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21 Nov 2008

Volume 129, Issue 19, Articles (19xxxx)

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back to top Theoretical Methods and Algorithms
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Precision shooting: Sampling long transition pathways

Michael Grünwald, Christoph Dellago, and Phillip L. Geissler

J. Chem. Phys. 129, 194101 (2008); http://dx.doi.org/10.1063/1.2978000 (8 pages) | Cited 8 times

Online Publication Date: 17 November 2008

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The kinetics of collective rearrangements in solution, such as protein folding and nanocrystal phase transitions, often involve free energy barriers that are both long and rough. Applying methods of transition path sampling to harvest simulated trajectories that exemplify such processes is typically made difficult by a very low acceptance rate for newly generated trajectories. We address this problem by introducing a new generation algorithm based on the linear short time behavior of small disturbances in phase space. Using this “precision shooting” technique, arbitrarily small disturbances can be propagated in time, and any desired acceptance ratio of shooting moves can be obtained. We demonstrate the method for a simple but computationally problematic isomerization process in a dense liquid of soft spheres. We also discuss its applicability to barrier-crossing events involving metastable intermediate states.
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82.30.Qt Isomerization and rearrangement
82.20.Wt Computational modeling; simulation
82.20.Db Transition state theory and statistical theories of rate constants
82.20.Fd Collision theories; trajectory models
82.20.Pm Rate constants, reaction cross sections, and activation energies
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Optimized effective potentials from arbitrary basis sets

Tim Heaton-Burgess and Weitao Yang

J. Chem. Phys. 129, 194102 (2008); http://dx.doi.org/10.1063/1.2982799 (12 pages) | Cited 17 times

Online Publication Date: 17 November 2008

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We investigate the use of a regularized optimized effective potential (OEP) energy functional and L-curve procedure [ T. Heaton-Burgess, F. A. Bulat, and W. Yang, Phys. Rev. Lett. 98, 256401 (2007) ] for determining physically meaningful OEPs from arbitrary combinations of finite orbital and potential basis sets. The important issue of the manner in which the optimal regularization parameter is determined from the L-curve perspective is reconsidered with the introduction of a rigorous measure of the quality of the potential generated—that being, the extent to which the Ghosh–Parr exchange energy virial relation is satisfied along the L-curve. This approach yields nearly identical potentials to our previous work employing a minimum derivative condition, however, gives rise to slightly lower exact-exchange total energies. We observe that the ground-state energy and orbital energies obtained from this approach, either with balanced or unbalanced basis sets, yield meaningful potentials and energies which are in good comparison to other (a priori balanced) finite basis OEP calculations and experimental ionization potentials. As such, we believe that the regularized OEP functional approach provides a computationally robust method to address the numerical stability issues of this often ill-posed problem.
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71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
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Optimal alignment control of a nonpolar molecule through nonresonant multiphoton transitions

Kazuyuki Nakagami, Yoshihiko Mizumoto, and Yukiyoshi Ohtsuki

J. Chem. Phys. 129, 194103 (2008); http://dx.doi.org/10.1063/1.3010369 (9 pages) | Cited 4 times

Online Publication Date: 17 November 2008

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Alignment control of an ensemble of nonpolar molecules is numerically studied by means of optimal control simulation. A nitrogen molecule that is modeled by a quantum rigid rotor is adopted. Controlled rotational wave packets are created through nonresonant optical transitions induced by polarizability coupling. Optimal pulses are designed to achieve the alignment control at a specified time in the absence/presence of external static fields in zero- and finite-temperature cases, as well as to maintain an aligned state. When maintaining an aligned state over a specified time interval is chosen as a target, the control mechanism is primarily attributed to a dynamical one. Multiple optimal solutions that lead to virtually the same control achievement are found, which are consistent with the topology of the quantum control landscape.
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33.80.Wz Other multiphoton processes
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Bh General molecular conformation and symmetry; stereochemistry
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Coarse grained open system quantum dynamics

Ioannis Thanopulos, Paul Brumer, and Moshe Shapiro

J. Chem. Phys. 129, 194104 (2008); http://dx.doi.org/10.1063/1.3010370 (5 pages) | Cited 2 times

Online Publication Date: 17 November 2008

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We show that the quantum dynamics of a system comprised of a subspace Q coupled to a larger subspace P can be recast as a reduced set of “coarse grained” ordinary differential equations with constant coefficients. These equations can be solved by a single diagonalization of a general complex matrix. The method makes no assumptions about the strength of the couplings between the Q and the P subspaces, nor is there any limitation on the initial population in P. The utility of the method is demonstrated via computations in three following areas: molecular compounds, photonic materials, and condensed phases.
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03.65.Yz Decoherence; open systems; quantum statistical methods
02.10.Yn Matrix theory
03.65.Ge Solutions of wave equations: bound states
02.30.Hq Ordinary differential equations
03.65.Vf Phases: geometric; dynamic or topological
03.65.Fd Algebraic methods
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A noniterative perturbative triples correction for the spin-flipping and spin-conserving equation-of-motion coupled-cluster methods with single and double substitutions

Prashant U. Manohar and Anna I. Krylov

J. Chem. Phys. 129, 194105 (2008); http://dx.doi.org/10.1063/1.3013087 (10 pages) | Cited 25 times

Online Publication Date: 18 November 2008

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A noniterative N 7 triples correction for the equation-of-motion coupled-cluster method with single and double substitutions (CCSD) is presented. The correction is derived by second-order perturbation treatment of the similarity-transformed CCSD Hamiltonian. The spin-conserving variant of the correction is identical to the triples correction of Piecuch and co-workers [Mol. Phys. 104, 2149 (2006) ] derived within method-of-moments framework and is not size intensive. The spin-flip variant of the correction is size intensive. The performance of the correction is demonstrated by calculations of electronic excitation energies in methylene, nitrenium ion, cyclobutadiene, ortho-, meta-, and para-benzynes, 1,2,3-tridehydrobenzene, as well as C–C bond breaking in ethane. In all cases except cyclobutadiene, the absolute values of the correction for energy differences were 0.1 eV or less. In cyclobutadiene, the absolute values of the correction were as large as 0.4 eV. In most cases, the correction reduced the errors against the benchmark values by about a factor of 2–3, the absolute errors being less than 0.04 eV.
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31.15.bw Coupled-cluster theory
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Perturbative calculation of spin-orbit splittings using the equation-of-motion ionization-potential coupled-cluster ansatz

Kerstin Klein and Jürgen Gauss

J. Chem. Phys. 129, 194106 (2008); http://dx.doi.org/10.1063/1.3013199 (6 pages) | Cited 5 times

Online Publication Date: 18 November 2008

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Spin-orbit splittings for 2Π states are calculated within coupled-cluster (CC) theory via first-order degenerate perturbation theory. Using the equation-of-motion CC variant for ionization potentials (EOMIP-CC), the two components of the considered 2Π state are treated in a balanced way by generating both radical states via annihilation of one electron out of the CC wave function of the corresponding anion. We report on the implementation of the described approach within the CC singles and doubles approximation. To ensure computational efficiency, an atomic mean-field approximation for the spin-orbit integrals is used, resulting in a formulation in terms of one-electron transition-density matrices. Calculations for XH radicals (X = O, S, Se) lead to satisfactory agreement with experiment. For 2Π systems that within an EOMIP-CC treatment can only be reached from a triplet reference state (e.g., CF and O2+) the influence of spin contamination is found to be negligible.
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31.15.bw Coupled-cluster theory
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.15.xp Perturbation theory
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Magnetic exchange couplings from noncollinear spin density functional perturbation theory

Juan E. Peralta and Veronica Barone

J. Chem. Phys. 129, 194107 (2008); http://dx.doi.org/10.1063/1.3013602 (6 pages) | Cited 3 times

Online Publication Date: 19 November 2008

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We propose a method for the evaluation of magnetic exchange couplings based on noncollinear spin density functional calculations. The method employs the second derivative of the total Kohn–Sham energy of a single reference state, in contrast to approximations based on Kohn–Sham total energy differences. The advantage of our approach is twofold: It provides a physically motivated picture of the transition from a low-spin to a high-spin state, and it utilizes a perturbation scheme for the evaluation of magnetic exchange couplings. The latter simplifies the way these parameters are predicted using first principles: It avoids the nontrivial search for different spin states that needs to be carried out in energy difference methods, and it opens the possibility of “black-boxifying” the extraction of exchange couplings from density functional theory calculations. We present proof of concept calculations of magnetic exchange couplings in the H–He–H model system and in an oxovanadium bimetallic complex where the results can be intuitively rationalized.
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71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
71.15.Nc Total energy and cohesive energy calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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Optical absorption of small silver clusters: Agn, (n = 4–22)

M. Harb, F. Rabilloud, D. Simon, A. Rydlo, S. Lecoultre, F. Conus, V. Rodrigues, and C. Félix

J. Chem. Phys. 129, 194108 (2008); http://dx.doi.org/10.1063/1.3013557 (9 pages) | Cited 23 times

Online Publication Date: 20 November 2008

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We present a joint theoretical and experimental investigation of the absorption spectra of silver clusters Agn (4 ≤ n ≤ 22). The experimental spectra of clusters isolated in an Ar matrix are compared with the calculated ones in the framework of the time-dependent density functional theory. The analysis of the molecular transitions indicates that the s-electrons are responsible for the optical response of small clusters (n ≤ 8) while the d-electrons play a crucial role in the optical excitations for larger n values.
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78.40.Kc Metals, semimetals, and alloys
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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An efficient implementation for determining volume polarization in self-consistent reaction field theory

Marius J. Vilkas and Chang-Guo Zhan

J. Chem. Phys. 129, 194109 (2008); http://dx.doi.org/10.1063/1.3020767 (7 pages) | Cited 3 times

Online Publication Date: 20 November 2008

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An efficient algorithm of the surface and volume polarization for electrostatics (SVPE) method in self-consistent reaction field (SCRF) theory, denoted by SV(1)PE, has been proposed to simulate direct volume polarization potential with a single layer of point charges outside the solute cavity while the indirect effects of volume polarization on surface polarization are still simulated with multiple layers of point charges. The free energies of solvation calculated using the SV(1)PE algorithm (implemented in GAUSSIAN03) reproduce the corresponding values calculated using the standard SVPE implementation within an error of only ∼ 0.1% when the solute cavity is defined by the standard 0.001e/a03 solute charge isodensity contour. The SV(1)PE results are much less sensitive to the used cavity size in comparison with the well-established surface and simulated volume polarization for electrostatics [SS(V)PE] method which simulates volume polarization through an additional surface charge distribution on the cavity surface. The SCRF calculations using the SV(1)PE method are more efficient than those using the original SVPE method.
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82.30.Nr Association, addition, insertion, cluster formation
82.60.-s Chemical thermodynamics
82.20.Db Transition state theory and statistical theories of rate constants
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First-principles study of thermal and electron-activated dissociation of acetone on Si(001)

Jun-Ho Lee, Ji Young Lee, and Jun-Hyung Cho

J. Chem. Phys. 129, 194110 (2008); http://dx.doi.org/10.1063/1.3021075 (6 pages) | Cited 4 times

Online Publication Date: 20 November 2008

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Using first-principles density-functional calculations, we investigate the reaction of acetone on the Si(001) surface, which exhibits the conversion from a kinetically controlled reaction to thermodynamically controlled one by means of thermal anneal or the highly confined electron beam of the scanning tunneling microscopy (STM) tip. We identified the four different reaction pathways forming not only two kinds of di-σ structures on top of a single Si dimer (termed as the [2+2] cycloaddition structure) and across the ends of two adjacent Si dimers but also two bridge-bonded dissociative structures (termed the “end-bridge” and “dimer-bridge” structures) involving two adjacent Si dimers. Our calculated energy profiles for the reaction pathways show not only that formation of the [2+2] cycloaddition structure is kinetically favored because of its low-energy barrier, but also that, as temperature increases, the kinetically favored [2+2] cycloaddition structure is converted to the more thermodynamically stable end-bridge and dimer-bridge structures via an intermediate state where the O atom forms a dative bond to the down Si atom of the buckled dimer. In addition, we find that the Si–C bonding (antibonding) states of the [2+2] cycloaddition structure appear at about 1–2 (2–3) eV below (above) the Fermi level, in which injected holes (electrons) through the STM tip can be created (trapped) to give rise to a Si–C bond breakage. These results manifest that the kinetically controlled reaction of acetone on Si(001) is associated with the [2+2] cycloaddition structure, rather than the α-H cleavage structure proposed by a recent STM experiment.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.20.Db Transition state theory and statistical theories of rate constants
61.72.Cc Kinetics of defect formation and annealing
82.60.-s Chemical thermodynamics
68.43.Fg Adsorbate structure (binding sites, geometry)
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On the accuracy of density-functional theory exchange-correlation functionals for H bonds in small water clusters. II. The water hexamer and van der Waals interactions

Biswajit Santra, Angelos Michaelides, Martin Fuchs, Alexandre Tkatchenko, Claudia Filippi, and Matthias Scheffler

J. Chem. Phys. 129, 194111 (2008); http://dx.doi.org/10.1063/1.3012573 (14 pages) | Cited 52 times

Online Publication Date: 20 November 2008

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Second order Møller–Plesset perturbation theory at the complete basis set limit and diffusion quantum Monte Carlo are used to examine several low energy isomers of the water hexamer. Both approaches predict the so-called prism to be the lowest energy isomer, followed by cage, book, and cyclic isomers. The energies of the four isomers are very similar, all being within 10–15 meV/H2O. These reference data are then used to evaluate the performance of several density-functional theory exchange-correlation (xc) functionals. A subset of the xc functionals tested for smaller water clusters [ I. Santra et al., J. Chem. Phys. 127, 184104 (2007) ] has been considered. While certain functionals do a reasonable job at predicting the absolute dissociation energies of the various isomers (coming within 10–20 meV/H2O), none predict the correct energetic ordering of the four isomers nor does any predict the correct low total energy isomer. All xc functionals tested either predict the book or cyclic isomers to have the largest dissociation energies. A many-body decomposition of the total interaction energies within the hexamers leads to the conclusion that the failure lies in the poor description of van der Waals (dispersion) forces in the xc functionals considered. It is shown that the addition of an empirical pairwise (attractive) C6R−6 correction to certain functionals allows for an improved energetic ordering of the hexamers. The relevance of these results to density-functional simulations of liquid water is also briefly discussed.
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61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids
66.10.C- Diffusion and thermal diffusion
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Quantum trajectory calculations for bipolar wavepacket dynamics in one dimension

Kisam Park, Bill Poirier, and Gérard Parlant

J. Chem. Phys. 129, 194112 (2008); http://dx.doi.org/10.1063/1.3013630 (16 pages) | Cited 5 times

Online Publication Date: 20 November 2008

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Quantum trajectory methods (QTMs) hold great promise as a potential means of obtaining dynamical insight and computational scaling similar to classical trajectory simulations but in an exact quantum dynamical context. To date, the development of QTMs has been stymied by the “node problem”—highly nonclassical and numerically unstable trajectories that arise when the wavepacket density |ψ|2 exhibits substantial interference oscillations. In a recent paper, however [ B. Poirier, J. Chem. Phys. 128, 164115 (2008) ], a “bipolar decomposition,” ψ = ψ++ψ−, was introduced for one-dimensional (1D) wavepacket dynamics calculations such that the component densities |ψ±|2 are slowly varying and otherwise interference-free, even when |ψ|2 itself is highly oscillatory. The bipolar approach is thus ideally suited to a QTM implementation, as is demonstrated explicitly in this paper. Two model 1D benchmark systems exhibiting substantial interference are considered—one with more “quantum” system parameters and the other more classical-like. For the latter, more challenging application, synthetic QTM results are obtained and found to be extremely accurate, as compared to a corresponding fixed-grid calculation. Ramifications of the bipolar QTM approach for the classical limit and also for multidimensional applications, are discussed.
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82.20.Fd Collision theories; trajectory models
82.20.Wt Computational modeling; simulation
03.65.-w Quantum mechanics
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Nonlinear quantum time correlation functions from centroid molecular dynamics and the maximum entropy method

Francesco Paesani and Gregory A. Voth

J. Chem. Phys. 129, 194113 (2008); http://dx.doi.org/10.1063/1.3013365 (10 pages) | Cited 4 times

Online Publication Date: 20 November 2008

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A new approach for the calculation of nonlinear quantum time correlation functions within the path-integral centroid dynamics formalism is presented. This approach combines information on the real-time dynamics obtained from centroid molecular dynamics with classical operators with information on the corresponding imaginary-time dynamics obtained from path-integral molecular dynamics. Nonlinear quantum correlation functions in the high and low temperature regimes computed for model potentials are in remarkably good agreement with the corresponding exact results, suggesting that the method represents an effective and accurate approach for the investigation of general quantum time correlation functions in systems with many degrees of freedom.
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05.70.Ce Thermodynamic functions and equations of state
03.65.Ta Foundations of quantum mechanics; measurement theory
02.70.Ns Molecular dynamics and particle methods
05.45.-a Nonlinear dynamics and chaos
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Exact integral constraint requiring only the ground-state electron density as input on the exchange-correlation force −∂Vxc(r)/∂r for spherical atoms

N. H. March and Á. Nagy

J. Chem. Phys. 129, 194114 (2008); http://dx.doi.org/10.1063/1.3013808 (4 pages) | Cited 1 time

Online Publication Date: 20 November 2008

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Following some studies of ∫ n(r)∇V(r)dr by earlier workers for the density functional theory (DFT) one-body potential V(r) generating the exact ground-state density, we consider here the special case of spherical atoms. The starting point is the differential virial theorem, which is used, as well as the Hiller–Sucher–Feinberg [ Phys. Rev. A 18, 2399 (1978) ] identity to show that the scalar quantity paralleling the above vector integral, namely, ∫ n(r)∂V(r)/∂rdr, is determined solely by the electron density n(0) at the nucleus for the s-like atoms He and Be. The force −∂V/∂r is then related to the derivative of the exchange-correlation potential Vxc(r) by terms involving only the external potential in addition to n(r). The resulting integral constraint should allow some test of the quality of currently used forms of Vxc(r). The article concludes with results from the differential virial theorem and the Hiller–Sucher–Feinberg identity for the exact many-electron theory of spherical atoms, as well as for the DFT for atoms such as Ne with a closed p shell.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
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The CCSD(T) complete basis set limit for Ne revisited

Ericka C. Barnes, George A. Petersson, David Feller, and Kirk A. Peterson

J. Chem. Phys. 129, 194115 (2008); http://dx.doi.org/10.1063/1.3013140 (6 pages) | Cited 10 times

Online Publication Date: 21 November 2008

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Recent estimates of the CCSD(T)(FC) limit for the neon atom (−128.8690±0.001 and −128.8687±0.0005 hartree) are refined. Re-examination of the basis set convergence of the separate self-consistent field, MP2-αβ, MP2-αα, CCSD–MP2, and (T) components of the valence CCSD(T) energy gives a complete basis set limit of −128.869 236±0.000 02 hartree. This can now be used as an improved benchmark to calibrate more approximate calculations.
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31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry
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Analysis of polyatomic molecules using high resolution coherent two-dimensional spectroscopy: Application to nitrogen dioxide

Peter C. Chen and Kamilah Mitchell

J. Chem. Phys. 129, 194301 (2008); http://dx.doi.org/10.1063/1.3009265 (8 pages) | Cited 1 time

Online Publication Date: 17 November 2008

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The peak-sorting capabilities of high resolution coherent two-dimensional (2D) spectroscopy provide a new way of dealing with severe rotational congestion. This paper describes the application of this technique to the polyatomic molecule, NO2. NO2 is a primary component of photochemical smog and has a notoriously complex and congested spectrum that extends from the infrared to the ultraviolet regions. This spectrum is infamous for having an unusually high density of peaks and very few regular patterns. However, the coherent 2D spectrum of NO2 shows a network consisting of numerous X-shaped patterns that mark the locations of vibronic origins. This paper describes how peak sorting leads to the formation of such patterns and how peak coupling can be used to conduct a rotational analysis of congested areas in the visible spectrum of NO2.
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33.20.Ea Infrared spectra
33.20.Lg Ultraviolet spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Kf Visible spectra
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Melting scenario in metallic clusters

P. J. Hsu, J. S. Luo, S. K. Lai, J. F. Wax, and J.-L. Bretonnet

J. Chem. Phys. 129, 194302 (2008); http://dx.doi.org/10.1063/1.3009194 (15 pages) | Cited 7 times

Online Publication Date: 18 November 2008

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The isothermal Brownian-type molecular dynamics simulation has been applied to study the melting behavior of bimetallic clusters. It was found that the specific heat and Lindermann-like parameter customarily used in bulk system to describe solid-liquid transition show incongruity in the predicted melting temperature Tmelt. The underlying mechanisms that lead to the incompatibility of Tmelt separately deduced from these two quantities were analyzed further. To gain insight into the melting behavior, we calculated in addition the velocity autocorrelation function and its Fourier transform, the power spectrum, and extracted from them the Tmelt. It appears that the Tmelt inferred from the latter quantities is closer to that deduced from the principal peak position of specific heat. Two bimetallic clusters, namely, Ag1Cu13 and Au1Cu13, were selected for a thorough investigation. In the context of cluster morphology, we scrutinized the atomic distributions of Ag1Cu13, Au1Cu13, and Cu14 and effected a comparative study between a bimetallic cluster and a pure cluster so as to learn from comparison the differences in the thermal reaction of atoms, in particular, the impurity atom in the bimetallic cluster. On analyzing the dynamical data, we observed at a lower temperature (T⪡Tmelt) migrational relocation of atoms whose dynamics was superimposed at an intermediate temperature (T<Tmelt) by permutations between atoms, and at a higher temperature (T ≈ Tmelt), liquidlike or even gaslike behavior.
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64.70.dj Melting of specific substances
65.20.Jk Studies of thermodynamic properties of specific liquids
65.40.Ba Heat capacity
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Kinetic and theoretical study of the reaction of Cl atoms with a series of linear thiols

Andrés Garzón, José Albaladejo, Alberto Notario, Tomás Peña-Ruiz, and Manuel Fernández-Gómez

J. Chem. Phys. 129, 194303 (2008); http://dx.doi.org/10.1063/1.3012355 (11 pages) | Cited 2 times

Online Publication Date: 19 November 2008

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The reactions of Cl with a series of linear thiols: 1-propanethiol (k1), 1-butanethiol (k2), and 1-pentanethiol (k3) were investigated as a function of temperature (in the range of 268–379 K) and pressure (in the range of 50–200 Torr) by laser photolysis-resonance fluorescence. Only 1-propanethiol has previously been studied, but at 1 Torr of total pressure. The derived Arrhenius expressions obtained using our kinetic data were as follows: k1 = (3.97±0.44)×10−11 exp[(410±36)/T], k2 = (1.01±0.16)×10−10 exp[(146±23)/T], and k3 = (1.28±0.10)×10−10 exp[(129±25)/T] (in units of cm3 molecule−1 s−1). Moreover, a theoretical insight into mechanisms of these reactions has also been pursued through ab initio Möller–Plesset second-order perturbation treatment calculations with 6-311G** basis set. Optimized geometries have been obtained for transition states and molecular complexes appearing along the different reaction pathways. Furthermore, molecular energies have been calculated at QCISD(T) level in order to get an estimation of the activation energies. Finally, the nature of the molecular complexes and transitions states is analyzed by using kinetic-potential and natural bond orbital total energy decomposition schemes.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.80.-d Chemical analysis and related physical methods of analysis
82.50.-m Photochemistry
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Photodissociation dynamics of methyl formate at 193.3 nm: Branching ratios, kinetic-energy distributions, and angular anisotropies of products

Shih-Huang Lee

J. Chem. Phys. 129, 194304 (2008); http://dx.doi.org/10.1063/1.3020761 (8 pages) | Cited 4 times

Online Publication Date: 21 November 2008

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We investigated the photodissociation dynamics of methyl formate-d (CH3OC(O)D) at 193.3 nm in a molecular-beam apparatus using undulator radiation as an ionization source. We measured kinetic-energy distributions, spatial angular anisotropies, and branching ratios of all photofragments. Fractions of energy release into product translation were calculated from the kinetic-energy distributions. Four primary dissociation pathways to asymptotes CH3O(math 2E)+DCO(math 2A′), CH3O(math 2E)+DCO(math 2A″), CH3OCO(math 2A′)+D(2S), and CH3OD(math 1A′)+CO(X 1Σ+) were identified; their branching ratios were determined to be 0.73, 0.06, 0.13, and 0.08, respectively. The former two dissociation paths were discernible in the time-of-flight spectra of fragment CH3O with a signal at m/z = 29. Nominal products DCO (math 2A″) and CH3OCO (math 2A′) were unobservable as DCO in state math dissociated to D(2S)+CO(X 1Σ+) and internally hot CH3OCO (math 2A′) decomposed to CH3(math 2A2″)+CO2(math 1A1g). Products DCO and CH3O have angular anisotropy parameter β ≈ −0.37 but other products have nearly isotropic angular distributions with |β|<0.1. Nonadiabatic transitions might play an important role in fragmentation of methyl formate irradiated at 193.3 nm.
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82.50.Hp Processes caused by visible and UV light
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.80.Dx Analytical methods involving electronic spectroscopy
82.20.-w Chemical kinetics and dynamics
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The multiconfigurational-reference internally contracted configuration interaction/complete basis set study of the excited states of the trifluoride anion F3−

Jiří Czernek and Oldřich Živný

J. Chem. Phys. 129, 194305 (2008); http://dx.doi.org/10.1063/1.3020764 (4 pages) | Cited 2 times

Online Publication Date: 21 November 2008

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The multiconfigurational-reference internally contracted configuration interaction (MRCI)/aug-cc-pVQZ-based computational protocol was employed to search for the minima of the potential energy surface of the low-lying singlet and triplet electronic states of the trifluoride anion F3− in the D∞h, C∞v, C2v, and Cs symmetry groups. The math2 bound state was located (re = 1.8777 Å and θ = 103.39°), which was predicted to lie less than 1 eV above the X mathg+ ground state (re = 1.7382 Å) by both the MRCI and equation-of-motion coupled-cluster singles, doubles and triples approaches [the MRCI adiabatic excitation energy extrapolated to the complete basis set (CBS) limit was 0.91 eV]. The latter value is proposed as a reliable estimate of the singlet-triplet energy gap in F3−. The vertical transitions from the X mathg+ state were analyzed in terms of the reorganization of electrons leading to the excited states and the corresponding MRCI/CBS excitation energies.
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31.50.Df Potential energy surfaces for excited electronic states
34.80.Gs Molecular excitation and ionization
31.15.bw Coupled-cluster theory
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Intermolecular vibrations of different isotopologs of the water dimer: Experiments and density functional theory calculations

J. Ceponkus, P. Uvdal, and B. Nelander

J. Chem. Phys. 129, 194306 (2008); http://dx.doi.org/10.1063/1.3009620 (7 pages) | Cited 8 times

Online Publication Date: 21 November 2008

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Far infrared spectra of seven different isotopologs of the water dimer have been measured in neon matrices at 2.8 K. The experiments are interpreted with the aid of density functional theory calculations, in particular the calculated harmonic isotopic shifts were utilized. All six intermolecular vibrational modes of the water dimer and the fully deuterated water dimer are assigned based the isotopic shifts induced. 31 of a total of 42 intermolecular fundamental modes of the seven different H, D, and 18O containing water dimers have been experimentally observed and assigned accordingly. The overall agreement between the calculations and the experiments of all isotopologs results in a complete and consistent description of these modes.
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31.15.E- Density-functional theory
33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Jg Line and band widths, shapes, and shifts
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Heterogeneous dynamics of ionic liquids from molecular dynamics simulations

J. Habasaki and K. L. Ngai

J. Chem. Phys. 129, 194501 (2008); http://dx.doi.org/10.1063/1.3005372 (15 pages) | Cited 21 times

Online Publication Date: 17 November 2008

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Molecular dynamics simulations have been performed to study the complex and heterogeneous dynamics of ions in ionic liquids. The dynamics of cations and anions in 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO3) are characterized by van Hove functions and the corresponding intermediate scattering functions Fs(k,t) and elucidated by the trajectories augmented by the use of singular spectrum analysis (SSA). Several time regions are found in the mean squared displacement of the ions. Change in the slope in a plot of the diffusion coefficient against temperature is found at around 410 K in the simulation. Heterogeneous dynamics with the presence of both localized ions and fast ions capable of successive jumps were observed at long time scales in the self-part of the van Hove functions and in the trajectories. Non-Gaussian dynamics are evidenced by the self-part of the van Hove functions and wave number dependence of Fs(k,t) and characterized as Lévy flights. Successive motion of some ions can continue even after several nanoseconds at 370 K, which is longer than the onset time of diffusive motion, tdif. Structure of the long time dynamics of fast ions is clarified by the phase space plot of the successive motion using the denoised data by SSA. The continual dynamics are shown to have a long term memory, and therefore local structure is not enough to explain the heterogeneity. The motion connecting localized regions at about 370 K is jumplike, but there is no typical one due to local structural changes during jump motion. With the local motion, mutual diffusion between cation and anion occurs. On decreasing temperature, mutual diffusion is suppressed, which results in slowing down of the dynamics. This “mixing effect of cation and anion” is compared with the “mixed alkali effect” found in the ionics in the ionically conducting glasses, where the interception of paths by different alkali metal ions causes the large reduction in the dynamics [ J. Habasaki and K. L. Ngai, Phys. Chem. Chem. Phys. 9, 4673 (2007) , and references herein]. Although a similar mechanism of the slowing down is observed, strong coupling of the motion of cation and anion prevents complete interception unless deeply supercooled, and this explains the wide temperature region of the existence of the liquid and supercooled liquid states in the ionic liquid.
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61.20.Ja Computer simulation of liquid structure
66.10.Ed Ionic conduction
61.25.Em Molecular liquids
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Isomorphic classical molecular dynamics model for an excess electron in a supercritical fluid

Thomas F. Miller, III

J. Chem. Phys. 129, 194502 (2008); http://dx.doi.org/10.1063/1.3013357 (10 pages) | Cited 9 times

Online Publication Date: 17 November 2008

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Ring polymer molecular dynamics (RPMD) is used to directly simulate the dynamics of an excess electron in a supercritical fluid over a broad range of densities. The accuracy of the RPMD model is tested against numerically exact path integral statistics through the use of analytical continuation techniques. At low fluid densities, the RPMD model substantially underestimates the contribution of delocalized states to the dynamics of the excess electron. However, with increasing solvent density, the RPMD model improves, nearly satisfying analytical continuation constraints at densities approaching those of typical liquids. In the high-density regime, quantum dispersion substantially decreases the self-diffusion of the solvated electron. In this regime where the dynamics of the electron is strongly coupled to the dynamics of the atoms in the fluid, trajectories that can reveal diffusive motion of the electron are long in comparison to βℏ.
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61.20.Ja Computer simulation of liquid structure
66.10.cg Mass diffusion, including self-diffusion, mutual diffusion, tracer diffusion, etc.
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Rovibrational matrix elements of the quadrupole moment of N2 in a solid parahydrogen matrix

Adya P. Mishra and T. K. Balasubramanian

J. Chem. Phys. 129, 194503 (2008); http://dx.doi.org/10.1063/1.3020712 (6 pages) | Cited 1 time

Online Publication Date: 18 November 2008

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The present work pertains to the study of the rotational dynamics of N2 molecules solvated in a matrix of solid para-H2. It is shown that the mixing of the rotational states due to the anisotropic part of the N2–H2 pair potential in the solid gives rise to an additional 5.4% contribution to the intensity of quadrupole-induced double transitions involving N2–H2 pair. Hence the recently reported quadrupole moment matrix element of N2 in a solid para-H2 crystal [ A. P. Mishra and T. K. Balasubramanian, J. Chem. Phys. 125, 124507 (2006) ], which was deduced from a comparison of the theoretical intensity (with rotational mixing of states neglected) with the measured value is larger by ∼ 2.7%. The ground electronic state rovibrational matrix elements 〈v′J′|Q2(r)|vJ〉 of N2 molecule in a solid parahydrogen matrix for v,v′ ≤ 1 and J,J′ ≤ 4 have also been computed by taking into account the changes in the intramolecular potential of N2 due to the intermolecular interaction in the matrix. The computed quadrupole moment matrix elements agree well with a few available values (for v = v′ = 0) deduced from the observed transitions.
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33.20.Vq Vibration-rotation analysis
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.20.Sn Rotational analysis
34.20.Gj Intermolecular and atom-molecule potentials and forces
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Fluids of hard natural and Gaussian ellipsoids: A comparative study by integral equation theories

Aurélien Perera

J. Chem. Phys. 129, 194504 (2008); http://dx.doi.org/10.1063/1.3020337 (9 pages)

Online Publication Date: 18 November 2008

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The hard Gaussian overlap (HGO) model for ellipsoids is compared to the hard ellipsoid of revolution (HER) model, in the isotropic fluid phase and within the framework of the Percus–Yevick (PY) and hypernetted chain (HNC) integral equation theories. The former model is often used in place of the latter in many approximate theories. Since the HGO model slightly overestimates the contact distance when the two ellipsoids are perpendicular to each other, it leads to small differences in the Mayer function of the two models, but nearly none in the integrals of these functions and particularly for the second virial coefficients. However, it leads to notable differences in the pair correlation functions, as obtained by the Percus–Yevick and the hypernetted chain theories, especially at high densities. The prediction of the stability of the isotropic phase with respect to orientational order, at high densities, is notably influenced by these small differences. Both theories predict that, for same aspect ratios, the HGO model overestimates the ordering, when compared to the HER model. This explains why the PY approximation predicts ordering for the HGO model with aspect ratio of 1:3, while it does not for the HER model, in accordance with the very first integral equation results obtained for this system, and at variance with many opposite claims from subsequent publications that used the HGO model in place of the HER model.
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61.20.Gy Theory and models of liquid structure
64.30.-t Equations of state of specific substances
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