bluuebunny/crossref_metadata_2025 huggingface dataset:
Ground state properties of cesium dimers from a
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o pseudopotential approaches
Authors: Moullet I. , Andreoni Wanda , Giannozzi P.
Published: July 2002

We present the results of several calculations of the ground state of Cs2 and Cs+2 performed in the local-spin-density approximation of density functional theory, and using different approximations for the core electrons in the derivation of ab initio norm-conserving pseudopotentials. We investigate the influence of both core polarization and relativistic effects on the molecular bonding, which turns out to be of minor importance for the determination of the equilibrium characteristics. We find that in order to guarantee an accurate description within the one-electron scheme, one must avoid the usual ‘‘linear’’ approximation of the exchange-correlation functional in the derivation of the pseudopotentials. This introduces significant errors for Cs and most probably for all one-electron systems.
Picosecond multiphoton ionization of molecular clusters
Authors: Barton Smith D. , Miller John C.
Published: July 2002

High peak-power picosecond laser pulses have been used for the first time to effect nonresonant or resonant multiphoton ionization (MPI) of clusters generated in a supersonic nozzle expansion. The resultant ions are subsequently detected and characterized by time-of-flight mass spectroscopy. Specifically, we present preliminary results involving MPI of the cluster series (NO)n (n=1–4), and ArnI2 (n=1–10). Previous MPI studies of these two species using nanosecond lasers have not been successful, presumably due to fast dissociation channels. It is proposed that the present technique is a new and rather general ionization source for cluster studies which is complementary to electron impact but may, in addition, provide unique spectroscopic or dynamical information.
Mapping the van der Waals state space
Authors: Diósi L. , Lukács B. , Rácz A.
Published: July 2002

We show that the metric structures of the thermodynamic state space enables us to uniquely decide if two thermodynamic states belong to the same phase or not, even in the presence of a critical point. While the method cannot classify one state in itself into a definite phase, coherent regions can be identified in the state space. The construction is demonstrated on the van der Waals gas.
The H+D2 reaction: Quantum-state distributions at collision energies of 1.3 and 0.55 eV
Authors: Rinnen Klaus-Dieter , Kliner Dahv A. V. , Zare Richard N.
Published: July 2002

We have studied the H+D2 →HD+D reaction using thermal D2 (∼298 K) and translationally hot hydrogen atoms. Photolysis of HI at 266 nm generates H atoms with center-of-mass collision energies of 1.3 and 0.55 eV, both of which are above the classical reaction barrier of 0.42 eV. The rovibrational population distribution of the molecular product is measured by (2+1) resonance-enhanced multiphoton ionization (REMPI). The populations of all energetically accessible HD levels are measured. Specifically, we observe HD(v=0, J=0–15), HD(v=1, J=0–12), and HD(v=2, J=0–8). Of the available energy, 73% is partitioned into product translation, 18% into HD rotation, and 9% into HD vibration. Both the rotational and vibrational distributions are in remarkably good agreement with quasiclassical trajectory (QCT) calculations, though the calculated rotational distributions are slightly too hot. We discuss factors contributing to the success of the QCT calculations.
Thermodynamic curvature of a one-dimensional fluid
Authors: Ruppeiner George , Chance Jerry
Published: July 2002

We present investigations of the thermodynamic curvature in a one-dimensional hard-core square-well-potential continuum system of rods with a locus in the (T,P) plane across which the asymptotic decay of the correlation function crosses over from monotonic to oscillatory. For small temperatures, as the pressure is increased from zero, this system exhibits an abrupt pseudo-phase-transition from a gas-like phase of clusters of rods to a liquid-like phase. In the gas-like phase, where the correlation-function decay is monotonic, the thermodynamic curvature corresponding to the thermodynamics of the configuration integrals very nearly equals the correlation length for the long-range decay of the correlation function. In the liquid-like phase, where the correlation-function decay is oscillatory, the curvature is on the order of a rod length. These results are consistent with expectations based on a covariant thermodynamic fluctuation theory. During the pseudo-phase-transition, the curvature changes rapidly from gas-like to liquid-like behavior. During the rapid change, it overshoots to large negative values whose interpretation is yet unclear. Also yet unclear is the interpretation of the thermodynamic curvature based on the full thermodynamics, particularly at small pressures. In addition to examining the thermodynamic curvature, we reexamine the decay in the correlation function. We argue, on the basis of the amplitudes in the correlation-function series, that at small temperatures the crossover in the correlation-function decay from monotonic to oscillatory occurs effectively at the pseudo-phase-transition. This is consistent with the behavior of the thermodynamic curvature.
Time dependent quantum mechanics using picosecond time steps: Application to predissociation of HeI2
Authors: Das Sanjukta , Tannor David J.
Published: July 2002

Time-dependent quantum mechanics has proved to be a useful conceptual and calculational tool for a wide range of femtosecond time scale processes. In this paper we develop a method of extending time-dependent methodology to processes taking many picoseconds, through use of the interaction representation of quantum mechanics. Two implementations are presented, one in the energy frame and one in the coordinate frame; both of these implementations scale as N2 with the number of basis functions, as opposed to N3 for a matrix diagonalization. The interaction representation approach allows the use of time steps much greater than those that can be used in the Schrödinger representation, and is expected to be useful for the broad class of problems where the Hamiltonian can be partitioned into H0+V, where V≪H0. Converged quantum mechanical results are presented for the predissociation of T-shaped HeI2 from highly excited vibrational levels, for times up to 200 ps.
Eigenstate-resolved unimolecular reaction dynamics: Ergodic character of S formaldehyde at the dissociation threshold
Authors: Polik William F. , Guyer Dean R. , Miller William H. , Moore C. Bradley
Published: July 2002

The Stark level-crossing spectra of S0 D2CO are analyzed for evidence of energy level correlations and quantum ergodicity. The analysis for short and long range level correlations gives an apparent energy level spacing distribution which lies between the Poisson and GOE limits. However, the Stark level-crossing method diminishes any existing correlations. The true level spacing distribution must be closer to the Gaussian orthogonal ensemble (GOE) than to the Poisson limit. Complete distributions of S1−S0 coupling matrix elements and S0 dissociation rates are reported and subjected to statistical tests for ergodicity. The distribution of S1−S0 coupling matrix elements indicates that the dynamics of intramolecular vibrational redistribution of energy (IVR) is very nearly quantum ergodic. There is strong coupling and free flow of energy among vibrational degrees of freedom in this molecule above its dissociation threshold. The average S0 decay rate can be accounted for by RRKM theory with tunneling corrections. The large variation about the average is interpreted as quantum statistical fluctuations. These fluctuations serve as a probe of the transition state. The existence of several open decay channels is inferred, and the presence of a low frequency vibrational motion near the transition state geometry is suggested. The distribution of coupling matrix element phase angles is derived from lineshape asymmetries which arise from interfering decay channels. The phase angle distribution is shown to be consistent with the matrix element and decay rate distributions, thereby providing an independent verification of these quantities. All of these distributions are quantitatively consistent with the hypothesis that the wavefunction coefficients of the molecular eigenstates of S0 D2CO at its dissociation threshold are Gaussian random when projected onto the eigenfunctions of a separable Hamiltonian.
Atom–asymmetric top van der Waals complexes: Angular momentum coupling in Ar–H2O
Authors: Hutson Jeremy M.
Published: July 2002

A model for the vibrational states of atom–asymmetric top van der Waals complexes is developed, based on hindered rotation of the asymmetric top in the complex. The pattern of energy levels expected from such a model is described, and the dependence of the spectrum on features of the potential energy surface is explored. The selection rules arising from the model are presented. The model is used to interpret the infrared spectra of Ar–H2O reported by Cohen et al. [J. Chem. Phys. 89, 4494 (1988)], and preliminary information on the effective bending potential for Ar–H2O is obtained. The potential is very isotropic, varying by only about 40 cm−1 as a function of the angular coordinates. Predictions of band origins for other allowed infrared transitions are given.
The vibrational spectra of molecular ions isolated in solid neon. IV. NO+, NO−, ONNO+, and ONNO−
Authors: Jacox Marilyn E. , Thompson Warren E.
Published: July 2002

When a Ne:NO sample is codeposited at approximately 5 K with a beam of neon atoms that has been excited in a microwave discharge, photoionization and Penning ionization of the NO lead to the stabilization of both monomer and dimer ions. The most prominent infrared absorption, at 1619.2 cm−1, results from the reaction of NO− with NO or of an electron with (NO)2 to form the charge-delocalized trans-ONNO− species. The position of the totally symmetric NO-stretching absorption of trans ONNO− can be inferred from observation of a combination band. The infrared absorption of NO+ has been detected very close to the gas-phase band center. Other infrared absorptions have been tentatively assigned to NO−, cis ONNO−, trans ONNO, and the lowest energy isomer of ONNO+. Comparison of the gas-phase vibrational frequencies observed for the weakly bound species cis ONNO and N2O3 with the corresponding frequencies observed in this neon-matrix study suggests that the matrix shifts for the dimer ion absorptions in the mid-infrared probably amount to less than 1%.
Fluids in equilibrium with disordered porous materials. Integral equation theory
Authors: Fanti Lisa A. , Glandt Eduardo D. , Madden William G.
Published: July 2002

The Ornstein–Zernike equations previously introduced by two of the authors for a fluid in equilibrium within a quenched disordered matrix are solved numerically within the Percus–Yevick approximation. The structure of a fluid of hard spheres is reported for two types of microporous matrices: a sintered-type structure of mutually penetrable (randomly placed) obstacles or sites, and a packed bed of quenched hard spheres. The integral-equation results agree well with Monte Carlo simulation data also reported here. For the case of point obstacles, when both models coincide, the structure of the fluid is found to be insensitive to obstacle concentration. The structure of a hard-sphere fluid in a bed of other quenched hard spheres is found to be significantly different from that of the equilibrium binary mixture of the two types of particles. Pressures and bulk-pore partition coefficients are reported for beds of randomly placed obstacles. A sparse-matrix approximation is presented and compared with the full solution. These are the first results for the equilibrium properties of fluids in disordered substrates.
Deposition of mass selected gold clusters in solid krypton
Authors: Harbich W. , Fedrigo S. , Buttet J. , Lindsay D. M.
Published: July 2002

We report on the deposition of size selected gold dimers and trimers in krypton matrices. Gold cluster ions were produced by sputtering, mass selected in a quadrupole mass filter, and then codeposited with krypton on a CaF2 window in the presence of low energy electrons. Neutralized cluster samples were interrogated in situ by excitation and fluorescence spectroscopy. The spectra found for Au2 (absorption bands at 212, 250, 298, and 347 nm) are in reasonable accord with previous measurements made in Ar matrices. For the trimer we find four strong absorption bands centered at 233, 255, 282, and 308 nm, plus three weaker features lying further to the red. The dimer spectra are discussed with reference to gas phase measurements and recent molecular structure calculations.
Deviations from linear Stern–Volmer law in hopping quenching theory
Authors: Fedorenko S. G. , Burshtein A. I.
Published: July 2002

The differential and integral binary theories of hopping quenching were used to calculate the quenching kinetics, its stationary rate, and the luminescence quantum yield assuming Markovian random walk of excitation and dipole–dipole energy transfer to acceptors. It is shown that the integral theory results are not valid for high concentration of acceptors, however, its kernel (mass operator) is defined. On the contrary, the differential theory which is exact for immobile donors as well as so-called Burshtein model, which is appropriate for immobile acceptors, are at least the useful interpolations between binary ‘‘migration accelerated quenching’’ (MAQ) limit and multiparticle in principle ‘‘static quenching’’ limit.
Ab initio studies of the water dimer using large basis sets: The structure and thermodynamic energies
Authors: Kim Kwang S. , Mhin Byung Jin , Choi U-Sung , Lee Kuntack
Published: July 2002

Ab initio calculations with various large basis sets have been performed on the water dimer in order to study the structure, energetics, spectra, and electrical properties. As a reference system, the calculations of the water monomer were also performed. The second order Mo/ller-Plesset perturbation theory (MP2) using a large basis set (O:13s,8p,4d,2f/H:8s,4p,2d) well reproduces various water monomer experimental data except for the somewhat underestimated absolute energy and hyperpolarizability. The monomer energy calculated with the fourth-order Mo/ller–Plesset perturbation theory (MP4) with the above basis set is −76.407 hartrees, which is only 0.073 hartree above the experimental energy. To compare the theoretical and experimental dimer structures and thermal energies accurately, we summarized the quantum statistical thermodynamic quantities with corrections for anharmonic vibration, rotation, rotation–vibration coupling, and internal rotation. With the correction for the anharmonic binding potential and rotation, the predicted interoxygen distance of the dimer is 2.958 Å, which is so far the closest to the experimental value ∼2.976 Å. The predicted dimer dipole moment is 2.612 D, which is the first agreement with experiment (2.60–2.64 D). The predicted frequency shift of the dimer with respect to the monomer is in good agreement with experiment. With the MP2 calculation using the large basis set, the basis set superposition error correction (BSSEC) of the dimer is only 0.33 kcal/mol, which is by far the smallest among the MP2 results reported. Without BSSEC, the predicted binding energy, enthalpy, free energy, and entropy are all in good agreement with experiment within the error bounds, whereas with BSSEC, some of them seem to be slightly off the experimental error bounds. Nevertheless, the results with BSSEC can be more reliable than those without BSSEC.
Thermodynamic and stochastic theory for nonideal systems far from equilibrium
Authors: Ross John , Chu Xiaolin
Published: July 2002

The thermodynamic and stochastic theory of nonlinear chemical kinetics systems, possibly with multiple stable stationary states, is extended to nonideal species, either nonideal gases or nonideal solutions. The Brönsted theory of the kinetics of nonideal species is chosen for explicit formulation of this extension, but the development is similar for other choices.
Second-order flexoelectric effect in chiral nematic liquid crystals
Authors: Cronin D. W. , Petschek R. G. , Terentjev E. M.
Published: July 2002

A molecular–statistical model for the second-order flexoelectric effect in chiral nematics is developed, accounting for additional chiral terms in the pair intermolecular potential U(1,2). It is shown by symmetry analysis, that there are four independent chiral terms in the flexoelectric polarization, proportional to the square of director gradients, 𝒫∼λ∇n∇n. A molecular–statistical theory is developed for the second-order chiral flexoelectric constants λ. Their values, obtained in the mean-field approximation, depend on molecular dimensions, interaction parameters, and the degree of molecular chirality. Possible experimental situations are discussed where this effect can be observed.
Determination of an improved intermolecular global potential energy surface for Ar–H2O from vibration–rotation–tunneling spectroscopy
Authors: Cohen R. C. , Saykally R. J.
Published: July 2002

A new highly accurate and detailed intermolecular potential surface for Ar–H2O is derived by a direct nonlinear least squares fit to 37 far infrared, infrared, and microwave spectroscopic measurements. The new potential (denoted AW2) gives a much better description of the strong radial dependence of the anisotropic forces and of the binding energy than its predecessor, the AW1 surface [Cohen and Saykally, J. Phys. Chem. 94, 7991 (1990)]. The global minimum on the AW2 potential (De=142.98 cm−1) occurs at the position R=3.636 Å, θ=74.3°, and φ=0°. At these coordinates the argon is located in the monomer plane between the perpendicular to the C2 axis (θ=90°) and the hydrogen bonded geometry (θ=55°). This orientation of the minimum is opposite of that found in recent ab initio calculations of Bulski et al. [J. Chem. Phys. 94, 8097 (1991)] and Chalisinski et al. [J. Chem. Phys. 94, 2807 (1991)]. Both sets of authors find a minimum at an antihydrogen bonded geometry corresponding to an orientation Ar–OH (θ=125°).
Sudden structural change at an air/binary liquid interface: Sum frequency study of the air/acetonitrile–water interface
Authors: Zhang D. , Gutow J. H. , Eisenthal K. B. , Heinz T. F.
Published: July 2002

We report the first observation of a sharp structural change in an air/acetonitrile–water interface as the solution composition varies; the abruptness of which suggests an interfacial phase transition. The structural change is manifested by shifts in the frequency and in the polarization of the signal from the acetonitrile molecules in the interface observed using infrared+visible surface sum frequency generation.
Nonlinear normal modes and local bending vibrations of H+3 and D+3
Authors: Sadovskií D. A. , Fulton Nicholas G. , Henderson James R. , Tennyson Jonathan , Zhilinskií B. I.
Published: July 2002

The structure of bending overtones of the H3+ and D3+ molecular ions at the energies below the barrier to linearity is analyzed using energies and wave functions from full three-dimensional discrete variable representation calculations. The lowest-in-energy states of the vibrational polyads ν2=4,5,6 are shown to follow the localization pattern of local bending modes, three equivalent-by-symmetry principal periodic trajectories of the corresponding classical two-mode system near the equilibrium.
A random matrix/transition state theory for the probability distribution of state-specific unimolecular decay rates: Generalization to include total angular momentum conservation and other dynamical symmetries
Authors: Hernandez Rigoberto , Miller William H. , Moore C. Bradley , Polik William F.
Published: July 2002

A previously developed random matrix/transition state theory (RM/TST) model for the probability distribution of state-specific unimolecular decay rates has been generalized to incorporate total angular momentum conservation and other dynamical symmetries. The model is made into a predictive theory by using a semiclassical method to determine the transmission probabilities of a nonseparable rovibrational Hamiltonian at the transition state. The overall theory gives a good description of the state-specific rates for the D2CO→D2+CO unimolecular decay; in particular, it describes the dependence of the distribution of rates on total angular momentum J. Comparison of the experimental values with results of the RM/TST theory suggests that there is mixing among the rovibrational states.
Critical phenomena in mixtures. II. Synergistic and other effects near mixture critical points
Authors: Munoz F. , Chimowitz E. H.
Published: July 2002

In this paper we investigate the properties of near-critical mixtures as solvent media, with particular regard to the critical azeotrope. The results from Part I are used in conjunction with integral equation theory calculations to demonstrate that the critical azeotrope has a number of unique properties as a solvent medium. Volumetric and thermal effects related to the solvation of dilute solutes are greatly enhanced and these can be related to higher order divergences of these properties near a critical azeotrope. Finally, we consider synergistic solvent effects near a mixture critical point, showing that solvent mixtures exhibit cooperative behavior that affect solvation thermodynamics in interesting ways that have been little discussed to date.
Tests of thermodynamic theory of relative stability in one-variable systems
Authors: Wolff A. N. , Hjelmfelt A. , Ross J. , Hunt P. M.
Published: July 2002

Schlögl’s criterion for equistability of two stable stationary states of an inhomogeneous chemical kinetic system is valid only for single-variable cases dependent along a single spatial coordinate. We test the thermodynamic theory, developed in the preceding article for multivariable systems, in one dimension by comparison with calculations based on the deterministic reaction-diffusion equation for a cubic Schlögl model (single variable). This reaction-diffusion system is equivalent to a coupled multivariable system. The prediction of the thermodynamic theory of equistability approaches Schlögl’s result as the length of the system increases. We also test the theory for an optically bistable (ZnSe) system which has been studied experimentally; in this system, temperature is the variable, and there is transport by thermal conduction. Again, we find that the prediction of the multivariable thermodynamic theory approaches the result from the deterministic kinetic equation as the length of the system increases. Further, the single-variable thermodynamic theory of relative stability of homogeneous steady states agrees with experiments and with the prediction of the deterministic equation.
Adsorption and abstraction of hydrogen on polycrystalline diamond
Authors: Thoms Brian D. , Russell John N. , Pehrsson Pehr E. , Butler James E.
Published: July 2002

The processes of atomic hydrogen adsorption and abstraction on a diamond surface determine the fraction of sites available for reaction with carbon containing species during growth. The relative efficiencies of hydrogen atom adsorption and abstraction on a polycrystalline diamond surface were determined at surface temperatures of 80 and 600 °C using high resolution electron energy loss spectroscopy. Abstraction is seen to occur 0.05 times as efficiently as adsorption on a diamond surface at 80 °C. This is interpreted to indicate that the activation barrier to abstraction is higher than in analogous gas phase reactions. No change in either the adsorption or abstraction rate is seen for a diamond surface at 600 °C indicating that hydrogen atoms do not accommodate the surface during the reaction. Thus, abstraction proceeds via a generalized Eley–Rideal mechanism.
Automatic numerical integration techniques for polyatomic molecules
Authors: Pérez-Jordá José M. , Becke Axel D. , San-Fabián Emilio
Published: July 2002

We describe a new algorithm for the generation of 3D grids for the numerical evaluation of multicenter molecular integrals in density functional theory. First, we use the nuclear weight functions method of Becke [A. D. Becke, J. Chem. Phys. 88, 2547 (1988)] to decompose a multicenter integral ∫F(r) dr into a sum of atomic-like single-center integrals. Then, we apply automatic numerical integration techniques to evaluate each of these atomic-like integrals, so that the total integral is approximated as ∫F(r) dr≊∑iωiF(ri). The set of abscissas ri and weights ωi constitutes the 3D grid. The 3D atomic-like integrals are arranged as three successive monodimensional integrals, each of which is computed according to a recently proposed monodimensional automatic numerical integration scheme which is able to determine how many points are needed to achieve a given accuracy. When this monodimensional algorithm is applied to 3D integration, the 3D grids obtained adapt themselves to the shape of the integrand F(r), and have more points in more difficult regions.

The function F(r), which, upon numerical integration, yields the 3D grid, is called the generating function of the grid. We have used promolecule densities as generating functions, and have checked that grids generated from promolecule densities are also accurate for other integrands. Our scheme is very reliable in the sense that, given a relative tolerance ε, it generates 3D grids which are able to approximate multicenter integrals with relative errors smaller than ε for all the molecules tested in this work. Coarser or finer grids can be obtained using greater or smaller tolerances. For a series of 21 molecules, the average number of points per atom for ε=2.0⋅10−3, ε=2.0⋅10−4, ε=2.0⋅10−5, ε=2.0⋅10−6, and ε=2.0⋅10−7 is respectively 3141 (2.9⋅10−4), 10271 (2.4⋅10−5), 27184 (3.1⋅10−6), 72266 (1.9⋅10−7), and 164944 (5.2⋅10−9) (in parentheses are the maximum errors obtained when integrating the density). It is possible to reduce the number of points in the grid by taking advantage of molecular symmetry. It seems that our method achieves a given accuracy with fewer points than other recently proposed methods.
Rotational reorientation dynamics of disperse red 1 in polystyrene: α -relaxation dynamics probed by second harmonic generation and dielectric relaxation
Authors: Dhinojwala Ali , Wong George K. , Torkelson John M.
Published: July 2002

Using a novel experimental protocol involving second harmonic generation (SHG), the rotational, reorientation dynamics of disperse red 1 (DR1) chromophores doped at 2 wt. % in polystyrene (PS) have been monitored over 11 decades in time, from 10−6 s onwards. This nonlinear optical (NLO) technique allows quantification of the average rotational, reorientation time constant 〈τ〉 over a broad range of temperatures, 28 to 138 °C, above and below the glass transition temperature Tg. Good agreement was obtained between 〈τ〉’s determined from SHG and dielectric relaxation measurements. This is expected since both SHG and dielectric relaxation measurements of PS+2 wt. % DR1 monitor the rotational reorientation of DR1 chromophores. Values of 〈τ〉 fit well to a Williams–Landel–Ferry equation above Tg, indicating that the rotational reorientation of DR1 is coupled to the α relaxation of PS; below Tg the values of 〈τ〉 show an apparent Arrhenius temperature dependence with an activation energy of 45–50 kcal/mol. Above Tg, the SHG data are in agreement with the literature values of the temperature dependence of the α-relaxation dynamics for homopolymer PS obtained using NMR, viscosity, compliance, and photon correlation spectroscopy measurements; at Tg the absolute values of 〈τ〉 obtained by SHG are also in good agreement with the literature values obtained for relaxation times in PS using NMR, photon correlation spectroscopy, and enthalpy relaxation. Upon scaling the data using a reduced parameter, Tr/T, where Tr is close to the calorimetrically determined Tg, an excellent overlap of SHG results for the rotational reorientation dynamics of DR1 and thereby the α transition in PS, poly(ethyl methacrylate), and poly(isobutyl methacrylate) is observed below Tg; however, above Tg, the PS data show a much stronger temperature dependence, suggesting that PS may be a more ‘‘fragile’’ glass-former than the methacrylate-based polymers. Physical aging greatly shifts the relaxation to longer times, resulting in a significant increase in the temporal stability of SHG properties in these NLO polymers.
State-selective multireference coupled-cluster theory employing the single-reference formalism: Implementation and application to the H8 model system
Authors: Piecuch Piotr , Adamowicz Ludwik
Published: July 2002

The new state-selective (SS) multireference (MR) coupled-cluster (CC) method exploiting the single-reference (SR) particle-hole formalism, which we have introduced in our recent paper [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)], has been implemented and the results of the pilot calculations for the minimum basis-set (MBS) model composed of eight hydrogen atoms in various geometrical arrangements are presented. This model enables a continuous transition between degenerate and nondegenerate regimes. Comparison is made with the results of SR CC calculations involving double (CCD), single and double (CCSD), single, double, and triple (CCSDT), and single, double, triple, and quadruple (CCSDTQ) excitations. Our SS CC energies are also compared with the results of the Hilbert space, state-universal (SU) MR CC(S)D calculations, as well as with the MR configuration interaction (CI) results (with and without Davidson-type corrections) and the exact correlation energies obtained using the full CI (FCI) method. Along with the ground-state energies, we also analyze the resulting wave functions by examining some selected cluster components. This analysis enables us to assess the quality of the resulting wave functions. Our SS CC theory truncated at double excitations, which emerges through selection of the most essential clusters appearing in the full SR CCSDTQ formalism [SS CCSD (TQ) method] provides equally good results in nondegenerate and quasidegenerate regions. The difference between the ground-state energy obtained with the SS CCSD(TQ) approach and the FCI energy does not exceed 1.1 mhartree over all the geometries considered. This value compares favorably with the maximum difference of 2.8 mhartree between the SU CCSD energies and the FCI energies obtained for the same range of geometries. The SS CCSD(T) method, emerging from the SR CCSDT theory through selection of the most essential clusters, is less stable, since it neglects very important semi-internal quadruple excitations. Unlike the genuine multideterminantal SU CC formalism, our SS CC approach is not affected by the intruder state problem and its convergence remains satisfactory in nondegenerate and quasidegenerate regimes.
Polymer melt near a solid wall
Authors: Brazhnik Pavel K. , Freed Karl F. , Tang Hai
Published: July 2002

We develop a theory for the equilibrium concentration profile formed by a compressible polymer melt near a solid wall (or the mathematically equivalent incompressible polymer solution near a solid wall). The theory uses a Landau–Ginzburg free energy functional with a concentration dependent square gradient coefficient and an additional local contribution characterizing the influence of the wall. We introduce a mean-field algorithm for constructing the surface free energy contribution from the expression for the bulk free energy of lattice polymers. This algorithm automatically includes both energetic and entropic contributions with no adjustable parameters for lattice systems and can be applied for branched polymers as well. Approximate analytical solutions are provided for one-phase polymer density profiles at neutral, repulsive, and attractive walls. The approximate solutions reflect the behavior of the numerical solutions and display (only at lower polymer densities) a near wall linear variation of the polymer density which crosses over to an exponential approach to the bulk concentration. The numerically evaluated profiles for both athermal and nonathermal melts compare well with available Monte Carlo simulation data for a neutral wall. Physical arguments are presented which anticipate the existence of deviations between theory and simulations at higher densities. The use of lattice cluster theory free energy functions enables us for the first time to investigate the dependence of the density profile on the polymer architecture.
Reactions of simple hydrocarbons with Nb+n: Chemisorption and physisorption on ionized niobium clusters
Authors: Berg Christian , Schindler Thomas , Niedner-Schatteburg Gereon , Bondybey Vladimir E.
Published: July 2002

In a Fourier transform-ion cyclotron resonance mass spectrometer the gas phase reactivities of niobium clusters Nb+n (n=1–28) with molecular hydrogen, water, methane, ethane, n-propane, n-heptane, cyclohexane, acetylene, ethylene, allene, benzene, propene, toluene, xylene, and acetonitrile were investigated under single collision conditions as well as the reactivities of oxidized niobium cluster cations with ethylene and benzene. The reactions of larger clusters with a variety of unsaturated hydrocarbons are believed to proceed via long lived ‘‘physisorbed’’ addition intermediate complexes, which subsequently rearrange to form ‘‘chemisorbed,’’ extensively dehydrogenated final products. The overall reaction seems to proceed with near collision rates, almost independent of cluster size. In some cases also the physisorbed primary products are stabilized and detected. Their yields depend sensitively on the specific nature of the reactant, and on the niobium cluster size n. Fully saturated hydrocarbons unable to form the long lived complexes are unreactive with respect to the larger (n≳7) Nb+n clusters. Smaller clusters with n≤7 seem to react by a different, prompt reaction mechanism. The rate of this reaction steeply decreases, and the degree of product dehydrogenation increases with n.
Computer design of idealized β-motifs
Authors: Kolinski Andrzej , Galazka Wojciech , Skolnick Jeffrey
Published: July 2002

A lattice model of protein conformation and dynamics is used to explore the requirements for the de novo folding from an arbitrary random coil state of idealized models of four and six-member β-barrels. A number of possible conjectures for the factors giving rise to the structural uniqueness of globular proteins are examined. These include the relative role of generic hydrophilic/ hydrophobic amino acid patterns, the relative importance of the specific identity of the hydrophobic amino acids that form the core of the protein and the possible role played by polar groups in destabilizing alternative, misfolded conformations. These studies may also provide some insights into the relative importance of short range interactions, cooperative hydrogen bonding and tertiary interactions in determining the uniqueness of the native state, as well as the cooperativity of the folding process. Thus, these simulations may provide guidelines for the early stages of the protein design process. Possible applications to the general protein folding problem are also briefly discussed.
Reaction of Cl with vibrationally excited CH4 and CHD3: State-to-state differential cross sections and steric effects for the HCl product
Authors: Simpson William R. , Rakitzis T. Peter , Kandel S. Alex , Orr-Ewing Andrew J. , Zare Richard N.
Published: July 2002

The mechanism for the reaction of atomic chlorine with vibrationally excited methane is investigated by measurement of correlated state and scattering distributions using the method of core extraction (see preceding paper). Laser photolysis of molecular chlorine creates monoenergetic chlorine atoms (≳98% Cl 2P3/2) that react with vibrationally excited methane molecules prepared by linearly polarized infrared laser excitation. The resulting HCl product population distributions are determined by (2+1) resonance-enhanced multiphoton ionization (REMPI), and the differential cross section for each product rovibrational state is measured by core extraction. Approximately 30% of the product is formed in HCl(υ=1,J) with a cold rotational distribution; the remaining population is formed in HCl(υ=0,J) and is more rotationally excited. We observe a rich variation of the scattered flux that is dependent on the internal-energy state of the product. The HCl(υ=1) product is sharply forward scattered for low J and becomes nearly equally forward–backward scattered for high J; the HCl(υ=0,J) product is back and side scattered. The reactions of Cl with C–H stretch-excited methane (CH4) and C–H stretch-excited CHD3 are found to have similar angular and internal-state distributions. Observation of the spatial anisotropy of the HCl(υ=0, J=3) product shows that significant vibrational excitation of the methyl fragment does not occur.

The measured spatial anisotropy is most consistent with a model in which backscattered HCl(υ=0, J=3) is formed in coincidence with slight methyl vibrational excitation and the forward-scattered HCl(υ=0, J=3) is formed in coincidence with no methyl excitation. The approach of the attacking chlorine atom with respect to the C–H stretch direction can be varied by rotating the plane of polarization of the infrared excitation. A marked steric effect is observed in which Cl atoms approaching perpendicular to the C–H stretch preferentially yield forward-scattered HCl(υ=1) product. On the other hand, the reaction is weakly dependent on the rotational quantum state of CH4(υ3=1,J), and on the rotational polarization. The data are consistent with a model that has a widely open ‘‘cone of acceptance’’ in which the impact parameter controls the internal-state and scattering distributions of the HCl product.
Structure and IR-spectrum calculations for small SF6 clusters
Authors: Beu T. A. , Takeuchi K.
Published: July 2002

A new second order perturbation approach for evaluating the splittings and shifts of the vibrational bands of homogeneous molecular clusters, consistently treating degenerate normal modes, is described. The Hamiltonian of the system comprises harmonic and anharmonic intramolecular vibration terms, and the intermolecular potential. The anharmonic intramolecular contributions and the intermolecular potential are treated as a perturbation. A new site-site intermolecular potential model for SF6, featuring exchange, dispersion, electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine SF6 cluster structures up to the hexamer and, by means of the new line shift formalism, to calculate the corresponding IR-spectra in the region of the ν3 vibrational mode (at 947.968 cm−1). The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is confirmed to be the electrostatic one (implicitly the resonant dipole-dipole coupling). The theoretical spectra are shown to fairly describe the experimental evidence when considering only exchange, dispersion and electrostatic interactions. With the available atomic polarizabilities, induction seems to lead to a systematic redshift of the entire spectrum for all cluster sizes. The structure of the cluster vibrations is investigated in terms of the individual monomer vibrations and is correlated with the found geometrical cluster configurations.
Structure of chemically synthesized nanophase GaAs studied by nuclear magnetic resonance and x-ray diffraction
Authors: Potter L. D. , Guzelian A. A. , Alivisatos A. P. , Wu Y.
Published: July 2002

Nanophase GaAs produced by organometallic synthesis was studied by 71Ga, 69Ga, and 75As nuclear magnetic resonance (NMR) as well as x-ray diffraction. The structure of the samples synthesized below 250 °C is predominantly amorphous. Raising the temperature of synthesis (or post-synthesis annealing) above 280 °C improves significantly the crystallinity as evidenced by the appearance of a sharp bulklike 71Ga (and 69Ga) peak. In addition, a sharp peak shifted up-field also appears. Other NMR features of this up-field shifted peak are very similar to the bulklike peak including quadrupole interactions and spin–lattice and spin–spin relaxations. These results are consistent with the presence of stacking faults in nanocrystalline GaAs.
Matrix isolation study of the interaction of excited neon atoms with SiF4: Infrared spectra of SiF+3 and SiF−3
Authors: Jacox Marilyn E. , Irikura Karl K. , Thompson Warren E.
Published: July 2002

When a Ne:SiF4 mixture is codeposited at approximately 5 K with a beam of excited neon atoms, the infrared spectrum of the resulting deposit includes absorptions of SiF3, together with new absorptions which have been assigned to SiF+3 and SiF−3. The results of ab initio calculations of the structure and ground-state vibrational fundamentals of these two ion species are presented, and support the proposed assignments.
Tunneling currents in electron transfer reactions in proteins
Authors: Stuchebrukhov Alexei A.
Published: July 2002

A new theoretical method for the analysis of the superexchange coupling and localization of electron tunneling pathways in long distance electron transfer reactions is introduced. The new method allows one to examine spatial distribution of microscopic quantum mechanical tunneling currents flowing through individual atoms, or to evaluate the relative probability that the tunneling electron will pass through an individual atom, in the intervening medium between donor and acceptor in the course of an electron transfer reaction. It is shown how the interatomic tunneling currents introduced in this paper can be calculated using methods of quantum chemistry. The method provides a rigorous theoretical framework for the description of the tunneling process in long-range electron transfer reactions in proteins. The relation of the present theory of tunneling currents to the theory of pathways of Beratan and Onuchic is discussed.
TTF–TCNE a charge transfer π–molecular crystal with partial ionic ground state: Optical properties and electron-molecular vibrations interaction
Authors: Meneghetti M. , Pecile C.
Published: July 2002

The electronic ground state structure of the charge transfer molecular complex TTF–TCNE is investigated on the basis of its electronic and vibrational spectra. Highly oriented polycrystalline films and the spectra of the fully deuterated complex, TTF-d4–TCNE, allow one to obtain a full exploitation of the spectra. Using the vibrational frequencies as local probes of the electronic structure one finds a value of 0.5±0.1 for the degree of charge transfer of this molecular solid. This partial degree of charge transfer and the alternation of self-dimers of TTF and TCNE along the one-dimensional electronic π-structure reveal themselves in the vibrational spectra and particularly in the charge transfer vibronic resonances present in the infrared and Raman spectra. These resonances and the electronic spectrum related to the charge transfer excitations are understood on the basis of a Holstein–Hubbard model which allows the determination of the electron-intramolecular vibration coupling constants of TTF and TCNE.
The correlation between hydrogen bond tunneling dynamics and the structure of benzoic acid dimers
Authors: Brougham D. F. , Horsewill A. J. , Ikram A. , Ibberson R. M. , McDonald P. J. , Pinter-Krainer M.
Published: July 2002

We report a correlation between the rate of incoherent tunneling associated with proton transfer in hydrogen bonds and the structure of aromatic carboxylic acid dimers. The compressibility of the hydrogen bond in benzoic acid, specifically the oxygen–oxygen distance r(O⋅⋅O), has been measured as a function of hydrostatic pressure up to 3.2 kbar using neutron powder diffraction. All data were recorded at a temperature of 5 K. Using previously published pressure dependence NMR measurements, we have investigated the relationship between the dynamics in the quantum regime and r(O⋅⋅O) in the hydrogen bonds of benzoic acid. The incoherent tunneling rate increases exponentially with decreasing r(O⋅⋅O). This behavior is attributed to the increase in the tunneling matrix element as the potential wells and the localized eigenfunctions of the double minimum potential which characterize the system are brought into closer proximity. There is a quantitative agreement between this study, in which the hydrogen bonds are compressed by the application of pressure, and the behavior exhibited by two benzoic acid derivatives with different oxygen–oxygen distances at ambient pressure.
Experimental study of the O–H ring vibrations of the methanol trimer
Authors: Huisken Friedrich , Kaloudis Michael , Koch Martin , Werhahn Olav
Published: July 2002

The O–H ring vibrations of the methanol trimer have been measured employing molecular beam depletion spectroscopy in combination with mass spectrometric detection and kinematic size selection. Three vibrational bands (at 3434, 3472, and 3503 cm−1, respectively) have been identified as being due to the methanol trimer. The observation of three bands is consistent with the asymmetric ring structure predicted by theoretical calculations.
Vibrational relaxation and geminate recombination in the femtosecond-photodissociation of triiodide in solution
Authors: Kühne Thomas , Vöhringer Peter
Published: July 2002

The dynamics of product vibrational deactivation and subsequent geminate recombination of diiodide ions with atomic iodine following 400-nm photolysis of triiodide in ethanol solution has been studied using femtosecond transient absorption spectroscopy. The excess vibrational energy of the diatomic product was found to decay on two distinct time scales. An ultrafast subpicosecond component, which accounts for the dissipation of most of the energy that is initially deposited into fragment vibrations, is followed by thermalization near the bottom of the I−2 potential on a time scale of several picoseconds. The former process is associated with recoil of the fragments in the exit channel of the potential energy surface relevant to bond breakage whereas the latter process represents relaxation in the asymptotic limit where interaction between the atom–diatom fragments becomes negligible. Transient product vibrational distributions are determined for delay times larger than the dephasing time of nuclear coherences in the diiodide product ions, thereby providing new information about the mechanism for bond fission. These product distributions are translated into energy-time profiles which are analyzed by a master-equation approach using various model functions for the power spectrum of solvent forces acting on the I−2 vibrational coordinate. The dynamics of geminate recombination are found to exhibit a strongly nonexponential character and are interpreted with a simple diffusion model that takes the initial stages of bond breakage and recoil of the fragments into account.
Binding energy of the ring form of (H2O)6: Comparison of the predictions of conventional and localized-orbital MP2 calculations
Authors: Pedulla J. Marc , Vila Fernando , Jordan K. D.
Published: July 2002

The binding energy of the ring form of (H2O)6 is calculated by means of the MP2 and localized-orbital MP2 (LMP2) methods. The LMP2 method is found to be effective at reducing basis set superposition error in the electron correlation contribution to the binding energy. The inclusion of f and g functions on the O atoms and d and f functions on the H atoms leads to an increase of about 3.2 kcal/mol in the stability of the ring form of (H2O)6. Our best estimate of the binding energy is −44.3 kcal/mol. Of this, three-body interactions contribute −11.60 kcal/mol, and the four- , five- , and six-body interactions combined contribute −2.0 kcal/mol. Although inclusion of electron correlation energy is crucial for obtaining an accurate value of the two-body interactions, the net effect of electron correlation on the three- and higher-body interactions is only about 0.2 kcal/mol. Based on these results, a computationally efficient strategy for obtaining accurate binding energies of hydrogen-bonded clusters is proposed.