bluuebunny/crossref_metadata_2025 huggingface dataset:
Brownian motion of reacting charged particles in ionization tracks
Authors: Sano Hisatake , Baird James K.
Published: February 2003

We consider an ensemble of fixed positive ions and Brownian electrons interacting by way of a Coulomb potential in an ionization track. Screening of one electron by another is taken into account exactly, so that no mean-field approximation is necessary. The electrons are allowed to react with the positive ions, so as to produce a continuous neutralization of charge in the track. Focusing our attention on the simplest example of two electrons in the field of a divalent cation, we derive a Smoluchowski equation for the pair density. A single electron may neutralize one of the valences of the cation or escape completely from the track. Taking note of this, we construct from the Smoluchowski equation some boundary value problems satisfied by the probabilities for the four possible joint events allowed a pair of electrons. We present a variation method for finding approximate analytic solutions to these boundary value problems. In the case, however, of two noninteracting particles executing Brownian motion outside a perfectly absorbing sink, we find exact solutions for the pair density and the four joint event probabilities.
Interchain contributions to soliton properties in polyacetylene
Authors: Baughman R. H. , Moss G.
Published: February 2003

The energetic degeneracy of the phase-shifted trans polyacetylene chain segments on opposite sides of a soliton center is removed by interchain interactions, which are shown to have important consequences for soliton physics. This energy difference (ESL), the solition-lattice energy, has been calculated from available structural data as a sum of atom-centered, bond-centered, and electrostatic contributions. While ESL is small per C2H2 length of phase-shifted chain (roughly 4 cal/mol for P21/n and 14 cal/mol for P21/a), the resultant total energy cannot generally be ignored since it diverges with increasing separation between soliton and antisoliton or between soliton and chain segment end. Consequences of the interchain interactions are major changes in the equilibrium concentration of soliton pairs, localization of solitons in the proximity of a stationary defect which shifts bond alternation (for chains much longer than the soliton width), and a predicted temperature dependence of phase coherence in bond alternation. Analysis of experimental data provides upper limit estimates for the average chain conjugation length and a lower limit estimate for ESL (greater than 3 cal/mol), which is consistent with the energy calculations based on intermolecular potentials.
On rotation of CH3F isolated in rare gas matrices
Authors: Jones L. H. , Swanson B. I.
Published: February 2003

We report herein some new infrared spectral observations on the CF stretching mode of CH3F isolated in krypton matrices. It is shown that extensive annealing removes the asymmetry which had been tentatively assigned to an envelope of rotational structure. Our results can be explained with a two-site model and without invoking free or hindered rotation, except in the limit of libration. (AIP)
Conditions for the definition of a strictly diabatic electronic basis for molecular systems
Authors: Mead C. Alden , Truhlar Donald G.
Published: February 2003

A strictly diabatic electronic basis is defined as one for which all components of the nuclear momentum coupling vanish. We examine the possibility that such a basis may exist, and we find that, in general, it does not. The only important exception is for diatomic states of the same symmetry. We also consider some conditions for the definition of an approximately diabatic electronic basis. For molecular systems with three or more nuclei, one can obtain useful approximate diabatic basis sets if the transverse (solenoidal) part of the coupling is negligible; this may occur, for example, if the part of the coupling due to the internuclear-distance dependence of the configurational wave functions is negligible as compared to that due to the internuclear-distance dependence of the configurational coefficients. We derive a criterion showing that such approximations may be useful and accurate if the role of the coupling is important over regions of sufficiently small linear dimensions.
A simple CI model for the valence CI bands in CO+2 and isovalent ions. Ionization potentials of CO2 in the 20–30 eV range
Authors: England Walter B.
Published: February 2003

The qualitatively correct Mulliken–Walsh Aufbau rules are used to derive a simple CI model for the energy differences between the valence CI bands and the valence MO bands in CO+2 and isovalent ions. The essence of the model is that the valence CI bands form multiplet terms, and at least one member of the multiplet has the same symmetry as, and interacts weakly with, at least one MO ion level. In this case, a common set of orbitals may be derived and modest CI calculations simultaneously provide accurate term splittings and at least one accurate energy separation between one term and one known lower energy MO ion state. CI calculations which include roughly 2000 or fewer spin and space adapted configurations are used to determine all valence CI band ionization potentials of CO2 in the 20–30 eV range. Agreement with observed peaks is typically very good.
The Thomas–Fermi–Scott–Schwinger expansion and the Schrödinger ground state energy of complex neutral atoms
Authors: Aguilera-Navarro V. C. , de Llano M. , Rojo O. , Verardi S. L. L.
Published: February 2003

The large Z Thomas–Fermi–Scott–Schwinger expansion for the total nonrelativistic binding energy of a neutral atom is used, together with the Bohr energy formula valid at Z=1, to construct various Padé approximants among which the two-point [3//3] approximant gives an overall improved representation for the energy for Z from 1 to 290, and very especially for the closed shell values Z=10, 18, 36, 54, 86, 118, 168, and 290. The presence of oscillatory terms for open shells in the large Z expansion is also confirmed.
Vibrationally adiabatic models for reactive tunneling
Authors: Skodje Rex T. , Truhlar Donald G. , Garrett Bruce C.
Published: February 2003

The approximation of vibrational adiabaticity in curvilinear natural collision coordinates is investigated for tunneling in three-atom collinear reactions. A validity criterion is derived which limits the adiabatic approximation to systems with small reaction-path curvature. A general formalism is developed for systems which satisfy this criterion. A one-dimensional Schrödinger equation is proposed which is sufficiently flexible so as to be adaptable to many different models of tunneling. We present three new methods for including reaction-path curvature effects on multidimensional tunneling in reactive systems: a method based on a quantum mechanical vibrational average (VA) over degrees of freedom transverse to the minimum-energy path; a method (called DA for dynamical-path vibrational- average) that includes internal centrifugal effects in the description of the transverse vibrational motion (in this method the vibrational average is approximated as a quantal vibrational average about the dynamical path along which the Born–Oppenheimer force cancels the internal centrifugal force); and a semiclassical optical potential (SOP) method based on the Feshbach formalism translated into an adiabatic representation with reaction-path curvature providing the coupling mechanism between the explicit and implicit spaces. These models are compared, both formally and numerically, to each other and to four other methods that have been proposed previously, including the small-curvature (SC) approximation that we have proposed in a recent communication. The VA and SOP methods are shown to provide generalizations of phase average (PA) and second-order (SO) methods proposed earlier by Miller and co-workers. The difference is that vibrations are treated quantum mechanically in the VA and SOP methods but classically and harmonically in the PA and SO methods; the quantum mechanical methods have the advantage that anharmonicity can be included more straightforwardly. The DA, SO, and SOP methods, although they include internal centrifugal effects more fully than the VA and PA methods, do not offer significant improvement in accuracy. The numerical results clearly support the physical interpretation of the collapse of the vibrational wave function about a least-action path. The most successful methods are the Marcus–Coltrin path (MCP) and SC approximations. These methods, especially the SC approximation because it is more general, are recommended for future applications.
Statistical mechanics of the ripple phase in lipid bilayers
Authors: Pearce Paul A. , Scott H. L.
Published: February 2003

The essential features of lipid molecule interactions responsible for the ripple phase in lecithin bilayers are identified and incorporated into a general statistical mechanical model. As a first approximation, the model is reduced to the axial next-nearest neighbor Ising (ANNNI) model. This model exhibits an intermediate ripple phase, over a narrow temperature range, between a high-temperature (chain melted) disordered phase and a low-temperature (chain tilted) ordered phase. In this way the ripples are found to arise from an interplay between the attractive van der Waals forces and the action of the headgroup to hinder level packing of the lipid molecules.
Intermolecular potential well depths from the temperature dependence of the absorption spectra of gas-phase complexes in HCl–noble gas mixtures
Authors: Boom E. W. , van der Elsken J.
Published: February 2003

Far infrared spectra of Ar–HC1 and Xe–HCl van der Waals molecules have been determined at various temperatures. A method is developed for analyzing the temperature dependence of the intensities of the absorption lines. Apart from information on the relative positions of the energy levels involved in the observed transitions, the data yield an estimate of D0, the dissociation energy of the complex: 184±24 cm−1 for Ar–HC1 and 339±33 cm−1 for Xe–HCl.
Electron spin-echo detection of nuclear quadrupole resonance in photoexcited triplets
Authors: Bowman Michael K. , Norris James R.
Published: February 2003

Electron spin-echo experiments have been performed on the photoexcited triplet state of phenazine in a fluorene single crystal host. The relatively strong nuclear modulation observed contains the nuclear quadrupole resonance spectrum of the nitrogen nuclei in the phenazine triplet. Detailed analysis gives a coupling of 4.10+/−0.07 MHz for e2qQ/h and 0.22+/−0.05 for η. The principal axes of the nuclear quadrupole interaction coincide with the principle axes of the zero field splitting. Electron spin-echo detection of nuclear quadrupole resonance is not limited to molecules where the quadrupole and the zero field splitting axes coincide and should therefore be a useful probe of the electronic and physical structure of excited triplets of complex organic and biological molecules.
Visible and near-infrared fluorescence spectra of Fe atoms in argon matrices
Authors: McKenzie Martin T. , Epting Michael A. , Nixon Eugene R.
Published: February 2003

Argon laser and dye laser excitations of Fe atoms at dilute concentrations in solid argon matrices gave rise to about 50 fluorescence bands spanning the region from 24 600–11 300 cm−1. Most of the bands have half-widths of 3–10 cm−1. The bands all correspond to transitions terminating in levels of either the a 5D or the a 5F quintet. Spacings of the levels within these two multiplets are the same in the matrix as in the gaseous atom. The fluorescence band peak intensities to the red of the laser line vary as the first power of the laser intensity. The peak intensities of fluorescence bands to the blue of the laser excitation vary as about the 1.2 power of the laser intensity and are significantly dependent upon matrix concentration.
State-to-state reaction dynamics of HF(v = 2) with Ba and Sr
Authors: Torres-Filho Afranio , Pruett J. Gary
Published: February 2003

HF gas molecules in v = 2 are produced in a beam-gas reaction system using an intracavity, multiply resonant HF chemical laser to consecutively pump HF(v = 0) using the P1 (4) and P2 (3) HF lasing lines. A beam of Ba or Sr reacts with the perturbed reagent gas to form the metal monohalide. The internal state distribution of the MF product is probed using pulsed laser induced fluorescence. In both reactions, products are seen with internal energies out to nearly the exothermic limit, but fractional vibrational energy releases are small and apparently decrease with increasing reagent vibration. State-to-state cross sections for v = 2 reagents, relative to those for v = 1 and v = 0 are reported.
Temperature dependence of the rate constant and the branching ratio for the reaction Cl+HO2
Authors: Lee Yuan-Pern , Howard Carleton J.
Published: February 2003

The rate constant and branching ratio for the reaction have been measured in a discharge-flow system with far infrared laser magnetic resonance detection of HO2, OH, and ClO. Chlorine atom concentrations were determined by titration with O3 and detected as ClO. Measurements were made at low pressure (0.9–1.5 Torr) and at temperatures between 250 and 420 K under pseudo-first-order conditions. The total rate constant showed no significant temperature dependence k1 = (4.2±0.7)×10−11 cm3 molecule−1 s−1. k1a = (1.8 ±0.5)×10−11exp[(170±80)/T] cm3 molecule−1 s−1 , and k1b = (4.1±0.8)×10−11exp[(−450±60)/T] cm3 molecule−1 s−1, the latter corresponds to about 20% of k1 at 298 K. The errors are 95% confidence limits and include a factor for possible systematic errors.
Negative ion chemistry and the electron affinity of SF6
Authors: Streit Gerald E.
Published: February 2003

An extensive study of negative ion–molecule chemistry involving SF6 and SF−6 utilizing the room temperature flowing afterglow technique is reported. Based on a series of limit determinations, the recommended value of E.A.(SF6) is 1.0±0.2 eV. The discrepancy between this value and several previous determinations of E.A. (SF6) is discussed in terms of systematic experimental conditions and the formation of an excited state of SF−6. A large number of reaction rate coefficients as well as qualitative observations of reaction mechanisms are presented.
Dynamical instability and external perturbations: Bimolecular collisions in laser fields
Authors: Taylor Ronald D. , Brumer Paul
Published: February 2003

The detailed dynamics of F+H2, and to a lesser extent H+H2, in the presence of laser fields are examined in order to establish the relationship between field-free unstable trajectories and the effect of external perturbations. Weak fields are found to alter the dynamics of individual unstable trajectories and, in particular, to induce transitions amongst the unstable trajectory manifold. As a result, the field has little effect, on the average, on gross collision properties. Intense fields substantially alter the qualitative stable/ unstable structure of phase space and hence the reaction probability. An interesting high field limit is also discussed.
Estimation of 〈P〉 and 〈P−1〉 from atomic electron densities: A comment
Authors: Gadre Shridhar R. , Pathak Rajeev K.
Published: February 2003

Momentum density functions for various atomic systems are disussed. (AIP)
Comments on ‘‘The effect of solvent environment on molecular electronic oscillator strengths’’
Authors: Abe Takehiro
Published: February 2003

Comments are made on the theoretical treatment of solvent effects on molecular electronic oscillator strengths by A. B. Myers and R. R. Birge [J. Chem. Phys. 73, 5314 (1980)]. There are doubts about their theoretical and experimental expressions for the oscillator strength. (AIP)
Pseudo-Gruneisen parameter of molten salts and their binary mixtures
Authors: Pandey J. D. , David Alec D. M.
Published: February 2003

The Flory’s statistical theory has been applied to evaluate the pseudo-Gruneisen parameter for pure molten salts and their isothermal composition dependant binary system. The agreement between the calculated and experimental values was found to be good. The experimental values of the pseudo-Gruneisen parameter (Γ) were evaluated by substituting experimental values of specific heat ratio (Γ) and thermal expansion coefficient (α) in the equation of state relationship Γ = (γ−1/Tα). (AIP)
Transverse modes in polymer gels
Authors: Deutch J. M. , Johnson David Linton
Published: February 2003

Explicit expressions are given for the frequencies of the transverse modes of a polymer gel model developed previously. The limiting cases of weak and strong frictional coupling are separately examined for application to depolarized light scattering. Application to transverse elastomechanical disturbance is also discussed.
Solubility of solids in supercritical gases
Authors: Vezzetti David J.
Published: February 2003

A lattice gas model is used to discuss the solubility of a solid in a supercritical gas. The model isotherms have qualitatively the correct form over essentially the entire range of gas densities and quantitative agreement is good provided the critical temperature is not approched too closely. The effects of the model parameters on the shape of the isotherms are simply interpreted.
Elastodynamics of gels
Authors: Johnson David Linton
Published: February 2003

The mechanical properties of gels are analyzed in terms of models which treat the three-dimensional displacements of the solid and fluid parts separately and on an equal footing, with no assumptions regarding smallness of concentration or even weakness of the solid skeleton frame. Thus, a unified description of the normal modes in a gel is presented; parameters deduced from measurement on a given mode are shown to influence the properties of the others. It is still assumed, however, that the fluid and solid components can be described by their bulk densities, elastic moduli, etc. This picture is discussed in light of experimental results.
Neutron vibrational spectroscopy of hydrogen and deuterium on Raney nickel
Authors: Cavanagh R. R. , Kelley R. D. , Rush J. J.
Published: February 2003

Incoherent neutron inelastic scattering has been applied to the study of the chemisorption of hydrogen and deuterium on Raney nickel (a high surface area nickel powder). The binding sites have been identified by comparing the vibrational frequencies and intensities provided by the inelastic scattering with calculations for model structures of different geometries, force constants, and symmetries. We conclude that, on Raney nickel above 150 K, hydrogen chemisorbs predominantly in sites of threefold symmetry. The ’’best fit’’ corresponds to a Ni–H bond distance of 1.88±0.03 Å and a force constant of 0.58±0.03 mdyn/Å. In addition, we have observed lateral adsorbate interactions, deuterium isotope shifts, and temperature dependent binding sites.
The equilibrium structures of the NN–HF and OC–HF complexes
Authors: Benzel Mark A. , Dykstra Clifford E.
Published: February 2003

Large basis set SCF and extensively correlated wave functions have been used to study the potential energy surfaces of the NN–HF and OC–HF complexes. These calculations establish the linearity of the equilibrium structures of the complexes and provide accurate estimates of the well depths. Comparison of results at different levels reveals serious deficiencies in describing these systems with either small basis sets or with the neglect of electron correlation effects.
Phase III of CD4: Reconciliation of inelastic neutron scattering and heat capacity measurements of rotational tunneling levels
Authors: White M. A.
Published: February 2003

Concurrent analyses of the rotational tunneling levels of CD4 in phase III from the inelastic neutron scattering spectra and the Schottky heat capacity anomaly allow the resolution of apparent discrepancies between the two methods, and yield the conclusion that the complete nuclear spin species conversion does not occur in CD4. Based on T-T transitions only, calculation of Cp in the region of the Schottky anomaly discriminates between several proposed assignments of the rotational tunneling levels. (AIP)
Comment on ‘‘Approximate formulas for Franck–Condon factors’’
Authors: Waldenstro/m S. , Naqvi K. Razi
Published: February 2003

If is contended that Nicholls4 formulas fro Frank-Condon factors can be deduced from Wagner’s1 derivation of Franck-Condon factors of displaced oscillations. (AIP)
Approximate formulas for Franck–Condon factors: A reply
Authors: Nicholls R. W.
Published: February 2003

The derivation of formulae for Franck-Condon factors is discussed. The calculations of Wanger1 and Ansbacher2 are discussed in detail. (AIP)
Reactive paths in the diffusion limit
Authors: van der Zwan G. , Hynes James T.
Published: February 2003

The character of the reactive motion in multidimensional passage over a chemical barrier in the diffusion limit is examined. Frictional coupling between the reactive and nonreactive modes that are defined by the potential forces leads to an enhancement of the rate constant. This effect arises from a bias of the reactive motion towards the direction of least friction, and is modulated by the potential forces. A detailed explanation of this effect is presented and an application to a simple model of isomerization in polymers is given.
Inelastic high-energy electron scattering from a hydrogen molecule. I. X–B and X–E transitions
Authors: Kol/os Wl/odzimierz , Monkhorst Hendrik J. , Szalewicz Krzysztof
Published: February 2003

Generalized oscillator strengths have been computed for high-energy electron scattering by H2. The optically allowed transitions from the electronic ground state (X 1Σ+g) to the B 1Σ+u state and the optically forbidden transitions to the E 1Σ+g state have been considered. Accurate wave functions expanded in explicitly correlated Gaussian functions were used to describe the electronic states of H2. The dependencies of the form factors on the molecular orientation, the momentum transfer, and the internuclear separation are discussed. For the X–B transitions the form factors have been integrated over the internuclear separation with accurate vibrational wave functions. A comparison of the obtained values with those computed assuming the Franck–Condon approximation has shown that the latter are not always reliable. In the region of a very small momentum transfer, the present results are found to be very close to the accurate values obtained for optical transition probabilities to the B state with wave functions depending explicitly on odd and even powers of the interelectronic distance.
Molecular orbital studies of dimethyl diselenide
Authors: Renugopalakrishnan V. , Wyssbrod Herman R. , Walter Roderich , Druyan M. E.
Published: February 2003

Results from theoretical studies of dimethyl diselenide (H3C–Se–Se–CH3) by semiempirical and ab initio molecular orbital (MO) methods are reported. The CNDO/2 method was employed for the semiempirical studies, and the restricted Hartree–Fock (RHF) method, in which an augmented STO-3G basis set was used, for the ab initio studies. Total energy E was calculated as a function of a chosen internal coordinate (bond length, bond angle or torsion angle) by the CNDO/2 method while all other internal coordinates were held at fixed values. In addition, a geometric optimization in which selected internal coordinates were permitted to vary simultaneously was performed. The heights of the cis and trans torsional (rotational) barriers were calculated by the CNDO/2 method and found to be greatly exaggerated. More realistic values were calculated by the RHF method, in which an augmented STO-3G basis set was used. The heights calculated by both MO methods were compared to the corresponding ones calculated by similar methods for the sulfur isolog dimethyl disulfide (H3C–S–S–CH3).
A semiempirical method for the calculation of potential energy surfaces of Hn systems
Authors: Tanaka Noboru , Nomura Okio
Published: February 2003

The multiple exchange integrals play a central role in the nonionic valence bond formulation of any number of hydrogen atoms Hn. These integrals are proved to be reduced to two basic types which are expressed in terms of diatomic parameters with the use of the Mulliken approximtion. Thus, the potential energy surface of Hn can be calculated easily.
Mössbauer spectra and related electronic properties of some cubic spinel systems of the type MxFe3−xO4
Authors: Bhaduri Monmatha
Published: February 2003

Studies of the room temperature Mössbauer spectra of MxFe3−xO4 (where M = Cu, Cd, or Ti; and O⩽x⩽1) together with the measurements of unit cell dimension and electrical conductivity indicate that a significant change in the electronic properties of each system occurs around the intermediate composition (x∼0.5). The samples with low values of x(<0.5) retain the characteristics of Fe3O4, while those with high values of x(>0.6) behave as semiconductors as the sixth 3d electron of Fe2+ ion becomes localized.
Comparison of transition from low to high density transport behavior for ions and neutral molecules in simple fluids
Authors: Gee Norman , Huang Sam S.-S. , Wada Toshinori , Freeman Gordon R.
Published: February 2003

For cation mobility μ in several simple fluids, the transition from normal gas to liquid type behavior occurs in the density region 0.5≲n/nc≲1.9, which corresponds to the viscosity region 0.6≲η/ηc≲2.5. The subscript c refers to the critical fluids and the other fluids are the coexistence vapor and liquid. The fluids range from monatomic to polyatomic molecules: Ar, Xe, N2, CH4. The semihydrodynamic equation provides a reasonable interpretation of the variation of ημ, or the equivalent ηDe/kT for neutral molecules, with η only in the normal gas regime, where n/nc<0.4 and η/ηc<0.5. Under these circumstances, nμ or nDe/kT is nearly constant. In calculating the polarization interaction contribution to the ion scattering cross section, the largest axis of polarizability of the molecule should be used. As n (or η) increases in the region 0.6<n/nc<1.6 (or 0.7<η/ηc<1.9) the value of nμ (or ημ) increases by ∼60%, whereas nDe/kT (or ηDe/kT) for neutral molecules decreases by ∼20%. The ratio (De/kTμ)n has values 5–6 in the dilute gases; the polarization interaction and clustering make the ion scattering cross sections greater than those of neutral molecules. At higher densities the ratio (De/kTμ)n decreases, due to the destructive interference of polarization interactions of an ion with more than one molecule at a time, and to a decrease of the special importance of the molecules clustered about the ion, compared to about a neutral molecule, in the dense fluid. In the dense liquid (De/kTμ)n = 2.5 in methane and xenon, and 1.7 in argon.
Vibrationally induced nuclear spin relaxation of quadrupolar nuclei
Authors: Brown R. J. C. , Colpa J. P.
Published: February 2003

Nuclear spin relaxation times are calculated for quadrupolar nuclei situated at the center of octahedral and tetrahedral molecules. Vibrational distortion of such a molecule generates an electric field gradient at the center, which does not average to zero when the molecule is in an excited state of a degenerate vibration. The electric field gradient causes quadrupolar relaxation because of randomness in the vibrational state of the molecule. The theory relates the nuclear spin relaxation time to constants derived from the high resolution infrared spectrum of the molecule in the gas phase.
The structure of H2S⋅HF and the stereochemistry of the hydrogen bond
Authors: Viswanathan R. , Dyke T. R.
Published: February 2003

The radiofrequency and microwave spectra of H2S⋅HF and deuterium substituted analogs have been studied by molecular beam electric resonance spectroscopy. Rotational constant data, electric dipole moments, and nuclear hyperfine interactions were obtained from the spectra. The structure determined from this data has H2S as the proton acceptor. The hydrogen bond is linear to within ±10°. The S⋅⋅⋅F distance is 3.249 Å. The plane of the H2S is oriented perpendicular to the hydrogen bond axis within conservative error limits of ±10°. The right angle structure of H2S⋅HF compared to the tetrahedral orientation of the proton acceptors in (H2O)2, (HF)2, and HF⋅HCl shows a striking analogy to the geometries of first and second row hydrides. This result is discussed in terms of traditional chemical concepts.
Intersystem crossing and predissociation of haloaromatics
Authors: Huppert D. , Rand S. D. , Reynolds A. H. , Rentzepis P. M.
Published: February 2003

The mechanism for the dissociation of several haloaromatic molecules has been investigated by means of absorption and emission picosecond spectroscopy. It is found that the rates of intersystem crossing and dissociation increase from chloro to bromo to iodo substituents. It is observed that the position of the halo substituent and temperature influence these rates. A model for the dissociation mechanism is proposed which is consistent with the absorption and emission data presented.
Raman study of the phase transitions and ionic motions in dioxygenyl hexafluoroarsenate O+2 AsF−6
Authors: Griffiths J. E. , Sunder W. A.
Published: February 2003

Evidence for three stable phases I, II, III of dioxgenyl hexafluoroarsenate has been found by a temperature dependent Raman study (0–2000 cm−1) of the polycrystalline material. The I/II transition is of the disorder to order type and occurs at −3 °C. The II/III transition occurs at −82 °C. Both anion and cation motions have rotational character in phase I which becomes damped in phase II. In phase III, there is little evidence for large amplitude ionic motions; linewidth changes with temperature in this phase correlate with expected temperature dependent variations in lattice dimensions. The structures and spectra in the phases of O2AsF6 are similar to those in NOAsF6 and different from those in KAsF6.
External heavy atom effect, exciton–phonon coupling, and triplet energy transfer in a novel crystalline complex between naphthalene and 1,4-diiodotetrafluorobenzene
Authors: Law Kam S. , Prasad Paras N.
Published: February 2003

The crystalline complex naphthalene: 2(1,4-diiodotetrafluorobenzene), the formation of which is being reported here for the first time, was investigated by a combination of low temperature Raman spectroscopy and electronic spectroscopy. The Raman investigation reveals that this crystalline complex is ordered but stabilized, primarily, by the van der Waals interaction. The phonon bands of the two components in the lattice are found to be largely decoupled. The liquid helium temperature electronic spectroscopy suggests the absence of any low-lying charge-transfer band. The lowest triplet state in the complex is that of naphthalene which shows a strong manifestation of the external heavy-atom effect due to the iodine atoms. Because of this heavy-atom effect, the S0 → T1 absorption of naphthalene, in this complex, can be seen with a 50% absorption in a crystal of 0.5 mm thickness. This triplet-exciton transition is found to have a very narrow exciton bandwidth, and it shows a strong phonon sideband in contrast to what is observed for the triplet transition of naphthalene in neat and isotopically mixed crystals. Yet, the fluorescence of naphthalene in the complex does not show such an enhancement of the phonon-sideband intensity. A theoretical description is presented, whereby both the strong heavy-atom effect and the selective increase of the exciton-phonon coupling for the triplet transition of naphthalene in the complex are explained by mixing of this transition with the triplet transition of 1,4-diiodotetrafluorobenzene.
Reaction rates of hydroxyl radical with nitric acid and with hydrogen peroxide
Authors: Marinelli William J. , Johnston Harold S.
Published: February 2003

The rates of the reactions HO+HNO3 → H2O+NO3, (1), HO+H2O2 → H2O+HOO (2) have been studied by laser flash photolysis of reactants and resonance fluorescence of hydroxyl radicals. The recently reported high rate constants at room temperature for both reactions and the negative activation energy for Reaction (1) at low temperature have been confirmed. Results obtained here are: k1 = 1.52×10−14 exp(644/T) cm3 molecule−1 s−1 from 218–363 K and k2 = 1.81×10−12 cm3 molecule−1 s−1 at 298 K. These two reactions have been examined by transition-state theory; (1) is assigned a cyclic and (2) a chainlike transition state. Even with no potential energy barrier, the reaction coordinate of (1) involves a quantum-mechanical, temperature independent frequency; and with this model the low pre-exponential factor and negative activation energy of Reaction (1) can be explained.
Isotope exchange and collisional association in the reactions of CH3+ and its deuterated analogs with H2, HD, and D2
Authors: Smith David , Adams Nigel G. , Alge Erich
Published: February 2003

The rate coefficients and product ion distributions have been determined at 80, 205, and 295 K for the reactions of CH3+, CH2D+, CHD2+, and CD3+ with H2, HD, and D2 using a SIFT apparatus. Two types of reactions were observed, those involving isotope (H/D) exchange and those in which ion–molecule collisionally stabilized association occurs. Both exoergic and endoergic isotope exchange occurred and often more than one product was observed. Example reactions are (1) CH3++HD⇄CH2D++H2, (2) CD3++H2→CHD2++HD, CH2D++D2,CH2D3+. The exchange reactions are exoergic when H atoms in the reactant ion are exchanged for D atoms from the reactant neutral. As expected, the reverse (endoergic) rate coefficients kr decrease with decreasing temperature, whereas the forward (exoergic) rate coefficients kf increase with decreasing temperature. From the kf and kr for some of these reactions van’t Hoff plots have been constructed and thus ΔH° and ΔS° have been obtained. Using the experimentally determined values for ΔH°, the difference in the strengths of the C–H and C–D bonds in CH3+ like ions has been determined to be 1.56 kcal mol−1. Only collisional association can be observed in the CH3++H2 and CD3+ +D2 reactions. In several reactions, however, association is observed to compete with endoergic isotope exchange and at 80 K it is the dominant channel in reactions such as (2). At 80 K the ternary association rate coefficient k3 is smallest for the CH3++H2 reaction and largest for the CD3++H2 reaction (rather than the CD3++D2 reaction). This is tentatively explained not only in terms of the increased densities of states in the deuterated intermediate complexes which increase their lifetimes against unimolecular decomposition (and hence increase k3), but also to ’’isotopic refrigeration’’ which results from the rearrangement of H and D atoms in the CH5+ like complexes.
Homogeneous nucleation of ethanol
Authors: Schmitt J. L. , Adams G. W. , Zalabsky R. A.
Published: February 2003

The authors have used an expansion cloud chamber to measure homogeneous nucleation rate as a function of temperature and supersaturation in ethanol. Nucleation rates from 102 to 105 drops/cm3 sec were measured in the temperature range 252–272 °K. An empirical functional fit to the data is used to extrapolate for comparison with other data in the literature at a nucleation rate of 1 drop/cm3 sec and to compare with the predictions of the classical Becker–Doring homogeneous nucleation theory. A full listing of the data and thermodynamics constants used in the reduction of the data is given in an appendix.
Monte Carlo evaluation of path integrals: Quantal intramolecular degrees of freedom in solution
Authors: Stratt Richard M.
Published: February 2003

Standard many-body classical Monte Carlo methods are extended so as to allow the examination of quantal internal degrees of freedom of molecules in solution by using discretized path integral representations of the solute. It is shown how, in a variety of cases, the classical structure of the path integral helps in choosing an appropriate reference system and how recently developed methods for evaluating path integrals make practical calculations possible. The concepts are illustrated on a model of a two-state system in solution.
Diffusion thermoeffect measurements of heats of transport in ternary liquid toluene–chlorobenzene–bromobenzene mixtures at 25 and 35 °C
Authors: Platt G. , Vongvanich T. , Fowler G. , Rowley R. L.
Published: February 2003

Heats of transport in ternary nonelectrolyte liquid mixtures have been directly measured for the first time. Diffusion thermoeffect measurements have been made for ternary mixtures of toluene–chlorobenzene– bromobenzene at 25 ° and 35 °C. A new boundary sharpening technique was used in addition to a liquid gate withdrawal method. Both techniques yield consistent, accurate values of the heat of transport, but the new cell provides versatility in allowable densities and compositions of the initial phases. Heats of transport were obtained by nonlinear least-squares fitting of calculated to experimental temperature differences measured at positions symmetric about the initial diffusional interface. The two independent heats of transport, obtained by performing two runs with different starting composition gradients for each mean composition, were each fit to a polynomial in temperature and composition. The resultant analytical expression is valid only in the region bounded by 0.1⩽wi⩽0.6(i=1,2) and w3⩾0.1, where the composition is far enough from the binary limits that both of the two heats of transport remain defined and independently determinable. The results obtained for the heats of transport show a direct dependence on temperature and a strong dependence on the relative amount of the heavy component, bromobenzene. These facts are consistent with a proposed interpretive model of the diffusion thermoeffect.
Slow vibrational relaxation in picosecond iodine recombination in liquids
Authors: Nesbitt David J. , Hynes James T.
Published: February 2003

A characteristic time scale of order 100 ps has been observed by several workers in picosecond spectroscopy experiments on iodine recombination in liquid CCl4 and other solvents. This time scale has been thought to be associated with the diffusive relative motion, prior to actual recombination, of photodissociated I atoms. In this paper, we point out that extensive vibrational deactivation of nascent iodine molecules formed in the recombination is required for spectroscopic detection of I2. We propose that slow vibrational relaxation of nascent I2 may play an important role in such recombination studies. We examine the relaxation of highly vibrationally excited I2 in liquid CCl4. Both vibrational-translational (VT) and vibrational–vibrational (VV) energy transfer mechanisms are considered. We estimate that the characteristic time scale for the vibrational relaxation of I2 in CCl4 is of order 100 ps. VV energy transfer is found to play a critical role in the relaxation. Possible experimental avenues for testing our conclusions are suggested.
The effect of heavy metallic ions on fatty acid monolayers at the air–water interface
Authors: Miyano K. , Abraham B. M. , Xu S. Q. , Ketterson J. B.
Published: February 2003

We have studied the effect of Ba2+ and Pb2+ ions on the pressure-area behavior of fatty acid (Cn−1H2n−1COOH) monolayers at the air–water interface for n = 15–21.
Evidence for two different transition states in the reaction of NO+O3→NO2+O2
Authors: Ende D. van den , Stolte S. , Cross J. B. , Kwei G. H. , Valentini J. J.
Published: February 2003

Chemiluminescent cross sections have been obtianed for NO2*. (AIP)
NMR study of the behavior of hydrogen in vanadium hydride (2). Superstructures and diffusion of hydrogen at high hydrogen concentration in β-VHx
Authors: Hayashi Shigenobu , Hayamizu Kikuko , Yamamoto Osamu
Published: February 2003

The behavior of hydrogen in vanadium hydride at high hydrogen concentration in the bct-lattice phase was studied by 1H NMR spectroscopy. Three kinds of hydrogen were observed in VH0.77, where some kinds of superstructures of hydrogen are formed. Their structures are proposed by analogy with the β1-V2H superstructure, and the occupation of the tetrahedral site is suggested. (AIP)
Propensity rules for rotationally inelastic collisions of symmetric top molecules or linear polyatomic molecules with structureless atoms
Authors: Alexander Millard H.
Published: February 2003

Using an asymptotic expansion of the angular momentum coupling coefficients which appear in the infinite-order-sudden formulation of collisions between symmetric top molecules and structureless atoms [S. Green, J. Chem. Phys. 70, 816 (1979)], we demonstrate that in certain cases, transitions which conserve the ’’parity index’’ of the rotational wave function will be strongly favored over those in which the parity index changes. This propensity rule will apply both to transitions which are inelastic in J but elastic in K, the projection of J along the molecular axis, as well as to transitions which are elastic in J but inelastic in K. Identical propensity rules will emerge within a Born treatment of the collision dynamics. These rules are completely general and independent of the particular form of the atom–molecule potential. The propensity toward conservation of the parity index will also be apparent in collisions of slightly asymmetric tops and l-doubled linear polyatomic molecules. A simple criteria is developed to determine the minimum value of J for which these propensity rules will be valid. Examination of accurate close-coupling and coupled-states cross sections for the NH3–He and H2CO–He systems confirms the accuracy of the new propensity rules. These rules can be used to predict intensity changes in four-level double resonance experiments; excellent agreement is obtained in a comparison with available experimental data for collisions of both HCN and NH3 with He, Ar, Xe, and H2.
Electron diffraction studies of hot molecules. I. Observed and calculated thermal expansions of SF6, CF4, and SiF4
Authors: Goates Steven R. , Bartell Lawrence S.
Published: February 2003

A new method is described for the rapid heating (10−5–10−6 s) of gas molecules for study by electron diffraction. Laser irradiation of the tip of a micronozzle makes it possible to determine structures, amplitudes of vibrations, and aspects of anharmonicity of molecules at temperatures much higher than those at which decomposition occurs in conventional oven nozzles. The vibrations and thermal expansions of SF6, CF4, and SiF4 have been investigated up to 1700, 1600, and 1200 K, respectively. Clear evidence for effects of anharmonicity was observed in amplitudes of vibration as well as mean bond lengths. Various models proposed for the treatment of increases in bond length have been assessed, among which an anharmonic Urey–Bradley field accounted well for results. Comparisons are made with the predictions of Heenan and Robiette based on spectroscopic analyses. The diffraction approach offers a promising method for augmenting spectroscopy in the investigation of intramolecular forces.
Invariant imbedding solution of driven (inhomogeneous) and homogeneous Schrödinger equations
Authors: Singer Sherwin , Freed Karl F. , Band Yehuda B.
Published: February 2003

The method of invariant imbedding is generalized and extended to treat the solution of the inhomogeneous (driven) multichannel Schrödinger equation arising from two potential problems with one of the potentials being weak (as in photodissociation, photoionization, etc.). The method directly propagates the physical transition amplitudes of interest and is consequently extremely stable for situations where a number of other integration methods for the driven equations are found to be numerically unstable. We also present a simplified derivation of the invariant imbedding equations for the homogeneous Schrödinger scattering equations. A transformation is introduced enabling the propagation of symmetric matrices only. A simple analytically solvable two-channel driven equations model is introduced to exhibit the origins of numerical instabilities in a number of other integration methods for the driven equations in situations when an asymptotically open channel is badly closed in the Franck-Condon region.
Electron correlation effects on the structure of all 3d
n4s
m valence states of Ti, V, and Cr and their ions as studied by quasidegenerate many-body perturbation theory
Authors: Lee Yoon S. , Freed Karl F.
Published: February 2003

The role of electron correlation on the electronic structure of the first row transition metal atoms Ti, V, and Cr and their ions is studied using the ab initio quasidegenerate many-body perturbation theory based upon an effective valence shell Hamiltonian formalism. Second order ℋv calculations yield good agreement with experiment for the excitation energies of all the 3dn4sm valence states of Ti, V, and their ions. The agreement is not as good for Cr and its ions. The contributions of various classes of excited configurations to the excitation energies of all valence states are studied in detail. The role of 3s and 3p core excitations and of excitations into the 4f virtual orbital are substantial in the second order ℋv treatment. This is argued to be a general feature of transition metal electronic structure, and these effects are expected to be of similar importance in calculations employing other methods. Relativistic effects are estimated by comparison between relativistic and nonrelativistic self-consistent field calculations and are found to be 0.1–0.4 eV for the neutral atoms, perhaps contributing substantially to discrepancies between theory and experiment.
How well does the Hartree–Fock model predict equilibrium geometries of transition metal complexes? Large-scale LCAO–SCF studies on ferrocene and decamethylferrocene
Authors: Lüthi H. P. , Ammeter J. H. , Almlöf J. , Faegri K.
Published: February 2003

Large scale ab initio LCAO–SCF calculations performed on ferrocene show that the Hartree–Fock model is unable to account for the experimentally observed metal to ring distance. The present results, using basis sets of better than triple zeta quality, show that both the equilibrium geometry and the orbital energies have converged already at the double zeta level (metal-ring distance;1.89 Å, i.e., 15% larger than the experimental value of 1.65 Å). A comparative calculation on decamethylferrocene yields essentially the same results. These findings raise some doubts as to the adequacy of the Hartree–Fock model for predictions of equilibrium geometries of transition metal complexes in general.
Strong nonadiabatic effects and conical intersections in molecular spectroscopy and unimolecular decay: C2H4+
Authors: Köppel H. , Cederbaum L. S. , Domcke W.
Published: February 2003

The importance of nonadiabatic effects in small polyatomic molecules is discussed. It is pointed out that the interaction between different molecular electronic states can in general not be described in terms of a single vibrational mode. Rather, totally symmetric modes which modulate the electronic energy separation must also be taken into account. The inclusion of these modes leads to a multidimensional intersection of the adiabatic potential energy surfaces and to a dramatic enhancement of the nonadiabatic effects. In the presence of several totally symmetric modes, those modes that have a minor influence on the vibronic coupling problem by themselves can still strongly enhance the nonadiabatic effects. The importance of the multimode effects is demonstrated for the second band in the photoelectron (PE) spectrum of ethylene. This band is well separated energetically (2 eV and more) from all other bands in the spectrum. It is found that in this band none of the ∼1000 calculated lines can be understood within the adiabatic approximation. The line structure is highly erratic and cannot be explained by any decoupling of the modes nor by a ’’broadening’’ of the adiabatic vibrational levels. It is concluded that strong nonadiabatic effects constitute a more common phenomenon than is usually believed.