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22 Dec 2005

Volume 123, Issue 24, Articles (24xxxx)

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Gold behaves as hydrogen: Prediction on the existence of a new class of boron-containing radicals, AuBX (X = F,Cl,Br)

Tapan K. Ghanty

J. Chem. Phys. 123, 241101 (2005); http://dx.doi.org/10.1063/1.2137316 (4 pages) | Cited 7 times

Online Publication Date: 23 December 2005

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In this Communication we have reported the prediction of a new class of compounds, AuBX (with X = F,Cl,Br), using the results obtained from ab initio quantum-chemical calculations. We have compared their electronic structures, bonding, and stability with that of the recently discovered HBX radicals and demonstrated an excellent one-to-one quantitative correspondence between the structures, nature of bonding, and stability of AuBX radicals with the corresponding HBX radicals, which is of considerable significance. Comparison has also been made with the radicals containing other coinage metal atoms, viz., CuBX and AgBX. Structurally they are found to be quite similar to the HBX radicals. However, the stability in terms of some of the bond dissociation energy values differs considerably from the corresponding values in HBX or AuBX species. This feature is attributed to the unusually high relativistic effects in gold. The present results suggest that AuBX radicals are stable enough to be prepared experimentally in analogy with the experimentally observed HBX radicals. The gold-hydrogen analogy demonstrated here quantitatively would motivate further research to predict gold analogs of novel hydride species and vice versa.
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31.15.A- Ab initio calculations
33.15.Fm Bond strengths, dissociation energies
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Quasirelativistic theory equivalent to fully relativistic theory

Werner Kutzelnigg and Wenjian Liu

J. Chem. Phys. 123, 241102 (2005); http://dx.doi.org/10.1063/1.2137315 (4 pages) | Cited 74 times

Online Publication Date: 27 December 2005

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The Dirac operator in a matrix representation in a kinetically balanced basis is transformed to a quasirelativistic Hamiltonian matrix, that has the same electronic eigenstates as the original Dirac matrix. This transformation involves a matrix X, for which an exact identity is derived, and which can be constructed either in a noniterative way or by various iteration schemes, without requiring an expansion parameter. The convergence behavior of five different iteration schemes is studied numerically, with very promising results.
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31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
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back to top Theoretical Methods and Algorithms

Pump-dump iterative squeezing of vibrational wave packets

Bo Y. Chang and Ignacio R. Sola

J. Chem. Phys. 123, 244101 (2005); http://dx.doi.org/10.1063/1.2139091 (9 pages) | Cited 4 times

Online Publication Date: 28 December 2005

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The free motion of a nonstationary vibrational wave packet in an electronic potential is a source of interesting quantum properties. In this work we propose an iterative scheme that allows continuous stretching and squeezing of a wave packet in the ground or in an excited electronic state, by switching the wave function between both potentials with π pulses at certain times. Using a simple model of displaced harmonic oscillators and delta pulses, we derive the analytical solution and the conditions for its possible implementation and optimization in different molecules and electronic states. We show that the main constraining parameter is the pulse bandwidth. Although in principle the degree of squeezing (or stretching) is not bounded, the physical resources increase quadratically with the number of iterations, while the achieved squeezing only increases linearly.
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37.10.Mn Slowing and cooling of molecules
37.10.Pq Trapping of molecules
37.10.Vz Mechanical effects of light on atoms, molecules, and ions
33.15.Mt Rotation, vibration, and vibration-rotation constants

Mixed quantum-classical Redfield master equation

Mohamad Toutounji

J. Chem. Phys. 123, 244102 (2005); http://dx.doi.org/10.1063/1.2140270 (8 pages) | Cited 10 times

Online Publication Date: 28 December 2005

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Redfield master equation is derived from mixed quantum-classical Liouville equation using product initial conditions. Simple two-level system example is given and comparison with Fermi golden rule is made.
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03.65.Sq Semiclassical theories and applications
02.30.Rz Integral equations
02.60.Nm Integral and integrodifferential equations
31.15.xg Semiclassical methods

Relativistic two-component calculations of electronic g-tensors that include spin polarization

Irina Malkin, Olga L. Malkina, Vladimir G. Malkin, and Martin Kaupp

J. Chem. Phys. 123, 244103 (2005); http://dx.doi.org/10.1063/1.2135290 (16 pages) | Cited 25 times

Online Publication Date: 30 December 2005

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The first two-component relativistic density-functional approach for the calculation of electronic g-tensors is reported that includes spin polarization using noncollinear spin-density functionals. The method is based on the relativistic Douglas-Kroll-Hess Hamiltonian and has been implemented into the ReSpect program package. Using three self-consistent-field calculations with orthogonal orientations of total magnetization J, the full g-matrix may be obtained. In contrast to previous spin-restricted two-component treatments, results with the new approach agree excellently with spin-polarized one-component calculations for light-atom radicals. Additionally, unlike one-component approaches, the method also reproduces successfully the negative Δg‖-values of heavy-atom math radicals and the negative Δg⊥ components in cysteinyl. The new method removes effectively the dilemma existing up to now regarding the simultaneous inclusion of spin polarization and higher-order spin-orbit effects in g-tensor calculations. It is straightforwardly applicable to higher than doublet spin multiplicities and has been implemented with hybrid functionals.
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31.15.E- Density-functional theory
31.15.xr Self-consistent-field methods
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions
33.35.+r Electron resonance and relaxation
33.15.Pw Fine and hyperfine structure
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Quantum wave-packet calculation of reaction probabilities, cross sections, and rate constants for Li+H2+ reaction

Fahrettin Gogtas

J. Chem. Phys. 123, 244301 (2005); http://dx.doi.org/10.1063/1.2145927 (5 pages) | Cited 7 times

Online Publication Date: 23 December 2005

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The Li+H2+(υ,j)→LiH(υ′,j′)+H+ reactive scattering has been studied by using quantum real wave-packet method. The state-to-state and state-to-all reaction probabilities for the entitled collision have been calculated. The probabilities show a smooth variation for all initial rotational quantum states. The J-shifting approximation has been employed to estimate the integral cross sections and thermal rate constants have been calculated.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.20.Ej Quantum theory of reaction cross section
82.20.Hf Product distribution
82.20.Pm Rate constants, reaction cross sections, and activation energies

Density-functional theory structures of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complexes for ions across the lanthanide series

Lidia Smentek, B. Andes Hess, Jason P. Cross, H. Charles Manning, and Darryl J. Bornhop

J. Chem. Phys. 123, 244302 (2005); http://dx.doi.org/10.1063/1.2139997 (9 pages) | Cited 7 times

Online Publication Date: 23 December 2005

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The use of organically chelated lanthanides in diagnosis and treatment is a rapidly growing field in medicine. In order to gain a deeper understanding into the properties of these chelates, particularly spectroscopic, density-functional calculations have been performed on a series of lanthanide ions chelated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. Based on the results of these calculations, it has been concluded that the local symmetry experienced by the chelated lanthanide ion may be treated as being axial, which will make the interpretation of their spectroscopic properties greatly simplified. It has also been suggested that the so-called “capping” water molecule at the ninth coordination position of the lanthanide is hydrogen bonded to the acetate oxygens of the sidearms, rather than coordinated as the ninth ligand of the lanthanide.
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31.15.E- Density-functional theory
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Fm Bond strengths, dissociation energies

A computational investigation of copper-doped germanium and germanium clusters by the density-functional theory

Jin Wang and Ju-Guang Han

J. Chem. Phys. 123, 244303 (2005); http://dx.doi.org/10.1063/1.2148949 (12 pages) | Cited 23 times

Online Publication Date: 23 December 2005

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The geometries, stabilities, and electronic properties of Gen and CuGen (n = 2–13) clusters have been systematically investigated by using density-functional approach. According to optimized CuGen geometries, growth patterns of Cu-capped Gen or Cu-substituted Gen+1 clusters for the small- or middle-sized CuGen clusters as well as growth patterns of Cu-concaved Gen or Ge-capped CuGen−1 clusters for the large-sized CuGen clusters are apparently dominant. The average atomic binding energies and fragmentation energies are calculated and discussed; particularly, the relative stabilities of CuGe10 and Ge10 are the strongest among all different sized CuGen and Gen clusters, respectively. These findings are in good agreement with the available experimental results on CoGe10− and Ge10 clusters. Consequently, unlike some transition metal (TM)Si12, the hexagonal prism CuGe12 is only low-lying structure; however, the basketlike structure is located as the lowest-energy structure. Different from some TM-doped silicon clusters, charge always transfers from copper to germanium atoms in all different sized clusters. Furthermore, the calculated highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) gaps are obviously decreased when Cu is doped into the Gen clusters, together with the decrease of HOMO-LUMO gaps, as the size of clusters increases. Additionally, the contribution of the doped Cu atom to bond properties and polarizabilities of the Gen clusters is also discussed.
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36.40.Cg Electronic and magnetic properties of clusters
36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Qv Stability and fragmentation of clusters
31.15.E- Density-functional theory
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
34.70.+e Charge transfer

Damped and thermal motion of laser-aligned hydrated macromolecule beams for diffraction

D. Starodub, R. B. Doak, K. Schmidt, U. Weierstall, J. S. Wu, J. C. H. Spence, M. Howells, M. Marcus, D. Shapiro, A. Barty, and H. N. Chapman

J. Chem. Phys. 123, 244304 (2005); http://dx.doi.org/10.1063/1.2137313 (7 pages) | Cited 9 times

Online Publication Date: 23 December 2005

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We consider a monodispersed Rayleigh droplet beam of water droplets doped with proteins. An intense infrared laser is used to align these droplets. The arrangement has been proposed for electron- and x-ray-diffraction studies of proteins which are difficult to crystallize. This paper considers the effect of thermal fluctuations on the angular spread of alignment in thermal equilibrium, and relaxation phenomena, particularly the damping of oscillations excited as the molecules enter the field. The possibility of adiabatic alignment is also considered. We find that damping times in a high-pressure gas cell as used in x-ray-diffraction experiments are short compared with the time taken for molecules to traverse the beam and that a suitably shaped field might be used for electron-diffraction experiments in vacuum to provide adiabatic alignment, thus obviating the need for a damping gas cell.
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87.15.M- Spectra of biomolecules
87.14.E- Proteins
87.15.Ya Fluctuations
87.15.B- Structure of biomolecules
37.10.Vz Mechanical effects of light on atoms, molecules, and ions

Characterization of the electronic structure of C50Cl10 by means of soft x-ray spectroscopies

Barbara Brena and Yi Luo

J. Chem. Phys. 123, 244305 (2005); http://dx.doi.org/10.1063/1.2137317 (4 pages) | Cited 2 times

Online Publication Date: 27 December 2005

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The electronic structure of the last synthesized fullerene molecule, the C50Cl10, has been characterized by theoretical simulation of x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and near-edge x-ray-absorption fine structure. All the calculations were performed at the gradient-corrected and hybrid density-functional theory levels. The combination of these techniques provides detailed information about the valence band and the unoccupied molecular orbitals, as well as about the carbon core orbitals.
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36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.E- Density-functional theory
33.60.+q Photoelectron spectra
33.20.Rm X-ray spectra

Geometries and excited-state dynamics of van der Waals dimers and higher clusters of 1-cyanonaphthalene

Takashige Fujiwara, Ricardo Campos Ramos, Marek Z. Zgierski, and Edward C. Lim

J. Chem. Phys. 123, 244306 (2005); http://dx.doi.org/10.1063/1.2141613 (8 pages) | Cited 1 time

Online Publication Date: 27 December 2005

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Mass-selected resonant two-photon ionization and infrared-ultraviolet double-resonance spectroscopies are combined with correlated (second Møller-Plesset perturbation) quantum chemistry calculation to probe electronic spectra and ground-state geometries of the jet-cooled dimer and higher clusters of 1-cyanonaphthalene. The results indicate that the dimer and trimer have stacked geometries, consistent with the highly efficient, rapid excimer formation that follows photoexcitation of the ground-state clusters.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Cg Electronic and magnetic properties of clusters
33.20.Ea Infrared spectra
33.20.Lg Ultraviolet spectra
31.15.xp Perturbation theory
33.15.Bh General molecular conformation and symmetry; stereochemistry

Experimental and theoretical studies of the conformational structures of the mixed clusters of 1-cyanonaphthalene with water

Takashige Fujiwara, Ricardo Campos Ramos, Marek Z. Zgierski, and Edward C. Lim

J. Chem. Phys. 123, 244307 (2005); http://dx.doi.org/10.1063/1.2141614 (9 pages) | Cited 2 times

Online Publication Date: 27 December 2005

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A concerted experimental (mass-selective, double-resonance laser spectroscopic technique) and theoretical (correlated quantum chemistry calculation) study of hydrogen-bonded clusters of 1-cyanonaphthalene (CNN) with water has been carried out to probe geometrical structures of the conformational isomers. The structures of the two low-energy conformers of CNN–H2O and CNN–(H2O)2, calculated at the MP2/cc-pVDZ level of theory, are consistent with the mass-selective infrared-ultraviolet double-resonance spectra and the partially resolved rotational band contours of the S1←S0 origin bands. The facile loss of a neutral water molecule from the cluster ion of CNN–(H2O)2, relative to that of CNN–H2O, is in accord with the proposed structures of the clusters.
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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.Lg Ultraviolet spectra
33.15.Fm Bond strengths, dissociation energies
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.A- Ab initio calculations

Structures and dynamics of protonated ammonia clusters

Antony Fouqueau and Markus Meuwly

J. Chem. Phys. 123, 244308 (2005); http://dx.doi.org/10.1063/1.2128669 (12 pages) | Cited 9 times

Online Publication Date: 27 December 2005

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The structures and infrared spectra of protonated ammonia clusters NH4+(NH3)n, for n ⩽ 8, are investigated using density functional-theory (DFT) calculations and semiempirical DFT/molecular dynamics simulations. For n<5 the clusters are found to be mostly stable up to 100 K, while the larger clusters (n ≥ 5) isomerize. Temperature effects are taken into account by performing ab initio molecular dynamics simulations with the computationally tractable self-consistent charges density functional tight-binding method. The infrared spectra at 10 K for the most stable isomers for n = 3–8 compare qualitatively with predissociation experiments, and using a common scaling factor almost quantitative agreement is found. For n ≥ 6 the notion of multiple isomers present under the experimental conditions is supported. Of the 13 stable structures for n = 8 only three are found to survive at 100 K. All other clusters isomerize. Cluster structures are inferred from the analysis of the cumulative radial distribution function of the ammonia molecules surrounding the NH4+ core. The infrared spectra are found to be typical for the structure of the clusters, which should help to relate the experimentally measured infrared spectra to the number and identity of the contributing isomers. For clusters that reorganize to a more stable isomer during the dynamics, the infrared spectrum is generally similar to that of the stable isomer itself. The clusters are found to preferably form globular structures, although chain-like arrangements are also among the low-energy configurations.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Sx Diffusion and dynamics of clusters
33.20.Ea Infrared spectra
31.15.E- Density-functional theory
31.15.A- Ab initio calculations
31.15.bu Semi-empirical and empirical calculations (differential overlap, Hückel, PPP methods, etc.)
31.15.xv Molecular dynamics and other numerical methods
33.15.Bh General molecular conformation and symmetry; stereochemistry

Proton and deuteron position preferences in water clusters: An ab initio study

David J. Anick

J. Chem. Phys. 123, 244309 (2005); http://dx.doi.org/10.1063/1.2139669 (10 pages) | Cited 1 time

Online Publication Date: 28 December 2005

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In order to explore the effect of H-to-D substitution on the zero-point energy (ZPE) of water clusters, Hessians were computed for a database of 53 optimized (H2O)n clusters, 5 ⩽ n ⩽ 21, at the B3LYP/6−311++G** level. The 53 clusters contained 1524 protons, which were sorted into 18 categories according to the type of their donor O and (if not free) acceptor O. Letting ΔZPE{H*} denote the change in ZPE when the proton H* is replaced by D, mean values for ΔZPE{H*} for the H-bonded categories ranged from −2172 cal mol−1 for H* in a DDAA–DDAA bond to −2118 for H* in a DAA–DDA bond. Mean value for H* free on DAA (respectively, DA) was −2018 (respectively, −1969). For DAA–DDA bonds, and for short H bonds in general, there was a strong inverse correlation between ∣ΔZPE{H*}∣ and the O–H* distance. ΔZPE for multiple H-to-D substitutions was additive, except for a cooperativity effect of −13.7 to −19.7 cal mol−1 when two substituted protons were in the same H2O unit and a much smaller cooperativity when one proton’s donor was the other’s acceptor. Implications of these data include a relative preference for D to occupy H bonded rather than free positions in finite water clusters, a value of 3.82 for the disproportionation equilibrium constant of mixed ice at 150 K, increased occupation by H at surface positions of mixed ice, and a larger average coordination number for liquid D2O than for liquid H2O.
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61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids

Electron propagator theory study of N-/O-methylglycine conformers

Shan Xi Tian

J. Chem. Phys. 123, 244310 (2005); http://dx.doi.org/10.1063/1.2139996 (7 pages) | Cited 5 times

Online Publication Date: 29 December 2005

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The low-lying conformers of N-/O-methylglycine are studied by ab initio calculations at the B3LYP, MP3, and MP4(SDQ) levels of theory with the aug-cc-pVDZ basis set. The conformers having the intramolecular hydrogen bonds N–H⋯OC or O–H⋯N are more stable than the others. Vertical ionization energies for the valence molecular orbitals of each conformer predicted with the electron propagator theory in the partial third-order quasiparticle approximation are in good agreement with the experimental data available in the literatures. The relative energies of the conformers and comparison between the simulated and the experimental photoelectron spectra demonstrate that there are at least three and two conformers of N- and O-methylglycine, respectively, in the gas-phase experiments. The intramolecular hydrogen bonding O–H⋯N effects on the molecular electronic structures are discussed for the glycine methyl derivatives, on the basis of the ab initio electronic structure calculations, natural orbital bond, and atoms-in-molecules analyses. The intramolecular hydrogen bonding O–H⋯N interactions hardly affect the electronic structures of the O-NH2–CH2–C(O)–O–CH3 and α-methylated NH2–CH2–C(CH3)OOH conformers, while the similar intramolecular interactions lead to the significantly lower-energy levels of the highest occupied molecular orbitals for the N-(CH3–NH–CH2–COOH) and β-methylated (NH2–CH2–CH2–COOH) conformers.
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31.15.A- Ab initio calculations
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.E- Density-functional theory
31.15.xp Perturbation theory
33.15.Fm Bond strengths, dissociation energies
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.60.+q Photoelectron spectra

Vibrational predissociation spectroscopy of the (H2O)6–21− clusters in the OH stretching region: Evolution of the excess electron-binding signature into the intermediate cluster size regime

Nathan I. Hammer, Joseph R. Roscioli, Joseph C. Bopp, Jeffrey M. Headrick, and Mark A. Johnson

J. Chem. Phys. 123, 244311 (2005); http://dx.doi.org/10.1063/1.2134701 (7 pages) | Cited 34 times

Online Publication Date: 29 December 2005

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We report vibrational predissociation spectra of the (H2O)n− cluster ions in the OH stretching region to determine whether the spectral signature of the electron-binding motif identified in the smaller clusters [ Hammer et al. Science 306, 675 (2004) ] continues to be important in the intermediate size regime (n = 7−21). This signature consists of a redshifted doublet that dominates the OH stretching region, and has been traced primarily to the excitation of a single water molecule residing in a double H-bond acceptor (AA) binding site, oriented with both of its H atoms pointing toward the excess electron cloud. Strong absorption near the characteristic AA doublet is found to persist in the spectra of the larger clusters, but the pattern evolves into a broadened triplet around n = 11. A single free OH feature associated with dangling hydrogen atoms on the cluster surface is observed to emerge for n ≥ 15, in sharp contrast to the multiplet pattern of unbonded OH stretches displayed by the H+∙(H2O)n clusters throughout the n = 2−29 range. We also explore the vibration-electronic coupling associated with normal-mode displacements of the AA molecule that most strongly interact with the excess electron. Specifically, electronic structure calculations on the hexamer anion indicate that displacement along the −OH2 symmetric stretching mode dramatically distorts the excess electron cloud, thus accounting for the anomalously large oscillator strength of the AA water stretching vibrations. We also discuss these vibronic interactions in the context of a possible relaxation mechanism for the excited electronic states involving the excess electron.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Tp Vibrational analysis
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.70.Jg Line and band widths, shapes, and shifts
33.15.Fm Bond strengths, dissociation energies
31.15.-p Calculations and mathematical techniques in atomic and molecular physics
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions
31.15.vj Electron correlation calculations for atoms and ions: excited states

A calculation of the rovibronic energies and spectrum of the math math electronic state of SiH2

R. Guérout, P. R. Bunker, Per Jensen, and W. P. Kraemer

J. Chem. Phys. 123, 244312 (2005); http://dx.doi.org/10.1063/1.2139676 (8 pages)

Online Publication Date: 30 December 2005

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The math math electronic state of silylene (SiH2) is the second excited singlet state of the molecule and, like the analogous math state of methylene (CH2), it is quasilinear with symmetry math at linearity. This state dissociates to Si(math)+H2(math). At equilibrium, the math state of SiH2 has an energy that we calculate to be 0.71 eV above that of the dissociation products. However, there is a barrier to dissociation that allows quasibound rovibrational levels to occur, and some have been observed recently [ Y. Muramoto et al., J. Chem. Phys. 122, 154302 (2005) ]. Starting with our analytical ab initio potential-energy surface, we adjusted it in a fitting to the experimental term values in order to determine the optimum potential-energy function in the bound region. This potential has a C2v equilibrium structure with a SiH bond length of 1.459 Å and a bond angle of 165.4°; the barrier to linearity is only 129 cm−1. Using the optimized potential-energy surface we calculate math-state term values, and using our calculated y and z dipole moment surfaces, we simulate the rotation-vibration spectrum of the state in order to assist in the detection of the matrix isolation spectrum.
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33.20.Vq Vibration-rotation analysis
31.15.A- Ab initio calculations
33.15.Mt Rotation, vibration, and vibration-rotation constants
31.50.Df Potential energy surfaces for excited electronic states
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Dj Interatomic distances and angles
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.20.-t Molecular spectra
33.15.Fm Bond strengths, dissociation energies
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Derivation of a microscopic theory of barriers and activated hopping transport in glassy liquids and suspensions

Kenneth S. Schweizer

J. Chem. Phys. 123, 244501 (2005); http://dx.doi.org/10.1063/1.2137701 (13 pages) | Cited 49 times

Online Publication Date: 23 December 2005

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A recently proposed microscopic activated barrier hopping theory [ K. S. Schweizer and E. J. Saltzman, J. Chem. Phys. 119, 1181 (2003) ] of slow single-particle dynamics in glassy liquids, suspensions, and gels is derived using nonequilibrium statistical mechanics. Fundamental elements underlying the stochastic nonlinear Langevin equation description include an inhomogeneous liquid or locally solid-state perspective, dynamic density-functional theory (DDFT), a local equilibrium closure, and a coarse-grained free-energy functional. A dynamic Gaussian approximation is not adopted which is the key for avoiding a kinetic ideal glass transition. The relevant excess free energy is of a nonequilibrium origin and is related to dynamic force correlations in the fluid. The simplicity of the approach allows external perturbations to be rather easily incorporated. Dynamic heterogeneity enters naturally via mobility fluctuations associated with the stochastic barrier-hopping process. The derivation both identifies the limitations of the theory and suggests new avenues for its systematic improvement. Comparisons with ideal mode-coupling theory, alternative DDFT approaches and a field theoretic path-integral formulation are presented.
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61.20.Gy Theory and models of liquid structure
72.20.Ee Mobility edges; hopping transport
72.80.-r Conductivity of specific materials
65.20.-w Thermal properties of liquids
82.70.Kj Emulsions and suspensions
82.70.Gg Gels and sols

Homogeneous nucleation of n-nonane and n-propanol mixtures: A comparison of classical nucleation theory and experiments

A. I. Gaman, I. Napari, P. M. Winkler, H. Vehkamäki, P. E. Wagner, R. Strey, Y. Viisanen, and M. Kulmala

J. Chem. Phys. 123, 244502 (2005); http://dx.doi.org/10.1063/1.2138703 (12 pages) | Cited 12 times

Online Publication Date: 23 December 2005

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The homogeneous nucleation rates for n-nonane–n-propanol vapor mixtures have been calculated as a function of vapor-phase activities at 230 K using the classical nucleation theory (CNT) with both rigorous and approximate kinetic prefactors and compared to previously reported experimental data. The predicted nucleation rates resemble qualitatively the experimental results for low n-nonane gas phase activity. On the high nonane activity side the theoretical nucleation rates are about three orders of magnitude lower than the experimental data when using the CNT with the approximate kinetics. The accurate kinetics improves the situation by reducing the difference between theory and experiments to two orders of magnitude. Besides the nucleation rate comparison and the experimental and predicted onset activities, the critical cluster composition is presented. The total number of molecules is approximated by CNT with reasonable accuracy. Overall, the classical nucleation theory with rigorous kinetic prefactor seems to perform better. The thermodynamic parameters needed to calculate the nucleation rates are revised extensively. Up-to-date estimates of liquid phase activities using universal functional activity coefficient Dortmund method are presented together with the experimental values of surface tensions obtained in the present study.
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64.60.Q- Nucleation
65.20.-w Thermal properties of liquids
68.03.Cd Surface tension and related phenomena

Molecular-dynamics simulations of methane hydrate dissociation

Niall J. English, J. K. Johnson, and Charles E. Taylor

J. Chem. Phys. 123, 244503 (2005); http://dx.doi.org/10.1063/1.2138697 (12 pages) | Cited 26 times

Online Publication Date: 23 December 2005

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Nonequilibrium molecular-dynamics simulations have been carried out at 276.65 K and 68 bar for the dissolution of spherical methane hydrate crystallites surrounded by a liquid phase. The liquid was composed of pure water or a water-methane mixture ranging in methane composition from 50% to 100% of the corresponding theoretical maximum for the hydrate and ranged in size from about 1600 to 2200 water molecules. Four different crystallites ranging in size from 115 to 230 water molecules were used in the two-phase systems; the nanocrystals were either empty or had a methane occupation from 80% to 100% of the theoretical maximum. The crystal-liquid systems were prepared in two distinct ways, involving constrained melting of a bulk hydrate system or implantation of the crystallite into a separate liquid phase. The breakup rates were very similar for the four different crystal sizes investigated. The method of system preparation was not found to affect the eventual dissociation rates, despite a lag time of approximately 70 ps associated with relaxation of the liquid interfacial layer in the constrained melting approach. The dissolution rates were not affected substantially by methane occupation of the hydrate phase in the 80%–100% range. In contrast, empty hydrate clusters were found to break up significantly more quickly. Our simulations indicate that the diffusion of methane molecules to the surrounding liquid layer from the crystal surface appears to be the rate-controlling step in hydrate breakup. Increasing the size of the liquid phase was found to reduce the initial delay in breakup. We have compared breakup rates computed using different long-range electrostatic methods. Use of the Ewald, minimum image, and spherical cut-off techniques led to more rapid dissociation relative to the Lekner method.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Pm Rate constants, reaction cross sections, and activation energies
64.75.-g Phase equilibria
64.70.D- Solid-liquid transitions
61.20.Ja Computer simulation of liquid structure

Quantification of global orientational order in organic solids by magic-angle spinning deuterium NMR with rotor synchronization

Magesh Nandagopal and Marcel Utz

J. Chem. Phys. 123, 244504 (2005); http://dx.doi.org/10.1063/1.2132274 (8 pages)

Online Publication Date: 23 December 2005

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A new method for the characterization of orientational order in organic solids based on magic-angle spinning NMR spectroscopy is introduced. The method is related to the rotor-synchronized magic-angle spinning experiment proposed by Harbison and Spiess [Chem. Phys. Lett. 124, 128 (1986) ], but exploits the anisotropy of the deuterium quadrupolar coupling instead of the carbon-13 chemical shielding anisotropy. Magic-angle spinning provides a sensitivity advantage over pseudostatic techniques; using the deuterium quadrupolar coupling makes the method applicable to systems that do not exhibit large carbon chemical shift anisotropies, such as aliphatic polymers. Due to the magnitude of the deuterium quadrupolar coupling, a large number of spinning sidebands can be reliably observed, allowing for a precise determination of the orientational distribution function. Experimental data are analyzed in terms of Wigner matrix basis functions as well as the conjugate orthogonal functions framework. Unidirectionally cold-drawn poly(ethylene) is used as an example to demonstrate the method.
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76.60.-k Nuclear magnetic resonance and relaxation
61.50.-f Structure of bulk crystals

Theoretical prediction of the coordination number, local composition, and pressure-volume-temperature properties of square-well and square-shoulder fluids

Jiawen Hu and Zhenhao Duan

J. Chem. Phys. 123, 244505 (2005); http://dx.doi.org/10.1063/1.2140271 (12 pages) | Cited 6 times

Online Publication Date: 27 December 2005

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By assuming a Boltzmann distribution for the molecular equilibrium between local and bulk environments, a general model is derived for the prediction of coordination numbers and local compositions of square-well and square-shoulder fluids. The model has no empirical parameter fitted from the data of square-well and square-shoulder fluids, but is valid from the low-density limit to the high-density limit. The applicable width of well or shoulder covers the commonly used range varying from 1.0 to 2.0. The model can accurately predict the coordination numbers of pure square-well and square-shoulder fluids, so the equation of state derived from it is superior to other equations of state based on the existing coordination number models. The model also accurately predicts the local compositions of mixtures in wide ranges of density and size ratio (1.0–8.0), as well as the configuration energy of lattice gases and highly nonideal lattice mixtures. It is remarkable that the model correctly predicts temperature-dependent coordination numbers and local compositions for both equal- and unequal-sized mixtures at close packing, which cannot be predicted by the existing coordination number models. Our derivation demonstrates that the energy parameters in local composition models should represent the potential difference of a molecule between the local and bulk environments, not the pair-interaction potential, and depend on the system conditions and different kinds of pair-interaction parameters. This result is very useful for the development of local composition and activity coefficient models and the mixing rules of equations of state.
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64.10.+h General theory of equations of state and phase equilibria
82.80.-d Chemical analysis and related physical methods of analysis

Effective chromophore potential, dissipative trajectories, and vibrational energy relaxation: Br2 in Ar matrix

M. Gühr and N. Schwentner

J. Chem. Phys. 123, 244506 (2005); http://dx.doi.org/10.1063/1.2138028 (12 pages) | Cited 10 times

Online Publication Date: 28 December 2005

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The intramolecular wave packet dynamics on the electronic B (math) potential of Br2 in solid argon is induced and interrogated by femtosecond pump-probe spectroscopy. An effective potential of the chromophore in the solid is derived from the wave packet period for different excitation photon energies. Deep in the potential well, it is consistent with vibrational energies from wavelength-resolved spectra. It extends to higher energies, where the vibrational bands merge to a continuum, and even beyond the dissociation limit, thus quantifying the cage effect of the argon matrix. This advantage of pump-probe spectroscopy is related to a reduced contribution of homogeneous and inhomogeneous line broadenings. The vibrational energy relaxation rates are determined by a variation of the probe window spatial position via the probe quantum energy. A very large energy loss in the first excursion of the wave packet is observed near the dissociation limit. This strong interaction with the argon matrix is directly displayed in an experimental trajectory.
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33.20.Kf Visible spectra
33.80.Be Level crossing and optical pumping
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
33.70.Jg Line and band widths, shapes, and shifts

Molecular-dynamics study of anomalous volumetric behavior of water-benzene mixtures in the vicinity of the critical region

Shun-ichi Ikawa

J. Chem. Phys. 123, 244507 (2005); http://dx.doi.org/10.1063/1.2145749 (7 pages) | Cited 4 times

Online Publication Date: 29 December 2005

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Molecular-dynamics simulations of water-benzene mixtures at 573 K and pressures in the 85–140 bars range have been performed to examine local structure and dynamics of the mixtures, which exhibit anomalously large volume expansion on mixing as recently found by in situ near-infrared measurements. Fractional charges for a simple-point-charge-type potential of water were adjusted so as to reproduce liquid densities and the gas-to-liquid transition pressure of neat water at 573 K. A Lennard-Jones-type potential for benzene was used and the Lorentz-Berthelot combination rule was applied to the water-benzene interaction. Simulations with a N-P-T ensemble of 800-molecule system have been performed and the results reproduce well the anomalous volumetric behavior of the mixtures with the mole fraction of benzene in the 0.3–0.8 range. Pair distribution functions, coordination numbers, and self-diffusion coefficients for the mixtures are calculated, and it is suggested that the local structure around water molecules undergoes drastic change by dissolution of benzene in the vicinity of the critical region, but that around benzene molecules seems to be understood as that of ordinary liquid mixtures.
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61.20.Ja Computer simulation of liquid structure
61.25.Em Molecular liquids
64.75.-g Phase equilibria
64.70.F- Liquid-vapor transitions
66.10.C- Diffusion and thermal diffusion

Kinetic model for binary homogeneous nucleation in the H2O–H2SO4 system: Comparison with experiments and classical theory of nucleation

A. Sorokin, X. Vancassel, and P. Mirabel

J. Chem. Phys. 123, 244508 (2005); http://dx.doi.org/10.1063/1.2141511 (6 pages) | Cited 4 times

Online Publication Date: 29 December 2005

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A kinetic model to predict nucleation rates in the sulfuric acid-water system is presented. It allows calculating steady-state nucleation rates and the corresponding time lag, using a direct solution of a system of kinetic equations that describe the populations of sub- and near-critical clusters. This kinetic model takes into account cluster-cluster collisions and decay of clusters into smaller clusters. The model results are compared with some predictions obtained with the classical nucleation theory (CNT) and also with available measurement data obtained in smog chambers or flow tubes. It is shown that in the case of slow nucleation processes, the kinetic model and the CNT as used by Shugard et al. [J. Chem. Phys. 75, 5298 (1974) ] give the same results. However, in the case of intensive nucleation, a large part of the nucleation flux is due to cluster-cluster collisions and the CNT underestimates the nucleation rates.
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64.60.Q- Nucleation
64.70.F- Liquid-vapor transitions
82.70.Rr Aerosols and foams
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