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14 Jun 2007

Volume 126, Issue 22, Articles (22xxxx)

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Simulated surface tensions of common water models

Feng Chen and Paul E. Smith

J. Chem. Phys. 126, 221101 (2007); http://dx.doi.org/10.1063/1.2745718 (3 pages) | Cited 13 times

Online Publication Date: 12 June 2007

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Initial simulated values of the surface tension for the SPC/E water model have indicated excellent agreement with experiment. More recently, differing values have been obtained which are significantly lower than previous estimates. Here, we attempt to explain the differences between the previous studies and show that a variety of simulation conditions can affect the final surface tension values. Consistent values for the surface tensions of six common fixed charge water models (TIP3P, SPC, SPC/E, TIP4P, TIP5P, and TIP6P) are then determined for four temperatures between 275 and 350 K. The SPC/E and TIP6P models provide the best agreement with experiment.
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68.03.Cd Surface tension and related phenomena
61.20.Ja Computer simulation of liquid structure

Infrared spectra of organic monolayer films in a standing wave measured by photon-trap spectroscopy

Kazuhiro Egashira, Akira Terasaki, and Tamotsu Kondow

J. Chem. Phys. 126, 221102 (2007); http://dx.doi.org/10.1063/1.2748747 (3 pages) | Cited 7 times

Online Publication Date: 12 June 2007

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Photon-trap spectroscopy, a generalized scheme of cavity ringdown spectroscopy, is applied to measure an infrared spectrum of the C–H stretching vibration of alkylsiloxane monolayer films grafted on a silicon substrate. A continuous-wave laser beam is introduced into a high-finesse Fabry-Pérot cavity containing the substrate placed exactly normal to the light beam to minimize optical losses. The lifetime of the light trapped in the cavity is measured to detect optical absorption sensitively. The results show clear dependence of the absorbance on the location of the monolayers with respect to a standing wave formed in the cavity; the absorbance is practically zero when the monolayers on both the surfaces are adjusted at nodes, whereas it is maximized at antinodes. The present experiment is materialized on the basis of the principles established by our previous study [ Terasaki et al., J. Opt. Soc. Am. B 22, 675 (2005) ].
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78.30.Jw Organic compounds, polymers
68.55.-a Thin film structure and morphology
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
79.20.Ds Laser-beam impact phenomena
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back to top Theoretical Methods and Algorithms

Adaptive explicit-implicit tau-leaping method with automatic tau selection

Yang Cao, Daniel T. Gillespie, and Linda R. Petzold

J. Chem. Phys. 126, 224101 (2007); http://dx.doi.org/10.1063/1.2745299 (9 pages) | Cited 14 times

Online Publication Date: 12 June 2007

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The existing tau-selection strategy, which was designed for explicit tau leaping, is here modified to apply to implicit tau leaping, allowing for longer steps when the system is stiff. Further, an adaptive strategy that identifies stiffness and automatically chooses between the explicit and the (new) implicit tau-selection methods to achieve better efficiency is proposed. Numerical testing demonstrates the advantages of the adaptive method for stiff systems.
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82.20.Uv Stochastic theories of rate constants

Vibrational modes in partially optimized molecular systems

A. Ghysels, D. Van Neck, V. Van Speybroeck, T. Verstraelen, and M. Waroquier

J. Chem. Phys. 126, 224102 (2007); http://dx.doi.org/10.1063/1.2737444 (13 pages) | Cited 19 times

Online Publication Date: 12 June 2007

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In this paper the authors develop a method to accurately calculate localized vibrational modes for partially optimized molecular structures or for structures containing link atoms. The method avoids artificially introduced imaginary frequencies and keeps track of the invariance under global translations and rotations. Only a subblock of the Hessian matrix has to be constructed and diagonalized, leading to a serious reduction of the computational time for the frequency analysis. The mobile block Hessian approach (MBH) proposed in this work can be regarded as an extension of the partial Hessian vibrational analysis approach proposed by Head [Int. J. Quantum Chem. 65, 827 (1997)] . Instead of giving the nonoptimized region of the system an infinite mass, it is allowed to move as a rigid body with respect to the optimized region of the system. The MBH approach is then extended to the case where several parts of the molecule can move as independent multiple rigid blocks in combination with single atoms. The merits of both models are extensively tested on ethanol and di-n-octyl-ether.
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33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.15.Bh General molecular conformation and symmetry; stereochemistry

An exact formulation of hyperdynamics simulations

L. Y. Chen and N. J. M. Horing

J. Chem. Phys. 126, 224103 (2007); http://dx.doi.org/10.1063/1.2737454 (6 pages) | Cited 6 times

Online Publication Date: 12 June 2007

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We introduce a new formula for the acceleration weight factor in the hyperdynamics simulation method, the use of which correctly provides an exact simulation of the true dynamics of a system. This new form of hyperdynamics is valid and applicable where the transition state theory (TST) is applicable and also where the TST is not applicable. To illustrate this new formulation, we perform hyperdynamics simulations for four systems ranging from one degree of freedom to 591 degrees of freedom: (1) We first analyze free diffusion having one degree of freedom. This system does not have a transition state. The TST and the original form of hyperdynamics are not applicable. Using the new form of hyperdynamics, we compute mean square displacement for a range of time. The results obtained agree perfectly with the analytical formula. (2) Then we examine the classical Kramers escape rate problem. The rate computed is in perfect agreement with the Kramers formula over a broad range of temperature. (3) We also study another classical problem: Computing the rate of effusion out of a cubic box through a tiny hole. This problem does not involve an energy barrier. Thus, the original form of hyperdynamics excludes the possibility of using a nonzero bias and is inappropriate. However, with the new weight factor formula, our new form of hyperdynamics can be easily implemented and it produces the exact results. (4) To illustrate applicability to systems of many degrees of freedom, we analyze diffusion of an atom adsorbed on the (001) surface of an fcc crystal. The system is modeled by an atom on top of a slab of six atomic layers. Each layer has 49 atoms. With the bottom two layers of atoms fixed, this system has 591 degrees of freedom. With very modest computing effort, we are able to characterize its diffusion pathways in the exchange-with-the-substrate and hop-over-the-bridge mechanisms.
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68.35.Fx Diffusion; interface formation
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation

Two-state model based on the block-localized wave function method

Yirong Mo

J. Chem. Phys. 126, 224104 (2007); http://dx.doi.org/10.1063/1.2746026 (9 pages) | Cited 1 time

Online Publication Date: 12 June 2007

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The block-localized wave function (BLW) method is a variant of ab initio valence bond method but retains the efficiency of molecular orbital methods. It can derive the wave function for a diabatic (resonance) state self-consistently and is available at the Hartree-Fock (HF) and density functional theory (DFT) levels. In this work we present a two-state model based on the BLW method. Although numerous empirical and semiempirical two-state models, such as the Marcus-Hush two-state model, have been proposed to describe a chemical reaction process, the advantage of this BLW-based two-state model is that no empirical parameter is required. Important quantities such as the electronic coupling energy, structural weights of two diabatic states, and excitation energy can be uniquely derived from the energies of two diabatic states and the adiabatic state at the same HF or DFT level. Two simple examples of formamide and thioformamide in the gas phase and aqueous solution were presented and discussed. The solvation of formamide and thioformamide was studied with the combined ab initio quantum mechanical and molecular mechanical Monte Carlo simulations, together with the BLW-DFT calculations and analyses. Due to the favorable solute-solvent electrostatic interaction, the contribution of the ionic resonance structure to the ground state of formamide and thioformamide significantly increases, and for thioformamide the ionic form is even more stable than the covalent form. Thus, thioformamide in aqueous solution is essentially ionic rather than covalent. Although our two-state model in general underestimates the electronic excitation energies, it can predict relative solvatochromic shifts well. For instance, the intense π→π* transition for formamide upon solvation undergoes a redshift of 0.3 eV, compared with the experimental data (0.40–0.5 eV).
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82.20.Db Transition state theory and statistical theories of rate constants
82.20.Wt Computational modeling; simulation
82.30.Nr Association, addition, insertion, cluster formation

On the performance of local, semilocal, and nonlocal exchange-correlation functionals on transition metal molecules

A. Ramírez-Solís

J. Chem. Phys. 126, 224105 (2007); http://dx.doi.org/10.1063/1.2739539 (12 pages) | Cited 2 times

Online Publication Date: 13 June 2007

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The lowest singlet-triplet transition (X 1Σ+‐3Σ+) of AgI has been used to study systematically the performance of local [local density approximation (LDA)], semilocal [generalized gradient approximation (GGA)], and nonlocal (semiempiric hybrid and meta)-type exchange-correlation functionals on a transition metal molecule where dynamic electronic correlation effects are essential. Previous benchmark ab initio calculations showed that the triplet ground state possesses a shallow well in the Franck-Condon region before becoming repulsive at longer internuclear distance [ A. Ramírez-Solís, J. Chem. Phys. 118, 104 (2003) ]. Several density functional theory (DFT) descriptions are compared with the benchmark complete active space self-consistent-field+averaged coupled pair functional results, using the same relativistic effective core potentials and optimized Gaussian basis sets. A rather unreliable performance of exchange-correlation functionals was found when ascending the various rungs in DFT Jacob’s ladder for this complex molecule. While some of the simpler (LDA and GGA) functionals correctly predict the presence of a short-distance maximum for the 3Σ+ state, more sophisticated hybrid and meta-functionals lead to totally repulsive or oscillating curves for the ground triplet state. A thorough discussion addressing the local versus nonlocal character of the exchange and correlation effects on the triplet potential curve is presented. The author concludes that any new efforts directed at producing more accurate exchange-correlation functionals must take into account the more complex electronic structure arising in transition metal molecules, whether these efforts follow the dominant pragmatic semiempiric trend or the more philosophically correct nonempiric pathway to develop better exchange-correlation functionals; only then will the Kohn-Sham version of DFT make the necessary improvements to correctly describe the electronic structure of complex transition metal systems.
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31.15.E- Density-functional theory
31.15.vn Electron correlation calculations for diatomic molecules
31.50.Df Potential energy surfaces for excited electronic states

Efficient multiparticle sampling in Monte Carlo simulations on fluids: Application to polarizable models

Filip Moučka, Michael Rouha, and Ivo Nezbeda

J. Chem. Phys. 126, 224106 (2007); http://dx.doi.org/10.1063/1.2745293 (8 pages) | Cited 7 times

Online Publication Date: 13 June 2007

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A novel Monte Carlo simulation scheme based on biased simultaneous displacements of all particles of the system has been developed. The method is particularly suited for systems with nonadditive interactions and its efficiency is demonstrated by its implementation for the polarizable Stockmayer fluid. Performance of the method is compared with both the standard one-particle move method and an unbiased multiparticle scheme by computing the mean squared displacements, rotation relaxation, and the speed of equilibration (translational order parameter). It is shown that the proposed biased method is about a factor of 10 faster, for the system considered, when compared with the other schemes.
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61.20.Ja Computer simulation of liquid structure

Computing Fukui functions without differentiating with respect to electron number. I. Fundamentals

Paul W. Ayers, Frank De Proft, Alex Borgoo, and Paul Geerlings

J. Chem. Phys. 126, 224107 (2007); http://dx.doi.org/10.1063/1.2736697 (13 pages) | Cited 23 times

Online Publication Date: 13 June 2007

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By using perturbations in the molecular external potential, the authors deduce the Fukui function from the change in Kohn-Sham orbital energies, avoiding the troublesome differentiation of the density with respect to electron number. Though this paper focuses on the Fukui function, the same general technique can be used to compute the functional derivative of any observable with respect to the external potential. In this paper, the method is used to compute the Fukui function for the beryllium atom and the formaldehyde molecule. The follow-up paper (part II) addresses the problem of computing condensed reactivity indicators.
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31.15.E- Density-functional theory

Computing Fukui functions without differentiating with respect to electron number. II. Calculation of condensed molecular Fukui functions

Nick Sablon, Frank De Proft, Paul W. Ayers, and Paul Geerlings

J. Chem. Phys. 126, 224108 (2007); http://dx.doi.org/10.1063/1.2736698 (6 pages) | Cited 14 times

Online Publication Date: 13 June 2007

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The Fukui function is a frequently used DFT concept in the description of a system’s regioselective preferences to undergo electrophilic, nucleophilic, or radical attacks. Until now, this function has usually been evaluated using finite difference approximations. The first paper in this series proposed a method for obtaining the Fukui function by a direct calculation of the functional derivative of the chemical potential with respect to the external potential. This paper extends the method to condensed Fukui functions and applies it to an extensive testing set of molecules. Results are promising, which demonstrates the usefulness of the new formalism.
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31.15.E- Density-functional theory

Efficient binomial leap method for simulating chemical kinetics

Xinjun Peng, Wen Zhou, and Yifei Wang

J. Chem. Phys. 126, 224109 (2007); http://dx.doi.org/10.1063/1.2741252 (9 pages) | Cited 5 times

Online Publication Date: 13 June 2007

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The binomial τ-leaping method of simulating the stochastic time evolution in a reaction system uses a binomial random number to approximate the number of reaction events. Theory implies that this method can avoid negative molecular numbers in stochastic simulations when a larger time step τ is used. Presented here is a modified binomial leap method for improving the accuracy and application range of the binomial leap method. The maximum existing population is first defined in this approach in order to determine a better bound of the number reactions. To derive a general leap procedure in chemically reacting systems, in this method a new sampling procedure based on the species is also designed for the maximum bound of consumed molecules of a reactant species in reaction channel. Numerical results indicate that the modified binomial leap method can be applied to a wider application range of chemically reacting systems with much better accuracy than the existing binomial leap method.
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82.20.Fd Collision theories; trajectory models
82.20.Uv Stochastic theories of rate constants

Energies of the first row atoms from quantum Monte Carlo

M. D. Brown, J. R. Trail, P. López Ríos, and R. J. Needs

J. Chem. Phys. 126, 224110 (2007); http://dx.doi.org/10.1063/1.2743972 (6 pages) | Cited 29 times

Online Publication Date: 14 June 2007

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All-electron variational and diffusion quantum Monte Carlo calculations of the ground state energies of the first row atoms (from Li to Ne) are reported. The authors use trial wave functions of four types: single-determinant Slater-Jastrow wave functions, multideterminant Slater-Jastrow wave functions, single-determinant Slater-Jastrow wave functions with backflow transformations, and multideterminant Slater-Jastrow wave functions with backflow transformations. At the diffusion quantum Monte Carlo level and using their multideterminant Slater-Jastrow wave functions with backflow transformations, they recover 99% or more of the correlation energies for Li, Be, B, C, N, and Ne, 97% for O, and 98% for F.
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31.15.-p Calculations and mathematical techniques in atomic and molecular physics
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Quadrupole moment function and absolute infrared quadrupolar intensities for N2

Hui Li and Robert J. Le Roy

J. Chem. Phys. 126, 224301 (2007); http://dx.doi.org/10.1063/1.2739524 (9 pages) | Cited 7 times

Online Publication Date: 11 June 2007

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High level ab initio methods have been used to calculate values of the quadrupole moment of the ground X 1Σg+ state of N2 on a dense radial mesh spanning the interval of 0.8–12.1 a.u. Detailed convergence tests indicate that the resulting equilibrium values of the quadrupole moment ϴe = −1.1273 a.u. and its first radial derivative  dϴ(R)/dR∣e = 0.9604 a.u. have absolute uncertainties of 0.3% and 0.8%, respectively, and are more accurate than the best experimental values of these quantities. The calculated quadrupole moment function, together with a recently reported accurate analytic empirical potential energy function [ Le Roy et al., J. Chem. Phys. 125, 164310 (2006) ], is used to generate values of the radial matrix elements determining the absolute intensities of infrared vibration-rotation transitions of ground-state N2, which take full account of vibration-rotation interactions. These results should improve the reliability of the interpretations of N2 contributions to infrared atmospheric spectra.
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33.20.Ea Infrared spectra
33.20.Vq Vibration-rotation analysis
33.70.Fd Absolute and relative line and band intensities
31.15.A- Ab initio calculations
31.50.Bc Potential energy surfaces for ground electronic states
33.15.Mt Rotation, vibration, and vibration-rotation constants

Photoinduced evaporation of mass-selected aniline+(water)n (n = 4–20) clusters

Sang Hwan Nam, Hye Sun Park, Mi Ae Lee, Nu Ri Cheong, Jae Kyu Song, and Seung Min Park

J. Chem. Phys. 126, 224302 (2007); http://dx.doi.org/10.1063/1.2738945 (9 pages) | Cited 8 times

Online Publication Date: 11 June 2007

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Photofragmentation of mass-selected aniline+(water)n (An+Wn, n = 4–20) clusters is investigated over photon energies ranging from 1.65 to 4.66 eV by linear tandem time-of-flight mass spectrometry. The aniline ring turns out to survive irradiation of photons, and most of the absorbed photon energy flows to the hydrogen-bonding networks to be used up for liberation of water molecules. The average number of ejected water molecules measured as a function of photon energy reveals that the loss of water molecules is a photoevaporation process. The distributions of internal energies for parent ions and binding energies of water molecules are estimated from the plots of photofragment branching ratio versus photon energy, which give nice Gaussian fits. Also, density functional theory calculations are performed to obtain optimized structures of isomers for An+Wn clusters and binding energies. The authors find that the An+W6 cluster has a highly symmetric structure and its binding energy in An+W6→An+W5+W stands out. This is in line with the experimental results showing that n = 6 is a magic number in the mass distribution and An+W6 is relatively stable in metastable decay.
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36.40.Wa Charged clusters
36.40.Qv Stability and fragmentation of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.E- Density-functional theory
33.15.Fm Bond strengths, dissociation energies
33.80.Gj Diffuse spectra; predissociation, photodissociation

Stabilities of multiply charged dimers and clusters of fullerenes

Henning Zettergren, Henning T. Schmidt, Peter Reinhed, Henrik Cederquist, Jens Jensen, Preben Hvelplund, Shigeo Tomita, Bruno Manil, Jimmy Rangama, and Bernd A. Huber

J. Chem. Phys. 126, 224303 (2007); http://dx.doi.org/10.1063/1.2743433 (8 pages) | Cited 8 times

Online Publication Date: 12 June 2007

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The authors find even-odd variations as functions of r ( ⩽ 7) for multiple ionization of van der Waals dimers in slow Xe30++[C60]2([C60C70])→⋯+[C60]2r+([C60C70]r+) electron-transfer collisions. This even-odd behavior is in sharp contrast to the smooth one for fullerene monomers and may be related to even-odd effects in dimer ionization energies in agreement with results from an electrostatic model. The kinetic energy releases for dimer dissociations [predominantly yielding intact fullerenes [C60]2r+→C60r1++C60r2+ in the same (r1 = r2) or nearby (r1 = r2±1) charge states] are found to be low in comparison with the corresponding model results indicating that internal excitations of the separating (intact) fullerenes are important. Experimental appearance sizes for the heavier clusters of fullerenes [C60]nr+ (n>3 and r = 2–5) compare well with predictions from a new nearest-neighbor model assuming that r unit charges in [C60]nr+ are localized to r C60 molecules such that the Coulomb energy of the system is minimized. The system is then taken to be stable if (i) two (singly) charged C60 are not nearest neighbors and (ii) the r C60+ molecules have binding energies to their neutral nearest neighbors which are larger than the repulsive energies for the (r−1) C60+–C60+ pairs. Essential ingredients in the nearest-neighbor model are cluster geometries and the present results on dimer stabilities.
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36.40.Mr Spectroscopy and geometrical structure of clusters
34.70.+e Charge transfer
34.50.Gb Electronic excitation and ionization of molecules
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Real or artifactual symmetry breaking in the BNB radical: A multireference coupled cluster viewpoint

Xiangzhu Li and Josef Paldus

J. Chem. Phys. 126, 224304 (2007); http://dx.doi.org/10.1063/1.2746027 (7 pages) | Cited 22 times

Online Publication Date: 12 June 2007

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The ground state of the linear BNB radical has been examined via the recently developed reduced multireference coupled cluster method with singles and doubles that is perturbatively corrected for triples [RMR CCSD(T)] using the correlation consistent basis sets (cc-pVXZ, X = D, T, and Q). Similar to earlier results that were based on the single reference CCSD(T) and BD(T) approaches, the RMR CCSD(T) method also predicts an asymmetric structure with two BN bonds of unequal length, even though the MR effects significantly reduce the barrier height. The computed frequencies for the symmetric and antisymmetric stretching modes agree reasonably well with the experimental data.
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31.15.bw Coupled-cluster theory
33.15.Dj Interatomic distances and angles
33.15.Bh General molecular conformation and symmetry; stereochemistry

Spectroscopic constants and potential energy curves of yttrium carbide (YC)

Bingbing Suo and Krishnan Balasubramanian

J. Chem. Phys. 126, 224305 (2007); http://dx.doi.org/10.1063/1.2743015 (8 pages) | Cited 1 time

Online Publication Date: 12 June 2007

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The potential energy curves of the low-lying electronic states of yttrium carbide (YC) and its cation are calculated at the complete active space self-consistent field and the multireference single and double excitation configuration interaction (MRSDCI) levels of theory. Fifteen low-lying electronic states of YC with different spin and spatial symmetries were identified. The X 4Σ− state prevails as the ground state of YC, and a low-lying excited A 4Π state is found to be 1661 cm−1 higher at the MRSDCI level. The computations of the authors support the assignment of the observed spectra to a B 4Δ(Ω = 7/2)←A 4Π(Ω = 5/2) transition with a reinterpretation that the A 4Π state is appreciably populated under the experimental conditions as it is less than 2000 cm−1 of the X 4Σ− ground state, and the previously suggested 4Π ground state is reassigned to the first low-lying excited state of YC. The potential energy curves of YC+ confirm a previous prediction by Seivers et al. [J. Chem. Phys. 105, 6322 (1996)] that the ground state of YC+ is formed through a second pathway at higher energies. The calculated ionization energy of YC is 6.00 eV, while the adiabatic electron affinity is 0.95 eV at the MRSDCI level. The computed ionization energy of YC and dissociation energy of YC+ confirm the revised experimental estimates provided by Seivers et al. although direct experimental measurements yielded results with greater errors due to uncertainty in collisional cross sections for YC+ formation.
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31.50.-x Potential energy surfaces
31.15.xr Self-consistent-field methods
31.15.vj Electron correlation calculations for atoms and ions: excited states
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Comparative study of charge division in substituted benzene cations

Kang Taek Lee, Jiha Sung, Kwang Jun Lee, Hyung Min Kim, Kyu Young Han, Young Dong Park, and Seong Keun Kim

J. Chem. Phys. 126, 224306 (2007); http://dx.doi.org/10.1063/1.2737455 (5 pages)

Online Publication Date: 13 June 2007

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A recently proposed phenomenon of charge division in a molecular cation [ K. T. Lee et al., J. Am. Chem. Soc. 129, 2588 (2007) ] was examined in a number of molecules by experiment and theory. We investigated the spatial distribution of electrostatic charge in the cation of the following benzene derivatives: n-propylbenzene (PB), 3-phenylpropionic acid (PPA), 2-phenylethyl alcohol (PEAL), and 2-phenylethylamine (PEA). A density functional theory calculation indicated that the positive charge was divided into two cationic charge cores in both conformers of PEA+, while it is localized mainly on the phenyl group in PB+, PPA+, and PEAL+. This finding was experimentally verified by the characteristic range of electronic transition of these species reflected in the fragmentation pattern of the mass spectra. The degree of charge division in PEA+ was slightly less than in the cationic conformers of L-phenylalanine in its subgroup II. The charge distribution in a phenyl-containing cation is suggested to depend on whether there exists a functional group that can act as a competing charge core against the phenyl ring.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Ta Mass spectra

Anharmonicity and tunneling effects in revisited vibrational O(1s) photoelectron spectrum of water gas phase

Stéphane Carniato

J. Chem. Phys. 126, 224307 (2007); http://dx.doi.org/10.1063/1.2736700 (12 pages) | Cited 3 times

Online Publication Date: 13 June 2007

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The authors have revisited the description of the core-hole ionization dynamics of the oxygen atom in water by re-exploiting the high-resolution, vibrationally resolved, XPS photoelectron spectrum of gas phase at the O(1s) edge. The agreement between theory and experiments is mainly controlled by (i) the description of the tunneling behavior near the barrier top (linear H–O–H conformation) of wave functions with high vibrational quanta, and (ii) the relative displacement of the potential-energy minimum of the O(1s) final state with respect to the ground state one. Accurate change in bond angle between the neutral and core-ionized states is essential to account for the Franck-Condon factors. The O(1s) photoelectron spectrum of water is well reproduced by the molecular ab initio calculations based on density functional theory and Franck-Condon factors calculations in a double-well (2×W) simulation of the bending motion.
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33.15.Bh General molecular conformation and symmetry; stereochemistry
33.60.+q Photoelectron spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.15.E- Density-functional theory
33.80.Eh Autoionization, photoionization, and photodetachment

Rotationally resolved electronic spectra of 9,10-dihydrophenanthrene. A “floppy” molecule in the gas phase

Leonardo Alvarez-Valtierra and David W. Pratt

J. Chem. Phys. 126, 224308 (2007); http://dx.doi.org/10.1063/1.2732753 (10 pages) | Cited 5 times

Online Publication Date: 13 June 2007

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Rotationally resolved fluorescence excitation spectra of several bands in the S1←S0 electronic spectrum of 9,10-dihydrophenanthrene (DHPH) have been observed and assigned. Each band was fit using rigid rotor Hamiltonians in both electronic states. Analyses of these data reveal that DHPH has a nonplanar configuration in its S0 state with a dihedral angle between the aromatic rings (φ) of ∼ 21.5°. The data also show that excitation of DHPH with UV light results in a more planar structure of the molecule in the electronically excited state, with φ ∼ 8.5°. Three prominent Franck-Condon progressions appear in the low resolution spectrum, all with fundamental frequencies lying below 300 cm−1. Estimates of the potential energy surfaces along each of these coordinates have been obtained from analyses of the high resolution spectra. The remaining barrier to planarity in the S1 state is estimated to be ∼ 2650 cm−1 along the bridge deformation mode and is substantially reduced by excitation of the molecule along the (orthogonal) ring twisting coordinate.
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33.50.Dq Fluorescence and phosphorescence spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.50.Df Potential energy surfaces for excited electronic states

Ab initio design of picosecond infrared laser pulses for controlling vibrational-rotational excitation of CO molecules

Thomas Herrmann, Qinghua Ren, Gabriel G. Balint-Kurti, and Frederick R. Manby

J. Chem. Phys. 126, 224309 (2007); http://dx.doi.org/10.1063/1.2738469 (13 pages) | Cited 4 times

Online Publication Date: 14 June 2007

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Optimal control of rovibrational excitations of the CO molecule using picosecond infrared laser pulses is described in the framework of the electric-nuclear Born-Oppenheimer approximation [ G. G. Balint-Kurti et al., J. Chem. Phys. 122, 084110 (2005) ]. The potential energy surface of the CO molecule in the presence of an electric field is calculated using coupled cluster theory with a large orbital basis set. The quantum dynamics of the process is treated using a full three dimensional treatment of the molecule in the laser field. The detailed mechanisms leading to efficient control of the selected excitation processes are discussed.
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33.80.-b Photon interactions with molecules
31.15.A- Ab initio calculations
31.15.bw Coupled-cluster theory
31.50.Df Potential energy surfaces for excited electronic states
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Transition between collapsed state phases and the critical swelling of a hydrogen bonding gel: Poly(methacrylic acid-co-dimethyl acrylamide)

Yaşar Yılmaz

J. Chem. Phys. 126, 224501 (2007); http://dx.doi.org/10.1063/1.2743960 (5 pages) | Cited 4 times

Online Publication Date: 11 June 2007

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Transition between collapsed state phases and discontinuous volume phase transition for a hydrogen bonding gel, poly(methacrylic acid-co-dimethyl acrylamide), were observed by using both the volume measurements and fluorescence intensity of the pyranine fluoroprobe (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt) bonded to the gel by means of electrostatic interactions. In the collapsed state, while there is no appreciable change in the volume of the gel, a considerable variation in the fluorescence intensity occurred around 30 °C signaling a second order phase transition between collapsed state phases, from relatively frozen to a fluctuating phase. Our analysis of the data around 30 °C indicates that the critical point of gel volume transition belongs to the so-called mean-field universality class, as predicted in Onuki [Phys. Rev. A 38, 2192 (1988)] and by Golubovic and Lubensky [Phys. Rev. Lett. 63, 1082 (1989)] . The relaxation time for the equilibrium swelling critically depends on the temperature and diverges near 60 °C, where both fluorescence intensity and the volume of the gel change drastically and indicate the discontinuous volume phase transition. The swelling kinetics of the critical gel during the discontinuous volume phase transition can be modeled best with the first term in the expansion of the Li-Tanaka equation for a long initial period of the swelling time.
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61.25.H- Macromolecular and polymers solutions; polymer melts
64.70.-p Specific phase transitions
78.55.-m Photoluminescence, properties and materials

Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density-functional theory

Christos N. Likos, Bianca M. Mladek, Dieter Gottwald, and Gerhard Kahl

J. Chem. Phys. 126, 224502 (2007); http://dx.doi.org/10.1063/1.2738064 (18 pages) | Cited 49 times

Online Publication Date: 11 June 2007

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We demonstrate the accuracy of the hypernetted chain closure and of the mean-field approximation for the calculation of the fluid-state properties of systems interacting by means of bounded and positive pair potentials with oscillating Fourier transforms. Subsequently, we prove the validity of a bilinear, random-phase density functional for arbitrary inhomogeneous phases of the same systems. On the basis of this functional, we calculate analytically the freezing parameters of the latter. We demonstrate explicitly that the stable crystals feature a lattice constant that is independent of density and whose value is dictated by the position of the negative minimum of the Fourier transform of the pair potential. This property is equivalent with the existence of clusters, whose population scales proportionally to the density. We establish that regardless of the form of the interaction potential and of the location on the freezing line, all cluster crystals have a universal Lindemann ratio Lf = 0.189 at freezing. We further make an explicit link between the aforementioned density functional and the harmonic theory of crystals. This allows us to establish an equivalence between the emergence of clusters and the existence of negative Fourier components of the interaction potential. Finally, we make a connection between the class of models at hand and the system of infinite-dimensional hard spheres, when the limits of interaction steepness and space dimension are both taken to infinity in a particularly described fashion.
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61.20.-p Structure of liquids
64.70.D- Solid-liquid transitions

Investigation of the local composition enhancement and related dynamics in supercritical CO2-cosolvent mixtures via computer simulation: The case of ethanol in CO2

Ioannis Skarmoutsos, Dimitris Dellis, and Jannis Samios

J. Chem. Phys. 126, 224503 (2007); http://dx.doi.org/10.1063/1.2738476 (10 pages) | Cited 1 time

Online Publication Date: 11 June 2007

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The supercritical mixture ethanol-carbon dioxide (EtOH–CO2) with mole fraction of ethanol XEtOH ≅ 0.1 was investigated at 348 K, by employing the molecular dynamics simulation technique in the canonical ensemble. The local intermolecular structure of the fluid was studied in terms of the calculated appropriate pair radial distribution functions. The estimated average local coordination numbers and mole fractions around the species in the mixture reveal the existence of local composition enhancement of ethanol around the ethanol molecules. This finding indicates the nonideal mixing behavior of the mixture due to the existence of aggregation between the ethanol molecules. Furthermore, the local environment redistribution dynamics have been explored by analyzing the time correlation functions (TCFs) of the total local coordination number (solvent, cosolvent) around the cosolvent molecules in appropriate parts. The analysis of these total TCFs in the auto-(solvent-solvent, cosolvent-cosolvent) and cross-(solvent-cosolvent, cosolvent-solvent) TCFs has shown that the time dependent redistribution process of the first solvation shell of ethanol is mainly determined by the redistribution of the CO2 solvent molecules. These results might be explained on the basis of the CO2–CO2 and EtOH–CO2 intermolecular forces, which are sufficiently weaker in comparison to the EtOH–EtOH hydrogen bonding interactions, creating in this way a significantly faster redistribution of the CO2 molecules in comparison with EtOH. Finally, the self-diffusion coefficients and the single reorientational dynamics of both the cosolvent and solvent species in the mixture have been predicted and discussed in relationship with the local environment around the species, which in the case of the EtOH molecules seem to be strongly affected.
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61.20.Ja Computer simulation of liquid structure
61.25.-f Studies of specific liquid structures
66.10.C- Diffusion and thermal diffusion

The laboratory glass transition

Prabhat K. Gupta and John C. Mauro

J. Chem. Phys. 126, 224504 (2007); http://dx.doi.org/10.1063/1.2738471 (9 pages) | Cited 25 times

Online Publication Date: 11 June 2007

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The phenomenology of the laboratory glass transition is examined in the enthalpy landscape framework. It is shown that a generic description of the glassy state based on partitioning of the phase space caused by the finiteness of the time of observation explains all universal features of glass transition. Using this description of glass, which is referred to as the extrinsically constrained liquid, expressions are derived for properties of glass and for property changes at the laboratory glass transition. A model enthalpy landscape is used to illustrate the basic concepts of this description. Additional new consequences of this description, such as the role of complexity in glass transition and the zero residual entropy of a glass, are discussed.
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64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
65.20.-w Thermal properties of liquids
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