bluuebunny/crossref_metadata_2025 huggingface dataset:
Generalized oscillator strengths for 7.4 eV excitation of H2O at 300, 400, and 500 eV kinetic energy. Singlet-triplet energy differences
Authors: Lassettre Edwin N. , Skerbele Ausma
Published: February 2004

Generalized oscillator strengths f have been determined as a function of scattering angle for the 7.4 eV excitation of H2O vapor by studying the scattering of electrons with kinetic energies of 300, 400, and 500 eV. Over the angle range studied, 2°–7°, f vs (ΔP)2 (ΔP the momentum change) lie approximately on a common curve at all three kinetic energies. Extrapolation to zero momentum change gives the value f0 =0.060±0.006 for the limiting oscillator strength. Singlet-triplet energy differences are calculated using the generalized oscillator strengths and a recently developed theory by Lassettre and Dillon.
Raman spectroscopic study of vibrational lifetimes of a tricyclic hydrocarbon system in the liquid state
Authors: Baglin F. G. , Thomas Rasma B. , Fickes G. N.
Published: February 2004

The Fourier transformed spectra of a totally symmetric A1 vibration in a large organic molecule, C8H12, is reported and interpreted. Equivalence of the widely used Rakov-Sykora deconvolution procedure to a purely experimental technique offered recently by Griffiths is discussed. Using Raman spectroscopy at various temperatures, a comparison of the intrinsic vibrational lifetimes and the reorientational relaxation times is effected that allows us to propose a mechanism for intramolecular energy transfer for this A1 normal coordinate of C8H12 in the liquid state.
Derivation of and comments on Bonaccorsi-Scrocco-Tomasi potentials for electrophilic additions
Authors: Anderson Alfred B.
Published: February 2004

The Bonaccorsi-Scrocco-Tomasi electrostatic potential fields in molecules are shown to have the same form as potential surfaces derived by assuming that the Hellmann-Feynman force theorem holds for a rigid atom interacting with the rigid molecule. By using the Anderson-Parr theory of vibrational force constants, which assumes the same interaction model, the analytic form of corrections which make a Bonaccorsi-Scrocco-Tomasi potential exact near equilibrium in a diatomic molecule is shown to be proportional to |Rα|−1, where Rα is the position of nucleus α.
Collision dynamics of three interacting atoms: Permutational symmetry for identical nuclei
Authors: Micha David A.
Published: February 2004

Expressions have been obtained for scattering probability amplitudes of direct, atom-exchange, and dissociative processes in atom-diatom collisions involving two or three identical nuclei. This is done in terms of free states written as products of electronic, nuclear coordinate, and nuclear spin wavefunctions. Symmetrized amplitudes are re-expressed in terms of unsymmetrized ones and nuclear spin weights are factored out. The Faddeev equations for identical nuclei interacting through sums of pairs of spin-dependent potentials are uncoupled and used to expand amplitudes in powers of pair transition operators. It is found that a spectator-type mechanism contributes to the direct processes in the single collision approximation. The corresponding probability amplitudes are explicitly written down.
Interpretation and utilization for crystal stucture determination of ESR spectra of single crystals of nitroxide free radicals
Authors: Capiomont Annie , Chion Bernadette , Lajzerowicz-Bonneteau Janine , Lemaire Henri
Published: February 2004

Since nitroxide free radicals are spin labelled molecules, we use their resonance spectra to get information about their crystallographic structure. ESR spectra of nitroxide single crystals (pyrroline, piperidine, and pyrrolidine) show only a single line. From the evolution of line position with crystal orientation we try to obtain information about the orientations of g tensors. Generally, we calculate the angles defining individual cc > NO groups orientations with an accuracy better than 5 deg. Principal g values are given for several nitroxides.
Depolarized Brillouin spectrum of liquid CS2
Authors: Enright G. , Stoicheff B. P.
Published: February 2004

The depolarized doublet found in the Brillouin spectra of many molecular liquids has been observed in liquid CS2. This doublet is best resolved at 162°K (near the freezing point); it broadens rapidly with increasing temperature and is unresolved at temperatures higher than 192°K. In this temperture range, the half-widths increase from 11 to 19 GHz, yielding orientational relaxation times of 14×10−12 to 8×10−12 sec, respectively. The observed spectra are in good agreement with the predictions of hydrodynamic (Rytov) and microscopic (Keyes and Kivelson) theories.
Polarization study of electronic transitions in molecules in a stretched PVA matrix using their projected three-dimensional vibrational space
Authors: Gangakhedkar N. S. , Namjoshi A. V. , Tamhane P. S. , Chaudhuri N. K.
Published: February 2004

A method is described for determining the direction of an optical transition in the chain molecules or guest molecules in a stretched polymer matrix with a quantitative elegance comparable to that of polarized single crystal spectroscpy (PSCS). The method consists in using the three-dimensional vector (ray) space formed by the vibrational and electronic transition moments of the molecules. It determines the polarization ratios for four of these transitions: Three of them are linearly independent and constitute a monoclinic system of axes; the remaining one is the transition whose direction in the vibrational vector space of the molecules is to be determined. The method leads to the determination of an orientation parameter K that is described in terms of the molecular orientation distribution function of the relevant molecules in the stretched polymer matrix, without assuming any explicit form of that function. It has been applied to three aromatic molecules incorporated as guests in the PVA matrix and gives directions of electronic transitions in them that differ in most cases from those determined previously on the basis of assumptions regarding their molecular orientation distribution function in the stretched PVA matrix and the identity of their molecular orientation axis. These directions compare well with those obtained in mixed crystals using PSCS. Better agreement with SCF-calculated theoretical results is also obtained.
Rotational energy for spherical tops. II. Triply-degenerate fundamental
Authors: Michelot F. , Moret-Bailly J. , Fox K.
Published: February 2004

The Hamiltonian for a triply-degenerate fundamental of spherical top molecules is extended to sixth order of approximation. Formulas for the energy levels are given.
Comment on determination of the interaction potential between Ar and HCl
Authors: Green Sheldon
Published: February 2004

To test the accuracy of the Gordon-Kim theory of intermolecular forces, predicted and experimental values are compared for Ar–HCl. The method appears to accurately predict the short-range repulsive forces and also the position (but possibly not the depth) of the potential well.
Stoichiometry, Mössbauer spectrum, and magnetic susceptibility of the one-dimensional conductor iridium carbonyl chloride
Authors: Ginsberg A. P. , Cohen R. L. , DiSalvo F. J. , West K. W.
Published: February 2004

Carbon and chlorine analyses of the linear chain compound Ir(CO)3Cl confirm the report by Krogmann et al. that it is partially oxidized. The results are consistent with the formulation Ir(CO)3−xCl1+x(x = 0.08 ± 0.02) or Ir(CO)3Cl1+x(x = 0.10 ± 0.03). The latter formulation is preferred because recoilless absorption measurements of the 73 keV 193Ir γ-ray show no indication of a line attributable to a species other than partially oxidized Ir(CO)3Cl units. Presumably the ``extra'' chloride occurs in disordered interchain positions. The Mössbauer absorption is a temperature independent (1.8 – 35 °K) doublet with isomer shift − 0.03 ± 0.01 mm/sec relative to iridium metal and quadrupole splitting 2.11 ± 0.02 mm/sec. For 4.3 ≤ T < 25 °K, the molar magnetic susceptibility corrected for ligand and metal core diamagnetism, χM′, follows the Curie law χM′ = 38 × 10−6 + (6.79 × 10−4)/T; for 25 < T ≤ 280 °K, the susceptibility is given by χM′ = 25 × 10−6 + (1.03 × 10−3)/T. The susceptibility and Mössbauer results indicate that in the range 1.8–280 °K the charges arising from partial oxidation of Ir(CO)3Cl chains are not localized on individual atoms. The results are therefore inconsistent with a model requiring transition from a metallic state to a smallbandgap magnetic Mott insulator, but can be accounted for in terms of the interrupted metal strand model or the one-dimensional disorder model. For the latter, the Curie contribution to the susceptibility leads to an estimate that the disorder ``localized'' states are spread over ∼ 25 Ir atoms. If the Curie component of the susceptibility is assigned to paramagnetic structure defects, the results are also consistent with the model in which a high-temperature metallic state undergoes a continuous transition to a low temperature Peierls band insulator. The origin of the discontinuity at T ≈ 25 °K in the slope of the T−1 dependence of χM′ is at present unknown.
Magnetic susceptibility of the one-dimensional electron gas; application to BDP(TCNQ)2
Authors: Kahn A. H. , Candela G. A. , Walatka V. , Perlstein Jerome H.
Published: February 2004

The magnetic susceptibility of the noninteracting electron gas has been studied on the basis of free electrons and the tight-binding model, with application to the properties of organic conducting materials. Theoretical curves of susceptibility versus temperature are presented. The gradual transition from low temperature Pauli paramagnetism to the high temperature Curie law region is shown. In the transition region, the one-dimensional systems show a peak in the susceptibility of the order of 110% of the low temperature limit. The magnetic susceptibility of the compound BDP(TCNQ)2 is interpreted in terms of a one-dimensional tight-binding model with an energy bandwidth of 0.07 eV.
Quantum correction factors for classical small angle scattering by inverse power potentials
Authors: Anderson Roger W.
Published: February 2004

An integral expression is derived for a quantum correction factor that gives the semiclassical differential elastic scattering cross section as a multiple of the Kennard small angle classical scattering cross section. The correction factor is evaluated numerically using the Landau-Lifshitz approximation for inverse power potentials with s =4, 5, 6, and 8. The quantum correction is compared at very small angles with existing semiclassical small angle formulas. The relevance of this theory to the small angle elastic scattering found in reactive systems where the reaction cross section is determined by the collisions which surmount the centrifugal barrier is considered.
Nonradiative vibrational relaxation of diatomic molecules isolated in solid rare-gas matrices
Authors: Diestler D. J.
Published: February 2004

An approximate quantum-theoretical expression for the rate of nonradiative vibrational relaxation of a diatomic molecule embedded in a rare-gas matrix is derived. The rare-gas matrix is assumed to have the normal structure of the crystalline rare gas, the diatomic being a substitutional impurity. Since the forces acting between the atoms of the crystal are much weaker than the force binding the atoms of the diatomic, the vibrational modes of the crystal lattice are assumed not to interact with the internal diatomic vibrational mode in zero order. The rotation of the diatomic is neglected. The lattice modes corresponding to each diatomic state, which are determined in the harmonic approximation, are assumed parallel (i.e., harmonic potentials for each mode are simply displaced with different force constants). The rate of nonradiative transition of the diatomic impurity between vibrational levels α and β, Wα→β, is calculated from the Fermi ``golden rule'' expression thermally averaged over the initial states of the lattice. By approximating the perturbative coupling matrix element responsible for the relaxation process by its value at the initial equilibrium configuration of the lattice, the rate expression is effectively simplified to a vibrational overlap integral, which is reduced to an exponential time integral and then evaluated by the saddle-point method. The explicit dependence of Wα→β upon the temperature and upon various microscopic parameters characterizing the system is demonstrated. Experimental data for the relaxation of CO in an Ar matrix in the temperature range 6–18 K are fitted quite well by the theoretical expression.
Twist elastic constant of a cholesteric polypeptide liquid crystal
Authors: Duke Russell W. , DuPré Donald B.
Published: February 2004

We have determined the critical magnetic field for the cholesteric to nematic transition of a pure lyotropic liquid crystal solution of an α-helical polypeptide, poly-γ-benzyl-L-glutamate, by direct microscopic observation of the changing pitch of the structure as a function of magnetic field strength. The course of the transition is well accounted for by the theory of deGennes. An independent measurement of the anisotropy of diamagnetic susceptibility allows the determination of κ22, the twist elastic constant of the medium.
Variational time-dependent perturbation scheme based on the hydrodynamic analogy to Schrödinger's equation
Authors: Askar Attila , Demiralp Metin
Published: February 2004

A variational principle correct through the second order is presented for time-dependent perturbations about a bound state. The formulation is based on the hydrodynamic analogy to quantum mechanics and is obtained by linearization of a general principle within the spirit of the acoustic approximation in classical fluid mechanics. The method requires the knowledge of the wavefunction for the unperturbed state only, unlike in the standard procedure of expansion in terms of a complete set of unperturbed wavefunctions. In dealing with the changes in the amplitude and phase of the wavefunction which are nonoscillatory, the method is expected to offer advantages over the usual variational perturbation methods which utilize the oscillatory wavefunctions directly. The method is applied to the calculation of the dynamic polarizability of the hydrogen atom and reasonable accuracy is obtained for an overly simple choice of basis functions.
Solvent shifts and excited-state potentials in rare-gas mixtures
Authors: Saxton Michael J. , Deutch J. M.
Published: February 2004

Solvent shifts of the vacuum ultraviolet spectrum of xenon in gaseous argon are used to obtain the excited-state Ar+Xe(3P1) potential. This potential is used to calculate the absorption and emission shifts of xenon in liquid argon, and satisfactory agreement with experiment is obtained. A new approximate scaling method is proposed to obtain the radial distribution functionf (rdf) for the mixture. This scaling uses readily available rdf's for hard sphere mixtures and pure fluids to obtain the rdf of the mixture. The emission and absorption shifts in the gas are sensitive to different regions of the excited-state potential, so that by combining observed shifts in absorption and emission one should be able to obtain accurate excited-state potentials.
Phenomenological manifestations of quantum-mechanical tunneling. II. Effect on Arrhenius pre-exponential factors for primary hydrogen kinetic isotope effects
Authors: Stern Marvin J. , Weston Ralph E.
Published: February 2004

The effect of quantum-mechanical tunneling on the Arrhenius pre-exponential factor AQ in reactions involving primary protium-deuterium kinetic isotope effects is investigated as a function of temperature. It is shown that large tunneling factors can either raise or lower AQ considerably or leave it essentially unchanged from its value in the absence of tunneling, depending on the details of the reaction system involved and on the temperature range under consideration. It is concluded that experimental AQ factors can be used as quantitative indicators of quantum-mechanical tunneling only in conjunction with model calculations. Experimental values of AQ inside the ``no-tunneling'' range, 2−1 ≤ AQ ≤ √ 2, do not necessarily indicate the absence of large tunneling factors.
Phenomenological manifestations of quantum-mechanical tunneling. III. Effect on relative tritium-deuterium kinetic isotope effects
Authors: Stern Marvin J. , Weston Ralph E.
Published: February 2004

The effect of quantum-mechanical tunneling on relative tritium-deuterium kinetic isotope effects is investigated with the aid of model calculations. It is shown that even when tunneling is important, such relative effects, defined as r=ln(kH/kT) [ln(kH/kD)]−1, have values not very different from their values in the absence of tunneling. Thus, for the types of reactions commonly used to investigate tunneling, the limits 1.33 ≤ r ≤ 1.58 are not likely to be exceeded. There is no direct general correlation between the magnitude of r and the extent of tunneling; the temperature dependence of r is qualitatively the same for a wide range of reaction models and barrier heights.
EPR of γ-irradiated crystals of DL-threonine at room temperature
Authors: Close David M. , Anderson Roy S.
Published: February 2004

The room temperature EPR spectra of γ-irradiated crystals of DL-threonine, CH3CH(OH)CH(N+H3)CO2−, and of deuterated DL-threonine, CH3CH(OD)CH(N+D3)CO2−, have been studied. Both crystals show complex spectra resulting from the overlap of signals from two different paramagnetic species. In the deuterated crystal, the EPR spectrum was simplified by uv photobleaching to remove one paramagnetic species. The remaining, stable radical has been identified as CH3Ċ(OD)CH(N+D3)CO2−, resulting from an H abstraction. Similarly, the stable radical remaining in the uv photobleached normal crystal was identified as CH3Ċ(OH)CH(N+H3)CO2−. The observed hyperfine couplings are AβH=32.8 G,  AmethylH =19.1 G, AβN =4.4 G, and AOHH [[inverted lazy s]] 3 G. Precise determination of g values has not been possible. Using the known crystal structure of DL-threonine and the measured hyperfine coupling constants, we have calculated the orientation of the damaged molecule in the host crystal.
Effective potential for water-ion interactions in prenucleation embryos
Authors: Briant C. L. , Burton J. J.
Published: February 2004

The energies of prenucleation embryos containing one to four water molecules and an ion were calculated using molecular dynamics. Previously proposed potentials give at best qualitative agreement with experiment. The energy of the embryo depends strongly on the ionic radius and the distance of the negative partial charge from the oxygen atom. By varying these two quantities, an effective potential was developed that gives excellent agreement with experiment. This potential is correct for both the ion-water and water-water interactions in the embryo.
On the vibrational structure in photoelectron spectra by the method of Green's functions
Authors: Cederbaum L. S. , Domcke W.
Published: February 2004

The density of states functions, which describe the vibrational structure in PE spectra, are calculated with the help of Green's functions. In addition to the exact solution of the one-body problem, two different approaches to calculate reorganization and many-body effects are presented. In all cases no data of the ion are required, and all transitions except those between linear and bent configurations are included. It is shown that the density of states is well approximated by using only the first-order coupling constants in the one-particle approximation. This means that already a simple calculation becomes a powerful tool in interpreting PE spectra.
Calculation of dipolar nuclear magnetic relaxation times in molecules with multiple internal rotations. II. Theoretical results for anisotropic over-all motion of the molecule, and comparison with 13C relaxation times in n -alkanes and n -alkyl bromides
Authors: Levine Y. K. , Birdsall N. J. M. , Lee A. G. , Metcalfe J. C. , Partington P. , Roberts G. C. K.
Published: February 2004

The theory of dipolar nuclear magnetic relaxation in molecules with multiple internal motions is extended to the case where the motion of the molecule as a whole is anisotropic, with no restriction on the magnitudes of the correlation times involved. Numerical calculations of the 13C relaxation times T1 and T2 for 13C–1H relaxation in a hydrocarbon chain attached to an axially symmetric prolate ellipsoid are presented. Effects of the anisotropic motion of the molecule can be seen as far as five carbons along the chain, even when the motion about the C–C bonds in the chain is a factor of 10 faster than the fastest motion of the molecule as a whole. Two cases have been considered in which the motion about the long axis of the ellipsoid is fast [(ωC + ωH)2 / 6DZ2 ≪ 1] or slow [(ωC + ωH)2 / 6DZ2 > 1] compared to the highest relevant Larmor frequency. The effects on T1 and T2 of changes in the axial ratio of the ellipsoid and in the angle between the first bond in the chain and the long axis of the ellipsoid, which are markedly different in these two cases, can be explained in terms of the effectiveness of the different motions in the system for relaxation. 13C spin-lattice relaxation times have been determined for all the resolved resonances in n -alkanes from C6 to C18 and in n -alkyl bromides from C4 to C15. Using the theoretical treatment developed in the first part of the paper, these relaxation times have been used to calculate the diffusion coefficients for the various motions of these molecules (considered as axially symmetric prolate ellipsoids with internal motion). In the shorter-chain n -alkanes (C6 to C10), where the resonances of all the carbons can be resolved, it is possible to calculate the two diffusion coefficients of the molecules as a whole and the diffusion coefficients for motion about each C–C bond in the chain. The diffusion coefficients about all the bonds in the chain (except that to the terminal methyl) are the same, being in the range 0.1−0.2 × 1011 sec−1 (about a factor of 10 slower than the motion of the whole molecule about its long axis). The motion about the terminal bond is a factor of 2–6 faster. In longer chain alkanes (C12–C18), where not all the resonances are resolved, only the diffusion coefficients about the four bonds nearest the terminal methyl can be obtained. Comparison of the present results with those obtained earlier with dipalmitoyl lecithin bilayers indicate that the region of the hydrocarbon chain near the terminal methyl group has essentially identical motional freedom in the bilayer and in the simple n -alkanes, while the region near the glycerol moiety in the bilayer is motionally much more restricted.
Number of atoms formed per ion pair in tritium gas. Correlation with molecular processes
Authors: Jones W. M. , Dever D. F.
Published: February 2004

The number of atoms formed per ion pair by the β particles in tritium gas was determined to be M0 = 5.1±0.3 by capture of the atoms on a surface of molybdenum trioxide at 22, 40, 60, and 100°C and from 8 to 211 torr. Correction was made for a decrease in the measured number of atoms per ion pair with increasing pressure. The effect was interpreted as resulting from a competition between capture of atoms by MoO3 and surface recombination, with the former dominating at low pressure. Atoms are formed by T2+ + T2 → T3+ + T (R1) and T3+ + e− → T2 + T (R2), by T3+ + e− → 3T (R2′) to at most a minor extent, and by excitation of triplet and subsequently quenched singlet molecular states. The number of atoms resulting from dissociative recombination of H3+ (nr) was found to be approximately one by reconsidering the conditions of an experiment of Corrigan. This value of nr and previous calculations of the numbers of triplet and singlet excitations per ion pair were used to deduce M0 = 4.8, in reasonable agreement with experiment.
Classical dynamics: The study of vibrational and rotational excitation in Li+, H2
Authors: Brown N. J.
Published: February 2004

A classical study of translational, rotational, and vibrational energy exchange in the systen Li+, H2 is presented. The potential surface used was the ab initio Lester surface appropriate for rotational and vibrational energy transfer studies. The effect of initial energy configuration and orientation variables on the energy transfer processes was studied. A central force calculation of the deflection function was made for comparison with that obtained from an anisotropic potential. The primary mode of energy transfer to the internal degrees of freedom is from translational energy.
Infrared spectroscopic evidence for matrix isolated ``nitrosyl hypofluorite,'' an isomer of nitryl fluoride
Authors: Smardzewski Richard R. , Fox William B.
Published: February 2004

The in situ photolysis of molecular fluorine and NO2 molecules in nitrogen matrices at 8°K yields the FN-bonded nitryl fluoride, FNO2, as the primary reaction product. However, controlled fluorine atom diffusion at 20°K, in the absence of photolytic radiation, results in the formation of the highly reactive, FO-bonded, nitrosyl hypofluorite, ONOF. Infrared absorptions at 1716.4, 1199.9, 702.3, and 411.9 cm−1 were assigned to this species, which was shown from 15N and 18O isotopic studies to contain two nonequivalent oxygen atoms. The ONOF molecule undergoes intramolecular photorearrangement at wavelengths below 400 nm to produce the more stable FNO2 form.
Proton magnetic resonance absorption and relaxation of dimethylamine in the condensed and clathrate deuterate phases
Authors: Khanzada A. W. K. , McDowell C. A. , Raghunathan P.
Published: February 2004

Proton magnetic resonance second moment and spin-lattice relaxation data are reported for polycrystalline solid dimethylamine and dimethylamine clathrate deuterate from 77 K up to the melting points of the respective compounds. The second moment results for solid dimethylamine indicate a hindered rotation of the methyl groups around their threefold axes, while the proton T1 data show a broad distribution of reorientational correlation times among the methyl groups. A quantitative interpretation of this distribution based on the Davidson-Cole distribution function is given, and a reorientational energy barrier of 2.70±0.20 kcal mole−1 is obtained from the relaxation data using a Davidson-Cole distribution parameter δ=0.8. In the dimethylamine clathrate deuterate, a pseudothreefold axis (c-axis) reorientation of dimethylamine is observed at higher temperatures. Proton spin-lattice relaxation in this case is nonexponential over most of the temperature range studied, and an analysis of the ``null'' time (t0) data in terms of the Hilt-Hubbard theory leads to an activation energy barrier of 2.26 ± 0.02 kcal mole−1 for reorientation around the c axis. Measurements of the rotating frame spin-lattice relaxation (T1ρ) in the temperature region above the t0 minimum for the clathrate disclose the onset of slow tumbling and lattice diffusion processes, governed by an activation energy of 14.0 ±0.6 kcal mole−1.
Semiclassical treatment of bound state systems. II. Trajectory calculations using the exact quantum momentum
Authors: Starkschall George , Light John C.
Published: February 2004

Equations of motion based upon the exact solution of the Milne equation are used to perform trajectory calculations. Using the trajectory information we calculate charge densities for N fermions in a harmonic oscillator potential in a semiclassical approximation. The charge densities so obtained are in almost exact agreement with exact calculation within the outermost density maxima. Outside this region the differences between the approximate and exact densities are essentially independent of the number of fermions in the system.
Rotational excitation in the small-angle scattering of protons from diatomic molecules
Authors: Udseth H. , Giese Clayton F. , Gentry W. Ronald
Published: February 2004

Data for the small-angle scattering of protons from several polar and nonpolar molecules in the energy range 10 to 30 eV indicate that the presence of a dipole moment in the neutral molecule greatly enhances the rotational excitation process in ion-molecule collisions. Within our experimental energy resolution, no rotational excitation was observable for proton collisions with H2, HD, D2, or N2. There is, however, some evidence for a small amount of rotational excitation of the dipolar molecule CO, and for HCl and HF large amounts of rotational excitation were observed. The inelasticity was found to be greater for HF than for HCl, consistent with the relative dipole moments and moments of inertia of the two molecules. The average excitation energies for HF measured at fixed scattering angle are qualitatively in agreement with a classical perturbation calculation for pure rotational excitation. For HCl, the classical perturbation calculation gives a rotational excitation energy much lower than the average observed translational energy loss, indicating that concurrent vibrational excitation, though not resolved in the presence of rotational excitation, may be relatively more important for HCl than for HF.
Angular correlations in linear copolymers from the compositional dependence of their dipole moments. I. Random copolymerization
Authors: Work Richard N.
Published: February 2004

A theory is developed, without recourse to a detailed model, for evaluating certain linear combinations of the average cosines of the angles between dipole moments of pairs of structural units that are closely spaced along the chain contour. Measurements of the compositional dependence of the effective dipole moment per structural unit together with a knowledge of the compositional dependence of the probabilities for various types of sequences of structural units provide the required information. The theory offers a possibility for testing the validity of certain restrictive assumptions that are implicit in earlier treatments of the problem. Additionally, the theory provides a basis for determining unequivocally from experimental measurements whether angular correlations exist among the structural units that contribute to the orientational part of the polarization and whether the angular correlation parameters, if they exist, depend on the types of structural units involved.
Monte Carlo study of ring polymers
Authors: Naghizadeh J. , Sotobayashi H.
Published: February 2004

Nonintersecting ring polymers of step number (chain length) 6–118 have been produced by the Monte Carlo method on a tetrahedral lattice. The mean square radius of gyration and its standard deviation for each sample have been determined. A log-log plot of the mean square radius of gyration 〈S2〉 vs N, the number of steps, gives the usual straight line with a slope of 1.218. The deviation of this exponent from reported values for the mean square end to end distance of linear polymers (1.200) lies within the margin of error and is unlikely to be due to the ring geometry of the polymer. The data are also compared with similar results on linear polymers; in particular, the ratio of the two radii of gyration 〈S2〉lin/〈S2〉ring in the presence of excluded volume is discussed, and its asymptotic value is compared with the value predicted from existing theory.
Far-from-equilibrium phenomena at local sites of reaction
Authors: Bimpong-Bota K. , Ortoleva P. , Ross J.
Published: February 2004

We investigate some far-from-equilibrium effects of localized nonlinear reaction sites, such as electrode surfaces or catalytic membranes, on an otherwise stable reacting-diffusing bulk medium. We obtain an integral equation as the formal solution of the partial differential equations describing reaction and diffusion in the total system, in which the nonlinearities of the local reaction sites are fully retained; the kernel of that integral equation is a propagator function which describes the linearized bulk dynamics in the absence of the localized reactions. Analytical solutions to the integral equations are found for several mechanisms which demonstrate the existence of a variety of typically far-from-equilibrium phenomena. The equations for steady states are derived and used to show the existence of multiple steady states for a model system. A determinantal condition for the stability of steady states is obtained and applied to another model system consisting of a localization of the Prigogine-Lefever mechanism with no bulk reactions. The system is found to have oscillatory instability whose frequency depends on transport processes in the bulk as well as on the parameters of the localized reactions. Waves and dissipative space structures on a planar localized reaction site are shown to obey ordinary integral equations which are solved, in the case of waves, for two model cases. The waves are shown to exist only in a region about the plane of the local reaction. Propagation occurs due to bulk transport processes coupling both local and bulk reaction in the vicinity of the reactive surface. Waves are shown to propagate along the membrane or surface only for certain ranges of values of the wavevector. A threshold wave phenomenon is shown to exist for one case, such that over the entire range of allowed wavevectors waves exist only beyond a minimal amplitude about a stable steady state. Symmetry-breaking instabilities in a system with two equivalent localized sites are considered and we show that these can occur only for intermediate values of separation of the local sites.
On an electron diffraction study of the structure of the dibromomethylene radical
Authors: Ivey R. Charles , Schulze Paul D. , Leggett Tom L. , Kohl Denis A.
Published: February 2004

The molecular structures of two states of the dibromomethylene radical have been observed by the technique of gas phase electron diffraction. The radicals were produced by the thermal decomposition of tetrabromomethane utilizing an oven nozzle within the scattering chamber. The decomposition resulted in about 60% conversion to dibromomethylenes and lesser amounts of hexabromomethane, tribromomethyl radicals, and molecular bromine. The two structures observed for dibromomethylene were a near linear form, assigned as the triplet state, and a bent form assigned as the singlet state. An average carbon-bromine bond distance of 1.74 Å was observed for both states.
Deuterium quadrupole coupling in solid complexes of diethyl ether, acetone, and mesitylene with chloroform-d
Authors: Ragle J. L. , Minott G. , Mokarram M.
Published: February 2004

The deuterium quadrupole coupling constant in the 1:1 solid compound of CDCl3 with diethyl ether was measured to be 149.1±0.5 kHz at 77°K. The corresponding value for CDCl3 at liquid nitrogen temperature is 166.9±0.1 kHz. This shift in field gradient is discussed in terms of the charge distribution of the acceptor molecule and the shifts in charge in the donor molecule which accompany formation of the C–D⋯O hydrogen bond which is expected to be a salient feature of the structure of this compound. Measurements on the 1:1 compounds of CDCl3 with acetone and mesitylene are also briefly discussed in terms of the electronic structures of the complexes. Values of the deuteron quadrupole coupling constant are closely related to shifts which occur in vibrational force constants for CDCl3 complexes.
Rotational analysis of ν6 of DCF3 and ν3 + ν6 of HCF3
Authors: Lockett P. , Wilt P. M.
Published: February 2004

Experimental data of moderately high resolution for the ``pseudoparallel'' bands ν6 of DCF3 and ν3+ν6 of HCF3 are reported. Analysis of ν6 is straightforward and results in values of ν0, α6C and the conclusion that q6>0. ν3+ν6 is perturbed by Fermi resonance with ν5. Analysis in this case yields values of ν0, α3,6C, the anharmonic potential constant |k356| and the conclusion q6>0. The effect of this Fermi resonance on the J = 1←0 pure rotational spectrum of HCF3 is discussed.
Determination of the structure of ArHF
Authors: Harris Stephen J. , Novick Stewart E. , Klemperer William
Published: February 2004

Radio-frequency and microwave spectra of K = 0 states of ArHF in the ground vibrational state have been measured by molecular beam electric resonance spectroscopy. The rotational constant is (B+C)/2=3065.719 MHz; the component of the dipole moment along the a inertial axis is μa = 1.332 D. From the centrifugal distortion constant, DJ = 72.1 kHz, the stretching frequency of the van der Waals bond is estimated to be 42.1 cm−1. In analogy with ArHCl the internal Ar–F–H angle θ is expected to be acute and the average value of the angle is 48.20°. The equilibrium configuration is likely to be near linear with atomic arrangement ArHF. The vibrationally averaged Ar–F distance is 3.540 Å.
Molecular motions of fluorobenzene-d5 in the dense fluid region
Authors: DeZwaan J. , Finney R. J. , Jonas J.
Published: February 2004

The angular position correlation time τθ and the angular momentum correlation time τJ have been determined in fluorobenzene-d5 in the dense fluid region in the temperature range T/Tc from 0.5 to 1.1 and the density range ρ/ρc from 0.5 to 3.1. The correlation times are calculated from the experimental NMR fluorine and deuterium spin-lattice relaxation times measured at temperatures 30–350°C and pressures 1–3500 bar. Since the densities were also determined it is possible to separate the effects of temperature and density on τθ and/or τJ. Our finding that the angular momentum correlation time τJ increases with increasing temperature at constant density is of particular interest. As τJ is related to the time between collisions, the cell model and the Enskog theory are used to calculate the theoretical τJ. In order to obtain the correct prediction of the observed temperature changes of τJ at constant density, a temperature dependence of the hard sphere diameter is assumed. In agreement with our earlier results we find that the reorientational motion of fluorobenzene-d5 is well described by the extended J-diffusion model. A brief discussion of the density and temperature effects on τθ and viscosity is also presented.
Deuterium effect on the quenching of aromatic hydrocarbon triplet excited states by oxygen
Authors: Benson Robert , Geacintov Nicholas E.
Published: February 2004

By using ultraviolet light flash excitation, the triplet (phosphorescence) decay time τ of a series of protonated (p) and deuterated (d) aromatic hydrocarbons dissolved in a fluffy freeze-dried polystyrene matrix was studied as a function of oxygen pressure [O2] at 298°K. The quenching constants γ were calculated from linear plots of τ−1 as a function of [O2] and the ratios of γ(p)/γ(d) are 1.2±0.1 for anthracene and 1,2-benzanthracene, 1.9±0.2 for pyrene, 1.4±0.2 for chrysene, and 2.5±0.4 for naphthalene. These results are interpreted in terms of Franck-Condon factors and the energy transfer quenching model proposed by Kawaoka, Khan, and Kearns in which the 1Δg and Σg+1 excited states of oxygen are generated, and in which the remaining energy is dissipated by the vibrational modes of the quenched aromatic molecules. In pyrene and in naphtalene, the 1Δg pathway appears to be more important than in chrysene and dominates over the Σg+1 mechanism. No conclusions can be reached about the preferred mode of energy transfer to oxygen in anthracene and 1,2-benzanthracene.
An electron diffraction investigation of the reaction intermediate in the gas-phase bromination of ethylene
Authors: Leggett T. L. , Kennerly R. E. , Kohl D. A.
Published: February 2004

The addition of bromine to ethylene was studied in the gas phase by electron diffraction. Atomic bromine and an excess of ethylene were mixed just prior to introduction into the scattering region by a dual-sample inlet system. The atomic bromine was produced from molecular bromine by thermolysis in an oven adjacent to the mixing region. Diffraction patterns recorded at each of three different oven temperatures were analyzed. The radial distribution curve for one pattern had the normal distances attributed to ethylene and bromine plus a Br–Br peak corresponding to the Π03'μ state of Br2. The radial distribution curve for a second pattern exhibited a peak at 1.7 Å in addition to the ethylene and bromine distances. The radial distribution curve for a third pattern, recorded at the highest oven temperature, again showed the 1.7 Å peak and ethylene peaks but no bromine peak. To account for the scattering power and still preserve stoichiometry, it was necessary to assign the 1.7 Å peak to a carbon-bromine pair with a double weight. This implies that the intermediate in the bromination of ethylene has a bromine atom symmetrically bonded to each of the two carbon atoms.
4He atomic beam scattering from clean surfaces of silicon and germanium
Authors: Houston D. E. , Frankl D. R.
Published: February 2004

A modulated, nozzle type 4He atomic beam has been used to investigate scattering from clean, room temperature surfaces of silicon and germanium. The surfaces are prepared by cleaving in situ under ultrahigh vacuum, the pressure being in the low 10−9 Torr range. The beam can be operated with the source gas at room temperature or cooled with liquid nitrogen, resulting in a wavelength of ∼0.57 and ∼1.07 Å, respectively. The scattered beam is subspecularly directed with a maximum intensity less than 1% of the incident beam intensity.
Dimer models for the antiferroelectric transition in copper formate tetrahydrate
Authors: Allen G. R.
Published: February 2004

This paper develops the lattice statistical theory of the antiferroelectric transition in copper formate tetrahydrate in terms of dimer and 8-vertex models. The simplest dimer model is shown to be inappropriate for describing the transition, primarily because the hydrogens order into the wrong ground-state configuration. In the context of the dimer model, we introduce two exactly solvable modifications by postulating additional, physically plausible short-range interactions in the hydrogen bonding. The first model, which results from asymmetrically weighting the dimer positions, does have the observed ``frozen'' ground state and residual short-range order above Tc, but the transition is continuous. The second model introduces repulsive interactions between nearest-neighbor parallel dimer positions via the Baxter formulation of the dimer model in terms of the 8-vertex model. A suitable lattice transformation enables us to develop the model in terms of a generalized KDP model. This model has a first-order transition with a maximum allowable transition entropy of 0.876 cal/deg · mole, in excellent agreement with that determined from specific heat measurements. The higher energy hydrogen configurations are of the order of kT, indicative of the nature of the thermal excitations.
Crystal structure determination of disodium acetylide, Na2C2, by neutron diffraction
Authors: Atoji Masao
Published: February 2004

A neutron powder-diffraction study of disodium acetylide has been made in which the intensity data were deduced from careful, repeated measurements so as to attain high statistical accuracy. An extensive least squares refinement was carried out in the structure determination. The resultant data confirmed essential aspects of Föppl's structure obtained using the x-ray powder method. As regards the atomic position parameters, the neutron values are an order of magnitude higher in accuracy than those from x-ray diffraction, hence revealing the interatomic characteristics to a much greater extent. The structure is tetragonal D4h20 − I41/a c d) and the unit cell, a = 6.743(2) and c = 12.674(3), encases eight Na2C2 units. The −C≡C− bond distance is 1.200±0.006 Å which is insignificantly different from 1.191 Å in CaC2 and 1.204 Å in acetylene. The Na+ ions and C22− groups are arranged in a distorted anti-CaF2 type packing. The neighboring Na+⋯C− distances are 2.623, 2.648, and 2.774 Å, indicating a close packing of C22− ions around Na+. However, the closest nonbonding C⋯C distance is 4.050 Å being much longer than the 3.40 Å in CaC2. A comparative survey is given on the interionic configuration in Na2C2 and related compounds.
Perturbation calculations of the square well system in the solid phase
Authors: Rosenfeld Y. , Thieberger R.
Published: February 2004

The perturbation expansion scheme of Zwanzig is applied to the square well system in the solid phase. The Monte Carlo method is used to calculate the reference system of hard spheres and some isotherms of the square well system. First order perturbation theory reproduces the machine calculated ``quasiexperimental'' equation of state, to a fair degree of accuracy.
Free radical formation in x-irradiated histidine HCl
Authors: Ngo Frank Quoc-hai , Budzinski Edwin E. , Box Harold C.
Published: February 2004

ENDOR spectroscopy was used to identify three primary radicals in single crystals of histidine hydrochloride x irradiated at 4.2°K. Oxidation leads to decarboxylation producing the radical R–CH2–CH–NH3+, where R stands for the protonated imidazole ring. Another oxidation product is produced by removal of an electron from the imidazole ring. The reduction process yields a carboxyl anion by addition of an electron to the carbonyl oxygen. ENDOR spectroscopy was also used to characterize more completely the final hydrogen-adduct radical that is obtained on warming the crystals to room temperature.
Microwave spectrum of (fluoromethyl)germane, CH2FGeH3
Authors: Krisher Lawrence C. , Watson William A. , Morrison John A.
Published: February 2004

Microwave spectra of three isotopic species of CH2FGeH3 have been investigated in the region 8000–30 000 MHz. The height of the potential barrier hindering the germyl group (GeH3) internal rotation was determined to be 1390±40 cal/mole from measured A-E frequency differences, using a PAM calculation through fourth-order perturbations. Dipole moment components μa = 1.00 D, μb = 1.30 D, giving a resultant μ = 1.64±0.03 D, were obtained from Stark effect measurements. A technique for assigning R-branch asymmetric rotor transitions is discussed.
Catalysis of radical recombination in flames by iron
Authors: Jensen D. E. , Jones G. A.
Published: February 2004

Flame photometric measurements of H and OH radical concentrations in fuel-rich laminar atmospheric-pressure H2/O2/N2 flames with and without added iron have been made by the Li/Na comparison method. The observed catalysis of radical recombination by iron is quantitatively interpreted in terms of the following homogeneous gas-phase reaction cycle, with associated forward and backward rate coefficients in milliliter · molecule · second units: FeOH + H⇌FeO + H2:kf = 5 × 10−11 exp(−800/T),kb = 1.1 × 10−9 exp(−6200/T);FeO + H2O⇌Fe(OH)2:kf = 9 × 10−12,kb = 1.0 × 1013 exp(−35 500/T);Fe(OH)2 + H⇌FeOH + H2O:kf = 1.1 × 10−10 exp(−300/T),kb = 1.1 × 10−11 exp(−12 000/T).

The interpretation also includes contributions from the effectively balanced reaction Fe+H2O⇄FeOH+H; equilibrium constant = 66 exp(−16 100/T).
Potentials and continuum spectra of Rb-noble gas molecules
Authors: Drummond D. L. , Gallagher Alan
Published: February 2004

Rubidium vapor is optically excited by either of the atomic resonance lines in the presence of typically 400 Torr of He, Ne, Ar, Kr, or Xe. Normalized emission spectra of the far wings of these resonance lines are then measured as a function of temperature. These far wings, extending as much as 1000 Å from the resonance lines, are interpreted as molecular continuum radiation of Rb-noble gas molecules. Using the quasistatic theory of line broadening, or the classical Franck-Condon principle, these spectra are analyzed to yield the ground and excited state molecular potentials.