bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear optical properties of p-nitroaniline: An ab initio time-dependent coupled perturbed Hartree–Fock study
Authors: Karna Shashi P. , Prasad Paras N. , Dupuis Michel
Published: July 2002

For p-nitroaniline the ab initio method with a double-zeta basis set which includes semidiffuse polarization functions has been used to calculate the dipole moment μ, frequency-dependent linear polarizability α, and nonlinear hyperpolarizabilities β and γ using the time-dependent coupled perturbed Hartree–Fock approach. The computation procedure used here yields information on the dispersion behavior of all the tensor components of polarizability and various hyperpolarizability terms. The largest dispersion effect is observed for the diagonal components of the polarizability and hyperpolarizability tensors along the long in-plane axis. The magnitudes of the various hyperpolarizability terms which describe the various second-order nonlinear processes show the following trend: β(−2ω;ω,ω) ≳β(0;ω,−ω)=β(−ω;0,ω) ≳β(0;0,0), with β(−2ω;ω,ω) exhibiting the largest frequency dispersion. The various second hyperpolarizability terms which describe the various third-order nonlinear optical processes show the following trend: γ(−3ω;ω,ω,ω) ≳γ(−2ω;0,ω,ω) ≳γ(−ω;ω,−ω,ω) ≳γ(−ω;0,0,ω) ≊ γ(0;0,ω,−ω) ≳γ(0;0,0,0). Again γ(−3ω;ω,ω,ω) shows the largest dispersion effect.

The results of existing semiempirical calculations on p-nitroaniline are compared with that of the present ab initio calculation, and the problem due to the arbitrary parametrization procedure adopted in the past for semiempirical calculation is discussed. The computed values of the first resonance energy, the dipole moment, and the polarizability are in good agreement with the respective values experimentally observed, within the spread of the existing experimental data. In contrast, the computed β and γ values are considerably smaller than the respective experimentally determined values. We attribute this discrepancy to two sources. First, in the theoretical calculation electron correlation has been neglected, and the basis set used, although large, may not still be adequate. Second, there is a considerable spread in the reported experimental values for a given nonlinear coefficient making any comparison between the theory and the experiment difficult.
A mechanism of temperature dependent electron transfer reactions in biological systems
Authors: Cartling Bo
Published: July 2002

A mechanism contributing to the temperature dependence of electron transfer reactions in biological systems, based on conformational control of electron transfer, is considered. It is shown that such a mechanism can account for the observed temperature dependence of electron transfer between cytochrome c and the special pair of bacteriochlorophylls in the reaction center of several photosynthetic bacteria. It is also shown that a parallel path model, recently proposed as an alternative to the vibronic coupling analysis because of criticism against the latter, cannot explain the observations on some of these systems. In those systems for which a complete structure has been determined, a specific structural component is suggested to be involved in the conformational control of electron transfer. The reaction center of the photosynthetic bacterium Rhodopseudomonas viridis is shown to have a conformational state at an energy above the ground state close to the experimental high temperature activation energy of electron transfer and with an enhanced electron transfer capability compared to the ground state in accordance with the proposed mechanism. Suggestions for further experimental and theoretical investigations of the mechanism are provided.
Influence of polymerization irregularities on the electronic properties of polythiophene
Authors: dos Santos D. A. , Brédas J. L.
Published: July 2002

The results of quantum chemical calculations on the electronic structure of chains of poly(2,5-thienylene), i.e., polythiophene, along which some of the α–α′(2–5′) linkages are replaced randomly by α–β′(2–4′) linkages are presented. The randomness in the α–β′ linkage positions is taken into account via the negative factor counting technique, on the basis of Fock matrices calculated by the valence effective Hamiltonian method. The eigenfunctions of the upper occupied levels are explicitly calculated using the inverse iteration technique and the degree of localization is determined via the inverse participation number approach. It is shown that a random distribution in the α–β′ linkages leads to a localization of the upper occupied levels within segments containing exclusively α–α′ linkages; more extended levels start to appear some 0.3 eV below the HOMO level for α–β′ linkage concentrations of about 6%. On the contrary, when the α–β′ linkages occur regularly, delocalization remains effective even for the upper occupied levels.
Structure of methyl cyanoformate from microwave spectroscopy and a
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i
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o calculations
Authors: Durig J. R. , Groner P. , Lin Jie , van der Veken B. J.
Published: July 2002

The microwave spectra of seven isotopic species of methyl cyanoformate, CD3OC(O)CN, CD2HOC(O)CN, 13CH3OC(O)CN, CH318OC(O)CN, CH3O13C(O)CN, CH3OC(O)13CN and CH3OC(O)C15N have been measured and assigned. A barrier to internal rotation of 407±1 cm−1 (1164±3 cal/mol) has been obtained for seven symmetric internal rotor species from the internal rotation analyses. The rotational constants from nine isotopic species have been used to determine both rs and r0 structural parameters for the s-trans conformer (methyl group trans to the cyanide group). All independent structural parameters could be determined. Optimized structures were calculated for the s-trans conformer by ab initio Hartree–Fock gradient calculations with both the 3-21G and 6-31G* basis sets as well as with MP2/6-31G* calculations. With the 6-31G* basis set, the barrier to interconversion of the s-trans form to the s-cis form was calculated to be 4055 cm−1 (11.59 kcal/mol) with the s-trans conformer being more stable by 2364 cm−1 (6.76 kcal/mol). The barrier to internal rotation of the methyl group was calculated to be 442 cm−1 (1264 cal/mol).
Fine tuning of the band gap in conjugated polymers via control of block copolymer sequences
Authors: Meyers F. , Heeger A. J. , Brédas J. L.
Published: July 2002

We present the results of the Austin-Model 1 (AM1) geometry optimizations and valence effective Hamiltonian (VEH) band-structure calculations aimed at determining the evolution of the geometric structure and electronic properties (ionization potentials, electron affinities, band gaps) as a function of unit cell content in two sets of regular block copolymers. The first set consists of poly(p-phenylene vinylene)/poly(2,5-dimethoxy-1, 4-phenylene vinylene) copolymers, the second set of poly(p-phenylene)/polyacetylene copolymers. In the latter case, the band gap can be tuned over the whole visible range, depending on the extent of the block copolymer sequences. Special attention is paid to the possible occurrence of localization phenomena.
A molecular dynamics study of freezing in a confined geometry
Authors: Ma Wen-Jong , Banavar Jayanth R. , Koplik Joel
Published: July 2002

The dynamics of freezing of a Lennard-Jones liquid in narrow channels bounded by molecular walls is studied by computer simulation. We quantify the time development of ordering and observe a novel freezing mechanism. The liquid forms layers and subsequent in-plane ordering within a layer is accompanied by a sharpening of the layer in the transverse direction. The effects of channel size, the methods of quench, the liquid–wall interaction and the roughness of walls on the freezing mechanism are elucidated. Comparison with recent experiments on freezing in confined geometries is presented.
Optical spectroscopy of the dimer system Cs3Yb2Br9
Authors: Hehlen Markus P. , Güdel Hans U.
Published: July 2002

Single crystals of Cs3Yb2Br9 were grown using the Bridgman technique. Well-resolved absorption, luminescence, excitation, and Raman measurements down to 5 K are reported and analyzed. The crystal-field splitting of Yb3+ in Cs3Yb2Br9 was determined and can be rationalized in terms of a trigonally distorted YbBr63− octahedron. Under near-infrared (NIR) cw-laser excitation of the 2F7/2→2F5/2 transitions, Cs3Yb2Br9 shows up-conversion luminescence in the green spectral region. Conversely, NIR 2F5/2→2F7/2 luminescence can be excited at twice the energy of the 2F7/2→2F5/2 transitions in the green spectral region. These effects are shown to result from cooperative processes within the Yb2Br93− dimers. The cooperative dimer-absorption oscillator strength of 4.0×10−12 corresponds to the square of the NIR absorption oscillator strength of 1.9×10−6, as expected for weak coupling within the Yb2Br93− dimer. By a numerical deconvolution the cooperative dimer luminescence spectrum can be quantitatively correlated with the NIR luminescence spectrum in which reabsorption effects are important.
A van der Waals intermolecular potential for (O2)2
Authors: Bussery B. , Wormer P. E. S.
Published: July 2002

First-order (electrostatic and exchange) contributions to the O2(3Σg−)–O2(3Σg−) interaction energy are computed ab initio and represented by a spherical expansion. The spin average energy as well as the Heisenberg exchange coupling parameter are fitted as a function of the O2 orientations and the intermolecular distance. The second-order polarization energy is evaluated through an analytical angular-dependent term for which the effective isotropic coefficient C6 is given by the treatment recently proposed by Cambi et al. for a generalized correlation in terms of polarizability. The resulting potential is in good agreement with the available experimental data for the gas phase (second virial coefficients) and for the ordered α phase of the solid oxygen. The structure of the van der Waals molecule (O2)2 is discussed. Its energy is lowest for the parallel planar D2h geometry for the singlet (ΔEmin=−221 K at Re=6.1 a0) and triplet (ΔEmin=−201 K at Re=6.2 a0) states. The lowest energy for the quintet state (ΔEmin=−181 K at Re=6.2 a0) is found for a crossed D2d structure. The (staggered) parallel and T-shaped structures are slightly higher in energy. The (O2)2 is indeed a weakly bound molecule with hindered rotation around its van der Waals bond. The barrier for internal O2 rotations around z (φ angle) is estimated to be 50 K for the singlet, 27 K for the triplet, and 9 K for the quintet state.
Conformational properties and apparent dissociation constants of titrating polyelectrolytes: Monte Carlo simulation and scaling arguments
Authors: Ullner Magnus , Jönsson Bo , Widmark Per-Olof
Published: July 2002

Titrating polyelectrolytes with up to 2000 monomers are studied by Monte Carlo simulations. The polyelectrolyte, regarded to be a polyacid in a salt-free solution, is modelled with harmonic bonds between monomers that can be either neutral or negatively charged. The charges are allowed to fluctuate in a grand canonical ensemble and charged monomers interact via a Coulomb potential. Scaling laws expressing the root-mean-square end-to-end distance and the apparent dissociation constant as functions of the number of monomers and degree of ionization are obtained from a simple Flory approach. Comparisons with the simulation results show that these scaling laws describe a universal behavior for long and highly charged chains.
A direct ab initio dynamics approach for calculating thermal rate constants using variational transition state theory and multidimensional semiclassical tunneling methods. An application to the CH4+H↔CH3+H2 reaction
Authors: Truong Thanh N.
Published: July 2002

We present a new methodology, called ‘‘direct ab initio dynamics,’’ for calculations of thermal rate constants and related properties from first principles. The new method is based on full variational transition state theory plus multidimensional semiclassical tunneling transmission coefficients with the potential energy information to be calculated from an accurate level of ab initio electronic structure theory. To make this approach practical, we propose the use of a focusing technique to minimize the number of electronic structure calculations, while still preserving the accuracy of the dynamical results. We have applied this method to study detailed dynamics of the hydrogen abstraction reaction, CH4+H↔CH3+H2, and obtained excellent agreement with the available experimental data for both the forward and reverse rate constants for a range of temperatures from 300 to 1500 K. In these calculations, the potential energy surface was calculated at the quadratic configuration interaction including single and double excitation (QCISD) level of theory using the triple-zeta plus polarizations 6-311G(d,p) basis set.
Rotationally resolved ultraviolet spectroscopy of indole, indazole, and benzimidazole: Inertial axis reorientation in the S1(1L
b)←S transitions
Authors: Berden Giel , Meerts W. Leo , Jalviste Erko
Published: July 2002

Rotationally resolved laser induced fluorescence excitation spectra of the S1(1Lb)←S0 origin bands of indole, indazole, and benzimidazole have been measured. From these spectra, the rotational constants in both electronic states have been determined. The spectra of all three molecules exhibit ‘‘anomalous’’ rotational line intensities. These intensity perturbations are a result of the reorientation, upon electronic excitation, of the inertial axes of the molecule. Intensity analysis of the rotational lines yielded information about the inertial axis reorientation, and the direction of the transition moment vector for each molecule.
Relaxation of spatially heterogeneous dynamic domains in supercooled ortho-terphenyl
Authors: Cicerone Marcus T. , Ediger M. D.
Published: July 2002

A photobleaching technique has been used to observe rotational dynamics of dilute probe molecules in supercooled o-terphenyl (OTP). The nonexponential rotational relaxation of the probe molecules is shown to be due, at least in part, to the presence of spatial heterogeneity in the host dynamics. Under appropriate photobleaching conditions, a nonequilibrium distribution of probe molecule mobilities can be created by preferentially bleaching the more mobile probe molecules in a sample. Near Tg this nonequilibrium distribution is observed to return to an equilibrium distribution of relaxation times over times on the order of 103 τc, where τc is the average rotational correlation time of a probe molecule. The time required to return to equilibrium is interpreted as a structural relaxation time for dynamic heterogeneities in OTP.
Medium and long range correlations in the electrolyte LiCl⋅4H2O: Transition to the glass regime
Authors: Prével B. , Jal J. F. , Dupuy-Philon J. , Soper A. K.
Published: July 2002

New data on the structure of water in the aqueous system LiCl⋅RH2O, with R=4, are presented for the liquid, supercooled liquid, and glass states. The results are compared to an earlier study of the system LiCl⋅6H2O [J. Chem. Phys. 103, 1886 (1995)]. Many of the qualitative trends seen for R=6 are similar for R=4, but there is evidence that the water structure is even more severely disrupted than for R=6. In the liquid state the distribution functions appear to be dominated by the packing of hydrated ions, rather than by hydrogen bonding forces. The latter partly reassert themselves in the supercooled and glass states. H/D isotope substitution throws some light on the first structure diffraction peak (FSDP) at 0.5 Å−1 which is correlated with the nonhydrogenated components. This peak is not therefore the signature of long range water concentration fluctuations, but must be due to some weak periodicity in the underlying longer range order.
Structural and vibrational properties of fullerenes and nanotubes in a nonorthogonal tight-binding scheme
Authors: Menon Madhu , Richter Ernst , Subbaswamy K. R.
Published: July 2002

A simple, computationally efficient method for the evaluation of structural and vibrational properties of carbon is presented. The scheme is based on the generalized tight-binding molecular dynamics technique applicable to covalent systems. The force constants for the evaluation of vibrational modes are obtained by employing analytic second derivatives of the electronic structure Hamiltonian matrix elements. This method, while providing better accuracy than conventional schemes, greatly expedites the determination of vibrational modes for large size clusters. The efficacy of the method is demonstrated by application to fullerenes and nanotubes. Good agreement is obtained with experiment for bond lengths and vibrational frequencies for these systems.
The isotropic–nematic phase transition in uniaxial hard ellipsoid fluids: Coexistence data and the approach to the Onsager limit
Authors: Camp Philip J. , Mason Carl P. , Allen Michael P. , Khare Anjali A. , Kofke David A.
Published: July 2002

The isotropic–nematic (I-N) phase transition in hard ellipsoid fluids has been studied by computer simulation, using the Gibbs–Duhem integration technique introduced by Kofke; and theoretically, using Onsager theory and the Parsons–Lee improvement. In the simulations, the I-N coexistence line is mapped out in the P–x plane, where P is the pressure and x is the elongation, by numerically integrating a Clapeyron-like first-order differential equation, using constant-pressure simulation data for the two coexisting phases. The elongation range 5≤x≤20 has been studied, using independent starting points provided by chemical potential calculations and thermodynamic integration of the equation of state at x=5,20, plus a direct Gibbs ensemble simulation at x=20. The Onsager–Parsons–Lee theory has been applied to the I-N phase transition for aspect ratios up to x=1000, affording an accurate investigation of the approach to the Onsager limit for this model. This involved the numerical computation of the orientation-dependent second virial coefficient in a way that avoids expansions in Legendre polynomials, so as to be accurate at high elongation. Over the elongation range studied here, agreement between simulation and the Parsons–Lee theory is good.
Electron binding energies of TCNQ and TCNE
Authors: Zakrzewski V. G. , Dolgounitcheva O. , Ortiz J. V.
Published: July 2002

Ab initio electron propagator calculations on tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE) produce accurate predictions of vertical ionization energies and electron affinities. Plots of Feynman–Dyson amplitudes associated with each ionization process represent how the electron distribution changes from initial to final states. Calculated electron detachment energies of the TCNQ dianion imply that two states of the TCNQ anion are bound with respect to the neutral molecule. Configuration interaction calculations on the TCNQ anion confirm this result.
Thermodynamic length, time, speed, and optimum path to minimize entropy production
Authors: Diósi L. , Kulacsy K. , Lukács B. , Rácz A.
Published: July 2002

In addition to the Riemannian metricization of the thermodynamic state space, local relaxation times offer a natural time scale, too. Generalizing existing proposals, we relate a thermodynamic time scale to the standard kinetic coefficients of irreversible thermodynamics. The notion of thermodynamic speed is generalized to higher dimensions. Criteria for minimum entropy production in slow, slightly irreversible processes are discussed. Euler–Lagrange equations are derived for optimum thermodynamic control for fixed clock time period as well as for fixed thermodynamic time period. It is emphasized that the correct derivation of the principle of constant thermodynamic speed, proposed earlier by others, requires the entropy minimization at fixed thermodynamic time instead of clock-time. Most remarkably, optimum paths are Riemannian geodesics which would not be the case had we used ordinary time. To interpret thermodynamic time, an easy-to-implement stepwise algorithm is constructed to realize control at constant thermodynamic speed. Thermodynamic time is shown to correspond to the number of steps, and the sophisticated task of determining thermodynamic time in real control problems is achieved by measuring ordinary intensive variables.
Electro-osmosis: Velocity profiles in different geometries with both temporal and spatial resolution
Authors: Söderman Olle , Jönsson Bengt
Published: July 2002

A theoretical framework for the description of the phenomenon of electro-osmosis is developed. The main emphasis of the work is to develop relations that describe the time and spatial resolution of the velocity of the liquid in contact with a charged surface when a train of electric field pulses are applied parallel to the surface. The work is motivated by the recent development of NMR detected electrophoresis to a powerful tool in the field of colloid chemistry. In this approach one employs pulsed electric fields, and the process of electro-osmosis is a complication. In developing this framework, we make use of results from electro-osmosis outside a single charged plane and in a slot, when the electric field is applied as a step-function. Results for both planar and cylindrical geometries are presented. In both cases we present results without and with effects due to counterflow taken into account. Finally, we compare the results of our theoretical description with some recently published velocity profiles obtained from NMR imaging techniques.
Dynamics of OH and OD radical reactions with HI and GeH4 as studied by infrared chemiluminescence of the H2O and HDO products
Authors: Butkovskaya N. I. , Setser D. W.
Published: July 2002

The infrared chemiluminescence of vibrationally excited H2O and HDO from the highly exothermic reactions of OH and OD radicals with HI and GeH4 was observed in the 2200–5500 cm−1 range. The experiments utilized a fast-flow reactor with 0.3–1 Torr of Ar carrier gas at 300 K; the OH(OD) radicals were produced via the H(D)+NO2 reaction and the H or D atoms were generated by a discharge in a H2(D2)/Ar mixture. The H2O and HOD vibrational distributions were determined by computer simulation of the emission spectra in the 2200–3900 cm−1 range. The total vibrational energy released to H2O and HOD molecules is, respectively, 〈fv〉=0.36 and 0.41 from HI and 〈fv〉=0.46 and 0.51 from GeH4. These values are significantly smaller than for the reactions of OH and OD with HBr, 〈fv〉=0.61 and 0.65. The populations of the O–H stretching vibration of HOD and the collisionally coupled ν1 and ν3 stretching modes of H2O decrease with increasing vibrational energy. In contrast, the vibrational distribution from the HBr reaction is inverted. The bending mode distributions in all stretching states of H2O and HOD extend to the thermodynamic limit of each reaction. A surprisal analysis was made for H2O(HOD) distributions from the title reactions and compared with that for OH(OD)+HBr. The surprisal analysis tends to confirm that the dynamics for the HI and GeH4 reactions differ from the HBr reaction. The HI reaction may proceed mainly via addition-migration, while the GeH4 reaction may involve both direct abstraction and addition-migration. A rate constant for the OH+GeH4→H2O+GeH3 reaction was evaluated by comparing the H2O emission intensities with that of the OH+HBr→H2O+Br reaction, kGeH4/kHBr=6.5±0.9. Secondary kinetic-isotope effects, kOH/kOD=1.4±0.1, 1.0±0.2, and 1.3±0.2, were determined for reactions of OH and OD with GeH4, HI, and HBr, respectively, by comparing the relative H2O and HOD emission intensities.
Rotational and spin-orbit effects in the dynamics of O(3Pj)+hydrocarbon reactions. I. Experimental results
Authors: Sweeney Gillian M. , Watson Allister , McKendrick Kenneth G.
Published: July 2002

Laser photolysis of NO2 has been combined with laser-induced fluorescence detection of the nascent OH product to investigate the dynamics of the reactions of O(3P) with a series of saturated hydrocarbons. We confirm previous observations of very low fractions of OH rotational energy release for the higher homologues such as isobutane and cyclohexane. By photolysing NO2 at shorter wavelengths to produce translationally hotter O(3P) atoms, we have been able to extend the measurements to the previously unstudied parent members of the series, methane and ethane. Similar low levels of rotational energy release were found, suggesting that the previously proposed strong collinear constraint also applies to CH4 and C2H6. No preference for either OH Λ-doublet component was observed, also consistent with a collinearly constrained reaction. The OH spin-orbit states were found to be nonstatistically populated. In part II we present a detailed discussion of the potential source of the spin-orbit propensities.
Comment on “Diffusive reaction rates from Brownian dynamics simulations” [J. Chem. Phys. 97, 5682 (1992)]
Authors: Agmon Noam , Zaloj Veaceslav
Published: July 2002

Luty, McCammon, and Zhou have derived the distribution function of absorption points on the surface of a sphere for diffusing particles emanating from a given point outside the sphere. This problem has an electrostatic analog involving a point charge outside a grounded metallic sphere. The desired distribution is, consequently, given by the ratio of the induced surface charge distribution to the total induced charge. Since the isomorphism is valid for bodies of arbitrary shape, it might prove useful in generalizing the result to nonspherical objects.
Pseudolattice theory of strong electrolyte solutions
Authors: Varela L. M. , Garcia M. , Sarmiento F. , Attwood D. , Mosquera V.
Published: July 2002

The quadratic concentration dependence of the logarithmic activity coefficient and relative partial molar enthalpies of concentrated strong electrolyte solutions can be explained by assuming the solution to have a pseudolattice structure and taking short-range interactions into account through a hard-core potential with an attractive 1/r6 tail. The pseudolattice model is shown to reproduce the experimental data of activity coefficients and relative partial molar enthalpies with a great degree of accuracy over a large range of concentration. The validity of the short-range potential is explicitly tested with experimental data for a number of substances.
Structure and interaction of weakly charged polyelectrolyte brushes: Self-consistent field theory
Authors: Zhulina E. B. , Borisov O. V.
Published: July 2002

A self-consistent field analytical theory is developed to analyze the equilibrium structure of weakly charged polyelectrolyte brushes in salt-free solutions. In contrast to previous studies based on the local electroneutrality approximation valid for sufficiently strongly charged or densely grafted (“osmotic”) brushes, we consider a general case comprising both limits of weakly and strongly charged systems without any a priori assumptions about the distribution of counterions. The analytical expressions for various structural properties of free and confined polyelectrolyte brushes are obtained and compared to existing scaling predictions. The elastic response and structural rearrangement in the polyelectrolyte brush under compression are analyzed.
The frequency-dependent conductivity of a saturated solution of ZnBr2 in water: A molecular dynamics simulation
Authors: Löffler Gerald , Schreiber Hellfried , Steinhauser Othmar
Published: July 2002

The first part of this paper reviews the theory of the calculation of the frequency-dependent dielectric properties (i.e., conductivity and dielectric constant) of ionic solutions from computer simulations. Based on a 2.2-ns molecular dynamics simulation, the second part presents a detailed analysis of the various contributions to the frequency-dependent conductivity of a saturated solution of ZnBr2 in water. We find evidence for two separate relaxation channels in the frequency-dependent conductivity, and a very low value for the static (i.e., zero frequency) conductivity, which is consistent with the high degree of ion association and the prevalence of electrically neutral ion clusters that we observe in this system.
Microwave rotation-tunneling spectroscopy of the water–methanol dimer: Direct structural proof for the strongest bound conformation
Authors: Stockman Paul A. , Blake Geoffrey A. , Lovas Frank J. , Suenram Richard D.
Published: July 2002

Rotation-tunneling a-type spectra of CH3OH⋯H2O and CH3OD⋯D2O were recorded between 18 and 60 GHz using direct absorption microwave spectroscopy, and for CH3OH⋯H2O, CH313OH⋯H2O, CH3OH⋯DOH, CD3OH⋯H2O, and CH3OD⋯D2O between 7 and 24 GHz using a Fourier-transform microwave spectrometer. Because CH3OH and H2O are capable of both accepting and donating hydrogen bonds, there exists some question as to which donor–acceptor pairing of the molecules is the lowest energy form. This question is further emphasized by the ambiguity and variety present in previous experimental and computational results. Transitions arising from the methyl torsional A state were assigned in each of the studied isotopomers, and for the A and E states in CH3OH⋯H2O. While the measured components of the dipole moment for the parent (H,12C,16O) isotopomer—μa=7.956±0.03×10−30 C m (2.385±0.008 D), μb=3.636±0.02×10−30 C m (1.090±0.006 D), μc=0.43±0.47×10−30 C m (0.13±0.14 D), where the errors correspond to 1σ uncertainties—are consistent with either conformation, the fit of the structure to the rotational constants demonstrates unambiguously that the lower-energy conformation formed in supersonically cooled molecular beams corresponds to a water–donor, methanol–acceptor complex. The results and implications for future work are also discussed in terms of the permutation-inversion theory presented by Hougen and Ohashi [J. Mol. Spectros. 159, 363 (1993)].
Path integral grand canonical Monte Carlo
Authors: Wang Qinyu , Johnson J. Karl , Broughton Jeremy Q.
Published: July 2002

We derive the real-space path integral formulations of Widom’s test particle method and grand canonical Monte Carlo (GCMC). We apply these simulation methods to hydrogen and neon at temperatures ranging from 30 to 120 K. In addition, in order to explore configuration space both efficiently and ergodically, our implementation involves multiple time step hybrid Monte Carlo. Agreement between experiment and simulation for chemical potentials (Widom) and densities (GCMC) is very good over the entire temperature range, even when quantum effects are large.
One dimensional surface diffusion. II. Density dependence in a corrugated potential
Authors: Krylov S. Yu. , Prosyanov A. V. , Beenakker J. J. M.
Published: July 2002

A kinetic theory is presented for the transport (collective) diffusion of molecules residing in a one dimensional periodic potential and relaxing to a bath formed by the substrate. Observing that the behavior of the system on a macroscopic scale cannot depend on the detailed rapidly varying motion of the molecules on the microscopic scale of the potential, and assuming that the molecules bound in the well can contribute only indirectly to the macroscopic transport, a kinetic equation for the unbound molecules is obtained. This allows one to derive an expression for the low density limit of the transport diffusion coefficient D0. In a second part of the paper the density dependence of D is studied following an Enskog-like approach, where the density dependence originates from the finite size of the molecules, combined with molecule-phonon relaxation. The density dependence is found to be the result of two effects: (i) the finite size introduces an additional contribution to the driving force, resulting in an increase of D with increasing density, (ii) the collisions of the free with the bound molecules increase the momentum loss to the substrate and result in a decrease of D. The resulting expression for D as a function of the occupancy (θ) shows under reasonable assumptions for the kinetic parameters a dependence close to 1/(1−θ), in agreement with the universal behavior observed in nanochannels of zeolites.
Nonresonant dielectric hole burning spectroscopy of supercooled liquids
Authors: Schiener B. , Chamberlin R. V. , Diezemann G. , Böhmer R.
Published: July 2002

The nonexponential response of propylene carbonate and glycerol near their glass transitions could be selectively altered using nonresonant spectral hole burning (NSHB) experiments. This observation provides evidence of the existence of a distribution of relaxation times in these supercooled liquids. NSHB is based on a pump, wait, and probe scheme and uses low-frequency large amplitude electrical fields to modify the dielectric relaxation. The temporal evolution of the polarization of the sample is then measured subsequent to a small voltage step. By variation of a recovery time inserted between pump and probe, the refilling of the spectral features could be monitored and was found to take place on the time scale set by the peak in the distribution. The recovery time and pump frequency dependences of the spectral modifications were successfully simulated using a set of coupled rate equations.
Role of guest-host intermolecular forces in photoinduced reorientation of dyed liquid crystals
Authors: Marrucci L. , Paparo D. , Maddalena P. , Massera E. , Prudnikova E. , Santamato E.
Published: July 2002

An experimental study of the photoinduced molecular reorientation of dyed liquid crystals for a set of guest-host combinations is reported. We find large variations in the magnitude of the effect for different dyes but also for different hosts, with polar hosts resulting often significantly more effective than nonpolar ones. The data are interpreted in terms of a kinetic mean-field model for the dye molecule rotational dynamics and interaction with the liquid crystal host. The results point to a significant variation of guest-host intermolecular forces upon photoinduced electronic excitation of dye molecules. This force variation is reflected in a variation of dye molecule physical parameters such as the rotational friction coefficient and the orientational mean-field energy.
Dissociative recombination and excitation of N2+: Cross sections and product branching ratios
Authors: Peterson J. R. , Le Padellec A. , Danared H. , Dunn G. H. , Larsson M. , Larson A. , Peverall R. , Strömholm C. , Rosén S. , af Ugglas M. , van der Zande W. J.
Published: July 2002

The absolute dissociative recombination and absolute dissociative excitation rate coefficients and cross sections have been determined for N2+ and electrons for collision energies between 10 meV and 30 eV. The ion storage ring CRYRING has been used in combination with an imaging technique with a position-and-time-sensitive detector. Information is retrieved on the ion beam vibrational state populations and on the product branching in the dissociative recombination process at 0 eV collisions. A hollow cathode ion source has been used to lower the vibrational excitation in the ion beam; a more traditional hot-cathode ion source was used as well. The most important findings are the following. The rate coefficient for an N2+ ion beam (46%, v=0, 27% v=1) versus electron temperature (K) is α(Te)=1.75(±0.09)×10−7(Te/300)−0.30 cm3 s−1. The dissociative recombination rate is found to be weakly dependent on the N2+ vibrational level. At 0 eV collision energy, the v=0 product branching is found to be 0.37(8):0.11(6):0.52(4) for N(4S)+N(2D):N(2P)+N(4S):N(2D)+N(2D) fragments. The dissociative recombination cross section does not have a high-energy peak as was found in a number of lighter molecular systems. The dissociative excitation signal starts only slightly above the energy threshold for dissociation, and peaks near 25 eV. From the dissociative excitation data and literature data, information is retrieved on the dissociative ionization of N2+. The comparison of these results with earlier DR measurements is extensively discussed.
Optical absorption of nanoscale colloidal silver: Aggregate band and adsorbate-silver surface band
Authors: Fang Y.
Published: July 2002

The absorption band (band A) due to the aggregate effect of nanoscale silver particles in silver sol prepared by a modification of the procedure of the original silver sol used in surface-enhanced Raman scattering (SERS) was observed near 435 nm without the presence of any foreign molecules. This band eventually stabilized at 435 nm and was not significantly shifted by added molecules and chlorions, which differs from the properties of the well-known absorption band of the original silver sol appearing in the longer wavelength region upon addition of foreign molecules. A new absorption band (band B) appeared on the longer wavelength side of the 435 nm aggregate band with the addition of pyridine to the modified silver sol. This band can be associated with chemisorption of molecules on colloidal silver and probably to the charge–transfer transition band between silver particles and adsorbed molecules. The effects of aggregate and chemisorption are clearly separated and identified to band A and band B, respectively.
Solvation forces between rough surfaces
Authors: Frink Laura J. Douglas , van Swol Frank
Published: July 2002

We investigate the role of surface roughness on solvation forces and solvation free energies. Roughness is introduced by dividing a surface into an array of square tiles that are then randomly displaced in the direction perpendicular to the wall. The integrated wall strength of these tiled surfaces is independent of the surface roughness and hence this class of rough walls is ideally suited for isolating roughness effects. We use grand canonical Monte Carlo simulations of a Lennard-Jones fluid confined in a slit pore with rough walls to generate the solvation interactions as a function of roughness, tile size, and surface area. The simulation data are compared to a simple superposition approximation of smooth wall solvation interactions (obtained from simulation or density functional theory), based on a distribution of wall separations. We find that this approximation provides a surprisingly accurate route to the solvation interaction of rough surfaces. In general, increased roughness leads to a reduction of oscillations in the solvation forces and surface free energies. However, nonmonotonic behavior of the oscillation amplitude with roughness can be observed for finite surfaces. The washing out of the oscillations found for large surface roughness produces a solvation force that exhibits a broad repulsive peak with separation. The broad repulsion is a consequence of the resistance to squeezing out fluid from the smallest gaps between two opposing rough surfaces. It is as much a reflection of packing effects as are the solvation oscillations for perfectly smooth pores. In addition, we present results for patterned and undulating surfaces produced by an analogous modification of the one-body external field for smooth walls. Finally, we discuss the implications of our results for a number of experimental systems including self-assembled monolayers, microporous materials, protein solutions, and DNA crystals.
A density functional study of the electronic structure of sodalite
Authors: Thomson Kendall T. , Wentzcovitch Renata M.
Published: July 2002

We have conducted a first principles density functional theory (DFT) calculation to explore the electronic structure of sodalite at various stages of Al-substitution. By calculating the electronic structure of both substituted and unsubstituted frameworks, with and without the presence of extra-framework atoms, we show that Al-substitution and cation compensation essentially affect the electronic structure only at the upper valence band edge (i.e., the frontier orbitals of reactivity theory). In addition, we show that the equilibrium positions of the extra-framework cations are located in the vicinity of the frontier orbitals which are preferentially localized near aluminum.
Reorientations in supercooled glycerol studied by two-dimensional time-domain deuteron nuclear magnetic resonance spectroscopy
Authors: Böhmer R. , Hinze G.
Published: July 2002

The method of stimulated echoes was used to investigate the reorientational mechanism in the selectively deuterated glass-former glycerol, C3D5(OH)3 about 15 K above its calorimetric glass temperature. The reorientation process is fully isotropic. This enables an accurate determination of the decay constant, T1Q, of the quadrupolar spin order in the regime of ultraslow motion. The knowledge of this time constant has made it possible to reliably determine the rotational correlation function. The experimentally obtained evolution time-dependent correlation functions are compared with those from a simulation procedure involving a distribution of molecular jump angles. It is found that in glycerol small angles in the 2°–3° range dominate. They are accompanied by a small, but significant, fraction of larger jump angles.
Long-ranged relaxations in poly(ethylene oxide) melts: Evidence for network behavior
Authors: Walkenhorst R. , Selser J. C. , Piet G.
Published: July 2002

Two slow, long-ranged relaxations in melts of poly(ethylene oxide) (PEO) were studied using the noninvasive dynamic light-scattering technique of photon correlation spectroscopy (PCS). Measurements were made for two PEO samples, one with a molecular weight distinctly above the entanglement value and a second sample with a molecular weight distinctly below the entanglement value. Based on several lines of evidence, it is concluded that the faster of these relaxations, which can be identified with the “cluster” or “Fischer” mode observed earlier in other homopolymer melts, results from the gel-like behavior of a physical network formed by the polymer in the melt, while larger-scale rearrangements in the polymer network structure are responsible for the slower relaxation.
A consistent correlation approach to single file diffusion with reaction
Authors: Okino Miles S. , Snurr Randall Q. , Kung Harold H. , Ochs Joshua E. , Mavrovouniotis Michael L.
Published: July 2002

A method to efficiently simulate diffusion and reaction in a single-file system is presented. By considering all possible configurations of M species in a length N one-dimensional pore, a deterministic model consisting of (M+1)N variables can be constructed for the system. The order of the system can then be significantly reduced by considering only pairs of adjacent cells, or (M+1)2(N−1) doublets. This lumped model is able to capture the most important correlations between cells when the dominant mode of transport is through single-site hops. Extensions of this method for higher dimensional pores and more complex molecular interactions are discussed. The results of the approximation are compared to results of the full deterministic model, and new situations are investigated. The implications of single-file behavior are discussed for reversible reactions and molecules of different mobilities.
Rigorous and rapid calculation of the electron repulsion integral over the uncontracted solid harmonic Gaussian-type orbitals
Authors: Ishida Kazuhiro
Published: July 2002

A rigorous general formula for calculating the electron repulsion integral (ERI) over the uncontracted solid harmonic (SH) Gaussian-type orbitals (GTOs) can be derived by the use of the “reducing mixed solid harmonics” defined in this paper. A general algorithm can be obtained inductively from this formula with the use of the “mixed solid harmonics” also defined in this paper. This algorithm is named as accompanying coordinate expansion (ACE) b1k1. This ACE-b1k1 is capable of computing very fast SH-ERIs. The floating-point operation (FLOP) count assessment is shown for the (LL|LL) class of SH-ERIs (L=2–5). It is found that the present ACE-b1k1 algorithm is the fastest among all algorithms in the literature for the ERI over the uncontracted SH-GTOs.
Variational nonequilibrium thermodynamics of reaction-diffusion systems. I. The information potential
Authors: Gaveau Bernard , Moreau Michel , Toth Janos
Published: July 2002

In this work, we consider the nonequilibrium thermodynamics of a reaction-diffusion system at a given temperature, using the Master equation. The information potential is defined as the logarithm of the stationary state. We compare the approximations, given by the Fokker–Planck equation and the Wentzel-Kramers-Brillouin method directly applied to the Master equation, and prove that they lead to very different results. Finally, we show that the information potential satisfies a Hamilton–Jacobi equation and deduce general properties of this potential, valid for any reaction-diffusion system, as well as a unicity result for the regular solution of the Hamilton–Jacobi equation. A second article (Paper II), in the same series, will develop a path integral approach and an estimation of the chemical rate constants in this general context.
Forward–backward initial value representation for the calculation of thermal rate constants for reactions in complex molecular systems
Authors: Wang Haobin , Thoss Michael , Miller William H.
Published: July 2002

The semiclassical (SC) initial value representation (IVR) provides a potentially practical way for including quantum effects into classical molecular dynamics simulations. The forward–backward (FB) version of the IVR provides an especially attractive way for calculating time correlation functions, in particular the reactive flux correlation function which determines chemical reaction rates. This paper presents a further analysis and development of the FB-IVR approach. Applications show that it is feasible and accurate for a reaction coordinate coupled to up to 40 degrees of freedom.
Exponential and logarithmic spectral diffusion in single molecule fluorescence
Authors: Osad’ko I. S. , Yershova L. B.
Published: July 2002

A dynamical theory for two-photon autocorrelation functions (AF) which takes into account interaction of a chromophore with phonons and with huge number of tunneling systems (TSs) of a cooled polymer is developed. The interaction with one extrinsic TS, which relates to the chromophore with its nearest environment and has undergone light-induced tunneling, and the interaction with a huge number of intrinsic TSs inherent in a polymer, which have undergone spontaneous tunneling, is considered. Relaxation of these TSs results in both exponential and logarithmic spectral diffusion. Both types of spectral diffusion manifest themselves in AF. Influence of temperature on AF is studied. Experimental data for AF in low temperature polymers are treated on the basis of our theory.
Is Mo/ller–Plesset perturbation theory a convergent ab initio method?
Authors: Leininger Matthew L. , Allen Wesley D. , Schaefer Henry F. , Sherrill C. David
Published: July 2002

Recent studies have seriously questioned the use of higher-order Mo/ller–Plesset perturbation theory (MPn) in describing electron correlation in atomic and molecular systems. Here we first reinvestigate with improved numerical techniques previously controversial and disturbing MPn energetic series for Ne, F−, HF, BH, C2 and N2. Conspicuously absent in previous work is research on the convergence of MPn spectroscopic constants, and thus complete MPn (energy, re, ωe) series were then computed for (BH, HF, CN+, C2 and N2) through the high orders (MP25, MP21, MP13, MP39 and MP19) within the correlation consistent family of basis sets. A persistent, slowly decaying ringing pattern in the C2 energy series was tracked out to MP155. Finally, new energy series were generated and analyzed through MP167 for Cl− and MP39 for Ar and HCl. The MPn energy and property series variously display rapid or slow convergence, monotonic or oscillatory decay, highly erratic or regular behavior, or early or late divergence, all depending on the chemical system or the choice of one-particle basis set. For oscillatory series the spectroscopic constants computed from low-order MPn methods are often more accurate, with respect to the full configuration interaction (FCI) limit, than those computed via high-order MPn theory.