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21 Jul 2006

Volume 125, Issue 3, Articles (03xxxx)

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Resolving the emission times of solute and solvent four-wave mixing signals by spectral interferometry

Andrew M. Moran, Rene A. Nome, and Norbert F. Scherer

J. Chem. Phys. 125, 031101 (2006); http://dx.doi.org/10.1063/1.2217940 (4 pages) | Cited 7 times

Online Publication Date: 20 July 2006

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Electric field-resolved transient grating measurements are used to distinguish the four-wave mixing signal emission from a resonant solute and a non-resonant solvent. The two components of the solution (i.e., solute and solvent) emit signal fields at different times with respect to the arrival of the probe pulse to the sample. This gives rise to a recurrence in the temporal profile of the total signal field. We show that the origin of this interference is the difference in relaxation time scales of the holographic gratings associated with the solute and solvent. The grating of the resonant solute relaxes on the time scale of a few picoseconds due to depopulation of its excited electronic state, whereas the electronic polarizability response of the solvent relaxes on the femtosecond time scale. This separability of responses is a general phenomenon that is particularly useful for studying weakly absorbing solute dynamics in polarizable solvents.
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42.40.My Applications
42.40.Eq Holographic optical elements; holographic gratings
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
07.60.Ly Interferometers
42.40.Kw Holographic interferometry; other holographic techniques
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back to top Theoretical Methods and Algorithms

The stress tensor of a molecular system: An exercise in statistical mechanics

S. Morante, G. C. Rossi, and M. Testa

J. Chem. Phys. 125, 034101 (2006); http://dx.doi.org/10.1063/1.2214719 (11 pages) | Cited 9 times

Online Publication Date: 18 July 2006

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We prove that conservation of the stress tensor is a consequence of the invariance of the partition function under canonical diffeomorphisms. From this observation a simple and general derivation of the formula which gives the local expression of the stress tensor of a molecular system in terms of its microscopic degrees of freedom readily follows. The derivation is valid in the canonical as well as the microcanonical ensemble. It works both in the classical and in the quantum mechanical settings and for arbitrary boundary conditions. In particular, if periodic boundary conditions are assigned to the system, the usual minimal-image prescription is naturally born out for mathematical consistency. An interesting outcome of our general analysis is that only in the case of a short-range interaction potential a truly local formula for the stress tensor can exist.
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31.10.+z Theory of electronic structure, electronic transitions, and chemical binding

Calculation of origin-independent optical rotation tensor components in approximate time-dependent density functional theory

Mykhaylo Krykunov and Jochen Autschbach

J. Chem. Phys. 125, 034102 (2006); http://dx.doi.org/10.1063/1.2210474 (10 pages) | Cited 23 times

Online Publication Date: 18 July 2006

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We outline an implementation of the origin-independent optical rotation tensor, which includes electric dipole-magnetic dipole and electric dipole-electric quadrupole polarizability. The method is based on approximate time-dependent density functional theory. We utilize time-periodic magnetic-field-dependent basis functions as well as a modified velocity-gauge formulation of dynamic polarizability tensors in order to obtain a gauge-origin independence. To ensure gauge-origin independence of the results within a given numerical accuracy, density fit coefficient derivatives are employed. A damping constant has been introduced into the linear response equations to treat both resonance and nonresonance regions of optical activity. We present calculations for trans-2,3-dimethyloxirane and derivatives thereof as well as calculations for androst-4,17-dien-3-one. In the Appendix, we derive the equivalence between the common-gauge origin and gauge-including atomic orbitals formulations for the optical rotation tensor in time-dependent DFT.
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33.55.+b Optical activity and dichroism
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
31.15.E- Density-functional theory

Fundamental importance of the Coulomb hole sum rule to the understanding of the Colle-Salvetti wave function functional

Xiao-Yin Pan, Viraht Sahni, and Lou Massa

J. Chem. Phys. 125, 034103 (2006); http://dx.doi.org/10.1063/1.2217731 (6 pages) | Cited 1 time

Online Publication Date: 19 July 2006

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In this paper we consider the general form of the correlated-determinantal wave function functional of Colle and Salvetti (CS) for the He atom. The specific form employed by CS is the basis for the widely used CS correlation energy formula and the Lee-Yang-Parr correlation energy density functional of Kohn-Sham density functional theory. We show the following: (i) The key assumption of CS for the determination of this wave function functional, viz., that the resulting single-particle density matrix and the Hartree-Fock theory Dirac density matrix are the same, is equivalent to the satisfaction of the Coulomb hole sum rule for each electron position. The specific wave function functional derived by CS does not satisfy this sum rule for any electron position. (ii) Application of the theorem on the one-to-one correspondence between the Coulomb hole sum rule for each electron position and the constraint of normalization for approximate wave functions then proves that the wave function derived by CS violates charge conservation. (iii) Finally, employing the general form of the CS wave function functional, the exact satisfaction of the Coulomb hole sum rule at each electron position then leads to a wave function that is normalized. The structure of the resulting approximate Coulomb holes is reasonably accurate, reproducing both the short- and the long-range behavior of the hole for this atom. Thus, the satisfaction of the Coulomb hole sum rule by an approximate wave function is a necessary condition for constructing wave functions in which electron-electron repulsion is represented reasonably accurately.
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31.15.E- Density-functional theory
31.15.xr Self-consistent-field methods

Reversible measure-preserving integrators for non-Hamiltonian systems

Gregory S. Ezra

J. Chem. Phys. 125, 034104 (2006); http://dx.doi.org/10.1063/1.2215608 (14 pages) | Cited 6 times

Online Publication Date: 20 July 2006

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We present a systematic method for deriving reversible measure-preserving integrators for non-Hamiltonian systems such as the Nosé-Hoover thermostat and generalized Gaussian moment thermostat (GGMT). Our approach exploits the (non-Poisson) bracket structure underlying the thermostat equations of motion. Numerical implementation for the GGMT system shows that our algorithm accurately conserves the thermostat energy function. We also study position and momentum distribution functions obtained using our integrator.
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45.05.+x General theory of classical mechanics of discrete systems
02.30.Hq Ordinary differential equations
02.60.Lj Ordinary and partial differential equations; boundary value problems

Including quantum subsystem character within classical equilibrium simulations

Maria A. Gomez and Patricia Peart

J. Chem. Phys. 125, 034105 (2006); http://dx.doi.org/10.1063/1.2216710 (7 pages)

Online Publication Date: 20 July 2006

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A mixed quantum/classical density matrix approximation is derived. The density matrix makes use of quantum subsystem vibrational wave functions. The diagonal of the density matrix can be used as an equilibrium distribution in Monte Carlo simulations. The approximate distribution compares well with the path integral distribution for a model system. Since it includes quantum subsystem information, it performs much better than the quadratic Feynman-Hibbs distribution. These types of distributions can aid in including quantum vibrational information in otherwise classical simulations.
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61.20.Ja Computer simulation of liquid structure

A new ligand field approach to linear transition metal dihalides

M. Vogel, W. Weber, and W. Wenzel

J. Chem. Phys. 125, 034106 (2006); http://dx.doi.org/10.1063/1.2209686 (6 pages)

Online Publication Date: 20 July 2006

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We have performed theoretical multiconfigurational calculations of the molecular energy levels based on two axial ligand field models and determined the model parameters to reproduce ab initio energies of TiCl2, TiF2, VCl2, and VF2. We develop two extensions to ligand field theory (LFT) for linear transition metal dihalides, which are incorrectly described by standard LFT. The standard LFT is augmented (1) by including the ligand induced hybridization of the d(σ) orbital with the 4s orbital or (2) by using a different radial part for the d(σ) orbital. Both models reproduce the energies of the first electronic states in very good agreement with numerical multireference configuration interaction results. Furthermore the model parameters are very close to experimental known Racah parameters describing the term energies of Ti2+ and V2+.
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31.15.A- Ab initio calculations
31.15.vq Electron correlation calculations for polyatomic molecules
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

Dynamics of radiation induced isomerization for HCN–CNH

Kyungsun Na, Christof Jung, and L. E. Reichl

J. Chem. Phys. 125, 034301 (2006); http://dx.doi.org/10.1063/1.2216704 (11 pages) | Cited 6 times

Online Publication Date: 17 July 2006

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We have analyzed the dynamics underlying the use of sequential radiation pulses to control the isomerization between the HCN and the CNH molecules. The appearance of avoided crossings among Floquet eigenphases as the molecule interacts with the radiation pulses is the key to understanding the isomerization dynamics, both in the adiabatic and nonadiabatic regimes. We find that small detunings of the incident pulses can have a significant effect on the outcome of the isomerization process for the model we consider.
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82.30.Qt Isomerization and rearrangement
33.80.Be Level crossing and optical pumping
82.50.-m Photochemistry
82.20.-w Chemical kinetics and dynamics

Infrared spectroscopy of Li(NH3)n clusters for n = 4–7

Tom E. Salter, Victor A. Mikhailov, Corey J. Evans, and Andrew M. Ellis

J. Chem. Phys. 125, 034302 (2006); http://dx.doi.org/10.1063/1.2208349 (10 pages) | Cited 13 times

Online Publication Date: 17 July 2006

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Infrared spectra of Li(NH3)n clusters as a function of size are reported for the first time. Spectra have been recorded in the N–H stretching region for n = 4→7 using a mass-selective photodissociation technique. For the n = 4 cluster, three distinct IR absorption bands are seen over a relatively narrow region, whereas the larger clusters yield additional features at higher frequencies. Ab initio calculations have been carried out in support of these experiments for the specific cases of n = 4 and 5 for various isomers of these clusters. The bands observed in the spectrum for Li(NH3)4 can all be attributed to N–H stretching vibrations from solvent molecules in the first solvation shell. The appearance of higher frequency N–H stretching bands for n ≥ 5 is assigned to the presence of ammonia molecules located in a second solvent shell. These data provide strong support for previous suggestions, based on gas phase photoionization measurements, that the first solvation shell for Li(NH3)n is complete at n = 4. They are also consistent with neutron diffraction studies of concentrated lithium/liquid ammonia solutions, where Li(NH3)4 is found to be the basic structural motif.
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36.40.Mr Spectroscopy and geometrical structure of clusters
78.30.C- Liquids
82.50.-m Photochemistry
61.20.Gy Theory and models of liquid structure

Unimolecular processes in CH2OH below the dissociation barrier: O–H stretch overtone excitation and dissociation

Jie Wei, Boris Karpichev, and Hanna Reisler

J. Chem. Phys. 125, 034303 (2006); http://dx.doi.org/10.1063/1.2216703 (8 pages) | Cited 3 times

Online Publication Date: 17 July 2006

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The OH-stretch overtone spectroscopy and dynamics of the hydroxymethyl radical, CH2OH, are reported in the region of the second and third overtones, which is above the thermochemical threshold to dissociation to H+CH2O (D0 = 9600 cm−1). The second overtone spectrum at 10 484 cm−1 is obtained by double resonance IR-UV resonance enhanced multiphoton ionization (REMPI) spectroscopy via the 3pz electronic state. It is rotationally resolved with a linewidth of 0.4 cm−1 and displays properties of local-mode vibration. No dissociation products are observed. The third overtone spectra of CH2OH and CD2OH are observed at ∼ 13 600 cm−1 by monitoring H-atom photofragments while scanning the excitation laser frequency. No double resonance REMPI spectrum is detected, and no D fragments are produced. The spectra of both isotope analogs can be simulated with a linewidth of 1.3 cm−1, indicating dissociation via tunneling. By treating the tunneling as one dimensional and using the calculated imaginary frequency, the barrier to dissociation is estimated at about 15 200 cm−1, in good agreement with theoretical estimations. The Birge-Sponer plot is linear for OH-stretch vibrations 1ν1–4ν1, demonstrating behavior of a one-dimensional Morse oscillator. The anharmonicity parameter derived from the plot is similar to the values obtained for other small OH containing molecules. Isomerization to methoxy does not contribute to the predissociation signal and the mechanism appears to be direct O–H fission via tunneling. CH2OH presents a unique example in which the reaction coordinate is excited directly and leads to predissociation via tunneling while preserving the local-mode character of the stretch vibration.
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82.50.Bc Processes caused by infrared radiation
82.50.Hp Processes caused by visible and UV light
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Dx Analytical methods involving electronic spectroscopy
82.30.Qt Isomerization and rearrangement
82.20.Hf Product distribution

Energy pooling in multiple ionization and Coulomb explosion of clusters by nanosecond-long, megawatt laser pulses

P. Sharma, R. K. Vatsa, S. K. Kulshreshtha, J. Jha, D. Mathur, and M. Krishnamurthy

J. Chem. Phys. 125, 034304 (2006); http://dx.doi.org/10.1063/1.2217370 (7 pages) | Cited 11 times

Online Publication Date: 17 July 2006

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We report the results of experiments that establish the possibility of bringing about multiple ionization and Coulomb explosion of molecular clusters with nanosecond laser pulses at intensities as small as 109 W cm−2. We demonstrate several new facets of the laser-cluster interaction in the low-intensity, long-pulse domain: (i) The choice of laser wavelength for a given cluster species is very crucial. (ii) Excited electronic states play a very important role in the ionization dynamics. (iii) When field ionization is insignificant and ponderomotive energies are very small, it is energy pooling rather than inverse bremsstrahlung that determines how clusters absorb energy from the optical field.
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36.40.-c Atomic and molecular clusters
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
33.80.Eh Autoionization, photoionization, and photodetachment
33.80.Gj Diffuse spectra; predissociation, photodissociation

Anharmonic vibrational levels of the two cyclic isomers of SiC3

Roberto Linguerri, Pavel Rosmus, and Stuart Carter

J. Chem. Phys. 125, 034305 (2006); http://dx.doi.org/10.1063/1.2209693 (7 pages) | Cited 6 times

Online Publication Date: 17 July 2006

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Using coupled-cluster approach full six-dimensional analytic potential energy surfaces for two cyclic SiC3 isomers [C–C transannular bond (I) and Si–C transannular bond (II)] have been generated and used to calculate anharmonic vibrational wave functions. Several strong low-lying anharmonic resonances have been found. In both isomers already some of the fundamental transitions cannot be described within the harmonic approximation. Adiabatic electron affinities and ionization energies have been calculated as well. The Franck-Condon factors for the photodetachment processes c-SiC3−(I)→c-SiC3(I) and c-SiC3−(II)→c-SiC3(II) are reported.
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33.20.Tp Vibrational analysis
31.15.bw Coupled-cluster theory
31.50.-x Potential energy surfaces
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.80.Eh Autoionization, photoionization, and photodetachment

H(D)-atom yields in the quenching of Xe(6s[3/2]1) by methane, ethane, ethene, ethyne, and their deuterated isotopologues

Hironobu Umemoto

J. Chem. Phys. 125, 034306 (2006); http://dx.doi.org/10.1063/1.2213256 (7 pages) | Cited 3 times

Online Publication Date: 18 July 2006

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The yields for the production of H(D) atoms in the reactions of Xe(6s[3/2]1) with simple hydrocarbons and their deuterated variants were determined. Xe(6s[3/2]1) was produced by two-photon laser excitation of Xe(6p[1/2]0) followed by concomitant amplified spontaneous emission. H(D) atoms are detected using a vacuum-ultraviolet laser-induced fluorescence (LIF) technique. The H(D)-atom yields were evaluated from the LIF intensities and the overall rate constants for the quenching, which were determined from the temporal profile measurements of the resonance fluorescence from Xe(6s[3/2]1). H/D isotope effects were observed not only in the overall rate constants but also in the H(D)-atom yields. The yields for CH4, C2H4, and C2H2 were determined to be 0.89, 1.43, 1.03, respectively, while those for CD4, C2D4, and C2D2 were found to be smaller; 0.63, 0.86, and 0.79, respectively. The H/D yield ratio for CH2D2 was 1.76. The presence of the isotope effects both in the rate constants and the yields suggests that electronic-to-electronic energy transfer processes and abstractive processes are competing.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.80.Dx Analytical methods involving electronic spectroscopy
82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Tr Kinetic isotope effects including muonium

Laser control of reactions of photoswitching functional molecules

Hiroyuki Tamura, Shinkoh Nanbu, Toshimasa Ishida, and Hiroki Nakamura

J. Chem. Phys. 125, 034307 (2006); http://dx.doi.org/10.1063/1.2213963 (10 pages) | Cited 13 times

Online Publication Date: 18 July 2006

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Laser control schemes of reactions of photoswitching functional molecules are proposed based on the quantum mechanical wave-packet dynamics and the design of laser parameters. The appropriately designed quadratically chirped laser pulses can achieve nearly complete transitions of wave packet among electronic states. The laser parameters can be optimized by using the Zhu-Nakamura theory of nonadiabatic transition. This method is effective not only for the initial photoexcitation process but also for the pump and dump scheme in the middle of the overall photoswitching process. The effects of momentum of the wave packet crossing a conical intersection on the branching ratio of products have also been clarified. These control schemes mentioned above are successfully applied to the cyclohexadiene/hexatriene photoisomerization (ring-opening) process which is the reaction center of practical photoswitching molecules such as diarylethenes. The overall efficiency of the ring opening can be appreciably increased by using the appropriately designed laser pulses compared to that of the natural photoisomerization without any control schemes.
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82.50.-m Photochemistry
82.30.Qt Isomerization and rearrangement
33.80.-b Photon interactions with molecules

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(π,π*) excited state

Juan Yang, Martin Wagner, Katsuhiko Okuyama, Kevin Morris, Zane Arp, Jaebum Choo, Niklas Meinander, Ohyun Kwon, and Jaan Laane

J. Chem. Phys. 125, 034308 (2006); http://dx.doi.org/10.1063/1.2208616 (9 pages) | Cited 3 times

Online Publication Date: 18 July 2006

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The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(π,π*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (ν45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (ν43) and ring-twisting (ν44) vibrations. Despite some interaction with ν43 and ν44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(π,π*) state coumaran is quasiplanar with a barrier to planarity of 34 cm−1 and with energy minima at puckering angles of ±14°. The corresponding ground state (S0) values are 154 cm−1 and ±25°. As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the π→π* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm−1 for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(π,π*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.
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33.50.Dq Fluorescence and phosphorescence spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Lg Ultraviolet spectra
33.15.Dj Interatomic distances and angles

Ionization spectra and electronic decay in small iodide clusters: Fully relativistic results

Markus Pernpointner, Stefan Knecht, and Lorenz S. Cederbaum

J. Chem. Phys. 125, 034309 (2006); http://dx.doi.org/10.1063/1.2222363 (8 pages) | Cited 3 times

Online Publication Date: 19 July 2006

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Singly ionized systems in high-lying energetic final states can stabilize themselves via various electronic decay mechanisms. With increasing system size interatomic and intermolecular processes dominate over intra-atomic (Auger) decay channels. For the small (HI)2 and (LiI)2 clusters fully relativistic ionization spectra are calculated and the subsequent electronic decay of the cations is investigated. Due to the presence of the iodine atom a fully relativistic description is mandatory and was performed by the algebraic diagrammatic construction technique in its four-component form. The lifetimes of the singly ionized final states are estimated by the application of Weisskopf-Wigner [ Z. Phys. 63, 54 (1930) ] theory.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.80.Eh Autoionization, photoionization, and photodetachment
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors
31.30.J- Relativistic and quantum electrodynamic (QED) effects in atoms, molecules, and ions

Intermediate state polarization in multiphoton ionization of HCl

A. I. Chichinin, P. S. Shternin, N. Gödecke, S. Kauczok, C. Maul, O. S. Vasyutinskii, and K.-H. Gericke

J. Chem. Phys. 125, 034310 (2006); http://dx.doi.org/10.1063/1.2218336 (14 pages) | Cited 14 times

Online Publication Date: 20 July 2006

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The paper presents the detailed theoretical description of the intermediate state polarization and photofragment angular distribution in resonance enhanced multiphoton ionization (REMPI) of molecules and the experimental investigation of these effects in the E math and V math states of HCl populated by two-photon transitions. It is shown that the intermediate state polarization can be characterized by the universal parameter b which is in general a complex number containing information about the symmetry of the two-photon excitation and possible phase shifts. The photofragment angular distribution produced by one- or multiphoton excitation of the polarized intermediate state is presented as a product of the intermediate state axis spatial distribution and the angular distribution of the photofragments from an unpolarized intermediate state. Experiments have been carried out by two complementary methods: REMPI absorption spectroscopy of rotationally resolved (E,v′ = 0←X,v″ = 0) and (V,v′ = 12←X,v″ = 0) transitions and REMPI via the Q(0) and Q(1) rotational transitions followed by three-dimensional ion imaging detection. The values of the parameter b determined from experiment manifest the mostly perpendicular nature of the initial two-photon transition. The experimentally obtained H+ -ion fragment angular distributions produced via the Q(1) rotational transition show good agreement with theoretical prediction.
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32.80.Rm Multiphoton ionization and excitation to highly excited states
33.80.Gj Diffuse spectra; predissociation, photodissociation
33.15.Mt Rotation, vibration, and vibration-rotation constants

Real space pseudopotential calculations for copper clusters

Shen Li, M. M. G. Alemany, and James R. Chelikowsky

J. Chem. Phys. 125, 034311 (2006); http://dx.doi.org/10.1063/1.2216698 (7 pages) | Cited 10 times

Online Publication Date: 20 July 2006

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Neutral and anion clusters of copper, Cun (n = 3–11), are examined using real space pseudopotentials constructed within the local spin density approximation. We predict the ground state structure for each cluster, the binding energy, and the corresponding photoelectron spectra, which we compare to experiment. We find strong final state effects in the photoelectron spectra, especially for the smaller clusters. The binding energy as a function of cluster size tracks well with the measured values, although the magnitude of the binding energy exceeds the experimental values by ∼ 20%, as expected for the local spin density approximation.
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36.40.Mr Spectroscopy and geometrical structure of clusters
36.40.Wa Charged clusters
31.15.E- Density-functional theory
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
33.60.+q Photoelectron spectra

A Hirshfeld partitioning of polarizabilities of water clusters

A. Krishtal, P. Senet, M. Yang, and C. Van Alsenoy

J. Chem. Phys. 125, 034312 (2006); http://dx.doi.org/10.1063/1.2210937 (7 pages) | Cited 24 times

Online Publication Date: 21 July 2006

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A new Hirshfeld partitioning of cluster polarizability into intrinsic polarizabilities and charge delocalization contributions is presented. For water clusters, density-functional theory calculations demonstrate that the total polarizability of a water molecule in a cluster depends upon the number and type of hydrogen bonds the molecule makes with its neighbors. The intrinsic contribution to the molecular polarizability is transferable between water molecules displaying the same H-bond scheme in clusters of different sizes, and geometries, while the charge delocalization contribution also depends on the cluster size. These results could be used to improve the existing force fields.
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36.40.Mr Spectroscopy and geometrical structure of clusters
31.15.E- Density-functional theory
33.15.Kr Electric and magnetic moments (and derivatives), polarizability, and magnetic susceptibility
33.15.Fm Bond strengths, dissociation energies
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Phase coexistence in polydisperse multi-Yukawa hard-sphere fluid: High temperature approximation

Yu. V. Kalyuzhnyi and S. P. Hlushak

J. Chem. Phys. 125, 034501 (2006); http://dx.doi.org/10.1063/1.2212419 (11 pages) | Cited 1 time

Online Publication Date: 17 July 2006

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High temperature approximation (HTA) is used to describe the phase behavior of polydisperse multi-Yukawa hard-sphere fluid mixtures. It is demonstrated that in the frames of the HTA the model belongs to the class of “truncatable free energy models,” i.e., the models with thermodynamical properties (Helmholtz free energy, chemical potential, and pressure) defined by the finite number of generalized moments. Using this property we were able to calculate the complete phase diagram (i.e., cloud and shadow curves as well as binodals) and size distribution functions of the coexisting phases of several different models of polydisperse fluids. In particular, we consider polydisperse one-Yukawa hard-sphere mixture with factorizable Yukawa coefficients and polydisperse Lennard-Jones (LJ) mixture with interaction energy parameter and/or size polydispersity. To validate the accuracy of the HTA we compare theoretical results with previously published results of more advanced mean spherical approximation (MSA) for the one-Yukawa model and with the Monte Carlo (MC) computer simulation results of [ Wilding et al. J. Chem. Phys. 121, 6887 (2004) ; Phys. Rev. Lett. 95, 155701 (2005) ] for the LJ model. We find that overall predictions of the HTA are in reasonable agreement with predictions of the MSA and MC, with the accuracy range from semiquantitative (for the phase diagram) to quantitative (for the size distribution functions).
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61.20.Ja Computer simulation of liquid structure
64.60.F- Equilibrium properties near critical points, critical exponents
65.20.-w Thermal properties of liquids
82.70.-y Disperse systems; complex fluids

A molecular dynamics simulation study of buckyballs in water: Atomistic versus coarse-grained models of C60

Niharendu Choudhury

J. Chem. Phys. 125, 034502 (2006); http://dx.doi.org/10.1063/1.2217442 (7 pages) | Cited 3 times

Online Publication Date: 17 July 2006

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A simple coarse-grained water-C60 model potential is proposed to study the solvation behavior of C60 in water. Extensive molecular dynamics simulations are performed to compare the hydration behavior of the coarse-grained C60 with that of its atomistic counterpart. In the coarse-grained description, we model the C60 as a spherical solute and its interaction with water is represented by a simple two-body central potential as obtained from a coarse graining of the interactions of a water molecule with all the atoms of the C60. On the other hand, the atomistic model takes into account a discrete nature of all the atoms of C60 explicitly. Molecular dynamics simulations are carried out in an isothermal-isobaric ensemble at normal temperature and pressure. Various spatial and orientational correlations of the water around the two model C60s are compared. The coarse-grained model is shown to reproduce the hydration behavior of the C60 quite well. The simplicity and the computational economy of the coarse-grained model will allow for simulations of self-assembly processes of a much larger system over a longer period of time.
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61.20.Ja Computer simulation of liquid structure
82.30.Nr Association, addition, insertion, cluster formation

Vapor-liquid equilibria from the triple point up to the critical point for the new generation of TIP4P-like models: TIP4P/Ew, TIP4P/2005, and TIP4P/ice

C. Vega, J. L. F. Abascal, and I. Nezbeda

J. Chem. Phys. 125, 034503 (2006); http://dx.doi.org/10.1063/1.2215612 (9 pages) | Cited 37 times

Online Publication Date: 18 July 2006

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The vapor-liquid equilibria of three recently proposed water models have been computed using Gibbs-Duhem simulations. These models are TIP4P/Ew, TIP4P/2005, and TIP4P/ice and can be considered as modified versions of the TIP4P model. By design TIP4P reproduces the vaporization enthalpy of water at room temperature, whereas TIP4P/Ew and TIP4P/2005 match the temperature of maximum density and TIP4P/ice the melting temperature of water. Recently, the melting point for each of these models has been computed, making it possible for the first time to compute the complete vapor-liquid equilibria curve from the triple point to the critical point. From the coexistence results at high temperature, it is possible to estimate the critical properties of these models. None of them is capable of reproducing accurately the critical pressure or the vapor pressures and densities. Additionally, in the cases of TIP4P and TIP4P/ice the critical temperatures are too low and too high, respectively, compared to the experimental value. However, models accounting for the density maximum of water, such as TIP4P/Ew and TIP4P/2005 provide a better estimate of the critical temperature. In particular, TIP4P/2005 provides a critical temperature just 7 K below the experimental result as well as an extraordinarily good description of the liquid densities from the triple point to the critical point. All TIP4P-like models present a ratio of the triple point temperature to the critical point temperature of about 0.39, compared with the experimental value of 0.42. As is the case for any effective potential neglecting many body forces, TIP4P/2005 fails in describing simultaneously the vapor and the liquid phases of water. However, it can be considered as one of the best effective potentials of water for describing condensed phases, both liquid and solid. In fact, it provides a completely coherent view of the phase diagram of water including fluid-solid, solid-solid, and vapor-liquid equilibria.
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64.70.F- Liquid-vapor transitions
64.60.F- Equilibrium properties near critical points, critical exponents
64.70.D- Solid-liquid transitions

Orientational order in high density dipolar hard sphere fluids

J.-J. Weis and D. Levesque

J. Chem. Phys. 125, 034504 (2006); http://dx.doi.org/10.1063/1.2215614 (5 pages) | Cited 10 times

Online Publication Date: 19 July 2006

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Taking advantage of recent estimates, by one of us, of the critical temperature of the isotropic-ferroelectric transition of high density dipolar hard spheres, we performed new Monte Carlo simulations in the close vicinity of these estimates and applied histogram reweighting methods to obtain refined values of the critical temperatures from the crossing of the fourth-order cumulant for different system sizes. The ferroelectric line is determined in the density range ρ* = 0.80–0.95, and the onset of columnar ordering is located.
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77.80.B- Phase transitions and Curie point
61.20.Ja Computer simulation of liquid structure

Observation of hydrogen in deuterated methane hydrate by maximum entropy method with neutron powder diffraction

Akinori Hoshikawa, Naoki Igawa, Hiroki Yamauchi, and Yoshinobu Ishii

J. Chem. Phys. 125, 034505 (2006); http://dx.doi.org/10.1063/1.2215606 (6 pages) | Cited 5 times

Online Publication Date: 19 July 2006

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The crystal structure of deuterated methane hydrate (structure I, space group: Pmmathn) was investigated by neutron powder diffraction at temperatures of 7.7–185 K. The scattering amplitude density distribution was examined by a combination of Rietveld method and maximum entropy method (MEM). The distribution of the D atoms in both D2O and CD4 molecules was clarified from three-dimensional graphic images of the scattering amplitude density. The MEM results showed that there were low-density sites for the D atom of D2O in a particular location within the D2O cage at low temperatures. The MEM provided more reasonable results because of the decrease in the R factor that is attainable by this method. Accordingly, the low-density sites for the D atom of D2O probably exist within the D2O cage. This suggests that a spatial disorder of the D atom of D2O occurs at these sites and that hydrogen bonds between D2O molecules become partially weakened. With regard to the CD4 molecules, there were high-density sites for the D atom of CD4, and the density distribution of the C and D atoms was observed separately in the scattering amplitude density image. Consequently, the C–D bonds of CD4 were not observed clearly because the CD4 molecules had an orientational disorder. The D atoms of CD4 were displaced from the line between the C and O atoms, and were located near the face center of the polygon in the cage. Accordingly, the D atoms of CD4 were not bonded to specific O atoms. This result is consistent with the hydrophobicity of the CD4 molecule. We also report the difference between the small and the large cages in the density distribution map and the temperature dependence of the scattering amplitude density.
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61.66.Hq Organic compounds
61.50.Lt Crystal binding; cohesive energy

Study on the intermolecular interaction of C60 and simulations on the orientational properties of C60 in crystals

Yukiumi Kita, Kei Wako, Hiromitsu Goto, Takeshi Naito, Hidemi Kawai, and Isamu Okada

J. Chem. Phys. 125, 034506 (2006); http://dx.doi.org/10.1063/1.2215601 (11 pages) | Cited 3 times

Online Publication Date: 20 July 2006

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We developed the new intermolecular interaction model of C60 with the quantitative accuracy for the molecular orientational properties in crystals. The energy difference (ΔE) and the activation barrier (Ebarrier) between the two stable orientations (P and H orientations) in crystals are in the values of +14.7 and +260 meV in our model, respectively; these values are in fairly good agreement with the experimental values (ΔE ≃ +11 meV, Ebarrier = +235–+290 meV in experiments). The relaxation calculation for C60 crystals using our model revealed that there is the reversal of the stable orientations between the P and H orientations under the high H-orientation occupancy (pH) in crystals, when pH>0.83, ΔE<0. From the molecular dynamics calculations for C60 crystals using our model, it is found that the phase transition is induced at TC = 200–260 K, which is consistent with the experimental value of 260 K. Immediately below TC, we found a great variety of molecular rotational jumps involving that between the P and H orientations every about 10−9 s due to the thermal activation. In the high temperature phase (>TC), all molecules rotate irregularly like in Brownian motion involving the rotational “slumber” for ≃ 10−12–10−11 s.
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61.48.-c Structure of fullerenes and related hollow and planar molecular structures
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
64.70.K- Solid-solid transitions
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