bluuebunny/crossref_metadata_2025 huggingface dataset:
Ten low-lying electronic states of Pd3
Authors: Balasubramanian K.
Published: July 2002

Multiconfiguration self consistent field (MCSCF) followed by multireference singles plus doubles configuration interaction (MRSDCI) calculations, including up to two million configurations, are carried out on low-lying electronic states of Pd3. In addition, spin-orbit effects are included using a relativistic configuration interaction (RCI) method. There are at least ten low-lying electronic states in the absence of spin-orbit coupling (and more when spin-orbit effects are included) of 1A2, 3B2, 3A1, 1A1, 3A2, 3B1, 5A2, 5B1 symmetry with isosceles triangle structures. The calculated density of states is in accord with the recently observed photoelectron spectrum of Pd−3. The Pd3 trimer was found to be more stable than the dimer with an atomization energy of 124 kcal/mol in the ground state with respect to the separated Pd(3D) atoms. The Mulliken population analysis of the MRSDCI natural orbitals reveal that the apex atom of the isosceles triangle has a considerably larger s population in comparison to the base atoms, while the base atoms have larger d populations in most of the electronic states. All the electronic states of Pd3 exhibit small hybridization of 5p orbitals of the palladium atoms.
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o study of the infrared spectra of linear Cn clusters (n=6–9)
Authors: Martin J. M. L. , François J. P. , Gijbels R.
Published: July 2002

The harmonic frequencies, infrared intensities, and isotopic substitution bands of C6 have been evaluated at the MP2/6-31G* level. They confirm previous assignments of the 1952 and 1197 cm−1 matrix IR bands to C6. Isotopic substitution spectra at the MP2/6-31G* and HF/4-21G levels, as well as those with a newly developed ad hoc modified neglect of diatomic overlap (MNDO) method are shown to be in good agreement with experiment for C4, C5, and C6. On the basis of RHF/4-21G, RHF/6-31G*, MP2/4-21G, MP2/6-31G*, and ad hoc MNDO data, including theoretical frequencies, IR intensities, and isotopic substitution spectra, it is shown with virtual certainty that the 2128 and 1893 cm−1 matrix IR bands belong to C7. A recent assignment of the very intense 1997 cm−1 matrix IR feature to C8 is contradicted on the same grounds, and an alternative assignment to C9 put forward.
Photoelectron spectroscopy of metal cluster anions: Cu−n, Ag−n, and Au−n
Authors: Ho Joe , Ervin Kent M. , Lineberger W. C.
Published: July 2002

Negative ion photoelectron spectra of Cu−n, Ag−n(n=1–10), and Au−n (n=1–5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6–9 meV. The metal cluster anions are prepared in a flowing afterglow ion source with a cold cathode dc discharge. In the spectra of Cu−2, Ag−2, and Au−2, the M2 X 1Σ+g←M−2 X 2Σ+u transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a 3Σ+u triplet states of Cu2 and Ag2 are also observed. Using experimental and theoretical data, we assign the major features in the Cu−3 and Ag−3 spectra to the transition from the linear ground state of the anion (M−31Σ+g) to an excited linear state of the neutral (M3 2Σ+u). The Au−3 spectrum is attributed to a two-photon process, photodissociation followed by photodetachment of the Au− or Au−2 fragment. For larger clusters, we measure the threshold and vertical detachment energies as a function of size. Trends in the electron affinities and excited state energy levels as a function of cluster size and composition are discussed in terms of simple models.
Molecular conformation and disjoining pressure of polymeric liquid films
Authors: Mate C. Mathew , Novotny V. J.
Published: July 2002

Atomic force microscopy, angle resolved x-ray photoelectron spectroscopy, and ellipsometry are applied to study the conformation of fluorocarbon polymers in molecularly thin liquid films, 5–130 Å thick, on solid surfaces. The combination of these techniques shows that the physisorbed polymers at the solid surface have an extended, flat conformation. In addition, the disjoining pressure of these liquid films is determined from atomic force microscopy measurements of the distance needed to break the liquid meniscus that forms between solid surface and force microscope tip. For a monolayer thickness of ∼7 Å, the disjoining pressure is ∼5 MPa, indicating strong attractive interaction between the polymer molecules and the solid surface. The disjoining pressure decreases with increasing film thickness in a manner consistent with a strong attractive van der Waals interaction between the liquid molecules and the solid surface.
Diffusion of Brownian particles with hydrodynamic interaction and hard core repulsion
Authors: Cichocki B. , Felderhof B. U.
Published: July 2002

We study diffusion in a dilute system of Brownian particles with hydrodynamic interaction and hard core repulsion. We assume that the hard core has a radius larger than the hydrodynamic one. The short-time and long-time self-diffusion coefficient, as well as the collective diffusion coefficient, are calculated exactly to first order in the volume fraction. We show that hydrodynamic interactions are important even when the hard core radius is several times larger than the hydrodynamic one.
Extension of Gaussian-1 (G1) theory to bromine-containing molecules
Authors: McGrath Mark P. , Radom Leo
Published: July 2002

Bromine bases suitable for computing G1 energies of bromine-containing molecules have been derived. Our recommended procedure for calculating such energies adopts the G1 steps previously introduced by Pople and co-workers with the following modifications: (i) second-order Mo/ller–Plesset (MP2) geometry optimizations use the polarized, split-valence SV4P basis set for bromine along with 6-31G(d) for first- and second-row atoms; (ii) fourth-order Mo/ller–Plesset (MP4) and QCISD(T) energies are calculated with our new bromine bases along with supplemented 6-311G and McLean–Chandler (MC) 6-311G bases for first- and second-row atoms; and (iii) bromine atomic spin–orbit corrections are explicitly taken into account. G1 energies have been calculated for a selection of simple processes involving bromine-containing molecules. The results obtained are within 0.1 eV of experiment except for the ionization energy of Br2, where the inclusion of molecular spin–orbit corrections is necessary to achieve 0.1 eV accuracy.
Diffusion of finite-sized Brownian particles in porous media
Authors: Kim In Chan , Torquato S.
Published: July 2002

The effective diffusion coefficient De for porous media composed of identical obstacles of radius R in which the diffusing particles have finite radius βR (β≥0) is determined by an efficient Brownian motion simulation technique. This is accomplished by first computing De for diffusion of ‘‘point’’ Brownian particles in a certain system of interpenetrable spherical obstacles and then employing an isomorphism between De for this interpenetrable sphere system and De for the system of interest, i.e., the one in which the Brownian particles have radius βR. [S. Torquato, J. Chem. Phys. 95, 2838 (1991)]. The diffusion coefficient is computed for the cases β=1/9 and β=1/4 for a wide range of porosities and compared to previous calculations for point Brownian particles (β=0). The effect of increasing the size of the Brownian particle is to hinder the diffusion, especially at low porosities. A simple scaling relation enables one to compute the effective diffusion coefficient De for finite β given the result of De for β=0.
Water in weak interactions: The structure of the water–nitrous oxide complex
Authors: Zolandz D. , Yaron D. , Peterson K. I. , Klemperer W.
Published: July 2002

The rotational spectra of H2O–N2O, D2O–N2O, and HDO–N2O have been observed using molecular beam electric resonance techniques at both zero and nonzero electric fields. H2O–N2O is nonrigid with respect to internal rotation of the water subunit. Rotational constants in MHz for the spatially antisymmetric tunneling state are A=12 605.001(77), B=4437.978(32), and C=3264.302(32). Rotational constants for the spatially symmetric tunneling state are A=12 622.595(203), B=4437.422(47), C=3264.962(47). These together with the rotational constants of the other isotopomers are consistent with a planar, T-shaped arrangement of the heavy atoms of the complex, with the distance between the centers of mass of the two subunits, Rc.m., equal to 2.91(2) Å or a distance of 2.97(2) Å from the H2O oxygen to the central nitrogen of N2O. The measured dipole moments of the two tunneling isomers are identical; μa = 1.480(2) and μb = 0.31(2) D. The values of these dipole moment components indicate an in-plane equilibrium tilt of about 20° between the C2v axis of water and the N–O weak bond. This tilt suggests a second interaction may exist between a hydrogen on water and the N2O subunit. The rotational constants suggest that the N2O unit is tilted by about 9° from perpendicular to the N–O weak bond. The barrier for the tunneling interchange of the water protons is estimated to be 235(10) cm−1. Quadrupole coupling constants eqQaa for the outer and inner nitrogen of N2O are 0.371(130) and 0.128(45) MHz, respectively. Electrostatic models applied to water–N2O and water–CO2 predict hydrogen bonded structures rather than the experimentally observed Lewis base structures.
Properties of deposited size-selected clusters: Reactivity of deposited silicon clusters
Authors: Bower J. Eric , Jarrold Martin F.
Published: July 2002

The room temperature oxidation of deposited size-selected silicon clusters (Sin, n=10, 13, and 40–50) has been examined using x-ray photoelectron spectroscopy (XPS). The size-selected clusters were deposited as positive ions on an amorphous carbon substrate at 5 eV. Silicon cluster sticking probabilities (determined from Rutherford backscattering measurements) were 95%±5%. XPS spectra were recorded both before and after exposure to O2. The unoxidized clusters showed no significant (≥0.2 eV) core levels shifts relative to bulk silicon. Oxygen adsorption was monitored by following the evolution of the O1s and Si2p core levels. While the results are qualitatively similar to those obtained for bulk silicon surfaces, the O2 sticking coefficients are much smaller (∼0.001 compared to ∼0.1). The O2 sticking coefficients are similar to those observed for the larger clusters in the gas phase, however, the spectacular variations in the reactivity of the smaller clusters in the gas phase are not reproduced. All the deposited clusters have similar O2 sticking coefficients. A number of observations suggest that there are substantial cluster–cluster interactions on the substrate (possibly leading to coalescence and sintering).
Surface scattering of x rays in thin films. Part II. Experiments on thin soap films
Authors: Daillant J. , Bélorgey O.
Published: July 2002

The diffuse scattering of x rays by black-soap films has been investigated. The results were analyzed using a treatment of surface scattering developed in a companion paper. We show that the fluctuations of the surfaces limiting the film are correlated at wavelengths on the order of 100 nm. The results are consistent with the Derjagin–Landau–Verwey–Overbeek theory when applicable. Reflectivity results have been reinterpreted within this frame, showing that the Newton black film can be seen as a single fluctuating membrane.
Surface chemical reactions studied via ab initio-derived molecular dynamics simulations: Fluorine etching of Si(100)
Authors: Weakliem Paul C. , Carter Emily A.
Published: July 2002

Previous isothermal dynamics simulations of the interaction of F with Si(100) failed to predict any reaction beyond saturation of the surface dangling bonds. We show that this lack of reactivity was due to the overly repulsive nature of the empirical potential employed. We used the method of simulated annealing to fit a new analytic interaction potential to data from ab initio configuration interaction calculations. This potential was then utilized in isothermal molecular dynamics simulations to explore the mechanism by which fluorine begins to etch silicon. Calculated adsorption and reaction probabilities, as a function of both fluorine coverage and structure, reveal that the buildup of the fluorosilyl layer occurs via several competing reactions and that it does not follow a well defined reaction sequence. This competition creates disorder in the adsorbed fluorosilyl layer, which is shown to be an important precursor to continued reaction. Idealized ordered surface structures are shown to be unstable relative to highly disordered structures for coverages of more than 1.25 ML of fluorine.
The electron affinity of platinum
Authors: Gibson N. D. , Davies B. J. , Larson D. J.
Published: July 2002

The electron affinity of platinum has been measured to be 17 125(9) cm−1. A pulsed dye laser beam perpendicular to an ion beam photodetached the Pt−. The relative cross section near threshold was measured by detecting the fast neutrals. The threshold was determined by fitting the cross section to the Wigner law.
Dynamics of molecules with internal degrees of freedom by multiple time-step methods
Authors: Watanabe Masakatsu , Karplus Martin
Published: July 2002

In molecular dynamics simulations of flexible molecules, the treatment of motions that take place on different time scales (e.g., bond stretching and bond angle bending vs dihedral angle librations and overall translation and rotation) or of forces that vary differently with distance (e.g., bonding forces vs electrostatic forces) is a major consideration. The most rapidly varying quantity limits the integration time step, while the most slowly varying quantity usually determines the simulation time required. The reversible multiple time-step methods introduced by Tuckerman, Berne, and Martyna [J. Chem. Phys. 97, 1990 (1992)] allow one to use different integration time steps in the same simulation so as to treat the time development of the slow and fast motions most effectively. As a first step toward extending multiple time-step methods to macromolecules, we investigate their utility for determining the dynamics of n-alkanes as neat liquids and in aqueous solution. Because the dynamics is done in Cartesian rather than internal coordinates, the different time steps are implemented in terms of hard and soft forces acting on individual atoms; e.g., hard forces are contributed by bond stretching (and bond angle bending) terms and soft forces by the remaining intramolecular and intermolecular (van der Waals and electrostatic) terms in the additive empirical potential function used to describe these systems. Integrations with and without a Nose–Hoover thermostat are performed. Comparisons are made with ordinary single time step molecular dynamics and dynamics with constraints applied to the bonds. The stability of the integration methods and the dynamic properties are considered. It is shown that stable integrations that yield agreement with standard dynamics can be achieved with a saving of up to a factor of 5 in computer time.
Photoelectron spectroscopy of Cu−n clusters: Comparison with jellium model predictions
Authors: Cha Chia-Yen , Ganteför G. , Eberhardt W.
Published: July 2002

We present a comparison of the electronic level structure of Cu−n clusters with the jellium model using photoelectron spectroscopy of metal cluster anions. The spectra are recorded at an energy resolution of 30 meV using photon energies of up to 6.4 eV. We obtain a well resolved picture of the electronic structure of the 4s derived electronic states in the energy region between the localized 3d derived states and the highest occupied molecular orbital. The observed features can be assigned to the 1s, 1p, and 1d shells predicted by the jellium model if ellipsoidal distortions and effects like shake-up processes, multiplet splittings and the s–d hybridization are taken into consideration.
Effects of electron correlation in the calculation of nuclear magnetic resonance chemical shifts
Authors: Gauss Jürgen
Published: July 2002

Using second-order many-body perturbation theory [MBPT(2)] and the gauge-including atomic orbital (GIAO) ansatz, electron correlation effects are investigated in the calculation of NMR chemical shieldings and shifts. A thorough discussion of the theory, aspects of the implementation as well as the computational requirements of the GIAO-MBPT(2) method are presented. The performance of the GIAO-MBPT(2) approach is tested in benchmark calculations of 13C, 15N, and 17O chemical shifts. Comparison with available experimental gas phase NMR data shows that GIAO-MBPT(2) improves in all cases considered here over the GIAO results obtained at the Hartree–Fock self-consistent-field (HF-SCF) level. Correlation effects turn out to be particularly important for molecules with multiple bonds, e.g., carbonyl or cyano compounds, and it seems that GIAO-MBPT(2) slightly overestimates these effects for difficult cases having relatively large correlation contributions of 30 to 110 ppm. For CO, N2, N2O, additional calculations with large basis sets are presented to check the accuracy of the GIAO-MBPT(2) method and the geometry dependence of the calculated chemical shieldings is analyzed.
Hybrid simulations of solvation effects on electronic spectra: Indoles in water
Authors: Muiño Pedro L. , Callis Patrik R.
Published: July 2002

Solute–solvent interactions and dynamics are simulated with a fully molecular hybrid method consisting of a semiempirical quantum mechanical method with singly excited configurations for the solute and classical molecular dynamics (MD) for the solvent (H2O). The interactions are purely electrostatic, with the solute being polarizable and sharing its charge information with the MD at 5 fs intervals. The solvent charges are fixed and the results are not sensitive to the point charges used. For the solute, the results depend on the dipole moment much more than on the point charge magnitudes leading to a given dipole. This method is applied to the spectral shifts, dynamics, linewidths, and free energies of indole and 3-methylindole (3MI) in water at 300 K, including the effect of geometry changes and clarifications concerning vertical vs 0–0 transition predictions. Large fluorescence Stokes shifts are predicted, in fair agreement with observed values. The 1La excited state dipole is calculated to be about 12 D after solvent relaxation following excitation. This increase of about 5 D above that calculated in vacuum is caused by the solvent reaction field, and approximately doubles the calculated shift compared to that using the vacuum dipoles. There does not seem to be a need to invoke a solute–solvent excited state charge transfer complex (exciplex) to account for the large shifts. About 50% of the Stokes shift occurs in ∼15 fs with a Gaussian response function, and the remainder is approximately an exponential with τ=400 fs. The fast component is created by small rotational deviations in the trajectories of a few nearby waters. The change in free energy of solvation upon excitation is found to be half the sum of the absorption and fluorescence shifts.
Pressure and temperature dependent viscosity of two glass forming liquids: Glycerol and dibutyl phthalate
Authors: Cook Richard L. , King H. E. , Herbst Chris A. , Herschbach Dudley R.
Published: July 2002

The pressure and temperature dependent viscosities of two glass forming liquids, glycerol and dibutyl phthalate (DBP), have been studied in the range P=0–3 GPa, T=0–125 °C, and η=101–1010 cP. These studies were made using a combination of a rolling-ball and a centrifugal-force diamond anvil cell viscometer. The majority of the results extend up to viscosities of 107 cP, with those at 22.5 °C going to 1010 cP. The overall precision of the data are approximately 10% or better throughout. This level of precision allows us to define a viscosity surface which can then be extrapolated to the glass transition along both temperature and pressure cuts. The T-dependence of viscosity is larger for glycerol than DBP but the P-dependence smaller for glycerol than for DBP, whereas the T-dependence is much more pressure sensitive for DBP. These data provide an assessment of the T-dependence of an isothermal model (free volume), the P-dependence of an isobaric model (Vogel–Tammann–Fulcher) and by extension that for isochoric conditions. Fragility parameters are evaluated for these three isometric conditions. For glycerol and (less conclusively) DBP under isobaric conditions, the fragility increases markedly at high pressure. Under isochoric conditions, the fragility for both glycerol and DBP increases with increasing density. This is dramatic for DBP, which goes from a strong to an intermediate-strength liquid. For the isothermal model, we derive a new measure of fragility. Using this, DBP shows a trend common to several liquids, a decrease in fragility with increasing temperature. Glycerol, however, becomes more fragile over the same temperature range. For glycerol, the trends towards increased fragility at elevated pressure and temperature are consistent with diminished hydrogen bonding under those conditions. The P-dependence of the glass transition is also determined over a wide range of T. The slope, dTg/dP, is positive with the pressure dependence for glycerol being considerably smaller than for DBP; both are nonlinear, tending to saturate in temperature at high pressures.
On superexchange electron-transfer reactions involving three paraboloidal potential surfaces in a two-dimensional reaction coordinate
Authors: Tang Jau , Norris James R.
Published: July 2002

A stochastic Liouville theory is presented for the superexchange electron-transfer reactions involving three paraboloidal potential surfaces in a two-dimensional reaction coordinate. Its close relationship with the spin–boson model for photosynthesis is discussed. A triangle representation is used to explain the relationship between the reorganization energy and the degree of correlation for the solvent fluctuations experienced by the donor, the intermediate, and the acceptor. Explanations for a small reorganization energy for an efficient superexchange mechanism in natural photosynthesis are offered.
Triplet-state photoexcitations of oligothiophene films and solutions
Authors: Janssen R. A. J. , Smilowitz L. , Sariciftci N. S. , Moses D.
Published: July 2002

We present studies of steady-state photoinduced absorption (PIA) spectroscopy on photoexcitations in a series of well-defined α-oligothiophene (Tn, n=6, 7, 9, and 11) films and solutions. The PIA spectra and the excited state lifetimes are consistent with the signatures of a photoexcited triplet state. The PIA spectra consist of a strong vibronically resolved subgap absorption, which is readily observed in solid-state films and in solutions at ambient and cryogenic temperatures. The transition energy is linearly dependent on the reciprocal chain length and shifts to lower energy for longer oligomers. Variation of the modulation frequency and the pump intensity under matrix-isolated conditions reveals that the photoexcitation is created via an intrachain mechanism and decays nonradiatively with monomolecular kinetics. In solid films we find a significant contribution of a bimolecular decay process to the relaxation rate.
Relativistic all-electron ab initio calculations of ground and excited states of LiHg including spin–orbit effects
Authors: Gleichmann Matthias M. , Hess Bernd A.
Published: July 2002

We present the results of relativistic all-electron ab initio calculations on LiHg including spin–orbit effects. Kinematical relativistic effects were taken into account by use of the spin-free no-pair Hamiltonian obtained from a second-order Douglas–Kroll transformation, the spin–orbit coupling by use of a relativistically corrected spin–orbit operator also based on the Douglas–Kroll transformation, which was used for the first time within molecular calculations. From these calculations, the dissociation and transition energies of the ground state and seven excited states of the LiHg excimer as well as their spectroscopic parameters were determined. The results are in good agreement with the existing experimental data.
Real time path integral methods for a system coupled to an anharmonic bath
Authors: Ilk Gregory , Makri Nancy
Published: July 2002

We propose an efficient path integral scheme for calculating the quantum dynamics of an arbitrary one-dimensional system coupled nonlinearly to many anharmonic noninteracting ‘‘bath’’ degrees of freedom. The starting point is an improved discretization of the path integral in terms of numerically constructed propagators [Chem. Phys. Lett. 193, 435 (1992)]. The resulting influence functional is comprised of one-dimensional correlation functions with step-structured time-dependent potentials and therefore is similar in structure to that employed in the spin-boson calculations of Coalson [J. Chem. Phys. 86, 995 (1987)]. In the present case, though, the influence functional is nonlinear and is computed using numerical iterative wave function propagation methods. Numerical tests on a system coupled to ten anharmonic oscillators demonstrate the efficiency of the proposed scheme, which requires numerical effort that scales only linearly with the number of anharmonic bath degrees of freedom.
State-selective multi-reference coupled-cluster theory employing the single-reference formalism: Application to an excited state of H8
Authors: Alexandrov Vadim , Piecuch Piotr , Adamowicz Ludwik
Published: July 2002

The state-selective (SS) multi-reference (MR) coupled-cluster (CC) method exploiting the single-reference (SR) formalism [P. Piecuch, N. Oliphant, and L. Adamowicz, J. Chem. Phys. 99, 1875 (1993)] is applied to the first excited totally symmetric singlet state of a prototype molecular system composed of eight hydrogen atoms. Minimum basis set is employed and various geometries are considered. The SS CC energies are compared with the results of the state-universal (SU) MR CC calculations involving single and double excitations (CCSD) as well as with the exact energies obtained using the full configuration interaction method. Comparison is also made with the results of the standard SR CCSD calculations. In both nondegenerate and quasidegenerate regions, our SS CC theory truncated at double excitations [SS CCSD(TQ) method] provides much better description of the first excited state than the genuine multi-determinantal SU CCSD formalism.
Exact and approximate nonlinear least-squares inversion of dielectric relaxation spectra
Authors: Macdonald J. Ross
Published: July 2002

Three weighted, complex nonlinear least-squares methods for the deconvolution of dielectric or conducting system frequency-response data are described and applied to synthetic data and to dielectric data of n-pentanol alcohol, water, and glycerol. The first method represents a distribution of relaxation times or transition rates by an inherently discrete function. Its inversion accuracy and resolution power are shown to be limited only by the accuracy of the data when the data themselves arise from a discrete distribution involving an arbitrary number of spectral lines. It is shown that those inversion methods employed here which allow the relaxation times to be free variables are much superior to those where these quantities are fixed. Furthermore, free-τ methods allow unambiguous discrimination between discrete and continuous distributions, even for data with substantial errors. Contrary to previous conclusions, discrete distributions were determined for both n-pentanol alcohol and water. A complex, continuous distribution estimate was obtained for glycerol. Algorithms for all approaches are incorporated in a readily available computer program. Serious problems with some previous dielectric inversion methods are identified. Finally, several possibilities are mentioned that may allow greater inversion resolution to be obtained for complex nonlinear least-squares estimation of continuous distributions from noisy data.
Computer simulation of liquid/liquid interfaces. I. Theory and application to octane/water
Authors: Zhang Yuhong , Feller Scott E. , Brooks Bernard R. , Pastor Richard W.
Published: July 2002

Statistical ensembles for simulating liquid interfaces at constant pressure and/or surface tension are examined, and equations of motion for molecular dynamics are obtained by various extensions of the Andersen extended system approach. Valid ensembles include: constant normal pressure and surface area; constant tangential pressure and length normal to the interface; constant volume and surface tension; and constant normal pressure and surface tension. Simulations at 293 K and 1 atm normal pressure show consistent results with each other and with a simulation carried out at constant volume and energy. Calculated surface tensions for octane/water (61.5 dyn/cm), octane/vacuum (20.4 dyn/cm) and water/vacuum (70.2 dyn/cm) are in very good agreement with experiment (51.6, 21.7, and 72.8 dyn/cm, respectively). The practical consequences of simulating with two other approaches commonly used for isotropic systems are demonstrated on octane/water: applying equal normal and tangential pressures leads to an instability; and applying a constant isotropic pressure of 1 atm leads to a large positive normal pressure. Both results are expected for a system of nonzero surface tension. Mass density and water polarization profiles in the liquid/liquid and liquid/vapor interfaces are also compared.
Temperature dependence of hydrogen bonding in neat, liquid formamide
Authors: Ludwig R. , Weinhold F. , Farrar T. C.
Published: July 2002

Temperature dependent quadrupole coupling parameters (QCP), χQ, and quadrupole coupling asymmetry parameters, ηQ, of neat, liquid formamide were calculated. These calculations are based on standard ab initio self-consistent field (SCF) methods at the 6–31G* level for eight different clusters and a quantum cluster equilibrium (QCE) model of liquids. The cluster sizes varied from one to six molecules and include linear and cyclic structures. The equilibrium populations of the different formamide clusters were calculated for temperatures between 270 and 400 K. These calculations indicate that at low temperatures a six-membered ring of formamide molecules involving hydrogen bonding between the trans amide deuteron and the oxygen is the dominant (95%) species. At higher temperatures the cluster populations change and the six-membered ring is gradually replaced, primarily by a linear tetramer. The temperature dependence of the calculated QCP values for nitrogen, oxygen and two of the deuterium nuclei (the carbonyl and trans deuterons) are all in good agreement with the experimentally measured NMR results.
Coupled-cluster calculations of nuclear magnetic resonance chemical shifts
Authors: Gauss Jürgen , Stanton John F.
Published: July 2002

Theory and implementation of the gauge-including atomic orbital (GIAO) ansatz for the gauge-invariant calculation of nuclear magnetic resonance chemical shifts are described for the coupled-cluster singles and doubles (CCSD) approach. Results for the shielding constants of the hydrides HF, H2O, NH3, and CH4 as well as for a few multiply bonded systems such as CO, N2, and HCN demonstrate the importance of higher-order correlation corrections, as good agreement with experiment is only obtained at the CCSD level and to some extent at partial fourth-order many-body perturbation theory [SDQ-MBPT(4)] with the latter slightly overestimating correlation effects due to single and double excitations. For relative chemical shifts, GIAO-CCSD calculations provide in difficult cases (e.g., CO and CF4) more accurate results than previous GIAO-MBPT(2) calculations. But, it seems that it is often more important to include rovibrational effects (as well as possible molecule–solvent interactions) than higher-order correlation corrections. Despite that, GIAO-CCSD proves to be a powerful tool for the accurate calculation of NMR chemical shifts. Its capabilities as well as its limitations are demonstrated in shielding calculations for formaldehyde, diazomethane, and ozone. At least for the latter, the description provided by the CCSD ansatz is not sufficient and even higher excitations need to be considered.
An instanton approach to intramolecular hydrogen exchange: Tunneling splittings in malonaldehyde and the hydrogenoxalate anion
Authors: Smedarchina Zorka , Siebrand Willem , Zgierski Marek Z.
Published: July 2002

Calculations of hydrogen tunneling splittings are reported based on a combination of the instanton approach with quantum-chemically computed potentials and force fields. The splittings are due to intramolecular hydrogen transfer in symmetric double-minimum potentials in molecules such as malonaldehyde and the hydrogenoxalate anion. Potential-energy curves along the tunneling coordinates and harmonic force fields at the stationary points are calculated at the HF/6-31G** and HF/6-31+G** level of theory, and combined to yield a complete multidimensional surface. All modes that are displaced between the equilibrium configuration and the transition state are included in the calculation. In the formalism, these modes are linearly coupled to the tunneling mode, the couplings being proportional to the displacements in dimensionless units. These couplings modify the instanton trajectory and subject it to fluctuations. It is argued that within the accuracy of the available potential-energy surfaces, direct calculations of the instanton trajectory can be avoided and that the dynamics can be expressed with adequate accuracy in terms of the classical action integral calculated for the one-dimensional potential along the reaction coordinate with corrections for the coupled modes. In addition, the fluctuations of the coupled modes which control the preexponential factor in the instanton rate equation are included in the adiabatic approximation. These approximations greatly simplify the tunneling dynamics and permit its combination with real rather than model molecular potentials. It is shown that this approach accounts satisfactorily for the zero-point level splittings in malonaldehyde and its monodeuterated isotopomer. Moreover, it yields a detailed picture of the effect of various skeletal modes, both symmetric and antisymmetric, on the observed splittings. The calculations are extended to produce predicted zero-point level splittings for the hydrogenoxalate anion for which no experimental splittings are available as yet.
2H nuclear magnetic resonance study of supercooled toluene: Slow and fast processes above and below the glass transition
Authors: Hinze G. , Sillescu H.
Published: July 2002

2H-NMR spin-lattice relaxation times T1 of phenyl ring deuterated toluene have been measured as a function of temperature (75–290 K) at Larmor frequencies between 13.8 and 55.8 MHz. The results are interpreted by assuming a Cole–Davidson rotational correlation time distribution for the α process dominating T1 at temperatures down to about 20 K above the glass transition (Tg=117 K) and one further process at lower temperatures. The latter is analyzed below Tg using a model with temperature dependent librational angles ΔΦ<13° and mean correlation times τ≳10−9 s. Self-diffusion coefficients are determined at temperatures down to 136 K (D=1.5×10−14 m2 s−1) using a static field gradient 1H-NMR method. The product of D and the mean rotational correlation time shows a tendency to increase at the lowest temperatures.
A complete basis set model chemistry. V. Extensions to six or more heavy atoms
Authors: Ochterski Joseph W. , Petersson G. A. , Montgomery J. A.
Published: July 2002

The major source of error in most ab initio calculations of molecular energies is the truncation of the one-electron basis set. Extrapolation to the complete basis set second-order (CBS2) limit using the N−1 asymptotic convergence of N-configuration pair natural orbital (PNO) expansions can be combined with the use of relatively small basis sets for the higher-order (i.e., MP3, MP4, and QCI) correlation energy to develop cost effective computational models. Following this strategy, three new computational models denoted CBS-4, CBS-q, and CBS-Q, are introduced. The mean absolute deviations (MAD) from experiment for the 125 energies of the G2 test set are 2.0, 1.7, and 1.0 kcal/mol, respectively. These results compare favorably with the MAD for the more costly G2(MP2), G2, and CBS-QCI/APNO models (1.6, 1.2, and 0.5 kcal/mol, respectively). The error distributions over the G2 test set are indistinguishable from Gaussian distribution functions for all six models, indicating that the rms errors can be interpreted in the same way that experimental uncertainties are used to assess reliability.

However, a broader range of examples reveals special difficulties presented by spin contamination, high molecular symmetry, and localization problems in molecules with multiple lone pairs on the same atom. These characteristics can occasionally result in errors several times the size expected from the Gaussian distributions. Each of the CBS models has a range of molecular size for which it is the most accurate computational model currently available. The largest calculations reported for these models include: The CBS-4 heat of formation of tetranitrohydrazine (91.5±5 kcal/mol), the CBS-4 and CBS-q isomerization energies for the conversion of azulene to naphthalene (ΔHcalc=−35.2±1.0 kcal/mol, ΔHexp=−35.3±2.2 kcal/mol), and the CBS-Q heat of formation of SF6 (ΔHcalc=−286.6±1.3 kcal/mol, ΔHexp=−288.3±0.2 kcal/mol). The CBS-Q value for the dissociation energy of a C–H bond in benzene (113.1±1.3 kcal/mol) is also in agreement with the most recent experimental result (112.0±0.6 kcal/mol). The CBS-QCI/APNO model is applicable to the prediction of the C–H bond dissociation energies for the primary (100.7±0.7 kcal/mol calc.) and secondary (97.7±0.7 kcal/mol calc., 97.1±0.4 kcal/mol exp.) hydrogens of propane.
Ammonia adsorption on MgO(100): A density functional theory study
Authors: Nakajima Y. , Doren D. J.
Published: July 2002

The properties of ammonia adsorbed at flat, step and kink sites on a MgO(100) surface have been investigated using density functional theory. A finite MgO cluster is used to represent the MgO(100) surface. Bonding is primarily determined by electrostatic interactions, which can have long range effects on these ionic surfaces. To account for such effects, cluster models of various sizes have been compared and the effects of surrounding the cluster by an array of fixed point charges have been explored. Several different structures have been found for ammonia bound at the various surface sites. The calculated adsorption energy on a flat surface is 19–21 kcal/mol, which is in reasonable agreement with experimental estimates of 13–17 kcal/mol at low coverage. Binding at kink sites is strongly favored over binding on the flat surface and dissociation is feasible only at kink sites. The electrostatic environment has a much greater effect on adsorption at step and kink sites than on the flat surface. The reaction path for diffusion of ammonia on a flat MgO(100) surface is explored. The nature of the ammonia–MgO interactions, the effects of the electrostatic environment, and considerations in the choice of cluster models are discussed.
The CC3 model: An iterative coupled cluster approach including connected triples
Authors: Koch Henrik , Christiansen Ove , Jo/rgensen Poul , Sanchez de Merás Alfredo M. , Helgaker Trygve
Published: July 2002

An alternative derivation of many-body perturbation theory (MBPT) has been given, where a coupled cluster parametrization is used for the wave function and the method of undetermined Lagrange multipliers is applied to set up a variational coupled cluster energy expression. In this variational formulation, the nth-order amplitudes determine the energy to order 2n+1 and the nth-order multipliers determine the energy to order 2n+2. We have developed an iterative approximate coupled cluster singles, doubles, and triples model CC3, where the triples amplitudes are correct through second order and the singles amplitudes are treated without approximations due to the unique role of singles as approximate orbital relaxation parameters. The compact energy expressions obtained from the variational formulation exhibit in a simple way the relationship between CC3, CCSDT-1a [Lee et al., J. Chem. Phys. 81, 5906 (1984)] CCSDT-1b models [Urban et al., J. Chem. Phys. 83, 4041 (1985)], and the CCSD(T) model [Raghavachari et al., Chem. Phys. Lett. 157, 479 (1989)]. Sample calculations of total energies are presented for the molecules H2O, C2, CO, and C2H4. Comparisons are made with full CCSDT, CCSDT-1a, CCSDT-1b, CCSD(T), and full configuration interaction (FCI) results. These calculations demonstrate that CC3 and CCSD(T) give total energies of a similar quality. If results obtained by CC3 and CCSD(T) differ significantly, neither method can be trusted. In contrast to CCSD(T), time-dependent response functions can be obtained for CC3.
Theoretical investigation of the autoionization process in molecular collision complexes: Computational methods and applications to He*(23S)+H(12S)
Authors: Movre M. , Meyer W.
Published: July 2002

The first complete ab initio treatment is applied to the autoionization process in the He*(2s3S)+H(1s) collisional complex. The autoionizing resonance state is defined through Feshbach projection based on orbital occupancy, and the corresponding potential is determined from multireference–configuration interaction (MR-CI) calculations with an accuracy of about 10 meV. The energy-dependent coupling with the continuum is derived from a compact (L2) molecular orbital (MO) without any phase information being lost. This “Penning MO” is projected onto the states of the continuum electron for energies that comply with the resonance condition thus providing the l-dependent coupling elements in local approximation. The continuum electron functions are calculated within the static-exchange approximation for up to 25 coupled angular momentum channels. The nuclear dynamics calculation is based on a complex Numerov algorithm and uses a converged set of seven complex coupling matrix elements. Weighting with experimental collision energy distributions finally gives the angle-dependent, as well as the angle-integrated, electron spectra for Penning and associative ionization processes. The results are discussed with respect to previous, either partial or model studies, and are compared with the recent most detailed experimental study of the angular-dependent Penning ionization electron spectra. The close agreement of theory and experiment demonstrates the adequacy of the local complex potential approach, as well as the importance of electron angular momentum transfer so far neglected in theoretical treatments.
Kinetics over a wide range of temperature (13–744 K): Rate constants for the reactions of CH(ν=0) with H2 and D2 and for the removal of CH(ν=1) by H2 and D2
Authors: Brownsword Richard A. , Canosa André , Rowe Bertrand R. , Sims Ian R. , Smith Ian W. M. , Stewart David W. A. , Symonds Andrew C. , Travers Daniel
Published: July 2002

Rate constants were determined for the reactions of CH(X2Π,ν=0) with H2 and D2 and for the relaxation of CH(X2Π,ν=1) by H2 and D2. The method, employing pulsed laser photolysis to generate CH radicals and laser-induced fluorescence to observe their rate of removal, was implemented between 744 and 86 K in heated and cryogenically cooled cells and from 295 to 13 K in a Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The rate constants for the reaction of CH(ν=0) with D2 were determined from 13 to 584 K and those for the removal of CH(ν=1) by H2 and D2 from 23 to 584 K. These rate constants show no dependence on total pressure and a mild negative temperature dependence, and they are clearly related closely to the rate of capture to form a strongly bound CH3* or CHD2* energized collision complex. The rate constants for the reaction of CH(ν=0) with H2 were measured from 53 to 744 K. By contrast, their values depend in a complex fashion on temperature and total pressure, the latter effect having been studied from 4 to 400 Torr in heated and cooled cells and from 0.28 to 4.5 Torr at 53 K in the CRESU apparatus. It is clear that reaction proceeds via a CH3* complex which, favored by high temperatures and low total pressures, can dissociate to give CH2+H, CH(X2Π)+H2→CH2+H; ΔrH2980=+(12.0±5.2) kJ mol−1 or, especially at low temperatures and high total pressures, may be stabilized by collisions to yield CH3 radicals CH(X2Π)+H2+M→CH3+M; ΔrH2980 =−(450.7±2.0) kJ mol−1. Careful analysis of the pressure-dependent fall-off behavior yields the following expressions for the rate constants (in the case of the association reaction, in the limit of low pressure): k(1a)=(3.1±0.7)×10−10 exp(−(1650±100)/T) cm−3 molecule−1s−1, k(1b)0[Ar]=(5.2±1.5)×10−30(T/298)−1.6±0.22[Ar] cm3 molecule−1 s−1. Comparison of the rate data obtained for the reaction of CH(ν=0) with D2 and for the removal of CH(ν=1) by H2 and D2 is consistent with all three processes occurring through the formation of a CH3* (CHD2*) complex whose rate of formation is independent of the vibrational state of the CH. Consequently the rate constant for the reaction in the limit of high pressure can be estimated from that for the removal of CH(ν=1) by H2 yielding: k(1b)∞=(1.60±0.11)×10−10(T/298)−0.08±0.05 cm3 molecule−1 s−1.
Morphology of porous media studied by nuclear magnetic resonance
Authors: Allen S. G. , Stephenson P. C. L. , Strange J. H.
Published: July 2002

The filling processes of water and cyclohexane in porous silica (40 Å, 60 Å and 112 Å pore size samples) were studied using T2 nuclear magnetic resonance (n.m.r.) experiments. The silica pores contained water or cyclohexane and the experiments were performed at room temperature and at filling fractions ranging from 0.02 to 1.0 (that is, completely full). Two distinct processes were observed which depended on the hydrophilicity of the silica surface (or the surface adhesion of the liquid). Water was found to collect in small puddles in the silica interstices, and to form a surface layer over the silica before the remaining pore volume was filled. Water in a surface-treated porous silica and cyclohexane in regular porous silica appeared to completely fill the smaller before the larger pores, and not form a separate surface-coating layer. This work also presents the techniques used to calculate quantitative information about the filling process; specifically, determination of the volume to surface-area ratio of the liquid puddles as well as the number of these puddles, is demonstrated.