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1 May 2003

Volume 118, Issue 17, pp. 7733-8117

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Dual frequency 2D-IR of peptide amide-A and amide-I modes

I. V. Rubtsov, J. Wang, and R. M. Hochstrasser

J. Chem. Phys. 118, 7733 (2003); http://dx.doi.org/10.1063/1.1570398 (4 pages) | Cited 19 times

Online Publication Date: 15 April 2003

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Dual frequency two-dimensional infrared (2D-IR) spectra were acquired with three IR fields, two at 3 and one at 6 μm from two pulses, creating a joint nonlinear response from both the amide-A (N–H) and the amide-I transitions of two model peptides AcAlaOMe and caprolactam. The 2D-IR spectra yield anharmonicities and correlations of inhomogeneous distributions. A positive correlation between the fundamental N–H mode frequency and its anharmonicity is found for the dipeptides. The amide-I frequencies are correlated with N–H mode frequencies and anticorrelated with the amide-I/N–H mode coupling. Structural implications of these results are discussed in relation to the anharmonic potential surface. © 2003 American Institute of Physics.
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33.20.Ea Infrared spectra
33.20.Tp Vibrational analysis
33.15.Mt Rotation, vibration, and vibration-rotation constants
87.15.M- Spectra of biomolecules

Chemical reactivity of metcar Ti8C12, nanocrystal Ti14C13 and a bulk TiC(001) surface: A density functional study

Ping Liu, José A. Rodriguez, Hua Hou, and James T. Muckerman

J. Chem. Phys. 118, 7737 (2003); http://dx.doi.org/10.1063/1.1570397 (4 pages) | Cited 18 times

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Density functional calculations show that the “magic” structures of titanium carbide nanoparticles display an unexpected high reactivity toward CO, NH3, and H2O when compared to the bulk TiC(001) surface. In spite of the large C/Ti ratio in Ti8C12, our results show that the bulk surface is much more tightly bonded than the nanocluster, and thus the nanocluster has a much higher chemical activity. © 2003 American Institute of Physics.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
61.46.-w Structure of nanoscale materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
31.15.E- Density-functional theory
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back to top Theoretical Methods and Algorithms

Blinking molecules: Determination of photophysical parameters from the intensity correlation function

Gerhard C. Hegerfeldt and Dirk Seidel

J. Chem. Phys. 118, 7741 (2003); http://dx.doi.org/10.1063/1.1563615 (6 pages) | Cited 4 times

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An explicit expression is given for the correlation function of blinking systems, i.e., systems exhibiting light and dark periods in their fluorescence. We show through the example of terrylene in a crystalline host that it is possible to determine by means of this explicit expression photophysical parameters, like Einstein coefficients and the mean light and dark periods. In addition we obtain further parameters like the frequency of the various intensity periods and the probability density of photons scattered off the host crystal. It turns out that this approach is simpler and allows greater accuracy than previous procedures.© 2003 American Institute of Physics.
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33.50.Dq Fluorescence and phosphorescence spectra
33.70.Ca Oscillator and band strengths, lifetimes, transition moments, and Franck-Condon factors

Monte Carlo simulations using sampling from an approximate potential

Lev D. Gelb

J. Chem. Phys. 118, 7747 (2003); http://dx.doi.org/10.1063/1.1563597 (4 pages) | Cited 29 times

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A simulation algorithm is proposed in which the potential energy function used in a Monte Carlo simulation is replaced with one that is less expensive to evaluate, coupled with a correction step based on the difference between the two potentials. This can result in a substantial reduction in computational cost. A formal derivation of the appropriate sampling criteria is given, as well as estimates of the possible improvements in code performance. The method is demonstrated on the Lennard-Jones fluid at several state points, where speedups of as much as fourfold are achieved with negligible loss in precision. © 2003 American Institute of Physics.
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61.43.Bn Structural modeling: serial-addition models, computer simulation
61.20.Ja Computer simulation of liquid structure
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling

Geometry optimizations with the coupled-cluster model CC2 using the resolution-of-the-identity approximation

Christof Hättig

J. Chem. Phys. 118, 7751 (2003); http://dx.doi.org/10.1063/1.1564061 (11 pages) | Cited 96 times

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An implementation of the gradient for the second-order coupled-cluster singles-and-doubles model CC2 is reported, which employs the resolution-of-the-identity (RI) approximation for electron repulsion integrals. The performance of the CC2 model for ground state equilibrium geometries and harmonic frequencies is investigated and compared with experiment and other ab initio methods. It is found that CC2 equilibrium geometries have a similar accuracy to those calculated with second-order Møller–Plesset perturbation theory (MP2), but the bond lengths are larger. In particular, double and triple bonds and bonds in electron-rich compounds are elongated by 0.5–1.5 pm. Thereby CC2 slightly outperforms MP2 for single bonds, in particular in electron-rich compounds, but for strong double and triple bonds CC2 is somewhat inferior to MP2. The results for harmonic frequencies go in parallel with the results for equilibrium structures. The error introduced by the RI approximation is found to be negligible compared to the remaining one-electron basis set error, if optimized auxiliary basis sets are used. Typically, the RI error in bond lengths is of the order of 10−3 pm and the error in angles 10−3–10−2 deg. Applications are reported for the geometry of trans-azobenzene and for the geometry and harmonic frequencies of cis,trans-1,4-difluorobutadiene. © 2003 American Institute of Physics.
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31.15.bw Coupled-cluster theory
31.15.A- Ab initio calculations
33.15.Bh General molecular conformation and symmetry; stereochemistry
31.15.xp Perturbation theory
33.15.Dj Interatomic distances and angles

A novel path sampling method for the calculation of rate constants

Titus S. van Erp, Daniele Moroni, and Peter G. Bolhuis

J. Chem. Phys. 118, 7762 (2003); http://dx.doi.org/10.1063/1.1562614 (13 pages) | Cited 94 times

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We derive a novel efficient scheme to measure the rate constant of transitions between stable states separated by high free energy barriers in a complex environment within the framework of transition path sampling. The method is based on directly and simultaneously measuring the fluxes through many phase space interfaces and increases the efficiency with at least a factor of 2 with respect to existing transition path sampling rate constant algorithms. The new algorithm is illustrated on the isomerization of a diatomic molecule immersed in a simple fluid. © 2003 American Institute of Physics.
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82.20.Pm Rate constants, reaction cross sections, and activation energies
82.20.Db Transition state theory and statistical theories of rate constants
82.30.Qt Isomerization and rearrangement

An improved 6-31G∗ basis set for first-row transition metals

Alexander V. Mitin, Jon Baker, and Peter Pulay

J. Chem. Phys. 118, 7775 (2003); http://dx.doi.org/10.1063/1.1563619 (8 pages) | Cited 36 times

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We propose a modification to the popular 6-31G∗ basis set, which has recently been extended to cover first-row transition metals [Rassolov et al., J. Chem. Phys. 109, 1223 (1998)]. As demonstrated by a number of calculations, the existing basis performs poorly for many transition metals, particularly those toward the end of the series (Co, Ni, and especially Cu). The reason for this lies primarily with the 3D shell, which lacks a sufficiently diffuse exponent. A reoptimization of the D-shell exponents and coefficients by a two-step procedure, keeping the rest of the basis unchanged, corrects the problem, giving a basis set that performs uniformly well across the entire first-row transition metal series from scandium to copper. © 2003 American Institute of Physics.
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31.15.E- Density-functional theory

Converting Kohn–Sham eigenenergies into electron binding energies

Julius Jellinek and Paulo H. Acioli

J. Chem. Phys. 118, 7783 (2003); http://dx.doi.org/10.1063/1.1560134 (14 pages) | Cited 11 times

Online Publication Date: 15 April 2003

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A new accurate scheme for converting the Kohn–Sham eigenenergies into electron binding energies is formulated. The accuracy of the scheme is illustrated in applications to ten atoms and three molecules. © 2003 American Institute of Physics.
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71.15.Mb Density functional theory, local density approximation, gradient and other corrections
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy
back to top Gas Phase Dynamics and Structure: Spectroscopy, Molecular Interactions, Scattering, and Photochemistry

The water–hydroxyl radical complex: A matrix isolation study

Anders Engdahl, Gunnar Karlström, and Bengt Nelander

J. Chem. Phys. 118, 7797 (2003); http://dx.doi.org/10.1063/1.1563608 (6 pages) | Cited 27 times

Online Publication Date: 15 April 2003

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The water–hydroxyl radical complex was prepared by irradiating peroxy radicals in hydrogen-doped argon matrices. The low water content of the matrices made it possible to observe the fundamental bands of the complexed water molecule. The experimental results are compared with the results from ab initio calculations. The complex rotates around the O–O axis in the matrix. © 2003 American Institute of Physics.
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33.20.Ea Infrared spectra

Pure rotational spectrum of the NCCS radical studied by Fourier-transform microwave spectroscopy

Masakazu Nakajima, Yoshihiro Sumiyoshi, and Yasuki Endo

J. Chem. Phys. 118, 7803 (2003); http://dx.doi.org/10.1063/1.1564045 (5 pages) | Cited 11 times

Online Publication Date: 15 April 2003

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Pure rotational transitions of the NCCS radical, showing resolved fine and hyperfine splittings, have been observed by Fourier-transform microwave spectroscopy in a discharged supersonic jet of acetonitrile and carbon disulfide. Since the transitions have been observed at frequencies corresponding to the even multiples of the rotational constant, it is concluded that the NCCS radical has a bent structure in the ground electronic state, math 2A′, and the Ka = 0 ladder of the radical has been observed under the jet-cooled condition. Precise molecular constants, including the hyperfine constants of the nitrogen nucleus, are determined by a least-squares fit for the observed transition frequencies using a standard asymmetric top Hamiltonian. The determined rotational constant is compared with results of high-level ab initio calculations in order to confirm the spectral carrier to be the bent NCCS radical. © 2003 American Institute of Physics.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Sn Rotational analysis

Reactions of gold cluster cations Aun+ (n = 1–12) with H2S and H2

Ko-ichi Sugawara, Frank Sobott, and Andrei B. Vakhtin

J. Chem. Phys. 118, 7808 (2003); http://dx.doi.org/10.1063/1.1564057 (9 pages) | Cited 26 times

Online Publication Date: 15 April 2003

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The reactions of gold cluster cations Aun+ (n = 1–12) with H2S and H2 have been studied using Fourier-transform ion-cyclotron resonance (FT–ICR) mass spectrometry. The cluster cations were produced by laser ablation of a gold rod in He atmosphere, and their reactions were observed at room temperature and low total pressures of 10−7–10−5 Torr. Initial products of the reactions with H2S were mainly AuSH+ for n = 2, AunS+ for n = 4–8 and 10, and AunSH2+ for n = 9, 11, and 12. No reactions of Au+ and Au3+ with H2S were observed. Even n cluster cations were more reactive than adjacent odd n clusters. The particularly low reactivity at n = 1, 3, 9, and 11 is consistent with the low ionization potential of Aun and the weak binding energy of Aun+–Au. Further sulfuration reactions of AunS+ proceeded to give AunSm+ and finally stopped at AunSm+xH2+ when H2 release did not occur. The maximum number of sulfur atoms m+x increased with the cluster size up to n = 8, while the sulfuration reaction stopped at early stages for n ≥ 9. In another series of experiments, no reaction of Aun+ (n = 1–12) with H2 gas pulses introduced into the FT–ICR cell was observed. To investigate the stability of gold hydride clusters, laser ablation of gold in a H2/He mixture was performed. The hydride cluster cations AunHm+ were produced for n = 1–7, while bare Aun+ clusters were the main products for n ≥ 8. There is a distinct border between n = 7 and 8, as the structure of Aun+ changes from planar for n ⩽ 7 to three-dimensional for n ≥ 8, suggesting the stability of hydride cluster cations with planar gold frameworks. © 2003 American Institute of Physics.
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82.33.Fg Reactions in clusters
36.40.Jn Reactivity of clusters
33.15.Ta Mass spectra
33.15.Ry Ionization potentials, electron affinities, molecular core binding energy

Doppler-free high resolution laser spectroscopy of the Na2 C 1Πu state: Perturbation and predissociation

Md. Humayun Kabir, Toyoki Shinano, and Shunji Kasahara

J. Chem. Phys. 118, 7817 (2003); http://dx.doi.org/10.1063/1.1563606 (6 pages) | Cited 2 times

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High resolution absorption spectra of the C 1Πu←X 1Σg+ transition of the 23Na2 molecule have been measured in the range 29 680–30 950 cm−1 by exploiting the technique of Doppler-free UV-visible optical–optical double resonance polarization spectroscopy. The molecular constants of the C 1Πu(v = 0–10) levels are determined, and the Rydberg–Klein–Rees potential energy curve is calculated. A number of C 1Πu(v = 0–9,Je and f ) levels were found to be perturbed. The energy shifts of e and f parity levels of the C 1Πu state were studied and identified as originating from perturbations between the C 1Πu and 3 3Σu+ states. Remarkable line broadenings were observed only in the perturbed region and identified as the lifetime broadening effect originating from indirect (accidental) predissociation through the two diabatic 3Σu+ states. © 2003 American Institute of Physics.
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33.40.+f Multiple resonances (including double and higher-order resonance processes, such as double nuclear magnetic resonance, electron double resonance, and microwave optical double resonance)
33.70.Jg Line and band widths, shapes, and shifts

High level theoretical study of the structure and rotational barriers of trans-stilbene

S. P. Kwasniewski, L. Claes, J.-P. François, and M. S. Deleuze

J. Chem. Phys. 118, 7823 (2003); http://dx.doi.org/10.1063/1.1563617 (14 pages) | Cited 32 times

Online Publication Date: 15 April 2003

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The relative energies of stationary points on the potential energy surface of trans-stilbene have been accurately determined using Hartree–Fock, second and third-order Møller–Plesset (MP2, MP3), as well as Coupled Clusters theories with single and double excitations (CCSD), together with a perturbative estimate of connected triple excitations [CCSD(T)], in conjunction with basis sets of increasing size, containing up to 1130 basis functions. A focal point analysis has been carried out in order to determine how the energy differences and rotational barriers approach convergence, enabling extrapolation of the CCSD(T) results to a near-complete basis set. The investigated saddle points pertain to independent rotations of the phenyl rings about the single C–C bond, and to pedalling motions described by a twofold rotation of the central ethylene bond about the longitudinal axis of the molecule. The benchmark calculations presented in this study lead to the conclusion that, in the nonrelativistic limit and within the frozen core approximation, trans-stilbene in vacuum is a strictly planar molecule in its absolute energy minimum form, in sharp contrast with many previous theoretical studies. This point has been ultimately confirmed by an MP2 geometry optimization using the aug-cc-pVDZ basis set. At last, the energy of cis-stilbene relative to the trans-isomer is accurately evaluated.© 2003 American Institute of Physics.
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33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.15.xr Self-consistent-field methods
31.15.bw Coupled-cluster theory

Photoassociation of ultracold K atoms: Observation of high lying levels of the 1g ∼ 1 1Πg molecular state of K2

M. Pichler, H. M. Chen, H. Wang, W. C. Stwalley, A. J. Ross, F. Martin, M. Aubert-Frécon, and I. Russier-Antoine

J. Chem. Phys. 118, 7837 (2003); http://dx.doi.org/10.1063/1.1563609 (9 pages) | Cited 6 times

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Very high-lying vibrational levels of the 1g ∼ 1 1Πg electronic state of K2 have been observed in a photoassociation experiment using an ion detection scheme. The photoassociation measurements have been treated together with laser induced fluorescence data recorded by Fourier transform spectrometry to construct a pointwise potential curve for this electronic state. Several vibrational levels were observed by both techniques, so the dissocation energy can be deduced without extrapolation, from the sum of binding energies measured in photoassociation and vibrational energies established with respect to the potential minimum (defined by the fluorescence data), giving De = 1290.292±0.002 cm−1 (1 standard deviation). The potential curve reproduces over 2000 term values (up to v = 138) with a root mean square error of 0.0025 cm−1. Nevertheless, small differences are found between the rotational constants generated from the potential curve and the effective rotational constants deduced from binding energy measurements at very high v. The smoothness of the outermost part of the pointwise potential energy curve has been investigated through fits to a Hund’s case (c) asymptotic model. © 2003 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
82.50.-m Photochemistry
37.10.De Atom cooling methods
37.10.Gh Atom traps and guides
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
32.50.+d Fluorescence, phosphorescence (including quenching)
82.20.Kh Potential energy surfaces for chemical reactions
33.15.Fm Bond strengths, dissociation energies
33.15.Mt Rotation, vibration, and vibration-rotation constants

Mixed quantum-classical study of energy transfer in a Na+ collision with a peptide

Ming L. Wang and J. Z. H. Zhang

J. Chem. Phys. 118, 7846 (2003); http://dx.doi.org/10.1063/1.1563610 (11 pages) | Cited 1 time

Online Publication Date: 15 April 2003

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We use a mixed quantum-classical (MQC) approach to study energy transfer to a peptide’s torsional modes through collision with an atomic ion. In this study, the torsional motion of the peptide is treated quantum mechanically while the translational motion of the attacking atomic ion is treated classically. The MQC approach is applied to study the excitations of torsional modes of GLY-ALA dipeptide by colliding the peptide with a sodium ion Na+. In particular, we study the energy transfer to torsional motions around the Cα–C and Cα–N bond and rotation of the CH3 side chain. Calculation shows that the efficiency of energy transfer is strongly dependent on initial orientation of the colliding partners. For the majority of approaching angles, the energy transfer to torsional mode is rather small. However, for certain attacking angles, the torsional mode is strongly excited and the Na+ is found to be trapped near the peptide. The average ratios of energy transfer to torsional motions along Cα–C, Cα–N and the side chain CH3 are about 58%, 27%, and 17%, respectively. © 2003 American Institute of Physics.
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34.50.-s Scattering of atoms and molecules
87.15.K- Molecular interactions; membrane-protein interactions

Study of vibrational interactions in DCO+ by millimeter-wave spectroscopy and determination of the equilibrium structure of the formyl ion

Luca Dore, Sabina Beninati, Cristina Puzzarini, and Gabriele Cazzoli

J. Chem. Phys. 118, 7857 (2003); http://dx.doi.org/10.1063/1.1564042 (6 pages) | Cited 7 times

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Rotational transitions of DCO+ in vibrationally excited states were measured in the region 360–440 GHz. The states investigated are: 0 2 0 (with l = 0,2), 0 3 0 (with l = 1,3), 0 4 0 (with l = 0,2,4), 0 11 1, and 1 00 0. This study allowed us to carefully analyze the vibrational interaction affecting the first C–D stretching state 1 00 0, which was confirmed to be due to the 0 11 1 state. Taking account of the perturbation effect on the rotational constants of the involved states allowed to derive an accurate equilibrium structure of HCO+, which resulted to be rCH = 1.092 04 Å and rCO = 1.105 58 Å. © 2003 American Institute of Physics.
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33.20.Bx Radio-frequency and microwave spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Mt Rotation, vibration, and vibration-rotation constants

Pulsed discharge nozzle cavity ringdown spectroscopy of cold polycyclic aromatic hydrocarbon ions

Ludovic Biennier, Farid Salama, Louis J. Allamandola, and James J. Scherer

J. Chem. Phys. 118, 7863 (2003); http://dx.doi.org/10.1063/1.1564044 (10 pages) | Cited 57 times

Online Publication Date: 15 April 2003

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The gas-phase electronic absorption spectra of the naphthalene (C10H8+) and acenaphthene (C12H10+) cations have been measured in the visible range in a free jet planar expansion in an attempt to collect data in an astrophysically relevant environment. The direct absorption spectra of two out of four bands measured of the gas-phase cold naphthalene cation along with the gas-phase vibronic absorption spectrum of the cold acenaphthene cation are reported for the first time. Direct absorption spectra of their van der Waals complexes with argon are also reported for the first time. The study has been carried out using the ultrasensitive and versatile technique of cavity ringdown spectroscopy (CRDS) coupled to a pulsed discharge slit nozzle (PDN). The new PDN-CRDS set up is described and its characteristics are evaluated. The direct-absorption spectra of the polycyclic aromatic hydrocarbon (PAH) ions are discussed and compared to the gas-phase and solid-phase data available in the literature. The analysis of the results show that cold, free flying PAH ions are generated in the argon discharge primarily through soft Penning ionization. This enables the intrinsic band profiles to be measured, a key requirement for astrophysical applications. © 2003 American Institute of Physics.
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33.20.Kf Visible spectra
33.20.Wr Vibronic, rovibronic, and rotation-electron-spin interactions

Infrared spectroscopy of the isomers of magnesium–HCN formed in helium nanodroplets: Comparisons with ab initio calculations

P. L. Stiles, D. T. Moore, and R. E. Miller

J. Chem. Phys. 118, 7873 (2003); http://dx.doi.org/10.1063/1.1562160 (9 pages) | Cited 7 times

Online Publication Date: 15 April 2003

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High-resolution infrared spectra are reported for two isomers of the magnesium–HCN binary complex, corresponding to the magnesium bonding at the nitrogen and hydrogen ends of the molecule. Stark spectra are also reported for these complexes, from which the corresponding dipole moments are determined. Ab initio calculations confirm that the potential energy surface has two minima, consistent with the experimentally determined structures. The wave functions of the two dimensional intermolecular coupled cluster singles doubles (triples) potential energy surface, calculated with the collocation method, are also reported for the magnesium–HCN system and are used to calculate the vibrationally averaged dipole moment for both isomers, which are then compared to experiment. © 2003 American Institute of Physics.
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33.20.Ea Infrared spectra
33.15.Bh General molecular conformation and symmetry; stereochemistry
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)
31.50.-x Potential energy surfaces
33.57.+c Magneto-optical and electro-optical spectra and effects

Nonadiabatic coupling effects on the short time signal in four-wave mixing experiments

J. P. Lavoine and A. J. Boeglin

J. Chem. Phys. 118, 7882 (2003); http://dx.doi.org/10.1063/1.1542882 (6 pages) | Cited 1 time

Online Publication Date: 15 April 2003

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The transient four-wave mixing signal from a three-level system coupled to a bath of harmonic oscillators is calculated in the short-pulse limit and analyzed at short times. The two excited states of the system are coupled by a constant nonadiabatic interaction V which is treated exactly. This allows an examination of the influence of V on the dynamics in the excited levels at short times. We show that the positions of the minima of the potential energy surfaces of the excited states with respect to the one of the fundamental state play an important role in the relaxation processes. We also show that for high nonadiabatic coupling it is possible to minimize the effect of the bath and hence to minimize, at short times, the coherence losses of the system. The identification of the molecular parameters required to take advantage of this effect should be of interest in the selection of materials for applications in nonlinear optics. © 2003 American Institute of Physics.
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42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
42.50.-p Quantum optics
back to top Condensed Phase Dynamics, Structure, and Thermodynamics: Spectroscopy, Reactions, and Relaxation

Determination of the friction coefficient of a Brownian particle by molecular-dynamics simulation

F. Ould-Kaddour and D. Levesque

J. Chem. Phys. 118, 7888 (2003); http://dx.doi.org/10.1063/1.1563593 (4 pages) | Cited 15 times

Online Publication Date: 15 April 2003

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By using the formula first derived by Kirkwood, the friction coefficient of a solvated Brownian particle is determined from the integration on time of the autocorrelation function of the force that the solvent exerts on this particle. Extensive molecular dynamics simulations show that above a definite size of the studied systems the value of the integral defining the friction coefficient goes to a quasiconstant value (a plateau) when the upper bound on time increases. The minimal value of the system size where the integral exhibits this asymptotic behavior, rises with the Brownian particle size. From the plateau, a reliable estimate of the friction coefficient is obtained. © 2003 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure
05.40.Jc Brownian motion

Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate

Shigeo Sasaki, Shinsuke Hori, Tetsuji Kume, and Hiroyasu Shimizu

J. Chem. Phys. 118, 7892 (2003); http://dx.doi.org/10.1063/1.1563600 (6 pages) | Cited 18 times

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Visual observations through a microscope and in situ Raman scattering measurements of a synthesized nitrogen hydrate have been performed at pressures up to 6 GPa and 296 K. High-pressure transformations have been found at 0.85 and 1.45 GPa. The cubic structure II (sII) nitrogen hydrate initially transforms to the hexagonal structure (sH) at 0.85 GPa and finally forms the orthorhombic dihydrate (sO) above 1.45 GPa. The sO phase of nitrogen hydrate exists up to at least 6 GPa. A variety of Raman spectra composed of three peaks have been sometimes observed in sII phase below 0.50 GPa, which implies that the guest nitrogen molecules doubly occupy the large hexakaidecahedron cages. Two Raman bands of the guest nitrogen vibrations with nearly equal intensities appearing in sH phase suggest that five nitrogen molecules are filling in extra large icosahedron cages. © 2003 American Institute of Physics.
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78.30.Hv Other nonmetallic inorganics
07.60.Pb Conventional optical microscopes
62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
61.66.Fn Inorganic compounds

Tunneling currents in long-distance electron transfer reactions. V. Effective one electron approximation

Alexei A. Stuchebrukhov

J. Chem. Phys. 118, 7898 (2003); http://dx.doi.org/10.1063/1.1563616 (9 pages) | Cited 9 times

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In this paper we further develop the formalism of tunneling currents for the description of the tunneling transition in long-distance bridge-mediated electron transfer reactions introduced in our previous work [A. A. Stuchebrukhov, Adv. Chem. Phys. 118, 1 (2001)]. Here we consider the Hartree–Fock picture of electron tunneling in a many-electron system and, based on the corresponding orbitals analysis of the tunneling process, we introduce an effective one-electron approximation. In this picture, the (electron or hole) tunneling is described by a single pair of orbitals, as in a true one-electron theory, yet all the polarization and exchange effects characteristic for many-electron treatment are retained in it. The pair of tunneling orbitals, which is different from the usual HOMO’s of the donor and acceptor complexes, is found in a self-consistent way in a special orthogonalization procedure. This picture results in much simplified formulas for current density and for interatomic currents, as well as for the tunneling matrix element, and provides a simplified way of thinking about electron tunneling in many-electron systems. © 2003 American Institute of Physics.
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73.40.Gk Tunneling
34.70.+e Charge transfer
71.70.-d Level splitting and interactions
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

Monte Carlo simulations for the phase behavior of symmetric nonadditive hard sphere mixtures

Kamakshi Jagannathan and Arun Yethiraj

J. Chem. Phys. 118, 7907 (2003); http://dx.doi.org/10.1063/1.1563595 (5 pages) | Cited 15 times

Online Publication Date: 15 April 2003

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Computer simulation results are presented for the phase behavior of a symmetric binary mixture of nonadditive hard spheres. In this model, the hard sphere diameters are given by σAA = σBB = λd and σAB = d. At high densities and for small enough λ, this hard sphere mixture exhibits a fluid–fluid phase separation into an A-rich and a B-rich phase. Semigrand ensemble simulations are performed for the critical point and the phase behavior of this model for various values of λ. The results for the critical density are significantly different from previous simulation estimates. A comparison of our simulation results to existing theories shows that none of the theories are accurate for the location of the critical point, over the entire range of λ. © 2003 American Institute of Physics.
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61.20.Ja Computer simulation of liquid structure
61.20.Gy Theory and models of liquid structure
02.70.Uu Applications of Monte Carlo methods
64.60.-i General studies of phase transitions

Thermophysical properties of liquid refractory metals: Comparison between hard sphere model calculation and electrostatic levitation measurements

Takehiko Ishikawa, Paul-François Paradis, Toshio Itami, and Shinichi Yoda

J. Chem. Phys. 118, 7912 (2003); http://dx.doi.org/10.1063/1.1564050 (9 pages) | Cited 24 times

Online Publication Date: 15 April 2003

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Thermophysical properties of molten refractory metals (titanium, zirconium, hafnium, and niobium) have been measured using a containerless method. Using an in-house developed electrostatic levitator, the density, the heat capacity, the entropy, the surface tension, and the viscosity of liquid phases have been measured over a wide temperature range. The measured data showed good agreement with theoretical calculations based on the hard sphere model. © 2003 American Institute of Physics.
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65.20.-w Thermal properties of liquids
61.25.Mv Liquid metals and alloys
68.03.Cd Surface tension and related phenomena
66.20.-d Viscosity of liquids; diffusive momentum transport

Aqueous Na+Cl− pair association from liquidlike to steamlike densities along near-critical isotherms

A. A. Chialvo and J. M. Simonson

J. Chem. Phys. 118, 7921 (2003); http://dx.doi.org/10.1063/1.1564052 (9 pages) | Cited 13 times

Online Publication Date: 15 April 2003

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An extensive molecular-based study of ion-pair formation in near-critical dilute aqueous NaCl solutions is performed along three near- (super- and sub-) critical isotherms and from liquidlike to steamlike densities. The study encompasses the determination of the ion-pair association constant via potential of mean force calculations. The main goal is to find answers to some relevant questions regarding the thermodynamic and corresponding microscopic behavior of the ion-pair formation at steamlike densities, where experimental data are extremely difficult to obtain accurately. A direct comparison is made between simulation, theoretical developments, and experiment to aid the interpretation of experimental data and their macroscopic modeling. © 2003 American Institute of Physics.
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82.30.Nr Association, addition, insertion, cluster formation
64.70.F- Liquid-vapor transitions
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