Showing papers in "Journal of Chemical Physics in 1990"

An all‐electron numerical method for solving the local density functional for polyatomic molecules

Abstract: A method for accurate and efficient local density functional calculations (LDF) on molecules is described and presented with results The method, Dmol for short, uses fast convergent three‐dimensional numerical integrations to calculate the matrix elements occurring in the Ritz variation method The flexibility of the integration technique opens the way to use the most efficient variational basis sets A practical choice of numerical basis sets is shown with a built‐in capability to reach the LDF dissociation limit exactly Dmol includes also an efficient, exact approach for calculating the electrostatic potential Results on small molecules illustrate present accuracy and error properties of the method Computational effort for this method grows to leading order with the cube of the molecule size Except for the solution of an algebraic eigenvalue problem the method can be refined to quadratic growth for large molecules

A Simple Measure of Electron Localization in Atomic and Molecular-Systems

Abstract: We introduce in this work a new approach to the identification of localized electronic groups in atomic and molecular systems. Our approach is based on local behavior of the Hartree–Fock parallel‐spin pair probability and is completely independent of unitary orbital transformations. We derive a simple ‘‘electron localization function’’ (ELF) which easily reveals atomic shell structure and core, binding, and lone electron pairs in simple molecular systems as well.

Dynamics of entangled linear polymer melts: A molecular‐dynamics simulation

Abstract: We present an extensive molecular‐dynamics simulation for a bead spring model of a melt of linear polymers. The number of monomers N covers the range from N=5 to N=400. Since the entanglement length Ne is found to be approximately 35, our chains cover the crossover from the nonentangled to the entangled regime. The Rouse model provides an excellent description for short chains N

Molecular dynamics with electronic transitions

Abstract: A method is proposed for carrying out molecular dynamics simulations of processes that involve electronic transitions. The time dependent electronic Schrodinger equation is solved self‐consistently with the classical mechanical equations of motion of the atoms. At each integration time step a decision is made whether to switch electronic states, according to probabilistic ‘‘fewest switches’’ algorithm. If a switch occurs, the component of velocity in the direction of the nonadiabatic coupling vector is adjusted to conserve energy. The procedure allows electronic transitions to occur anywhere among any number of coupled states, governed by the quantum mechanical probabilities. The method is tested against accurate quantal calculations for three one‐dimensional, two‐state models, two of which have been specifically designed to challenge any such mixed classical–quantal dynamical theory. Although there are some discrepancies, initial indications are encouraging. The model should be applicable to a wide variety of gas‐phase and condensed‐phase phenomena occurring even down to thermal energies.

Coupled cluster response functions

Abstract: The linear and quadratic response functions have been determined for a coupled cluster reference state From the response functions, computationally tractable expressions have been derived for excitation energies, first‐ and second‐order matrix transition elements, transition matrix elements between excited states, and second‐ and third‐order frequency‐dependent molecular properties

The melting behavior of organic materials confined in porous solids

Abstract: The solid–liquidphase transition temperatures and heats of fusion ΔH f of nonpolar organic solids confined in the pores of controlled pore glasses were measured by differential scanning calorimetry. The pore diameters d were in the range of 40–730 A and the organics studied were cis‐decalin, trans‐decalin, cyclohexane, benzene, chlorobenzene, naphthalene, and heptane. In accordance with previous reports on studies of primarily inorganic materials, the melting point of the pore solidT(d) decreased with decreasing pore diameter. In addition, a large reduction in the bulk enthalpy of fusion ΔH f of the pore solid was measured, which apparently has not been studied in detail by other workers. A linear correlation was found between the melting point depression (ΔT m ) and the reciprocal diameter, as predicted by theories of solidification in a capillary. The calculated values of the solid–liquid interfacial energy σsl were in reasonable agreement with values reported in the literature based on other methods of measurement.

Gaussian‐1 theory of molecular energies for second‐row compounds

Abstract: The Gaussian‐1 theoretical procedure is extended and tested on compounds containing second‐row atoms (Na–Cl). This is a composite procedure based on ab initio molecular orbital theory, utilizing large basis sets (including diffuse‐sp, double‐d, and f‐polarization functions) and treating electron correlation by Mo/ller–Plesset perturbation theory and by quadratic configuration interaction. Total atomization energies for a set of 24 species agree with accurate experimental data to an accuracy of better than 3 kcal/mol in most cases, SO2 being the notable exception. Similar agreement is achieved for ionization energies, electron affinities, and proton affinities. The method is used to assess experimental data for a number of other compounds having less accurate atomization energies.

Liquid to solidlike transitions of molecularly thin films under shear

Abstract: We have measured the shear forces between two molecularly smooth solid surfaces separated by thin films of various organic liquids. The aim was to investigate the nature of the transitions from continuum to molecular behavior in very thin films. For films whose thickness exceeds ten molecular diameters both their static and dynamic behavior can usually be described in terms of their bulk properties, but for thinner films their behavior becomes progressively more solidlike and can no longer be described, even qualitatively, in terms of bulk/continuum properties such as viscosity. The solidlike state is characterized by the ordering of the liquid molecules into discrete layers. The molecular ordering is further modified by shear, which imposes a preferred orientation. All solidlike films exhibit a yield point or critical shear stress, beyond which they behave like liquid crystals or ductile solids undergoing plastic deformation. Our results on five liquids of different molecular geometry reveal some very co...

Population transfer between molecular vibrational levels by stimulated Raman scattering with partially overlapping laser fields. A new concept and experimental results

Abstract: The feasibility of a novel technique for efficient and selective population transfer from a thermally populated level 1 via an intermediate state 2 to level 3 is experimentally demonstrated. It is shown for sodium dimers that the process of on‐ or near‐resonance stimulated Raman scattering with only partially overlapping laser beams is, in particular, useful for the selective population of high vibrational levels of particles in a molecular beam. This is achieved when the interaction with the Stokes laser, coupling levels 2 and 3, begins earlier than the interaction with the pump laser. The phenomenon, which is closely related to the formation of ‘‘trapped states,’’ is quantitatively explained using the basis of eigenstates of molecules strongly coupled to the radiation fields. The similarity and difference to related techniques such as rapid adiabatic passage phenomena in two‐level systems, off‐resonant stimulated Raman scattering, or stimulated emission pumping is briefly discussed.

Excitation energies from the coupled cluster singles and doubles linear response function (CCSDLR). Applications to Be, CH+, CO, and H2O

Abstract: The linear response function for a coupled cluster singles and doubles wave function is used to calculate vertical electronic energies for the closed shell system Be, CH+, CO, and H2O. It is shown that excitations of single electron replacement character can be described accurately in such an approach. Improved convergence is obtained using a preconditioned form of the coupled cluster linear response matrix.

Kinetics of swelling and shrinking of gels

Abstract: The kinetics of swelling and shrinking of gels is theoretically generalized and experimentally studied. A new relation, in addition to the differential equation developed by Tanaka and Fillmore, is formulated to solve the kinetics of gels having arbitrary shape. Using our new theory, we provide explicit solutions for long cylinder and large disk gels. These solutions predict that the effective diffusion constants of long cylinder and large disk gels are 1.5 and 3 times smaller than that of a spherical gel. The relaxation times of long cylinder and large disk gels with small shear modulus μ are approximately 2.0 and 5.7 times longer than that of a sphere, where the diameters of the cylinder and the sphere and the thickness of the disk are the same. The theory also concludes that the experimentally measured kinetics of a long cylinder and disk gels along the z axis and the radial axis are the same. The results have been excellently confirmed by the experiments.

Photoelectron spectroscopy of metal cluster anions : Cu−n, Ag−n, and Au−n

Abstract: Negative ion photoelectron spectra of Cu−n, Ag−n(n=1–10), and Au−n (n=1–5) are presented for electron binding energies up to 3.35 eV at an instrumental resolution of 6–9 meV. The metal cluster anions are prepared in a flowing afterglow ion source with a cold cathode dc discharge. In the spectra of Cu−2, Ag−2, and Au−2, the M2 X 1Σ+g←M−2 X 2Σ+u transitions are vibrationally resolved. We analyze these spectra to yield the adiabatic electron affinities, vibrational frequencies, bond length changes, and dissociation energies. The a 3Σ+u triplet states of Cu2 and Ag2 are also observed. Using experimental and theoretical data, we assign the major features in the Cu−3 and Ag−3 spectra to the transition from the linear ground state of the anion (M−31Σ+g) to an excited linear state of the neutral (M3 2Σ+u). The Au−3 spectrum is attributed to a two‐photon process, photodissociation followed by photodetachment of the Au− or Au−2 fragment. For larger clusters, we measure the threshold and vertical detachment energies...

Relaxation of conformers and isomers in seeded supersonic jets of inert gases

Abstract: We have studied the relaxation of conformers and the formation/relaxation of isomeric, weakly bonded dimers in pulsed supersonic expansions of seeded inert gases (He, Ne, Ar, Kr). The relaxation was determined from the intensity of a rotational transition for the higher energy species as a function of carrier gas composition, using the Balle/Flygare Fourier transform microwave spectrometer. Of thirteen molecules with rotational conformers which we examined, those with barriers to internal rotation greater than 400 cm−1 did not relax significantly in any of the carriers. The higher energy forms of ethyl formate, ethanol, and isopropanol, with smaller barriers, were not relaxed by He; those of ethanol and isopropanol were somewhat relaxed by Ne; and all were completely relaxed by as little as 5 to 20 mole percent of Ar or Kr in He or Ne. The relaxation in He or Ne is first order in the concentration of added Ne, Ar, or Kr as well as in the concentration of the high energy conformer. The pseudo first‐order r...

Photoelectron spectroscopy of hydrated electron cluster anions, (H2O)−n=2–69

Abstract: Etude de la variation de l'energie de photodetachement vertical en fonction de n, en vue de distinguer les agregats a electron de surface ou a electron interne solvate

Kinetic nucleation theory: A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor

Abstract: The ‘‘kinetic theory’’ of homogeneous nucleation developed by Katz and Wiedersich is extended to derive a new expression for the rate of nucleation from an ideal supersaturated vapor. Compared to the classical expression for the nucleation rate, the new expression has a slightly different dependence on supersaturation, and a substantially different dependence on temperature. A comparison of the new expression with experimental data on nucleation rates of several organic liquids indicates that in some but not all cases the new expression gives much closer agreement with the data than does the classical expression. Discrepancies between the theory and the data are ascribed mainly to the physical assumptions of the theory presented, which are the same as in the classical theory—particularly, that the physical properties of microscopic clusters are the same as those of the bulk liquid.

Investigation of the free energy functions for electron transfer reactions

Abstract: The free energy functions for electron transfer reactions in solution are explored using a previously developed microscopic simulation approach that provides a clear definition of these functions and the variable (the reaction coordinate) used as their argument. The issue of whether the curvatures of the two functions (which correspond to states with nonpolar and polar solutes) are different is given special attention. It is found, in contrast to some previous suggestions, that the curvatures of the two functions are quite similar, even when one would expect differences due to dielectric saturation effects, and that Marcus’ approximation (and, in fact, the linear response theory inherent in this approximation) provides a valid description of the solvent’s role in electron transfer reactions over a wide range of conditions. The present study demonstrates that direct simulations of the reactant and product states do not provide the data needed for determination of the free energy functions in high energy re...

Molecular-dynamics simulation of aqueous mixtures : methanol, acetone, and ammonia

Abstract: Effective pair‐potential models, parametrized to the properties of the pure liquids, have been used in molecular‐dynamics simulations of aqueous (binary) mixtures containing methanol, ammonia, or acetone. Results are reported for thermodynamic and structural properties, self‐diffusion coefficients, and reorientational correlation times. There is fair agreement with a wide variety of experimental data. The pattern of hydrogen bonding and the distribution of hydrogen‐bond lifetimes in the simulated mixtures have been investigated. The observed anomalous behavior of methanol and acetone solutions appears to be related to specific features of the hydrogen bonding—namely, the ability of these molecules to exhibit enhanced acceptor character. As a consequence of the assumed intermolecular potentials, the balance between the competing effects of hydrophobic hydration of methyl groups and hydrogen bonding to oxygen atoms is tipped towards the latter. A number of interesting structural effects have been noted. In ...

Theory and simulations of homonuclear spin pair systems in rotating solids

Abstract: The theory of nuclear magnetic resonance (NMR) on a solid sample containing pairs of coupled homonuclear spins‐1/2, rotating in a large magnetic field, is presented. The time dependence introduced by the sample rotation, in conjunction with the spin–spin coupling, makes it appear that each of the central two levels in the four‐level system split into a pair of ‘‘virtual states.’’ Each of the eight possible single‐quantum coherences between the virtual states and the two outer levels in general contribute to the spectrum, although four of these contributions are forbidden unless a rotational resonance occurs (matching of an integer multiple of the spinning speed with the difference in isotropic shifts). Analytical line shapes for the case of vanishing shift anisotropy are given and techniques for numerical simulation in the general case demonstrated. The theory of Zeeman magnetization exchange in the presence of zero‐quantum dephasing is presented.

Fluctuation effects in a symmetric diblock copolymer near the order–disorder transition

Abstract: The thermodynamic and dynamic properties of a partially deuterated poly(ethylene‐propylene)–poly(ethylethylene)(PEP–PEE) diblock copolymer containing 55% by volume PEP were characterized above and below the order–disorder transition (ODT) by small‐angle neutron scattering (SANS) and rheological measurements, respectively. Both experimental techniques produced unambiguous evidence of composition fluctuations well above TODT(T−TODT≲50 °C) in the disordered state, which increase in magnitude as the weak first‐order transition is approached. Based on the SANS results, which are nearly predicted by a recent fluctuation theory, we conclude that the (equilibrium) instantaneous morphology in the disordered state closely resembles a spinodally decomposed binary mixture. Below TODT, long‐range order can be obtained by the application of a shear field as evidenced by the resulting highly anisotropic (one‐dimensional) SANS pattern. As the ODT is approached in the shear‐oriented ordered state, an isotropic scattering ...

Studies of the temperature-dependent phase behavior of long chain n-alkyl thiol monolayers on gold

Abstract: The influences of temperature on the infrared spectra of a docosylthiol (C22) monolayer on a gold substrate are reported. The data suggest that this adsorbate exhibits a complex phase state at 300 K in which gauche conformations concentrated at the chain termina figure prominently. The detailed temperature dependence of the spectral band intensities is found to be very similar in many regards to that of bulk hydrocarbon crystals. At low temperature, band splittings are observed which suggest the lowest energy structure for this adsorbate is one containing two chains per unit cell. A simple lattice argument, consistent with steric models and low‐energy electron diffraction data, is presented to explain these observations.

Comparison of coupled-cluster methods which include the effects of connected triple excitations

Abstract: The 'coupled cluster single, double, and triple' (CCSDT) excitation model has been used to ascertain electron correlation energies for 14 different molecules representing a variety of chemical bonds, in conjunction with several methods of this type 'CCSDT-x', which include only an approximate treatment of connected triple excitations; these methods encompass CCSDT-1a, -1b, -2, -3, and -4, as well as the novel CCSD(T). While all methods treat the effects of connected triple excitations iteratively, CCSD(T) approaches then perturbationally. For the 14 molecules considered, the CCSD(T) method's average error relative to CCSDT is substantially lower than any of the CCSDT-x methods.

Molecular‐thermodynamic approach to predict micellization, phase behavior and phase separation of micellar solutions. I. Application to nonionic surfactants

Abstract: We present a detailed description of a molecular‐thermodynamic approach which consists of blending a molecular model of micellization with a thermodynamic theory of phase behavior and phase separation of isotropic (surfactant–water) micellar solutions. The molecular model incorporates the effects of solvent properties and surfactant molecular architecture on physical factors which control micelle formation and growth. These factors include (i) hydrophobic interactions between surfactant hydrocarbon chains and water, (ii) conformational effects associated with hydrocarbon‐chain packing in the micellar core, (iii) curvature‐dependent interfacial effects at the micellar core–water interface, and (iv) steric and electrostatic interactions between surfactant hydrophilic moieties. The free energy of micellization gmic is computed for various micellar shapes Sh and micellar‐core minor radii lc. The ‘‘optimum’’ equilibrium values, l*c, Sh*, and g*mic, are obtained by minimizing gmic with respect to lc and Sh. The deduced ‘‘optimum’’ micellar shape Sh* determines whether the micelles exhibit two‐dimensional, one‐dimensional, or no growth. These results are then utilized in the thermodynamic theory to predict a broad spectrum of micellar solution equilibrium properties as a function of surfactant concentration and temperature. These properties include (1) the critical micellar concentration, (2) the micellar size distribution, (3) the critical surfactant concentration for the onset of phase separation, and (4) other thermodynamic properties such as the osmotic compressibility. The proposed molecular‐thermodynamic approach provides an excellent description of a wide range of experimental findings in aqueous solutions of nonionic surfactants belonging to the polyoxyethylene glycol monoether and glucoside families.

Hydrodynamic transport coefficients of random dispersions of hard spheres

Abstract: Accurate values for the hydrodynamic transport properties of random dispersions of hard spheres have been determined by numerical simulation. The many‐body hydrodynamic interactions are calculated from a multipole‐moment expansion of the force density on the surface of the solid particles; the singular lubrication forces are included exactly for pairs of particles near contact. It has been possible to calculate the transport properties of small periodic systems, at all packing fractions, with uncertainties of less than 1%; but for larger systems we are limited computationally to lower order, and therefore less accurate, moment approximations to the induced force density. Nevertheless, since the higher‐order moment contributions are short range they are essentially independent of system size and we can use small system data to correct our results for larger systems. Numerical calculations show that this is a reliable and accurate procedure. The ensemble‐averaged mobility tensors are strongly dependent on s...

Molecular theory of curvature elasticity in surfactant films

Abstract: We develop a microscopic‐level formulation for the curvature elasticity of monolayer and bilayer systems of typical surfactant molecules. It is argued that both the bending and saddle‐splay force constants k and k are determined primarily by the conformational entropy of the flexible hydrocarbon chain rather than by the electrostatic interactions associated with hydrophilic head groups. A priori estimates of the chain contributions are made for the first time, without the use of any adjustable parameters. Both k and k are shown to be calculable wholly from the conformational statistics describing the planar film. In particular, these constants are expressed in terms of the derivatives and moments of the lateral pressure profile characterizing chain packing in the unbent layers. By considering the dependence of the curvature elasticity on chain length, area per molecule, and composition in mixed films, we are able to account for the order‐of‐magnitude variations in k observed in a variety of different surfactant systems. The replacement of long chain molecules by short ones is shown to be especially efficient in lowering the bending energy from 10’s of kBT to kBT. The effect of ‘‘free’’ vs ‘‘blocked’’ exchange are also presented and contrasted with the case of fixed area‐per‐molecule bending deformation. Finally, monolayer vs bilayer results are compared and the calculated signs and magnitudes of k and k are discussed in the context of planar bilayer stability.

The coupled‐cluster single, double, and triple excitation model for open‐shell single reference functions

Abstract: The CCSDT model for general single determinant reference functions for open and closed‐shell electronic states has been implemented for the first time and has been used to compute the electron affinity of the F atom, the CH2, 3B1–1A1 energy difference, and the ionization potentials of 1A1 CH2. The results compare very well with FCI and are markedly superior to those of simpler coupled‐cluster methods.

Studies of alkaline earth and transition metal M++ gas phase ion chemistry

Abstract: A breakthrough into the hitherto inaccessible alkaline earth and transition metal M++ gas phase ion chemistry is reported. Ions M++(L)n, where M++(Mg++, Ca++, Sr++, Ba++, Mn++, Fe++, Co++, Ni++, and Zn++) and L=H2O could be produced. The hydrate equilibria M++(H2O)n−1+H2O=M++(H2O)n (n−1, n), were determined for Mg++, Ca++, Sr++, Mn++, and Co++. These lead to successive ion–H2O binding energies for high n, i.e., n=8–13 which are in the 15 kcal/mol range. The above hydrates and many other ion–ligand complexes could be produced by transferring the ions from liquid solution into the gas phase by means of electrospray. The ions were detected with a triple quadrupole mass spectrometer. The much stronger inner shell ion–ligand interactions can be studied by collision‐induced dissociation in the triple quadrupole. Single ligand loss gives way to charge reduction at low n. Thus the M++(H2O)n give MOH+(H2O)k+H3O+ at a low n. The n for which reduction occurs decreases as the second ionization energy of M decreases. Ligands such as DMSO and DMF lead to charge reduction at a lower n than that observed for H2O. For these ligands, the charge reduction occurs via simple charge transfer, i.e., Cu++(DMSO)3=Cu+(DMSO)2+DMSO+.

Hydrogen desorption from the monohydride phase on Si(100)

Abstract: The kinetics of the thermal recombinative desorption of hydrogen from the monohydride phase on the Si(100) surface has been studied by laser‐induced thermal desorption (LITD). A rate law that is first order in the atomic hydrogen coverage with an activation energy of 45 kcal/mol gives an accurate fit to the data over a temperature range of 685–790 K and a coverage range of 0.006 to 1.0 monolayer. A new mechanism is proposed to explain these surprising results, namely, that the rate limiting step of the reaction is the promotion of a hydrogen atom from a localized bonding site to a delocalized band state. The delocalized atom then reacts with a localized atom to produce molecular hydrogen which desorbs. Evidence to support these conclusions comes from isotopic mixing experiments. Studies of recombinative desorption from other surfaces of silicon, which had been assumed to obey second‐order kinetics, are discussed in the light of these results.

The morphology of symmetric diblock copolymers as revealed by neutron reflectivity

Abstract: The specular reflectivity of neutrons has been used to characterize quantitatively the microphase separated morphology of symmetric, diblock copolymers of polystyrene (PS), and polymethylmethacrylate (PMMA), as a function of the total molecular weight of the copolymer where either block is perdeuterated It is shown that the hyperbolic tangent function, as opposed to a linear or cosine‐squared function, most closely describes the concentration gradient at the interface between the lamellar copolymer microdomains The effective width of the interface is found to be independent of the molecular weight of the copolymer blocks and has a value of 50±3 A This interface is also found to be identical to that between PS and PMMA, homopolymers However, using measured values of the Flory–Huggins interaction parameter for PS and PMMA, current theoretical treatments cannot describe the observed widths of the interface

Brute force in molecular reaction dynamics: A novel technique for measuring steric effects

Abstract: We present the theoretical foundation of a novel technique for the orientation of ∑‐molecules and symmetric tops with permanent electric dipole moment. The method bases on the adiabatic transformation of free rotational states into those of librational oscillations taking place during the passage of a molecule into a strong electric field. Several examples of calculated spatial distributions of the molecular axis are given. The computational results demonstrate that in connection with highly relaxed supersonic nozzle beams only moderately strong electric fields are required to generate a marked orientation of the axis. In a first application of this technique we studied steric effects of the reaction K+CH3I→KI+CH3 at two elevated collision energies (0.79 and 1.24 eV). All observed steric effects could be rationalized in terms of a simple impulsive reaction model. We find that backward scattered products are more likely formed if the approaching atoms face the I‐end of CH3I—in agreement with low energy results—while forward scattered products are favorably formed if the CH3‐end is encountered.

Reducing Quasi-Ergodic Behavior in Monte Carlo Simulations by J-Walking: Applications to Atomic Clusters

Abstract: A method is introduced that is easy to implement and greatly reduces the systematic error resulting from quasi‐ergodicity, or incomplete sampling of configuration space, in Monte Carlo simulations of systems containing large potential energy barriers. The method makes possible the jumping over these barriers by coupling the usual Metropolis sampling to the Boltzmann distribution generated by another random walker at a higher temperature. The basic techniques are illustrated on some simple classical systems, beginning for heuristic purposes with a simple one‐dimensional double well potential based on a quartic polynomial. The method’s suitability for typical multidimensional Monte Carlo systems is demonstrated by extending the double well potential to several dimensions, and then by applying the method to a multiparticle cluster system consisting of argon atoms bound by pairwise Lennard‐Jones potentials. Remarkable improvements are demonstrated in the convergence rate for the cluster configuration energy, ...