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14 Sep 2009

Volume 131, Issue 10, Articles (10xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 131, 105101 (2009); http://dx.doi.org/10.1063/1.3216550 (9 pages)

Christopher Forrey and M. Muthukumar
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A converse approach to the calculation of NMR shielding tensors

T. Thonhauser, D. Ceresoli, Arash A. Mostofi, Nicola Marzari, R. Resta, and David Vanderbilt

J. Chem. Phys. 131, 101101 (2009); http://dx.doi.org/10.1063/1.3216028 (4 pages) | Cited 9 times

Online Publication Date: 8 September 2009

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We introduce an alternative approach to the first-principles calculation of NMR shielding tensors. These are obtained from the derivative of the orbital magnetization with respect to the application of a microscopic, localized magnetic dipole. The approach is simple, general, and can be applied to either isolated or periodic systems. Calculated results for simple hydrocarbons, crystalline diamond, and liquid water show very good agreement with established methods and experimental results.
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76.60.-k Nuclear magnetic resonance and relaxation
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Cr Saturation moments and magnetic susceptibilities
71.70.Jp Nuclear states and interactions
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Antifreeze protein NMR sensor to detect water molecular reorientation in the surface of ice

Yougang Mao, Wei Lin, and Yong Ba

J. Chem. Phys. 131, 101102 (2009); http://dx.doi.org/10.1063/1.3223181 (3 pages) | Cited 1 time

Online Publication Date: 10 September 2009

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The century-old puzzle of the existence of a quasiliquid layer in the surface of ice was revisited using ice-surface binding type I antifreeze proteins with specific side chain 13C labeled methyl groups. Variable temperature dependent 13C spin lattice relaxation NMR experiments were carried out below 0 °C to probe the water molecular reorientations surrounding the methyl groups. The result shows that water molecular reorientations in ice surface are much more dynamic than those in bulk ice. This study demonstrates that antifreeze proteins can be used as a sensor to detect the dynamics of water in the surface of ice.
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76.60.Es Relaxation effects
82.80.-d Chemical analysis and related physical methods of analysis
68.15.+e Liquid thin films
82.56.Na Relaxation
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back to top Theoretical Methods and Algorithms

Advanced multiple time scale molecular dynamics

Igor P. Omelyan

J. Chem. Phys. 131, 104101 (2009); http://dx.doi.org/10.1063/1.3212922 (12 pages) | Cited 1 time

Online Publication Date: 8 September 2009

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A novel method for integrating the equations of motion in molecular dynamics simulations of many-particle systems is proposed. It is based on canonical transformations of the phase space and high-accuracy reversible decompositions of the time evolution propagator into terms with different time scales. This allows one to efficiently overcome the limitations imposed on the efficiency of the integration in standard multiple time stepping algorithms. As is demonstrated for a particular case of the Lennard-Jones fluids, the precision of the simulations within the method presented can be increased by several orders of magnitude already at typical computational costs and time steps.
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61.20.Ja Computer simulation of liquid structure
61.20.Gy Theory and models of liquid structure

Coarse grained simulations of a small peptide: Effects of finite damping and hydrodynamic interactions

Uwe Winter and Tihamér Geyer

J. Chem. Phys. 131, 104102 (2009); http://dx.doi.org/10.1063/1.3216573 (7 pages) | Cited 7 times

Online Publication Date: 9 September 2009

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In the coarse grained Brownian dynamics (BD) simulation method the many solvent molecules are replaced by random thermal kicks and an effective friction acting on the particles of interest. For BD the friction has to be so strong that the particles’ velocities are damped much faster than the duration of an integration timestep. Here we show that this conceptual limit can be dropped with an analytic integration of the equations of damped motion. In the resulting Langevin integration scheme our recently proposed approximate form of the hydrodynamic interactions between the particles can be incorporated conveniently, leading to a fast multiparticle propagation scheme, which captures more of the short-time and short-range solvent effects than standard BD. Comparing the dynamics of a bead-spring model of a short peptide, we recommend to run simulations of small biological molecules with the Langevin type finite damping and to include the hydrodynamic interactions.
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87.15.H- Dynamics of biomolecules
87.14.E- Proteins
36.20.-r Macromolecules and polymer molecules

Accurate treatment of nonbonded interactions within systematic molecular fragmentation

Matthew A. Addicoat and Michael A. Collins

J. Chem. Phys. 131, 104103 (2009); http://dx.doi.org/10.1063/1.3222639 (9 pages) | Cited 11 times

Online Publication Date: 9 September 2009

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The accuracy of the systematic fragment approach to the estimation of molecular electronic energies is enhanced by a significantly improved treatment of nonbonded interactions between molecular fragments. Distributed electrostatic interactions, pairwise dispersion interactions, and many-body induction are evaluated from ab initio calculations of small molecular fragments. The accuracy of the complete approach is reported for a large sample of typical neutral organic molecules.
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31.15.ae Electronic structure and bonding characteristics
34.20.Gj Intermolecular and atom-molecule potentials and forces

A comparison of new methods for generating energy-minimizing configurations of patchy particles

Eric Jankowski and Sharon C. Glotzer

J. Chem. Phys. 131, 104104 (2009); http://dx.doi.org/10.1063/1.3223834 (8 pages) | Cited 1 time

Online Publication Date: 9 September 2009

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Increasingly complex particles are pushing the limits of traditional simulation techniques used to study self-assembly. In this work, we test the use of a learning-augmented Monte Carlo method for predicting low energy configurations of patchy particles shaped like “Tetris®” pieces. We extend this method to compare it against Monte Carlo simulations with cluster moves and introduce a new algorithm—bottom-up building block assembly—for quickly generating ordered configurations of particles with a hierarchy of interaction energies.
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05.10.Gg Stochastic analysis methods (Fokker-Planck, Langevin, etc.)
02.70.Uu Applications of Monte Carlo methods
02.50.Fz Stochastic analysis
02.70.Rr General statistical methods

Comparison study of finite element and basis set methods for finite size scaling

Edwin Antillon, Winton Moy, Qi Wei, and Sabre Kais

J. Chem. Phys. 131, 104105 (2009); http://dx.doi.org/10.1063/1.3207909 (6 pages)

Online Publication Date: 9 September 2009

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We compare two methods of obtaining critical parameters for a quantum Hamiltonian using a finite size scaling approach. A finite element and basis set method were used in conjunction with the finite size scaling to obtain the critical parameters for the Hulthen potential. The critical parameters obtained analytically were the coupling constant λc = ½, the critical exponents for the energy α = 2 and for the “correlation length” ν = 1. The extrapolated results for finite size scaling with the basis set method are λc = 0.499 99, α = 1.9960, and ν = 0.999 10. The results for the finite element solutions are λc = 0.501 84, α = 1.999 93, and ν = 1.000 79 for the linear interpolation and λc = 0.500 00, α = 2.000 11, and ν = 1.000 32 for the Hermite interpolation. The results for each method compare very well with the analytical results obtained for the Hulthen potential. However, the finite element method is easier to implement and may be combined with ab initio and density functional theory to obtain quantum critical parameters for more complex systems.
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03.65.Ge Solutions of wave equations: bound states
02.60.Ed Interpolation; curve fitting
05.70.Ce Thermodynamic functions and equations of state
05.70.Jk Critical point phenomena

Quantum Monte Carlo study of Jastrow perturbation theory. I. Wave function optimization

Hongjun Luo, Wolfgang Hackbusch, and Heinz-Jürgen Flad

J. Chem. Phys. 131, 104106 (2009); http://dx.doi.org/10.1063/1.3220631 (13 pages) | Cited 4 times

Online Publication Date: 10 September 2009

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We have studied an iterative perturbative approach to optimize Jastrow factors in quantum Monte Carlo calculations. For an initial guess of the Jastrow factor we construct a corresponding model Hamiltonian and solve a first-order perturbation equation in order to obtain an improved Jastrow factor. This process is repeated until convergence. Two different types of model Hamiltonians have been studied for both energy and variance minimization. Our approach can be considered as an alternative to Newton’s method. Test calculations revealed the same fast convergence as for Newton’s method sufficiently close to the minimum. However, for a poor initial guess of the Jastrow factor, the perturbative approach is considerably more robust especially for variance minimization. Usually only two iterations are sufficient in order to achieve convergence within the statistical error. This is demonstrated for energy and variance minimization for the first row atoms and some small molecules. Furthermore, our perturbation analysis provides new insight into some recently proposed modifications of Newton’s method for energy minimization. A peculiar feature of the analysis is the continuous use of cumulants which guarantees size-consistency and provides least statistical fluctuations in the Monte Carlo implementation.
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31.15.vq Electron correlation calculations for polyatomic molecules
31.15.xp Perturbation theory

A spatial averaging approach to rare-event sampling

J. D. Doll, J. E. Gubernatis, Nuria Plattner, Markus Meuwly, P. Dupuis, and H. Wang

J. Chem. Phys. 131, 104107 (2009); http://dx.doi.org/10.1063/1.3220629 (8 pages) | Cited 2 times

Online Publication Date: 10 September 2009

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We describe a method for treating the sparse or rare-event sampling problem. Our approach is based on the introduction of a family of modified importance functions, functions that are related to but easier to sample than the original statistical distribution. We quantify the performance of the approach for a series of example problems using an asymptotic convergence analysis based on transition matrix methods.
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05.30.-d Quantum statistical mechanics
02.10.Yn Matrix theory
02.50.-r Probability theory, stochastic processes, and statistics

Atoms in boxes: From confined atoms to electron-atom scattering

Meta van Faassen

J. Chem. Phys. 131, 104108 (2009); http://dx.doi.org/10.1063/1.3223281 (7 pages)

Online Publication Date: 11 September 2009

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We show that both confined atoms and electron-atom scattering can be described by a unified basis set method. The central idea behind this method is to place the atom inside a hard potential sphere, enforced by a standard Slater type basis set multiplied by a cutoff factor. For confined atoms, where the wall is placed close to the atomic nucleus, we show how the energy of the highest occupied atomic orbital and the static polarizability of helium and neon atoms evolve with the confinement radius. To our knowledge, these are the first confined atom polarizability calculations that include correlation for many-electron atoms, through the use of time-dependent density-functional theory. By placing the atom in a large spherical box, with a wall outside the electron density, we obtain scattering phase shifts using a recently developed method [ M. van Faassen, A. Wasserman, E. Engel, F. Zhang, and K. Burke, Phys. Rev. Lett. 99, 043005 (2007) ]. We show that the basis set method gives identical results to previously obtained phase shifts for e-H and e-He+ scattering.
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34.80.Dp Atomic excitation and ionization
32.30.-r Atomic spectra
31.15.E- Density-functional theory

Multiple quantum NMR dynamics of spin-½ carrying molecules of a gas in nanopores

S. I. Doronin, A. V. Fedorova, E. B. Fel’dman, and A. I. Zenchuk

J. Chem. Phys. 131, 104109 (2009); http://dx.doi.org/10.1063/1.3231692 (7 pages) | Cited 6 times

Online Publication Date: 14 September 2009

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We consider the multiple quantum (MQ) NMR dynamics of a gas of spin carrying molecules in nanocavities. MQ NMR dynamics is determined by the residual dipole-dipole interactions, which are not averaged completely due to the molecular diffusion in nanopores. Since the averaged nonsecular Hamiltonian describing MQ NMR dynamics depends on only one coupling constant, this Hamiltonian commutes with the square of the total spin angular momentum math2. We use the basis of common eigenstates of math2 and the projection of I on the external magnetic field for investigation of MQ NMR dynamics. This approach allows us to study MQ NMR dynamics in systems consisting of several hundreds of spins. The analytical approximation of the stationary profile of MQ coherences is obtained. The analytical expressions for MQ NMR coherence intensities of the five-spin system in a nanopore are found. Numerical investigations allow us to find the dependencies of intensities of MQ coherences on their orders (the profiles of MQ coherences) in systems consisting of 600 spins and even more. It is shown that the stationary MQ coherence profile in the considered system is an exponential one.
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33.25.+k Nuclear resonance and relaxation

Extended ensemble approach for deriving transferable coarse-grained potentials

J. W. Mullinax and W. G. Noid

J. Chem. Phys. 131, 104110 (2009); http://dx.doi.org/10.1063/1.3220627 (13 pages) | Cited 14 times

Online Publication Date: 14 September 2009

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Coarse-grained (CG) models provide a computationally efficient means for investigating biological and soft-matter processes that evolve on long time scales and large length scales. The present work introduces an extended ensemble framework for calculating transferable CG potentials that accurately reproduce the structure of atomistic models for multiple systems. This framework identifies a generalized potential of mean force (PMF) as the appropriate CG potential for reproducing the structural correlations of an atomistic extended ensemble. A variational approach is developed for calculating transferable potentials that provide an optimal approximation to this PMF. Calculations for binary mixtures of alkanes and alcohols demonstrate that the extended ensemble potentials provide improved transferability relative to potentials calculated for a single system.
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33.15.Bh General molecular conformation and symmetry; stereochemistry
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Accurate ab initio quartic force fields for NH2− and CCH− and rovibrational spectroscopic constants for their isotopologs

Xinchuan Huang and Timothy J. Lee

J. Chem. Phys. 131, 104301 (2009); http://dx.doi.org/10.1063/1.3212560 (15 pages) | Cited 7 times

Online Publication Date: 9 September 2009

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A series of high-quality, purely ab initio, quartic force fields (QFFs), computed using a procedure we recently proposed, is reported for NH2− and CCH−. The singles and doubles coupled-cluster method with a perturbational estimate of the effects of connected triple excitations, denoted CCSD(T), was used with TZ, QZ, and 5Z quality basis sets and was combined with extrapolation to the one-particle basis-set limit, core-correlation effects, scalar relativistic effects, and higher-order correlation effects to yield accurate QFFs. A “best-guess” reference geometry was determined at the CCSD(T)/5Z level of theory. Analytical transformation removes nonzero gradients to facilitate a second-order perturbation theory spectroscopic analysis. The QFF is transformed into Morse/cosine coordinates in order to perform exact vibrational configuration interaction computations. Equilibrium structures, vibrational frequencies, rotational constants, and selected spectroscopic constants are reported in comparison with experimental values and previous theoretical studies. Higher-order correlation effects are found comparable to core-correlation effects in magnitude, e.g., ∼ 10 cm−1 for fundamentals, but are of opposite sign. For CCH−, a thorough discussion is presented on effective rotational constants B0. It is concluded that the “best” QFF should incorporate all the small corrections mentioned above. Correspondingly, the best vibrational fundamentals of CCH− are estimated at 502.0 cm−1 (ν2), 1800.9 cm−1 (ν3), and 3204.3 cm−1 (ν1), while the best vibrational fundamentals of NH2− are at 3118.5 cm−1 (ν1), 1447.8 cm−1 (ν2), and 3186.5 cm−1 (ν3). Excellent agreement with high-resolution experiments has been obtained for fundamentals—e.g., 1–3 cm−1 deviation for the symmetric and antisymmetric stretches of NH2−, 3121.93 cm−1 (ν1) and 3190.29 cm−1 (ν3), respectively. Isotopic effects are studied and presented to aid future experimental analyses. The present study should facilitate future characterizations of NH2− and CCH− from astronomical observations or other high-resolution laboratory studies.
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31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
33.20.Tp Vibrational analysis
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.bw Coupled-cluster theory
31.15.xp Perturbation theory
33.20.Vq Vibration-rotation analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Laser control of conical intersections: Quantum model simulations for the averaged loss-gain strategies of fast electronic deactivation in 1,1-difluoroethylene

Jesús González-Vázquez, Leticia González, Ignacio R. Sola, and J. Santamaria

J. Chem. Phys. 131, 104302 (2009); http://dx.doi.org/10.1063/1.3223998 (5 pages) | Cited 2 times

Online Publication Date: 9 September 2009

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The enhancing and inhibition of population transfer via a conical intersection is demonstrated with quantum model calculations on the 1,1-difluoroethylene system. Averaged loss-gain strategies are achieved using strong laser pulses, which either trap the wave packet in the excited state, or accelerate the wave packet in the vicinity of the conical intersection.
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33.80.-b Photon interactions with molecules
31.50.Df Potential energy surfaces for excited electronic states
31.15.ae Electronic structure and bonding characteristics

Interatomic Coulombic decay and its dynamics in NeAr following K-LL Auger transition in the Ne atom

Ph. V. Demekhin, Y.-C. Chiang, S. D. Stoychev, P. Kolorenč, S. Scheit, A. I. Kuleff, F. Tarantelli, and L. S. Cederbaum

J. Chem. Phys. 131, 104303 (2009); http://dx.doi.org/10.1063/1.3211114 (11 pages) | Cited 5 times

Online Publication Date: 9 September 2009

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We analyze in detail the accessible relaxation pathways via electron emission of the Ne2+Ar states populated via the K-LL Auger decay of Ne+(1s−1)Ar. In particular, we concentrate on the “direct” interatomic Coulombic decay (ICD) of the Ne2+(2s−12p−1)Ar weakly bound doubly ionized states into the manifold of the Ne2+(2p−2)–Ar+(3p−1) repulsive triply ionized ones. To carry out the present study the potential energy curves of the NeAr ground state, the core ionized state Ne+(1s−1)Ar, the relevant dicationic and tricationic states, and the corresponding ICD transition rates have been computed using accurate ab initio methods and basis sets. The total and partial ICD electron spectra are computed within the framework of the time-dependent theory of wave packet propagation. Thereby, the impact of nuclear dynamics accompanying the electronic decay on the computed ICD-electron spectra is investigated in detail.
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33.80.Eh Autoionization, photoionization, and photodetachment
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.15.ap Polarizabilities and other atomic and molecular properties
34.50.Gb Electronic excitation and ionization of molecules
31.50.Bc Potential energy surfaces for ground electronic states

Metal coordination converts the tub-shaped cyclo-octatetraene into an aromatic molecule: Electronic states and half-sandwich structures of group III metal-cyclo-octatetraene complexes

Jung Sup Lee, Yuxiu Lei, Sudesh Kumari, and Dong-Sheng Yang

J. Chem. Phys. 131, 104304 (2009); http://dx.doi.org/10.1063/1.3224127 (7 pages) | Cited 4 times

Online Publication Date: 10 September 2009

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Group III (Sc, Y, and La) metal-(1,3,5,7-cyclo-octatetraene) (COT) complexes were produced in a laser-vaporization molecular beam source and studied by pulsed-field-ionization zero-electron-kinetic-energy (ZEKE) spectroscopy. Adiabatic ionization energies and metal-ligand stretching frequencies were measured from the ZEKE spectra. Metal-ligand bonding and low-lying electronic states of the neutral and ionized complexes were analyzed by combining the spectroscopic measurements with the molecular orbital treatment and density functional theory calculations. The ionization energies and metal-ligand stretching frequencies of these complexes are in the order of Sc>Y>La. The ground electronic state of the neutral complexes is 2A1, whereas the ground state of the ions is 1A1. The molecular symmetry is C8v in both neutral and ionic ground states. Although free COT is a nonaromatic molecule with a tublike structure, coordination of the group III metal atoms converts the tub-shaped molecule into a planar, aromatic structure. This conversion is induced by a two-electron transfer from the metal atoms to the ligand upon the formation of the complexes.
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33.80.Eh Autoionization, photoionization, and photodetachment
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.E- Density-functional theory
37.20.+j Atomic and molecular beam sources and techniques
34.70.+e Charge transfer
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis

Synthesis of interstellar 1,3,5-heptatriynylidyne, C7H(X 2Π), via the neutral-neutral reaction of ground state carbon atom, C(3P), with triacetylene, HC6H (X 1Σg+)

B. J. Sun, C. H. Huang, M. F. Tsai, H. L. Sun, L. G. Gao, Y. S. Wang, Y. Y. Yeh, Y. H. Shih, Z. F. Sia, P. H. Chen, R. I. Kaiser, and A. H. H. Chang

J. Chem. Phys. 131, 104305 (2009); http://dx.doi.org/10.1063/1.3212625 (13 pages)

Online Publication Date: 11 September 2009

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The reaction of ground-state carbon atom with a polyyne, triacetylene (HC6H) is investigated theoretically by combining ab initio calculations for predicting reaction paths, RRKM theory to yield rate constant for each path, and a modified Langevin model for estimating capturing cross sections. The isomerization and dissociation channels for each of the five collision complexes are characterized by utilizing the unrestricted B3LYP/6-311G(d,p) level of theory and the CCSD(T)/cc-pVTZ calculations. Navigating with the aid of RRKM rate constants through web of ab initio paths composed of 5 collision complexes, 108 intermediates, and 20 H-dissociated products, the most probable paths, reduced to around ten species at collision energies of 0 and 10 kcal/mol, respectively, are identified and adopted as the reaction mechanisms. The rate equations for the reaction mechanisms are solved numerically such that the evolutions of concentrations with time for all species involved are obtained and their lifetimes deduced. This study predicts that the five collision complexes, c1–c5, would produce a single final product, C7H (p1)+H, via the most stable intermediate, carbon chain HC7H (i1); namely, C+HC6H→HC7H→C7H+H. Our investigation indicates that the title reaction is efficient to form astronomically observed C7H in cold molecular clouds, where a typical translational temperature is 10 K.
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82.20.Db Transition state theory and statistical theories of rate constants
82.20.Pm Rate constants, reaction cross sections, and activation energies
98.58.Db Molecular clouds, H2 clouds, dense clouds, and dark clouds
82.30.Qt Isomerization and rearrangement
82.20.Fd Collision theories; trajectory models
95.30.Ft Molecular and chemical processes and interactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Geometric bonding effects in the X 2A1, A 2Σu+, and B 2Πg states of Li2F

Kris W. A. Wright, Daniel E. Rogers, and Ian C. Lane

J. Chem. Phys. 131, 104306 (2009); http://dx.doi.org/10.1063/1.3216373 (8 pages) | Cited 1 time

Online Publication Date: 11 September 2009

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Published ab initio and pseudopotential calculations for the dialkali halide systems suggest that the preferred colinear geometry is for the metal to approach the metal end of the alkali halide. Here, ab initio calculations on the Li2F system reveal that the well depth on the halide side in this radical is much deeper and is a local saddle point associated with the ionic nonlinear global minima. Although many features of the pseudopotential surfaces are confirmed, significant differences are apparent including the existence of a linear excited A 2Σu+ state instead of a triangular one, a considerably deeper global minimum some 50% lower in energy and a close approach between the X 2A1 and the A 2Σu+ states, with the A 2Σu+ minimum 87 kJ mol−1 below the ground state asymptote. All the results can be rationalised as the avoided crossings between a long range, covalent potential dominant within the LiLiF geometry and an ionic state that forms the global minimum. Calculations on the third 2A′ potential indicate that even for Li+LiF collisions at ultracold temperatures the collision dynamics could involve as many as three electronic states.
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31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations
33.80.Be Level crossing and optical pumping
34.50.-s Scattering of atoms and molecules

Collisional depolarization of NO(A) by He and Ar studied by quantum beat spectroscopy

M. Brouard, H. Chadwick, Y.-P. Chang, R. Cireasa, C. J. Eyles, A. O. La Via, N. Screen, F. J. Aoiz, and J. Kłos

J. Chem. Phys. 131, 104307 (2009); http://dx.doi.org/10.1063/1.3212608 (15 pages) | Cited 11 times

Online Publication Date: 11 September 2009

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Zeeman and hyperfine quantum beat spectroscopies have been used to measure the total elastic plus inelastic angular momentum depolarization rate constants at 300 K for NO(A 2Σ+) in the presence of He and Ar. In the case of Zeeman quantum beats it is shown how the applied magnetic field can be used to allow measurement of depolarization rates for both angular momentum orientation and alignment. For the systems studied here, collisional loss of alignment is more efficient than loss of orientation. In the case of NO(A) with He, and to a lesser extent NO(A) with Ar, collisional depolarization is found to be a relatively minor process compared to rotational energy transfer, reflecting the very weak long-range forces in these systems. Detailed comparisons are made with quantum mechanical and quasiclassical trajectory calculations performed on recently developed potential energy surfaces. For both systems, the agreement between the calculated depolarization cross sections and the present measurements is found to be very good, suggesting that it is reasonable to consider the NO(A) bond as frozen during these angular momentum transferring collisions. A combination of kinematic effects and differences in the potential energy surfaces are shown to be responsible for the differences observed in depolarization cross section with He and Ar as a collider.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Dx Analytical methods involving electronic spectroscopy
78.47.jm Quantum beats
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Kh Potential energy surfaces for chemical reactions

Photodetachment, photofragmentation, and fragment autodetachment of [O2n(H2O)m]− clusters: Core-anion structures and fragment energy partitioning

Daniel J. Goebbert and Andrei Sanov

J. Chem. Phys. 131, 104308 (2009); http://dx.doi.org/10.1063/1.3224135 (9 pages) | Cited 1 time

Online Publication Date: 11 September 2009

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Building on the past studies of the O2n− and O2−(H2O)m cluster anion series, we assess the effect of the strong hydration interactions on the oxygen-core clusters using photoelectron imaging and photofragment mass spectroscopy of [O2n(H2O)m]− (n = 1–4, m = 0–3) at 355 nm. The results show that both pure-oxygen and hydrated clusters with n ≥ 2 form an O4− core anion, indicated in the past work on the pure-oxygen clusters. All clusters studied can be therefore described in terms of O4−(H2O)m(O2)n−2 structures, although the O4− core may be strongly perturbed by hydration in some of these clusters. Fragmentation of these clusters yields predominantly O2− and O2−(H2O)l (l<m) anionic products. The low-electron kinetic energy O2− autodetachment features, prominent in the photoelectron images, signal that the fragments are vibrationally excited. The relative intensity of photoelectrons arising from O2− fragment autodetachment is used to shed light on the varying degree of fragment excitation resulting from the cluster fragmentation process depending on the solvent conditions.
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33.80.Eh Autoionization, photoionization, and photodetachment
36.40.-c Atomic and molecular clusters
33.60.+q Photoelectron spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Radiative lifetimes of NO A 2Σ+(v′ = 0,1,2) and the electronic transition moment of the A 2Σ+−X 2Π system

Thomas B. Settersten, Brian D. Patterson, and William H. Humphries, IV

J. Chem. Phys. 131, 104309 (2009); http://dx.doi.org/10.1063/1.3227520 (11 pages) | Cited 2 times

Online Publication Date: 11 September 2009

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Improved measurements of the radiative lifetimes of NO A 2Σ+(v′ = 0,1,2) are presented and used to update the absolute electronic transition moment for the NO γ bands. The pressure-dependent fluorescence decay rate was measured in a low-pressure, room-temperature, flow cell containing dilute mixtures of NO in N2 using time-resolved laser-induced fluorescence excited with a picosecond laser and detected with a microchannel-plate photomultiplier tube. Fluorescence decay rates were determined using an analysis procedure that accounted for the electronic response of the detection system and measurement noise. Radiative lifetimes were determined from an extrapolation of the measured decay rates to zero pressure. In comparison with prior measurements of these radiative lifetimes, the improved experimental approach and analysis procedure result in a significant improvement in the measurement precision. The accuracy of the fluorescence decay-rate measurements was confirmed by independent measurements using time-correlated single-photon counting and time-resolved probing of laser-excited population in A 2Σ+ using 266 nm photoionization and charge detection. The measured radiative lifetimes are 192.6±0.2 ns for v′ = 0, 186.2±0.4 ns for v′ = 1, and 179.4±0.7 ns for v′ = 2. The measured lifetimes are shown to be in outstanding agreement with those predicted by an electronic transition moment that is identical in form to the function recommended by Luque and Crosley [J. Chem. Phys. 111, 7405 (1999)] after appropriate rescaling. This rescaling does not affect the agreement of the transition moment function with the previously reported vibrational branching ratios and improves agreement with previously reported absolute oscillator strength measurements. Based on the rescaled transition moment, updated values of absolute transition probabilities in the NO A 2Σ+−X 2Π system are presented.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.50.Hv Radiationless transitions, quenching
33.50.Dq Fluorescence and phosphorescence spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

An experimental and theoretical study on vibrational structure in the math–math transition of CH2CHS

Masakazu Nakajima, Timothy W. Schmidt, Yoshihiro Sumiyoshi, and Yasuki Endo

J. Chem. Phys. 131, 104310 (2009); http://dx.doi.org/10.1063/1.3224146 (7 pages) | Cited 2 times

Online Publication Date: 14 September 2009

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An electronic excitation spectrum of the math–math transition of the CH2CHS (vinylthio, thiovinoxy) radical was observed in the range of 21 800–23 400 cm−1 by fluorescence depletion spectroscopy. Vibrational assignments for the depletion spectrum were made with the help of a theoretical excitation spectrum computed from results of ab initio calculations. The previously observed emission spectrum [ M. Nakajima et al., J. Chem. Phys. 126, 044307 (2007) ] was also reassigned based on the present theoretical study. Rotational band contours observed in the depletion spectrum show a large variation due to a vibrational-level dependence of the excited state lifetime. However, no obvious vibrational mode dependence of the lifetime was observed.
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33.50.Dq Fluorescence and phosphorescence spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
31.15.A- Ab initio calculations

Chemical dynamics of the formation of the ethynylsilylidyne radical (SiCCH(X 2Π)) in the crossed beam reaction of ground state silicon atoms (Si(3P)) with acetylene (C2H2(X 1∑g+))

R. I. Kaiser and X. Gu

J. Chem. Phys. 131, 104311 (2009); http://dx.doi.org/10.1063/1.3224150 (6 pages) | Cited 2 times

Online Publication Date: 14 September 2009

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The reaction dynamics of ground state silicon atoms (Si(3P)) with the acetylene molecule (C2H2(X 1∑g+)) were investigated at a collision energy of 101.6±1.6 kJ mol−1 under single collision conditions in a crossed molecular beam machine. We found that the reaction dynamics proceeded via an addition of the silicon atom to the π-electrons of the acetylene molecule at a single carbon atom forming a Cs symmetric SiC2H2(X 3A″) intermediate. The latter either emitted a hydrogen atom leading to the linear SiCCH(X 2Π) product or underwent a hydrogen migration to the SiCCH2(X 3A2) isomer prior to the decomposition of the latter to SiCCH(X 2Π) plus atomic hydrogen. The overall reaction to form the SiCCH(X 2Π) plus atomic hydrogen products was determined to be highly endoergic by 84±6 kJ mol−1. No evidence of a molecular hydrogen elimination channel was found at this collision energy. Our study predicts that this endoergic neutral-neutral reaction can lead to the formation of an organosilicon transient species, i.e., the linear SiCCH(X 2Π) radical, in high-temperature environments like in the circumstellar envelope of the carbon star IRC+10 216, where temperatures of a few 1000 K exist close to the central star.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.33.Tb Atmospheric chemistry

Infrared spectroscopy of the protonated nitrogen dimer: The complexity of shared proton vibrations

Allen M. Ricks, Gary E. Douberly, and Michael A. Duncan

J. Chem. Phys. 131, 104312 (2009); http://dx.doi.org/10.1063/1.3224155 (8 pages) | Cited 2 times

Online Publication Date: 14 September 2009

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The proton-bridged dimers of nitrogen, e.g., N2–H+–N2 and N2–D+–N2, are produced in a pulsed-discharge supersonic nozzle source, mass selected in a reflectron time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy using the method of messenger atom tagging with argon. Both complexes are studied from 700–4000 cm−1. These spectra reproduce the high frequency vibrations seen previously but discover many new vibrational bands, particularly those in the region of the shared proton modes. Because of the linear structure of the core ions, simple vibrational spectra are expected containing only the antisymmetric N–N stretch and two lower frequency modes corresponding to proton stretching and bending motions. However, many additional bands are detected corresponding to various combination bands in this system activated by anharmonic couplings of the proton motions. The anharmonic coupling is stronger for the H+ system than it is for the D+ system. Using anharmonic proton vibrations computed previously and combinations of computed harmonic frequencies, reasonable assignments can be made for the spectra of both isotopomers. However, advanced anharmonic computational treatments are needed for this system to confirm these assignments.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.50.Bc Processes caused by infrared radiation
82.80.Gk Analytical methods involving vibrational spectroscopy
82.20.Tr Kinetic isotope effects including muonium

An ab initio study of the (H2O)20H+ and (H2O)21H+ water clusters

Tomasz Kuś, Victor F. Lotrich, Ajith Perera, and Rodney J. Bartlett

J. Chem. Phys. 131, 104313 (2009); http://dx.doi.org/10.1063/1.3231684 (6 pages) | Cited 5 times

Online Publication Date: 14 September 2009

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The study of the minimum Born–Oppenheimer structures of the protonated water clusters, (H2O)nH+, is performed for n = 20 and 21. The structures belonging to four basic morphologies are optimized at the Hartree–Fock, second-order many-body perturbation theory and coupled cluster level, with the 6–31G, 6-31G∗, and 6-311G∗∗ basis sets, using the parallel ACES III program. The lowest energy structure for each n is found to be the cagelike form filled with H2O, with the proton located on the surface. The cage is the distorted dodecahedron for the 21-mer case, and partially rearranged dodecahedral structure for the 20-mer. The results confirm that the lowest energy structure of the magic number n = 21 clusters corresponds to a more stable form than that of the 20-mer clusters.
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31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
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