bluuebunny/crossref_metadata_2025 huggingface dataset:
Group VI trimers (Se3, Te3, and Po3). Electronic states and potential energy surfaces
Authors: Balasubramanian K. , Dai Dingguo
Published: July 2002

The potential energy surfaces of several electronic states of Se3, Te3, and Po3 are computed. We employed the ab initio complete active space multiconfiguration self-consistent field followed by multireference configuration interaction techniques which included up to 1.7 million configurations. Our computations reveal two nearly degenerate electronic states with D3h(1A′1) and C2v(1A1) symmetries as candidates for the ground states, although the C2v minimum was found to be favored at the highest level of theory. Our computations facilitate the assignment of the observed spectra of Se3 and Te3 in the visible region by Andrews and co-workers to the 1B2– X1A1 systems. Our computed Te values were found to be in excellent agreement with experiment. We computed the dipole moments, dissociation and atomization energies for all three trimers and the vertical ionization energies for the two minima of Se3. The analysis of the periodic trends revealed significant relativistic effects for Po3.
Difference-frequency laser spectroscopy of the 3ν2+ν3 band of C2H
Authors: Yan Wen-Bin , Amano T.
Published: July 2002

A spectrum of C2H 2Π–2Σ+ transition centered at 2928 cm−1 has been measured in the gas phase using a high resolution difference-frequency infrared laser spectrometer. The C2H molecules are produced in a hollow cathode discharge through a mixture of C2H2 (∼20 mTorr) and H2 (∼500 mTorr). The discharge amplitude modulation technique is used to enhance sensitivity and selectivity. About 125 lines are rotationally analyzed and accurate molecular constants are determined through a least-squares fit. The band is assigned to the 3ν2+ν3 combination band. The assignment is consistent with the observed 2Π–2Σ+ symmetry and is in good agreement with the recent theoretical calculation of the vibrational frequency and the spin–orbit coupling constant. The relatively large spin–orbit coupling constant of the (0 3 1) state is indicative of strong vibronic interaction between the Ã and X̃ states. A small perturbation appears near the end of observed Q-branch series, which is probably caused by a Σ vibronic state. New assignments of the progressions (0 v2 0) and (0 v2 1) toward higher v2 are proposed based on the existing gas phase and rare gas matrix data with the help of recent ab initio theoretical predictions.
Tests of nonlocal kinetic energy functionals
Authors: Lacks Daniel J. , Gordon Roy G.
Published: July 2002

The accuracies of various nonlocal kinetic energy functionals are evaluated by considering the kinetic energies of rare gas atoms and the kinetic energy contributions to the interaction energy (ΔEk) for pairs of rare gas atoms. The most accurate kinetic energy functionals are those that combine the density gradient factors of accurate exchange energy functionals with the Thomas–Fermi (TF) kinetic energy functional; these functionals, including a new one proposed in this paper, give total atomic kinetic energies to within 1%, as well as accurate values of ΔEk. Other gradient-corrected functionals give good results for total kinetic energies, but give poor results for ΔEk.
Dynamics in supercooled glycerol by high resolution stimulated Brillouin gain spectroscopy
Authors: Grubbs W. Tandy , MacPhail Richard A.
Published: July 2002

We have used high resolution stimulated Brillouin gain spectroscopy to probe the dynamics of glycerol over the temperature ranges 146 to 305 K and 401 to 534 K, which include both the supercooled liquid and glass regimes. The high resolution and large spectral range of the technique have allowed us to resolve Brillouin peaks at low temperatures (146 K) with widths as narrow as 13 MHz and shifts as large as 17.3 GHz. A comparison of the observed Brillouin shifts and linewidths with predictions based on previous work at lower frequencies indicates that the main dispersion in our data arises from the primary (α) structural relaxation processes. However, this comparison also reveals that additional relaxation processes, perhaps associated with the secondary (β) processes, contribute to the Brillouin linewidths both above and below the glass transition. Our results also show a distinct kink in the temperature dependence of the speed of sound at 187 K, the glass transition temperature.
Brownian relaxation of rotators in a driving field
Authors: Cisl/o J. , Dudek M. R.
Published: July 2002

We describe analytically the relaxation process of Brownian motion of rotators subjected to external field E, which is suddenly switched on at time t=0. The field E is constant, E=E0, or time dependent, E=E0 sin ωt. We make detailed calculations, respectively, for one dipole in the field E and two interacting dipoles in the field E. However, the method involving any finite number of rotators is described. In particular, we derive analytical formulas for the first dipole moment and compare the numerical results with the computer experiment.
Observation of doubly excited Rydberg states of N2O by positive ion–negative ion coincidence spectroscopy
Authors: Yoshida Hiroaki , Mitsuke Koichiro
Published: July 2002

Ion-pair formation processes of N2O are studied in detail using synchrotron radiation in the 24–34 eV photon energy range. Positive ion–negative ion coincidence (PINICO) spectroscopy makes it possible to distinguish dissociation into three bodies, O−+N++N, from dissociation into two bodies, O−+N+2. The photodissociation efficiency curves for the two processes are measured. Several resonance-like peaks in the curves are assigned to doubly excited Rydberg states of N2O from an analysis of kinetic energy release in dissociation.
Optical–optical double resonance polarization spectroscopy of highly excited states of 23Na39K
Authors: Kasahara Shunji , Ikoma Heiji , Katô Hajime
Published: July 2002

The Doppler-free high resolution spectrum of the transitions to highly excited states of the 23Na39K molecule was measured by using the technique of the optical–optical double resonance polarization spectroscopy. Several highly excited states 1Σ+, 1Π, and 1Δ were found, and the molecular constants for these states were determined. The vibrational numbering of the 1Δ state (G 1Δ state) was estimated, and the potential energy curve was calculated by the Rydberg–Klein–Rees method. Many perturbed lines were observed. The indirect perturbation between 1Σ+ and 1Δ levels through the L-uncoupling interaction with a distant 1Π state was identified. The hyperfine splittings were observed in transition lines to a 3Π state, which is perturbed by a 1Σ+ state.
Is there a potential minimum corresponding to singlet methylnitrene? A study of the CH3N to CH2NH rearrangement on the lowest singlet state potential energy hypersurface
Authors: Richards Claude , Meredith Cynthia , Kim Seung-Joon , Quelch Geoffrey E. , Schaefer Henry F.
Published: July 2002

Ab initio molecular electronic structure methods have been used to examine the rearrangement of methylnitrene (CH3N) to methyleneimine (CH2NH) on the lowest-lying singlet surface. Geometries have been optimized and harmonic vibrational frequencies obtained for both methylnitrene and methyleneimine as well as for the transition states connecting them. Basis sets up to triple zeta plus double polarization plus f functions (TZ2P+f ) in size were used. Equilibrium geometries have been determined at the self-consistent field (SCF) and the single and double excitation configuration interaction (CISD) levels of theory. Because the 1E state of CH3N exhibits a Jahn–Teller splitting, the resulting 1A′ and 1A″ states have been examined in order to determine their respective roles on the potential energy surface. After identifying and analyzing a larger 1A′–1A″ splitting artifact due to symmetry breaking, the true classical Jahn–Teller splitting is predicted to be <0.01 kcal mol−1. This theoretical investigation is the first to consider the effects of electron correlation in conjunction with extended basis sets on the energetics of the methylnitrene 1,2-hydrogen shift. It is found that the conclusion of most previous lower level studies—that there is no barrier to rearrangement on the 1A′ surface—appears to be justified. If Cs symmetry is enforced, there is a sizable barrier (∼30 kcal mol−1) on the 1A″ potential surface.
Basis set convergence of atomic axial tensors obtained from self-consistent field calculations using London atomic orbitals
Authors: Bak Keld L. , Jo/rgensen Poul , Helgaker Trygve , Ruud Kenneth , Jo/rgen Aa. Jensen Hans
Published: July 2002

Hartree–Fock calculations of atomic axial tensors (AATs) and atomic polar tensors (APTs) are presented for HF, H2O, NH3, CH4, cyclopropane, carbodiimide, allene, and oxirane. London atomic orbitals are used for the axial tensors. The basis set convergence of the London AATs is found to be considerably faster than for the distributed origins (DO) approach. Indeed, the convergence of the London axial tensors is comparable to that of the polar tensors, indicating that the differential intensities of vibrational circular dichroism can be calculated as accurately as the total intensities of infrared spectroscopy. Dipole and rotational strengths are reported for NHDT, trans-cyclopropane-1,2-d2, carbodiimide, allene-1,3-d2, and trans-oxirane-2,3-d2.
Reversible first-order transition between two H2O amorphs at ∼0.2 GPa and ∼135 K
Authors: Mishima Osamu
Published: July 2002

Transition between the low- and high-density amorphous H2O was observed with a piston-cylinder apparatus from 77 K to 140 K, almost the glass-transition temperature of the amorphs. The transition was found to occur reversibly and abruptly at ∼135 K and ∼0.2 GPa with a volume change of 0.20±∼0.01 cm3/g and some hysteresis. This confirmed the transition to be first order. Absence of intermediate amorphous states between the low-density and high-density amorphs suggested existence of two substantially different liquid phases near the glass-transition temperature.
Spectra and relaxation of Hg atoms and molecules in low temperature matrices. I. CH4, CD4, and mixed CH4/Ar, CD4/Ar matrices
Authors: Crépin C. , Tramer A.
Published: July 2002

The absorption spectra in the 3P1←1S0 range and those of the prompt and long-lived emission components as well as their decay times are recorded for Hg atoms isolated in CH4, CD4, and mixed CH4/Ar matrices at T≂12 K using the dye-laser excitation. The results are compared to those obtained in the previous work for Ar, Kr, and Xe matrices. The rate of the 3P1■3P0 intramultiplet relaxation is only slightly increased, while that of the 3P0→1S0 emission shows an important increase due to the coupling to internal modes of methane molecules. The evidence of this coupling is the vibrational structure of the emission spectrum involving nontotally symmetric ν2(E), ν3(F), and ν4(F) normal modes of CH4 or CD4 molecules.
Comparison of transition state theory with quantum scattering theory for the reaction Li+HF→LiF+H
Authors: Yang C.-Y. , Klippenstein S. J. , Kress J. D. , Pack R. T , Parker G. A. , Laganà A.
Published: July 2002

The validity of transition state theory is examined for the bimolecular reaction of Li+HF→LiF+H. Accurate three-dimensional quantum scattering theory calculations of the cumulative reaction probability are reported for energies ranging from threshold (0.255 eV) up to 0.600 eV and a total angular momentum J of 0. Transition state theory estimates of the effect of both the entrance and exit channels on the cumulative reaction probability are reported for the same energy range and J value. The transition state theory results are found to provide an accurate description of the smoothed energy dependence of the cumulative reaction probabilities with a maximum disagreement between the two calculations of about 25% arising at the highest energy considered of 0.6 eV.
Structural properties of mixtures of highly asymmetrical electrolytes and uncharged particles using the hypernetted chain approximation
Authors: Forciniti Daniel , Hall Carol K.
Published: July 2002

We have solved the Ornstein–Zernike equation in the hypernetted chain (HNC) approximation for several mixtures of electrolytes and uncharged hard spheres. The mixtures that we studied range from a simple restricted primitive model plus hard spheres of the same size to highly asymmetric electrolytes plus hard spheres of different sizes. We monitored the radial distribution function and thermodynamic properties. We find that the presence of neutral particles changes the nature of the interaction between the charged particles. We also find a strong correlation between nonelectrostatic and electrostatic contributions to the free energy of the mixtures. Potential applications of this approach to the study of protein solubility in solutions of nonionic polymers are discussed.
Thermodynamic approach to nonequilibrium chemical fluctuations
Authors: Vlad Marcel O. , Ross John
Published: July 2002

A generalized thermodynamic description of one-variable complex chemical systems is suggested on the basis of the Ross, Hunt, and Hunt (RHH) theory of nonequilibrium processes. Starting from the stationary solution of a chemical Master Equation, two complimentary, related sets of generalized state functions are introduced. The first set of functions is derived from a generalized free energy F̌X, and is used to compute the moments of stationary and non-Gaussian concentration fluctuations. Exact expressions for the cumulants of concentration are derived; a connection is made between the cumulants and the fluctuation–dissipation relations of the RHH theory. The second set of functions is derived from an excess free energy φ(x); it is used to express the conditions of existence and stability of nonequilibrium steady states. Although mathematically distinct, the formalisms based on the F̌X and φ(x) functions are physically equivalent: both lead to the same type of differential expressions and to similar global equations. A comparison is made between the RHH and Keizer’s theory of nonequilibrium processes. An appropriate choice of the integration constants occurring in Keizer’s theory is made for one-variable systems. The main differences between the two theories are: the constraints for the two theories are different; the stochastic and thermodynamic descriptions are global in RHH, whereas Keizer’s theory is local. However, both theories share some common features. Keizer’s fluctuation–dissipation relation can be recovered by using the RHH approach; it is valid even if the fluctuations are nonlinear.

If the thermodynamic constraints are the same, then Keizer’s theory is a first-order approximation of RHH; this approximation corresponds to a Gaussian description of the probability of concentration fluctuations. Keizer’s theory is a good approximation of RHH in the vicinity of a stable steady state: near a steady state the thermodynamic functions of the two theories are almost identical; the chemical potential in the stationary state is of the equilibrium form in both theories. Keizer’s theory gives a very good estimate of the absolute values of the peaks of the stationary probability density of RHH theory. Away from steady states the predictions of the two theories are different; the differences do not vanish in the thermodynamic limit. The shapes of the tails of the stationary probability distributions are different; and hence the predictions concerning the relative stability are different for the two theories.
Symmetry-adapted perturbation theory potential for the HeK+ molecular ion and transport coefficients of potassium ions in helium
Authors: Moszynski Robert , Jeziorski Bogumil/ , Diercksen Geerd H. F. , Viehland Larry A.
Published: July 2002

The interaction potential for the HeK+ system has been computed as the sum of attractive and repulsive contributions due to the electrostatic, exchange, induction, and dispersion interactions using the symmetry-adapted perturbation theory and a high-level treatment of electron correlation. The zero of the theoretical potential occurs at 4.704 bohr and the minimum occurs at 5.418 bohr where the potential value is −0.779 mhartree. The potential supports 36 bound rovibrational levels, and the ground state of the HeK+ molecular ion is bound by 125.1 cm−1. For all interatomic distances the ab initio potential agrees very well with the empirical potential obtained by direct inversion of the K+ mobilities in gaseous helium and disagrees with the potential obtained from the ion–beam scattering cross section data. The ab initio potential has been used to compute the transport coefficients of potassium ions in helium gas over a wide range of temperature and reduced field strength. A very good agreement of the calculated transport coefficients with the preponderance of the existing experimental data is observed. This agreement leads us to believe that the present interaction potential is accurate to within a few percent at all separations between 4.0 and 10.0 bohr and, consequently, represents the most accurate potential available for the HeK+ system.
Optical spectroscopy of platinum and palladium containing poly-ynes
Authors: Wittmann H. F. , Friend R. H. , Khan M. S. , Lewis J.
Published: July 2002

We present a comparative study of two isostructural platinum and palladium poly-ynes, poly[trans(bis(tributylphosphine))M(1,4) phenylenediethynylene], where M represents either platinum or palladium. Optical excitations are associated with the π-molecular orbitals, and with strong spin–orbit coupling provided by the transition metal, both singlet and triplet excitations can be measured. The platinum and palladium polymers show similar low-temperature photoluminescence and photoinduced absorption spectra, with a blue-shift for the palladium polymer of around 5% for electronic transitions with respect to the platinum polymer. For Pt (Pd) we see strong singlet absorption peaks at 3.26 eV (3.49 eV), and weaker triplet absorption peaks at 2.43 eV (≊2.5 eV) at room temperature. Strong photoluminescence from triplet to ground state peaks at 2.38 eV (2.5 eV), and weak photoluminescence from the excited singlet peaks at 3.06 eV (3.2 eV) at low temperatures. Photoinduced absorption that peaks at 1.49 eV (1.72 eV) is due to transitions within the triplet manifold. Triplet lifetimes at low temperatures of 30 and 200–600 μs are found for the platinum and palladium polymers, respectively.
Characterization of the potential energy surface of the HO2 molecular system by a density functional approach
Authors: Barone Vincenzo
Published: July 2002

The potential energy surface for the ground electronic state of the HO2 system has been characterized using extended basis sets with a recently introduced density functional incorporating gradient corrections and some Hartree–Fock exchange. All the structural, thermodynamic and spectroscopic properties of the hydroperoxide radical and of its molecular fragments (OH, O2) are in close agreement with experiment. The saddle points for HO2 isomerization and OO–H dissociation, together with the hydrogen bonded OH–O structure, have been fully characterized. Refined post Hartree–Fock computations have been performed to further validate density functional results. The two series of quantum mechanical computations are in good agreement and suggest some refinement of the most recent semiempirical surfaces developed for dynamical studies. This task can be made easier by the force fields of all the stationary points computed in the present work. These findings together with the very favorable scaling of the computations with the number of electrons suggest that the density functional approach is a promising theoretical tool for the study of reactions involving large, chemically significant species.
Luminescence of atomic magnesium in inert low temperature solids. I. Argon and krypton
Authors: McCaffrey John G. , Ozin Geoffrey A.
Published: July 2002

Absorption and luminescence spectra have been recorded in the 200–700 nm range for atomic magnesium isolated in solid Ar and Kr at 12 K. Strong absorptions occurring in the near UV at 285 nm, showing a threefold splitting, are identified as the solid phase equivalent of the gas phase 3p 1P1←3s 1S0 Mg atom transition. Evidence of multiple site trapping of Mg atoms in Ar and Kr matrices formed at 12 K has been obtained from annealing studies in absorption, but especially in luminescence spectroscopy. The single emission band of Mg/Ar, centered at 297.6 nm, exhibits a radiative lifetime of 1.12 ns and is thereby assigned as singlet 3p 1P1→3s 1S0 Mg atom fluorescence. The luminescence exhibited by the Mg/Kr system is more complex than the Mg/Ar system in that a weak visible band at 472 nm occurs as well as several bands in the UV having nanosecond lifetimes. The richness of the Mg/Kr UV spectra has been examined with annealing and time-resolved measurements and identified as arising from multiple trapping site effects, with at least three spectrally distinct sites identified. Efficient resonant radiative energy transfer is demonstrated to be occurring between two of these sites and an average separation between the sensitizor and activator sites is calculated to be 60 nm at a Mg:Ar dilution ratio of 3:104. Annealing of Mg/Kr samples to 45 K was found to remove all but one site which exhibits emission at 297.6 nm and a very weak band at 472.6 nm. The former, having a radiative lifetime of 1.25 ns, is assigned as 3p 1P1→3s 1S0 Mg atom fluorescence; the latter with a radiative lifetime of 8.9 ms, is assigned as 3p 3P1→3s 1S0 Mg atom phosphorescence.
Analysis of certain factors in the direct dynamics method: Variational rate constant of the NH3+OH→NH2+H2O reaction
Authors: Espinosa-García J. , Corchado J. C.
Published: July 2002

The minimum energy path for the NH3+OH→NH2+H2O reaction was traced and the coupling between the reaction coordinate and normal modes was analyzed along it. Following the ‘‘direct dynamics’’ methodology, the rate constants were calculated for the temperature range 200–4000 K using the variational transition-state theory with two approximations to calculate the tunneling effect. With respect to the construction of the minimum energy path, the influence of the number of points calculated along this path, the scale factor in the Hessian matrix, and different ways of scaling the potential energy surface were analyzed. As this reaction presents a large reaction-path curvature, the small curvature approximation was not suitable for including the tunneling effect and a fitted Eckart potential was used. The variational effects were concluded to be negligible for this reaction, and good agreement with experimental values was found for T≥300 K when using the PSAC4/A1 curve (scaled by 0.8 and shifted to s=0), the Hessian matrix scaled by 0.9, and the fitted Eckart factor for the tunneling correction.
A metal-insulator transition in a supported monolayer of a group 10 metal
Authors: Burdett Jeremy K. , Sevov Slavi C.
Published: July 2002

Electronic structure calculations are used to show that a free monolayer of Ni with the structure of W(110) is metallic but becomes nonmetallic when ‘‘coordinated’’ to a layer of a nonmetal. This is in a good agreement with various experimental results for Ni monolayers supported on W(110) and on W(110)/O.
Ab initio vibrational levels for HO2 and vibrational splittings for hydrogen atom transfer
Authors: Barclay V. J. , Dateo Christopher E. , Hamilton I. P.
Published: July 2002

We calculate vibrational levels and wave functions for HO2 using the recently reported ab initio potential energy points of Walch and Duchovic [J. Chem. Phys. 94, 7068 (1991)] as fit by Dateo (unpublished). There is intramolecular hydrogen atom transfer when the hydrogen atom tunnels through a T-shaped saddle point separating the two equivalent equilibrium geometries, and correspondingly, the vibrational levels are split. We focus on vibrational levels and wave functions with significant splitting. The first three vibrational levels with splitting greater than 2 cm−1 are (1,5,0), (0,7,1), and (0,8,0), where v2 is the O–O–H bend quantum number. We discuss the dynamics of hydrogen atom transfer; in particular, the O–O distances at which hydrogen atom transfer is most probable for these vibrational levels.
Photoelectron spectroscopy of the multiphoton ionization and above threshold ionization of HI at 532 and 355 nm
Authors: Zietkiewicz C. J. , Gu Y.-Y. , Farkas A. M. , Eden J. G.
Published: July 2002

Multiphoton ionization and fragmentation of the hydrogen iodide molecule have been examined at 355 and 532 nm and optical field intensities of 1011−5×1012 W cm−2 by photoelectron spectroscopy and time-of-flight mass spectrometry. At 532 nm, scission of the H–I bond is favored over direct photoionization of the molecule. Three and four photon ionization of HI at 355 nm (ℏω≊3.5 eV), however, reveals clear vibrational progressions associated with the HI+ 2Π3/2 and 2Π1/2 ion product states. Analysis of these spectra yields vibrational frequencies for the 2Π3/2 and 2Π1/2 states that agree to better than 8% with those determined by Böwering et al. [Chem. Phys. Lett. 189, 467 (1992)] from single photon ionization experiments. Above threshold ionization (ATI) of both HI and atomic iodine are also observed.
Two-state linear curve crossing problems revisited. IV. The best analytical formulas for scattering matrices
Authors: Zhu Chaoyuan , Nakamura Hiroki
Published: July 2002

Based on the achievements of the previous three papers of this series, the best working formulas for scattering matrix are obtained for both the Landau–Zener (LZ) and the nonadiabatic tunneling (NT) case: two formulas valid at b2≥0 and b2≤0 in the LZ case, and three formulas valid atb2≤−1, −1≤b2≤1 and b2≥1 in the NT case, where b2 represents the effective energy. Simple and compact formulas which work far better than the LZ formula are proposed for nonadiabatic transition probability by one passage of crossing point for both the LZ and NT cases. Furthermore, compact expressions are derived, for the first time, for the nonadiabatic tunneling probability at b2≤1, i.e., at energies lower than the bottom of the upper adiabatic potential. All the formulas proposed here can be usefully utilized at any coupling strength, namely the validity range has been very much expanded compared to the previous formulas by employing certain empirical corrections. Besides, these formulas have convenience to enable an extension to general curved potentials.
Self diffusion of nuclear spins in a porous medium with a periodic microstructure
Authors: Dunn Keh-Jim , Bergman David J.
Published: July 2002

A Fourier approach is developed for evaluating the diffusion eigenstates and the diffusion propagator of a periodic, fluid filled porous medium, and is applied to the calculation of the pulsed-field-gradient-spin-echo amplitude M(k,t). The method is most effective for long times t, but works quite well down to times that are short enough so that asymptotic short time approximations of the diffusion process are still valid. The main advantage of the method is that it is applicable regardless of the value of the porosity or the shape of the periodic pore space. It is used to calculate M(k,t) for a number of examples of periodic porous media with porosities as low as 10%.
Orbital-invariant second-order many-body perturbation theory on parallel computers: An approach for large molecules
Authors: Bernholdt David E. , Harrison Robert J.
Published: July 2002

The equations for the second-order many-body perturbation theory [MBPT(2)] energy are derived in an orbital-invariant representation, analogous to that obtained with the method of self-consistent electron pairs of Meyer. This formulation is well suited to take advantage of the localized nature of interactions in large chemical systems in order to reduce the computational effort required to study them. This formulation of the MBPT(2) method also lends itself to implementation on parallel computers. We describe a scalable implementation in which the key data are distributed across the parallel computer rather than being replicated. Portability to both shared- and distributed-memory computer architectures is provided through the use of a subroutine library implementing a ‘‘global array’’ programming model. We demonstrate that this approach is scalable even for relatively small chemical systems.
Concentration polydispersity in anisotropically swollen polymer networks
Authors: Geissler Erik , Horkay Ferenc , Hecht Anne-Marie
Published: July 2002

Unlike polymer solutions, the component chains in swollen networks have limited freedom of translation. In such gels radiation is scattered by two types of concentration fluctuation: dynamic thermal agitation of the polymer segments, which confers osmotic properties on the gel; and static fluctuations, due to elastic constraints frozen in by crosslinks when the gel is formed. In small angle scattering experiments, these two components of the scattering spectra are separable. In this Communication, the mean square amplitude of the static concentration fluctuations 〈δφ2〉s in polydimethyl siloxane networks swollen anisotropically in octane is determined from small angle x-ray scattering. Although the characteristic length of these fluctuations is strongly anisotropic, 〈δφ2〉s remains approximately constant. This finding provides a mechanism for the ‘‘butterfly’’ effect observed in small angle neutron scattering spectra.
The microwave spectrum and structure of the pyridine–CO complex
Authors: Bettens R. P. A. , Bauder A.
Published: July 2002

Rotational spectra of five isotopomers of the pyridine–CO complex with D, 13C, 15N, and 18O have been measured over the 8–18 GHz frequency range. Rotational constants, centrifugal distortion constants, and 14N quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. Two different structures have been found to be compatible with the moments of inertia of all isotopomers. Both have CO lying in the plane of pyridine approaching simultaneously N and ortho-H but with two different orientations of CO.
Variable-structure cell model of two-dimensional liquids and glasses
Authors: Somer Frank L. , Kovac Jeffrey
Published: July 2002

Using a recently developed cell theory of liquids and glasses, a simple model calculation of the equilibrium and quenched properties of two-dimensional systems is presented. The particular model chosen is essentially a Bernal liquid, so that the distribution of molecular coordination numbers plays a central role. The basic model of Bernal is simplified, however, in that the Bernal polygons available to the system are restricted to a few basic types and is further modified to allow for small-amplitude vibrations of the molecules about their lattice sites. Quenching is then effected by freezing the coordination-number distribution (which is determined by a single ordering parameter, in the present, simplified treatment) at its ‘‘glass-transition’’ value. The equilibrium and quenched thermodynamic functions are calculated analytically, and substantial qualitative agreement with well-known characteristics of real liquids and glasses is obtained.
The 3 1Πg and 3 1Δg states of 39K2 studied by optical–optical double resonance spectroscopy
Authors: Kim J. T. , Wang H. , Bahns J. T. , Stwalley W. C.
Published: July 2002

The 3 1Πg and 3 1Δg states of K2 have been observed for the first time via the intermediate levels of the A 1Σ+u and B 1Πu states using optical–optical double resonance (OODR) excitation spectroscopy. The absolute vibrational numberings are obtained by comparing the calculated Franck–Condon (FC) factors with the excitation intensities and resolved fluorescence spectra. The Rydberg–Klein–Rees (RKR) potential curves are determined with the fitted molecular constants. The dependence of the molecular constants on the principal quantum numbers for the bonding and antibonding orbitals and the quantum defects are discussed. The adiabatic and diabatic dissociation limits for those states are given based on comparison with theoretical calculations.
Free energy and surface tension of arbitrarily large Mackay icosahedral clusters
Authors: McClurg Richard B. , Flagan Richard C. , Goddard W. A.
Published: July 2002

We present a model for predicting the free energy of arbitrarily large Mackay icosahedral clusters. van der Waals clusters are experimentally observed to be particularly stable at magic numbers corresponding to these structures. Explicit calculations on the vibrational states were used to determine the spectrum of fundamental frequencies for smaller (≤561) icosahedral clusters. The scaled cumulative frequency distribution function rapidly approaches a limiting function for large clusters. This function was used to predict zero-point energies and vibrational free energies for larger clusters (≳561 atoms). Combining these predictions with correlations for the moment of inertia and for the minimum potential energy of large clusters leads to free energies of arbitrary large clusters. The free energies are used to predict the chemical potential and surface tension as a function of size and temperature. This connects macroscopic properties to the microscopic atomic parameters.
Theoretical analysis of vibronic structure of absorption spectrum of fulvene
Authors: Negri Fabrizia , Zgierski Marek Z.
Published: July 2002

Ab initio and semiempirical quantum-chemical methods are used to study the lowest two valence excited states of fulvene. A new assignment is proposed for ground state frequencies and the Franck–Condon structure of the S0→S1, S2 transitions is obtained with the help of calculated geometries and vibrational force-fields of the electronic states involved. It is shown to account very satisfactorily for the observed vibronic structure of the absorption spectra of fulvene. It is demonstrated that, contrary to previous assumptions, CH2 out-of-plane vibrational modes play no role in formation of the vibronic structures of the two transitions.
Collision dynamics for an asymmetric top rotor and a linear rotor: Coupled channel formalism and application to H2O–H2
Authors: Phillips Timothy R. , Maluendes Sergio , Green Sheldon
Published: July 2002

The close coupling formalism and the approximate coupled states formalism for collisions of an asymmetric top rigid rotor and a linear rigid rotor are reviewed. Calculations for excitation of H2O by H2 using a recent accurate, ab initio potential energy surface are presented.
Polymer chain dynamics in dilute solutions under Couette flow: Dynamic light scattering from polystyrenes in a good solvent
Authors: Tsunashima Yoshisuke
Published: July 2002

Dynamics of flexible linear polymers in dilute solution under circular Couette flow was examined through dynamic light scattering measurements for two polystyrene samples (Mw=1.92×105 and 7.10×105) in benzene at 25 °C and at a fixed scattering angle, 75.6°. The scattered light from the sample solution, which solution was put into a gap between an inner rotating and an outer stationary glass cylinders of a rotating cylinder viscometer and was set under given shear flow, was measured with the homodyne method. For the lower molecular-weight sample, the obtained correlation function A(t) was of single exponential decay (the decay rate Γ1) over all the shear gradients γ measured at γ=0–5.2 s−1. For the higher molecular-weight sample, however, A(t) was of double exponential decays (the decay rates Γ1 and Γ2) at lower γ, and of single exponential (the decay rate Γ1) at higher γ. Any sigmoidal decay feature was not detected in A(t), so that the effect of the polymer convection on A(t) was excluded completely. The translational diffusion coefficient of the polymer D1 at given shear gradient was thus evaluated distinctly from Γ1. The obtained D1 increased with the increase of γ and gave the relation D1∝γ1/2.

This D1–γ1/2 relation could not be explained by the hydrodynamic descriptions of polymers so far proposed theoretically for the dilute solution (the so-called macroscopic description), where the diffusion-type equation for the polymer configurational distribution function has been used with the assumptions that the polymer segments travel in a continuous viscous medium of a constant friction coefficient and that the hydrodynamic interactions can be described approximately by the Oseen tensor, a solution of a linearized Navier–Stokes equation of motion. It is suggested that the present results obtained in Couette flow, the flow being a rigorous solution of the Navier–Stokes equation, could not be explained without taking into account the microscopic descriptions for polymer dynamics, i.e., the hydrodynamic coupling between the polymer and the solvent. Moreover, the intramolecular relaxation motions were found to be suppressed by the shear flow if the shear gradient became larger than 0.8 s−1. This suppression is caused by the decrease in the amplitudes of the normal-mode type of internal relaxation motions, not by the extinction of their intrinsic internal frequencies.
On the contribution of electron transfer reactions to the quenching of tryptophan fluorescence
Authors: Goldman Carla , Pascutti Pedro G. , Piquini Paulo , Ito Amando S.
Published: July 2002

We present a theoretical calculation of rates for excited state electron transfer (ET) reactions in zwitterionic tryptophan for each of its six rotameric conformers. Within the Born–Oppenheimer approximation, we calculate the electronic part of the rate, using the renormalized perturbation expansion (RPE) method. The vibrational part is extracted from experimental data, assuming that at neutral pH, the observed fluorescence decay profile is composed of two exponentials corresponding to two competing populations in the sample; one in which fluorescence is accompanied by ET and other that is not. Complementary data, namely, ionization potentials and charge distributions are obtained using a combination of classical molecular mechanics simulation to determine the geometry of the six rotamers, and semiempirical (INDO/S) molecular orbital calculation. Our results indicate that conformers perpχ2g− and antiχ2g− give the main contribution to ET in the time scale of fluorescence.
Thermochemistry of disulfur decafluoride, S2F10
Authors: Irikura Karl K.
Published: July 2002

Ab initio calculations, using a slightly modified version of the second-order Gaussian-2 procedure, have been performed to determine the thermochemistry of disulfur decafluoride, S2F10. The results include ΔfH0°(S2F10)=−1880 kJ/mol and ΔfH298°(S2F10)=−1903 kJ/mol, with an estimated uncertainty of 8 kJ/mol (2σ). This implies sulfur–sulfur bond strengths D0°(F5S–SF5)=195±8 kJ/mol and D0°(F5S–SF5)=198±8 kJ/mol. For comparison, second-order Gaussian-2 calculations of S2 and S8 yield S–S bond energies in error by only −6 kJ/mol and +3 kJ/mol, respectively. The torsional barrier in S2F10 is calculated to be 40±5 kJ/mol, indicating that torsional motion is negligible at 298 K. Minor reassignments are made in the experimental vibrational spectrum. Some comparisons between conventional and density-functional calculations are also made.
Clustering of CF4 on graphite as studied by molecular dynamics simulation
Authors: Kawai Tetsuya , Nakamura Nobuo
Published: July 2002

Molecular dynamics simulations of CF4 monolayer physisorbed on graphite are performed. The results of the simulations indicate that 2×2 commensurate phase is attained at the surface for coverage of 0.8 but it contains a large uncovered region. A CF4 molecule is adsorbed with its three fluorine atoms, librates about the axes parallel to the graphite surface and rotates about the axis perpendicular to the plane. The frequency of the molecular translational diffusion increases on heating and the melting of two-dimensional solid is recognized around 70 K.
Magnetic field effects on recombination fluorescence in liquid iso-octane
Authors: Saik Vladimir O. , Ostafin Agnes E. , Lipsky Sanford
Published: July 2002

The 123.6 nm photoionization of deuterated isooctane at −10 °C in the presence of hexafluorobenzene has been studied by examining the effect of a magnetic field to alter the quantum yield of recombination fluorescence. This fluorescence results from geminate recombination of hexafluorobenzene anions with isooctane positive ions. The use of a deuterated as contrasted to a protonated alkane makes the intensity of the recombination fluorescence much more sensitive to the magnetic field and permits observation of two maxima in the fluorescence yield at field strengths of 0 and 411 G and a possible third maximum at 822 G. The theory of the hyperfine induced spin evolution predicts these resonances at selected multiples of the C6F−6 hyperfine constant of 137 G. Utilizing the diffusion theory of geminate recombination in a Coulomb field, the experimental magnetic field spectrum is found to be well predicted over most of the range of magnetic field strengths studied (up to 2.5 kG) by a simple, one parameter, exponential radial probability density of initial scavenged geminate pair separation distances.
Disposal of reactant vibrational excitation in adiabatically endothermic reactions. I. H+D2(v″=1, j″=2)→HD(v′, j′)+D
Authors: Lanzisera Dominick V. , Valentini James J.
Published: July 2002

We report absolute partial and total cross sections for the H+D2(v″=1, j″=2)→HD(v′, j′)+D reaction at Erel=1.3 eV. Addition of D2 reactant vibrational energy increases the total reactive cross section from 1.2 to 2.5 Å2. That a similar amount of increased collision energy does not increase the cross section to such an extent distinguishes reactant vibrational energy from reactant translational energy. The average rotational energy for the HD product increases from 0.25 to 0.44 eV, but the effect is caused entirely by increased rotational energy in the v′=0 vibrational ground state. Reactant vibrational energy does not enhance the rotational energy for v′=1 and only modestly enhances HD vibrational energy. The average vibrational energy 〈Ev〉 is 0.10 eV for the v″=0 reaction and 0.16 eV for the v″=1 reaction. These results contrast with those of the D+H2(v″=j″=1)→HD(v′,j′)+H reaction at ∼1.4 eV, in which the vibrational energy of the HD product is three times as great for the v″=1 reaction as for the v″=0 reaction. This difference in reactions may be explained by the reactant H2 vibrational energy, as opposed to the reactant D2 vibrational energy, exceeding one quantum of vibration of the product HD. There is no specific or selective channeling of reactant vibration into product rotation in the present case, but reactive trajectories that allow channeling into v′=0, high j′ quantum states are enhanced upon the addition of D2 vibrational energy.
Integral equation theory for uncharged liquids: The Lennard-Jones fluid and the bridge function
Authors: Duh Der-Ming , Haymet A. D. J.
Published: July 2002

For the Lennard-Jones fluid, a new approximation for the bridge function is introduced and tested. The approximation is semi-phenomenological in nature. The structure predicted by the new approximation, in the form of the pair correlation function g(r), agrees extremely well with recent computer simulations for large systems, over the full range of density and temperature. The thermodynamic properties of the Lennard-Jones fluid are predicted and are in better agreement with computer simulations than earlier theories. The gas-liquid phase diagram predicted by this work is displayed and discussed.
Thermal and vibrational-state selected rates of the CH4+Cl↔HCl+CH3 reaction
Authors: Duncan Wendell T. , Truong Thanh N.
Published: July 2002

We present direct ab initio dynamics studies of thermal and vibrational-state selected rates of the hydrogen abstraction CH4+Cl↔CH3+HCl reaction. Rate constants were calculated within the canonical variational transition state theory formalism augmented by multidimensional semiclassical tunneling corrections. A vibrational diabatic model was used for vibrational-state selected rate calculations, particularly for exciting the CH4 symmetric stretching and umbrella bending modes. The potential energy information was calculated by a combined density functional and molecular orbital approach. Becke’s half-and-half (BH&H) nonlocal exchange and Lee–Yang–Parr (LYP) nonlocal correlation functionals (BH&HLYP) were used with the 6-311G(d,p) basis set for determining structures and frequencies at the stationary points and along the minimum energy path (MEP). Energetics information was further improved by a series of single point spin-projected fourth-order Mo/ller–Plesset perturbation theory (PMP4(SDTQ)) calculations using the 6-311+G(2df,2pd) basis set. We found that the calculated thermal rate constants have reasonable agreement with experimental results for both the forward and reverse reactions. Our results also predict that exciting the CH4symmetric stretching mode will greatly enhance the hydrogen atom transfer rate. Surprisingly, exciting the CH4 umbrella bend mode is also predicted to have a noticeable enhancement factor at room temperature.
Isotropic–nematic phase transitions in aqueous solutions of weakly charged, rodlike polyelectrolytes
Authors: Chen Shing Bor , Koch Donald L.
Published: July 2002

The transition from an isotropic to a nematic liquid crystal phase in a solution of charged rodlike molecules is studied under conditions in which the characteristic interaction energy between two rods is small compared with the thermal energy, i.e., w′=Q2λ/(εkBTL2)≪1. Here, Q and L are the charge and length of the molecules, λ is the Debye screening length for the small counterions, ε is the permittivity of the solution, kB is Boltzmann’s constant, and T is the absolute temperature. A truncated virial expansion that includes only two-rod correlations and a Debye–Huckel-like theory that incorporates many-rod correlations are developed. The electrostatic interactions cause a small perturbation to the phase transition for hard rods as long as w′≪d/λ where d is the rod diameter. In the region of parameter space defined by d/λ≪w′≪λ/L, the orientation-independent component of the electrostatic repulsion leads to a lowering of the concentration difference between the phases and a decrease in the molecular alignment in the nematic phase relative to the results for solutions of uncharged rods. Within this region, the many-rod correlations incorporated in the Debye–Huckel theory decrease the importance of the anisotropic electrostatic interactions and prevent a transition to a highly aligned phase predicted by the virial theory. For w′≫d/λ and w′≫λ/L, the anisotropic electrostatic interactions do become important and a highly aligned nematic phase for which the order parameter is asymptotically close to one is achieved.
Induced oscillations in an electron transfer reaction in the presence of a bichromatic electromagnetic field
Authors: Evans Deborah G. , Coalson Rob D. , Dakhnovskii Yuri
Published: July 2002

The effect of a bi-(multi-)chromatic electromagnetic field on electron transfer dynamics in a polar solvent is examined. Whereas the electron population dynamics is characterized by simple exponential decay in the presence of a monochromatic field, a bichromatic field gives rise to large induced coherent oscillations in electronic density between the reactant and product states. This oscillation is driven indefinitely by the applied field despite the strongly dissipative environment. The dependence of the dynamics on the reaction heat and the laser field properties is explored using an analytical expression derived within the noninteracting blip approximation. The oscillations depend on the field properties: both cosinelike and more complicated behavior can be obtained. For a trichromatic driving force, aperiodic (chaotic) oscillations are found. Ideal conditions and possibly appropriate chemical systems for the experimental verification of this phenomenon are discussed.