bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural changes during stress relaxation in simple liquids
Authors: Picu R. C. , Weiner J. H.
Published: July 2002

We investigate the structural changes that accompany the stress relaxation in simple liquids by means of nonequilibrium molecular dynamics (NEMD) simulations. It is shown that during elongational deformation of the system, the statistical distribution of interacting neighbors about a given atom in the system, as measured by the mean interatomic distance in an arbitrary direction, becomes ellipsoidal with the largest semiaxis oriented in the stretch direction. Also, the mean number of particles in the distribution is smaller in that direction than normal to the stretch axis. Therefore, the return to isotropy consists on the one hand of a return of the spatial distribution of neighbors to the spherical shape and, on the other hand, of an isotropization of the number density distribution. These two processes, which take place with widely different rates, are shown to be closely related to the two modes of stress relaxation characteristic for the simple liquids. The first of these, which is the fastest and corresponds to the most significant stress decay, is shown to be related to the fast return of the distribution of neighbors to the spherical shape. The second mode is intimately related to the isotropization of the density distribution on the sphere through a quasidiffusional process.
Density-functional thermochemistry. V. Systematic optimization of exchange-correlation functionals
Authors: Becke Axel D.
Published: July 2002

A systematic procedure for refining gradient corrections in Kohn–Sham exchange-correlation functionals is presented. The procedure is based on least-squares fitting to accurate thermochemical data. In this first application of the method, we use the G2 test set of Pople and co-workers to generate what we believe to be an optimum GGA/exact-exchange density-functional theory (i.e., generalized gradient approximation with mixing of exactly computed exchange).
Theory of desorption induced by electronic transitions. II. The strong interaction regime
Authors: Seideman Tamar , Guo Hua
Published: July 2002

The dynamics of desorption induced by electronic transitions is studied using a recently formulated quantum mechanical method [J. Chem. Phys. 106, 417 (1997)]. We consider two qualitatively different model Hamiltonians representing the limits of desorption induced by a single and by multiple electronic transitions and at the same time the limits of resonance-mediated and direct nuclear dynamics. The photodesorption probability of NH3/Cu induced by low-intensity, nanosecond pulses is dominated by a resonant component and determined by the competition of desorption with vibrational relaxation. The probability is linear in the excitation intensity but highly nonlinear in the nonradiative coupling. The photodesorption of NO/Pd induced by intense, femtosecond pulses involves complex electronic dynamics and its vibrational dynamics is mostly direct. Multiple transitions to an ionic state are shown formally and numerically. These give rise to several thresholds in the time-resolved desorption probability. A power-law fluence dependence is found, consistent with observations.
Effects of chemical impurities on the adsorption of polymer chains from a semidilute solution
Authors: Zajac Richard , Chakrabarti Amitabha
Published: July 2002

Results are presented from a series of lattice Monte Carlo simulations of the adsorption and desorption of polymer chains from semidilute solution onto a solid/liquid interface, where either the chains or the adsorbing surface contain chemical impurities. We compare these results with our previous results for chemically pure systems with regard to the growth, equilibrium structure, and dynamics of the adsorbed layers. Generally, the presence of impurities causes a reduction in the total adsorbance, but does not gravely affect the de Gennes-type self-similar grid found for the pure system. In one case, however, the impurities are found to increase the total adsorbance. We find some correspondence between the static chain characteristics for cases of impurities within the chains and impurities within the surface. There are, however, visible differences between the dynamics of the two cases, particularly in their growth and exchange kinetics. Dramatic changes in behavior are shown to arise when the chain impurities impede the “reeling in” motion of the chains onto the surface, altering the chains’ relaxations. We compare our results with previous theoretical predictions for cases of random copolymer adsorption, and adsorption onto heterogeneous surfaces.
The rotational spectrum and molecular properties of bromine dioxide, OBrO
Authors: Müller Holger S. P. , Miller Charles E. , Cohen Edward A.
Published: July 2002

The rotational spectrum of the OBrO radical has been observed in the gas phase over the solid products of the O+Br2 reaction. Spectra have been measured for both O79BrO and O81BrO in their (000), (010), and (020) vibrational states in selected regions between 88 and 627 GHz spanning the quantum numbers 1⩽N⩽61 and 0⩽Ka⩽14. The spectra are well described by a Hamiltonian which includes centrifugal distortion effects for fine and hyperfine terms. The molecular structure, the dipole moment, and the harmonic force field have been derived, and they, as well as fine and hyperfine structure constants, are compared with data of related molecules and electron spin resonance data from OBrO isolated in cryogenic salt matrices.
Acetonitrile: A critical test case for solvent induced hyperpolarizabilities obtained by the reaction field model
Authors: Norman Patrick , Luo Yi , Ågren Hans
Published: July 2002

The acetonitrile molecule is used as a critical test case for studying solvation effects on static and dynamic hyperpolarizabilities within the reaction field model. With due consideration of a parameter free cavity, local field factors, and inertial contributions, the hyperpolarizability of acetonitrile in solutions can be obtained, with about the same accuracy as in the gas phase.
OH stretch overtone spectroscopy and transition dipole alignment of HOD
Authors: Fair Joanna R. , Votava Ondrej , Nesbitt David J.
Published: July 2002

Photoacoustic spectroscopy in room-temperature H2O/HOD/D2O cells has been used to probe the 3νOH and 4νOH overtone bands in HOD via excitation with a single-mode (160 MHz) injection-seeded optical parametric oscillator (OPO) and pulsed dye laser, respectively. Transitions corresponding to HOD are isolated by comparison between H2O/HOD/D2O and pure H2O overtone spectra and analyzed with a Watson asymmetric top Hamiltonian. Least-squares fits yield the vibrational dependence of Av, Bv, and Cv rotational constants as well as a Birge–Sponer analysis of the vibrational overtone series for up to vOH=4. Relative intensities of a-type and b-type bands are reported for 3νOH and 4νOH which indicate that the transition dipole moment vector tilts away from the OH bond with increasing νOH excitation. This observation is qualitatively inconsistent with simple local-mode, bond–dipole model predictions of the vibrational transition moments. However, the data are in excellent agreement with full 3D quantum variational calculations based on the H2O potential of Polyansky et al. [J. Chem. Phys. 105, 6490 (1996)] and dipole moment surface of Gabriel et al. [J. Chem. Phys. 99, 897 (1993)]. These results highlight the relative importance of electrical versus mechanical anharmonicity effects in preparing highly excited vibrational states of HOD via laser overtone pumping.
The hydrogen atom abstraction reaction CO++H2→HCO++H: Translational and internal energy dependence of the integral cross section
Authors: Knott W. J. , Proch D. , Kompa K. L.
Published: July 2002

The widely accepted model descriptions of ion–molecule reactions suggest identical rate constants for the title reaction and the analogous, isoelectronic system N2++H2, which has been the topic of a previous publication from this laboratory. A comparison of both data sets, however, reveals substantial disagreement which we attribute to the absence of energetic resonances between the reagents which characterized the N2+/H2 system. Resonantly enhanced photoionization was exploited to accomplish the state-specific preparation of CO+ (X 2Σ+, v+=0,1). A monoenergetic beam of vibrationally selected ions, tuned within the range 0.01 eV⩽Ecm⩽10 eV, transits a scattering chamber which confines the neutral, H2. The title reaction was the only channel observed—no evidence of charge transfer or dissociative ionization could be detected. The hydrogen atom transfer turns out to proceed directly. The reactive cross section fails to show the E−1/2 dependence indicative of collision determined processes. The experimental data are perfectly mimicked by an expanded Langevin model which includes additional attractive potential terms. In contrast to the N2+/H2 case, vibrational excitation does not affect the rate of the reaction.
Post-Hartree–Fock study on FOCl+ and FClO+
Authors: Francisco Joseph S.
Published: July 2002

Equilibrium geometries have been optimized and harmonic vibrational frequencies obtained for each of the cationic structures of FOCl+ and FClO+ at various levels of theory, employing second-order Mo/ller–Plesset perturbation interaction theory (MP2), singles and doubles excitation configuration interaction theory (CISD), and coupled-cluster theory with perturbative triple excitations [CCSD(T)]. Of the ions, FClO+ has the lowest energy structure, with an energy separation of 25.7 kcal mol−1. The adiabatic ionization potential for HOCl is used to calibrate the values for FOCl and FClO. From the calibration the probable uncertainty in the adiabatic potentials for the formation of FClO+ and FOCl+ is estimated. The adiabatic ionization potentials for the formation of FClO+ and FOCl+ are estimated as 246.9±3 kcal mol−1 and 267.7±3 kcal mol−1, respectively.
Mode-coupling theory of self-diffusion in diblock copolymers I. General derivation and qualitative predictions
Authors: Guenza Marina , Tang Hai , Schweizer Kenneth S.
Published: July 2002

A microscopic theory of self-diffusion in diblock copolymer melts and solutions has been developed based on polymeric mode-coupling methods formulated at the level of the time and space correlated interchain excluded volume and chi-parameter forces. Equilibrium structural correlations are determined via microscopic liquid state integral equation or coarse-grained field theoretic methods. The specific dynamical consequences of self-assembly are predicted to depend rather sensitively on temperature, degree of polymerization, copolymer composition and concentration, and local block friction coefficients. The dominant physical effect for entangled diblocks is the retardation of the relaxation time of the interchain excluded volume forces due to the thermodynamically-driven segregation of blocks into microdomains, resulting in suppression of translational motion. Analytic analysis in the long chain limit allows the derivation of new scaling laws relating the self-diffusion constant and chain degree of polymerization and solution concentration. Potential limitations for real copolymer materials associated with the structurally and dynamically isotropic description adopted by the theory are discussed.
The role of a molecular phase in two-pathway excitation schemes
Authors: Seideman Tamar
Published: July 2002

The origin and potential implications of a molecular phase in coherent two-pathway excitation are analyzed by means of a time-independent, quantum mechanical theory. A unified model is developed describing unimolecular decay into several continua of general nature, where each breakup reaction takes place both directly and via a manifold of resonances. It is found that the molecular phase, observable in two pathway excitation experiments, correlates with the single-pathway fragmentation line shape, in agreement with recent measurements. We propose the measurement of molecular phases as a route to molecular properties which are silent in conventional observables.
Vibrational state control of bimolecular reactions
Authors: Kreher Christoph , Rinnenthal Jan Leo , Gericke Karl-Heinz
Published: July 2002

The influence of rotation and vibration on the reactivity and the dynamics of the reaction X+HCN(ν1,ν2,ν3,J)→HX+CN(v,J) with X=H, Cl has been studied. The HCN molecule is prepared in a specific rovibrational level by IR/VIS overtone excitation in the wavelength region 6500–18 000 cm−1. The H atoms are generated by laser photolysis of CH3SH at 266 nm, the Cl atoms are formed in the photodissociation of Cl2 at 355 nm. The CN products are probed quantum state specifically by laser-induced fluorescence (LIF). For low rotational states of HCN, the reactivity of Cl and H is independent of the initial rotational state. However, an enhancement in reactivity of the Cl+HCN reaction is observed when the time of rotation becomes comparable to the passing time of the Cl atom. The reaction of Cl as well as of the H atom with HCN shows strong mode specific behavior, implying a simple direct reaction mechanism, which is also supported from Rice–Ramsperger–Kassel–Marcus (RRKM) calculations. An increase in CH stretch vibration increases both the reaction rate and the CN product vibration. Channeling energy in CN stretch vibration has only a minor effect on the reactivity and the CN product vibration even decreases. Trajectory calculations of the H+HCN system agree with the experimental results. The dependence of reaction rates on reactant approach geometry is investigated by preparing aligned reactants using linear polarized light. The CN signal is markedly influenced by the prepared alignments (steric effect). The experimental results suggest that the reaction of hydrogen and chlorine atoms with vibrationally excited HCN proceeds mainly via a collinear transition state, but the cone of acceptance is larger for chlorine atoms.
Comparative studies of the lowest singlet states of (O2)2 including ab initio calculations of the four excited states dissociating into O2(1Δg)+O2(1Δg)
Authors: Bussery-Honvault B. , Veyret V.
Published: July 2002

Ab initio calculations have been performed for the four singlet states of (O2)2 dissociating into O2(1Δg)+O2(1Δg). They are compared with previous semi-ab initio perturbation calculations. A surprisingly good agreement in the potential behavior is observed between these two complementary treatments. After the recent work of Minaev et al., these are the first ab initio calculations of the excited states of the dimer, corrected for basis set superposition errors (BSSE) and evaluated in a size consistent way. Though a similar behavior of the potential with the intermolecular distance has been observed by Minaev et al., discrepancies remain in the binding energies of the dimer.
On the calculation of thermodynamic properties of electrolyte solutions from Kirkwood–Buff theory
Authors: Behera Raghunath
Published: July 2002

Earlier workers discussed a methodological difficulty of using Kirkwood–Buff theory for ionic solutions, arising from the non-invertibility of the matrix ∂ρ/∂μ for ionic species. We point out that there exists no such difficulty if the method of generalized inverse is used. It is found that the problem of non-uniqueness of the generalized inverse does not affect the thermodynamic properties, and is related to our inability to split the chemical potential of salt into that of its constituent ions, uniquely.
The kinkiness of cumulenones: H2C3O, H2C4O, and H2C5O
Authors: East Allan L. L.
Published: July 2002

The floppiest bending or “kinking” modes of propadienone (H2CCCO), butatrienone (H2CCCCO), and pentatetrenone (H2CCCCCO) are investigated with high-quality ab initio methods. The potential surface for bending at the β-carbon atoms of these species is very flat for the C4 and C5 species, and together with zero-point energy contributions from the other vibrations, it is concluded that the barriers to linearity of their carbon chains are less than 1 kJ mol−1, if they exist at all. Comparisons are made to published experimental values for rotational constants and floppy-mode barrier heights (from gas-phase microwave spectra) and vibrational fundamentals (from matrix infrared spectra). Consensus is achieved for propadienone and butatrienone, and the arguments for infrared detection of synthesized pentatetrenone in an argon matrix are strongly supported by our calculations.
Development of a potential function for describing the properties of HCN clusters
Authors: Cabaleiro-Lago Enrique M. , Ríos Miguel A.
Published: July 2002

We developed potential functions for (HCN)2 and used them to study larger aggregates. The functions are based on ab initio perturbation calculations for the dimer and on molecular properties also estimated from ab initio calculations involving two different basis sets. The overall energy is resolved into electrostatic, repulsion, induction, and dispersion components. The electrostatic contribution is represented by a multipole expansion at several sites. The repulsion function is of the exponential type and includes atomic anisotropy. The dispersion is described by means of a London-type formula. Finally, the induction contribution is expressed in terms on polarizabilities distributed among the atoms. The proposed functions accurately reproduce the characteristics of the HCN dimer and the two minimum-energy forms of the trimer. Also, they predict various properties (e.g., the dependence of the dipole moment and interaction energy on cluster size) of larger aggregates.
Variety of wetting behaviors at the free surface of isotropic liquid crystals
Authors: Lucht R. , Bahr Ch. , Heppke G. , Goodby J. W.
Published: July 2002

We present an experimental study of the wetting behavior at the free surface of the isotropic phase of various liquid-crystal compounds. For the first time we observe a prewetting transition above a bulk nematic—isotropic transition. Above smectic-A–isotropic transitions of nonpolar compounds, we observe complete wetting, partial wetting with pretransitional increase of surface order, and partial or nonwetting without any pretransitional increase. Above the smectic-A–isotropic transition of a polar (cyano-substituted) compound, a series of single layering transitions occurs at the surface which has already been observed by x-ray reflectivity studies. These layering transitions are not observed in the nonpolar smectic compounds.
Phase diagram of a lyotropic mixture sodium bis (2 ethylhexyl) sulfosuccinate/dodecanol/water: Reverse micellar, cylindrical, lamellar, and sponge phases
Authors: Magalhães M. , Pusiol D. , Ramia M. E. , Figueiredo Neto A. M.
Published: July 2002

A combined study of flow-induced optical birefringence, x-ray diffraction, optical observations, electrical measurements and 23Na NMR is reported with the aim to characterize the phase diagram of the ternary lyotropic mixture of sodium bis (2 ethylhexyl) sulfosuccinate, dodecanol, and water. One of the most interesting features of this phase diagram is the existence of a bicontinuous structure associated to an inverted sponge phase in the amphiphilic-rich region of it. This phase is surrounded by a micellar isotropic inverted phase. Besides these phases, a lamellar and a cylindrical (triclinic) phase were observed in the phase diagram.
Far-infrared and combination-band spectra of the ring-puckering and ring-flapping vibrations of phthalan: A failure of the one-dimensional model
Authors: Klots Timothy , Sakurai Sachie , Laane Jaan
Published: July 2002

The far-infrared spectra of phthalan showing bands resulting from the ring-puckering and ring-flapping vibrations have been recorded and analyzed. The observation and assignment of the puckering fundamental at 30.9 cm−1 and of numerous hot-band, double quantum jump, and triple quantum jump transitions have provided the basis for generating a detailed and self-consistent energy map for the vibrational states. In addition, sum, difference, and hot bands associated with the C–O symmetric stretching mode also provide confirmation of the assignments. Previous far-infrared, electronic, and dispersed fluorescence spectra assignments are shown to be erroneous whereas a previous microwave study is consistent with the present results. The ring-puckering energy spacings show considerable irregularities and these can be ascribed to the puckering/flapping interaction. Consequently, a one-dimensional potential energy function can only do a mediocre job of fitting the observed vibrational data.
Structural transitions in metal ion-doped noble gas clusters: Experiments and molecular dynamics simulations
Authors: Prekas Dimitris , Lüder Christian , Velegrakis Michalis
Published: July 2002

We present the size distributions of metal ion-doped noble gas clusters of the form M+Xn (M=Mg, K and X=Ar, Kr, Xe) studied with time-of-flight mass spectrometry. All the recorded spectra exhibit magic number patterns, which change gradually from the familiar icosahedral sequence N=n+1=13,19,23,26,29,32 to another one that exhibits the magic numbers N=9,10,11,17,21,24,26,27,30, as the atomic size ratio of the metal ion to the noble gas atom decreases. Furthermore, as the cluster size N increases, the new sequence seems to convert again to the icosahedral one at some critical cluster size. Molecular dynamics simulations using pairwise additive Lennard-Jones potentials are performed in order to investigate the stability and the geometrical structure of these systems as a function of radii ratio, interaction energy, and cluster size. The results obtained are in very good agreement with the experimental ones and indicate that when the size of the dopant is comparable to that of the noble gas atoms then the clusters exhibit icosahedral geometries, while for smaller ratios, clusters having a geometry based on a capped square antiprism (CSA) are more stable.
Determination of alkyl chain tilt angles in Langmuir monolayers: A comparison of Brewster angle autocorrelation spectroscopy and x-ray diffraction
Authors: Lautz C. , Fischer Th. M. , Weygand M. , Lösche M. , Howes P. B. , Kjaer K.
Published: July 2002

Brewster angle autocorrelation spectroscopy (BAAS), a combination of Brewster angle microscopy and the photocorrelation technique is a new method for the quantitative characterization of Langmuir monolayers. In this paper we compare tilt angle measurements in octadecanol monolayers performed using BAAS and grazing incidence x-ray diffraction (GIXD). BAAS offers the possibility of tilt angle measurements in Langmuir monolayers—without any fitted parameters—with a precision similar to GIXD, but ten times faster. GIXD, conversely, offers detailed microscopic information not available from BAAS. The swiftness of BAAS is exploited for a detailed analysis of the phase transitions from the L2′ phase to the LS (Rotator I) and LS (Rotator II) phases. The L2′/LS (Rot I) transition is of first order whereas the L2′/LS (Rot II) transition is more probably of weak first order than of second order.
An association theory for the formation of ion oligomers: Extensions from the low density limit
Authors: Kraska Thomas
Published: July 2002

A recently published ion association theory which includes the formation of trimers and tetramers has been further developed by accounting for density-dependent properties. This theory is developed in the framework of a primitive model and based on the completely dissociated reference system. The association constants of the equilibria between the ions and the clusters are related to an integral over the pair distribution function of the ionic fluid by an association balance. In order to calculate the fraction of trimers and larger ion clusters use has been made of the Kirkwood superposition approximation. For all calculations the dielectric constant of the pure solvent has been employed. In this paper an extension of the theory has been accomplished by replacing properties of the low density limit by density-dependent properties. The exponential low density pair correlation function has been replaced by the analytically available general mean spherical approximation expression. The covolumes of the clusters have been approximated with a perturbation approach for an effective Bjerrum distance. The ion clusters are modeled as hard spherocylinders. For the charged clusters the Coulomb interaction is described with the mean spherical approximation expression. Finally, the model is applied to some 1–1 electrolyte solutions in water. In the applications the dielectric constant of pure water has been used at a given temperature.
State-to-state vibrational energy transfer in S1 p-difluorobenzene at intermediate state densities: A change in propensity rules
Authors: Mudjijono  , Lawrance Warren D.
Published: July 2002

Collision-induced vibrational energy transfer has been studied from two vibrational levels at intermediate state density in S1 p-difluorobenzene in a supersonic free jet expansion at ∼30–40 K. Transfer was studied from the 7181 (Evib=751 cm−1) and 84 (Evib=733 cm−1) states where ρvib is ∼0.4 states per cm−1. Data were obtained for He, Ne, H2, and D2 for both levels and also for Ar and Kr for 7181. There is some doubt concerning the influence of predissociation of van der Waals complexes on the spectra for these latter partners. The data analysis for 7181 is quantitative for all collision partners. For 84 the analysis is quantitative for H2 and D2 but qualitative for He and Ne because of poor signal levels. The state-to-state propensity rules in this region of the vibrational manifold are compared with those observed at lower state densities, particularly those from 82. The main feature to emerge is a lack of predictability of the major relaxation pathways. There is a clear increase in the importance of transfers involving multiple changes in vibrational quanta for all situations studied and at times such transfers totally dominate. This occurs in spite of the possibility for loss of one quantum of ν8, which is a very efficient channel in transfer from 82. Collision partners that show similar state-to-state branching ratios for 82 show quite different branching ratios for 7181 and for 84.
Proton and deuteron spin-lattice relaxation times in HD–Ar mixtures: A combined experimental and theoretical study
Authors: Sabzyan Hassan , Power William P. , McCourt Frederick R. W.
Published: July 2002

A combined experimental and theoretical study of the nuclear magnetic resonance (NMR) spin-lattice relaxation times for the proton and deuteron of HD in HD–Ar mixtures is presented. Spin-lattice relaxation times for the proton and deuteron of HD in HD–Ar mixtures have been measured over the temperature range 180–420 K at several densities and mole fractions, and extrapolation to infinite dilution has been carried out. Theoretical values of the spin-lattice relaxation times associated with the HD–Ar interaction have been calculated using the XC(fit) potential energy surface obtained by Bissonnette et al. [J. Chem. Phys. 105, 2639 (1996)], transformed to allow displacement of the center-of-mass of the HD molecule from its center-of-force. Both experimental and theoretical results show that the density-dependence of the deuteron relaxation times lies in the linear regime, while that of the proton lies in the non-linear regime. The experimental and theoretical results for the relaxation times of the proton are in excellent agreement. The corresponding results for the deuteron are in good agreement (within a few percent); the agreement is, however, not as good as it is for the proton. These results indicate that the transformed XC(fit) potential energy surface represents the anisotropic part of the HD–Ar interaction rather accurately. It is argued that the improved quality of the XC(fit) potential energy surface for the heteronuclear HD–Ar interaction is due to the large contribution that the isotropic part of the homonuclear potential surface makes to the anisotropic part of the heteronuclear potential surface.
High-resolution threshold photoelectron spectra of molecular oxygen in the 18–24 eV region
Authors: Tanaka T. , Yoshii H. , Morioka Y. , Hayaishi T. , Ito K. , Hall R. I.
Published: July 2002

A high-resolution threshold photoelectron spectrum of molecular oxygen has been studied between 18 and 24 eV using the penetrating field technique and a synchrotron radiation source. Nine ionic states are observed, and three of them for the first time. Higher vibrational levels of the b 4Σg− state and the 2Φu state are also studied. Furthermore, the doublet structure of the 2Φu state is resolved for the first time. Two new states with quartet multiplicity are assigned to the 4Πg and the 4Πu states which dissociate to the limit, [O(3P)+O+(2D)], at 22.059 eV. Also we tentatively assign another newly observed band to the 2Πu state. The long vibrational progression of the 3 2Πu state is clearly observed in the region 22.3–23.8 eV with doublet splitting which increases with increasing of the vibrational quantum number.
Dispersion solute-solvent coupling in electron transfer reactions. I. Effective potential
Authors: Matyushov Dmitry V. , Ladanyi Branka M.
Published: July 2002

Theories of electron transfer (ET) reactions and optical spectra in condensed phases consider electronic transitions between instantaneous Born-Oppenheimer energies of the intramolecular electronic states which depend on the system nuclear configuration. With the aim of constructing a molecular description of the solvent effect on these phenomena, we consider in the present paper a system composed of a polar polarizable solute immersed in a solvent of polar polarizable molecules. The instantaneous free energies are defined in terms of partial partition functions obtained by averaging over the electronic degrees of freedom of the solute and the solvent. Electronic polarizabilities of the solvent molecules are modelled as quantum Drude oscillators. For the solute, two models are considered: (i) the Drude oscillator and (ii) the two-state solute. The former enables us to derive the solute-solvent dispersion potential with account for the effects of nonlocal polarizability coupling in the solvent and the many-body solute-solvent dispersion contributions. These effects are analyzed using equilibrium theories of nonpolar liquids. The two-state description of the solute involves redistribution of the electron density between the two localized sites. The instantaneous adiabatic (in contrast to diabatic in the Drude oscillator model) free energy can be derived in this case under the only restriction of the quantum character of the solvent electronic excitations. It leads to the ET matrix element renormalized from its vacuum value due to the equilibrium field of the electronic solvent polarization and the instantaneous field of the permanent solvent dipoles. The theory predicts some useful relations which can be applied to treating the solvent effect on transition moments of optical spectra. The equilibrium ET matrix element is found to depend on the orientation of the solute diabatic transition dipole in the solute molecular frame and the spectral shift due to solvation by permanent and induced dipoles. This offers an interesting phenomenon of self-localization of the transferred electron (zero ET matrix element). Finally, the comparison of two derivations performed enables us to write down the diabatic instantaneous free energies which can be used for a molecular formulation of the effect of the solvent and the solute energy gap on ET rates.
Long-range van der Waals forces between alkali clusters and atoms
Authors: Kresin V. V. , Tikhonov G. , Kasperovich V. , Wong K. , Brockhaus P.
Published: July 2002

We study the long-range attractive force between neutral sodium clusters Nan (2⩽n⩽20) and the alkali atoms Li, Na, and K. Absolute cross sections for the scattering of a beam of clusters by atomic vapor are measured, and are shown to arise from the van der Waals dispersive interaction V=−C6/r6. These cross sections are extremely large (up to thousands of Å2 in the center-of-mass frame). Their magnitudes are in good agreement with predictions based on the London theory of dispersion forces; the large strength of the interaction is a consequence of the high cluster polarizabilities. In addition, we evaluate the contribution of the higher-order potential term −C8/r8 to the scattering cross section and show that it can become quite significant for collisions involving large clusters (n∼102–103).
Vibronic structure in triatomic molecules: The hydrocarbon flame bands of the formyl radical (HCO). A theoretical study
Authors: Serrano-Andrés Luis , Forsberg Niclas , Malmqvist Per-Åke
Published: July 2002

A theoretical study of the vibrational structure of the X̃ 2A′ ground and B̃ 2A′ excited states of the formyl radical, HCO, and its deuterated form, DCO, has been performed. The potential energy surfaces have been computed by means of a multiconfigurational perturbative method, CASPT2. The computed geometries and the harmonic and anharmonic frequencies are successfully compared to the available experimental information. The vibrational intensities of the transition B̃ 2A′↔X̃ 2A′ have been computed both for absorption and emission. The results lead to accurate determinations of several structural parameters and some reassignments of the vibrational transitions of the so-called hydrocarbon flame bands of the formyl radical.
Cavity formation free energies for rigid chains in hard sphere fluids
Authors: Stamatopoulou Argyroula , Ben-Amotz Dor
Published: July 2002

Excluded-volume-anisotropy (EVA) model predictions for the cavity formation free energies of rigid linear polyatomic chains dissolved in hard sphere fluids are tested against Monte Carlo Widom insertion simulation measurements performed as a function of chain length (1⩽N⩽6), the ratio of the chain bead diameter to the solvent diameter (0⩽σ/σS⩽3), and solvent density (0.1⩽ρσS3⩽0.8). The results reveal a linear dependence of cavity formation energy on chain length for N⩾2. This allows extrapolation to chain lengths larger than can be measured by direct insertion. EVA predictions are found to be in good agreement with direct simulation results as well as long chain length extrapolations (up to N=50). As an illustration of potential practical application of these results, the EVA model is used to predict the cavity formation free energy of n-hexane dissolved in water and in the pure n-hexane liquid as a function of temperature and pressure, throughout the liquid temperature range.
The millimeter wave spectrum of silver monoxide, AgO
Authors: Steimle T. , Tanimoto M. , Namiki K. , Saito S.
Published: July 2002

The pure rotational spectra of AgO107 and AgO109 were recorded in the 117–380 GHz spectral region using a dc-sputtering absorption cell. The Ag107(I=1/2) and Ag109(I=1/2) magnetic hyperfine parameters are interpreted in terms of plausible electronic configuration contributions to the X 2Πi state. It is shown that the determined unusual sign of the Λ-doubling and Fermi contact parameters implies that the X 2Πi state is dominated by a three open shell configuration. A comparison with isovalent CuO is made.
Non-Abelian point group symmetry in direct second-order many-body perturbation theory calculations of NMR chemical shifts
Authors: Kollwitz Markus , Häser Marco , Gauss Jürgen
Published: July 2002

A scheme that fully exploits molecular point group symmetry in direct second-order many-body perturbation theory chemical shift calculations based on gauge-including atomic orbitals is presented and implemented. Representative calculations for the molecules B4(tBu)4 (Td symmetry) and [AlCp]4 (D2d symmetry) involving more than 600 basis functions demonstrate the applicability of the developed program to large symmetric molecules (which could be otherwise not treated) and indicate the importance of electron correlation effects for the reliable prediction of NMR chemical shifts in larger molecules.
Submillimeter-wave spectral lines of negative ions (SH− and SD−) identified by their Doppler shift
Authors: Civiš S. , Walters A. , Tretyakov M. Yu , Bailleux S. , Bogey M.
Published: July 2002

For the first time submillimeter-wave transitions of an anion have been firmly identified. We observed one transition of SH− (J=1←0, 564 421.869 MHz) and two of SD− (J=1←0, 292 359.129 MHz; J=2←1, 584 629.519 MHz) created in the positive column of an electric discharge set up in a mixture of H2S and argon. By studying the Doppler shift brought about by the motion of charged species in the electric field of the discharge we were able to distinguish between positively and negatively charged ions and neutrals. We have hence shown that velocity modulation techniques commonly used for infrared spectroscopy could also be successfully applied for the study of ions in the submillimeter-wave region.
The ultrafast photochemical ring-opening reaction of 1,3-cyclohexadiene in cyclohexane
Authors: Pullen Stuart H. , Anderson Neil A. , Walker Larry A. , Sension Roseanne J.
Published: July 2002

The ring-opening reaction of 1,3-cyclohexadiene in cyclohexane solution and the subsequent photoproduct cooling dynamics have been investigated by using two-color transient absorption kinetic measurements and novel time-resolved absorption spectroscopy in the 260–300 nm spectral region. The initial photoproduct in this reaction, s-cis,Z,s-cis-1,3,5-hexatriene (cZc-HT) is formed on a ∼250 fs time scale. Spectra deduced for time delays very close to zero, as well as calculated Rice–Ramsperger–Kassel–Marcus unimolecular reaction rates, provide strong evidence that the quantum yield for the reaction is determined before any relaxation occurs on the ground state. Upon formation, the vibrationally excited hexatriene photoproduct is able to isomerize around C–C single bonds freely. As a result, the evolution observed in the transient absorption measurements represents a combination of rotamer population dynamics and thermalization due to energy transfer to the solvent. Three distinct time scales for relaxation are observed. These time scales correspond approximately to the development of an evolving equilibrium of Z-HT rotamers (1–5 ps), vibrational cooling and thermal equilibration with the surroundings (10–20 ps), and activated isomerization of trapped cZt-HT to tZt-HT (≫100 ps).
The exact quantum mechanical kinetic energy operator in internal coordinates for vibration of a hexatomic molecule
Authors: Rempe Susan B. , Watts R. O.
Published: July 2002

The full nonrelativistic quantum mechanical vibrational J=0 kinetic energy operator for a hexatomic molecule, (A,B)-C-D-(E,F), expressed in valence stretch, bend, and torsion internal coordinates is given explicitly. The derivation of the operator is reviewed and some of its properties and possible applications discussed.
Al
3
O
y
(y=0–5) clusters: Sequential oxidation, metal-to-oxide transformation, and photoisomerization
Authors: Wu Hongbin , Li Xi , Wang Xue-Bin , Ding Chuan-Fan , Wang Lai-Sheng
Published: July 2002

Photoelectron spectra of a series of Al3Oy− clusters (y=0–5) are presented at several photon energies: 532, 355, 266, and 193 nm. The electron affinities and low-lying electronic states of the Al3Oy clusters are reported. The photoelectron spectra clearly reveal a sequential oxidation behavior and how the electronic structure of the clusters evolves from that of a metal cluster at Al3 to that of a complete oxide cluster at Al3O5: Two valence electrons of Al3 are observed to be transferred to each additional O atom until Al3O5, where all the nine valence electrons of Al3 are transferred to the five O atoms. The anion, Al3O5−, which can be viewed as (Al3+)3(O2−)5, is found to be a closed shell cluster, yielding an extremely high electron affinity for Al3O5 (4.92 eV). The electron affinities of the remaining clusters are: 1.90 (Al3), 1.57 eV (Al3O), 2.18 eV (Al3O2), 2.80 eV (Al3O3), and 3.58 eV (Al3O4). An electronic excited state of Al3− is also observed at 0.40 eV above the Al3− ground state. Isomers are observed for all the oxide clusters with lower electron affinities. Particularly, vibrational structures are observed for the two isomers of Al3O3−, as well as a photoisomerization process between the two isomers. The structure and bonding of the oxide clusters are discussed based on the experimental data and the known structures for Al3 and Al3O.
A nonequilibrium statistical grand-canonical ensemble: Description in terms of flux operators
Authors: Madureira Justino R. , Vasconcellos Áurea R. , Luzzi Roberto
Published: July 2002

In the domain of Statistical Mechanics of nonequilibrium-nonlinear (dissipative) systems based on a generalized Gibbs–Boltzmann ensemble formalism, it may be highlighted the so-called Nonequilibrium Statistical Operator Method, and, particularly, Zubarev’s approach. We report here a detailed analysis of a case consisting in a generalized nonequilibrium grand-canonical ensemble. Its construction requires to introduce besides the traditional densities of energy and the particle number their nonconserving-dissipative fluxes of all order. The description is quite appropriate to provide a framework for the construction of a nonclassical thermo-hydrodynamics, which is briefly described.
Electric field gradient, generalized Sternheimer shieldings and electric field gradient polarizabilities by multiconfigurational SCF response
Authors: Rizzo Antonio , Ruud Kenneth , Helgaker Trygve , Jaszuński Michał
Published: July 2002

The electric field gradient (EFG) at the nuclei, the generalized Sternheimer shielding constants and the EFG electric dipole polarizabilities are computed for eight small molecules employing multiconfigurational self-consistent field wave functions and the corresponding linear and quadratic response functions. The molecules studied are H2, N2, CO, HF, C2H2, HCl, HCN, and HNC, all of which are linear. For the hydrogen molecule, full configuration-interaction results for the properties are also reported. The dependence of the computed quantities on the basis set and the electron-correlation treatment is analyzed.
Spectroscopic studies of luminescent and ionically conducting Eu[N(CF3SO2)2]3-PPG4000 complexes
Authors: Ferry Anders , Furlani Maurizio , Franke Axel , Jacobsson Per , Mellander Bengt-Erik
Published: July 2002

Alternating current impedance, Fourier transform Raman/infrared (IR), and luminescence excitation (continuous and time-resolved) measurements have been conducted on solutions of poly(propylene glycol) (MW 4000) complexed with Eu[N(CF3SO2)2]3 salt, EuTFSI3, along with differential scanning calorimetry (DSC) studies. From observed frequency shifts of characteristic internal anionic vibrational modes (Raman and IR), we conclude that the salt is solvated by the polymer host. The TFSI anions, however, interact extensively with Eu3+ cations at all concentrations investigated. Ion–polymer interactions are manifested as changes in characteristic vibrational modes of the polymer. Continuous and time-resolved site-selective luminescence data give, respectively, evidence for two different types of chemical environments for solvated Eu3+ cations. In particular, the strongly forbidden nondegenerate 5D0–7F0 transition exhibits a structured two-component profile in the spectra. DSC data show that the glass transition temperature, Tg, is only marginally affected by the introduction of a relatively high concentration of salt into the host matrix, whereas the resulting polymer–salt complex is of rubbery character, distinctly different from the pure host polymer, which is a viscous liquid at room temperature. The present findings are interpreted in terms of a phase-segregated microstructure. This conjecture is supported by previous studies on PPG4000-based electrolytes indicating microscopic phase anomalies.
Definitive ab initio structure for the X̃ 2A′H2PO radical and resolution of the P–O stretching mode assignment
Authors: Wesolowski Steven S. , Johnson Eric M. , Leininger Matthew L. , Crawford T. Daniel , Schaefer Henry F.
Published: July 2002

Previous ab initio studies of the X̃ 2A′H2PO radical have reported dramatically differing P–O bond distances when using spin-restricted wave functions predicting two artifactual isomers of H2PO: a singly bonded oxygen-centered radical and a doubly bonded phosphorus-centered radical. We show that large basis sets coupled with high levels of dynamical electron correlation are required to correctly describe the P–O bond in H2PO as well as the unpaired electron density as evidenced by the Fermi contact terms and anisotropic components of the 31P, 1H, and 17O hyperfine splitting (hfs) constants. The optimized geometry, harmonic vibrational frequencies, and hfs constants of H2PO were determined at several coupled-cluster levels of theory using both spin-restricted (ROHF) and spin-unrestricted (UHF) Hartree–Fock reference wave functions. The geometrical parameters at the coupled-cluster level with single, double, and perturbatively applied triple substitutions [CCSD(T)] using Dunning’s correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ) are r(P–O)=1.492 Å; r(P–H)=1.410 Å; ∠(HPH)=102.63°; ∠(HPO)=114.92°. These are in excellent agreement with those derived from recent gas phase microwave data, with the surprising exception of the P–H distance which deviates 0.02 Å from experiment. The value of the P–O harmonic stretching frequency at the CCSD(T) level within the cc-pVQZ basis set is 1190 cm−1, in good agreement with the experimental fundamental frequency of 1147 cm−1 obtained by Withnall and Andrews and in constrast to previous speculation that this experimental band may have been misassigned. Hyperfine splitting constants determined at the TZ2P(f,d)/UHF-CCSD(T) level are in very good agreement with experimental values with an average deviation of 23 MHz.
A study of O2(a 1Δg) with photoelectron spectroscopy using synchrotron radiation
Authors: Barr Jonathan D. , De Fanis Alberto , Dyke John M. , Gamblin Stuart D. , Morris Alan , Stranges Stefano , West John B. , Wright Timothy G. , Wright Andrew E.
Published: July 2002

The atmospherically important species O2(a 1Δg) has been studied by photoelectron spectroscopy using vacuum ultraviolet radiation from a synchrotron as the photon source. Constant-ionic-state (CIS) spectra, recorded for vibrational levels of O2+(X 2Πg) v+=0,1,2,3 accessed from O2(a 1Δg) v″=0, exhibit intense signals in the photon energy region 14.0–15.5 eV which are shown to arise from autoionization from a Rydberg state with an O2+(C 2Φu) core. On the basis of the results obtained and earlier evidence derived from vacuum ultraviolet absorption spectroscopy, this state is assigned as a (C 2Φu,3sσg) 1Φu Rydberg state. Photoelectron spectra recorded for O2(a 1Δg) at positions of strong resonances have allowed extended vibrational structure to be obtained in the first photoelectron band. The relative vibrational component intensities in the resonant photoelectron spectra are in good agreement with computed relative intensities obtained via Franck–Condon calculations, confirming the vibrational numbering of the resonances in the 1Φu state. Competition between autoionization and predissociation in the 1Φu Rydberg state is discussed on the basis of the results obtained. Weaker structure is observed in CIS spectra recorded in the photon energy regions 12.5–13.5 and 15.0–20.0 eV. Suggestions are made for the nature of the highly excited states of O2 associated with this structure, based on available ionization energies and spectroscopic constants of known ionic states accessible from O2(a 1Δg). For example, two broad bands centered at ≈16.4 and ≈17.75 eV are assigned to excitation to Rydberg states arising from the configurations (D 2Δg,3pπu) and (D 2Δg,4pπu), respectively.