bluuebunny/crossref_metadata_2025 huggingface dataset:
A theoretical characterization of the quartet states of the SO+ molecular ion
Authors: Ornellas Fernando R. , Borin Antonio Carlos
Published: July 2002

The quartet states of the SO+ molecular ion are described theoretically using the internally contracted multireference configuration interaction approach and natural orbitals generated from a state-averaged density matrix. Correlation-consistent polarized-valence quadruple-zeta atomic functions are used in the expansion of the one-electron basis. Potential energy curves are presented for all the states, and solutions of the radial Schrödinger equation allowed the determination of vibrational energy differences and spectroscopic constants. For the b 4Σ− state, this study corroborates the available experimental data and extends the spectroscopic information to regions not yet accessed experimentally; an alternative explanation for the predissociation mechanism is also suggested. For the a 4Π state, our data and analysis are indicative that the vibrational spectroscopic constants derived from the photoelectron spectra might be underestimated. It also leaves open the possibility that the experimental vibrational level numbering might have to be increased by one unit. Transition probabilities as given by the Einstein A coefficients, and Franck–Condon factors are also provided to help analyze the experimental data. Of immediate relevance to the direct ion-fragment spectroscopy, this study predicts the existence of a new bound Π4 state in the energy range of photons used in these experiments. This new state crosses the b 4Σ− curve very close to where it was supposed to be crossed by the 1 4Σ+, and its repulsive side runs almost parallel to this latter state. Our theoretical prediction places the 1 4Σ+ state still lower than it was inferred experimentally. For the a 4Π–1 4Σ+ transition we have also computed the transition moment function and showed that its constancy assumed in the simulation of the experimental intensity data is not valid.
Dissociative photoionization of SO2 from 16 to 120 eV
Authors: Masuoka Toshio , Chung Youngmin , Lee Eun-Mee , Samson James A. R.
Published: July 2002

Using synchrotron radiation as a continuum light source, dissociative photoionization of SO2 has been studied in the photon-energy region of 16–120 eV. Ion branching ratios were obtained by analyzing time-of-flight mass spectra and were converted to the absolute partial cross sections for the production of SO2+, SO+, S+(O2+), O+, and SO2+ by using the reported total absorption cross sections of SO2. Ion branching ratios were differentiated with respect to the incident photon energy. The results obtained by this analytical photoion spectroscopy show dissociation pathways of the SO2+ and SO22+ ions, some of which are first observed in the present study. These pathways are discussed by comparing them with the reported electronic states of the ions.
Differential scanning calorimetry, x-ray diffraction and F19 nuclear magnetic resonance investigations of the crystallization of InF3-based glasses
Authors: Franco R. W. A. , Tambelli C. C. , Magon C. J. , Donoso J. P. , Messaddeq Y. , Ribeiro S. J. L. , Poulain M.
Published: July 2002

Results of differential scanning calometry (DSC), x-ray diffraction (XRD), and F19 nuclear magnetic resonance (NMR) of InF3-based glasses, treated at different temperatures, ranging from glass transition temperature (Tg) to crystallization temperature (Tc), are reported. The main features of the experimental results are as follows. DSC analysis emphasizes several steps in the crystallization process. Heat treatment at temperatures above Tg enhances the nucleation of the first growing phases but has little influence on the following ones. XRD results show that several crystalline phases are formed, with solid state transitions when heated above 680 K. The F19 NMR results show that the spin–lattice relaxation, for the glass samples heat treated above 638 K, is described by two time constants. For samples treated below this temperature a single time constant T1 was observed. Measurements of the F19 spin–lattice relaxation time (T1), as a function of temperature, made possible the identification of the mobile fluoride ions. The activation energy, for the ionic motion, in samples treated at crystallization temperature was found to be 0.18±0.01 eV.
Substrate mediated photochemistry on a molecular surface: OCS on Ag clusters
Authors: Brown L. A. , Rayner D. M.
Published: July 2002

The photochemistry of carbonyl sulfide (OCS) adsorbed to small silver clusters is shown to exhibit a striking odd–even dependence on the number of Ag atoms in the cluster. OCS is found to desorb nondissociatively from even numbered silver clusters. In contrast, on odd silver clusters, a new product channel corresponding to AgnS is observed. Parallels are found with the photochemistry of adsorbates on extended surfaces. Cross-section measurements for OCS desorption from Ag10 and wavelength-dependent measurements of OCS dissociation on Ag9 both indicate that electronic excitation of the cluster initiates chemistry, analogous to substrate mediated surface photochemistry. The size dependence is reasonably explained in terms of a charge-transfer mechanism involving an ion-pair state of the AgnOCS complex.
Inter-system crossing catalyzed by solute encounters
Authors: Gopich I. V. , Burshtein A. I.
Published: July 2002

We studied the reversible transitions between two levels of excited molecule stimulated by encounters with other solutes. The inter-system crossing results in either quenching or trapping of excitation depending on the relationship between the lifetimes of the levels. In the particular case of an immobile excited molecule and equal lifetimes, an exact solution of the problem was given which is valid for arbitrary concentration of solutes. We compared the fluorescence quantum yield obtained with this solution, superposition approximation and integral encounter theory. Our main conclusion is that for equal lifetimes the latter has an advantage, even in small concentration limits and is out of competition when the lifetime of the primary excited level is shorter than another.
Gaussian-2 (G2) theory for third-row elements: A systematic study of the effect of the 3d orbitals
Authors: Duke Brian J. , Radom Leo
Published: July 2002

The importance of the inclusion of the 3d orbitals on third-row atoms in the correlation space in G2 theory has been systematically examined through calculations on the third-row G2 test set. Compared with standard G2, this G2(d) approach gives better agreement with experiment for the evaluation of ionization energies, a slightly poorer agreement for atomization energies, and much the same agreement for the very small sub-set of electron affinities and proton affinities. Overall, there is only slightly better agreement with experiment. However, when mixing of the 3d orbitals of the third-row atom with valence orbitals on the adjacent atoms is strong, inclusion of the 3d orbitals in the correlation space becomes a prerequisite to obtaining reliable results. Standard G2 theory is unsuitable in these circumstances. Similar conclusions pertain for the more economical G2(MP2)(d) method and for the full G2(QCI)(d) method. Inclusion of the 3d orbitals in the correlation space greatly increases the computer time required for a G2 calculation so some simple additive corrections to the G2 energy to approximate the effect of this inclusion have been investigated. These additivity methods generally underestimate the effect of the 3d orbitals but give reasonable agreement with the full G2(d) calculations in most cases. They cannot be used, however, in situations where the 3d orbital mixing is strong.
Cluster variation study of the underpotential deposition of Cu on Au(111) in the presence of bisulfate
Authors: Huckaby Dale A. , Legault Marc D. , Blum L.
Published: July 2002

A cluster variation method is developed to study the phase transitions and the structures of phases which occur at the fluid-crystal interface during the underpotential deposition of a metal on an electrode in the presence of an anion, such as bisulfate. In addition to the possibility of first-order condensation phase transitions occurring during the deposition of a metal, the steric repulsion of adsorbed anions can also cause an order-disorder transition. Using clusters containing six adsorption sites, the method is applied to the underpotential deposition of copper on (111) gold in the presence of bisulfate. In order to fix a constant in the expression for the entropy, the effect of the hard-core exclusion of a pair of first-neighbor bisulfates, in addition to the effect of finite interactions, is calculated exactly in the limit of high temperature. The cluster method yields two coupled adsorption isotherms for copper and bisulfate in terms of their activities and coverages. The resulting isotherms show an order-disorder transition due to the hard-core exclusion of neighboring bisulfate ions, as well as two first-order phase transitions in the copper and bisulfate coverages which correspond to the two spikes in the experimental voltammogram. The cluster method also gives the local structure of the phases which occur as the voltage is changed.
Implications of parity conservation for superpositions of chiral states
Authors: Maierle Christopher S. , Harris Robert A.
Published: July 2002

In this paper, we consider how parity conservation limits the type of superpositions of chiral states that can be created and detected. We also show how spontaneous emission can cause the left and right hands of the superposition to become coupled to different states of the radiation field. This coupling, aside from being an interesting effect in its own right, helps to enforce the restrictions that parity conservation places upon superpositions of chiral states. Finally, we outline an analogy that exists between aspects of this work and the Einstein, Podolsky, Rosen paradox.
Nuclear magnetic resonance-paramagnetic relaxation enhancements: Influence of spatial quantization of the electron spin when the zero-field splitting energy is larger than the Zeeman energy
Authors: Abernathy S. M. , Miller J. C. , Lohr L. L. , Sharp R. R.
Published: July 2002

Dissolved paramagnetic ions generally provide an efficient mechanism for the relaxation of nuclear spins in solution, a phenomenon called the nuclear magnetic resonance-paramagnetic relaxation enhancement (NMR-PRE). Metal ions with electron spins S⩾1 exhibit rich NMR relaxation phenomena originating in the properties of the zero-field splitting (zfs) interaction, which vanishes for spin-12 ions but which is nonzero for S⩾1 ions in site symmetry lower than cubic. For S⩾1 ions in the vicinity of the zfs-limit, i.e., at magnetic-field strengths low enough that the zfs energy exceeds the Zeeman energy, the NMR-PRE depends strongly on the detailed structure of the electron spin energy levels as well as on the spatial quantization of the spin motion. It is shown theoretically and experimentally that the NMR-PRE produced by integer spins can be influenced strongly by the small intradoublet zero-field splittings, i.e., the splittings between the components of the non-Kramers doublets, which are produced by noncylindrical components of the crystal field potential. These small splittings produce relatively low-frequency oscillations in the dipolar field associated with 〈Sẑ〉 (the spin component along the molecule-fixed ẑ axis). These motions decouple the nuclear spin from the electron spin, thereby depressing, in some cases very strongly, the NMR-PRE. The presence of a relatively small Zeeman field, comparable in magnitude to the intradoublet spacing but small compared to the larger interdoublet zfs splittings, causes a major change in the spin wave functions which has profound effects on the motions of the electron spin. When the Zeeman energy exceeds the small zfs splitting, the oscillatory motion of 〈Sẑ〉 damps out, with the result that the electron spin couples more effectively to the nuclear spin, providing a more efficient NMR relaxation pathway. NMR-PRE data are presented for the S=1 complex Ni(II)(o-pda)2Cl2 (o-pda=ortho-phenylenediamine) which confirm the importance of the splitting of the mS=±1 non-Kramers doublet on the NMR relaxation efficiency. The zfs E-parameter was measured from the NMR data to be |E|=0.26 cm−1. The S=2 spin system, Mn(III)-tetraphenylporphyrin sulfonate, exhibits a related phenomenon which arises from the effects of a small zfs splitting, Δε±2, of the mS=±2 non-Kramers doublet that is caused by a fourfold rotational component of the crystal field potential. The splitting Δε±2 was measured from NMR data to be 0.20 cm−1.
Doorway state enhanced intramolecular vibrational energy redistribution in the asymmetric =CH2 hydride stretch of methyl vinyl ether
Authors: Cupp Sam , Lee Chung Yi , McWhorter David , Pate Brooks H.
Published: July 2002

The microwave-infrared double-resonance capabilities of an electric-resonance optothermal spectrometer have been used to assign the high resolution (5 MHz) infrared spectrum of the asymmetric =CH2 stretch of the cis conformer of methyl vinyl ether near 3130 cm−1. This vibrational state is anharmonically coupled to a near-resonant bath state by a 0.69 cm−1 matrix element resulting in two vibrational bands separated by about 1.44 cm−1. The two mixed states resulting from this interaction are further coupled to other near-resonant bath states with an average matrix element of about 0.01 cm−1. The coupled state density increases weakly with the total angular momentum, J, however, the intramolecular vibrational energy distribution (IVR) rate is approximately independent of the total angular momentum quantum number. Therefore, the rotationally mediated coupling mechanisms are weaker than the anharmonic terms in the redistribution process. A two-state analysis of the strong coupling, which includes a phenomenological IVR rate constant, suggests that the IVR rate in the two mixed states is dominated by the contribution from the coupled dark state. From the deconvolution of the IVR rates to remove the contribution from the dark state, the IVR lifetime of the asymmetric =CH2 stretch is determined to be 660 ps.
On the resonant coherent two-dimensional Raman scattering
Authors: Cho Minhaeng
Published: July 2002

A theoretical description of the resonant coherent two-dimensional Raman scattering (ReCOTRAS) process is presented. In comparison to the third-order coherent Raman scattering, ReCOTRAS measurement in frequency domain is a two-dimensional experiment probing two vibrational coherence states simultaneously. By using three external fields with frequencies and wave vectors as (ω1,ω2,ω3) and (k1,k2,k3), it is found that the ReCOTRAS field is generated with the frequency and the wave vector of 3ω1−ω2−ω3 and 3k1−k2−k3, respectively. It is shown that this experiment can provide two-dimensional vibrational spectrum that contains crucial informations on the vibrational mode-couplings in both the ground and the excited states. Unlike its off-resonant analogue, this method can be used to study the vibrational interactions in the electronic excited state.
Vibrational relaxation of azide ion in water: The role of intramolecular charge fluctuation and solvent-induced vibrational coupling
Authors: Morita Akihiro , Kato Shigeki
Published: July 2002

Vibrational energy relaxation of azide anion in water was investigated with molecular dynamics simulation. The Landau–Teller formula without the solute electronic polarization exceedingly underestimated the relaxation rate of the antisymmetric stretching mode, and thus various relaxation mechanisms were comprehensively evaluated to elucidate the fast relaxation. As a result, the direct relaxation to the vibrational ground state and the intramolecular vibrational redistribution (IVR) to the symmetric stretching mode have equally dominant contributions to the decay rate. Intramolecular charge fluctuation induced by the solute vibrational motion greatly enhanced the friction by the solvent to reproduce the experimental rate fairly well. The isotope effect of H2O and D2O was also elucidated along the present mechanism.
Temperature dependent vibrational lifetimes in supercritical fluids near the critical point
Authors: Myers D. J. , Chen Shirley , Shigeiwa Motoyuki , Cherayil Binny J. , Fayer M. D.
Published: July 2002

Vibrational relaxation measurements on the CO asymmetric stretching mode (∼1980 cm−1) of tungsten hexacarbonyl (W(CO)6) as a function of temperature at constant density in several supercritical solvents in the vicinity of the critical point are presented. In supercritical ethane, at the critical density, there is a region above the critical temperature (Tc) in which the lifetime increases with increasing temperature. When the temperature is raised sufficiently (∼Tc+70 °C), the lifetime decreases with further increase in temperature. A recent hydrodynamic/thermodynamic theory of vibrational relaxation in supercritical fluids reproduces this behavior semiquantitatively. The temperature dependent data for fixed densities somewhat above and below the critical density is in better agreement with the theory. In fluoroform solvent at the critical density, the vibrational lifetime also initially increases with increasing temperature. However, in supercritical CO2 at the critical density, the temperature dependent vibrational lifetime decreases approximately linearly with temperature beginning almost immediately above Tc. The theory does not reproduce this behavior. A comparison between the absolute lifetimes in the three solvents and the temperature trends is made.
Generalized Langevin dynamics simulations of NaCl electrolyte solutions
Authors: Canales M. , Sesé G.
Published: July 2002

The stochastic simulation method called generalized Langevin dynamics has been employed in the study of NaCl aqueous solutions at several ionic concentrations. Only the ion pairs have been considered explicitly in the simulations. Every ion follows a generalized Langevin equation in which the total acceleration has a stochastic term, an integral time-dependent friction term, and a deterministic part that takes into account the interaction with the other ions in the system. Different solvent-averaged potentials and memory kernels have been tested in order to obtain a realistic behavior of the systems. The screening of electrostatic forces between ions has been analyzed too. The electric field around Na+ and Cl− ions has been calculated and the screening has been analyzed in terms of the structural features of the ionic surroundings.
The calculation of kinetic isotope effects based on a single reaction path
Authors: Fast Patton L. , Corchado José C. , Truhlar Donald G.
Published: July 2002

In this paper we propose a new method for calculating kinetic isotope effects without calculating a separate reaction path for each isotopically substituted species. The new method yields reasonable kinetic isotope effects from calculations using the same reaction path for all isotopic variations. The method is tested by carrying out variational transition state theory calculations, including multidimensional tunneling contributions, for the reactions OH+H2→H2O+H, CH3+H2→CH4+H, and H+H2→H2+H and nine deuterium-substituted isotopologs of these reactions. The results are very encouraging.
Optical spectroscopy of the Ag+ ion in NaF: Experimental results and analysis of manifestations of the Jahn–Teller effect
Authors: Bill H. , Hollingsworth G. J. , McClure D. S. , Moine B. , Pedrini Ch.
Published: July 2002

The 4d10→4d95s1 transitions of the Ag+ ion in single crystals of NaF were investigated by one- and two-photon spectroscopy and luminescence experiments. The one-photon absorption spectrum extends from 238 to below 180 nm. At low temperature, several bands show resolved fine structure, but compared to NaF:Cu+, there is much less, and the lines are broader. The Jahn–Teller effect is much larger in Ag+ than in Cu+ and has been identified in three of the excited states. Trapping in Jahn–Teller minima is shown to change the emission kinetics radically compared to Cu+. The off-center force from the d9p states has much less effect on Ag+ than on Cu+. All of the evidence shows that Ag+ is more strongly coupled to the lattice than is Cu+.
Room-temperature fluorescence characteristics of single dye molecules adsorbed on a glass surface
Authors: Weston Kenneth D. , Carson Paul J. , Metiu Horia , Buratto Steven K.
Published: July 2002

Measurements of the total fluorescence and fluorescence spectra from single carbocyanine dye molecules (DiIC12) as a function of time reveal a wide range of phenomena. Discrete jumps in the fluorescence intensity from single molecules on a glass surface have been observed with correlation times spanning several orders of magnitude (1 ms–10 s). We propose a model for these fluctuations in which two or more ground state potential minima are accessed by twists of the chromophore backbone that alter the quantum efficiency of emission as well as the emission spectrum. Monte Carlo simulations based on this model are shown which qualitatively match experimental data. In addition, we observed emission spectra which range in shape from narrow, well-separated vibronic bands to a broad, featureless band. The distribution of emission parameters from different molecules, not obtained from ensemble measurements, indicates an abundance of distinct nanoenvironments of the glass surface sampled by the adsorbed molecules.
On the perturbative estimates of the correlation energy from localized orbitals in periodic systems
Authors: Reinhardt P. , Malrieu J.-P.
Published: July 2002

Starting with self-consistent fields (SCFs), localized orbitals should facilitate the calculation of the correlation energy in extended, and in particular periodic, systems. This idea is exploited on model ring systems (H4n+2). It is shown that for insulating materials [(H2)2n+1, presenting a large gap in the band structure], most of the energy lowering brought by the orders larger than 2 in the canonical many-body perturbation expansion are due to the local-hole–local-particle interaction and that the localized Epstein–Nesbet second-order energies are close to the best correlation-energy estimates. The situation is completely different for small-gap (metalliclike) systems, such as cyclic H4n+2, where the localized second-order approach misses a large fraction of the correlation energy, involving the propagation of the holes and of the particles and implying specific higher-order diagrams.
High-resolution infrared spectroscopy of jet-cooled allyl radical (CH2–CH–CH2): In-phase (ν1) and out-of-phase (ν13) antisymmetric CH2 stretching vibrations
Authors: Uy Dairene , Davis Scott , Nesbitt David J.
Published: July 2002

High-resolution infrared spectra of jet-cooled allyl (CH2–CH–CH2) radical in the CH2 stretching region are obtained in a slit jet discharge spectrometer. Over 200 transitions are observed in the 3100–3140 cm−1 region, which have been rigorously assigned to “in-phase” (ν1) and “out-of-phase” (ν13) coupling of antisymmetric CH stretch excitations on each of the equivalent CH2 subunits. Origins for the two bands are determined to be 3113.98 488(89) cm−1 (ν1) and 3110.59 857(36) cm−1 (ν13), respectively. The data are reasonably well fit to semirigid asymmetric rotor behavior for the Ka⩽2 levels populated in the slit jet at Trot≈20 K, though not to within the experimental precision, which provides some indication of perturbations in the upper states. The sign and magnitude of the +3.4 cm−1 splitting between“in-phase” and“out-of-phase” excitations are in good qualitative agreement with a simple electrostatic model of dipole–dipole coupling between the two “local mode” CH2 groups. Due to sub-Doppler resolution (Δν≈70 MHz) in the slit jet expansion, quantum-state-dependent excess broadening of the rovibrational transitions is observed, which can be ascribed to spin–rotation interactions. Based on a least squares analysis of the high-resolution line shapes, the data are consistent with a spin rotation constant of εaa≈−67(25) MHz.
Nuclear magnetic resonance relaxation measurements in electrolyte solutions containing LiCF3SO3
Authors: Williamson M. J. , Southall J. P. , Ward I. M.
Published: July 2002

The characteristic features of ionic mobility in polymer gel electrolytes are closely related to those shown by the incorporated liquid electrolytes and as such much research is concentrated on the liquid electrolytes in the absence of the polymer host. In this paper, longitudinal relaxation (T1) measurements of H1, Li7, and F19 nuclei in tetraglyme/LiCF3SO3 and N,N-dimethyl-formamide/LiCF3SO3 liquid electrolytes are reported. From comparisons with measurements of self-diffusion coefficients and solution viscosity it is concluded that in general there are contributions to T1 from both translational and rotational motions.
Electron intracule densities and Coulomb holes from energy-derivative two-electron reduced density matrices
Authors: Cioslowski Jerzy , Liu Guanghua
Published: July 2002

Application of the energy-derivative formalism to two-electron reduced density matrices produces a robust approach to the approximate evaluation of electron intracule densities I(R) and Coulomb holes in atoms and molecules. The versatility of this approach, which makes routine calculations of correlated I(R) feasible at any level of electronic structure theory, is demonstrated by results of selected MP2 calculations. The MP2/(20s10p10d) values of I(0) are within 10% of their “exact” counterparts in systems such as H−, He, Li+, Be2+, Li, and Be. Quantitative reproduction of the exact I(R) is found to be contingent upon the inclusion of Gaussian primitives with high angular momenta in the basis sets.
van der Waals resonances in cumulative reaction probabilities for the F+H2, D2, and HD reactions
Authors: Takayanagi Toshiyuki , Kurosaki Yuzuru
Published: July 2002

We present accurate quantum cumulative reaction probabilities for the F+H2, D2, and HD reactions with a special emphasis on resonances associated with quasibound states localized in the reactant van der Waals region of the potential energy surface. The accurate ab initio potential surface of Stark and Werner and the less accurate 5SEC-W surface developed by Truhlar and co-workers have been employed. The van der Waals resonance states which occur in the tunneling region can dissociate into the product channel, while the van der Waals resonance states at the classical region dissociate into the reactant channel. This indicates that prereaction effectively occurs in the tunneling region. It has been found that the van der Waals resonance states can be accurately assigned with the eigenvalues calculated with one-dimensional adiabatic potential energy curves.
Multiquantum electron paramagnetic resonance transitions excited in a two-level system by tetrachromatic microwave irradiation
Authors: Jeleń Małgorzata , Froncisz Wojciech
Published: July 2002

A method to excite multiquantum transitions in a two-level system by application of continuous irradiation with a four-frequency microwave field is presented. The generated signal is composed of many intermodulation sidebands at different frequencies, which arise from the multiquantum transitions. These processes involve an odd number of photons, that are absorbed from, or emitted to two, three, or four fields. The correspondence between the frequency spectrum of the signal and possible pathways of multiquantum processes is found with recourse to the many mode Floquet theory. The reported experiments demonstrate the multiquantum origin of the detected intermodulation sidebands and their properties.
Ab initio MO studies of van der Waals molecule (N2)2: Potential energy surface and internal motion
Authors: Wada Akira , Kanamori Hideto , Iwata Suehiro
Published: July 2002

The equilibrium structure, potential energy surface and van der Waals (vdW) mode vibration of (N2)2 have been studied with high levels of ab initio calculations. The most stable structure is found to be a 45 deg. canted parallel structure of C2h. On the other hand, neither T-shape of C2v nor cross-shape of D2d is a stable structure, but they are transition state structures, which are contrary to the previous calculations. The out-of-plane motion changing from the 45 deg. canted parallel structure of C2h to the cross-shape of D2d has a higher barrier than the in-plane motion. The binding energy of two N2 monomers is about 80 cm−1 and the fundamental frequency of vdW stretching mode is estimated at 22 cm−1. A small energy difference of 5 cm−1 between the C2h and C2v structures implies that two N2 molecules move coherently like a gear rotation in the plane. This internal freedom of motion should make the rotational energy states extremely modulated, and therefore, a very complex spectrum pattern could be expected.
The calculation of accurate O17 hyperfine coupling constants in the hydroxyl radical: A difficult problem for current quantum chemical methods
Authors: Wetmore Stacey D. , Eriksson Leif A. , Boyd Russell J.
Published: July 2002

The hyperfine coupling constants (HFCCs) in the hydroxyl radical are investigated through comparison of results obtained from a variety of quantum chemical methods. The couplings obtained from the multi-reference configuration interaction (MRCI) wave function, built upon the restricted open-shell Hartree–Fock (ROHF) reference determinant, are investigated in terms of the basis set, the configuration selection energy threshold, and the size of the reference space. Overall results which converge to the experimental couplings are obtained for hydrogen, but not for oxygen. In particular, the MRCI method shows no improvement over density functional theory (the B3LYP functional), for the calculation of Aiso(17O). On the other hand, results in excellent agreement with experiment are obtained through the use of the quadratic configuration interaction (QCISD) method based on the unrestricted HF (UHF) reference determinant with the identical basis sets. Examination of UHF and ROHF based coupled-cluster methods, CCSD and CCSD(T), indicates that once a high enough level of electron correlation is included, the oxygen HFCC is independent of the form of the reference determinant. Unlike the ROHF-CCSD method, which yields reliable results once the effects of triple excitations have been taken into account, the MRCI wave function cannot easily be adjusted to account for the inadequacies of the ROHF reference determinant in order to accurately predict O17 HFCCs.
Simulation of self-associating polymer systems. I. Shear-induced structural changes
Authors: Khalatur P. G. , Khokhlov A. R. , Mologin D. A.
Published: July 2002

We present the results of nonequilibrium molecular dynamics (NEMD) studies of self-associating polymer systems composed of flexible telechelic chains with associating end-groups (“stickers”). Formation of micellar aggregates, their structure and structural characteristics of associative polymer network (micellar gel) are studied under the influence of external shearing forces. When the association energy εc is quite large (εc⩾εc*, where εc* is a critical association energy corresponding to the gelation transition at rest), the spatial organization of the system as a whole is characterized by a typical network architecture with bridging chains connecting different micellar aggregates. The shearing forces cause only a slight perturbation in the structural properties of the sol (at εc<εc*). However, when the association energy εc becomes quite large (εc≳εc*), we observe sharp structural variations as the shear force is increased. At sufficiently strong attraction between stickers, the shear flow facilitates development of the aggregation process: even at rather weak shearing forces, the cluster-size distribution functions become considerably wider when the association energy is close to εc*. Thus, aggregation processes become considerably more pronounced under shear, as compared to the same system at rest. As the shear force increases, the content of looplike chains decreases while the fraction of bridgelike chains rises. This process is accompanied by stretching of the chains. However, under a condition of high shear, many chains belonging to the associative network tend to be coiled up. This leads to the redistribution of structural elements in favor of loop chains. This exchange process can decrease the internal stress in the associative network which is mainly determined by the number and spatial topology of elastically active bridgelike chains.
Electronic structure of vanadium tetramer ion studied by optical absorption spectroscopy
Authors: Minemoto Shinichirou , Terasaki Akira , Imoto Hideo , Kondow Tamotsu
Published: July 2002

The optical absorption spectrum of a vanadium tetramer ion, V4+, was obtained by measuring a photodissociation efficiency of an ion complex, V4+Ar, as a function of the photon energy of the laser pulse used for the photodissociation. The optical absorption spectrum thus obtained was simulated by a density functional calculation to search for the most probable geometric structure which reproduces the measured spectrum. The analysis showed that V4+ is most likely to have a “distorted” tetrahedral structure with C2v symmetry. The ground electronic state of V4+ was found to be a low spin state, A12. The relatively broad spectral profile is explained in terms of the distortion related to a low-energy vibration. Geometry optimization of the Amsterdam density functional (ADF) calculation also predicts C2v symmetry for the structure of V4+.
Dynamics of polarization relaxation at solid–liquid interface
Authors: Chandra A. , Senapati S. , Sudha D.
Published: July 2002

Dynamics of polarization relaxation at the solid–liquid interface is investigated theoretically by using time dependent density functional theory. The time dependence of the building up of solvent polarization next to a newly charged solid surface is calculated by solving both Markovian and non-Markovian equations of motion for density and momentum variables. The initial inhomogeneity of the medium is taken into account and it is calculated by using a weighted density functional theory. Explicit numerical results are obtained for the relaxation of solvent polarization at varying distances from the solid surface and the dynamics of interfacial relaxation is compared with that of the bulk. A slowing down of the rate of relaxation is found for interfacial molecules. However, the relaxation rate changes nonmonotonically with distance from the surface which can be attributed to the layering of solvent molecules in the interfacial region.
Electronic structure of large systems: Coping with small gaps using the energy renormalization group method
Authors: Baer Roi , Head-Gordon Martin
Published: July 2002

A newly developed energy renormalization-group method for electronic structure of large systems with small Fermi gaps within a tight-binding framework is presented in detail. A telescopic series of nested Hilbert spaces is constructed, having exponentially decreasing dimensions and electrons, for which the Hamiltonian matrices have exponentially converging energy ranges focusing to the Fermi level and in which the contribution to the density matrix is a sparse contribution. The computational effort scales near linearly with system size even when the density matrix is highly nonlocal. This is illustrated by calculations on a model metal, a small radius carbon-nanotube and a two-dimensional puckered sheet polysilane semiconductor.
Theoretical study of CH4 photodissociation on Pd and Ni(111) surfaces
Authors: Akinaga Yoshinobu , Taketsugu Tetsuya , Hirao Kimihiko
Published: July 2002

Photofragmentations of a methane molecule adsorbed on Pd and Ni(111) surfaces have been studied by means of density functional theory (DFT) and ab initio molecular orbital calculations. The metal surfaces were represented approximately by finite metal clusters Mn (n=1, 7, 10). The CH4−3s Rydberg excited state is found to be stabilized by about 2.0 and 1.5 eV through the physisorption on Pd and Ni metal surfaces, respectively. This stabilization can be understood as the results of the electron transfer from adsorbates to metal surfaces through an overlap between the CH4 Rydberg orbital and the metal s orbital. Potential energies of the ground and several excited states for the H3C⋯HMn system as functions of the C⋯H distance suggest that the charge transfer states lead to the fragmentation of CH4 to CH3 and H. The CH4 photodissociation for Pd and Ni(111) surfaces occurs through a direct excitation and the mechanism is basically the same as what we found for the CH4/Pt(111) system [J. Chem. Phys. 107, 415 (1997)].
New statistical mechanical treatment of systems near surfaces. IV. Surface and surface-induced capillary transitions in polymer solutions
Authors: Chhajer Mukesh , Gujrati P. D.
Published: July 2002

We apply a recently developed analytic but approximate method to study surface and surface-induced capillary transitions in a solution of polydisperse linear and/or branched polymers confined between two infinite but identical surfaces. We use an equilibrium polymerization model where various densities in the system are controlled by the corresponding activities. The bulk region, i.e., the central region between the surfaces may or may not be in the bulk equilibrium state (see the text). We find a line of first-order transitions, commonly known as a prewetting transition line, passing through the phase separation point in the bulk equilibrium state, at which the surface undergoes a first-order transition. For a certain range around the bulk equilibrium transition point, the bulk region undergoes a stable-metastable transition. This presents us with the possibility of being able to prepare the system in a bulk metastable state, no matter how far apart the two surfaces are. This range is found to be identical to the range of metastability for the bulk equilibrium state. Outside of this range, the transition line behaves like a prewetting transition line and, as usual, terminates at surface critical points, one on each side of the above range.
Inelastic neutron scattering of large molecular systems: The case of the original benzylic amide [2]catenane
Authors: Caciuffo Roberto , Esposti Alessandra Degli , Deleuze Michael S. , Leigh David A. , Murphy Aden , Paci Barbara , Parker Stewart F. , Zerbetto Francesco
Published: July 2002

The inelastic neutron scattering (INS) spectrum of the original benzylic amide [2]catenane is recorded and simulated by a semiempirical quantum chemical procedure coupled with the most comprehensive approach available to date, the CLIMAX program. The successful simulation of the spectrum indicates that the modified neglect of differential overlap (MNDO) model can reproduce the intramolecular vibrations of a molecular system as large as a catenane (136 atoms). Because of the computational costs involved and some numerical instabilities, a less expensive approach is attempted which involves the molecular mechanics-based calculation of the INS response in terms of the most basic formulation for the scattering activity. The encouraging results obtained validate the less computationally intensive procedure and allow its extension to the calculation of the INS spectrum for a second, theoretical, co-conformer, which, although structurally and energetically reasonable, is not, in fact, found in the solid state. The second structure was produced by a Monte Carlo simulated annealing method run in the conformational space (a procedure that would have been prohibitively expensive at the semiempirical level) and is characterized by a higher degree of intramolecular hydrogen bonding than the x-ray structure. The two alternative structures yield different simulated spectra, only one of which, the authentic one, is compatible with the experimental data. Comparison of the two simulated and experimental spectra affords the identification of an inelastic neutron scattering spectral signature of the correct hydrogen bonding motif in the region slightly above 700 cm−1. The study illustrates that combinations of simulated INS data and experimental results can be successfully used to discriminate between different proposed structures or possible hydrogen bonding motifs in large functional molecular systems.
Vibrational second hyperpolarizability of CH4−nFn molecules with n=0–4
Authors: Quinet Olivier , Champagne Benoı̂t
Published: July 2002

The frequency-dependent vibrational second hyperpolarizability of CH4−nFn molecules with n=0–4 has been computed for the most common nonlinear optical (NLO) processes by adopting the perturbation approach due to Bishop and Kirtman [J. Chem. Phys. 95, 2646 (1991)]. These calculations have been performed by using the Sadlej atomic basis set with the Hartree-Fock technique as well as with the Mo/ller-Plesset second order perturbation theory (MP2) procedure. The inclusion of electron correlation and of the first-order mechanical and electrical anharmonicities turn out to be of quantitative importance for most quantities. In particular, it permits us to improve the agreement with the experimental data for the difference between the anisotropic dc-Kerr and mean electric-field-induced second harmonic generation (ESHG) vibrational second hyperpolarizability of CF4. With the exception of the small ESHG vibrational second hyperpolarizability the infinite optical frequency method turns out to be a satisfactory approximation for evaluating the vibrational NLO responses.
Ultrafast multiphoton ionization dynamics and control of NaK molecules
Authors: Davidsson Jan , Hansson Tony , Mukhtar Emad
Published: July 2002

The multiphoton ionization dynamics of NaK molecules is investigated experimentally using one-color pump–probe femtosecond spectroscopy at 795 nm and intermediate laser field strengths (about 10 GW/cm2). Both NaK+ and Na+ ions are detected as a function of pulse separation time, pulse intensities, and strong pulse–weak pulse order. To aid in the analysis, the potential energy curves of the two lowest electronic states of NaK+ and the electronic transition dipole moment between them are calculated by the GAUSSIAN94 UCIS method. Different ionization pathways are identified by Franck-Condon analysis, and vibrational dynamics in the A 1Σ+ and 3 1Π states, as well as in the ground state, is observed. Further, the existence of a highly excited (above the adiabatic ionization limit) neutral state of NaK is proposed. By changing the strong pulse–weak pulse order of the pulses, the ionization pathways for production of both ions can be varied and thus controlled.
α and β process in the glass forming electrolyte solution LiCl–6H2O
Authors: Maurin P. O.
Published: July 2002

α and β relaxation time of glass forming solution LiCl–6H2O have been measured in three states; liquid, supercooled liquid, and glass. The β process presents an original behavior which is analyzed. By using the Adam Gibbs and the Coupling Rearranging Region theory, we try to describe the α process. The predictive model of Perez based on these two theories give excellent results and a coherent interpretation of the observed phenomenon.
Formation of subsurface oxygen at Ru(0001)
Authors: Böttcher Artur , Niehus Horst
Published: July 2002

The conditions for formation of subsurface oxygen on the Ru(0001) surface have been studied using thermal desorption spectroscopy, low energy electron diffraction (LEED) and specular helium scattering. The incorporation of oxygen has been performed via dissociative chemisorption of three molecular carriers of atomic oxygen: NO2, O2 and N2O. The rates for oxygen dissolution can be related to the initial dissociative sticking probability of the molecules on the bare Ru surface. For sample temperatures below 800 K, oxygen penetration into the subsurface region starts only when oxygen molecules impinge on the saturated adsorbed layer characterized by the 1×1 O LEED pattern, indicative of 1 monolayer. A thermally induced transformation of this chemisorbed 1×1 O phase into subsurface oxygen could not be caused even at temperatures close to the onset of oxygen desorption. Oxygen incorporation into the subsurface region by passing through the 1×1 O adsorbed layer, however, shows Arrhenius-type behavior. For impinging O2 molecules, the onset of subsurface oxygen formation appears at 550 K and the entire process is characterized by a rather low activation energy of about 0.5 eV. Deposition of alkali metals on the Ru(0001) surface does not enhance the probability for oxygen dissolution. The resulting oxygen content is substantially reduced and this effect strongly depends on the coverage of alkali–metal atoms. In contrast structural defects on the Ru surface, as generated by Ar+ sputtering and characterized by specular He scattering, act as promoters for oxygen accommodation. The onset for oxygen penetration on a rough surface already begins at about 350 K and the resulting oxygen content can be directly related to the surface roughness.
Experimental and theoretical studies on the permeation of argon through matrices of acrylic polymers containing 1,3-dioxane groups in their structure
Authors: Laguna Mari-Fe , Guzmán Julio , Saiz Enrique , Riande Evaristo
Published: July 2002

The permeation of argon gas through membranes of poly(cis/trans 2-phenyl-5-ethyl-5acryloxymethyl-1,3-dioxacyclohexane) (PAEDP) has been measured in the vicinity of the glass transition temperature of this polymer (∼Tg≈50 °C). Both the permeation and the diffusion coefficients show only a slight dependence on temperature while the membrane remains in glassy state, but exhibit a sharp increase with temperature in the rubbery state. Theoretical calculations of the diffusion coefficient were performed according to the transition-state approach, i.e., assuming that the diffusant path is independent of the structural relaxation in the polymeric matrix, as a function of the smearing factor Δ and temperature. Reasonably good agreement among theoretical and experimental values of the diffusion coefficient was obtained. Theoretical calculations were also performed for poly(cis/trans 2-phenyl-5-ethyl-5-methacryloxymethyl-1,3-dioxacyclohexane) (PMAEDP), the methacrylate analog of PAEDP, which indicate that the diffusion coefficient of glassy PMAEDP is lower than that of glassy PAEDP when the same temperature is taken as the basis of comparison, due to the higher values of Tg in methacrylate than in acrylate polymers which, in turn is a consequence of the rigidity conferred to the polymeric chain by the methyl group.
Response to “Comment on ‘Dynamics of solvated ion in polar liquids: An interaction-site-model description’ ” [J. Chem. Phys. 110, 1833 (1999)]
Authors: Hirata Fumio , Chong Song-Ho
Published: July 2002

We reply to the comments by Biswas and Bagchi on our paper concerning dynamics of solvated ion in polar liquids. Some of the disagreement between us is merely due to the ambiguity in definitions traditionally used in the field, and to the difference in our attitude in science. For those, we have made the definition and our stand point clear. We also clarify the relation between our microscopic results and the macroscopic expression for the Stokes friction.