bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of chemical dynamics by restricting intramolecular vibrational relaxation
Authors: Goswami Debabrata , Warren Warren S.
Published: July 2002

We address the issue of localization of bond energy in a molecule by stopping intramolecular vibrational relaxation (IVR). We show through model calculations that appropriate frequency sweeps permit selective locking over a well-defined range of resonance frequencies, with little excitation outside that range. We also propose a modified version of an adiabatic half passage experiment that will perform photon locking without complications from inhomogeneities or partial excitation of other transitions for a bright state coupled to a finite number of dark states.
Hyperfine interactions in the ground states of titanium monoxide and mononitride
Authors: Fletcher D. A. , Scurlock C. T. , Jung K. Y. , Steimle T. C.
Published: July 2002

A comparative study of the hyperfine interactions in the X 2Σ+ state of TiN and the X 3Δ state of TiO has been performed. The 48Ti14N(I=1) hyperfine structure was determined from the analysis of 19 components of the N=1–0 and N=2–1 pure rotational transitions recorded using the pump/probe microwave-optical double resonance technique. The 47Ti(I=5/2) hyperfine structure of X 2Σ+ TiN was determined from an analysis of the high resolution optical spectrum of the (0,0) A 2Π3/2–X 2Σ+ band system. The resulting parameters are (in MHz) B(48Ti14N)=18 589.3513(13), D(48Ti14N)=0.026 31(18), γ(48Ti14N)=−52.2070(13), bF(N)=18.480(3), c(N)=0.166(7), eQq0(N)=−1.514(8), CI(N)=0.0137(12), bF(47Ti) =−558.8(11), c(47Ti)=−15(5), and eQq0(47Ti)=62(16). An analysis of the (0,0) band of the B 3Π–X 3Δ system of 47Ti16O produced the X 3Δ hyperfine parameters (in MHz): a(47Ti) =−54.7(21), (bF+2c/3)(47Ti)=−231.6(60), and eQq0(47Ti)=−49(31). An interpretation based upon the predicted nature of the bonding in TiO and TiN is given.
Hartree–Fock limit magnetizabilities from London orbitals
Authors: Ruud Kenneth , Helgaker Trygve , Bak Keld L. , Jo/rgensen Poul , Jensen Hans Jo/rgen Aa.
Published: July 2002

Molecular magnetizabilities have been calculated at the Hartree–Fock level for a series of diamagnetic molecules: H2O, NH3, CH4, PH3, H2S, CO2, CSO, CS2, and C3H4. Gauge invariance is imposed by the use of London atomic orbitals. The results are compared to those obtained with the IGLO (individual gauge for localized orbitals) method and are found to converge faster to the basis set limit. Magnetizabilities obtained from basis sets of different quality never differ by more than 4% for the London method, compared to 20% for IGLO. The Hartree–Fock limit may be approached using London basis sets of moderate size, in contrast to calculations of molecular polarizabilities which require large basis sets to be reliable. Comparison with experiment shows that the Hartree–Fock method overestimates experimental susceptibilities by 5%–10%.
First-principles-derived dynamics of F2 reactive scattering on Si(100)-2×1
Authors: Carter Lawrence E. , Khodabandeh Shervin , Weakliem Paul C. , Carter Emily A.
Published: July 2002

We have simulated via molecular dynamics the interaction of F2 with the clean Si(100)-2×1 reconstructed surface. Using a Stillinger–Weber-type many-body potential with the Si–F interactions refit to ab initio data, we find that both vibrational and translational excitation of the incident F2 can lead to increased reactivity, but they do so in different ways. The dominant reaction channels are (a) F-atom abstraction, leading to the formation of one Si–F bond while the remaining fluorine atom is ejected away from the surface, and (b) dissociative chemisorption, where both fluorine atoms in the incident F2 molecule form Si–F bonds on the surface. Nonreactive scattering is almost never observed. As a result, enhanced reactivity is mainly characterized by an increase in dissociative chemisorption at the expense of F-atom abstraction and by a corresponding increase in the initial reaction probability S0. We find S0 ranges from 0.57±0.04 for the lowest excitation energies to 0.78±0.04 for the largest translational excitation of 20.9 kcal/mol. For cases where F-atom abstraction occurs, the exit velocities of fluorine atoms ejected from the surface are found to be independent of the incident F2 energy and with kinetic temperatures much higher than the surface temperature, suggesting that the exiting fluorine atom does not equilibrate with the surface, yet loses memory of its initial state. Finally, for dissociative chemisorption trajectories, we find that the adsite location of the two fluorine atoms is strongly dependent on the incident orientation.
Intermolecular structure in a single component polymer glass: Towards high resolution measurements of the sidechain pair correlation function
Authors: Marcus A. H. , Fayer M. D. , Curro John G.
Published: July 2002

Electronic excitation transport among interacting polymer molecules lightly tagged with chromophore substituents is theoretically examined as a function of tagged polymer concentration in the polymeric solid. The results are compared to experimental data obtained in a previous study [Macromolecules 26, 3041 (1993)]. The dependence of time-resolved fluorescence observables on intermolecular polymer structure is of primary interest. A theory is presented which describes excitation transport for both donor–donor (DD) and donor–trap (DT) systems. For the case of DD transport, the theory is based on a first order cumulant approximation to the transport master equation. For DT transport, the theory does not involve approximations and is an exact representation of the assumed model. In both cases, the model makes use of the Flory ‘‘ideality’’ postulate by depicting the intramolecular segmental distribution as a Gaussian with a second moment that scales linearly with chain size. The only adjustable parameter in the treatment is the form of the intermolecular segmental pair distribution function g(r). The model is found to be extremely sensitive to the behavior of g(r). Comparisons to experimental data indicate that g(r) is primarily made up of hard core interactions between the chromophore sites. The DT calculations display a higher sensitivity to the form of g(r) than the corresponding DD calculations. For purposes of comparison, the analysis is applied to a DT system in which every polymer chain has chromophore tags. The sensitivity of the method for 100% tagged systems to g(r) is comparable to the analysis for systems with only some of the chains tagged.
Quasiclassical trajectory study of collisional energy transfer in toluene systems. I. Argon bath gas: Energy dependence and isotope effects
Authors: Lim Kieran F.
Published: July 2002

Experimental studies of collisional energy transfer from highly vibrationally excited toluene to various bath gases have recently been reported [Toselli and Barker, J. Chem. Phys. 97, 1809 (1992), and references therein]. A quasiclassical trajectory investigation for toluene in argon bath gas at 300 K for initial internal energies E′=41 000, 30 000, and 15 000 cm−1 is reported here. Collisional energy transfer is almost linearly dependent on E′. Predictions of energy transfer quantities are very sensitive to the average well depth of the assumed individual pairwise potentials, but is less sensitive to the detailed shape. Qualitative and quantitative agreement with experiment is obtained where the overall well depth is physically realistic. Isotope studies using 40Ar and pseudohelium (4Ar) bath gases indicate that energy transfer is independent of the mass of the bath-gas collider, but perdeuteration increases 〈ΔE2〉1/2 by 13% over the undeuterated values.
Femtosecond polarization spectroscopy: A density matrix description
Authors: Ziegler L. D. , Fan R. , Desrosiers A. E. , Scherer N. F.
Published: July 2002

A density matrix treatment of the time evolution of the third order polarization response describing the optical heterodyne detected (OHD) transient birefringence and dichroism excited by ultrafast pulses is given. The relationship between frequency domain (Raman scattering) and time domain (pump–probe) spectroscopies is revealed by this pathway explicit description. Constructive and destructive interferences between time evolution density matrix pathways account for the respective strong birefringent and weak dichroic ground state nuclear response when the pulses are electronically nonresonant. However, for electronically resonant chromophores, the dichroic response is larger than the corresponding birefringent response due to constructive and destructive interferences respectively between density matrix time evolution histories. No such interferences contribute to spontaneous Raman scattering. The relative magnitude of the resonant dichroic and birefringent responses is pulse width dependent in the fast pulse limit and dependent on the relative rates of optical dephasing and ground state nuclear motion in the rapid optical dephasing limit. The spatial interpretation of the ground and excited state OHD polarization responses is given within the context of this polarization approach and the familiar Maker–Terhune notation. These relationships between time and frequency domain spectroscopies are illustrated by the observed OHD birefringence and dichroism and the spontaneous Raman spectra of both a nonresonant liquid (chloroform) and a resonant solution (I2 in n-hexane).
A theoretical study of CO/Cu(100)
Authors: Bauschlicher Charles W.
Published: July 2002

The convergence of the binding energy, geometry, and vibrational frequencies for CO/Cu(100) is studied using a cluster model containing up to 38 Cu atoms. A large basis set is used for the Cu atom at the adsorption site and for the CO molecule. Electron correlation, which is included at the modified coupled-pair functional (MCPF) level, is found to increase the binding energy by about 1 eV. The binding energy and geometry are in very good agreement with experiment for the largest Cu clusters considered. The MCPF binding energies for the smaller clusters can be either larger or smaller than experiment. Unlike the binding energies, the shift in the CO vibrational frequency, relative to free CO, is quite independent of the cluster size or even the inclusion of electron correlation.
The ultraviolet effect on the alanine radicals and its resolution enhanced electron paramagnetic resonance powder pattern
Authors: Cabral-Prieto A. , Ureña-Nuñez F. , Jiménez-Domínguez H.
Published: July 2002

This study reports the electron paramagnetic resonance powder-line structure of γ-ray-induced DL-alanine radicals, and its changes when these radicals are exposed to ultraviolet (UV) light. Deconvolution results show that the resolution enhanced powder pattern of UV-free and UV-exposed alanine radicals may consist of five pairs of superimposed quintets on quartets, and a doublet whose line intensity is proportional to that of the quartets. From the analysis of such resolution enhanced powder patterns, one may expect that the four alanine radicals per unit cell could always be magnetically grouped by pairs, irrespective of any crystalline axis orientation with respect to the magnetic field. Additionally, as the γ-ray-irradiated powdered DL-alanine is UV-exposed, a line broadening effect concomitant to the signal loss of the strongest resonance line is observed. These spectral UV-time-dependent variations, as well as the near constancy of the total spectral area, might reject the coexistence of two alanine radical populations, but may suggest a recombination process among the alanine radicals of a single radical species population that is very stable at atmospheric conditions but UV-triggered.
Electronic and nuclear chemical reactivity
Authors: Cohen M. H. , Ganduglia-Pirovano M. V. , Kudrnovský J.
Published: July 2002

The local softness and the Fukui function emerge from density functional theory as measures of local electronic reactivity. We obtain here an exact linear integral relation between the Fukui functions of insulators or molecules and the probability density of the frontier orbitals of Kohn–Sham theory. The same linear map holds between the local softness and the local Kohn–Sham density of states at the Fermi level for metals. The kernel in those relations is the inverse of the transpose of the potential response function (PRF) of Kohn–Sham theory. The PRF has the form of the static Hartree dielectric function with an exchange and correlation interaction added to the bare Coulomb interaction. The exact static dielectric function also has the Hartree form, but with a renormalized polarization propagator. The map is norm preserving for systems with energy gaps such as insulators and molecules and norm reducing or screening for systems with a finite density of states above the ground state such as normal metals and Anderson insulators. Nuclear reactivities are defined in analogy with the more familiar electronic reactivities and are more directly relevant to reaction pathways. The former are linearly related to the frontier orbital densities or the local density of states through a kernel which is just the electron–nuclear Coulomb force screened by the PRF. This shows explicitly how the frontier orbital density or the local density of states drives the nuclear reactivity. The limitations of such definitions of chemical reactivity are discussed and directions for improvement indicated.
Boltzmann entropy, relative entropy, and related quantities in thermodynamic space
Authors: Eu Byung Chan
Published: July 2002

The conventional solution methods for the Boltzmann kinetic equation such as the Chapman–Enskog method or the moment method provide a thermodynamic branch of the distribution function evolving through macroscopic variables under the functional hypothesis. Such a distribution function is different in nature from the phase-space distribution function obtained by directly solving the Boltzmann kinetic equation subject to initial and boundary conditions in the phase space without the functional hypothesis. The Boltzmann entropy is an information entropy enumerated with the phase-space distribution function in the phase space, whereas the compensation function introduced in the previous work is a representation of the former in thermodynamic space. The two quantities are not generally the same. By using the concept of relative entropy which is the difference between the two quantities, we examine their relations and significance for the mathematical structure of thermodynamics of irreversible processes. By using the balance equations for the compensation function and the relative entropy, we investigate the limiting behavior of the rate of relative entropy as the thermodynamic branch of the distribution function becomes convergent in the sense of means (i.e., weakly converges) to the phase-space distribution function. The time derivative of the relative entropy does not vanish in the limit, but tends to a limit associated with energy dissipation. Such a limit represents a contraction of information as the description of irreversible processes is made in the thermodynamic space contracted from the phase space of 1023 particles. On the basis of such results, it is indicated that, as was found in earlier work, a thermodynamic theory of irreversible processes can be erected on the compensation function since it is an integral of a one-form in the thermodynamic space whereas the Boltzmann entropy is not.
Accurate three-dimensional quantum probabilities and collision lifetimes of the H+O2 combustion reaction
Authors: Pack Russell T , Butcher Eric A. , Parker Gregory A.
Published: July 2002

Accurate 3D quantum state-to-state reaction probabilities and collision lifetimes for the H+O2→OH+O combustion reaction for total angular momentum J=0 are reported. The reaction probabilities are dominated by resonances, many of which overlap. The total reaction probability is not enhanced by vibrational or rotational excitation of the reactants. The first accurate final state distributions are presented, and probability is found to spread out into all open product vibrational and rotational channels, with a rather strong tendency to produce highly excited product rotational states. The first calculated collision lifetimes are presented, and resonances with lifetimes of 1–10 ps are found at most energies. Whether the reaction behaves statistically is discussed, and future extensions needed are also discussed.
Hyper-Rayleigh scattering studies of first order hyperpolarizability of tricyanovinylthiophene derivatives in solution
Authors: Pauley M. A. , Wang C. H. , Jen Alex K.-Y.
Published: July 2002

First order hyperpolarizabilities, β, for donor–acceptor substituted heteroaromatic compounds are measured by the hyper-Rayleigh scattering technique. Hyper-Rayleigh scattering utilizes the mechanism of number density fluctuations and enables the second harmonic generation signal of nonlinear optical chromophores to be generated as scattered light. Effects of substituting different electron donating groups to the parent heteroaromatic compounds and different solvents on the hyperpolarizability is investigated using this technique.
Dipole-bound excited states of the I−⋅CH3CN and I−⋅(CH3CN)2 ion–molecule complexes: Evidence for asymmetric solvation
Authors: Dessent Caroline E. H. , Bailey Christopher G. , Johnson Mark A.
Published: July 2002

Dipole-bound excited states are reported for the I−⋅CH3CN and I−⋅(CH3CN)2 cluster ions, located just below their vertical electron detachment energies (determined using negative ion photoelectron spectroscopy). The absorption cross sections for excitation to these states are observed to increase with increasing dipole moments of the solvent molecules in the I−⋅M series (M=methyl iodide, acetone, acetonitrile). Photoexcitation at the peak of the transition to the dipole-bound state results exclusively in the dipole-bound fragment ion, M−. The photoelectron spectrum of the CH3CN− fragment was also recorded by sequential two-photon absorption in the I−⋅CH3CN parent, indicating that the excess electron is indeed weakly bound (≤10 meV) with very little intramolecular distortion evident upon electron detachment. The I−⋅(CH3CN)2 cluster displays two absorption bands, one below each of the two features in the photoelectron spectrum. The most intense band correlates with the weaker, lower binding energy photoelectron band. Excitation of this strong absorption band results in production of both CH3CN− and (CH3CN)−2 fragments, while excitation of the weaker absorption band only results in electron detachment. We interpret these results in the context of two structural isomers for I−⋅(CH3CN)2: one with the solvent molecules surrounding the I− and another with both solvent molecules on the same side, asymmetrically solvating I− in a configuration with a large electric dipole moment in the neutral cluster over this anionic geometry.
Application of an efficient asymptotic analysis method to molecule–surface scattering
Authors: Mowrey R. C. , Kroes G. J.
Published: July 2002

An improved method for performing asymptotic analysis developed by Balint-Kurti et al. [J. Chem. Soc. Faraday Trans. 86, 1741 (1990)] was used with the close-coupling wave packet (CCWP) method. S-matrix elements are computed from the time dependence of the wave packet amplitude at a dividing surface in the asymptotic region. The analysis technique can be combined in a natural way with the use of an optical potential to absorb the scattered wave function beyond the dividing surface and with a technique in which the initial wave function is brought in on a separate, one-dimensional grid, thereby allowing the use of a smaller grid. The use of the method in conjunction with the Chebyshev and short-iterative Lanczos propagation techniques is demonstrated for a model problem in which H2 is scattered from LiF(001). Computed S-matrix elements are in good agreement with those obtained using a time-independent close-coupling method.
Reactivity kernels, the normal modes of chemical reactivity, and the hardness and softness spectra
Authors: Cohen Morrel H. , Ganduglia-Pirovano M. V. , Kudrnovský J.
Published: July 2002

Chemical reactivity theory provides a basis for predicting the reactive proclivities of molecular or condensed systems. The frontier-orbital concepts of Fukui, as generalized by Parr and collaborators within the framework of density-functional theory, were developed further by us in a previous paper (I) [J. Chem. Phys. 101, 8988 (1994)]. Nevertheless, five aspects of the theory still require further development; the reactivities are defined as local responses to global stimuli instead of nonlocal responses to local stimuli; there are ambiguities associated with the existence of energy bands in condensed systems; the theory is static and does not properly incorporate the internal dynamics of the reacting systems; the theory focuses on responses without a corresponding definition of chemical stimuli; and no connection is made with the potential energy surface and the reaction pathway. In the present paper, we concentrate on gapless systems extended in at least one dimension, metals, semimetals, and insulators. We show that taking as measures of chemical reactivity the reactivity kernels of Nalewajski [J. Chem. Phys. 78, 6112 (1983); 81, 2088 (1984); 89, 2831 (1985)] and of Berkowitz and Parr [J. Chem. Phys. 88, 2554 (1988)] in the form introduced by the latter meets the first two requirements. We find an explicit expression for the softness kernel which is a natural generalization of that found for the local softness in I.

The response functions entering it are calculable via the Kohn–Sham formalism. As recognized by Nalewajski and Koniński [Z. Naturforsch. 42a, 451 (1987)], there is an infinite set of modes of chemical reactivity with corresponding spectra of softness and hardness eigenvalues. The softness spectrum has an upper bound corresponding to the most reactive mode or set of degenerate modes [Z. Naturforsh. 42a, 451 (1987)] and a vanishing lower bound. The softness kernel itself can be expressed simply in terms of the static dielectric function. We generalize the nuclear softness introduced in I to a nuclear softness kernel analogous to the electronic softness kernel and find a simple expression for it in terms of the dielectric function. The present theory provides a basis for the remaining developments needed.
Quantum-state distributions for the HD product of the direct reaction of H(D)/Cu(111) with D(H) incident from the gas phase
Authors: Rettner C. T. , Auerbach D. J.
Published: July 2002

We have determined the internal-state distribution for the HD product of the reaction of gas-phase D atoms with H atoms chemisorbed on Cu(111) and for the corresponding reaction of H atoms with chemisorbed D atoms. In the case of D-on-H, the populations of the vibrational states v=0, 1, 2, and 3 are comparable, while that for v=4 is considerably smaller, giving a mean vibrational energy of ∼0.7 eV. The vibrational state distribution for H-on-D is similar, but in this case there is a clear peak at v=1, even less population in v=4, and a somewhat smaller mean vibrational energy of ∼0.6 eV. The mean rotational energy falls with increasing v in both cases, ∼0.5 eV for v=0 to <0.2 eV for v=4, with an overall mean rotational energy of ∼0.4 eV. The rotational distributions are distinctly narrower for H-on-D than for D-on-H. The maximum internal energy observed is ∼2.3 eV, consistent with the total energy available to the product. Results are consistent with recent calculations.
New model for electron-impact ionization cross sections of molecules
Authors: Hwang W. , Kim Y.-K. , Rudd M. E.
Published: July 2002

A theoretical model for electron-impact ionization cross sections, which has been developed primarily for atoms and atomic ions, is applied to neutral molecules. The new model combines the binary-encounter theory and the Bethe theory for electron-impact ionization, and uses minimal theoretical data for the ground state of the target molecule, which are readily available from public-domain molecular structure codes such as GAMESS. The theory is called the binary-encounter Bethe (BEB) model, and does not, in principle, involve any adjustable parameters. Applications to 19 molecules, including H2, NO, CH2, C6H6, and SF6, are presented, demonstrating that the BEB model provides total ionization cross sections by electron impact from threshold to several keV with an average accuracy of 15% or better at the cross section peak, except for SiF3. The BEB model can be applied to stable molecules as well as to transient radicals.
Bifurcation in rotational spectra of nonlinear AB2 molecules
Authors: Kozin Igor N. , Pavlichenkov I. M.
Published: July 2002

A classical microscopic theory of rovibrational motion at high angular momenta in symmetrical nonlinear molecules AB2 is derived within the framework of small oscillations near the stationary states of a rotating molecule. The full-dimensional analysis including stretching vibrations has confirmed the existence of the bifurcation predicted previously by means of the rigid-bender model [see B. I. Zhilinskii and I. M. Pavlichenkov, Opt. Spectrosk. (USSR) 64, 413 (1988)]. The formation of fourfold energy clusters resulting from the bifurcation has been experimentally verified for H2Se and it has been demonstrated in fully-dimensional quantum mechanical calculations carried out with the MORBID computer program. We show in the present work that apart from the level clustering, the bifurcation produces physically important effects including molecular symmetry-breaking and a transition from the normal mode to the local mode limit for the stretching vibrations due to the centrifugal forces. The application of the present theory with realistic molecular potentials to the H2Te, H2Se, and H2S hydrides results in predictions of the bifurcation points very close to those calculated previously. However for the lighter H2O molecule we find that the bifurcation occurs at higher values of the total angular momentum than obtained in previous estimations. The present work shows it to be very unlikely that the bifurcation in H2O will lead to clustering of energy levels. This result is in agreement with recent variational calculations.
On the influence of the ionic charge on the mean force potential of ion-pairs in water
Authors: Guàrdia E. , Padró J. A.
Published: July 2002

Mean force potentials [W(r)] for the Na2+–Cl− and Na2+–Na2+ ion-pairs in water are calculated in order to investigate the effects of the cation electric charge on the ion-pair association. The constrained molecular dynamics simulation method is used. W(r) for Na2+–Cl− shows two minima corresponding to the contact and solvent-separated ion pairs with dissociation–association barriers which are markedly higher than those for Na+–Cl−. Unlike for Na+–Na+, W(r) for Na2+–Na2+ does not show any noticeable minimum.
Theoretical study of the unimolecular dissociation HO2→H+O2. II. Calculation of resonant states, dissociation rates, and O2 product state distributions
Authors: Dobbyn Abigail J. , Stumpf Michael , Keller Hans-Martin , Schinke Reinhard
Published: July 2002

Three-dimensional quantum mechanical calculations have been carried out, using a modification of the log-derivative version of Kohn’s variational principle, to study the dissociation of HO2 into H and O2. In a previous paper, over 360 bound states were found for each parity, and these are shown to extend into the continuum, forming many resonant states. Analysis of the bound states close to the dissociation threshold have revealed that HO2 is a mainly irregular system and in this paper it is demonstrated how this irregularity persists in the continuum. At low energies above the threshold, these resonances are isolated and have widths that fluctuate strongly over more than two orders of magnitude. At higher energies, the resonances begin to overlap, while the fluctuations in the widths decrease. The fluctuations in the lifetimes and the intensities in an absorption-type spectrum are compared to the predictions of random matrix theory, and are found to be in fair agreement. The Rampsberger–Rice–Kassel–Marcus (RRKM) rates, calculated using variational transition state theory, compare well to the average of the quantum mechanical rates. The vibrational/rotational state distributions of O2 show strong fluctuations in the same way as the dissociation rates. However, their averages do not agree well with the predictions of statistical models, neither phase space theory (PST) nor the statistical adiabatic channel model (SACM), as these are dependent on the dynamical features of the exit channel. The results of classical trajectory calculations agree well on average with those of the quantum calculations.
Photodissociation study on Ca+(H2O)n, n=1–6: Electron structure and photoinduced dehydrogenation reaction
Authors: Sanekata Masaomi , Misaizu Fuminori , Fuke Kiyokazu
Published: July 2002

The photodissociation spectra of size-selected Ca+(H2O)n (n=1–6) ions are examined in the wavelength region from 330 to 1440 nm by monitoring the total yield of the fragment ions. The absorption bands exhibit redshifts as large as ∼16 000 cm−1 with respect to the 2P–2S resonance line of the free Ca+ ion and are explained by the shift of this transition as a result of hydration. The converging trend in the spectral shifts at n∼6 is discussed in relation to the filling of the first solvation shell for Ca+ undergoing the sdσ hybridization. We also discuss the possible contribution of charge-transfer character in the observed transitions in conjunction with our recent results on the photoelectron spectra of Na−(NH3)n. The mass spectra of the fragment ions show the existence of two dissociation channels: The evaporation of water molecules and the dehydrogenation reaction to produce the hydrated CaOH+ ions. The evaporation process is the dominant decay channel for n=1, while the reaction is the only one for n≥3. The former process is found to take place from the higher vibrational levels in the ground state being populated through the fast internal conversion induced by the presence of the low-lying 2D-type states. As for n=1, the reaction takes place only in an excited state which has the Ca+p orbital aligned on the intermolecular axis. The state-specific reaction for n=1 is explained in terms of charge-transfer interaction between the Ca+ ion and the water molecule. On the other hand, the reactivity for the larger clusters drastically increases with increasing cluster size. These reaction features are discussed in comparison with those for Mg+(H2O)n reported previously.
Interlayer interactions in two-dimensional systems: Second-order effects causing ABAB stacking of layers in graphite
Authors: Yoshizawa Kazunari , Kato Takashi , Yamabe Tokio
Published: July 2002

The role of second-order perturbations in interlayer interactions of two-dimensional electronic systems is examined. The general features of interlayer interactions are deduced from a simple two-layer model. The preferred motion is discussed specifically for the stacking of layers in graphite, using a transition-density analysis for extended systems. The well-known ABAB stacking of layers in graphite is proposed to be a consequence of orbital interactions between layers rather than of generally accepted van der Waals forces.
The condensed phases of carboranes
Authors: Gamba Z. , Powell B. M.
Published: July 2002

An atom–atom Lennard-Jones (LJ) model for the intermolecular potential of carborane molecules (B10C2H12) is developed and used in a series of classical constant-pressure molecular dynamics simulations of p-, m- and o-carborane crystals, at several temperatures and zero pressure. The potential model reproduces many structural and dynamical properties of these crystals, in particular the high temperature orientationally disordered plastic phase common to the three compounds. This work stresses the need of further experimental data to improve the potential model.
A new model for lattice systems
Authors: Aranovich G. L. , Donohue M. D.
Published: July 2002

A new model is derived for lattice systems (lattice gas and binary mixtures of monomers). This model is based on a generalization to three dimensions of the Ono–Kondo equations for the density profile near a flat surface. The internal energy is calculated and compared with previous models. Unlike many previous theories, this new model has the correct limiting behavior at infinite dilution, at high densities, when the interchange energy goes to zero and for the lattice gas. In addition, it displays the correct behavior for systems with very strong interactions (such as hydrogen bonds) in that it predicts that the energy saturates to a constant value at a low density. For one-component, monomer systems, the new theory also describes simulation data for square-well (off-lattice) molecules better than previous theories.
Theoretical investigation of structural and thermodynamic properties of lanthanum carbides LaCn (n=2–6)
Authors: Roszak S. , Balasubramanian K.
Published: July 2002

Theoretical studies of monolanthanum carbides, LaCn for n=2–6, are presented. The fan structures were found as ground states in most cases studied. The computed enthalpies of formation of LaCn and atomization energies of these species are close to the corresponding experimental data. The agreement is even closer when experimental Gibbs energy functions are corrected using theoretical ground state structures and partition functions. The La–C bond is strongly ionic due to electronic charge transfer from lanthanum to carbon atoms.
Theoretical study of the electronic states of Rh5
Authors: Majumdar D. , Balasubramanian K.
Published: July 2002

Geometries and energy separations of 32 low-lying electronic states of Rh5 with different structures have been investigated. Complete active space multi-configuration self-consistent field method followed by large scale multireference singles + doubles configuration interaction computations that included up to 2.09 million configurations were used. Three nearly degenerate electronic states, namely, 2B2(C2v) with a distorted trigonal bipyramid, 4A2(C2v) with a distorted trigonal bipyramid, and 4A2(C2v) with a distorted tetragonal geometry were found as candidates for the ground state of Rh5. The calculated ionization potential from the ground state structure was found to be within the observed experimental range. The atomization and dissociation energies have also been computed and the results are compared with the smaller clusters.
Shaping molecular beams with intense light
Authors: Seideman Tamar
Published: July 2002

We discuss the possibility of manipulating the center-of-mass motion of molecules using the nonlinear interaction of a moderately intense, long-pulse laser field with the molecular polarizability tensor. Recent theoretical work demonstrating the possibility of focusing and trapping molecules is extended to consider the effects of circularly and elliptically polarized light and the effect of nonspherical laser optics. The aberrations affecting the quality of the “molecular lens” are analyzed and the means by which they can be minimized are discussed. Molecular focusing is extended to a general field of molecular optics; the possibilities of steering, reflecting, and collimating molecular beams are illustrated. Application of the mechanical force of light to disperse and separate species according to their mass, velocity, or quantum state is proposed.
Fast, accurate semiempirical molecular orbital calculations for macromolecules
Authors: Dixon Steven L. , Merz Kenneth M.
Published: July 2002

A detailed review of the semiempirical divide-and-conquer (D&C) method is given, including a new approach to subsetting, which involves dual buffer regions. Comparisons are drawn between this method and other semiempirical macromolecular schemes. D&C calculations are carried out using a basic 32 Mbyte memory workstation on a variety of peptide systems, including proteins containing up to 1960 atoms. Aspects of storage and SCF convergence are addressed, and parallelization of the D&C algorithm is discussed.
Theoretical study of the interaction of benzene with Rh+ and Rh2+ cations
Authors: Majumdar D. , Roszak S. , Balasubramanian K.
Published: July 2002

Extensive ab initio calculations have been carried out on the benzene–Rh+ (Rh+–C6H6) complex, the Rh+(C6H6)2 sandwich complex and the Rh2+⋯ benzene interaction products, viz., Rh2+–C6H6 and Rh2+–C6H4. The “physisorbed” benzene–Rh+ was found to be in a 3A2 state with C6v symmetry. The ground state of the Rh+(C6H6)2 sandwich complex is 3A2 with C2v symmetry. Although the optimization of the ground state of the sandwich complex was carried out in C2v symmetry, no symmetry breaking distortion in the structure of the benzene moiety was found in the final optimized geometry. The relative strength of binding of benzene with the Rh+ cation in these two complexes has also been compared. The interaction of Rh2+ and benzene has been found to favor the formation of two different products consistent with the recent experiment of Bondybey and co-workers. The probability of the formation of the chemisorbed Rh2+C6H4 species with respect to the other cleaved product resulting from the physisorbed Rh2+C6H6 species, benzene–Rh+, has been discussed in the light of energy calculations.
Dynamics of glass-forming liquids. III. Comparing the dielectric α- and β-relaxation of 1-propanol and o-terphenyl
Authors: Hansen C. , Stickel F. , Berger T. , Richert R. , Fischer E. W.
Published: July 2002

We have measured the dielectric relaxation of the glass-former 1-propanol for temperatures between 65 and 350 K in the frequency range 10−2 to 2⋅1010 Hz and the photon correlation spectro-scopy decays near Tg. Attributing the strong Debye-type process of 1-propanol to distinct -OH group effects leaves two faster processes related to the structural relaxation which can be identified as α-relaxation and Johari–Goldstein type β-relaxation characteristic of nonhydrogen-bonding supercooled liquids. From the temperature dependent relaxation times τ(T) regarding the three distinct loss peaks, we can specify an α-β-bifurcation temperature Tβ, which coincides with characteristic qualitative changes in the τ(T) behavior, as also observed for ortho-terphenyl and other glass-forming liquids. This assignment is confirmed by the correla-tion times derived from incoherent quasielastic light-scattering data obtained from the simultaneously measured photon-correlation spectroscopy.
Six dimensional quantum dynamics study for dissociative adsorption of H2 on Cu(111) surface
Authors: Dai Jiqiong , Light John C.
Published: July 2002

In this letter we present preliminary results of full six dimensional quantum dynamics calculations for dissociative adsorption of a hydrogen molecule on a Cu(111) surface. We utilize the time-dependent wave-packet approach to simulate the dissociation process on a full dimensional LEPS potential energy surface which has incorporated the latest ab initio data [Hammer et al. Phys. Rev. Lett. 73, 1400 (1994)]. We use a novel partitioning of the angular momentum operator in the split-operator method so that a direct product DVR can be rigorously implemented. The most interesting observation in the present rigorous quantum dynamics study is the site-averaged effect, i.e., the averaged dissociation probability of the four dimensional calculations over the three symmetric impact sites strongly resembles the exact dissociation probability of the six dimensional calculations. In accord with the low dimensional calculations, initial vibrational excitation of H2 effectively reduces the translational threshold energy. The rotational orientation effect observed in the four dimensional studies remains in the present full dimensional dynamics with the cartwheel orientation yielding dramatically lower dissociative efficiency than the energetically equivalent helicopter orientation. We focus on normal incident scattering. The diffractive scattering and more detailed results will be presented in a later paper.
Pure translation instantaneous normal modes: Imaginary frequency contributions vanish at the glass transition in CS2
Authors: Li Wu-Xiong , Keyes T.
Published: July 2002

While the connection between imaginary frequency instantaneous normal modes (INM) and barrier crossing in liquids is well established, nondiffusive modes exist which must be removed in theories of the self-diffusion constant, D. It is shown that in molecular systems rotation is the primary source of such modes. A new type of pure translation (TR) INM is introduced, describing center-of-mass motion. TR densities of states, 〈ρTR(ω)〉, are obtained at several T for CS2. The TR Im-ω modes vanish at the glass transition and their fraction, fuTR, has a T dependence identical to that of D. Correspondingly, the Re-ω density looses the usual INM linear low-ω dependence and attains conventional ω2 behavior. It is proposed that the unmodified TR INM have a negligible nondiffusive character and naturally describe translational dynamics in molecular systems. Some general ideas are given for how INM properties, including the importance of nondiffusive modes, vary among different materials.
Interpretation of the dynamic heat capacity observed in glass-forming liquids
Authors: Simon Sindee L. , McKenna Gregory B.
Published: July 2002

Slow structural relaxations can complicate the interpretation of thermodynamic measurements on glass-forming liquids. Here we demonstrate using model calculations that structural recovery can lead to an apparent frequency-dependent heat capacity in ac calorimetry experiments. The model is shown to describe the complex heat capacity data reported in the literature for glycerol and poly(vinyl acetate). Importantly, the model does not invoke a complex heat capacity; rather, only static heat capacities are used. The analysis further suggests that ac calorimetry should provide a powerful way of testing models of structural recovery.
Neutron diffraction study of (ND4)2SeCl6, (ND4)2PtCl6, and (ND4)2PtBr6 crystals: The origin of the strong deuterium substitution effect on the phase transitions
Authors: Yamamuro Osamu , Okishiro Kenji , Matsuo Takasuke , Onoda-Yamamuro Noriko , Oikawa Kenichi , Kamiyama Takashi , Kume Yoshio , Izumi Fujio
Published: July 2002

Neutron powder diffraction experiments of (ND4)2SeCl6, (ND4)2PtCl6, and (ND4)2PtBr6 were performed to investigate the mechanism of the strong deuterium substitution effect on the phase transitions of the (NH4)2MX6 family. The isotope effect is strong in the first and second compounds and weak in the third. The data were collected in the d-spacing range 0.5–4.3 Å by using a time-of-flight powder diffractometer VEGA installed at the pulsed cold neutron source in KEK. The intensity data of high-temperature phases (HTP) were measured at the temperatures corresponding to 1.3Tc (Tc: transition temperature), and the low-temperature phases (LTP) at 4 K. The HTPs of the three compounds have an antifluorite type cubic structure (a≈9.8 Å and Z=4) as previously reported while the LTPs of (ND4)2PtCl6 and (ND4)2PtBr6 are tetragonal with a tentative space group P42/n and unit cell dimensions similar to those of the HTPs. The LTP of (ND4)2SeCl6 may have an orthorhombic structure with a larger unit cell. The Rietveld refinement and Fourier analysis for the HTPs revealed that the D atoms in (ND4)2SeCl6 and (ND4)2PtCl6 are broadly distributed around the crystallographic threefold rotation axis with three weak overlapping maxima, while that of (ND4)2PtBr6 is more strongly localized at three positions away from the threefold rotation axis. These results indicate that the transitions are due to orientational ordering of the ND4+ ions and that the strong isotope effects may be quantum effects associated with the ND4+ motion in a shallow rotational potential.
Influence of temperature and viscosity on anthracene rotational diffusion in organic solvents: Molecular dynamics simulations and fluorescence anisotropy study
Authors: Jas Gouri S. , Wang Yan , Pauls Steven W. , Johnson Carey K. , Kuczera Krzysztof
Published: July 2002

Molecular dynamics simulations and fluorescence anisotropy decay measurements are used to investigate the rotational diffusion of anthracene in two organic solvents—cyclohexane and 2-propanol—at several temperatures. Molecular dynamics simulations of 1 ns length were performed for anthracene in cyclohexane (at 280, 296, and 310 K) and in 2-propanol (at 296 K). The calculated time constants for reorientation of the short in-plane axis were 7–9 and 11–16 ps at 296 K in cyclohexane and 2-propanol, respectively, in excellent agreement with corresponding fluorescence depolarization measurements of 8 and 14 ps. The measured rotational reorientation times and the calculated average rotational diffusion coefficients varied in accord with Debye–Stokes–Einstein theory. Their magnitudes were close to values predicted for an ellipsoid of shape and size equivalent to an anthracene molecule, and exhibited predictable variation with external conditions—increasing with temperature and decreasing with solvent viscosity. However, analysis of the calculated rotational diffusion coefficients for the individual molecular axes gave a more complex picture. The diffusion was highly anisotropic and changes in temperature and solvent type led to nonuniform variation of the diffusion coefficients. The nature of these changes was rationalized based on analysis of variation of solvation patterns with temperature and solvent.
Direct time-resolved measurement of anharmonic lattice vibrations in ferroelectric crystals
Authors: Brennan Ciaran J. , Nelson Keith A.
Published: July 2002

Coherent lattice vibrations are driven beyond the harmonic limit through impulsive stimulated Raman scattering excitation with intense femtosecond pulses. High overtones of the fundamental lattice vibrational frequency and wave vector are observed in the time-dependent responses of the ferroelectric crystals lithium tantalate and lithium niobate. The results provide controlled experimental access to anharmonic regions of lattice potential energy surfaces, including the “soft” mode or “collective reaction coordinate” paths along which ions move from their initial lattice positions toward the positions they occupy in new lattice structures, i.e., new domain orientations or crystalline phases. The observations represent a significant extension of coherent optical control over material behavior, and a step toward “collective mode-selective chemistry” through which cooperative structural rearrangements might be induced.
Molecular dynamics simulation with an ab initio potential energy function and finite element interpolation: The photoisomerization of cis-stilbene in solution
Authors: Berweger Christian D. , van Gunsteren Wilfred F. , Müller-Plathe Florian
Published: July 2002

An interpolation scheme for potential energy surfaces is presented. It employs a regular grid and finite element interpolation. The aim is the reduction of the computational expense for molecular dynamics simulation with a quantum chemical potential energy function. The methods used are described in detail. The feasibility is demonstrated and the efficiency and accuracy are evaluated for the photoisomerization of cis-stilbene in supercritical argon, using an ab initio configuration- interaction treatment for the first electronically excited state of the stilbene molecule and classical force fields for the solvent–solute interactions (quantum mechanical/molecular mechanical molecular dynamics). The number of required quantum chemical calculations of energy and gradients was substantially reduced compared to a simulation not using the interpolation scheme. On the other hand, the impact on the accuracy is insignificant.