniques over previously available methods for these properties.