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28 Aug 2010

Volume 133, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 133, 084101 (2010); http://dx.doi.org/10.1063/1.3478526 (6 pages)

Dongshan Wei and Feng Wang
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Communication: Mass-analyzed velocity map imaging of thermal photofragments from C60

Hideki Katayanagi and Koichiro Mitsuke

J. Chem. Phys. 133, 081101 (2010); http://dx.doi.org/10.1063/1.3475515 (4 pages) | Cited 2 times

Online Publication Date: 24 August 2010

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The velocity distributions of the fragments produced by dissociative photoionization of C60 have been measured in the extreme UV region for the first time, by using a flight-time resolved velocity map imaging technique combined with a high-temperature molecular beam and synchrotron radiation. Values of the average kinetic energy release were estimated at six different photon energies with respect to five reaction steps of sequential C2 ejection, starting from C602+→C582++C2 to C522+→C502++C2. The translational temperatures of the fragment ions were found to be lower than those obtained by laser multiphoton absorption of C60. The kinetic energies released in the first to fourth steps increase with increasing hν and reach 0.35–0.5 eV at hν = 102 eV, reflecting statistical redistribution of the excess energy in the transition state, whereas that in the fifth step leading to C502+ was exceptionally small.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Jn Reactivity of clusters
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.20.Lg Ultraviolet spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Ta Mass spectra
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Communication: Mode-selective vibrational excitation induced by nonequilibrium transport processes in single-molecule junctions

Rainer Härtle, Roie Volkovich, Michael Thoss, and Uri Peskin

J. Chem. Phys. 133, 081102 (2010); http://dx.doi.org/10.1063/1.3474464 (4 pages) | Cited 1 time

Online Publication Date: 26 August 2010

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In a nanoscale molecular junction at finite bias voltage, the intramolecular distribution of vibrational energy can strongly deviate from the thermal equilibrium distribution and specific vibrational modes can be selectively excited in a controllable way, regardless of the corresponding mode frequency. This is demonstrated for generic models of asymmetric molecular junctions with localized electronic states, employing a master equation as well as a nonequilibrium Green’s function approach. It is shown that the applied bias voltage controls the excitation of specific vibrational modes by tuning the efficiency of vibrational cooling processes due to energy exchange with the leads.
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85.65.+h Molecular electronic devices
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
73.23.-b Electronic transport in mesoscopic systems
68.35.Ja Surface and interface dynamics and vibrations
73.20.-r Electron states at surfaces and interfaces
81.07.Nb Molecular nanostructures
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Communication: Feshbach resonances in the water molecule revealed by state-selective spectroscopy

Maxim Grechko, Pavlo Maksyutenko, Thomas R. Rizzo, and Oleg V. Boyarkin

J. Chem. Phys. 133, 081103 (2010); http://dx.doi.org/10.1063/1.3472312 (4 pages) | Cited 3 times

Online Publication Date: 26 August 2010

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We employ triple-resonance vibrational overtone excitation to access quasibound states of water from several fully characterized bound states of the molecule. Comparison of the measured dissociation spectra allows a rigorous assignment of rotational quantum numbers J, nuclear spin and parity, and a tentative vibrational characterization of the observed resonances. Their asymmetrical shapes (Fano profiles) reflect interference of dipole moments for transitions to these resonances with that to the dissociative continuum. The assignments and Fano profile parameters of the resonances stand as a benchmark for the extension of accurate quantum-mechanical calculations to activated complexes of water. The narrow widths of some of these resonances indicate that water molecules may survive for as long as up to 60 ps in states above the dissociation threshold. We consider the possible implication of such long-lived states for the kinetics of water dissociation and the OH+H association reaction.
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33.20.Tp Vibrational analysis
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Hp Processes caused by visible and UV light
82.30.Nr Association, addition, insertion, cluster formation
33.70.Jg Line and band widths, shapes, and shifts
33.50.Dq Fluorescence and phosphorescence spectra
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Communication: An extended model of liquid bridging

F. Saija, F. Aliotta, M. E. Fontanella, M. Pochylski, G. Salvato, C. Vasi, and R. C. Ponterio

J. Chem. Phys. 133, 081104 (2010); http://dx.doi.org/10.1063/1.3483690 (4 pages) | Cited 3 times

Online Publication Date: 26 August 2010

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Recent phenomenological studies have drawn attention to an appealing effect, observed for the first time in 1893, today known as water-bridge. The phenomenon has been ascribed to unknown properties of water. We report some experimental results showing that, contrary to a widely common belief, the phenomenon is not to be related with water neither with a property of hydrogen bonded networks. Using a very simple model, we show that the liquid bridge phenomenon is originated by electrostatic effects and can be reproduced in any dense fluid with no respect of its peculiar molecular properties. This basic approach is able to reproduce many of the experimentally observed features of the bridge formation. In perspective of future investigations, the possible phenomena responsible of the bridge stability, after its formation, are briefly discussed.
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61.50.Lt Crystal binding; cohesive energy
82.30.Rs Hydrogen bonding, hydrophilic effects
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Mimicking coarse-grained simulations without coarse-graining: Enhanced sampling by damping short-range interactions

Dongshan Wei and Feng Wang

J. Chem. Phys. 133, 084101 (2010); http://dx.doi.org/10.1063/1.3478526 (6 pages) | Cited 3 times

Online Publication Date: 23 August 2010

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The damped-short-range-interaction (DSRI) method is proposed to mimic coarse-grained simulations by propagating an atomistic scale system on a smoothed potential energy surface. The DSRI method has the benefit of enhanced sampling provided by a typical coarse-grained simulation without the need to perform coarse-graining. Our method was used to simulate liquid water, alanine dipeptide folding, and the self-assembly of dimyristoylphosphatidylcholine lipid. In each case, our method appreciably accelerated the dynamics without significantly changing the free energy surface. Additional insights from DSRI simulations and the promise of coupling our DSRI method with Hamiltonian replica-exchange molecular dynamics are discussed.
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61.20.Ja Computer simulation of liquid structure
87.15.ap Molecular dynamics simulation
87.15.Cc Folding: thermodynamics, statistical mechanics, models, and pathways
87.15.R- Reactions and kinetics
87.14.E- Proteins
87.14.Cc Lipids

Time-dependent auxiliary density perturbation theory

Javier Carmona-Espíndola, Roberto Flores-Moreno, and Andreas M. Köster

J. Chem. Phys. 133, 084102 (2010); http://dx.doi.org/10.1063/1.3478551 (10 pages) | Cited 3 times

Online Publication Date: 23 August 2010

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The recently developed auxiliary density perturbation theory is extended to time-dependent perturbations. As its static counterpart, it is based on auxiliary density functional theory in which the Coulomb and exchange-correlation potentials are expressed through one auxiliary function density. As in the case of static perturbations a noniterative alternative to the corresponding coupled perturbed Kohn–Sham method is formulated. The new methodology is validated by local and gradient corrected dynamical polarizability calculations. Comparison with experiment indicates that for low frequencies reliable dynamical polarizabilities are obtained. Our discussion also shows that the computational performance of time-dependent auxiliary density perturbation theory is similar to the previously described static approach. In order to demonstrate the potential of this new methodology, dynamic polarizabilities of C60, C180, and C240 are calculated.
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31.15.E- Density-functional theory
31.15.xp Perturbation theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
36.40.-c Atomic and molecular clusters

Exact nonadditive kinetic potentials for embedded density functional theory

Jason D. Goodpaster, Nandini Ananth, Frederick R. Manby, and Thomas F. Miller, III

J. Chem. Phys. 133, 084103 (2010); http://dx.doi.org/10.1063/1.3474575 (10 pages) | Cited 15 times

Online Publication Date: 23 August 2010

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We describe an embedded density functional theory (DFT) protocol in which the nonadditive kinetic energy component of the embedding potential is treated exactly. At each iteration of the Kohn–Sham equations for constrained electron density, the Zhao–Morrison–Parr constrained search method for constructing Kohn–Sham orbitals is combined with the King-Handy expression for the exact kinetic potential. We use this formally exact embedding protocol to calculate ionization energies for a series of three- and four-electron atomic systems, and the results are compared to embedded DFT calculations that utilize the Thomas–Fermi (TF) and the Thomas–Fermi–von Weisacker approximations to the kinetic energy functional. These calculations illustrate the expected breakdown due to the TF approximation for the nonadditive kinetic potential, with errors of 30%–80% in the calculated ionization energies; by contrast, the exact protocol is found to be accurate and stable. To significantly improve the convergence of the new protocol, we introduce a density-based switching function to map between the exact nonadditive kinetic potential and the TF approximation in the region of the nuclear cusp, and we demonstrate that this approximation has little effect on the accuracy of the calculated ionization energies. Finally, we describe possible extensions of the exact protocol to perform accurate embedded DFT calculations in large systems with strongly overlapping subsystem densities.
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32.50.+d Fluorescence, phosphorescence (including quenching)
31.15.E- Density-functional theory
31.15.bt Statistical model calculations (including Thomas-Fermi and Thomas-Fermi-Dirac models)

Alchemical derivatives of reaction energetics

Daniel Sheppard, Graeme Henkelman, and O. Anatole von Lilienfeld

J. Chem. Phys. 133, 084104 (2010); http://dx.doi.org/10.1063/1.3474502 (7 pages) | Cited 2 times

Online Publication Date: 24 August 2010

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Based on molecular grand canonical ensemble density functional theory, we present a theoretical description of how reaction barriers and enthalpies change as atoms in the system are subjected to alchemical transformations, from one element into another. The change in the energy barrier for the umbrella inversion of ammonia is calculated along an alchemical path in which the molecule is transformed into water, and the change in the enthalpy of protonation for methane is calculated as the molecule is transformed into a neon atom via ammonia, water, and hydrogen fluoride. Alchemical derivatives are calculated analytically from the electrostatic potential in the unperturbed system, and compared to numerical derivatives calculated with finite difference interpolation of the pseudopotentials for the atoms being transformed. Good agreement is found between the analytical and numerical derivatives. Alchemical derivatives are also shown to be predictive for integer changes in atomic numbers for oxygen binding to a 79 atom palladium nanoparticle, illustrating their potential use in gradient-based optimization algorithms for the rational design of catalysts.
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82.20.-w Chemical kinetics and dynamics
82.60.Cx Enthalpies of combustion, reaction, and formation
82.30.Nr Association, addition, insertion, cluster formation
82.33.Hk Reactions on clusters
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Molecular dynamics simulation with weak coupling to heat and material baths

Hossein Eslami, Fatemeh Mojahedi, and Jalil Moghadasi

J. Chem. Phys. 133, 084105 (2010); http://dx.doi.org/10.1063/1.3474951 (7 pages) | Cited 1 time

Online Publication Date: 25 August 2010

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A method for performing molecular dynamics simulation in the grand canonical ensemble is developed. The molecular dynamics, with coupling to an external bath, simulation method of [ Berendsen et al., J. Chem. Phys. 81, 3684 (1984) ] is extended for this purpose. Here the physical system of interest consists of real indistinguishable particles plus one fractional particle, whose potential energy of interaction with the rest of particles is scaled by a coupling parameter, ranging dynamically between zero and one. This coupling changes the number of particles in the system gradually and dynamically, depending on the target values of the excess chemical potential, temperature, and volume. A nonlinear scaling scheme has been adopted to scale the potential energy of interaction of the fractional particle with the rest of the system. The method has been employed to predict the density of compressed Lennard-Jones fluid, compatible with the target values of temperature and the excess chemical potential, over a wide range of temperatures and densities. The method has further been applied to do molecular dynamics simulation in the grand canonical ensemble for water and to predict its vapor-liquid phase coexistence point. The results obtained using this method are in complete agreement with previously reported results in the literature.
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61.20.Ja Computer simulation of liquid structure
65.20.Jk Studies of thermodynamic properties of specific liquids

Vapor-liquid nucleation of argon: Exploration of various intermolecular potentials

Matthew J. McGrath, Julius N. Ghogomu, Narcisse T. Tsona, J. Ilja Siepmann, Bin Chen, Ismo Napari, and Hanna Vehkamäki

J. Chem. Phys. 133, 084106 (2010); http://dx.doi.org/10.1063/1.3474945 (11 pages) | Cited 3 times

Online Publication Date: 26 August 2010

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The homogeneous vapor-liquid nucleation of argon has been explored at T = 70 and 90 K using classical nucleation theory, semiempirical density functional theory, and Monte Carlo simulations using the aggregation-volume-bias algorithm with umbrella sampling and histogram-reweighting. In contrast with previous simulation studies, which employed only the Lennard-Jones intermolecular potential, the current studies were carried out using various pair potentials including the Lennard-Jones potential, a modified Buckingham exponential-six potential, the Barker–Fisher–Watts pair potential, and a recent ab initio potential developed using the method of effective diameters. It was found that the differences in the free energy of formation of the critical nuclei between the potentials cannot be explained solely in terms of the difference in macroscopic properties of the potentials, which gives a possible reason for the failure of classical nucleation theory.
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64.60.Q- Nucleation
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
64.70.F- Liquid-vapor transitions

Renner–Teller intersections along the collinear axes of polyatomic molecules: H2CN as a case study

Anita Das, Debasis Mukhopadhyay, Satrajit Adhikari, and Michael Baer

J. Chem. Phys. 133, 084107 (2010); http://dx.doi.org/10.1063/1.3479399 (11 pages) | Cited 9 times

Online Publication Date: 31 August 2010

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The tetra-atomic C2H2+ cation is known to form Renner–Teller-type intersections along its collinear axis. Not too long ago, we studied the nonadiabatic coupling terms (NACTs) of this molecule [ G. J. Halász et al., J. Chem. Phys. 126, 154309 (2007) ] and revealed two kinds of intersections. (i) By employing one of the hydrogens as a test particle, we revealed the fact that indeed the corresponding (angular) NACTs produce topological (Berry) phases that are equal to 2π, which is a result anticipated in the case of Renner–Teller intersections. (ii) However, to our big surprise, repeating this study when one of the atoms (in this case a hydrogen) is shifted from the collinear arrangement yields for the corresponding topological phase a value that equals π (and not 2π). In other words, shifting (even slightly) one of the atoms from the collinear arrangement causes the intersection to change its character and become a Jahn–Teller intersection. This somewhat unexpected novel result was later further analyzed and confirmed by other groups [e.g., T. Vertesi and R. Englman, J. Phys. B 41, 025102 (2008) ]. The present article is devoted to another tetra-atomic molecule, namely, the H2CN molecule, which just like the C2H2+ ion, is characterized by Renner–Teller intersections along its collinear axis. Indeed, we revealed the existence of Renner–Teller intersections along the collinear axis, but in contrast to the C2H2+ case a shift of one atom from the collinear arrangement did not form Jahn–Teller intersections. What we found instead is that the noncollinear molecule was not affected by the shift and kept its Renner–Teller character. Another issue treated in this article is the extension of (the two-state) Berry (topological) phase to situations with numerous strongly interacting states. So far the relevance of the Berry phase was tested for systems characterized by two isolated interacting states, although it is defined for any number of interacting states [ M. V. Berry, Proc. R. Soc. London, Ser. A 392, 45 (1984) ]. We intend to show how to overcome this limitation and get a valid, fully justified definition of a Berry phase for an isolated system of any number of interacting states (as is expected).
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33.25.+k Nuclear resonance and relaxation
31.30.Gs Hyperfine interactions and isotope effects
03.65.Ta Foundations of quantum mechanics; measurement theory
02.40.Re Algebraic topology

Global and local Voronoi analysis of solvation shells of proteins

Gregor Neumayr, Tibor Rudas, and Othmar Steinhauser

J. Chem. Phys. 133, 084108 (2010); http://dx.doi.org/10.1063/1.3471383 (13 pages) | Cited 3 times

Online Publication Date: 31 August 2010

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This paper presents the structure and dynamics of hydration shells for the three proteins: ubiquitin, calbindin, and phospholipase. The raw data derived from molecular dynamics simulations are analyzed on the basis of fully atomistic Delaunay tesselations. In order to cope with the high numerical effort for the computation of these Voronoi shells, we have implemented and optimized an intrinsically periodic algorithm. Based on this highly efficient Voronoi decomposition, a variety of properties is presented: three dimensional water and ion nuclear densities as well as the geometrical packing of water molecules are discussed. Thereby, we develop Voronoi interface surface area, the Voronoi analog of the well known solvent accessible surface area. The traditional radial distribution functions are resolved into Voronoi shells as a transient device to the new concept of shell-grained orientational order. Thus, we analyze the donor-acceptor property as well as the amount of dielectric screening. Shell dynamics is described in terms of mean residence times. In this way, a retardation factor for different shells can be derived and was compared to experimental values. All these results and properties are presented both at the global protein level as well as at the local residue level.
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87.15.B- Structure of biomolecules
87.15.ap Molecular dynamics simulation
31.15.xv Molecular dynamics and other numerical methods
87.14.ej Enzymes

Dynamical effects in ab initio NMR calculations: Classical force fields fitted to quantum forces

Mark Robinson and Peter D. Haynes

J. Chem. Phys. 133, 084109 (2010); http://dx.doi.org/10.1063/1.3474573 (9 pages) | Cited 3 times

Online Publication Date: 31 August 2010

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NMR chemical shifts for an L-alanine molecular crystal are calculated using ab initio plane wave density functional theory. Dynamical effects including anharmonicity may be included by averaging chemical shifts over an ensemble of structural configurations generated using molecular dynamics (MD). The time scales required mean that ab initio MD is prohibitively expensive. Yet the sensitivity of chemical shifts to structural details requires that the methodologies for performing MD and calculating NMR shifts be consistent. This work resolves these previously competing requirements by fitting classical force fields to reproduce ab initio forces. This methodology is first validated by reproducing the averaged chemical shifts found using ab initio molecular dynamics. Study of a supercell of L-alanine demonstrates that finite size effects can be significant when accounting for dynamics.
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33.25.+k Nuclear resonance and relaxation
31.15.ae Electronic structure and bonding characteristics
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
33.20.Tp Vibrational analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Comparing modeling and measurements of the output power in chemical oxygen-iodine lasers: A stringent test of I2 dissociation mechanisms

K. Waichman, B. D. Barmashenko, and S. Rosenwaks

J. Chem. Phys. 133, 084301 (2010); http://dx.doi.org/10.1063/1.3480397 (7 pages) | Cited 5 times

Online Publication Date: 23 August 2010

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A parametric study of the output power of supersonic chemical oxygen-iodine lasers (COILs) is carried out, applying a kinetic-fluid dynamics model calculations as well as an analytical model and comparing the results to experimental studies. The I2 dissociation mechanism recently suggested by Azyazov et al. [J. Chem. Phys. 130, 104306 (2009)] , which was previously used for comparison of model calculations to measurements of the small signal gain [ K. Waichman et al., J. Appl. Phys. 106, 063108 (2009) ], is applied here for a similar, but more sensitive, comparison of the laser output power. The dependence of the power on iodine flow rate and on mirror transmission is studied for low and high pressure COILs, respectively. Good agreement between the calculated and measured power is obtained for both low and high pressure COILs only when the processes suggested by Azyazov et al. are included in the calculations. This is different from the situation for the gain where for high pressure COILs, the calculated values were insensitive to the assumed dissociation mechanism, although for low pressure the measurements were reproduced only by applying the Azyazov et al. mechanism. We believe that the results of the present work strongly support the application of this mechanism for modeling the COIL operation.
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42.55.Ks Chemical lasers
42.60.By Design of specific laser systems
47.40.Ki Supersonic and hypersonic flows
47.11.-j Computational methods in fluid dynamics

Photoionization of iodine atoms: Angular distributions and relative partial photoionization cross-sections in the energy region 11.0–23.0 eV

Marie Eypper, Fabrizio Innocenti, Alan Morris, John M. Dyke, Stefano Stranges, John B. West, and George C. King

J. Chem. Phys. 133, 084302 (2010); http://dx.doi.org/10.1063/1.3469798 (9 pages)

Online Publication Date: 24 August 2010

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Relative partial photoionization cross-sections and angular distribution parameters, β, have been measured for the first, I+(3P2)←I(2P3/2), and fourth, I+(1D2)←I(2P3/2), (5p)−1 photoelectron (PE) bands of atomic iodine, by performing angle-resolved constant-ionic-state (CIS) measurements on these PE bands in the photon energy range 11.0–23.0 eV. Three Rydberg series, two ns and one nd series, which converge to the I+ 3P1 limit at 11.33 eV and four Rydberg series, two ns and two nd series, which converge to the I+ 1D2 limit at 12.15 eV were observed in the first PE band CIS spectra. The fourth band CIS spectrum showed structure in the 12.9–14.1eV photon energy range, which is also seen in the first band CIS spectra. This structure arises from excitation to ns and nd Rydberg states that are parts of series converging to the I+ 1S0 limit we reported on earlier, as well as 5s→5p excitations in the photon energy range 17.5–22.5 eV. These atomic iodine CIS spectra show reasonably good agreement with the equivalent spectra obtained for atomic bromine. The β-plots for the first PE band recorded up to the I+ 3P1 and I+ 1D2 limits only show resonances corresponding to some of the 5p→nd excitations observed in the first band CIS spectra scanned to the I+ 1D2 limit (12.15 eV). These plots are interpreted in terms of an angular momentum transfer model with the positive values of β obtained on resonances corresponding to parity allowed jt = 1 and 3 channels and the off-resonance negative β values corresponding to parity unfavored channels, where jt is the quantum number for angular momentum transfer between the molecule, and the ion and photoelectron. The β-plots recorded for iodine are significantly different from those obtained for atomic bromine. Comparison of the experimental CIS spectra and β-plots with available theoretical results highlights the need for higher level calculations which include factors such as configuration interaction in the initial and final states, relativistic effects including spin-orbit interaction, and autoionization via resonant Rydberg states.
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32.80.Fb Photoionization of atoms and ions
32.80.Ee Rydberg states

Negative ion photoelectron spectroscopy of the copper-aspartic acid anion and its hydrated complexes

Xiang Li, Haopeng Wang, Kit H. Bowen, Ana Martínez, Jean-Yves Salpin, and Jean-Pierre Schermann

J. Chem. Phys. 133, 084303 (2010); http://dx.doi.org/10.1063/1.3466923 (6 pages) | Cited 1 time

Online Publication Date: 24 August 2010

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Negative ions of copper-aspartic acid Cu(Asp)− and its hydrated complexes have been produced in the gas phase and studied by anion photoelectron spectroscopy. The vertical detachment energies (VDE) of Cu(Asp)− and Cu(Asp)−(H2O)1,2 were determined to be 1.6, 1.95, and 2.20 eV, respectively. The spectral profiles of Cu(Asp)−(H2O)1 and Cu(Asp)−(H2O)2 closely resembled that of Cu(Asp)−, indicating that hydration had not changed the structure of Cu(Asp)− significantly. The successive shifts to higher electron binding energies by the spectra of the hydrated species provided measures of their stepwise solvation energies. Density functional calculations were performed on anionic Cu(Asp)− and on its corresponding neutral. The agreement between the calculated and measured VDE values implied that the structure of the Cu(Asp)− complex originated with a zwitterionic form of aspartic acid in which a copper atom had inserted into the N–H bond.
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33.60.+q Photoelectron spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
31.15.E- Density-functional theory

Photophysics of fluorinated benzene. I. Quantum chemistry

T. Mondal and S. Mahapatra

J. Chem. Phys. 133, 084304 (2010); http://dx.doi.org/10.1063/1.3465555 (12 pages) | Cited 3 times

Online Publication Date: 25 August 2010

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The electronic structure of energetically low-lying excited singlet states of fluorobenzene molecules is investigated here. Increasing fluorine substitution alters the nature of the excited electronic states and the so-called perfluoro effect is observed for penta- and hexafluorobenzene. Detailed quantum chemistry calculations are carried out at the equation-of-motion coupled-cluster singles and doubles level of theory to establish the potential energy surfaces of the low-lying electronic states of mono-, di- (ortho- and meta-), and pentafluorobenzene molecules. A sequence of low-energy conical intersections among the electronic potential energy surfaces is established. It is found that increasing fluorine substitution lowers the energy of the πσ∗ electronic state and leads to conical intersections between the S1 and S2 electronic states of pentafluorobenzene. Existence of numerous conical intersections among the excited electronic states of these molecules forms the mechanistic details underlying their nonradiative internal conversion. In particular, the slow and biexponential fluorescence emission in pentafluorobenzene is attributed to the existence of low-lying S1-S2 conical intersections. The electronic structure data are analyzed in detail and the coupling mechanism among various electronic excited states of mono-, di-, and pentafluorobenzene molecules is established.
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31.15.bw Coupled-cluster theory
33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion)
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.50.Df Potential energy surfaces for excited electronic states

Photophysics of fluorinated benzene. II. Quantum dynamics

T. Mondal and S. Mahapatra

J. Chem. Phys. 133, 084305 (2010); http://dx.doi.org/10.1063/1.3465557 (13 pages) | Cited 1 time

Online Publication Date: 25 August 2010

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Nuclear dynamics in the coupled electronic states of mono-, di-(ortho and meta), and pentafluorobenzene molecules is investigated here. Attempts are specifically made to understand the complexity and broadening of the recorded gas phase electronic absorption spectra of these molecules. Justification is also provided for the low quantum yield of fluorescence emission with increasing number of fluorine substitutions. The nuclear dynamics is simulated from first principles both by time-independent and time-dependent quantum mechanical methods. Potential energy surfaces of the low-lying excited electronic states of these molecules constructed in Paper I [ Mondal and Mahapatra, J. Chem. Phys. 133, 084304 (2010) ] are employed for the purpose. Theoretical results presented in this paper are compared with the available experimental data and the agreement between the two is found to be excellent. While structured electronic absorption bands are observed for the S1 state of mono- and difluorobenzene molecules, the same for the pentafluorobenzene is broad and structureless. Occurrence of S1-S2 conical intersections in pentafluorobenzene leads to a nonradiative internal conversion of the S1 state in ∼ 165 fs and contributes to the broadening of the S1←S0 absorption band and a biexponential decay of its fluorescence emission.
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33.70.Jg Line and band widths, shapes, and shifts
33.50.Dq Fluorescence and phosphorescence spectra
03.65.-w Quantum mechanics
31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations
33.50.Hv Radiationless transitions, quenching
33.20.-t Molecular spectra

Absorption in the Q-band region by isolated ferric heme+ and heme+(histidine) in vacuo

Jean Ann Wyer and Steen Brøndsted Nielsen

J. Chem. Phys. 133, 084306 (2010); http://dx.doi.org/10.1063/1.3474998 (5 pages) | Cited 4 times

Online Publication Date: 27 August 2010

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Absorption by heme proteins is determined by the heme microenvironment that is often vacuumlike (hydrophobic pocket). Here we provide absorption spectra in the Q-band region of isolated ferric heme+ and heme+(histidine) ions in vacuo to be used as references in protein biospectroscopy. Ions were photoexcited in an electrostatic storage ring and their decay monitored in time. Both ions display a triple band structure with maxima at 500, 518, and 530 nm. Previous attempts to study four-coordinate Fe(III)-heme+ were hampered by the strong affinity of Fe3+ for water and anions. Absorption at higher wavelengths is also measured, which is ascribed to charge-transfer transitions from the porphyrin to the iron. Finally, our data serve to benchmark theoretical calculations.
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87.15.-v Biomolecules: structure and physical properties
33.20.-t Molecular spectra
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
33.80.-b Photon interactions with molecules

Magnetostructural relations from a combined ab initio and ligand field analysis for the nonintuitive zero-field splitting in Mn(III) complexes

Rémi Maurice, Coen de Graaf, and Nathalie Guihéry

J. Chem. Phys. 133, 084307 (2010); http://dx.doi.org/10.1063/1.3480014 (12 pages) | Cited 6 times

Online Publication Date: 30 August 2010

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The zero-field splitting (ZFS) of a model monometallic Mn(III) complex is theoretically studied as function of a systematic symmetry lowering. First, we treat the octahedral case for which the standard math⋅math⋅math model Hamiltonian cannot be applied due to a zero-field splitting in the absence of anisotropy induced by the spin-orbit coupling between the two spatial components of the 5Eg state at second-order of perturbation. Next, the symmetry is lowered to D4h and D2h and the anisotropic spin Hamiltonian is extracted using effective Hamiltonian theory. A simple relation is derived between the ratio E/|D| and the applied rhombic and axial distortions. Moreover, it is shown that close to Oh symmetry, the orbital mixing due to spin-orbit coupling can be accurately described with Stevens fourth-order operators. The calculated tendencies are interpreted within a refined Racah plus ligand field model and it is shown that the ZFS parameters in Mn(III) complexes follow special rules that are nonintuitive compared to other dn configurations. Finally, some angular distortions are applied to study their effect on the anisotropy.
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71.15.-m Methods of electronic structure calculations
75.30.Gw Magnetic anisotropy
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
75.10.Dg Crystal-field theory and spin Hamiltonians

The van der Waals potential of the magnesium dimer

P. Li, W. Xie, and K. T. Tang

J. Chem. Phys. 133, 084308 (2010); http://dx.doi.org/10.1063/1.3479392 (9 pages) | Cited 2 times

Online Publication Date: 31 August 2010

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The ground state van der Waals potential of the magnesium dimer is described by the Tang–Toennies potential model, which requires five essential parameters. Among them, the three dispersion coefficients C6, C8, and C10 are available from accurate ab initio calculations. The other two are the Born–Mayer parameters in A exp(−bR). In this paper, we show that A and b can be determined from the self-consistent Hartree–Fock calculations and the experimental dissociation energy D0. The predicted well depth De and equilibrium distance Re are in nearly perfect agreement with the experiment. In fact, the entire potential energy curve, which is given by a single analytic function, is in excellent agreement with the pointwise potential energies constructed from the spectroscopic measurements in the interval of 6a0–14a0 and in good agreement with the experimental repulsive potential determined from Franck–Condon factors of the bound-free transitions for R less than 6a0. The reduced potential of Mg2 is analyzed in terms of its components, and the number of terms in the dispersion series necessary for convergence is investigated.
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34.20.Cf Interatomic potentials and forces
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.15.Fm Bond strengths, dissociation energies
31.15.ap Polarizabilities and other atomic and molecular properties
31.15.xr Self-consistent-field methods

HY⋯N2 and HXeY⋯N2 complexes in solid xenon (Y = Cl and Br): Unexpected suppression of the complex formation for deposition at higher temperature

Leonid Khriachtchev, Salla Tapio, Markku Räsänen, Alexandra Domanskaya, and Antti Lignell

J. Chem. Phys. 133, 084309 (2010); http://dx.doi.org/10.1063/1.3472976 (7 pages) | Cited 6 times

Online Publication Date: 31 August 2010

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The 1:1 complexes of HY and HXeY (Y = Cl and Br) with nitrogen are characterized by FTIR spectroscopy in a Xe matrix. These complexes show small blue shifts of the HY and H–Xe stretching frequencies with respect to the monomers (ca. +10 cm−1). In the HXeY⋯N2 synthesis procedure, a HY/N2/Xe matrix with HY⋯N2 complexes is first photolyzed at 193 nm to yield isolated H and Y⋯N2 fragments. At the second step, annealing at ca. 40 K activates mobility of H atoms and promotes the H+Xe+Y⋯N2 reaction. It is quite remarkable that the HY⋯N2 and consequently HXeY⋯N2 complexes are observed in Xe matrices deposited at relatively low temperature (below ca. 35 K). For Xe matrices deposited above ca. 40 K, HY molecules do not form a complex with nitrogen and the HXeY⋯N2 complex does not appear after photolysis and annealing; however, this observation is not explained in this article.
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33.20.Ea Infrared spectra
33.70.Jg Line and band widths, shapes, and shifts
82.50.Bc Processes caused by infrared radiation

Electronic-resonance-enhanced coherent anti-Stokes Raman scattering of nitric oxide: Saturation and Stark effects

Ning Chai, Robert P. Lucht, Waruna D. Kulatilaka, Sukesh Roy, and James R. Gord

J. Chem. Phys. 133, 084310 (2010); http://dx.doi.org/10.1063/1.3474702 (20 pages) | Cited 1 time

Online Publication Date: 31 August 2010

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A theoretical analysis of electronic-resonance-enhanced (ERE) coherent anti-Stokes Raman scattering (CARS) of NO is described. The time-dependent density-matrix equations for the nonlinear ERE-CARS process are derived and manipulated into a form suitable for direct numerical integration. In the ERE-CARS configuration considered in this paper, the pump and Stokes beams are far from electronic-resonance. The visible 532 and 591 nm laser beams are used to excite Q-branch Raman resonances in the vibrational bands of the X 2Π electronic state of NO. An ultraviolet probe beam at 236 nm is used to excite P-, Q-, or R-branch transitions in the (v′ = 0, v″ = 1) band of the A 2Σ+−X 2Π electronic system of NO molecule. Experimental spectra are obtained either by scanning the ultraviolet probe beam while keeping the Stokes frequency fixed (probe scans) or by scanning the Stokes frequency while keeping the probe frequency fixed (Stokes scans). The calculated NO ERE-CARS spectra are compared with experimental spectra, and good agreement is observed between theory and experiment in terms of spectral peak locations and relative intensities. The effects of saturation of the two-photon Raman-resonant Q-branch transitions, the saturation of a one-photon electronic-resonant P-, Q-, or R-branch transitions in the A 2Σ+−X 2Π electronic system, and the coupling of these saturation processes are investigated. The coupling of the saturation processes for the probe and Raman transitions is complex and exhibits behavior similar to that observed in the electromagnetic induced transparency process. The probe scan spectra are significantly affected by Stark broadening due to the interaction of the pump and Stokes radiation with single-photon resonances between the upper vibration-rotation probe level in the A 2Σ+ electronic levels and vibration-rotation levels in higher lying electronic levels. The ERE-CARS signal intensity is found to be much less sensitive to variations in the collisional dephasing rates under saturation conditions.
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42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
02.30.Hq Ordinary differential equations
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
36.20.Ng Vibrational and rotational structure, infrared and Raman spectra

Photodissociation of nitromethane cluster anions

Daniel J. Goebbert, Dmitry Khuseynov, and Andrei Sanov

J. Chem. Phys. 133, 084311 (2010); http://dx.doi.org/10.1063/1.3479586 (7 pages) | Cited 2 times

Online Publication Date: 31 August 2010

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Three types of anionic fragments are observed in the photodissociation of nitromethane cluster anions, (CH3NO2)n−, n = 1–6, at 355 nm: NO2−(CH3NO2)k, (CH3NO2)k−, and OH− (k<n). The fragmentation trends are consistent with the parent clusters containing a monomer-anion core, CH3NO2−, solvated by n−1 neutral nitromethane molecules. The NO2−(CH3NO2)k and OH− fragments formed from these clusters are described as core-dissociation products, while the (CH3NO2)k− fragments are attributed to energy transfer from excited CH3NO2− into the solvent network or a core-dissociation—recombination (caging) mechanism. As with other cluster families, the fraction of caged photofragments shows an overall increase with increasing cluster size. The low-lying A2A′ and/or B2A′ electronic states of CH3NO2− are believed responsible for photoabsorption leading to dissociation to NO2− based fragments, while the C2A″ state is a candidate for the OH− pathway. Compared to neutral nitromethane, the photodissociation of CH3NO2− requires lower energy photons because the photochemically active electron occupies a high energy π∗ orbital (which is vacant in the neutral). Although the electronic states in the photodissociation of CH3NO2 and CH3NO2− are different, the major fragments, CH3+NO2 and CH3+NO2−, respectively, both form via C–N bond cleavage.
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82.50.Hp Processes caused by visible and UV light
33.80.Gj Diffuse spectra; predissociation, photodissociation
36.40.Jn Reactivity of clusters
82.30.Nr Association, addition, insertion, cluster formation
82.33.Fg Reactions in clusters
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Melting of Sn at high pressure: Comparisons with Pb

Beate Schwager, Marvin Ross, Stefanie Japel, and Reinhard Boehler

J. Chem. Phys. 133, 084501 (2010); http://dx.doi.org/10.1063/1.3481780 (3 pages)

Online Publication Date: 23 August 2010

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Measurements for Sn, made using the laser-heated diamond cell, are reported that extend the melting curve to 68 GPa and 2300 K. Initially the melting temperature of Sn increases linearly with increasing pressure (dT/dP ∼ 40 K/GPa) and near 38 GPa (2200 K) the melting curve flattens (dT/dP ∼ 0), indicating a zero volume phase change at melting. The results are in good agreement with previously reported shock melting studies. In comparison to Sn the melting curve of Pb is relatively linear to 100 GPa, the highest pressure at which measurements have been made.
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64.70.dj Melting of specific substances
62.50.-p High-pressure effects in solids and liquids
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