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28 Jan 2012

Volume 136, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Chem. Phys. 136, 041101 (2012); http://dx.doi.org/10.1063/1.3678309 (4 pages)

José J. Plata, Antonio M. Márquez, and Javier Fdez. Sanz
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Communication: Improving the density functional theory+U description of CeO2 by including the contribution of the O 2p electrons

José J. Plata, Antonio M. Márquez, and Javier Fdez. Sanz

J. Chem. Phys. 136, 041101 (2012); http://dx.doi.org/10.1063/1.3678309 (4 pages)

Online Publication Date: 23 January 2012

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Density functional theory (DFT) based approaches within the local-density approximation or generalized gradient approximation frameworks fail to predict the correct electron localization in strongly correlated systems due to the lack of cancellation of the Coulomb self-interaction. This problem might be circumvented either by using hybrid functionals or by introducing a Hubbard-like term to account for the on site interactions. This latter DFT+U approach is less expensive and therefore more practical for extensive calculations in solid-state computational simulations. By and large, the U term only affects the metal electrons, in our case the Ce 4f ones. In the present work, we report a systematic analysis of the effect of adding such a U term also to the oxygen 2p electrons. We find that using a set of Uf = 5 eV and Up = 5eV effective terms leads to improved description of the lattice parameters, band gaps, and formation and reduction energies of CeO2.
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71.20.Ps Other inorganic compounds
71.27.+a Strongly correlated electron systems; heavy fermions
61.66.Fn Inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
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Communication: Crystallite nucleation in supercooled glycerol near the glass transition

Hai-Feng Yuan, Ted Xia, Marie Plazanet, Bruno Demé, and Michel Orrit

J. Chem. Phys. 136, 041102 (2012); http://dx.doi.org/10.1063/1.3681292 (4 pages)

Online Publication Date: 24 January 2012

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Heterogeneity and solid-like structures found near the glass transition provide a key to a better understanding of supercooled liquids and of the glass transition. However, the formation of solid-like structures and its effect on spatial heterogeneity in supercooled liquids is neither well documented nor well understood. In this work, we reveal the crystalline nature of the solid-like structures in supercooled glycerol by means of neutron scattering. The results indicate that inhomogeneous nucleation happens at temperatures near Tg. Nevertheless, the thermal history of the sample is essential for crystallization. This implies such structures in supercooled liquids strongly depend on thermal history. Our work suggests that different thermal histories may lead to different structures and therefore to different length and time scales of heterogeneity near the glass transition.
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64.60.qj Studies of nucleation in specific substances
64.70.dg Crystallization of specific substances
64.70.pm Liquids
61.25.Em Molecular liquids
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Communication: A chemically accurate global potential energy surface for the HO + CO → H + CO2 reaction

Jun Li, Yimin Wang, Bin Jiang, Jianyi Ma, Richard Dawes, Daiqian Xie, Joel M. Bowman, and Hua Guo

J. Chem. Phys. 136, 041103 (2012); http://dx.doi.org/10.1063/1.3680256 (4 pages)

Online Publication Date: 24 January 2012

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We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ∼35 000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data.
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82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.33.Vx Reactions in flames, combustion, and explosions
31.15.bw Coupled-cluster theory
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Communication: Correlation of the instantaneous and the intermediate-time elasticity with the structural relaxation in glassforming systems

F. Puosi and D. Leporini

J. Chem. Phys. 136, 041104 (2012); http://dx.doi.org/10.1063/1.3681291 (4 pages)

Online Publication Date: 26 January 2012

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The elastic models of the glass transition relate the increasing solidity of the glassforming systems with the huge slowing down of the structural relaxation and the viscous flow. The solidity is quantified in terms of the instantaneous shear modulus G∞, i.e., the immediate response to a step change in the strain. By molecular-dynamics simulations of a model polymer system, one shows the virtual absence of correlations between the instantaneous elasticity and the structural relaxation. Instead, a well-defined scaling is evidenced by considering the elastic response observed at intermediate times after the initial fast stress relaxation. The scaling regime ranges from sluggish states with virtually pure elastic response on the picosecond time scale up to high-mobility states where fast restructuring events are more apparent.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.de Elastic moduli
62.20.fq Plasticity and superplasticity
64.70.pj Polymers
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
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back to top Theoretical Methods and Algorithms

The coupled cluster singles, doubles, and a hybrid treatment of connected triples based on the split virtual orbitals

Jun Shen, Zhuangfei Kou, Enhua Xu, and Shuhua Li

J. Chem. Phys. 136, 044101 (2012); http://dx.doi.org/10.1063/1.3678008 (9 pages)

Online Publication Date: 23 January 2012

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We have proposed a simple strategy for splitting the virtual orbitals with a large basis set into two subgroups (active and inactive) by taking a smaller basis set as an auxiliary basis set. With the split virtual orbitals (SVOs), triple or higher excitations can be partitioned into active and inactive subgroups (according to the number of active virtual orbitals involved), which can be treated with different electron correlation methods. In this work, the coupled cluster (CC) singles, doubles, and a hybrid treatment of connected triples based on the SVO [denoted as SVO-CCSD(T)-h], has been implemented. The present approach has been applied to study the bond breaking potential energy surfaces in three molecules (HF, F2, and N2), and the equilibrium properties in a number of open-shell diatomic molecules. For all systems under study, the SVO-CCSD(T)-h method based on the unrestricted Hartree–Fock (UHF) reference is an excellent approximation to the corresponding CCSDT (CC singles, doubles, and triples), and much better than the UHF-based CCSD(T) (CC singles, doubles, and perturbative triples). On the other hand, the SVO-CCSD(T)-h method based on the restricted HF (RHF) reference can also provide considerable improvement over the RHF-based CCSD(T).
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31.15.bw Coupled-cluster theory
31.15.vj Electron correlation calculations for atoms and ions: excited states
31.15.xr Self-consistent-field methods
31.50.Df Potential energy surfaces for excited electronic states
33.15.Fm Bond strengths, dissociation energies

Prediction of reaction barriers and force-induced instabilities under mechanochemical conditions with an approximate model: A case study of the ring opening of 1,3-cyclohexadiene

Adrian Bailey and Nicholas J. Mosey

J. Chem. Phys. 136, 044102 (2012); http://dx.doi.org/10.1063/1.3678010 (11 pages)

Online Publication Date: 23 January 2012

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Mechanochemistry, the use of mechanical stresses to activate chemical reactions, has emerged as a topic of significant interest. The present study examines the use of an approximate model for the prediction of reaction barriers under mechanochemical conditions using the ring opening of 1,3-cyclohexadiene along conrotatory and disrotatory directions as a specific test case. To do this, reaction barriers are evaluated using quantum chemical methods with an external force applied between various pairs of atoms. The results show that the consequent effects on the barrier exhibit a significant dependence on the locations of the atoms used to apply the external force, and in some cases, force-induced instabilities occur that alter the fundamental nature of the reaction pathway. The ability of an approximate model based on a second-order expansion of the force-modified potential energy with respect to nuclear coordinates to reproduce this behavior is then assessed. Good agreement between the results obtained through the quantum chemical calculations and approximate model is attained when force-induced instabilities do not occur. In addition, a strategy for predicting when such instabilities occur is presented and found to yield results that are in qualitative agreement with the quantum chemical calculations. Finally, the response of the system to the external force is interpreted in terms of the parameters entering the model, which correspond to interatomic distances and stiffnesses, and possibly sheds lights on ways to design molecules that exhibit a desired chemical response to mechanochemical conditions.
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82.20.Kh Potential energy surfaces for chemical reactions

Generalized essential energy space random walks to more effectively accelerate solute sampling in aqueous environment

Chao Lv, Lianqing Zheng, and Wei Yang

J. Chem. Phys. 136, 044103 (2012); http://dx.doi.org/10.1063/1.3678220 (10 pages)

Online Publication Date: 23 January 2012

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Molecular dynamics sampling can be enhanced via the promoting of potential energy fluctuations, for instance, based on a Hamiltonian modified with the addition of a potential-energy-dependent biasing term. To overcome the diffusion sampling issue, which reveals the fact that enlargement of event-irrelevant energy fluctuations may abolish sampling efficiency, the essential energy space random walk (EESRW) approach was proposed earlier. To more effectively accelerate the sampling of solute conformations in aqueous environment, in the current work, we generalized the EESRW method to a two-dimension-EESRW (2D-EESRW) strategy. Specifically, the essential internal energy component of a focused region and the essential interaction energy component between the focused region and the environmental region are employed to define the two-dimensional essential energy space. This proposal is motivated by the general observation that in different conformational events, the two essential energy components have distinctive interplays. Model studies on the alanine dipeptide and the aspartate-arginine peptide demonstrate sampling improvement over the original one-dimension-EESRW strategy; with the same biasing level, the present generalization allows more effective acceleration of the sampling of conformational transitions in aqueous solution. The 2D-EESRW generalization is readily extended to higher dimension schemes and employed in more advanced enhanced-sampling schemes, such as the recent orthogonal space random walk method.
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02.70.Ns Molecular dynamics and particle methods
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
02.50.-r Probability theory, stochastic processes, and statistics

Molecular properties via a subsystem density functional theory formulation: A common framework for electronic embedding

Sebastian Höfener, André Severo Pereira Gomes, and Lucas Visscher

J. Chem. Phys. 136, 044104 (2012); http://dx.doi.org/10.1063/1.3675845 (16 pages)

Online Publication Date: 24 January 2012

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In this article, we present a consistent derivation of a density functional theory (DFT) based embedding method which encompasses wave-function theory-in-DFT (WFT-in-DFT) and the DFT-based subsystem formulation of response theory (DFT-in-DFT) by Neugebauer [J. Neugebauer, J. Chem. Phys. 131, 084104 (2009)10.1063/1.3212883] as special cases. This formulation, which is based on the time-averaged quasi-energy formalism, makes use of the variation Lagrangian techniques to allow the use of non-variational (in particular: coupled cluster) wave-function-based methods. We show how, in the time-independent limit, we naturally obtain expressions for the ground-state DFT-in-DFT and WFT-in-DFT embedding via a local potential. We furthermore provide working equations for the special case in which coupled cluster theory is used to obtain the density and excitation energies of the active subsystem. A sample application is given to demonstrate the method.
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31.15.bw Coupled-cluster theory
31.15.vj Electron correlation calculations for atoms and ions: excited states

Increasing the applicability of density functional theory. II. Correlation potentials from the random phase approximation and beyond

Prakash Verma and Rodney J. Bartlett

J. Chem. Phys. 136, 044105 (2012); http://dx.doi.org/10.1063/1.3678180 (8 pages)

Online Publication Date: 26 January 2012

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Density functional theory (DFT) results are mistrusted at times due to the presence of an unknown exchange correlation functional, with no practical way to guarantee convergence to the right answer. The use of a known exchange correlation functional based on wave-function theory helps to alleviate such mistrust. The exchange correlation functionals can be written exactly in terms of the density-density response function using the adiabatic-connection and fluctuation-dissipation framework. The random phase approximation (RPA) is the simplest approximation for the density-density response function. Since the correlation functional obtained from RPA is equivalent to the direct ring coupled cluster doubles (ring-CCD) correlation functional, meaning only Coulomb interactions are included, one can bracket RPA between many body perturbation theory (MBPT)-2 and CCD with the latter having all ring, ladder, and exchange contributions. Using an optimized effective potential strategy, we obtain correlation potentials corresponding to MBPT-2, RPA (ring-CCD), linear-CCD, and CCD. Using the suitable choice of the unperturbed Hamiltonian, Kohn-Sham self-consistent calculations are performed. The spatial behavior of the resulting potentials, total energies, and the HOMO eigenvalues are compared with the exact values for spherical atoms. Further, we demonstrate that the self-consistent eigenvalues obtained from these consistent potentials used in ab initio dft approximate all principal ionization potentials as demanded by ionization potential theorem.
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31.15.eg Exchange-correlation functionals (in current density functional theory)
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
31.15.bw Coupled-cluster theory

The electric double layer structure around charged spherical interfaces

Zhenwei Yao, Mark J. Bowick, and Xu Ma

J. Chem. Phys. 136, 044106 (2012); http://dx.doi.org/10.1063/1.3681147 (4 pages)

Online Publication Date: 30 January 2012

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We derive a formally simple approximate analytical solution to the Poisson-Boltzmann equation for the spherical system via a geometric mapping. Its regime of applicability in the parameter space of the spherical radius and the surface potential is determined, and its superiority over the linearized solution is demonstrated.
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82.45.-h Electrochemistry and electrophoresis

Coherently controlled molecular junctions

Uri Peskin and Michael Galperin

J. Chem. Phys. 136, 044107 (2012); http://dx.doi.org/10.1063/1.3676047 (7 pages)

Online Publication Date: 30 January 2012

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Within a generic model, we discuss the possibility of coherent control of charge fluxes in unbiased molecular junctions. The control is induced by resonances between the Rabi frequency due to a pumping laser field and internal characteristic frequencies of pre-designed molecular donor-bridge-acceptor complexes. Two models are considered: a coherently controlled molecular charge pump and a molecular switch. The study generalizes previous consideration of light induced current [M. Galperin and A. Nitzan, Phys. Rev. Lett. 95, 206802 (2005)10.1103/PhysRevLett.95.206802] and of a molecular electron pump [R. Volkovich and U. Peskin, Phys. Rev. B 83, 033403 (2011)10.1103/PhysRevB.83.033403] and accounts for the coherently driven charge transport in an unbiased molecular junction with symmetric coupling to leads. Numerical examples demonstrate the feasibility of the control mechanism for realistic junctions parameters.
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85.65.+h Molecular electronic devices
84.30.Jc Power electronics; power supply circuits

Parallel density matrix propagation in spin dynamics simulations

Luke J. Edwards and Ilya Kuprov

J. Chem. Phys. 136, 044108 (2012); http://dx.doi.org/10.1063/1.3679656 (7 pages)

Online Publication Date: 31 January 2012

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Several methods for density matrix propagation in parallel computing environments are proposed and evaluated. It is demonstrated that the large communication overhead associated with each propagation step (two-sided multiplication of the density matrix by an exponential propagator and its conjugate) may be avoided and the simulation recast in a form that requires virtually no inter-thread communication. Good scaling is demonstrated on a 128-core (16 nodes, 8 cores each) cluster.
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03.67.Lx Quantum computation architectures and implementations

Robust interpolation between weak- and strong-correlation regimes of quantum systems

Jerzy Cioslowski

J. Chem. Phys. 136, 044109 (2012); http://dx.doi.org/10.1063/1.3679657 (5 pages) | Cited 1 time

Online Publication Date: 31 January 2012

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A robust interpolation between the weak- and strong-correlation regimes of quantum systems is presented. It affords approximants to the function E(ω) describing the dependence of the total energy (or other observable) on the coupling parameter ω that measures the correlation strength. The approximants conform to truncations of the asymptotic expansions of E(ω) at the ω → 0 and ω → ∞ limits with arbitrary (but given) numbers of terms. In addition, depending on the number of fitted parameters, they either reproduce or optimally (in the least-square or maximum-error sense) approximate the exact E(ω) at any given number of values of the coupling strength. Numerical tests demonstrate the high accuracy of even the low-order approximate expression for E(ω). The approximants, which do not suffer from spurious poles, possess a wide range of applicability that stems from their capability of accurately reproducing not only E(ω) but also its derivatives with respect to ω. They are equally useful for interpolation between the low- and high-temperature limits of energy and other quantities associated with various models of statistical thermodynamics. The new interpolation scheme is not applicable to the cases where the weak- and strong-correlation asymptotics involve non-analytic functions of ω or expressions dependent on logarithm of the coupling strength. Excluded are also the cases where the weak- and strong-correlation asymptotics pertain to de facto different states, e.g., the ground state of a homogeneous electron gas in three dimensions.
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05.30.Fk Fermion systems and electron gas
05.70.-a Thermodynamics
03.65.Yz Decoherence; open systems; quantum statistical methods
back to top Advanced Experimental Techniques

Influence of structure in heterodyne electrophoretic light scattering

Martin Medebach

J. Chem. Phys. 136, 044201 (2012); http://dx.doi.org/10.1063/1.3678315 (9 pages)

Online Publication Date: 26 January 2012

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Heterodyne light scattering is employed to probe the electrophoretic flow behavior of a concentrated, deionized model colloidal suspension. Both for the fluid and crystalline regime, the power spectra are well described as velocity distributions, which let assume that the power spectra are dominated by the self-dynamic structure factor. Furthermore, all measured power spectra show an increase of the frequency integrated intensity with the electric field. Since the number of particles should remain constant this finding is unexpected. These effects are rationalized on the basis of a new theoretical model including the influence of the structure factor and of the mobility polydispersity on the power spectra.
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82.70.Dd Colloids
82.70.Kj Emulsions and suspensions
78.30.C- Liquids
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.20.Ea Infrared spectra
82.45.-h Electrochemistry and electrophoresis
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Experimental verification of strong rotational dependence of fluorescence and predissociation yield in the b 1Πu(v = 1) level of 14N2

C. Y. Robert Wu, D. L. Judge, M.-H. Tsai, Y.-C. Lin, T.-S. Yih, J.-I. Lo, H.-S. Fung, Y.-Y. Lee, B. R. Lewis, A. N. Heays, and S. T. Gibson

J. Chem. Phys. 136, 044301 (2012); http://dx.doi.org/10.1063/1.3676781 (3 pages)

Online Publication Date: 23 January 2012

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New, rotationally resolved fluorescence-excitation spectra confirm coupled-channel Schrödinger-equation predictions of strong rotational dependence of the fluorescence and predissociation yields in the b(v = 1) level of 14N2.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.50.Dq Fluorescence and phosphorescence spectra

Gas-phase photodissociation of CH3COCN at 308 nm by time-resolved Fourier-transform infrared emission spectroscopy

Yu-Ying Yeh, Meng-Hsuan Chao, Po-Yu Tsai, Yuan-Bin Chang, Ming-Tsang Tsai, and King-Chuen Lin

J. Chem. Phys. 136, 044302 (2012); http://dx.doi.org/10.1063/1.3674166 (10 pages)

Online Publication Date: 23 January 2012

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By using time-resolved Fourier-transform infrared emission spectroscopy, the fragments of HCN(v = 1, 2) and CO(v = 1–3) are detected in one-photon dissociation of acetyl cyanide (CH3COCN) at 308 nm. The S1(A″), 1(nO, π*CO) state at 308 nm has a radiative lifetime of 0.46 ± 0.01 μs, long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of Ar collision-induced internal conversion is estimated to be (1–7) × 10−12 cm3 molecule−1 s−1. The measurements of O2 dependence exclude the production possibility of these fragments via intersystem crossing. The high-resolution spectra of HCN and CO are analyzed to determine the ro-vibrational energy deposition of 81 ± 7 and 32 ± 3 kJ/mol, respectively. With the aid of ab initio calculations, a two-body dissociation on the energetic ground state is favored leading to HCN + CH2CO, in which the CH2CO moiety may further undergo secondary dissociation to release CO. The production of CO2 in the reaction with O2 confirms existence of CH2 and a secondary reaction product of CO. The HNC fragment is identified but cannot be assigned, as restricted to a poor signal-to-noise ratio. Because of insufficient excitation energy at 308 nm, the CN and CH3 fragments that dominate the dissociation products at 193 nm are not detected.
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82.50.Bc Processes caused by infrared radiation
33.20.Ea Infrared spectra
33.50.Hv Radiationless transitions, quenching
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.20.Pm Rate constants, reaction cross sections, and activation energies

Phase-only shaped laser pulses in optimal control theory: Application to indirect photofragmentation dynamics in the weak-field limit

Chuan-Cun Shu and Niels E. Henriksen

J. Chem. Phys. 136, 044303 (2012); http://dx.doi.org/10.1063/1.3678013 (6 pages)

Online Publication Date: 23 January 2012

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We implement phase-only shaped laser pulses within quantum optimal control theory for laser-molecule interaction. This approach is applied to the indirect photofragmentation dynamics of NaI in the weak-field limit. It is shown that optimized phase-modulated pulses with a fixed frequency distribution can substantially modify transient dissociation probabilities as well as the momentum distribution associated with the relative motion of Na and I.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
82.50.Nd Control of photochemical reactions
82.53.-k Femtochemistry

Vibrational energy relaxation of benzene dimer and trimer in the CH stretching region studied by picosecond time-resolved IR-UV pump-probe spectroscopy

Ryoji Kusaka, Yoshiya Inokuchi, and Takayuki Ebata

J. Chem. Phys. 136, 044304 (2012); http://dx.doi.org/10.1063/1.3676658 (8 pages)

Online Publication Date: 23 January 2012

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Vibrational energy relaxation (VER) of the Fermi polyads in the CH stretching vibration of the benzene dimer (Bz2) and trimer (Bz3) has been investigated by picosecond (ps) time-resolved IR-UV pump-probe spectroscopy in a supersonic beam. The vibrational bands in the 3000–3100 cm−1 region were excited by a ps IR pulse and the time evolutions at the pumped and redistributed (bath) levels were probed by resonance enhanced multiphoton ionization with a ps UV pulse. For Bz2, a site-selective excitation in the T-shaped structure was achieved by using the isotope-substituted heterodimer hd, where h = C6H6 and d = C6D6, and its result was compared with that of hh homodimer. In the hd heterodimer, the two isomers, h(stem)d(top) and h(top)d(stem), show remarkable site-dependence of the lifetime of intracluster vibrational energy redistribution (IVR); the lifetime of the Stem site [h(stem)d(top), 140–170 ps] is ∼2.5 times shorter than that of the Top site [h(top)d(stem), 370–400 ps]. In the transient UV spectra, a broad electronic transition due to the bath modes emerges and gradually decays with a nanosecond time scale. The broad transition shows different time profile depending on UV frequency monitored. These time profiles are described by a three-step VER model involving IVR and vibrational predissociation: initial → bath1(intramolecular) → bath2(intermolecular) → fragments. This model also describes well the observed time profile of the Bz fragment. The hh homodimer shows the stepwise VER process with time constants similar to those of the hd dimer, suggesting that the excitation-exchange coupling of the vibrations between the two sites is very weak. Bz3 also exhibited the stepwise VER process, though each step is faster than Bz2.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Ea Infrared spectra
33.20.Kf Visible spectra
33.80.Eh Autoionization, photoionization, and photodetachment
36.40.Cg Electronic and magnetic properties of clusters

Mid-infrared spectroscopy of molecular ions in helium nanodroplets

Xiaohang Zhang, Nils B. Brauer, Giel Berden, Anouk M. Rijs, and Marcel Drabbels

J. Chem. Phys. 136, 044305 (2012); http://dx.doi.org/10.1063/1.3678011 (10 pages) | Cited 2 times

Online Publication Date: 24 January 2012

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High resolution IR spectra of aniline, styrene, and 1,1-diphenylethylene cations embedded in superfluid helium nanodroplets have been recorded in the 300–1700 cm−1 range using a free-electron laser as radiation source. Comparison of the spectra with available gas phase data reveals that the helium environment induces no significant matrix shift nor leads to an observable line broadening of the resonances. In addition, the IR spectra have provided new and improved vibrational transition frequencies for the cations investigated, as well as for neutral aniline and styrene. Indications have been found that the ions desolvate from the droplets after excitation by a non-evaporative process in which they are ejected from the helium droplets. The kinetic energy of the ejected ions is found to be ion specific and to depend only weakly on the excitation energy.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis

Microwave, infrared and Raman spectra, r0 structural parameters, ab initio calculations and vibrational assignment of 1-fluoro-1-silacyclopentane

James R. Durig, Savitha S. Panikar, Daniel A. Obenchain, Brandon J. Bills, Patrick M. Lohan, Rebecca A. Peebles, Sean A. Peebles, Peter Groner, Gamil A. Guirgis, and Michael D. Johnston

J. Chem. Phys. 136, 044306 (2012); http://dx.doi.org/10.1063/1.3673889 (10 pages)

Online Publication Date: 25 January 2012

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The microwave spectrum (6500–18 500 MHz) of 1-fluoro-1-silacyclopentane, c-C4H8SiHF has been recorded and 87 transitions for the 28Si, 29Si, 30Si, and 13C isotopomers have been assigned for a single conformer. Infrared spectra (3050-350 cm−1) of the gas and solid and Raman spectrum (3100-40 cm−1) of the liquid have also been recorded. The vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twist form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but much lower energy than the planar conformer. By utilizing the microwave rotational constants for seven isotopomers (28Si, 29Si, 30Si, and four 13C) combined with the structural parameters predicted from the MP2(full)/6–311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the twist conformer. The heavy atom distances in Å are: r0(SiC2) = 1.875(3); r0(SiC3) = 1.872(3); r0(C2C4) = 1.549(3); r0(C3C5) = 1.547(3); r0(C4C5) = 1.542(3); r0(SiF) = 1.598(3) and the angles in degrees are: ∠CSiC = 96.7(5); ∠SiC2C4 = 103.6(5); ∠SiC3C5 = 102.9(5); ∠C2C4C5 = 108.4(5); ∠C3C5C4 = 108.1(5); ∠F6Si1C2 = 110.7(5); ∠F6Si1C3 = 111.6(5). The heavy atom ring parameters are compared to the corresponding rs parameters. Normal coordinate calculations with scaled force constants from MP2(full)/6–31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values, and infrared band contours. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.
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33.20.Bx Radio-frequency and microwave spectra
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
33.20.Tp Vibrational analysis
33.20.Sn Rotational analysis
33.20.Fb Raman and Rayleigh spectra (including optical scattering)
33.20.Ea Infrared spectra

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the Cl(2P, 2P3/2) + CH4 → HCl + CH3 and H + CH3Cl reactions

Gábor Czakó and Joel M. Bowman

J. Chem. Phys. 136, 044307 (2012); http://dx.doi.org/10.1063/1.3679014 (18 pages)

Online Publication Date: 25 January 2012

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We report a high-quality, ab initio, full-dimensional global potential energy surface (PES) for the Cl(2P, 2P3/2) + CH4 reaction, which describes both the abstraction (HCl + CH3) and substitution (H + CH3Cl) channels. The analytical PES is a least-squares fit, using a basis of permutationally invariant polynomials, to roughly 16 000 ab initio energy points, obtained by an efficient composite method, including counterpoise and spin-orbit corrections for the entrance channel. This composite method is shown to provide accuracy almost equal to all-electron CCSD(T)/aug-cc-pCVQZ results, but at much lower computational cost. Details of the PES, as well as additional high-level benchmark characterization of structures and energetics are reported. The PES has classical barrier heights of 2650 and 15 060 cm−1 (relative to Cl(2P3/2) + CH4(eq)), respectively, for the abstraction and substitution reactions, in good agreement with the corresponding new computed benchmark values, 2670 and 14 720 cm−1. The PES also accurately describes the potential wells in the entrance and exit channels for the abstraction reaction. Quasiclassical trajectory calculations using the PES show that (a) the inclusion of the spin-orbit corrections in the PES decreases the cross sections by a factor of 1.5–2.5 at low collision energies (Ecoll); (b) at Ecoll ≈ 13 000 cm−1 the substitution channel opens and the H/HCl ratio increases rapidly with Ecoll; (c) the maximum impact parameter (bmax) for the abstraction reaction is ∼6 bohr; whereas bmax is only ∼2 bohr for the substitution; (d) the HCl and CH3 products are mainly in the vibrational ground state even at very high Ecoll; and (e) the HCl rotational distributions are cold, in excellent agreement with experiment at Ecoll = 1280 cm−1.
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82.20.Kh Potential energy surfaces for chemical reactions
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
82.60.-s Chemical thermodynamics
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory

Ultraviolet photodissociation dynamics of the phenyl radical

Yu Song, Michael Lucas, Maria Alcaraz, Jingsong Zhang, and Christopher Brazier

J. Chem. Phys. 136, 044308 (2012); http://dx.doi.org/10.1063/1.3679166 (10 pages)

Online Publication Date: 26 January 2012

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Ultraviolet (UV) photodissociation dynamics of jet-cooled phenyl radicals (C6H5 and C6D5) are studied in the photolysis wavelength region of 215–268 nm using high-n Rydberg atom time-of-flight and resonance enhanced multiphoton ionization techniques. The phenyl radicals are produced from 193-nm photolysis of chlorobenzene and bromobenzene precursors. The H-atom photofragment yield spectra have a broad peak centered around 235 nm and are in good agreement with the UV absorption spectra of phenyl. The H + C6H4 product translational energy distributions, P(ET)’s, peak near ∼7 kcal/mol, and the fraction of average translational energy in the total excess energy, 〈fT〉, is in the range of 0.20–0.35 from 215 to 268 nm. The H-atom product angular distribution is isotropic. The dissociation rates are in the range of 107–108 s−1 with internal energy from 30 to 46 kcal/mol above the threshold of the lowest energy channel H + o-C6H4 (ortho-benzyne), comparable with the rates from the Rice–Ramsperger–Kassel–Marcus theory. The results from the fully deuterated phenyl radical are identical. The dissociation mechanism is consistent with production of H + o-C6H4, as the main channel from unimolecular decomposition of the ground electronic state phenyl radical following internal conversion of the electronically excited state.
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33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
82.50.Hp Processes caused by visible and UV light
32.80.Rm Multiphoton ionization and excitation to highly excited states

Unimolecular thermal decomposition of phenol and d5-phenol: Direct observation of cyclopentadiene formation via cyclohexadienone

Adam M. Scheer, Calvin Mukarakate, David J. Robichaud, Mark R. Nimlos, Hans-Heinrich Carstensen, and G. Barney Ellison

J. Chem. Phys. 136, 044309 (2012); http://dx.doi.org/10.1063/1.3675902 (11 pages)

Online Publication Date: 26 January 2012

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The pyrolyses of phenol and d5-phenol (C6H5OH and C6D5OH) have been studied using a high temperature, microtubular (μtubular) SiC reactor. Product detection is via both photon ionization (10.487 eV) time-of-flight mass spectrometry and matrix isolation infrared spectroscopy. Gas exiting the heated reactor (375 K–1575 K) is subject to a free expansion after a residence time in the μtubular reactor of approximately 50–100 μs. The expansion from the reactor into vacuum rapidly cools the gas mixture and allows the detection of radicals and other highly reactive intermediates. We find that the initial decomposition steps at the onset of phenol pyrolysis are enol/keto tautomerization to form cyclohexadienone followed by decarbonylation to produce cyclopentadiene; C6H5OH → c-C6H6 = O → c-C5H6 + CO. The cyclopentadiene loses a H atom to generate the cyclopentadienyl radical which further decomposes to acetylene and propargyl radical; c-C5H6 → c-C5H5 + H → HC≡CH + HCCCH2. At higher temperatures, hydrogen loss from the PhO–H group to form phenoxy radical followed by CO ejection to generate the cyclopentadienyl radical likely contributes to the product distribution; C6H5O–H → C6H5O + H → c-C5H5 + CO. The direct decarbonylation reaction remains an important channel in the thermal decomposition mechanisms of the dihydroxybenzenes. Both catechol (o-HO–C6H4–OH) and hydroquinone (p-HO–C6H4–OH) are shown to undergo decarbonylation at the onset of pyrolysis to form hydroxycyclopentadiene. In the case of catechol, we observe that water loss is also an important decomposition channel at the onset of pyrolysis.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Qt Isomerization and rearrangement
82.80.Rt Time of flight mass spectrometry
33.20.Ea Infrared spectra
33.80.Eh Autoionization, photoionization, and photodetachment
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions

The ultraviolet photodissociation of CS2: The S(1D2) channel

M. Brouard, E. K. Campbell, R. Cireasa, A. J. Johnsen, and W.-H. Yuen

J. Chem. Phys. 136, 044310 (2012); http://dx.doi.org/10.1063/1.3678007 (15 pages)

Online Publication Date: 26 January 2012

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The photodissociation of CS2 has been investigated using velocity-map ion imaging of the S(1D2) atomic photofragments following excitation at 193  nm and at longer wavelengths close to the S(1D2) channel threshold. The experiments probe regions both above and below the energetic barrier to linearity on the 1Σu+(1B2) potential energy surface. The imaging data in both regions indicate that the electronic angular momentum of the S(1D2) atom products is unpolarized, but also reveal different dissociation dynamics in the two regions. Excitation above the barrier to linearity yields an inverted CS(1Σ+) vibrational population distribution, whereas the long-wavelength state-to-state results following excitation below the barrier reveal CS(1Σ+)(v, J) coproduct state distributions which are consistent with a statistical partitioning of the energy. Below the barrier, photofragment excitation spectra point to an enhancement of the singlet channel for K = 1, relative to K = 0, where K is the projection of the angular momentum along the principal axis, in agreement with previous work. However, the CS cofragment product state distributions are found to be insensitive to K. It is proposed that dissociation below the barrier to linearity occurs primarily on a surface with a significant potential energy well and without an exit channel barrier, such as that for the ground electronic state. However, oscillatory structure is also observed in the kinetic energy release distributions, which is shown to be consistent with a mapping of parent molecule bending motion. This could indicate the operation of competing direct and indirect dissociation mechanisms below the barrier to linearity.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
31.50.Df Potential energy surfaces for excited electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Lg Ultraviolet spectra
33.20.Tp Vibrational analysis
33.80.Be Level crossing and optical pumping

17O excess transfer during the NO2 + O3 → NO3 + O2 reaction

Tesfaye Ayalneh Berhanu, Joël Savarino, S. K. Bhattacharya, and Willliam C. Vicars

J. Chem. Phys. 136, 044311 (2012); http://dx.doi.org/10.1063/1.3666852 (9 pages)

Online Publication Date: 27 January 2012

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The ozone molecule possesses a unique and distinctive 17O excess (Δ17O), which can be transferred to some of the atmospheric molecules via oxidation. This isotopic signal can be used to trace oxidation reactions in the atmosphere. However, such an approach depends on a robust and quantitative understanding of the oxygen transfer mechanism, which is currently lacking for the gas-phase NO2 + O3 reaction, an important step in the nocturnal production of atmospheric nitrate. In the present study, the transfer of Δ17O from ozone to nitrate radical (NO3) during the gas-phase NO2 + O3 → NO3 + O2 reaction was investigated in a series of laboratory experiments. The isotopic composition (δ17O, δ18O) of the bulk ozone and the oxygen gas produced in the reaction was determined via isotope ratio mass spectrometry. The Δ17O transfer function for the NO2 + O3 reaction was determined to be: Δ17O(O3*) = (1.23 ± 0.19) × Δ17O(O3)bulk + (9.02 ± 0.99). The intramolecular oxygen isotope distribution of ozone was evaluated and results suggest that the excess enrichment resides predominantly on the terminal oxygen atoms of ozone. The results obtained in this study will be useful in the interpretation of high Δ17O values measured for atmospheric nitrate, thus leading to a better understanding of the natural cycling of atmospheric reactive nitrogen.
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82.33.Tb Atmospheric chemistry
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Tr Kinetic isotope effects including muonium
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