bluuebunny/crossref_metadata_2025 huggingface dataset:
Local state spectrum of end substituted polyenes
Authors: Malysheva Lyuba I. , Onipko Alexander I.
Published: July 2002

Starting from the electronic part of the Su–Schrieffer–Heeger [Phys. Rev. B 22, 1099 (1980)] Hamiltonian with a strict account to the end effects, it is shown that the π-electron spectrum of substituted polyenes is determined by solutions of the Lennard-Jones type equation, where each end group is represented by a single energy dependent parameter associated with the diagonal component of the end-group Green function referred to the binding site. The conditions of the appearance/disappearance of in-gap and out-of-band local π-electron states are found in the form of analytical relations between the conjugated bridge length, the C–C bond alternation parameter, and parameters of end-groups included in the above mentioned Green functions. These relations determine a set of critical parameters (defined explicitly), at which the local state spectrum undergoes qualitative transformations. On the basis of the obtained results we propose a certain procedure that allows us to predict the number of in-gap and out-of-band states in any substituted polyene provided the π-electronic structure of end groups and the bridge parameters are known. Throughout the discussion, several conclusions are made concerning the dependence of the electron localization on the finite-length and bond-alternation effects.
The thermal conductivity of methane in the critical region
Authors: Sakonidou E. P. , van den Berg H. R. , ten Seldam C. A. , Sengers J. V.
Published: July 2002

We have measured the thermal conductivity of methane at temperatures from 308 K down to 190.585 K, which is just 21 mK above the critical temperature, and at densities up to 14 mol L−1. The data were obtained with an improved guarded parallel-plate cell with a new cryostat that was built especially for measurements in the critical region of methane. The new experimental data have a higher accuracy than those reported previously in the literature and enable us to examine the validity of the currently available theoretical description of the asymptotic and nonasymptotic behavior of the thermal conductivity of fluids in the critical region. Equations for the thermal conductivity of methane in a wide range of temperatures and densities are also presented.
Microwave spectroscopy and interaction potential of the long-range He⋯Kr+ ion: An example of Hund’s case (e)
Authors: Carrington Alan , Pyne Christopher H. , Shaw Andrew M. , Taylor Susie M. , Hutson Jeremy M. , Law Mark M.
Published: July 2002

We have observed a microwave spectrum of the HeKr+ ion in which all of the observed levels lie within a few cm−1 of either the first or second dissociation limit. We use an ion beam technique in which HeKr+ ions, formed by electron impact, are mass analyzed. Passage of the ion beam through an electric field lens results in selective fragmentation of energy levels lying close to dissociation. Kr+ ions formed in the lens are separated from all other ions by means of an electrostatic analyzer, and are detected with an electron multiplier. Microwave radiation induces transitions which result in population transfer and produce detected changes in the electric field-induced Kr+ fragment ion current. Additional transitions have been detected by a microwave–microwave double resonance method, and we have also made extensive use of the Zeeman effects produced by small applied coaxial magnetic fields to identify the J quantum numbers of the levels involved. Coupled channel calculations of the bound states of the He⋯Kr+ ion are carried out, fully including all the couplings between different electronic states correlating with He+Kr+ (2P3/2 and 2P1/2). The calculations allow the spectra to be assigned to pure rotational transitions involving levels in the X, A1, and A2 states that lie within 2.5 cm−1 of the dissociation limits. Because of a systematic near degeneracy between vibrational levels in the X and A1 states, the long-range He⋯Kr+ ion provides a very good example of Hund’s case (e) in the form introduced by Mulliken, in which there are no projection quantum numbers onto the interatomic axis. Mulliken’s case (e) is rather different from the Rydberg case (e) described by Lefebvre–Brion, and this is the first time that Mulliken’s case (e) has been observed. The spectra allow the interaction potential for He⋯Kr+ to be determined accurately, for the first time, by least-squares fitting of potential parameters to the experimental line frequencies and g factors. The resulting interaction potential (designated MAL1) is compared with that previously determined for He⋯Ar+: the He⋯Kr+ potential is significantly shallower, because the long-range ion-induced dipole C4 coefficient is the same for the two systems but the larger Kr+ ion prevents the He atom approaching as close.
Mode-coupling theory of the slow dynamics of polymeric liquids: Fractal macromolecular architectures
Authors: Fuchs Matthias , Schweizer Kenneth S.
Published: July 2002

Recently a mode coupling theory for the dynamics of solutions and melts of entangled linear chain polymers has been developed. We report the extension of this approach to macromolecular architectures different from linear chains. Specifically, this work addresses recent experimental findings on melts of ring shaped polymers, small spherical micro-networks, and linear chains in two dimensions. The mechanical and dielectric response, diffusion, and molecular relaxation times of macromolecules modeled by fractal mass distributions are studied. The distribution is chosen to be Gaussian and then is uniquely determined from the experimentally measured scaling of macromolecular size (Rg) with degree of polymerization (N), i.e., Rg∝Nν. The exponent ν and the spatial dimension d determine the large N scaling of the transport coefficients and the exponents describing intermediate time anomalous diffusion. Within the theory, entanglement corrections to the single polymer Rouse dynamics are effective for ν<2/d only. There, we find D∝N2dν−5 for the diffusion coefficient and that the ratio DτD/Rg2 is almost constant, where τD is the terminal relaxation time. Using independent input from equilibrium liquid state theories, the magnitude and scaling with macromolecular density and segment length of the dynamical properties is determined. It is also found that macromolecular interpenetration requires progressively higher densities and consequently entanglements become less effective with fractal dimension 1/ν approaching the spatial dimension.
Theory of desorption induced by electronic transitions: A time-independent quantum mechanical approach
Authors: Seideman Tamar
Published: July 2002

A new method is developed for study of photon- or electron-stimulated desorption from noninsulator surfaces. Concise and physically transparent, closed form expressions for the observables in a desorption experiment are given in terms of matrix elements of the interaction in stationary bases. The method is nonperturbative and spans the limits of weak to strong substrate–adsorbate interactions. We propose the possibility of relaxing several approximations frequently adopted in theoretical studies of desorption induced by electronic transitions. In particular, continuous decay of the transient electronic excitation produced by hot substrate carriers and the possibility of multiple electronic transitions follow naturally from the formulation and need not be introduced ad hoc. The description of time-delayed desorption, resulting from occurrence of predesorption resonances, entails neither conceptual nor computational difficulty and vibrational relaxation is consistently accounted for. The method is tested by application to an empirical model of the NH3/Cu photodesorption dynamics.
Continuum estimates of rotational dielectric friction and polar solvation
Authors: Maroncelli Mark
Published: July 2002

Dynamical solvation data recently obtained with the probe solute coumarin 153 are used to test the reliability of dielectric continuum models for estimating dielectric friction effects. In particular, the predictions of the Nee–Zwanzig theory of rotational dielectric friction are examined in some detail. The analysis undertaken here uncovers an error made in virtually all previous applications of the Nee–Zwanzig formalism. The error involves neglect of the solvent’s electronic polarizability when calculating dielectric friction constants. In highly polar solvents the effect of this neglect is shown to be minor, so that the results of past studies should not be appreciably altered. However, in weakly polar and especially in nondipolar solvents, the proper inclusion of electronic polarizability terms is essential. The equivalence between the Nee–Zwanzig theory of dielectric friction and more general continuum treatments of polar solvation dynamics is also demonstrated. This equivalence enables the use of solvation data to test the reliability of the Nee–Zwanzig description of electrical interactions between a solute and solvent that form the core of this and related continuum theories of dielectric friction. Comparisons to experimental data show that, with the important exception of nondipolar solvents, such continuum treatments provide reasonably accurate (±40%) predictors of time-dependent solvation and/or dielectric friction.
Modeling orientational randomization in zeolites: A new probe of intracage mobility, diffusion and cation disorder
Authors: Auerbach Scott M. , Metiu Horia I.
Published: July 2002

We have performed kinetic Monte Carlo simulations of benzene orientational randomization (BOR) and diffusion in Na-Y zeolite for various Na(II) occupancies and Na(II) spatial patterns. Full Na(II) occupancy gives BOR rates controlled by intracage motion, whereas half Na(II) occupancy gives BOR rates sensitive to both intracage and intercage motion, but insensitive to particular Na(II) spatial patterns. Alternatively, BOR with one quarter Na(II) occupancy demonstrates qualitative sensitivity to different Na(II) spatial patterns. Calculated diffusion coefficients vary weakly with decreasing Na(II) occupancy until ca. one Na(II) per supercage. Diffusion coefficients and mean square displacements reveal no information about intracage motion, and are insensitive to different spatial patterns of Na(II) cations. Our computational results thus suggest that measuring orientational randomization in zeolites can provide important information regarding intracage motion, diffusion and cation disorder.
The local pressure in a cylindrical liquid–vapor interface: A simulation study
Authors: Mareschal M. , Baus M. , Lovett R.
Published: July 2002

The equilibrium force distribution, or the local pressure in an interface as defined in a companion paper, has been determined from molecular dynamics simulations of a Lennard-Jones fluid. Both a cylindrical and a planar interface are considered. Limited evidence is found that the surface tension could be independent of the curvature of the interface.
Vibrational mode and frequency dependence of the photofragmentation of the methoxy radical
Authors: Powers David E. , Pushkarsky Michael B. , Miller Terry A.
Published: July 2002

The photodissociation process for the jet-cooled methoxy radical at energies above the origin of the Ã state has been probed by several techniques: laser induced fluorescence, fluorescence temporal decay, and fluorescence depletion spectroscopy. A threshold for this process is clearly established. Above that threshold the ratio for the competing processes of photon emission and bond fission vary strongly. This variation is a function not only of the total excitation energy but is also strongly mediated by the nature of the vibrational motion involved. It is found that excitation of the ν3 mode (or other components in Fermi resonance with it) strongly promotes the breaking of the C–O bond.
Fokker–Planck equation for the one-molecule distribution function in polymer mixtures and its solution
Authors: Curtiss C. F. , Byron Bird R.
Published: July 2002

It is shown how a Fokker–Planck equation in the phase space of a single polymer molecule in a multicomponent mixture can be obtained from the Liouville equation in the phase space of a mixture of polymeric liquids. This result is a generalization of the Schieber–Öttinger equation for a dilute solution of a single polymer species in a solvent, or the Öttinger–Petrillo equation for nonisothermal systems. The Fokker–Planck equation is solved as a series in powers of a small parameter ε, thereby displaying quantitatively the deviation of the velocity distribution from the Maxwellian. It is then shown how moments of the singlet distribution function needed for the evaluation of the transport coefficients can be obtained. In addition, expressions for the first three moments of the Brownian force are developed. It is further shown how the present discussion is related to the Curtiss–Bird theory for multicomponent diffusion. Throughout the development the polymer molecules are modeled as arbitrary bead-spring structures, with all inter-bead forces (representing both intra- and intermolecular forces) derivable from a potential and directed along the bead–bead vectors. These models can describe flexible chain macromolecules, ring-shaped polymers, starlike polymers, and branched polymers.
Cauchy moments and dispersion coefficients using coupled cluster linear response theory
Authors: Hättig Christof , Christiansen Ove , Jo/rgensen Poul
Published: July 2002

Expressions for the even Cauchy moments for nonvariational methods have been derived using the time-averaged quasienergy Lagrangian technique. The expressions obtained require the solution of linear equations but do not involve a sum over individual excited-state contributions. An implementation is reported for the coupled cluster models CCS, CC2, and CCSD and calculations have been performed for the Cauchy moments and the Verdet and Cotton–Mouton constants of the Ne atom and for the C6 dispersion coefficient of the Ne2 dimer.
New operators for calculation of indirect nuclear spin–spin coupling constants
Authors: Chipman Daniel M. , Rassolov Vitaly A.
Published: July 2002

The Fermi contact interaction, which makes an important contribution to indirect nuclear spin–spin coupling constants, is sensitive to the electronic behavior at the positions of the coupled nuclei. Unfortunately, these regions are often not described well in commonly used molecular wave functions. It is shown that the delta function operators which formally represent this interaction in such second-order properties can be replaced by certain global operators in a manner analogous to that previously derived for first-order properties. It is further argued that two specific choices of the new operators that have previously been demonstrated to be very useful for the calculation of isotropic hyperfine coupling constants may also be valuable in improving the accuracy of indirect nuclear spin–spin coupling constant calculations. Illustrative calculations are presented for HD with encouraging results.
The effects of molecular association on mutual diffusion in acetone
Authors: Chan T. C. , Ma N. L. , Chen Nong
Published: July 2002

Limiting mutual diffusion coefficients of aromatic compounds in acetone have been measured at 298.2 K by using the chromatographic peak-broadening method. The data of the polar and nonpolar pseudoplanar solutes are compared, and the effects of molecular association on diffusion as well as the solvation numbers are determined. It is found that the effects of hydrogen bonding are such that –OH>–NH2>–SH, and that the solvation numbers are approximately equal for solutes containing the same polar group. Using ab initio molecular orbital theory, molecular solute–acetone interaction energies have been calculated. There exists a nearly linear relationship between the interaction energy and the effect of solute–solvent association on diffusion. The applicability of the rough-hard-sphere theory to the diffusion of associated molecules is discussed.
Variational energy band theory for polarons: Mapping polaron structure with the global-local method
Authors: Brown David W. , Lindenberg Katja , Zhao Yang
Published: July 2002

In this paper we revisit from a contemporary perspective a classic problem of polaron theory in one dimension using a new variational approach generalizing that of Toyozawa, based on delocalized trial states including mixed gobal and local exciton-phonon correlations. Polaron structure is represented by variational surfaces giving the optimal values of the complete set of exciton and phonon amplitudes for every value of the joint exciton-phonon crystal momentum κ. Characteristic small polaron, large polaron, and nearly free phonon structures are identified, and the manner in which these compete and/or coexist is examined in detail. Through such examination, the parameter space of the problem is mapped, with particular attention given to problematic areas such as the highly quantum mechanical weak-coupling regime, the highly nonlinear intermediate-coupling regime, and to the self-trapping transition that may be said to mark the onset of the strong-coupling regime. Complete energy bands are presented in illustrative cases, and the principal trends in the ground-state energy, polaron bandwidth, and effective mass are identified. The internal structure of our variational Bloch states is examined for qualities that might reflect the typical characteristics of solitons, finding some intriguing qualitative comparisons, but little that bears close scrutiny.
Langmuir monolayers as disordered solids: Strain-state calculations applied to stearic acid
Authors: Swanson David R. , Luty Tadeusz , Eckhardt Craig J.
Published: July 2002

This paper presents a calculational procedure to determine the equilibrium phase for a given surface pressure π. The monolayer is treated as orientationally free tails grafted to a two-dimensional net formed by the head groups of the amphiphilic molecules. The head groups form a subsystem with translational degrees of freedom characterized by strain variables in the plane of the surface, and the tail groups compose a subsystem characterized by rotational degrees of freedom. The order in the monolayer derives indirectly from the crystalline head groups through translational–rotational coupling. A stress–strain relation is derived which shows the energetically most favorable path for reorientation of the molecules due to a two-dimensional strain. This set of strain states for a given symmetry (phase) allows a contribution to the strain-state partition function to be computed. It is then straightforward to calculate the strain-state contribution to the free energy for a given phase and estimate the transition temperature between phases.
The structure of supercritical heavy water as studied by neutron diffraction
Authors: Bellissent-Funel M.-C. , Tassaing T. , Zhao H. , Beysens D. , Guillot B. , Guissani Y.
Published: July 2002

Neutron diffraction experiments of heavy water at several supercritical states (380<T<500 °C and 200<P<900 bar) corresponding to densities ranging from 0.2 to 0.7 g/cm3 are performed. They allow the static structure factor and the pair correlation function to be derived once absorption, multiple scattering, and inelasticity corrections have been made. The experimental results are compared with previous data obtained by Postorino et al. [Nature 366, 668 (1993)] and with new molecular dynamics simulations using the extended simple point charge (SPCE) potential. They lead to the conclusion that hydrogen bonding is still present in supercritical water. This conclusion does not agree with the previous findings of Postorino et al. but looks rather in agreement with the results of computer simulations.
Calculation, with the inclusion of vibrational corrections, of the dc-electric-field-induced second-harmonic-generation hyperpolarizability of methane
Authors: Bishop David M. , Sauer Stephan P. A.
Published: July 2002

Calculations of γ̄(−2ω;ω,ω,0) for CH4 are reported for a number of frequencies (ω). This mean frequency-dependent second hyperpolarizability governs the nonlinear optical process: dc-electric-field-induced second-harmonic generation. Two levels of calculation have been carried out: one is based on the random phase approximation and the other on the multiconfigurational random phase approximation using a complete active space self-consistent-field wavefunction. This is the first calculation of γ̄(−2ω;ω,ω,0) that has been made for CH4 using electron-correlated wavefunctions. Included in the calculations are corrections for vibrational effects of which the zero-point vibrational-averaging correction is both dominant and significant (13–18% of the electronic values). Comparison with experiment shows that not all the electron correlation is captured by our treatment. The dispersion properties are also discussed.
NiH
2
has a singlet ground state
Authors: Barron Joshua R. , Kelley Amy R. , Liu Ruifeng
Published: July 2002

Contrary to previous ab initio and semi-empirical studies, density functional theory and high level ab initio calculations predict that the ground state of NiH2 is a bent singlet state. The linear triplet Δg3 state, predicted as the ground state by all previous calculations, is found a few kcal/mol higher.
Measurement and ab initio modeling of the inelastic neutron scattering of solid N-methylformamide
Authors: Bouř Petr , Tam Cheok N. , Sopková Jana , Trouw Frans R.
Published: July 2002

Vibrational motions of solid N-methylformamide (NMF) and its N-deuterated analogue are investigated using the inelastic neutron scattering (INS) technique at 15 K. The force field for obtaining the normal vibrational modes of the crystal is based on a quantum chemical calculation and a subsequent transfer of a harmonic force field of a smaller pentameric segment to a fragment of 11 NMF molecules. Two types of hydrogen bonds present in crystalline NMF are also modeled with dimers. The distinct bonding leads to a splitting of the N-hydrogen wagging mode in the spectrum. Although the hydrogen bonding has a profound effect on vibrational frequencies, the results indicate that an occurrence of a double-well potential for bonded hydrogen proposed previously is unlikely. Instead, a limited electronic conjugation along the hydrogen bonds in crystalline NMF is observed. Unlike in previous models, we simulate the relative INS intensity of each vibrational transition separately, which leads to a substantial improvement of the overall profile of the intensity pattern. The modeling allows one to assign most of observed INS bands to vibrational modes and the overall spectral profile that reproduced by the simulation compares well with the experiment.
Molecular potential energy surfaces by interpolation in Cartesian coordinates
Authors: Thompson Keiran C. , Jordan Meredith J. T. , Collins Michael A.
Published: July 2002

We present a new method for expressing a molecular potential energy surface (PES) as an interpolation of local Taylor expansions. By using only Cartesian coordinates for the atomic positions, this method avoids redundancy problems associated with the use of internal coordinates. The correct translation, rotation, inversion, and permutation invariance are incorporated in the PES via the interpolation method itself. The method is most readily employed for bound molecules or clusters and is demonstrated by application to the vibrational motion of acetylene.
Assessment of complete basis set methods for calculation of enthalpies of formation
Authors: Curtiss Larry A. , Raghavachari Krishnan , Redfern Paul C. , Stefanov Boris B.
Published: July 2002

Three complete basis set models of Petersson et al. [J. Chem. Phys. 104, 2598 (1996)], CBS-Q, CBS-q, and CBS-4, have been assessed on the G2 neutral test set of 148 molecules [J. Chem. Phys. 106, 1063 (1997)]. The average absolute deviations with experiment of the calculated enthalpies of formation from the three CBS methods are 1.57 kcal/mol (CBS-Q), 2.13 kcal/mol (CBS-q), and 3.06 kcal/mol (CBS-4). The maximum deviations of the methods are 11.2, 10.3, and 14.4 kcal/mol. respectively. The most accurate method, CBS-Q, has an average absolute deviation similar to that of G2 theory. The three CBS methods have also been assessed on a 40 molecule set using isodesmic bond separation reactions to calculate enthalpies of formation. There is a significant improvement in the accuracy of the enthalpies compared to those calculated using atomization energies, although not as much as for G2 theory. In a test on naphthalene, enthalpies calculated using the CBS methods have large deviations. The CBS-Q method has a deviation of 28.7 kcal/mol and, surprisingly, the deviation increases to 34.3 kcal/mol when isodesmic bond separation reaction energies are used.
Coupled-cluster calculations of structure and vibrational frequencies of ozone: Are triple excitations enough?
Authors: Watts John D. , Bartlett Rodney J.
Published: July 2002

Coupled-cluster calculations with full inclusion of singles, doubles, and triples (CCSDT) with a double-zeta plus polarization and correlation-consistent polarized valence triple-zeta basis sets have been used to calculate the structure and harmonic vibrational frequencies of the ozone molecule. These results have been compared with those of more approximate CC methods, and the effects of the different terms in the triple excitation equation are analyzed. The effect of basis set extension on the CCSDT results has been estimated. In the limit of a large basis set, it appears that the complete CCSDT method will give smaller bond lengths than experiment and an asymmetric stretching frequency about 50 cm−1 above the experimental value. That is, it would appear that connected quadruple excitations are needed for quantitative calculations of the structure and frequencies of ozone.
Stimulated emission at 10 μm of O3 trapped in low temperature matrices: Threshold measurements and quantum yield evaluation
Authors: Jasmin D. , Brosset P. , Dahoo R. , Gauthier-Roy B. , Abouaf-Marguin L.
Published: July 2002

The recently reported fluorescence, emitted near 10 μm by laser excited ozone molecules trapped in xenon matrices, is actually a stimulated emission. It is observed, at the 2ν3→ν3 frequency, only if the laser pulse, which excites the ν1+ν3 level, contains a sufficient amount of energy. A similar emission is found in the other rare gas matrices and in nitrogen, above a laser energy threshold which is matrix dependent. Above the threshold, the emitted signal grows linearly with the laser pulse energy. An evaluation of the quantum yield is obtained by comparison with the literature data on CO fluorescence. The characteristics of the observed emission (low directionality, laser energy dependence, shortening, and amplification when compared to spontaneous emission) are discussed.
Analytic integral equation theory for the critical properties of homopolymer fluids
Authors: Chatterjee Avik P. , Schweizer Kenneth S.
Published: July 2002

We apply the analytic version of the polymer reference interaction site model theory to determine the critical properties of homopolymer fluids. The Gaussian thread model is used throughout, together with a Yukawa form for the attractive interaction between chain segments. Atomiclike as well as molecular closures are employed, and results are presented using both the compressibility and free-energy route approaches to the thermodynamics. Predictions derived based on different closure approximations for the chain length (N) dependence of the theta and critical temperatures, and of the critical density, are compared with the results of simulations of the liquid-vapor equilibrium in homopolymer systems, as well as with experimental results for the demixing transition in polymer solutions. The large N asymptotic scaling laws, and finite size corrections, for the critical properties depend strongly on the closure employed for treating attractive interactions, and for all cases studied significant deviations from the mean-field Flory–Huggins lattice theory are found. The importance of simultaneously including fluctuation effects associated with both the repulsive and attractive interactions is demonstrated. Model calculations are also presented for the liquid-vapor spinodal and coexistence curves.
Nonadiabatic dynamics in polyatomic systems studied by femtosecond time-resolved photoelectron spectroscopy
Authors: Blanchet Valérie , Stolow Albert
Published: July 2002

We investigate the use of time-resolved photoelectron spectroscopy for studying nonadiabatic polyatomic dissociation dynamics. In particular, we emphasize the importance of the electronic structure of the ionization continuum in interpreting the results and provide an experimental example of these effects in the dissociation dynamics of the NO dimer.
Spin–orbit branching in the photofragmentation of HCl at long wavelength
Authors: Lambert H. Mark , Dagdigian Paul J. , Alexander Millard H.
Published: July 2002

We report a new experimental determination of the branching ratio for formation of ground state Cl(2P3/2) and spin–orbit excited Cl(2P1/2) fragments subsequent to excitation of the HCl molecule to the repulsive A 1Π electronic state. Our work extends previous experimental measurements well into the long-wavelength tail of the A 1Π←X 1Σ+ absorption profile. The branching into the spin–orbit excited Cl(2P1/2) fragment is found to reach a maximum near 220 nm, before falling to zero at threshold. Our new measured branching fractions are reproduced nearly quantitatively by new calculations which supplement those reported previously [M. H. Alexander, B. Pouilly, and T. Duhoo, J. Chem. Phys. 99, 1752 (1993)].
The polymerization of actin: Study by small angle neutron scattering
Authors: Ivkov Robert , Forbes Jeffrey G. , Greer Sandra C.
Published: July 2002

We report measurements of small angle neutron scattering from solutions of rabbit muscle G-actin at 3.00 mg/mL in D2O buffer solution, with [Ca2+]=0.52 mM and with [KCl]=15, 8.9, and 5.4 mM. We observe the onset of the polymerization of G-actin to F-actin as the temperature is increased. The polymerization takes place on a time scale of 30–45 min for each temperature jump of 2 °C–3 °C. As the temperature is increased further, the average size of the polymers increases, and the characteristic length scale (or correlation length), ξ, of the F-actin in the dilute solution grows: ξ is about 10 Å below Tp, and about 70 Å a few degrees above Tp. The transition is sharper for lower concentrations of KCl. For the sample with [KCl]=8.9 mM, we observe a peak in ξ at about 2 °C above Tp, which indicates a crossover into the semidilute regime. The transition is essentially reversible, but shows evidence of incomplete depolymerization on cycling. We are unable to apply the available theoretical model for reversible polymerization to rabbit muscle actin because of a lack of information on the enthalpy and entropy of polymerization. However, our observations for rabbit muscle actin are quite analogous to observations on the equilibrium polymerization of α-methylstyrene [A. P. Andrews, K. P. Andrews, S. C. Greer, F. Boué, and P. Pfeuty, Macromolecules 27, 3902 (1994)].
Photodissociation study of CH3Br in the first continuum
Authors: Gougousi Theodosia , Samartzis Peter C. , Kitsopoulos Theofanis N.
Published: July 2002

The photolysis of CH3Br is studied in the energy region of the A band between 4.94 and 5.76 eV using ion imaging. Velocity distributions for both the bromine-atom and methyl-radical photofragments are determined. Our results indicate that transitions to the 3Q0 and 3Q1 states dominate the absorption cross section and the partial cross section to each state is determined. The [Br*]/[Br] branching ratio is found to be strongly dependent on the excitation energy varying between 0.6 and 1.5. Both the bromine-atom and the methyl-radical translational energy distributions suggest that the vibrational distribution in the nascent CH3 is nonstatistical with appreciable excitation in the v2 umbrella mode. The lifetime of the A band is estimated at τ=120±40 fs.
Phase diagram of argon clusters
Authors: Rytkönen A. , Valkealahti S. , Manninen M.
Published: July 2002

Systems containing both argon cluster with number of atoms N=100, 200, or 400 and argon vapor were studied with constant energy molecular dynamics simulations. The vapor pressure versus temperature phase diagram including the melting temperatures was determined as a function of cluster particle number. The melting temperature of the cluster was determined with a new method based on the nearest-neighbor exchange of atoms, and it was found to approach the bulk melting temperature linearly as a function of N−1/3. The vapor pressure versus temperature curve approached the corresponding bulk curve as the particle number of the cluster increased. At temperatures lower than the melting temperatures the clusters were found to contain an icosahedral core.
An HREELS study of π* and σ* negative ion resonances of c(8×2) benzoate on Cu(110)
Authors: Chen Q. , Frederick B. G. , Richardson N. V.
Published: July 2002

For the c(8×2) benzoate on Cu(110) system, multiple unoccupied states have been measured through negative ion resonances by high-resolution electron-energy-loss spectroscopy (HREELS). Based on the azimuthal and polar angle dependences, we assign contributions from B2 and A1 states to a resonant feature in the 〈001〉 direction and contributions from B1 and A1 states in the 〈110〉 direction. The B2 state is observed close to 4.7 eV and the B1 feature is centered at 6.5 eV. The angular and azimuthal dependence of the C–H stretch intensity in HREELS at resonance is consistent with their assignment and the results of ab initio calculations. Overtone intensity shows an exponential decay as a function of overtone quantum number. From the overtone frequencies, an anharmonicity of 0.0272 is obtained which leads to a dissociation energy of the C–H bond of about 81 kcal/mol.
Investigation of the phase behavior in a compressible polymer-solvent system using the full Tompa model
Authors: Keskin Mustafa , Gençaslan Mustafa
Published: July 2002

Critical lines and the phase behavior of a compressible polymer-solvent system are investigated using of the full Tompa model for varying numbers of segments in each polymer chain. Critical lines are displayed in the density–density and the combined P,T and T,x planes for the “right corner” and also “left corner” cases. The crossing point, in a point at which two critical lines meet and exchange branches, is also calculated analytically. The P,T diagrams are discussed using the Scott and van Konynenburg phase diagram classifications. It is found that the right corner case corresponds to the transition from type II to type III classifications. On the other hand, the left corner case does not correspond to any of the six main phase diagram classes.
Density functional study of molecular crystals: Polyethylene and a crystalline analog of bisphenol-A polycarbonate
Authors: Montanari B. , Ballone P. , Jones R. O.
Published: July 2002

Density functional calculations have been performed on two types of molecular crystal: (a) crystalline (orthorhombic) polyethylene comprising covalently bonded parallel chains with weak interchain interactions, and (b) a crystalline analog of bisphenol-A polycarbonate with a unit cell containing two molecules with 59 atoms each. The local density approximation for the exchange-correlation energy overestimates the strength of the intermolecular bonds in both, and the Becke–Perdew functional (gradient corrected) gives no intermolecular binding in the former and a very weak bond in the latter. The functional of Perdew, Burke, and Ernzerhof leads to binding in both molecules.
Calculation of the nuclear Fukui function and new relations for nuclear softness and hardness kernels
Authors: De Proft Frank , Liu Shubin , Geerlings Paul
Published: July 2002

Calculations of Cohen’s nuclear Fukui function and softness are presented for a sample set of diatomic molecules. The obtained results were interpreted using Berlin’s theorem of binding and antibinding molecular regions. Moreover, new relations among the nuclear reactivity descriptors were derived within the four ensembles of density functional theory; a definition was provided for the nuclear hardness kernel, in accordance with Cohen’s nuclear softness kernel. It turned out that local hardness can be connected with this nuclear hardness kernel, strengthening the idea that local hardness should be considered as a nuclear reactivity index, whereas local softness is an electronic reactivity index.
Experimental and computational study of neutral xenon halides (XeX) in the gas phase for X=F, Cl, Br, and I
Authors: Schröder Detlef , Harvey Jeremy N. , Aschi Massimiliano , Schwarz Helmut
Published: July 2002

We report a combined experimental and theoretical study of the xenon monohalide radicals XeX• (X=F, Cl, Br, and I) together with their cationic and anionic counterparts XeX+ and XeX−. In brief, the XeX+ cations are characterized by reasonably strong chemical bonds with significant charge-transfer stabilization, except for X=F. In contrast, the neutral XeX• radicals as well as the XeX− anions can mostly be described in terms of van der Waals complexes and exhibit bond strengths of only a few tenths of an electron volt. For both XeX• and XeX− the fluorides (X=F) are the most strongly bound among the xenon halides due to significant covalency in the neutral radical, and to the large charge density on fluoride in the XeX− anion, respectively. Mass spectrometric experiments reveal the different behavior of xenon fluoride as compared to the other halides, and in kiloelectron-volt collisions sequential electron transfer according to XeX+→XeX•→XeX− can be achieved allowing one to generate neutral XeX• radicals with lifetimes of at least a few microseconds for X=F and I.
Stability of binary clusters of fullerene molecules
Authors: Garcı́a R. , Rey C. , Gallego L. J.
Published: July 2002

Using Girifalco’s potential for describing the interaction between fullerene molecules, we performed molecular dynamics simulations to obtain the ground-state structures of (C70)n−x(C60)x clusters (11⩽n⩽22; 0⩽x⩽n−1). For all values of x, (C70)13−x(C60)x clusters have a closed-shell icosahedral structure that is more stable than those of neighboring (C70)n−x(C60)x clusters; and for 2⩽x⩽10, (C70)19−x(C60)x clusters have a double icosahedral structure that exhibits similarly enhanced relative stability for 2⩽x⩽9. The discrepancy between this latter result and experimental findings for ionized fullerene clusters, which suggest that [(C70)19−x(C60)x]+ is more stable than the homologous 18- and 20-member clusters for all x, is tentatively attributed to the ionization-induced alteration of the intermolecular potential.
Equivalence of the Kobrak–Rice photoselective adiabatic passage and the Brumer–Shapiro strong field methods for control of product formation in a reaction
Authors: Kobrak Mark N. , Rice Stuart A.
Published: July 2002

It is shown that the photoselective adiabatic passage approach to control of product formation in a chemical reaction proposed by Kobrak and Rice may be viewed as a high-field limit of the strong field Brumer–Shapiro scheme. It is further shown that the adiabatic passage representation provides new insight into the nature of control over chemical reactions in strong fields, and that it correctly predicts the results of physically realistic simulations of the sodium dimer.
Cross sections for radicals from electron impact on methane and fluoroalkanes
Authors: Motlagh Safa , Moore John H.
Published: July 2002

Molecular and atomic radicals from electron-impact dissociation of methane and a variety of fluoroalkanes are detected mass spectometrically as organotellurides produced by the reaction of the radicals at the surface of a tellurium mirror. The radicals detected include CH3 from CH4; CF3 from CF4 and CHF3; CHF2 from CHF3 and CH2F2; CH2F from CH3F; and CF3 and C2F5 from C2F6 and C3F8 produced by electron impact at energies between 10 eV and 500 eV. Relative cross sections are measured. These are placed on an absolute scale by comparison with related measurements. For the collision energies relevant to processing plasmas, 10–30 eV, it is shown that dissociation into neutrals rather than dissociative ionization is mainly responsible for the production of molecular radicals.
Mode-specific tunneling splittings in 9-hydroxyphenalenone: Comparison of two methods for direct tunneling dynamics
Authors: Fernández-Ramos Antonio , Smedarchina Zorka , Zgierski Marek Z. , Siebrand Willem
Published: July 2002

A benchmark comparison is presented of two direct dynamics methods for proton tunneling, namely variational transition-state theory with semiclassical tunneling corrections (VTST/ST) and the instanton method. The molecules chosen for the comparison are 9-hydroxyphenalenone-d0 and -d1, which have 64 vibrational degrees of freedom and show large tunneling splittings for the zero-point level and several vibrationally excited levels of the electronic ground state. Some of the excited-level splittings are larger and some smaller than the zero-level splitting, illustrating the multidimensional nature of the tunneling. Ab initio structure and force field calculations at the Hartree–Fock/6-31G** level are carried out for the two stationary points of the tunneling potential, viz. the equilibrium configuration and the transition state. The VTST/ST calculations are based on both the small- and the large-curvature approximation; the additional quantum-chemical calculations required at intermediate points of the potential are performed at the semiempirical modified neglect of differential overlap (MNDO)/H2 level. The VTST/ST computations use the MORATE 6.5 code developed by Truhlar and co-workers. The instanton dynamics calculations are based on the method we previously developed and applied to tropolone, among others. It uses the transition state rather than the equilibrium configuration as reference structure and approximates the least action analytically. The computations use our “dynamics of instanton tunneling” (DOIT) code. It is found that the large-curvature approximation and the instanton method both reproduce the observed zero-level splitting of the d0 isotopomer if the calculated barrier is reduced by a factor 0.87. With this adjusted barrier, the instanton method also reproduces the zero-level and excited-level splittings of the d1 isotopomer. However, both the small- and the large-curvature approximations severely underestimate all these splittings. These methods, which use relatively inflexible trajectories, do not handle the isotope effect well and also are not developed to the point where they can deal satisfactorily with vibronic level splittings. In addition, there is a striking difference in efficiency between the two methods: the MORATE 6.5 code took 40 h on an R8000 workstation to perform the dynamics calculations, whereas the DOIT code took less than 1 min and produced superior results. The main reason for this superior performance is ascribed to the effective use made of the least-action principle by the instanton method and to the avoidance of the adiabatic approximation, which is not valid for modes with a frequency equal to or lower than the tunneling-mode frequency.
Order fluctuations of the director in nematic thermotropic liquid crystals studied by nuclear magnetic resonance dipolar relaxation
Authors: Zamar R. C. , Anoardo E. , Mensio O. , Pusiol D. J. , Becker S. , Noack F.
Published: July 2002

Larmor frequency dependent NMR studies of dipolar order relaxation in liquid crystals have seldom been tried in the past. Using conventional static magnetic field techniques, the experiment cannot be extended to the low Larmor frequency (νL) regime due to limitations in the signal-to-noise ratio of the dipolar echo. In this work, we present an experimental study of the dipolar relaxation time in the frequency range 103–7×107 Hz in nematic thermotropic liquid crystals. To extend the study to such low frequencies, we used the Jeener–Broekaert pulse sequence combined with fast field-cycling NMR technique. For frequencies higher than 105 Hz, the dipolar relaxation time T1D(νL) follows the νL1/2-law that is characteristic of order fluctuations of the director (OFD) in nematics. In contrast, the Zeeman relaxation is driven by faster and less correlated motions, specially in the MHz frequency range. The relaxation of dipolar energy was measured to be remarkably faster than the one predicted by the usual semiclassical model of isolated spin pairs. Conceivably, the failure of the usual two-spin model should be sought in the absence of multispin interactions and multispin correlations. We propose that the OFD are the dominant relaxation mechanism for the dipolar order, even in the MHz regime. This result turns T1D(νL) experiment in a useful NMR technique for the study of slow molecular dynamics in mesophases.
Coupling of internal rotation of methyl group with proton transfer in the S1 state of 5-methyltropolone
Authors: Nishi Kaori , Sekiya Hiroshi , Kawakami Hiroko , Mori Akira , Nishimura Yukio
Published: July 2002

The fluorescence excitation and hole-burning spectra in the S1–S0 region of 5-methyltropolone have been measured in a supersonic free jet. It has been found that the electronic origin of 5-methyltropolone splits into four components corresponding to the transitions between the sublevels of the zero-point levels in the S0 and S1 states originating from coupled motion of the internal rotation of the methyl group and proton transfer. The tunneling rate of the zero-point level in the S1 state is one order smaller than that of tropolone due to this coupling, but the excitation of the internal rotational levels promotes proton tunneling.
Concentration-profile- and molecular-size-dependent instabilities associated with the free diffusion and self-assembling of amphiphiles along a solid–fluid–fluid meniscus
Authors: Haidara Hamidou , Vonna Laurent , Schultz Jacques
Published: July 2002

We report surfactant-induced instabilities along the wetting meniscus of an oil–water–solid system which are discussed through density fluctuations of the interfacial molecular film, coupled with variations of the meniscus shape according to Laplace’s relation γJ∼(Δρg)z, where J=(1/R1+1/R2). The results show that two critical parameters exist which govern these instabilities and can be tuned to either magnify or suppress them. These are (i) the initial flux close to the meniscus and, (ii) the size and structure of the amphiphiles. The first parameter determines through the interfacial adsorption amount (∂Γ/∂t) both the magnitude and rate of the meniscus reconformation ∼∂(γJ)/∂t. The higher ∂(γJ)/∂t, the higher the inertia of the moving meniscus fluid (ρv2/2) and the more the interfacial film is compressed beyond its equilibrium position and enters the unstable regime. The molecular size and structure are found to determine the stability and response of the film to the oscillations of the meniscus. A description of these instabilities, their concentration-profile- and molecular-size-dependent behaviors is then proposed.
Picosecond infrared pump–probe spectrum of D2O adsorbed at acid OD group of zeolite
Authors: Fujino T. , Kashitani M. , Onda K. , Wada A. , Domen K. , Hirose C. , Ishida M. , Goto F. , Kano S. S.
Published: July 2002

Spectroscopic and dynamic features of the vibrationally excited D2O/zeolite system have been investigated by two-color infrared–infrared pump–probe experiment. The frequency- and delay-scanned probe intensities were measured by tuning the pump laser to the OD stretching bands of the D2O molecule hydrogen bonded to the acidic OD group of mordenite zeolite. Two types of pump-induced signals were observed: the ones which have the population lifetime of 43±5 ps and display frequency shift by the pumping frequency, and the others, which have the lifetime of about 15 ps and exhibit no such frequency shift. Possible origins of the signals are discussed.