bluuebunny/crossref_metadata_2025 huggingface dataset:
Configurational dependence of the optical properties of π-conjugated polymers. I. Strong disorder limit
Authors: Schweizer Kenneth S.
Published: July 2002

The dependence of the optical properties (ππ* energy gap, polarizability, oscillator strength) of polyenes on chain length, degree of bond alternation, and backbone configuration are studied theoretically. For all-trans polyenes a modified form of Hückel theory is proposed which is analytically simple and well behaved in the high polymer limit. Comparison with exact valence bond calculations for short polyene chains and the infinite chain limit reveals that the method is quantitatively accurate for the polarizability and qualitatively reliable for other properties and degrees of bond alternation. Modification of the optical properties by the presence of conjugation-disrupting defects along the polymer backbone is addressed by combining molecular orbital and statistical mechanical techniques. The specific case when the rotational defects severely localize the π electrons (strong disorder) is treated in detail. The disordered polymer configurations are classified by the defect density or mean conjugation length as the primary order parameter. The effects of conjugation length fluctuations and both long and short range defect interactions on the optical properties are determined. Experimental implications for π-conjugated polymers in condensed phases are briefly discussed.
Methoxycarbene and methylhydroxycarbene: Energies, structures, vibrational frequencies, and unimolecular reactivities
Authors: Yadav Janardan S. , Goddard John D.
Published: July 2002

In the light of recent experimental interest in the spectroscopy and photochemistry of matrix isolated oxycarbenes, ab initio calculations have been employed to investigate two prototypical oxycarbenes, CH3COH and CH3OCH, derived from acetaldehyde. Singlet state methylhydroxymethylene, CH3COH, exists as trans and cis isomers lying at 61.8 and 64.2 kcal mol−1 above ground state acetaldehyde. Partial CO double bond character as evidenced by a barrier to internal rotation from trans to cis of 27.3 kcal mol−1 accounts for the existence of geometric isomers. CH3COH (S0) faces a barrier of 27.7 kcal mol−1 to the Woodward–Hoffmann ‘‘forbidden’’ 1,2-hydrogen shift leading to ground state acetaldehyde. Trans and cis singlet methoxycarbenes, CH3OCH, are 73.6 and 77.3 kcal mol−1 less stable than S0 acetaldehyde. The large trans to cis rotational barrier of 30.4 kcal mol−1 is again evidence for a partial CO double bond. The barrier to the 1,2-methyl shift from trans-methoxycarbene to CH3CHO is 28.3 kcal mol−1. These carbenes, substituted with electron donating groups, OH or OCH3, are singlet stable. The singlet–triplet gaps are 23.0 kcal mol−1 for CH3COH and 21.3 kcal mol−1 for CH3OCH. Triplet CH3COH lies at 84.8 kcal mol−1 relative to ground state acetaldehyde which is 8.0 kcal mol−1 greater than the triplet acetaldehyde origin. The barrier for the triplet state 1,2-hydrogen shift from CH3COH to CH3CHO is calculated to be 33.3 kcal mol−1. Although all these methylene isomers lie high in energy in comparison with acetaldehyde, the transition state for the rearrangement to singlet CH3COH is lower than the energies of ∼95 kcal mol−1 employed in many gas phase studies of acetaldehyde photochemistry. It should be possible to

isolate the trans- and cis-CH3COH species in an inert matrix following acetaldehyde photolysis and to characterize these methylenes by their vibrational spectra as predicted herein. Ion cyclotron double resonance experiments are suggested that could afford an experimental measure of the relative thermochemical stability of CH3COH with respect to CH3CHO which is calculated to be 61.8 kcal mol−1.
Laser spectroscopy, rotational assignment, and perturbation analysis of the A 1Σ+u– X 1Σ+g Ca2 red system
Authors: Hofmann R. Thomas , Harris David O.
Published: July 2002

We measured the Ca2 red system using filtered laser excitation to simplify the spectrum. The transition is assigned A 1Σ+u −X 1Σ+g, where X is the previously studied van der Waals ground state and A correlates adiabatically to 1S+1D calcium atoms. The A state suffers many homogeneous and heterogeneous perturbations, which are assigned to interactions with components of c 3Πu. Following deperturbation of the band spectra, we find that the A state dissociation energy, D′e is 8694±2 cm−1, with a vibrational frequency, ωe, of 126.73±0.11 cm−1, and an equilibrium internuclear separation, Re, of 3.63 Å. The analysis shows c 3Πu, which correlates to 1S+3P atoms, is bound by at least 1170 cm−1, with ωe≥67.7 cm−1 and Re≤4.25 Å. The assignment of the previously analyzed green system of Ca2 is revised to B 1Σ+u– X 1Σ+g, where B correlates adiabatically to 1S+1P atoms. The new assignment gives B a dissociation energy of 5783 cm−1, and resolves some confusion about the origin of the Ca2 red and green systems.
Electron localization in liquid water: A computer simulation study of microscopic trapping sites
Authors: Schnitker Jürgen , Rossky Peter J. , Kenney-Wallace Geraldine A.
Published: July 2002

Sites at which excess electrons in pure water at 10 °C could become initially localized and subsequently rapidly solvated are identified by scanning a molecular dynamics simulation of pure equilibrium ST2 water with a test charge. Very favorable sites with respect to both volume and electrostatic potential are found in relatively high number density. Although the observed distribution of large cavities spans a wide range with respect to the electrostatic potential at the cavity centers, the calculated potential energy at the most favorable sites is almost as large as the apparent hydration energy of the electron in the equilibrated state.
ESR investigation of the photoisomerization of OClO to ClOO in an H2SO4 glass: Magnetophotoselection and kinetics
Authors: Adrian F. J. , Bohandy J. , Kim B. F.
Published: July 2002

The photoisomerization of OClO (ClO2) to ClOO in a sulfuric acid glass at 20 to 77 K has been reinvestigated using electron spin resonance (ESR) to observe the reactant and product and to quantitatively determine their relative concentrations. Polarized light photolysis yields a partial orientation of the remaining ClO2 molecules (magnetophotoselection) and shows that the optical transition dipole moment lies in the molecular plane and is perpendicular to the ClO2 rotational symmetry axis, as expected for the well known Ã(2A2)←X̃(2B1) transition of ClO2. Here, however, the photolysis yields only ClOO, in contrast to the gas phase where ClO is observed suggesting photodissociation. The photoisomerization kinetics are nonexponential and suggest that random variations among the ClO2 trapping sites in the H2SO4 glass have a considerable effect on the probability that a photoexcited ClO2 molecule will isomerize.
Vibrational predissociation spectroscopy of binary HF–base complexes
Authors: Kolenbrander Kirk D. , Lisy James M.
Published: July 2002

The vibrational predissociation spectra of N2–HF, acetylene–HF, ethylene–HF, and cyclopropane–HF in the HF stretching region have been measured using a molecular beam apparatus equipped with a mass spectrometer and a LiNbO3 optical parametric oscillator (OPO). The spectra are not dominated by homogeneous broadening. The HF stretching frequencies of N2–HF, C2H2–HF, C2H4–HF, and C3H6–HF are 3918.2(1), 3793.4(2), 3781(1), and 3761(1)cm−1, respectively. Individual rotational lines have been resolved for the N2–HF, C2H2–HF, and C2H4–HF complexes using the OPO in high resolution (0.06 cm−1 FWHM) mode. The lower limits to the predissociation lifetime obtained from the widths of these lines vary from 35 to 88 ps. An upper limit to these lifetimes of ∼2 μs was obtained from the observed depletion of the molecular beam.
On the construction of perturbation integrals for the description of reactive molecular collisions
Authors: Horn Thomas R. , Eno Larry
Published: July 2002

We continue our investigation of the relative effectiveness of conventional and quasiadiabatic (QA) perturbation schemes in the determination of reactive transition probabilities. The simple problem of particle reflection and transmission at a one-dimensional potential barrier (loosely based on the minimum energy path interaction of the H+H2 system) is again adopted for our calculations. By examining the accumulation with reaction coordinate of exact and conventional perturbative probability integrals over a wide range of energies and for both low and high system masses (on a molecular scale), we have been able to account for our recent observations with regard to the high mass ineffectiveness of the conventional scheme. The effectiveness of a QA based scheme at high mass, also observed in our earlier work, is here give more substance by the development of a procedure for optimizing parameters of the (QA) representation. The procedure tempers maximization of the QA transmission (with respect to parameter variation) by also driving for a balance between its diabatic and nonadiabatic component contributions. Comparing for the optimized QA parameters and at a high system mass, exact and QA accumulating probability integrals, we find very good agreement.
Vibrational relaxation of hydrogen fluoride by HF dimers in a laser excited nozzle beam
Authors: Knuth E. L. , Rubahn H.-G. , Toennies J. P. , Wanner J.
Published: July 2002

An HF nozzle beam and a cw HF laser beam in a coaxial and antiparallel arrangement facilitated exciting HF molecules vibrationally in v=1,J up to the saturation limit. A beam consisting exclusively of monomeric HF was generated by heating the beam source to 600 K and was confirmed using a mass spectrometer. Energy-flux measurements made using a bolometer indicate that the vibrational energy distribution initiated by the laser is conserved, confirming that V–T energy transfer to monomers is slow. The maximum measured flux of HF(v=1,J) molecules was 1.5×1017 molecules s−1 sr−1 at 670 mbar stagnation pressure. Beams consisting of a mixture of monomeric and polymeric HF were generated using appropriate combinations of source temperature and pressure. In the presence of dimers, the excited monomers relax rapidly by a fast V–V transfer and subsequent predissociation of the metastable dimers. The loss of beam energy due to energy transport by the departing dimer fragments was detected bolometrically. The laser-induced depletion of dimers was determined over a wide source pressure range at T0=300 K using a mass spectrometer. From a kinetic model of these dimer-depletion data, the product of cross section and transfer probability for V–V energy transfer from HF (v=1) monomers to dimers was deduced to be 14 Å2. The terminal-dimer-concentration equation for HF nozzle beams was refined. An analysis of the energy fluxes measured by the bolometer with no laser excitation of the beam revealed that, for an HF nozzle beam, the flow from the source to the detector is, in general, nonisenthalpic due to the onset of dimerization. Mach-number focusing enriches the dimer mole fractions detected by the bolometer. An analysis of the excess enthalpy due to the presence of polymers in the beam results in a new method for the experimental determination of the dissociation energy of HF dimers, determined to be 0.36 eV.
Quantitative absorption spectroscopy and dissociation dynamics of I2 van der Waals complexes with He, Ar, Kr, and Xe
Authors: Goldstein N. , Brack T. L. , Atkinson G. H.
Published: July 2002

Noble gas (He, Ar, Kr, and Xe) van der Waals (vdW) complexes of I2 formed in axiosymmetric, pulsed jet expansions were studied by absorption using intracavity laser spectroscopy (ILS). Absorption spectra of I2He, I2Ar, I2Ar2, and I2Ar3 as well as previously undetected I2Kr, I2Kr2, I2Kr3, I2Xe, and I2Xe2 were recorded using B(3Π+0u)←X transitions (Figs. 2–4 and Table I). The formation efficiencies of nine I2–noble gas vdW complexes were studied as a function of expansion conditions (Table II). Comparisons of absorption and laser-induced fluorescence data provide an analysis of excited-state dissociation mechanisms for the overall He, Ar, Kr, and Xe chemical series and as a function of vibrational level populated in the B(3Π+0u) state for I2He and I2Ar. The competition between the electronic and vibrational predissociation of the vdW complex is discussed with respect to absorption data.
Theory of polyatomic photodissociation in the reactive infinite order sudden approximation: Application to the Rydberg states of H3
Authors: Kulander Kenneth C. , Light John C.
Published: July 2002

The photodissociation of the Rydberg states of H3 and H2D via their ground molecular electronic states are treated using the reactive infinite order sudden approximation to calculate the final scattering state. A new method for performing the sudden calculations is proposed and tested for the reactive scattering of H+H2 in the J=0 total angular momentum state. Fragment vibrational state distributions and isotopic branching ratios for the photodissociation process are presented. The line shape of the emission from the Rydberg state in the far blue wing is determined for a number of initial vibrational states of the molecule. A strong isotope effect is seen in the photodissociation of H2D.
Order–disorder transitions of π-conjugated polymers in condensed phases. I. General theory
Authors: Schweizer Kenneth S.
Published: July 2002

A novel statistical mechanical theory for the order–disorder transformations of conjugated polymers in dilute solution and neat amorphous films is proposed. Single chain configurational statistics and the conformation-dependent polymer–solvent interactions are included in the basic formulation. Abrupt transition phenomena arise from the energetic stabilization of the rod-like conformation via the interaction of the delocalized electrons of the polymer backbone with the surrounding polarizable solvent. The theory is developed in detail for π-conjugated polymers (polyenes and polydiacetylenes) in dilute solution and makes a significant number of qualitative and quantitative predictions. Available experimental data are consistent with the theoretical picture and a variety of additional experiments are suggested to critically test the proposed ideas. The statistical mechanical formalism is generalized to allow for intramolecular hydrogen bonding, polydispersity, solvent mixtures, defect correlations, and substituent disorder. These effects have quantitative consequences but do not change the qualitative physics of the proposed three-dimensional solvation mechanism for the coupled electronic-conformational transitions. Preliminary application of the theory to amorphous films is briefly considered.
Neutron scattering study of molecular motions in the naphthalene–tetracyanobenzene complex
Authors: Czarniecka K. , Janik J. M. , Janik J. A. , Krawczyk J. , Natkaniec I. , Wasicki J. , Kowal R. , Pigoń K. , Otnes K.
Published: July 2002

Neutron diffraction measurements performed with the naphthalene–tetracyanobenzene complex showed the occurrence of the phase transition at ∼70 K in the thermal expansion of lattice parameters. Incoherent inelastic neutron scattering measurements showed many similarities between the phonon density of states of the complex on one hand and of the pure naphthalene on the other. There are also differences, which indicate a coupling between the external and the internal modes in the complex. Quasielastic neutron scattering showed an occurrence of fast stochastic jumps of naphthalene molecules in the complex. The two positions between which these rotational jumps occur differ by ∼36° and a residence time is of the order of 8 ps at room temperature.
Dispersed fluorescence of jet-cooled tryptophan: Excited state conformers and intramolecular exciplex formation
Authors: Rizzo Thomas R. , Park Yong D. , Levy Donald H.
Published: July 2002

The dispersed fluorescence of the amino acid tryptophan has been measured in the environment of a cold, supersonic free jet. Analysis of the region of the spectrum near the electronic origin indicates that the electronic excitation spectrum contains features which arise from various ground state conformers of tryptophan, confirming our previous assignment of these features. Under the conditions of our experiment the conformers do not interconvert in the excited state during the fluorescence liftime. Analysis of the dispersed emission spectrum of one conformer reveals broad red-shifted fluorescence which exists even when the electronic origin transition is excited. This broad red-shifted fluorescence is produced by the formation of an intramolecular exciplex involving excited state proton transfer to form a zwitterion. Molecules which do not have the ability to form a zwitterion do not exhibit this behavior, and deuterated trytophan shows broad fluorescence in an amount consistent with a slower proton transfer rate. The significance of these results for understanding the excited state photophysics of tryptophan in solution is discussed.
Dirac scattered-wave study of trigonal bipyramidal silver clusters Ag5q+ (q=0, 2–4)
Authors: Arratia-Perez Ramiro , Malli Gulzari L.
Published: July 2002

The electronic structure of the neutral and cationic pentaatomic silver bare clusters is investigated by the Dirac scattered-wave (DSW) method. The results indicate that there is significant 5s1/2–4d5/2 hybridization in the bonding molecular orbitals, due to relativistic effects. Molecular hyperfine interactions (hfi) are calculated for the paramagnetic species Agq+5 (q=0, 2, and 4) through a  first-order perturbation to the Dirac Hamiltonian. The ground state (2E′) orbital degeneracy of Ag5 in D3h geometry is removed by spin-orbit interaction leading to Kramers degeneracy, and consequently the D3h geometry of Ag5 will not distort due to Jahn–Teller effect. It is found that the hyperfine coupling constants calculated by using a four-component wave function for the Ag2+5 and Ag4+5 clusters differ significantly from previously computed hfi using a second-order perturbation to the Schrödinger Hamiltonian. First ionization potentials and excitation energies are predicted for all the species as calculated by the spin-restricted transition state method.
Molecular beam scattering measurements of differential cross sections for D+H2(v=0)→HD+H at Ec.m.=1.5 eV
Authors: Götting Rüdiger , Mayne Howard R. , Toennies J. Peter
Published: July 2002

The reaction D+H2→HD+H has been studied in a crossed molecular beam scattering experiment at a most probable collision energy of Ec.m.=1.5 eV. Angular and time-of-flight distributions of the HD product have been measured over a wide range of angles. The experimental data are compared with quasiclassical trajectory (QCT) calculations on the LSTH potential hypersurface. The QCT calculations fit the experimental data quite satisfactorily. The sensitivity of the experiment to the different properties of the cross section has been investigated.
The effect of the heat of association on homogeneous nucleation rates in methanol vapor
Authors: Strey R. , Schmeling T. , Wagner P. E.
Published: July 2002

Homogeneous nucleation rates in methanol vapor were measured using the expansion pulse technique. Our results show an apparently inverted temperature dependence. The obtained data are consistent with earlier expansion chamber investigations, reported in the literature, however, strong deviations from corresponding diffusion chamber data are observed. In the present paper we show that these deviations can be explained by considering vapor association during adiabatic expansion. The homogeneous nucleation rates are found to be drastically reduced due to the latent heat released during the formation of subcritical clusters. Thereby the severe discrepancies between expansion chamber experiments for methanol and the corresponding diffusion chamber data are now essentially resolved. Furthermore, an apparently anomalous behavior of ethanol vapor, observed in expansion chambers at higher temperatures, is now qualitatively understood.
State selected ion–molecule reactions by a TESICO technique. XII. Internal energy dependence of the relative cross section and mechanism branching of the reaction CH+4(v) +CH4 →CH+5 +CH3 and its isotopic variants
Authors: Herman Zdenek , Tanaka Kenichiro , Kato Tatsuhisa , Koyano Inosuke
Published: July 2002

Total cross sections of the title reaction studied at collision energies 0.6–3 eV by the TESICO method were found to decrease by about 1.6 times as the internal energy of the reactant ion is increased from 0.1 to 1 eV. Branching ratios of collisional mechanisms were derived from isotope labeling studies: with decreasing total initial energy (sum of collision energy and reactant ion internal energy) from 4 to 0.9 eV the fraction of the dominant proton transfer mechanism slightly decreases, the fraction of H-atom transfer is almost constant, and the fraction of intermediate complex formation increases from about 5% to 15%. Significant isotope effects were observed in the impulsive proton and H-atom transfer mechanisms depending on whether the transferred particle was H or D. The charge transfer channel represents about 10%–50% of the chemical reaction channel.
Absolute intensities of infrared-active fundamentals and combination bands of gaseous PuF6 and NpF6
Authors: Person W. B. , Kim K. C. , Campbell G. M. , Dewey H. J.
Published: July 2002

Results are presented for the absolute integrated molar absorption coefficients of the infrared-active fundamental and binary combination bands of PuF6 and of NpF6 between 1400 and 450 cm−1. The low resolution band contours of the combination bands ν1+ν3, ν2+ν3, ν3+ν5, ν2+ν6, and ν2+ν4 are discussed. From band center positions, the six harmonic wave numbers were obtained and Coriolis constant ζi (i=3, 4, 5, and 6) estimated. For PuF6, the six normal mode vibrations appear to group into one ‘‘local mode’’ stretching at a mean value of 589±50 cm−1 and one local mode bending at 195±5 cm−1. The role of the heavy central atom and the magnitudes of the vibrational force constants are discussed in comparison with UF6. The measured intensity for ν3 is discussed in comparison with previously predicted values.
EPR observations of trivalent titanium in orthophosphate single crystals
Authors: Abraham M. M. , Boatner L. A. , Aronson M. A.
Published: July 2002

The ground state spectroscopic properties of the 3d1 electronic configuration ion Ti3+ have been investigated using EPR spectroscopy. Trivalent titanium was incorporated as a dilute impurity in single crystals of the tetragonal, zircon-structure hosts ScPO4, YPO4, and LuPO4. The EPR spectrum of Ti3+ was observed in the as-grown orthophosphate crystals, and low temperature irradiations were not required to produce the trivalent state. The EPR results show that Ti3+ occupies a substitutional cation site in the host orthophosphate single crystals. Axial spin-Hamiltonian parameters were determined at 77 K, and these results are compared to those obtained previously for the 4d1 and 5d1 configuration ions Zr3+ and Hf 3+ in the same host crystals. Titanium-doped orthophosphate crystals containing isotopically enriched 47Ti or 49Ti were also grown and employed in determinations of the hyperfine parameters. The g-values observed here for Ti3+, and determined previously for Zr3+ and Hf 3+ are not accounted for by the published second-order expressions indicating that additional coupling mechanisms are operative.
Experimental determination of scattering induced angular momentum alignment in state selected rotational transitions
Authors: Treffers M. A. , Korving J.
Published: July 2002

Measurements of angular momentum alignment in molecular beam scattering are presented for scattering of Na2 by He, Ne, and Ar. A mechanism for the production of this alignment is discussed, based on mJ conservation for quantization along the momentum transfer vector. The experimental results differ in several aspects from the predictions of this simple mechanism. It is found that the differential cross sections are strongly dependent on mJ. Furthermore, it is found that the principal axis of the orientational distribution does not coincide with the momentum transfer vector. The discrepancy is attributed to a nonstrict mJ conservation in the collision. These experiments are therefore expected to provide a sensitive probe to the nonsudden aspects of molecular scattering.
Electronic excitation transport on isolated, flexible polymer chains in the amorphous solid state randomly tagged or end tagged with chromophores
Authors: Peterson Kristen A. , Fayer M. D.
Published: July 2002

The transport of electronic excitations among chromophores which are randomly tagged or end tagged on finite flexible polymer chains in the amorphous solid state is described. First, the previously reported first order cumulant expansion treatment of excitation transport among chromophores randomly distributed in solution is extended to treat finite volume systems with nonrandom chromophore distributions. The method is demonstrated by considering chromophores randomly distributed in a finite sphere. The results are in good agreement with a previous treatment of this problem using a density expansion. The probability of finding the initially excited chromophore, still excited at time t, Gs(t), is calculated. Gs(t) is directly related to fluorescence depolarization and other observables. Gs(t) is then calculated for a finite flexible polymer chain in a polymer blend which is randomly tagged in low concentration with chromophores. The method permits any form of the polymer pair correlation function to be used. A specific model, a Gaussian segment distribution function, is employed, and an explicit time domain expression is obtained. This expression is compared to previous theoretical treatments which did not contain adequate descriptions of the pair correlation function. Greatly improved agreement with experiment is obtained. Excitation transfer on an end tagged coil is a limiting case of the random tagged problem. In addition, the energy transport aspect of the end tagged problem is treated exactly and the two approaches are compared.
Photodissociation of vibrationally excited OCS
Authors: Zittel P. F. , Masturzo D. E.
Published: July 2002

A two-step photodissociation method has been used to measure the photodissociation cross section of vibrationally excited OCS(ν1) at λ=249 nm. The method employs an NH3 laser to pump OCS molecules to the ν1=1 vibrational state, followed by photodissociation by a KrF excimer laser and mass spectrometric analysis of the chemically scavenged photodissociation products. The experiments determine the photodissociation cross section ratio σ d(ν1)/σ d295 K =4.5±0.9, where σ d295 K is the 249 nm photodissociation cross section of thermal (295 K) OCS. An analysis of our previously reported data for the ν2=2 vibrational state of OCS gives the photodissociation cross section ratio σ d(2ν2)/σ d295 K =8.4±1.1. The measured cross section ratios are used in an analysis of temperature dependent photoabsorption spectra to obtain the photoabsorption cross section ratios σ a(0)/σ a295 K =0.41±0.04 and σ a(ν2)/σ a295 K =2.5±0.6 for the vibrationless and ν2=1 states of OCS, respectively. Isotopically selective, two-step photodissociation through the OCS(ν1) vibrational state was used to separate 33S or 34S isotopes with enrichment factors of β=2.2.
Collision kernels for the Waldmann–Snider equation
Authors: Blackmore Robert
Published: July 2002

A collision kernel for the Waldmann–Snider collision operator has been derived for a rigid rotor in a monatomic heat bath. The general form of the collision kernel is reduced to a classical Boltzmann form when the rotor is restricted to the ground rotational state. A similar form for the collision kernel is obtained with the use of the spherical approximation for the scattering matrix and by considering only elastic collisions. In addition, an outline of how this may be applied to line shape calculations is given for arbitrary gas pressures.
Inelastic incoherent neutron scattering study of the methyl rotation in various methyl halides
Authors: Prager M. , Stanislawski J. , Häusler W.
Published: July 2002

The inelastic incoherent neutron scattering technique has been applied in an energy range from 0.4 μeV to 60 meV to measure the rotational excitations of the methyl groups in polycrystals of CH3I, CH2DI, CD3I, CH3Br, CD3Br, and CH3Cl in a temperature range 5<T[K]<190. Tunnel splittings of the librational groundstate of 2.44 and 0.9 μeV were found for CH3I and CH3Br, respectively. Excitations to the higher excited librational states are identified via the temperature dependence and the isotope effects of the modes in the meV regime. The rotor–rotor coupling between neighboring methyl groups is found to be small. Thus single particle rotational potentials were derived from the observed eigenenergies. The potential strength increases going from the iodide via the bromide to the chloride. In CH3Br the rotational potential does not change on deuteration. In CH3I a very unusual increase in the rotational potential with deuteration is found. Besides the methyl librations, large amplitude motions around axes perpendicular to the molecular symmetry axis are observed at energies comparable to the methyl libration. The coupling of the methyl rotor to lattice modes modulating the strength of the orientational potential in CH3I gives rise to a peculiar temperature dependence of the tunnel splitting.
Vibrational and translational energy effects in the abstraction and exchange reactions of NH+3 with D2
Authors: Winniczek J. W. , Braveman A. L. , Shen M. H. , Kelley S. G. , Farrar J. M.
Published: July 2002

We present a study of the abstraction and exchange reactions of NH+3 with D2 yielding NH3D++D and NH2D++HD, respectively. The translational energy of these experiments was fixed at 0.50 eV, while the vibrational energy of the ammonia cations was selected at values between 3.3 and 4.9 eV by a charge exchange technique. The flux distributions for the two distinct reaction products were quite different, indicating different precursors to their formation. The abstraction reaction products were backward scattered in the center-of-mass reference frame, while sharp forward–backward scattering of the exchange products suggested the participation of a transient intermediate living a fraction of a rotational period. The angular distribution for exchange became more symmetric about 90° with decreasing reagent vibrational energy, further supporting the participation of an intermediate complex. The product kinetic energy distributions for both processes broadened with increasing vibrational energy. This observation is consistent with partitioning of the total available energy of the incoming reagents into exchange product degrees of freedom as one would expect for a transient complex. The participation of the vibrational energy in the abstraction process is consistent with vibration–translation energy transfer from the ν2 bending mode of NH+3 to relative translation of the products.
Microscopic observation of ordered colloids in sedimentation equilibrium and important role of Debye-screening length. I. Heavy and monodispersed polystyrene type spheres (specific gravity=1.50) in aqueous and aqueous methanol suspensions
Authors: Okubo Tsuneo
Published: July 2002

A metallurgical microscope is used to directly observe the ordered structures of heavy and monodispersed polystyrene type spheres (specific gravity= 1.50) in sedimentation equilibrium. The interparticle distances (2Dexp) increase with time after the latex sample is set into the cell, and reach an equilibrium value after seven days. 2Dexp continues to decrease as the amount of spheres fed increases and from these data Young’s modulus of the ordered lattices is estimated to be ∼100 dyn cm−2. The hexagonal ordering is observable not only in the deionized state but also in the presence of sodium hydroxide (10−6–10−3 M), and 2Dexp decreases with increasing alkali concentration. The results show that electrostatic interparticle repulsion and the elongated Debye-screening length around the spheres are essential for the ordering.
Use of a nonlocal density approximation for transformation from electron density to electron momentum density
Authors: Gadre Shridhar R. , Chakravorty Subhas J.
Published: July 2002

A new method to extract atomic electron momentum density from the corresponding electron density has been proposed. The method is based upon the nonlocal density approximation (NLDA) due to Gunnarsson, Jonson, and Lundqvist; and to Alonso and Girifalco. The reduced first order density matrix Γ(r/r′) is estimated from the atomic electron density ρ (r) by use of an averaged density distribution, ρ̃ (r). The Γ(r′/r′+r) thereby obtained is integrated out to obtain the autocorrelation function B(r) enabling evaluation of electron momentum density γ(p) and 〈pn〉 values. The method has been developed for spherically symmetric densities and has been tested out for beryllium, nitrogen, neon, and argon atoms. The 〈pn〉 values and momentum densities thus obtained are in a very good agreement with the corresponding Hartree–Fock counterparts.
Self-consistent mode-coupling theory of electrical conductivity and incoherent excitation transport in disordered media
Authors: Loring Roger F. , Franchi Daniel S. , Mukamel Shaul
Published: July 2002

We present a new method for calculating transport properties of an excitation such as an electron, exciton, or vibron in a disordered medium, in which the dynamics are governed by a Pauli master equation. A hierarchy of self-consistent mode-coupling equations for the master equation propagator is developed, and the first member of the hierarchy is explicitly constructed. This equation can be used to calculate the propagator for an excitation transfer rate with any dependence on the intermolecular separation. The resulting propagator is exactly correct to lowest order in the density and in the ordered lattice limit. Extensive calculations are presented for the site percolation problem, for which a crossover in time to fractal dynamics is observed. The spectral dimension of a percolation cluster in three dimensions is predicted to be 4/3, in agreement with the conjecture of Alexander and Orbach and with recent simulations.
Forward and inverse functional variations in elastic scattering
Authors: Guzman Robert , Rabitz Herschel
Published: July 2002

This paper considers the response of various types of elastic collision cross sections to functional variations in the intermolecular potential. The following cross sections are considered differential, total, effective diffusion, and effective viscosity. A very simple expression results for the diffusion and viscosity cross sections at high energy relating the variations to the classical deflection function. Attention is first given to the forward sensitivity densities δσ(E)/δV(R) [i.e., the functional derivative of cross sections σ(E) with respect to the potential surface V(R)]. In addition inverse sensitivity densities δV(R)/δσ(E) are obtained. These inverse sensitivity densities are of interest since they are the exact solution to the infinitesimal inverse scattering problem. Although the inverse densities do not in themselves form an inversion algorithm, they do give a quantitative measure of the importance of performing particular measurements for the ultimate purpose of inversion. In addition, the degree to which different regions of a potential surface are correlated to a given set of cross sections are calculated by means of the densities {δV(R)/δV(R′)}. The overall numerical results contain elements which are physically intuitive as well as perplexing. This latter interesting and unexpected behavior is a direct result of allowing for unconstrained cross section ↔ potential response, as well as the presence of quantum interference processes. The present focus on elastic scattering is simply for the purpose of illustration of the functional variation technique which has broad applicability in all types of scattering processes.
Constructing approximately diabatic states from LCAO-SCF-CI calculations
Authors: Desouter-Lecomte M. , Dehareng D. , Lorquet J. C.
Published: July 2002

We consider here two approaches which have been proposed in the literature to obtain diabatic states from ab initio calculations. First, by calculating explicitly the coupling vector g=〈ψ2‖∇ψ1〉 which describes the nonadiabatic interaction between two adiabatic states ψ1 and ψ2. Second, by some extrapolation process of the wave functions obtained at a particular reference point. The coupling vector g is the sum of three contributions: g=gCI+gLCAO+gAO. The first two represent the change in character of the adiabatic states in the region of nonadiabatic coupling due to the variation of the CI and LCAO coefficients, whereas gAO results from the translation of the atomic orbitals with the moving nuclear centers. Criteria have been given to recognize when it is possible to transform a set of CI wave functions into a pair of useful diabatic states. A particularly favorable situation is obtained when the interacting electronic states are doubly excited with respect to each other. Within the two-state approximation, gCI is strictly irrotational and never gives rise to problems. One can expect situations in which this is also true for gLCAO. However, that part of the coupling represented by gAO can never be described as a rotation of two diabatic functions. Nevertheless, the latter contribution can frequently be neglected, at least when the nonadiabatic coupling is strong. The theory of the electron translation factors (ETF’s) provides further insight into the problem and confirms our conclusions.
Basis set quality vs size. Approximate Gaussian-type orbital (GTO) wave functions for first row transition metal atoms
Authors: Faegri Knut , Speis Hans Josef
Published: July 2002

Energy optimized basis sets ranging from (12, 6, 4) to (16, 11, 8) have been derived for the atoms Sc–Zn. The accuracy of these basis sets compared to numerical calculations range from 603 to 5 mhartree. The varying number of d electrons makes it difficult to establish uniform quality criteria through the series. The largest s2dn optimized basis sets also describe s1dn+1 states reasonably well, but should be augmented by diffuse d functions to describe s0dn+2 states.
Electron spin resonance (ESR) investigation of the structure of methyl radical trapping sites in methanol glass
Authors: Doba T. , Ingold K. U. , Reddoch A. H. , Siebrand W. , Wildman T. A.
Published: July 2002

Measurements are reported of ESR spectra of methyl radicals trapped in methanol glasses. In these spectra, forbidden lines appear as satellites of the lines of the methyl quartet as a result of dipolar coupling of the unpaired electron with protons of neighboring methanol molecules. The relative intensity of the satellites is used to study the structure of the sites where the radicals are trapped. Comparison of intensities observed in CH3OH, CH3OD, CHD2OD, CD3OH, and CD3OD indicates a structure that is locally similar to the (disordered) β-phase crystal structure of methanol, with the methyl radical replacing a methanol molecule and occupying a position close to its methyl position. The resulting methyl–methyl distances are compared with those deduced from the observed rate constants of the hydrogen abstraction reaction taking place at the trapping sites. If volume changes due to cooling and phase transitions are taken into account, the distances obtained in the two experiments are found to be compatible. This confirms earlier conclusions that methanol glass has many structural features in common with the β-phase crystal.
Investigation of variable angle sample spinning (VASS) NMR of quadrupolar nuclei. I. Theory
Authors: Lefebvre F. , Amoureux J.-P. , Fernandez C. , Derouane E. G.
Published: July 2002

Variable angle sample spinning NMR of quadrupolar nuclei in the solid state has been theoretically investigated in order to determine the optimal angles to achieve high-resolved and quantitative spectra. The earlier literature contributions have been included in a more general model. Complete analytical expressions were derived and are reported for all possible cases. It is concluded that magic angle spinning (at 54.7°) is to be used whenever the dipolar interactions and/or the chemical shift anisotropy dominate the spectra. For the other cases, θ=43.5° (which would allow an easy discrimination of the quadrupolar frequency and the asymmetry parameter), and θ=60°–70° (where the linewidth is half of that obtained for MAS conditions) are proposed as preferred spinning angles.
Comment on the pair correlation function in an inhomogeneous liquid: A test of the Fischer approximation
Authors: Harris Jonathan , Rice Stuart A.
Published: July 2002

Using the results of Monte Carlo simulations of cesium, we demonstrate that the simple point local density approximation is inaccurate in predicting the behavior of the transverse pair correlation function in the liquid–vapor interface of a simple metal. Our calculations demonstrate that a local density approximation, such as the one proposed by Fischer and Methfessel, which uses the point density averaged over a region the size of the ion, more accurately describes the behavior of the transverse pair correlation function.
Structure, vibrational spectrum, and energetics of the CH+5 ion. A theoretical investigation
Authors: Komornicki Andrew , Dixon David A.
Published: July 2002

A set of large-scale ab initio molecular orbital calculations on the CH+5 potential energy surface is presented. The final calculations were done with very large polarized basis sets and correlation corrections were included at the CI(SDQ) level. The geometries were gradient optimized, while the harmonic force fields and infrared intensities were also determined analytically at the SCF level. None of the modes are predicted to have an extremely large intensity. The frequency of the torsional mode for rotation of the H2 about the pseudo threefold axis of the CH+3 group is found to be extremely low. This makes the exact energy content of the ion difficult to define precisely. The proton affinity of CH4 has been calculated at the CI(SDQ) level including zero point energy and thermodynamic corrections. The calculated value is 129.0 kcal/mol as compared to an experimental value of 132.4 kcal/mol. It is suggested that the experimental value is somewhat too high.
Torsional dynamics of molecules on barrierless potentials in liquids. II. Test of theoretical models
Authors: Ben-Amotz Dor , Harris C. B.
Published: July 2002

The theoretical predictions of three theories for large amplitude dynamics on a barrierless excited state potential are tested. The predicted forms for the time dependent integrated excited state population are compared with experimentally measured excited state absorption decays of crystal violet in various solvents. We have generalized the early theory of Oster–Nishijima [J. Am. Chem. Soc. 78, 1581 (1956)] to allow for arbitrary placement of the initial excited state population relative of the nonradiative sinks which couple the flat excited state potential to the ground state. An analytic form for decay of the integrated excited state population is derived for this generalized flat potential model. In addition we have found that the predictions of Forster–Hoffmann [Z. Phys. Chem. NF 75, 63 (1971)] cannot be made to fit the experimentally measured excited state decays of crystal violet. Finally we show that the theory of Bagchi–Fleming–Oxtoby [J. Chem. Phys. 78, 7375 (1983)] can fit the decays observed in various solvents fairly well. The best fits, however, are obtained using the generalized flat potential model.
Fiber structure of space of molecular orbitals within LCAO method
Authors: Biel J. , Chatterjee R. , Lulek T.
Published: July 2002

A method for the construction and classification of single-electron molecular orbitals and spin orbitals by applying the theory of a fiber bundle has been developed. The molecule is described in terms of the vector bundle with the set of labels of constituent atoms and a space of atomic orbitals being respectively the base and a standard fiber. The configuration space of the molecule is that of all cross sections of this fiber bundle. It has been demonstrated that the factorization of the molecular symmetry group action in the space of all cross sections into the positional and one-center factor representations allows one to obtain some new classification indices related to these factors. In some important cases it yields a unique classification scheme of molecular orbitals, based entirely on the symmetry group of the molecule.
Rotational relaxation in a gear network
Authors: Zwanzig Robert
Published: July 2002

The time correlation function of total polarization of a network of interacting gears randomly placed on a square lattice is investigated. Near the percolation point, the tcf decays very slowly.
Crystallization and glass formation processes in liquid sodium: A molecular dynamics study
Authors: Watanabe M. S. , Tsumuraya K.
Published: July 2002

Crystallization and glass formation processes are studied for the sodium system with 108 atoms by means of molecular dynamics simulation with constant volume. The equilibrium liquids, with four different initial conditions, are cooled with four different rates. The processes are analyzed by means of Voronoi polyhedra, diffusion coefficients, pair-distribution functions, and mean-square displacements. Embryo formation process, being accompanied by the abrupt slope increment of the displacement, appears with incubation period prior to nucleation and growth processes. The temperatures for the embryo formation and crystallization decrease with increasing cooling rate. The diffusion mechanism in supercooled liquid before the embryo formation is the same as that in equilibrium liquid. The critical cooling rate, which separates glass formation and crystallization processes, is greater than 8.00×1013 K s−1.
A density functional treatment of the hard dumbbell freezing transition
Authors: McCoy John D. , Singer Sherwin J. , Chandler David
Published: July 2002

We present the first implementation of our density functional theory [J. Chem. Phys. 85, 5971, 5977 (1986)] to investigate a fluid–solid phase transition. In this theory, designed specifically for polyatomic systems, the entropy functional with bonding constraints is treated exactly, and approximations are generated by truncating expansions of the intermolecular interaction part of the free-energy density functional. We examine the theory resulting from the quadratic truncation of the interaction free energy, and determine the resulting phase diagram for hard dumbbell molecules. The results for short bond lengths are in accord with known trends from experiment and simulation. However, the theory predicts no plastic crystal transition for hard dumbbells with a bond length that might characterize nitrogen, for which the experimental β phase is a plastic crystal. Reasons for this behavior are discussed.
Upper and lower bounds for the rate of diffusion-controlled reactions
Authors: Richards Peter M. , Torquato S.
Published: July 2002

Recent upper and lower bounds established by the authors for the rate to diffuse to traps in a random medium are compared in a consistent manner by taking account of how the trap-free volume affects various definitions of the rate constant.
Systematic trends in van der Waals interactions: Atom–atom and atom–surface cases
Authors: Ihm G. , Cole Milton W. , Toigo Flavio , Scoles G.
Published: July 2002

Correlations are discussed for van der Waals interactions [V(x)] of two types: between two atoms and between an atom and a surface. Letting ε be the well depth and Cn be the asymptotic dispersion coefficient [such that V(x)∼−Cnx−n], it is shown that (Cn/ε)1/n is proportional to the equilibrium distance and to the decay length of the repulsive part of the interaction. A simple model which describes and unifies these correlations is presented.