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14 Dec 2011

Volume 135, Issue 22, Articles (22xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 135, 224504 (2011); http://dx.doi.org/10.1063/1.3660208 (6 pages)

Vojtěch Spiwok and Blanka Králová
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Communication: Branching ratio measurements in the predissociation of 12C16O by time-slice velocity-map ion imaging in the vacuum ultraviolet region

Hong Gao, Yu Song, Lei Yang, Xiaoyu Shi, Qingzhu Yin, C. Y. Ng, and William M. Jackson

J. Chem. Phys. 135, 221101 (2011); http://dx.doi.org/10.1063/1.3669426 (4 pages) | Cited 1 time

Online Publication Date: 14 December 2011

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The first direct branching ratio measurement of the three lowest energy dissociation channels of CO that produce C(3P) + O(3P), C(1D) + O(3P), and C(3P) + O(1D) is reported. Rotational resolved carbon ion yield spectra for two Π bands (W(3sσ)1Π (v′ = 3) at 108 012.6 cm−1 and 1Π(v′ = 2) at 109 017 cm−1) and two Σ bands ((4sσ)1Σ+(v′ = 4) at 109 452 cm−1 and (4pσ)1Σ+(v′ = 3) at 109 485 cm−1) of CO were obtained. Our measurements show that the branching ratio in this energy region is strongly dependent on the electronic and vibrational energy but it is independent or just weakly dependent on the parity and rotational energy levels. To our knowledge, this is the first time that the triplet channel producing O(1D) has been experimentally observed and this is also the first time that a direct measurement of the branching ratio for the different channels in the predissociation of CO in this energy region has been made.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.30.Gs Hyperfine interactions and isotope effects
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Ni Vacuum ultraviolet spectra
33.20.Sn Rotational analysis
33.20.Tp Vibrational analysis
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back to top Theoretical Methods and Algorithms

Diffusion in periodic two-dimensional channels formed by overlapping circles: Comparison of analytical and numerical results

Inti Pineda, Marco-Vinicio Vazquez, Alexander M. Berezhkovskii, and Leonardo Dagdug

J. Chem. Phys. 135, 224101 (2011); http://dx.doi.org/10.1063/1.3664179 (5 pages)

Online Publication Date: 8 December 2011

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We study two-dimensional diffusion in a channel formed by periodic overlapping circles. Periodic variation of the channel width leads to the slowdown of diffusion along the channel axis. There are several approximate approaches, which allow one to analyze the slowdown. We use these approaches to derive five expressions for the effective diffusion coefficient of a point Brownian particle in the channel. To check the accuracy of the expressions we compare their predictions with the effective diffusion coefficient obtained from Brownian dynamics simulations.
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05.60.-k Transport processes
05.40.Jc Brownian motion

Interaction energies of large clusters from many-body expansion

Urszula Góra, Rafał Podeszwa, Wojciech Cencek, and Krzysztof Szalewicz

J. Chem. Phys. 135, 224102 (2011); http://dx.doi.org/10.1063/1.3664730 (19 pages)

Online Publication Date: 8 December 2011

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In the canonical supermolecular approach, calculations of interaction energies for molecular clusters involve a calculation of the whole cluster, which becomes expensive as the cluster size increases. We propose a novel approach to this task by demonstrating that interaction energies of such clusters can be constructed from those of small subclusters with a much lower computational cost by applying progressively lower-level methods for subsequent terms in the many-body expansion. The efficiency of such “stratified approximation” many-body approach (SAMBA) is due to the rapid convergence of the many-body expansion for typical molecular clusters. The method has been applied to water clusters (H2O)n, n = 6, 16, 24. For the hexamer, the best results that can be obtained with current computational resources in the canonical supermolecular method were reproduced to within about one tenth of the uncertainty of the canonical approach while using 24 times less computer time in the many-body expansion calculations. For (H2O)24, SAMBA is particularly beneficial and we report interaction energies with accuracy that is currently impossible to obtain with the canonical supermolecular approach. Moreover, our results were computed using two orders of magnitude smaller computer resources than used in the previous best calculations for this system. We also show that the basis-set superposition errors should be removed in calculations for large clusters.
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36.40.Jn Reactivity of clusters
31.15.bw Coupled-cluster theory

Orbital-optimized third-order Møller-Plesset perturbation theory and its spin-component and spin-opposite scaled variants: Application to symmetry breaking problems

Uğur Bozkaya

J. Chem. Phys. 135, 224103 (2011); http://dx.doi.org/10.1063/1.3665134 (13 pages) | Cited 1 time

Online Publication Date: 9 December 2011

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In this research, orbital-optimized third-order Møller-Plesset perturbation theory (OMP3) and its spin-component and spin-opposite scaled variants (SCS-OMP3 and SOS-OMP3) are introduced. Using a Lagrangian-based approach, an efficient, quadratically convergent algorithm for variational optimization of the molecular orbitals (MOs) for third-order Møller-Plesset perturbation theory (MP3) is presented. Explicit equations for response density matrices, the MO gradient, and Hessian are reported in spin-orbital form. The OMP3, SCS-OMP3, and SOS-OMP3 approaches are compared with the second-order Møller-Plesset perturbation theory (MP2), MP3, coupled-cluster doubles (CCD), optimized-doubles (OD), and coupled-cluster singles and doubles (CCSD) methods. All these methods are applied to the O4+, O3, and seven diatomic molecules. Results demonstrate that the OMP3 and its variants provide significantly better vibrational frequencies than MP3, CCSD, and OD for the molecules where the symmetry-breaking problems are observed. For O4+, the OMP3 prediction, 1343 cm−1, for ω6 (b3u) mode, where symmetry-breaking appears, is even better than presumably more reliable methods such as Brueckner doubles (BD), 1194 cm−1, and OD, 1193 cm−1, methods (the experimental value is 1320 cm−1). For O3, the predictions of SCS-OMP3 (1143 cm−1) and SOS-OMP3 (1165 cm−1) are remarkably better than the more robust OD method (1282 cm−1); the experimental value is 1089 cm−1. For the seven diatomics, again the SCS-OMP3 and SOS-OMP3 methods provide the lowest average errors, |Δωe| = 44 and |Δωe| = 35 cm−1, respectively, while for OD, |Δωe| = 161 cm−1and CCSD |Δωe| = 106 cm−1. Hence, the OMP3 and especially its spin-scaled variants perform much better than the MP3, CCSD, and more robust OD approaches for considered test cases. Therefore, considering both the computational cost and the reliability, SCS-OMP3 and SOS-OMP3 appear to be the best methods for the symmetry-breaking cases, based on present application results. The OMP3 method offers certain advantages: it provides reliable vibrational frequencies in case of symmetry-breaking problems, especially with spin-scaling tricks, its analytic gradients are easier to compute since there is no need to solve the coupled-perturbed equations for the orbital response, and the computation of one-electron properties are easier because there is no response contribution to the particle density matrices. The OMP3 has further advantages over standard MP3, making it promising for excited state properties via linear response theory.
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31.15.bw Coupled-cluster theory
33.20.Tp Vibrational analysis

Sensitivity field for nonautonomous molecular rotors

A. V. Akimov and N. A. Sinitsyn

J. Chem. Phys. 135, 224104 (2011); http://dx.doi.org/10.1063/1.3667196 (5 pages) | Cited 1 time

Online Publication Date: 9 December 2011

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We propose a numerical approach to quantify the control of a nonautonomous molecular rotor motion. Unlike straightforward molecular dynamics simulations in an explicitly time-dependent framework, our method is based on the theory of geometric phases. This theory allows us to define a sensitivity field (SF) in control parameter space that characterizes average motion of a molecule induced by a cyclic perturbation. We show that the SF can be obtained using only equilibrium free energy sampling techniques. A density plot of the SF quantifies response of a molecule to an arbitrary cyclic adiabatic evolution of parameters. For demonstration, we numerically find the SFs for two surface mounted molecular rotor molecules that can be driven, in practice, by strong time-dependent electric fields of a STM tip.
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85.65.+h Molecular electronic devices
84.50.+d Electric motors

Geometric integration in Born-Oppenheimer molecular dynamics

Anders Odell, Anna Delin, Börje Johansson, Marc J. Cawkwell, and Anders M. N. Niklasson

J. Chem. Phys. 135, 224105 (2011); http://dx.doi.org/10.1063/1.3660689 (11 pages)

Online Publication Date: 12 December 2011

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Geometric integration schemes for extended Lagrangian self-consistent Born-Oppenheimer molecular dynamics, including a weak dissipation to remove numerical noise, are developed and analyzed. The extended Lagrangian framework enables the geometric integration of both the nuclear and electronic degrees of freedom. This provides highly efficient simulations that are stable and energy conserving even under incomplete and approximate self-consistent field (SCF) convergence. We investigate three different geometric integration schemes: (1) regular time reversible Verlet, (2) second order optimal symplectic, and (3) third order optimal symplectic. We look at energy conservation, accuracy, and stability as a function of dissipation, integration time step, and SCF convergence. We find that the inclusion of dissipation in the symplectic integration methods gives an efficient damping of numerical noise or perturbations that otherwise may accumulate from finite arithmetics in a perfect reversible dynamics.
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31.15.xr Self-consistent-field methods
31.15.xp Perturbation theory
31.15.xv Molecular dynamics and other numerical methods

Spin-orbit relativistic long-range corrected time-dependent density functional theory for investigating spin-forbidden transitions in photochemical reactions

Ayako Nakata, Takao Tsuneda, and Kimihiko Hirao

J. Chem. Phys. 135, 224106 (2011); http://dx.doi.org/10.1063/1.3665890 (9 pages)

Online Publication Date: 12 December 2011

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A long-range corrected (LC) time-dependent density functional theory (TDDFT) incorporating relativistic effects with spin-orbit couplings is presented. The relativistic effects are based on the two-component zeroth-order regular approximation Hamiltonian. Before calculating the electronic excitations, we calculated the ionization potentials (IPs) of alkaline metal, alkaline-earth metal, group 12 transition metal, and rare gas atoms as the minus orbital (spinor) energies on the basis of Koopmans’ theorem. We found that both long-range exchange and spin-orbit coupling effects are required to obtain Koopmans’ IPs, i.e., the orbital (spinor) energies, quantitatively in DFT calculations even for first-row transition metals and systems containing large short-range exchange effects. We then calculated the valence excitations of group 12 transition metal atoms and the Rydberg excitations of rare gas atoms using spin-orbit relativistic LC-TDDFT. We found that the long-range exchange and spin-orbit coupling effects significantly contribute to the electronic spectra of even light atoms if the atoms have low-lying excitations between orbital spinors of quite different electron distributions.
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31.15.aj Relativistic corrections, spin-orbit effects, fine structure; hyperfine structure
32.50.+d Fluorescence, phosphorescence (including quenching)
82.50.-m Photochemistry

Electrostatic embedding in large-scale first principles quantum mechanical calculations on biomolecules

Stephen J. Fox, Chris Pittock, Thomas Fox, Christofer S. Tautermann, Noj Malcolm, and Chris-Kriton Skylaris

J. Chem. Phys. 135, 224107 (2011); http://dx.doi.org/10.1063/1.3665893 (13 pages)

Online Publication Date: 12 December 2011

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Biomolecular simulations with atomistic detail are often required to describe interactions with chemical accuracy for applications such as the calculation of free energies of binding or chemical reactions in enzymes. Force fields are typically used for this task but these rely on extensive parameterisation which in cases can lead to limited accuracy and transferability, for example for ligands with unusual functional groups. These limitations can be overcome with first principles calculations with methods such as density functional theory (DFT) but at a much higher computational cost. The use of electrostatic embedding can significantly reduce this cost by representing a portion of the simulated system in terms of highly localised charge distributions. These classical charge distributions are electrostatically coupled with the quantum system and represent the effect of the environment in which the quantum system is embedded. In this paper we describe and evaluate such an embedding scheme in which the polarisation of the electronic density by the embedding charges occurs self-consistently during the calculation of the density. We have implemented this scheme in a linear-scaling DFT program as our aim is to treat with DFT entire biomolecules (such as proteins) and large portions of the solvent. We test this approach in the calculation of interaction energies of ligands with biomolecules and solvent and investigate under what conditions these can be obtained with the same level of accuracy as when the entire system is described by DFT, for a variety of neutral and charged species.
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87.15.kp Protein-ligand interactions
87.15.kr Protein-solvent interactions
87.14.E- Proteins

Efficient exploration of reaction paths via a freezing string method

Andrew Behn, Paul M. Zimmerman, Alexis T. Bell, and Martin Head-Gordon

J. Chem. Phys. 135, 224108 (2011); http://dx.doi.org/10.1063/1.3664901 (9 pages)

Online Publication Date: 13 December 2011

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The ability to efficiently locate transition states is critically important to the widespread adoption of theoretical chemistry techniques for their ability to accurately predict kinetic constants. Existing surface walking techniques to locate such transition states typically require an extremely good initial guess that is often beyond human intuition to estimate. To alleviate this problem, automated techniques to locate transition state guesses have been created that take the known reactant and product endpoint structures as inputs. In this work, we present a simple method to build an approximate reaction path through a combination of interpolation and optimization. Starting from the known reactant and product structures, new nodes are interpolated inwards towards the transition state, partially optimized orthogonally to the reaction path, and then frozen before a new pair of nodes is added. The algorithm is stopped once the string ends connect. For the practical user, this method provides a quick and convenient way to generate transition state structure guesses. Tests on three reactions (cyclization of cis,cis-2,4-hexadiene, alanine dipeptide conformation transition, and ethylene dimerization in a Ni-exchanged zeolite) show that this “freezing string” method is an efficient way to identify complex transition states with significant cost savings over existing methods, particularly when high quality linear synchronous transit interpolation is employed.
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82.20.-w Chemical kinetics and dynamics

Milestoning with transition memory

Alexander T. Hawk and Dmitrii E. Makarov

J. Chem. Phys. 135, 224109 (2011); http://dx.doi.org/10.1063/1.3666840 (9 pages)

Online Publication Date: 13 December 2011

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Milestoning is a method used to calculate the kinetics and thermodynamics of molecular processes occurring on time scales that are not accessible to brute force molecular dynamics (MD). In milestoning, the conformation space of the system is sectioned by hypersurfaces (milestones), an ensemble of trajectories is initialized on each milestone, and MD simulations are performed to calculate transitions between milestones. The transition probabilities and transition time distributions are then used to model the dynamics of the system with a Markov renewal process, wherein a long trajectory of the system is approximated as a succession of independent transitions between milestones. This approximation is justified if the transition probabilities and transition times are statistically independent. In practice, this amounts to a requirement that milestones are spaced such that trajectories lose position and velocity memory between subsequent transitions. Unfortunately, limiting the number of milestones limits both the resolution at which a system's properties can be analyzed, and the computational speedup achieved by the method. We propose a generalized milestoning procedure, milestoning with transition memory (MTM), which accounts for memory of previous transitions made by the system. When a reaction coordinate is used to define the milestones, the MTM procedure can be carried out at no significant additional expense as compared to conventional milestoning. To test MTM, we have applied its version that allows for the memory of the previous step to the toy model of a polymer chain undergoing Langevin dynamics in solution. We have computed the mean first passage time for the chain to attain a cyclic conformation and found that the number of milestones that can be used, without incurring significant errors in the first passage time is at least 8 times that permitted by conventional milestoning. We further demonstrate that, unlike conventional milestoning, MTM permits milestones to be spaced such that trajectories do not have enough time to lose their velocity memory between successively crossed milestones.
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33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xv Molecular dynamics and other numerical methods

A scheme to interpolate potential energy surfaces and derivative coupling vectors without performing a global diabatization

Christian Evenhuis and Todd J. Martínez

J. Chem. Phys. 135, 224110 (2011); http://dx.doi.org/10.1063/1.3660686 (15 pages) | Cited 1 time

Online Publication Date: 13 December 2011

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Simulation of non-adiabatic molecular dynamics requires the description of multiple electronic state potential energy surfaces and their couplings. Ab initio molecular dynamics approaches provide an attractive avenue to accomplish this, but at great computational expense. Interpolation approaches provide a possible route to achieve flexible descriptions of the potential energy surfaces and their couplings at reduced expense. A previously developed approach based on modified Shepard interpolation required global diabatization, which can be problematic. Here, we extensively revise this previous approach, avoiding the need for global diabatization. The resulting interpolated potentials provide only adiabatic energies, gradients, and derivative couplings. This new interpolation approach has been integrated with the ab initio multiple spawning method and it has been rigorously validated against direct dynamics. It is shown that, at least for small molecules, constructing an interpolated PES can be more efficient than performing direct dynamics as measured by the total number of ab initio calculations that are required for a given accuracy.
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31.50.-x Potential energy surfaces
31.15.xv Molecular dynamics and other numerical methods
31.15.A- Ab initio calculations

Thermodynamic integration from classical to quantum mechanics

Scott Habershon and David E. Manolopoulos

J. Chem. Phys. 135, 224111 (2011); http://dx.doi.org/10.1063/1.3666011 (6 pages)

Online Publication Date: 13 December 2011

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We present a new method for calculating quantum mechanical corrections to classical free energies, based on thermodynamic integration from classical to quantum mechanics. In contrast to previous methods, our method is numerically stable even in the presence of strong quantum delocalization. We first illustrate the method and its relationship to a well-established method with an analysis of a one-dimensional harmonic oscillator. We then show that our method can be used to calculate the quantum mechanical contributions to the free energies of ice and water for a flexible water model, a problem for which the established method is unstable.
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03.65.Ge Solutions of wave equations: bound states
05.70.Ce Thermodynamic functions and equations of state

The problem of hole localization in inner-shell states of N2 and CO2 revisited with complete active space self-consistent field approach

Alexandre B. Rocha and Carlos E. V. de Moura

J. Chem. Phys. 135, 224112 (2011); http://dx.doi.org/10.1063/1.3666016 (8 pages) | Cited 1 time

Online Publication Date: 13 December 2011

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Potential energy curves for inner-shell states of nitrogen and carbon dioxide molecules are calculated by inner-shell complete active space self-consistent field (CASSCF) method, which is a protocol, recently proposed, to obtain specifically converged inner-shell states at multiconfigurational level. This is possible since the collapse of the wave function to a low-lying state is avoided by a sequence of constrained optimization in the orbital mixing step. The problem of localization of K-shell states is revisited by calculating their energies at CASSCF level based on both localized and delocalized orbitals. The localized basis presents the best results at this level of calculation. Transition energies are also calculated by perturbation theory, by taking the above mentioned MCSCF function as zeroth order wave function. Values for transition energy are in fairly good agreement with experimental ones. Bond dissociation energies for N2 are considerably high, which means that these states are strongly bound. Potential curves along ground state normal modes of CO2 indicate the occurrence of Renner-Teller effect in inner-shell states.
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33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
33.15.Fm Bond strengths, dissociation energies
31.50.Bc Potential energy surfaces for ground electronic states
31.15.xr Self-consistent-field methods
31.15.xp Perturbation theory

Boiling point determination using adiabatic Gibbs ensemble Monte Carlo simulations: Application to metals described by embedded-atom potentials

Lev D. Gelb and Somendra Nath Chakraborty

J. Chem. Phys. 135, 224113 (2011); http://dx.doi.org/10.1063/1.3665457 (7 pages)

Online Publication Date: 14 December 2011

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The normal boiling points are obtained for a series of metals as described by the “quantum-corrected Sutton Chen” (qSC) potentials [S.-N. Luo, T. J. Ahrens, T. Çağın, A. Strachan, W. A. Goddard III, and D. C. Swift, Phys. Rev. B 68, 134206 (2003)]. Instead of conventional Monte Carlo simulations in an isothermal or expanded ensemble, simulations were done in the constant-NPH adabatic variant of the Gibbs ensemble technique as proposed by Kristóf and Liszi [Chem. Phys. Lett. 261, 620 (1996)]. This simulation technique is shown to be a precise tool for direct calculation of boiling temperatures in high-boiling fluids, with results that are almost completely insensitive to system size or other arbitrary parameters as long as the potential truncation is handled correctly. Results obtained were validated using conventional NVT-Gibbs ensemble Monte Carlo simulations. The qSC predictions for boiling temperatures are found to be reasonably accurate, but substantially underestimate the enthalpies of vaporization in all cases. This appears to be largely due to the systematic overestimation of dimer binding energies by this family of potentials, which leads to an unsatisfactory description of the vapor phase.
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61.20.Ja Computer simulation of liquid structure
61.25.Mv Liquid metals and alloys
64.70.fh Boiling and bubble dynamics
65.20.Jk Studies of thermodynamic properties of specific liquids
back to top Advanced Experimental Techniques

Ultra-sensitive high-precision spectroscopy of a fast molecular ion beam

Andrew A. Mills, Brian M. Siller, Michael W. Porambo, Manori Perera, Holger Kreckel, and Benjamin J. McCall

J. Chem. Phys. 135, 224201 (2011); http://dx.doi.org/10.1063/1.3665925 (8 pages)

Online Publication Date: 13 December 2011

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Direct spectroscopy of a fast molecular ion beam offers many advantages over competing techniques, including the generality of the approach to any molecular ion, the complete elimination of spectral confusion due to neutral molecules, and the mass identification of individual spectral lines. The major challenge is the intrinsic weakness of absorption or dispersion signals resulting from the relatively low number density of ions in the beam. Direct spectroscopy of an ion beam was pioneered by Saykally and co-workers in the late 1980s, but has not been attempted since that time. Here, we present the design and construction of an ion beam spectrometer with several improvements over the Saykally design. The ion beam and its characterization have been improved by adopting recent advances in electrostatic optics, along with a time-of-flight mass spectrometer that can be used simultaneously with optical spectroscopy. As a proof of concept, a noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) setup with a noise equivalent absorption of ∼2 × 10−11 cm−1 Hz−1/2 has been used to observe several transitions of the Meinel 1–0 band of N2+ with linewidths of ∼120 MHz. An optical frequency comb has been used for absolute frequency calibration of transition frequencies to within ∼8 MHz. This work represents the first direct spectroscopy of an electronic transition in an ion beam, and also represents a major step toward the development of routine infrared spectroscopy of rotationally cooled molecular ions.
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37.20.+j Atomic and molecular beam sources and techniques
33.15.Mt Rotation, vibration, and vibration-rotation constants
07.75.+h Mass spectrometers
07.57.Ty Infrared spectrometers, auxiliary equipment, and techniques
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Coherent control of molecular torsion

Shane M. Parker, Mark A. Ratner, and Tamar Seideman

J. Chem. Phys. 135, 224301 (2011); http://dx.doi.org/10.1063/1.3663710 (7 pages) | Cited 1 time

Online Publication Date: 8 December 2011

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We propose a coherent, strong-field approach to control the torsional modes of biphenyl derivatives, and develop a numerical scheme to simulate the torsional dynamics. By choice of the field parameters, the method can be applied either to drive the torsion angle to an arbitrary configuration or to induce free internal rotation. Transient absorption spectroscopy is suggested as a probe of torsional control and the usefulness of this approach is numerically explored. Several consequences of our ability to manipulate molecular torsional motions are considered. These include a method for the inversion of molecular chirality and an ultrafast chiral switch.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.55.+b Optical activity and dichroism
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Infrared absorption of gaseous benzoylperoxy radical C6H5C(O)OO recorded with a step-scan Fourier-transform spectrometer

Barbara Golec, Jin-Dah Chen, and Yuan-Pern Lee

J. Chem. Phys. 135, 224302 (2011); http://dx.doi.org/10.1063/1.3664304 (10 pages)

Online Publication Date: 8 December 2011

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A step-scan Fourier-transform infrared spectrometer coupled with a multipass absorption cell was utilized to monitor the transient species produced in gaseous reactions of benzoyl radical, C6H5CO, with O2. C6H5CO was produced either from photolysis of acetophenone, C6H5C(O)CH3, at 248 nm, or from photolysis of a mixture of benzaldehyde, C6H5CHO, and Cl2 at 355 nm. Two intense bands near 1830 and 1226 cm−1 are assigned to the C=O stretching (ν6) and the C−C stretching mixed with C−H deformation (ν13) modes, and two weaker bands near 1187 and 1108 cm−1 are assigned to the ν14 (C−H deformation) and ν16 (O−O stretching /C−H deformation) modes of C6H5C(O)OO, the benzoylperoxy radical. These observed vibrational wave numbers and relative infrared intensities agree with those reported for syn-C6H5C(O)OO isolated in solid Ar and values predicted for syn-C6H5C(O)OO with the B3LYP/cc-pVTZ method. The simulated rotational contours of the two intense bands based on rotational parameters predicted with the B3LYP/cc-pVTZ method fit satisfactorily with experimental results.
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33.20.Ea Infrared spectra
33.70.Fd Absolute and relative line and band intensities
31.15.E- Density-functional theory
33.15.Mt Rotation, vibration, and vibration-rotation constants
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
33.20.Tp Vibrational analysis

Assessing computationally efficient isomerization dynamics: ΔSCF density-functional theory study of azobenzene molecular switching

Reinhard J. Maurer and Karsten Reuter

J. Chem. Phys. 135, 224303 (2011); http://dx.doi.org/10.1063/1.3664305 (10 pages)

Online Publication Date: 8 December 2011

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We present a detailed comparison of the S0, S1 (n → π*) and S2 (π → π*) potential energy surfaces (PESs) of the prototypical molecular switch azobenzene as obtained by Δ-self-consistent-field (ΔSCF) density-functional theory (DFT), time-dependent DFT (TD-DFT) and approximate coupled cluster singles and doubles (RI-CC2). All three methods unanimously agree in terms of the PES topologies, which are furthermore fully consistent with existing experimental data concerning the photo-isomerization mechanism. In particular, sum-method corrected ΔSCF and TD-DFT yield very similar results for S1 and S2, when based on the same ground-state exchange-correlation (xc) functional. While these techniques yield the correct PES topology already on the level of semi-local xc functionals, reliable absolute excitation energies as compared to RI-CC2 or experiment require an xc treatment on the level of long-range corrected hybrids. Nevertheless, particularly the robustness of ΔSCF with respect to state crossings as well as its numerical efficiency suggest this approach as a promising route to dynamical studies of larger azobenzene-containing systems.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
31.15.xr Self-consistent-field methods
31.15.bw Coupled-cluster theory
31.50.Bc Potential energy surfaces for ground electronic states
31.50.Df Potential energy surfaces for excited electronic states
82.30.Qt Isomerization and rearrangement

High-resolution threshold photoelectron study of the propargyl radical by the vacuum ultraviolet laser velocity-map imaging method

Hong Gao, Yuntao Xu, Lei Yang, Chow-Shing Lam, Hailing Wang, Jingang Zhou, and C. Y. Ng

J. Chem. Phys. 135, 224304 (2011); http://dx.doi.org/10.1063/1.3664864 (7 pages) | Cited 2 times

Online Publication Date: 8 December 2011

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By employing the vacuum ultraviolet (VUV) laser velocity-map imaging (VMI) photoelectron scheme to discriminate energetic photoelectrons, we have measured the VUV-VMI-threshold photoelectrons (VUV-VMI-TPE) spectra of propargyl radical [C3H3(math2B1)] near its ionization threshold at photoelectron energy bandwidths of 3 and 7 cm−1 (full-width at half-maximum, FWHM). The simulation of the VUV-VMI-TPE spectra thus obtained, along with the Stark shift correction, has allowed the determination of a precise value 70 156 ± 4 cm−1 (8.6982 ± 0.0005 eV) for the ionization energy (IE) of C3H3. In the present VMI-TPE experiment, the Stark shift correction is determined by comparing the VUV-VMI-TPE and VUV laser pulsed field ionization-photoelectron (VUV-PFI-PE) spectra for the origin band of the photoelectron spectrum of the math+−math transition of chlorobenzene. The fact that the FWHMs for this origin band observed using the VUV-VMI-TPE and VUV-PFI-PE methods are nearly the same indicates that the energy resolutions achieved in the VUV-VMI-TPE and VUV-PFI-PE measurements are comparable. The IE(C3H3) value obtained based on the VUV-VMI-TPE measurement is consistent with the value determined by the VUV laser PIE spectrum of supersonically cooled C3H3(math2B1) radicals, which is also reported in this article.
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33.60.+q Photoelectron spectra
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Copper doping of small gold cluster cations: Influence on geometric and electronic structure

Sandra M. Lang, Pieterjan Claes, Ngo Tuan Cuong, Minh Tho Nguyen, Peter Lievens, and Ewald Janssens

J. Chem. Phys. 135, 224305 (2011); http://dx.doi.org/10.1063/1.3664307 (12 pages)

Online Publication Date: 9 December 2011

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The effect of Cu doping on the properties of small gold cluster cations is investigated in a joint experimental and theoretical study. Temperature-dependent Ar tagging of the clusters serves as a structural probe and indicates no significant alteration of the geometry of Aun+ (n = 1–16) upon Cu doping. Experimental cluster–argon bond dissociation energies are derived as a function of cluster size from equilibrium mass spectra and are in the 0.10–0.25 eV range. Near-UV and visible light photodissociation spectroscopy is employed in conjunction with time-dependent density functional theory calculations to study the electronic absorption spectra of Au4-mCum+ (m = 0, 1, 2) and their Ar complexes in the 2.00−3.30 eV range and to assign their fragmentation pathways. The tetramers Au4+, Au4+·Ar, Au3Cu+, and Au3Cu+·Ar exhibit distinct optical absorption features revealing a pronounced shift of electronic excitations to larger photon energies upon substitution of Au by Cu atoms. The calculated electronic excitation spectra and an analysis of the character of the optical transitions provide detailed insight into the composition-dependent evolution of the electronic structure of the clusters.
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31.15.ee Time-dependent density functional theory
33.20.Kf Visible spectra
33.20.Lg Ultraviolet spectra
36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Vz Optical properties of clusters

High-resolution Fourier-transform infrared spectroscopy of the ν6 and Coriolis perturbation allowed ν10 modes of ketenimine

Michael K. Bane, Evan G. Robertson, Christopher D. Thompson, Dominique R. T. Appadoo, and Don McNaughton

J. Chem. Phys. 135, 224306 (2011); http://dx.doi.org/10.1063/1.3664624 (6 pages)

Online Publication Date: 9 December 2011

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High-resolution FTIR spectra of the short lived species ketenimine have been recorded in the region 700–1300 cm−1 and over 1500 transitions of the ν10 and ν6 modes have been assigned. Effective rotational and centrifugal distortion parameters for the v10 = 1 and v6 = 1 (excluding Ka = 5) states were determined by co-fitting transitions, and treating strong a- and c-axis Coriolis interactions between them. Other perturbations attributed to interactions with the v8 = 2 and v12 = 1 + v8 = 1 dark-states were also observed and treated. The ν10 transitions are predicted to be inherently very weak, but are enhanced by an intensity stealing effect with the highly IR active ν6 mode. A mechanism for this intensity stealing in ketenimine is also detailed.
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33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants

The infrared spectrum of NN⋯CO+ trapped in solid neon

Warren E. Thompson and Marilyn E. Jacox

J. Chem. Phys. 135, 224307 (2011); http://dx.doi.org/10.1063/1.3666046 (7 pages)

Online Publication Date: 13 December 2011

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Codeposition of a Ne:N2:CO = 200:1:1 mixture at 4.3 K with a beam of very pure neon atoms excited to their energy levels between 16.6 and 16.85 eV leads to stabilization in the resulting solid of sufficient NNCO+ for detection of its NN- and CO-stretching vibration fundamentals. Detailed isotopic substitution studies and density functional calculations for the various isotopologues support the identification of NNCO+ and permit estimation of the positions of two of its low-frequency fundamentals. A sufficient concentration of NOCN is also stabilized in the neon matrix for detection of its NO-stretching vibrational fundamental
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33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.30.Gs Hyperfine interactions and isotope effects
31.15.E- Density-functional theory

Field-free molecular alignment control by phase-shaped femtosecond laser pulse

Shian Zhang, Chenhui Lu, Tianqing Jia, Zhenrong Sun, and Jianrong Qiu

J. Chem. Phys. 135, 224308 (2011); http://dx.doi.org/10.1063/1.3666850 (4 pages)

Online Publication Date: 14 December 2011

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In this paper, we theoretically show that the field-free molecular alignment can be controlled by shaping the femtosecond laser pulse with a periodic phase step modulation, involving the maximum degree and temporal structure of the molecular alignment. We show that the molecular alignment can be completely suppressed or reconstructed as that by the transform-limited laser pulse, the temporal structure of the alignment transient can be controlled with a desired shape, and the molecular alignment and antialignment for any temporal structure can be switched. Furthermore, we also show that both the degree and direction of the molecular alignment at a fix time delay can be continuously modulated.
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33.80.Wz Other multiphoton processes

Rotational spectra and properties of complexes B⋯ICF3 (B = Kr or CO) and a comparison of the efficacy of ICl and ICF3 as iodine donors in halogen bond formation

Susanna L. Stephens, Nicholas R. Walker, and Anthony C. Legon

J. Chem. Phys. 135, 224309 (2011); http://dx.doi.org/10.1063/1.3664314 (8 pages) | Cited 1 time

Online Publication Date: 14 December 2011

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The ground-state rotational spectra of two weakly bound complexes B⋯ICF3 (B = Kr or CO) formed by trifluoroiodomethane have been observed in pulsed jets by using two types of Fourier-transform microwave spectroscopy (chirped-pulse and Fabry-Perot cavity). Both complexes exhibit symmetric-top type spectra, thus indicating that the Kr atom in Kr⋯ICF3 and both the C and O atoms in OC⋯ICF3 lie along the C3 axis of ICF3. The rotational constant B0, the centrifugal distortion constants DJ and DJK, and the iodine nuclear quadrupole coupling constant χaa(I) were determined for each of the isotopologues 84Kr⋯ICF3, 86Kr⋯ICF3, 16O12C⋯ICF3, 16O13C⋯ICF3, and 18O12C⋯ICF3. Interpretation of the spectroscopic constants reveals that the carbon atom of CO is adjacent to I and participates in the weak bond in OC⋯ICF3. Simple models based on unperturbed component geometries lead to the distances r(Kr⋯I) = 3.830(1) Å and r(C⋯I) = 3.428(1) Å in Kr⋯ICF3 and OC⋯ICF3, respectively, and to the quadratic force constants for stretching of the weak bond kσ = 2.80 N m−1 and 3.96 N m−1, respectively. The distances r(Z⋯I) (Z is the acceptor atom in B), the kσ values, and the angular geometries of the pair of complexes B⋯ICF3 and B⋯ICl for a given B are compared when B = Kr, CO, H2O, H2S, or NH3. The comparison reveals that the iodine bond in B⋯ICF3 is systematically longer and weaker than that of B⋯ICl, while the angular geometry of the B⋯I moiety is isomorphic in B⋯ICF3 and B⋯ICl for a given B. It is concluded that −CF3 is less effective than −Cl as an electron-withdrawing group when attached to an I atom and that the angular geometries of the B⋯ICF3 can be predicted by means of a simple rule that holds for many hydrogen- and halogen-bonded complexes.
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33.20.Bx Radio-frequency and microwave spectra
33.25.+k Nuclear resonance and relaxation
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Density-scaling and the Prigogine–Defay ratio in liquids

R. Casalini, R. F. Gamache, and C. M. Roland

J. Chem. Phys. 135, 224501 (2011); http://dx.doi.org/10.1063/1.3664180 (5 pages)

Online Publication Date: 8 December 2011

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The term “strongly correlating liquids” refers to materials exhibiting near proportionality of fluctuations in the potential energy and the virial pressure, as seen in molecular dynamics simulations of liquids whose interactions are comprised primarily of van der Waals forces. Recently it was proposed that the Prigogine–Defay ratio, Π, of strongly correlating liquids should fall close to unity. We verify this prediction herein by showing that the degree to which relaxation times are a function T/ργ, the ratio of temperature to density with the latter raised to a material constant (a property inherent to strongly correlating liquids) is reflected in values of Π closer to unity. We also show that the dynamics of strongly correlating liquids are governed more by density than by temperature. Thus, while Π may never strictly equal 1 for the glass transition, it is approximately unity for many materials, and thus can serve as a predictor of other dynamic behavior. For example, Π ≫ 1 is indicative of additional control parameters besides T/ργ.
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61.20.Ja Computer simulation of liquid structure
64.70.P- Glass transitions of specific systems
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