bluuebunny/crossref_metadata_2025 huggingface dataset:
Chain statistics in micelles and bilayers: Effects of surface roughness and internal energy
Authors: Szleifer I. , Ben-Shaul A. , Gelbart W. M.
Published: July 2002

A recently developed mean field (‘‘single-chain’’) theory for amphiphile chain organization and thermodynamics in micellar aggregates is applied to rotational isomeric state, model chains. The theory provides explicit, simple expressions for the probability distribution of chain conformations and related molecular and thermodynamic properties applicable to aggregates of arbitrary geometries. Bond order parameter profiles calculated from the theory for a planar bilayer, assuming a compact hydrophobic core, show very good agreement with experimental data and molecular dynamics simulations. For small spherical micelles comparison between theory and experiment suggest the existence of a somewhat rough (few angstroms wide) hydrocarbon–water interfacial region. Cylindrical aggregates reveal intermediate behavior. The extent of ‘‘surface roughness’’ is introduced into the theory via a density profile of the hydrophobic core which decreases gradually from the bulk liquid (compact core) density to zero. A series of calculations is presented to analyze the effects of internal chain (gauche/trans) energy and micellar geometry on the conformational and thermodyamic properties of the hydrocarbon chains. It is found that the internal energy plays only a secondary role, compared to the primary role of the packing constraints. (This is qualitatively consistent with our previous findings for approximate, ‘‘cubic,’’ model chains.) The conformational free energy cost associated with chain packing in aggregates is shown to depend on the micellar geometry (i.e., on the curvature of, and the average area per head group at, the hydrocarbon–water interface) and to be comparable with the surface (head group) contributions treated exclusively in the prevailing theories of surfactant self-assembly. Finally, a ‘‘corresponding-states’’ behavior is demonstrated for packed chains (in planar bilayers) by referencing all thermodynamic functions and configurational properties to those of the associated ‘‘free’’ chain.
The Enskog theory for multicomponent mixtures. IV. Thermal diffusion
Authors: Kincaid J. M. , Cohen E. G. D. , López de Haro M.
Published: July 2002

Using the revised Enskog theory we derive equations for the thermal diffusion ratios kTi and thermal diffusion factors αij of multicomponent hard-sphere mixtures for systems in mechanical equilibrium. The first ten Enskog approximations to the thermal diffusion factor αij[N] (N=1,2,...,10) are evaluated numerically for a variety of system parameter choices appropriate to binary and ternary mixtures. We find that the first Enskog approximation does not vanish; the sequence of Enskog approximations converges most rapidly when the mass of the spheres are nearly equal. The seventh Enskog approximation was estimated to lie within about 1% of the exact value for all choices of system parameters. A comprehensive numerical study of the properties of the thermal diffusion factor for binary mixtures is given, including special mixtures such as the dusty gas, the Lorentz, quasi-Lorentz and Rayleigh gases, and isotopic mixtures. Particular emphasis is placed on showing how the properties of α12 change as the density is increased. The most striking difference between the low-density (Boltzmann) and high-density values for α12 is that the region of the mass ratio—diameter ratio plane for which α12 is either strictly positive or negative, as a function of composition, is much smaller at high densities. Also, at higher densities α12 is not a monotonic function of the mole fraction. For ternary mixtures in which two of the components are isotopes we show how the properties of the third component affect the separation of the two isotopes.
Classical and semiclassical mechanics of strongly resonant systems: A Fourier transform approach
Authors: Martens Craig C. , Ezra Gregory S.
Published: July 2002

The Fourier transform approach to EBK quantization, previously applied to nonresonant systems with up to four degrees of freedom [J. Chem. Phys. 83, 2990 (1985)], is extended to the case of strongly resonant classical motion. The classical mechanics of systems with 3:4, 1:2, and 1:1 resonances is examined in detail from the Fourier transform point of view, and the results of nonlinear resonance analysis used to interpret numerical trajectory Fourier spectra. Calculation of classical actions and numerical construction of the angle parametrization of invariant tori is described, and the relation between spectral frequency assignments and the choice of good action-angle variables investigated. It is shown that correct quantization conditions for arbitrary resonant motion can be determined by direct numerical evaluation of Maslov indices. Semiclassical eigenvalues are reported for the 3:4, 1:2, and 1:1 resonant systems.
Liquid–glass transition: A molecular dynamics study of the sodium system
Authors: Watanabe M. S. , Tsumuraya K.
Published: July 2002

Glass formation process of the sodium system is studied by means of molecular dynamics simulation with constant volume. An equilibrium liquid with 864 atoms is cooled with the rate of 8.00×1013 K s−1 to form the glassy state. A second peak splitting of pair-distribution function (PDF) disappears at high temperatures in the glassy state region and appears clearly with decreasing temperature. The disappearance is attributed to the thermal vibration effect of atoms in the glass. The harmonic vibration model is valid in the glassy state. The Wendt–Abraham parameter, the ratio of the first minimum to the first maximum in the PDF, is 0.08 at the glass transition temperature. The value is equal to the one for the soft-core system, which is lower than the value 0.14 for the Lennard-Jones and the Gaussian-core system. The constant-volume specific heat is found to be 3 Rg in the glassy state, where Rg is the gas constant. The difference of the specific heat between the supercooled liquid and the glassy state is Rg.
Origins of the poor convergence of many-body perturbation theory expansions from unrestricted Hartree–Fock zeroth-order descriptions
Authors: Lepetit M. B. , Pélissier M. , Malrieu J. P.
Published: July 2002

Besides unavoidable discontinuities of the finite-order potential curves derivatives at the symmetry-breaking point, the UHH Möller–Plesset perturbation series show very slow convergence at intermediate and large interatomic distances. This bad behavior has two origins: (i) the MP denominators tend towards a constant value instead of vanishing so that each contribution is decreasing too rapidly with the interatomic distance; this remark condemns the use of MP zeroth-order Hamiltonian and forces to adopt an Epstein–Nesbet definition; (ii) the coupling between the singly and doubly excited determinants is very strong at intermediate distances, and the effect of the singly excited configurations, which only appears at the fourth order, slows down the convergence rate. These trends are both analytically demonstrated and numerically illustrated in the model minimal basis sets He++2 and N2 problems. A proposal to include the effect of singly excited determinants through a low-dimensional effective intermediate-type Hamiltonian has significantly improved the second-order results. By the way it is demonstrated that the perturbation from a singlet symmetry-broken HF solution (charge instability) necessarily diverges at intermediate distance.
A polarizable model for water using distributed charge sites
Authors: Sprik Michiel , Klein Michael L.
Published: July 2002

An algorithm is proposed for treating many-body polarization effects that is suitable for molecular dynamics simulations of polar fluids. As an application of the procedure we have augmented an existing point charge model for water. Using reasonable parameters, the characteristic water structure and liquid binding energy can be reproduced. The resulting effective dipole moment of a water molecule in water is found to be 2.85 D.
HNSi; ν1 emission band by high resolution Fourier transform spectroscopy
Authors: Elhanine M. , Farrenq R. , Guelachvili G.
Published: July 2002

The emission spectrum of the fundamental vibration–rotation ν1 band (NH stretch) of iminosilicon HNSi has been observed near 2.7 μm by high resolution Fourier transform spectroscopy from a mixture of N2+SiH4 excited in a radio frequency discharge. This is the first spectroscopic observation of this molecule in the gas phase. Molecular constants σ, B, D, H are reported. Calculated microwave transitions which should be helpful for the investigation and the possible identification of HNSi in astrophysical sources are also given.
Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the Ã(A″) state
Authors: Loison Jean-Christophe , Kable Scott H. , Houston Paul L. , Burak Itamar
Published: July 2002

Photofragment excitation (PHOFEX) spectra of the jet-cooled formyl (HCO and DCO) radical have been measured by monitoring laser-induced fluorescence of the CO fragment. The following Ã(A″)←X̃(A′) vibronic transitions were measured: (v1,v2,v3)←(0,0,0) where for HCO v2=6–16 with v1,v3=0; v2=7–12 with v1=1, v3=0; v2=9–12 with v1=0, v3=1 and for DCO v2=14–18 with v1,v3=0. The PHOFEX technique provides a powerful method for discriminating against strong transitions so as to allow assignment and measurement of many weaker Franck–Condon bands. The linewidths of the diffuse transitions lie in the 16–150 cm−1 (FWHM) range; they increase strongly with K′ (the projection of the total angular momentum onto the a axis) and decrease slightly with v′2 (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper Ã-state lifetimes ranging from 70 to 700 fs. These data are in excellent agreement with a dynamical model of HCO/DCO curve crossing developed by R. N. Dixon [Mol. Phys. 54, 333 (1985)].
Polarizabilities of the alkali halide dimers. II
Authors: Guella T. , Miller Thomas M. , Stockdale J. A. D. , Bederson B. , Vušković L.
Published: July 2002

We have measured the average electric dipole polarizabilities of twenty alkali halide dimers, using a molecular beam electric deflection method. This is a continuation of the earlier measurements of five alkali halide dimers [R. Kremens et al., J. Chem. Phys. 81, 1676 (1984)]. We compare our measurements with a simple combination of bond (via a ‘‘spring’’ model) and effective ionic polarizabilities, using literature values for the required molecular constants. Agreement between our measured and calculated values is reasonable, in most cases, considering the relative crudeness of the model.
The B←X electronic spectrum of N+2–Ne
Authors: Bieske E. J. , Soliva A. M. , Maier J. P.
Published: July 2002

The electronic spectrum of N+2–Ne has been measured in the region corresponding to the B 2∑+u←X 2∑+g origin and 1–0 transitions of N+2. Spectra were obtained by irradiating a mass selected population of N+2–Ne and monitoring the production of N+2 as a function of wavelength. Low temperature N+2–Ne spectra exhibit several well resolved bands. From the shift of the N+2–Ne origin with respect to that of free N+2 it is apparent that the complex dissociation energy D0 is 146.5 cm−1 greater in the B state than the X state. Pronounced changes in the complex’s spectrum occur as the effective temperature is increased. The hottest spectra resemble a broadened and truncated N+2 spectrum. The breaking off at the high energy end of the spectrum at elevated temperatures allows us to establish a rough ground-state dissociation energy of 300 cm−1. Other conclusions resulting from this work are that the equilibrium geometry of the N+2–Ne molecule is probably linear in X and B electronic states, that the ΔG1/2 for the low frequency stretch in the B state is 104 cm−1, and that the N–N stretching motion is affected only very weakly by the presence of the Ne atom.
Precursor state in the chemisorption of CO on supported palladium clusters
Authors: Henry C. R. , Chapon C. , Duriez C.
Published: July 2002

It is demonstrated that physisorption of CO molecules on a MgO (100) surface, supporting epitaxially grown Pd particles, acts as a precursor state for CO chemisorption on the particles. The adsorption rate of CO on the Pd particles is measured, at zero coverage by a molecular beam technique, as a function of the substrate temperature and for different particle sizes. A kinetic model describing the adsorption, desorption, diffusion, and capture by the clusters of CO molecules is given. Comparison with the experimental data gives the adsorption probability: 0.5±0.05 and the saddle energy for surface diffusion: 0.25±0.05 eV of CO molecules on MgO.
Absorption band oscillator strengths of N2 transitions between 95.8 and 99.4 nm
Authors: Stark G. , Smith Peter L. , Huber K. P. , Yoshino K. , Stevens M. H. , Ito K.
Published: July 2002

Photoabsorption cross sections for seven electric dipole-allowed N2 bands, at wavelengths between 95.8 and 99.4 nm, have been measured with an instrumental resolution of 6.2×10−4 nm. Integrated cross sections and band oscillator strengths are presented for the c4′ 1Σu+(v′=0)–X  1Σg′(v ″=0) and c3 1Πu(v′=0)–X  1Σg+(v″=0) transitions, as well as for five bands (v′=0,1,2,3,4) in the b  1Πu(v′)–X  1Σg+(0) progression. The results are compared with earlier, lower resolution absorption measurements, electron scattering measurements, and calculations based on a deperturbation analysis of the excited states.
Theoretical vibrational and rotational energies and intensities of the HNSi and DNSi molecules
Authors: Chong Delano P. , Papoušek Dusan , Chen Yit-Tsong , Jensen Per
Published: July 2002

Using the demon density functional program, 104 points on the potential-energy surface for the ground electronic state of the HNSi molecule were calculated. An analytic function was fitted through these points and used in two different computer programs (called Morse oscillator–rigid bender internal dynamics and triatom) to calculate the vibration–rotational energies of the HNSi and DNSi molecules. Other analytic functions were fitted through the calculated dipole-moment data to compute the vibrational transition moments and the vibrationally averaged dipole moments. The computed fundamentals ν1, ν2, and ν3 for HNSi from the triatom program are 3466, 413, and 1172 cm−1, respectively, in reasonable agreement with the observed values of 3588, 523, and 1198 cm−1. Agreement is similar for the Morse oscillator–rigid bender internal dynamics program as well as for DNSi.
The nematic to isotropic transition of a liquid crystal in porous media
Authors: Dadmun M. D. , Muthukumar M.
Published: July 2002

An experimental study of the effect of disorder on the nematic to isotropic (N–I) transition of a liquid crystal is reported. Differential scanning calorimetry is used to monitor the N–I transition of p-azoxyanisole in a series of controlled pore glasses. An increase and a decrease in the N–I transition temperature is observed with varying pore sizes. The direction of the temperature shift depends on the pore size. The observed increase in the transition temperature can be attributed to surface induced ordering, while the observed decrease can be explained as a result of the interruption of the orientational correlation of the liquid crystalline order due to confinement in pores. These results agree, qualitatively, with Monte Carlo results showing that quenched disorder can lower the transition temperature, round and lower the heat capacity peak, and change the order of the transition.
A simple model for the swelling of polymer networks
Authors: Painter Paul C. , Shenoy Suresh L.
Published: July 2002

The Flory–Rehner approach to the solvent swelling of polymer networks is modified so as to abandon the affine deformation assumption. Instead, it is assumed that swelling occurs by a process of disinterspersion of cross-link junctions, and relationships between chain expansion and segment concentration are established using the ideas of de Gennes. Using this approach, we also examine osmotic deswelling, aspects of phase equilibria, and the maximum observed in differential swelling measurements.
The two-determinant coupled-cluster method for electric properties of excited electronic states: The lowest 1B1 and 3B1 states of the water molecule
Authors: Balková Anna , Bartlett Rodney J.
Published: July 2002

A recently proposed two-determinant coupled-cluster method for open-shell singlet states is implemented to study the dipole moments, polarizabilities, and excitation energies for the lowest singlet and triplet B1 states of the water molecule. Selected electric properties for the B1 electronic states are determined and results compared with the previous complete active space self-consistent field (CASSCF) study. The effect of dynamic correlation on the convergence of the dipole moments and polarizabilities is critically examined, including a comparison of the corresponding coupled-cluster and exact full configuration interaction (CI) values in selected orbital spaces. As a result, an optimal description of the reference space as well as a balanced treatment of dynamic and nondynamic correlation on the single and double excitation level are found to be superior to an extensive treatment of only the nondynamic correlation effects. Vertical and adiabatic excitation energies for the B1 excited states are investigated and found to be in close agreement with experiment and the best theoretical values.
Collision of alkali ions with C60/C70: Insertion, thermionic emission, and fragmentation
Authors: Wan Zhimin , Christian James F. , Basir Yousef , Anderson Scott L.
Published: July 2002

Interactions of Li+, Na+, and K+ with C60 molecules have been studied over the collision energy range from 0 to 150 eV. For Li+ and Na+, insertion of the alkali to form the endohedral [Li@C60]+ and [Na@C60]+ species is observed, with appearance energies of ∼6 and ∼18 eV, respectively. At higher collision energies several processes are observed. For Na+, the hot [Na@C60]+ appears to relax mainly by loss of C2 units from the fullerene cage, yielding [Na@C60−2n]+. For [Li@C60]+, escape of Li+ dominates over C2 loss. For K+, very little [K@C60]+ is observed, and the main endohedral species observed are fragments [K@C60−2n]+ (n=1–7), which begin to appear at ∼40 eV. Pure fullerene fragment ions (C60−2n+) are also observed, with cross sections that increase sharply at ∼30 eV for all three projectile ions. These ions are believed to result primarily from thermionic emission of collisionally excited neutrals. For K+, we also examined collisions with C70, obtaining results similar to those for C60. Changes in the endohedral insertion mechanism with projectile size are discussed, as is the competition between C2 evaporation, thermionic emission, and IR emission.
Nuclear dynamics of several decaying overlapping electronic states: A time-dependent formulation
Authors: Cederbaum L. S. , Tarantelli F.
Published: July 2002

The nuclear dynamics accompanying the excitation of a polyatomic target to a manifold of decaying overlapping electronic states and the subsequent decay to final electronic states are discussed. Nonadiabatic interactions within the manifold of decaying states as well as of final states are included. The cross sections for the excitation and for the production of final states are derived. The wave packet dynamics are investigated for both the excitation and the decay and related to the corresponding cross sections. Particular attention is paid to core levels of systems with equivalent atoms. The time-dependent formulation is also used to compute the gross features of the observed spectra via a spectral moment expansion. An interesting example is investigated where the decaying as well as the final states have extremely localized electron vacancies.
Calculation of the electron affinities of the second row atoms: Al–Cl
Authors: Woon David E. , Dunning Thom H.
Published: July 2002

The electron affinities (EAs) of aluminum through chlorine have been calculated with multireference single and double excitation configuration interaction calculations (MRSD-CI) with the augmented correlation consistent basis sets of Woon and Dunning [J. Chem. Phys. 98, 1358 (1993)]. The orbital space for the complete active space (CAS) wave functions is systematically expanded beyond the Hartree–Fock wave function by inclusion of additional s, p, and d orbitals. This approach provides a well balanced treatment of the neutral and ionic charge states and led to accurate EAs for hydrogen and the first row atoms [R. A. Kendall, T. H. Dunning, Jr., and R. J. Harrison, J. Chem. Phys. 96, 6796 (1992)]. The best MRSD-CI values for the EAs (in eV) of the second row atoms are Al, 0.440; Si, 1.413; P, 0.689; S, 2.023; and Cl, 3.606. Inclusion of spin–orbit effects in the calculated EAs yields (experimental values in parentheses): Al, 0.437 (0.441); Si, 1.395 (1.385); P, 0.700 (0.746); S, 2.019 (2.077); and Cl, 3.570 (3.613). The amenability of the correlation consistent basis sets for extrapolating to the complete basis set limit has also been exploited in this work. The neutral and anion energies are extrapolated separately in order to account for the different convergence rates in the energies of the two species. The extrapolated values of the EAs, including an empirical spin–orbit correction, are Al, 0.438; Si, 1.395; P, 0.713; S, 2.047; and Cl, 3.596. Additional calculations suggest that the remaining error is a measure of the residual imbalance in the MRSD-CI treatments of the neutral and anion. Extrapolated EAs for the first row atoms are also reported.
Analysis of some integrals arising in the atomic four-electron problem
Authors: King Frederick W.
Published: July 2002

An analysis is presented for the evaluation of the one-center integrals of the form ∫ri1rj2rk3rl4rm12 rn13rp14rq23rs24 rt34e−ar1−br2−cr3−dr4 dr1 dr2 dr3 dr4 which arise in the solution of the atomic four-electron problem when a Hylleraas expansion is employed. A majority of the cases that arise can be solved by a reduction of the four-electron integrals to integrals arising in the three-electron problem. A second general approach is developed which is able to handle almost all cases that arise in practical calculations.
The coupled three-dimensional wave packet approach to reactive scattering
Authors: Marković Nikola , Billing Gert D.
Published: July 2002

A recently developed scheme for time-dependent reactive scattering calculations using three-dimensional wave packets is applied to the D+H2 system. The present method is an extension of a previously published semiclassical formulation of the scattering problem and is based on the use of hyperspherical coordinates. The convergence requirements are investigated by detailed calculations for total angular momentum J equal to zero and the general applicability of the method is demonstrated by solving the J=1 problem. The inclusion of the geometric phase is also discussed and its effect on the reaction probability is demonstrated.
Colloids dispersed in polymer solutions. A computer simulation study
Authors: Meijer Evert Jan , Frenkel Daan
Published: July 2002

We report a computer-simulation study of a simple model for a colloid dispersed in a polymer solution. The simulations were performed using a computational scheme that allows simulation at constant osmotic pressure of the polymers. We present results for the polymer-induced interaction, the equation-of-state, and the phase diagram. The simulations show that when the radius of gyration of the polymers Rg is sufficiently large compared to the diameter of the colloidal particles σcol (2Rg/σcol≥0.45), addition of polymer induces a colloidal ‘‘liquid–vapor’’ phase separation. For shorter polymers, only the solid–fluid transition is observed. In addition, we find that the nonpairwise additivity of the polymer-induced attraction between the colloidal particles has a pronounced effect on the equation-of-state and the phase behavior of a colloid–polymer mixture. The perturbation theory of Lekkerkerker et al. [Europhys. Lett. 20, 559 (1992)] is found to perform well, except at low densities.
Nonlinear molecular properties using biorthogonal response approach
Authors: Vaval Nayana , Ghose Keya Basu , Pal Sourav
Published: July 2002

In this paper, we report the use of extended coupled cluster functional of Arponen, Bishop, and co-workers to implement a stationary biorthogonal response approach. The objective of this is to calculate nonlinear molecular properties like hyperpolarizability, etc. in a more convenient way.
Reversing-pulse electric birefringence of disklike suspension in the low electric field region: An extension of the ion-fluctuation model
Authors: Yamaoka Kiwamu , Tanigawa Masato , Sasai Ryo
Published: July 2002

A rigorous theory is formulated for the reversing-pulse electric birefringence (RPEB) for the ionized macroion of cylindrical symmetry, according to the idea of the coupled rotational (with the molecular relaxation time τθ) and ion-atmosphere dynamics of rodlike macroion, developed previously by Szabo et al. [J. Chem. Phys. 85, 7472 (1986)]. The present theory is based on the interaction of applied electric pulse with two electric dipole moments of the macroion; one is due to the ion-atmosphere polarizability α3 with a single relaxation time τI for the distortion of ion-atmosphere along the longitudinal axis, as originally proposed, and the other is newly considered as being due to the intrinsic molecular, or instantaneously field-induced, polarizability anisotropy Δα′ between the longitudinal and transverse axes of the macroion. The RPEB expressions were derived with the electric and hydrodynamic parameters (α3/Δα′ and τI/τθ) for the low-field region. Theoretical curves were calculated with appropriate values to those parameters. The curves show some new features, i.e., either a maximum or a minimum in the buildup and reverse processes, that were not revealed in the Szabo et al. theory. The present theory was used to analyze an experimental RPEB signal of montmorillonite clay suspension. The contribution of the permanent electric dipole moment of the disklike clay particle to field orientation was ruled out.
Experimental determination of ΔH
f(HOBr) and ionization potentials (HOBr): Implications for corresponding properties of HOI
Authors: Ruscic B. , Berkowitz J.
Published: July 2002

The photoion yield curves of HOBr+ and Br+ from HOBr are presented. The adiabatic I.P. of HOBr is found to be 10.638±0.003 eV. Autoionizing structure in HOBr+ is tentatively assigned and leads to an adiabatic I.P. of the first excited (A L2 A′) state of ∼11.46 eV. The 0 K threshold of Br+ from HOBr (13.915±0.018 eV) implies D0(HO–Br)≤2.101±0.018 eV≡48.45±0.42 kcal/mol. Together with auxiliary thermochemistry, this value yields ΔHf2980(HOBr) ≥ −13.43 ± 0.42 kcal/mol, in excellent agreement with a recent ab initio value −14.2±1.6 kcal/mol. The resulting proton affinity of BrO is 163.8 kcal/mol. Trends observed in the properties of OX and HOX (X=F, Cl, Br) are utilized to infer new values for ΔH0f(OI) and ΔH0f (HOI). Predictions are made for I.P.(HOI). Proton affinities of OX are seen to increase with heavier X, while the X 2A″–A 2A′ splitting decreases.
The initial vibrational state distribution of HCN X̃ 1Σ+(v1,0,v3) from the reaction CN(2Σ+)+C2H6→HCN+C2H5
Authors: Bethardy G. A. , Northrup F. J. , Macdonald R. Glen
Published: July 2002

The reaction of the cyano radical (CN) with ethane was studied using time-resolved infrared absorption spectroscopy to monitor individual rovibrational states of the HCN product. A method is described that can be used to determine the initial vibrational state distribution at pressures of several Torr. This technique was applied to the title reaction to determine that the vibrational states of HCN(v1,0,v3), where v1, v3=0, 1, and 2, were not directly populated in the title reaction to any significant extent. The initial vibrational energy content of the CN radical was also varied but did not influence the initial population in the HCN vibrational levels probed in this experiment. The time dependence of HCN(v1,0,v3) was followed and interpreted in terms of bimolecular rate constants for vibrational relaxation with ethane. The title reaction is mode specific in its energy disposal in that at least every HCN product appears to have at least one quantum of bending excitation, likely in combination with stretching vibrations.
On the Jahn–Teller effect on Mn2+ in zinc-blende ZnS crystal
Authors: Stavrev Krassimir K. , Zerner Michael C.
Published: July 2002

The excited states of Mn2+ in zinc-blende ZnS crystal have been studied by the intermediate neglect of differential overlap model parametrized for spectroscopy within the restricted open-shell Hatree–Fock configuration interaction approximation. The effect of the Jahn–Teller (JT) active e-mode on the first excited state (a 4T1) has been examined with respect to energy barriers and geometric distortions. The dynamical JT effect has been studied and the excited state absorption (ESA) profiles for different e-mode conformations have been calculated. Good reproduction of the experimental ESA spectrum has been obtained for nuclear arrangements corresponding to small S–Mn–S angle distortions. The off-center τ2-mode vibrations were found to promote the JT coupling by reducing the barriers between the stable structures.
Neutral rare-gas containing charge-transfer molecules in solid matrices. II. HXeH, HXeD, and DXeD in Xe
Authors: Pettersson Mika , Lundell Jan , Räsänen Markku
Published: July 2002

Photolysis of hydrogen halides (and some other hydrogen containing small molecules) in solid Xe yields in a two step process charged centers, one of them being XeHXe+. Annealing of the irradiated doped solids produces, in addition to H–Xe–Y (Y=Cl, Br, or I) species characterized by us previously, a fairly strong doublet at 1181 and 1166 cm−1 and a weak absorption at 701 cm−1. Deuterated precursors yield a doublet at 846 and 856 cm−1. Also peaks belonging to mixed H/D form are found, indicating that the absorbing species contains two H/D atoms. The new species responsible for these absorptions are assigned as neutral linear centrosymmetric HXeH, HXeD, and DXeD. The nature of the bonding can be understood in terms of the resonance between the two ionic forms HXe+H− and H−Xe+H, analogously to the valence bond description of the well known XeF2. The pseudopotential (LANL1DZ) ab initio calculations at the MP2 level are in good agreement with the observed spectra.
A local model for the hardness kernel and related reactivity parameters in density functional theory
Authors: Fuentealba P.
Published: July 2002

The local density approximation is used to construct a model for the reactivity parameters defined in density functional theory. Starting from a model for the hardness kernel, equations are derived to compute the softness kernel, local softness and local hardness, global softness and global hardness, the Fukui function and also the electric dipole polarizability. A very simple functional is used to test the equations in some atomic systems. The results are promising. Extensions and applications of the model are discussed.
Theoretical study of the CH4+F→CH3+FH reaction. II. Semiempirical surfaces
Authors: Corchado J. C. , Espinosa-García J.
Published: July 2002

We present two semiempirical surfaces for the CH4+F→CH3+FH reaction. One is based on the PM3 semiempirical molecular orbital theory, using parameters specifically calculated for this reaction (SRP method), and the other is based on the analytic function J1 for the CH4+H→CH3+H2 reaction, slightly modified (MJ1 surface). To calibrate the first surface we chose as reference data the reactant and product experimental properties, while to fit the second, we also used ab initio calculated saddle-point information. Experimental rate constants were not used in the calibration because of their uncertainty. Because of the flattening of these surfaces in the saddle-point zone, the variational effects are important and the location of the transition state is concluded to be due to entropy effects. The kinetic isotope effects (KIEs) at different temperatures were also analyzed showing reasonable agreement with the experimental value for both surfaces. The factor analysis of the KIEs indicates an inverse tunneling contribution originated by the behavior of the VGa curve. The strengths and the weaknesses of these two surfaces, along with the ab initio reaction path studied previously, were also analyzed.
The continuous configurational Boltzmann biased direct Monte Carlo method for free energy properties of polymer chains
Authors: Sadanobu Jiro , Goddard III William A.
Published: July 2002

We develop here a highly efficient variant of the Monte Carlo method for direct evaluation of the partition function, free energy, and other configurational dependent physical properties for long polymer chains. This method (CC–BB) combines continuous configurational biased sampling with Boltzmann factor biased enrichment. To illustrate the efficiency and to validate the bias correction for weighting the torsion and chain enrichments, we applied this model to isolated single chains using a united atom force field. For a 50 monomer polymer chain CC–BB with 400 chains leads to an accuracy of 0.1% in the free energy whereas simple sampling direct Monte Carlo requires about 109 chains for this accuracy. This leads to cost savings by a factor of about 350 000. CC–BB is easily extended to multichain systems, to the condensed state, to more realistic force fields, and to evaluating the mixing free energy for polymer blends.
Mechanism of NH2+CO2 formation in OH+HNCO reaction: Rate constant evaluation via ab initio calculations and statistical theory
Authors: Sengupta Debasis , Nguyen Minh Tho
Published: July 2002

Portions of the [CH2NO2] potential energy surface related to the OH+HNCO reaction were calculated by means of ab initio molecular orbital theory at the QCISD(T)/6-311++G(d,p) level based on UMP2/6-31G(d,p) optimized geometries. Of all possible three channels considered, the hydrogen abstraction turns out to be the dominant reaction channel. The addition to C atom requires activation energy slightly larger than that of the abstraction but smaller than that of the N addition, in contrast to the H+HNCO reaction. The structural and energetic parameters for the channels thus characterized were further utilized for the calculation of rate constants in the framework of a quantum statistical theory (QRRK). The contributions of the individual reaction channel towards the total rate constant have been examined. Although the OH+HNCO→NH2+CO2 reaction is more exothermic than the hydrogen abstraction OH+HNCO→H2+NCO, it is confirmed that rate constant for CO2 loss is much lower than that of H2O-elimination. The standard heat of formation of the adduct HNC(OH)O is estimated to be ΔHf298=−41.1±3 kcal/mol.
Effective rotational Hamiltonian for molecules with two periodic large-amplitude motions
Authors: Groner Peter
Published: July 2002

Starting from a general Hamiltonian for a molecule with two periodic large-amplitude motions, an effective rotational Hamiltonian is developed for bound states of the internal motions. It contains two types of matrix elements: (i) The internal motion energy, expressed as two-dimensional Fourier series, and transformed to a reference axes system. (ii) Matrix elements of rotational operators in the reference axes system multiplied by two-dimensional Fourier series. The spectroscopic parameters are the coefficients of the Fourier series (integrals of localized functions), the internal motion parameters ρ, and the angles between the vectors ρ and the reference axes. Expressions are given for cases with higher symmetry. The effectiveness of the Hamiltonian is demonstrated by fitting the rotational spectrum of the ground state of dimethylether (356 frequencies involving levels up to J=25) to experimental precision. The data of five vibrational excited states can be fit to almost the same precision.
Folding and misfolding of designed proteinlike chains with mutations
Authors: Tiana G. , Broglia R. A. , Roman H. E. , Vigezzi E. , Shakhnovich E.
Published: July 2002

We study the impact of mutations (changes in amino acid sequence) on the thermodynamics of simple proteinlike heteropolymers consisting of N monomers, representing the amino acid sequence. The sequence is designed to fold into its native conformation on a cubic lattice. It is found that quite a large fraction, between one-half and one-third of the substitutions, which we call “cold errors,” make important contributions to the dynamics of the folding process, increasing folding times typically by a factor of 2, the altered chain still folding into the native structure. Few mutations (“hot errors”), have quite dramatic effects, leading to protein misfolding. Our analysis reveals that mutations affect primarily the energetics of the native conformation and to a much lesser extent the ensemble of unfolded conformations, corroborating the utility of the “energy gap” concept for the analysis of folding properties of proteinlike heteropolymers.
Density functional theory studies of meso-alkynyl porphyrins
Authors: Wang Zhiqiang , Day Paul N. , Pachter Ruth
Published: July 2002

Geometry optimizations and electronic structure calculations are reported for porphyrins with meso-acetylene substituents using density functional theory (DFT). The addition of the acetylene group alters the nearest C–C bond length in the porphyrin ring by 0.01 to 0.02 Å, but the molecule retains its D2h symmetry. The calculated electronic structures clearly show that the acetylene group contributes to the π-electron conjugation along the porphyrin ring for the HOMO and LUMO, and reduces significantly the HOMO-LUMO gap, being consistent with experimental results [H. L. Anderson, Tetrahedron Lett. 33, 101 (1992)]. The core ionization potential (IP) of the central nitrogen calculated using the so-called local density approximation effective core (LDAEC) method, is increased by 0.24 eV, indicating a more positive electrostatic potential than in PH2. LDA-ΔSCF results for the first three valence IPs indicate changes of 0.1 to 0.2 eV relative to those of the free-base porphin, with the gap between the first and second IPs being over twice as large in the acetylated molecule. The protective trimethylsilyl (TMS) group further reduces the HOMO-LUMO gap, but by a small amount.
Ag
5
is a planar trapezoidal molecule
Authors: Haslett T. L. , Bosnick K. A. , Moskovits M.
Published: July 2002

The Raman spectrum of Ag5, mass-selected from a sputtered jet of silver cluster ions, neutralized and matrix isolated in solid argon, is reported. Analysis of the frequencies and intensities of the 7 vibrational modes recorded suggests unequivocally that the molecule has a planar trapezoidal structure. No other reasonable structure is consistent with the observations. Nor is there evidence for more that one structure coexisting with the planar trapezoid. The observation is in accord with the conclusions of most quantum computations on Ag5. The silver pentamer is found to photofragment into the dimer and trimer when irradiated with 458 or 466 nm laser light.
Polyelectrolyte adsorption onto an initially-bare solid surface of opposite electrical charge
Authors: Sukhishvili Svetlana A. , Granick Steve
Published: July 2002

We contrast the adsorption, over a wide range of pH and ionic strength, of polyelectrolyte chains with different fractions of charged segments but similar degree of polymerization. The system was a cationic polymer, poly(1,4 vinyl)pyridine (PVP), with 14%, 48%, and 98% quaternized repeat units, adsorbed from aqueous solution (D2O or H2O) onto a single silicon oxide substrate at 25 °C. Measurements were based on Fourier transform infrared spectroscopy in attenuated total reflection (FTIR-ATR). In the first phase of this study, we varied the surface charge density by changing pH and showed that attraction of PVP to the surface was electrostatic. The amount adsorbed of charged (quaternized) PVP segments was nearly the same regardless of the overall fraction of charged segments in the chain. In addition, polymer adsorption appeared to enhance the dissociation of silanol groups on the solid surface. In a second phase of this study, the ionic strength was varied systematically under conditions of high negative surface charge density (high pH), focusing on 98% quaternized PVP. Strong chemical specificity was found; the polyelectrolyte was insoluble in KI above a low salt concentration, but soluble in NaCl, signifying that the anions, Cl− and I−, competed with the negatively-charged surface for association with the polyelectrolyte. At the same time, the cations, Na+ and K+, competed with the polyelectrolyte for access to the limited surface area. The mass adsorbed increased strongly with increasing salt concentration and, for polymer in aqueous NaCl, passed through a maximum with subsequent decrease, reflecting a greater abundance of loops and tails at intermediate ionic strength and ultimately complete desorption of the chains when the salt concentration was very high. The maximum in mass adsorbed occurred at very high ionic strength (1 molar NaCl), indicating competitive adsorption of Na+ with charged segments of the polymer. Direct measurements of the infrared dichroism of pyridinium rings of the adsorbed PVP confirmed the presence of a relatively flattened state at low ionic strength and nearly isotropic orientation otherwise. In the third phase of this study, we studied the competitive adsorption to surfaces of high negative charge of Na+ and a monomeric analog of the PVP repeat unit, the 1,4-dimethylpyridinium ion (P+). A tentative quantitative estimate of the effective surface sticking energy of P+ relative to Na+ ions indicated a decrease from 7kBT in low-ionic-strength buffer solution to 4.5kBT in 0.5 M NaCl. These numbers appear to exceed the weak-adsorption limit in which facile equilibration of the adsorbed layer should be expected.
Theoretical study of the mechanism of electron transfer at photosynthetic reaction centers. I. Singlet excited states of free base porphin
Authors: Kitao Osamu , Ushiyama Hiroshi , Miura Nobuaki
Published: July 2002

Free base porphin is a key unit in the electron transfer reaction at photosynthetic reaction centers. For the electron transfer reaction, the transfer integral of the rate constant depends strongly on the quality of the wave functions of porphin-based chromophores. Therefore, we need a stable method for calculating the wave functions of optically allowed excited states of the porphin. We developed such a method and verified its stability by calculating the wave functions for an ethylene molecule. We confirmed that the optically allowed excited states required the entire amount of valence molecular orbitals for the active space to adequately describe the wave function with molecular in–out polarization. We applied our calculation method to investigate the wave function of free base porphin. Our ab initio calculation used 4-31 G plus d polarization functions for the carbon and nitrogen atoms, and Rydberg 2d basis functions on the center of each pyrrol ring simultaneously. We also proposed an assignment for the controversial B and N bands. This assignment is reasonably consistent with experimental data within 0.36 eV for Q and B bands, and 0.75 eV for N bands. The reproduction accuracy of the lower excitation spectra up to B band confirms the reliability of the transfer integral part to calculate the rate constants of the electron transfer reaction at photosynthetic reaction centers.
Dissipative particle dynamics: Transport coefficients
Authors: Evans G. T.
Published: July 2002

The self-diffusion coefficient and the shear viscosity are calculated for a system evolving under dissipative particle dynamics in two and three dimensions. The self-diffusion coefficient agrees with that Marsh et al. although the shear viscosity has corrections absent in previous theories. These corrections arise from a subtle combination of convective and dissipative effects. When the present theory is compared with computer simulations of two dimensional fluids, it appears that the present work portrays albeit qualitatively the deviation of the dissipative and kinetic parts of the shear viscosity from the low friction limits depicted in the Marsh et al. work.
Longitudinal dielectric properties of molecular liquids: Molecular dynamics simulation studies of CH3CN, C6H6, and CO2
Authors: Perng Baw-Ching , Ladanyi Branka M.
Published: July 2002

Molecular dynamics (MD) simulations of εL(k,ω), the frequency (ω) and wave vector (k) dependent longitudinal component of the dielectric permittivity tensor, a quantity of importance in several theories of solvation dynamics and charge transfer reactions, is reported for three molecular liquids: CH3CN, CO2, and C6H6, represented by nonpolarizable model potentials. In order to study dielectric properties of nondipolar fluids we use, instead of the conventional approach which relates εL(k,ω) to longitudinal dipole density fluctuations, a more general approach of Raineri and co-workers which expresses this quantity in terms of charge density fluctuations. The two formulations are compared in the case of acetonitrile to assess the model dependence of εL(k,ω). We find that at finite k, 1/εL(k), where εL(k)=εL(k,0) is the static longitudinal permittivity, exhibits several similar features for all three liquids: A partial cancellation between single-molecule and pair charge density fluctuation correlations at small k, their constructive interference at intermediate k and the lack of molecular pair correlation contributions at large k. We also find that the extended reference interaction site model (XRISM) integral equations provide an excellent approximation to εL(k) of all three liquids. We use the fact 1/εL(k) is a polynomial in k2 at small k to determine the static dielectric constant ε0=εL(k=0) of acetonitrile and obtain a value in good agreement with ε0 evaluated by more conventional methods. We find that intermolecular correlations contribute the most to the dielectric properties of CH3CN and the least to those of CO2. In the range of k most relevant to solvation (k≲1 Å−1), the pair component of the charge–charge time correlation function Φqq(k,t) is negative, partially cancelling the positive single-molecule component. The extent of cancellation varies with k and the strength of intermolecular electrostatic interactions, leading to significant qualitative differences in the behavior of Φqq(k,t) for polar and nondipolar liquids: In this k range, Φqq(k,t) in acetonitrile decays more slowly as k increases, while the opposite k-ordering is seen in the two nondipolar liquids. We use our results for εL(kmin,ω), where kmin is the smallest wave vector accessible in our simulation, to calculate the far-IR (infrared) absorption coefficient α(ω) of acetonitrile and find that it agrees well with α(ω) obtained from the transverse permittivity component, εT(kmin,ω), indicating that the bulk limit for this quantity has been reached.