bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effects of cooperative intermolecular interactions on the electronic delocalization in mixed-valence biferrocenium trihalide compounds
Authors: Kambara Takeshi , Hendrickson David N. , Dong Teng-Yuan , Cohn Michelle J. 
Published: July 2002

Intramolecular electron transfer in the mixed-valence complexes in the solid state shows qualitatively different behavior from that in an isolated complex. The ‘‘extra’’ electron in a given mixed-valence complex in the solid state may be localized at lower temperature and delocalized at higher temperature, even though the extra electron may be delocalized in the isolated molecule at any temperature. The solid state environment has an essential effect on the intramolecular electron-transfer rate in mixed-valence complexes. In order to clarify the mechanism of any possible environmental effects, a model is proposed for binuclear mixed-valence biferrocenium trihalides. A binuclear mixed-valence biferrocenium cation has two localized electronic states, [FeIIAFeIIIB] and [FeIIIAFeIIB], which are coupled to an out-of-phase combination of symmetric ligand–metal stretching modes, one on each metallocene unit. An intramolecular electronic interaction α induces the electron transfer between the two vibronic states of the mixed-valence cation. Each trihalide counterion is also mixed valence, also has two electronic states,
[X−A- - -Y–XB] and [XA–Y- - -X−B], and an intramolecular electronic interaction β that induces charge oscillation in the trihalide anion. If the anion has only one molecular form, the charge oscillation due to the anion may be induced by another mechanism: Each anion may move between the two stable lattice positions as a whole. In addition to these two intramolecular interactions, cation–cation and cation–anion intermolecular interactions are considered in the context of a molecular field approximation. Energies and wave functions of the crystals are obtained. The electronic localization in the mixed-valence cation may be induced by the intermolecular interactions j, when j is large compared to α. The localized state becomes more unstable with increasing temperature, that is, the extra electron transfers more and more rapidly between the two iron ions in the cation, and the mixed-valence cation becomes delocalized at temperatures above a critical temperature TC. This electronic localization–delocalization transition occurs cooperatively and is a phase transition. The charge oscillation (X−A- - - Y–X−B ⇄ XA–Y - - -X−B) in the counteranions and/or the whole anion moving modulates the electron transfer rate in the cation. The present model consistently explains the various observed data from Mössbauer spectra, heat capacity measurements, x-ray crystallography, and infrared spectroscopy.
Adsorbate islanding in surface reactions: A combined Monte Carlo-lattice gas approach
Authors: Silverberg M. , Ben-Shaul A. 
Published: July 2002

The effects of reactant aggregation and lateral interactions on bimolecular surface reactions A+B→AB are studied using a theoretical model combining Monte Carlo simulations with (modified) lattice gas approximations, with special reference to temperature programmed reactions. A and B are chemisorbed species yielding a rapidly desorbing product AB. We are particularly interested in systems where one reactant (A) tends to organize in ordered domains, as a result of strong A–A attractive forces. Our modeling scheme consists of two stages. First, A is randomly adsorbed at a temperature lower than the critical temperature of the gas–solid transition, and phase separation (i.e., A aggregation) starts taking place. This process is modeled by (conserved) Monte Carlo dynamics. As in previous studies, we find that after a while the A atoms form finite, metastable, often highly ramified islands, whose further growth is a very slow process. At this stage, or earlier, B is adsorbed on the A-free lattice sites, T is raised (linearly) and reaction begins. It is assumed that A is stationary while B is highly mobile and that B diffusion is much faster than A+B reaction. Thus, the distribution of B on A-free sites is always in equilibrium, but it depends on A–B and B–B interactions, as well as on coverage, temperature, and, in particular, the given distribution of A atoms. To calculate the B distribution we formulate and employ two lattice gas models, which are appropriately modified versions of the mean-field and the quasichemical approximations. To follow the time-space evolution of the system we use Monte Carlo methods to simulate the slow processes, A aggregation and AB formation, and (one of the) lattice gas models for B distribution. Calculations of thermal desorption spectra are performed for A and B adsorbed on a 500×500 square lattice (with periodic boundary conditions) assuming nearest-neighbor
lateral interactions wAA, wBB, and wAB. The results reveal nontrivial kinetic behaviors, which depend strongly on initial conditions (coverages, dosing sequence) and interaction parameters. These are reflected in the temperature programmed spectra and the apparent (coverage dependent) activation energies derived from them. It is shown, for example, that the major effect of A–B interactions on the rate is via their influence on local concentration of B’s near A’s. B–B interactions affect directly the reaction activation barrier. A–A interactions show both ‘‘topological’’ and ‘‘energetic’’ effects.
State-to-state rotational energy transfer in methane (13CD4) from infrared double-resonance experiments with a tunable diode laser
Authors: Foy B. , Hetzler J. , Millot G. , Steinfeld J. I. 
Published: July 2002

An infrared double resonance laser spectroscopic technique is used to study state-resolved rotational and vibrational energy transfer in the isotopically substituted methane molecule,13CD4 . Molecules are prepared in a selected rovibrational state by CO2 laser pumping, with the quantum numbers v, J, and Cn completely specified. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v′, J′, Cn′ ) are carried out using time-resolved tunable diode laser absorption spectroscopy. The depopulation rates due to collisions between methane and the rare gases are on the order of the Lennard-Jones collision frequencies. Self-relaxation is slightly more efficient than the Lennard-Jones estimate. The rather small relaxation rates are characteristic of a short-range potential, or ‘‘strong-collision’’ regime, expected for a molecule without a dipole moment. The state-to-state energy transfer measurements reveal a dramatic selectivity of rotational energy transfer pathways with respect to the fine-structure rotational states Cn . Relaxation occurs through a surprisingly small subset of the energetically accessible pathways. Also suggested is a preference for population transfer to occur within the initial vibrational angular momentum sublevel of the ν4 (F2 ) vibrational state, which has three sublevels in consequence of Coriolis interaction. This preference can be formulated as a propensity for Δ(R−J)=0 transitions. We deduce that large changes of J(ΔJ∼5) can occur in single collisions between methane molecules, based on a simple kinetic model of the data. This is also characteristic of
short-range collisions in which it is likely that no single multipolar interaction dominates. Collisional relaxation between the ν4 and ν2 bending vibrations proceeds more slowly than rotational relaxation, but as fast as transfer among the closely grouped stretching and bend-overtone levels, measured previously in CH4 . No evidence for rotationally specific V–V transfer is found. We discuss an exhaustive spectroscopic analysis of 13CD4 that provides unambiguous spectral assignments for use in detecting vibrationally excited molecules (v4 =1) in specific rotational states.
Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. II. Temperature dependence of steady state and time-resolved luminescence
Authors: McCaffrey John G. , Ozin Geoffrey A. 
Published: July 2002

The visible emission resulting from photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in the solid methanes is analyzed with spectral and time resolution. Two emission bands are observed in both solid methane (CH4) and solid perdeuteromethane (CD4) at 516 and 560 nm. The measured radiative lifetimes of the two bands are 12 and 19 ms, respectively, and independent of isotopic variation of the host. On the basis of their spectral position and radiative lifetimes, the emission features are assigned to the spin-forbidden 3P–1S intercombination band of atomic magnesium. Emission spectra recorded at various temperatures in the range 12–33 K were found to show reversible temperature dependence whereby the intensity of the higher energy 516 nm band decreased monotonically with a concomitant increase in that of the lower energy 560 nm band. Time-resolved measurements of the two bands in Mg/CH4 and Mg/CD4 show that the radiative lifetime of the 516 nm band decreases with increasing temperature while that of
the 560 nm band remains constant but showing a rise time component. The magnitude of this rise time is correlated with that of the decreasing lifetime of the 516 nm band. With a combination of the temperature dependence observed in the radiative lifetimes and spectral intensities of the 516 and 560 nm emission bands in the Mg/CH4 and Mg/CD4 systems, dynamical information is extracted which allows for the creation of a simple kinetic model and an identification of the mechanism giving rise to this temperature dependent phenomenon. The experimental data is found to exhibit linear behavior on an Arrhenius plot indicating an activated process of population interconversion between the emitting levels. In solid methane the barrier in this activated process is 177 and 125 cm−1 in solid perdeuteromethane. The origin of this process is discussed in terms of (a) structural phase transitions of the solid methanes and (b) the effect of strong Jahn–Teller coupling between phonon modes of the solid methane host and the spin triplet electronic state of the atomic magnesium guest yielding symmetry-distinct minima on this surface. The lengthened values of the measured radiative lifetimes in the solid methanes, indicate population equilibration amongst the total degeneracy of the triplet state J sublevels of atomic magnesium in these solids. Because of the extremely low temperatures at which the experiment is conducted, extensive orbital reduction factors are implied to achieve such equilibration conditions. This is a manifestation of very strong Jahn–Teller coupling, a necessary condition for the stabilization of symmetry-distinct minima on the triplet spin state surface and the mechanism proposed to explain the presence of the temperature dependent emission features.
Rovibrational energy transfer from the (0, K′=0) and (72,K′=0) levels of S1 glyoxal in crossed beam collisions with H2
Authors: Butz Kirk W. , Du Hong , Krajnovich Douglas J. , Parmenter Charles S. 
Published: July 2002

Crossed molecular beams have been used to study rotationally and vibrationally inelastic scattering from the (72, K′=0) and the (00, K′=0) levels of 1Au (S1) trans-glyoxal in collision with H2. The center-of-mass collision energy was approximately 80 meV (650 cm−1). Laser excitation was used to select each of the initial vibrational levels with rotational states limited to J′=0–10, K′=0. Dispersed fluorescence was used to monitor quantitatively the final rotational and vibrational levels populated in single collision scattering. Vibrationally inelastic scattering from each initial level has an absolute cross section large enough to allow the process to be observed easily with resolution of accompanying ΔK rotational state changes. Although many final vibrational levels are energetically accessible and in principle observable, only those reached by ±1 quantum changes in the lowest frequency mode, the OHC–CHO torsion ν7, are seen. From the initial level 72, the cross sections for Δυ7=+1 and Δυ7=−1 are nearly identical, mirroring the symmetry of UP and DOWN vibrational transitions recently observed in I*2 inelastic scattering from H2, D2, and He. The total cross section for pure rotationally inelastic scattering from 00 is about nine times larger than that for rovibrationally inelastic scattering, whereas the two cross sections are nearly equivalent for 72. With resolution of ΔK rotational state changes, it is seen that the cross sections for these processes scale approximately exponentially with the amount of energy ΔE transferred between translational motion and the internal degrees of freedom of the glyoxal molecule. For the initial level (00, K′=0), not only do these rotational and rovibrational cross sections obey the same energy scaling law, but where similar ΔE occurs, they are the same size. In contrast, the ΔK rotationally state-resolved cross sections for transitions from (72, K′=0) fall off much more rapidly in the vibrationally elastic (Δυ7=0) channel than in the vibrationally inelastic
(Δυ7=±1) channels. Consequently, some rovibrational transitions have cross sections comparable to the pure rotational transitions with the same value of ΔK. These relationships contradict the conventional rule of thumb prescribing large rotational and small vibrational cross sections.
Experimental and theoretical total state-selected and state-to-state absolute cross sections. I. The H+2(X,v′)+Ar reaction
Authors: Liao C. -L. , Xu R. , Flesch G. D. , Baer M. , Ng C. Y. 
Published: July 2002

Total state-selected and state-to-state absolute cross sections for the reactions, H+2(X̃,v′=0–4)+Ar→H2(X,v) +Ar+(2P3/2,1/2) [reaction (I)], ArH++H [reaction (II)], and H++H+Ar [reaction (III)], have been measured in the center-of-mass collision energy (Ec.m.) range of 0.48–100 eV. Experimental state-selected cross sections for reactions (I) and (II) measured at Ec.m.=0.48–0.95 eV are in agreement with those reported previously by Tanaka, Kato, and Koyano [J. Chem. Phys. 75, 4941 (1981)]. The experiment shows that prominent features of the cross sections for reactions (I) and (II) are governed by the close resonance of the H+2(X̃,v′=2)+Ar and H2(X,v=0)+Ar+(2P1/2) vibronic states. At Ec.m.≤3 eV, the vibrational state-selected cross section for the charge transfer reaction (I) is peaked at v′=2.
The enhancement of the charge transfer cross section for v′=2 as compared to other v′ states of reactant H+2 increases as Ec.m. is decreased. The state-to-state cross sections for reaction (I),measured at Ec.m.≤3 eV, show that the enhancement for the charge transfer cross section for v′=2 is due to the preferential population of Ar+(2P1/2). At Ec.m.=0.48–0.95 eV and v′=2, nearly 80% of the charge transfer product Ar+ ions are formed in the 2P1/2 state. However, at Ec.m.&gt;5 eV, the intensity for charge transfer product Ar+(2P3/2) is greater than that for Ar+(2P1/2). Contrary to the strong vibrational dependence of the cross section for reaction (I), the cross section for reaction (II) is only weakly dependent on the vibrational state of H+2. At Ec.m.≤3 eV, the cross section for the formation of ArH+ is the lowest for v′=2 compared to other v′ states, an observation attributed to the competition of the nearly resonant Ar+(2P1/2)+H2(X,v=0) charge transfer channel. The cross section for reaction (II) decreases with increasing Ec.m..
At Ec.m.≥20 eV, the cross sections for the formation of ArH+ become negligible compared to those for Ar+. The appearance energies for the collision-induced dissociation H+2(X̃,v′=0–4) are consistent with the thermochemical threshold for reaction (III). The cross sections the formation of H+ are ≤20% of those for H+2. Theoretical state-to-state cross sections for reaction (I) at Ec.m.=19.3 and 47.6 eV calculated using the nonreactive infinite-order sudden approximation are found to be in fair agreement with experimental results.
Theory of liquid-state activated barrier crossing: The instantaneous potential and the parabolic model
Authors: Adelman Steven A. , Muralidhar R. 
Published: July 2002

This paper gives a theoretical treatment of liquid-phase activated barrier crossing that is valid for chemical reactions which occur on typical (e.g., high activation barrier) potential-energy surfaces. This treatment is based on our general approach [S. A. Adelman, Adv. Chem. Phys. 53, 61 (1983)] to problems in liquid-phase chemical dynamics. We focus on the early-time regime [times short compared to the relaxation time of 〈F̃(t)F̃〉0, the fluctuating force autocorrelation function of the reaction coordinate] in which the solvent is nearly ‘‘frozen.’’ This regime has been shown to be important for the determination of the rate constant in the molecular-dynamics simulations of model aqueous SN2 reactions due to Wilson and co-workers. Our treatment is based on a generalized Langevin equation of motion which naturally represents the physics of the early-time regime.
In this regime the main features of the reaction dynamics are governed by the instantaneous potential WIP[y,F̃], which accounts for the cage confinement forces which dominate the liquid-phase effects at early times, rather than by the familiar potential of mean force. The instantaneous potential is derived from the t→0 limit of the equation of motion and its properties are developed for both symmetric and nonsymmetric reactions. The potential is then shown to account for both the early-time barrier recrossing processes found to determine the transmission coefficient κ in the SN2 simulations and the dependence of these processes on environmental fluctuations modeled by F̃. Making the parabolic approximation for the gas-phase part of WIP[y,F̃] yields the following result for the transmission coefficient: κ=ω−1PMFx+=ω−1PMFωMIP[1+ω−2 MIPΘ̂(x+)]1/2≠ ω−1PMFω MIP[1+ (1)/(2) ω−2MIPΘ̂(ωMIP)], where ωMIP and ωPMF are, respectively, the barrier frequencies of WIP[y,F̃=0] and of the potential of mean force, and where Θ̂(x+)=∫∞0 exp(−x+t)Θ(t)dt with Θ(t)≡(kBT)−1〈F̃(t)2F〉0. This result for κ, which is equivalent to a result of Grote and Hynes, but which more naturally represents the physics of the early-time regime, permits a straightforward interpretation of the variation of the transmission coefficients for the model SN2 systems.
Characteristic properties of the nuclear magnetic resonance–paramagnetic relaxation enhancement arising from integer and half-integer electron spins
Authors: Sharp Robert R. 
Published: July 2002

The influence of zero field splitting (zfs) interactions on the magnetic field dispersion profile of the nuclear magnetic resonance–paramagnetic relaxation (NMR–PRE) (i.e., the enhancement of nuclear magnetic relaxation rates that is produced by paramagnetic solute species in solution) has been explored systematically for S=1, 3/2, 2, and 5/2 spin systems using recently developed theory. To facilitate comparison of results for different spin values, the theory was expressed in a reduced form with Larmor frequencies in units of ωD (the uniaxial zfs parameter D in rad s−1), and correlation times and spin relaxation times in units of ωD−1. For S=1, the functional form of the profile can be described in terms of five types of qualitative features. Two of these are characteristic of Zeeman-limit [Solomon, Bloembergen, and Morgan (SBM)] theory and result from the magnetic field dependence of the spin energy level splittings. The remaining three have no analog in Zeeman-limit theory and arise from a change in the quantization axis of the electron spin precessional motion which, in the zfs limit, lies along molecule-fixed coordinate axes, and, in the Zeeman limit, lies along the external field direction.
The reduced field dispersion profiles for the integer spin systems S=1 and S=2 were found to be very similar to each other, the principal difference being that the midfield positions of the requantization features (types 2, 3, and 4) are shifted for S=2 relative to S=1, the magnitude and sign of the shift depending on the position of the nuclear spin in the molecular coordinate frame. For half-integer spins, the dispersion profiles exhibit, in addition to the five features characteristic of integer spins, a sixth type of feature, which is centered somewhat to low field of ωSτc=1, where τc is the dipolar correlation time. The type-6 feature results from field-dependent level splitting of the mS=±1/2 Kramers doublet. It is present when ωDτc≥1. These theoretical predictions have been examined by means of reinterpretations of the NMR–PRE data for tris-(acetylacetonato)–metal complexes of V(III) (S=1), Cr(III) (S=3/2), Mo(III) (S=3/2), Mn(III) (S=2), and Fe(III) (S=5/2). As predicted, type-6 features are absent for the integer spin complexes, for which the T1 field dispersion profiles are nearly field independent.
The experimental profiles were successfully simulated quantitatively by the generalized theory, but not by Zeeman-limit theory. For the half-integer spin systems, the predicted zfs-related type-6 features appear to be present in the profiles, particularly for Mo(acac)3, for which the data deviate significantly from the Zeeman-limit profile in a manner that is explained by the generalized theory.
Transient changes in polarizability for centrosymmetric linear molecules interacting at long range: Theory and numerical results for H2...H2, H2...N2, and N2...N2
Authors: Li X. , Hunt K. L. C. 
Published: July 2002

Transient, collision-induced changes in polarizability Δα on the subpicosecond time scale affect Rayleigh and rototranslational Raman scattering by diatomic molecules in dense gases and liquids, induced birefringence, impulsive stimulated scattering, and dielectric and refractivity virial coefficients. For pairs of D∞h molecules, this work gives the long-range contributions to Δα complete through order R−6 in the intermolecular separation R, including the first- and second-order dipole–induced-dipole (DID) interactions, higher-multipole induction, effects of the nonuniformity in the local field, hyperpolarization, and dispersion. We have used spherical tensor methods to cast Δα into the symmetry-adapted form needed for spectroscopic line shape analysis.
The polarization mechanisms included here give rise to isotropic rototranslational Raman scattering and to simultaneous rotational transitions on two interacting molecules; both are collision-induced phenomena. Transitions with ΔJ up to ±4 are produced by the R−5 and R−6 polarization mechanisms treated in this work (and the isotropic part of the pair potential). For the pairs H2...H2, H2...N2, and N2...N2, we have used ab initio results for permanent multipoles and susceptibilities to evaluate the classical induction contributions to Δα. For the dispersion contributions, we have derived exact results in the form of integrals of the polarizability and γ hyperpolarizability over imaginary frequencies, and we have approximated these numerically in terms of the static α and γ values, together with van der Waals energy coefficients. For the pairs and configurations studied, the first-order DID terms give the dominant contributions to Δα; the agreement between these terms and the full long-range results tends to be better for the anisotropic collision-induced polarizability ΔαM2 than for the isotropic part Δα00, particularly for T-shaped pairs.
The relative contributions to Δα from the other polarization mechanisms reach as high as 29% for second-order DID terms, 27% for E-tensor terms (higher-multipole induction and local field nonuniformity), 21% for dispersion, and 8% for hyperpolarization effects, for R values ∼0.5–1.0 a.u. outside the isotropic van der Waals minimum.