bluuebunny/crossref_metadata_2025 huggingface dataset:
A coupled channel study of HN2 unimolecular decay based on a global a
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i
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o potential surface
Authors: Koizumi Hiroyasu , Schatz George C. , Walch Stephen P.
Published: July 2002

In this paper values of the unimolecular decay lifetimes of several vibrational states of HN2 based on an accurate coupled channel dynamics study using a global analytical potential surface that was derived from previously reported ab initio calculations are reported. The surface was developed by fitting the N–N stretch part to a Morse function, with parameters that are represented by a two-dimensional spline function in terms of the H to N2 center of mass distance and approach angle. This surface reproduces the ab initio points with a root mean square error of 0.08 kcal/mol for energies below 20 kcal/mol. Modifications to the potential that describe the effect of improving the basis set in the ab initio calculations are also provided. Converged coupled channel calculations have been done for the ground rotational state of HN2 to determine lifetimes of the lowest ten vibrational states. We find that only the ground vibrational state (000) and first excited bend (001) have lifetimes longer than 1 ps. The best estimates of the lifetimes of these states are 3×10−9 and 2×10−10 s, respectively. Variation of these results with quality of the ab initio calculations is less than or equal to a factor of 5.
Characterization of the bifurcated structure of the water dimer
Authors: Marsden Colin J. , Smith Brian J. , Pople John A. , Schaefer Henry F. , Radom Leo
Published: July 2002

The bifurcated structure of the water dimer has been the subject of considerable uncertainty with respect to its vibrational characterization. We have considered this question at the self-consistent-field (SCF) level of theory using finite basis sets that allow a close approach to the Hartree–Fock limit. As one approaches the Hartree–Fock limit, the bifurcated structure is predicted to be a true transition state, with one imaginary vibrational frequency, ω12(B2)∼200i cm−1.
Infinite order sudden calculation of Raman Q-branch linewidths for H2O+H2O
Authors: Agg P. J. , Clary D. C.
Published: July 2002

The infinite order sudden (IOS) approximation is extended to calculate rate constants for rotationally inelastic scattering between two H2O molecules. A modified version of the breathing sphere (BS) approximation, which has previously been shown to work reasonably well for N2+N2 rotationally inelastic scattering, is also extended to H2O+H2O scattering. The rate constants obtained using both approximations are used to determine Raman Q-branch spectral linewidths. In order to calculate such linewidths, it is necessary to sum the rate constants for any initial rotational level over all final inelastic rotational levels. While the IOS linewidths are in reasonable agreement with experiment over a range of temperatures, the IOS-BS linewidths are much too small.
On liquid-film thickness measurements with the atomic-force microscope
Authors: Forcada Mikel L. , Jakas Mario M. , Gras-Martí Alberto
Published: July 2002

We provide a theoretical description of some recent measurements where the thicknesses of solid-supported thin lubricant films are measured using the atomic-force microscope [Mate et al., J. Chem. Phys. 90, 7550 (1989)]. In our model, the differences observed between the thicknesses measured with the force microscope and ellipsometric thicknesses are explained by the appearance of an instability in the liquid film. Our calculations also predict the dependence of these differences on the thickness of the film.
NO2 jet cooled visible excitation spectrum: Vibronic chaos induced by the X̃ 2A1–Ã 2B2 interaction
Authors: Delon A. , Jost R. , Lombardi M.
Published: July 2002

Significant improvements have been obtained on measurements of the NO2 jet cooled excitation spectrum in the 16 300–18 502 cm−1 range, previously obtained by Smalley et al. [J. Chem. Phys. 63, 4977 (1975)], Persch et al. [Ber. Bunsenges. Phys. Chem. 92, 312 (1988)], and Hiraoka et al. [J. Mol. Spectrosc. 126, 427 (1987)]. The improvements concern first the rotational analysis, owing to a better resolution (150 MHz) and absolute precision (500 MHz), and second the completeness and purity of the resulting vibronic sequence, owing to a better sensitivity. As a result, 159 vibronic energy levels have been observed in the 16 500–18 500 cm−1 energy range, where 210±10 are expected. A detailed comparison with previous results is presented. The statistical analysis of the corresponding energy spacings shows that long range correlations up to 50 mean levels spacings are present, confirming the chaotic behavior of this set of vibronic levels. Furthermore, we analyze the observed rovibronic interactions (or rotational perturbations) that are responsible for the very irregular rotational behavior of the visible absorption spectrum of NO2 at room temperature.
Stationary points on the potential energy surfaces of (SO2)2 and (SO2)3
Authors: Bone Richard G. A. , Le Sueur C. Ruth , Amos Roger D. , Stone Anthony J.
Published: July 2002

The equilibrium structure and interconversion tunneling of the van der Waals dimer of sulphur dioxide is investigated. Results with the electrostatic model are compared with those obtained ab initio at the self-consistent field level and with second order Mo/ller–Plesset perturbation theory. This complex is shown to be a difficult problem for theoretical chemistry. We have located six stationary points on the dimer surface, two of which are probably transition states. The lowest energy region is very flat at all levels of theory but dispersion forces are likely to be responsible for the observed Cs symmetry global minimum. Substantial vibrational averaging must be invoked to explain the observed dipole moment. The tunneling splittings can be explained by a single motion proceeding via a centrosymmetric transition state and analogous to the internal rotation of acetylene dimer. A model potential gives a value of 56 cm−1 for the barrier, within the range predicted ab initio. We have also investigated three cyclic structures of the SO2 trimer, two of which are minima. The global minimum is probably nonsymmetric.
Determination of the rapid energy redistribution in a transition state by using molecular rotation as a clock and translational energy release as an energy monitor: The photodissociation of iodobenzene
Authors: Hwang Hyun Jin , El-Sayed Mostafa A.
Published: July 2002

State-selective photofragment translational spectroscopy is used to determine the spatial and velocity distributions of the ground state iodine atoms produced from the photodissociation of iodobenzene at 304.67 nm. In addition to an alkyl iodide type fast dissociation, a slow dissociation was observed. From the observed velocity dependence of the spatial anisotropy of the latter dissociation and the theoretical dissociation time dependence of the spatial anisotropy obtained from the rotational correlation function, we were able to determine the rate of energy redistribution to the phenyl ring modes during the dissociation process.
Microscopic theory of anchoring transitions at the surfaces of pure liquid crystals and their mixtures. I. The Fowler approximation
Authors: Teixeira P. I. C. , Sluckin T. J.
Published: July 2002

We have generalized earlier work on anchoring of nematic liquid crystals by Sullivan, and Sluckin and Poniewierski, in order to study transitions which may occur in binary mixtures of nematic liquid crystals as a function of composition. Microscopic expressions have been obtained for the anchoring energy of (i) a liquid crystal in contact with a solid aligning surface; (ii) a liquid crystal in contact with an immiscible isotropic medium; (iii) a liquid crystal mixture in contact with a solid aligning surface. For (iii), possible phase diagrams of anchoring angle versus dopant concentration have been calculated using a simple liquid crystal model. These exhibit some interesting features including re-entrant conical anchoring, for what are believed to be realistic values of the molecular parameters. A way of relaxing the most drastic approximation implicit in the above approach is also briefly discussed.
A microscopic mechanism for shock-wave generation in pulsed-laser-heated colloidal suspensions
Authors: Löwen Hartmut , Madden Paul A.
Published: July 2002

The kinetics of the heat and mass transport involved in vapor bubble formation around a colloidal particle which has been heated rapidly to high temperatures are examined with a theoretical model. It is argued that the likely mechanism of bubble formation on the nanosecond time scale is a spinodal decomposition of the liquid at the particle surface to the low density (vapor) phase. This process is shown to give rise to extremely rapid changes in the density and pressure fields of the fluid. The existence of such rapid events has been invoked to explain experimental observations of acoustic shocks generated in laser-pulse-heated colloidal suspensions.
Measurement of relative state-to-state rate constants for the reaction D+H2(v, j)→HD(v′, j′)+H
Authors: Adelman David E. , Shafer Neil E. , Kliner Dahv A. V. , Zare Richard N.
Published: July 2002

We have measured state-to-state integral rate constants for the reaction D+H2(v,j) →HD(v′=0,1,2;j′)+H, in which the H2 reagent was either in the ground state, H2(v=0,j), or prepared in the first excited vibrational state, H2(v=1, j=1), by stimulated Raman pumping. Translationally hot D atoms were produced via UV photolysis of DI, generating two center-of-mass collision energies corresponding to the two I atom spin–orbit states. Resonance-enhanced multiphoton ionization and time-of-flight mass spectrometry were employed to detect the nascent HD product in a quantum-state-specific manner. Two experimental geometries were used: (1) a probe-laser-induced geometry, in which the same laser both initiated the reaction, by photolysis of DI, and detected the HD and (2) an independent-photolysis-source geometry, in which photolysis of DI was carried out by an independent laser. We find that vibrational excitation of the H2 reagent results in substantial HD rotational excitation for each product vibrational state, a shift in the vibrational product state distribution such that the rates for the reaction D+H2(v=1, j=1) into HD(v′=0) and HD(v′=1) are comparable, and somewhat surprisingly, almost no change in the total rate into HD(v′=0,1,2;j′). The experimental results are consistent with a model in which internal energy is conserved, i.e., internal energy of the reagents appears as internal energy of the products, while relative translational energy of the reagents appears primarily as translation of the products. Good to excellent agreement is found between the experiment and recent quantum-mechanical scattering calculations of Neuhauser, Judson, and Kouri. Minor discrepancies persist, however, between theory and experiment for some product rotational distributions.
Nonlocal polarizability density of a model system: A homogeneous electron gas at T=0
Authors: Nimalakirthi R. , Hunt K. L. C.
Published: July 2002

The nonlocal polarizability density α(r,r′;ω) gives the polarization induced at a point r in a quantum mechanical system, due to a perturbing field of frequency ω that acts at the point r′, within linear response; thus it reflects the distribution of polarizability in the system. In order to gain information about the nature and functional form of α(r,r′;ω), in this work we analyze the nonlocal polarizability density of a well-characterized system, a homogeneous electron gas at zero temperature. We establish a connection between the static, longitudinal component of the nonlocal polarizability density in position space and the dielectric function ε(k,0), and then use the connection to obtain results at three levels of approximation to ε(k,0): We compare the Thomas–Fermi (TF), random phase approximation (RPA), and Vashishta–Singwi (VS) forms. At TF level, we evaluate the nonlocal polarizability density analytically, while within the RPA we obtain asymptotic analytical results. The RPA and VS results are similar, and qualitatively distinct from the TF results, which diverge as ‖r−r′‖ approaches zero. Within the RPA, we find two long-range components in αL(r,r′;0): The first is a monotonically decreasing component that arises from charge screening in the electron gas, and varies as ‖r−r′‖−3; the second is an oscillatory component with terms of order ‖r−r′‖−n (n≥3) associated with Friedel oscillations in the electron density. These results indicate the possibility of long-range, intramolecular terms in the nonlocal polarizability densities of individual molecules.
Fourier transform dispersed fluorescence spectroscopy: Observation of new vibrational levels in the 5000–8000 cm−1 region of ã 1A1 CH2
Authors: Hartland Gregory V. , Qin Dong , Dai Hai-Lung
Published: July 2002

Dispersed fluorescence spectra from the CH2 b̃ 1B1 state to highly excited vibrational levels of the ã 1A1 state were recorded by a new technique, Fourier transform dispersed fluorescence spectroscopy. The spectra obtained clearly show the advantages of using a Fourier transform spectrometer for dispersing fluorescence, namely emission over a wide spectral range can be efficiently detected with high sensitivity and resolution. These advantages allow four new vibrational levels in the CH2 ã 1A1 state to be observed; the (2,0,0) and (0,5,0) vibrational overtones and the (1,2,0) and (1,3,0) combination bands. The vibrational term values for these levels are given, along with the harmonic frequencies and anharmonicities for the v1 and v2 modes. From the (0,5,0) term value an improved estimate of the barrier height to linearity in the CH2 ã 1A1 state is made.
A study of low temperature heat capacity anomalies in bimetallic alloy clusters using J-walking Monte Carlo methods
Authors: Lopez Gustavo E. , Freeman David L.
Published: July 2002

Heat capacities are calculated as a function of temperature for bimetallic clusters composed of six palladium and seven nickel atoms using Monte Carlo techniques both with and without J-walking. By applying a simulated annealing strategy, the minimum energy configurations at 0 K are identified for a series of interatomic interaction strengths. A significant dependence of the spatial arrangement of atoms on the strength of the Pd–Ni interaction is observed. Calculations of the heat capacity as a function of temperature show the presence of a ‘‘melting’’ coexistence region similar to that observed in pure clusters. For the alloy clusters, low temperature heat capacity anomalies are observed arising from isomerizations that are reminiscent of order–disorder transitions known to occur in some bulk alloy materials. These low temperature heat capacity anomalies are observable only when the J-walking algorithm is used.
Rotator phases of the normal alkanes: An x-ray scattering study
Authors: Sirota E. B. , King H. E. , Singer D. M. , Shao Henry H.
Published: July 2002

We present results of a detailed x-ray scattering study on the rotator phases of normal alkanes: CH3–(CH2)n−2–CH3 (20≤n≤33). We have characterized a new tilted rotator phase and determined the temperature and chain length dependence of the distortion, tilt, and azimuthal order parameters which characterize the time-space averaged structures of the five rotator phases. We have shown that there is no strong even–odd chain length effect on the phase diagram within the rotator phases and have shown the continuity of that phase diagram in the 26-27 carbon vicinity.
Analytic energy gradients for the equation-of-motion coupled-cluster method: Implementation and application to the HCN/HNC system
Authors: Stanton John F. , Gauss Jürgen
Published: July 2002

A recently developed theory for calculating analytic first derivatives of the energy for excited states treated by the equation-of-motion coupled-cluster (EOM-CC) method has been implemented and applied to study potential energy surfaces in the HCN/HNC system. The EOM-CC singles and doubles (EOM-CCSD) approximation is used to predict equilibrium structures, energies, dipole moments, harmonic vibrational frequencies and infrared intensities of HCN and HNC isomers in both the Ã 1A″ and B̃ 1A′ excited states. In addition, the activated complex for HCN→HNC isomerization in these states is investigated theoretically for the first time.
Force balance and force relay in molecular interactions: An analysis based on nonlocal polarizability densities
Authors: Liu P.-H. , Hunt K. L. C.
Published: July 2002

We have recently derived new results for dispersion, induction, and hyperpolarization forces, using nonlocal polarizability densities to characterize the changes in electronic charge density induced by molecular interactions. In this work, we prove that the fundamental physical requirement of force balance for two interacting molecules A and B is satisfied within the nonlocal response theory, order by order. An explicit proof is needed because of differences in the molecular properties that determine the forces on A and B. For example, at first order the force on A depends on the polarizability density of A, αA(r,r′;ω=0), while the first-order force on B depends on its polarizability density; and for distinct species A and B, there is no relation between αA(r,r′;ω=0) and αB(r,r′;ω=0). We show that force balance is derivable from a condition that we term ‘‘force relay.’’ Epstein has previously derived this condition for molecules in fixed external fields, assuming that the electronic state adjusts adiabatically to the perturbation: then the force of the external field on the nth order term in the electronic charge density equals the force on the nuclei due to the (n+1)st order correction to the electronic charge density. Our work generalizes the condition to external fields that are modified by and correlated with the changes in the electronic charge distribution, as for two interacting molecules with negligible charge overlap. Force relay is guaranteed by relations that we have established among permanent charge densities, linear response tensors, and nonlinear susceptibilities. All of the results stem from a hypervirial theorem applied to the electronic momentum operator, and hence from translational invariance. The results are not limited to the framework of the polarizability density theory, but also hold for the standard perturbation theory of interactions between nonoverlapping molecules, and for the Hellmann–Feynman theory of intermolecular forces.
Crystal nucleation and growth from the undercooled liquid: A nonclassical piecewise parabolic free-energy model
Authors: Bagdassarian Carey K. , Oxtoby David W.
Published: July 2002

An undercooled liquid exhibits crystalline fluctuations, some of which grow into crystal of macroscopic dimension, while smaller fluctuations disappear. We present a model which allows for exact analytic characterization of the inhomogeneous critical nucleus, the smallest fluctuation which will give rise to crystal growth, in terms of a single spatially varying order parameter for the degree of crystallinity. The model is built around the square-gradient approximation for the free energy with a simple double-parabolic form for the homogeneous component. We study the radius, free energy of formation, and profile of the critical nucleus as functions of the liquid undercooling and compare these with results from an earlier nonclassical theory and from the classical capillarity approximation. The time evolution of the order parameter is described by a phase-field equation which is easily solved numerically for growth dynamics of initially supercritical fluctuations or for the regression of subcritical profiles.
Determination of phase diagrams for the hard-core attractive Yukawa system
Authors: Hagen M. H. J. , Frenkel D.
Published: July 2002

The phase diagram of a system consisting of hard particles with an attractive Yukawa interaction is computed by Monte Carlo simulation. From the results of these simulations we can estimate that the liquid–vapor coexistence curve disappears when the range of the attractive part of the Yukawa potential is less than approximately one-sixth of the hard-core diameter. The results of the simulations are compared with predictions based on first order perturbation theory.
Semiclassical Green function calculation of four wave mixing in polarizable clusters and liquids
Authors: Leegwater Jan A. , Mukamel Shaul
Published: July 2002

A theory for the nonlinear optical response of an assembly of four level atoms representing an s–p transition, with dipole–dipole interactions is developed. Simulation results for two-pulse ultrafast four wave mixing measurements in atomic clusters and in liquids demonstrate that the electronic density of states can be obtained by combining absorption and four wave mixing experiments. We show that a sensitive direct probe for interatomic interactions and correlations is provided by reversing the pulse time ordering in a photon echo sequence. The effect of nuclear motions on the linear absorption spectrum is investigated.
A theoretical study of the chemical bonding in M(CO)x (M=Cr, Fe, and Ni)
Authors: Persson B. Joakim , Roos Björn O. , Pierloot Kristine
Published: July 2002

The equilibrium structure and bond energies of the transition metal complexes Ni(CO)x (x=1–4), Fe(CO)5, and Cr(CO)6 have been studied using the complete active space (CAS)SCF method and second-order perturbation theory (CASPT2). It is shown that the major features of the electronic structure are properly described by a CASSCF wave function based on an active space comprising the bonding and antibonding orbitals directly involved in the metal–ligand bond. Remaining correlation effects are dealt with in the second, CASPT2, step. The computed energies have been corrected for basis set superposition errors (BSSE) and relativistic corrections have been added. Resulting bond distances and bond energies are in agreement with experimental data, when available: Cr(CO)6, r(Cr–C)=1.91(1.91) Å, D0=148(153) kcal/mol; Fe(CO)5, rax(Fe–C) =1.79(1.81) Å, req(Fe–C)=1.80(1.83) Å, D0=133(137) kcal/mol; Ni(CO)4, r(Ni–C)=1.83(1.83) Å, D0=139(138) kcal/mol (experimental values within parentheses). Some excited states were computed for Fe(CO)5. The first charge transfer (CT) state was located at 4.8 eV in agreement with an intense band found at 5.0 eV.
The intermolecular dynamics of liquid water
Authors: Castner E. W. , Chang Y. J. , Chu Y. C. , Walrafen G. E.
Published: July 2002

We present the complete intermolecular dynamical spectrum of liquid water, by merging the data sets from femtosecond nonlinear-optical polarization spectroscopy with the depolarized, Bose–Einstein corrected Raman spectrum to cover the frequency range from 0–1200 cm−1. The impulse response function for liquid water at room temperature is calculated, including all of the intermolecular motions.
Potential energy surface for carbon-dioxide activation by V+: A guided ion beam study
Authors: Sievers M. R. , Armentrout P. B.
Published: July 2002

A guided ion beam tandem mass spectrometer is used to measure the kinetic energy dependence of the V+(5D) + CO2 reaction and a reverse pathway, VO+(3∑−) + CO. Two intermediates along these reaction pathways, V+(CO2) and OV+(CO), are examined by threshold collision-induced dissociation experiments with Xe. Thermochemical analyses of the cross sections obtained in these systems allow the measurement of D0(OV+–O) = 3.06±0.40 eV, D0(V+–CO2) = 0.75±0.04 eV, D0(OV+–CO) = 1.05±0.10 eV, and speculative characterization of electronic excitation energies for two states of VO+. Combined with literature information on the electronic states of V+ and VO+, these data enable the potential energy surfaces for this reaction system to be mapped out in some detail. We find that coupling between surfaces of different spin is observed, but that spin conservation plays an important role in both the forward and reverse reactions.
Mixed amphiphilic bilayers: Bending elasticity and formation of vesicles
Authors: Porte G. , Ligoure C.
Published: July 2002

In this article, we report on a theoretical investigation of the bending elasticity of fluid bilayers consisting of two amphiphilic species. According to the ideas of Safran et al. [Phys. Rev. A 43, 1071 (1991)], one expects that the corresponding composition degree of freedom couples to curvature playing so a significant role in the bending properties of the mixed membrane. We show that these properties can be interpreted in terms of the bending moduli of the individual constituting monolayers provided that these are calculated at fixed chemical potential μ and not at fixed composition. A simple general theory is built up, designed so that the μ moduli can be calculated in a consistent way. It is shown that the composition degree of freedom softens the mean curvature modulus κ but leaves the Gaussian modulus κ̄ unaffected. Finite thickness effects are also considered. They do not modify κ, whereas their influence on κ̄ just has the same value as formerly calculated for the case of no composition degree of freedom. Finally, this general approach is applied to more specific models of mixed membranes: (i) bilayers made of surfactant and cosurfactant; (ii) initially neutral membranes weakly charged by addition of a small amount of an ionic surfactant; and (iii) bilayers covered with a brush of adsorbed end-functionalized polymers. New insights are also obtained on well known experimental facts such as the existence of spherulite textures in lamellar phases and spontaneous formation of vesicles and liposomes.
Chemisorption and oxidation reactions of nickel group cluster anions with N2, O2, CO2, and N2O
Authors: Hintz Paul A. , Ervin Kent M.
Published: July 2002

Reactions of nickel (Ni−n, n=3–10), palladium (Pd−n, n=3–8), and platinum (Pt−n, n=3–7) cluster anions are investigated in a flow tube reactor. Rate coefficients are measured for reactions with N2, O2, CO2, and N2O. Reactions with O2, CO2, and N2O have rates that are greater than 10% of the collision rate for most clusters of four atoms or larger, while N2 reactions generally exhibit much lower reaction efficiencies. All the reactions studied show a strong dependence on cluster elemental composition. Many of the palladium cluster reactions are significantly faster than the corresponding nickel and platinum cluster reactions, while Ni−n and Pt−n have similar rate coefficients. Pt−6 is observed to have anomalously low rate constants for reactions with N2, CO2, and N2O compared to neighboring platinum clusters sizes and the nickel and palladium hexamers. N2, CO2, and O2 reactions are generally association reactions with varying degrees of cluster fragmentation observed. N2O reactions result in sequential addition of O atoms to the cluster. The extent of cluster fragmentation for the various reagents can be correlated with the estimated exothermicities of the adsorption processes.
An ab initio molecular orbital study of potential energy surface of the NH2+NO2 reaction
Authors: Mebel A. M. , Hsu C.-C. , Lin M. C. , Morokuma K.
Published: July 2002

Potential energy surface of the reaction of NH2 with NO2 has been studied at the QCISD(T)/6-311G(d,p)//MP2/6-311G(d,p)+ZPC[MP2/6-311G(d,p)] and GAUSSIAN−2 (G2) levels of calculation. The reaction is shown to give three different groups of products. H2NO+NO can be produced by two different channels: (i) the barrierless association of the reactants to form H2NNO2 1, followed by the nitro–nitrite rearrangement into H2NONO 3 and the ON bond scission and (ii) the association of H2N with ONO directly forming 3 without barrier, followed by the dissociation 3. The barrier for the nitro–nitrite rearrangement at the transition state (TS) 2, 31.2 kcal/mol with respect to 1, is 20.8 kcal/mol lower than the reactants at the best G2 level. The TS 2 is found to lie significantly lower and to have much tighter structure than those previously reported. The thermodynamically most stable N2O+H2O products can be formed from 1 by the complex mechanism (iii), involving 1,3-hydrogen shift from nitrogen to oxygen, rotation of the OH bond, H shift from one oxygen to another and migration of the second H atom from N to O leading to elimination of H2O. The rate-determining step is the 1,3-H shift at TS 4 which is 12.5 kcal/mol lower than NH2+NO2, but 8.3 kcal/mol higher than the barrier for the nitro–nitrite isomerization at TS 2 at the G2 level. N2+H2O2 cannot be formed in the reaction, but several channels are shown to produce N2+2OH. All of them have as the rate-determining step the second 1,3-hydrogen shift from nitrogen to oxygen at TS 11 or 16, lying by 6.9 kcal/mol higher than NH2+NO2, and are not expected to compete with the reaction mechanisms producing H2NO+NO and N2O+H2O.
Surface segregation in conformationally asymmetric polymer blends: Incompressibility and boundary conditions
Authors: Wu David T. , Fredrickson Glenn H. , Carton Jean-Pierre
Published: July 2002

Recent experiments, analytical theory, and simulations have raised and examined the possibility of entropically driven segregation effects in conformationally asymmetric polymer blends. We consider herein a model of surface segregation in a molten blend of two polymers with different flexibilities as characterized by the pure-component parameter β2=R2g/Vmol, where Rg is the radius of gyration and Vmol is the molecular volume of a polymer chain. Analytic solutions to the self-consistent field equations are presented for small deviations of the conformational asymmetry parameter ε=(βA/βB)2 from unity. Even in the absence of enthalpic interactions with the wall, we find an effective exchange surface potential of entropic origin, which can be understood in terms of an imperfect screening of the wall by the self-consistent potential. We find that the more flexible component segregates to the surface, in qualitative agreement with an earlier density functional calculation, but with a different parameterization of the surface potential. For weak conformational asymmetry, the magnitude of the segregation is found to be proportional to (ε−1), and inversely proportional to the bulk screening length of the total monomer density. Our analysis indicates that unlike single-component melts, where reflecting boundary conditions are appropriate, molten blends near a surface are described by an effective mixed boundary condition on the polymer Green’s function G(z,z′;s,s′) of the form ∂zG∝UG, where U is the strength of the surface potential. In the perturbative limit, ‖ε−1‖≪1, this proves equivalent to effective constant flux boundary conditions.
The heats of formation of HNO and of DNO
Authors: Dixon Richard N.
Published: July 2002

The heat of formation ΔfHo (0 K) of HNO, deduced from spectroscopic measurements, is 110.02±0.25 kJ mol−1; and for DNO is 106.85±0.25 kJ mol−1.
Analytical potential energy surface for the NH3+H↔NH2+H2 reaction: Application of variational transition-state theory and analysis of the equilibrium constants and kinetic isotope effects using curvilinear and rectilinear coordinates
Authors: Corchado J. C. , Espinosa-Garcı́a J.
Published: July 2002

The potential energy surface (PES) for the gas-phase NH3+H↔NH2+H2 reaction is constructed with suitable functional forms to represent the stretching and bending modes, and using as calibration criterion the reactant and product experimental properties and the ab initio saddle point properties. This surface is then used to calculate rate constants with variational transition-state theory over the temperature range 300–2000 K. While the forward rate constants agree with experimental results, the reverse ones are lower by factors of between 4 and 6. Since the same PES is used and these rates are related by detailed balance, this disagreement could indicate an uncertainty in the few available experimental studies for the reverse reaction. We also provide a detailed analysis of the equilibrium constants and of the kinetic isotope effects and compare the results of this analytical PES with earlier ab initio reaction-path calculations. Finally, for the vibrational frequency calculations, we analyze the consequences of the choice of different coordinate systems (curvilinear or rectilinear) on various kinetic magnitudes.
A quasiclassical trajectory study of product state distributions from the CN+H2→HCN+H reaction
Authors: Bethardy G. A. , Wagner Albert F. , Schatz George C. , ter Horst Marc A.
Published: July 2002

An extensive quasiclassical trajectory study of the dynamics of the CN+H2→HCN+H reaction has been undertaken on two of the potential energy surfaces reported by ter Horst, Schatz, and Harding [J. Chem. Phys. 105, 558 (1996)] with the goal of converging product state distributions. The effect of zero-point energy violations on the behavior of the reactive cross section near threshold has been examined leading to an improved estimate of the thermal rate constant on ter Horst–Schatz–Harding potential energy surface 3 (3.01±0.24×10−14 cm3/s at 300 K). The calculated HCN vibrational product state distribution is not statistical and exhibits a systematic over population in the stretching vibrations of the ground state bend manifold indicating that the –C–N does not behave like a “spectator bond” in this reaction. There is also significant population in modes with bending excitation, but these vibrations are under populated relative to prior statistical expectations. The sensitivity of the distribution on the size of the barrier and its location in the entrance channel has been undertaken by comparing results on the ter Horst–Schatz–Harding potential energy surfaces 2 and 3. Similar to the case of exoergic atom-diatom reactions, it is found that the earlier barrier on ter Horst–Schatz–Harding potential energy surface 3 gives rise to more excitation in the –C–H stretching vibration. The rotational distributions of the HCN product appear similar to the thermal distribution of CN reagents from which they are born indicating that the abstraction of the light H atom perturbs the rotational motion of the cyano radical very little. The dependence of the average HCN rotational quantum number, 〈J〉, on the bending quantum number, v2, exhibits an interesting alternation such that the points for even values of v2 are larger than those for odd. There is a corresponding alternation in the dependence of the average scattering angle, 〈θ〉, on v2 in the opposite sense. These observations suggest that for the odd bending states (which are primarily l=1) the energy diverted into exciting motion perpendicular to the reaction path at the transition state is not available to excite product rotation or to produce reactive trajectories with large impact parameters which lead to small scattering angles.
Phase diagrams of systems of particles interacting via repulsive potentials
Authors: Rascón C. , Velasco E. , Mederos L. , Navascués G.
Published: July 2002

We use a recently developed density-functional perturbation theory, which has been applied successfully to predict phase diagrams of systems of attractive particles, to describe the phase diagram of particles interacting via repulsive potentials. We consider potentials composed of a hard-sphere core plus a repulsive term. Specifically, we have investigated square shoulder and repulsive Yukawa terms. We show that, when the range of the interaction is very short, the shoulder potential leads to solid–solid coexistence involving two face-centered cubic structures, in analogy to an attractive square-well potential. Comparison with simulation results shows that the theory is quantitatively correct. If the range of the potentials is sufficiently long, we also find that a body-centered cubic structure can be stabilized. By considering the phase behavior at zero temperature, we argue that several triple points, involving coexistence of fluid and/or solid phases, may occur. A repulsive Yukawa term also shows a region of body-centered cubic stability but, contrary to the square shoulder and attractive Yukawa cases, there is no isostructural solid-solid coexistence. The role of the functional dependence of the interaction potential on particle separation at short distances is discussed and shown to be crucial to generate a solid-solid transition in systems of repulsive particles. Available computer simulation results for this system indicate that the density-functional approximation for the hard-sphere system used in this work, as well as all other currently available approximations, although qualitatively correct, may be overestimating the stability of the body-centered cubic phase.
The isotropic–nematic transition for the hard Gaussian overlap fluid: Testing the decoupling approximation
Authors: Padilla P. , Velasco E.
Published: July 2002

The isotropic–nematic phase transition in a fluid of moderately long molecules interacting via a hard Gaussian overlap potential is studied using the decoupling approximation and computer simulation. Molecules of length-to-breadth ratios equal to 3 and 5, thought to set the relevant range of molecular elongations in real nematic liquid crystals, are considered. The results of the theory (pressure, order parameter, and location of the phase transition) and of several of its extensions, are compared with those from computer simulation, and their relative accuracy assessed. We first study the standard decoupling approximation, a resummed Onsager virial expansion where only the (exact) second virial coefficient, B2, is retained, and consider two different mappings to perform the resummation: a fluid of equivalent hard spheres and the isotropic phase of the hard Gaussian overlap fluid. Whereas the former mapping predicts a phase transition already in qualitative agreement with simulation, the mapping to the isotropic phase predicts a transition in closer agreement with the simulation result, shifting the location of the transition to lower pressures. However, the transition is overestimated in both cases, which seems to indicate a poor representation of angular correlations. In order to incorporate higher-order correlations, an approximate method is proposed to evaluate the B3 and B4 virial coefficients in the nematic phase numerically. This new information allows us to address two points: (i) the convergence of the virial series for short molecules, and (ii) the performance of extended decoupling approximation theories, incorporating the third and the fourth virial coefficients. As expected, inclusion of the high-order virial coefficients improves the results of the corresponding truncated virial expansion for the largest elongation considered, and provides quantitative agreement with the simulations, indicating a fast convergence of the virial series. The standard decoupling approximation provides results of similar accuracy. Also, the extended decoupling approximation including B3 improves these results, though the extension to B4 degrades the coexistence data slightly, which might indicate that the latter misrepresents to some extent the importance of angular correlations. In contrast, for molecules with a length-to-breadth ratio of 3, the truncated virial expansion is still inaccurate, whereas the extended decoupling approximation theories perform better, providing almost quantitative agreement with the simulations. As a result of our findings, we conclude that in order to improve the standard decoupling approximation for fluids of short molecules, it is essential to resum the virial series using knowledge of the B3 virial coefficient and also the B4 coefficient for the shortest molecules forming nematic phases.
Molecular dynamics simulation of polymer crystallization through chain folding
Authors: Yamamoto Takashi
Published: July 2002

Detailed molecular mechanism of polymer crystallization into the chain folded lamella is investigated by molecular dynamics simulation. Though the crystallization of polymer is generally a very slow process that seems to defy every computational approach, simplifications of the computational model are found to improve the computational efficiency and to accelerate the crystallization itself. A relatively short polymer model made up of 500 beads connected by harmonic springs is adopted here, and in addition the polymer is assumed to be strongly adsorbed on the growth surface. It is shown that the polymer molecule crystallizes into a neat chain folded lamella within several nanoseconds. The crystallization process is found to be divided into three stages: the initial stage for the local chain ordering into small clusters, the intermediate stage for the coalescing of the clusters into small lamellae, and the late stage for the completion of a single lamella. In the crystallization process of our model polymer, the polymer diffusion along the chain and the subsequent lamella thickening are found to be of great significance. The thickening phenomena of our model lamella is also investigated, and it is found that the lamella thickens rather quickly, driven mainly by the van der Waals attraction between beads.
Density functional calculations of molecular g-tensors in the zero-order regular approximation for relativistic effects
Authors: van Lenthe Erik , Wormer Paul E. S. , van der Avoird Ad
Published: July 2002

A method has been developed for the calculation of the g-tensor of Kramers doublet open shell molecules, which uses the spinor of the unpaired electron of the paramagnetic molecule, obtained from a density functional calculation. Spin–orbit coupling is taken into account variationally using the zeroth-order regular approximation (ZORA) to the Dirac equation. The problem of gauge dependence is solved by using gauge including atomic orbitals (GIAO’s). The method gives fair agreement with experimental values for the g values of some small test molecules NO2, HCO, and TiF3.
X-ray diffraction studies of freezing and melting of water confined in a mesoporous adsorbent (MCM-41)
Authors: Morishige K. , Nobuoka K.
Published: July 2002

In order to study the freezing/melting behavior of pore water, we performed x-ray diffraction measurements of water confined inside the cylindrical pores of two kinds of siliceous MCM-41 with different pore size and one kind of aluminosilicate MCM-41 as a function of temperature. The results show that its freezing/melting behavior is not affected by the incorporation of Al into the pore wall and the hysteresis effect between freezing and melting is very small or negligible. On cooling the water in the middle of the pores with a pore diameter of 4.2 nm, that is, the free water freezes abruptly around 232 K to give rise to cubic ice while the water confined in the pores with a pore diameter of 2.4 nm freezes very gradually at lower temperatures. The diffraction profile after the freezing of the free water suggests that the interfacial water confined between the surface of the pore wall and the frozen phase of the free water consists of randomly displaced water molecules.
Ab initio calculations of radiationless transitions between excited and ground singlet electronic states of ethylene
Authors: Hayashi M. , Mebel A. M. , Liang K. K. , Lin S. H.
Published: July 2002

General expressions for internal conversion (IC) rate constant calculations have been derived by taking into account displacements, distortions, and rotation (mixing) of normal modes. The electronic part of the rate constant has been computed through the ab initio calculations of vibronic coupling. The corresponding expressions for the simplest two-mode case as well as for the general n-mode case have been derived. We demonstrate the effect of rotated (mixed) normal modes on the IC rate constants based on a model consisting of one promoting and two mixed modes. The dynamics of excited states of C2H4 has been investigated based on the internal conversion mechanism. The calculated rate of internal conversion show that the lifetimes of the excited π–3p and π–π* states of C2H4 are on the picosecond scale. We predict that if the molecule is excited to a Rydberg π–3p state, it relaxes to the ground state via the cascade mechanism, π–3p→π–3s(1B3u)→π–π*(1B1u)→1Ag.
Reversible work of formation of an embryo of a new phase within a uniform macroscopic mother phase
Authors: Debenedetti Pablo G. , Reiss Howard
Published: July 2002

A thermodynamically consistent formalism is derived for calculating the reversible work needed to form a small amount of a new phase (embryo) within a uniform macroscopic mother phase. The treatment goes beyond the classic work of Gibbs, who solved the problem for the particular case in which the embryo is in equilibrium with the mother phase, constituting a so-called critical nucleus. The formalism results in a new expression for the reversible work of embryo formation, the extrema conditions for which yield the correct conditions of equilibrium between the critical nucleus and the mother phase, as well as Gibbs’ result for the reversible work needed to form the critical nucleus. The new expression for the work of embryo formation differs from the one commonly used in the nucleation literature. In order to extend the Gibbsian formalism to noncritical nuclei, it is necessary to introduce a constraint that prevents the free transfer of matter between the embryo and the mother phase. The present approach is valid in the limit in which curvature contributions to the interfacial energy can be neglected.
Benchmark configuration interaction spectroscopic constants for X 1Σg+ C2 and X 1Σ+ CN+
Authors: Leininger Matthew L. , Sherrill C. David , Allen Wesley D. , Schaefer Henry F.
Published: July 2002

Explicit full configuration interaction (FCI) computations in a double-ζ plus polarization (DZP) basis, involving as many as 105 million Slater determinants, have been performed to sample the potential energy curves of X 1Σg+ C2 and X 1Σ+ CN+ for benchmark purposes. Quartic force fields have been determined at the optimized structures, and sets of anharmonic spectroscopic constants [re, ωe, Be, D̄e, αe, and ωexe] have been ascertained. Analogous results obtained from high-level but inexact correlation treatments establish a CISDTQ<CCSDT<CISDTQPH≈FCI series of increasing accuracy for the notorious X 1Σg+ C2 and X 1Σ+ CN+ multireference systems. The data also reveal that recent schemes for CI+PT extrapolations to the FCI limit are quite accurate, to within 0.4 mEh, 0.001 Å, and 4 cm−1 in the total energy, re, and ωe, respectively. Whether such schemes approximate FCI curves with sufficient smoothness to reproduce the anharmonic data obtained here is elevated as a challenge for future work. © 1998 American Institute of Physics.
Guided ion beam studies of the reactions of Crn+ (n=1–18) with CO2: Chromium cluster oxide bond energies
Authors: Griffin James B. , Armentrout P. B.
Published: July 2002

The kinetic energy dependence of the reactions of Crn+ (n=1–18) with CO2 are studied in a guided ion beam mass spectrometer. The primary product ions are CrnO+, which then decompose by sequential loss of chromium atoms as the kinetic energy is increased. Simple collision-induced dissociation to form the Crn−1+ product ions is also observed. Large cluster ions, n⩾9, form the CrnCO2+ adduct at low kinetic energies. For many cluster sizes, the cross section for the primary reaction, Crn++CO2→CrnO++CO, exhibits an interesting bimodal energy behavior that is discussed in some detail. Crn+–O bond energies are measured and found to compare well with measurements obtained from guided ion beam studies of the Crn++O2 systems. The trends in this thermochemistry are discussed and compared to bulk phase oxidation values.