bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of intramolecular quantum modes on free energy relationships for electron transfer reactions
Authors: Ulstrup Jens , Jortner Joshua
Published: February 2003

A general quantum mechanical description of exothermic electron transfer reactions is formulated by treating such reactions as the nonradiative decay of a ’’supermolecule’’ consisting of the electron donor, the electron acceptor, and the polar solvent. In particular, the role of the high-frequency intramolecular degrees of feedom on the free energy relationship for series of closely related reactions was investigated for various model systems involving displacement of potential energy surfaces, frequency shift, and anharmonicity effects. The free energy plots are generally found to pass through a maximum and to be asymmetric with a slower decrease in the transition probability with increasing energy of reaction. For high-frequency intramolecular modes this provides a rationalization of the experimental observation of ’’activationless’’ regions. Isotope effects are discussed as also are the oscillatory free energy relationships, predicted for low temperatures and high frequencies, and which are analogous to the vibrational structure in optical transitions.
Conductivity relaxation in positive dielectric liquid crystals
Authors: Schadt M. , von Planta C.
Published: February 2003

Measurements are reported of the frequency dependence of the conductivities σ⊥ and σ∥ in some nematic liquid crystals with strong positive dielectric anisotropy which are nematic at room temperature. The conductivity relaxation which causes σ∥ to increase already at frequencies <100 Hz in highly deionized liquid crystals are shown to be quantitatively in agreement with the dispersion of the orientation polarization of the molecules. The magnitude and sign of the conduction anisotropy σ∥−σ⊥ is shown to be strongly frequency dependent. The relaxation times τ⊥ and τ∥ due to the respective rotation of the molecules in a plane perpendicular and parallel to their long axes were determined.
A photoion–photoelectron coincidence (PIPECO) study of fragmentation rates and kinetic energy release in energy selected metastable ions
Authors: Tsai Bilin P. , Werner Arthur S. , Baer Tomas
Published: February 2003

The absolute fragmentation rates and kinetic energy releases associated with the unimolecular dissociation reactions of energy selected halo-hydrocarbon ions have been investigated by photoion–photoelectron coincidence (PIPECO) spectroscopy in which nominally zero kinetic energy electrons are detected in coincidence with their corresponding fragment ions. The reactions investigated were HCl loss from C2H5Cl+ and ClCH2CH2Cl+, Cl loss from C3H3Cl+ (propargyl chloride) and Br loss from C3H3Br+ (propargyl bromide). Although the kinetic energy released in the HCl loss from C2H5Cl+ reaction is small, the other three reactions have large energy release values associated with them. We found that X loss from C3H3X+ proceeds via two paths yielding different isomeric forms of C3H3+ with markedly different kinetic energy release values. The most unusual feature in these complex dissociation reactions is that the rate of fragmentation cannot be described by a single exponential decay function but rather must be expressed as the sum of at least two exponentials. Possible explanations for two-component decay rates and their implications for the statistical theory of unimolecular decay are discussed.
The one-dimensional CH3-quantumrotator in solid 4-methyl-pyridine studied by inelastic neutron scattering
Authors: Alefeld B. , Kollmar A. , Dasannacharya B. A.
Published: February 2003

The phenomenon of tunneling through potential barriers, representing motions which are classically forbidden, has been studied for the case of rotational motion of a methyl group in solid 4-methyl-pyridine. A sixfold hindrance potential of 11±3 meV is inferred from a comparison of the directly observed transitions between the three lowest levels and theoretical calculations.
Use of semiclassical collision theory to compare analytic fits to the interaction potential for vibrational excitation of H2 by He
Authors: Duff James W. , Truhlar Donald G.
Published: February 2003

Vibrational transition probabilities for strongly classically forbidden single-quantum and two-quantum transitions calculated by two semiclassical methods involving real-valued trajectories are compared to quantum mechanical close coupling for various analytic fits to the ab initio interaction potential for the He–H2 system. The final-value-representation integral expression from classical S matrix theory and the classical-trajectory forced-quantum-oscillator method are found to be in semiquantitative agreement with the quantum mechanical calculation even for transition probabilities as small as about 10−6. Further, the semiclassical methods reproduce the important trends in the results as functions of the interaction potential. The reliability of these semiclassical calculations allows one to determine the region of the potential energy surface which is sufficient for calculation of vibrational excitation probabilities. The important region for the present calculations is in the classically allowed region of the potential energy surface and at the fairly high total energy of 0.14 hartree includes the range of H2 distances 1.195–1.467 bohr for the forced quantum oscillator methods and the wider range 0.927–2.000 bohr for the more accurate final value representation. The region which must be known for an accurate calculation is more restricted than previous discussions had suggested. At lower energies, an even more restricted range of the potential energy surface contributed to the semiclassical calculations.
Deuteron and proton magnetic resonance studies of AlCl3⋅6D2O and AlCl3⋅6H2O
Authors: El Saffar Z. M. , Peterson E. M.
Published: February 2003

The angular dependence of the deuteron magnetic resonance line splittings recorded in one plane of rotation of a single crystal of AlCl3⋅6D2O have been used to establish the quadrupole coupling tensor and determine the deuteron positional coordinates. These coordinates are found to be 0.073, 0.187, 0.143; 0.202, 0.204, and 0.087. Proton relaxation times in polycrystalline AlCl3⋅6H2O have been measured as a function of temperature to evaluate the dynamic parameters associated with molecular motion. Twofold motion of H2O with an activation energy of 10.0±0.5 kcal/mole is found to dominate the relaxation process below 385 °K. A second motion with an activation energy of 17.0±1.0 kcal/mole appears above 385 °K and is likely to be translational diffusion of the water molecules.
Effect of electron correlation on the H2CO-He interaction potential
Authors: Garrison Barbara J. , Lester William A.
Published: February 2003

A previously reported Hartree-Fock (HF) interaction potential between H2CO(1A1) and He(1s) is modified through a series of configuration interaction (CI) calculations. The CI contribution is described by a three-term (l=0,1,2) Legendre polynomial expansion in the angle τ formed by the direction of incidence of He and the CO bond of formaldehyde. No significant azimuthal angle dependence is obtained. Correlation is found to have little effect in the strongly anisotropic repulsive region of the interaction potential but dominates the well and long-range regions. The maximum well depth is attained for in-plane approaches of He and lies in the range 35–40 °K for arbitrary τ at center of mass separation of 7.5 a.u. The CI contribution in the region of the minimum is beleived accurate to ∼20%.
Vibrational relaxation in H2–CO and D2–CO mixtures, measured via stimulated Raman-ir fluorescence
Authors: Matsui H. , Resler E. L. , Bauer S. H.
Published: February 2003

Rate constants for vibration–vibration and vibration–translation energy transfers were measured by exciting the hydrogenic species in a stimulated Raman cavity and recording the subsequent rise and decay of ir fluorescence from the admixed CO at 4.7 μm. Under the justifiable assumption that the intraspecies redistribution of vibrational energy is very rapid, the time variation of fluorescence intensities and their dependence on the sample composition can be accounted for by the following processes: CO(v)+H2→CO+H2, kC,H=14.3±0.7 sec−1⋅torr−1; H2(v)+H2→H2+H2, kH,H=4.3±0.1; D2(v)+D2→D2+D2, kD,D=0.9±0.3; H2(v)+CO→H2+CO(v), kvH,C=4.5{+2.1−1.5; D2(v)+CO→D2+CO(v), kvD,C=138±25. The ratio (kvH,C/kvD,C) is surprisingly small in view of the near resonant condition for Δv (H2) =−1 coupled with Δv (CO) =+2.
Exciton–charge carrier interactions in the electroluminescence of crystalline anthracene
Authors: Wittmer M. , Zschokke-Gränacher I.
Published: February 2003

The interactions of triplet excitons with trapped charge carriers are studied in anthracene single crystals. The triplet excitons are generated by the recombination of charge carriers through double injection. The lifetime of the triplet excitons is studied from the decay of the phosphorescence intensity in a transient measurement. The interaction rate constant for the trapped charge carriers is of the same order of magnitude as previous published data measured with single injection of electrons. It is further shown that the rate constant varies with different conditions of crystal growth. The simultaneous investigation of double injection delayed fluorescence reveals also a strong quenching of the delayed fluorescence from the excited singlet state by trapped charge carriers. Magnetic field measurements show that this quenching is partly due to an increased dissociation of the triplet pair state through the presence of charge carriers. The possibility of enhanced spin relaxation of the triplets in the pair state is also discussed.
Short-range order in liquid Li–Pb alloys
Authors: Ruppersberg Henner , Egger Hans
Published: February 2003

The neutron diffraction patterns of liquid lithium lead alloys containing 0, 17, 34, 48, 61, 80, and 100 at.% 7Li were measured. The long-wavelength limit of the partial structure factors aij(k) and of the concentration–concentration correlation function SCC(k) is calculated. These values agree well with the values calculated from activity data. For the 80 at.% alloy the total SCC(k) function was measured. It shows a pronounced first peak at the k value of a prepeak observed with x rays. The Fourier tranform of SCC(k) is related to the short-range order-size coefficients which have been introduced by Warren et al. (Ref. 19) to describe the local order of solid solutions. The data for the liquid Li–Pb alloys indicate a preference for unlike nearest neighbors and a reduction of the distance between unlike nearest neighbors as compared with the mean distance of the pure components. This short-range order is probably due to a partially saltlike bonding between lithium and lead ions. The over-all structure of the liquid alloys is similar to the structure of a hard sphere system.
Raman spectra of gases. XVIII.  Internal rotational motions in ethylamine and ethylamine-d2
Authors: Durig J. R. , Li Y. S.
Published: February 2003

The Raman spectra of gaseous CH3CH2NH2 and CH3CH2ND2 have been recorded from 100 to 600 cm−1. Both ground and excited states of the methyl and amino torsional modes for the gauche conformer have been observed. The overtone of the methyl torsion has also been observed for the gauche conformer. The methyl barriers in the gauche conformers of the CH3CH2NH2 and CH3CH2ND2 molecules were found to be 3.71±0.05 and 3.59±0.05 kcal/mole, respectively. The apparent difference in the barrier heights for these two isotopic species is attributed to a difference in the coupling between the methyl and amino torsions in these two molecules. The potential function for internal rotation around the C–N bond in CH3CH2ND2 was determined and the following potential constants found: V1=279±16 cm−1, V3=707±4 cm−1, V6=−12±3 cm−1. The energy difference between the potential energy minima of the gauche and trans conformations is 207 cm−1 (0.592 kcal/mole) with the trans being the more stable. The calculated gauche–gauche barrier is 779 cm−1 (2.23 kcal/mole) and the trans–gauche barrier is 572 cm−1 (1.64 kcal/mole). A similar potential function was determined for the CH3CH2NH2 isomer but the fit to this one-dimensional model was not as good as that obtained for the deuterium compound probably because of the greater coupling in the light molecule.
Paramagnetic defects in irradiated ferroelectric (NH4)2SO4
Authors: Bailey C. E. , Cuesta-Barro R. C.
Published: February 2003

EPR studies of x-ray damaged crystals of (NH4)2SO4 show that long lived defects, NH3+ ions, are produced in two inequivalent sites with each site having two magnetically inequivalent orientations. All sites have approximately the same isotropic hyperfine splittings which are found to be 19.0 G for nitrogen and −24.6 G for hydrogen. The anisotropic components are different for the two inequivalent sites indicating different amounts of restricted rotation for the two sites. A second transient defect is identified as SO3−.
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o valence bond calculations of the potential energy surface for H+H2
Authors: Norbeck J. M. , Certain P. R.
Published: February 2003

Valence bond calculations of the potential energy surface of H+H2 have been done. In all the calculations the hydrogen molecule bond length was kept at 1.4 a0. The isotropic potential V0(r) and the leading anisotropic potential V2(R) were obtained. Our caclulated V0 is in good agreement with the experimentally determined V0 reported by Gengenbach, Hahn, and Toennies. Our calculations predict that the triangular arrangement of H2+H is favored over the linear configuration at long distances of R with alignment to the linear geometry at approximately 3.1 a0. A populaion analysis of our wavefunction is also presented.
Inelastic light scattering from isotropic MBBA
Authors: Clark Noel A. , Liao York
Published: February 2003

The spectrum of light scattered from the isotropic phase of p-methoxybenzylidene p- (n-butylaniline) (MBBA) has been studied over a 50 °C temperature range above the nematic–isotropic transition temperature (46.2 °C). Light scattered through a wave vector K?2.7×105 cm−1 with frequency shifts f in the range 1 GHz<f<15 GHz shows two major contributions: a polarized shifted (Brillouin) peak of f∼6 GHz and a depolarized 1/(f)2 tail of an unshifted line. The adiabatic sound speed and the sound attenuation coefficient in the vicinity of 6 GHz have been determined from the Brillouin splitting and width, respectively, and compared to values measured at 23, 150, and 400 MHz. It is found that at 6 GHz substantial parts of the bulk and shear viscosities have relaxed out. Dispersion in the sound speed is also observed and used along with the linewidth measurements to estimate the relaxation frequency fr, which is found to be ∼2.5 GHz. The total dispersion and relaxation frequency are similar to those found in benzene. The ratio of peak Brillouin intensity to 1/f2 intensity is analyzed and shows the local dielectric anisotropy to be weakly temperature independent for 46<T<97 °C.
Piezoelectricity and long-range order in NH4Cl
Authors: Bruins Douglas E. , Garland Carl W.
Published: February 2003

The temperature dependence of the piezoelectric constant d14 of NH4Cl has been measured in the vicinity of the order–disorder phase transition. An analysis of data obtained in the ordered phase yields β=0.134 for the critical exponent associated with the order parameter. Above the transition temperature, there is an anomalous ’’tail’’ of nonzero d14 values that indicates a persistence of long-range order in the ’’disordered’’ phase. The values of d14 in this tail are well described by the empirical expression d14=const×exp(−σT/Tc) with σ=335. This anomalous behavior is essentially identical to that observed previously for the second-harmonic scattering intensity.
Bonded and nonbonded interactions in saturated hydrocarbons
Authors: Kao James W. H. , Chung-Phillips Alice
Published: February 2003

The enthalpy of formation (ΔH°f) of an alkane is expressed as a linear function of eight structure parameters (N) that are essential in characterizing stable conformations of the molecule. Using the least squares criterion, energy parameters (X) which correspond to the linear coefficients of the structure parameters are calculated from experimental ΔH°f. For acyclic alkanes and polycyclic alkanes with six-membered rings, 80 liquids and 63 gases are used. The mean and root-mean-square deviations between the calculated and experimental ΔH°f are, respectively, ±0.18 and ±0.24 kcal mole−1 for liquids and ±0.20 and ±0.28 kcal mole−1 for gases. The resulting energy parameters may be used for predicting the unknown ΔH°f of higher alkanes and for estimating enthalpy differences between certain conformers. For the latter application, those involving the 1,4 and 1,5 interactions represented by the gauche CCCC (XG), gauche–gauche prime CCCCC (XV), and anti–anti CCCCC (XW) interactions are particularly useful. The energy parameter for ring strain (XR) appears necessary for bringing the predicted ΔH°f of gaseous adamantane and diamantane into reasonable agreement with available experimental data.
Validity of approximate methods in molecular scattering: Thermal HCl–He collisions
Authors: Green Sheldon , Monchick L.
Published: February 2003

Accurate close coupling scattering calculations are presented for thermal energy HCl–He collisions. The interaction potential is obtained from the Gordon–Kim electron gas model, adjusted to have the correct long-range multipole form. A variety of phenomenological cross sections are computed from the close coupling S matrix, and these are compared with results from several commonly employed approximate methods. In particular, it is found that the total integral, total differential, and gas kinetic cross sections are accurately predicted by the central field approximation which retains just the spherical average of the interaction. Integral inelastic cross sections are represented quite accurately by the coupled states approximation of McGuire and Kouri, but only qualitatively by the effective potential method of Rabitz. Pressure broadening cross sections from the close coupling calculation are in much better agreement with experiment than either Anderson theory calculations or the classical trajectory study of Gordon. NMR spin–lattice relaxation cross sections are also presented.
The microwave spectrum of HO2 near 65 GHz
Authors: Beers Yardley , Howard Carleton J.
Published: February 2003

Using a Zeeman-modulated cavity spectrometer with a 10 sec time constant and a phase locked klystron, we have observed in the products of a discharge-flow system, Zeeman components of the six allowed zero-field lines at 65 070±2, 65 082±2, 65 098±2, 65 373±2, 65 397±2, and 65 401±2 MHz, and of one forbidden zero-field line at 65 369±4 MHz. The Q of the Fabry–Perot cavity is about 10 000, and the magnetic field was swept from 0–30 G. Chemical tests indicate that the observed lines are due to HO2. They have been assigned and least-squares fitted using a simple theoretical model to yield a value of 65 185±2 MHz for the 101–000 asymmetric rotor transition frequency of HO2, a value of −208±2 MHz for the linear combination (εbb+εcc)/2 of elements of the electron spin–molecular rotation interaction tensor, a value of −28±2 MHz for the nuclear spin–electron spin Fermi contact interaction parameter σ, and a value of +4±2 MHz for the spin–spin tensor interaction parameter λ. These constants are in excellent agreement with three less precise constants obtained from an earlier laser magnetic resonance study and have been confirmed by recent more accurate measurements of Saito.
Electron paramagnetic resonance of Eu2+ in the alkali iodides
Authors: Rubio O. J. , Murrieta S. H. , Muñoz P. E. , Boldú O. J. , Aguilar S. G.
Published: February 2003

The electron paramagnetic resonance of divalent europium impurity in the isomorphic cubic hosts of NaI, KI and RbI has been observed at X-band frequencies. The spin-Hamiltonian parameters for EU2+ in orthorhombic sites of NaI, KI, and RbI have been determined at 94 and 300 K. The dominant second order spin-Hamiltonian parameter b02 for the observed symmetry was found to be positive for all sodium, potassium and rubidium iodides at liquid helium temperatures. The behavior of the b02 term with lattice size in the alkali iodide series is discussed on the basis of the one observed for the chlorides and bromides. Considerable disagreement between the spin-Hamiltonian parameters for Eu2+ in KI obtained in this work and those previously reported by Poret and Lambert is noted.
Vibrational energy transfer in molecular collisions: An information theoretic analysis and synthesis
Authors: Procaccia I. , Levine R. D.
Published: February 2003

State-to-state rate constants for vibrational energy transfer (both V–V and V–T) are characterized using an ’’exponential gap’’ representation. The collisions analyzed include experimental results for both V–T and V–V transfer rates, trajectory computations for V–T transfer rates (via both inelastic and reactive, atom exchange, collisions) and quantal computations for V–V transfer rates. The results are analyzed in terms of the surprisal, a measure of deviation of the actual rate from the prior (or reference) rate. The prior rates are computed on the basis of the assumption that (at a given collision energy) all final states are equally probable. For almost all cases the surprisal could be characterized by a single parameter. The magnitude of this parameter (and its temperature dependence) could be predicted using bulk averages only. Such predictions are demonstrated for both experimental and trajectory computed rates. The link between the microscopic dynamics and macroscopic kinetics is forged by the use of sum rules (which provide constraints on the rates) and of the principle of maximal entropy (which is used to determine the least biased set of rate constants subject to the constraints).
Liquid crystal phase diagrams: A study of seven thermotropic materials
Authors: Keyes P. H. , Weston H. T. , Lin W. J. , Daniels W. B.
Published: February 2003

We report and discuss the pressure–temperature phase diagrams for seven materials having liquid crystal phases: N- (p-methoxybenzylidene) -p-butylaniline (MBBA), N- (p-ethoxybenzylidene) -p-butylaniline (EBBA), N- (p-methylbenzylidene) -p-butylaniline (MBA), cholesteryl oleyl carbonate (COC), 4,4′-bis (heptyloxy) -azoxybenzene (HAB), cholesteryl nonanoate (CN), and N- (p-cyanobenzylidene) -p-octyloxyaniline (CBOOA).
Higher-order decoupling of the electron propagator
Authors: Redmon Lynn Tyner , Purvis George , Öhrn Yngve
Published: February 2003

The decoupling of the electron propagator in the superoperator formalism is extended to include a description of processes corresponding to simultaneous electron ionization (or electron attachment) and double excitation, observed in electron spectrometry, by including products of five spin–orbital field operators in the inner projection basis. Although the terms thus introduced are fourth order in the electron interaction, an argument based on the self-consistent solution of the second-order equations suggests that these fourth-order terms are more important than the third-order terms. The argument is substantiated by numerical applications to the neon atom. A calculation using third-order shifts is compared with a calculation using the suggested fourth-order terms, but without third-order terms.
Concentration fluctuations in chemical reactions
Authors: Keizer Joel
Published: February 2003

A theory of microscopic fluctuations, which is based on generalized fluctuation–dissipation assumptions, is used to describe concentration fluctuations in a uniform system undergoing chemical reactions. The theory is shown to agree with the linear Langevin-type theory of concentration fluctuations near equilibrium, and provides a method for calculating the time evolution of an arbitrary initial probability distribution for a system far from equilibrium. These results are easily compared to the birth and death (master equation) theory of chemical reactions, and recent work on the asymptotic solutions of these master equations indicates that in the limit of a macroscopic system, the two approaches are identical.
Parallel polarizabilities of diatomic molecules via the partial-wave SCF procedure
Authors: McCullough E. A.
Published: February 2003

The results of polarizability calculations on several small diatomic molecules using the recently implemented partial-wave self-consistent-field (PWSCF) method are reported. These calculations were done with the sigma orbital version of the PWSCF program so that only αzz, the polarizability tensor component parallel to the internuclear axis, could be computed. The results are compared with matrix SCF (MSCF) results and with exact results, where available. (AIP)
Detection of hydrogen atom beams by stimulated surface ionization
Authors: Gordon R. J. , Hsu D. S. Y. , Lee Y. T. , Herschbach D. R.
Published: February 2003

The flux of a hydrogen beam has been measured by ionization of an alkali metal coating on a tungsten filament. It is suggested that this method is generally applicable to extending the range of surface ionization detectors. (AIP)
Proton and deuteron relaxation in aqueous solutions of gadolinium (III)
Authors: Reuben Jacques
Published: February 2003

In previous proton relaxation studies of the gadolinium ion, the estimation of the mean residence time of a proton in the aqueous complex has been ambiguous. It is shown that this ambiguity can be resolved by comparison of deuteron and proton relaxation times. The method is applied to aqueous solutions of GdCl3. (AIP)
Laser induced fluorescence from NH2(2A1). State selected radiative lifetimes and collisional de-excitation rates
Authors: Halpern J. B. , Hancock G. , Lenzi M. , Welge K. H.
Published: February 2003

Time resolved fluorescence from the first excited (2A1) state of NH2 has been observed following excitation of the radical in its ground state by means of a pulsed tunable dye laser. Specific rotational levels within a number of vibronic states were populated, decay rates measured as a function of total pressure for a variety of added gases, and zero pressure lifetimes and collisional de-excitation rates evaluated. Measured zero pressure lifetimes are good approximations to the vibrational state radiative lifetimes, typically 10 μsec for the (0, 9, 0) state. Collisional de-excitation rate constants were measured as 1.0×10−9 cm3 molecule−1⋅sec−1 for NH3, independent of vibronic state, and for the Σ (0, 9, 0) level were found for other gases in the ratio NH3:CO:H2:N2:CH4:Ar:He=1.0:0.47:0.46:0. 40:0.30:0.152:0.145. Using excitation by a tunable cw dye laser, steady state spectra of NH2 have been obtained and collisional energy transfer observed within the (2A1) excited electronic state of NH2. Transfer was observed both within the initially populated vibronic state and to other such states within the same overall vibrational level. The symmetric or antisymmetric character of the rotational level remained unchanged in collision, i.e., only a↔a and s↔s transfer occurred.
Vibrational spectra and torsional barrier of cyclopentadienyl beryllium borohydride, C5H5BeBH4
Authors: Coe Douglas A. , Nibler Joseph W. , Cook Thomas H. , Drew Douglas , Morgan George L.
Published: February 2003

Infrared and Raman spectra are reported for liquid and solid C5H5BeCl, C5H5BeBH4, and C5H5BeBD4 and vibrational assignments of most of the C5H5, BeX, and skeletal modes are made. Conclusive evidence for a double hydrogen bridge configuration is found for C5H5BeBH4 and, in contrast to Be(BH4)2, this structure is unchanged in going from gas to liquid to solid. The C5H5 frequencies are in close accord with those of (C5H5)2Fe while the vibrations of the BeBH4 group are analogous to those reported earlier for the +BeBH4 ion in solid Be(BH4)2. Observation of several transitions for the skeletal torsional motion of C5H5BeBH4 implies that free internal rotation about the symmetry axis does not occur and calculations yield a value of 500 cm−1 for the tenfold torsional barrier. Raman polarization data are consistent with a low energy orientation which places the BeH2bB bridging plane and a Cs molecular symmetry plane in coincidence.
An equations of motion approach for open shell systems
Authors: Yeager D. L. , McKoy V.
Published: February 2003

A straightforward scheme is developed for extending the equations of motion formalism to systems with simple open shell ground states. Equations for open shell random phase approximation (RPA) are given for the cases of one electron outside of a closed shell in a nondegenerate molecular orbital and for the triplet ground state with two electrons outside of a closed shell in degenerate molecular orbitals. Applications to other open shells and extension of the open shell EOM to higher orders are both straightforward. Results for the open shell RPA for lithium atom and oxygen molecule are given.
Method of measurement of nonzero average dipolar spin coupling in molten polymers
Authors: Cohen-Addad J. P.
Published: February 2003

The present paper concerns a problem more general than that related to NMR studies of molten polymer systems. It aims at defining a method of measurement of the nonzero average dipolar spin coupling in the case where (i) spin–spin relaxation rates due to nucleus motion, (ii) field inhomogeneities, and (iii) chemical shifts are of the same order of magnitude as the nonzero average dipolar coupling, in frequency units. It is shown that by using pulse sequences giving rise to appropriate response functions of the transverse magnetization, contributions (i), (ii), and (iii) can be eliminated. As an example, the experimental determination of the nonzero average dipolar coupling is considered, using a cis-1,4-polybutadiene sample.
An experimental and mechanistic study of the reactions of COF2 with H2 and with CO
Authors: Gangloff H. J. , Milks D. , Maloney K. L. , Adams T. N. , Matula R. A.
Published: February 2003

The decomposition of carbonyl fluoride (COF2), the reactions of carbonyl fluoride with carbon monoxide (CO), and the reactions of carbonyl fluoride with hydrogen (H2) in excess argon were studied behind reflected shock waves. Kinetic data were obtained in the temperature and total pressure ranges 2600–3300 °K and 2.5–9.0 atm, respectively, for the COF2 decompostion study, in the temperature and total pressure ranges 2400–3000 °K and 3.3–7.7 atm, respectively, for the COF2–CO study, and in the temperature and total pressure ranges 1900–2700 °K and 2–12 atm, respectively, for the COF2–H2 study. Under the experimental conditions, the bimolecular reaction between COF2 and CO was found to be negligible with respect to the decomposition of COF2. An empirical relationship for the over-all kinetics of the COF2–H2 reaction was determined, and a combined analytical and experimental study was conducted to determine a plausible mechanism for the COF2–H2 reaction. Under the experimental conditions, the dominant reaction steps in this mechanism were determined to be COF2+M →k1 COF+F+M, COF+M →k2 CO+F+M, H2+M → H+H+M, F+H2 → HF+H, H+COF2 →18 COF+HF, H+COF →20 CO+HF. In these studies an experimental value of k1 was obtained as k1 = 1014.25±0.24 exp(−77 200 ± 3300/RT) cc/mole⋅sec. The rate constant for Reaction 2 was derived based on experimental data, a steady state analysis, and an estimate of the ratio of the rate constants of Reactions 18 and 20. The derived value for k2 is given as k2 = 1015.0 exp(−49 000/RT) cc/mole⋅sec. The heat of formation of COF at absolute zero was determined to be approximately equal to −66 kcal/mole relative to the JANAF standard state definition.
The oxidation and reduction of succinic acid by ionizing radiation
Authors: Budzinski Edwin E. , Box Harold C.
Published: February 2003

The oxidation of succinic acid was observed by ESR–ENDOR spectroscopy in single crystals x irradiated at 4.2 °K. Oxidation products result from loss of hydrogen from the saturated carbon atoms as well as from the carboxyl groups. Reduction is a more specific process; electrons are captured by carbonyl oxygen atoms. The competition for electrons between succinic acid and fumaric acid in doped crystals was examined at 4.2 °K.
Critical temperatures of continuous spin models and the free energy of a polymer
Authors: Gerber Paul R. , Fisher Michael E.
Published: February 2003

The critical temperature Tc(n,d) of a classical n-component spin model with a general continuous spin distribution on a d-dimensional hypercubic lattice is expanded in inverse powers of d to order 1/d3. The general result differs significantly from the special case of fixed spin length owing to changes in the graphical structure of the high temperature expansion. In the limit n→0 the model becomes identical to the general self-interacting random walk or polymer problem: The Boltzmann factors for self-intersections in the walks correspond to the moments of the spin distribution functions, and Tc(0,d) yields the polymer free energy.
Some further comments on correlation-function formulas for polymer intrinsic viscosity
Authors: Yamakawa Hiromi , Tanaka Genzo
Published: February 2003

A correlation-function formalism for the intrinsic viscosity of polymers is reinvestigated. It is shown that the nonprojected part of the total momentum flux for the solution may be neglected for long enough chains and that the diffusion-force term does not appear in the flux for the polymer in the diffusion limit.
Emission spectra of XeBr, XeCl, XeF, and KrF
Authors: Brau C. A. , Ewing J. J.
Published: February 2003

The emission spectra of XeBr, XeCl, XeF, and KrF at high pressure are reported and discussed. The spectra were obtained by observing spontaneous emission from electron beam excited mixtures of argon containing lesser amounts of xenon with the halogens or krypton with fluorine. The emitting state in these species is best described as an ionic species Xe+X− or Kr+F−. The wavelengths of these emission bands are in good agreement with a theoretical model in which the ionic binding energy of the noble gas halide ion pair is roughly equal to that of the nearest alkali halide. Our high pressure spectra imply that the lowest potential energy curve for XeF is bound.
New semiclassical treatments of rotational and vibrational transitions in heavy-particle collisions. I. H–H2 and He–H2 collisions
Authors: McCann K. J. , Flannery M. R.
Published: February 2003

Two new semiclassical methods—the multistate orbital treatment and the multichannel eikonal treatment—are proposed for the description of rotational and vibrational excitation in heavy-particle collisions. The first method includes appropriate trajectories determined from a certain optical potential designed to couple the response of the internal structure, which is described by a quantal multistate expansion, to the orbit for the relative motion and vice versa. While this approach is, in general, valid when the quantal imprecision in the classical trajectories is small (as for heavy particles) the second method based on the use of a straight-line eikonal for the relative motion, of different local momenta in the various channels and of a multistate expansion for the internal motions, is valid for scattering mainly about the forward direction. These procedures are applied to representative rotational transitions in H–H2 and He–H2 collisions at 0.25–1.5 eV and yield angular distributions and integral cross sections in very close accord with corresponding quantal results. The methods are particularly valuable at higher impact energies when the inclusion of the resulting many rotational and vibrational channels by full quantal treatments is prohibitively difficult. Various approximate schemes—the perturbed-rotating-atoms approximation and the effective potential method—are also investigated.
Proton-decoupled carbon-13 relaxation in 13CH2 and 13CH3 spin systems
Authors: Werbelow Lawrence G. , Grant David M.
Published: February 2003

The transient behavior of the carbon magnetization in proton decoupled spectra is discussed in detail for 13CH2 and 13CH3 spin systems. The influence of multispin dipolar cross correlation effects is shown to influence the decoupled inversion-recovery experiment and lead to a predicted biexponential recovery of the carbon magnetization. It is rationalized that for methyl relaxation, the interference effects will play an inconsequential role. However, in the methylene case, it is demonstrated that there may arise many possible instances when it will become necessary to consider these correlation effects in much greater detail. It is also shown that if (1) extreme narrowing arguments are invalid or (2) other relaxation mechanisms compete with dipolar interactions, then the NOE enhancement factors deviate from those predicted from conventional treatments. For many conceivable situations, the inferred relaxation rate will underestimate the sum of the dipolar and nondipolar contributions whereas the usage of Overhauser enhancements will lead to an overestimation of the relative importance of nondipolar contributions. Again, this facet of cross correlation effects appears to be of more general concern in CH2 than CH3 studies.
Representations of molecular force fields. I. Ethane: A
b i
n
i
t
i
o and model, harmonic and anharmonic
Authors: Bartell L. S. , Fitzwater Susan , Hehre Warren J.
Published: February 2003

The quadratic and selected cubic force constants for ethane have been computed, using single determinant molecular orbital wavefunctions at the 4-31G level, with a view to testing and extending model consistent force fields (CFF) for ’’molecular mechanics’’ calculations. Results agree semiquantitatively with experiment, but experimental force constants of sufficient reliability to provide a definitive comparison are not yet available. In a comparison with the most rational general CFF available, that of Ermer and Lifson, the most significant discrepancies found to occur are those for certain stretch–bend couplings assumed to be zero in the CFF but shown to be appreciable by quantum calculation. It is observed that these couplings, but not the stretch–stretch couplings, are well accounted for by a steric interaction model. The ab initio cubic constants examined display the same pattern of conformity with a steric model. Bend–bend–bend and bend–bend–stretch but not all stretch–stretch–stretch interactions agree with those of the steric model. The partial success of the steric model shows that it is possible to represent a large number of interaction constants, quadratic and higher order, by a small number of parameters in molecular mechanics. The failure of the steric model to account for predominantly stretching interactions confirms that ’’classical’’ nonbonded interactions as embodied in conventional Urey–Bradley fields are not the only major contributors to off-diagonal force constants. An alternative model, the anharmonic model of Warshel, as modified by Kirtman et al., was found to account well for pure stretches but not for bends or stretch–bend interactions.
Positron–hydrogen scattering by the Faddeev approach
Authors: Mandal Puspajit , Saha B. C. , Sil N. C.
Published: February 2003

An approximate form of the Faddeev equations has been employed to investigate the e+–H scattering problem with the inclusion of the couplings of the 1s, 2s, and 2p states of the hydrogen atom and the ground state of the Ps atom. As no experimental measurements have been reported until now, the present predictions with and without the inclusion of the Ps formation have been compared with other existing theoretical findings. In the low energy region the effects of the Ps-formation channel on the elastic cross sections have been found to be quite important. The present values for the Ps-formation cross section are slightly smaller than the FBA results. The results for 1s–2s and 1s–2p excitations are seen to be influenced considerably in the low energy region by the inclusion of the Ps-formation channel whereas from E?150 eV these results are very close to the corresponding FBA values.
Macromolecular dimensions obtained by an efficient Monte Carlo method without sample attrition
Authors: Wall Frederick T. , Mandel Frederic
Published: February 2003

The statistical dimensions of macromolecular chains of fixed contour length can be rapidly calculated by Monte Carlo methods applied to a model consisting of dynamic self-avoiding random chains on a lattice. This ’’slithering snake’’ model involves moving the head of a chain one space in a lattice with all other elements of the chain moving forward along the old contour. Possible moves of the head are selected at random, but if such a move is precluded by double occupancy, the old configuration is retained, with head and tail interchanged, and then counted as if a move were made. This technique gives unbiased statistical results except for the effect of double cul-de-sacs. The method can also be applied to interacting chains, either free or confined to a box. Calculations have been made for 10-link chains on a square planar lattice for two different concentrations in infinite space and for two concentrations in a small box.
Heat capacity, entropy, enthalpy, and Gibbs energy of 242PuF4 from 10 to 350 °K
Authors: Osborne Darrell W. , Flotow Howard E. , Fried Sherman M. , Malm John G.
Published: February 2003

Heat capacity measurements have been made on a 20.9 g sample of 242PuF4 from 10 to 350 °K. The measurements from 20 to 350 °K were made by intermittent electrical heating with corrections based on the observed temperature drifts. Heat capacities determined in this way were compared with temperature drifts without electrical heating in the range 20 to 56 °K to evaluate the radioactive self-heating rate as (1.214±0.004) ×10−4 W/g Pu. From 10 to 20 °K heat capacities were obtained from this value of the radioactive self-heating rate and the temperature drifts without electrical heating. The heat capacity curve had the normal sigmoid shape. At 298.15 °K the heat capacity C°p, entropy S °, enthalpy H °−H °0, and tempered Gibbs energy (G °−H °0)/T are, respectively (116.19±0.29) J °K−1⋅ mole−1, (147.25±0.37) J °K−1⋅ mole−1, (22 385±56) J mole−1, and −(72.17±0.18) J °K−1⋅ mole−1. The changes in various thermodynamic functions for the formation of PuF6(g) from PuF4(c) and F2(g) are evaluated at 550 and 298.15 °K.
A semiclassical, nonperturbative approach to collision-induced transitions between rotational levels for the N2–Ar system
Authors: Mehrotra Suresh C. , Boggs James E.
Published: February 2003

A semiclassical and nonperturbative approach in which coupling between translational and rotational degrees of freedom is also taken into account is developed to compute rotationally inelastic transition probabilities of linear molecule–atom collisions. This approach is applied to N2–Ar collisions. The transfer of rotational, translational, and interaction energies is also computed during collisions. The results show that if the coupling between rotational and translational degrees of freedom is neglected, the computed transition probabilities change significantly for strong collisions, although there are no significant changes in the corresponding trajectories.
Reactions of modulated molecular beams with pyrolytic graphite. III. Hydrogen
Authors: Balooch M. , Olander D. R.
Published: February 2003

The reaction of hydrogen and pyrolytic graphite was studied by modulated molecular beam–mass spectrometric methods. Because molecular hydrogen does not react with graphite (within the detection limits of the technique), an atomic hydrogen reactant beam was generated by thermal dissociation of H2 in an effusion oven. For surface temperatures up to 800 °K, methane was the sole product. Acetylene was observed at temperatures above 1000 °K. Between 800 and 1000 °K, no carbon gasification occured; the surface acted only to recombine H atoms to form H2. The data were analyzed in terms of a model in which methane is formed by sequential addition of H atoms to CHn (n=0,1,2,3) and acetylene is formed by surface combination of two CH groups. The agreement between the model and the molecular beam data is very good. The model also predicts previous high pressure (1 atm H2) carbon gasification rates determined in conventional kinetic experiments.
Oxygen–oxygen pair correlation function for computer-simulated liquid water
Authors: O’Reilly D. E.
Published: February 2003

The oxygen–oxygen pair correlation function g00 for computer-simulated water is calculated for the ice Ic–VII quasilattice model of liquid water and compared with the experiments of Stillinger and Rahman (SR) both at 1 atm and at a density corresponding to a pressure of 22 kbar. The agreement between model and experiment is good and the basic differences between the SR water and real water are described.
Pressure dependence of rotational relaxation times T1 and T2 in the J=0−1 transition of OCS
Authors: Coy Stephen L.
Published: February 2003

Time-domain experiments on the OCS J=0−1 ΔM=0 microwave transition have been used to measure the pressure dependence of two relaxation times: for radiation from a coherently excited sample, 1/T2=37.66±0.10 μsec−1⋅Torr−1, and for the population difference between the J=0, M=0, and J=1, M=0, states, 1/T1=38.10±0.18 μsec−1⋅Torr−1. These results imply that not all collisions induce dipole-allowed transitions. These values are consistent with previous linewidth values, beam maser experiments, and four-level double resonance experiments.
Analysis of Monte Carlo results on the kinetics of lattice polymer chains with excluded volume
Authors: Hilhorst H. J. , Deutch J. M.
Published: February 2003

Monte Carlo calculations by Verdier et al. on the kinetics of polymer chains on a lattice have shown a large increase of relaxation times in the presence of excluded volume restrictions, i.e., when two beads of the chain cannot occupy the same lattice site. We show that these long relaxation times must be attributed to the specific choice of the kinetics rather than to the intrinsic nature of the excluded volume interaction. A simplified analytic model which preserves the essential characteristics of the kinetics of Verdier’s model reproduces qualitatively the Monte Carlo results for the realxation of the squares of the Rouse coordinates.
Study of the temperature dependence of chlorine-35 nuclear quadrupole resonance in ClF5
Authors: Weulersse J. M. , Virlet J. , Guibé L.
Published: February 2003

The 35Cl nuclear quadrupole resonance frequency νq has been studied as a function of temperature between 4.2 and 135 °K. No break is seen on either the νq(T) curve or its derivative in spite of a phase change observed in the fluorine NMR at 117 °K. The quadrupole coupling constant observed is discussed in terms of the occupancy of the 3p orbitals on chlorine as determined by molecular orbital calculations and compared to the value found by microwave spectroscopy. The temperature dependence of νq(T) is interpreted in the light of Bayer’s theory and a value of the lowest libration frequency is obtained (νl?30 cm−1) which is also found consistent with the value of the relaxation time, T1, of 35Cl nuclei.
The effect of pressure on the near-neighbor distance in water
Authors: Whalley E.
Published: February 2003

When liquid water is compressed, the O−−O hydrogen bonds tend to contract because of the direct effect of the pressure, and the O−−O−−O angles tend to deviate more from the tetrahedral. The coupling between the O−−O stretching and O−−O−−O angle bending is such that the change of the angles tends to increase the bond lengths. The net effect of pressure is the resultant of the direct contraction and the indirect expansion. It is suggested that the actual contraction can be measured by the effect of pressure on the O–H or O–D stretching frequencies of HDO in dilute solution in D2O or H2O. The application of this suggestion to literature data shows that the bond contracts only about 0.3 of the rate that the hydrogen bond in ice contracts. No doubt the contraction due to the direct effect of pressure is largely compensated by the expansion due to the distortion of the O−−O−−O angles.