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28 Feb 2010

Volume 132, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 132, 084501 (2010); http://dx.doi.org/10.1063/1.3308499 (7 pages)

Herman Cho, Wibe A. de Jong, and Chuck Z. Soderquist
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Communications: A nonperturbative quantum master equation approach to charge carrier transport in organic molecular crystals

Dong Wang, Liping Chen, Renhui Zheng, Linjun Wang, and Qiang Shi

J. Chem. Phys. 132, 081101 (2010); http://dx.doi.org/10.1063/1.3328107 (4 pages) | Cited 4 times

Online Publication Date: 22 February 2010

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We present a nonperturbative quantum master equation to investigate charge carrier transport in organic molecular crystals based on the Liouville space hierarchical equations of motion method, which extends the previous stochastic Liouville equation and generalized master equation methods to a full quantum treatment of the electron-phonon coupling. Diffusive motion of charge carriers in a one-dimensional model in the presence of nonlocal electron-phonon coupling was studied, and two different charge carrier diffusion mechanisms are observed for large and small average intermolecular couplings. The new method can also find applications in calculating spectra and energy transfer in various types of quantum aggregates where the perturbative treatments fail.
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72.20.Fr Low-field transport and mobility; piezoresistance
71.38.-k Polarons and electron-phonon interactions
63.20.kd Phonon-electron interactions
02.30.Rz Integral equations
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Communications: Adsorption of element 112 on the gold surface: Many-body wave function versus density functional theory

Andréi Zaitsevskii, Christoph van Wüllen, and Anatoly V. Titov

J. Chem. Phys. 132, 081102 (2010); http://dx.doi.org/10.1063/1.3336403 (4 pages) | Cited 7 times

Online Publication Date: 23 February 2010

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The applicability of the relativistic density functional theory (RDFT) with conventional generalized gradient and hybrid exchange-correlation functionals to the description of the interactions of element 112 (Cn) and its lighter homolog Hg with a gold surface is assessed. The comparison of Cn–Au (Hg–Au) bond properties for two simple models of adsorption complexes on Au(111) surface obtained by RDFT and accurate many-body calculations indicates a strong underestimation of binding energies by conventional RDFT schemes. This effect provides a possible explanation of the discrepancies between the RDFT-based theoretical and experimental data concerning the thermochromatographic registration of the α-decay chain element 114→Cn.
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68.43.Mn Adsorption kinetics
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
68.43.Fg Adsorbate structure (binding sites, geometry)
68.43.Bc Ab initio calculations of adsorbate structure and reactions
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Communications: Development and characterization of a source of rotationally cold, enriched para-H3+

Brian A. Tom, Andrew A. Mills, Michael B. Wiczer, Kyle N. Crabtree, and Benjamin J. McCall

J. Chem. Phys. 132, 081103 (2010); http://dx.doi.org/10.1063/1.3322827 (4 pages) | Cited 3 times

Online Publication Date: 25 February 2010

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In an effort to develop a source of H3+ that is almost entirely in a single quantum state (J = K = 1), we have successfully generated a plasma that is enriched to ∼ 83% in para-H3+ at a rotational temperature of 80 K. This enrichment is a result of the nuclear spin selection rules at work in hydrogenic plasmas, which dictate that only para-H3+ will form from para-H2, and that para-H3+ can be converted to ortho-H3+ by subsequent reaction with H2. This is the first experimental study in which the H2 and H3+ nuclear spin selection rules have been observed at cold temperatures. The ions were produced from a pulsed solenoid valve source, cooled by supersonic expansion, and interrogated via continuous-wave cavity ringdown spectroscopy.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.80.Ha Analytical methods involving rotational spectroscopy
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back to top Theoretical Methods and Algorithms

Efficient computation of free energy of crystal phases due to external potentials by error-biased Bennett acceptance ratio method

Pankaj A. Apte

J. Chem. Phys. 132, 084101 (2010); http://dx.doi.org/10.1063/1.3308622 (8 pages)

Online Publication Date: 22 February 2010

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Free energy of crystal phases is commonly evaluated by thermodynamic integration along a reversible path that involves an external potential. However, this method suffers from the hysteresis caused by the differences in the center of mass position of the crystal phase in the presence and absence of the external potential. To alleviate this hysteresis, a constraint on the translational degrees of freedom of the crystal phase is imposed along the path and subsequently a correction term is added to the free energy to account for such a constraint. The estimation of the correction term is often computationally expensive. In this work, we propose a new methodology, termed as error-biased Bennett acceptance ratio method, which effectively solves this problem without the need to impose any constraint. This method is simple to implement and it does not require any modification to the path. We show the applicability of this method in the computation of crystal-melt interfacial energy by cleaving wall method [ R. L. Davidchack and B. B. Laird, J. Chem. Phys. 118, 7651 (2003) ] and bulk crystal-melt free energy difference by constrained fluid λ-integration method [ G. Grochola, J. Chem. Phys. 120, 2122 (2004) ] for a model potential of silicon.
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65.40.G- Other thermodynamical quantities
68.03.Cd Surface tension and related phenomena

Electronic excitation energies in solution at equation of motion CCSD level within a state specific polarizable continuum model approach

Marco Caricato, Benedetta Mennucci, Giovanni Scalmani, Gary W. Trucks, and Michael J. Frisch

J. Chem. Phys. 132, 084102 (2010); http://dx.doi.org/10.1063/1.3314221 (7 pages) | Cited 10 times

Online Publication Date: 23 February 2010

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We present a study of excitation energies in solution at the equation of motion coupled cluster singles and doubles (EOM-CCSD) level of theory. The solvent effect is introduced with a state specific polarizable continuum model (PCM), where the solute-solvent interaction is specific for the state of interest. Three definitions of the excited state one-particle density matrix (1PDM) are tested in order to gain information for the development of an integrated EOM-CCSD/PCM method. The calculations show the accuracy of this approach for the computation of such property in solution. Solvent shifts between nonpolar and polar solvents are in good agreement with experiment for the test cases. The completely unrelaxed 1PDM is shown to be a balanced choice between computational effort and accuracy for vertical excitation energies, whereas the response of the ground state CCSD amplitudes and of the molecular orbitals is important for other properties, as for instance the dipole moment.
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31.70.Dk Environmental and solvent effects
31.15.bw Coupled-cluster theory
33.70.Jg Line and band widths, shapes, and shifts
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

Quantum dynamics of the H+CH4→H2+CH3 reaction in curvilinear coordinates: Full-dimensional and reduced dimensional calculations of reaction rates

Gerd Schiffel and Uwe Manthe

J. Chem. Phys. 132, 084103 (2010); http://dx.doi.org/10.1063/1.3304920 (10 pages) | Cited 14 times

Online Publication Date: 23 February 2010

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Full-dimensional quantum dynamics calculations for the H+CH4→H2+CH3 reaction using curvilinear coordinates are presented. A curvilinear coordinate system to describe reactions of the type X+YCH3→XY+CH3 is developed which facilitates efficient calculations using the multiconfigurational time-dependent Hartree (MCTDH) approach. To describe the bending motion of the X and Y atoms relative to the axis defined by the CH3 fragment, coordinates based on stereographic projection are introduced. These coordinates yield a kinetic energy operator free of singularities within the dynamically relevant region. Employing this curvilinear coordinate system, full-dimensional and reduced dimensional MCTDH calculations study the cumulative reaction probability (for J = 0) and the thermal rate constant for the H+CH4 reaction on the Jordan-Gilbert potential energy surface [J. Chem. Phys. 102, 5669 (1995) ]. The full-dimensional results agree very well with previous full-dimensional MCTDH results which used transition state based normal coordinates. The results of our eight-dimensional (8D) calculations are in reasonable agreement with the full-dimensional ones. They deviate significantly from older 8D results of Zhang et al. [J. Chem. Phys. 127, 234213 (2007)] but agree well with more recent results from the same group.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Xr Quantum effects in rate constants (tunneling, resonances, etc.)
82.20.Pm Rate constants, reaction cross sections, and activation energies

Identifying and correcting non-Markov states in peptide conformational dynamics

Dmitry Nerukh, Christian H. Jensen, and Robert C. Glen

J. Chem. Phys. 132, 084104 (2010); http://dx.doi.org/10.1063/1.3328781 (6 pages) | Cited 4 times

Online Publication Date: 23 February 2010

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Conformational transitions in proteins define their biological activity and can be investigated in detail using the Markov state model. The fundamental assumption on the transitions between the states, their Markov property, is critical in this framework. We test this assumption by analyzing the transitions obtained directly from the dynamics of a molecular dynamics simulated peptide valine-proline-alanine-leucine and states defined phenomenologically using clustering in dihedral space. We find that the transitions are Markovian at the time scale of ≈ 50 ps and longer. However, at the time scale of 30–40 ps the dynamics loses its Markov property. Our methodology reveals the mechanism that leads to non-Markov behavior. It also provides a way of regrouping the conformations into new states that now possess the required Markov property of their dynamics.
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87.15.hp Conformational changes
02.50.Ga Markov processes
87.14.ef Peptides
87.15.ap Molecular dynamics simulation
87.15.B- Structure of biomolecules

Size consistency of explicit functionals of the natural orbitals in reduced density matrix functional theory

N. N. Lathiotakis, N. I. Gidopoulos, and N. Helbig

J. Chem. Phys. 132, 084105 (2010); http://dx.doi.org/10.1063/1.3324699 (7 pages) | Cited 7 times

Online Publication Date: 23 February 2010

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We report a size-inconsistency problem for several functionals within reduced density matrix functional theory. Being explicit functionals of the natural orbitals and occupation numbers, instead of the one-body reduced density matrix, many of the approximate functionals are not invariant under unitary transformations in the subspace of degenerate occupation numbers. One such transformation mixes the degenerate natural orbitals of identical independent subsystems, delocalizing them. Noninvariance under this transformation results in size inconsistency for some of the approximations while others avoid this pathology by favoring orbital localization.
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31.15.E- Density-functional theory

Møller–Plesset perturbation theory gradient in the generalized hybrid orbital quantum mechanical and molecular mechanical method

Jaewoon Jung, Yuji Sugita, and S. Ten-no

J. Chem. Phys. 132, 084106 (2010); http://dx.doi.org/10.1063/1.3329370 (6 pages) | Cited 3 times

Online Publication Date: 23 February 2010

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An analytic gradient expression is formulated and implemented for the second-order Møller–Plesset perturbation theory (MP2) based on the generalized hybrid orbital QM/MM method. The method enables us to obtain an accurate geometry at a reasonable computational cost. The performance of the method is assessed for various isomers of alanine dipepetide. We also compare the optimized structures of fumaramide-derived [2]rotaxane and cAMP-dependent protein kinase with experiment.
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87.15.ag Quantum calculations
87.15.B- Structure of biomolecules
87.14.ef Peptides
87.14.ej Enzymes

A smoothing monotonic convergent optimal control algorithm for nuclear magnetic resonance pulse sequence design

Ivan I. Maximov, Julien Salomon, Gabriel Turinici, and Niels Chr. Nielsen

J. Chem. Phys. 132, 084107 (2010); http://dx.doi.org/10.1063/1.3328783 (9 pages) | Cited 5 times

Online Publication Date: 23 February 2010

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The past decade has demonstrated increasing interests in using optimal control based methods within coherent quantum controllable systems. The versatility of such methods has been demonstrated with particular elegance within nuclear magnetic resonance (NMR) where natural separation between coherent and dissipative spin dynamics processes has enabled coherent quantum control over long periods of time to shape the experiment to almost ideal adoption to the spin system and external manipulations. This has led to new design principles as well as powerful new experimental methods within magnetic resonance imaging, liquid-state and solid-state NMR spectroscopy. For this development to continue and expand, it is crucially important to constantly improve the underlying numerical algorithms to provide numerical solutions which are optimally compatible with implementation on current instrumentation and at same time are numerically stable and offer fast monotonic convergence toward the target. Addressing such aims, we here present a smoothing monotonically convergent algorithm for pulse sequence design in magnetic resonance which with improved optimization stability lead to smooth pulse sequence easier to implement experimentally and potentially understand within the analytical framework of modern NMR spectroscopy.
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07.57.Pt Submillimeter wave, microwave and radiowave spectrometers; magnetic resonance spectrometers, auxiliary equipment, and techniques
02.30.Yy Control theory

Statistics of tethered self-avoiding chains under spherical confinement and an external force

Nabil Laachi and Kevin D. Dorfman

J. Chem. Phys. 132, 084108 (2010); http://dx.doi.org/10.1063/1.3330916 (7 pages) | Cited 1 time

Online Publication Date: 23 February 2010

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We compute the partition function of self-avoiding chains tethered inside a confining sphere using Monte Carlo simulations on a three-dimensional lattice. Two cases are considered: (i) single-tethered chains with one end anchored and one end free and (ii) double-tethered chains where both ends are tethered at a distance equal to the diameter of the sphere. The self-avoidance, confinement, and tethering constraints dramatically decrease the number of allowed configurations when compared with an unconstrained random coil, thereby affecting the sampling method used in the Monte Carlo procedure. The effect of an external applied force and the bias it introduces in the partition function are also investigated. Our method involves a decomposition of the partition function into the product of several terms that can be evaluated independently. For short chains, we demonstrate the validity of our approach through a direct evaluation of the partition function using an exact enumeration of the appropriate paths on the lattice. In the case of long chains, scaling laws for the behavior of the partition function are identified.
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05.50.+q Lattice theory and statistics (Ising, Potts, etc.)
05.10.Ln Monte Carlo methods

Gradient-based multiconfiguration Shepard interpolation for generating potential energy surfaces for polyatomic reactions

Oksana Tishchenko and Donald G. Truhlar

J. Chem. Phys. 132, 084109 (2010); http://dx.doi.org/10.1063/1.3310296 (6 pages) | Cited 4 times

Online Publication Date: 23 February 2010

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This paper describes and illustrates a way to construct multidimensional representations of reactive potential energy surfaces (PESs) by a multiconfiguration Shepard interpolation (MCSI) method based only on gradient information, that is, without using any Hessian information from electronic structure calculations. MCSI, which is called multiconfiguration molecular mechanics (MCMM) in previous articles, is a semiautomated method designed for constructing full-dimensional PESs for subsequent dynamics calculations (classical trajectories, full quantum dynamics, or variational transition state theory with multidimensional tunneling). The MCSI method is based on Shepard interpolation of Taylor series expansions of the coupling term of a 2×2 electronically diabatic Hamiltonian matrix with the diagonal elements representing nonreactive analytical PESs for reactants and products. In contrast to the previously developed method, these expansions are truncated in the present version at the first order, and, therefore, no input of electronic structure Hessians is required. The accuracy of the interpolated energies is evaluated for two test reactions, namely, the reaction OH+H2→H2O+H and the hydrogen atom abstraction from a model of α-tocopherol by methyl radical. The latter reaction involves 38 atoms and a 108-dimensional PES. The mean unsigned errors averaged over a wide range of representative nuclear configurations (corresponding to an energy range of 19.5 kcal/mol in the former case and 32 kcal/mol in the latter) are found to be within 1 kcal/mol for both reactions, based on 13 gradients in one case and 11 in the other. The gradient-based MCMM method can be applied for efficient representations of multidimensional PESs in cases where analytical electronic structure Hessians are too expensive or unavailable, and it provides new opportunities to employ high-level electronic structure calculations for dynamics at an affordable cost.
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82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Localized Hartree product treatment of multiple protons in the nuclear-electronic orbital framework

Benjamin Auer and Sharon Hammes-Schiffer

J. Chem. Phys. 132, 084110 (2010); http://dx.doi.org/10.1063/1.3332769 (8 pages) | Cited 2 times

Online Publication Date: 24 February 2010

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An approximation for treating multiple quantum nuclei within the nuclear-electronic orbital (NEO) framework for molecular systems is presented. In the approximation to NEO-Hartree–Fock, the nuclear wave function is represented by a Hartree product rather than a Slater determinant, corresponding to the neglect of the nuclear exchange interactions. In the approximation to NEO-density functional theory, the nuclear exchange-correlation functional is chosen to be the diagonal nuclear exchange interaction terms, thereby eliminating the nuclear self-interaction terms. To further enhance the simplicity and computational efficiency, the nuclear molecular orbitals or Kohn–Sham orbitals are expanded in terms of localized nuclear basis sets. These approximations are valid because of the inherent localization of the nuclear orbitals and the numerical insignificance of the nuclear exchange interactions in molecular systems. Moreover, these approximations lead to substantial computational savings due to the reduction in both the number of integrals that must be calculated and the size of the matrices that must be diagonalized. These nuclear Hartree product approximation (HPA) methods scale linearly with the number of quantum protons and are highly parallelizable. Applications to a water hexamer, glycine dimer, and 32-water cluster, where all hydrogen nuclei are treated quantum mechanically, illustrate the accuracy and computational efficiency of the nuclear HPA methods. These strategies will facilitate the implementation of explicitly correlated NEO methods for molecular systems with multiple quantum protons.
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31.15.xr Self-consistent-field methods
36.40.-c Atomic and molecular clusters
31.15.E- Density-functional theory

Compatibility between shape equation and boundary conditions of lipid membranes with free edges

Z. C. Tu

J. Chem. Phys. 132, 084111 (2010); http://dx.doi.org/10.1063/1.3335894 (6 pages) | Cited 1 time

Online Publication Date: 24 February 2010

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Only some special open surfaces satisfying the shape equation of lipid membranes can be compatible with the boundary conditions. As a result of this compatibility, the first integral of the shape equation should vanish for axisymmetric lipid membranes, from which two theorems of nonexistence are verified: (i) there is no axisymmetric open membrane being a part of torus satisfying the shape equation; (ii) there is no axisymmetric open membrane being a part of a biconcave discodal surface satisfying the shape equation. Additionally, the shape equation is reduced to a second-order differential equation while the boundary conditions are reduced to two equations due to this compatibility. Numerical solutions to the reduced shape equation and boundary conditions agree well with the experimental data [ A. Saitoh et al., Proc. Natl. Acad. Sci. U.S.A. 95, 1026 (1998) ].
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87.16.D- Membranes, bilayers, and vesicles
87.10.Ed Ordinary differential equations (ODE), partial differential equations (PDE), integrodifferential models
87.14.Cc Lipids

Extraction of state-to-state reactive scattering attributes from wave packet in reactant Jacobi coordinates

Zhigang Sun, Hua Guo, and Dong H. Zhang

J. Chem. Phys. 132, 084112 (2010); http://dx.doi.org/10.1063/1.3328109 (11 pages) | Cited 11 times

Online Publication Date: 25 February 2010

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The S-matrix for a scattering system provides the most detailed information about the dynamics. In this work, we discuss the calculation of S-matrix elements for the A+BC→AB+C, AC+B type reaction. Two methods for extracting S-matrix elements from a single wave packet in reactant Jacobi coordinates are reviewed and compared. Both methods are capable of extracting the state-to-state attributes for both product channels from a single wave packet propagation. It is shown through the examples of H+HD, Cl+H2, and H+HCl reactions that such reactant coordinate based methods are easy to implement, numerically efficient, and accurate. Additional efficiency can be gained by the use of a L-shaped grid with two-dimensional fast Fourier transform.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.20.Fd Collision theories; trajectory models
34.50.-s Scattering of atoms and molecules
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Search for Br∗ production in the D+DBr reaction

Jianyang Zhang, Justin Jankunas, Nate C.-M. Bartlett, Noah T. Goldberg, and Richard N. Zare

J. Chem. Phys. 132, 084301 (2010); http://dx.doi.org/10.1063/1.3319717 (7 pages) | Cited 4 times

Online Publication Date: 22 February 2010

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Deuterium bromide (DBr) is expanded from a pulsed jet into a vacuum and a synchronized pulsed laser causes photodissociation of some of the DBr molecules to produce primarily ( ∼ 85%) ground-state bromine atoms (math3/2) and fast D atoms. The latter collide with the cold DBr molecules and react to produce molecular deuterium (D2) via two possible channels, the adiabatic channel D2+Br(math3/2) and the nonadiabatic channel D2+Br∗(math1/2), which are asymptotically separated in energy by the spin-orbit splitting (0.457 eV) of the bromine atom. Ion images are recorded for D2(v′ = 1, J′ = 16, 18–21), D2(v′ = 2, J′ = 6,7, 10–12, 14–16), and D2(v′ = 3, J′ = 2–5) for various collision energies. For the nonadiabatic production of spin-orbit-excited Br∗ in the D+DBr reaction for the conditions studied we estimate that this channel contributes 1% or less.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

Valence ionized states of iron pentacarbonyl and η5-cyclopentadienyl cobalt dicarbonyl studied by symmetry-adapted cluster-configuration interaction calculation and collision-energy resolved Penning ionization electron spectroscopy

Ryoichi Fukuda, Masahiro Ehara, Hiroshi Nakatsuji, Naoki Kishimoto, and Koichi Ohno

J. Chem. Phys. 132, 084302 (2010); http://dx.doi.org/10.1063/1.3319778 (12 pages) | Cited 3 times

Online Publication Date: 23 February 2010

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Valence ionized states of iron pentacarbonyl Fe(CO)5 and η5-cyclopentadienyl cobalt dicarbonyl Co(η5-C5H5)(CO)2 have been studied by ultraviolet photoelectron spectroscopy, two-dimensional Penning ionization electron spectroscopy (2D-PIES), and symmetry-adapted cluster-configuration interaction calculations. Theory provided reliable assignments for the complex ionization spectra of these molecules, which have metal-carbonyl bonds. Theoretical ionization energies agreed well with experimental observations and the calculated wave functions could explain the relative intensities of PIES spectra. The collision-energy dependence of partial ionization cross sections (CEDPICS) was obtained by 2D-PIES. To interpret these CEDPICS, the interaction potentials between the molecules and a Li atom were examined in several coordinates by calculations. The relation between the slope of the CEDPICS and the electronic structure of the ionized states, such as molecular symmetry and the spatial distribution of ionizing orbitals, was analyzed. In Fe(CO)5, an attractive interaction was obtained for the equatorial CO, while the interaction for the axial CO direction was repulsive. For Co(η5-C5H5)(CO)2, the interaction potential in the direction of both Co–C–O and Co–Cp ring was attractive. These anisotropic interactions and ionizing orbital distributions consistently explain the relative slopes of the CEDPICS.
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31.15.vq Electron correlation calculations for polyatomic molecules
34.50.Gb Electronic excitation and ionization of molecules
33.60.+q Photoelectron spectra

On the nature of BCcarbene bonding in a stable neutral diborene

Z. Liu

J. Chem. Phys. 132, 084303 (2010); http://dx.doi.org/10.1063/1.3326225 (7 pages) | Cited 3 times

Online Publication Date: 23 February 2010

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We report the bonding interactions within [R(H)BB(H)] and [R] (R≕C(NHCH)2) as a ligand in a newly synthesized stable neutral diborene. By using theoretical analyses, we have found the nature of the BCcarbene bonding, and, more importantly, the key to realize multiple bonds for chemical elements. With character of almost equal covalency and ionicity, the stabilizing orbital interaction term, ΔEorb, of BCcarbene, is mainly given by σ-symmetry orbital interactions; the donor-acceptor interaction is weak and contributes small to ΔEorb. In the weak donor-acceptor interaction, the B→Ccarbene π backdonation is stronger than the B←Ccarbene σ donation. Thus, in effect, the bond emerges in the B(δ+)Ccarbene(δ−) dipole. Inspection of the correlation lines of the orbital correlation diagram for the BCcarbene bonding indicates that the strength of the bonding orbitals in the central BB unit is weakened due to the coordination of the carbenes, and the center is unstabilized by the carbene ligand. This is contrary to the conventional view on the mechanism of coordination and the Dewar–Chatt–Duncanson model. However this unstabilizing effect should be responsible for the stability of the BB double bond in the stable neutral diborene. This is because the very short bond lengths arising from multiple bonds will lead to a very strong Pauli repulsion, and, ultimately, destruction of chemical bonds. It can therefore be concluded that, actually, to prevent the very short bond lengths is the true reason for the successful realization of multiple bonds for main-group elements such as boron.
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31.15.E- Density-functional theory
33.15.Dj Interatomic distances and angles
33.15.Fm Bond strengths, dissociation energies

The HOOH UV spectrum: Importance of the transition dipole moment and torsional motion from semiclassical calculations on an ab initio potential energy surface

Greg T. Drozd, Ann Melnichuk, and Neil M. Donahue

J. Chem. Phys. 132, 084304 (2010); http://dx.doi.org/10.1063/1.3317438 (7 pages) | Cited 1 time

Online Publication Date: 23 February 2010

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The absorption cross section of HOOH, a starting point for larger ROOH, was calculated using the “Wigner method.” Calculations use the Wigner transform of ground state wave functions and classical approximations for excited state wave functions. Potential energy and transition dipole moment surfaces were calculated using the equation-of-motion coupled-cluster singles and doubles method over an extended Franck–Condon region. The first two O–O stretches and the first five HOOH torsional levels are included. This study also addresses two fundamental questions about ROOH photodissociation. The long wavelength math1A:math1B excited state preference has been measured from dynamics experiments, but a Franck–Condon overlap explanation has not been directly verified. A moderate barrier to HOOH torsional motion and excited state dynamics affect the temperature dependence in the UV spectrum. Based on these initial findings for HOOH, photodissociation of large ROOH cannot be eliminated as an important factor for ozone and particulate matter production seen in both ambient and laboratory studies.
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33.20.Lg Ultraviolet spectra
33.20.Tp Vibrational analysis
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.50.Df Potential energy surfaces for excited electronic states
31.15.ag Excitation energies and lifetimes; oscillator strengths
31.15.bw Coupled-cluster theory

Seven-degree-of-freedom, quantum scattering dynamics study of the H2D++H2 reaction

Dunyou Wang, Zhen Xie, and Joel M. Bowman

J. Chem. Phys. 132, 084305 (2010); http://dx.doi.org/10.1063/1.3329730 (7 pages) | Cited 2 times

Online Publication Date: 24 February 2010

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A quantum scattering dynamics, time-dependent wavepacket propagation method is applied to study the reaction of H2D++H2→H3++HD on the Xie–Braams–Bowman potential energy surface. The reduced-dimensional, seven-degree-of-freedom approach is employed in this calculation by fixing one Jacobi and one torsion angle related to H2D+ at the lowest saddle point geometry of D2d on the potential energy surface. Initial state selected reaction probabilities are presented for various initial rovibrational states. The ground state reaction probability shows no threshold for this reaction, in other words, this reaction can occur without an activation barrier. The vibrational excitation shows that the stretching motion of H+–HD only has a small effect on the reaction probability; the vibrational excitation of HD in H2D+ hinders the reactivity. By contrast, rotational excitation of H+–HD greatly enhances the reactivity with the reaction probability increased double or triple at high rotational states compared to the ground state. Reactive resonances, seen in all the initial state selected reaction probabilities, are also found in the integral cross section for the ground state of H2D+ and H2. The thermal rate coefficient is also calculated and is found to be in semiquantitative agreement with experiment; however, quantum scattering approaches including more degrees of freedom, especially including all the angles, are necessary to study this reaction in the future.
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82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.30.Gs Hyperfine interactions and isotope effects
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Tr Kinetic isotope effects including muonium
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Probing the oxygen environment in UO22+ by solid-state math nuclear magnetic resonance spectroscopy and relativistic density functional calculations

Herman Cho, Wibe A. de Jong, and Chuck Z. Soderquist

J. Chem. Phys. 132, 084501 (2010); http://dx.doi.org/10.1063/1.3308499 (7 pages) | Cited 1 time

Online Publication Date: 22 February 2010

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A combined theoretical and solid-state math nuclear magnetic resonance (NMR) study of the electronic structure of the uranyl ion UO22+ in (NH4)4UO2(CO3)3 and rutherfordine (UO2CO3) is presented, the former representing a system with a hydrogen-bonding environment around the uranyl oxygens and the latter exemplifying a uranyl environment without hydrogens. Relativistic density functional calculations reveal unique features of the U–O covalent bond, including the finding of math chemical shift anisotropies that are among the largest for oxygen ever reported (>1200 ppm). Computational results for the oxygen electric field gradient tensor are found to be consistently larger in magnitude than experimental solid-state math NMR measurements in a 7.05 T magnetic field indicate. A modified version of the Solomon theory of the two-spin echo amplitude for a spin-5/2 nucleus is developed and applied to the analysis of the math echo signal of U math22+.
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33.25.+k Nuclear resonance and relaxation
31.15.E- Density-functional theory
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
33.15.Fm Bond strengths, dissociation energies

Mesoscopic nonequilibrium thermodynamics approach to non-Debye dielectric relaxation

Humberto Híjar, J. G. Méndez-Bermúdez, and Iván Santamaría-Holek

J. Chem. Phys. 132, 084502 (2010); http://dx.doi.org/10.1063/1.3314728 (14 pages) | Cited 2 times

Online Publication Date: 22 February 2010

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Mesoscopic nonequilibrium thermodynamics is used to formulate a model describing nonhomogeneous and non-Debye dielectric relaxation. The model is presented in terms of a Fokker–Planck equation for the probability distribution of noninteracting polar molecules in contact with a heat bath and in the presence of an external time-dependent electric field. Memory effects are introduced in the Fokker–Planck description through integral relations containing memory kernels, which in turn are used to establish a connection with fractional Fokker–Planck descriptions. The model is developed in terms of the evolution equations for the first two moments of the distribution function. These equations are solved by following a perturbative method from which the expressions for the complex susceptibilities are obtained as a function of the frequency and the wave number. Different memory kernels are considered and used to compare with experiments of dielectric relaxation in glassy systems. For the case of Cole–Cole relaxation, we infer the distribution of relaxation times and its relation with an effective distribution of dipolar moments that can be attributed to different segmental motions of the polymer chains in a melt.
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77.22.Gm Dielectric loss and relaxation
05.10.Gg Stochastic analysis methods (Fokker-Planck, Langevin, etc.)
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
65.20.De General theory of thermodynamic properties of liquids, including computer simulation

Matrix isolation and computational study of isodifluorodibromomethane (F2CBr–Br): A route to Br2 formation in CF2Br2 photolysis

Lisa George, Aimable Kalume, Patrick Z. El-Khoury, Alexander Tarnovsky, and Scott A. Reid

J. Chem. Phys. 132, 084503 (2010); http://dx.doi.org/10.1063/1.3319567 (8 pages) | Cited 5 times

Online Publication Date: 22 February 2010

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The photolysis products of dibromodifluoromethane (CF2Br2) were characterized by matrix isolation infrared and UV/Visible spectroscopy, supported by ab initio calculations. Photolysis at wavelengths of 240 and 266 nm of CF2Br2:Ar samples ( ∼ 1:5000) held at ∼ 5 K yielded iso-CF2Br2 (F2CBrBr), a weakly bound isomer of CF2Br2, which is characterized here for the first time. The observed infrared and UV/Visible absorptions of iso-CF2Br2 are in excellent agreement with computational predictions at the B3LYP/aug-cc-pVTZ level. Single point energy calculations at the CCSD(T)/aug-cc-pVDZ level on the B3LYP optimized geometries suggest that the isoform is a minimum on the CF2Br2 potential energy surface, lying some 55 kcal/mol above the CF2Br2 ground state. The energies of various stationary points on the CF2Br2 potential energy surface were characterized computationally; taken with our experimental results, these show that iso-CF2Br2 is an intermediate in the Br+CF2Br→CF2+Br2 reaction. The photochemistry of the isoform was also investigated; excitation into the intense 359 nm absorption band resulted in isomerization to CF2Br2. Our results are discussed in view of the rich literature on the gas-phase photochemistry of CF2Br2, particularly with respect to the existence of a roaming atom pathway leading to molecular products.
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82.50.Hp Processes caused by visible and UV light
31.15.A- Ab initio calculations
31.50.-x Potential energy surfaces
33.20.Kf Visible spectra
33.20.Lg Ultraviolet spectra
33.20.Ea Infrared spectra
82.30.Qt Isomerization and rearrangement

Plasticization and antiplasticization of polymer melts diluted by low molar mass species

Evgeny B. Stukalin, Jack F. Douglas, and Karl F. Freed

J. Chem. Phys. 132, 084504 (2010); http://dx.doi.org/10.1063/1.3304738 (11 pages) | Cited 6 times

Online Publication Date: 22 February 2010

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An analysis of glass formation for polymer melts that are diluted by structured molecular additives is derived by using the generalized entropy theory, which involves a combination of the Adam–Gibbs model and the direct computation of the configurational entropy based on a lattice model of polymer melts that includes monomer structural effects. Our computations indicate that the plasticization and antiplasticization of polymer melts depend on the molecular properties of the additive. Antiplasticization is accompanied by a “toughening” of the glass mixture relative to the pure polymer, and this effect is found to occur when the diluents are small species with strongly attractive interactions with the polymer matrix. Plasticization leads to a decreased glass transition temperature Tg and a “softening” of the fragile host polymer in the glass state. Plasticization is prompted by small additives with weakly attractive interactions with the polymer matrix. However, the latter situation can lead to phase separation if the attractive interactions are sufficiently strong. The shifts in Tg of polystyrene diluted by fully flexible short oligomers (up to 20% mass of diluent) are evaluated from the computations, along with the relative changes in the isothermal compressibility at Tg (a softening or toughening effect) to characterize the extent to which the additives act as antiplasticizers or plasticizers. The theory predicts that a decreased fragility can accompany both antiplasticization and plasticization of the glass by molecular additives. The general reduction in the Tg of polymers by molecular additives is rationalized by analyzing the influence of the diluent’s properties (cohesive energy, chain length, and stiffness) on glass formation in fluid mixtures and the variation of fragility is discussed in relation to changes in the molecular packing in diluted polymer melts. Our description of constant temperature glass formation upon increasing the diluent concentration directly leads to the Angell equation (τα ∼ A exp{B/(ϕ0,p−ϕp)}) for the structural relaxation time as function of the polymer concentration, where the extrapolated “zero mobility concentration” ϕ0,p calculated from the theory scales linearly with the inverse polymerization index N.
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81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
82.35.Jk Copolymers, phase transitions, structure
64.70.pj Polymers
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
81.40.Cd Solid solution hardening, precipitation hardening, and dispersion hardening; aging

Quantum hydrodynamics of electron gases

Radomir Slavchov and Roumen Tsekov

J. Chem. Phys. 132, 084505 (2010); http://dx.doi.org/10.1063/1.3328126 (2 pages)

Online Publication Date: 23 February 2010

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Electron gases in metals are described as quantum charged Newtonian viscous fluids experiencing Ohmic Darcy friction on the solid lattice ions as well. The dispersion relation of the electron acoustic waves is derived, which shows the existence of new quantum diffusion processes. The electric double layer near a metal surface is studied, which exhibits a new quantum oscillatory-decaying behavior different from the Friedel oscillations.
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71.10.Ca Electron gas, Fermi gas
73.30.+y Surface double layers, Schottky barriers, and work functions
62.65.+k Acoustical properties of solids
05.30.Ch Quantum ensemble theory
05.30.Fk Fermion systems and electron gas
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