bluuebunny/crossref_metadata_2025 huggingface dataset:
Volume fraction dependence and reorganization in cluster–cluster aggregation processes
Authors: van Garderen H. F. , Dokter W. H. , Beelen T. P. M. , van Santen R. A. , Pantos E. , Michels M. A. J. , Hilbers P. A. J.
Published: July 2002

Off-lattice diffusion limited cluster aggregation simulations in two dimensions have been performed in a wide volume fraction range between 0.001 and 0.60. Starting from a system of 10 000 monomers with radius 0.5, that follow Brownian trajectories, larger aggregates are generated by bond formation between overlapping aggregates. No rings are present in the nonaged structures. The influence of the initial monomer volume fraction on the fractal properties of the gels is studied and interpreted by calculation of small angle scattering structure factor patterns to find the fractal dimension. It is found that an increase of the volume fraction leads to the development of two distinct fractal regions. The fractal dimension at short length scale shows the diffusion limited cluster aggregation value of 1.45 up to the correlation length, while the long range fractal dimension gradually increases from 1.45 to 2.00, the Euclidean dimension of the simulation space. It is shown that high volume fractions lead to changes in the local morphology of the monomers as a function of the volume fraction with corresponding changes in the high-q region of the structure factor pattern. A new method to simulate aging of aggregates at modest (low pH and temperature) reaction conditions by an off-lattice nonbreaking reorganization process, called shaking, is presented. The shaking process induces small changes in the local mass distribution, causing ring formation, but does not affect the radius of gyration of the aggregates.

It is found that shaking leads to the formation of a polydisperse collection of new primary scatterers with a volume fraction dependent radius between 6 and 8 initial monomer radii. For low volume fractions the fractal dimension decreases approximately by 0.2, due to the formation of the new primary scatterers. The shaking process induces changes in the fractal properties at length scales up to ∼40. A simple theory that relates the volume fraction with the correlation length and describes the influence of aging on fractal properties such as monomer radius, fractal dimension, and correlation length is developed. The diffusion limited cluster aggregation simulation results of nonaged and aged gels are compared with this theory. Satisfactory quantitative agreement between simulated and theoretical results is found.
A modification of the Gaussian-2 approach using density functional theory
Authors: Bauschlicher Charles W. , Partridge Harry
Published: July 2002

The quadratic configuration interaction calculation in the Gaussian-2 second-order Mo/ller–Plesset perturbation theory approach, G2(MP2), is replaced by a coupled-cluster (CC) singles and doubles calculation including a perturbational estimate of the triples excitations. In addition, the self-consistent-field (SCF) and MP2 geometry optimizations and SCF frequency calculation in the G2(MP2) approach are replaced by a density functional theory geometry optimization and frequency calculation [using the Becke three parameter hybrid functional with the Lee–Yang–Parr non-local correlation functional (B3LYP)] in the proposed G2(B3LYP/MP2/CC) approach. This simplification does not affect the average absolute deviation from experiment, but decreases the maximum error compared with the G2(MP2) approach. The G2(B3LYP/MP2/CC) atomization energies are compared with those obtained using the B3LYP approach, and the G2(B3LYP/MP2/CC) model is found to be more reliable, even if the B3LYP calculations are performed using a large basis set.
Thermostats for molecular fluids undergoing shear flow: Application to liquid chlorine
Authors: Travis Karl P. , Daivis Peter J. , Evans Denis J.
Published: July 2002

In this article we show that atomic thermostats which have been used in the past for nonequilibrium molecular dynamics (NEMD) simulations of molecular fluids were incorrectly formulated. The error stems from an incorrect assumption made about the form of the streaming angular velocity. This assumption is incorrect even in the linear regime. One spurious effect of this atomic thermostat is the generation of a nonsymmetric pressure tensor. We outline a general method, based on a variational principle, for calculating the position and orientation dependent streaming velocity. Using this streaming velocity we develop an atomic thermostat for molecular fluids which does not bias the positional or orientational distribution functions for the fluid. The new atomic thermostat is validated in NEMD simulations of liquid chlorine undergoing planar Couette flow.
Monte Carlo analysis of conformational transitions in superhelical DNA
Authors: Sun Hongzhi , Mezei Mihaly , Fye Richard , Benham Craig J.
Published: July 2002

Metropolis–Monte Carlo algorithms are developed to analyze the strand separation transition in circular superhelical DNA molecules. Moves that randomize the locations of unpaired regions are required in order to diminish correlations among the sampled states. This approach enables accurate simulations to be performed in reasonable computational times. Sufficient conditions to guarantee the formal correctness of the complete algorithm are proven to hold. The computation time required scales at most quadratically with molecular length, and is approximately independent of linking difference. Techniques are developed to estimate the sample size and other calculation parameters needed to achieve a specified accuracy. When the results of Monte Carlo calculations that use shuffling operations are compared with those from statistical mechanical calculations, excellent agreement is found. The Monte Carlo methodology makes possible calculations of transition behavior in cases where alternative approaches are intractable, such as in long molecules under circumstances where several runs of open base pairs occur simultaneously. It also allows the analysis of transitions in cases where the base pair separation energies vary in complex manners, such as through near-neighbor interactions, or the presence of modified bases, abasic sites, or bound molecules.
Phase transition of the hydrogen-bonded crystals and ice
Authors: Fujii Katsuhiko
Published: July 2002

The order–disorder phase transition for KH2PO4 (KDP) and ice VIII is studied in terms of the dipole–hydrogen coupling model. The effective interactions between hydrogens are obtained by averaging out the dipole’s degrees of freedom. It is found that the bonding energy of ice VIII obtained in the present theory compares with the hydrogen bond energy determined by Whalley. The isotope effect is explained in the difference of bonding energies between KDP and DKDP, without the tunneling energy. It is shown that the energy levels excited from the states with stable hydrogen bond in ice VIII are considerably higher than those in KDP, and the concentration of ionic defect in ice VIII is much smaller than that in KDP. Our result can explain the well-known fact that the ice rule condition is conserved in ice VIII, on the contrary broken in KDP.
Linearly polarized probes of surface chirality
Authors: Verbiest Thierry , Kauranen Martti , Maki Jeffery J. , Teerenstra M. N. , Schouten A. J. , Nolte R. J. M. , Persoons André
Published: July 2002

We present a new nonlinear optical technique to study surface chirality. We demonstrate experimentally that the efficiency of second-harmonic generation from isotropic chiral surfaces is different for excitation with fundamental light that is +45° and −45° linearly polarized with respect to the p-polarized direction.
Electronic dephasing of APT in glassy films of water from 5 to 100 K: Implications for H-bonding liquids
Authors: Reinot T. , Kim W.-H. , Hayes J. M. , Small G. J.
Published: July 2002

Nonphotochemical hole burning was used to characterize the linear electron–phonon coupling and measure the temperature dependence of the pure electronic dephasing of Al-phthalocyanine tetrasulphonate (APT) in unannealed and annealed hyperquenched glassy films of water (HGW). Below about 10 K, the dephasing is dominated by coupling to the intrinsic two-level systems (TLSint) of HGW. This dephasing is a factor of 5× faster for unannealed HGW due to its higher TLSint number density. For annealed HGW, the pure electronic dephasing time (i.e., that dephasing associated with the zero-phonon line), T2*, is 6.4 ns at 5 K, the slowest dephasing time yet reported for a molecular glassy system at this temperature. At higher temperatures, dephasing due to exchange coupling with pseudolocalized modes at 50 and 180 cm−1, which correlate well with the transverse and longitudinal acoustic modes of water, becomes dominant. The exchange coupling mechanism is based on diagonal quadratic electron–phonon coupling. At 100 K, for example, the pure electronic dephasing times (T2*) are close to 1 ps in value for both types of film. Whereas the transverse acoustic mode is Franck–Condon active (S∼0.5), the 180 cm−1 mode is silent (S<0.02). The determination of the electron–phonon coupling parameters and static inhomogeneous broadening (400 cm−1) of APT’s origin band allowed for simulation of the burn temperature dependence of the overall hole profile (zero-phonon hole plus phonon-sideband holes). Comparison with experimental profiles shows that the hole profile theory of Hayes et al. [J. Phys. Chem. 98, 7337 (1994)] captures the main features of the temperature dependence.

The S value of the transverse acoustic mode is used to quantitatively explain the burn temperature dependence of the saturated intensity of the zero-phonon hole and its demise at temperatures just above 100 K (due to Franck–Condon forbiddeness). In view of the essentially complete understanding of the electron–phonon coupling and pure electronic dephasing of APT in HGW attained in this work, the data are used for extrapolation to ice (0° C) and water (at temperatures not far above 0° C) in order to connect with recent photon echo studies of optical coherence loss of dye molecules in liquids. The extrapolation predicts an ‘‘average’’ T1-dephasing time of ∼0.1 ps due to multiphonon (Brownian oscillator) transitions associated with the transverse acoustic mode and subpicosecond pure electronic dephasing due to exchange coupling with the longitudinal mode. It is suggested that the marriage of hole burning and photon echo techniques in studies of glass forming liquids should be a powerful approach to understanding optical coherence loss in liquids.
General spin adaptation of open-shell coupled cluster theory
Authors: Nooijen Marcel , Bartlett Rodney J.
Published: July 2002

A general scheme for the spin adaptation of open-shell coupled-cluster theory is presented, and the generalization to genuine multireference cases is briefly discussed. In our formulation the wave operator is parameterized such that it automatically commutes with the spin operators, and the correlated wave function is hence a spin eigenfunction provided the reference state is. We employ an exponential form for the wave operator, which is assumed to be normal ordered with respect to a closed-shell vacuum state. The excitation operators can be expressed in terms of generators of the unitary group, and the number of independent coefficients is only marginally larger than in the closed-shell case: Open-shell orbitals occur both as creation and as annihilation operators. Using our formalism we are able to obtain spin–orbital based equations, which are expressed in terms of second quantized matrix elements of the similarity transformed Hamiltonian. The explicit form of the similarity transformed Hamiltonian generated by normal ordered exponential operators is presented in an accompanying paper [M. Nooijen, J. Chem. Phys. 104, 2638 (1996) preceding paper], and is crucial to the present formulation.
Comparison of linearly and circularly polarized probes of second-order optical activity of chiral surfaces
Authors: Maki Jeffery J. , Verbiest Thierry , Kauranen Martti , Elshocht Sven Van , Persoons André
Published: July 2002

We compare two nonlinear-optical techniques for measuring chirality of isotropic surfaces. One technique measures the difference in the efficiency of surface second-harmonic generation between the cases of left-hand and right-hand circularly polarized fundamental light and the other measures the difference in the efficiency between the cases of fundamental light linearly polarized −45° and +45° with respect to the p-polarized direction. A nonzero difference in either case indicates chirality, where no difference should be measured for an isotropic achiral surface. We examine the complementariness of these two techniques theoretically and experimentally. In experiments utilizing thin films of a chiral poly(isocyanide), we measure the second-harmonic response as a function of the state of polarization of the fundamental light when manipulated by both quarter and half waveplates. We extract the circular and linear differences from these more complete measurements and find the largest circular and linear differences to be 23% and 118%, respectively.
Relativistic and correlation effects on molecular properties. II. The hydrogen halides HF, HCl, HBr, HI, and HAt
Authors: Visscher L. , Styszyñski J. , Nieuwpoort W. C.
Published: July 2002

A benchmark study of a number of four-component relativistic correlation methods is presented. Bond lengths, harmonic frequencies, and dissociation energies of the molecules HF, HCl, HBr, HI, and HAt are calculated at various levels of theory, using both the Schrödinger and the Dirac–Coulomb–(Gaunt) Hamiltonian. The inclusion of relativity leads to a weakening of the bond, giving a decrease in the calculated harmonic frequencies and dissociation energies of the hydrogen halides. The effect on the bond length is small. These trends are explained by considering the relativistic change in hybridization induced by the spin–orbit coupling.
Calculation of nuclear magnetic shieldings. X. Relativistic effects
Authors: Fukui H. , Baba T. , Inomata H.
Published: July 2002

The lowest order relativistic effect theory for nuclear magnetic shieldings was derived from a two-component positive energy Hamiltonian. It was shown that the previous relativistic shielding theory based on the two-component Hamiltonian is not gauge invariant and the new terms have to be added to make a result gauge invariant. The presented theory is gauge invariant to the order of (Z/137)4 where Z is the atomic number of the heaviest atom in the molecule. One of the new contributions to the relativistic magnetic shieldings is a second-order perturbation term due to the combination of the spin-orbit interaction and the Fermi-contact interaction. A numerical estimation for this term was performed for the four hydrogen halides, HF, HCl, HBr, and HI. The computational results showed that the contribution of this term to the hydrogen shieldings is negligibly small, but the contribution to the halogen atoms is considerable.
A density-functional study of the intermolecular interactions of benzene
Authors: Meijer Evert Jan , Sprik Michiel
Published: July 2002

We have tested the performance of three frequently used density functionals (LDA, LDA+B, and LDA+B+LYP) in a study of the intermolecular interactions of benzene. Molecular geometries are satisfactory, with the gradient-corrected density functionals yielding slightly better results. The quadrupole moment is significantly underestimated by all three functionals. LDA performs fortuitously comparatively well for both binding energies and geometries of the dimer and the solid, whereas in LDA+B, and LDA+B+LYP the dimer interaction is purely repulsive, leading to the complete absence of cohesion in the solid. These results are consistent with density-functional theory calculations for noble gas dimers. However, when the dispersion energy calculated from a model potential is included, LDA fails. Binding energies are overestimated by unacceptable amounts, and intermolecular distances are too small. In contrast, dispersion corrected LDA+B and LDA+B+LYP perform reasonably well, although discrepancies are still large when measured on the thermal energy scale at room temperature.
In-plane structure of the liquid–vapor interfaces of dilute bismuth:gallium alloys: X-ray-scattering studies
Authors: Lei Ning , Huang Zhengqing , Rice Stuart A. , Grayce Christopher J.
Published: July 2002

The in-plane structure functions in the liquid–vapor interfaces of two bismuth–gallium alloys (0.18 at. % Bi and 0.07 at. % Bi) have been studied by grazing incidence x-ray diffraction. The higher concentration alloy exhibits, at 25, 43, and 77 °C, segregation of Bi into nearly complete monolayers atop the bulk alloy; at each temperature the Bi monolayer has the structure of a two-dimensional supercooled liquid. The intensity distribution of the diffuse x-ray scattering at qz=1.0 and 1.5 Å−1 is used to calculate the surface tension of the alloy at 77 °C; the value obtained is 570±30 dyn/cm. This value, which is very different from the value obtained by extrapolating the surface tension of liquid Bi to 77 °C, is related to the structure of the liquid–vapor interface. We have compared experimental grazing incidence x-ray diffraction studies on the liquid–vapor interface of the 0.07% alloy with integral-equation theoretical calculations on a two-dimensional hard-disk fluid mixture. The results suggest that the segregated partial monolayer may consist of dimers and trimers in equilibrium with a small amount of two-dimensional liquid bismuth.
Structure and solvation forces in confined films: Linear and branched alkanes
Authors: Gao Jianping , Luedtke W. D. , Landman Uzi
Published: July 2002

Equilibrium structures, solvation forces, and conformational dynamics of thin confined films of n-hexadecane and squalane are investigated using a new grand canonical ensemble molecular dynamics method for simulations of confined liquids. The method combines constant pressure simulations with a computational cell containing solid surfaces and both bulk and confined liquid regions in equilibrium with each other. For both molecular liquids layered density oscillations in the confined films are found for various widths of the confining gap. The solvation force oscillations as a function of the gap width for the straight chain n-hexadecane liquid are more pronounced exhibiting attractive and repulsive regions, while for the branched alkane the solvation forces are mostly repulsive, with the development of shallow local attractive regions for small values of the gap width. Furthermore, the nature of the transitions between well-formed layered configurations is different in the two systems, with the n-hexadecane film exhibiting solid-like characteristics portrayed by step-like variations in the number of confined segments occurring in response to a small decrease in the gap width, starting from well-layered states of the film. On the other hand the behavior of the squalane film is liquid-like, exhibiting a monotonic continuous decrease in the number of confined segments as the gap width is decreased. These characteristics are correlated with structural properties of the confined films which, for n-hexadecane, exhibit enhanced layered ordering and in-plane ordered molecular arrangements, as well as with the relatively high tendency for interlayer molecular interdigitation in the squalane films. Reduced conformational (trans-guache) transition rates in the confined films, compared to their bulk values, are found, and their oscillatory dependence on the degree of confinement is analyzed, showing smaller transition rates for the well-formed layered states of the films.
Statistical modeling of collision-induced dissociation thresholds
Authors: Rodgers M. T. , Ervin Kent M. , Armentrout P. B.
Published: July 2002

Analysis of the energy dependence of the cross sections for collision-induced dissociation reactions has permitted the determination of quantitative thermodynamic information for a variety of ionic clusters. As such clusters become larger, the rate at which the decomposition occurs becomes comparable to the instrumental time available for observing the reaction. A method for incorporating statistical theories for energy-dependent unimolecular decomposition in this threshold analysis is reviewed and updated. The revision relies on the fact that for most ionic clusters, the transition state is a loose association of the products that can be located at the centrifugal barrier. This permits a straightforward estimation of the molecular parameters needed in statistical theories for the transition state. Further, we also discuss several treatments of the adiabatic rotations of the dissociating cluster. The various models developed here and previously are compared and used to analyze a series of data for Li+(ROH) complexes, where ROH=methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, s-butanol, and t-butanol. The trends in the bond energies derived by these various models are compared and their accuracy evaluated by comparison with relative values determined by equilibrium methods.
First principles implementation of solvent effects without outlying charge error
Authors: Baldridge Kim , Klamt Andreas
Published: July 2002

In this work, we present a sound modified implementation of the noniterative approach for the inclusion of solvent effects for molecular-shaped cavities in conventional ab initio gas phase computations. This model will serve as the starting point from which to add additional influences such as nonelectrostatic and correlation effects, as well as optimization of cavity radii. The improvement over recent dielectric continuum models is (a) the utilization of distributed multipoles up to hexadecapoles for the generation of the potential on the cavity boundary to the dielectric and the calculation of surface charges, and (b) an accurate assessment of outlying charge effects. The new method is tested in calculations of solvation energies and geometries for a number of neutral and charged molecules.
The hindered rotor density-of-states interpolation function
Authors: McClurg Richard B. , Flagan Richard C. , Goddard III William A.
Published: July 2002

We construct an approximation to the partition function for hindered rotors based entirely on their asymptotic behavior and no fitting parameters. The approximant is shown to be quite accurate in all temperature ranges. Explicit auxiliary functions are derived for the Helmholtz free energy, internal energy, heat capacity, and entropy. We apply this function to estimating the heat capacity and unimolecular dissociation rate for ethane.
Calculation of ionization energies, electron affinities, electronegativities, and hardnesses using density functional methods
Authors: De Proft Frank , Geerlings Paul
Published: July 2002

The performance of two exact exchange methods is tested in the calculation of ionization energies, electron affinities, electronegativities, and hardnesses using Dunning’s correlation consistent basis sets. Comparison is made to experiment and other density functional methods, including the local density approximation and two gradient corrected functionals. The obtained electronegativities and hardnesses are also compared with high level coupled cluster results. Both the exact exchange methods show an excellent performance in the calculation of all four properties, yielding mean absolute deviations from experiment below 0.20 eV for all basis sets.
Resonant x-ray scattering beyond the Born–Oppenheimer approximation: Symmetry breaking in the oxygen resonant x-ray emission spectrum of carbon dioxide
Authors: Cesar Amary , Gel’mukhanov Faris , Luo Yi , Ågren Hans , Skytt Per , Glans Peter , Guo Jinghua , Gunnelin Kerstin , Nordgren Joseph
Published: July 2002

Although resonant x-ray scattering of molecules fulfills strict electronic symmetry selection rules, as now firmly proven by spectra of diatomic molecules, the accumulated body of data for polyatomic molecules indicates that an apparent breaking of these rules represents the common situation rather than the exception. The CO2 molecule provides a good example of symmetry breaking, with the oxygen x-ray emission spectra showing strong intensity for transitions that are forbidden by the parity selection rule. We present time-independent and time-dependent theories for frequency-dependent resonant x-ray scattering beyond the Born–Oppenheimer approximation in order to explore under what circumstances one can anticipate symmetry breaking in the spectra of polyatomic molecules. The theory starts out from the Kramers–Heisenberg dispersion relation and is generalized for vibrational degrees of freedom and for nonadiabatic coupling of the electronic (vibronic) states, including the frequency dependency of the scattering cross section. Different limiting cases and few-level models are considered. The symmetry breaking is proven to be the result of pseudo-Jahn–Teller-like vibronic coupling between near-degenerate core-excited states. Thus vibronic interaction over the antisymmetric vibrational mode between the “bright” 1σg−12πu1 and “dark” 1σu−12πu1 intermediate states of CO2 allows transitions otherwise forbidden. The measurements and theory demonstrate that the symmetry-selective character of the resonant x-ray emission is strongly frequency dependent. The strong intensity of “dipole-forbidden” transitions in the π* oxygen K spectrum at resonance is reduced monotonically with the detuning of the excitation energy from resonance, and the spectra become “symmetry purified.” Simulations with full vibronic coupling predict this feature of the x-ray scattering experiment and a few-level model explains the energy dependence of the symmetry selection and the symmetry purification at large detuning energies in the limit of narrowband photon excitation.
Investigation of the temperature dependence of dielectric relaxation in liquid water by THz reflection spectroscopy and molecular dynamics simulation
Authors: Ro/nne Cecilie , Thrane Lars , Åstrand Per-Olof , Wallqvist Anders , Mikkelsen Kurt V. , Keiding So/ren R.
Published: July 2002

We report measurements of the real and imaginary part of the dielectric constant of liquid water in the far-infrared region from 0.1 to 2.0 THz in a temperature range from 271.1 to 366.7 K. The data have been obtained with the use of THz time domain reflection spectroscopy, utilizing ultrashort electromagnetic pulses generated from a photoconductive antenna driven by femtosecond laser pulses. A Debye model with an additional relaxation time is used to fit the frequency dependence of the complex dielectric constants. We obtain a fast (fs) and a Debye (ps) relaxation time for the macroscopic polarization. The corresponding time correlation functions have been calculated with molecular dynamics simulations and are compared with experimental relaxation times. The temperature dependence of the Debye relaxation time is analyzed using three models: Transition state theory, a Debye–Stoke–Einstein relation between the viscosity and the Debye time, and a model stating that its temperature dependence can be extrapolated from a singularity of liquid water at 228 K. We find an excellent agreement between experiment and the two latter models. The simulations, however, present results with too large statistical error for establishing a relation for the temperature dependence.
Polymerized C60 clusters
Authors: Tast F. , Malinowski N. , Billas I. M. L. , Heinebrodt M. , Branz W. , Martin T. P.
Published: July 2002

Clusters produced by co-evaporization of C60 and graphite in a low pressure inert gas condensation cell are characterized by photoionization time-of-flight mass spectrometry. The recorded mass spectra suggest that fullerene molecules can be linked by carbon atoms to form polymers C60(CC60)n−1. The dimer C121 is especially abundant, however we find evidence for polymers containing up to n=7 C60 molecules. Photo-fragmentation measurements of preselected clusters confirm the proposed multicage structure. At moderate fragmentation laser intensities the polymers are found to dissociate into the constituent monomers. At high laser intensities a second minor C2-loss dissociation channel can be observed, indicating the presence of large single cage fullerenes formed by coalescence of the C60 molecules.
The picosecond timescale relaxation of photoexcited quaterphenyl in solution
Authors: Matousek P. , Parker A. W. , Towrie M. , Toner W. T.
Published: July 2002

Time-resolved resonance Raman and transient absorption spectra of photoexcited S1 quaterphenyl in solution have been measured in a single series of experiments over a range of probe wavelengths at various time delays and solvent temperatures. Increases of 0.4% in the energy of the 0–0 electronic resonance transition to the higher (Sn) state and 0.1% in some vibrational frequencies are observed to take place on a 17 picosecond timescale following photoexcitation, and electronic and vibrational bandwidths both reduce by a few percent. Comparisons of measured Raman excitation profiles with profiles calculated from the transient absorbance spectra are used to interpret the time dependence of Stokes resonance Raman band intensities. The electronic resonance shift and width change and relaxation of Franck–Condon displacements all contribute. All parameters vary with bath temperature, but bandshifts are small on cooling and the 0–0 resonance shifts to the red. The change in S0–Sn resonance frequency is taken to imply a change in S0–S1 potential and to be a solvation effect which is also responsible for the displacement and Raman frequency shifts. The anti-Stokes Raman band at 766 cm−1 shows additional intensity changes due to population relaxation on two distinct timescales: <1 ps and ∼17 ps. The fast component is attributed to intramolecular redistribution of v>1 excitation energy and the slow component to the decay of a hot v=1 population with an average excess energy of ∼60 cm−1 per molecule. This is much smaller than the initial excess photoexcitation energy of ∼5000 cm−1 but corresponds to a temperature much greater than indicated by the bandwidth changes, implying a non-equilibrium distribution of vibrational energy whose decay is not limited by thermal diffusion. The slow component of the population relaxation matches approximately with the change in potential in both energy and timescale but no causal connection is identified. This experiment links the dynamics of Raman frequency shifts observed in an excited state molecule directly to a change in electronic potential. It is suggested a similar mechanism may operate in other systems, such as stilbene.
Electronic absorption spectra of carbon chain anions C2n− (n=4–7) in neon matrices
Authors: Freivogel Patrick , Grutter Michel , Forney Daniel , Maier John P.
Published: July 2002

Electronic transitions, (2)2Π←X 2Π (n=4–7), 2Σ←X 2Π (n=4,5) in addition to the known one C 2Π←X 2Π (n=4–7) of linear carbon anions C2n− have been observed in neon matrices. Absorption spectra were measured after mass-selected deposition of the anions with excess of neon at 5 K. The assignments are based on mass-selection, wavelength dependence of the 000 transition on size, photobleaching behavior, and ab initiocalculations. Vibrational frequencies in the excited electronic states have been inferred. The band system attributed previously to the 3Σu−←X 3Σg− transition of C10, C12, and C14 is shown to be the (2)2Π←X 2Π transition of their anions. Another set of weak bands is assigned to the 3Σu−←X 3Σg− system of C10 but none were discernible for C12 or C14. This is experimental evidence for the predicted change from linear to nonlinear geometries for the even numbered carbon species larger than C10.
The Gibbs–Thomson effect and intergranular melting in ice emulsions: Interpreting the anomalous heat capacity and volume of supercooled water
Authors: Johari G. P.
Published: July 2002

Calculations for the Gibbs–Thomson effect and the intergranular melting of the ice droplets in (water) emulsions at temperatures below 273.16 K show that water and ice coexist at thermodynamic equilibrium in an apparently frozen emulsion. The fraction of water at this equilibrium increases on heating, which alters further the thermodynamic properties of the emulsion. As some of the ice in the emulsion has already melted, the increase in the enthalpy, H, and heat capacity, Cp, and the decrease in the volume measured on the normal melting at 273.16 K, are less than the values anticipated. The ratio of this increase in H, or Cp, on melting of the emulsion to the corresponding value for pure ice, underestimates the emulsion’s water content which, when used for scaling the difference between the Cp of the unfrozen and frozen emulsion at lower temperatures, as in earlier studies, leads to a larger Cp of supercooled water than the actual value. Similar scaling of the corresponding difference between the volume leads to higher volume, or lower density, than the actual value. A formalism for this premelting effect is given for both the adiabatic and differential scanning calorimetry (DSC), and its magnitude is calculated. New experiments show that the rise in the DSC signal, or equivalently in the apparent Cp observed on heating the frozen emulsion, occurs over a temperature range much wider than the Gibbs–Thomson effect and intergranular melting predict, for which reasons are given. It is shown that Cp of the dispersant phase is also affected by the melting of ice droplets. There are four consequences of the premelting effects for all finely dispersed materials, for frozen water emulsions below 273.16 K: (i) water and ice coexist in the emulsion, (ii) its apparent Cp will increase with increase in the heat input used to measure it, (iii) the apparent Cp will increase with decrease in the average size of the droplets, and (iv) the apparent Cp will decrease on annealing the frozen emulsion for a period long enough to allow the ice-grain growth in the frozen droplets. Calorimetry of emulsions has verified consequences (i) and (iv). The corresponding effects on the molar volume are briefly discussed. A substantial fraction of the anomalously high Cp and volume of supercooled water is due to the observed premelting effects.
Three-flavor instantaneous normal mode formalism: Diffusion, harmonicity, and the potential energy landscape of liquid CS2
Authors: Li Wu-Xiong , Keyes T. , Sciortino Francesco
Published: July 2002

Instantaneous normal modes are classified by their one-dimensional potential energy profiles, U(q), into single well (SW), double well (DW), and shoulder potential (SH) modes. It is proposed that the resulting three-flavor description replace the current two-flavor division into real or imaginary frequency modes, and that the DW modes replace Im ω in theories of diffusion. Calculations of the three-flavor densities of states are presented for normal and supercooled liquid CS2, and the self-diffusion constant, D(T), is related to the DW modes. Indicators of strength/fragility are given based on the relative numbers of different mode types, and indicators of harmonicity are constructed by comparison of simulated instantaneous normal modes properties with the predictions of the harmonic approximation. It is found that the SW modes are harmonic in an intermediate Re ω range 20 ps−1>ω>2.5 ps−1, and the anharmonicity at high and low ω is explained in terms of the potential energy “landscape.” DW modes are remarkably harmonic over the full range of Im ω. The T dependence of the diffusion constant is also interpreted in terms of the landscape, as manifested primarily in the properties of the energy barriers to diffusion. Diffusion is clearly associated with the strongly T-dependent crossing of barriers with ω∼3 ps−1.
In situ studies of thiol self-assembly on gold from solution using atomic force microscopy
Authors: Xu Song , Cruchon-Dupeyrat Sylvain J. N. , Garno Jayne C. , Liu Gang-Yu , Kane Jennings G. , Yong Tseh-Hwan , Laibinis Paul E.
Published: July 2002

The kinetics and mechanism for the solution-phase adsorption of n-alkanethiols onto gold to form self-assembled monolayers (SAMs) have been monitored in situ using atomic force microscopy (AFM). Time-dependent AFM images reveal detailed structural information about the adsorbed layer during its growth. In 2-butanol, CH3(CH2)17SH molecules initially adsorb on gold with the molecular axis of their hydrocarbon chains oriented parallel to the surface. As the surface coverage increases to near saturation, a two-dimensional phase transition occurs and produces islands composed of molecules with their hydrocarbon axis oriented ∼30° from the surface normal. Continued exposure to the thiol solution results in a greater number of these islands and the growth of these nuclei until a SAM is formed with a commensurate (∛×∛)R30° structure. The growth of the lying-down phase follows a first-order Langmuir adsorption isotherm, while the phase transition is best described by a second-order reaction. The kinetics of the self-assembly process also depends on the chain length of the alkanethiol and the cleanness of the gold surface. Longer-chained thiols, such as CH3(CH2)17O(CH2)19SH, formed complete SAMs more rapidly than did shorter-chained thiols, such as CH3(CH2)17SH. The physisorbed, lying-down phase for CH3(CH2)17O(CH2)19SH was less homogeneous and its two-dimensional phase transition was more complicated than for CH3(CH2)17SH and CH3(CH2)21SH, as the CH3(CH2)17O(CH2)19SH molecules adopt multiple conformations. Of these, the two dominant ones are an all-trans, and another where the hydrocarbon chain adopts an all-trans conformation except for a gauche bond on both sides of the ether unit. These conformers coexist on the surface during the initial adsorption and its transition to the standing-up phase, but change to the all-trans structure in the complete SAM.
Classical trajectory and statistical adiabatic channel study of the dynamics of capture and unimolecular bond fission. IV. Valence interactions between atoms and linear rotors
Authors: Maergoiz A. I. , Nikitin E. E. , Troe J. , Ushakov V. G.
Published: July 2002

The addition of atoms to linear molecules forming linear or nonlinear adducts is treated using standardized valence potentials. The dynamics is analyzed with a combination of classical trajectory (CT) and statistical adiabatic channel (SACM) calculations. For classical adiabatic conditions, the two approaches coincide. The transition from adiabatic to nonadiabatic dynamics is investigated using CT calculations. The low-temperature adiabatic quantum range is studied by SACM. Thermal capture rate constants are represented in analytical form. Thermal rigidity factors are expressed in terms of molecular parameters such as the frequencies of transitional bending modes, the bond dissociation energy, the rotational constant of the linear fragment, and the ratio of the looseness and Morse parameters α/β of the potential-energy surface. The final rate expressions are of simple form suitable for direct practical applications.
Bond length alternation and aromaticity in large annulenes
Authors: Choi Cheol Ho , Kertesz Miklos
Published: July 2002

Properties of [4n] and [4n+2]annulenes were studied as a function of n for up to [66]annulene using Hartree–Fock and density functional theory in the generalized gradient approximation (DFT-GGA). In the 4n+2 series a “transition” from delocalized to localized structures occurs at 4n+2=30. Various indices of aromaticity, including NMR chemical shifts, bond localization, and aromatic stabilization energy (ASE) were monitored. π-bond localization occurs not due to a dramatic decrease of ASE as n increases, but rather as a result of a pseudo-Jahn–Teller (PJT) effect that sets in as the HOMO-LUMO gap decreases with increasing size. The NMR measures of aromaticity (difference between inner and outer 1H chemical shielding constants and the nucleus-independent chemical shifts, NICS) are reduced in the localized structures in comparison to the delocalized ones. The gradual nature of this “transition” is also implied by the relatively large values of the NMR measures of aromaticity that approach zero only gradually for larger size annulenes. Therefore intermediate size annulenes, such as [30]annulene are predicted to have a localized structure and aromatic properties at the same time showing the delocalized structure is not a necessary condition to be aromatic.
Macroscopic theory for equilibrium properties of ionic-dipolar mixtures and application to an ionic model fluid
Authors: Weiss V. C. , Schröer W.
Published: July 2002

An electrostatic theory for the equilibrium properties of a mixture of equisized ions and dipoles is developed by solving the Debye–Hückel differential equation for the cases of a charge and a dipole, respectively, in a dielectric continuum that contains free charges. This theory yields expressions for the potentials of mean force of ion-ion, ion-dipole, and dipole-dipole interactions. Furthermore, it leads to a formula for the dielectric constant that modifies Onsager’s result by accounting not only for the dipoles, but also for the free ions, and resembles the expression obtained by Adelman and Chen. The theory is applied to calculate thermodynamic properties of the model system of equisized charged hard spheres in a dielectric continuum (restricted primitive model), approximating dipolar ion pairs as spheres. We thus augment the work of Fisher and Levin, who added a shielded dipole-ion interaction term to the traditional Debye–Hückel theory, for a shielded dipole-dipole interaction and a density-dependent dielectric constant. The approximate evaluation of this theory yields a narrowing of the coexistence curve compared to the Fisher–Levin theory along with a considerable decrease of the critical density; however, it does not lead to the dramatic increase of the critical temperature that was observed by Guillot and Guissani who modified the Fisher–Levin theory by including unscreened dipole-dipole interactions adopting the original Onsager theory.
Simulation of the athermal coarsening of composites structured by a biaxial field
Authors: Martin James E. , Anderson Robert A. , Tigges Chris P.
Published: July 2002

We report the results of a computer simulation of the evolution of structure in a two component fluid consisting of a liquid phase and a dispersed colloidal phase subjected to a biaxial field. A biaxial field, such as a rotating field, can induce the organization of polarizable particles into two-dimensional sheets, in contrast with the essentially one-dimensional columns formed in a uniaxial field. Our primary objective is to explore the kinetics of coarsening, the emergence of structure, and the anisotropy in materials properties. Using an efficient, linear-N simulation method we report studies of systems of N=10 000 particles over the concentration range of φ=10–50 vol %. We present a variety of methods of characterizing the structures that emerge, including the two dimensional pair correlation, velocity correlations, microcrystallinity, optical attenuation, dipolar interaction energy, conductivity, and permittivity. The anisotropies that we compute are generally inverted relative to those found in materials structured by a uniaxial field.
Theory of the diffusion-influenced substrate binding rate to a buried and gated active site
Authors: Zhou Huan-Xiang
Published: July 2002

The effects of stochastic gating on the diffusion-influenced substrate binding rate to a buried active site are studied. An approximation introduced by Samson and Deutch [J. Chem. Phys. 68, 285 (1978)] is shown to be equivalent to making the constant-flux approximation on the entrance to the active site. The constant-flux approximation is then extended to the case where the entrance to the active site is stochastically gated because of conformational fluctuations of the enzyme. The stochastically gated rate constant, ksg, is found to be given by the relation 1/ksg=1/k+wo/wc(wo+wc)ĥ(wo+wc), where k is the rate constant in the absence of gating, ĥ(s) is the Laplace transform of the total flux across the entrance after the substrate is started from an equilibrium distribution outside the entrance, and wo and wc are the transition rates between the open and closed gating states. This relation reduces to an approximate relation derived earlier for a more restrictive situation, where the reactivity within the active site is gated. The leading term in the expansion of sĥ(s) for large s is DA〈exp(−βU)〉(s/D)1/2/2, where D is the diffusion coefficient of the substrate, A is the total area of the entrance, and 〈exp(−βU)〉 is the average Boltzmann factor on the entrance. The time scale of conformational fluctuations, ∼ a few picoseconds, is much shorter than the time scale of diffusion, so this leading term is useful for estimating (wo+wc)ĥ(wo+wc). A further consequence of the disparity in time scales is that the value of (wo+wc)ĥ(wo+wc) is much larger than k. As a result the decrease of the rate constant due to gating is relatively small (unless the entrance to the active site is closed nearly all the time). This suggests that a buried and gated active site may play the important role of controlling enzyme specificity without sacrificing efficiency.
Surface roughening: Kinetics, adsorbate-induced effects, and manifestation in catalytic reactions
Authors: Zhdanov V. P. , Kasemo B.
Published: July 2002

Using the Monte Carlo (MC) technique, we have analyzed the kinetics of roughening of a clean surface at T>TR (TR is the roughening temperature), the kinetics of relaxation from a roughened to a flat surface at T<TR, the effect of adsorption on surface roughening, and the influence of adsorbate-induced roughening on the kinetics of the catalytic 2A+B2→2AB reaction. (i) For a clean surface, the results were obtained by employing the standard restricted-solid-on-solid (RSOS) model with surface diffusion occurring via jumps to nearest-neighbor and next-nearest-neighbor sites. The kinetics of surface roughening at T>TR and of surface relaxation at TTR increases with time as w2∝ln t. (ii) Complementing the RSOS model by terms describing adsorption, we demonstrate that adsorbate-induced surface roughening may occur with significant lowering of TR, due, respectively, to indirect next-nearest-neighbor adsorbate–substrate repulsion, attractive nearest- neighbor adsorbate–substrate lateral interaction, and/or repulsive adsorbate-adsorbate lateral interaction. In all these cases, the ratio of TRnear adsorbate saturation and for the clean surface, respectively, is given by an analytical expression that is a simple combination of the corresponding interactions. The coverage dependence of TR in MC simulations is nonlinear and slightly different compared to the mean-field-type prediction. (iii) The effect of adsorbate-induced roughening on a catalytic reaction can be dramatic even at weak roughening, with qualitative and quantitative changes in the kinetics. For example, the absolute conversion rate at maximum rate, and the reactant mixture corresponding to this maximum can be very different on the flat and roughened surface. The behavior of the adsorbate coverages near the points corresponding to the kinetic phase transitions can change character qualitatively.