bluuebunny/crossref_metadata_2025 huggingface dataset:
Large electric dipole moments on polydiacetylenes in toluene: Results from birefringence experiments
Authors: Zacher Robert A.
Published: July 2002

We report the results of a series of electric field induced birefringence experiments on polydiacetylenes (PDA) in toluene which demonstrate the presence of large electric dipole moments when the molecule is in the rod-like conformation. We show that an induced dipole moment cannot account for the saturation of the birefringence which was observed at relatively low values of the applied electric field. This result is in conflict with earlier models of the alignment process. Using a sinusoidally varying field, we demonstrate a frequency dependence in the birefringence signal which is the characteristic signature of orientation by a permanent dipole moment. The magnitude of the dipole moment is calculated from the value of the saturation birefringence. The degree of orientation of the polymers is estimated from a measurement of the Kerr constant. The degree of orientation is found to be rather low. The apparent permanent dipole moment is thought to arise from an anisotropic net charge which is found on the extended conformation of the PDAs in solvent solutions.
Fine-structure selectivity in polyatomic reaction products: CN(X 2Σ+,v=0,N=0,1)+O2→NCO(X̃ 2Π3/2, 0010,J,e/f)+O
Authors: Sonnenfroh David M. , Macdonald R. Glen , Liu Kopin
Published: July 2002

The reaction CN+O2→NCO+O has been investigated in a crossed-molecular beam experiment at a collision energy of 0.76 kcal/mole. Internal state distributions of the ground state NCO(X̃ 2Π3/2, 0010,J,e/f) products have been measured by laser-induced fluorescence. The rotational excitation is small. There is an observed preference in the population of the K doublet fine-structure states, 1f>1e, and this preference is even stronger for low J states.
Trajectory studies of OH vibrational excitation propensities in the reaction of O(1D) with H2
Authors: Goldfield Evelyn M. , Wiesenfeld John R.
Published: July 2002

The reaction of excited oxygen atoms with hydrogen molecules has been explored using classical trajectory methods, with particular emphasis on those features of the dynamics responsible for the OH vibrational distribution. Plots of final OH vibrational action vs. initial H2 phase are analyzed. A Monte Carlo ensemble of 6300 trajectories is also studied. Two methods are used to classify trajectories: (1) the amount of exponential separation of nearby trajectories and (2) the number of minimum bond length exchanges during the course of the trajectory. Trajectories which undergo different numbers of minimum exchanges are found to have markedly different propensities for producing OH in a given vibrational state. Direct insertions are found to play an important role in the reaction; two very direct types of trajectories are identified, one preferentially populating OH in v=0; the other leading to highly vibrationally excited OH (v=4 or 5). Slightly less direct trajectories are found to result preferentially in moderately excited OH(v=2). The disposition of energy in OH vibration is found to be highly correlated with the manner in which the complex dissociates; the most important patterns of dissociation are identified and described.
Geometrical arrangement of water molecules in copper–aquoion complex—Influence on shifts and relaxivities in nuclear magnetic resonance
Authors: Sahoo N. , Das T. P.
Published: July 2002

The unrestricted Hartree–Fock cluster procedure has been applied to the Cu2+–(H2O)6 complex to study both its geometry and the hyperfine properties of protons and 17O nuclei in the water molecules of the first hydration sphere. The geometry was studied to understand the Jahn–Teller effect expected for this complex and to explain the observed tetragonal distortion of an elongated nature from extended x-ray absorption fine structure measurements involving four of the H2O molecules being closer to the Cu2+ ion than the other two. The hyperfine properties were studied to attempt to explain observed data in this respect from nuclear magnetic resonance measurements. Our results for the situation in which there is a hole in the dx2–y2 type of state do explain the elongated nature of the tetragonal distortion. The calculated electronic wave functions provide a satisfactory explanation of the 17O hyperfine interaction. For the protons, our results demonstrate the validity of the point ion model commonly used for the dipolar interaction between protons of the hydrated H2O molecules and the unpaired spins in explaining proton nuclear magnetic resonance relaxation data for aquoions. For the proton contact interaction however, theory appears to lead to a higher value than experiment. Possible sources that can lead to an improvement in the predicted proton hyperfine interaction constant are discussed.
Cluster calculations of CO chemisorbed on the bridge site of Pd(100)
Authors: Pacchioni Gianfranco , Bagus Paul S.
Published: July 2002

The interaction of CO chemisorbed on the bridge site of the Pd(100) surface has been investigated by means of ab initio relativistic and nonrelativistic Hartree–Fock and MCSCF calculations. Pd2 and Pd8 clusters were used to determine the Pd/CO chemisorption properties. The interaction energy has been decomposed into different contributions arising from intraunit polarization and interunit effects including charge transfer and covalent bonding. Besides the classical π back bonding found for other transition metal surfaces, the CO σ donation to the partially occupied 4dσ–5sp metal hybrid orbitals significantly contributes to the chemisorption energy. By increasing the cluster size from Pd2 to Pd8, the average Pd atomic configuration becomes more d9s1-like, with consequent increase of the σ repulsion between the metal 5sp and the CO 5σ charge distributions. However, this increased repulsion is largely compensated by the increased metal polarization resulting in a strong surface bond.
Broad vibrational overtone linewidths in the 7νOH band of rotationally selected NH2OH
Authors: Luo X. , Fleming P. R. , Seckel T. A. , Rizzo T. R.
Published: July 2002

Infrared–optical double-resonance spectroscopy of the 7νOH vibrational overtone level of NH2OH reveals 14 cm−1 wide spectral features. The product state distribution of the OH fragment subsequent to overtone excitation indicates that the 7νOH level of NH2OH is ∼128 cm−1 above the N–O bond dissociation energy. Comparison to HOOH overtone spectra at a similar excess energy suggests that the broad NH2OH linewidths result from vibrational state mixing at the 7νOH level and not from inhomogeneous structure or lifetime broadening of the dissociating molecules. The observation of 14 cm−1 overtone linewidths for a molecule the size of NH2OH suggests that the broad vibrational overtone transition linewidths in larger polyatomics may contain a substantial homogeneous component.
Electron-stimulated production of NO2(g) from O2 coadsorbed with NO on Pt(111)
Authors: Orlando T. M. , Burns A. R. , Stechel E. B. , Jennison D. R.
Published: July 2002

Using laser resonance-enhanced ionization spectroscopy, we have detected O(3PJ=2,1,0) and NO X 2Π3/2,1/2 (ν=5) above a 6–350 eV electron beam-irradiated Pt(111) surface containing coadsorbed O2 and NO at 90 K. Both product yields have the same chemisorbed NO coverage dependence at saturation O2 precoverage as well as the same ≊10 eV excitation threshold. We conclude that both O(3PJ) and NO(ν=5) are laser-induced photodissociation fragments of NO2(g). This is established by the observation of identical O(3P2) and NO(ν=5) time-of-flight distributions that correspond to NO2 desorption from the surface. The NO2(g) is probably the reaction product of a collision between an O atom (created by electron-stimulated dissociation of adsorbed O2) and NO(a). We correlate the 10 eV NO2 production threshold with the dissociative ionization of the 3σg molecular bonding orbital of O2(a).
Critical fluctuations and relaxation phenomena in the isotopic blend polystyrene/deuteropolystyrene investigated by small angle neutron scattering
Authors: Schwahn D. , Hahn K. , Streib J. , Springer T.
Published: July 2002

For the isotopic blend of polystyrene (PS) PS/deutero PS (M=106 dalton), the Flory–Huggins interaction parameter χ(φ,T) was determined by means of the structure factor S(Q) measured by small angle neutron scattering (SANS). From χ(φ,T), the miscibility gap and the corresponding spinodal line was evaluated with a critical temperature of 130 °C. Decomposition in the miscibility gap is extremely slow due to the critical slowing down and due to a glass transition 30 K below Tc. Furthermore, the transition of the homogeneous blend between two temperatures was investigated by SANS. The interdiffusion constant of the two isotopic species was determined from these experiments. Our results approach the Cahn–Hilliard–Cook theory only in the limit of small wavenumbers of the fluctuations. Three weeks after annealing the sample, demixing was observed in the predicted two-phase region.
Diode laser probing of the low frequency vibrational modes of baths of CO2 and N2O excited by relaxation of highly excited NO2
Authors: Chou James Z. , Hewitt Scott A. , Hershberger John F. , Flynn George W.
Published: July 2002

Quenching of highly excited vibrational states of NO2 in baths of CO2 and N2O has been investigated. Dilute NO2 mixtures were excited by a pulse from an excimer pumped dye laser operating at 495 nm. Various vibrational modes of the bath gases were probed with continuous wave IR diode lasers. Less than 20% of the energy initially placed in the NO2 by the dye laser is taken up by the vibrational degrees of freedom of the CO2 or N2O baths. For N2O, the three different vibrational modes (ν1=1285 cm−1, ν2=589 cm−1, ν3=2223 cm−1) take up almost equal amounts of energy from NO2, but the number of vibrational quanta produced in the bath is found to increase with decreasing vibrational frequency. Similar results are found for CO2 except that the ν1 and ν2 modes cannot be studied separately for this bath gas due to rapid ν1↔ν2 intermode equilibration.
Fourier transform emission spectroscopy: The B 4Σ−–X 4Σ− transition of BC
Authors: Fernando W. T. M. L. , O’Brien L. C. , Bernath P. F.
Published: July 2002

The gas-phase emission spectrum of BC was recorded using a high resolution Fourier transform spectrometer. The BC radical was produced by sputtering in a composite-wall hollow cathode discharge lamp. The Δv=0 vibrational sequence of the B 4Σ−– X 4Σ− transition near 5590 Å was rotationally analyzed. A set of spectroscopic constants were derived for the 0–0, 1–1, 2–2, and 3–3 vibrational bands, including re =1.491 16(34) Å for the X 4Σ− state and re =1.460 23(29) Å for the B 4Σ− state.
The Gibbs equation from maximum entropy
Authors: Nettleton R. E.
Published: July 2002

The maximum entropy formalism maximizes an entropy functional of a phase density ρ(x) subject to conditions that the internal energy and other state variables, such as heat flux Ji and viscous pressure pij, have specified values. By differentiating the entropy thus calculated, one obtains a Gibbs equation. It is shown in the case of a dilute gas that the same Gibbs equation is consistent with the H theorem without additional conditions on the thermodynamic forces. In general, the thermodynamic pressure is not the trace of the momentum flux while the entropy flux, Js, has a nonclassical form. Depending on conditions imposed in maximizing the entropy, these nonclassical features can be eliminated. We find a nonuniqueness which, in fluids not highly dilute, does not appear to affect prediction of observable effects. There is a statistical basis for apparently differing forms of extended thermodynamics.
Classical dynamics studies of the unimolecular decomposition of nitromethane
Authors: Rice Betsy M. , Thompson Donald L.
Published: July 2002

Classical trajectories are used to investigate the unimolecular decomposition of nitromethane on three model potential energy surfaces. The surfaces differ mainly in the barrier height for the isomerization of nitromethane to methyl nitrite. The energies at the barrier to isomerization for the three surfaces are 216.4, 55.1, and 47.6 kcal/mol. Three primary decomposition pathways are observed: CH3NO2 →CH3NO°2 →CH3+NO2, CH3NO°2 →CH3ONO° →CH3O+NO, and CH3NO°2 →CH3ONO° →CH3+NO2. The dynamics results also show that there are two mechanisms for isomerization of nitromethane to methyl nitrite: (a) A two-step process of dissociation of nitromethane to CH3+NO2 and subsequent recombination of these radicals to form methyl nitrite and (b) a one-step process of concerted C–N bond breaking and C–O bond formation. Product energy distributions indicate that the products formed from C–N bond scission in nitromethane and C–O bond scission in methyl nitrite are indistinguishable. The branching ratio for the potential energy surface with the barrier height of 47.6 kcal/mol is close to the experimentally determined value near the threshold energy of the reactions, while the branching ratio for the potential energy surface with a barrier of 55.1 kcal/mol is an order of magnitude smaller than the experimental value. Isomerization via the dissociation–recombination mechanism occurs only on the surface with a large barrier height (216.4 kcal/mol) that prevents direct isomerization. Although isomerization on this surface is rare, the results of trajectories calculated on this surface indicate that this type of isomerization mechanism is feasible, even under collision-free conditions.
The silyl anion (SiH−3): Harmonic vibrational frequencies and infrared intensities predicted at the SCF, CISD, and CCSD levels of theory with substantial basis sets
Authors: Shen Mingzuo , Xie Yaoming , Schaefer Henry F.
Published: July 2002

A b initio quantum mechanical methods have been applied to obtain harmonic vibrational frequencies and infrared intensities for the silyl anion (C3vSiH−3) in the gas phase. The best results in this study agree well with the limited experimental data of Ellison, although new experiments are required for an assessment of our predictions. The theoretical methods employed are the self-consistent field (SCF), configuration interaction with single and double excitations (CISD), and coupled cluster with single and double excitations (CCSD) methods. Basis sets used include double-zeta plus polarization (DZP), triple-zeta plus double polarization (TZ2P), and additional diffuse functions on the silicon atom [DZP+diff(Si), TZ2P+diff(Si)]. The umbrella mode of the silyl anion is most sensitive to basis set quality and electron correlation. Our best predicted vibrational frequency [CISD with TZ2P+diff(Si)] for this particular mode is 890 cm−1. At all levels of theory, the infrared (IR) intensities have been obtained. The planar transition state (D3hSiH−3) has also been studied with respect to its geometry and energy relative to the pyramidal structure.
Critical behavior of the intrinsic viscosity of poly(vinylalcohol) solutions near the gelation point
Authors: Takigawa Toshikazu , Urayama Kenji , Masuda Toshiro
Published: July 2002

The critical behavior of intrinsic viscosity near the gelation point was experimentally examined for poly(vinylalcohol) (PVA) solutions. The constant of the Huggins equation for viscosity (k′) for PVA clusters was higher than that of linear PVA. The intrinsic viscosity ([η]) for PVA clusters was smaller than that of linear PVA at lower polymer concentrations, at which PVA solutions were prepared and cooled. The critical exponent of [η] was found to be 0.20. The weak divergence of [η] resembles the critical behavior of Zimm clusters predicted by the 3D percolation theory, or that of Rouse clusters by the classical theory of the Flory–Stockmeyer type. The critical behavior of [η] of PVA solutions, however, may be considered to be close to Zimm clusters of the 3D percolation theory rather than that of Rouse clusters by the classical theory, because Zimm clusters are more preferable than Rouse clusters for the description of the critical behavior of [η].
Surface temperature dependence of the steric effect in the scattering of oriented t
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t-butyl chloride and fluoroform molecules by graphite(0001)
Authors: Ionov Stanislav I. , LaVilla Michael E. , Mackay R. Scott , Bernstein Richard B.
Published: July 2002

The scattering of focused beams of hexapole-oriented t-BuCl and CHF3 molecules from a graphite (0001) surface has been studied over the surface temperature range 165 < Ts<730 K. The detected number densities of the surface-scattered beams are compared for parallel vs antiparallel incident orientation of the molecular dipole with respect to the surface normal. From the fractional difference in scattered signals, i.e., the so-called steric effect, and from the scattering angular distributions of the unoriented molecules, the relative difference in scattering probabilities for opposite molecular orientations is obtained. For both molecules the magnitude of the relative difference in scattering (the steric effect) increases as Ts decreases. At the lowest surface temperature of 165 K the relative difference in scattering probability reaches +1.5 for t-BuCl (and −0.8 for CHF3), approaching the largest possible magnitude of 2.0, as compared to +0.3 (and −0.4 for CHF3) at the highest temperatures. At all temperatures the steric effect for directly (near-specularly) scattered molecules depends linearly on the degree of molecular orientation. From the angular distribution measurements, it is found that the inelastic translational energy loss in the ‘‘direct’’ scattering process increases as Ts decreases. Finally, the temperature dependence of the scattering probability of unoriented beams of t-BuCl and CHF3 has been obtained.
Association reactions at low pressure. III. The C2H+2/C2H2 system
Authors: Anicich Vincent G. , Sen Atish D. , Huntress Wesley T. , McEwan Murray J.
Published: July 2002

The association reactions, C4H+2+C2H2 and C4H+3+C2H2 have been examined at pressures between 8×10−8 and 1×10−4 Torr at 298 K in an ion cyclotron resonance mass spectrometer. Association occurred via two different mechanisms. At pressures below ∼2×10−6 Torr, the association was bimolecular having rate coefficients k2=2.7×10−10 cm3 s−1 and 2.0×10−10 cm3 s−1 for C4H+2 and C4H+3, respectively. At pressures above ∼2×10−6 Torr, termolecular association was observed with rate coefficients, k3=5.7×10−23 cm6 s−1 and 1.3×10−23 cm6 s−1 for C4H+2 and C4H+3, respectively, when M=C2H2. The termolecular rate constants with N2, Ar, Ne, and He as the third body, M, are also reported. We propose that the low pressure bimolecular association process was the result of radiative stabilization of the complex and the termolecular association process was the result of collisional stabilization. Elementary rate coefficients were obtained and the lifetime of the collision complex was ≥57 μs for (C6H+4)* and ≥18 μs for (C6H+5)*. At pressures below 1×10−6 Torr, ∼11% of the (C6H+4)* were stabilized by photon emission and the remaining ∼89% reverted back to reactants, while ∼24% of the (C6H+5)* were stabilized by photon emission and the remaining ∼76% reverted back to reactants. The ionic products of the C2H+2+C2H2 reaction, C4H+2 and C4H+3, were found to be formed with enough internal energy that they did not react by the radiative association channel until relaxed by several nonreactive collisions with the bath gas.
Theoretical studies of the first- and second-row transition-metal mono- and dicarbonyl positive ions
Authors: Barnes Leslie A. , Rosi Marzio , Bauschlicher Charles W.
Published: July 2002

A b initio calculations have been carried out on the first- and second-row transition-metal mono- and dicarbonyl positive ions. The bonding in these systems is discussed in detail. Trends in the series of mono- and dicarbonyl ions and between the first- and second-row transition metals are explained in terms of a dominantly electrostatic bonding interaction and differences in metal ion state separations, ionization potentials, and s and d orbital sizes. Dissociation energies are presented and a detailed comparison is made with experimental data. Where reliable experimental data exists, agreement with the theoretical results is generally good. An exception is Mo(CO)+1,2, where the computed binding energies are much smaller than the experimental values.
On the electronic structure of electrides
Authors: Rencsok R. , Kaplan T. A. , Harrison J. F.
Published: July 2002

We explore a mechanism for the remarkable charge isolation of the localized, or trapped, electrons found in the crystalline electrides Cs(18C6)2 and Cs(15C5)2. 133Cs NMR measurements show only ≊ 0.05% atomic character of the spin density at the Cs nucleus, consistent with many features of the structure and measured properties which indicate that the localized electron distribution is centered at the anion vacancies. The optical absorption data suggest that the localized electrons, which give rise to the Curie-law spin susceptibility, must penetrate appreciably into the crown ethers, (18C6) and (15C5), which encapsulate the Cs. We suggest that the large reduction of the spin density at the Cs nucleus is due to a Coulomb barrier resulting from negative charge on the oxygens. A crude model, one electron moving in two spherical charged shells surrounding the Cs core, illustrates the mechanism and accounts accurately for the ratio of spin densities at the Cs nucleus found in the 18C6 and the 15C5 electrides. Hartree–Fock calculations for an idealized model of an isolated Cs(18C6)2 molecule, namely Li(9C3)2, tend to support the mechanism.
On the coupling between the intrinsic angular momentum of molecules and the fluid vorticity
Authors: Xu Hong , Ryckaert Jean-Paul
Published: July 2002

Nonequilibrium molecular dynamics simulations of diatomic liquids (modeling nitrogen and chlorine at specific state points) subject to a pure rotational flow of vorticity ωH are considered. After such flow has been switched on at t=0, the transient behavior of the mean angular velocity of the molecules from zero at equilibrium towards its nonequilibrium stationary value ωH is monitored. The relaxation time of this transient experiment is a measure of the intensity of the coupling between the intrinsic (spin) angular momentum of the molecules and the fluid vorticity. Using linear response theory, this relaxation time is shown to be directly related to the vortex viscosity transport coefficient. Numerical estimates of the latter are compared to results obtained previously by other techniques.
Exact classical simulation of hydrogen migration on Ni(100): The role of fluctuations, recrossing, and multiple jumps
Authors: Zhang Zhenyu , Haug Kenneth , Metiu Horia
Published: July 2002

We examine the migration of a classical hydrogen atom adsorbed on a Ni(100) surface, in a temperature range when the motion consists of jumps between lattice sites. We view these jumps as isomerization reactions and calculate exactly their rate constants by using the correlation function theory. We examine in detail the effect of lattice motion, lattice fluctuations and lattice distortion on the jumping rates and test the accuracy of the transition state theory. We propose a new approximation to the rate constant which includes all the effects incorporated in the transition state theory and calculates approximately the dynamic correction due to recrossing. We find that the magnitude of the diffusion coefficient is affected by multiple jumps.
Molecular-dynamics simulation for a model nonadiabatic proton transfer reaction in solution
Authors: Borgis Daniel , Hynes James T.
Published: July 2002

It is shown how a dynamical theory for proton transfer rates in solution can be implemented in a molecular-dynamics simulation for a model reaction system. The reaction is in the nonadiabatic limit, in which the transfer occurs via quantum tunneling of the proton. The importance of the coupling of the proton to the solvent and to an intramolecular vibration is illustrated, and the simulation results are successfully compared with analytic rate-constant expressions in several limiting regimes.
Theoretical vibrational study of FX...NH3 (X=H, D, Li) complexes
Authors: Bouteiller Y. , Latajka Z. , Ratajczak H. , Scheiner S.
Published: July 2002

This paper presents the first ab initio attempt to construct the stretching fundamentals νFX and νF...N (X=H, D, Li) in the FX...NH3 complexes taking into account the mechanical anharmonicity. A potential-energy surface V(rFX,RF...N) grid was generated at the self-consistent-field and second-order Mo/ller–Plesset levels. The coefficients fitting the potential-energy surface up to the fourth order have been used to compute the νFX and νF...N stretching modes. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The results are compared with the available experimental data.
Ornstein–Zernike formalism for reaction rates in random media
Authors: Given James A. , Stell George
Published: July 2002

A useful, general model for the study of diffusion-controlled reactions in random media consists of spherical inclusions of the reactive phase dispersed randomly in a background matrix through which the reagents diffuse. We develop Ornstein–Zernike equations for the material correlation functions of such media. These functions are used both to characterize random media and to determine bounds on the rate of chemical reactions occurring in them. The Ornstein–Zernike equations are solved readily by using standard closures for any degree of correlation among elements of the reactive phase. This allows us to obtain bounds on reaction rates in a large class of random morphologies. In particular, we show that the hypernetted-chain (HNC) closure gives the exact material correlation functions when elements of the reactive phase have uncorrelated positions. On the other hand, the mean-spherical approximation (MSA) gives exact material correlation functions when the reactive phase is dispersed in nonoverlapping inclusions. Our formalism gives the first general method for calculating specific surface and volume fraction in an arbitrarily correlated two-phase medium. The approximations that we have developed for these quantities prove to be essentially exact in the cases for which one has simulation data for comparison and should be highly accurate in the general case.
Communal entropy of bulky molecules. Pressure–volume–temperature relations of triphenylchloromethane
Authors: Naoki Motosuke , Tanioka Toshiya , Karasawa Koji , Kinoshita Akihiro
Published: July 2002

Pressure–volume–temperature relations of the liquid, the supercooled liquid, and the crystal of triphenylchloromethane (TPCM) are presented. The excess entropy of the liquid and glass over the crystal at constant volume has been determined and compared with the previous result for o-terphenyl (o-TP). While the excess entropy of TPCM in the liquid is larger than that of o-TP by about 2R, where R is the gas constant, the communal entropy (defined as the increment of the excess entropy in the liquid over that in the glass) of these molecules is almost the same. This is evidence that the difference of the entropy in the liquid is mainly due to the structural contributions, which depend on individual molecules and are frozen-in by the vitrification. The communal entropy of these asymmetrical, bulky molecules depends on volume and temperature and seems to approach 3R at the zero-density extreme.
Search for a percolation transition in a lattice polymer model
Authors: Tuthill G. F. , Glover D. J.
Published: July 2002

We report the results of Monte Carlo simulation studies of a lattice model, derived from the n=0 vector model, of noncrossing polymers in two and three dimensions. We have searched for evidence of a phase transition characterized by single-chain percolation in the model’s dense phase—the phase in which multiple chains occupy a large fraction of the lattice. Extrapolation methods were used on a series of samples of increasing size in order to probe the large system limit. Our results suggest that in the thermodynamic limit the transition takes place only when the statistical weight for chain ends vanishes.
Photodissociation and charge transfer photodynamics in crystalline krypton doped with F2 and Xe
Authors: Kunttu H. , Sekreta E. , Apkarian V. A.
Published: July 2002

Long-range migration of F atoms upon photodissociation of F2 is demonstrated in mixed F2:Xe:Kr solids by monitoring arrival of F atoms at Xe sites. The impulsive migration probabilities are wavelength dependent. At excess energies above 2.4 eV migration lengths spanning 15 lattice sites are observed. Migration is not observed for excess energies below 1.9 eV. The photodynamics of charge transfer states in F:Xe:Kr solids is presented. Both diatomic Xe+F−, and mixed triatomic (KrXe)+F− exciplexes are observed. The XeF(C) and (KrXe)+F− states are strongly coupled and decay radiatively with a lifetime of 80 ns. These states are populated via excited vibrations of XeF(B) and XeF(D). A crossing between XeF(D) and KrXeF potentials is identified. Above this crossing the lower manifold of charge transfer states are directly populated, while a delay of 10 ns is observed below the crossing. XeF(B, v=0) remains uncoupled from the rest of charge transfer states. Implications with respect to solid state exciplex lasers are discussed.
Reaction rate constants of NO+(a)+NO and Ar++NO measured by an optical detection method
Authors: Ma Guang , Suto Masako , Lee L. C.
Published: July 2002

The NO γ emission is observed from the reaction of NO+(a)+NO. The emission provides a new detection method for studying the NO+(a) reaction kinetics at thermal energy without electric field. The NO+(a) is produced by photoionization of NO at 76.5 nm as well as by the reaction of Ar++NO, where Ar+ is also produced by photoionization of Ar. The vibrational population distributions of NO(A) resulted from the ion–molecule reactions are measured and used to discuss the mechanisms for the production of the emission. The reaction rate constants are determined from the decay rates of the emission intensity as a function of time. The reaction rate constants of NO+(a)+NO and Ar++NO at room temperature are measured to be (5.8±0.7)×10−10 and (2.1±1.0)×10−10 cm3/s, respectively. The reaction rate constant of NO+(a)+Ar at thermal energy is estimated to be about 10−12 cm3/s.
A
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n
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t
i
o study of hydrogen adsorption on Be (0001)
Authors: Marino M. M. , Ermler W. C.
Published: July 2002

Hydrogen adsorption on the (0001) beryllium surface is modeled using a Be45 cluster containing seven layers. The ab initio Hartree–Fock calculations employ effective core potentials and full D3h point group symmetry. Six low-lying electron configurations are investigated and the effect of electron spin coupling on the adsorption process is discussed. The adsorption of H on Be is calculated to be stable by 20 kcal/mol and lowers the work function by 1.3 eV.
A theoretical study of low-lying electronic states of aminonitrene, phosphinonitrene, and phosphinocarbene
Authors: Hoffmann Mark R. , Kuhler Kathleen
Published: July 2002

The recently formulated multiconfiguration-based unitary coupled electron pair approximation (UCEPA) is compared with multireference configuration interaction (MR-CISD) calculations, including all single and double excitations, for the molecules in this study. The electronic states of the molecules in this study are not only of experimental interest, but represent a challenge to any formalism to accurately predict the energy separations of the low-lying electronic states. The equilibrium geometries and fundamental vibrational frequencies of the three lowest electronic states (i.e., 1A1, 3A″, and 1A″) of aminonitrene H2N2, and phosphinonitrene, H2PN, have been determined using a split-valence basis with polarization functions on the heavy atoms and a small complete active space self-consistent-field (CASSCF) description of the active space. Both MR-CISD and UCEPA calculations have been performed at the equilibrium structures using larger basis sets to accurately determine the relative energetics of the electronic states. The equilibrium geometries and vibrational frequencies of the two lowest electronic states (i.e., 1A′ and 3A″) of phosphinocarbene, H2PCH, have been determined using a larger than double zeta basis set, augmented with polarization and diffuse functions, and a CASSCF description of the active space. Both MR-CISD and UCEPA calculations were performed on the equilibrium structures and predict that the singlet lies between 10.4 and 11.8 kcal/mol lower in energy than the triplet. The use of a generalized valence bond (GVB) reference function within UCEPA is introduced and is shown to be a useful approximation.
Velocity autocorrelation function of Lennard-Jones fluids
Authors: Leegwater Jan A.
Published: July 2002

A kinetic equation describing tagged particle motion in a fluid of particles interacting through continuous potentials is used to calculate the velocity autocorrelation function. The kinetic equation takes into account only binary collisions. It is an extension the Lorentz–Boltzmann equation to higher densities, by inserting a factor g(r) in such a way that in the hard sphere limit the Lorentz–Enskog equation is obtained. The velocity autocorrelation function of a fluid consisting of Lennard-Jones particles calculated using this theory compares well with simulation results, even for remarkably high densities. Close to the triple point the theory fails due to the neglect of perturbed binary collisions and correlated collisions. Approximate expressions for the collision operator are given for r−n interactions and a closed expression for the self-diffusion coefficient in terms of the pair distribution function is presented.
Structures, barriers for internal rotation and inversion, vibrational frequencies, and thermodynamic functions of CH2FCHF, CHF2CHF, and CF3CHF radicals: An a
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o study
Authors: Chen Yonghua , Rauk Arvi , Tschuikow-Roux E.
Published: July 2002

The equilibrium geometries, rotational and inversion barriers, and harmonic vibrational frequencies for α, β difluoroethyl (CH2FCHF), α, β, β trifluorethyl (CHF2CHF), and α, β, β, β tetrafluoroethyl (CF3CHF) radicals have been determined by ab initio molecular orbital techniques using the gaussian 86 system of programs at the unrestricted Hartree–Fock (UHF)/6–31G* level of theory. Three conformational minima in the potential energy surface were found for the CH2FCHF, and CHF2CHF radicals, while only one was found for the CF3CHF radical. The radical centers for all three species are nonplanar. The barriers of rotation about the C–C bond and the inversion barriers of the radical center were located for each compound by analytical methods. Vibrational frequencies, as well as moments of inertia for overall and internal rotation are reported for each species. Calculated heat capacities, entropies, and enthalpy and free energy functions are tabulated as a function of temperature. Several isodesmic/homodesmic reactions have been studied for the purpose of obtaining theoretical heats of formation of the three radicals for which experimental values are not available. The theoretical heats of formation (ΔH0f,298 ) thus evaluated are −56.3 kcal/mol, −108.0 kcal/mol, and −164.5 kcal/mol for CH2FCHF, CHF2CHF, and CF3CHF, respectively. These data are used to evaluate ΔH0f,T, ΔG0f,T, and Kf,T for all three radicals as a function of temperature.
O(3P) attack on boranes. II. B5H9
Authors: Cheng H-Z. , Bauer S. H.
Published: July 2002

When B5H9 is injected into a stream of He that is carrying O(3P) atoms (approximately 100/1), at a total pressure of 5–15 Torr, a blue-green flame develops. The major chemiluminescent species is BO(A 2Π). While its translational and rotational temperatures are ≊350 K, the vibrational temperature in the A state is high, ≊3800 K. From among the many products of this reaction, the OH radical can be most easily quantitated by measuring the intensity of its laser-induced fluorescence. The central streamline from a flow-tube reactor was extracted into an evacuated plenum via a pinhole. The time-intensity profile was calibrated using C2H6 for the fuel. Check runs were made with B2H6. A multistep mechanism was developed for B5H9+O(3P) that simulates the shape as well as the magnitude of the OH concentration over a reactor residence time 0.5–10 ms. Less than a dozen crucial reactions were identified by means of an extended sensitivity analysis. Breakdown schemes for the oxidation of B2H6 and B5H9 have been developed.
Spectroscopic studies of Sb3+ color centers in alkali halide single crystals
Authors: Choi K. O. , Lee S. W. , Bae H. K. , Jung S. H. , Chang C. K. , Kang J. G.
Published: July 2002

The emission from KCl:Sb3+ and KI:Sb3+ excited in the A-absorption band was measured as a function of exciting photon energy and temperature. The A-band excitation produced two emission bands for KCl:Sb3+ and a single band for KI:Sb3+. The definitive assignment of these bands is presented in terms of the adiabatic potential energy surface (APES), in which the effect of the spin–orbit interaction (SO) on the Jahn–Teller (JT) interaction coupling to the Eg mode is taken into account. The polarization spectrum and the angular dependence of polarization ratio of the A-band emission were also studied to determine the symmetry axes of the Sb3+ –vacancies complex. The results indicate that the anisotropy is associated with the relaxed excited state (RES) of Sb3+. It is also found that the JT interaction coupling to the T2g mode and the vacancies, situated in the next-nearest-neighbor (nnn) and the nearest-neighbor (nn) positions to the Sb3+ ion, give rise to an additive perturbation.
Model calculations of electronic excitation of Li adsorbed on LiF(001)
Authors: Shima Yaakov , Bowman Joel M.
Published: July 2002

An efficient method to solve the three-dimensional Schrödinger equation of an adsorbed atom on a rigid corrugated surface is presented. Formulas for calculating the Franck–Condon coefficients on two potential surfaces are given. Numerical results are obtained for a model of Li adsorbed on the LiF(001) surface.
A Comment on: ‘‘High temperature deviations from Rice–Ramsperger–Kassel–Marcus unimolecular rate coefficients expressions’’
Authors: Schranz H. W. , Nordholm S.
Published: July 2002

The validity of the traditional solution to the strong collision master equation in thermal unimolecular reactions is discussed in a manner which significantly extends the analysis of Oref and Tardy [J. Chem. Phys. 91, 205 (1989)].
19F nuclear spin relaxation by intermolecular magnetic dipole coupling. CF4 and SiF4 in oxygen gas
Authors: Jameson Cynthia J. , Jameson A. Keith , Hwang Joseph K.
Published: July 2002

The 19F spin relaxation in CF4 in oxygen gas and in SiF4 in oxygen gas has been measured as a function of density, temperature, and magnetic field. The spin–rotation (intramolecular) and the nuclear spin dipole–electron spin dipole interactions (intermolecular) are the dominant mechanisms. The field dependence of the dipolar relaxation rates is characterized for CF4–O2 and SiF4–O2, and compared with those for other spherical top-O2 systems. Agreement of theoretical estimates with the observed field dependence is satisfactory. The temperature dependences of the collision efficiencies for the CF4–O2 and SiF4–O2 intermolecular relaxation have been determined. The magnitudes are found to be roughly three times that for hard spheres.
Excitation of symmetric and asymmetric stretch in a symmetric triatomic molecule: A time-dependent (collinear) analysis of the IHI− photodetachment spectrum
Authors: Engel Volker
Published: July 2002

We present time-dependent quantum calculations for the collinear IHI−→IHI transition, which has been studied before by time-independent methods by Schatz [J. Chem. Phys. 90, 4847 (1990)] and Bowman and Gazdy [J. Phys. Chem. 93, 5129 (1989)]. The mass ratio of the atoms involved determines the time scales of the symmetric stretch on the one hand and the asymmetric stretch on the other. Both modes are excited in the transition complex. We demonstrate how resonances in the spectrum can be related to the motion of the ionic wave function on a potential energy surface for IHI. The dynamical picture is complementary to the one which is obtained from the stationary state calculations. The system under consideration differs in many aspects from other examples of symmetric triatomic molecules. The dynamics corresponding to structures in the spectrum reveals the peculiarities of heavy–light–heavy systems.
The pressure dependence of the thermal diffusion factors of the systems He–n–propane, He–octafluorocyclobutane (C4F8), and CH4–C4F8 at 300 K
Authors: Dunlop Peter J. , Bignell C. M. , Bell T. N.
Published: July 2002

Thermal diffusion factors, αT, are reported for the systems He–nC3H8, He–C4F8, and CH4–C4F8 at 300 K as functions of concentration and pressure; excess second virial coefficients are also reported for each system. The αT values are extrapolated to zero pressure and the theory of Oost, Los, Cauwenbergh, and van Dael is used to predict the pressure dependence of αT at each concentration for comparison with the experimental results. The data for CH4–C4F8 do not agree with the theory.
Direct observation of electric double layers at solution surfaces by means of electron spectroscopy
Authors: Moberg R. , Bökman F. , Bohman O. , Siegbahn H. O. G.
Published: July 2002

Solutions of fatty acid potassium salts in formamide have been investigated using electron spectroscopy in the angular resolved mode. The variable surface sensitivity thus achieved allows details of the electric double layers formed at the solution surfaces to be investigated. The cation distribution is found to vary as a function of solution concentration. The simple diffuse layer theory based on the Poisson–Boltzmann equations is found inadequate in describing the observed features. The data suggest that structural changes occur at the higher concentrations forming closer bound cation states at the surface. These findings qualitatively confirm recent theoretical model predictions by other workers.
Rotation–vibration spectra of icosahedral molecules. III. Rotation energy level spectra for half-integral angular momentum icosahedral molecules
Authors: Harter William G. , Reimer Tyle C.
Published: July 2002

Icosahedral spinor representation theory is developed for analyzing half-integral J eigenlevels of rotational tensor Hamiltonians for molecules such B12H−212,C20H20, and C60. Large half-integral J eigenlevels for general sixth rank icosahedral tensors are discussed using different approximations and quantization schemes. Interpretations are made concerning some of the qualitative and semiquantitative spectral structure that is revealed.
A bifurcation sequence to stationary spatial patterns in a nonuniform chemical model system with equal diffusion coefficients
Authors: Röhricht B. , Horsthemke W.
Published: July 2002

Numerical studies of the Gray–Scott model with equal diffusion coefficients show that a simple kinetic scheme can produce a transition sequence from a simple spatial state to stable stationary structures in an open spatial reactor with external gradients. The system displays multistability between periodic states, diffusion-induced aperiodic states and different steady patterns. The main upshot of the investigation is that external gradients greatly facilitate the self-organization of chemical structures.
Electronic structure and lifetime of an excimer state of HeF
Authors: Petsalakis I. D.
Published: July 2002

Theoretical calculations have been carried out on the few lowest electronic states of HeF in an effort to identify a possible excimer state. The results show that the 2 2Σ+ state is bound with a minimum at 2.0 bohr. The calculated predissociation rates of the υ=0, N=1 level of the 2 2Σ+ state are lower by two orders of magnitude than the rate of the radiative transition 2 2Σ+, which has a vertical transition energy of 9.4 eV.
Quantum-mechanical isomerization of a rotationally and vibrationally excited triatomic model molecule
Authors: Jiang Jun , Hutchinson John S.
Published: July 2002

The effects of rotational excitation on the isomerization dynamics of a model triatomic molecule are analyzed via a quantum-mechanical analysis. The model is chosen to represent approximately hydrogen cyanide undergoing a 1,2 hydrogen shift isomerization. A new method is developed which combines elements of the discrete variable representation–distributed Gaussian basis-function method with a set of parameter-dependent zeroth-order basis functions. Our results indicate that molecular rotation has a significant effect on unimolecular reaction probability, particularly when changes in the rotational quantum number produce or eliminate critical Fermi resonance interactions.
Recalculation of the infrared continuum spectrum of the lowest energy triplet transitions in K2
Authors: Ligare Martin , Edmonds J. Brent
Published: July 2002

The observation and identification of the spectra arising from transitions between the lowest energy triplet electronic states of diatomic potassium molecules were made by Huennekens et al. [J. Chem. Phys. 80, 4794 (1984)]. In this letter we recalculate theoretical spectra for these transitions using quasistatic line broadening theory and the recently published ab initio potential energy curves of Jeung and Ross [J. Phys. B 21, 1473 (1988)]. The calculated satellite of the 3Σ+g–3Σ+u transition occurs at 1.105 μm while the satellite is experimentally observed at 1.096 μm. This improved agreement both solidifies the original identification of Huennekens et al. and indicates the accuracy of the recent potential energy curves of Jeung and Ross for the low energy triplet states.
Methyl rotational excitations in p-xylene: A test of pair interaction potentials
Authors: Prager M. , David W. I. F. , Ibberson R. M.
Published: July 2002

The rotational potential of methyl groups in p-xylene is calculated from transferable pair interaction potentials on the basis of the low temperature crystal structure. Various parametrizations of pair potentials are used. Rotational states in these potentials are calculated and compared to tunneling and librational transitions observed by inelastic neutron scattering. None of the pair interaction potentials can describe the rotational modes correctly. The best results are obtained with Williams IVb parameters. Improvement could be achieved by diminishing the hydrogen–hydrogen interaction strength at short distances.
Theoretical study of the reaction mechanism for the interaction of Si+ with disilane
Authors: Al-Laham Mohammad A. , Raghavachari Krishnan
Published: July 2002

The reaction mechanism for the interaction of Si+ with disilane has been studied by means of accurate ab initio molecular orbital techniques including polarized basis sets, effects of electron correlation, and zero-point corrections. There are two main accessible channels for the reaction, via Si+ insertion into the Si–Si or the Si–H bonds. While both are exothermic and lead to the same products, the Si–Si insertion channel is the lower energy pathway. The insertion is followed by 1,2-H shift and H2 elimination reactions. The reaction leads to the formation of two Si3H+4 isomers, a noncyclic isomer, H3Si–SiH–Si+, and a cyclic isomer, c(HSi–SiH2–SiH)+, with no overall activation barriers. Formation of silane and Si2H+2 is calculated to be significantly exothermic. Other fragmentation channels leading to the ions Si2H+5, Si2H+4, and Si2H+3 are calculated to be endothermic. Our results are in good agreement with the available experimental results. We compare our results to those from previous studies of the interaction of Si+ with silane and methylsilane.