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7 Jul 2008

Volume 129, Issue 1, Articles (01xxxx)

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Announcement: Online manuscript submission and peer review via Peer X-Press

Mark M. Cassar

J. Chem. Phys. 129, 010201 (2008); http://dx.doi.org/10.1063/1.2963991 (1 page)

Online Publication Date: 7 July 2008

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Abstract Unavailable
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01.30.-y Physics literature and publications
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Announcement: Communications and Letters to the Editor

Branka M. Ladanyi

J. Chem. Phys. 129, 010202 (2008); http://dx.doi.org/10.1063/1.2960530 (1 page)

Online Publication Date: 7 July 2008

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Abstract Unavailable
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01.10.Cr Announcements, news, and awards
01.30.-y Physics literature and publications
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Predictive first-principles simulations of strain-induced phenomena at water-silica nanotube interfaces

Yao He, Chao Cao, S. B. Trickey, and Hai-Ping Cheng

J. Chem. Phys. 129, 011101 (2008); http://dx.doi.org/10.1063/1.2953457 (4 pages)

Online Publication Date: 2 July 2008

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Via ab initio simulation, we study the behavior of interfaces of water with silica nanotubes. We find profound differences between zero and finite tensile strains and between unconfined (exterior) versus confined (interior) water. For these distinct cases, we characterize the fracture dynamics of the silica nanotube and the underlying physical mechanisms.
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68.08.-p Liquid-solid interfaces
61.46.Fg Nanotubes
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.25.Mn Fracture/brittleness

Accurate solid-state band gaps via screened hybrid electronic structure calculations

Edward N. Brothers, Artur F. Izmaylov, Jacques O. Normand, Verónica Barone, and Gustavo E. Scuseria

J. Chem. Phys. 129, 011102 (2008); http://dx.doi.org/10.1063/1.2955460 (4 pages) | Cited 28 times

Online Publication Date: 2 July 2008

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The band energy differences of solids calculated with screened hybrid density functionals, such as the functional of Heyd–Scuseria–Ernzerhof (HSE), reproduce experimental band gaps with a high degree of accuracy. This unexpected result is here rationalized by observing that band energy differences obtained from generalized Kohn–Sham calculations with screened (short-range) Hartree–Fock–type exchange approach the excitation energies obtained via time-dependent density functional calculations with the corresponding unscreened functional. The latter are expected to be the accurate predictions of the experimental optical absorption spectra. While the optimum screening parameter (ω) is system dependent, the HSE standard value of ω = 0.11 bohr−1 represents a reasonable compromise across diverse systems.
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71.20.-b Electron density of states and band structure of crystalline solids
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

A hierarchical construction scheme for accurate potential energy surface generation: An application to the F+H2 reaction

Bina Fu, Xin Xu, and Dong H. Zhang

J. Chem. Phys. 129, 011103 (2008); http://dx.doi.org/10.1063/1.2955729 (4 pages) | Cited 9 times

Online Publication Date: 3 July 2008

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We present a hierarchical construction scheme for accurate ab initio potential energy surface generation. The scheme is based on the observation that when molecular configuration changes, the variation in the potential energy difference between different ab initio methods is much smaller than the variation for potential energy itself. This means that it is easier to numerically represent energy difference to achieve a desired accuracy. Because the computational cost for ab initio calculations increases very rapidly with the accuracy, one can gain substantial saving in computational time by constructing a high accurate potential energy surface as a sum of a low accurate surface based on extensive ab initio data points and an energy difference surface for high and low accuracy ab initio methods based on much fewer data points. The new scheme was applied to construct an accurate ground potential energy surface for the FH2 system using the coupled-cluster method and a very large basis set. The constructed potential energy surface is found to be more accurate on describing the resonance states in the FH2 and FHD systems than the existing surfaces.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions

Stability and interface quality of GeO2 films grown on Ge by atomic oxygen assisted deposition

Alessandro Molle, Sabina Spiga, and Marco Fanciulli

J. Chem. Phys. 129, 011104 (2008); http://dx.doi.org/10.1063/1.2955446 (5 pages) | Cited 6 times

Online Publication Date: 3 July 2008

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The composition of GeO2 films grown on Ge has been studied for different molecular deposition processes and after exposure to ambient air. The stoichiometry, the interaction with moisture, and the interfacial details of the films are shown to be dramatically process dependent.
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68.55.at Other materials
68.55.Nq Composition and phase identification
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
61.66.Bi Elemental solids
61.66.Dk Alloys
68.35.Ct Interface structure and roughness

Femtosecond real-time probing of transition state dynamics in a surface photoreaction: Methyl desorption from CH3I on MgO(100)

Mihai E. Vaida, Peter E. Hindelang, and Thorsten M. Bernhardt

J. Chem. Phys. 129, 011105 (2008); http://dx.doi.org/10.1063/1.2953578 (4 pages) | Cited 4 times

Online Publication Date: 3 July 2008

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A novel experimental approach to the investigation of surface adsorbate reaction dynamics is presented. The direct time-resolved monitoring of the surface reaction transition state and product formation dynamics were accomplished via pump-probe mass spectrometry. As an example, methyl iodide molecules adsorbed at submonolayer coverage on an ultrathin magnesia film on Mo(100) were photoexcited to the A-band by ultrafast laser pulse irradiation. Employing time-delayed multiphoton ionization the dynamics of the dissociative methyl iodide transition state and of the emerging methyl photoproduct could be detected with femtosecond resolution. The reaction times deduced from the temporal evolution of the methyl ion mass signal indicate a strong interaction of the methyl fragment with the substrate surface prior to desorption.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.43.Nr Desorption kinetics
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Hf Product distribution
82.80.Dx Analytical methods involving electronic spectroscopy
82.50.Pt Multiphoton processes
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back to top Theoretical Methods and Algorithms

Inclusion of explicit electron-proton correlation in the nuclear-electronic orbital approach using Gaussian-type geminal functions

Arindam Chakraborty, Michael V. Pak, and Sharon Hammes-Schiffer

J. Chem. Phys. 129, 014101 (2008); http://dx.doi.org/10.1063/1.2943144 (13 pages) | Cited 9 times

Online Publication Date: 1 July 2008

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The nuclear-electronic orbital explicitly correlated Hartree-Fock (NEO-XCHF) approach for including electron-proton correlation in mixed nuclear-electronic wavefunctions is presented. A general ansatz for the nuclear-electronic wavefunction that includes explicit dependence on the nuclear-electronic distances with Gaussian-type geminal functions is proposed. Based on this ansatz, expressions are derived for the total energy and the electronic and nuclear Fock operators for multielectron systems. The explicit electron-proton correlation is incorporated directly into the self-consistent-field procedure for optimizing the nuclear-electronic wavefunction. This approach is computationally practical for many-electron systems because only a relatively small number of nuclei are treated quantum mechanically, and only electron-proton correlation is treated explicitly. Electron-electron correlation can be included by combining the NEO-XCHF approach with perturbation theory, density functional theory, and multiconfigurational methods. Previous nuclear-electronic orbital-based methods produce nuclear densities that are too localized, resulting in abnormally high stretching frequencies and inaccuracies in other properties relying on these densities. The application of the NEO-XCHF approach to the [He–H–He]+ model system illustrates that this approach includes the significant electron-proton correlation, thereby leading to an accurate description of the nuclear density. The agreement between the proton densities obtained with the NEO-XCHF and grid-based methods validates the underlying theory and the implementation of the NEO-XCHF method.
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31.15.xr Self-consistent-field methods

The influence of thermostats and manostats on reverse nonequilibrium molecular dynamics calculations of fluid viscosities

Thomas J. Müller, Michael Al-Samman, and Florian Müller-Plathe

J. Chem. Phys. 129, 014102 (2008); http://dx.doi.org/10.1063/1.2943312 (8 pages) | Cited 4 times

Online Publication Date: 1 July 2008

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Reverse nonequilibrium molecular dynamics to calculate the shear viscosity of Lennard-Jones liquids was extended to simulations at constant number of particles, constant volume, and constant pressure using a Berendsen thermostat and a Berendsen manostat. Using additional systems such as water and hexane, we also report on the performance of shear viscosity calculations of systems with electrostatic and nontrivial intramolecular interactions when a manostat is applied. We compare the shear viscosities of simulations using no coupling, only temperature coupling, and temperature and pressure coupling and characterize discrepancies, where observed. From this, we deduce guidelines for when and how manostats can be usefully applied in reverse nonequilibrium simulations.
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66.20.Cy Theory and modeling of viscosity and rheological properties, including computer simulation
61.20.Ja Computer simulation of liquid structure

Rotationally invariant ab initio evaluation of Coulomb and exchange parameters for DFT+U calculations

Nicholas J. Mosey, Peilin Liao, and Emily A. Carter

J. Chem. Phys. 129, 014103 (2008); http://dx.doi.org/10.1063/1.2943142 (13 pages) | Cited 19 times

Online Publication Date: 2 July 2008

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Conventional density functional theory (DFT) fails for strongly correlated electron systems due to large intra-atomic self-interaction errors. The DFT+U method provides a means of overcoming these errors through the use of a parametrized potential that employs an exact treatment of quantum mechanical exchange interactions. The parameters that enter into this potential correspond to the spherically averaged intra-atomic Coulomb (U) and exchange (J) interactions. Recently, we developed an ab initio approach for evaluating these parameters on the basis of unrestricted Hartree–Fock (UHF) theory, which has the advantage of being free of self-interaction errors and does not require experimental input [ Mosey and Carter, Phys. Rev. B 76, 155123 (2007) ]. In this work, we build on that method to develop a more robust and convenient ab initio approach for evaluating U and J. The new technique employs a relationship between U and J and the Coulomb and exchange integrals evaluated using the entire set of UHF molecular orbitals (MOs) for the system. Employing the entire set of UHF MOs renders the method rotationally invariant and eliminates the difficulty in selecting unambiguously the MOs that correspond to localized states. These aspects overcome two significant deficiencies of our earlier method. The new technique is used to evaluate U and J for Cr2O3, FeO, and Fe2O3. The resulting values of U-J are close to empirical estimates of this quantity for each of these materials and are also similar to results of constrained DFT calculations. DFT+U calculations using the ab initio parameters yield results that are in good agreement with experiment. As such, this method offers a means of performing accurate and fully predictive DFT+U calculations of strongly correlated electron materials.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.27.+a Strongly correlated electron systems; heavy fermions
75.30.Et Exchange and superexchange interactions
71.70.Gm Exchange interactions
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
75.50.Ee Antiferromagnetics

Second order coalescence conditions of molecular wave functions

David P. Tew

J. Chem. Phys. 129, 014104 (2008); http://dx.doi.org/10.1063/1.2945900 (7 pages) | Cited 12 times

Online Publication Date: 2 July 2008

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Kato’s cusp condition gives the exact first order dependence of molecular wave functions on interparticle separation near the coalescence of two charged particles. We derive conditions correct to second order in interparticle separation, which concern second order derivatives of the wave function at the coalescence point. For identical particle coalescence, we give equations correct to third order. In addition to a universal, particle dependent term, a system and state dependent term arises in the higher order conditions, which we interpret as an effect of Coulombic screening. We apply our analysis to the standard orbital-based methods of quantum chemistry and discuss the implications for Jastrow- and R12-type correlation factors.
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31.15.V- Electron correlation calculations for atoms, ions and molecules
31.15.xr Self-consistent-field methods

Essential energy space random walks to accelerate molecular dynamics simulations: Convergence improvements via an adaptive-length self-healing strategy

Lianqing Zheng and Wei Yang

J. Chem. Phys. 129, 014105 (2008); http://dx.doi.org/10.1063/1.2949815 (9 pages) | Cited 9 times

Online Publication Date: 3 July 2008

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Recently, accelerated molecular dynamics (AMD) technique was generalized to realize essential energy space random walks so that further sampling enhancement and effective localized enhanced sampling could be achieved. This method is especially meaningful when essential coordinates of the target events are not priori known; moreover, the energy space metadynamics method was also introduced so that biasing free energy functions can be robustly generated. Despite the promising features of this method, due to the nonequilibrium nature of the metadynamics recursion, it is challenging to rigorously use the data obtained at the recursion stage to perform equilibrium analysis, such as free energy surface mapping; therefore, a large amount of data ought to be wasted. To resolve such problem so as to further improve simulation convergence, as promised in our original paper, we are reporting an alternate approach: the adaptive-length self-healing (ALSH) strategy for AMD simulations; this development is based on a recent self-healing umbrella sampling method. Here, the unit simulation length for each self-healing recursion is increasingly updated based on the Wang–Landau flattening judgment. When the unit simulation length for each update is long enough, all the following unit simulations naturally run into the equilibrium regime. Thereafter, these unit simulations can serve for the dual purposes of recursion and equilibrium analysis. As demonstrated in our model studies, by applying ALSH, both fast recursion and short nonequilibrium data waste can be compromised. As a result, combining all the data obtained from all the unit simulations that are in the equilibrium regime via the weighted histogram analysis method, efficient convergence can be robustly ensured, especially for the purpose of free energy surface mapping.
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02.70.Ns Molecular dynamics and particle methods
02.60.-x Numerical approximation and analysis
02.50.-r Probability theory, stochastic processes, and statistics
05.20.-y Classical statistical mechanics
05.70.Ce Thermodynamic functions and equations of state

Self-consistent implementation of a nonlocal van der Waals density functional with a Gaussian basis set

Oleg A. Vydrov, Qin Wu, and Troy Van Voorhis

J. Chem. Phys. 129, 014106 (2008); http://dx.doi.org/10.1063/1.2948400 (8 pages) | Cited 21 times

Online Publication Date: 3 July 2008

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Nearly all common density functional approximations fail to properly describe dispersion interactions responsible for binding in van der Waals complexes. Empirical corrections can fix some of the failures but cannot fully grasp the complex physics and may not be reliable for systems dissimilar to the fitting set. In contrast, the recently proposed nonlocal van der Waals density functional (vdW-DF) was derived from first principles, describes dispersion interactions in a seamless fashion, and yields the correct asymptotics. Implementation of this functional is somewhat cumbersome: Nonlocal dependence on the electron density requires numerical double integration over the space variables and functional derivatives are nontrivial. This paper shows how vdW-DF can be implemented self-consistently with Gaussian basis functions. The gradients of the energy with respect to nuclear displacements have also been derived and coded, enabling efficient geometry optimizations. We test the vdW-DF correlation functional in combination with several exchange approximations. We also study the sensitivity of the method to the basis set size and to the quality of the numerical quadrature grid. For weakly interacting systems, acceptable accuracy in semilocal exchange is achieved only with fine grids, whereas for nonlocal vdW-DF correlation even rather coarse grids are sufficient. The current version of vdW-DF is not well suited for pairing with Hartree–Fock exchange, leading to considerable overbinding.
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31.15.es Applications of density-functional theory (e.g., to electronic structure and stability; defect formation; dielectric properties, susceptibilities; viscoelastic coefficients; Rydberg transition frequencies)
34.20.Gj Intermolecular and atom-molecule potentials and forces
34.20.Cf Interatomic potentials and forces

Molecular dynamics in the isothermal-isobaric ensemble: The requirement of a “shell” molecule. III. Discontinuous potentials

Mark J. Uline and David S. Corti

J. Chem. Phys. 129, 014107 (2008); http://dx.doi.org/10.1063/1.2949799 (17 pages) | Cited 3 times

Online Publication Date: 7 July 2008

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Based on the approach of Gruhn and Monson [Phys. Rev. E 63, 061106 (2001) ], we present a new method for deriving the collisions dynamics for particles that interact via discontinuous potentials. By invoking the conservation of the extended Hamiltonian, we generate molecular dynamics (MD) algorithms for simulating the hard-sphere and square-well fluids within the isothermal-isobaric (NpT) ensemble. Consistent with the recent rigorous reformulation of the NpT ensemble partition function, the equations of motion impose a constant external pressure via the introduction of a shell particle of known mass [ M. J. Uline and D. S. Corti, J. Chem. Phys. 123, 164101 (2005) ; 123, 164102 (2005) ], which serves to define uniquely the volume of the system. The particles are also connected to a temperature reservoir through the use of a chain of Nosé-Hoover thermostats, the properties of which are not affected by a hard-sphere or square-well collision. By using the Liouville operator formalism and the Trotter expansion theorem to integrate the equations of motion, the update of the thermostat variables can be decoupled from the update of the positions of the particles and the momentum changes upon a collision. Hence, once the appropriate collision dynamics for the isobaric-isenthalpic (NpH) equations of motion is known, the adaptation of the algorithm to the NpT ensemble is straightforward. Results of MD simulations for the pure component square-well fluid are presented and serve to validate our algorithm. Finally, since the mass of the shell particle is known, the system itself, and not a piston of arbitrary mass, controls the time scales for internal pressure and volume fluctuations. We therefore consider the influence of the shell particle algorithm on the dynamics of the square-well fluid.
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61.20.Ja Computer simulation of liquid structure
65.20.-w Thermal properties of liquids

Piris natural orbital functional study of the dissociation of the radical helium dimer

M. Piris, J. M. Matxain, and J. M. Ugalde

J. Chem. Phys. 129, 014108 (2008); http://dx.doi.org/10.1063/1.2950094 (6 pages) | Cited 11 times

Online Publication Date: 7 July 2008

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We have investigated the dissociation behavior of the radical helium dimer He2+ using the Piris natural orbital functional (PNOF). This system is particularly challenging to be described by standard density functionals. The restricted open formulation of the PNOF-2, as well as the PNOF-2 energy plus the extended Koopmans’ vertical ionization potential calculations of the neutral helium dimer, have been tested for calculating the ground-state energies of He2+ as a function of the internuclear distance. For comparison, we present the dissociation curve obtained with the diffusion Monte Carlo method. The dissociation energies, equilibrium bond lengths, and rovibrational levels are reported. The obtained potential energy curves indicate that PNOF-2 yields a correct and accurate dissociation behavior for the helium radical dimer.
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34.80.Ht Dissociation and dissociative attachment
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.15.Fm Bond strengths, dissociation energies
33.15.Dj Interatomic distances and angles
31.50.-x Potential energy surfaces
31.15.E- Density-functional theory

Daubechies wavelets as a basis set for density functional pseudopotential calculations

Luigi Genovese, Alexey Neelov, Stefan Goedecker, Thierry Deutsch, Seyed Alireza Ghasemi, Alexander Willand, Damien Caliste, Oded Zilberberg, Mark Rayson, Anders Bergman, and Reinhold Schneider

J. Chem. Phys. 129, 014109 (2008); http://dx.doi.org/10.1063/1.2949547 (14 pages) | Cited 32 times

Online Publication Date: 7 July 2008

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Daubechies wavelets are a powerful systematic basis set for electronic structure calculations because they are orthogonal and localized both in real and Fourier space. We describe in detail how this basis set can be used to obtain a highly efficient and accurate method for density functional electronic structure calculations. An implementation of this method is available in the ABINIT free software package. This code shows high systematic convergence properties, very good performances, and an excellent efficiency for parallel calculations.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
71.20.-b Electron density of states and band structure of crystalline solids

Self-interaction correction and the optimized effective potential

T. Körzdörfer, S. Kümmel, and M. Mundt

J. Chem. Phys. 129, 014110 (2008); http://dx.doi.org/10.1063/1.2944272 (12 pages) | Cited 13 times

Online Publication Date: 7 July 2008

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Self-interaction is one of the most substantial problems in present-day density functional theory. A widely used approach to overcome this problem is the self-interaction correction proposed by Perdew and Zunger. However, the thus given functional not only depends on the orbitals explicitly but is also variant under unitary transformation of the orbitals. In this manuscript, we present a generalized version of the optimized effective potential equation which is able to deal with both problems in one go. Calculations for molecules exemplify the approach.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
31.15.E- Density-functional theory
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Theoretical study of one-photon and two-photon absorption properties of perylene tetracarboxylic derivatives

Yang Zhao, Ai-Min Ren, Ji-Kang Feng, and Chia-Chung Sun

J. Chem. Phys. 129, 014301 (2008); http://dx.doi.org/10.1063/1.2938374 (10 pages) | Cited 8 times

Online Publication Date: 1 July 2008

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The geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the perylene tetracarboxylic derivatives (PTCDs) were studied theoretically by using density functional theory (DFT) and Zerner’s intermediate neglect of differential overlap (ZINDO) methods. The results revealed that increasing the number of naphthalene nucleus, extending the conjugated length on long axis, increasing the strength of donor group on lateral side, decreasing the ΔEH-L (energy gap between the highest occupied orbital and the lowest unoccupied orbital) and keeping the conjugation effect and inductive effect along the same molecular axis are the efficient ways to enlarge TPA cross section of PTCDs compounds. The results that PTCDs compounds exhibited extremely large TPA cross section of around 800–1100 nm (near infrared region) shed light into the significance of the PTCDs compounds for applications in TPA labeling materials in vivo.
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33.80.Wz Other multiphoton processes
31.15.E- Density-functional theory
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
33.20.Ea Infrared spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
42.65.-k Nonlinear optics

Mixed quantum/classical investigation of the photodissociation of NH3(math) and a practical method for maintaining zero-point energy in classical trajectories

David Bonhommeau and Donald G. Truhlar

J. Chem. Phys. 129, 014302 (2008); http://dx.doi.org/10.1063/1.2943213 (15 pages) | Cited 8 times

Online Publication Date: 1 July 2008

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The photodissociation dynamics of ammonia upon excitation of the out-of-plane bending mode (mode ν2 with n2 = 0,…,6 quanta of vibration) in the math electronic state is investigated by means of several mixed quantum/classical methods, and the calculated final-state properties are compared to experiments. Five mixed quantum/classical methods are tested: one mean-field approach (the coherent switching with decay of mixing method), two surface-hopping methods [the fewest switches with time uncertainty (FSTU) and FSTU with stochastic decay (FSTU/SD) methods], and two surface-hopping methods with zero-point energy (ZPE) maintenance [the FSTU/SD+trajectory projection onto ZPE orbit (TRAPZ) and FSTU/SD+minimal TRAPZ (mTRAPZ) methods]. We found a qualitative difference between final NH2 internal energy distributions obtained for n2 = 0 and n2>1, as observed in experiments. Distributions obtained for n2 = 1 present an intermediate behavior between distributions obtained for smaller and larger n2 values. The dynamics is found to be highly electronically nonadiabatic with all these methods. NH2 internal energy distributions may have a negative energy tail when the ZPE is not maintained throughout the dynamics. The original TRAPZ method was designed to maintain ZPE in classical trajectories, but we find that it leads to unphysically high internal vibrational energies. The mTRAPZ method, which is new in this work and provides a general method for maintaining ZPE in either single-surface or multisurface trajectories, does not lead to unphysical results and is much less time consuming. The effect of maintaining ZPE in mixed quantum/classical dynamics is discussed in terms of agreement with experimental findings. The dynamics for n2 = 0 and n2 = 6 are also analyzed to reveal details not available from experiment, in particular, the time required for quenching of electronic excitation and the adiabatic energy gap and geometry at the time of quenching.
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82.50.-m Photochemistry
82.20.Kh Potential energy surfaces for chemical reactions

Energy-dependent dynamics of large-ΔE collisions: Highly vibrationally excited azulene (E = 20 390 and 38 580 cm−1) with CO2

Liwei Yuan, Juan Du, and Amy S. Mullin

J. Chem. Phys. 129, 014303 (2008); http://dx.doi.org/10.1063/1.2943668 (11 pages) | Cited 3 times

Online Publication Date: 2 July 2008

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We report the energy dependence of strong collisions of CO2 with highly vibrationally excited azulene for two initial energies, E = 20 390 and 38 580 cm−1. These studies show that both the distribution of transferred energy and the energy transfer rates are sensitive to the azulene energy. Highly excited azulene was prepared in separate studies by absorption of pulsed excitation at λ = 532 or 266 nm, followed by rapid radiationless decay from S1 or S4 to vibrationally excited levels of the ground electronic state. The appearance of scattered CO2 (0000) molecules with Erot>1000 cm−1 was monitored by high-resolution transient IR absorption at λ = 4.3 μm. The average rotational and translational energies of the scattered CO2 molecules double when the azulene energy is increased by a factor of 2. The rate of energy transfer in strong collisions increases by nearly a factor of 4 when the azulene energy is doubled. The energy transfer probability distribution function for ΔE>3000 cm−1 at each initial energy is an exponential decay with curvature that correlates with the energy dependence of the state density, in excellent agreement with predictions from GRETCHEN, a model based on Fermi’s golden rule to describe collisional quenching of highly excited molecules.
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34.50.Ez Rotational and vibrational energy transfer
33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Do ionic and hydrophobic probes sense similar microenvironment in Triton X-100 nonionic reverse micelles?

G. B. Dutt

J. Chem. Phys. 129, 014501 (2008); http://dx.doi.org/10.1063/1.2946705 (6 pages) | Cited 2 times

Online Publication Date: 1 July 2008

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Rotational diffusion of two structurally similar ionic probes, rhodamine 110 and fluorescein, has been examined in nonionic reverse micellar system of Triton X-100/benzene–n-hexane/water as a function of mole ratio of the water to surfactant, W. This study has been undertaken to find out whether ionic and hydrophobic probes experience similar microenvironment in these reverse micelles. Experimental results indicate that, from W = 0 to 3, the average reorientation time, which is a measure of the microviscosity experienced by the probe molecule, increases by 90% and 40% for rhodamine 110 and fluorescein, respectively, and from W = 3 to 8, it decreases by 20% for both the probes. The increase in the average reorientation time with W has been rationalized on the basis of the flexible oxyethylene chains of the TX-100 surfactant being hydrogen bonded by the water molecules, which makes the core region less fluid. However, once the hydration of the oxyethylene chains is complete, further addition of water results in formation of water droplet; which renders the micelle-water interface in the core region less compact leading to a marginal decrease in the average reorientation time of the probe molecules. These explanations are consistent with the location of the probes and the structure of the Triton X-100/benzene–hexane/water reverse micelles. To compare how the microenvironment experienced by these ionic probes is different from the hydrophobic ones, results from our earlier work [ J. Phys. Chem. B 108, 7944 (2004) ] have been considered. Such a comparison revealed that both ionic and hydrophobic probes experience similar microenvironment in these reverse micelles until the hydration of the oxyethylene chains is complete. In case of hydrophobic probes, however, the onset of water droplet formation does not alter their microenvironment, which is due to their location in the reverse micellar cores.
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82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions)
82.70.Dd Colloids
83.80.Qr Surfactant and micellar systems, associated polymers

Shear thinning and shear dilatancy of liquid n-hexadecane via equilibrium and nonequilibrium molecular dynamics simulations: Temperature, pressure, and density effects

Huan-Chang Tseng, Jiann-Shing Wu, and Rong-Yeu Chang

J. Chem. Phys. 129, 014502 (2008); http://dx.doi.org/10.1063/1.2943314 (20 pages) | Cited 7 times

Online Publication Date: 1 July 2008

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Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in both isochoric-isothermal (NVT) and isobaric-isothermal (NPT) ensemble systems. Under steady state shearing conditions, thermodynamic states and rheological properties of liquid n-hexadecane molecules have been studied. Between equilibrium and nonequilibrium states, it is important to understand how shear rates (math) affect the thermodynamic state variables of temperature, pressure, and density. At lower shear rates of math<1×1011 s−1, the relationships between the thermodynamic variables at nonequilibrium states closely approximate those at equilibrium states, namely, the liquid is very near its Newtonian fluid regime. Conversely, at extreme shear rates of math>1×1011 s−1, specific behavior of shear dilatancy is observed in the variations of nonequilibrium thermodynamic states. Significantly, by analyzing the effects of changes in temperature, pressure, and density on shear flow system, we report a variety of rheological properties including the shear thinning relationship between viscosity and shear rate, zero-shear-rate viscosity, rotational relaxation time, and critical shear rate. In addition, the flow activation energy and the pressure-viscosity coefficient determined through Arrhenius and Barus equations acceptably agree with the related experimental and MD simulation results.
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61.20.Ja Computer simulation of liquid structure
65.20.Jk Studies of thermodynamic properties of specific liquids
66.20.-d Viscosity of liquids; diffusive momentum transport
83.60.Fg Shear rate dependent viscosity

Metal clusters that freeze into high energy geometries

Martin F. Jarrold, Baopeng Cao, Anne K. Starace, Colleen M. Neal, and Oscar H. Judd

J. Chem. Phys. 129, 014503 (2008); http://dx.doi.org/10.1063/1.2939579 (10 pages) | Cited 11 times

Online Publication Date: 1 July 2008

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Heat capacities measured for isolated aluminum clusters show peaks due to melting. For some clusters with around 60 and 80 atoms there is a dip in the heat capacities at a slightly lower temperature than the peak. The dips have been attributed to structural transitions. Here we report studies where the clusters are annealed before the heat capacity is measured. The dips disappear for some clusters, but in many cases they persist, even when the clusters are annealed to well above their melting temperature. This indicates that the dips do not result from badly formed clusters generated during cluster growth, as originally suggested. We develop a simple kinetic model of melting and freezing in a system consisting of one liquidlike and two solidlike states with different melting temperatures and latent heats. Using this model we are able to reproduce the experimental results including the dependence on the annealing conditions. The dips result from freezing into a high energy geometry and then annealing into the thermodynamically preferred solid. The thermodynamically preferred solid has the higher freezing temperature. However, the liquid can bypass freezing into the thermodynamically preferred solid (at high cooling rates) if the higher energy geometry has a larger freezing rate.
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65.40.Ba Heat capacity
64.70.dj Melting of specific substances
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
81.40.Gh Other heat and thermomechanical treatments

Selective polarization transfer using a single rf field

Eddy R. Rey Castellanos, Dominique P. Frueh, and Julien Wist

J. Chem. Phys. 129, 014504 (2008); http://dx.doi.org/10.1063/1.2939572 (9 pages)

Online Publication Date: 1 July 2008

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NMR is a popular and mature technique used in fields as diverse as chemistry, biology, or material science. One reason for this versatility lies in its ability to correlate the nuclei that are present in one molecule to another. This provides the researcher with correlation maps allowing for studies of the molecules at an atomic level. Selective experiments allow isolation of one such correlation to focus on spins of interest. This leads to a savings in precious experimental time by reducing the dimension of the experiment, which in turn may enable one to record more elaborate experiments that would otherwise not be amenable within reasonable acquisition times. Here, we present an alternative method to selectively transfer magnetization using a single rf field. This technique, which we call single field polarization transfer, allows to obtain longitudinal two-spin order of two scalar-coupled spins when only one of them is irradiated. The method is easy to implement and does not depend on stringent conditions, such as Hartmann–Hahn matching for selective cross-polarization transfers or very long inversion pulses and identification of coupling satellites in selective population inversion experiments.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
76.60.-k Nuclear magnetic resonance and relaxation

The photophysics of all-trans polyenes from ttbP5, a nonphotolabile pentaene

Javier Catalán, Pilar Pérez, Henning Hopf, and Dagmar Klein

J. Chem. Phys. 129, 014505 (2008); http://dx.doi.org/10.1063/1.2952270 (7 pages)

Online Publication Date: 2 July 2008

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The all-trans pentaene, 3,12-di(tert-butyl)-2,2,13,13-tetramethyl-3,5,7,9,11-tetradecapentaene (ttbP5) fluoresces in two different regions of the visible spectrum. It produces an extremely weak emission in the gas phase that can also be detected in the condensed phase; such an emission exhibits a negligible Stokes shift with respect to the 1Ag→1Bu absorption transition and can in principle be assigned to the 1Bu→1Ag emission of the compound. ttbP5 also exhibits a second fluorescence emission at approximately 520 nm in both the gas phase and the condensed phase. The emission in the condensed phase increases in strength and structure, with no change in spectral position, as the solvent viscosity increases by effect of the solution temperature being lowered. The spectral behavior of this pentaene (ttbP5) is different enough from that reported [ J. Catalán et al., J. Chem. Phys. 128, 104504 (2008) ] for its tetraene counterpart (ttbP4) to warrant a separate analysis in order to facilitate a better understanding of the way the photophysics of these polyenes changes as their chain is lengthened.
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33.50.Dq Fluorescence and phosphorescence spectra
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