bluuebunny/crossref_metadata_2025 huggingface dataset:
Ab initio studies of the reactions of M(S,1 P,3 and P1) with SiH4 (M=Cd, Hg)
Authors: Luna-Garcı́a H. , Castillo S. , Ramı́rez-Solı́s A.
Published: July 2002

The reactions of Cd(1S:5s2,3,1P:5s15p1) and Hg(1S:6s2,3,1P:6s16p1) with SiH4 have been studied through multiconfiguration self-consistent-field (MCSCF) (with relativistic effective core potentials) followed by extensive variational and perturbational second-order multireference Möller–Plesset configuration interaction by perturbation selected by iterative process (CIPSI) calculations using extended Gaussian basis sets. It was found that both metal atoms in their P(ns3np11) state break the Si–H bond of silane spontaneously, leading directly to the MH+SiH3 final products, in agreement with the experimental results of this reaction for Cd. One important qualitative difference between the Cd and Hg(3P) reactions is that for the former an unstable intermediate was found, whereas for the latter no intermediate exists at all. Again, for both atoms, the P(ns1np11) state is also inserted in the Si–H bond and the corresponding interaction surface shows an avoided crossing with the lowest-lying X1A′ potential surface, adiabatically correlated with the M(1S:ns2)+SiH4 reactants. This interaction leads eventually to the MH+SiH3 products. The structure of these HMSiH3 intermediates, diabatically correlated with the M(1P:ns1np1)+SiH4 reactants, was carefully studied, as well as the dissociation channels leading to the MH+SiH3 and H+MSiH3 products. Accurate energy differences between all these species are also reported. The theoretical results obtained for the mercury reaction are discussed in light of the very recent experimental results of Legay-Sommaire and Legay [J. Phys. Chem. A 102, 8579 (1998)] for the insertion of Hg(3P:4s14p1) in SiH4 over N2 and rare gas matrices. Our results confirm their conclusion that the photochemical insertion of Hg(3P) into the Si–H bond of silane proceeds without any activation barrier.
Nonstatistical effects in the unimolecular dissociation of the acetyl radical
Authors: Peña-Gallego Angeles , Martı́nez-Núñez Emilio , Vázquez Saulo A.
Published: July 2002

Classical trajectory and statistical variational efficient microcanonical sampling transition state theory calculations were carried out to investigate the dissociation dynamics of the acetyl radical. For this purpose, an analytical potential function was developed based on ab initio and experimental data reported in the literature. This potential function reproduces reasonably well the geometries, frequencies, and energies of the stationary points of the ground state potential energy surface. The dynamics of the reaction was shown to be intrinsically non-Rice–Ramsperger–Kassel–Marcus (RRKM) at high energies and particularly at 65.9 kcal/mol, at which experimental work showed evidence for nonstatistical behavior. On the other hand, initial excitations of normal modes 507 (CCO bend), 1079 (CC stretch), 1504 (CH3 umbrella vibration), and 1939 (CO stretch) enhance significantly the rate of reaction; specifically, excitation of the CO stretch gives a rate coefficient an order of magnitude higher than the rate obtained under random initial conditions. These mode specific effects are explained in terms of a restricted intramolecular vibrational redistribution (IVR). Under statistical initial conditions, the classical trajectory calculations showed a normal isotope effect at the two lowest energies studied, and a slight inverse isotope effect at 65.9 kcal/mol, a result that can be explained with the presence of a methyl free-rotor at the transition state. In contrast, upon initial excitation of the CC and CO stretches and CCO bending at 65.9 kcal/mol, the calculations predicted a normal isotope effect, which agrees with the experimental findings.
Computer simulations and integral equation theory for the structure of salt-free rigid rod polyelectrolyte solutions: Explicit incorporation of counterions
Authors: Shew Chwen-Yang , Yethiraj Arun
Published: July 2002

The static structure of salt-free solutions of rod-like polyelectrolyte solutions is studied using Monte Carlo simulations and integral equations. The polymer reference interaction site model theory with the reference Laria–Wu–Chandler closure is found to be in good agreement with the simulations for the polyion–polyion and counterion–counterion correlation functions, and in qualitative agreement with the simulations for the counterion–polyion correlation functions. The effect on the polyion concentration on the counterion distribution is found to be important at low concentrations and becomes more significant when the Bjerrum length or degree of polymerization are increased. The theory is in almost quantitative agreement with the simulations for the static structure factor. The theory predicts that all three partial structure factors display low angle peaks at roughly the same wave vectors. The position and intensity of the peak in the polyion structure factor is insensitive to the Bjerrum length. These predictions are consistent with recent experiments.
Density functional study of intramolecular ferromagnetic interaction through m-phenylene coupling unit. III. Possibility of high-spin polymer
Authors: Mitani Masaki , Takano Yu , Yoshioka Yasunori , Yamaguchi Kizashi
Published: July 2002

Because it has been well-known that the effective exchange interaction of spins is much larger within a molecule than between molecules, it is very important to design high-spin polymers linking high-spin molecules with each other as a constituent unit to obtain molecule-based ferromagnetic materials with high transition temperature. Experimental efforts to synthesize such polymers extended in one- or two-dimensions have been made in recent years while theoretical treatment of infinite polymers has been behind in the sense that no study of electronic and magnetic structure calculations by ab initio periodic approaches exists until now. We examined the magnetic properties in m-phenylene and related molecules with organic radicals by density functional and molecular orbital methods as monomer units of high-spin polymers in Part I and Part II of this series, since it has already been confirmed experimentally and theoretically that m-phenylene coupling unit leads to the ferromagnetic coupling between spins. In Sec. III, we examine the possibility of ferromagnetic polymers in which organic radicals are arranged thorough m-phenylene bridge periodically in one dimension. For this purpose, ab initio crystal orbital methods based on density functional and Hartree–Fock approximations are employed for m-phenylene polymers with four species of C, CH, N, and NH+ radical groups as spin sources. It is shown that these polymers have the high-spin ground states and the density functional approaches can yield reasonable results which are comparable with the experiment. Implications of the calculated results are also discussed in relation to ferromagnetic polymers with different coupling units.
Role of Al–O2 chemistry in the laser-induced vaporization of Al films in air
Authors: Pangilinan G. I. , Russell T. P.
Published: July 2002

Evidence for the prominent role of atypical reactions, and enhanced vaporization arising from resonant optical absorption of the exciting beam, in the pulsed-laser heating of thin films is presented. Time-resolved emission of Al and the reaction product AlO are monitored in the laser vaporization of Al films in air. Optical emission is observed up to 200 microseconds after an 8 μs heating pulse, indicative of long chemical lifetimes. Furthermore, increased emission from both Al and AlO up to 100 μs after the heating pulse, are observed when the laser wavelength is tuned to resonance with AlO transitions. Since the electronic resonance with the reaction product AlO affects the subsequent emission of Al, these measurements underscore the importance of physical and chemical processes peculiar to laser-induced vaporization, and ablation.
Resonances in the Ba…FCH3+hν→BaF+CH3 reaction probability
Authors: Skowronek S. , Jiménez J. B. , González Ureña A.
Published: July 2002

By measuring both the Ba…FCH3 photodepletion and the BaF action spectra, the Ba…FCH3+hν→BaF+CH3 reaction probability, PR(E), has been determined over the 16 065–16 340 cm−1 energy range. PR(E) shows a peak structure with an energy spacing of 10.9 cm−1 that may be related to the internal motion of the transition state of the reaction.
Scalar and vector properties of the NO(v′=0) produced from the reaction O(1D)+N2O→NO+NO
Authors: Tsurumaki Hiroshi , Fujimura Yo , Kajimoto Okitsugu
Published: July 2002

We have measured the product state-selected differential cross-section (DCS), and the rotational angular momentum polarization, together with the energy distributions for the reaction O(1D)+N2O→NO+NO by utilizing Doppler-resolved polarization spectroscopy. The reaction dynamics of the vibrational channel forming the product NO(v′=0) is discussed based on both the scalar and vector properties. The product rotational and center-of-mass translational energy distributions are described as Boltzmann distributions with Trot≈10 000 K and Ttr≈13 000 K, respectively. These energy distributions are close to statistical predictions. The product DCS has substantial intensities over the whole angular range with a slight preference for backward scattering. The product rotational angular momentum vector j′ does not have a noticeable angular correlation with either k or k′ (the relative velocity vectors of the reactant and product, respectively). This nearly isotropic angular distribution of j′ indicates that both in-plane and out-of-plane motions of the collisional ONNO complex contribute to the product rotation to almost the same degree. Considering that this reaction has no potential well deep enough for the formation of a long-lived complex, these nearly statistical scalar and isotropic vector properties suggest that the energy redistribution among the internal modes of the collisional ONNO complex efficiently takes place. It implies that there are strong couplings among the internal modes.
Simulation of UV photodissociation of I2−(CO2)n: Spin-orbit quenching via solvent mediated electron transfer
Authors: Delaney N. , Faeder J. , Parson R.
Published: July 2002

We simulate the 395 nm photodissociation of I2− embedded in clusters of 6 to 22 CO2 molecules. In the isolated molecule, photodissociation at this wavelength leads exclusively to spin-orbit excited iodine (I*) plus I−. In the larger clusters we observe efficient electronic relaxation, leading both to dissociated products containing ground-state iodine and to recombined products containing I2−. The time scale and cluster size dependence of the spin-orbit quenching process agree well with experimental determinations of Sanov et al. (companion paper). The simulation trajectories show that spin-orbit quenching occurs by resonant charge transfer from solvated I− to a nascent I* atom. A model derived from the theory of electron transfer reactions in solution illustrates that this resonance arises when the I spin-orbit energy is compensated by the difference between the solvation energies of the ion and the neutral.
Autoionizing Rydberg states of NO in strong electric fields
Authors: Warntjes J. B. M. , Robicheaux F. , Bakker J. M. , Noordam L. D.
Published: July 2002

We report on an investigation on autoionization of Rydberg electrons of the nitric oxide molecule in strong, static electric fields. The excitation was done via various rotational states of the A 2Σ+ intermediate state (v′=0) and with polarization parallel or perpendicular to the electric field. The splitting of the autoionizing Rydberg states into overlapping Stark manifolds is resolved for excitation above the saddlepoint created by the field. We observe that the competing decay between photoionization and predissociation can lead to an incorrect interpretation of threshold energy. The photoionization spectrum of NO Rydberg series attached to various rotational thresholds is very similar owing to weak rotational coupling. The experimental results are accurately simulated by quantum calculations based on multichannel quantum defect theory (MQDT). A more intuitive formulation of the average behavior of the photoionization cross section is developed that accounts for the suppression of ionization near the threshold due to competing predissociating channels.
Electron attachment to oxygen clusters studied with high energy resolution
Authors: Matejcik S. , Stampfli P. , Stamatovic A. , Scheier P. , Märk T. D.
Published: July 2002

Highly monochromatized electrons (with energy distributions of less than 30 meV FWHM) are used in a crossed beam experiments to investigate electron attachment to oxygen clusters (O2)n at electron energies from approximately zero eV up to several eV. At energies close to zero the attachment cross section for the reaction (O2)n+e→(O2)m− (for m=1, 2, and 3) rises strongly with decreasing electron energy compatible with s-wave electron capture to (O2)n. Peaks in the oxygen attachment cross sections present at higher energies (≈80 meV, 193 meV, 302 meV) can be ascribed to vibrational levels of the anion populated by attachment of an electron to a single oxygen molecule within the target cluster via a direct Franck–Condon transition from the ground vibrational state v=0 to a vibrational excited state v′=7,8,9,… of the anion produced. The vibrational structures observed here for the first time can be quantitatively accounted for by model calculations using a microscopic model to examine the attachment of an electron to an oxygen molecule inside a cluster. This involves (i) molecular dynamics simulations to calculate the structure of neutral clusters prior to the attachment process and (ii) calculation of the solvation energy of an oxygen anion in the cluster from the electrostatic polarization of the molecules of the cluster. The occurrence of this polarization energy at the surface of larger clusters explains the appearance of an s-wave capturing cross section at 0 eV and the slightly smaller spacings (compared to the monomer case) between the peaks at finite energy, as observed experimentally. The relative transition probabilities from the ground state of the neutral oxygen molecule to the different vibrational levels of the anion are obtained by calculating the corresponding Franck–Condon factors thereby resulting in a reasonable theoretical fit to the observed yields of negatively charged oxygen molecules and clusters.
Electron transfer in a slow relaxation bath: Coherence and nonexponential kinetics
Authors: Hornbach Matthew J. , Dakhnovskii Yuri
Published: July 2002

A fast reaction of an electron transfer in a slow relaxation environment is considered. In the framework of the spin-boson approach, the time-dependent probability, P(t), is shown to be essentially nonexponential at short times. P(t) exhibits coherent oscillations of the electron density with rather different types of decay, depending on the parameter values. The fast decay corresponds to large values of the reaction heat and small values of the transition matrix element. The slow decay, determined by P(t)≃cos(Δt)/t, corresponds to large values of the electronic matrix element and small barriers. The general expression for P(t) is interpreted as free coherent oscillations of the electron density. The heat of the reaction is treated as a random Gaussian variable centered at the value ε−Er, where ε and Er are the bias and reorganization energy, respectively. It is pointed out that such nonexponential behavior can be observed for fast reactions in matrices of inert gases or electron transfer in bacterial photosynthetic centers or photoexcited mixed-valence compounds.
A Monte Carlo simulation and symmetric Poisson–Boltzmann study of a four-component electrolyte mixture
Authors: Reščič J. , Vlachy V. , Outhwaite C. W. , Bhuiyan L. B. , Mukherjee A. K.
Published: July 2002

The structure and thermodynamics of a four-component electrolyte mixture consisting of two electrolytes with a common ion, and a neutral species are studied using the canonical Metropolis Monte Carlo simulations. The steric effects of the neutral species are seen to shape substantially the equilibrium fluid properties, an important result being the existence of an attractive force between like-charged ions at sufficiently high concentrations of the neutral species. Parallel calculations performed utilizing a symmetric Poisson–Boltzmann theory reveal the usefulness of the classical theory; it is able to predict (a) the simulation profiles and the thermodynamics to a very reasonable degree of accuracy up to moderate solution concentrations, and (b) the attractive interaction between like-charge ions under the influence of high contents of neutral particles. These are useful results since the generalizations of the more formal statistical mechanical theories to four or more components require very extensive numerical work.
Comment on “A new algorithm for reverse Monte Carlo simulations” [J. Chem. Phys. 109, 2624 (1998)]
Authors: Tóth Gergely , Pusztai László , Baranyai András
Published: July 2002

The purpose of the reverse Monte Carlo method is to help in processing experimental data obtained from x-ray or neutron diffraction measurements and setup three-dimensional models plausible with chemical evidence and the results of the measurement. Although the “inverse theorem” provides certain theoretical footage to the method, it says nothing about the correct numerical details of the RMC algorithm. Unfortunately, the latter quality opens up opportunity for methodological studies with results of questionable values. Thus, we felt obliged to point to the problematic features of the paper of da Silva et al. [J. Chem. Phys. 109, 2624 (1998)] with the aim to draw the attention of the entire scientific community dealing with structural modeling. The debate can be very useful in clarifying the problematic details of the frequently criticized reverse Monte Carlo technique.
The photodissociation of HNCO in the S1 band: A five-dimensional classical trajectory study
Authors: Klossika Jörg-Joachim , Schinke Reinhard
Published: July 2002

We present a classical trajectory study of the photodissociation of HNCO in the S1 electronic state using a five-dimensional potential energy surface with all atoms confined to a plane. The potential energy surface has been determined by ab initio calculations (multi-reference configuration–interaction method, triple-zeta basis set). Because any coupling to other electronic states is neglected in our calculations, direct comparison with experimental data is limited and basically restricted to energies well above the HN(a 1Δ)+CO channel. Nevertheless, some aspects are described in a realistic way and help to interpret experimental results. In particular, the calculated vibrational–rotational state distributions of the diatomic fragments are in good agreement with the measured ones. Moreover, our calculations support the most recently published data on the H-atom quantum yield for photolysis with 193-nm photons. It is found that pre-excitation of the HN stretching mode by several quanta significantly enhances the lifetime with respect to breaking the NC bond, which may partly explain the drastic increase of the H-atom quantum yield in the vibrationally mediated photodissociation.
Retrapping and solvation dynamics after femtosecond UV excitation of the solvated electron in water
Authors: Assel M. , Laenen R. , Laubereau A.
Published: July 2002

We report on a novel investigation of the solvated electron with excitation into the continuum band. The subsequent localization process of quasifree electrons in neat water is studied by femtosecond probe spectroscopy in the spectral range between 580 nm and 990 nm. Excitation is achieved by a pump pulse at 310 nm promoting equilibrated solvated electrons to well-defined levels in the continuum band approximately 0.7 eV above the band edge. The subsequent retrapping and solvation of the electron occurs via two observed intermediates with time constants of τ2=300±50 fs and τs=1.0±0.2 ps. The absorption bands of the two intermediates are derived by the help of a 4-level energy scheme. Comparison with investigations of the solvated electron after excitation with 2 eV visible pulses gives strong evidence that the second intermediate in the UV-excitation experiment is identical to the modified ground state s″ occupied after excitation in the visible. The present study with excitation of the solvated electrons to continuum states sheds also new light on the generation process of localized electrons in neat water. Our data present strong evidence that the so-called “wet electron” is the solvated electron in a modified, hot ground state.
Photodissociation of 1,1-difluoroethene (CH2CF2) at 193 nm monitored with step-scan time-resolved Fourier-transform infrared emission spectroscopy
Authors: Lin Shiaw-Ruey , Lee Yuan-Pern
Published: July 2002

We measured time-resolved vibration–rotational emission of HF (Hartree–Fock) at various intervals (5–500 μs) after photolysis of 1,1-difluoroethene (CH2CF2) at 298 K with an excimer laser at 193 nm by means of a step-scan Fourier-transform spectrometer. Emission of HF(v) with 1⩽v⩽5 was observed, with intensity maxima at varied intervals after photolysis. Temporal profiles of HF(v) fit satisfactorily with a kinetic model consisting of nascent production of HF(v) followed by quenching of HF(v) by parent molecules. Measurements of rates of quenching at varied partial pressure of CH2CF2 yield bimolecular rate coefficients of quenching of HF(v) by CH2CF2:kqvII/10−12 cm3 molecule−1 s−1=1.07±0.10, 2.95±0.22, 13.5±0.9, and 45.2±4.1 for v=1–4; listed errors represent one standard deviation. The nascent vibrational distribution of HF is (0.365±0.014):(0.255±0.017):(0.177±0.015):(0.134±0.014):(0.069±0.012) for v=1–5, respectively, consistent with previous results. By adding Cl2 into the system, we observed weak emission of HCl(v), 1⩽v⩽4, upon photolysis at 193 nm. Such observation indicates that production of H, followed by reaction of H with Cl2 to form HCl(v) takes place as a minor channel. Incorporation of this H-elimination channel into the kinetic model yields a branching ratio of 0.10±0.03 relative to the HF-elimination channel. Bimolecular rate coefficients of quenching of HCl(v) by CH2CF2 and vibrational distribution of HCl from the reaction H+Cl2 are also determined. The F-elimination channel was not observed; estimated upper limit for the branching ratio is 2%.
Transition probabilities and electronic transition moments of the A 2Σ+–X 2Π and D 2Σ+–X 2Π systems of nitric oxide
Authors: Luque Jorge , Crosley David R.
Published: July 2002

Dispersed fluorescence scans of the A–X(4,v″) and D–X(0,v″) progressions of nitric oxide, after two-photon excitation, are used to determine the electronic transition moments of these band systems. The measured collision free lifetimes of 206±7 ns for A 2Σ+, v′=0, and 18±1 ns for D 2Σ+, v′=0 are used to place transition probabilities on an absolute basis. The branching ratio for D→X is 3.0±0.3 times that for D→A and more than 30 times that for D→C. The ratio of two-photon absorption cross sections for D–X(0,0)/A–X(4,0) is 6±2 and the photoionization probability from D, v′=0 is larger than from A, v′=4 at 375 nm. Also, the two-photon excitation of A–X(4,0) has an unusual intensity distribution, probably due either to interference between intermediate states in the excitation or to anomalies in the photoionization step.
Quasiresonant vibration–rotation transfer: A kinematic interpretation
Authors: McCaffery Anthony J.
Published: July 2002

A kinematic interpretation for quasiresonant vibration–rotation collisional transfer (QRT) is outlined based on the angular momentum (AM) theory. QRT provides a particularly stringent test since as rotational AM increases, energy decreases (or vice versa). We demonstrate using velocity-AM plots for (A) 1∑u Li2–Ne that although experimentally spectacular, in kinematic terms it constitutes only a slightly unusual energetic constraint to the linear-to-angular momentum conversion.
Photoabsorption of the peroxide linkage defect in silicate glasses
Authors: Stefanov Boris B. , Raghavachari Krishnan
Published: July 2002

First-principles quantum chemical techniques on cluster models have been used to investigate the photoabsorption of the peroxide linkage defect in silicate glasses. The effects of geometry, basis sets, and cluster size have been considered carefully to derive converged values for the low-lying excitation energies. The lowest singlet-to-singlet transition is a weak absorption at 5.5 eV. A stronger valence absorption occurs at 6.8 eV.
Nonadditive intermolecular forces in Arn–HF van der Waals clusters: Effects on the HF vibrational frequency shift
Authors: Hutson Jeremy M. , Liu Suyan , Moskowitz Jules W. , Bačić Zlatko
Published: July 2002

The effects of nonadditive forces on Arn–HF van der Waals clusters are investigated for n=2, 3, 4, and 12. The pair potentials operating in these systems are accurately known. Earlier models of nonadditive forces in Ar2–HF, including nonadditive dispersion, induction, and overlap distortion, are generalized to handle clusters of arbitrary size. Calculations of vibrational frequency shifts (redshifts) are then performed and compared with experiment. The geometries of the clusters are first optimized by simulated annealing; the Arn cage is then held fixed, and the resulting five-dimensional Schrödinger equation is solved for the hindered rotational and translational motion of the HF molecule in the field of the Ar atoms. The nonadditive potentials are found to account remarkably well for the observed frequency shifts.
Guided ion beam investigation of the reaction CO++CO: C–O bond activation and C–C bond formation
Authors: Lu Wenyun , Tosi Paolo , Bassi Davide
Published: July 2002

We have investigated six different endothermic channels in the reaction of CO+ ions with neutral CO. For each ionic product we have measured the kinetic energy dependence of the integral cross section and inferred the neutral products by the reaction energetics. The onset of the process producing C+, O, and CO, has been identified by a feature of the integral cross section located at about 8.5 eV. Measurements of the product isotopic ratio suggest that C+ originates from both the CO+ ion and the neutral CO molecule. For the reaction channels producing C2++O2 and C2O++O, respectively, measurements of the reaction thresholds allow us to estimate the heats of formation of these two ionic products, ΔfH0(C2+)=19.8±0.2 eV and ΔfH0(C2O+)=14.7±0.2 eV. These values are in good agreement with recent independent estimations. Finally, we re-evaluated the dissociation energy of C2+(X 4Σg−), D0(C+–C)=6.2±0.2 eV.
Comment on “Monte Carlo simulations of smectic phase transitions in flexible-rigid-flexible molecules” [J. Chem. Phys. 110, 12183 (1999)]
Authors: Fung B. M. , Bräuniger Thomas
Published: July 2002

A recent publication on Monte Carlo simulations of phase transitions in liquid crystals suggests that the nematic-smectic A phase transition is continuous and the smectic A-smectic C transition is discontinuous, but the authors conclude that their calculations do not agree with experimental observations. However, recent experimental results show that both the nematic to smectic A and the smectic A to smectic C transitions can be either continuous or discontinuous. Therefore, it is suggested that a careful examination of experimental results may be useful for future computer simulations of liquid crystal ordering.
Spontaneous swelling of layered nanostructures by a polymer melt
Authors: Lee Jae Youn , Baljon Arlette R. C. , Loring Roger F.
Published: July 2002

Polymer layered silicate nanocomposites may be formed by annealing layered silicate particles with a polymer melt. Polymer molecules leave the bulk melt and intercalate between the silicate layers, producing a structure in which polymers are confined on the nanometer scale by the silicate layers. We report here molecular dynamics simulations of this formation process, which is modeled by the flow of polymer from a bulk melt into a slit whose walls are maintained at constant pressure and whose surfaces are decorated by grafted short hydrocarbon chains. The results are compared with x-ray diffraction studies of the intercalation of high molecular weight polymers into organically modified silicates, and with a previous simulation of the flow of polymer molecules from a bulk melt into a slit of fixed dimension.
Quantum Monte Carlo study of the H− impurity in small helium clusters
Authors: Casalegno Mose’ , Mella Massimo , Morosi Gabriele , Bressanini Dario
Published: July 2002

We report ground state energies and structural properties for small helium clusters (4He) containing a H− impurity computed by means of variational and diffusion Monte Carlo methods. Except for 4He2H− that has a noticeable contribution from collinear geometries where the H− impurity lies between the two 4He atoms, our results show that 4HeNH− clusters have a compact 4HeN subsystem that binds the H− impurity on its surface. The results for N⩾3 can be interpreted invoking the different features of the minima of the He–He and He–H− interaction potentials.
On the divergent behavior of Møller–Plesset perturbation theory for the molecular electric dipole moment
Authors: Larsen Helena , Halkier Asger , Olsen Jeppe , Jørgensen Poul
Published: July 2002

We have recently shown [J. Chem. Phys. 110, 7127 (1999)] that the Møller–Plesset perturbation theory series diverges for the equilibrium dipole and quadrupole moments of HF when the basis set contains diffuse functions. Here, we show that the divergence, as for the energy, is caused by highly excited diffuse back-door intruder states. The HF study is extended to include a stretched geometry, where divergence is also observed in a basis set that does not contain diffuse functions. For BH and CH2, a very slow monotonic convergence is observed. The different convergent/divergent behaviors are qualitatively reproduced by a simple two-state model. The slow convergence or divergence is more pronounced for the dipole moment than for the energy. These observations seriously question the use of higher-order Møller–Plesset perturbation theory for calculations of simple electric properties.
Theoretical characterization of the excited-state structures and properties of phenol and its one-water complex
Authors: Fang Wei-Hai
Published: July 2002

The structures and properties of phenol and its complex were characterized at the Hartree–Fock (HF), the second-order Moller–Plesset perturbation theory (MP2), and complete active space self-consistent field (CASSCF) levels for the ground state (S0) and at the configuration interaction with single excitation (CIS) and CASSCF levels for the excited electronic state (S1). The intermolecular interaction has little influence on the structures of phenol and water. However, a significant change is found in the properties upon complex, and this has been discussed in detail. A comparison with the experimental findings shows that the present calculations provide a good description of the nature of phenol and its complex in S0 and S1.
Generalized van der Waals density functional theory for nonuniform polymers
Authors: Patra Chandra N. , Yethiraj Arun
Published: July 2002

A density functional theory is presented for the effect of attractions on the structure of polymers at surfaces. The theory treats the ideal gas functional exactly, and uses a weighted density approximation for the hard chain contribution to the excess free energy functional. The attractive interactions are treated using a van der Waals approximation. The theory is in good agreement with computer simulations for the density profiles at surfaces for a wide range of densities and temperatures, except for low polymer densities at low temperatures where it overestimates the depletion of chains from the surface. This deficiency is attributed to the neglect of liquid state correlations in the van der Waals term of the free energy functional.
Comparing the vibrational properties of low-energy modes of a molecular and an atomic adsorbate: CO and O on Pt(111)
Authors: Engström Ulrika , Ryberg Roger
Published: July 2002

In this work we present a situation where it has been possible to compare the vibrational properties of the low-energy modes of a molecular and an atomic adsorbate on a metal surface. We have, by infrared spectroscopy, studied well-ordered structures of carbon monoxide chemisorbed in two different sites and atomic oxygen in a third site, all three on the same surface, with the same relative coverage and studied under identical conditions. We are able to present a unique test of the antiabsorption mechanism and its applicability to transition metals like platinum. For atomic oxygen we observe a dip in the absorptance spectra associated with the frustrated translation, while in contrast no such dip was observed for the frustrated rotations of CO. This emphasizes that the magnitude of the broadband decrease in reflectivity and the character of the adsorbate motion is very important for the strength of the effect. We also present the first study on the temperature dependence of the antiabsorption, the results being in agreement with theory. For the metal–adsorbate stretch vibrational modes we make relative comparisons between the observed integrated absorptance for the different species. We find a good correspondence between the relative values of the oscillating charge and the binding energy by making simple estimations on the bond geometry and the screening by the metal electrons.
Vibrational coordinates and their gradients: A geometric algebra approach
Authors: Pesonen Janne
Published: July 2002

The gradients of vibrational coordinates are needed in order to form the exact vibrational kinetic energy operator of a polyatomic molecule. The conventional methods used to obtain these gradients are often quite laborious. However, by the methods of geometric algebra, the gradients for any vibrational coordinate can be easily calculated. Examples are given, and special attention is directed to ring coordinates.
Regular and irregular vibrational states: Localized anharmonic modes and transition-state spectroscopy of Na3
Authors: Wright Nicholas J. , Hutson Jeremy M.
Published: July 2002

We have calculated the lowest 900 vibrational energy levels and wave functions for the quartet (14A2′) state of Na3. The equilibrium geometry of the trimer is triangular, but the calculations include many states that lie above the barrier to linearity. Most of the high-lying states are irregular, but there are a few relatively localized states embedded in the irregular bath. The localized modes observed include a “horseshoe” mode and a symmetric stretch centered on the linear transition state. The density of states and couplings are such that in most cases the “horseshoe” character is spread over several bath states, while the symmetric stretch states exist in a purer form. The localized states could be observed in laser-induced fluorescence, stimulated-emission pumping or ion photodetachment spectroscopy from a state with a linear equilibrium geometry.
Molecular dynamics simulation of supersaturated vapor nucleation in slit pore
Authors: Yasuoka K. , Gao G. T. , Zeng X. C.
Published: July 2002

Molecular dynamics simulations of nucleation of Lennard-Jones vapor confined in a slit pore have been performed. The walls of the slit pore are structureless walls; each wall interacts with vapor molecules via Lennard-Jones 9–3 potential. The rate of nucleation in the steady state is determined by analyzing time evolution of the cluster size distribution. At the same vapor density and temperature, the nucleation rate in the slit pore is higher than in the homogeneous vapor [K. Yasuoka and M. Matsumoto, J. Chem. Phys. 109, 8451 (1998)], irrespective of the strength of attraction between the wall and vapor molecules. However, this attraction strongly affects the process of nucleus formation: if the attraction is weak (a drying wall), nuclei tend to form in the middle of the pore, whereas if the attraction is strong (a wetting wall), the nucleus formation originates from two sources, the surface diffusion of adsorbed molecules and deposition of clusters formed in the middle of the pore.
Time moment analysis of first passage time, time lag and residence time problems via Taylor expansion of transmission matrix
Authors: Chen Jenn-Shing , Chang Wen-Yih
Published: July 2002

Taylor expansion (with respect to the Laplace variable, s) of the transmission matrix, T(s), has been developed for the diffusion transport with position-dependent diffusivity, D(x) and partition coefficient, K(x). First, we find the relation between the expansion coefficients of the matrix elements and the moments of the first passage times by connecting them to Ĵ(s), the Laplace transform of the escaping flux, J(t). The moments can be formulated by repeated integrals of K(x) and [D(x)K(x)]−1 from solving the backward diffusion equation subject to appropriate initial and boundary conditions. In this way, Taylor expansion coefficients of T11(s), T21(s), and T22(s) are expressed in terms of the repeated integrals. Further application of the identity det[T(s)]=1 leads to the Taylor expansion T12(s). With the knowledge of the Taylor expansion of T(s), the formulation of the time moments for diffusion problems with position dependent D(x) and K(x) subject to various initial and boundary conditions is then just a simple, algebraic manipulation. Application of this new method is given to the membrane permeation transport and mean residence time problem.
Self-organization and mismatch tolerance in protein folding: General theory and an application
Authors: Fernández Ariel , Berry R. Stephen
Published: July 2002

The folding of a protein is a process both expeditious and robust. The analysis of this process presented here uses a coarse, discretized representation of the evolving form of the backbone chain, based on its torsional states. This coarse description consists of discretizing the torsional coordinates modulo the Ramachandran basins in the local softmode dynamics. Whenever the representation exhibits “contact patterns” that correspond to topological compatibilities with particular structural forms, secondary and then tertiary, the elements constituting the pattern are effectively entrained by a reduction of their rates of exploration of their discretized configuration space. The properties “expeditious and robust” imply that the folding protein must have some tolerance to both torsional “frustrated” and side-chain contact mismatches which may occur during the folding process. The energy-entropy consequences of the staircase or funnel topography of the potential surface should allow the folding protein to correct these mismatches, eventually. This tolerance lends itself to an iterative pattern-recognition-and-feedback description of the folding process that reflects mismatched local torsional states and hydrophobic/polar contacts. The predictive potential of our algorithm is tested by application to the folding of bovine pancreatic trypsin inhibitor (BPTI), a protein whose ability to form its active structure is contingent upon its frustration tolerance.
Comment on “Effect of gravity on contact angle: A theoretical investigation” [J. Chem. Phys. 109, 3651 (1998)]
Authors: Blokhuis E. M.
Published: July 2002

In a recent theoretical analysis by Ward and Sasges [J. Chem. Phys. 109, 3651 (1998)] the experimentally observed difference in contact angle at the upper and lower meniscus in a capillary is ascribed to the height dependence of the three surface tensions that determine the contact angle through Young’s law. We argue that their analysis contains an error and that in equilibrium the contact angles should be equal.
Electron-hydrogen bonds and OH harmonic frequency shifts in water cluster complexes with a group 1 metal atom, M(H2O)n (M=Li and Na)
Authors: Tsurusawa Takeshi , Iwata Suehiro
Published: July 2002

The harmonic vibrational frequencies for M(H2O)n (M=Li and Na) are calculated with ab initio molecular orbital (MO) methods. Three types of isomers, surface, quasivalence and semi-internal, exhibit the characteristic frequency shifts ΔνOH. The calculated spectral patterns are related to the geometric conformations around the localized electron {e} in the cluster. The downward shifts of OH frequencies are strongly correlated with the lengthening of OH bond distances, as is well known in the hydrogen bonds. The magnitude of the downward shift is as large as that for the hydrogen bond. The correlation of the shifts with the distance between the center of the electron {e} and the hydrogen atom reveals two types of interaction schemes. The characteristics of the structural unit OH{e}HO in M(H2O)n and its OH modes are common with those found in water anion clusters. The interaction between the localized electron and OH bonds can be called an electron-hydrogen bond.
Photophysical properties of multiply phenylated C70 derivatives: Spectroscopic and quantum-chemical investigations
Authors: Coheur P.-F. , Cornil J. , dos Santos D. A. , Birkett P. R. , Liévin J. , Brédas J. L. , Walton D. R. M. , Taylor R. , Kroto H. W. , Colin R.
Published: July 2002

The photophysics of six multiply phenylated C70 derivatives [C70Ph2, C70Ph4, C70Ph6 (two regioisomers), C70Ph8 and C70Ph10] have been investigated by means of steady state spectroscopy in cyclohexane solution and quantum-chemical calculations derived from semiempirical Hartree–Fock approaches. There is good agreement between the measured absorption spectra and the INDO/SCI calculated excitation energies for each derivative. The foregoing results and some additional ground state properties calculated at the AM1 level have been used to predict that the perturbation of the π electronic system of the cage results in significant changes in the photophysics of the fullerene derivatives. The effects of conjugation and spatial localization of the HOMO and LUMO orbitals on the energy of the first electronic transition are discussed in greater detail.
The short-time dynamics of molecular reorientation in liquids. II. The microscopic mechanism of rotational friction
Authors: Jang Joonkyung , Stratt Richard M.
Published: July 2002

At short times, the dynamics of the rotational relaxation of linear molecules dissolved in liquids is governed by the instantaneous rotational friction, a quantity one can specify in complete molecular detail for each liquid configuration. Having the ability to construct such a friction is not only useful for the insight it provides into rotational dynamics, it means that it is possible to think about the superficially very different processes of rotational relaxation, vibrational population relaxation and solvation in a common language. In particular, the ability to understand the friction in molecular terms allows us to compare the actual solvent molecules participating and the actual solvent motions involved in all of these relaxation processes. In this paper we carry out a detailed study of the rotational friction felt by a homonuclear diatomic molecule dissolved in an atomic fluid, contrasting the results for a variety of solute sizes and thermodynamic states. We find remarkable levels of similarity among all three kinds of relaxation. While there are some detailed differences in the geometry of the relevant solvent motions, all three processes seem to be controlled by a small number of nearby solvents. Possibly as a result, the influence spectra (the spectral densities) of all three are virtually identical. The invariance of these findings, and indeed of the mechanistic details, to solute size and thermodynamic conditions suggests that there is a real universality to solution dynamics that comes into play when sharply varying forces are involved.
Exciton transport in molecular aggregates probed by time and frequency gated optical spectroscopy
Authors: Chernyak Vladimir , Minami Tatsuya , Mukamel Shaul
Published: July 2002

The signatures of exciton relaxation in time-resolved fluorescence spectroscopy of molecular aggregates excited by a short pulse are expressed as an overlap of a Doorway wavepacket representing the exciton density matrix with a Window wavepacket which describes the time and frequency resolved detection. Transport and relaxation of the excitons are accounted for using the Redfield equations for the density matrix, and the complete temporal and spectral profiles of the excitation pulse and detection gate are incorporated using Wigner spectrograms. The spread in the off diagonal elements of the density matrix in the chromophore (real space) representation provides a natural measure of the relevant exciton coherence size.
Fluorescence lifetimes of 9-(N-carbazolyl)-anthracene: Effects of intramolecular vibrational redistribution and electronic transitions in coupled bright and dark states
Authors: Brackhagen O. , Busse H. , Giraud-Girard J. , Manz J. , Oppel M.
Published: July 2002

The fluorescence lifetimes of 9-(N-carbazolyl)-anthracene (C9A) in selected vibronic states |1̃ l〉 are calculated by means of a simple model. Specifically, the states |1̃ l〉 are excited to the bright electronic state S1 which is coupled to a dark state X, as well as to the l-th vibrational excitation of the torsional mode of C9A. The model takes into account the torsional moments of inertia of C9A, the empirical diabatic torsional potentials, the potential couplings, the dipole couplings, and the vibrational couplings between the torsion and the other vibrations of C9A. The corresponding model parameters are fit to the experimental spectra and fluorescence lifetimes of Monte et al. [J. Chem. Phys. 98, 2580 (1993)]. Three competing processes are described by the model, i.e.: (i) fluorescence from doublets of states |1̃ l,+〉 and |1̃ l,−〉 with + and − parity; (ii) intramolecular vibrational redistribution IVR from the torsion to the other vibrational modes; and (iii) the coupling between states S1 and X. The resulting fluorescence lifetimes decrease systematically from ca. 20 ns for |1̃ 0〉 to ca. 6 ns for |1̃ 50〉, with an exceptional decrease to ca. 7 ns for intermediate states such as |1̃ 24〉, with energies close to the crossings of the diabatic potentials of the bright and dark states S1 and X. These systematic and exceptional trends agree well with the experimental results of Monte et al., and they are explained and interpreted as consequences of the three processes (i)–(iii) within our model. Accordingly, the fluorescence lifetimes contain some information about the intramolecular dynamics, e.g., IVR from the torsion to the other vibrations should be faster in the dark state X than in the bright state S1, where it proceeds on the extraordinarily long time scale of several ns.
Transport properties of liquid hydrogen fluoride
Authors: Balucani Umberto , Bertolini Davide , Tani Alessandro , Vallauri Renzo
Published: July 2002

The dynamical properties of liquid hydrogen fluoride are investigated by a molecular dynamics study of the correlation functions relevant for a generalized hydrodynamics description of transport coefficients. The results are compared with the corresponding ones in liquid water in order to understand the role of hydrogen bonding in the two systems. The different behavior can ultimately be attributed to the arrangement of the molecules, which form irregular chains in HF and a tetrahedral network in water. For the two systems, the differences between experimentally measurable quantities are also pointed out and discussed.
Comment on “Polarizabilities of the rare-gas homonuclear diatoms” [J. Chem. Phys. 111, 6316 (1999)]
Authors: Hohm Uwe
Published: July 2002

The collision-induced polarizability anisotropy γ(R) of the rare-gas homonuclear diatoms is represented in terms of the Cauchy moments S(−2k) of the single atoms. Except for neon, very good agreement is found between this new representation and other analytic forms given in the literature.