bluuebunny/crossref_metadata_2025 huggingface dataset:
The imidazole π cation and barbital π anion trapped in a cocrystalline complex x-irradiated at 12 K: An ESR–ENDOR study
Authors: Kar Leela , Bernhard William A.
Published: February 2003

The predominant free radicals trapped in single crystals of the 1:1 intermolecular complex of imidazole and 5,5-diethylbarbituric acid (barbital) x-irradiated at 12 K have been identified by ESR and ENDOR. The electron abstraction and electron addition products are found to be the imidazole π cation and the barbital π anion, respectively. The π cation provides experimental evidence of evenly distributed unpaired electron density at positions C2, C4, and C5 of the five membered imidazole ring. In the π anion the unpaired electron density is localized primarily on C4 of barbital. It is suggested that π anions are trapped in barbital in preference to imidazole because barbital has a higher cross section for electron capture than imidazole. On the other hand, π cations are trapped in imidazole in preference to barbital because the barbital π cation has a higher cross section for destruction than the imidazole π cation.
Influence of rotation on the vibrational predissociation of the van der Waals molecule HeI2
Authors: Beswick J. A. , Delgado-Barrio G.
Published: February 2003

We apply a rotational decoupling scheme related to the infinite order sudden approximation to treat vibrational predissociation of the triatomic van der Waals molecule HeI2. The potential surface is described by the sum of three Morse potentials between individual bonds. The total rate for vibrational predissociation as a function of vibrational excitation is compared with the results obtained for the colinear and T-shaped models. Final rotational distributions for the I2 fragment are also obtained.
Macromolecules in nonhomogeneous velocity gradient fields. II
Authors: Aubert James H. , Prager Stephen , Tirrell Matthew
Published: February 2003

Bead-spring model macromolecules in nonhomogeneous flows migrate relative to the local solvent velocity at the center of mass. We show that this is the result of the net viscous forces acting on all the beads. The analogy between these results and continuum mechanical results on suspensions of neutrally buoyant rigid particles is discussed. Detailed comparisons of this model with the available experimental data in the concentric cone geometry is made. Models with linear springs are found to over-predict the migration somewhat, but all qualitative features of the phenomenon are well represented. This provides a new means of testing macromolecular models.
Hole burning, softon–phonon scattering and anomalously fast dephasing of impurity electronic transitions in glasses
Authors: Hayes John M. , Stout Roland P. , Small Gerald J.
Published: February 2003

Solid state persistent hole burning data for impurity molecules embedded in glasses are presented which show that the pure dephasing associated with the impurity electronic transition is orders of magnitude faster than found in crystalline host media. This anomalously fast dephasing is explained in terms of a new theory which takes into account the two level systems structure of glasses and softon–phonon scattering involving very low frequency phonons. As a result, the high temperature limit for the scattering where the dephasing frequency is linear in T occurs at temperatures of only a few Kelvin.
Dynamics of the reaction of N+ with H2. V. Reactive and nonreactive scattering of N+(3P) at relative energies below 3.6 eV
Authors: Hansen Steven G. , Farrar James M. , Mahan Bruce H.
Published: February 2003

We have measured product velocity vector distributions for the processes N+(3P)(H2,H)NH+ and N+(3P)(H2,H2)N+ in the initial relative energy ranges of 0.98–3.60 eV and 0.66–2.50 eV, respectively, using the crossed beam technique. At energies below about 1.9 eV the predominance of a long-lived NH2+ complex is inferred from isotropic reactive scattering and a backscattered peak in the nonreactive distributions. Above 1.9 eV there is still a substantial interaction between all three atoms. The dynamics are adequately explained by a mechanism which involves accessing the deep 3B1 potential well through an avoided crossing with the 3A2 surface when the symmetry is relaxed from C2v to Cs. The reaction of electronically excited metastable ions, probably N+(1D), is seen as a forward peak in the reactive distributions.
Intramolecular vibrational relaxation in aryl alkyl ketones as revealed by excess energy dependence of T1→S intersystem crossing rate
Authors: Hirata Y. , Lim E. C.
Published: February 2003

T 1(nπ*)→S0 intersystem crossing in dilute vapors of aryl alkyl ketones has been studied as a function of excess vibrational energy (in T1) to determine the relative efficiency of intramolecular vibrational energy redistribution. The excess energy dependence of the nonradiative decay rate, as measured by α=d ln k/dE, is strongly molecular structure dependent. Molecules with a large alkyl group exhibit very large α for S0→S1(nπ*) excitation and relatively small α for S0→S2(ππ*) excitation. Molecules with a small alkyl group (acetophenone) exhibit an intermediate value of α, which is independent of the mode of excitation. These results indicate that the rate and the extent of communication between carbonyl and ring vibrations are significantly smaller for molecules with a bulky alkyl group. The possible origin of this ’’size-dependent’’ vibrational relaxation is discussed.
Time dependent nonlinear shear stress effects in simple liquids: A molecular dynamics study
Authors: Heyes D. M. , Kim J. J. , Montrose C. J. , Litovitz T. A.
Published: February 2003

The effects of large amplitude shearing rates on 108 particle amorphous Lennard-Jones systems have been examined. A resolution of the structural and dynamical properties reveals that there is a tendency for shear to reorganize the liquid into layers to facilitate flow and hence reduce shear viscosity. The first steps have been made to determine those physical phenomena that are necessary to develop a theory of time dependent nonlinear shear viscoelastic effects. The form of the distortion giving rise to nonlinear shear stresses is well understood although its magnitude is intimately linked with parallel structural changes in an, at present, intractable way.
Amplitude of liquid–vapor interfacial oscillations
Authors: Cole Milton W.
Published: February 2003

The root mean square amplitude δ of the oscillation of a liquid–vapor interface is computed. The general results for δ reduce to known forms in the classical and quantum limits. Special attention is directed toward the dependence on gravitational constant and lateral size L0 of the interface.
On chemical bond definitions; comparisons to computer study on dimer formation
Authors: Zurek W. H. , Schieve W. C.
Published: February 2003

Dimer mole fractions are calculated for the hard disk square well dilute gas from three alternative definitions of chemical bond (negative energy, proximity, and ’’exact,’’ including both stable and metastable dimers). Resulting theoretical predictions, based on phase–space averages are confronted with computer experiment, in which dimers can be identified dynamically from the history of interparticle collisions. Proximity and negative energy definitions give (respectively) upper and lower bounds on the molecular dynamics dynamical dimer mole fractions which fall closest to the exact prediction. It is concluded that for the purposes of both theoretical studies and computer experiments on clustering and nucleation it may be advantageous to use proximity and negative energy definitions simultaneously.
CARS spectra of transient molecular states in the photoisomerization of cycloheptatrienes
Authors: Luther K. , Wieters W.
Published: February 2003

The isomerization of CHT into toluene and Me-CHT into xylene, ethylbenzene, etc. is studied using a Nd-YAG laser.(AIP)
The temperature dependence of the surface photovoltage of nickel phthalocyanine films
Authors: Dahlberg S. C. , Musser M. E.
Published: February 2003

The temperature dependence of the initial photovoltage due to a laser pulse and the subsequent exponential decay of this signal have been measured for heat cleaned NiPc films in UHV between 293 and 400 K. A theoretical model involving an exp−0.416 eV/kT dependence for the rate of exciton recombination at the surface gives a reasonable fit to the data with only four adjustable parameters. (AIP)
Effect of viscosity on fluorescence of anthracenes in solution
Authors: Rice Janet , McDonald Daniel B. , Ng Lay-Keow , Yang N. C.
Published: February 2003

The fluorescence quantum yields of 9-alkylated anthracenes vary appreciably with solvent viscosity at 22 °C, while those of anthracene and 9,10-dimethyl-anthracene do not. The increase in fluorescence quantum yield in a more viscous solvent is due to a decrease in the rate of nonradiative decay. (AIP)
Raman intensities of stretch oriented polyethylene and perdeuteropolyethylene. I. Experiments
Authors: Masetti G. , Abbate S. , Gussoni M. , Zerbi G.
Published: February 2003

Two papers (I and II) are published jointly on the experimental determination of Raman intensities and calculations of intensity parameters of stretch-oriented polyethylene and perdeuteropolyethylene in different scattering geometries and polarization directions. In Paper I the reasons of the work, the basic principles of the method and the experimental techniques are described and the integrated relative Raman intensities are reported. A comparison of the data in various geometries allows to judge the validity of the experimental data. The vibrational assignment based on experiments is verified as a background for the calculations of intensities which follow in Paper II.
A simple model for internal motion of DNA based upon EPR studies in the slow motion region
Authors: Robinson B. H. , Forgacs G. , Dalton L. R. , Frisch H. L.
Published: February 2003

We develop a simplified theory of the EPR response from a spin-labeled intercalator bound to DNA by solving the stochastic Liouvillle equation in which the internal motions of the base pairs of the DNA are considered lattice variables. The dynamics are treated by a discrete version of the continuously elastic rod. The discrete model consists of harmonically coupled base pairs or disks which undergo Brownian rotational motion in solution. Correlation functions are obtained from the Langevin equation rather than from the Fokker–Planck equation. Apparent rotational times from the linear response theory results are compared with available experimental data. Reasonable agreement between the predictions of this simplified theory and experiment is obtained.
A new functional form for representing vibrational eigenenergies of diatomic molecules. III. Application to D2 and HD ground states
Authors: Beckel Charles L. , Kwong Robin B.
Published: February 2003

The authors apply the proposed functional form that relates vibrational energy level to quantum number to the experimental ΔE (ν + 1/2) for the ground states of D2 and HD, making use of the isotopic masses and the concept of mass-reduced quantum numbers. (AIP)
Effect of potential wells on rotational rainbow structure
Authors: Bentley John
Published: February 2003

In connection with rotationally inelastic cross section calculations for the atom-rigid rotor system in the infinite-order-sudden approximation, the author generalizes the rotational rainbow parameter to include potentials with wells, and examine some of its consequences. (AIP)
Photodissociation of methyl nitrite in the vacuum ultraviolet. II. Excitation energy dependence of the NO fluorescent fragments
Authors: Lahmani F. , Lardeux C. , Solgadi D.
Published: February 2003

The production efficiency of NO excited fragments in A 2Σ+ v′=0,1,2,3, C 2Π v′=0, D 2Σ+ v′=0 states resulting from the photolysis of methyl nitrite in the far UV has been measured as a function of incident wavelength using the synchrotron radiation of the electron storage ring of Orsay as the source of excitation. The experimental energy threshold to produce each NO excited state is in agreement with a bond dissociation energy of CH3O–NO of 1.8 eV. The excitation spectra of electronically excited NO A 2Σ+, C 2Π, D 2Σ+ are discussed in relation with the absorption spectrum of the parent molecule. The vibrational energy distribution in NO A 2Σ+ v′=0,1,2 can be explained by a statistical energy partition between CH3O and NO products provided some restrictions on the activated modes of the CH3O fragment. NO A 2Σ+ v′=3 population shows an anomalous behavior which can be indicative of a half-collision effect due to the interaction of NO C 2Π and D 2Σ+ primary product with CH3O.
Sudden rotation approximation for nonresonant transitions in an intense laser field
Authors: Leasure Steven C. , Wyatt Robert E.
Published: February 2003

A quantum mechanical study is made of rotational transitions within the vibrational manifold of a diatomic molecule which is in an intense laser field whose frequency is comparable to the vibrational frequency. An analytic iteration scheme for the time dependent propagator in the sudden approximation is implemented through the third order. The approximations are discussed and an improved scheme is suggested. The third order SRA results for HF rotational transitions are in excellent agreement with exact numerical results.
The repulsive forces in dense argon
Authors: Ross Marvin
Published: February 2003

Shock wave compression curves for liquid argon were computed using a number of recently proposed pair potentials. The results are compared to experiment and used to evaluate the accuracy of the repulsive potential. Recent molecular beam results are found to be in excellent agreement with the shock data.
Mass and orientation effects in dissociative collisions between rare gas atoms and alkali halide molecules
Authors: Tully F. P. , Cheung N. H. , Haberland H. , Lee Y. T.
Published: February 2003

The collision induced dissociation of alkali halide molecules to ion pairs upon impact with hyperthermal rare gas atoms has been investigated using the crossed molecular beam method. Relative total cross sections for the dissociation of CsI, CsBr, RbI, and KI to ion pairs upon collision with xenon and krypton have been measured over a relative collision energy range from threshold to 10 and 8 eV, respectively. In addition, complete angular and energy distributions of both dissociated ions from Xe+CsI, CsBr, and RbI collisions and from Kr+CsI and CsBr collisions have been obtained at several collision energies within the above energy range. Mass, collision orientation, and energy dependence effects observed throughout this work define two limiting case dissociation mechanisms for the Xe(Kr)+MX→Xe(Kr)+M++X− processes. The dominant dissociation configuration consists of the rare gas atom incident on the light atom end of the alkali halide molecule in a near collinear collision. The less preferred dissociation mechanism results when the rare gas atom is incident in a near collinear configuration on the heavy atom end of the alkali halide molecule. Experimental measurements of the percentage of energy transfer from the relative kinetic energy between Xe(Kr) and MX to the relative motion of M+–X− range as high as 95%; these percentage energy transfers correlate well with the predictions of an impulsive collision model. Three-dimensional classical trajectory calculations using realistic interaction potentials have been performed and they verify the dynamical interpretation suggested by the experiments.
Tunneling states of a tetrahedral molecule in a tetrahedral field
Authors: Ozaki Yoshiaki , Kataoka Yosuke , Okada Kenkichi , Yamamoto Tsunenobu
Published: February 2003

Accuracies of the rotational eigenvalues and eigenvectors of a tetrahedral molecule in a tetrahedral field are examined. Calculations are made in two ways: one is in terms of a set of free rotor functions and the other in the use of the pocket state method by Hüller and Kroll [J. Chem. Phys. 63, 4495 (1975)]. If the basis set consists of the free rotor functions up to the eighth order, the former method is superior to the latter as long as the potential strength does not exceed 40B, B being the rotational constant. Hüller and Raich’s criterion for the accuracy of the free rotor function method [J. Chem. Phys. 71, 3851 (1979)] is shown to have limited application.
Pseudopotential spin-density functional calculation of the ground state properties of Na2 and Na2+
Authors: Car R. , Meuli R. A. , Buttet J.
Published: February 2003

The potential wnergy curves for the ground states of Na2 and Na2+ are calculated within the local spin-density approximation, using model pseudopentials to simulate core effects. Our results compare very well with the experimental data and the results of much more elaborate configuration interaction calculations. We also find that at intermediate nuclear separations, where the local density approximation breaks down, the local spin-density formalism still allows a good description of the binding energy curve.
Piezo- and elasto-optic properties of liquids under high pressure. III. Results on twelve more liquids
Authors: Chen C. C. , Vedam K.
Published: February 2003

Precision optical interferometric measurements at high pressures have been made on twelve liquids. The liquids studied are chloroform, carbon disulfide, o-xylene, m-xylene, 1-3-5-trimethyl benzene, cumene, methyl cyclohexane, ethyl bromide, ethyl iodide, isobutyl alcohol, ethyl acohol, and deuterium oxide. The piezo- and elasto-optic properties of these liquids are found to be similar to those of other liquids described in Papers I and II of this series. For all these liquids, the data reveal pronounced nonlinear piezo-optical (Δn vs P) behavior, but linear elasto-optical (Δn vs ε, when ε is the Eulerian strain) relationship. Thus, the Eulerian representation of strain is advantageous in describing the elasto-optic behavior of liquids, irrespective of the shape, size, and composition of the molecules of the liquid. The linear relationship between Δn and Eulerian strain can be used to discriminate between the various equations of state. It may also provide a simple empirical expression for the computation of density from the accurately measured values of the changes in refractive index of liquids. The various expressions prevalent in the literature for the relationship between refractive index and density were analyzed by least-squares analysis technique. It is shown that the Drude and Lorentz–Lorenz equations (which are most widely used in the literature on such studies) give either too low or too high values of Δn. Kirkwood–Brown and Omini equations which were derived using detailed statistical mechanical approach, appear to yield values in moderately good agreement with experimental values.
On the calculation of classical time-autocorrelation functions: Resolution of the Liouville operator
Authors: Allen J. W. , Diestler D. J.
Published: February 2003

An alternate method for resolving the classical Liouville operator L into ’’raising’’ and ’’lowering’’ suboperators is introduced. A given resolution corresponds to describing the dynamical variable of interest in terms of a specified set of dynamical functions, i.e., within a specified basis. The Gram–Schmidt basis, in which the basis functions are orthogonal, generates a particularly simple resolution of L, which is utilized in conjunction with the Zassenhaus formula for the propagator exp(Lt) to construct a variety of useful approximate expressions for the time-autocorrelation function (TACF) of the variable of interest. These approximate formulas, which constitute partial summations of the Maclaurin series expansions of the TACF, are used to analyze the computer-simulation data of Levesque and Verlet [Phys. Rev. A 2, 2514 (1970)] for the single-particle velocity TACF φ of a Lennard-Jones liquid. This analysis provides a firm theoretical basis for the phenomenological memory function used by Levesque and Verlet to summarize and correlate the data for φ throughout the liquid state. The generalized Langevin equation for φ with the memory function approximated by Zassenhaus formulas is solved numerically for φ to obtain good fits to the simulated data through intermediate times. Some implications for model memory functions are discussed.
Modified Smoluchowski equation and a unified theory of the diffusion-controlled recombination
Authors: Berlin Yu. A. , Cordier P. , Delaire J. A.
Published: February 2003

A unified theory of the diffusion-controlled recombination based on the modified Smoluchowski equation is developed which takes into account simultaneously the diffusion transport, the chemical reaction, and the physical interaction between reactants. The stationary solution of this equation suggests a method for calculating the physical quantities characterizing the reactivity of active particles in the homogeneous and geminate processes with no need of any information about the solution of the corresponding time-dependent boundary value problem. A correlation between escape probability and apparent rate constant is established. Some results of the general theory are used for the theoretical description of ion pair neutralization and reaction between free ions. The validity of the physical model used in the present paper is briefly discussed.
Rotational relaxation of OCS in liquid n-heptane: Viscosity and free volume dependence
Authors: Goulay A. M. , Vincent-Geisse J.
Published: February 2003

The first rank tensor rotational relaxation time τ1R of OCS in liquid n-heptane was determined at various temperatures by isotropic Raman and infrared and was interpreted as a function of the viscosity η as well as the free volume of the solvent. The Debye–Stokes–Einstein relation provided a qualitative account of the data, although the experimental slope of τ1R vs η/T was found to be much smaller than the theoretical value. On the other hand, on the basis of a simple liquid cell model, it was established and experimentally verified that the reduced width of the orientational band contour (Δω1/2)1R (I/kT)1/2 is a linear function of the relative free volume of the solvent.
Two-photon spectroscopy of diphenylbutadiene. The nature of the lowest-lying 1A
g*−ππ* state
Authors: Bennett James A. , Birge Robert R.
Published: February 2003

A two-photon excitation spectrum of all-trans 1,4-diphenyl-1,3-butadiene is taken in EPA solvent glass at 77 K. The system origin of the lowest-lying 1Ag*− ππ* state is observed at 27 900±20 cm−1, 130 cm−1 below the system origin of the strongly allowed 1Bu*+ state. The vibronic analysis and the Franck–Condon intensity distribution indicate considerable bond order reversal in the 1Ag*− state localized in the polyene portion of the molecule. Configurational analysis using PPP–SCF–MO–CISD procedures predicts that the bond order reversal is primarily associated with a LUMO←HOMO double excitation. Although the bond order reversal is polyene localized, the electronic transition is calculated to be highly delocalized with 55% of the ’’optical’’ electron density on the phenyl groups. A single intermediate state approximation based on the strongly allowed 1Bu*+ state is demonstrated to adequately describe the two-photon absorptivity and polarization properties of the lowest-lying 1Ag*− state. The remarkably short intrinsic lifetime of fluorescence is shown to be associated with strong vibronic coupling of the 1Ag*− state with the nearby 1Bu*+ state.
Spectroscopic studies of the products of reactions of yttrium and scandium atoms with halogen molecules. I. The origin of chemiluminescence
Authors: Brayman Howard C. , Fischell David R. , Cool Terrill A.
Published: February 2003

Observations of laser induced fluorescence from the products of the reactions of yttrium and scandium atoms with halogen molecules have shown that chemiluminescence does not originate from electronically excited metal monohalide molecules, as previously suggested, but instead arises from electronically excited dihalide molecules,MX2*. Production of the metal dihalides appears to require the formation of vibrationally excited metal monohalides, MX°, in a precursor reaction. Radiative lifetime measurements for chemiluminescent bands are presented.
Spectroscopic studies of the products of reactions of yttrium and scandium atoms with halogen molecules. II. Laser induced fluorescence from yttrium and scandium monohalides
Authors: Fischell David R. , Brayman Howard C. , Cool Terrill A.
Published: February 2003

Excitation spectra from the monohalides of yttrium and scandium were recorded with the laser induced fluorescence method. Spectroscopic constants and radiative lifetimes were determined for several previously unobserved electronic states. Computer generated spectral simulations were used for the determination of spectroscopic constants and Franck–Condon factors associated with the fluorescence band systems.
Circular intensity differential scattering of light by helical structures. I. Theory
Authors: Bustamante Carlos , Maestre Marcos F. , Tinoco Ignacio
Published: February 2003

General expressions for the fields and intensities of scattered electromagnetic radiation by helical structures whose optical properties are described by a uniaxial polarizability along the tangent to the helix are obtained. Analytical expressions for the circular intensity differential of scattering (CIDS) as a function of the helix parameters and the wavelength of light are derived. It is shown that a sufficient condition for the existence of differential scattering for right and left circularly polarized light is the existence of an asymmetric polarizability. The choice of a uniaxial polarizability is found to give rise to form CIDS. For incident plane-polarized light, the scattered fields are found to be generally elliptically polarized. The Stokes parameters describing the state of polarization of the scattered radiation are derived. These results are expected to be relevant both as a new method to characterize chiral regions in macromolecular structures as well as to describe the light scattering properties of cholesteric and twisted nematic liquid crystals. The theory can also describe the radiation modes of helical antennas.
The gas phase molecular structure of carbonyl chlorofluoride, 0=CFCl
Authors: Oberhammer Heinz
Published: February 2003

The structure of carbonyl chlorofluoride has been determined by joint analysis of electron diffraction intensities and microwave rotational constants. The following geometric parameters (rav values) were derived: C=O=1.173 (2) Å, C–F=1.334 (2) Å, C–Cl=1.725 (2) Å, uOCF=123.7° (2) and uOCCl=127.5° (3). Error limits are 3σ values of the least squares analysis. A force field for this molecule was obtained by fitting the vibrational frequencies of the parent isotopic species. The experimental geometric parameters derived in this analysis agree very well with the values predicted by ab initio molecular orbital calculations using the gradient method.
Analytical reaction dynamics: Origin and implications of trapped periodic trajectories
Authors: Child M. S. , Pollak Eli
Published: February 2003

An analytical theory for the origin and dynamical implications of multiple trapped periodic trajectories on reactive surfaces is developed, and compared with numerical calculations. The dynamical motion is visualized in an orthogonal curvilinear coordinate system determined by the forms of the trapped trajectories, a device which leads naturally to the introduction of a generating function to determine the number and positions of possible trapped trajectories at any given energy. The connection between this function and the potential surface is examined in detail. This shows that the pattern of trapped trajectories may be deduced from knowledge of the combined variation of the potential energy and the transverse vibrational frequency along the reaction coordinate. This generating function is used to show that the lines of the trapped trajectories correspond to turning points of dynamical flux with respect to position along the reaction coordinate. It also provides a static explanation for the recently observed alternate repulsive and attractive character of successive trapped trajectories.
Linewidths and high resolution structure of the benzene 1B2u←1A1g two photon transition by multiphoton ionization–supersonic beam techniques
Authors: Aron Kenneth , Otis Charles , Demaray R. Ernest , Johnson Philip
Published: February 2003

The two photon spectra of the 1B2u state of benzene-h6 and benzene-d6 have been examined in a supersonic molecular beam/multiphoton ionization experiment. The effect of channel three nonradiative decay on the resonance linewidths was followed through energies greater than 6000 cm−1 above the origin, 3000 cm−1 past the fluorescence cutoff region. The linewidth data suggest a leveling off of the channel three broadening at energies greater than T00+6000 cm−1. A new progression in the benzene-h6 spectrum was revealed adjacent to the 1410 1n0 progression (n=1 to 5) which had previously been obscured in the rotational envelope of these bands. New structure and well resolved expected structure is seen in the benzene-d6 spectrum as the 1410 1n0 progression is followed out to n=5.
Long range tRNA–tRNA electrostatic interactions in salt-free and low-salt tRNA solutions
Authors: Patkowski A. , Gulari Erdogan , Chu B.
Published: February 2003

An ordering of salt-free and low-salt tRNA solutions due to long-range electrostatic tRNA–tRNA interactions has been studied by means of intensity and linewidth measurements of scattered light as well as small-angle x-ray scattering. From the position of the Bragg peak in small-angle x-ray scattering data an apparent intermacromolecular distance in the ordered tRNA clusters was calculated, while the angular asymmetry and photon correlation measurements of scattered light supply us with estimates of the tRNA cluster size. Based on an isolated hard sphere approximation, static and hydrodynamic radii of the tRNA clusters were always different, pointing to a more complicated pattern of macromolecular dynamics in the presence of long-range electrostatic interactions. The order, as well as the tRNA cluster size, decreases with decreasing tRNA concentration and/or increasing counterion concentration in solution. In tRNA solutions of NaCl concentration lower than 0.1 M the electrostatic tRNA–tRNA interactions are substantial and have to be taken into account in conformational studies of the macromolecule.
Collisional relaxation of vibrationally excited OCS in rare gas mixtures
Authors: Mandich M. L. , Flynn G. W.
Published: February 2003

Following excitation of the OCS bending overtone 2ν2, with a Q-switched CO2 laser, time-dependent fluorescence from the symmetric stretching fundamental ν1, has been observed for mixtures of OCS with 4He, Ne, Ar, Kr, and Xe. Analysis of the exponential rise and fall of fluorescence provides relaxation rates corresponding to the collisional coupling of the ν1 and ν2 manifolds and the overall vibration–translation/rotation relaxation for collisions of OCS with the various rare gases. For the process 2ν2→ν1, the experimentally derived probabilities per collision are OCS–Ne?1.7×10−4, OCS–Ar 1.2±0.4×10−4, OCS–Kr 0.8±0.2×10−4 and OCS–Xe 1.0±0.2×10−4. For the vibration–translation/rotation relaxation, the probabilities per collision are OCS–Ne 1.4±0.8×10−4, OCS–Ar 0.3±0.3×10−4, OCS–Kr 0.2±0.2 ×10−4, and OCS–Xe 0.1±0.1×10−4. Model calculated probabilities, using a spherically symmetric, short-range repulsive interaction potential, qualitatively agree with these experimental probabilities. If in the case of OCS–4He, only the path 2ν2→ν1 is assumed to couple the bending and stretching (ν1) modes, then the experimental probabilities are 6±2×10−4, for the process 2ν2→ν1 and 10 ± 3×10−4 for the vibration–translation/rotation process. This intermode vibration– vibration probability, however, is unexpectedly large when compared to both the model calculations and the experimental values for the other OCS–rare gas mixtures. A cyclic kinetic path allowing for both 2ν2 and ν2 to couple to ν1 in OCS–4He collisions is suggested by the model calculations and is also consistent with the OCS–4He experimental data. Assuming the complete kinetic cycle results in OCS–4He probabilities with the following ranges: 0–8×10−4 for the process 2ν2→ν1, 0–1×10−4 for the process ν2→ν1 and 5–13×10−4 for the vibration–translation/rotation process, depending on the relative contribution (which cannot be ascertained from these experiments) of the two competing steps filling the ν1 level. The cyclic path kinetics of OCS–4He system exemplifies the general phenomenon of cyclic mechanisms which may be important in promoting rapid vibration–vibration and vibration–translation/rotation energy transfer, particularly for complex or highly excited molecules.
131Xe nuclear quadrupole coupling and the rotational spectra of XeHCl
Authors: Keenan M. R. , Buxton L. W. , Campbell E. J. , Balle T. J. , Flygare W. H.
Published: February 2003

Rotational spectra have been assigned for the 129XeH35Cl, 129XeH37Cl, 129XeD35Cl, 131XeH35Cl and 132XeH35Cl van der Waals molecules by employing pulsed microwave Fourier transform spectroscopy in a Fabry–Perot cavity with a pulsed supersonic nozzle as the molecular source. The rotational constants, centrifugal distortion constants, and Cl nuclear quadrupole coupling constants obtained from the spectra are used to determine the structure of XeHCl and to gain information on the intermolecular potential binding of Xe to HCl. From the spectrum of 131XeH35Cl and an analysis of the centrifugal distortion in 129XeH35Cl, 131XeH35Cl, and 132XeH35Cl the 131Xe nuclear quadrupole coupling constant in XeHCl is found to be −4.9±0.2 MHz. Using the known nuclear quadrupole moment of 131Xe, the electric field gradient at the Xe nucleus is calculated. The observed field gradient is discussed in terms of Sternheimer shielding and formation of the weak Xe–HCl van der Waals bond.
Vibrational optical activity in perturbed degenerate modes: Concepts and model calculations in 1-substituted haloethanes
Authors: Nafie Laurence A. , Polavarapu Prasad L. , Diem Max
Published: February 2003

Vibrational optical activity arising from perturbed degenerate modes (PDM) is defined in terms of the chiral perturbation of symmetric chemical groups containing degenerate pairs of modes. This approach predicts that each such degenerate pair of vibrations will give rise to a couplet signal having equal and opposite VOA intensity for the two modes. The PDM formalism is developed explicitly for the case of a chemical group of C3v symmetry in vibrational circular dichroism (VCD), which complements earlier descriptions of degeneracy in Raman optical activity (ROA). Calculations using the fixed partial charge (FPC) model for VCD and the atom dipole interaction (ADI) model of ROA were carried out for the molecules (R)-1-bromo-1-chloro-1-fluoroethane and (S)-1-chlor-1-fluoroethane-1-d1 (and d0) as simple examples of molecules with a methyl group in a chiral environment. To further analyze the nature of the chiral perturbation of the methyl group, VOA calculations for bromochlorofluoroethane were carried out for perturbations of charge or polarizability, mass, geometry and potential energy, applied individually and in combination to the symmetric parent molecule 1,1,1-trichloroethane. In most cases degenerate mode pairs gave calculated VOA couplets of nearly equal and opposite intensity. The signs and magnitudes of these couplets are discussed in relation to the perturbations applied. It is found that different types of degenerate vibrations behave differently thus reducing the scope of generalizations regarding the effect of the chiral perturbations. However, a more consistent picture emerges if the net sign of the combined VOA from each degenerate mode pair is used and a strong correlation of these signs to the sign of the methyl torsion mode is apparent. The significance of these results to actual VOA observation and interpretation is discussed.
Matrix isolated copper and silver halide clusters
Authors: Martin T. P. , Schaber Helmut
Published: February 2003

Infrared absorption spectra are reported for matrix isolated (CuCl)n, (CuBr)n, (AgCl)n, (AgBr)n and for mixed clusters with composition CunClmBrn–m. Silver halide clusters with n=1 to 3 are already present in the vapor in equilibrium with the melt. The monomer and dimer forms of the copper halides could be produced only by using a two-stage oven to partially dissociate the predominant trimer. The stable configurations and corresponding infrared active vibrational frequencies are calculated and compared with experiment. By means of this comparison we come to the following tentative conclusions concerning the structure of these clusters: The silver dimers appear to be rhombic with an X–M–X angle differing considerably from 90°. Both the Cu and the Ag halide trimers are deformed six-rings. The Cu halide tetramer appears to be an eight-ring, not a cube. Larger clusters, (MX)6,7,8, formed within the matrix at high halide to argon concentrations also appear to have planar structures.
Absorption and emission spectra of Na atoms trapped in a Ne matrix
Authors: Wright J. J. , Balling L. C.
Published: February 2003

The optical absorption and emission spectra of Na atoms trapped in a Ne matrix at 3°K are reported. From these spectra, the Na-Ne interaction is deduced (AIP).
Vibrational spectra of carbon monoxide chemisorbed on alumina-supported nickel particles: A tunneling spectroscopy study
Authors: Kroeker R. M. , Kaska W. C. , Hansma P. K.
Published: February 2003

Tunneling spectroscopy is used to study the chemisorption of CO on small nickel particles. The particles are grown from vapor on a thermally oxidized aluminum substrate. Carbon monoxide is observed to chemisorb in at least four distinct ways, giving CO stretching frequencies of 256.5, 246, 222, and 207 meV. The 256.5 meV species has low frequency modes at 45.5 and 59.5 meV. The low frequency modes of the remaining three species are not resolved sufficiently for identification. The frequencies of all modes were found to have some coverage dependence. The largest shift is seen in a CO stretching vibration that moves from 200 to 207 meV with increasing coverage. The effect of hydrogen coadsorption is examined and evidence for the presence of oxygen on the particles is presented. The reaction of hydrogen with the CO chemisorbed on these nickel particles is found to produce very little surface hydrocarbon in contrast to results found previously for rhodium particles under similar conditions.
SiH: Λ doubling and ‘‘core polarization’’
Authors: Cooper D. L. , Richards W. G.
Published: February 2003

A b initio computation of the Λ-doublet splitting in the lowest rotational level of SiH (X 2Π) predicts a radioastronomical frequency of 3000 MHz which may be compared with the most recent terrestrial spectroscopic value of 2968 MHz. In this calculation no adjustment has been made for the effects of ’’core polarization’’ which can be handled satisfactorily providing a wave function of high accuracy is employed.
The polarization model applied to NH3, its cation NH+4, and its polymers
Authors: Turner Paul J. , David Carl W.
Published: February 2003

The polarization model has been successfully extended to predict stable geometries and potential energies for NH3, NH+4, (NH3)2, and NH3–NH4+. Calculated values for the inversion barrier and dipole moment derivatives for the ammonia monomer coincide closely with experimental data.
The total Hartree–Fock energy–eigenvalue sum relationship in atoms and ions
Authors: Castro Eduardo A. , Villata Laura , Fernández Francisco M.
Published: February 2003

A correct relationship between orbital SCF energies and total SCF energy for atoms and positive atomic ions is deduced. We report the results of the application of such relationship for the various isoelectronic series presented by Rao and Sen [J. Chem. Phys. 70, 586 (1979)]. Different calculations are compared and the goodness of each method is comparatively discussed.
The alchemy configuration interaction method. I. The symbolic matrix method for determining elements of matrix operators
Authors: Liu B. , Yoshimine M.
Published: February 2003

The symbolic matrix method which gives compact representation and efficient determination of expressions for the Hamiltonian and other matrix operators arising in configuration interaction (CI) calculations is presented. With this method, the computing and storage requirements for matrix expressions become insignificant compared to the total requirements of a CI calculation. The efficiency is achieved by taking advantage of analogies between expressions of different matrix elements to reduce drastically the number of expressions determined explicitly. The symbolic matrix method is completely general, unrestricted by the type of operators considered, or by the choice of n-particle basis. It can take full advantage of any point group symmetry, and the ordered interacting spaces to reduce the dimension of the n-particle basis. In addition, the method provides a basis for a general direct CI method which will be presented in a forthcoming paper. A comparison with the graphical unitary group approach is provided.
Near degenerate rearrangement between the radical cations of formaldehyde and hydroxymethylene
Authors: Osamura Yoshihiro , Goddard John D. , Schaefer Henry F. , Kim Kwang S.
Published: February 2003

Motivated by the recent experiments of Berkowitz, a systematic theoretical study of the ion isomerization H2CO+→HCOH+ has been carried out. Structures and vibrational frequencies for H2CO+, the transition state, and the cis and trans isomers of HCOH+ have been determined at the double zeta basis set self-consistent-field (SCF) level of theory. Equilibrium geometries were also predicted from SCF theory using a double zeta plus polarization (DZ+P) basis set. Final energetics were pinned down using DZ+P configuration interaction, involving a total of 16 290 configurations. The most reliable theoretical results suggest that trans-HCOH+ lies 5.5 kcal above H2CO+. Zero-point vibrational energy corrections do not change this H2CO+–HCOH+ separation. Similarly cis-HCOH+ is predicted to lie 4.1 kcal above the trans isomer, and the barrier to rotation between the two HCOH+ isomers is ∼18 kcal. The barrier to H2CO+→HCOH+ rearrangement is predicted to be 49.0 kcal, or 44.4 kcal after correction for zero-point vibrational energies is made. The relationship between this cationic 1,2-hydrogen shift and the corresponding neutral rearrangement is discussed in terms of qualitative molecular orbital theory.
Deuterium NMR study of molecular order and conformation in the nematic phase of p-azoxyanisole
Authors: Dong Ronald Y. , Lewis J. , Tomchuk E. , Wade Chas. G. , Bock E.
Published: February 2003

A single orientation order parameter tensor is shown to be sufficient for the entire molecule in the nematic phase of perdeuterated p-azoxyanisole by examining all the deuteron line splittings in the manner suggested by Bos and Doane. Two order parameters describing the molecular orientation order and the time averaged conformation of the molecule are measured. The relative conformation parameters are shown to be temperature independent within experimental error.
Order–disorder phase transitions in 9-hydroxyphenalenone
Authors: Abrahams S. C. , Bair H. E. , Haddon R. C. , Stillinger F. H. , Svensson C.
Published: February 2003

A heat capacity study of 9-hydroxyphenalenone reveals three phase transitions at 254.9, 379.8, and 385.5 K, followed by melting at 475.5 K. The observed entropy change at each transition can be fitted by an orientational disorder model. The broad transition at 379.8 K is ascribed to the onset of multiple disorder in both symmetry-independent molecules. Complete deuteration of the hydroxyl groups increases the lower phase transition temperature by 7.6 K, decreases that of both upper phase transitions by 2.4 K and decreases the melting point by 2.3 K. The effect of deuteration on the transition temperatures appears to be linearly dependent on the extent of isotopic replacement.
Molecular dynamics studies of the structure of water at high temperatures and density
Authors: Impey R. W. , Klein M. L. , McDonald I. R.
Published: February 2003

The effect of changes in temperature and density on the structure of water has been studied by molecular dynamics using the analytical potential of Matsuoka, Clementi, and Yoshimine. Significant structural changes are found to occur, particularly at high density. Implications for the interpretation of neutron scattering experiments are discussed.
Theoretical studies of relaxation processes in excited state impurity molecules: Application to naphthalene dimers
Authors: Wertheimer Reiner , Silbey Robert
Published: February 2003

We present a theoretical study for the relaxation modes of multilevel impurities embedded in a low temperature matrix. The equations of motion for the relevant macrovariables are derived, and the limits of validity carefully delineated. The theory is applied to triplet excitations of naphthalene dimers; the results provide the relations between the microscopic coupling and relaxation mechanisms and the observed spectroscopic data (phosphorescence, EPR, population relaxation, and energy transfer). Comparison is made with relaxation models developed previously, and the limitations of those are discussed.
Laser-induced fluorescence measurement of the internal and fine structure states distributions of OD(X 2Π,v,J,N) from D+NO2
Authors: Murphy Edmond J. , Brophy John H. , Arnold Graham S. , Dimpfl William L. , Kinsey James L.
Published: February 2003

The relative populations of rotational states in v=0, 1, 2, of OD produced in the crossed molecular beam reaction D+NO2 and in v=0 of OH from H+NO2 were measured by laser-induced fluorescence. High degrees of rotational and vibrational excitation are observed. These distributions are analyzed using an information theoretic approach which incorporates constraints on the rotational degeneracy of the reaction products. The probability of production of fine structure states within v=0 of the OD product is also reported. The reaction produces a nearly statistical spin state distribution. However, one of the members of each λ doublet is found to be preferentially populated. The implications of the experimental observations and the information theoretic analysis are discussed.
Effects of thermal-energy atomic and molecular collisions on lithium high-Rydberg quantum-state distributions
Authors: Kocher Carl A. , Shepard Chester L.
Published: February 2003

The distribution of principal quantum numbers for high-Rydberg states of lithium, produced in a thermal atomic beam by electron impact excitation, has been determined after passage of the beam through gaseous targets. Target densities were selected for an approximately tenfold reduction of the on-axis flux of Rydberg atoms. The state distribution, determined by electric field ionization, is markedly affected by a target of rapidly rotating polar molecules having rotational energy level separations comparable to the binding energy of the Rydberg valence electron. However, the distribution is not appreciably influenced by rare-gas or slowly rotating polar molecular targets, for which the Rydberg beam spreads as a result of momentum transfer.