bluuebunny/crossref_metadata_2025 huggingface dataset:
A contribution to the understanding of the structure of xenon hexafluoride
Authors: Crawford T. Daniel , Springer Kristen W. , Schaefer Henry F.
Published: July 2002

Three stationary points of symmetry C3v, C2v, and Oh on the potential energy surface of XeF6 have been located and characterized at the self-consistent field level of theory with a large basis set. At this level of theory, and contrary to results given earlier in the literature, two of these stationary points (C2v and Oh) are determined to be transition states, with harmonic vibrational frequencies leading to the third stationary point (C3v). In addition, second-order Mo/ller–Plesset perturbation theory, configuration interaction, and coupled-cluster energies have been determined at each of these optimized geometries. The C3v structure is predicted to lie lowest, followed by the C2v and then the Oh structure.
A simple and accurate equation of state for two-dimensional hard-body fluids
Authors: Maeso M. J. , Solana J. R.
Published: July 2002

A model relating the equation of state of two-dimensional linear hard-body fluids to the equation of state of the hard disk fluid is derived from the pressure equation in a similar way to that previously described for three-dimensional hard-body fluids. The equation of state reproduces simulation data practically within their accuracy for fluids with a great variety of molecular shapes.
Density-functional methods give accurate vibrational frequencies and spin densities for phenoxyl radical
Authors: Qin Yue , Wheeler Ralph A.
Published: July 2002

The structure, electronic spin density ratios, harmonic vibrational frequencies, and deuterium isotopic vibrational frequency shifts for phenoxyl radical, C6H5Ȯ, were calculated by using spin unrestricted Hartree–Fock (UHF), second-order Mo/ller–Plesset perturbation (UMP2), and various density functional (DF) and hybrid Hartree–Fock/density functional (HF/DF) methods with a 6–31G(d) basis set. Computed results are compared with experiment and with recently published complete active space self-consistent field (CASSCF) and unrestricted natural orbital-complete active space (UNO-CAS) calculations to evaluate the accuracy of DF and HF/DF methods for the highly delocalized, strongly correlated phenoxyl radical. Bond lengths and angles calculated by using DF and HF/DF methods appear as accurate as those obtained by using the UMP2 method. The correlation functional of Vosko, Wilk, and Nusair combined with Slater’s exchange functional (SVWN) gives a structure closest to the best available CASSCF calculations, with the structures from three parameter hybrid HF/DF methods (B3P86 and B3LYP) nearly as good. Electronic spin density ratios computed by using DF methods are closest to experimentally derived spin density ratios, UHF spin densities are the worst, and the hybrid HF/DF methods give intermediate quality spin density ratios. Unscaled vibrational frequencies from the DF and HF/DF methods are closer to experimentally measured frequencies than uniformly scaled UHF or UMP2 frequencies.

Unscaled frequencies calculated by using the B3LYP method, for example, differ from the nine experimentally observed frequencies by an average of only 22 cm−1. The vibrational mode of phenoxyl radical observed experimentally at 1552 cm−1 is confirmed to correspond to a C=C stretching vibration and the band observed at 1505 cm−1 is primarily a C–O stretch. Vibrational modes of phenoxyl radical shift to lower frequencies upon deuteration and the larger shift of the C=C stretch, compared to the C–O stretch, confirms their assignment. Thus density functional methods give electronic spin density ratios, vibrational frequencies, mode assignments, and isotopic frequency shifts in good agreement with experiment and with more expensive UNO-CAS//CASSCF calculations for phenoxyl radical.
Very large zero field splittings in the triplet state of an asymmetric top: Rotational analysis and Zeeman effects in the 820 nm ã 3A2– X̃ 1A1 band system of selenoformaldehyde
Authors: Joo Duck-Lae , Clouthier Dennis J. , Judge R. H. , Moule D. C.
Published: July 2002

High-resolution laser induced phosphorescence spectra of the 820 nm band system of selenoformaldehyde have been recorded with Doppler-limited resolution. Rotational analysis of the 000 bands of D2C80Se, H2C78Se, and H2C80Se revealed only P, Q, and R branches obeying ΔKa=0 selection rules with no evidence of spin splittings. However, these bands show pronounced Zeeman broadening which is linearly dependent on the upper state J value. These results are interpreted as the consequences of case (ab) coupling in the ã 3A2 state, in which there is a large zero field splitting due to substantial mixing of two of the spin components with the nearby 3A1(π,π*) state. The only other asymmetric top molecule known to exhibit case (ab) coupling in the triplet state is carbon disulfide.
The electronic structure of Cr2+,4+ in fluoride host materials
Authors: Pierloot K. , Van Praet E. , Vanquickenborne L. G.
Published: July 2002

The electronic absorption spectra of Cr4+ in Rb2CrF6 and Cr2+ in KCrF3 have been studied using ab initio molecular orbital methods. Near-degeneracy effects within the dn (n=2,4) manifold are treated using the complete active space (CAS) self-consistent field method, while dynamical correlation is dealt with using both the averaged coupled-pair functional (ACPF) method or second-order perturbation theory (CASPT2). The ground state electronic structure of the CrFn−6 (n=2,3,4) clusters in the ionic crystals Rb2CrF6, K2NaCrF6, KCrF3 was analyzed first using Bader’s theory of atoms in molecules. The topological analysis of the charge density is consistent with an ionic picture of the chemical bond in all three compounds, although the contribution of covalent effects clearly increases in the series CrF4−6<CrF3−6<CrF2−6. The ligand field strength exerted by the first coordination sphere of fluorine ligands increases within the same series. The present results indicate that the ligand field strength in the d2 system CrF2−6 has always been underestimated in the past. Indeed, our calculations do not confirm the earlier assignment of a band at 21 000 cm−1 as the 10Dq transition. Both the ligand field strength and the Jahn–Teller splitting of CrF4−6 in KCrF3 are reasonably well reproduced by the calculations. The composition of the excited triplet states was used as a guideline for the analysis of the complex quintet–triplet spectrum of this system.
Rate constant and reaction channels for the reaction of atomic nitrogen with the ethyl radical
Authors: Stief L. J. , Nesbitt F. L. , Payne W. A. , Kuo S. C. , Tao W. , Klemm R. B.
Published: July 2002

The absolute rate constant and primary reaction products have been determined at T=298 K for the atom–radical reaction N(4S)+C2H5 in a discharge flow system with collision-free sampling to a mass spectrometer. The rate constant measurements employed low energy electron impact ionization while the product study used dispersed synchrotron radiation as the photoionization source. The rate constant was determined under pseudo-first-order conditions by monitoring the decay of C2H5 or C2D5 as a function of time in the presence of excess N atoms. The result is k=(1.1±0.3)×10−10 cm3 molecule−1 s−1. For the reaction product experiments using photoionization mass spectrometry, products observed at 114 nm (10.9 eV) were CD3, D2CN and C2D4 for the N+C2D5 reaction. The product identification is based on the unambiguous combination of product m/z values, the shift of the m/z peaks observed for the N+C2D5 reaction products with respect to the N+C2H5 reaction products and the photoionization threshold measured for the major products. The observed products are consistent with the occurrence of the reaction channels D2CN+CD3(2a) and C2D4+ND(2c). Formation of C2D4 product via channel (2c) accounts for approximately 65% of the C2D5 reacted. Most, if not all, of the remaining 35% is probably accounted for by channel (2a). These rate constant and product results are compared with those for the N+CH3 reaction as well as other atom+C2H5 reactions. The role of the N+C2H5 reaction in the formation of HCN in the atmospheres of Titan and Neptune is briefly considered. In addition, the appearance energy for the formation of C2D+3 from C2D5 was determined from photoionization threshold measurements, AE0(C2D+3,C2D5)=239.5 kcal mol−1. From this, values are derived for the zero Kelvin heats of formation of C2D+3 (266 kcal mol−1) and C2D3 (71.6 kcal mol−1).
N2–Kr interaction: A multiproperty analysis
Authors: McCourt Frederick R. W. , ter Horst Marc A. , Jameson Cynthia J.
Published: July 2002

An earlier anisotropic N2–Kr potential energy surface [Chem. Phys. Lett. 88, 197 (1982)] of Lennard-Jones form, obtained by inversion of molecular beam differential scattering data, has been found to provide calculated transport–relaxation phenomena in good agreement with experiment, but leads to calculated second interaction virial coefficients which are significantly too low, and does not provide adequate agreement with recent microwave van der Waals spectral data. A modification of this surface to include a recent ab initio determination of the C6 dispersion coefficient, and to bring in the virial and microwave data, gives a new potential surface that is in good agreement with all available experimental data.
Irreducible dipole components of three interacting H2 molecules and the triple Q1 transition near 12 466 cm−1
Authors: Moraldi Massimo , Frommhold Lothar
Published: July 2002

We have previously obtained parameter-free models of the overlap-induced, irreducible, ternary dipole components of three interacting H2 molecules, which were successful in reproducing existing measurements of the ternary collision-induced absorption in the H2 fundamental band [Phys. Rev. A 49, 4508 (1994)]. Here, we use these models to calculate the intensity of the triple Q1 transition, which has been recently observed in compressed hydrogen gas in absorption near 12 466 cm−1. Simultaneous transitions in three interacting H2 molecules are thought to arise from irreducible, ternary dipole components. We calculate an intensity of the triple Q1 transition in reasonable agreement with the measurement, within a factor of two or three for the three temperatures for which measurements exist. This fact demonstrates once more that the so-called antiparallel dipoles-induced dipole model, which is based on Jansen’s one-effective electron approximation, is capable of describing ternary spectroscopic interactions in semi-quantitative detail. Other ternary, irreducible dipole components are shown to be less significant.
Theory of nematic phases in solutions of rodlike molecules with hydrogen bonds
Authors: Veytsman Boris A.
Published: July 2002

Nematic phases in dilute transitions of rodlike molecules with carboxylic acid functional groups are studied using an Onsager-like theory with the additional term in the free energy describing hydrogen bonding. It is shown that the hydrogen bonds with a proper geometry enhance nematic ordering. The behavior of the system turns out to be completely different in the cases when molecules can form only dimers and when then can form hydrogen-bonded ‘‘polymers.’’ In the latter case the molecules tend to form a completely ordered phase at low temperatures. The numbers of hydrogen bonds in the ordered phase and isotropic liquid are different. Therefore IR spectroscopy can be used for study of these systems.
Interaction of low energy electrons with gaseous and condensed perfluoro compounds
Authors: Weik Fritz , Illenberger Eugen
Published: July 2002

Formation of anions induced by collisions of low energy electrons (0–14 eV) with the perfluorinated compounds CF4, C2F6, C2F4, and C6F6 is studied (a) in the gas phase under collision-free conditions in a beam experiment, and (b) in the condensed phase where the electron beam interacts with the molecules deposited under ultrahigh vacuum conditions in definite amounts on a cold (20 K) metallic substrate. In the gas phase different fragment anions [F−, (M–F)−, CF−3] are formed via dissociative attachment (DA). From condensed molecules, desorption of anions is virtually restricted to F−. This can be explained by the polarization interaction of the excess charge and the desorption kinematics preferring desorption of light fragment ions. Below 10 eV F− desorption is induced by DA at or near the surface while above 10 eV a nonresonant signal from dipolar dissociation is observed. The present results indicate that the desorption cross section is essentially controlled by the amount of translational energy released to F− in the dissociation of the precursor anion (M−). In CF4 the precursor ion CF−4 is assigned as the T2 shape resonance with its pronounced σCF* antibonding nature. This results in F− with high kinetic energy release, and, consequently, a high desorption cross section. In C2F6 the decomposition of the transitory anion is less direct resulting in a comparatively low desorption yield. In C2F4 and C6F6, on the other hand, electron capture proceeds via the π* system associated with indirect (statistical) decomposition processes. This results in a very low desorption cross section for both compounds.
Analytical solution of the proper integral equations for interaction site fluids
Authors: Lue Leo , Blankschtein Daniel
Published: July 2002

We apply the Wiener–Hopf factorization technique to the Chandler–Silbey–Ladanyi equations with the Percus–Yevick closure for a general fluid composed of hard-sphere interaction sites. We analytically solve these equations for symmetric n-atomic tangent hard-sphere molecules (n≤4), and obtain closed analytical expressions for the equation of state and other thermodynamic properties of the fluid.
Hyperbolic tangent variational approximation for interfacial profiles of binary polymer blends
Authors: Lifschitz Masha , Freed Karl F. , Tang Hai
Published: July 2002

Contemporary theories of binary polymer blend interfaces incorporate such features of real polymer blends as compressibility, local correlations, monomer structure, etc. However, these theories require complicated numerical schemes, and their solutions often cannot be interpreted in a physically clear fashion. We develop a variational formalism for computing interfacial properties of binary polymer blends based on a hyperbolic tangent representation for the interfaces. While such an analysis is straightforward in the incompressible limit, the extension to compressible binary blends requires two distinct width parameters and nontrivial analysis. When the profile width parameters are chosen to minimize the excess free energy of a phase separated binary blend, then the interfacial properties computed from our simplified interfacial theory closely match those computed with the much more sophisticated (and computationally intensive) treatments. Significant attention is devoted to describing the interfacial properties of blends in the regime intermediate between the strong and the weak segregation limits as well as to extrapolating between these limits. The extension of the square gradient theory to the Tang–Freed quartic approximation provides a more precise definition of the weak segregation limit, but the treatment is found to overestimate both the interfacial tension and width in the strong segregation limit. The width parameters for the different components of a strongly asymmetric compressible blend vary to a lesser extent than an asymptotic analysis in the bulk suggests. This finding indicates that the central portion of the profile contributes the most in the minimization of the excess free energy with respect to the variational width parameters.
Improved expressions for the radial distribution function of hard spheres
Authors: Tang Yiping , Lu Benjamin C.-Y.
Published: July 2002

The solution of the first-order Ornstein–Zernike equation is applied to improve the Percus–Yevick radial distribution function (RDF) of hard spheres, where the direct correlation function is postulated to hold the Yukawa form outside the hard core. Thermodynamic consistency is imposed to determine the parameters in the postulation. Very simple analytical expressions for the Laplace transform of the RDF are obtained for hard spheres and hard sphere mixtures. The resulting RDFs are compared satisfactorily with computer simulation data.
Microscopic investigations of the single particle dynamics in colloidal crystals
Authors: Bongers J. , Versmold H.
Published: July 2002

In this paper we investigated the single particle dynamics in a colloidal crystal by means of video microscopy and digital imaging methods. To obtain the self term of the van Hove function Gs(x,τ) with a reasonable good statistical accuracy we analyzed up to two million particle displacements per time channel τ of Gs(x,τ), which corresponds to the analysis of 3000 video frames, each with approximately 1000 particles. It was found that the Gaussian approximation almost perfectly accounts for the experimental results, so that the time evolution of Gs(x,τ) can solely be described by the mean square displacement 〈x2(τ)〉. The measured mean square displacements were compared with the model of a Brownian particle bound in a harmonic potential [Physica A 157, 705 (1989)]. The experimental curves approach the plateau value 〈x2(τ→∞)〉 of the mean square displacement much slower than is predicted by the model. From the plateau value of the mean square displacement the Lindemann parameter was determined for various samples.
Molecular dynamics studies of Langmuir monolayers of F(CF2)20F
Authors: Schmidt Mark E. , Shin Seokmin , Rice Stuart A.
Published: July 2002

We report the results of molecular dynamics simulations of Langmuir monolayers of F(CF2)20F using both a united atom model and a recently developed anisotropic united atom model of the molecular force field. A comparison of our simulation results to experimental measures of the structure of Langmuir monolayers and lamellar crystals of F(CF2)20F indicates that the anisotropic united atom model provides the better representation. Simulations using the new model at 275 K generate an azimuthally disordered rotator phase monolayer, while simulations at 150 K generate a monolayer phase with azimuthal order. The relative intensities of peaks in the in-plane structure function can be used to distinguish between the azimuthally ordered and disordered phases. Our simulations also predict, in agreement with experiment, that at large average areas per molecule the monolayer breaks up into islands that have a close-packed structure at the center but are disordered at the periphery.
Resonance enhanced multiphoton ionization spectroscopy of the PF radical
Authors: Howe J. D. , Ashfold M. N. R. , Western C. M. , Hudgens J. W.
Published: July 2002

PF radicals in both their ground (X 3Σ−) and metastable (a 1Δ) electronic states have been produced by the gas phase reaction of F atoms with phosphine in a discharge flow reactor and detected by mass selective resonance enhanced multiphoton ionization (REMPI) spectroscopy in the wavelength range 410–225 nm. Analysis of the longer wavelength end of this spectrum (λ≳328 nm) has enabled identification and spectroscopic characterization of five hitherto unknown Rydberg states of this radical. These we label the h 1Σ+ (T0=57 324.9 cm−1), D 3Δ (T0=58 223.0 cm−1), E 3Σ− (T0=58 690 cm−1), i 1Δ (T0=59 881.6 cm−1), and j 1Σ− (T0=61 873.9 cm−1) states, respectively. [Origins, relative to the lowest rovibrational level of the ground (X 3Σ−) state are indicated by the numbers in brackets.] Quantum defect arguments suggest that all five states derive from the electronic configuration [2Π]4pπ1. Within the shorter wavelength region we identify two additional sets of poorly resolved resonances which we associate with a further two Rydberg states belonging to series converging to the ground state ionization limit, followed by two extensive progressions of bands. Possible assignments for these two progressions are considered.
Rotation of ammonium groups by infrared irradiation far from obvious absorption bands
Authors: Fei Suli , Yu G. S. , Li H. W. , Strauss Herbert L.
Published: July 2002

Direct irradiation of any of the four distinct N–D stretching bands of the ammonium nickel Tutton salt [(NH4)2Ni(H2O)6(SO4)2] that has been doped with a few percent deuterium, produces a persistent hole in that band. The hole and the accompanying antiholes indicate rotation of the ammonium ion. Indirect irradiation at up to a few hundred wave numbers away from the stretching bands—in a region of the spectrum that appears to be featureless background—also rotates the ion, although this requires considerably more power. The most effective indirect irradiation takes place at the positions of the combination bands between the stretches and the librations of the ammonium ions. Other effective irradiation positions are in the tails of the stretching bands or in combinations with other unassigned modes. Sufficient energy is transferred through the vibrational degrees of freedom to the librations without being dissipated to drive the rotation of the ammonium ion.
Atomic and radical densities in a hot filament diamond deposition system
Authors: Childs M. A. , Menningen K. L. , Anderson L. W. , Lawler J. E.
Published: July 2002

The gas phase densities of atomic carbon, atomic hydrogen, and methyl radicals are measured in a hot filament diamond deposition system by the use of absorption spectroscopy in the vacuum ultraviolet. The ratio of atomic to molecular hydrogen is also determined using the measured C and CH3 densities and the equilibrium constant for the hydrogen abstraction and recombination reactions that link the C and CH3 densities. The flux of C to the diamond surface is inadequate to explain the total growth rate, though C could have an important role in initiating growth. A model indicates that the hydrogen is dissociated at the filament and diffuses quickly throughout the rest of the system; gas phase reactions only have a small effect on the degree of dissociation. An increased input CH4 mole fraction results in lower hydrogen dissociation by poisoning the catalytic properties of the filament.
Kinetics of nonstationary, single species, bimolecular, diffusion-influenced irreversible reactions
Authors: Martinez Hernan L.
Published: July 2002

The extension to nonstationary situations of the statistical nonequilibrium thermodynamic theory of diffusion-influenced reactions is used to calculate the kinetics of the single species bimolecular chemical reactions. The method is based on the calculation of coupled dynamic equations for the average concentration and the radial distribution function. In particular, a detailed analysis is performed for the case of the reaction taking place in a one dimensional infinite medium. The single species bimolecular reactions (i.e., annihilation and coagulation) are found to have the same radial distribution function at all times in the low density limit, which implies that these reactions belong to the same spatial universality class under this criterion but not under the nearest-neighbor distance criterion. The rate of reaction depends on just one initial condition: the initial distribution of reactants, via g(r,0). For higher densities, the behavior is not universal and depends on the initial concentration of reactants, falling within the same universality class only if the initial concentration in the coagulation reaction is twice that of the annihilation reaction. This result agrees with what has previously been discussed in the literature using different approaches. The theory is checked against Monte Carlo simulations for the one dimensional case.
Ab initio molecular orbital study of the mechanism of photodissociation of trans-azomethane
Authors: Liu Ruifeng , Cui Qiang , Dunn Kevin M. , Morokuma Keiji
Published: July 2002

The mechanism of photodecomposition of trans-azomethane (CH3–N=N–CH3→2CH3⋅+N2) has been investigated with high level ab initio molecular orbital calculations. Potential surfaces of the low-lying electronic states were explored by state-average complete active space self-consistent-field (sa-CASSCF) and multireference configuration interaction with single and double excitation (MRCISD) methods. The calculated vertical excitation energies for S0→S1 and S0→T1 transitions are in good agreement with experiments. The lowest crossing point between the S0 and S1 surfaces, around which excited molecules would make efficient internal conversion to the ground state, is found to be asymmetrical with a CNNC dihedral angle of 92.8° and two CNN angles of 132.0° and 115.6°, respectively. Transition structures for both simultaneous and sequential C–N bond cleavages on the S0 surface were found. Though the activation energy of sequential C–N bond cleavage is about 7 kcal/mol higher than that of the simultaneous C–N bond cleavage, the Gibbs free energy of activation is lower above 0 °C, indicating that thermal decomposition of trans-azomethane is sequential. Photodissociation is expected to take place sequentially as well. In the sequential mechanism, dissociation of the first C–N bond on the S0 surface takes place endoergically without reverse barrier resulting in CH3N2 intermediate, which should decompose almost immediately over a barrier of less than 1 kcal/mol. Thus, the photodissociation reaction is highly asynchronous but is nearly concerted. This mechanism can explain two seemingly contradictory photodissociation experiments that two methyl radicals have very different translational as well as internal energies and that the velocity vectors of the three fragments are strongly correlated.
Femtosecond excitation dynamics of acetone: Dissociation, ionization, and the evolution of multiply charged elemental species
Authors: Buzza S. A. , Snyder E. M. , Card D. A. , Folmer D. E. , Castleman A. W.
Published: July 2002

Recent femtosecond pump–probe experiments have suggested that a stepwise dissociative mechanism is operative for acetone excited to Rydberg states and upper regions of the mixed singlet/triplet state. The present work focuses on the excitation of acetone and acetone clusters to the 3d (or perhaps 4s) electronic intermediate state in order to further explore the operative dissociation mechanisms and the effects of solvation (clustering). As reported herein, results from femtosecond pump–probe experiments suggest that the availability of additional vibrational modes in clusters, where internal energy may be dispersed, increases the fraction of acetyl intermediates which remain behind the barrier to dissociation into methyl and CO fragments. At progressively higher laser fluences, multiply charged elemental carbon and oxygen ions abruptly appear. Interestingly, the extent of their formation is observed to depend on both laser intensity and the relative time delay between the pump and probe laser beams responsible for their occurrence.
Transitions in two-dimensional patterns in a ferrocyanide–iodate–sulfite reaction
Authors: Li Ge , Ouyang Qi , Swinney Harry L.
Published: July 2002

Transitions in two-dimensional (2D) spatial patterns were investigated in a ferrocyanide–iodate–sulfite (FIS) reaction in a circular thin gel reactor. The state of the gel reactor was maintained by contact of one side of the gel with a continuously refreshed well-stirred reservoir. For long residence times of the chemicals in the reservoir, the gel reactor was in a spatially uniform state of low pH (about 4), while at short reservoir residence times the reactor was in a uniform state of high pH (about 7). At intermediate residence times the spatiotemporal 2D structures observed include a large low pH oscillating spot, small metastable high pH oscillating spots, shrinking rings, spirals that formed when the axisymmetry of shrinking rings was broken, self-replicating spots that either grew and divided or died from overcrowding, and highly irregular, stationary lamellae. Transitions among the different patterns were examined as a function of gel thickness (0.2–0.6 mm), reservoir residence time (0.6–4 min), and ferrocyanide concentration (12–80 mM). Iodate and sulfite concentrations were held fixed at 75.0 and 89.0 mM, respectively. Several transitions were examined in detail: from a stationary spot to an oscillating spot; from an oscillating spot to a shrinking ring or spirals; the onset of replicating spots; and the transition from a homogeneous state to lamellar patterns. The observed phenomena can all be described in terms of a parity-breaking front bifurcation (nonequilibrium Ising-Bloch bifurcation).
A density functional study of small copper clusters: Cun (n⩽5)
Authors: Calaminici P. , Köster A. M. , Russo N. , Salahub D. R.
Published: July 2002

Density functional calculations have been performed for small copper clusters, Cun (n≤5), using the linear combination of Gaussian-type orbitals density functional theory (LCGTO-DFT) approach. The calculations were of the all-electron type and local and nonlocal functionals were used. For each case, of both neutral and charged systems, several isomers have been considered in order to determine the lowest energy structures. The Jahn–Teller effect in Cu3 and Cu4 has been examined in detail. Bond lengths, equilibrium geometries, harmonic frequencies, adiabatic and vertical ionization potentials, adiabatic electron affinities, and binding energies are in reasonable agreement with experimental data, as well as with other theoretical results.
Determination of electroaffinity and ionization potential of conjugated polymers via Fowler–Nordheim tunneling measurements: Theoretical formulation and application to poly(p-phenylene vinylene)
Authors: Roman L. S. , Hümmelgen I. A. , Nart F. C. , Péres L. O. , de Sá E. L.
Published: July 2002

In this paper we present a method to obtain the electroaffinity and the ionization potential of conjugated polymers via Fowler–Nordheim tunneling current measurements. The method allows the measurement of the electroafinity and ionization potential for a sandwich type device, where the barriers at both sides of the polymer are different. The advantage of this method is that barrier height, the electroaffinity, and the ionization potential are obtained independent of the polymer film thickness and the electron (or hole) effective mass. We have used the proposed methodology to obtain the barrier heights and electroaffinity of poly (p-phenylene-vinylene) (PPV) in Metal/Polymer/Metal sandwich structures. For the present case the electrodes (Al/Au, Al/Ga, Al/In, Al/Sn) used lead only to electron and not hole tunneling current, thus only the electroaffinity has been obtained. The experimental value for the electroaffinity of PPV is of 3.87±0.04 eV.
Spectroscopy and predissociation dynamics of the Ã 1A′′ state of HNO
Authors: Pearson Jonathan , Orr-Ewing Andrew J. , Ashfold Michael N. R. , Dixon Richard N.
Published: July 2002

The spectroscopy and predissociation dynamics of the Ã 1A′′ state of HNO have been investigated by measurement of line positions and lifetime broadened linewidths in the cavity ring-down (CRD) spectrum. CRD spectroscopy is a technique better suited to studies of molecular predissociation than methods such as laser induced fluorescence in cases where the excited state dissociation lifetime is short compared to its fluorescence lifetime. The CRD spectrum extends well beyond the dissociation limit (we have identified transitions to rotational states lying up to 2400 cm−1 above the dissociation limit of 16450 cm−1). The lifetime-dependent Lorentzian components of the line shapes of numerous rovibrational features of the Ã 1A′′–X̃ 1A CRD absorption spectrum have been deconvoluted from the Doppler and laser line profiles to obtain lifetimes and predissociation rates for individual |v1v2v3〉|JK〉 states. Here, the labels v1, v2, and v3 denote the number of quanta of the N–H stretch, N=O stretch, and H–N–O bending vibrations, respectively. We have measured line broadening (of up to 0.3 cm−1) in transitions to six vibronic states above the predissociation threshold (the 100 and 020 states, for which the higher K levels are above the dissociation limit, and the 101, 030, 110, and 111 states). For three substates (100 K=5, 101 K=1 and 110 K=4) strongly J-dependent transition linewidths are seen. The 100 K=5 and 101 K=1 substates show maximum transition linewidths midway through the observed spectral transitions while the linewidths for transitions involving the 110 K=4 substate increase with J. Linewidths also generally increase with K. Some lines involving the 100 K=5 state are markedly asymmetric. Linewidths for transitions to states having excitation of the bending mode (the 101 and 111 states) are larger than those for which v3=0. These observations clarify the predissociation mechanism suggested by previous absorption and LIF studies. We attribute the primary predissociation mechanism to a-axis Coriolis coupling of Ã state levels to discrete quasibound highly vibrationally excited levels of the ground state which in turn are coupled to the electronic ground state continuum corresponding to dissociation to H(2S)+NO(X 2Π). Predissociation of Ã state levels with K=0 is probably caused by b-axis Coriolis coupling to such quasibound levels of the ground state. The variation of predissociation rates with J and K for the Ã 110 K=4, 5, and 6 substates cannot be accounted for by this mechanism and we propose the onset of predissociation to the continuum of the ã 3A′′ state. Interpretation of our experimental data is assisted by calculations performed using the potential energy surfaces of Guadagnini et al. [J. Chem. Phys. 102, 774 (1995)].
Classical and approximate quantum investigations of vibrational energy transfer in S1 p-difluorobenzene
Authors: Jordan Meredith J. T. , Clary David C.
Published: July 2002

A simple model potential energy surface is constructed and used in both quasiclassical trajectory calculations and quantum vibrational close-coupling, infinite order sudden approximation calculations of collision-induced vibrational energy transfer from four vibrational states of S1 p-difluorobenzene. Classical and quantum state-to-state cross sections are compared for excitation of the two lowest energy vibrational states and collision with He or Ar. Qualitatively, the same trends are seen in both sets of results. Classical cross sections, however, are significantly larger at very low collision energies as a consequence of the binning procedures used to determine classical final states and, in the case of the Ar collider, as a result of the possible breakdown of the sudden approximation. Rotational excitation of the p-difluorobenzene molecule is also investigated and found to have only small effects on the dominant energy transfer channels. The theoretical results are compared with recent experimental results of Mudjijono and Lawrance [J. Chem. Phys. 104, 7444 (1996)]. The classical results, for the He, Ne, Ar, and Kr collision partners, show good agreement with experiment, reproducing the major energy transfer channels and the experimental collision partner dependence. Quantum results agree well with experiment for the He collider and are also used to assign experimentally ambiguous product states and to investigate vibrational energy transfer channels that are not experimentally observable. The propensity toward the transfer of multiple quanta of vibrational energy is analyzed and, in general, found to increase with the intermolecular well depth and with the mass of the collision partner. The He collision partner, however, behaves anomalously.
On the control of excited state relative populations of enantiomers using circularly polarized pulses of varying durations
Authors: Salam A. , Meath W. J.
Published: July 2002

The possibility of controlling the populations of the left- versus right-handed excited states of enantiomers, such that the excited state populations vary from being equal to 100% left- or right-handed, through the use of pulsed circularly polarized lasers is discussed using a model two-level chiral molecular system.
Dipole and quadrupole plasmon resonances in large sodium clusters observed in scattered light
Authors: Kolwas K. , Demianiuk S. , Kolwas M.
Published: July 2002

We study the dependence of the optical properties of clusters as a function of cluster size. The Mie theory is used to describe intensities of light scattered by growing clusters in directions orthogonal to the direction of the incident light beam. We assigned the maxima in scattered intensities to secondary fields due to plasmons excited in clusters with appropriate radius. The maxima in measured intensities of scattered light in “perpendicular” and “parallel” polarization geometry are attributed to excitation of dipole plasmon when cluster radius approaches 55 nm and of quadrupole plasmon when cluster size approaches 118 nm, respectively.
Dynamic light scattering from binary mixtures of polyelectrolytes. II. Appearance of the medium polyelectrolyte mode upon mixing and comparison with experiments on binary mixtures of neutral polymers
Authors: Sedlák Marián
Published: July 2002

Dynamic light scattering experiments were performed on salt-free aqueous binary mixtures of sodium poly(styrene sulfonate) (NaPSS) with different molecular weights. Three diffusive modes were observed and investigated as a function of the mixture composition, total polymer concentration, and molecular weights of the components of the mixture. In addition to the fast and slow polyelectrolyte modes, which are typical for binary polyelectrolyte solutions, a third new mode, named the “medium mode,” was observed. To investigate the role of electrostatic interactions in the mechanism of this mode, similar experiments were performed on analogical binary mixtures of neutral polystyrenes with different molecular weights in benzene. Polystyrene molecular weights were chosen to match the contour lengths of NaPSS samples used. No “medium mode” was observed in this system. Two diffusive modes corresponding to the cooperative diffusion of component 1 and the cooperative diffusion of component 2, respectively, were measured. Experimental data on polystyrene mixtures were successfully fitted by existing theories on the dynamics of neutral binary mixtures. Regarding charged polyelectrolyte mixtures, it was concluded that the medium mode is of electrostatic origin, and its interpretation was discussed qualitatively within possibilities given by current understanding of salt-free polyelectrolytes and theories of light scattering from multicomponent media.
Molecular switching in one-dimensional finite periodic nonadiabatic tunneling potential systems
Authors: Nanbu Shinkoh , Nakamura Hiroki , Goodman Frank O.
Published: July 2002

A new idea of molecular switching is presented. The idea is based on the intriguing phenomenon of complete reflection, which occurs in a two-state potential curve crossing of the nonadiabatic tunneling type. Complete switching of transmission is theoretically possible in one-dimensional systems by introducing impurities in the system. The basic semiclassical theory is presented, and the phenomenon of complete reflection is clearly interpreted and numerically demonstrated. An idea of an energy filter to facilitate the switching efficiently is also introduced. The possibility of bound states in the continuum is also clarified. This new molecular switching is numerically realized by the wave packet propagation.
Solvated ion dynamics in the water-formamide mixtures using molecular dynamics simulations
Authors: Puhovski Yurii P. , Rode Bernd M.
Published: July 2002

Molecular dynamics computer simulations have been performed for Na+-water-formamide mixtures over the whole concentration range of the mixed solvent. It is shown that the Na+ cation is preferentially solvated by formamide even if only small amounts of formamide are present in the mixture. Specific cation solvent shell formation leads to a drastic change of the ion microdynamics in the mixture compared to pure solvents. A possibility of extension of the molecular theory of limiting ionic mobility to mixed solvents was tested and could not be confirmed by the simulation.
Intrinsic fluctuations and deterministic chemical chaos
Authors: Wang Hongli , Xin Houwen
Published: July 2002

The validity of phenomenological equations describing dissipative processes in chaotic regime was tested in this paper. We carried out a direct stochatic simulation of the chemical Lorenz model and found that in this chemical chaos model the deterministic attractor and Poincaré maps constructed from it differ drastically from their mesoscopic counterparts of stochastic simulation. These results indicated that, in contrast with previous studies of Williamowski–Rössler model, intrinsic fluctuations do have pronounced effect in the chaotic regime in the case of chemical Lorenz. The fact that the effect of intrinsic fluctuations on macroscopic dynamics differ from case to case proposes that the issue whether phenomenological equations are valid or not in the chaotic regime deserves further studies in the future.
The electron affinities of the perfluorocarbons C2Fn, n=1–6
Authors: King Rollin A. , Pettigrew Nathan D. , Schaefer Henry F.
Published: July 2002

Several independent density functional theory (DFT) methods have been employed to determine the molecular structures and total energies of C2Fn and C2Fn− (n=1–6). Three measures of neutral-anion separation are reported: the adiabatic electron affinity, the vertical electron affinity, and the vertical detachment energy of the anion. The first fluorine atom and fluoride ion dissociation energies as well as the harmonic vibrational frequencies of C2Fn and C2Fn− are also reported. Self-consistent Kohn–Sham orbitals were obtained using various exchange correlation functionals and a double-ζ plus polarization basis set augmented with diffuse s-type and p-type functions. Previously observed trends in the prediction of bond lengths by the DFT methods continue for the C2Fn/C2Fn− series. The BHLYP method, based upon the Becke half-and-half exchange functional and the Lee–Yang–Parr correlation functional, continues to predict the shortest and most accurate bond lengths of all of the DFT methods. More generally, the Hartree–Fock/DFT hybrid methods predict shorter and more accurate bond lengths than the pure DFT methods, as expected. As in previous studies, the BHLYP method predicts the lowest electron affinity values which are also those generally closest to available experiments. Agreement to within a few tenths of an electronvolt is typically achieved. The BHLYP adiabatic electron affinities are: C2F, 2.97 eV; C2F2, 1.26 eV; C2F3, 2.25 eV; C2F4, 0.21 eV; C2F5, 1.77 eV; and C2F6, <0.0 eV. Further, C2F6− is found to have a negative vertical detachment energy, while C2F2, C2F4, and C2F6 are predicted to have negative vertical electron affinities.
Total differential cross sections and differential energy loss spectra for He–C2H2 from an ab initio potential
Authors: Heijmen Tino G. A. , Moszynski Robert , Wormer Paul E. S. , van der Avoird Ad , Buck Udo , Ettischer Ingo , Krohne R.
Published: July 2002

State-to-state elastic and rotationally inelastic differential cross sections for He +C2H2 scattering were obtained from an ab initio potential computed by symmetry-adapted perturbation theory (SAPT) by means of converged close-coupling calculations. From these state-resolved data total differential cross sections at Ecm=71.3 meV and energy loss spectra at Ecm=62.0 and 102.9 meV were determined by transformation to the laboratory frame, and accounting for the experimental conditions via a Monte-Carlo averaging procedure. The results are in excellent agreement with experiment [U. Buck et al., J. Chem. Phys. 99, 3494 (1993)], which proves that the SAPT potential is indeed very accurate.
The hydrogen-bonding topologies of indole–(water)n clusters from resonant ion-dip infrared spectroscopy
Authors: Carney Joel R. , Hagemeister Fredrick C. , Zwier Timothy S.
Published: July 2002

A combination of resonant two-photon ionization, infrared-ultraviolet hole burning, and resonant ion-dip infrared spectroscopies are used to assign and selectively probe the hydrogen bonding topologies of indole–(water)n clusters with n=1,2. The indole–(water)1 complex is confirmed to possess the N−H⋯OH2 structure surmised from previous studies. However, the bands in the ultraviolet previously assigned to a π H-bound indole–water complex are shown to be due instead to the indole–(water)2 cluster in which the water dimer forms a H-bonded bridge between the N–H and aromatic π clouds of indole. The implications of this reassignment for our understanding of the influence of H-bonding solvents on indole’s fluorescence properties are discussed.
Chemical diffusivity and wave propagation in surface reactions: Lattice-gas model mimicking CO-oxidation with high CO-mobility
Authors: Tammaro M. , Evans J. W.
Published: July 2002

We analyze the spatiotemporal behavior in a lattice-gas model for the monomer-dimer reaction on surfaces. This model, which mimics catalytic CO-oxidation, includes a mobile monomer adspecies (representing CO), an immobile dissociatively adsorbed dimer species (representing O), and a finite reaction rate (for CO2 production). We characterize in detail the propagation of the chemical wave or reaction front produced when the stable reactive steady-state of the model displaces the metastable CO-poisoned state. In the regime of high CO-mobility, such propagation can be described directly within a “hydrodynamic” reaction-diffusion equation formalism. However, we show that the chemical diffusivity of CO is dependent on the O coverage, reflecting the percolative nature of CO-transport through a background of immobile O. We also emphasize that gradients in the coverage of immobile O induce a diffusive flux in the highly mobile CO. These features significantly influence wave propagation and reaction front structure. In addition, our analysis accounts for the feature that in this hydrodynamic regime, correlations persist in the distribution of adsorbed immobile O, and that these influence the reaction kinetics, the steady states, and the percolation and diffusion properties. To this end, we utilize a “hybrid” approach which incorporates a mean-field reaction-diffusion treatment of adsorbed CO, coupled with a lattice-gas treatment of adsorbed O [Tammaro et al., J. Chem. Phys. 103, 10277 (1995)].
High resolution spectroscopy of BaCH3(X̃ 2A1): Fine and hyperfine structure analysis
Authors: Xin J. , Robinson J. S. , Apponi A. J. , Ziurys L. M.
Published: July 2002

The pure rotational spectrum of BaCH3(X̃ 2A1) in its ground vibrational state has been recorded using millimeter/submillimeter direct absorption techniques, the first spectroscopic information obtained for this molecule. The radical was created using Broida-type oven/d.c. discharge methods by the reaction of barium vapor and Sn(CH3)4. Twenty-eight rotational transitions of the main isotopomer 138BaCH3 were recorded, as well as five for 136BaCH3 and three for the 137BaCH3 species. Being a prolate symmetric top, K ladder structure was observed in all transitions for BaCH3, as well as fine structure splittings which arise from the unpaired electron in the molecule. For the 137Ba isotopomer, hyperfine interactions were also resolved, arising from the spin of the barium nucleus. The complete data set has been analyzed with a 2A Hamiltonian, and rotational, spin-rotational, and magnetic hyperfine/nuclear quadrupole parameters accurately determined. The fine and hyperfine structure constants established from this study suggest a predominantly ionic bond for BaCH3, but with a considerable covalent component. Structural parameters for BaCH3 derived in this work are consistent with those of other alkaline earth monomethyl species.