bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigations of spectral broadening mechanisms in biomolecules: Cytochrome-c
Authors: Schomacker K. T. , Champion P. M.
Published: July 2002

We have studied the optical line shape of the intense π–π* ‘‘Soret’’ transition of ferrocytochrome-c as a function of temperature. Use of the linear electron–nuclear coupling strengths found from the transform analysis of the resonance Raman excitation profiles allows us to simulate the measured line shape changes using a multimode time correlator model that is exact at all temperatures. Four major sources of line broadening are considered: (1) Multimode broadening due to the observed Raman (Franck–Condon) active modes; (2) broadening due to a low frequency Franck–Condon active bath that is not observed directly in the resonance Raman spectrum; (3) inhomogeneous broadening, σ, modeled as a Gaussian distribution of 0–0 transition frequencies; (4) homogeneous broadening, Γ, due to exponential population decay of the excited electronic state. We find conclusive evidence that mechanism (2) is not important in determining the Soret band line shape and that mechanism (3) is constrained to an upper limit of σmax=200 cm−1. The results giving the best fits over the entire temperature range yield σ=100 cm−1, Γ=370 cm−1 and show that the thermal properties of the line shape are well described by the observed low frequency Raman modes. The resulting large value of Γ corresponds to ultrafast population decay times (10–20 fs) that are difficult to measure directly. As a result, we have expanded upon a light scattering sum rule that allows direct determination of Γ even in the presence of inhomogeneous broadening. Recent tests of this independent approach are also discussed. Finally, the analysis uncovers strong evidence for a set of weak temperature dependent transitions underlying the Soret band. These transitions are quantified via fitting to Gaussian line shapes and their temperature dependence is explicitly presented.
Anisotropic behavior of the 13C nuclear spin dynamics in a single crystal of l-alanine
Authors: Naito A. , McDowell C. A.
Published: July 2002

Strong anisotropic behavior of the 13C nuclear spin dynamics, namely, orientation dependent transient oscillation frequency of the spin locking cross polarization of the Cα carbon magnetization, the nuclear Overhauser enhancement, and the spin-lattice relaxation times for the CH3 carbon nucleus were observed in a single crystal of l-alanine. The spin locking cross-polarization dynamics, including the transient oscillation effects, are explained by considering the dipolar interaction between the a-carbon nucleus and its directly bonded proton, together with spin diffusion and spin-lattice relaxation in the rotating frame, in the equation of motion of the density operator. We found that by assuming that the CH3 group undergoes random jumps between three equilibrium positions, the application of relaxation theory yields equations which provide a satisfactory interpretation of the experimental observations. Thus we conclude that the orientation dependence of the nuclear Overhauser enhancement, and the spin-lattice relaxation time in the Zeeman frame of the CH3 carbon nucleus is caused by the anisotropic motion of the methyl group.
Effect of incidence kinetic energy and surface coverage on the dissociative chemisorption of oxygen on W(110)
Authors: Rettner C. T. , DeLouise L. A. , Auerbach D. J.
Published: July 2002

The dissociative chemisorption of oxygen on W(110) has been studied using molecular beam techniques. Chemisorption probabilities have been measured as a function of incidence angle, θi, and kinetic energy, Ei, and of surface coverage and temperature. In addition, angular scattering distributions have been measured for a range of conditions and LEED has been used to examine surface structure. The initial (zero coverage limit) sticking probability is found to depend strongly on the incidence energy, scaling with En=Ei cos2 θi. This probability is ∼10% at En =0.1 eV, rising to essentially unity above En =0.4 eV. At half a monolayer coverage of atomic oxygen, the sticking probability is close to zero up to a threshold of ∼0.25 eV, above which it rises to over 50% by 1.3 eV. In most cases, the sticking probability is found to fall roughly linearly with increasing surface coverage. However, a less-than-linear fall-off is observed for En ≥1 eV and for En ≤0.03 eV, the sticking probability actually rises with increasing coverage reaching a maximum at ∼0.2 ML. These results indicate that while dissociation may proceed via a classical precursor at the lowest energies, such a state can play little role

for En≥0.1 eV. For En≤0.3 eV, the chemisorption probability falls to less than 5% for a coverage of about 0.5 ML; however, this apparent saturation coverage rises to 0.75 ML at ∼0.25 eV and to about 1.0 ML at about 0.85 eV. These ‘‘favored’’ coverages of 0.5, 0.75, and 1.0 ML are found to be associated with p(2×1), p(2×2), and p(1×1) LEED patterns, respectively. Angular scattering distributions recorded with a differentially pumped rotatable mass spectrometer, revealed predominantly quasispecular peaks, and velocity distributions are also characteristic of direct-inelastic scattering. The variation of the sticking probability with En is analyzed to obtain barrier height distributions for the clean and half-covered surfaces and these results are used to predict the sticking probability as a function of coverage and also to predict the initial sticking probability for adsorption from ambient gas or an effusive molecular beam. Reasonable agreement is obtained with available data in each case.
The low-lying electronic excitations in long polyenes: A PPP-MRD-CI study
Authors: Tavan Paul , Schulten Klaus
Published: July 2002

A correct description of the electronic excitations in polyenes demands that electron correlation is accounted for correctly. Very large expansions are necessary including many-electron configurations with at least one, two, three, and four electrons promoted from the Hartree–Fock ground state. The enormous size of such expansions had prohibited accurate computations of the spectra for polyenes with more than ten π electrons. We present a multireference double excitation configuration interaction method (MRD-CI) which allows such computations for polyenes with up to 16 π electrons. We employ a Pariser–Parr–Pople (PPP) model Hamiltonian. For short polyenes with up to ten π electrons our calculations reproduce the excitation energies resulting from full-CI calculations. We extend our calculations to study the low-lying electronic excitations of the longer polyenes, in particular, the gap between the first optically forbidden and the first optically allowed excited singlet state. The size of this gap is shown to depend strongly on the degree of bond alternation and on the dielectric shielding of the Coulomb repulsion between the π electrons.
The rate constants for the H+H2 reaction and its isotopic analogs at low temperatures: Wigner threshold law behavior
Authors: Takayanagi Toshiyuki , Masaki Nobuyuki , Nakamura Kazutaka , Okamoto Makoto , Sato Shin , Schatz George C.
Published: July 2002

Variational transition-state theory rate constants for the H+H2 reaction and its isotopic analogs at very low temperatures have been calculated using the LSTH potential surface. The Wigner threshold law is found to hold quantitatively below 1 K and to within 34% on average at 5 K. The transition-state theory rate constants are found to be consistent with the requirements of the Wigner threshold law provided that a tunneling probability having the correct threshold behavior is used in calculating the transmission coefficient. Using these calculated rate constants, decay rates of H and D atoms in solid H2, D2, and HD were estimated and compared with experiment. The good agreement found suggests that these decays occur through quantum mechanical tunneling exchange reactions.
Expansion of a polymer chain with excluded volume interaction
Authors: Muthukumar M. , Nickel B. G.
Published: July 2002

We present explicit values of the mean square end-to-end distance 〈R2〉 of a linear flexible chain for all values of the excluded volume parameter z>0 in space dimension 3. Our results are obtained by performing series analyses of our earlier sixth-order perturbation calculation using Borel summation technique and a consideration of the second virial coefficient. The philosophy of our series analyses is essentially the same as that of Le Guillou and Zinn-Justin [Phys. Rev. B 21, 3976 (1980)]. Changes in 〈R2〉 values with perturbation theory order and with technical variations in the analysis allow us to estimate the accuracy of our results. We make comparisons with many of the previously known crossover formulas. The results of des Cloizeaux et al. [J. Phys. Lett. (Paris) 46, L595 (1985)], obtained by an entirely different analysis of the same sixth-order series, is in excellent agreement with that presented here and well within our estimate uncertainty. Our results can be summarized throughout the full range of z by the simple formula ln(〈R2〉/Ll)=0.1772 ln(1+7.524z+11.06z2), where z=(3/2πl)3/2 wL1/2 with L the chain length, l the Kuhn length, and wl2 the effective binary cluster integral for a pair of segments.
Scaling properties of chain interactions in amphiphilic aggregates
Authors: Viovy J. L. , Gelbart W. M. , Ben-Shaul A.
Published: July 2002

We outline a connection between recent mean-field theories of short-chain packing in micellar systems and earlier approaches developed for treating phase separation in polymer blends. These theories are easily unified on the basis of a common variational principle, thereby allowing a single route for deriving lateral pressures, local ordering, and thermodynamic properties. In this approach the search for conformational probability distribution functions is mapped into a constrained random walk problem, using the monomer propagator formalism first exploited by Edwards. As an application, the case of a compact (uniform density—‘‘dry’’) amphiphilic bilayer is studied in detail. We show that the ‘‘core’’ free energy can be described via a single reduced variable relevant to both short and long chains: convenient scaling relations are proposed and discussed.
Excitation of the OI (3s 5S–3p 5P; λ7774 Å) multiplet by electron impact on O2
Authors: Erdman P. W. , Zipf E. C.
Published: July 2002

We have measured the absolute cross section value and emission linewidths of the OI (3s5S0–3p5P; λ7774 Å) multiplet excited by electron impact on O2. The 3p5P cross section, which was measured from threshold to 400 eV, is large, with a value of 4.23×10−18 cm2 ±14% measured at 100 eV, hence this cascade feature constitutes a significant source of excitation for the OI (5S0) state. The multiplet’s emission line widths are highly Doppler broadened in dissociative excitation, and display two distinct kinetic energy distributions. These distributions indicate that both purely repulsive and discrete, bound, excited molecular states which then predissociate are involved in the dissociation process leading to the excitation of OI λ7774 Å. The magnitude of the measured cross section, and the fragment kinetic energy distribution inferred from the observed Doppler broadening both indicate that previous time-of-flight studies of the metastable OI (5S0) state require some reinterpretation.
Oxygen on Pd(100): Order, reconstruction, and desorption
Authors: Chang S.-L. , Thiel P. A.
Published: July 2002

We have investigated the temperature and coverage-dependent transformations of the ordered structures of oxygen on Pd(100). The four ordered structures are the chemisorbed c(2×2) and p(2×2) lattices, and the reconstructed p(5×5) and ((5)1/2×(5)1/2)R27° lattices. We present evidence that the p(5×5) reconstruction forms in an activated stepfrom c(2×2) regions. The onset of p(5×5) formation is associated with an increase in oxygen sticking coefficient. In thermal desorption, there are three states which can be correlated directly with the structure of the adsorbed phase during desorption: At lowest coverage, the α state shows the traits of second-order kinetics and is due to desorption from a disordered adlayer. At higher coverage, the β state appears and is due to desorption from a layer with c(2×2) order. There is a lower barrier to desorption in the β state than in the α state because of the repulsive second-nearest-neighbor interactions in the c(2×2). At highest coverage, the sharp and narrow γ state emerges. This is accompanied by decomposition of the ((5)1/2×(5)1/2)R27° reconstruction, in which cooperative stabilization of the reconstruction by oxygen atoms effectively creates strong quasiattractive oxygen–oxygen interactions. There are interesting similarities between the oxygen-stabilized reconstructions of Pd(100) and the initial stages of oxidation of Ni(100), as well as the oxygen-stabilized reconstructions of Pt(100). The data are obtained from low-energy electron diffraction coupled with a computer-interfaced Video camera, Auger electron spectroscopy, and thermal desorption spectroscopy.
The effect of heat-transfer law on performance of a two-heat-source endoreversible cycle
Authors: Chen Lixuan , Yan Zijun
Published: July 2002

In this paper it is shown that the optimal two-heat-source endoreversible cycle is the endoreversible Carnot cycle for a class of heat-transfer laws. The relation between optimal efficiency and power output of the cycle is derived. By use of this relation, we discuss the optimal performance of an endoreversible Carnot cycle for three common heat-transfer laws, and reveal the dependence of performance on the heat-transfer law and the heat-transfer coefficient. A number of new universal results are derived, and some major conclusions derived in several references are also deduced.
Theoretical studies of the complexes of benzene and pyrene with water and of benzene with formic acid, ammonia, and methane
Authors: Brédas J. L. , Street G. B.
Published: July 2002

We present ab initio quantum-chemical calculations including correlation effects via second-order Mo/ller–Plesset perturbation theory, of the structures of the complexes that benzene forms with water, ammonia, methane, and formic acid, and of the pyrene–water complex. We describe our results for a number of complex structures, including symmetric and nonsymmetric configurations, in which the aromatic molecule acts as proton acceptor. In some instances, configurations where benzene is the proton donor are also investigated. Series of calculations are performed in order to assess the energy required for the Lewis acid molecule to move on top of benzene or pyrene. Our results are compared to available experimental data on the geometries and relative binding energies of these complexes.
Interaction of oxygen with a Pd(110) surface. II. Kinetics and energetics
Authors: He Jian-Wei , Memmert Ulrich , Norton Peter R.
Published: July 2002

The interaction of oxygen with Pd(110) has been studied by thermal desorption spectroscopy (TDS), work function change (Δφ), and Auger electron spectroscopy (AES) at 100<T<900 K. Thermal desorption spectra reveal three major peaks, a low temperature one (α) being from molecular adsorbates and the other two (β1 and β2) from atomic oxygen. The β1 state is believed to be associated with subsurface oxygen. At 100 K, the population of molecular oxygen increases with increasing oxygen coverage. Above 200 K, all adsorbates are present as atomic oxygen. The rate of population of β1 decreases as T increases. This is explained as a kinetic effect due to the formation of the c(2×4) structure which hinders population of the β1 state. The oxygen induced work function change is found to depend strongly on both oxygen coverage and T. At a given T, Δφ is controlled by surface oxygen coverage, surface structural changes, as well as the formation of subsurface oxygen. A decrease of the intensity ratio of certain spectral features (I0 /IPd ) in the AE spectra is also observed, which might be associated with migration of surface oxygen to the subsurface region. The migration of oxygen to subsurface sites at low temperature is facilitated by the lateral displacements of surface Pd atoms which occurs on oxygen exposure at low T. A physical picture of the interaction of oxygen with a Pd(110) surface is tentatively proposed.
Reactions of H2 with He+ at temperatures below 40 K
Authors: Schauer M. M. , Jefferts S. R. , Barlow S. E. , Dunn G. H.
Published: July 2002

We have measured the rate coefficients for radiative (RCT) and dissociative (DCT) charge transfer from He+ to H2 at temperatures 15≲T≲40 K. We find the radiative charge transfer reaction to be essentially independent of temperature, while the rate coefficient for the dissociative reaction increases with increasing temperature in this temperature range. We compare our results with those of other experimenters and comment on sources of uncertainty unique to this experiment.
Order in chaos and the dynamics and kinetics of unimolecular conformational isomerization
Authors: De Leon N. , Marston C. Clay
Published: July 2002

A high degree of structure and therefore order in chaos is found to exist in the detailed dynamical pathways to conformational isomerization. It is shown that this structure can be used to determine the probabilities associated with the dynamical pathways to reaction, trapping, and back reaction. An earlier publication described the mediation of the dynamics of 3-phospholene by phase space structures we called ‘‘reactive islands’’ (RIS)21. In this paper we extend the physical and mathematical properties of RIS and develop the corresponding kinetic theory. RIS theory is applied to a model of a hindered rotor and 3-phospholene. It is shown that the RIS kinetic model accurately predicts trajectory simulations of conformer population decay. Comparisons with standard RRKM theory are included. A discussion on the extension of RIS theory to quantum reactive dynamics and its relevance to laboratory experiments is also included.
C
i
s–t
r
a
n
s isomerization of methyl nitrite
Authors: Preiskorn Aleksandra , Thompson Donald L.
Published: July 2002

Intramolecular vibrational energy redistribution and intramolecular cis→trans and trans→cis conversions have been investigated as a function of initial excitations of C–H local stretch modes and some bending normal modes in methyl nitrite by using quasiclassical trajectories. Harmonic and anharmonic valence force-field potential-energy surfaces with parameter values based on the available experimental and ab initio results were used in the calculations. The anharmonic potential gives rates of energy transfer and isomerization that are significantly faster than those for the harmonic potential. The rate of cis→trans isomerization is significantly larger than the rate of trans→cis. The energy flow out of excited C–H local stretch modes is irreversible and essentially complete in less than 0.5 ps.
Thermochemical properties of the lithium cluster Li5
Authors: Wu C. H.
Published: July 2002

The existence of the stable lithium cluster Li5 has been proved experimentally by mass spectrometry. From the measurement of the gaseous equilibria Li5(g)+Li2(g)=Li4(g)+Li3(g) and Li5(g)+Li(g)=Li4(g)+Li2(g), an atomization energy D00(Li5) =110.0±2.5 kcal/mol was obtained. The ionization potential (IP) of Li5(g) was determined as IP(Li5)=4.56±0.3 eV, from which the atomization energy of the molecule ion Li+5, D00(Li+5) =130.0±5.0 kcal/mol is obtained for the reaction Li+5→Li++4 Li. The heat of formation was determined as ΔH00(Li5) =82.5±2.0 kcal/mol. By extrapolation from experimental results, the properties of neutral and ion clusters of hexamer lithium Li6 were predicted.
Trapping-mediated dissociative chemisorption of ethane on Ir(110)-(1×2)
Authors: Mullins C. B. , Weinberg W. H.
Published: July 2002

Evidence is presented to support a trapping-mediated dissociative chemisorption mechanism for ethane interacting with an Ir(110)-(1×2) surface. The data were obtained from supersonic molecular-beam measurements with an incident kinetic energy Ei ranging between 1.2 and 24.1 kcal/mol, a surface temperature Ts between 154 and 500 K, and an incident angle θi between 0° and 45°. For Ei less than approximately 13 kcal/mol, the probability of dissociative chemisorption S0 decreases rapidly with increasing Ts. For a surface temperature of 154 K, S0 decreases with increasing Ei for 1.2≤Ei ≤13.4 kcal/mol, consistent with a trapping-mediated chemisorption mechanism. Indeed, the data also support quantitatively a kinetic model consistent with a trapping-mediated chemisorption mechanism. The difference in the activation energies for desorption and chemisorption from the physically adsorbed, trapped state Ed −Ec is 2.2±0.2 kcal/mol. In the trapping-mediated regime, S0 is found to be rather insensitive to incident angle, scaling with Ei  cos0.5 θi .
Vibrational level structure of highly excited SO2 in the electronic ground state. II. Vibrational assignment by dispersed fluorescence and stimulated emission pumping spectroscopy
Authors: Yamanouchi Kaoru , Takeuchi Shingo , Tsuchiya Soji
Published: July 2002

The dispersed fluorescence (DF) spectra for the fluorescence emitted from eight single rovibronic levels in the C̃ 1B2 state are observed. The vibrational quantum numbers for these levels are (v′1,v2,v′3) =(0,0,0), (0,1,0), (0,2,0), (0,0,2), (1,0,0), (1,1,0), (1,2,2), and (3,1,0). The 484 vibrational levels distributed between 4300 and 21 600 cm−1 in the electronic ground state X̃ 1A1 are identified based on the assignment of the 1388 transitions in the DF spectra. The vibrational level energy is expressed by an anharmonic expansion with 19 coefficients and the vibrational quanta for the three normal modes. The combination analysis of the DF and stimulated emission pumping spectra having the same upper rovibronic level clarifies the vibrational level structure in the vibrationally highly excited region above 17 000 cm−1.
Migration of liquid polymers on solid surfaces
Authors: Novotny V. J.
Published: July 2002

Migration of very thin (<30 nm) polymeric films on solid surfaces was investigated quantitatively using two techniques—scanning microellipsometry and scanning small spot x-ray photoemission spectroscopy. The surface diffusion coefficient Ds increases as the film thickness decreases down to 1 nm, however, below 1 nm, Ds becomes independent of polymer film thickness. The functional dependence of Ds with molecular weight M is described, over the limited range of M in the unentangled regime, by M−1.7 The activation energy for polymer segment hopping is about 41 kJ/mol, which is similar to values obtained from bulk self-diffusion of other polymers.
Reversible and irreversible thermochromic phase transitions in single crystals of polydiacetylenes substituted with alkyl-urethanes
Authors: Koshihara Shin-ya , Tokura Yoshinori , Takeda Kenji , Koda Takao , Kobayashi Akiko
Published: July 2002

Reversible transitions between the two spectroscopically distinct phases (A and B phases) have been investigated for a new family of polydiacetylenes (PDAs) substituted with side groups of alkyl-urethane [−(CH2)4OCONH(CH2)n−1CH3:n=1−10]. Measurements of optical spectra as well as x-ray and calorimetric studies have revealed the first-order-like phase transitions in a series of PDAs with n=1–10 which are associated with an n-dependent thermal hysteresis ranging from 0 to 60 K in width. It has been found that the thermochromic changes become irreversible in all these PDAs once the hydrogen bond chains in the side groups are interrupted by heating beyond the polymer melting temperature.
The translational energy dependence of the F+C2H4 →H+C2H3F reaction cross section near threshold
Authors: Robinson Gary N. , Continetti Robert E. , Lee Yuan T.
Published: July 2002

We have carried out crossed molecular beam studies of the substitution reaction F+C2 H4 →H+C2 H3 F in the collision energy range 0.8–2.5 kcal/mol using a velocity selected F atom beam. The relative substitution cross section is found to decrease with increasing collision energy indicating that if there is a potential energy barrier to F atom addition to C2 H4 it is much less than 0.8 kcal/mol and that the transition state for addition occurs early along the reaction coordinate. These results agree well with ab initio calculations. Although the product translational energy distributions are similar to those obtained from earlier work at higher collision energies, the C2 H3 F angular distributions suggest that a wider range of reactant approach geometries lead to products at lower energies.
Collision-induced population transfer in infrared absorption spectra. III. Temperature dependence of absorption in the Ar-broadened wing of CO2 ν3 band
Authors: Boissoles J. , Boulet C. , Hartmann J. M. , Perrin M. Y. , Robert D.
Published: July 2002

We present experimental and theoretical results on the absorption in the Ar-broadened wing of the infrared CO2 ν3 band. Measurements have been made in the 293–765 K temperature range for total gas pressures up to 60 bar and are in good agreement with previous determinations at room temperature. Calculations have been carried out by using a line by line coupling theory presented and tested previously [J. Chem. Phys. 89, 625 (1988) and 91, 2163 (1989)]. The theoretical approach accounts for both line mixing and the wave-number dependence of the relaxation operator. It enables correct modeling of the temperature dependence of absorption. The strong sublorentzian absorption resulting from competitions between negative and positive individual line contributions is analyzed.
Adsorption and diffusion of a Lennard-Jones vapor in microporous silica
Authors: MacElroy J. M. D. , Raghavan K.
Published: July 2002

The properties of a dilute Lennard-Jones vapor in contact with an adsorbing microporous medium are investigated using grand canonical ensemble Monte Carlo and molecular dynamics techniques. The bulk structure of the microporous system is modeled as an assembly of randomly distributed interconnected solid spheres, and vapor/surface interactions are treated in two ways: (i) using a smooth continuous interaction potential and (ii) using a molecular model for the surface structure of the solid. The microporous solid representation employed in these simulations is chosen to conform in realistic manner with the bulk and surface properties of silica gel. The results obtained from the simulations include equilibrium partition coefficients, diffusivities, and related microscopic properties. By comparing these results with available experimental data it is shown that the properties of simple nonpolar gases in microporous silica may be predicted with reasonable accuracy. This is particularly true when the molecular structure of the silica surface is taken into consideration.
High-resolution spectrum of the C–O stretch overtone band in methyl alcohol
Authors: Mukhopadhyay I. , Ozier I. , Lees R. M.
Published: July 2002

The Fourier-transform spectrum of the CH3OH overtone C–O stretching band (vCO=2←0) has been recorded in the range 1950–2060 cm−1, at a resolution of 0.004 cm−1. The spectrum is resolved into J multiplets, each displaying complicated substructure due to strong torsion–vibration–rotation effects. Within this structure, it has been possible so far to identify R and P branch transitions for K=0 and K=1 for both A and E torsional symmetry species for J≤24. The assignments are supported by combination differences derived from known ground-state frequencies. The identified branches have been analyzed in terms of effective state-dependent series-expansion parameters, and the leading terms in the torsion–rotation Hamiltonian have been derived for the second excited C–O stretching state. The vCO=2 torsional barrier height is obtained as 395.5±0.2 cm−1, and the effective B value as 23 671.9±1.3 MHz. In addition, the 34.946 cm−1 far-infrared laser line pumped by the 10R(48) CO2 laser line has been tentatively identified as the (nτK,J)v=(031+,16)2→(010+,15)2 transition, and its parent IR pump absorption as the P(031+,17) vCO=2←1 hot-band line.
Redetermination of the thermochemistry of gaseous UF5, UF2, and UF
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

Because of new studies questioning an earlier determination of the thermochemistry of UF5(g), we have reexamined the gaseous equilibrium Ag+UF5=AgF+UF4 by mass spectrometry over a broader temperature range, and have obtained more definitive results, leading to Δf H0298 (UF5,g)=−1929±10 kJ mol−1. Included was an independent thermochemical study of AgF which yielded Δf H0298 (AgF,g)=7.5±6 kJ mol−1 and D00 (AgF)=3.66±0.06 eV. Additionally, second-law measurements of the gaseous reaction U+UF2=2UF corroborated earlier results showing that D(U–F)>D(FU–F), while the reverse holds in the U–Cl and U–Br systems. A revised set of bond dissociation energies and enthalpies of formation for the gaseous UFn species is presented, consistent with all key thermochemical values. This new information is discussed in terms of other results in the literature.
Analytic energy derivatives in the numerical local-density-functional approach
Authors: Delley B.
Published: July 2002

Analytical energy gradients for numerical orbital expansions can be calculated using the same three-dimensional integration methods as for calculating the total energy in the local-density-functional approach. It is shown that in addition to Pulay corrections for expansion functions attached to the atomic sites correction terms for non-self-consistency of the auxiliary density can also be used with benefit. The usefulness of this approach is demonstrated in the calculation of equilibrium geometries of organic and inorganic molecules, radicals, and transition-metal compounds. The calculated structural parameters are in at least as good agreement with experimental data as structures obtained from standard ab initio methods. Excellent basis sets can be used at a comparably low computational cost.
Simple perturbation theory estimates of equilibrium constants from force fields
Authors: Truhlar Donald G. , Isaacson Alan D.
Published: July 2002

We propose and test a very simple method for calculating equilibrium constants from quartic force fields.
Potential nonrigidity of the NO3 radical
Authors: Stanton John F. , Gauss Jürgen , Bartlett Rodney J.
Published: July 2002

The equilibrium geometry and vertical excitation energies of the nitrate radical (NO3) have been determined at the coupled-cluster singles and doubles level. Unlike most previous theoretical methods used to study this problem, the present calculations circumvent the difficulties associated with symmetry-broken orbitals by using a reference function composed of orbitals obtained in a closed-shell self-consistent field (SCF) calculation for the NO3 anion. Although these orbitals do not satisfy the SCF equations for NO3 itself, they turn out to be more suitable for the correlation problem in the neutral molecule than the unrestricted Hartree–Fock solution for NO3. Nevertheless, our calculations agree with most previous studies in predicting that the high-symmetry D3h structure of NO3 is not a minimum on the potential energy surface. The potential near the D3h point is relatively flat and has seven stationary points: three (equivalent) C2v minima with one long and two short N–O bonds; three (equivalent) C2v transition states with two long and one short N–O bonds; and the D3h structure, which is unstable with respect to the in-plane degenerate mode (e′) and is consequently a saddle point of index two. Exchange of the oxygen positions via pseudorotation around the D3h stationary point is predicted to be an extremely facile process with a barrier height of ≊190 cm−1, suggesting that the molecule may be spectroscopically nonrigid, belonging to a molecule symmetry group which is isomorphic with D3h, as observed experimentally. Excitation energies are calculated for both the D3h structure and points on the pseudorotation pathway, in order to predict differences between values obtained from photodetachment spectroscopy of the NO3 anion and those determined by direct excitation of NO3.
Ordered morphologies in binary blends of diblock copolymer and homopolymer and characterization of their intermaterial dividing surfaces
Authors: Winey Karen I. , Thomas Edwin L. , Fetters Lewis J.
Published: July 2002

Binary blends of a lamellar poly(styrene-b-isoprene) diblock copolymer and homopolystyrenes of various molecular weights self-assembled into highly ordered morphologies upon solvent evaporation. The equilibrium blend morphologies were studied by transmission electron microscopy and small angle x-ray scattering. The type of ordered morphology (lamellae, the ordered bicontinuous double diamond morphology, cylinders on a hexagonal lattice, and spheres on a cubic lattice) observed in each blend was generally dictated by the overall polystyrene volume fraction in the blend, i.e., the homopolymer concentration. The blend morphology also depended on the homopolymer molecular weight at a single fixed composition. The intermaterial dividing surface between the polystyrene and the polyisoprene microdomains was characterized by the interfacial mean curvature (H) and the average area per copolymer junction (σj). H and σj increased as the homopolymer concentration increased and/or as the homopolymer molecular weight decreased. These trends were generally obeyed both between and within ordered morphology types. The size of the microdomains was readily controlled by the homopolymer molecular weight.
Stochastic theory of inhomogeneous spectroscopic line shapes reinvestigated
Authors: Kador L.
Published: July 2002

The inhomogeneous distribution of an ensemble of absorption or fluorescence lines in a disordered matrix can be described by a stochastic theory whose fundamental ideas have been known for many decades. Due to its very general principles, it can be applied to inhomogeneous effects of many different types, including inhomogeneous broadening in optical spectra and spectral diffusion in magnetic resonance and line-narrowed optical experiments. In the case of absorption and luminescence bands, it is often convenient to perform the so-called Gaussian approximation, which is valid in the limit that the density of the matrix molecules is high compared to the reciprocal volume of the cavity containing the absorbing or fluorescing center; this leads to the analytical result that the line shapes of the optical bands are Gaussian. Numerical calculations beyond the Gaussian approximation help to clarify its physical meaning and to interpret inhomogeneous bandwidths from a statistical point of view. Three types of intermolecular potentials are examined, namely, dipole–dipole, van der Waals, and a modified Lennard-Jones-type interaction.
Copolymers as amphiphiles in ternary mixtures: An analysis employing disorder, equimaxima, and Lifshitz lines
Authors: Holyst Robert , Schick M.
Published: July 2002

We study the phase diagram and scattering functions of a ternary mixture of A and B homopolymers, in equal concentrations, and AB diblock copolymer. We locate the disorder line, which marks the appearance of damped oscillatory decaying correlation functions in the system, and also the Lifshitz and equimaxima lines of the structure functions. At these lines, the oscillatory component becomes dominant in a given correlation so that the peak in scattering intensity is no longer at the zero wave vector. We find that while the Lifshitz line of the structure function of all A monomers is quite close to the disorder line, that of A monomers in the homopolymer only is far from it. This shows that the copolymer orders the homopolymers inefficiently, a behavior which is also reflected in the copolymer’s weak ability to solubilize them, and which contrasts with the amphiphilic solubilizers of oil and water. The disorder line and all Lifshitz lines meet at a Lifshitz tricritical point. As these ternary mixtures provide a rare opportunity to study this unusual point, we discuss its effects on the structure functions and surface tensions.
The titane molecule (TiH4): Equilibrium geometry, infrared and Raman spectra of the first spectroscopically characterized transition metal tetrahydride
Authors: Thomas J. Russell , Quelch Geoffrey E. , Seidl Edward T. , Schaefer Henry F.
Published: July 2002

The titanium tetrahydride molecule has been investigated using several levels of ab initio molecular electronic structure theory in conjunction with a basis set of size Ti(14s11p6d/10s8p3d), H(5s1p/3s1p). The self-consistent field, single and double excitation configuration interaction, and single and double excitation coupled cluster methods were used to predict the molecular structure, harmonic vibrational frequencies, infrared and Raman intensities. Results for all five isotopic species TiH4, TiH3D, TiH2D2, TiHD3, and TiD4 are presented. A careful comparison with the recent matrix isolation infrared results of Xiao, Hauge, and Margrave is made where experimental data exist. All four of the tentatively assigned experimental vibrational features are given strong support from theory.
Microscopic dynamics of fluids confined between smooth and atomically structured solid surfaces
Authors: Somers Susan A. , Davis H. Ted
Published: July 2002

Molecular dynamics and the grand canonical Monte Carlo techniques are employed to simulate the structure and dynamics of a fluid in a slit micropore at equilibrium and under Couette flow. Calculated quantities include the fluid’s density profiles, pair correlation functions, diffusion coefficients, normal pressure, stress tensor, and velocity profiles. Simulation results for fluids in equilibrium with the same bulk fluid, but confined by either atomically smooth or structured face centered cubic pore walls are compared. At the conditions considered, fluid in the contact layer next to structured walls exhibits enhanced fluid order which is not altered by flow for pores capable of accommodating two fully developed fluid layers (i.e., for pores wider than 2.5 molecular diameters across). At narrower pore widths, the equilibrium fluid structure is changed by flow and the fluid is more sensitive to shear-induced changes in the diffusivity and normal pressure. The layer average density profiles of the confined fluid are similar in the structured and smooth pores. However, in the fluid layers adjacent to the structured pore walls, the local density distributions are ordered with a periodicity reflecting that of the atomic structure of the pore walls. At the strongest fluid–wall interaction considered, the confined fluid does not solidify although the diffusion coefficient of the fluid confined by the structured walls is reduced by as much as a factor of 87 compared to the bulk phase.
The effect of density change on crystal growth rates from the melt
Authors: Oxtoby David W. , Harrowell Peter R.
Published: July 2002

When a liquid crystallizes, both its structure and its average density change. The former represents a nonconserved, and the later a conserved, order parameter. Including convective flow in the fluid leads to coupled equations for average density and structural order parameter that resemble those for nonconserved order parameters, and that allow steady-state solutions. We use a simple square-gradient free-energy functional to explore the solutions to these equations and relate them to experiments and computer simulations for the crystallization of atomic fluids such as argon, silicon, sodium, and lead. The effect of the density change on freezing is predicted to be small in most cases.
Induction time in transient nucleation theory
Authors: Shneidman Vitaly A. , Weinberg Michael C.
Published: July 2002

We derive an exact expression for the induction time associated with the Zeldovich nucleation equation in terms of rapidly convergent integrals. Also, we consider an asymptotic approximation of the exact expression which is obtained in the limit of a high nucleation barrier and derive explicit formulas for several widely employed nucleation models. We demonstrate that analytical results are in excellent agreement with numerical results of the present and previous studies.
A new quantum propagator for hard sphere and cavity systems
Authors: Cao J. , Berne B. J.
Published: July 2002

A new short time propagator for a particle interacting with a hard sphere or with a hard sphere cavity is derived. This new propagator is shown to be a much better approximation than the image approximations used in the literature and should prove useful for path integral or Green function Monte Carlo simulations. The simple example of the radial distribution function of a hard sphere gas is calculated using the new propagator as well as the older image approximation propagator and is shown to converge much more rapidly to the ab initio result.
Spontaneous pinning of domain growth during spinodal decomposition of off-critical polymer mixtures
Authors: Hashimoto Takeji , Takenaka Mikihito , Izumitani Tatsuo
Published: July 2002

Spinodal decomposition of off-critical mixtures of poly(styrene–random–butadiene) copolymer with polyisoprene or polybutadiene was investigated as a function of the composition w of the mixtures and of the phase separation temperature T by time-resolved light scattering. The coarsening behavior of the mixtures was characterized in terms of the time change of the maximum intensity Im and that of the magnitude of the scattering vector qm at which the intensity becomes maximum. It was found that the time changes of Im and qm for the off-critical mixtures studied here are effectively pinned at a certain time defined here as the ‘‘pinning’’ time. This spontaneous pinning of the growth of the unmixing structure was found to occur earlier when w is biased toward 0 or 1 at a given T and when the phase separation temperature is raised at a given w, for these systems which seemingly have upper critical solution temperature behavior.
On the choice of orbitals for symmetry breaking problems with application to NO3
Authors: Stanton John F. , Gauss Jürgen , Bartlett Rodney J.
Published: July 2002

The suitability of using approximate Brueckner determinants in coupled-cluster (CC) studies of orbital symmetry-breaking problems is explored. At least for the cases considered here [NO3(2A′2), N2(2Σg+) and NO2(2B2)], the Brueckner reference function, which by definition does not mix with single excitations in the CC wave function, preserves symmetry exactly. Furthermore, these solutions do not appear to be sensitive to the initial guess, as the same symmetric solutions are obtained starting with either symmetry-broken or symmetry-constrained initial guesses. The same behavior is not observed for Brueckner determinants obtained from configuration interaction calculations, where symmetry-broken solutions are found. An evaluation of the merits of basing CC calculations on Brueckner determinants [B–CC] and quasirestricted Hartree–Fock [QRHF] reference functions (which also can be chosen to preserve symmetry exactly) is presented. Calculations carried out on the NO3 radical predict a C2v equilibrium geometry at both the B–CC and QRHF–CC levels when triple excitations are not included. However, all methods which include an approximate treatment of triple excitation effects predict an extremely flat potential surface which slightly favors the symmetric [D3h] form of the molecule.
Neutron diffraction studies of ices III and IX on under-pressure and recovered samples
Authors: Londono J. D. , Kuhs W. F. , Finney J. L.
Published: July 2002

Detailed structural studies using powder neutron diffraction have been carried out on ices III and IX on both under-pressure and recovered samples. The incomplete hydrogen ordering in recovered ice IX first observed by La Placa et al. is confirmed, and our studies suggest similar orderings for the under-pressure and recovered phases. Sample history was observed to influence the lattice constants of ice IX, an effect which could relate to hydrogen ordering. There is an indication that ice III is not fully disordered, a conclusion which would be consistent with independent thermodynamic measurements.
A heuristic radial distribution function for hard disks
Authors: Yuste S. Bravo , Santos A.
Published: July 2002

We propose a model radial distribution function for hard disks that is interpolated between the Percus–Yevick distribution functions for hard rods and hard spheres. The model contains a mixing parameter and two scaling parameters, which are determined by imposing self-consistency with an extension to d=2 of the Carnahan–Starling equation of state. Comparison with computer simulation is carried out.
Combined diffusion coefficients in equilibrium mixtures of dissociating gases
Authors: Murphy A. B.
Published: July 2002

It is shown that diffusion due to concentration and temperature gradients in a mixture of two nonreacting homonuclear gases consisting of arbitrarily many species in chemical equilibrium may be described by one ordinary and one thermal diffusion coefficient. These coefficients replace the 1/2q(q−1) independent ordinary diffusion coefficients and the q−1 independent thermal diffusion coefficients required in previous treatments to describe diffusion in a q-species gas mixture. A sample calculation for a mixture of dissociating nitrogen and oxygen is presented.
Collisions of protic and aprotic gases with hydrogen bonding and hydrocarbon liquids
Authors: Saecker Mary E. , Nathanson Gilbert M.
Published: July 2002

We explore collisions of Ne, CH4, NH3, and D2O with glycerol, a hydrogen bonding liquid, and with squalane, a liquid hydrocarbon. The experiments are carried out by directing a molecular beam at a continuously renewed liquid surface in vacuum and monitoring the identity and velocity of the scattered products with mass spectroscopy. We observe both direct inelastic scattering and trapping desorption when the gases strike the liquids. The polyatomic gases thermalize readily at low collision energies but rebound more frequently as the incident energy increases. We find that impulsive energy transfer is extensive and depends only weakly on the type of gas or liquid; for encounters leading to direct scattering, the gases appear to undergo hard spherelike collisions with the CH2 and CH3 groups of squalane and the OH and CH2 groups of glycerol. The gases accommodate differently on the two liquids, however: Neon and methane equilibrate more efficiently on squalane, ammonia thermalizes equally well on each liquid, and water is trapped more frequently by glycerol. The differences in trapping probabilities are smaller than expected from their solubilities, but they roughly follow trends in the free energies and enthalpies of solvation. Our results suggest that thermal accommodation in gas–liquid collisions reflects both the mechanical roughness and softness of hydrocarbons and the strong attractive forces between protic gases and the OH groups of glycerol.
Photodissociation of CH2. II. Three-dimensional wave packet calculations on dissociation through the first excited triplet state
Authors: Kroes Geert-Jan , van Dishoeck Ewine F. , Beärda Robert A. , van Hemert Marc C.
Published: July 2002

Quantitative results on photodissociation of CH2(X̃ 3B1) through the first excited (1 3A1) triplet state, producing CH (X 2Π)+H(2S), are presented. A three-dimensional time dependent quantum mechanical method was adopted to perform the dynamics using ab initio potential energy surfaces and an ab initio transition dipole moment function. The calculations were performed for J=0, where J is the angular momentum associated with the overall rotation of the nuclei. Comparison with calculations in which the bending angle was kept fixed at its ground state equilibrium value shows that a two-dimensional treatment suffices for obtaining the absorption spectrum. On the other hand, a three-dimensional calculation is necessary for correctly predicting the final rotational state distribution of the CH fragment.