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28 Nov 2009

Volume 131, Issue 20, Articles (20xxxx)

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J. Chem. Phys. 131, 204302 (2009); http://dx.doi.org/10.1063/1.3246841 (8 pages)

Helen O. Leung, Mark D. Marshall, and Brent K. Amberger
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Universal nonexponential relaxation: Complex dynamics in simple liquids

David A. Turton and Klaas Wynne

J. Chem. Phys. 131, 201101 (2009); http://dx.doi.org/10.1063/1.3265862 (4 pages) | Cited 4 times

Online Publication Date: 30 November 2009

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The dynamics of the noble-gas liquids underlies that of all liquids making them an important prototypical model system. Using optical Kerr-effect spectroscopy we show that for argon, krypton, and xenon, both the librational and diffusional contributions to the spectrum are surprisingly complex. The diffusional relaxation appears as a stretched-exponential, such as widely found in studies of structured (e.g., glass-forming) liquids and as predicted by mode-coupling theory. We show that this behavior is remarkably similar to that measured in water and suggest that it is a fundamental or universal property.
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66.10.C- Diffusion and thermal diffusion
61.20.-p Structure of liquids
61.25.-f Studies of specific liquid structures
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back to top Theoretical Methods and Algorithms

Obtaining Hartree–Fock and density functional theory doubly excited states with Car–Parrinello density matrix search

Wenkel Liang, Christine M. Isborn, and Xiaosong Li

J. Chem. Phys. 131, 204101 (2009); http://dx.doi.org/10.1063/1.3266564 (6 pages) | Cited 3 times

Online Publication Date: 23 November 2009

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The calculation of doubly excited states is one of the major problems plaguing the modern day excited state workhorse methodology of linear response time dependent Hartree–Fock (TDHF) and density function theory (TDDFT). We have previously shown that the use of a resonantly tuned field within real-time TDHF and TDDFT is able to simultaneously excite both the α and β electrons to achieve the two-electron excited states of minimal basis H2 and HeH+ [ C. M. Isborn and X. Li, J. Chem. Phys. 129, 204107 (2008) ]. We now extend this method to many electron systems with the use of our Car–Parrinello density matrix search (CP-DMS) with a first-principles fictitious mass method for wave function optimization [ X. Li, C. L. Moss, W. Liang, and Y. Feng, J. Chem. Phys. 130, 234115 (2009) ]. Real-time TDHF/TDDFT is used during the application of the laser field perturbation, driving the electron density toward the doubly excited state. The CP-DMS method then converges the density to the nearest stationary state. We present these stationary state doubly excited state energies and properties at the HF and DFT levels for H2, HeH+, lithium hydride, ethylene, and butadiene.
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31.15.xr Self-consistent-field methods
31.15.ee Time-dependent density functional theory
34.80.-i Electron and positron scattering
31.50.Df Potential energy surfaces for excited electronic states

Semiclassical electron correlation operator

Vitaly A. Rassolov

J. Chem. Phys. 131, 204102 (2009); http://dx.doi.org/10.1063/1.3266578 (5 pages) | Cited 1 time

Online Publication Date: 24 November 2009

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The concept of the correlation operator, introduced 10 years ago as a possible method to model the electron correlation effects with single determinant wave functions [ Rassolov, J. Chem. Phys. 110, 3672 (1999) ], is revisited. We derive a semiclassical limit of the correlation operator in weakly correlated systems and give its coordinate space representation. Application of this operator to the atomic systems, such as computations of energies of the neutral atoms, energies of the cations, and spin states energy gaps, demonstrates capabilities and limitations of this concept.
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31.15.V- Electron correlation calculations for atoms, ions and molecules

Quantum mechanical corrections to simulated shock Hugoniot temperatures

Nir Goldman, Evan J. Reed, and Laurence E. Fried

J. Chem. Phys. 131, 204103 (2009); http://dx.doi.org/10.1063/1.3262710 (8 pages) | Cited 3 times

Online Publication Date: 25 November 2009

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We present a straightforward method for the inclusion of quantum nuclear vibrational effects in molecular dynamics calculations of shock Hugoniot temperatures. Using a Grüneisen equation of state and a quasiharmonic approximation to the vibrational energies, we derive a simple, postprocessing method for calculation of the quantum corrected Hugoniot temperatures. We have used our novel technique on ab initio simulations of shock compressed water and methane. Our results indicate significantly closer agreement with all available experimental temperature data for these two systems. Our formalism can be easily applied to a number of different shock compressed molecular liquids or solids, and has the potential to decrease the large uncertainties inherent in many experimental Hugoniot temperature measurements of these systems.
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64.30.-t Equations of state of specific substances
61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids

Comparative study of multireference perturbative theories for ground and excited states

Mark R. Hoffmann, Dipayan Datta, Sanghamitra Das, Debashis Mukherjee, Ágnes Szabados, Zoltán Rolik, and Péter R. Surján

J. Chem. Phys. 131, 204104 (2009); http://dx.doi.org/10.1063/1.3265769 (11 pages) | Cited 6 times

Online Publication Date: 25 November 2009

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Three recently developed multireference perturbation theories (PTs)—generalized Van Vleck PT (GVVPT), state-specific multireference PT (SS-MRPT), and multiconfiguration PT (MCPT)–are briefly reviewed and compared numerically on representative examples, at the second order of approximations. We compute the dissociation potential curve of the LiH molecule and the BeH2 system at various geometries, both in the ground and in the first excited singlet state. Furthermore, the ethylene twisting process is studied. Both Møller–Plesset (MP) and Epstein–Nesbet partition are used for MCPT and SS-MRPT, while GVVPT uses MP partitioning. An important thrust in our comparative study is to ascertain the degree of interplay of dynamical and nondynamical correlation for both ground and excited states. The same basis set and the same set of orbitals are used in all calculations to keep artifactual differences away when comparing the results. Nonparallelity error is used as a measure of the performance of the respective theories. Significant differences among the three methods appear when an intruder state is present. Additionally, difficulties arise (a) in MCPT when the choice of a pivot determinant becomes problematic, and (b) in SS-MRPT when there are small coefficients of the model function and there is implicit division by these coefficients, which generates a potential instability of the solutions. Ways to alleviate these latter shortcomings are suggested.
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31.15.xp Perturbation theory
31.50.Df Potential energy surfaces for excited electronic states
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Fm Bond strengths, dissociation energies

Symmetries of the master equation and long-lived states of nuclear spins

Alexander A. Karabanov, Christian Bretschneider, and Walter Köckenberger

J. Chem. Phys. 131, 204105 (2009); http://dx.doi.org/10.1063/1.3265852 (10 pages) | Cited 2 times

Online Publication Date: 25 November 2009

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In terms of the master equation of relaxation, long-lived states in systems of n interacting spin-1/2 nuclei are described as those with eigenvalues much less than the characteristic T1 rate. It is demonstrated that degeneracies of correlations between interacting dipole-dipole pairs are responsible for their presence. The case n = 3 is considered in full detail.
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33.25.+k Nuclear resonance and relaxation
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Fourier transform microwave spectroscopy and molecular structure of the 1,1-difluoroethylene-hydrogen fluoride complex

Helen O. Leung, Mark D. Marshall, Tasha L. Drake, Tadeuz Pudlik, Nazir Savji, and Daniel W. McCune

J. Chem. Phys. 131, 204301 (2009); http://dx.doi.org/10.1063/1.3250865 (8 pages) | Cited 5 times

Online Publication Date: 23 November 2009

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Fourier transform microwave rotation spectra in the 7–21 GHz region are obtained for the complex formed between 1,1-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted 13C species obtained in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used, in combination with information from the analogous complex, 1,1-difluoroethylene-acetylene, to determine a structure for CH2CF2–HF. This structure is similar to that obtained for vinyl fluoride-HF [ G. C. Cole and A. C. Legon, Chem. Phys. Lett. 400, 419 (2004) ] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,1-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. A comparison of the structures of 1,1-difluoroethylene complexes with the protic acids HF, HCl, and HCCH demonstrates that the hydrogen bond length increases with decreasing gas-phase acid strength, whereas a comparison of HF complexes with vinyl fluoride, 1,1-difluoroethylene, and 1,1,2-trifluoroethylene indicates that the nucleophilicity of the F atoms decreases with increasing fluorine substitution, but that the secondary interaction length is remarkably similar in all three complexes.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Pw Fine and hyperfine structure
33.15.Mt Rotation, vibration, and vibration-rotation constants
34.20.Gj Intermolecular and atom-molecule potentials and forces
33.15.Fm Bond strengths, dissociation energies
33.15.Dj Interatomic distances and angles

Fourier transform microwave spectroscopy and molecular structure of the trans-1,2-difluoroethylene-hydrogen fluoride complex

Helen O. Leung, Mark D. Marshall, and Brent K. Amberger

J. Chem. Phys. 131, 204302 (2009); http://dx.doi.org/10.1063/1.3246841 (8 pages) | Cited 3 times

Online Publication Date: 23 November 2009

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Guided by ab initio calculations, Fourier transform microwave rotation spectra in the 6.5–22 GHz region are obtained for the complex formed between trans-1,2-difluoroethylene and hydrogen fluoride, including the normal isotopomer and two singly substituted 13C species in natural abundance. Spectra are also obtained for the analogous three species formed using deuterium fluoride. Analysis of the spectra provides rotational and hyperfine constants that are used to determine a structure for trans-CHFCHF–HF. This structure is similar to that obtained for 1,1-difluoroethylene-HF [ H. O. Leung et al., J. Chem. Phys. 131, 204301 (2009) ] in that a primary, hydrogen bonding interaction exists between the HF donor and a F atom acceptor on the 1,2-difluoroethylene moiety, while a secondary interaction occurs between the F atom on the HF molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene and causes the hydrogen bond to deviate from linearity. Because the two F atoms and the two H atoms in trans-1,2-difluoroethylene form electrostatically equivalent pairs, the structure of the complex with HF provides insight into the contribution of steric effects to the observed geometries of fluoroethylene-protic acid complexes. A comparison of the observed hydrogen bond lengths and deviations from linearity in 1,1-difluoroethylene-HF and trans-1,2-difluoroethylene-HF suggests that the F atoms in trans-1,2-difluoroethylene are more nucleophilic than those in 1,1-difluoroethylene and that the H atoms are similarly more acidic. Ab initio calculations of electrostatic potentials mapped onto total electron density surfaces for these two molecules support these conclusions.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.A- Ab initio calculations
31.15.ae Electronic structure and bonding characteristics
31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
33.15.Pw Fine and hyperfine structure

Construction of theoretical hybrid potential energy curves for LiH(X 1Σ+)

Ian L. Cooper and Alan S. Dickinson

J. Chem. Phys. 131, 204303 (2009); http://dx.doi.org/10.1063/1.3259841 (9 pages) | Cited 1 time

Online Publication Date: 23 November 2009

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Various all-electron and valence-electron potential energy curves for LiH(X 1Σ+) are compared and assessed. Hybrid potential energy curves are constructed from all-electron potentials at short range and a valence-electron calculation otherwise. This approach provides for the X state of LiH an overall potential curve, which is ionic at equilibrium, and presents an avoided crossing with the excited A state, leading to neutral dissociation products. The classical turning points predicted by these purely theoretical hybrid potentials are compared with those of the experimentally based inverted-perturbation approach (IPA) potentials for both 7LiH and 7LiD. Predicted vibrational energy-level spacings show reasonable (≲1 cm−1) agreement with the corresponding IPA values. Rotation and vibration-rotation transition energies arising from the most accurate hybrid potential are shown to compare very favorably with recent high-resolution spectroscopic data on 7LiH and 7LiD.
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31.50.Df Potential energy surfaces for excited electronic states
34.50.Ez Rotational and vibrational energy transfer
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis

H atom formation from benzene and toluene photoexcitation at 248 nm

Tamás Kovács, Mark A. Blitz, Paul W. Seakins, and Michael J. Pilling

J. Chem. Phys. 131, 204304 (2009); http://dx.doi.org/10.1063/1.3262701 (12 pages) | Cited 5 times

Online Publication Date: 23 November 2009

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The technique of excimer laser excitation/Lyman alpha H atom laser induced fluorescence was used to investigate the formation of H atoms from the 248 nm photoexcitation of benzene and toluene. The H atom signal dependence on laser excitation energy demonstrated that it is produced from two photon photolysis of the aromatics; absorption of the first photon populates the bound 1B2u level followed by absorption from this level to a dissociative level, which produces H atoms, among other potential channels. Analysis of the data yields the second photon absorption cross section to produce H and is equal to 1.0 and 5.2×10−19 cm2 for benzene and toluene, respectively. In addition, the yield of H atoms was observed to be pressure dependent. This is because at sufficiently high pressures the nanosecond lifetime of the 1B2u state can be pressure quenched and hence may compete with the absorption of the second photon. The yields of H atoms were determined as a function of pressure for a range of the laser energies and with various collider gases. The analysis of these data allowed the total absorption cross section for the second photon to be determined and is equal to 2.8 and 1.7×10−17 cm2 for benzene and toluene, respectively. In addition, the rate constants for quenching 1B2u with various gases (He, Ar, N2, and O2) were determined. This large absorption coefficient for the second photon implies that with a pulsed laser source of 248 nm it is difficult to avoid aromatic photodissociation. We highlight a few previous studies that may need to be reevaluated in the light of the results from this study.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.50.Dq Fluorescence and phosphorescence spectra

Binary rototranslational hyper-Rayleigh spectra of H2–He gas mixture

J.-L. Godet, T. Bancewicz, W. Głaz, G. Maroulis, and A. Haskopoulos

J. Chem. Phys. 131, 204305 (2009); http://dx.doi.org/10.1063/1.3264691 (6 pages) | Cited 6 times

Online Publication Date: 23 November 2009

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The collision-induced rototranslational hyper-Rayleigh spectra of gaseous H2–He mixture are computed and discussed in the binary regime. As the input data we use our ab initio computed H2–He collision-induced first dipole hyperpolarizability tensor Δβ(R). Both the vector and the septor part of the H2–He hyper-Rayleigh spectra are evaluated at room temperature (T = 295 K). The spectra are calculated assuming the full quantum computations based on the Schrödinger equation of the relative translational motion in the isotropic H2–He potential as well as using semiclassical methods.
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33.20.Fb Raman and Rayleigh spectra (including optical scattering)
34.50.Ez Rotational and vibrational energy transfer
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility

The effect of intracluster photoelectron interactions on the angular distribution in I−⋅CH3I photodetachment

Matthew Van Duzor, Jie Wei, Foster Mbaiwa, and Richard Mabbs

J. Chem. Phys. 131, 204306 (2009); http://dx.doi.org/10.1063/1.3266936 (8 pages) | Cited 9 times

Online Publication Date: 23 November 2009

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I− and I−⋅CH3I velocity mapped photodetachment images are recorded over wavelengths between 270 and 370 nm. Spectral similarities, in conjunction with ab initio calculations show that the cluster anion comprises an atomic iodide anion moiety solvated by a relatively unperturbed CH3I molecule. Between 340–370 nm and at 280 nm, free I− is produced via a process analogous to dissociative electron attachment within the cluster anion. More strikingly, the photoelectron angular distribution for each species at a given electron kinetic energy is very different, despite detachment occurring from the iodide 5p orbital in each case. These observations reveal the effect of interaction of the photoelectron with the neutral cluster residue and are discussed in terms of resonances associated primarily with the CH3I molecule.
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33.80.Eh Autoionization, photoionization, and photodetachment
31.15.A- Ab initio calculations
33.60.+q Photoelectron spectra
36.40.Mr Spectroscopy and geometrical structure of clusters
33.80.Gj Diffuse spectra; predissociation, photodissociation

Heavy atom nitroxyl radicals. III. Identification of the Cl2P = S free radical in the gas phase by laser spectroscopy and ab initio calculations

Jie Yang, Xiaopeng Zhang, Dennis J. Clouthier, and Riccardo Tarroni

J. Chem. Phys. 131, 204307 (2009); http://dx.doi.org/10.1063/1.3266944 (6 pages) | Cited 5 times

Online Publication Date: 24 November 2009

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The dichlorothiophosphoryl (Cl2PS) free radical has been identified in the gas phase for the first time by a combination of laser-induced fluorescence and single vibronic level emission spectroscopy. High level ab initio calculations of the properties of the ground and first two excited states have been undertaken to aid in the interpretation of the data. The radicals were produced by an electric discharge through a dilute mixture of Cl3PS in high pressure argon at the exit of a pulsed supersonic expansion. An extensive band system was observed in the 760–560 nm region and has been assigned as the math 2A′-math 2A′ electronic transition in which an electron in the ground state π orbital is promoted to the π∗ orbital. Three excited state and four ground state vibrational frequencies and their chlorine isotope effects have been measured and found to be in good accord with the theoretical predictions. Theory indicates that the radical is nonplanar in the ground state with increased pyramidalization and a longer PS bond in the first (math 2A″ n-π∗) and second excited electronic states.
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33.50.Dq Fluorescence and phosphorescence spectra
31.15.ae Electronic structure and bonding characteristics
31.30.Gs Hyperfine interactions and isotope effects
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Fm Bond strengths, dissociation energies

S1/S2 excitonic splittings and vibronic coupling in the excited state of the jet-cooled 2-aminopyridine dimer

Philipp Ottiger, Samuel Leutwyler, and Horst Köppel

J. Chem. Phys. 131, 204308 (2009); http://dx.doi.org/10.1063/1.3266937 (11 pages) | Cited 6 times

Online Publication Date: 24 November 2009

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We analyze the vibronic band structure of the excitonically coupled S1←S0/S2←S0 excitations of the 2-aminopyridine (2AP) self-dimer (2AP)2, using a linear vibronic coupling model [ R. Fulton and M. Gouterman, J. Chem. Phys. 41, 2280 (1964) ]. The vibronic spectra of supersonically cooled (2AP)2 and its 13C-isotopomer were measured by two-color resonant two-photon ionization and UV/UV-depletion spectroscopies. In the C2-symmetric form of (2AP)2, the S1←S0 (1A←1A) transition is very weak, while the close-lying S2←S0 (1B←1A) transition is fully allowed. A single 12C/13C isotopic substitution breaks the symmetry of the dimer so that the (2AP)2−13C isotopologue exhibits both S1 and S2 electronic origins, which are split by 11 cm−1. In Fulton–Gouterman-type treatments, the linear vibronic coupling is mediated by intramolecular vibrational modes and couplings to intermolecular vibrations are not considered. For (2AP)2, a major vibronic coupling contribution arises from the intramolecular 6a′ vibration. However, the low-energy part of the spectrum is dominated by intermolecular shear (χ′) and stretching (σ′) vibrational excitations that also exhibit excitonic splittings; we apply a linear vibronic coupling analysis for these also. The respective excitation transfer integrals VAB are 50%–80% of that of the intramolecular 6a′ vibration, highlighting the role of intermolecular vibrations in mediating electronic energy exchange. The S1/S2 electronic energy gap calculated by the approximate second-order coupled-cluster method is ∼ 340 cm−1. This purely electronic exciton splitting is quenched by a factor of 40 by the vibronic couplings to the Franck–Condon active intramolecular vibrations.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.20.Lg Ultraviolet spectra
33.20.Tp Vibrational analysis
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Accurate analytic potentials for Li2(X 1Σg+) and Li2(A 1Σu+) from 2 to 90 Å, and the radiative lifetime of Li(2p)

Robert J. Le Roy, Nikesh S. Dattani, John A. Coxon, Amanda J. Ross, Patrick Crozet, and Colan Linton

J. Chem. Phys. 131, 204309 (2009); http://dx.doi.org/10.1063/1.3264688 (17 pages) | Cited 13 times

Online Publication Date: 25 November 2009

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Extensions of the recently introduced “Morse/long-range” (MLR) potential function form allow a straightforward treatment of a molecular state for which the inverse-power long-range potential changes character with internuclear separation. Use of this function in a direct-potential-fit analysis of a combination of new fluorescence data for 7,7Li2, 6,6Li2, and 6,7Li2 with previously reported data for the A(1Σu+) and X(1Σg+) states yields accurate, fully analytic potentials for both states, together with the analytic “adiabatic” Born–Oppenheimer breakdown radial correction functions which are responsible for the difference between the interaction potentials and well depths for the different isotopologues. This analysis yields accurate well depths of mathe = 8516.709(±0.004) and 8516.774(±0.004) cm−1 and scattering lengths of 18.11(±0.05) and 23.84(±0.05) Å for the ground-states of 7,7Li2 and 6,6Li2, respectively, as well as improved atomic radiative lifetimes of τ(2p) = 27.1018(±0.0014) ns for 7Li(2p) and 27.1024(±0.0014) ns for 6Li(2p).
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31.50.Df Potential energy surfaces for excited electronic states
32.70.Cs Oscillator strengths, lifetimes, transition moments
33.20.Tp Vibrational analysis
34.20.Cf Interatomic potentials and forces

Clusters of classical water models

Péter T. Kiss and András Baranyai

J. Chem. Phys. 131, 204310 (2009); http://dx.doi.org/10.1063/1.3266838 (14 pages) | Cited 9 times

Online Publication Date: 25 November 2009

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The properties of clusters can be used as tests of models constructed for molecular simulation of water. We searched for configurations with minimal energies for a small number of molecules. We identified topologically different structures close to the absolute energy minimum of the system by calculating overlap integrals and enumerating hydrogen bonds. Starting from the dimer, we found increasing number of topologically different, low-energy arrangements for the trimer(3), the tetramer(6), the pentamer(6), and the hexamer(9). We studied simple models with polarizable point dipole. These were the BSV model [ J. Brodholt et al., Mol. Phys. 86, 149 (1995) ], the DC model [ L. X. Dang and T. M. Chang, J. Chem. Phys. 106, 8149 (1997) ], and the GCP model [ P. Paricaud et al., J. Chem. Phys. 122, 244511 (2005) ]. As an alternative the SWM4-DP and the SWM4-NDP charge-on-spring models [ G. Lamoureux et al., Chem. Phys. Lett. 418, 245 (2006) ] were also investigated. To study the impact of polarizability restricted to the plane of the molecule we carried out calculations for the SPC-FQ and TIP4P-FQ models, too [ S. W. Rick et al., J. Chem. Phys. 101, 6141 (1994) ]. In addition to them, justified by their widespread use even for near critical or surface behavior calculations, we identified clusters for five nonpolarizable models of ambient water, SPC/E [ H. J. C. Berendsen et al., J. Phys. Chem. 91, 6269 (1987) ], TIP4P [ W. L. Jorgensen et al., J. Chem. Phys. 79, 926 (1983) ], TIP4P-EW [ H. W. Horn et al., J. Chem. Phys. 120, 9665 (2004) ], and TIP4P/2005 [ J. L. F. Abascal and C. Vega, J. Chem. Phys. 123, 234505 (2005) ]. The fifth was a five-site model named TIP5P [ M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910 (2000) ]. To see the impact of the vibrations we studied the flexible SPC model. [ K. Toukan and A. Rahman, Phys. Rev. B 31, 2643 (1985) ]. We evaluated the results comparing them with experimental data and quantum chemical calculations. The position of the negative charge in the models plays a crucial role. In this respect models with SPC geometry provided structures different from the TIP4P-type potentials, including polarizable ones. The TIP4P variants form configurations similar to one another. Results for TIP4P-EW and for TIP4P/2005 were especially close to each other in every respect. This is also true for the BSV and the DC pair. The charge-on-spring models (SWM4-DP and SWM4-NDP) are also very similar to each other, despite the sign exchange of charges on the spring particle and the oxygen. The spherical polarization of water is crucial. Due to the planar polarization of the SPC-FQ and the TIP4P-FQ models, they prefer planar arrangements contrary to other polarizable models and quantum chemical calculations. The tetrahedral geometry of TIP5P stabilizes additional clusters with peculiar geometries and small O–O distances. Inclusion of vibrations causes only insignificant changes in the characteristic geometries but decreases the internal energy relative to its reference rigid version. Comparing with quantum mechanical calculations the GCP model provided the best overall results.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Fm Bond strengths, dissociation energies
31.15.xv Molecular dynamics and other numerical methods

UV/visible spectroscopy of matrix-isolated hexa-peri-hexabenzocoronene: Interacting electronic states and astrophysical context

Gaël Rouillé, Mathias Steglich, Friedrich Huisken, Thomas Henning, and Klaus Müllen

J. Chem. Phys. 131, 204311 (2009); http://dx.doi.org/10.1063/1.3266939 (7 pages) | Cited 8 times

Online Publication Date: 25 November 2009

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Absorption spectra of hexa-peri-hexabenzocoronene isolated in rare-gas matrices are reported for the wavelength range between 200 and 500 nm. Measurements were carried out in neon and in argon at 5.8 and 12.0 K, respectively. Calculations based on semiempirical models and on density-functional theory were performed to assign the observed features. The electronically excited states involved in Clar’s α- and p-bands are identified as S1(B2u) and S2(B1u), respectively. Although the upper state associated with the β-band is found to be a 1E1u state, it remains undetermined whether it is S3 or S4. Structures in the β-band are interpreted as resulting from the interaction between the 1E1u state and the e2g vibrational manifold of S2(B1u). The new measurements are used to narrow down the wavelength ranges where the bands of hexa-peri-hexabenzocoronene should be found in the gas phase. A previous estimate of the interstellar abundance of this polycyclic aromatic hydrocarbon is discussed.
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33.20.Lg Ultraviolet spectra
33.20.Kf Visible spectra
95.30.Dr Atomic processes and interactions
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

Direct absorption spectroscopy of water clusters formed in a continuous slit nozzle expansion

A. Moudens, R. Georges, M. Goubet, J. Makarewicz, S. E. Lokshtanov, and A. A. Vigasin

J. Chem. Phys. 131, 204312 (2009); http://dx.doi.org/10.1063/1.3264576 (11 pages) | Cited 8 times

Online Publication Date: 25 November 2009

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In this article, we report on a Fourier transform infrared study of absorption bands belonging to small-sized water clusters formed in a continuous slit nozzle expansion of water vapor seeded in argon carrier gas. Clear signatures of free and H-bonded OH vibrations in water aggregates from dimer to pentamer are seen in our spectra. Following an increase in argon backing pressure, the position of the cluster absorption bands varies from those characteristics of isolated water aggregates in the gas phase to those known for clusters trapped in a static argon matrix. These variations can be interpreted in terms of sequential solvation of the water clusters by an increasing number of argon atoms attached to water clusters. Our measured spectra are in good agreement with those obtained previously either for free or Ar coated small-sized water clusters using pulsed slit-jet expansions. Our results are equally in accord with those originating from a variety of tunable laser based techniques using molecular beams or free jets or from the study of water aggregates embedded in rare gas matrices. Distinctions are reported, however, and discussed. Ab initio calculations have made it possible to speculate on the average size of an argon solvation shell around individual clusters as well as on the development of the OH stretch vibrational shifts in mixed (H2O)mArn clusters having different compositions and architectures.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.ae Electronic structure and bonding characteristics
33.15.Fm Bond strengths, dissociation energies

Structural and electronic properties of oxidized sodium clusters: A combined photoelectron and density functional study

Kiran Majer, Ma Lei, Christian Hock, Bernd von Issendorff, and Andrés Aguado

J. Chem. Phys. 131, 204313 (2009); http://dx.doi.org/10.1063/1.3267056 (10 pages) | Cited 5 times

Online Publication Date: 30 November 2009

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Oxygen-doped sodium cluster anions NanO2− with n = 41–148 have been studied by low temperature photoelectron spectroscopy and density functional theory (DFT), with a particular emphasis on those sizes where a spherical electron shell closing is expected. The experimental spectra are in good agreement with the electronic density of states of the DFT lowest energy structures. The cluster structures show segregation between an ionically bonded molecular unit located at the cluster surface and a metallic part. The DFT calculations reveal that each oxygen atom removes two electrons from the metallic electron gas in order to become an O2− dianion. A jellium model would therefore predict the electron shell closings to be shifted up by four sodium atoms with respect to pure Nan− cluster anions. The electron shell closings for NanO2− are located at n = 43, 61, 93, and 139, so the expected four-atom shift is observed only for the small clusters of up to n = 61, while a two-atom shift is observed for the larger clusters. The DFT calculations explain this departure from jellium model predictions in terms of a structural transition in the ionically bonded molecular unit.
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81.05.Bx Metals, semimetals, and alloys
64.70.kd Metals and alloys
71.20.Be Transition metals and alloys
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
79.60.Bm Clean metal, semiconductor, and insulator surfaces
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)

Further aspects on the control of photodissociation in light-induced potentials

Bo Y. Chang, Seokmin Shin, and Ignacio R. Sola

J. Chem. Phys. 131, 204314 (2009); http://dx.doi.org/10.1063/1.3266960 (6 pages) | Cited 4 times

Online Publication Date: 30 November 2009

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In this work we show how to control the photodissociation of a diatomic molecule in the frame of light-induced potentials for different shapes of the transition dipole moments. A sequence of a half-cycle or control pulse and a delayed pump pulse is used for achieving state-selective photodissociation with high yields. The effect of the control is to shift the photodissociation bands to higher frequencies. It is also possible to dissociate the molecule in a superposition of electronic states of the fragments, even when the photodissociation bands corresponding to the different electronic states of the products are largely separated. In this case one needs to engineer the sequence delaying the half-cycle pulse after the pump pulse and additionally turning off rapidly the control pulse. Depending on the shape of the dipole functions the duration of the pulses in the sequence must be constrained to shorter times as well. Finally we show that the control scheme affects the velocity of the fragments. Although broad kinetic energy distributions are always obtained when the half-cycle pulse is short, if the Stark effect implies a blueshifting in the energy of the electronic states, the distribution of the relative speed of the fragments will be redshifted.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.57.+c Magneto-optical and electro-optical spectra and effects
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Stabilizing a 22 karat nanogolden cage

Q. Wang, Q. Sun, and P. Jena

J. Chem. Phys. 131, 204501 (2009); http://dx.doi.org/10.1063/1.3266562 (5 pages)

Online Publication Date: 23 November 2009

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Since the discovery of C60 fullerene, considerable efforts have been devoted to find other elements with similar hollow cage structures. However, search for hollow metallic cages with a diameter similar to that of C60 fullerene has been elusive. We describe a procedure for the rational design of metallic cages by suitably choosing their size, composition, and charge state. A 22 karat nanogolden cage with a diameter of about 8.5 Å and consisting of 12 Al and 20 Au atoms is found to be metastable, which can be stabilized by embedding a Mn4 cluster. In contrast to bulk Mn, which is antiferromagnetic, and isolated Mn4 cluster, which is ferromagnetic with a giant magnetic moment of 20μB, the Mn4@Al12Au20 endohedral complex exhibits magnetic bistability with 0μB and 14μB configurations being energetically nearly degenerate. These results, based on density functional theory, open the door to design a novel class of endohedral complexes with possible applications.
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61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
75.50.Cc Other ferromagnetic metals and alloys
75.30.Cr Saturation moments and magnetic susceptibilities

The role of intramolecular barriers on the glass transition of polymers: Computer simulations versus mode coupling theory

Marco Bernabei, Angel J. Moreno, and Juan Colmenero

J. Chem. Phys. 131, 204502 (2009); http://dx.doi.org/10.1063/1.3266852 (15 pages) | Cited 6 times

Online Publication Date: 24 November 2009

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We present computer simulations of a simple bead-spring model for polymer melts with intramolecular barriers. By systematically tuning the strength of the barriers, we investigate their role on the glass transition. Dynamic observables are analyzed within the framework of the mode coupling theory (MCT). Critical nonergodicity parameters, critical temperatures, and dynamic exponents are obtained from consistent fits of simulation data to MCT asymptotic laws. The so-obtained MCT λ-exponent increases from standard values for fully flexible chains to values close to the upper limit for stiff chains. In analogy with systems exhibiting higher-order MCT transitions, we suggest that the observed large λ-values arise form the interplay between two distinct mechanisms for dynamic arrest: general packing effects and polymer-specific intramolecular barriers. We compare simulation results with numerical solutions of the MCT equations for polymer systems, within the polymer reference interaction site model (PRISM) for static correlations. We verify that the approximations introduced by the PRISM are fulfilled by simulations, with the same quality for all the range of investigated barrier strength. The numerical solutions reproduce the qualitative trends of simulations for the dependence of the nonergodicity parameters and critical temperatures on the barrier strength. In particular, the increase in the barrier strength at fixed density increases the localization length and the critical temperature. However the qualitative agreement between theory and simulation breaks in the limit of stiff chains. We discuss the possible origin of this feature.
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64.70.pj Polymers
61.25.hk Polymer melts and blends
61.20.Ja Computer simulation of liquid structure

Inquiry into thermodynamic behavior of hard sphere plus repulsive barrier of finite height

Shiqi Zhou and J. R. Solana

J. Chem. Phys. 131, 204503 (2009); http://dx.doi.org/10.1063/1.3265984 (13 pages) | Cited 3 times

Online Publication Date: 24 November 2009

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A bridge function approximation is proposed to close the Ornstein–Zernike (OZ) integral equation for fluids with purely repulsive potentials. The performance of the bridge function approximation is then tested by applying the approximation to two kinds of repulsive potentials, namely, the square shoulder potential and the triangle shoulder potential. An extensive comparison between simulation and the OZ approach is performed over a wide density range for the fluid phase and several temperatures. It is found that the agreement between the two routes is excellent for not too low temperatures and satisfactory for extremely low temperatures. Then, this globally trustworthy OZ approach is used to investigate the possible existence or not of a liquid anomaly, i.e., a liquid-liquid phase transition at low temperatures and negative values of the thermal expansion coefficient in certain region of the phase diagram. While the existence of the liquid anomaly in the square shoulder potential has been previously predicted by a traditional first-order thermodynamic perturbation theory (TPT), the present investigation indicates that the liquid-liquid phase transition disappears in the OZ approach, so that its prediction by the first-order TPT is only an artifact originating from the low temperature inadequacy of the first-order TPT. However, the OZ approach indeed predicts negative thermal expansion coefficients. The present bridge function approximation, free of adjustable parameters, is suitable to be used within the context of a recently proposed nonhard sphere perturbation scheme.
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61.20.Ja Computer simulation of liquid structure
65.20.-w Thermal properties of liquids
64.70.Ja Liquid-liquid transitions

Semiclassical nonlinear response functions for coupled anharmonic vibrations

Scott M. Gruenbaum and Roger F. Loring

J. Chem. Phys. 131, 204504 (2009); http://dx.doi.org/10.1063/1.3266566 (15 pages) | Cited 1 time

Online Publication Date: 25 November 2009

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Observables in linear and nonlinear infrared spectroscopy may be computed from vibrational response functions describing nuclear dynamics on a single electronic surface. We demonstrate that the Herman–Kluk (HK) semiclassical approximation to the quantum propagator yields an accurate representation of quantum coherence effects in linear and nonlinear response functions for coupled anharmonic oscillators. A considerable numerical price is paid for this accuracy; the calculation requires a multidimensional integral over a highly oscillatory integrand that also grows without bound as a function of evolution times. The interference among classical trajectories in the HK approximation produces quantization of good action variables. By treating this interference analytically, we develop a mean-trajectory (MT) approximation that requires only the propagation of classical trajectories linked by transitions in action. The MT approximation accurately reproduces coherence effects in response functions of coupled anharmonic oscillators in a regime in which the observables are strongly influenced by these interactions among vibrations.
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03.65.Ge Solutions of wave equations: bound states
03.65.Sq Semiclassical theories and applications

Hybrid quantum/classical simulation of the vibrational relaxation of the bend fundamental in liquid water

Adolfo Bastida, José Zúñiga, Alberto Requena, and Beatriz Miguel

J. Chem. Phys. 131, 204505 (2009); http://dx.doi.org/10.1063/1.3266834 (10 pages) | Cited 4 times

Online Publication Date: 25 November 2009

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The Ehrenfest method with quantum corrections is used to describe the vibrational relaxation of the bend fundamental in liquid water. All the vibrational degrees of freedom of the water molecules are described using quantum mechanics, while the remaining translational and rotational degrees of freedom are described classically. The relaxation time obtained compares well with experiment and with relaxation times calculated using other theoretical approximations. The presence of resonant intermolecular vibrational energy (VV) transfer is established with a maximum percentage of excited molecules, different from the initial one, of 9.2%. It is found through an effective kinetic fit that two VV transfers occur before relaxation of water to the vibrational ground state.
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63.50.-x Vibrational states in disordered systems
61.20.Gy Theory and models of liquid structure
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