bluuebunny/crossref_metadata_2025 huggingface dataset:
Power spectra as a diagnostic tool in probing statistical/nonstatistical behavior in unimolecular reactions
Authors: Chang Xiaoyen Y. , Sewell Thomas D. , Raff Lionel M. , Thompson Donald L.
Published: July 2002

The possibility of utilizing different types of power spectra obtained from classical trajectories as a diagnostic tool to identify the presence of nonstatistical dynamics is explored by using the unimolecular bond-fission reactions of 1,2-difluoroethane and the 2-chloroethyl radical as test cases. In previous studies, the reaction rates for these systems were calculated by using a variational transition-state theory and classical trajectory methods. A comparison of the results showed that 1,2-difluoroethane is a nonstatistical system, while the 2-chloroethyl radical behaves statistically. Power spectra for these two systems have been generated under various conditions. The characteristics of these spectra are as follows: (1) The spectra for the 2-chloroethyl radical are always broader and more coupled to other modes than is the case for 1,2-difluoroethane. This is true even at very low levels of excitation. (2) When an internal energy near or above the dissociation threshold is initially partitioned into a local C–H stretching mode, the power spectra for 1,2-difluoroethane broaden somewhat, but discrete and somewhat isolated bands are still clearly evident.

In contrast, the analogous power spectra for the 2-chloroethyl radical exhibit a near complete absence of isolated bands. The general appearance of the spectrum suggests a very high level of mode-to-mode coupling, large intramolecular vibrational energy redistribution (IVR) rates, and global statistical behavior. (3) The appearance of the power spectrum for the 2-chloroethyl radical is unaltered regardless of whether the initial C–H excitation is in the CH2 or the CH2Cl group. This result also suggests statistical behavior. These results are interpreted to mean that power spectra may be used as a diagnostic tool to assess the statistical character of a system. The presence of a diffuse spectrum exhibiting a nearly complete loss of isolated structures indicates that the dissociation dynamics of the molecule will be well described by statistical theories. If, however, the power spectrum maintains its discrete, isolated character, as is the case for 1,2-difluoroethane, the opposite conclusion is suggested. Since power spectra are very easily computed, this diagnostic method may prove to be useful.
A proposal to produce velocity-selected and state-selected molecular beams using the ballistic effect
Authors: Sindoni J. M. , Sharma R. D.
Published: July 2002

Theory of ballistic collisions is used to propose a method for the production of velocity-selected and state-selected molecular beams. The method is illustrated by describing the circumstances leading to the production of a desired N2 beam resulting from collisions of N2 with Li+ in crossed beams.
Statistical theory and structure factor of a semidilute solution of rodlike macromolecules interacting by van der Waals forces
Authors: van der Schoot Paul , Odijk Theo
Published: July 2002

The second and third virial coefficients are estimated for a semidilute suspension of macromolecular rods interacting by van der Waals forces. Whenever the attractive interaction becomes strong enough to be noticeable, it is no longer quantitatively correct to adopt a second virial approximation. Nevertheless, the statistical properties of the second virial fluid are investigated because it is a very convenient idealization of strongly interacting molecular fluids. Sufficient conditions for the local thermodynamic stability of the isotropic phase are set up. The coupling of orientational with translational degrees of freedom which arises from the anisotropic van der Waals interaction between two rods is particularly manifest in the structure factor. This is calculated analytically with the help of a variational theorem.
Radiative lifetimes of the excited electronic states of CuCl
Authors: Delaval J. M. , Schamps J. , Ramirez-Solis A. , Daudey J. P.
Published: July 2002

Spontaneous radiative lifetimes of the observed electronic states of CuCl have been calculated using a model including spin–orbit interaction mixings within the Cu+(3d94s)Cl−(3s23p6) structure. The required wave functions have been determined semiempirically and locations of the as-yet unobserved components have been estimated. For the A 3Σ1+, C 3Π0e, D 1Π1, E 1Σ0+, and F 3Δ1 components, the lifetimes calculated in the simple single-configuration pure-precession approximation with a slight modification in the Ω=0e block are in good agreement with earlier experimental measurements. On the contrary, for the B 3Π1 component, there is a marked discrepancy that cannot be reduced except by assigning very unlikely values to the off-diagonal spin–orbit parameters.
Direct observation of a nonequilibrium velocity distribution function in a system with a thermally activated chemical reaction
Authors: Gorecki Jerzy , Eu Byung Chan
Published: July 2002

The velocity distribution functions for the reactant and product of a thermally activated reaction A+A→B+B are calculated from molecular dynamics simulations. The results are compared with a simple phenomenological theory, which assumes that the temperature of the reactant A may be different from the temperature of the system as a whole. Good agreement between the two methods is observed.
Dissociation of state-selected complex ions studied by mass-selective pulsed field threshold ionization spectroscopy
Authors: Krause H. , Neusser H. J.
Published: July 2002

Pulsed field threshold ionization leads to a vibrationally resolved spectrum of the X̃ 2E1g state of the benzene cation. It is further used to investigate the state-selective dissociation of the benzene–Ar+ complex. Exclusive detection of threshold ions with a resolution of <9 cm−1 is achieved by kinetic energy analysis in a reflectron mass spectrometer.
The influence of molecular shapes on the relative stability of solid phases: Application to N2O
Authors: Kuchta Bogdan , Etters R. D. , LeSar Richard
Published: July 2002

It has been shown that the shape of molecules, represented by the calculated molecular charge distribution, is a valuable source of information about the nature of the potential between molecules. For solid N2O, calculations based on the Kihara and the isotropic and anisotropic site–site potential models have shown that details of the molecular shape affects the relative stability of cubic, tetragonal, and orthorhombic phases at various pressures. This and details of the utilization of experimental data to characterize the potential show that features of CO2 are also described. Also, a Monte Carlo calculation, using a random variable to simulate the end-to-end disorder of solid N2O, conclusively shows that the average structure is Pa3 at low pressure and Cmca above 4.75 GPa.
Ultrafast solvation dynamics: Molecular explanation of computer simulation results in a simple dipolar solvent
Authors: Bagchi B. , Chandra A.
Published: July 2002

Recently Perera and Berkowitz [J. Chem Phys. 96, 3092 (1992)] and Neria and Nitzan [J. Chem. Phys. 96, 5433 (1992)] carried out computer simulations of solvation dynamics of an ion in dense Stockmayer liquids. Both the simulations observed an ultrafast component which decays in a few tens of femtoseconds with a Gaussian time dependence. This is followed by a slow exponential-like decay with a time constant of a few picoseconds. It is shown here that many of the simulation results can be explained very well from a first principles theory with no adjustable parameter.
Quantum theory for free energies of electron transfer
Authors: Gehlen John N. , Chandler David
Published: July 2002

We consider the problem of electron transfer between two symmetric redox states for cases in which the interstate coupling can be strong and the coupled harmonic bath can be nonadiabatic. We utilize an adiabatic bath coupled to the charge transfer species as a reference system and treat the solvation effects of nonzero frequency Fourier modes approximately, yielding an analytical theory for the activation free energy in terms of the spectral density of the bath. The theory is exact for both slow and fast bath modes. For small interstate coupling, the theory agrees with the golden rule result. We test the theory’s accuracy at large couplings in the intermediate frequency regime by comparison with Monte Carlo simulation.
A study of basis set and electron correlation effects in the ab initio calculation of the electric dipole hyperpolarizability of ethene (H2C=CH2)
Authors: Maroulis George
Published: July 2002

We report finite-field self-consistent field (SCF) and many-body perturbation theory calculations of the dipole polarizability and hyperpolarizability of ethene. Large Gaussian-type basis sets have been used to ensure the determination of reliable, near Hartree–Fock values for all the independent components of the dipole hyperpolarizability tensor γαβγδ. The calculated SCF values of γ display strong basis set dependence. Electron correlation changes drastically the SCF picture of γαβγδ. The most important effect is the increase by nearly 70% of the component parallel to the double bond, γzzzz (molecular plane xz). It is found that the contribution from the fourth-order triple substitutions T4, dominates the fourth-order correction to the SCF value for almost all components of γ. The best SCF values obtained in this work are (in atomic units), γxxxx=3466, γyyyy=11 080, γzzzz=3351, γxxyy=2390, γyyzz=2936, γzzxx=1660, and γ̄=6374e4a04Eh−3. The estimated electron correlation corrections (Δ corr) are 431±79, −870±421, 2318±87, −134±101, −67±133, 240±62, and 392±236e4a04Eh−3, respectively. Albeit electron correlation affects strongly the individual tensor components of γ, the overall effect on the mean hyperpolarizability γ̄ does not exceed a few percent. The final estimate of γ̄=(68 ± 2)×102 e4 a04 Eh−3 is in essential agreement with the experimental results of Ward and Elliott.
Quantum and classical calculations of transport and relaxation cross sections in He–CO mixtures
Authors: Gianturco F. A. , Sanna N. , Serna S.
Published: July 2002

Rigorous quantum and classical calculations for transport and relaxation properties of the gaseous mixture of He with CO have been carried out over a broad range of temperatures. The interaction potential employed was the one recently suggested from molecular beam experiments (W. Dilling, Ph.D. thesis, University of Göttingen, Germany, 1985) and the calculations were carried out with the correct dynamical couplings between tensorial basis sets in the quantum case, leading to the usual close-coupled (CC) equations. Classical trajectory (CT) calculations were also carried out and compared with the CC results. A combined use of CC and CT results, together with an appropriate choice of partition functions, is found to produce rather good agreement with the experimental findings for diffusion and mixture viscosity coefficients. The present results suggest ways for further testing the original potential energy surface and for extending the multiproperty analysis of it started earlier on with the scattering data of Dilling and the pressure broadening data of Green, Boissoles, and Boulet [J. Quant. Spectrosc. Radiat. Transfer 39, 33 (1988)].
Correlation between redshifts and widths of the 0–0 band in the absorption spectra (1 1Bu←1 1Ag) of all-trans-β-carotene in solution
Authors: Torii Hajime , Tasumi Mitsuo
Published: July 2002

The wave numbers and widths of the 0–0 band in the electronic absorption spectra (1 1Bu←1 1Ag) of all-trans-β-carotene in various solvents at room temperature are analyzed. These quantities are obtained with sufficient precision from the broad absorption spectra by performing simulations with the parameters obtained in our previous work. The excess redshifts for various solvents (deviations from the linear relationship between the 0–0 wave number and the Bayliss parameter obtained for alkane solvents) are found to be correlated with the 0–0 band widths. To explain this correlation quantitatively, a theoretical model is developed by introducing a low-frequency collective solvent mode, which changes significantly the microscopic polarizability of solvent. This model is consistent with the experimental result that excess redshifts can be observed in nonpolar as well as polar solvents. It is pointed out that the small Stokes shifts observed for β-carotene in nonpolar solvents can also be explained by this model.
Superlattice structure at the surface of a monolayer of octadecanethiol self-assembled on Au(111)
Authors: Camillone Nicholas , Chidsey Christopher E. D. , Liu Gang-yu , Scoles Giacinto
Published: July 2002

We report direct evidence of a unit mesh containing more than one hydrocarbon chain at the surface of a self-assembled monolayer of long-chain n-alkanethiols. Our helium diffraction measurements for a monolayer of n-octadecanethiol on Au(111) are consistent with a rectangular primitive unit mesh of dimensions 8.68×10.02 Å containing four crystallographically distinct hydrocarbon chains. This packing arrangement can also be described as a c(4×2) superlattice with respect to the fundamental simple hexagonal [(√3×√3)R30°] array of lattice parameter 5.01 Å previously observed for monolayers of other n-alkanethiols on gold. No temperature-dependent phase behavior is observed in the temperature range where surface diffraction is measurable (30–100 K) and cycling up to temperatures as high as 50 °C caused no observable change in the diffraction. It is proposed that this larger unit mesh is the result of a patterned arrangement of rotations of the hydrocarbon chains about their molecular axes. This patterned arrangement must be different than the herringbone structure expected by simple analogy to bulk n-alkanes.
A molecular model for tilting phase transitions between condensed phases of Langmuir monolayers
Authors: Kaganer V. M. , Osipov M. A. , Peterson I. R.
Published: July 2002

A model of interacting rigid rods is proposed to describe tilting phase transitions in monolayers of freely rotating long-chain molecules with hexatic in-plane order. The model takes into account steric repulsion and van der Waals attraction between neighboring rods as well as the orientational entropy of individual rods, all within a mean field approximation limited to the unit cell. Two variants of the model are proposed, with different constraints on the polar molecular headgroups. In the first, the headgroups are grafted to a hexagonal close-packed (hcp) lattice, and in the second, the headgroup lattice deforms to accommodate to the tilt. For the monolayer on a solid substrate, tilt has two opposing actions on the internal energy. The decrease in the distance between rods acts to reduce the interaction energy, while the decrease in the overlapping length of the rods acts to increase it. As the area per molecule increases, the competition between these two effects drives the first-order phase transition U(untilted molecules)→NNN (collective tilt of the molecules in the direction of the next-nearest neighbor). This transition is present for both the fixed and the deformable lattices. For the monolayer on the water surface, the molecular tilt is accompanied by an increasing contact of the polar heads with the water. In this case, the effective interaction potential appears to be temperature dependent and under some circumstances can result in the first-order phase transition being replaced by a second-order one (U→NN) with the collective tilt in the direction of the nearest neighbor. The results obtained with the help of this model are compared with computer simulations and with experiment.
Phase separation dynamics in off-critical polymer blends
Authors: Brown Gregory , Chakrabarti Amitabha
Published: July 2002

A numeric integration of the spinodal decomposition process in an off-critical mixture of binary polymer blends is carried out to late times. The thermal fluctuation term in the evolution equation is found to be essential to avoid ‘‘freezing’’ of domain growth for such systems, an effect not present in small molecule systems. For deep quenches, late time domain growth follows the Lifshitz–Slyozov law and the scaling hypothesis is found to hold, although the scale invariant function differs in shape from the one obtained for a critical quench to the same temperature. For shallow quenches, although no freezing is observed, domain growth is found to be progressively slower as the spinodal curve is approached. For the model considered here, we do not find any evidence of the sharp transition in behavior that would be associated with a ‘‘transnodal’’.
Intramolecular vibrational energy redistribution and trapping in centro-symmetric chains by Fermi resonance
Authors: Talanina Irina B. , Collins Michael A.
Published: July 2002

A model of intramolecular vibrational energy redistribution (IVR) in regular chain or ring molecules has been studied under conditions where both Fermi resonance coupling between different modes and symmetric linear coupling between similar oscillators on neighboring groups are important. The relation between coupling parameters and the time scale for decay of an initial state, the rate of energy transfer through the molecule, and the appearance of energy localization is examined.
Resonance-enhanced one- and two-photon ionization of water molecule: Preliminary analysis by multichannel quantum defect theory
Authors: Vrakking Marcus J. J. , Lee Yuan T. , Gilbert Richard D. , Child Mark S.
Published: July 2002

Experimental results are presented for one- and two-photon ionization of the water molecule, obtained using a near transform-limited xuv laser. The single-photon ionization results show rotationally resolved autoionizing resonances corresponding to members of Rydberg series (nd←1b1; n=6–11) converging on the H2O+(100) vibrational state. The two-color (1+1) multiphoton ionization results show rotationally resolved structure corresponding to Rydberg series (nd←1b1; n≥6) converging on the H2O+(000) vibrational state. Typical linewidths below and above the H2O+(000) ionization threshold are 1 and 2 cm−1, respectively. The experimental results are simulated by multichannel quantum defect theory (MQDT). The main features in the spectrum are reproduced in a treatment of the rotational channel interactions with partial l mixing. It is argued that remaining discrepancies between experiment and theory arise from perturbative interactions between the (nd←1b1) levels and members of the (nd←3a1) Rydberg series. Also, it is argued that in the (1+1) multiphoton ionization spectra lines may be missing due to selective predissociation.
Molecular dynamics of photodissociation in matrices including nonadiabatic processes
Authors: Gersonde I. H. , Gabriel H.
Published: July 2002

Molecular dynamics calculations of the photodissociation of HCl and Cl2 in a Xe crystal have been performed using diatomics-in-molecules (DIM) Hamiltonians to describe the electronic interaction. Nonadiabatic transitions between the adiabatic states are explicitly taken into account in the equations of motion. It is shown that the transitions occur on a subpicosecond-time scale. They induce fast recombination of the fragments, thus leading to a reduction of the quantum yield of permanent dissociation. In addition to the octahedral and tetrahedral interstitial sites the DIM model predicts two further energy minima for Cl in a Xe crystal. One of them, a D2 site between two neighboring xenon atoms, is populated after dissociation of Cl2. In this case, our calculations led to recombination of the molecule when the system is annealed at temperatures below the onset of thermal diffusion of Cl atoms.
Hyperfine structure of the SiC radical
Authors: Mollaaghababa R. , Gottlieb C. A. , Thaddeus P.
Published: July 2002

The millimeter-wave rotational spectrum of the free 29SiC radical in the X3Π ground state, produced in a low-pressure glow discharge through SiH4 and CO, was detected with the same reactive-molecule spectrometer used earlier to detect SiC and Si13C. Eleven rotational transitions, all but two with resolved hyperfine structure, were measured between 195 GHz (J=4→5) and 372 GHz (J=8→9). Well-resolved hyperfine splittings in the three fine-structure ladders allow determination of the magnetic hyperfine constants to 2% or better.
Theoretical prediction of the vibrational spectrum of naphthalene in the first excited singlet state
Authors: Swiderek Petra , Hohlneicher Georg , Maluendes Sergio A. , Dupuis Michel
Published: July 2002

Complete harmonic force fields have been calculated for the ground state (S0) and the first excited singlet state (S1) of naphthalene using the multiconfiguration self-consistent field (MCSCF) approach. Identical calculations were performed for benzene to test the methodology with already available theoretical and empirical force fields. Two different basis sets were applied (STO-3G and near double-zeta) and all π-orbitals included in the active space. The geometries of ground and excited states were separately optimized. Following the ideas of Pulay, the force constants were scaled before calculating frequencies and normal modes. For the ground states the influence of correlation is discussed by comparison with Pulay’s results. Except for special vibrations where correlation effects turn out to be important, the use of Pulay’s scaling factors leads to a satisfactory description of the in-plane-vibrations. In the case of benzene the calculated frequency shifts between S0 and S1 are in complete qualitative agreement with experimental observations. In the case of naphthalene the new theoretical results suggest several revisions of earlier empirical assignments.
The rate of intramolecular vibrational energy relaxation of the fundamental C–H stretch in (CF3)3C–C≡C–H
Authors: Gambogi Joan E. , Lehmann Kevin K. , Pate Brooks H. , Scoles Giacinto , Yang Xueming
Published: July 2002

The high resolution spectrum of the fundamental C–H stretch in (CF3)3C–C≡C–H has been measured using optothermal detection of a collimated molecular beam. Only the Q branch was resolvable and was fit to a Lorentzian with a full width at half maximum of 2.76 GHz, corresponding to an IVR lifetime of 60 ps. The decrease in lifetime in comparison to (CH3)3C–C≡C–H is thought to be due to strong mixing between the C–F stretches and bends and the backbone C–C stretches and bends.
Quasirelativistic studies of molecular electric properties: Dipole moments of the group IVa oxides and sulfides
Authors: Kellö Vladimir , Sadlej Andrzej J.
Published: July 2002

Dipole moments of SiO, GeO, SnO, PbO, and of the corresponding sulfides are computed by using a variety of high-level correlated methods and polarized basis sets of Gaussian functions. The contribution of relativistic effects has been accounted for by the first-order quasirelativistic approach based on mass-velocity and Darwin terms. The results for sulfides are in good agreement with experiment while for oxides seem to show a progressive deterioration with the increase of the heavy atom nuclear charge. The discrepancies between the calculated and experimental dipole moments for oxides have been carefully analyzed for the PbO molecule in terms of the basis set choice and reliability of the calculated electron correlation and relativistic corrections. The pattern of relativistic corrections computed in the present study is compared with that which follows from recent quasirelativistic and fully relativistic calculations of Dyall. Some complementary calculations of the dipole moment of PbSe and PbTe seem to indicate that both the polarized basis sets and computational methods used in this study should not fail to predict correct values of the dipole moment of PbO and other oxides of the group IVa atoms. The rather unexpected discrepancies have no plausible explanation within the range of methods employed in our study and offer a challenge for more advanced, fully relativistic molecular calculations at the correlated level of approximation.
Orientation of a molecular fluid next to a hard wall: The Percus–Yevick theory
Authors: Quintana Jacqueline , Henderson D. , Haymet A. D. J.
Published: July 2002

The Percus–Yevick (PY) equation for inhomogeneous hard linear molecules is presented and solved. Calculations of the density profile are shown. Despite earlier failures of the PY approximation to predict some kinds of angular properties in homogeneous systems, here we find predictions which are qualitatively correct and quantitatively acceptable. In particular, we find that the molecules near the wall lie parallel to the wall.
Decoupling surface analysis of classical irregular scattering and clarification of its icicle structure
Authors: Someda Kiyohiko , Ramaswamy Ramakrishna , Nakamura Hiroki
Published: July 2002

Irregular scattering in molecular inelastic collision is analyzed classical mechanically by a novel method called ‘‘decoupling surface analysis.’’ Effective Hamiltonian of this analysis provides a phase space view of collision processes analogous to the Poincaré section of coupled-oscillator systems. In this phase space view irregular scattering occurs in a stochastic layer formed around separatrix connected to resonance structure of the effective Hamiltonian. This circumstance is parallel to that in the coupled-oscillator systems, in which stochastic motion is known to be connected to nonlinear resonance. The resonance structure in collision indicates trapping of classical trajectories in a certain dynamical well. The decoupling surface analysis suggests that the dynamical well is formed by a dip of stability exponents of trajectories as a function of time. By using a prototypical model exhibiting irregular scattering, a formal theoretical treatment is developed to analyze the structure of the fractal, termed icicle structure, observed in the plot of final vibrational action against the initial vibrational phase angle.
Electrostatic potential, polarization, shielding, and charge transfer in endohedral complexes of the C60, C70, C76, C78, C82, and C84 clusters
Authors: Cioslowski Jerzy , Raghavachari Krishnan
Published: July 2002

The theory of bonding in endohedral complexes is generalized to carbon clusters other than C60. Parameters that determine electronic properties of endohedral complexes, such as their complexation energies, shifts in ionization potentials, and dipole moments, are calculated for the C70, C76, C78 (2 isomers), C82 and C84 (2 isomers) host cages. In addition, conditions for the presence/absence of electron transfer in endohedral complexes of these carbon clusters are derived and orientational preferences of guests are determined. Trends in the calculated quantities associated with increasing host cage sizes are discussed.
Coexistence curve of a three-component microemulsion by a gradient method
Authors: Aschauer R. , Beysens D.
Published: July 2002

We describe a new gradient technique to determine the coexistence curve of partially miscible liquid mixtures. The method allows this determination to be done using a single sample and gives the proper slice (temperature-order parameter) of the coexistence surface, in contrast to the conventional methods. We employed this technique to determine the coexistence curve of a ternary microemulsion, consisting of water, benzene, and benzyldimethyl-n-hexadecyl ammonium chloride (BHDC), in a narrow temperature range close to the critical point. The value of the exponent β describing the coexistence curve was found to be 0.34±0.08.
Donor–acceptor substituted linear polyenes: Chain length dependent electronic structure of anthrylpolyenes
Authors: Gribi P. , Isenmann G. , Sigmund E. , Quapil G. , Holl N. , Port H.
Published: July 2002

We investigate the dependency of the electronic structure of methyl and pyridine substituted anthrylpolyenes on the polyene chain length. A systematic analysis of the experimental absorption spectra reveals a remarkable complexity of the electronic properties of the supermolecules. Mixed absorption bands are observed that cannot be assigned to transitions of the individual molecular components. Ground state semiempirical calculations show that the fragment’s molecular orbital symmetries are partially preserved due to a twisted biplanar molecular structure. A simple physical model is presented which is based on this special geometry, i.e. two partially decoupled π-electronic subsystems are considered. Both the experimental and semiempirical data on the electronic properties are reproduced quantitatively with regard to its dependency on chain length, state mixing and substitution effects.
ClF2: Structure and infrared spectra of a weakly bound triatomic molecule
Authors: Galbraith John Morrison , Vacek George , Schaefer Henry F.
Published: July 2002

Using sophisticated ab initio methods, the equilibrium structure of the ClF2 radical has been predicted to be bent, confirming previous experimental and theoretical work. The single, double and perturbative triple excitation coupled cluster level of theory along with a triple-ζ plus double polarization basis set gives the geometrical parameters re=1.756 Å and θe=151.8°. At the same level of theory, harmonic vibrational frequencies were predicted to be 536, 243, and 568 cm−1 for the symmetric stretch, bend and asymmetric stretch, respectively. In addition to this high level study of the harmonic vibrational frequencies, theoretical analyses of anharmonic terms have been included in order to help settle the controversy surrounding experimental assignments of the fundamental vibrational frequencies. This research supports Mamantov’s experimental assignment of the bending and symmetric stretching fundamental, over the experimental objections of Prochaska. The vibrational frequency splitting due to 37ClF2 was also studied indicating that only the splitting of the bending and asymmetric stretching modes should be readily observable experimentally. Dipole moments and infrared (IR) intensities were also predicted.
The surface free energy of solid–fluid interfaces: An inhomogeneous cell model description
Authors: Liu Xiang-Yang
Published: July 2002

Many important issues in the field of crystal growth and surface science can be interpreted satisfactorily on the basis of the surface free energy of solid–fluid interfaces. However, the values of energies are not always available either from theory or from experiment. In this paper, starting from the grand canonical partition function, a general expression of the surface free energy is derived within the framework of an inhomogeneous cell model. The results are applied to estimate surface free energies of metal-melt systems, which are compared with the experimental results of Turnbull. Our estimated results agree well in almost every case with Turnbull’s results. Based on our model, various factors which may affect the surface free energy are discussed.
Deuterium two-dimensional exchange nuclear magnetic resonance by rotor-synchronized magic angle spinning
Authors: Reichert D. , Olender Z. , Poupko R. , Zimmermann H. , Luz Z.
Published: July 2002

The method of two-dimensional exchange spectroscopy under condition of magic angle sample spinning (MAS) and synchronization of the mixing time with the rotor period is extended to spin I=1 nuclei. Theoretical equations are derived for the cross peak intensities as a function of the magnetic and kinetic parameters of the system and the method is demonstrated on a number of deuterated compounds. Dimethylmalonic acid–d6 is first used to illustrate the effect of rotor synchronization by the complete absence of cross peaks when no exchange takes place. The method is then applied to two dynamic systems, viz. dimethylsulfone–d6 and thiourea–C6D12 inclusion compound. The experimental results are compared with simulations as well as with analogous experiments on nonspinning samples. Since chemical shift effects are often negligible in deuterium NMR the time domain sampling can in principle be reduced to the number of the desired spinning sidebands, resulting in considerable time savings. The main advantage of the method is the gain in sensitivity at the expense of the characteristic ridge pattern of static two-dimensional exchange experiments. The gain in sensitivity may be sufficient for performing such experiments on deuterium in isotopically normal compounds. The feasibility of such experiments is demonstrated by the recording of a one-dimensional deuterium MAS spectrum of a nonenriched sample.
Fermi polyads and rovibronic spectrum of the X 2Πg state of BO2
Authors: Brommer Matthias , Rosmus Pavel
Published: July 2002

Using ab initio electronic structure calculations, the three dimensional potential energy functions have been generated for the X 2Πg electronic ground state of BO2 . Spin–rovibronic energy levels have been calculated up to 4000 cm−1, Ka=3 by a variational approach accounting for full dimensionality, anharmonicity, rotation–vibration, spin, and Renner–Teller coupling. Almost all of the approximately 70 experimentally known vibronic levels up to 4000 cm−1 have been reproduced to within 10 cm−1. All vibronic energy levels (J=P) up to 4000 cm−1 are given for the 11BO2 isotope. From the composition of the rovibronic wave functions for the Fermi polyads new assignments were made. The anharmonic resonances have been investigated in the framework of a complete three dimensional Brown–Joergensen Hamiltonian for quartic force fields in dimensionless normal coordinates.
Free surfaces of polymer blends. I. Theoretical framework and application to symmetric polymer blends
Authors: Hariharan Arvind , Kumar Sanat K. , Russell Thomas P.
Published: July 2002

In this paper we develop a general mean-field lattice model, in the spirit of the Scheutjens and Fleer theory of polymer solutions [J. M. H. M. Scheutjens and G. J. Fleer, J. Phys. Chem. 83, 1619 (1979); Macromolecules 18, 1882 (1985)], of an N-component polymer blend in the vicinity of an impenetrable surface. We consider the special case of N=3, model one component as noninteracting holes and obtain the bulk density of this system by using the Sanchez–Lacombe equation of state. The free surface of a polymer blend is then modeled by considering the interphase which forms when this ternary mixture is phase separated. One sample case corresponding to the free surface of a symmetric isotopic polystyrenelike blend is probed and we find that the deuterated component is partitioned to the air surface in agreement with experiment. The application of the concept of the Gibbs dividing surface relative to the polymer density profile at the free surface is then examined, and we show that all comparisons to experiment must be conducted relative to this hypothetical plane. Finally, comparisons of the concentration profiles predicted by this theory to existing square gradient theories of binary polymer blends, as well as Monte Carlo simulations, are performed to highlight the usefulness of these calculations to realistic systems.
Nuclear spin relaxation due to paramagnetic species in solution: Effect of anisotropy in the zero field splitting tensor
Authors: Sharp Robert R.
Published: July 2002

The NMR (nuclear magnetic resonance) paramagnetic relaxation enhancement (NMR-PRE) that is produced by paramagnetic solutes in solution has been investigated theoretically with respect to the influence of zero field splitting (zfs) interactions in the electron spin Hamiltonian, in particular with respect to the effects of anisotropy in the zfs tensor. These effects are a physical consequence of the influence of the zfs on the motion of the electron spin vector S̄. When the zfs energy is large compared to the Zeeman energy (the zfs limit), the precessional motion of S̄ is quantized in the molecule-fixed coordinate system that diagonalizes the zfs tensor. The uniaxial portion of the zfs tensor influences the NMR-PRE primarily through its influence on the quantization axes of S̄; the characteristic behavior of the NMR-PRE under the influence of a uniaxial zfs has been described in detail previously. Anisotropy in the zfs tensor induces oscillatory motion in Sz. This motion has a profound influence on the NMR-PRE, the major part of which normally arises from low frequency components of the local magnetic field that are associated with Sz, rather than from the rapidly precessing local fields that are associated with the transverse components S±. For this reason, the NMR-PRE is a sensitive function of zfs anisotropy, which acts to lower the NMR-PRE below the value that occurs in the uniaxial situation. The magnitude of this effect depends on the ratio (E/D) of the anisotropic and uniaxial zfs parameters, on the reduced dipolar correlation time, and on the location of the nuclear spin in the molecular coordinate frame. A second physical effect of zfs anisotropy on the NMR-PRE arises from a resonance between the electron spin precessional motion in the transverse plane with the precessional motion that is perpendicular to the transverse plane (the latter due to zfs anisotropy). Resonance of these motions, which occurs spin energy levels crossings, gives rise to low frequency transverse components of S̄ which result in a resonant increase in the NMR-PRE within a restricted range of E/D ratios.
Quantum dynamics of van der Waals clusters: Model results for He2Cl2 and Ne2Cl2 fragmentation
Authors: Le Quéré Frédéric , Gray Stephen K.
Published: July 2002

Wave packet calculations are carried out on a three degrees of freedom model for the fragmentation dynamics of X2Cl2 van der Waals clusters, with X=He and Ne. The model allows Cl2 to vibrate, but not rotate, with the X atoms restricted to motion in a plane perpendicular to and bisecting the Cl2 bond. The (X, Cl2 center, X) angle θ is also held fixed. Both X+XCl2 and 2X+Cl2 continua are explicitly included. Results for He2Cl2(v′) vibrational predissociation to 2He+Cl2(vF), with 7≤v′≤13 are presented, and found to be in qualitative accord with experiment. In particular, even though products Cl2(vF=v′−1) are energetically allowed for all v′ in the range, ≊90% of the products are estimated to be in vF=v′−2. Inspection of the wave packet confirms that a sequential mechanism He2Cl2(v′)→He+HeCl2(v′−1)→2He+Cl2(vF=v′−2) is operative. Theoretical lifetimes for Ne2Cl2(v′)→2Ne+Cl2(vF) are in rough agreement with experimental results, where available. The Cl2(vF) product distributions for Ne2Cl2(v′) are estimated to be similar to those obtained for He2Cl2(v′), with vF=v′−2 products being dominant over the entire v′ range. This is only partly in accord with experiment, which points to both vF=v′−1 and vF=v′−2 products being important when the v′−1 product channel is open (for v′≤10 in this case). We show that variation in θ can enhance production of vF=v′−1 products, and that this effect should be most important for Ne2Cl2(v′).
Induced infrared absorption of H2, HD, and D2 physisorbed on NaCl films
Authors: Dai David J. , Ewing George E.
Published: July 2002

Isotherms of H2, HD, and D2 on annealed NaCl films in the temperature range 28 K to 33 K are reported with an isosteric heat of adsorption of −3.4±0.8 kJ/mole. Infrared spectroscopy of H2, HD, and D2 induced by the strong electric fields at the NaCl film surface, reveals a number of features not reported before. Analysis of these spectroscopic features is consistent with molecule adsorption on defect sites and on smooth-face (100) surfaces of the film crystallites. The infrared bands of molecules adsorbed to smooth-face surfaces are red shifted by less than 1% from the gas phase fundamental vibrational frequencies for all hydrogen isotopes. A diffuse combination band is associated with the molecular tangential vibration along the surface. An analysis of both the thermodynamic and spectroscopic data with an extended Langmuir model of adsorption provides a consistent picture of the molecule physisorbed to the surface as a hindered rotor.
Electronic spectroscopy and excited state dynamics of the Ar⋅SH complex
Authors: Yang Min-Chieh , Salzberg Aldo P. , Chang Bor-Chen , Carter Christopher C. , Miller Terry A.
Published: July 2002

The laser excitation spectrum Ã (0,00,vS′)−X̃ (0,00,0), has been observed for the Ar⋅SH complex. Rotational constants and bond lengths have been obtained for vibrational levels of the Ã and X̃ states. A remarkable lengthening of the Ã state natural lifetime is observed upon Ar complexation.
A simulation approach to vibrational dynamics of the OH− ion in molten LiOH
Authors: Okazaki Susumu , Okada Isao
Published: July 2002

A new simulation approach to vibrational dynamics of molecules and ions in liquids is presented. The method is concerned with calculation of the time-dependent many-body vibrational potential of an oscillator reflecting the liquid structures, where frozen field, local mode, and one-particle potential are assumed. The equation of motion is solved for this time-dependent potential to include the dynamic effect on the resultant vibrational spectra. The simulation well reproduced the isotropic Raman spectrum of the OH− ion in a LiOH melt. The peak shift, the band broadening, and the asymmetric shape which are generally found for the liquid state spectra could be interpreted satisfactorily. The analysis showed that inhomogeneous broadening as well as motional narrowing plays an essential role in the vibrational dephasing. Fluctuation of the field on the oscillator and, hence, of the instantaneous transition frequency is large and very fast. The detailed relevant molecular processes are also discussed.
Infrared, diode laser spectroscopy of the Ar–N2O complex: Observation of the intermolecular bending mode in combination with the highest frequency intramolecular stretching mode
Authors: Hu T. A. , Chappell E. L. , Sharpe S. W.
Published: July 2002

Rotationally resolved vibrational spectra consisting of a-type transitions have been observed for the low-frequency, intermolecular bending mode in combination with the highest frequency, intramolecular stretching mode of Ar–N2O. Analysis of the spectral data places the origin of the combination band at 2256.1 cm −1 while the origin of the intramolecular stretching fundamental is at 2223.9 cm−1. The difference between these two origins is approximately 32.2 cm−1 and agrees well with our calculated frequency of 31.5 cm−1 for the intermolecular bending mode, which was obtained by analysis of the centrifugal distortion constants. In addition, argon–nitrous oxide exhibits an anomalously large inertial defect of 10.96 amu Å2 in the combination state. This indicates a breakdown in the assumption of separation between vibration and rotation. While much of the inertial defect in the ground state can be accounted for by including Coriolis interactions, that occurring in the combination state is only partially accounted for by a similar analysis. Small, but significant changes, are observed in both the radial and angular parameters for Ar–N2O when going from the ground to the combination state, indicating large amplitude motion. The combination band is approximately 200 times less intense than the high-frequency, stretching fundamental of Ar–N2O. In addition, over 400 new rovibrational transitions are assigned to the previously observed 101 intramolecular stretching fundamental of the complex, and the subsequent rotational analysis is found to be in close agreement with earlier studies. Data were taken on a newly built, rapid-scan, diode laser spectrometer that incorporates a 12 cm×200 μm pulsed slit-expansion nozzle.
Force between grafted polymer brushes
Authors: Dickman Ronald , Anderson Pearl E.
Published: July 2002

The force between a pair of planar, athermal grafted polymer brushes is determined in Monte Carlo simulations of the fluctuating bond lattice model. Monodisperse systems with chain lengths of 20 and 40 segments are studied at surface coverages ranging from 0.05 to 0.2. The variation of the force with chain length, surface coverage, and separation is in agreement with predictions of self-consistent field theory, except at large separations, when only the tails of the brushes are in contact. As the brush separation is reduced, the force per unit area approaches the pressure of the bulk polymer fluid. Density profiles of free and compressed brushes are analyzed.
Vibrations of pyrazine and its ion as studied by threshold ionization spectroscopy
Authors: Zhu Langchi , Johnson Philip
Published: July 2002

The pump–probe threshold ionization photoelectron spectra of pyrazine have been recorded using nine different vibrations of S1 as intermediate resonances. The extensive vibrational structure in these spectra of the ionic ground state have enabled the measurement of 12 of its vibrational frequencies and their assignments. Three new vibrational assignments of S1 are also made. MP2/6-31G* calculations of the vibrational frequencies of the neutral ground, S1, and the ionic ground state are compared with the experimental values, finding that certain vibrations of S1 and the ion which engage in extensive vibronic coupling are not properly determined by the calculated force field. Most vibrational frequencies are accurately reproduced, however. Variations in the complexity of the threshold ionization spectra with the level of S1 excitation indicate that internal vibrational relaxation is taking place at a very low energy in that state, possibly involving vibronic interactions and mixing with the triplet manifold.
Solvent effects on molecular spectra. I. Normal pressure and temperature Monte Carlo simulations of the structure of dilute pyrimidine in water
Authors: Zeng J. , Craw J. S. , Hush N. S. , Reimers J. R.
Published: July 2002

In this series, our aim is to develop a new scheme based upon a perturbation expansion of the weak intermolecular interactions for the solvent (solvatochromatic) shift of the center of an electronic absorption band in a condensed phase. It is tested by calculation of the shift of the 1(n,π*) absorption and fluorescence spectra of pyrimidine in water. Herein, NPT-ensemble Monte Carlo simulations are performed to determine the structure of dilute pyrimidine in water at 25 °C and 1 atm. pressure. Six different intermolecular effective pair potentials are used to produce a wide range of hydrogen-bond structures ranging from 0 to 2 hydrogen bonds per pyrimidine molecule, suitable for subsequent investigations into the correlation of spectral shift with solvent structure. One potential obtained using Kollman’s Lennard-Jones parameters combined with ab initio electrostatic-potential charges produces the most accurate potential function; this correctly reproduces the observed enthalpy of hydration and partial specific volume of pyrimidine, and produces a chemically reasonable description of the hydrogen-bond structure.
Crossed molecular beam study of the reaction O(3P)+allene
Authors: Schmoltner A. M. , Huang S. Y. , Brudzynski R. J. , Chu P. M. , Lee Y. T.
Published: July 2002

The reaction between ground state (3P) oxygen atoms and allene was studied under single collision conditions using the crossed molecular beams method. Product angular distributions and the translational energy distribution were determined for each channel. Two major reaction channels could be identified unambiguously: the formation of carbon monoxide and ethylene following oxygen atom attack on the central carbon atom, and the formation of allenyloxy (formyl–vinyl) radical and hydrogen atom following oxygen atom attack on the terminal carbon atom. In addition, at least one other reaction channel, which could be identified as the production of vinyl and formyl radicals, occurs. This channel involves the decomposition of acrolein which is formed by the addition of oxygen to the terminal carbon atom, followed by 1,2-hydrogen migration.