bluuebunny/crossref_metadata_2025 huggingface dataset:
Reduced equations of motion for semiclassical dynamics in phase space
Authors: Grad Jonathan , Yan Yi Jing , Haque Azizul , Mukamel Shaul
Published: July 2002

Time-dependent self-consistent equations for semiclassical dynamics in phase space are developed. The method is based on constructing a Gaussian density matrix, whose equations of motion are obtained by requiring that the first two moments of the coordinates and momenta have the correct time evolution. The method can yield, in principle, the exact values of these moments for all time. The present method can be applied for the time evolution of mixed states in phase space and may, therefore, be particularly useful for molecular dynamics in condensed phases. Raman excitation profiles in anharmonic molecules are calculated and show excellent agreement with exact calculations.
Multiple-quantum dynamics in NMR: A directed walk through Liouville space
Authors: Munowitz Michael , Pines Alexander , Mehring Michael
Published: July 2002

An approach to spin dynamics in systems with many degrees of freedom, based on a recognition of the constraints common to all large systems, is developed and used to study the excitation of multiple-quantum coherence under a nonsecular dipolar Hamiltonian. The exact equation of motion is replaced by a set of coupled rate equations whose exponential solutions reflect the severe damping expected when many closely spaced frequency components are superposed. In this model the evolution of multiple-quantum coherence under any bilinear Hamiltonian is treated as a succession of discrete hops in Liouville space, with each hop taking the system from a K-spin/n-quantum mode to a K′-spin/n′-quantum mode. In particular, for a pure double-quantum Hamiltonian the selection rules are ΔK=±1 and Δn=±2. The rate for each move depends on the number of Liouville states at the origin and destination, and on the total number of spins present. All rates are scaled uniformly by a factor dependent on the properties of the material, such as the dipolar linewidth, but otherwise the behavior predicted is universal for all sufficiently complicated systems. Results derived by this generic approach are compared to existing multiple-quantum data obtained from solids and liquid crystals.
Propagation method for the solution of the arrangement-channel coupling equations for reactive scattering in three dimensions
Authors: Schwenke David W. , Truhlar Donald G. , Kouri Donald J.
Published: July 2002

We present a new methodology for solving the three-dimensional reactive scattering problem. It is based upon a convenient choice of coordinates with the arrangement channels coupled either by the Fock method or by the wave function component form of arrangement-channel quantum mechanics. The resulting coupled ordinary differential equations are cast in a form suitable for use with standard nonreactive scattering codes. The method involves no matching of solutions from different arrangements and no numerical derivatives, interpolations of multidimensional functions, or nonlocal kernels.
Infrared fluorescence study of E–V energy transfer from Br*(4 2P1/2) to nitrosyl bromide
Authors: Sedlacek Arthur J. , Wight Charles A.
Published: July 2002

Electronic-to-vibration (E–V) energy transfer from excited Br(4 2P1/2) atoms to nitrosyl bromide has been investigated by an infrared fluorescence technique. The excited bromine atoms are generated by pulsed laser photolysis of Br2 at 500 nm. Approximately 50% of the quenching collisions of Br*+BrNO yield BrNO molecules excited in the 1800 cm−1 v1 stretching mode. Of these, six times as many BrNO molecules are produced in the v1=1 state as in v1=2, despite the fact that the latter is nearly resonant with the electronic energy of Br*. The total rate coefficient for removal of Br* by BrNO is determined to be (1.5±0.2)×10−11 cm3 molecule−1 s−1. Indirect comparison of our results with previous work suggests that quenching proceeds primarily by inelastic collisions; reactive quenching to form Br2+NO is probably a minor channel.
Optical–optical double resonance of the ICl–Ne complex: Binding energies in the E(0+), A(3Π1), and X(1Σ+) states
Authors: Drobits Janet C. , Lester Marsha I.
Published: July 2002

Sequential two-photon excitation of the ICl–Ne complex to ion pair states is achieved via long-lived intermediate levels in the A state. An entire manifold of van der Waals (vdW) vibrational levels is accessed in the E state. A Birge–Sponer analysis of the vdW stretching motion yields a fundamental vibrational frequency ωe =52.5±0.1 cm−1, anharmonicity ωexe =6.14±0.04 cm−1, and dissociation energy D0=87.6±0.8 cm−1 for ICl–Ne in the E(vE=1) state. Utilizing the spectroscopic shifts of the vdW complexes relative to bare ICl transitions, the vdW binding energies in the ground state, D0=48.2±0.5 cm−1, and all other optically accessed states are determined.
Determination of product electronic-state distributions in collision-induced dissociation experiments
Authors: Gislason Eric A. , Guyon Paul-Marie
Published: July 2002

For the collision-induced dissociation (CID) process H+2 +He→H++H+He, there are two degenerate product electronic states. We discuss the two potential energy surfaces for this system and describe for the first time a way to determine the distribution of products on the two surfaces from product velocity measurements. The procedure is remarkably simple but extremely general. High energy CID experiments are reexamined in light of this result. In addition, the analysis is applied to recent experiments in our laboratory. This shows that at Ec.m.=3.1 eV, CID of H+2 molecules, initially selected to be in vibrational level 0 or 1, gives products only on the excited electronic surface. This surprising result has important implications for any theory of CID. A simple mechanism is presented which can rationalize the experiments.
Theory of time-dependent polarization about an ion in an isotropic liquid
Authors: Friedrich Valerie , Kivelson Daniel
Published: July 2002

We have investigated the theory of polarization of a dipolar solvent by a newly created ion or free electron. As a reference system we have used the solvent together with the ‘‘uncharged ion,’’ and by means of statistical mechanics, we have examined the linear response of the system to the imposition of the ionic charge. The response depends upon the response at all k values, the low k behavior corresponding to continuum (universal) macroscopic dielectric effects, and the high k behavior corresponding to molecular (specific) phenomena which may have to be treated by quantum mechanics. We have focused on the quasicontinuum limits for which the low k behavior is assumed, but the sizes of ions and dipoles, and the velocities of ions are incorporated. We also look at some general properties of high k behavior; for example, because the polarization energy for small ions is independent of ion size, it is usually thought that solventbergs form about the ions, but, at least in part, this phenomenon can be explained by geometric considerations.
Integral equation for the structure of fluids in presence of three-body forces: Krypton, argon, and models for complex mixtures
Authors: Reatto L. , Tau M.
Published: July 2002

The modified hypernetted chain and the crossover integral equations for the radial distribution function are generalized to take into account three-body forces. Good agreement with the simulation results is found. The structure factor and its isothermal density derivative for krypton and argon are computed with a realistic two-body potential plus the Axilrod–Teller three-body interaction. For krypton at medium density a very good agreement with experiment if found. At higher densities we find significant deviations both for krypton and for argon. The effect of three-body forces becomes very significant when the two-body potential has a narrow attractive well as it is the case in micellar solutions or microemulsions. In particular the critical point can be shifted to a very low value of concentration and there is enhancement of concentration fluctuations for a given value of the correlation length.
Application of the close coupling wave packet method to long lived resonance states in molecule–surface scattering
Authors: Mowrey R. C. , Kouri D. J.
Published: July 2002

In this paper we describe the application of the close coupling wave packet (CCWP) method to the study of HD and H2 scattering from a Ag(111) surface. The presence of rotationally mediated selective adsorption (RMSA) resonances at low collision energies for the HD/Ag(111) system requires that the scattering wave function be propagated for extremely long times to allow the resonant states to decay. A new procedure is presented for obtaining both the magnitude and phase of the S-matrix elements from the scattered wave packet which drastically reduces the size of the spatial grid required. Both the transition probabilities and collision lifetimes obtained from CCWP calculations are in excellent agreement with those predicted by the CC method, demonstrating that both the moduli and phases of S-matrix elements can be obtained over a range of collision energies from the propagation of a single wave packet.
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o investigation of the spectroscopic properties of BCl, CS, CCl+, BF, CO, CF+, N2, CN−, and NO+
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

Spectroscopic properties have been calculated for the diatomics BCl, CS, CCl+, BF, CO, CF+, N2, CN−, and NO+ by Mo/ller–Plesset many-body perturbation theory through MP4SDQ (fourth order with single, double, and quadruple substitutions). Very good agreement of both rotational and vibrational constant predictions with experiment was found at the MP4SDQ level of theory with the extended basis sets used (66 CGTO’s). CI dipole moments, which appear to be accurate to at least 0.1 D, and electric field gradients, which yield eq0Q values accurate to within 1–2 MHz, were obtained.
Hindered rotation and site structure of CD4 trapped in rare gas solids
Authors: Jones Llewellyn H. , Ekberg Scott A.
Published: July 2002

High resolution infrared absorption spectra have been recorded for the ν4 vibrational mode of CD4 trapped in solid argon and krypton. As with CH4, evidence for two dominant sites was found: one with a cubic close-packed (CCP) environment and the other with a hexagonal close-packed (HCP) environment. The fine structure for the CCP site resembles that for a tetrahedral molecule rotating in an octahedral field: singlet peaks for P(1), Q(1), and R(0), and a doublet for R(1). The fine structure for the HCP site is more complex and is not assigned to specific rotational transitions. By high temperature annealing it was possible to eliminate the HCP site completely with increase in concentration of the more stable CCP site. The slow relaxation of nuclear spin found for CH4, from an F state (J=1) to an A state (J=0), was found to be two orders of magnitude faster for CD4.
The electronic structure of LaF: A multiconfiguration ligand field calculation
Authors: Schall Harold , Dulick Michael , Field Robert W.
Published: July 2002

A zero-free-parameter ligand field model is used to account for the observed low-lying (E<10 000 cm−1) electronic states of LaF. The electronic structure of LaF is represented as the effect of a nonpolarizable point F− ligand on the two valence electrons of a free La+ atomic ion. Free-ion configuration interaction (CI) effects (represented by a parametric fit of the free La+ energy levels to Fk,Gk Slater–Condon electrostatic and ζ spin–orbit parameters) and ligand-driven CI (treated by Bk0 ligand field radial integrals evaluated using Hartree–Fock La+ orbitals) are included in the model. A series of calculations is described, from the simplest ‘‘primitive LFT model’’ which includes the three lowest-lying and most important configurations (6s2, 5d6s, 5d2) to the most elaborate ‘‘balanced s-polarized LFT model’’ which includes six configurations (6s2, 5d6s, 5d2, 6s6p, 5d6p, and 6p2) in order to account properly for s–p polarization effects. The inclusion of ligand-driven CI (metal-centered orbital polarization) effects shows that simple electrostatic atomic-ion-in-molecule models are capable of accounting for the electronic structure of many highly polar diatomic molecules having more than a single metal-centered valence electron.
Dissociative chemisorption of H2 on Ni surfaces: Dependence on incident angles and rovibrational states
Authors: Lee Chyuan-Yih , DePristo Andrew E.
Published: July 2002

Using an effective medium plus two-body representation for the interaction potential and the GLE-ghost atom formulation for the dynamical simulation, we show that the dissociative chemisorption probability S0, increases with increasing vibrational energy, but decreases with increasing rotational energy for the H2/Ni system. At translational energies less than 0.4 eV, S0 at high incident polar angle is smaller than the value predicted by normal energy scaling. For the Ni(110) surface, S0 varies between two azimuthal incident directions: one parallel and one perpendicular to the troughs of the (110) face. The difference increases with increasing incident polar angle of the gas molecule. The physical mechanisms underlying these results are discussed.
The effect of dynamics on band shapes of hydrogen bonded complexes in solution
Authors: Johnson W. G. , Oxtoby David W.
Published: July 2002

We study the IR absorption of the νs (XH) mode of a hydrogen bonded species (XH---Y) in solution. Our theory assumes that the dephasing of the νs dipole transition is due to coupling to the low frequency stochastic νσ (XH---Y) stretching mode. Our approach is novel in that it uses Kubo’s stochastic Liouville equation to include both dynamics and Fermi resonance coupling of the νs levels with other intramolecular levels. Spectra assuming three bath models (Smoluchowski, Ornstein–Uhlenbeck, and Zwanzig) are compared and discussed. The theory gives good overall qualitative agreement with experiments involving moderate strength hydrogen bonds. It also demonstrates that the overall shape of the band can be analyzed in terms of parameters that affect primarily νs, and that dynamics should be included for a realistic description of these bands.
A vibrational study on a phase transition in tin(II) chloride dihydrate. The O–X stretchings (X=H or D)
Authors: Kanesaka Isao , Kyobashi Mariko , Kawai Kiyoyasu
Published: July 2002

The vibrational spectra of SnCl2⋅2H2O and its partially deuterated compound were measured in the O–X stretching region for both ordered and disordered phases. In the ordered phase some unexpected bands were found. They are explained in terms of hydrogen ion positional disorder present in the crystal. The Raman spectrum of SnCl2⋅2H2O was resolved in the disordered phase and direct information about disorder in hydrogen positions was obtained.
Theoretical study of the properties of BC and its positive ion in their ground and excited electronic states
Authors: Hirsch Gerhard , Buenker Robert J.
Published: July 2002

Potential curves have been calculated for the ground and low-lying states of the BC molecule and its positive ion by employing the MRD (multireference through double excitation)-CI method. Spectroscopic constants for 14 states are given, and the results are compared with those of the isoelectronic C+2 and BN+ molecules. Of particular technical interest is the occurrence of an artificial double minimum in the SCF treatment of the X 4Σ− ground state, corresponding to two different electronic configurations with quite distinctive dipole moment curves; use of natural orbitals in a subsequent CI treatment is shown to give a consistent description of the true dipole moment dependence on the BC internuclear distance.
Close-coupling scattering cross sections for Ar–O2 collisions at 97.0 meV
Authors: Bowers M. S. , Faubel M. , Tang K. T.
Published: July 2002

An exact quantum close-coupling calculation of rotationally inelastic scattering cross sections is performed for the best available anisotropic Ar–O2 potential. 110 coupled channels for 10 rotational states are included for the first 99 partial waves. The convergence is demonstrated with up to 156 coupled channels test calculations for selected partial waves. The results agree qualitatively with former IOS calculations of these cross sections and confirm an about 30% to 50% too small anisotropy of the employed potential.
Accurate three-dimensional quantum scattering calculations for F+H2→HF+H
Authors: Zhang John Z. H. , Miller William H.
Published: July 2002

A new approach for carrying out quantum scattering calculations for the fluorine and hydrogen reaction is reported. The method is based on Miller’s formulation of reactive scattering ( ref.1 ), and uses the S−matrix version of the Kohn variational principle (ref.2).(AIP)
The vibrational frequencies, the dissociation energy, and the electron affinity of nickel carbonyl
Authors: Blomberg Margareta , Brandemark Ulf , Johansson Jerry , Siegbahn Per , Wennerberg Jan
Published: July 2002

CPF, MCPF, and multireference CCI calculations have been done on nickel carbonyl and its positive and negative ions. For obtaining an accurate value of the dissociation energy De of Ni–CO it is important to correlate all the CO valence electrons. The difference in De with and without CO correlation is 17 kcal/mol which is about half of the total De. The best CCI calculation gave a dissociation energy of 33 kcal/mol in good agreement with experiments. The CPF and MCPF dissociation energies were 30 kcal/mol. The largest CCI calculation also gave a frequency shift between CO and NiCO in excellent agreement with experiments, 143 cm−1 compared to 142 cm−1. The effect of coupling the Ni–C and C–O motions is 20 cm−1 on the shift. The calculations on NiCO− show that the added electron enters into the σ symmetry and leads to an almost unchanged Ni–C bond length and a slight reduction of the dissociation energy. When NiCO is ionized on the other hand the removed electron in the σ symmetry causes a large increase of 0.8 a0 in the Ni–C bond length but does not change the dissociation energy.
Theoretical study of the electronic spectrum of SiH4
Authors: Larrieu C. , Liotard D. , Chaillet M. , Dargelos A.
Published: July 2002

A b initio SCF calculations of the first excited states of SiH4 are reported. Earlier experimental assignments of the three broad peaks in the 107–220 nm range are discussed in terms of the analysis of calculated charge transfer for each state. It is clearly seen that only the first band can be attributed to a pure 4s Rydberg transition, while the others result from an overlapping of diffuse 3d states with the first antibonding Si–H level. The shoulder at 118 nm is assigned to the 5s atomic transition.
On g factors and hyperfine structure in electronically excited states of NO2
Authors: Bylicki F. , Weber H. G. , Persch G. , Demtröder W.
Published: July 2002

The combined results of optical radio frequency double resonance and of hyperfine structure measurements in the spectral range around 593 nm show that the variations of hfs splittings within a vibronic band are due to the irregular behavior of the Landé factors in the excited state, rather than caused by variations of the hfs coupling constant OI. The observations can be explained by spin–orbit coupling and vibronic coupling.
Magnetic properties of t
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s-polyacetylene doped with FeCl3
Authors: Jones T. E. , Butler W. F. , Ogden T. R. , Gottfredson D. M. , Gullikson E. M.
Published: July 2002

Samples of trans-polyacetylene, (CH)x, were doped with the magnetic ion, FeCl−4, by immersion in nitromethane solutions of FeCl3. The resulting dopant levels ranged over two orders of magnitude. ESR spectra and spin–lattice relaxation times, T1, were measured for the undoped polymer and lightly doped polymer over the temperature range 1.5–120 K. The ESR spectra of trans-(CH)x lightly doped (<1 mol % Fe−4 ) with FeCl−4 exhibit a narrow (ΔH≊5–10 G at 4.2 K) g=2.003 signal which decreases in relative amplitude with increasing dopant level, and broad signals at g=2 (ΔH≊500 G), which increases with increasing dopant level, and at g=4.3 (ΔH≊200 G). The narrow signal is the intrinsic trans-(CH)x signal; the broad signals are attributable to iron. T1 for the narrow signal is approximately one-half of the value of undoped trans-(CH)x at 4.2 K. dc susceptibilities for all of the doped samples were measured over the temperature range of 1.5–300 K. The magnetically doped samples all exhibit Curie law behavior, and an analysis of the magnetization data yields an effective moment, Jeff, equal to 2.77, close to the value 5/2 expected for Fe(III).
Half and full collision matrix methods for scattering: Application to multichannel curve crossing
Authors: Band Yehuda B. , Mies Frederick H.
Published: July 2002

We present methods of numerically solving the multichannel Schrödinger equation by propagating exact first-order coupled equations for a specially designed half collision matrix X(r). The method requires choosing a convenient set of reference potentials with which to generate a pair of reference radial functions for each channel. It is easy to tailor the choice of basis to the nature of the exact multichannel interaction matrix in a given region of space, either to enhance the numerical efficiency of propagating the exact X(r), or, to facilitate the introduction of useful approximations. In particular, we define a classical half collision matrix Z(r) by neglecting rapidly oscillating terms in the propagation of X(r). This random phase approximation can only be justified in classically accessible regions of r, and results in a set of first order equations which imposes the unitarity of Z(r)Z°(r)=1 throughout the propagation. Fortunately, such regions often contain the dominant couplings, particularly for heavy collision partners, and the unitarity constraint generally can be utilized out to r=∞. We only begin to explore the consequences of this useful result in this paper. Because of the divergence of the reference functions, both the exact X(r) and the approximate Z(r) half collision amplitudes can only be propagated with confidence in classical regions of r. However, using the same basis, we derive a stable algorithm for propagating the full collision matrix Σ(r)=X(X*)−1 which asymptotically defines the scattering matrix. Since Σ(r) is always unitary, the first-order equations for Σ(r) can be propagated in all regions of r. The various methods are demonstrated by applying them to a multichannel curve crossing in atom–diatom vibrational energy transfer collisions. Exact results are obtained from both the complete propagation of Σ(r) and from the partial propagation of X(r) in classical allowed regions. Excellent results are obtained from the unitarized classical half collision matrix Z(r) when reasonable reference potentials are employed.
Atomic and molecular alignment from photoelectron angular distributions in (n+1) resonantly enhanced multiphoton ionization
Authors: Dubs Richard L. , McKoy V. , Dixit S. N.
Published: July 2002

Two distinct (n+1) REMPI techniques for obtaining the alignment of gas phase atoms and molecules from photoelectron angular distributions are presented. In both methods, the alignment is extracted from the angular distributions independently of the photoionization dynamics. The first method, which takes advantage of circular dichroism in the angular distributions (CDAD) has already been established experimentally as a useful probe of state alignment. The theory outlined in previous work is expanded here. The second method involves photoionization with light linearly polarized along the photoelectron collection direction and is presented here for the first time.
The interaction of O(1D2) with HCl: The initial vibrational distributions in the OH(2Π) produced by chemical reaction, and the HCl(1Σ+) produced by E–V energy transfer
Authors: Kruus E. J. , Niefer B. I. , Sloan J. J.
Published: July 2002

The initial vibrational distributions in both energetically accessible channels of the reaction between O(1D) atoms and HCl molecules are reported. The measurements were made using very fast time-resolved Fourier transform spectroscopy to observe the emission spectra of the products before their vibrational populations could be altered by collisional relaxation. Both the OH from the reaction and the HCl created in the E–V energy transfer process have strongly inverted vibrational distributions. The cross section of the former is found to be about 20 times larger than that of the latter. Although spin forbidden, the E–V process is fast, due to the fact that the 1A″ and 3A″ surfaces of the HOCl intermediate are nearly energetically degenerate over a large region of configuration space. The results suggest that the dynamics of the interaction are dominated by the relatively slow O–Cl collision. The rapid motion of the H atom, in response to changes in the potential created by the motion of the heavier atoms, permits the system to sample the singlet–triplet intersection many times during the collision. OH rotational deactivation is very fast in this system; the average probability for rotational energy transfer in the v′=3 level is more than four times larger than the gas kinetic collision probability. The time evolution of both vibrational distributions is also reported, and fast V–V energy transfer from vibrationally excited OH to ground state HCl is observed.
Measurement of Debye temperature from the zero-field splitting of Ni2+ in ZnSiF6⋅6H2O
Authors: Shrivastava Keshav N. , Rubenacker Gerald V. , Hutton Stuart L. , Drumheller John E. , Rubins R. S.
Published: July 2002

The variation of the zero-field splitting parameter D for Ni2+ in ZnSiF6⋅6H2O between 4 and 250 K has been used to calculate the temperature dependence of the Debye temperature θD(T) in ZnSiF6⋅6H2O. θD(T) was determined to be 143 K at a temperature of 30 K and 127 K at 250 K. There was a pronounced minimum of 105 K in θD(T) near 60 K. The behavior of θD(T) is qualitatively similar to that found for alkali halides from specific heat data.
Liquid-like ordered colloidal suspensions: The influence of the particle concentration
Authors: Härtl W. , Versmold H.
Published: July 2002

The density of dependence of the static structure factor S(Q) of liquid-like ordered colloidal suspensions is investigated by light scattering techniques. We found that variation of the particle number density even by a factor of 8 produced only a modest change of the height of the first peak of S(Q). In order to obtain a consistent theoretical description of S(Q) at various particle number densities, the effective charge Z*p of the particles and the Debye screening parameter κ* were calculated on the basis of the Poisson–Boltzmann–Cell (PBC) model and the Poisson–Boltzmann–Jellium (PBJ) model, respectively. The resulting Z*p and κ* values were used to calculate S(Q) via the rescaled mean spherical approximation (RMSA). It is found that a consistent description of the density dependence of S(Q) is possible with the PBC model. By contrast, the PBJ model predicts too weak a liquid-like order than observed experimentally.
Diffusion of H atoms on a Si(111) surface with partial hydrogen coverage: Monte Carlo variational phase-space theory with tunneling correction
Authors: Rice Betsy M. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

The diffusion of hydrogen atoms on a partially hydrogen-covered Si(111) surface has been studied by using Monte Carlo techniques with a potential-energy surface based on the available ab initio results and experimental data. The potential describes two kinds of binding sites, a covalent Si–H bond (top site) and an interstitial threefold bonding site (open site). Classical jump frequencies between the top and open sites were calculated using Monte Carlo variation phase-space theory with importance sampling at 300, 600, 900, and 1200 K. A new approach for treating tunneling through two-dimensional diffusional barriers is presented and used to calculate the phonon-assisted tunneling rates. This method assumes continuum-to-continuum WKB tunneling with classical Monte Carlo phase space averaging. Thermal diffusion coefficients are calculated using the jump frequencies. The diffusional barriers between the two binding sites on the equilibrium surface are 2.79 and 0.65 eV for top-to-open site and open-to-top site jumps, respectively. The calculated classical jump frequencies give Arrhenius parameters of A=1.3×1014 and 9.9×1013 s−1 Ea=2.72 and 0.59 eV for top-to-open and open-to-top site jumps, respectively. Monte Carlo techniques were used to compute the minimum energy path; the dynamical barrier is 2.64 eV for top-to-open site jumps. Tunneling rates were calculated at 300 K and estimated at higher temperatures. Due, in part, to the small width of the barrier, the tunneling rate at 300 K is 257 times larger than the classical value. Tunneling is important at room temperature, but its importance relative to the classical rate decreases with increasing temperature. The results indicate that surface phonons significantly enhance the tunneling rate.
A theoretical study of the potential energy surface for the reaction OH+CO→CO2+H
Authors: Aoyagi Mutsumi , Kato Shigeki
Published: July 2002

The mechanism of the reaction OH+CO→CO2+H is studied theoretically. The potential energy surface is constructed by using ab initio multiconfiguration (MC) SCF and configuration interaction (CI) calculations. The reaction surface Hamiltonian is derived to estimate the hydrogen tunneling probability. It is found that the first step is the trans addition of OH to CO and the trans–cis isomerization is the subsequent step to lead the hydrogen elimination from the HOCO. The rate constant is calculated by using the RRKM equation and the curved Arrhenius temperature dependence, experimentally observed, is obtained. We have found here that the effects of both the hydrogen tunneling from the HOCO to products and the backreaction from the HOCO to the reactants are essential to obtain this temperature dependence.
Structure, energetics, and vibrational spectra of beryllium borohydride isomers
Authors: Stanton John F. , Lipscomb William N. , Bartlett Rodney J.
Published: July 2002

Several structures of beryllium borohydride (BeB2H8) have been studied with techniques based on many-body perturbation theory (MBPT). Energies calculated at the MBPT(4) level with a basis set of 85 contracted Gaussian functions support the two structure hypothesis of Nibler, although a qualitative MBPT(4) asymmetric stretching potential indicates that the C3v double-minimum structure is unstable with respect to a more highly symmetric D3d model with two sets of equivalent triple hydrogen bridges. The theoretical free energy difference between the D3d structure and a diborane-like model having D2d symmetry is 1.1 kcal/mol, with the latter predicted to be more stable. Uncertainties in the theoretical force field and those resulting from the finite nature of the basis set, however, may change this value by several kcal/mol. Principal evidence for the two-structure proposal comes from MBPT(2) harmonic frequencies and infrared absorption intensities, which are found to be consistent with the published vibrational spectra. These are used in a tentative assignment of most of the observed features. Unfortunately, the experimentally observed dipole moment of 2.06 D cannot be explained at present.
Precipitation pattern formation
Authors: Falkovitz Meira S. , Keller Joseph B.
Published: July 2002

Precipitation patterns have been observed in initially uniform salt solutions. They cannot be explained by Ostwald’s supersaturation mechanism, which requires concentration gradients. A theory of them is presented based on the Cahn–Hillard equation for the salt concentration c. It shows that there is a range of initial concentrations which give rise to periodic precipitation patterns provided that the diffusion coefficient D(c) is negative for some range of c.
Electronic states of the XenHCl systems in gas and condensed phases
Authors: Last Isidore , George Thomas F.
Published: July 2002

Systems formed by one Cl, one H, and one or several Xe atoms are considered by the semiempirical diatomics-in-ionic-systems (DIIS) method, which takes into account the charge delocalization in ionic states and the coupling between the neutral and ionic states. Calculations are performed both for gas-phase systems, such as van der Waals complexes and clusters XenHCl (n=1,2,4,8,12) and ionic molecules (HXen)+Cl−, and for systems formed in Xe solids doped by HCl molecules. The calculations give the structure of the systems, dissociation energies, and energies of electronic transitions. The calculations show, in particular, the existence of the ground-state ionic molecule (HXe)+Cl−, the decrease of the HCl electronic excitation energy in clusters and solids, and the increase of the emission photon energy of the ionic HXe+nCl− systems as compared to the Xe+nCl− molecule.
The properties of the hydrogen-bonded dimer (CH3)3N⋅⋅⋅HCN from an investigation of its rotational spectrum
Authors: Rego C. A. , Batten R. C. , Legon A. C.
Published: July 2002

The ground-state rotational spectra of the three symmetric-top isotopomers (CH3)143N ⋅⋅⋅HC14N, (CH3)143 N⋅⋅⋅HC15N, and (CH3)143 N⋅⋅⋅DC15N of a hydrogen-bonded dimer formed from trimethylamine and hydrogen cyanide have been investigated by pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectroscopic constants B0, DJ, DJK, χ(14N1), and χ(14N2) have been determined from analyses of the spectra and for the isotopomer (CH3)143 N⋅⋅⋅HC14N take the values 1373.6236(2) MHz, 0.694(10) kHz, 98.52(2) kHz, −5.061(14) MHz, and −4.533(16) MHz, respectively. The spectroscopic constants have in turn been interpreted with the aid of a model of the dimer to yield the following molecular properties for the species (CH3)143 N⋅⋅⋅HC14N: r(N⋅⋅⋅C)=3.1035(6) Å, the subunit oscillation angles θav=13.4(2)°, and φav=9.1(4)° for the (CH3)3N and HCN subunits, respectively, and the hydrogen bond stretching force constant kσ=14.7(2) N m−1. The opportunity has been taken to determine a more accurate value χ0(14N)=−5.5024(25) MHz for the 14N-nuclear quadrupole coupling constant of free (CH3)143 N. The properties of the family of dimers R3N⋅⋅⋅HX, where R=CH3 or H and X=CN or CCH, are compared.
Phase equilibria in quadrupolar fluid binary mixtures
Authors: Scalise O. H. , Gianotti R. D. , Zarragoicoechea G. J. , Rodríguez A. E.
Published: July 2002

Phase equilibria in fluid binary mixtures of nonpolar–quadrupolar and quadrupolar–quadrupolar fluids is investigated by using a perturbation scheme recently reported. It is found that a molecular model with a Lennard-Jones function and a quadrupole–quadrupole interaction as the perturbing potential to the hard-sphere fluid reference system, is able to predict the shape of the coexistence curves observed in He–N2, H2–N2, and Ne–N2 mixtures.
Overtone spectra of C–H oscillators in cold molecules
Authors: Crofton M. W. , Stevens C. G. , Klenerman D. , Gutow J. H. , Zare R. N.
Published: July 2002

The C–H stretch overtone spectra of methane (5–0), ethylene (5–0 and 6–0), ethane (5–0 and 6–0), propyne (4–0 and 5–0 acetylenic and 5–0 methyl C–H stretches), allene (5–0), propane (5–0 and 6–0), cyclopropane (5–0 and 6–0), dimethyl ether (5–0), and isobutane (5–0) have been recorded at temperatures between 143 and 189 K, depending on the molecule. A comparison is made to the spectra obtained at room temperature, with the goal of improved understanding of the band shapes. The temperature dependence of most of the observed bands is found to be significantly less than that expected for ‘‘simple’’ bands. For these small to medium size hydrocarbons, the temperature independence of the overtone bands is found to correlate loosely with the density of states and with the degree of saturation. Other factors are important determinants of spectral widths and temperature independence as well, such as conformational inequivalence of the C–H oscillators, and the number and positions of the oscillators. It is concluded that the vast majority of hydrocarbon C–H stretch high overtone bands have upper states which are extensively mixed with other states. This is the case even for most of the relatively small hydrocarbons. This mixing produces a broadening effect and greatly increases the transition density, thereby diluting the oscillator strength of the rovibrational transitions from that of the zero-order approximation. The Fermi resonance type of interaction appears to be of greater importance than the Coriolis type in determining the appearance of the high overtone bands.
Multifrequency electron spin echo envelope modulation in S=1/2, I=1/2 systems: Analysis of the spectral amplitudes, line shapes, and linewidths
Authors: Lai Albert , Flanagan Heather L. , Singel David J.
Published: July 2002

The nuclear modulation effect in S=1/2, I=1/2 systems is analyzed with particular emphasis on the impact of variation of the external field strength. We show that the modulation depth parameter k reaches its maximum possible value when the Zeeman and hyperfine interactions are made equal in magnitude, or ‘‘matched,’’ and that this matching plays the dominant role in determining the ESEEM (electron spin echo envelope modulation) line shapes; the distinctive ESEEM spectral features are readily understood on this basis. Through multifrequency ESEEM, it is possible to interpret the features and to utilize them to measure hyperfine interactions. We discuss several experimental strategies, involving the field dependences of ESEEM amplitudes, line shapes, and linewidths, for determining hyperfine coupling constants.
The transition from nonadiabatic to solvent controlled adiabatic electron transfer kinetics: The role of quantum and solvent dynamics
Authors: Morillo M. , Cukier R. I.
Published: July 2002

We analyze the transition from nonadiabatic to solvent controlled adiabatic electron transfer kinetic behavior with emphasis on the inverted regime. By viewing the electron transfer process as dynamical motion on a donor surface followed by possible crossing to the acceptor surface, we are able to obtain a simple consecutive reaction expression for the overall rate in terms of rate constants for motion on the surfaces and crossing motion. When the crossing between surfaces is not localized to a point, we find that there are frictional effects on this crossing rate constant, in contrast to the localized case. Use of typical electron transfer parameters shows that these friction effects will only be in evidence for the inverted regime of electron transfer kinetics.
Spontaneous predissociation in BrCl: A possible role for shape resonances in threshold predissociation
Authors: Tellinghuisen Joel
Published: July 2002

A computational study of the spontaneous predissociation of the B 0+ (3 Π) state of BrCl indicates that this process is due to rotation–electronic coupling with a weakly bound state which intersects the B potential curve on its attractive branch near the first molecular dissociation limit. The state responsible for the predissociation is very likely the 1 Π state, the analog of the states responsible for this same process in I2 and Br2 . The sharp onset of predissociation is a threshold phenomenon and leads to a revised estimate of the ground state dissociation energy 𝒟e=18 244(5) cm−1. Broad shape resonances associated with the centrifugal barrier in the predissociative state can yield marked enhancement of the predissociative decay in favorable circumstances.
The solution of the hypernetted-chain and Percus–Yevick approximations for fluids of hard spherocylinders
Authors: Perera A. , Patey G. N.
Published: July 2002

The hypernetted-chain (HNC) and Percus–Yevick (PY) approximations are solved numerically for isotropic fluids of hard spherocylinders with length-to-breadth ratios ranging from 2 to 6. The theoretical results are compared with the available Monte Carlo data for the equation of state and the pair correlation function. The HNC theory was found to predict the existence of a nematic phase at densities in reasonable agreement with recent computer simulations.
Vibrational relaxation of ND3 trapped in a rare gas matrix. A comparative study with NH3
Authors: Lakhlifi A. , Girardet C.
Published: July 2002

The behavior of the vibrational relaxation time of ammonia molecule trapped in rare gas matrices with an isotopic substitution (ND3) is discussed within the framework of nonadiabatic coupling theory recently developed. The various factors which are subjected to changes in the H→D substitution are analyzed, and it is shown that the transfer of the vibrational energy to the orientational modes is here again the most efficient when compared to the multiphonon relaxation. The relaxation times are calculated to be 20 and 3 times longer for the deuterated species in argon and xenon matrices, respectively. When T is raised from 10 to 30 K, the vibrational relaxation efficiency is still enhanced in favor of NH3 since the efficiency rate factors become 40 (Ar) and 8 (Xe) smaller for ND3.
High pressure Raman study of the CO stretching mode in liquid N,N-dimethylacetamide
Authors: Thomas H. D. , Jonas J.
Published: July 2002

The Raman ν4 symmetric C=O stretching mode of N,N-dimethylacetamide (DMA) is reported as a function of pressure from 1 to 4000 bar within the temperature range 40 to 100 °C. As in acetone, the isotropic line width ω1/2 (ISO) decreases with increasing density, in contrast to the opposite trend usually found in other molecular liquids. The noncoincidence effect, δω, which is the frequency difference between the peak maxima of the isotropic and the anisotropic Raman bands, increases with increasing density. These trends indicate the presence of strong intermolecular dipolar interactions between DMA molecules. Two theories of the noncoincidence effect due to McHale and Logan are used to interpret the density dependence of δω for DMA. In addition, we also used these two theoretical models to calculate δω for acetone using the results obtained earlier in our laboratory. McHale’s theory can predict the correct density dependence for the noncoincidence effect when the dielectric screening factor is ignored. Logan’s theory can account for the density behavior of the noncoincidence effect in DMA and in acetone.
The photodissociation of formaldehyde: A coupled cluster study including connected triple excitations of the transition state barrier height for H2CO→H2+CO
Authors: Scuseria Gustavo E. , Schaefer Henry F.
Published: July 2002

A b initio molecular quantum mechanics has been applied to the unimolecular dissociation of H2CO. Basis sets as large as triple zeta plus double polarization (TZ+2P) were used in conjunction with complete optimization of all stationary point geometries. The classical barrier height is predicted with the TZ+2P basis set to be 101.9 (SCF), 95.0 (CISD), 90.4 (CCSD), and 86.8 kcal/mol (CCSDT-1). With correction for zero-point vibrational energies, the activation energy is predicted to be 81.4 kcal/mol, in good agreement with experimental estimates.
Infrared resonant desorption of butane from Al2O3(112̄0): Evidence for an ordered adlayer from vibrational mode selectivity
Authors: Tro N. J. , Arthur D. A. , George S. M.
Published: July 2002

The resonant desorption of butane from Al2O3(112̄0) was studied using tunable infrared radiation from an infrared free electron laser. Resonant desorption was induced by exciting C–H stretching vibrations in butane adlayers at coverages of θ=1–7 ML of Al2O3(112̄0) at 90 K. The resonant desorption spectrum did not duplicate the infrared absorption spectrum of solid butane. In contrast, the asymmetric C–H stretches produced a greater desorption yield than expected in comparison to the symmetric C–H stretches. The greater desorption efficiency of the asymmetric C–H stretches was attributed to the orientation of butane molecules in an ordered adlayer on Al2O3(112̄0). The proposed orientation in this adlayer is butane in an all-trans configuration with the carbon backbone perpendicular to the surface. The resonant desorption yield increased approximately linearly with laser energy and surface coverage. Isotope experiments were also performed with equal amounts of butane-h10 and butane-d10 on the surface. Equivalent amounts of protonated and deuterated species were observed to desorb after excitation of butane-h10. The results were all consistent with a thermal mechanism for the resonant desorption of butane from Al2O3(112̄0). In this thermal mechanism, vibrational energy deposited in the C–H stretching vibrations is thermalized quickly and leads to resonant heating of the butane adlayer and subsequent thermal desorption.
A classical functional sensitivity analysis of the collinear F+H2 reaction
Authors: Judson R. S. , Rabitz H.
Published: July 2002

The method of classical functional sensitivity analysis (CFSA) is applied to the collinear F+H2 reaction and its isotopic analogues. The effects of features in the Muckerman 5 (M5) potential upon product vibrational energy are examined using constant energy ensembles (1 and 5 kcal/mol) with H2(v=0) and H2(v=1). The same is done using thermal averaged ensembles at 300, 400, 1000, and 1500 K. Two significant results are reported. First, at low energies and at even relatively high temperatures (1000 K), the region of the potential to which the reactive product vibrational energy is most sensitive is the vicinity of the saddle point. In addition, the sensitivity function is smooth and nonoscillatory there. Second, at high temperatures and energies, the sensitivity function becomes oscillatory in magnitude and sign over the entire interaction region. The implications which these results have for methods which fit a potential to experimental data are discussed.
Molecular dynamics investigation of expanded water at elevated temperatures
Authors: Mountain Raymond D.
Published: July 2002

The structure of expanded states of TIP4P water has been examined over a range of densities running from 1000 to 100 kg/m3 and for a range of temperatures running from the coexistence temperature up to supercritical temperatures. The main result is that hydrogen bonding, as evidenced by the maximum in gOH (R) at 0.18 nm, persists to supercritical temperatures over the entire density range examined. For most liquid densities, the number of hydrogen bonds per molecule scales as a single function of the temperature but does not scale for dense vapor densities.
Effects of isotopic substitution on the critical behavior of aqueous solutions of n-dodecylhexaoxyethylene glycol monoether (C12E6)
Authors: Wilcoxon J. P. , Schaefer D. W. , Kaler E. W.
Published: July 2002

The critical behavior of a nonionic surfactant, n-dodecylhexaoxyethylene glycol monoether (C12 E6) in D2O, is examined using static and dynamic light scattering and small angle neutron scattering (SANS). Modification of the short-range attractive interactions responsible for phase separation by substitution of D2O for H2O has no effect on the measured universal critical exponents describing the divergence of the correlation length (ν=0.6±0.05) and the isothermal compressibility (γ=1.2±0.1) although such substitution results in a reduction of both the nonuniversal amplitudes and critical temperatures. The measured exponents are the same as those of binary molecular fluids. However, in contrast to binary fluids, significant deviations from the Ornstein–Zernike form are observed at intermediate length scales in SANS measurements near the critical point. Isotopic substitution also preserves the dynamic scaling of the Kawasaki form for the measured Rayleigh linewidth, as found previously for H2 O (Ref. 1). However, this scaling is restricted to a more limited range of distance from the critical point than the static scaling. A maximum in compressibility at the critical isochore is reflected in the concentration dependence of both the compressibility and diffusion coefficient even large distances (∼25 °C) from the coexistence curve. Estimates of micelle size, shape, and polydispersity are obtained.