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28 Dec 2008

Volume 129, Issue 24, Articles (24xxxx)

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Announcement: New location of main office of The Journal of Chemical Physics

Mark M. Cassar

J. Chem. Phys. 129, 240201 (2008); http://dx.doi.org/10.1063/1.3063089 (1 page) | Cited 1 time

Online Publication Date: 22 December 2008

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01.60.+q Biographies, tributes, personal notes, and obituaries
01.10.Cr Announcements, news, and awards
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Announcement: Appreciation to Associate Editor Branka M. Ladanyi for her service as Interim Editor

Mark M. Cassar

J. Chem. Phys. 129, 240202 (2008); http://dx.doi.org/10.1063/1.3068208 (1 page)

Online Publication Date: 22 December 2008

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01.60.+q Biographies, tributes, personal notes, and obituaries
01.10.Cr Announcements, news, and awards
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Anomalous melting behavior under extreme conditions: Hard matter turning “soft”

Gianpietro Malescio, Franz Saija, and Santi Prestipino

J. Chem. Phys. 129, 241101 (2008); http://dx.doi.org/10.1063/1.3050315 (4 pages) | Cited 13 times

Online Publication Date: 23 December 2008

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We show that a system of particles interacting through the exp-6 pair potential, commonly used to describe effective interatomic forces under high compression, exhibits anomalous melting features such as reentrant melting and a rich solid polymorphism, including a stable BC8 crystal. We relate this behavior to the crossover, with increasing pressure, between two different regimes of local order that are associated with the two repulsive length scales of the potential. Our results provide a unifying picture for the high-pressure melting anomalies observed in many elements and point out that, under extreme conditions, atomic systems may reveal surprising similarities with soft matter.
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64.70.D- Solid-liquid transitions
62.50.-p High-pressure effects in solids and liquids

Charge on a weak polyelectrolyte

Shengqin Wang, Steve Granick, and Jiang Zhao

J. Chem. Phys. 129, 241102 (2008); http://dx.doi.org/10.1063/1.3055596 (4 pages) | Cited 4 times

Online Publication Date: 31 December 2008

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Fluorescence measurements with single-molecule sensitivity are used to measure the hydrodynamic size and local pH of a weak polyelectrolyte, poly-2-vinyl pyridine end labeled with pH-sensitive dye, the polyelectrolyte having concentration so low (nanomolars) that molecular properties are resolvable only from fluorescence experiments and cannot be accessed by light scattering. We find that the local pH near the dye, inferred from its brightness, is consistently three orders of magnitude higher than the bulk pH. Upon varying the bulk pH, we measure the collapse point at which hydrophobic attraction overwhelms electrostatic repulsion between charged elements along the chain, and conclude that adding monovalent salt shifts this coil-to-globule collapse to higher pH than in the absence of salt. The influence of salt appears to shift the ionization equilibrium of this weak polyelectrolyte in the direction of the chain possessing enhanced electric charge at a given pH. Phenomenologically, this is opposite to the case for strong polyelectrolytes, although the mechanism differs.
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82.35.Rs Polyelectrolytes
82.45.Wx Polymers and organic materials in electrochemistry
82.80.Dx Analytical methods involving electronic spectroscopy
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Constructing diabatic states from adiabatic states: Extending generalized Mulliken–Hush to multiple charge centers with Boys localization

Joseph E. Subotnik, Sina Yeganeh, Robert J. Cave, and Mark A. Ratner

J. Chem. Phys. 129, 244101 (2008); http://dx.doi.org/10.1063/1.3042233 (10 pages) | Cited 14 times

Online Publication Date: 22 December 2008

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This article shows that, although Boys localization is usually applied to single-electron orbitals, the Boys method itself can be applied to many electron molecular states. For the two-state charge-transfer problem, we show analytically that Boys localization yields the same charge-localized diabatic states as those found by generalized Mulliken–Hush theory. We suggest that for future work in electron transfer, where systems have more than two charge centers, one may benefit by using a variant of Boys localization to construct diabatic potential energy surfaces and extract electronic coupling matrix elements. We discuss two chemical examples of Boys localization and propose a generalization of the Boys algorithm for creating diabatic states with localized spin density that should be useful for Dexter triplet-triplet energy transfer.
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31.50.Df Potential energy surfaces for excited electronic states

Ab initio calculation of interatomic decay rates of excited doubly ionized states in clusters

Přemysl Kolorenč, Vitali Averbukh, Kirill Gokhberg, and Lorenz S. Cederbaum

J. Chem. Phys. 129, 244102 (2008); http://dx.doi.org/10.1063/1.3043437 (12 pages) | Cited 9 times

Online Publication Date: 23 December 2008

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Recently, a computational technique for ab initio calculation of the interatomic and intermolecular nonradiative decay processes has been developed [ V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005) ]. It combines the Fano formalism with the Green’s function method known as the algebraic diagrammatic construction. The problem of normalization of continuum wave functions stemming from the use of the Gaussian basis sets is solved by using the Stieltjes imaging technique. In the present paper, the methodology is extended in order to describe the interatomic decay of excited doubly ionized states of clusters. The new computational scheme is applied to compute the interatomic decay rates of doubly ionized states formed by Auger relaxation of core vacancies in NeAr and MgNe van der Waals clusters.
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31.15.A- Ab initio calculations
36.40.Cg Electronic and magnetic properties of clusters
34.20.Gj Intermolecular and atom-molecule potentials and forces

Structure and reactions of carbon and hydrogen on Ru(0001): A scanning tunneling microscopy study

Tomoko K. Shimizu, Aitor Mugarza, Jorge I. Cerdá, and Miquel Salmeron

J. Chem. Phys. 129, 244103 (2008); http://dx.doi.org/10.1063/1.2991434 (7 pages) | Cited 2 times

Online Publication Date: 23 December 2008

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The interaction between carbon and hydrogen atoms on a Ru(0001) surface was studied using scanning tunneling microscopy (STM), density functional theory (DFT) and STM image calculations. Formation of CH species by reaction between adsorbed H and C was observed to occur readily at 100 K. When the coverage of H increased new complexes of the form of CH+nH (n = 1, 2, and 3) were observed. These complexes, never observed before, might be precursors for further hydrogenation reactions. DFT analysis reveals that a considerable energy barrier exists for the CH+H→CH2 reaction.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)

Variational and perturbative formulations of quantum mechanical/molecular mechanical free energy with mean-field embedding and its analytical gradients

Takeshi Yamamoto

J. Chem. Phys. 129, 244104 (2008); http://dx.doi.org/10.1063/1.3041381 (15 pages) | Cited 7 times

Online Publication Date: 24 December 2008

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Conventional quantum chemical solvation theories are based on the mean-field embedding approximation. That is, the electronic wavefunction is calculated in the presence of the mean field of the environment. In this paper a direct quantum mechanical/molecular mechanical (QM/MM) analog of such a mean-field theory is formulated based on variational and perturbative frameworks. In the variational framework, an appropriate QM/MM free energy functional is defined and is minimized in terms of the trial wavefunction that best approximates the true QM wavefunction in a statistically averaged sense. Analytical free energy gradient is obtained, which takes the form of the gradient of effective QM energy calculated in the averaged MM potential. In the perturbative framework, the above variational procedure is shown to be equivalent to the first-order expansion of the QM energy (in the exact free energy expression) about the self-consistent reference field. This helps understand the relation between the variational procedure and the exact QM/MM free energy as well as existing QM/MM theories. Based on this, several ways are discussed for evaluating non-mean-field effects (i.e., statistical fluctuations of the QM wavefunction) that are neglected in the mean-field calculation. As an illustration, the method is applied to an SN2 Menshutkin reaction in water, NH3+CH3Cl→NH3CH3++Cl−, for which free energy profiles are obtained at the Hartree–Fock, MP2, B3LYP, and BHHLYP levels by integrating the free energy gradient. Non-mean-field effects are evaluated to be <0.5 kcal/mol using a Gaussian fluctuation model for the environment, which suggests that those effects are rather small for the present reaction in water.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Db Transition state theory and statistical theories of rate constants
82.60.Lf Thermodynamics of solutions
82.20.Wt Computational modeling; simulation
82.20.Ej Quantum theory of reaction cross section

Implementation and performance of a domain-specific basis set incremental approach for correlation energies: Applications to hydrocarbons and a glycine oligomer

Joachim Friedrich and Michael Dolg

J. Chem. Phys. 129, 244105 (2008); http://dx.doi.org/10.1063/1.3043797 (8 pages) | Cited 17 times

Online Publication Date: 29 December 2008

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The fully automated implementation of the incremental scheme for CCSD energies has been modified for the usage of a domain-specific basis set. We find that the computational effort can be reduced significantly without loss of accuracy. It is shown explicitly in applications on hydrocarbons and the (glycine)4 oligomer that the error of the incremental expansion for the total energy is usually below 1 kcal/mol at third order. Furthermore, it is demonstrated that the proposed approach saves CPU time, random access memory, and disk space. Moreover, we show in various tests that the inherently parallel incremental calculations can be run on up to 50 CPUs without significant loss of computer time.
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31.15.bw Coupled-cluster theory
33.15.Bh General molecular conformation and symmetry; stereochemistry

Quantum Monte Carlo formulation of volume polarization in dielectric continuum theory

Claudio Amovilli, Claudia Filippi, and Franca Maria Floris

J. Chem. Phys. 129, 244106 (2008); http://dx.doi.org/10.1063/1.3043804 (12 pages) | Cited 2 times

Online Publication Date: 29 December 2008

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We present a novel formulation based on quantum Monte Carlo techniques for the treatment of volume polarization due to quantum mechanical penetration of the solute charge density in the solvent domain. The method allows to accurately solve Poisson’s equation of the solvation model coupled with the Schrödinger equation for the solute. We demonstrate the performance of the approach on a representative set of solutes in water solvent and give a detailed analysis of the dependence of the volume polarization on the solute cavity and the treatment of electron correlation.
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82.30.Nr Association, addition, insertion, cluster formation
82.60.Lf Thermodynamics of solutions
82.20.Yn Solvent effects on reactivity
82.20.Fd Collision theories; trajectory models

A note on the Pulay force at finite electronic temperatures

Anders M. N. Niklasson

J. Chem. Phys. 129, 244107 (2008); http://dx.doi.org/10.1063/1.3036203 (5 pages) | Cited 2 times

Online Publication Date: 29 December 2008

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Pulay’s original expression for the basis-set dependent adjustment term to the Hellmann–Feynman force in electronic structure theory, which occurs for nonorthogonal local basis-set representations, is based on the idempotency condition of a pure ensemble. At finite electronic temperatures with a fractional occupation of the states, the conventional expression of the Pulay force is therefore no longer valid. Here we derive a simple and computationally efficient expression for a generalized Pulay force, which is suitable for large-scale ab initio simulations at finite electronic temperatures using local nonorthogonal basis-set representations. The generalized Pulay force expression is given in terms of the temperature-dependent density matrix. For the construction of the density matrix, we propose a recursive Fermi operator expansion algorithm that automatically converges to the correct chemical potential.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

Application of improved virtual orbital based multireference methods to N2, LiF, and C4H6 systems

Sudip Chattopadhyay, Rajat K. Chaudhuri, and Uttam Sinha Mahapatra

J. Chem. Phys. 129, 244108 (2008); http://dx.doi.org/10.1063/1.3046454 (9 pages) | Cited 8 times

Online Publication Date: 29 December 2008

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The improved virtual orbital (IVO) complete active space configuration interaction (CASCI) based multiconfigurational quasidegenerate perturbation theory (MCQDPT) and its single-root version (termed as MRMPPT) are applied to assess the efficacy and the reliability of these two methods. Applications involve the ground and/or excited state potential energy curves (PECs) of N2, LiF, and C4H6 (butadiene) molecules, systems that are sufficiently complex to assess the applicability of these methods. The ionic-neutral curve crossing involving the lowest two 1Σ+ states of LiF molecule is studied using the IVO-MCQDPT method, while its single-root version (IVO-MRMPPT) is employed to study the ground state PEC for isomerization of butadiene and to model the bond dissociation of N2 molecule. Comparisons with the standard methods (full CI, coupled cluster with singles and doubles, etc.) demonstrate that the IVO-based MRMPPT and MCQDPT approaches provide smooth and reliable PECs for all the systems studied. The IVO-CASCI method is explored to enable geometry optimization for ground state of C4H6 using numerical energy gradients. The ground spectroscopic constants of N2 and LiF determined using the numerical gradient based IVO-CASCI method are in accord with experiment and with other correlated calculations. As an illustration, we may point out that the maximum deviation from the experiment in our estimated normal mode frequency of LiF is 34 cm−1, whereas for the bond length, the maximum error is just 0.012 Å.
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82.30.Qt Isomerization and rearrangement
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Fd Collision theories; trajectory models

Variational, V-representable, and variable-occupation-number perturbation theories

Brett I. Dunlap

J. Chem. Phys. 129, 244109 (2008); http://dx.doi.org/10.1063/1.3042143 (8 pages) | Cited 4 times

Online Publication Date: 30 December 2008

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Density-functional perturbation theory with variationally fitted Kohn–Sham (KS) potentials is described. Requiring the Fock matrix and density matrix to commute through each order of perturbation theory determines the off-diagonal elements of the density matrix, and thus the effect of changing occupation numbers in density-functional perturbation theory. At each order of perturbation theory, the change in occupation numbers at that order enters only the diagonal part of the density matrix. The theory contains no phases, and a limiting process relates the rest of the diagonal density matrix element, obtained from wave function perturbation theory, to the off-diagonal part, obtained by commutation. V-representable density-functional theory is most practical when the KS potential is expanded in a finite basis to create the Sambe–Felton (SF) potential of analytic density-functional theory. This reduces the dimensionality of perturbation theory from order N2 in the orbital basis to order N in the SF basis. Computing the (occupied-virtual)2, i.e., N4, sum over states once at the end of a self-consistent-field molecular orbital calculation removes the orbitals from all higher orders of perturbation theory. The rank-N2 iterative coupled-perturbed equations are replaced by rank-N matrix inversion, to fit variationally the perturbed SF potential at each order. As an example of the 2n+1 rule of perturbation theory, the variational, first-order potential is used to give precise second and third derivatives of the energy with respect to occupation number. The hardness and hyperhardness are computed for a standard set of molecules. Both are essentially independent of how the variational SF potential is constrained for four different constraint combinations. With variational fitting, the precision of derivatives and the fidelity of the fit to the SF potential are not related. Analytic derivatives are accurate to machine precision for any constraint and all fitting basis sets.
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31.15.ep Variational particle-number approach
02.10.Yn Matrix theory
31.15.xr Self-consistent-field methods
31.15.xp Perturbation theory
31.15.am Relativistic configuration interaction (CI) and many-body perturbation calculations
46.15.Cc Variational and optimizational methods
31.15.xt Variational techniques

Calculation of first and second static hyperpolarizabilities of one- to three-dimensional periodic compounds. Implementation in the CRYSTAL code.

Mauro Ferrero, Michel Rérat, Bernard Kirtman, and Roberto Dovesi

J. Chem. Phys. 129, 244110 (2008); http://dx.doi.org/10.1063/1.3043366 (13 pages) | Cited 23 times

Online Publication Date: 30 December 2008

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A computational scheme for the evaluation of the static first (β) and second (γ) hyperpolarizability tensors of systems periodic in 1D (polymers), 2D (slabs), 3D (crystals), and, as a limiting case, 0D (molecules) has been implemented, within the coupled perturbed Hartree–Fock framework (CPHF), in the CRYSTAL code, which uses a Gaussian type basis set. This generalizes to 2D and 3D the work by Bishop et al. (J. Chem. Phys. 114, 7633 (2001) ). CPHF is applied for β and γ (the polarizability tensor α is also reported for completeness) of LiF in different aggregation states: finite and infinite chains, slabs, and cubic crystal. Correctness of the computational scheme and its numerical efficiency are documented by the trend of β and γ for increasing dimensionality: for a finite linear chain containing N LiF units, the hyperpolarizability tends to the infinite chain value at large N, N parallel chains give the slab value when N is sufficiently large, and N superimposed slabs tend to the bulk value. High numerical accuracy can be achieved at relatively low cost, with a dependence on the computational parameters similar to that observed for field-free self-consistent field (SCF) calculations.
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33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.ap Polarizabilities and other atomic and molecular properties
31.15.xr Self-consistent-field methods
31.15.A- Ab initio calculations
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Spin-free intermediate Hamiltonian Fock space coupled-cluster theory with full inclusion of triple excitations for restricted Hartree Fock based triplet states

Monika Musial and Rodney J. Bartlett

J. Chem. Phys. 129, 244111 (2008); http://dx.doi.org/10.1063/1.3046453 (6 pages) | Cited 13 times

Online Publication Date: 31 December 2008

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The recently reported inclusion of the connected triples into the intermediate Hamiltonian realization of the Fock space coupled-cluster (IH-FS-CC) theory [ M. Musial and R. J. Bartlett, J. Chem. Phys. 129, 044101 (2008) ] is extended to produce the triplet states. This is done entirely in spatial orbitals on the basis of the double occupancy in the restricted Hartree Fock reference function. New equations for the triplet state amplitudes expressed in terms of the Goldstone diagrams are derived and implemented. Several applications show the usefulness of the IH-FS-CC scheme to describe the triplet states with the computational gains inherent to a spin-free implementation.
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31.15.bw Coupled-cluster theory
31.15.xr Self-consistent-field methods
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Enhanced identification and exploitation of time scales for model reduction in stochastic chemical kinetics

Carlos A. Gómez-Uribe, George C. Verghese, and Abraham R. Tzafriri

J. Chem. Phys. 129, 244112 (2008); http://dx.doi.org/10.1063/1.3050350 (16 pages) | Cited 1 time

Online Publication Date: 31 December 2008

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Widely different time scales are common in systems of chemical reactions and can be exploited to obtain reduced models applicable to the time scales of interest. These reduced models enable more efficient computation and simplify analysis. A classic example is the irreversible enzymatic reaction, for which separation of time scales in a deterministic mass action kinetics model results in approximate rate laws for the slow dynamics, such as that of Michaelis–Menten. Recently, several methods have been developed for separation of slow and fast time scales in chemical master equation (CME) descriptions of stochastic chemical kinetics, yielding separate reduced CMEs for the slow variables and the fast variables. The paper begins by systematizing the preliminary step of identifying slow and fast variables in a chemical system from a specification of the slow and fast reactions in the system. The authors then present an enhanced time-scale-separation method that can extend the validity and improve the accuracy of existing methods by better accounting for slow reactions when equilibrating the fast subsystem. The resulting method is particularly accurate in systems such as enzymatic and protein interaction networks, where the rates of the slow reactions that modify the slow variables are not a function of the slow variables. The authors apply their methodology to the case of an irreversible enzymatic reaction and show that the resulting improvements in accuracy and validity are analogous to those obtained in the deterministic case by using the total quasi-steady-state approximation rather than the classical Michaelis–Menten. The other main contribution of this paper is to show how mass fluctuation kinetics models, which give approximate evolution equations for the means, variances, and covariances of the concentrations in a chemical system, can feed into time-scale-separation methods at a variety of stages.
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82.39.-k Chemical kinetics in biological systems
82.39.Rt Reactions in complex biological systems
82.40.-g Chemical kinetics and reactions: special regimes and techniques
82.20.Uv Stochastic theories of rate constants
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Contribution of the Gouy phase to two-pathway coherent control of the photoionization and photodissociation of vinyl chloride

Vishal J. Barge, Zhan Hu, and Robert J. Gordon

J. Chem. Phys. 129, 244301 (2008); http://dx.doi.org/10.1063/1.3040269 (8 pages) | Cited 2 times

Online Publication Date: 22 December 2008

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The electric field of a light wave accumulates a π phase shift as it passes through a focus. We show here how this effect, known as the Gouy phase, may be used to control the branching ratio of a unimolecular reaction when the products are formed with different numbers of photons. We demonstrate this control method for the ionization and dissociation of vinyl chloride, using absorption of 177 and 532 nm photons to induce a pair of interfering paths. Excellent agreement between the observed and calculated phase shift as a function of the axial coordinate of the laser focus indicates that fragmentation occurs via a ladder switching mechanism. The axial dependence of the modulation depth is evidence of loss of coherence at higher internal temperatures of the molecule.
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82.50.Hp Processes caused by visible and UV light
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.20.Hf Product distribution
82.50.Nd Control of photochemical reactions

Gas-phase infrared multiple photon dissociation spectroscopy of isolated SF6− and SF5− anions

Jeffrey D. Steill, Jos Oomens, John R. Eyler, and Robert N. Compton

J. Chem. Phys. 129, 244302 (2008); http://dx.doi.org/10.1063/1.3036977 (8 pages) | Cited 3 times

Online Publication Date: 22 December 2008

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Resonantly enhanced multiple photon dissociation of gas-phase SF6− and SF5− is studied using tunable infrared light from the FELIX free electron laser. The photodissociation spectrum of the sulfur hexafluoride anion, producing SF5−, is recorded over the spectral range of 250–1650 cm−1. The infrared multiple photon dissociation cross section exhibits a strong, broad resonance enhancement at 675 cm−1 in agreement with the calculated value of ν3, one of the two IR-active fundamental vibrational modes predicted for the Oh-symmetry ion. Much weaker absorption features are observed in the spectral region of 300–450 cm−1 as well as at 580 cm−1 that are not easily assigned to the other IR-active fundamental of SF6− since these resonances are observed at a much higher energy than the calculated values for the IR-active ν4 mode. The potential role of binary combination bands is considered. Photodissociation from the sulfur pentafluoride anion produced only F−, but photodetachment was also observed through SF6 associative electron capture. The IR multiple photon dissociation spectrum of SF5− shows multiple resonances within the region of 400–900 cm−1 and agreement with calculations is clear, including the observation of three fundamental frequencies: ν1 at 780 cm−1, ν7 at 595 cm−1, and ν8 at 450 cm−1. Comparisons of the measured frequencies with ab initio and density functional theory calculations confirm an SF5− anion of C4v symmetry. Similar comparisons for SF6− are not inconsistent with an anion of Oh symmetry.
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33.80.Gj Diffuse spectra; predissociation, photodissociation
33.80.Wz Other multiphoton processes
31.15.es Applications of density-functional theory (e.g., to electronic structure and stability; defect formation; dielectric properties, susceptibilities; viscoelastic coefficients; Rydberg transition frequencies)
33.20.Ea Infrared spectra
31.15.ap Polarizabilities and other atomic and molecular properties

Interaction of NO(A 2Σ+) with rare gas atoms: Potential energy surfaces and spectroscopy

Jacek Kłos, Millard H. Alexander, Ramón Hernández-Lamoneda, and Timothy G. Wright

J. Chem. Phys. 129, 244303 (2008); http://dx.doi.org/10.1063/1.3040074 (12 pages) | Cited 11 times

Online Publication Date: 24 December 2008

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We present the results of an ab initio study of the interaction of electronically excited NO(A 2Σ+) with rare gas (Rg) atoms. The bound states of each NO(A)–Rg species are determined from potential energy surfaces calculated at the RCCSD(T) level of theory. Making use of the NO(X 2Π)–Rg vibrational wavefunctions, we then simulate electronic spectra. For NO–Kr and NO–Xe we obtain good qualitative agreement with the previously published experimental spectra. For NO–Ar, the shallowness of the surface gives rise to agreement that is less satisfactory, but a global scaling provides better qualitative agreement. The assignment of the spectra is far from straightforward and is only possible with guidance from the calculated energies and wavefunctions of the energy levels of the complex. Previous assignments are discussed in the light of this conclusion.
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34.50.-s Scattering of atoms and molecules
31.15.A- Ab initio calculations
34.20.-b Interatomic and intermolecular potentials and forces, potential energy surfaces for collisions
31.50.-x Potential energy surfaces

Binary-collision-induced longitudinal relaxation in gas-phase 83Kr

Zackary I. Cleveland and Thomas Meersmann

J. Chem. Phys. 129, 244304 (2008); http://dx.doi.org/10.1063/1.3029663 (6 pages) | Cited 2 times

Online Publication Date: 24 December 2008

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Density dependent NMR relaxation measurements of noble gases can provide complementary information to that obtained from relaxation studies of molecular gases. However, conventional noble gas NMR is typically hindered by low sensitivity or prohibitively long relaxation times. In this work, the low sensitivity of 83Kr (I = 9/2) was overcome by spin exchange optical pumping, and the quadrupolar interaction dominated 83Kr T1 times of 40–400 s enabled rapid collection of relaxation data. The density dependence of the 83Kr longitudinal relaxation in pure krypton was found to be about 1.6×10−3 amagat−1 s−1. Experiments were also performed in krypton mixtures containing either helium or nitrogen as a buffer gas. By varying the composition and the density of these mixtures, the density dependence of buffer gas induced relaxation and the relaxation efficiency of 83Kr-buffer gas collisions were determined. The results from these gas mixtures are compared with those from pure krypton.
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32.30.Dx Magnetic resonance spectra
33.80.Be Level crossing and optical pumping

Extended negative glow and “hollow anode” discharges for submillimeter-wave observation of CN−, C2H−, and C4H−

T. Amano

J. Chem. Phys. 129, 244305 (2008); http://dx.doi.org/10.1063/1.3043739 (7 pages) | Cited 3 times

Online Publication Date: 24 December 2008

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Three molecular anions, CN−, C2H−, and C4H−, have been detected in an extended negative glow discharge and a “hollow anode” discharge in the submillimeter-wave region. These electrical discharges have been unexpectedly found to be reasonable anion sources. The measurements have been extended up to 830 GHz, and the molecular constants have been improved significantly compared with those determined by microwave and millimeter-wave spectroscopy. The parent gas mixture of C2N2 or C2H2 of 2 mTorr and Ar or Ne buffer of 15 mTorr was used for the production of CN− or C2H− and C4H−. The number densities of the anions in the cell were estimated to be ∼ 1×109, ∼ 3×107, and ∼ 3×106 cm−3 for CN−, C2H−, and C4H−, respectively.
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33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Bx Radio-frequency and microwave spectra
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
95.30.Ky Atomic and molecular data, spectra, and spectral parameters (opacities, rotation constants, line identification, oscillator strengths, gf values, transition probabilities, etc.)

Pathways and reduced-dimension five-dimensional potential energy surface for the reactions H3++CO→H2+HCO+ and H3++CO→H2+HOC+

Hui Li, Tsuneo Hirano, Takayoshi Amano, and Robert J. Le Roy

J. Chem. Phys. 129, 244306 (2008); http://dx.doi.org/10.1063/1.3041494 (8 pages) | Cited 3 times

Online Publication Date: 29 December 2008

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To obtain theoretical insight regarding the stability and formation dynamics of the interstellar ions HCO+ and HOC+, stationary points and the associated vibrational frequencies on the full nine-dimensional potential energy surface for the electronic ground state have been calculated using coupled-cluster theory with both single and double substitutions (CCSD). The energetics were refined with a higher-level coupled-cluster method CCSD(T), with core-valence electron correlation treated at the complete basis set limit. To elucidate the formation mechanism and internal relaxation processes, the reaction paths for the reactions H3++CO→H2+HCO+ and H3++CO→H2+HOC+ were calculated at the second-order Møller–Plesset (MP2) level, and corresponding single-point energies were obtained at the higher CCSD(T)/aug-cc-pVTZ level. Based on the analysis of the main reaction processes, a reduced-dimension five-dimensional potential energy surface for this system was constructed from 128 440 ab initio points calculated at the CCSD(T)/aug-cc-pVTZ level.
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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.Db Transition state theory and statistical theories of rate constants

Renner–Teller coupled-channel dynamics of the N(2D)+H2 reaction and the role of the NH2 math 2A1 electronic state

Pablo Gamallo, Paolo Defazio, Miguel González, and Carlo Petrongolo

J. Chem. Phys. 129, 244307 (2008); http://dx.doi.org/10.1063/1.3046882 (5 pages) | Cited 12 times

Online Publication Date: 30 December 2008

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We present Renner–Teller (RT) and Born–Oppenheimer (BO) coupled-channel (CC) dynamics of the reaction 14N(2D)+1H2(X 1Σg+)→NH(X 3Σ−)+H(2S), considering both NH2 coupled electronic states, math 2B1 and math 2A1, and Coriolis interactions. We use the best available potential energy surfaces (PESs), and we obtain initial-state-resolved reaction probabilities, cross sections, and rate constants through the real wavepacket and flux methods, taking into account the nuclear-spin statistics for both electronic states. Contrasting RT-CC with more approximate results, we point out the role of RT and Coriolis couplings, and discuss the importance of the math 2A1 excited state on the initial-state-resolved dynamics and on the thermal kinetic rate. Confirming the previous results, RT couplings transfer partly the reactivity from math 2B1 to math 2A1, and CC calculations are necessary to obtain accurate high-energy cross sections. When H2 is initially rotating, RT couplings enhance strongly the electronic-state-resolved math 2A1 reactivity. Considering the nuclear-spin statistics for both electronic states, we find out that the math 2A1 state plays a significant role in the rotationally resolved dynamics of N(2D)+ortho-H2. However, the BO–math 2B1 approximation gives a thermal rate that is slightly smaller than the one obtained by the RT-CC calculations. This implies that this usual approximation is acceptable to calculate unresolved kinetic data of the title reaction. Our calculated rate constant values within the 213–300 K temperature interval are in excellent agreement with the experimental ones.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Kh Potential energy surfaces for chemical reactions
82.20.Pm Rate constants, reaction cross sections, and activation energies

Photoelectron imaging of atomic chlorine and bromine following photolysis of CH2BrCl

Linqiang Hua, Huan Shen, Changjin Hu, and Bing Zhang

J. Chem. Phys. 129, 244308 (2008); http://dx.doi.org/10.1063/1.3047756 (7 pages)

Online Publication Date: 30 December 2008

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Photoionization of chlorine and bromine atoms following photodissociation of CH2BrCl was studied in the wavelength range of 231–238 nm by photoelectron imaging technique. Final state-specific speed and angular distributions of the photoelectron were recorded. Analysis of relative branching ratios to different levels of Cl+ and Br+ revealed that the final ion level distributions are generally dominated by the preservation of the ion-core configuration of the intermediate resonant state. Some Jc numbers of the intermediate states were newly assigned according to this regulation. The configuration interaction between resonant states and the autoionization in the continuum were also believed to play an important role in the ionization process since some ions that deviate from the regulation mentioned ahead were observed. The angular distributions of the electrons were found to be well characterized by β2 and β4, although the ionization process of chlorine and bromine atoms involves three photons.
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82.50.Hp Processes caused by visible and UV light
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
32.80.Wr Other multiphoton processes
32.80.Zb Autoionization
32.80.Fb Photoionization of atoms and ions

Electron binding motifs in the (CS2)n− (n>4) cluster anions

Terefe Habteyes and Andrei Sanov

J. Chem. Phys. 129, 244309 (2008); http://dx.doi.org/10.1063/1.3046481 (4 pages) | Cited 1 time

Online Publication Date: 30 December 2008

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Photoelectron imaging spectroscopy of (CS2)n−, n>4, reveals a new state with an electron binding energy smaller than that of any of the corresponding CS2− and C2S4− states known to date. With support from ab initio calculations, two long-lived, metastable binding motifs with small electron binding energies are discussed for these clusters. The first is a solvent network permeating state, where the excess electron is delocalized over a number of linear CS2 molecules. The second is an excited 2B1 state of the core CS2− anion stabilized at a slightly bent geometry by the solvation interactions. Based on the observed solvation-induced shifts in binding energy, the second motif is favored.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.60.+q Photoelectron spectra
31.15.ae Electronic structure and bonding characteristics
82.30.Nr Association, addition, insertion, cluster formation
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