bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared spectrum of the water–carbon monoxide complex in the CO stretching region
Authors: Brookes Matthew D. , McKellar A. R. W.
Published: July 2002

The CO-stretching vibration–rotation spectra of water–carbon monoxide complexes (H2O–CO, D2O–CO, and HDO–CO) in the 2150 cm−1 region have been studied in a supersonic slit jet expansion using a rapid-scan tunable diode laser spectrometer. Accurate band origins and rotational parameters were determined in order to characterize the CO stretching dependence of the bonding in this system. The symmetric isotopes, H2O–CO and D2O–CO, exhibited two distinct nuclear spin components arising from proton- (deuteron-) exchange tunneling, as observed previously in the microwave spectrum. The tunneling splittings were found to increase by about 10% upon excitation of the CO stretching vibration, indicating a lowering of the tunneling barrier in the excited state. The D-bonded isomer, HOD–CO, of the unsymmetrical isotope was observed prominently, and the higher-energy H-bonded isomer, DOH–CO, was also detected for the first time. From the intensities of their spectra, the energy difference between these two forms of the complex was estimated to be 12.4±2.5 cm−1.
Photoabsorption and photoionization of propyne
Authors: Ho Grace H. , Lin Ming S. , Wang Yuan L. , Chang Ten W.
Published: July 2002

This work measured the absolute photoabsorption cross section of propyne (methylacetylene, CH3CCH) in the 6–30 eV energy range. The valence-shell oscillator strength of 16.97 and the static dipole polarizability of 35.82 atomic units (5.31 Å3) were also derived from the photoabsorption spectrum by the sum-rule method. In the photoionization measurement, the first ionization energy 10.369±0.007 eV of propyne and appearance energies for ion fragments were determined. In addition, new vibrational ν9 and ν10 progressions associated with the X 2E ion state of propyne and Rydberg transitions converged to the A 2E ion state were identified. In addition to the main ν3 progression with the vibrational interval of 1979±39 cm−1, the ν9 and ν10 vibrational structures of the X 2E ion state were resolved with vibrational frequencies of 586±74 cm−1 and 289±65 cm−1, respectively. Moreover, the absolute branching ratios, photoionization quantum yield, and partial photoionization cross sections for C3H4+, C3H3+, C3H2+, and C3H+ production in the 10.3–21.4 eV region were determined. Four shape resonance states were tentatively assigned at 11.2, 12.7, 14.8 and 15.8, and 16.7 eV. Furthermore, a CD3CCH isotopic measurement was taken to examine the reaction pathways of H-dissociative photoionization of propyne. According to the evolution of the C3D3+/C3D2H+ ratio, plausible decay mechanisms for H-dissociative photoionization of propyne were proposed.
Long-range, collision-induced dipoles of Td–D∞h molecule pairs: Theory and numerical results for CH4 or CF4 interacting with H2, N2, CO2, or CS2
Authors: Li X. , Champagne M. H. , Hunt K. L. C.
Published: July 2002

Compressed gases and liquids containing molecules of Td and D∞h symmetry absorb far-infrared radiation, due to transient dipole moments induced during molecular collisions. In earlier theoretical work on far-infrared absorption by CH4/N2 mixtures, good agreement was obtained between calculated and experimental spectra at low frequencies, but at higher frequencies—from 250 to 650 cm−1—calculated absorption intensities fell significantly below the experimental values. In this work, we focus on an accurate determination of the long-range, collision-induced dipoles of Td⋯D∞h pairs, including two polarization mechanisms not treated in the earlier line shape analysis: dispersion and nonuniformity in the local field gradient acting on the Td molecule. Since these mechanisms produce transitions with ΔJ=±3 or ±4 for CH4 and ΔJ=0 or ±2 for N2, their inclusion is expected to increase the calculated absorption intensities in the high frequency wings for CH4/N2 mixtures. This should improve agreement with the experimental spectra, and permit more accurate determination of anisotropic overlap terms in the collision-induced dipole. We give numerical values for the long-range dipole coefficients of CH4 or CF4 interacting with H2, N2, CO2, or CS2; the dipole coefficients have been derived with spherical-tensor methods and evaluated using single-molecule moments and susceptibilities from recent ab initio calculations or experiments. The dispersion dipoles are given rigorously in terms of integrals involving the imaginary-frequency polarizability α(iω) and the hyperpolarizabilities β(0;iω,−iω) and B(0;iω,−iω). To obtain numerical estimates for the dispersion dipoles, we have developed constant-ratio approximations that require only the static susceptibilities and C6 van der Waals coefficients.
Ultraviolet spectroscopy of water clusters: Excited electronic states and absorption line shapes of (H2O)n, n=2–6
Authors: Harvey Jeremy N. , Jung Joon O. , Gerber R. Benny
Published: July 2002

A semiempirical model is developed, based on ab initio calculations, to provide an analytic representation of excited-state potential energy surfaces for (H2O)n, n=2–6. Using quantum calculations of the ground vibrational states, the UV absorption spectra are computed by a semiclassical approximation, showing a strong blue-shift with extended blue tails relative to the monomer, but with an additional red tail in the case of the dimer absorption band. The nature of the excitonic states is discussed.
On the B←X transitions of expansion cooled silver halides
Authors: Stueber Guido J. , Foltin Martin , Bernstein Elliot R.
Published: July 2002

Fluorescence excitation and mass-resolved excitation spectroscopy of jet-cooled silver halide (AgX) bare molecules are explored for the B←X systems of AgCl, AgBr, and AgI. These spectra are compared to those from static gas measurements. The continuous-wave expansion molecular beam of Ar–He with AgX species is generated at a temperature of 1100 °C. For AgI two types of progressions are shown to appear in 31 000–33 000 cm−1 range. The first set of transitions is attributed to the B state and is quite similar to the other AgX molecule’s B←X transitions. The second set has been previously observed for high-temperature AgI vapor as a partly resolved series of features on a broad absorption. The nature of this state and whether it arises from a level crossing between the B surface and an unobserved state is not clear. We analyze two different possible explanations for the newly resolved B′ progression: The B and B′ progressions involve two different excited electronic states; and both progressions belong to one excited electronic state with a complicated single adiabatic potential surface. Computation of vibrational harmonic and anharmonic constants suggests that the potential shape of the B/B′ state is substantially different from that of a common anharmonic oscillator. The nature of the B/B′ surface must be resolved through detailed and accurate ab initio calculations. Ag ions produced by 1+2 photoionization of AgX at longer wavelengths are not fragmentation products of AgX ions, but rather arise from the dissociation of a neutral AgX in a highly excited state. This intermediate state at ∼62 000 cm−1 is accessed from the B state by absorption of a photon of approximately the same energy as required for the B←X AgX transition.
Density matrix renormalization group study of dimerization of the Pariser–Parr–Pople model of polyacetilene
Authors: Bendazzoli G. L. , Evangelisti S. , Fano G. , Ortolani F. , Ziosi L.
Published: July 2002

We apply the density matrix renormalization group method to the Pariser–Parr–Pople Hamiltonian and investigate the onset of dimerization. We deduce the parameters of the hopping term and the contribution of the σ bonds from ab initio calculations on ethylene. Denoting by Rij the C–C distances, we perform a variational optimization of the dimerization δ=(Ri,i+1−Ri−1,i)/2 and of the average bond length R0 for chains up to N=50 sites. The critical value of N at which the transition occurs is found to be between N=14 and N=18 for the present model. The asymptotic values for large N for R0 and δ are given by 1.408(3) Å and 0.036(0) Å.
Fokker-Planck equation and the grand molecular friction tensor for coupled rotational and translational motions of structured Brownian particles near structured surfaces
Authors: Peters Michael H.
Published: July 2002

The Fokker-Planck equation and molecular-based grand friction tensor are derived for the problem of rotational and translational motions of a structured Brownian particle, including the presence of a structured wall. Using the method of multiple time scales, it is shown that the time independent, grand friction tensor for the Brownian particle includes, in a general way, terms that account for the fluid molecular mediated interactions with the wall. The resulting Fokker-Planck equation has important applications in processes involving site-specific adsorption of macromolecules, such as affinity chromotography, biological separations, and numerous cellular processes involving attachment of macromolecules.
Observation of chloride-ion hydration in high-temperature liquid and supercritical water by spherical harmonic expansion analysis
Authors: Yamaguchi T. , Soper A. K.
Published: July 2002

The structure of the Cl− hydration shell in high-temperature (77 and 197 °C) and supercritical (375 °C) water at moderate densities (roughly twice the critical density of 0.356 g/cm3) as well as in water at an ambient temperature, is investigated by means of neutron diffraction with isotope substitution on Cl− in aqueous 3 and 9 m solutions of LiCl in D2O. These relatively high densities (compared to the critical density) are used here to maintain the salt solubility at a sufficiently high value that a measurable signal is obtained in the isotope difference. The data are subjected to an empirical potential structure refinement which enables individual site-site radial distribution functions to be estimated. The same structure refinement is also used to estimate the spherical harmonic coefficients of the Cl-water orientational pair correlation function, and these are used in turn to reconstruct the local environment around Cl− in solution. A marked disordering of the Cl− hydration shell is observed as the critical point is approached. The orientational correlation functions of the water-molecule dipole vectors around Cl− in ambient water show a strong peak at a distance of about 3.2 Å, and centered at an angle of about 126°±30°, with respect to the Cl-O(D2O) axis, confirming an almost linear Cl⋯D-O hydrogen bond but with a significant spread of orientations about this value. At the higher-temperatures the peak remains at about the same position, but becomes progressively weaker in amplitude and with a larger spread of angles, suggesting a gradual decrease in orientational correlation of the water molecules around Cl−. At the supercritical temperature, the peak is even weaker, and there is evidence for a broader hydration shell developing at a distance of r≈3.9 Å in which the water molecules are apparently randomly oriented: they apparently become more loosely packed than at lower temperatures, without being dispelled from the ion hydration shell.
Physical and mathematical content of coupled–cluster equations: Correspondence between coupled–cluster and configuration–interaction solutions
Authors: Jankowski K. , Kowalski K.
Published: July 2002

To gain more insight into the physical and mathematical content of the equations of the coupled–cluster (CC) method, comprehensive numerical studies have been performed for various geometries of the H4 model which belongs to the simplest and best understood among the realistic many–electron model systems. These studies are for the first time based on the knowledge of the complete sets of geometrically isolated solutions of the relevant equations that are obtained when using a special version of the homotopy methods. The equations of the CC method including two–electron excitations (CCD) both in the spin–orbital and spin–symmetry–adapted versions are considered. To establish the correspondence of the solutions attained with those of the configuration interaction (CID) method, we have for the first time solved the unabridged characteristic equations (CE) of T. P. Živković and H. J. Monkhorst [J. Math. Phys. 19, 1007 (1978)]. The complete sets of solutions to the spin–orbital and spin–symmetry–adapted versions of the CCD equations and CEs consist of 20 and 12 solutions, respectively. Their structures turned out to be unexpectedly simple, which makes it possible to understand the physical and mathematical significance of the individual solutions. Moreover, this simplicity rises hopes that the most serious irregularities previously found for low dimensional simulations of the CEs can be avoided for realistic systems. The present results confirm most of the findings of previous research. Attention is paid to studies of the impact of using mixed–symmetry cluster operators on the structure of solutions to the CC equations. The specific part played by the components of the cluster amplitudes corresponding to other spin–symmetry than that of the reference function has been demonstrated for the first time.
Fully converged integral cross sections of diatom-diatom reactions and the accuracy of the centrifugal sudden approximation in the H2+OH reaction
Authors: Zhang Dong H. , Lee Soo-Y.
Published: July 2002

The initial state selected time-dependent wave packet method has been extended to calculate integral cross sections for diatom-diatom chemical reactions without the CS (centrifugal sudden) approximation by including all important K (the projection of the total angular momentum on the body-fixed axis) blocks in the body-fixed frame. We report the first fully converged cross section for the ground rovibrational state of the title reaction and present a detail study of the accuracy of the CS approximation to the reaction. We find that for the ground rovibrational state the CS approximation works very well, but its accuracy deteriorates with increasing reagent rotational excitation. As expected, and as found in atom-diatom reactions, the CS approximation works much better in high energy region than in low energy region. In low energy region, the coupled channel cross sections are larger than the CS ones for all the rotationally excited states investigated here, in particular for the highly excited states. It is found the CS approximation gives rise to about 10% error in H2 or OH rotationally averaged rate constant. If simultaneous OH and H2 rotational excitation does not have a correlated effect on dynamics, the CS approximation introduces about 19% error in thermal rate constant for the reaction for low temperatures which is considerably larger than what is expected of a few percent.
Adsorption of bi-isonicotinic acid on rutile TiO2(110)
Authors: Patthey L. , Rensmo H. , Persson P. , Westermark K. , Vayssieres L. , Stashans A. , Petersson Å. , Brühwiler P. A. , Siegbahn H. , Lunell S. , Mårtensson N.
Published: July 2002

Bi-isonicotinic acid (2,2′-bipyridine–4,4′-dicarboxylic acid) is the ligand of several organometallic dyes, used in photoelectrochemical applications. Therefore the atomic scale understanding of the bonding of this molecule to rutile TiO2(110) should give insight into the crucial dye–surface interaction. High resolution x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), and periodic intermediate neglect of differential overlap (INDO) calculations were carried out on submonolayer bi-isonicotinic acid rutile TiO2(110). Data from multilayers is also presented to support the submonolayer results. For a multilayer, XPS shows that the carboxyl groups remain in the (pristine) protonated form, and NEXAFS show that the molecular plane is tilted by 57° with respect to the surface normal. For the submonolayer, the molecule bonds to the rutile TiO2(110) surface via both deprotonated carboxyl groups, with a tilt angle of 25°, and additionally an azimuthal orientation of 44° with respect to the [001] crystallographic direction. The adsorbant system was also investigated by quantum mechanical calculations using a periodic INDO model. The most stable theoretical adsorption geometry involves a twist around the molecular axis, such that the pyridine rings are tilted in opposite directions. Both oxygen atoms of each carboxyl group are bonded to five-fold coordinated Ti atoms (2M-bidentate), in excellent agreement with the experimental results.
Wetting of a chemically heterogeneous surface
Authors: Frink Laura J. Douglas , Salinger Andrew G.
Published: July 2002

Theories for inhomogeneous fluids have focused in recent years on wetting, capillary condensation, and solvation forces for model systems where the surface(s) is(are) smooth homogeneous parallel plates, cylinders, or spherical drops. Unfortunately natural systems are more likely to be heterogeneous both in surface shape and surface chemistry. In this paper we discuss the consequences of chemical heterogeneity on wetting. Specifically, a two-dimensional (2D) implementation of a nonlocal density-functional theory is solved for a striped surface model. Both the strength and range of the heterogeneity are varied. Contact angles are calculated, and phase transitions (both the wetting transition and a local layering transition) are located. The wetting properties of the surface are shown to be strongly dependent on the nature of the surface heterogeneity. In addition highly ordered nanoscopic phases are found, and the operational limits for formation of ordered or crystalline phases of nanoscopic extent are discussed.
Concentration dependence of the band profile parameters for the ν3(12C=O) Raman band of acetone in acetone–CCl4 binary mixtures: Experimental and Monte Carlo simulation results and their interpretation
Authors: Musso M. , Torii H. , Giorgini M. G. , Döge G.
Published: July 2002

The concentration dependence of specific band profile parameters, i.e., bandwidth and asymmetry, of the isotropic and anisotropic components of the ν3(12C=O) Raman band of acetone is investigated in the acetone–CCl4 binary mixtures. Emphasis is mainly placed upon the comparison of the results obtained from Monte Carlo simulations with the experimental results obtained by Raman spectroscopy. The results on the concentration dependence of the bandwidth are interpreted within the lineshape theory of Bratos and Tarjus, while the results on the concentration dependence of the band asymmetry are examined with the help of the theoretical predictions given by Knapp and by Knapp and Fischer. We have found a good qualitative agreement between all the results.
Physical and mathematical content of coupled-cluster equations. II. On the origin of irregular solutions and their elimination via symmetry adaptation
Authors: Jankowski K. , Kowalski K.
Published: July 2002

To establish the existence and origin of the nonalgebraic irregularities of solutions to coupled-cluster (CC) equations and to indicate ways of their elimination, we have revisited the two analytically solvable characteristic equations (CE) studied by Živković and Monkhorst [J. Math. Phys. 19, 1007 (1978)]. The results of these studies have strongly influenced the general conclusions concerning the possible types of singularities. We present some arguments that the most serious irregularities—the nonnormal and resonance ones—are a result of the special structures of the CEs considered. The CE employed for the demonstration of resonance solutions is not physically representable, which raises the hope that such solutions will not appear in quantum-chemical applications of the coupled-cluster method. It is proved that the presence of nonnormal solutions is a consequence of the existence of such passive diagonal blocks of the Hamiltonian matrix which share a common eigenvalue. Such blocks can be eliminated by taking into account the symmetry species of the basis functions involved, which is most effectively done by proceeding to a symmetry-adapted formulations. Therefore, one may eliminate or at least reduce the number of nonnormal solutions to the CC equations by proceeding to their symmetry-adapted versions.
Fracture of polymers
Authors: Persson B. N. J.
Published: July 2002

I present a model study of brittle fracture of polymers. I show that the relation v=v(F) between the crack-tip velocity v and the driving force F exhibit discontinuous transitions and hysteresis. For short polymers, at the onset of crack propagation the polymer chains separate by pulling out the molecular chains, while a discontinuous transition to a new state of rapid crack motion occurs at higher driving force, where the polymer chains break. In contrast to earlier atomistic studies of crack motion involving brittle materials such as silicon, for short polymers we find (at zero temperature) that the onset of crack motion is continuous, i.e., the crack-tip velocity v increases continuously from zero when F is increased above the critical value Fc necessary for starting crack motion. The difference is attributed to inertia, which is less important during pull-out of the molecular chains, than in models where the crack motion involves breaking strong short-ranged bonds. I also present a detailed study of the polymer chain pull-out process.
Exact solution for the singlet density distributions and second-order correlations of normal-mode coordinates for hard rods in one dimension
Authors: Barnes C. Daniel , Kofke David A.
Published: July 2002

We examine the distribution of normal-mode coordinates (defined via the eigenvectors of a chain of harmonic oscillators) for a system of purely repulsive hard rods in one dimension. We obtain an exact solution for the singlet density distribution, and separately for the covariances of the normal-mode coordinates. The hard-rod behavior is examined in terms of its deviation from the corresponding distributions for the system of harmonic oscillators. All off-diagonal covariances are zero in the hard-rod system, and the (on-diagonal) variances vary with the normal-mode wave number exactly as in the harmonic system. The detailed singlet normal-mode density distributions are very smooth but nonanalytic, and they differ from the (Gaussian) distributions of the corresponding harmonic system. However, all of the normal-mode coordinate distributions differ in roughly the same way when properly scaled by the distribution variance, and the differences vanish as 1/N in the thermodynamic limit of an infinite number of particles N.
Brownian dynamics of bead–rod–nugget–spring polymer chains with hydrodynamic interactions
Authors: Klaveness Elise , Elgsaeter Arnljot
Published: July 2002

With Brownian dynamics simulations of solutions containing complex biological macromolecules in mind we have formulated a polymer chain model consisting of a selectable sequence of spherical subunits (beads) and nonspherical subunits of arbitrary shapes (nuggets) connected by rigid rods, rigid ball–socket joints, or springs. Many physical properties of both subunits and connections are selectable. First a very general diffusion equation of polymer kinetic theory is employed. Next the equivalent Itô stochastic differential equation of motion is established and finally an effective Brownian dynamics simulation algorithm is presented. In all these steps it is assumed that the numerical values of the subunit steady state mobility tensor are obtainable. The important and previously unanswered question of how to include translational–rotational hydrodynamic interactions in Brownian dynamics simulations of polymer chains containing nonspherical subunits linked by rigid constraints is resolved rigorously here. This means that during the formal derivation of the presented Brownian dynamics simulation algorithm all subunit hydrodynamic interactions of the proposed bead–rod–nugget–spring polymer chain model have in principle been taken fully into account. In addition the model includes rigid constraints to fixed points in the laboratory coordinate system, solvent flow, external forces, excluded volume effects, and bending and torsional stiffness between polymer chain subunits. Bead–spring, bead–rod–spring, needle–spring, needle polymer chains and liquid crystals are all special cases of the bead–rod–nugget–spring polymer chain model. The validity of the algorithm presented is limited to the diffusion time domain.
Shrinking pattern and phase transition velocity of poly(
N
-isopropylacrylamide) gel
Authors: Suzuki A. , Yoshikawa S. , Bai G.
Published: July 2002

We have studied the shrinking phase transition of cylindrical poly(N-isopropylacrylamide) gels with submillimeter diameter. The macroscopic conformation change and the phase transition velocity were obtained during the heating process by two different methods. One is a continuous heating process with a constant temperature drift rate, and the other is an isothermal process after a steplike temperature increase beyond the transition point. In the former measurement, the phase transition can be controlled by the nucleation mechanism in the smaller temperature drift rates; at the transition point, after the fine pattern appears and disappears on the surface, for instance, the gel gradually and uniformly shrinks while keeping a smooth surface. On the other hand, at the larger temperature drift rates, the phase transition comes into the unstable region before being completed; after the fine pattern disappears, a coarse pattern appears on the surface, and the entire gel becomes opaque. The gel gradually becomes transparent with time from the surface layer to the core portion. These two processes, characterized by two types of surface pattern as well as the growth of a collapsed surface skin layer, can be clearly observed in the latter measurements, which depend on the degree of super-heating (how far the final temperature is from the transition point). The results are discussed qualitatively on the basis of the classical phase separation model of nucleation and spinodal decomposition, as well as the phase diagram of the present gel system.
Size-extensive calculations of static structure factors from the coupled cluster singles and doubles model
Authors: Watanabe Noboru , Ten-no Seiichiro , Pal Sourav , Iwata Suehiro , Udagawa Yasuo
Published: July 2002

The x-ray incoherent scattering factor S(q), which is also called the static structure factor, is very sensitive to electron correlation. In this study a method for calculating S(q) based on coupled cluster singles and doubles (CCSD) approach is developed and the computed S(q) of H2O, CH3OH, CH3CN, C6H6, and C6H12 are compared with experimental results. It is shown that the CCSD method improves theoretical S(q) of large molecules significantly compared with those by configuration interaction singles and doubles (CISD) previously employed.
A comparison of hydrogen-bonded and van der Waals isomers of phenol⋅⋅nitrogen and phenol⋅⋅carbon monoxide: An ab initio study
Authors: Chapman Darren M. , Müller-Dethlefs Klaus , Peel J. Barrie
Published: July 2002

The hydrogen-bonded and van der Waals isomers of phenol⋅⋅nitrogen and phenol⋅⋅carbon monoxide in their neutral electronic (S0) and cation ground state (D0) were studied using ab initio HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* methods. The hydrogen-bonded isomers have the ligand bound via the hydroxyl group of the phenol ring, while the van der Waals isomers studied have the ligand located above the aromatic ring. For both complexes, the hydrogen-bonded isomer was found to be the most stable form for both the S0 and the D0 states. For phenol⋅⋅carbon monoxide, twice as many isomers as compared to phenol⋅⋅nitrogen were found. The hydrogen-bonded isomer with the carbon end bonded to the hydroxyl group was the most stable structure for both the S0 and the D0 states.
Associating polyelectrolytes: Finite size cluster stabilization versus physical gel formation
Authors: Potemkin I. I. , Vasilevskaya V. V. , Khokhlov A. R.
Published: July 2002

The theory of phase equilibria in the solutions of associating polyelectrolyte telechelic chains proposed earlier is generalized to take into account the possibility of existence in the solutions of finite size thermodynamically stable clusters in addition to the formation of macroscopic physical gel. It is shown that in some region of polymer concentrations and energies of attraction between the end groups of the chain (stickers) the size distribution of chain clusters has a maximum, i.e., the solution is dominated by the clusters of optimum size. The stabilization of such a cluster is due to the interplay between the energy of attraction of stickers, on the one hand, and the Coulomb interaction together with the contribution from the translational entropy of counter ions, on the other hand. For small enough optimum clusters direct Coulomb interactions are more important, while for large optimum clusters the main reason for stabilization is connected with the translational entropy. This type of stabilization mechanism should be quite general for any polyelectrolyte system with attraction.
Analytical rebridging Monte Carlo: Application to cis/trans isomerization in proline-containing, cyclic peptides
Authors: Wu Minghong G. , Deem Michael W.
Published: July 2002

We present a new method, the analytical rebridging scheme, for Monte Carlo simulation of proline-containing, cyclic peptides. The cis/trans isomerization is accommodated by allowing for two states of the amide bond. We apply our method to five peptides that have previously been characterized by nuclear magnetic resonance methods. Our simulations achieve effective equilibration and agree well with experimental data in all cases. We discuss the importance of effective equilibration as well as the role of bond flexibility and solvent effects in the prediction of equilibrium properties.
Role of adsorbed nitrogen in the catalytic reduction of NO on rhodium surfaces
Authors: Zaera Francisco , Gopinath Chinnakonda S.
Published: July 2002

The role of surface nitrogen in the kinetics of the NO+CO conversion reaction on Rh(111) under steady-state catalytic conditions was explored by using collimated molecular beams and mass spectrometry detection. Two types of kinetically different nitrogen atoms were identified on the surface. The buildup of a critical nitrogen coverage was determined to be required for the start of the nitrogen recombination step to N2. This threshold coverage is quite large at low temperatures, amounting to over half a monolayer around 400 K, but decreases abruptly with increasing reaction temperature, and becomes almost insignificant above 600 K. The actual value of this coverage is quite insensitive to the ratio of NO to CO in the reaction mixture, but displays an inverse correlation with the steady-state reaction rate under most conditions. An additional small amount of nitrogen appears to be present on the surface during catalysis but to desorb rapidly after the removal of the gas-phase reactants. The NO reduction rate displays an approximately first-order dependence on the coverage of these labile N atoms. Isotope switching experiments indicated that the two types of kinetically different nitrogens are not likely to represent different adsorption sites, but rather similar adsorption states with adsorption energetics modified by their immediate surrounding environment on the surface. The data are explained here by a model in which the nitrogen atoms form surface islands and where the atoms at the perimeter of those islands react preferentially via N+N recombination to N2.
Application of interpolated potential energy surfaces to quantum reactive scattering
Authors: Collins Michael A. , Zhang Dong H.
Published: July 2002

The accuracy of an interpolation approach to molecular potential energy surfaces for quantum reactive scattering is demonstrated by comparison of the quantum reaction probability for a model surface and its interpolated approximation. Effective convergence of an ab initio surface with the size of the interpolation data set is demonstrated for the reaction BeH+H2 → BeH2+H.
Binding energy, structure, and annihilation properties of the positron-LiH molecule complex, studied with explicitly correlated Gaussian functions
Authors: Strasburger Krzysztof
Published: July 2002

The energy and two-photon annihilation rate of the complex of the LiH molecule with a positron have been calculated for the equilibrium geometry of LiH in the gas phase, using the explicitly correlated Gaussian basis set. The resulting energy of e+LiH (−8.104 850 hartree) is the lowest obtained to date. The binding energy of the positron in this system has been predicted to be at least 34.401 mhartree, and the annihilation rate has been estimated to be 1.26 ns−1. Maps of the electron, positron, and contact (electron–positron) densities have been prepared.
Diffusive and segmental dynamics in polymer gel electrolytes
Authors: Svanberg C. , Adebahr J. , Ericson H. , Börjesson L. , Torell L. M. , Scrosati B.
Published: July 2002

Dynamic light scattering data on a polymer gel electrolyte with a complex relaxation behavior is presented. The electrolyte consists of lithium perchlorate dissolved in an ethylene carbonate and propylene carbonate solution that is immobilized with poly(methyl methacrylate). We attribute the observed relaxation processes to two diffusive and one segmental relaxation processes based on the form of the time decay of the intermediate scattering function and the corresponding temperature and wave vector dependencies. The dynamic light scattering results are compared with the ionic conductivity, which reveals a close connection between the fast diffusive motion of the low molecular weight solvent within the gel and the ionic conductivity. This motion is strongly decoupled from and considerably faster than the segmental motion of the polymer matrix. The results indicate that the ionic transport occurs mainly within the low molecular weight solvent.
The gel that memorizes phases
Authors: Annaka Masahiko , Tokita Masayuki , Tanaka Toyoichi , Tanaka Shimei , Nakahira Takayuki
Published: July 2002

A poly(4-acrylamidosalicylic acid) gel exhibits multiple phases as characterized by distinct degrees of swelling; the gel can take one of four different swelling values, but none of the intermediate values. The multiple phase behavior appears as a result of the combination of hydrogen bonding and hydrophobic interaction between polymer segments. The gel has remarkable memory: The phase behavior of the gel depends on whether the gel has experienced the most swollen phase or the most collapsed phase in the immediate past. The information is stored and reversibly erased in the form of a macroscopic phase transition behavior. These phenomena are explained by a mean field theory where the number of hydrogen bonds is added to the equation of the state of a gel.
The Hartree product and the description of local and global quantities in atomic systems: A study within Kohn–Sham theory
Authors: Garza Jorge , Nichols Jeffrey A. , Dixon David A.
Published: July 2002

The Hartree product is analyzed in the context of Kohn–Sham theory. The differential equations that emerge from this theory are solved with the optimized effective potential using the Krieger, Li, and Iafrate approximation, in order to get a local potential as required by the ordinary Kohn–Sham procedure. Because the diagonal terms of the exact exchange energy are included in Hartree theory, it is self-interaction free and the exchange potential has the proper asymptotic behavior. We have examined the impact of this correct asymptotic behavior on local and global properties using this simple model to approximate the exchange energy. Local quantities, such as the exchange potential and the average local electrostatic potential are used to examine whether the shell structure in an atom is revealed by this theory. Global quantities, such as the highest occupied orbital energy (related to the ionization potential) and the exchange energy are also calculated. These quantities are contrasted with those obtained from calculations with the local density approximation, the generalized gradient approximation, and the self-interaction correction approach proposed by Perdew and Zunger. We conclude that the main characteristics in an atomic system are preserved with the Hartree theory. In particular, the behavior of the exchange potential obtained in this theory is similar to those obtained within other Kohn–Sham approximations.
DNA-linked metal nanosphere materials: Fourier-transform solutions for the optical response
Authors: Lazarides A. A. , Schatz G. C.
Published: July 2002

Methods are developed for modeling the optical properties of aggregates of large numbers of small metal nanospheres in a dielectric medium. Aggregates are modeled as systems of coupled dipoles, with the dipole polarizabilities for the spheres determined using Mie theory. Fast-Fourier-transform (FFT) and conjugate-gradient (CG) techniques are used to solve the electrodynamic equations for both ordered and disordered aggregates. Results are shown to match solutions arrived at by direct methods. The range of validity of the coupled-dipole approximation for modeling DNA-linked colloidal materials is established by comparison with coupled-multipole results. While the methods are applicable only to lattice gas aggregates and aggregates composed of nanospheres on cubic lattices, there are no restrictions as to aggregate shape.
Collisional activation of the endoergic hydrogen atom transfer reaction S−(2P)+H2→SH−+H
Authors: Rempala Katarzyna , Ervin Kent M.
Published: July 2002

Guided ion beam tandem mass spectrometry and ab initio calculations are used to investigate the reactions of S−(2P) with H2 and D2. The hydrogen atom transfer reaction to form SH− or SD− is observed at collision energies above the reaction endoergicity. Coupled-cluster theory calculations indicate that the hydrogen abstraction reaction occurs with no barrier along the C∞v collinear reaction path. The threshold energy yields the bond dissociation energy D0(S−−H)=373.1 ± 4.3 kJ/mol. Using literature values for electron affinities, we derive D0(S–H)=350.0±4.3 kJ/mol, in agreement with previous values. We also obtain gas phase acidities for H2S and SH, ΔacidH298(H2S)=1467.9±4.3 kJ/mol and ΔacidH298(SH)=1465.2±4.3 kJ/mol. The competition between hydrogen atom transfer and the exoergic associative detachment reaction forming SH2+e− is discussed. Calculations reveal that the transition state for insertion leading to associative detachment is a bent asymmetric SHH− species with an energy of 47 kJ/mol relative to S−+H2 reactants.
On the importance of exchange effects in three-body interactions: The lowest quartet state of Na3
Authors: Higgins J. , Hollebeek T. , Reho J. , Ho T.-S. , Lehmann K. K. , Rabitz H. , Scoles G. , Gutowski Maciej
Published: July 2002

Three-body interactions in a homonuclear van der Waals bound trimer (the 1 4A2′ state of Na3) are studied spectroscopically for the first time using laser induced emission spectroscopy on a liquid helium nanodroplet coupled with ab initio calculations. The van der Waals bound, spin polarized sodium trimers are prepared via pickup by, and selective survival in, a beam of helium clusters. Laser excitation from the 1 4A2′ to the 2 4E′ state, followed by dispersion of the fluorescence emission, allows for the resolution of the structure due to the vibrational levels of the lower state and for the gathering of precise information on the three-body interatomic potential. From previous experiments on Na2 we know that the presence of the liquid helium perturbs the spectra by a very small amount [see J. Higgins et al., J. Phys. Chem. 102, 4952 (1998)]. Ab initio potential energy calculations are carried out at 42 geometries of the lowest quartet state using the coupled cluster method at the single, double, and noniterative triple excitations level [CCSD(T)]. The full potential energy surface is obtained from the ab initio points using an interpolation procedure based on a Reproducing Kernel Hilbert Space (RKHS) methodology. This surface is compared to a second, constructed using an analytical model function for both the two-body interaction and the nonadditivity correction. The latter is calculated as the difference between the CCSD(T) points and the sum of the two-body interactions. The bound vibrational states are calculated using the two potential energy surfaces and are compared to the experimentally determined levels. The calculated bound levels are combined with an intensity calculation of the ν2″ mode of E′ symmetry derived from a Jahn–Teller analysis of the excited electronic state. The calculated frequencies of ν1″ and ν2″ are found to be 37.1 cm−1 and 44.7 cm−1, respectively, using the RKHS potential surface while values of 37.1 cm−1 and 40.8 cm−1 are obtained from the analytical potential. These values are found to be in good to fair agreement with those obtained from the emission spectrum and to be significantly different from any values calculated from additive potential energy surfaces. The 1 4A2′ Na3 potential energy surface is characterized by a D3h symmetry minimum of −850 cm−1 (relative to the three 3 2S Na atom dissociation limit) with a bond distance of 4.406 Å. This bond distance differs by about 0.8 Å from the value of 5.2 Å found for the sodium triplet dimer. This means that approximately 80% of the binding energy at the potential minimum is due to three-body effects. This strong nonadditivity is overwhelmingly due to the deformability of the valence electron density of the Na atoms which leads to a significant decrease of the exchange overlap energy in the trimer.
Prereactive evolution of monoalkenes excited in the 6 eV region
Authors: Mestdagh J. M. , Visticot J. P. , Elhanine M. , Soep B.
Published: July 2002

The time evolution of the first excited states of ethylene, and alkyl substituted ethylenes, isomers with formula C6H12, has been studied by the femtosecond pump probe method, using mass spectrometric detection, in the region of 6 eV (200 nm). Two cyclic alkenes of the formula C6H10 have also been studied. These systems exhibit a multi-exponential decay characterized by a very short time decay, ranging from 20 fs(ethylene) to 100 fs (trans hex-2-ene) and a longer decay, in the picosecond range follows for most of the alkyl isomers. The short time evolution is characteristic of wave packet motion on a steep potential surface. The initial motion has been identified as the torsion about the CC double bond resulting from excitation of the valence state. The evolution of the valence excited state of excited state ethylene [first studied by the group of Radloff, Chem. Phys. Lett. 288, 2044 (1997)] has been taken as a reference. The extremely rapid evolution, 20 fs, without any longer temporal component is explained by the disappearance of the wave packet from the Franck–Condon region into a conical intersection leading to the ground state surface by reference to the theoretical calculations of Ohmine [J. Chem. Phys. 83, 2348 (1985)]. This motion is essentially multidimensional to reach the funnel to the ground state; it combines the torsion about the CC double bond with a pyramidalization about one of the carbon atoms and/or H atom migration from one carbon to the other. Cyclic alkenes exhibit a similar behavior as ethylene with a single ultrashort decay that arises from this same mechanism. Also in the other substituted alkenes the short decay has been assigned to the wave packet motion away from the Franck–Condon region under the influence of the torsion about the double bond. The final longer decay could also be captured in the case of tetramethylethylene by a 800 nm probe as the internal conversion to the ground state via a funnel more difficult to reach. These measurements emphasize the role of conical intersections which could not be brought into evidence without time dependent methods.
Direct fifth-order electronically nonresonant Raman scattering from CS2 at room temperature
Authors: Blank David A. , Kaufman Laura J. , Fleming Graham R.
Published: July 2002

In this paper we report the measurement of the direct fifth-order electronically nonresonant intermolecular response from liquid CS2 at room temperature. We recently demonstrated that all previous attempts to measure the direct fifth-order response were dominated by third-order cascaded signals [J. Chem Phys. 111, 3105 (1999)]. Here we show that phase matching considerations can be used to provide substantial discrimination against the cascaded third-order signals, and the direct fifth-order response can be measured. The measured responses indicate that the intermolecular spectrum of liquid CS2 at room temperature is well described as homogenous. Comparisons of the data to simulations are discussed.
The matrix representations of g2. II. Representations in an su(3) basis
Authors: Le Blanc R. , Rowe D. J.
Published: July 2002

Irreducible representations of the real compact Lie algebra g2 are given in g2⊇ su(3) bases. A missing label is accounted for by the explicit construction of a g2⊇ su(3) basis of vector holomorphic functions. Analytical results are given for two multiplicity-free classes of irreps. It is also shown how vector coherent state (VCS) theory accommodates the decomposition of the nilpotent raising operator subalgebra of an arbitrary Lie algebra into a finite but arbitrary number of irreducible tensorial sets under transformations generated by a stability algebra.
Soliton solutions in an integrable chiral model in 2+1 dimensions
Authors: Ward R. S.
Published: July 2002

There is a modified SU(2) chiral model in 2+1 dimensions which is integrable. It admits multisoliton solutions, in which the solitons move at constant velocity, and pass through one another without scattering or changing shape.
Grassmann-valued fluid dynamics
Authors: Fatyga B. W. , Kostelecký V. Alan , Truax D. Rodney
Published: July 2002

Certain systems of nonlinear partial differential equations can be written in a simple form as a single Grassmann-valued partial differential equation. Equations describing compressible fluid flow are of this type. A method for finding soft solutions of the Grassmann-valued partial differential equation arising in this context is presented. The method is a generalization of the Lagrangian-coordinates approach to the case of Grassmann variables. Generally, solutions obtained by this method have the form of infinite series, whose expansion yields new relations among the unknown variables. In some simple cases, the series can be summed. The equivalence of the Grassmann solutions to the usual solutions is shown for these cases.
Avoided level crossings and Riemann sheet structure
Authors: Heiss W. D. , Steeb W.-H.
Published: July 2002

The Riemann sheet structure of the energy levels En(λ) of an N-dimensional symmetric matrix problem of the form H0+λH1 is discussed. It is shown that the singularities of the energy levels in the complex λ plane are related to avoided level crossings. It is argued that locally the sheet structure is like that of a two-, three-, or four-dimensional problem as far as two, three, or four adjacent levels are concerned. Expressions are given for adjacent levels displaying explicitly the Riemann sheet structure on a semiglobal footing.
Star-products and nonstandard quantization for Klein–Gordon equation
Authors: Dito Joseph
Published: July 2002

A study of star-quantization of the Klein–Gordon equation based on Poincaré group is presented. Several examples are discussed and the work is finished with some remarks on perturbation theory induced by a star-product.
Singularities of the one- and two-point mass gravitational lens
Authors: Witt Hans J. , Petters Arlie O.
Published: July 2002

A detailed study of when a change in the number of caustics and cusps occurs for one- and two-point mass gravitational lens lying on a single plane with continuously distributed matter and an external shear are presented herein. The equations for the positions of the cusps generated by such lens systems are investigated in detail. This method is based on a new approach using complex quantities and applies recent results on cusp counting.