bluuebunny/crossref_metadata_2025 huggingface dataset:
Collision Transfer of Vibrational Energy from Nitrogen and Methane to the Carbon Dioxide Molecule
Authors: Bauer H.-J. , Schotter R.
Published: February 2004

The sound absorption of CO2 has been measured at 95°C from 10 to 5 × 106 Hz/atm with 99%–92.5% N2 and at 80°C from 2 × 104 to 4 × 107 Hz/atm with 10%–90% CH4 admixed. N2 has been chosen because of its good resonance to the asymmetric valence-bond vibration of CO2 (ν3), while CH4 resonates approximately with the CO2 levels at about 1900°K. The results were analyzed by a modified “binary two-level” theory of the quantum exchange mechanism between the two molecules. From the CO2–N2 measurements, the reciprocal de-excitation probabilities of ν3 were obtained to be 1 / P = 58 000, 1 / P = 40 000···130 000 in collisions with N2 and CO2, respectively. The de-excitation of the first bending vibration level in collisions with nitrogen needs 1 / P = 58 000 collisions. The same analysis applied to the CO2–CH4 mixtures yielded the following transition probabilities: de-excitation of CO2 (0110) by CH4, 1 / P = 1000; resonance production of two states (0110) from one at 1900°K in collision with ground-state CO2 molecules, 1 / P = 150; quantum exchange from excited CH4 to the CO2 levels at 1900°K, 1 / P = 2600; de-excitation of CH4 in collisions with CO2, 1 / P>106. The fact that the relaxing molar heat capacity of the CO2–CH4 system exceeded its total vibrational heat capacity is explained as an influence of rotational–vibrational transitions, in which preferentially the vibration is de-excited and the rotation excited. Such processes also would explain the high efficiency of the (fast-rotating) methane for the de-excitation of the CO2 bending vibration.
Highly Anharmonic Crystal
Authors: Fixman Marshall
Published: February 2004

The free energy of the constrained hard-sphere crystal is evaluated by Hermite expansion techniques applied to the Boltzmann factors of the partition function. An independent-oscillator comparison potential generates a Hermite polynomial basis set in which exp( − 12Σjυij) is expanded. A first approximation results from retention of the first term, the Gaussian average. A second approximation results from retention and exponentiation of the succeeding two terms, and corresponds to a coupled-oscillator potential. For both approximations the pressure deviation from the molecular dynamics pressure is close to the statistical irregularities of the latter, for densities such that the unconstrained solid is thermodynamically stable. The free energy in second approximation, significantly improved over the first, is excessive by 0.2kT at high densities and by up to 0.3kT at low and intermediate densities, where the constraint is required to stabilize the lattice. The methods, readily applicable to smooth potentials (and more promising for them) are similar to those earlier applied to polymer theory. Related methods, based on expansions of the potential itself, have been devised by Koehler and others for weak or moderate crystal anharmonicity.
Electron Paramagnetic Resonance of Pu3+ in Cubic Symmetry Sites in CaF2, SrF2, and BaF2
Authors: Edelstein N. , Mollet H. F. , Easley W. C. , Mehlhorn R. J.
Published: February 2004

The g values of the ground Γ7 state of Pu3+ in cubic symmetry sites in CaF2, SrF2, and BaF2 are 1.297 ± 0.001, 1.250 ± 0.002, and 1.187 ± 0 004, respectively, as measured at T = 1.2°K by EPR techniques. By fitting the g values, estimates of the crystalline-field parameters are derived. It is shown that crystalline-field effects for Pu3+ in the alkaline-earth fluorides are the same order of magnitude as in the lanthanide series.
Theory of the Hydrogen Bond: Electronic Structure and Properties of the Water Dimer
Authors: Kollman Peter A. , Allen Leland C.
Published: February 2004

High-accuracy molecular-orbital calculations using essentially Hartree–Fock quality atomic orbitals as a basis have been carried out on different geometries of the water dimer. Different basis sets have been considered. The molecular-orbital approach is shown to well represent the geometry and heat of formation (− 5.3 kcal/mole) of the water dimer as well as general infrared spectral properties of the hydrogen bond. The individual molecular-orbital energies are shown to increase for the electron acceptor and to decrease for the electron donor. This trend in energies is proposed as a quantitative organizing principle for not only H-bond formation but all donor–acceptor interactions.
Ab Initio Predictions for Very Small Ions
Authors: Poshusta R. D. , Haugen J. A. , Zetik D. F.
Published: February 2004

Approximate ab initio calculations are made on all polyatomic unipositive ions with fewer than six electrons. Predictions are made regarding the stability of such ions. For those which are stable, estimates are made of binding energies, geometries, vibration frequencies, and center of charge. The wavefunctions are valence-bond configuration-interaction functions built form Gaussian orbitals. All possible valence-bond configurations within our chosen basis are used in order to eliminate a serious prejudice regarding geometry. BeH2+, LiHeH+, He3+, LiH3+, LiH2+, He2H+, HeH3+, H5+, HeH2+, H4+, and H3+ are found to be stable. (More accurate calculations on H4+ show that this ion is not stable relative to H3+ and H.) He2H2+, HeH4+, and H6+ are found to be unstable. Two stable isomers of BeH2+ are found: a stable triangular form and a metastable linear form. A semiclassical model is used to interpret and extrapolate our results.
Thermal Conductivity of Krypton and Xenon in the Temperature Range 350–1500°K
Authors: Saxena V. K. , Saxena S. C.
Published: February 2004

The thermal conductivity data for krypton and xenon are reported in the temperature range 350–1500°K. The conductivity column method is used for which all the necessary details are given in an earlier publication [J. Chem. Phys. 48, 5662 (1968)]. The new data agree well with the available data but the latter are limited to the highest temperature of 579°K only. Thus, the reported data above 579°K are novel. The measured conductivity values are well correlated by a quadratic polynomial in temperature over the entire range. The estimated uncertainty of our measurements is about ± 2%. The measured values are compared with the predictions of theory and also used for generating the viscosity and diffusion coefficients.
Localized Orbitals for Polyatomic Molecules. I. The Transferability of the C–H Bond in Saturated Molecules
Authors: Rothenberg Stephen
Published: February 2004

The concept of a transferable C–H bond orbital is investigated by applying the Edmiston–Ruedenberg localization criteria to ab initio SCF wavefunctions for methane, ethane, and methanol. The resulting C–H bond orbital is found to be practically identical in all of these molecules, including several conformations. A contour plot and a number of expectation values are given for the C–H bond orbital.
Radiative Lifetimes and Total Transition Probabilities for Some Polyatomic Species
Authors: Smith William Hayden
Published: February 2004

Radiative lifetimes have been measured in CS2+, SO2, C2N2, and N2O+ systems using the phase-shift technique and electron-beam excitation. The measured lifetimes ranged from 12.2 ± 2.5 nsec for the SO2 emission in the 2300–2700-Å range to 294 ± 30 nsec for the CS2+ B 2Σu+−X 2Πg−(i) transition. The transitions ascribed to SO2 and C2N2 show complications due to radiative cascading, while the CS2+ and N2O+ transitions observed were cascade free. These new data are discussed in light of prior spectroscopic results.
Recombination of Br Atoms by Flash Photolysis over a Wide Temperature Range. II. Br2 in He, Ne, Ar, Kr, N2, and O2
Authors: Ip J. K. K. , Burns George
Published: February 2004

Recombination rate constants of Br atoms in helium, neon, argon, krypton, nitrogen, and oxygen have been measured with an accuracy of approximately ± 10% from room temperature up to 1275°K using a flash photolysis technique. The temperature dependences of the rate constants can be expressed by the following equations: logkrHe = (9.066 ± 0.018) − (1.261 ± 0.043) log(T / 300), logkrNe = (9.170 ± 0.024) − (1.423 ± 0.057) log(T / 300), logkrAr = (9.381 ± 0.016) − (2.287 ± 0.125) log(T / 300) + (1.154 ± 0.194) log2(T / 300), logkrKr = (9.489 ± 0.021) − (2.77 ± 0.179) log(T / 300) + (1.473 ± 0.298) log2(T / 300), logkrN2 = (9.499 ± 0.034) − (2.208 ± 0.253) log(T / 300) + (0.889 ± 0.405) log2(T / 300), logkrO2 = (9.767 ± 0.021) − (1.583 ± 0.057) log(T / 300), where kr is in units of liter2 mole−2·second−1 and T is absolute temperature. At room temperature, the results for krHe, krAr, krN2, and krO2 agree with the literature; direct measurements of krNe and krKr have not been previously reported. The results are explained in terms of a mechanism, involving a complex intermediate BrM, where M is a third body. It is shown that the BrM complexes in bound and metastable states as well as BrM quasidimers may contribute to the recombination. A comparison of calculated and experimental rate data suggest that the interaction potentials between Br and M are several times larger than that between Kr and M. Although a satisfactory agreement between calculated and experimental rates could be obtained by setting steric factor for the reaction to P = 0.5, a better agreement is achieved if P is treated as a temperature-independent parameter. The efficiencies of Br and Br2 as third bodies in Br-atom recombination have also been studied. When the present data are combined with the available shock-wave data, the variation of krBr2 and krBr with temperature between 300 and 2985°K can be described bylogkrBr2 (liter2mole−2·sec−1) = (10.89 ± 0.85) − (3.01 ± 0.28) log(T / 300), logkrBr (liter2mole−2·sec−1) = (12.22 ± 2.00) − (4.3 ± 0.62) log(T / 300).
Negative Temperature Coefficient of Atomic Recombination Rate Constants from Flash Photolysis and Shock-Wave Data. Bromine
Authors: Ip J. K. K. , Burns George
Published: February 2004

The bromine atom recombination rate constants, kr,fl's, obtained in flash photolysis experiments between 300 and 1300°K (preceding paper) were compared with the dissociation rate constants of Br2, kd,sh's, obtained in shock-wave experiments between 1250 and 2300°K. It was found that the phenomenological equation kd,sh / kr,fl = K, where K is the equilibrium constant, is not obeyed if the kd,sh is measured by the method of absorption spectroscopy. On the other hand, if kd,sh is measured by the emission spectroscopy method [R. K. Boyd, G. Burns, T. R. Lawrence, and J. H. Lippiatt, J. Chem. Phys. 49, 3804 (1968)] the phenomenological equation is obeyed at 1300°K. The negative temperature coefficients from shock-wave experiments are considerably higher than those from flash photolysis at comparable temperatures. These conclusions are drawn by comparison of more than 200 measurements of kr,fl (preceding paper) with more than 200 measurements of kd,sh obtained by previous investigators. All these experimental findings are shown to be consistent with a simple model for dissociation of Br2 in shock waves. The model is based on the assumption that the Bethe–Teller law, which holds for a variety of vibrationally relaxing systems, is valid in the early stages of Br2 dissociation, which occurs from the uppermost vibrational levels. The model predicts that true phenomenological rate coefficients for the Br2 dissociation reaction, kd's, differ from the equilibrium rate constants, kd,eq's, and that the ratio of kd / kd,eq, taken at the same translational temperature, decreases rapidly with T. If argon is used as a third body, this ratio is 0.96 at 1300°K, while at 2300°K the nonequilibrium rate coefficient is only 33% of kd,eq.
Competitive Electron Attachment in Gaseous Halobenzene–Benzyl Chloride Mixtures. Evidence for Electron Transfer from the Chlorobenzene Anion to Benzyl Chloride
Authors: Warman John M. , Sauer Myran C. , Johnson G. R. A.
Published: February 2004

The yield of benzyl radicals formed by dissociative electron attachment to benzyl chloride in pulse-irradiated gaseous cyclohexane–benzyl chloride mixtures is reduced by the addition of halobenzenes. The results are interpreted in terms of competitive electron scavenging by the halobenzenes and the ratio of the rate constants, kC6H5X / kBZCl, for electron attachment at 120°C is found to be 20, 0.9, 0.6, and ≤ 0.005, for iodo-, bromo-, chloro-, and fluorobenzene, respectively. In the case of chlorobenzene, the data do not obey simple competition kinetics. This is explained in terms of the formation of a chlorobenzene anion which subsequently undergoes electron transfer to benzyl chloride in competition with ion recombination. The rate constant for electron transfer is estimated to be 5 × 10−11cm3/sec.
Model for Associative-Detachment Reactions of the Insertion Type
Authors: Fehsenfeld F. C. , Ferguson E. E.
Published: February 2004

Exothermic associative-detachment reactions of the bond-insertion type B−+AC → ABC+e or B+AC−→QBC+e may or may not be very fast at 300°K. It is proposed that the slow cases are those which have activation energies and that the occurrence of an activation energy is favored by low exothermicity ΔEr and by a large nuclear deformation energy ΔEnd. The nuclear deformation energy is defined as the energy required to increase the AC bond length to the value it would have in ABC. This simple criterion, ΔEnd > ΔEr, for existence of an activation energy accounts reasonably well for the five reactions of the insertion type which have so far been measured. Two of the measurements are reported here for the first time.
Translational Frequencies of Molecules in Naphthalene, Durene, and Anthracene Single Crystals at Low Temperatures
Authors: Hadni Armand , Wyncke Bruno , Morlot Guy , Gerbaux Xavier
Published: February 2004

Among the three translational frequencies of these monoclinic molecular crystals, two are expected when the electric field of the radiation is parallel to the monoclinic b axis, and one when the field is perpendicular to b. The infrared absorption spectra show that the last band is obtained in each case for an electric field parallel to the easy cleavage plane, at 66 cm−1 for naphthalene, 65 cm−1 for durene, 61 cm−1 for anthracene. This polarization is explained for naphthalene in terms of the tensor of polarizabilities of the molecule, and of its orientation in the crystal. For E parallel to the monoclinic axis the two expected absorption bands are found at 37 and 68 cm−1 for durene, 101 and 120 cm−1 for anthracene, and around 96 cm−1 for naphthalene where they have not been separated. All these bands are shifted towards higher frequencies when the crystal is cooled down to liquid-nitrogen temperature.
Thermal Quenching of Characteristic Fluorescence
Authors: Blasse G.
Published: February 2004

A model is proposed to explain qualitatively the dependence of the quenching temperature of a given fluorescent center on the choice of the host lattice. The model is tested on some actual cases.
Why is Condensed Oxygen Blue?
Authors: Tsai S. Cheng , Robinson G. W.
Published: February 2004

The blue color of condensed oxygen is mostly derived from double transitions. This paper presents a theoretical calculation of the intensity of the double transition (a 1Δg) (a 1Δg)←(X 3Σg−) (X 3Σg−), using a model based on a pair of oxygen molecules at a fixed separation of 3.81 Å. The intensity enhancement is assumed to be derived from the mixing (a 1Δg) (a 1Δg)∼∼(X 3Σg−) (B 3Σu−) and (a 1Δg) (1Δu)∼∼(X 3Σg−) (X 3Σg−). Matrix elements for these interactions are calculated using a π-electron approximation for the pair system. Good molecular wavefunctions are used for all but the perturbing B 3Σu− state, which is approximated in terms of ground-state orbitals. The largest contribution to the matrix elements arises from large intramolecular terms multiplied by intermolecular overlap integrals. The strength of the interaction depends not only on the intermolecular separation of the two oxygen molecules but also, as expected, on the relative orientation. Matrix elements are calculated for different orientations, and the angular dependence is fit to an analytical expression. The theory therefore not only predicts an intensity dependence on density but also one on phase at constant density. Agreement between theory and available experimental results is satisfactory considering the nature of the approximations, and indicates the essential validity of the over-all theoretical approach to this interesting intensity enhancement problem. A qualitative interpretation is also suggested of the anomalous (1-0) / (0-0) intensity ratio in the double transition on the basis of a dependence of intermolecular electronic interaction on vibrational amplitude.
Proton Magnetic Resonance Studies of 2-Methyl-2-Propanethiol
Authors: Smith George W.
Published: February 2004

Proton magnetic resonance (PMR) studies of 2-methyl-2-propanethiol reveal the existence of molecular reorientation (activation energy ∼ 3 kcal/mole) about the C–S axis in the low-temperature phase (phase IV). This type of molecular reorientation is in accord with Kondo's dielectric studies of phase IV. General molecular reorientation and self-diffusion occur in both phases II and I. The self-diffusion activation energies in the two phases (∼ 8.5 and 7.7 kcal/mole) are in agreement with estimates from the model of Andrew. Self-diffusion is found to be somewhat quenched upon transition from phase II to phase I; this behavior can be explained on the basis of a change of lattice structure from bcc (proposed by Guthrie and McCullough) to fcc (found experimentally by Kondo). The PMR studies can not distinguish between phases III and II. The rigid-lattice second moment (at T < 70°K) is in accord with a molecular structure based on standard bond lengths and angles.
De-excitation of Metastable Nitrogen Molecules in Collisions with N2 and NO
Authors: Sheridan J. R. , Peterson J. R.
Published: February 2004

The cross section for de-excitation of the A 3Σu+ state of N2 in collisions of a metastable molecular beam with NO and N2 molecules has been measured in the energy range 250–2200 eV. The two cross sections are roughly the same in magnitude, and decrease toward higher velocities approximately as (b–a logυ)2.
Internal Reorientations in K2ReH9 via Wide-Line and Pulsed Proton Resonance Studies
Authors: Farrar T. C. , Tsang Tung , Johannesen Rolf B.
Published: February 2004

Proton spin–lattice relaxation times (T1) and second moments (M2) have been measured as a function of temperature for K2ReH9. The activation energies for internal reorientation of ReH9= ions, and their rms errors, are 9.9 ± 0.4 and 25.0 ± 0.8 kJ/mole (2.4 ± 0.1 and 6.0 ± 0.2 kcal/mole), respectively, for type a and d sites. Our results suggest that the barriers are determined by nearest-neighbor ReH9=–ReH9= interactions.
Calculation of the Dipole Autocorrelation Function for a System of Weakly Coupled Electric Dipoles
Authors: Boon Jean Pierre , Köhler Michel
Published: February 2004

In this paper, we present a simple theoretical treatment of dipolar relaxation, essentially based on the autocorrelation formalism and derived from the general theory developed by Boon and Rice for the autocorrelation function of a dynamical variable. A simple model is introduced to derive analytical expressions for the dipolar autocorrelation function, from which other quantities, like the response function and the complex dielectric function, are calculated. These theoretical results are tested against the experimental data obtained from a molecular dynamics study by Bellemans, Köhler, and Gancberg on a two-dimensional system of electric dipoles on a rigid lattice, subject to dipole–dipole interactions. We also append some results concerning the memory function governing the evolution of the autocorrelation function as described by the master equation.
Interference Effects in the Optical Spectrum of Large Molecules
Authors: Jortner Joshua , Morris G. C.
Published: February 2004

Interference effects between intravalence and extravalence excitations in the optical spectrum of a large molecule are considered. The sharp antiresonances observed in the Rydberg spectrum of the naphthalene molecule are interpreted in terms of configuration interaction between zero-order Rydberg levels and an inhomogeneously broadened π→π* transition.
Lifetime and Motion of Triplet States in Molten Aromatic Systems
Authors: Baessler H. , Loelkes G. , Vaubel G.
Published: February 2004

First-order (T1) photocurrents in molten naphthalene and anthracene with or without an incorporated aromatic solute are caused by triplet states striking a metal anode. The identification of the active excited state with the triplet state is based on the fact that its lifetime is equal to the triplet lifetime inferred from delayed-fluorescence experiments. The analogy between T1 photocurrents and second-order (T2) photocurrents with respect to field dependence and quantum yield supports this interpretation because Gray et al. have shown that the T2 photocurrent is due to bimolecular triplet interaction in the bulk. Therefore we have used the time dependence of the T1 photocurrent for determination of triplet lifetimes. For various hydrocarbons the values are of the order of 10–100 msec. No temperature dependence is observed.
Molecular Integrals over Generalized Hermite–Gaussian Functions
Authors: Katriel J. , Adam G.
Published: February 2004

A generalized form of the Hermite–Gaussian functions is considered for molecular calculations. Explicit analytical formulas are obtained for the necessary integrals.
Padé Approximants for Radial Distribution Functions of the Gaussian Model
Authors: Carley David D.
Published: February 2004

Radial distribution functions (g) are computed for the Gaussian model of a fluid by approximating the density expansion with a Padé approximant g(m, n). Several choices of m and n are studied by comparing these calculated g's with those previously obtained using the Percus–Yevick and hypernetted chain integral equations, a Monte Carlo method, and simple truncation of the density expansion. It is found that, if a few terms in the density expansion are known, the range of densities for which this information is useful may be extended by the proper choice of m and n in the Padé approximant. A form of g(m, n) is presented which approximates g quite well for a large range of densities.
Tandem Mass-Spectrometer Study of D3O+ and Solvated Derivatives
Authors: DePaz M. , Leventhal J. J. , Friedman L.
Published: February 2004

The mass spectra of heavy water produced by electron impact on neutral gas at pressures up to 1 torr have been studied in a tandem mass spectrometer. The enthalpies of hydration of solvated D3O+ as well as the deuteron affinity of D2O were studied by measurement of kinetic-energy thresholds for the dissociation of the respective ions in collision with helium. Evidence was obtained for excited states in D3O+, D5O2+, D7O3+, D9O4+, etc., which survived for time periods in excess of 10−4 sec. Efforts to de-excite these ions with neutral D2O gas molecules in the high-pressure source were rarely successful, indicating difficulties in the interpretation of data based on the assumption of thermodynamic equilibrium in such ion sources. Values for the heat of association of two and three neutral heavy-water molecules were deduced from the threshold data. Relative cross sections for the attenuation of various ionic species were measured. The significant differences in the attenuation cross section point up problems in sampling steady-state concentrations of ions in a high-pressure ion source. The relative attenuation cross sections were used to test the validity of a point-charge–dipole interaction model for calculation of hydration enthalpies in the water systems. This model was shown to provide a very simple means of demonstrating an excellent correlation between experimentally determined enthalpies and cross-section data.
Nitrogen Isotope Effects in the Reduction of Nitrate, Nitrite, and Hydroxylamine to Ammonia. II. The MgO and CuSO4 Systems
Authors: Brown L. L. , Drury J. S.
Published: February 2004

Nitrogen kinetic isotope effects were used to study reaction mechanisms associated with reductions of NO3−, NO2−, and NH2OH in hot alkaline solutions that contained Fe(II). In the CuSO4 system the specific reaction rate constants varied in the order kNO2− > kNO3− > kNH2OH, rather than the order kNO2− > kNH2OH > kNO3− found previously in the Ag2SO4 system. When MgO was used, the order of the specific reaction rate constants was the same as that observed with Ag2SO4, but different mechanisms operated in the reduction of NO3− and NO2−, although not in the reduction of NH2OH. Reactions consistent with the observed isotope effects and the known chemistry of nitrogen were postulated for the most probable reaction paths in the various systems.
Barriers to Internal Rotation in Asymmetric Molecules: 3-Fluoropropene
Authors: Meakin Paul , Harris David O. , Hirota Eizi
Published: February 2004

The potential function hindering internal rotation in 3-fluoropropene has been determined using microwave and far-infrared spectroscopic data. It is given by the six potential constants in cm−1 (calories/mole): V1 = − 247 ± 30(− 707), V2 = 185 ± 25(530), V3 = 857 ± 15(2449), V4 = 188 ± 25(538), V5 = 7 ± 5(20), V6 = − 93 ± 10(− 265). From these the cis–gauche and gauche–trans–gauche barrier heights were determined to be 1090 + 75 − 25 cm−1 and 520 ± 40 cm−1, respectively. The spectra were interpreted using a one-dimensional model in which the only internal motion is the rotation about the bond connecting the top (–CH2F) to the frame (H2C=CH–); the various parameters in the Hamiltonian which was derived using the theory of internal rotation in asymmetric molecules developed by Quade and Lin were determined numerically and expressed as Fourier expansions in the internal coordinate α. In addition, the general theory was rederived and extended to include the case where neither the top nor the frame has a plane of symmetry. The large torsion–rotation interaction terms were reduced numerically by means of a Van Vleck transformation. The angle dependence of the reduced moment of inertia for the torsion was removed by a nonlinear coordinate transformation which yields a new Hamiltonian giving the same energy spectrum as the old but having a constant kinetic energy and a modified potential function. Finally, our potential is compared with that which was calcuated from the electrostatic model of Lowe and Parr.
Statistical Mechanics of Alternant Spin Waves
Authors: Abraham Douglas B.
Published: February 2004

The statistical mechanics of the Heisenberg antiferromagnetic ring of doublet sites with alternating exchange coupling is investigated by an approximate method based on the equivalence, established by the Jordan–Wigner transformation, of Pauli spin matrices and Fermi annihilation and creation operators. Mathematically, the theory is somewhat similar to the pseudospin approach of Soos, but the physical interpretation is quite different, being analogous to that in Anderson's theory of antiferromagnetism. Unlike Soos' work, the present formulation requires the sublattice magnetization to vanish at all temperatures, in agreement with the exact result of Mermin and Wagner. Different magnetic susceptibility valves are derived which are in better agreement with the numerical results of Duffy and Barr.
NMR Study of Molecular Motions near the Solid–Solid Transition in the Metal Hexafluorides
Authors: Rigny P. , Virlet J.
Published: February 2004

Fluorine-19 magnetic resonance and relaxation measurements on orthorhombic MoF6 and on cubic MoF6 and WF6 are reported. In the cubic plastic phases the molecules have rapid rotational and slow translational motions. The theory of Resing and Torrey has been applied to interpret the relaxation data and yields a diffusion coefficient of 0.52 × 10−8 cm2sec−1 for MoF6 and 0.36 × 10−8 cm2 sec−1 for WF6 at the transition point. Slow rotations of the molecules already occur in the orthorhombic phases; this is shown by the study of the shape of the resonance lines in the presence of both motion and anisotropic chemical-shift tensors. Accurate calculations of the contribution of these rotations to the longitudinal relaxation and to the second moments of the resonance line have been made. They show that in MoF6, which exhibits a solid–solid transition, the rotations of the molecules are quasi-isotropic, whereas in UF6, where no transition occurs, the rotations are slower and take place preferentially about a single C3 axis of the molecules. At high Larmor frequencies the relaxation in the plastic phases is probably due to the spin–rotation interaction, as in the liquid. A model for spin–rotational relaxation in a molecular solid is proposed.
Rotational Diffusion of Spherical-Top Molecules in Liquids
Authors: McClung R. E. D.
Published: February 2004

The extension of the classical rotational diffusion model proposed by Gordon in his study of linear molecules is applied to the evaluation of the reorientational correlation functions, their Fourier transforms, and correlation times for fluids composed of spherical-top moelcules. The reorientational correlation times τθ(j), describing the time correlation of a jth-order spherical harmonic, are expressed as functions of the correlation time τJ for the angular momentum of the molecules. These expressions are valid for all values of ω̄τJ, where ω̄ is the root-mean-square angular frequency (kT / I)1 / 2 for a spherical-top molecule with moment of inertia I. In the Debye limit (ω̄τJ ≪ 1), τθ(j) ≫ τJ, and τθ(j) are proportional to τJ−1 / ω̄2; in the perturbed-free-rotor limit (ω̄τJ ≫ 1), τθ(j) is proportional to τJ and is of comparable magnitude. τθ(j) goes through a minimum value when τθ(j), τJ, and ω̄−1 are approximately equal. Magnetic spin relaxation due to motional modulation of anisotropic spin–rotational interactions is also considered in the framework of this extended diffusion model. The resulting expression for the spin relaxation times involves a correlation function containing both the time correlation of the orientation of the molecule and the angular momentum of the molecule simultaneously. This correlation function and hence the spin-relaxation times, can be calculated on the basis of the extended diffusion model for all values of ω̄τJ without assuming any separability or independence of the correlations of the molecular orientation and angular momentum. In the Debye limit, the expression for the spin-relaxation times reduces to that of Hubbard, and in the perturbed-free-rotor limit, it reduces to the result previously obtained by Bloom et al.
Viscosity of Nitrogen, Helium, Hydrogen, and Argon from − 100 to 25°C up to 150–250 atm
Authors: Gracki J. A. , Flynn G. P. , Ross J.
Published: February 2004

The range of a previously developed capillary-flow viscometer has been extended to − 100°C, and the techniques of obtaining and determining the attainment of steady state have been improved. The viscosities of nitrogen, helium, hydrogen, and argon have been measured by an absolute method from − 100 or − 90° to 25°C and up to 150–175 atm (250 atm for N2 at 25°C). The accuracy is estimated to be 0.1%–0.2%, but is somewhat worse for Ar at − 100°C. A number of empirical and theoretical analyses of the viscosity–density dependence have been made; the evidence is inconclusive as to the presence of a logarithmic term in the density expansion. Correlations of the second and the third virial coefficients of viscosity have been made.
Magnetic Structure of HoAg2
Authors: Atoji Masao
Published: February 2004

The tetragonal body-centered HoAg2 has been shown by neutron powder diffraction to become an anti-ferromagnet with the Néel temperature of 5.7°K. The magnetic spin alignment is represented by a static longitudinal wave propagating along the a axis of the face-centered alternative of the chemical lattice. The wavelength of this static spin wave is 1.21 times (the face-centered a spacing), being incommensurable with the lattice repetition. Below 4.6°K, a linear–transverse spin wave being polarized in the c-axis direction develops with the propagation direction and wavelength identical to the coexisting longitudinal wave. The maximum and rms moments in Bohr magnetons extrapolated to 0°K are, respectively, 7.5 and 5.3 for the longitudinal wave and 5.0 and 3.5 for the transverse wave. The maximum composite moment at 0°K is 9.0 ± 0.2 μB, while the free Ho3+ ion should yield 10 μB.
Structural Studies of Polyethers, [–(CH2)m–O–]n. VII. Infrared Spectra of Polyoxacyclobutane
Authors: Makino Daisuke , Kobayashi Masamichi , Tadokoro Hiroyuki
Published: February 2004

Polarized infrared spectra of the three modifications of polyoxacyclobutane [–(CH2)3–O–]n have been measured in the region from 4000 to 80 cm−1. The selection rules for modifications I, II, and III are derived under the factor groups isomorphous to C2υ, Cs, and D2, respectively. The normal vibrations have been calculated for these modifications using the GF matrix method with the Urey–Bradley-type force field, and the vibrational assignments of the absorption bands have been given. For modification I (planar zigzag form), the CH2 rocking vibrations split into three modes which correspond to the modes with the phase differences of 3π / 4, 2π / 4, and π / 4 of a simply coupled oscillator model consisting of three methylene units, respectively. The observed frequencies have been found to fit the calculated dispersion curve for the rocking–twisting branch (ν8) of the polymethylene chain. Similar behavior in the rocking modes occurs also in the cases where the O–CH2–CH2–CH2–O group assumes the TG (modification II) or GG (modification III) conformation. Thus, the difference in the skeletal conformation gives rather little effects on the rocking modes of the polyoxacyclobutane molecule.
Analysis of the Cubic Crystal-Field Parameters for Gd3+ in the Alkaline Earth Fluorides
Authors: O'Hare J. M. , Detrio J. A. , Donlan V. L.
Published: February 2004

The optical spectra of the alkaline-earth fluorides doped with Ge3+ (MeF2:Gd3+, where Me represents Ca, Sr, or Ba) at octahedral sites are analyzed to determine the effect of lattice size on the cubic crystal-field parameters. The point-charge model predicts a linear dependence of the first-order crystal-field splitting on B04 which in turn varies inversely with the Gd3+–F− separation to the fifth power (R−5). The slopes of plots of log b40 vs logR yield values of − 2.1, − 2.7, and 0 for the S7 / 28, P7 / 26, and P5 / 26 multiplets, respectively. This apparent discrepancy is resolved by the inclusion of crystal-field interactions in all orders between the S8 and 6(P, I, D) multiplets. The log(A40〈r4〉) vs logR plot is found to be − 1.8 which agrees with the EPR result to within experimental error. The disagreement with the point-charge model is examined.
Significant Structure Theory Applied to Surface Tension of Binary Liquid Mixtures
Authors: Kim Sung Wan , Eyring Henry , Lee Yong-Teh
Published: February 2004

The surface tension of binary liquid mixtures, argon–nitrogen, oxygen–nitrogen, and oxygen–argon at 83.85°K were calculated by applying the significant-structure theory of liquids. In this paper, we calculate the contribution of successive layers to the surface tension. The calculations show that the top three layers of liquid mixture contribute to the surface tension. The curves obtained are in excellent agreement with experiment.
Mössbauer Study of IF5 and IF7
Authors: Bukshpan S. , Goldstein C. , Soriano J. , Shamir J.
Published: February 2004

The Mössbauer effect in 129I was used to investigate the high valence states of iodine in IF5 and IF7. The quadrupole couplings were found to be + 1073 ± 2 Mc/sec and − 148 ± 4 Mc/sec, respectively. From these values the character of the I–F bond in these compounds was deduced.
Optical Absorption and Fluorescence Spectra of Single-Crystal TbF3
Authors: Krupka D. C. , Guggenheim H. J.
Published: February 2004

Absorption and fluorescence measurements on a single crystal of TbF3 have been used to prepare an energy-level scheme for the Stark components of the 7F multiplet and the 5D4 level of Tb3+ in this crystal. The results are consistent with Cs site symmetry for the rare-earth ion. In addition a limited set of measurements indicate that the 5D4 level decays with lifetimes of 3.9 ± 0.4, 0.46 ± 0.05, and 0.30 ± 0.05 msec at 4.2, 77, and 300°K, respectively.
NH4Cl, a Mixed Conductor
Authors: Kröger F. A.
Published: February 2004

Pure and barium-doped single crystals of NH4Cl were grown from alkaline and acid aqueous solutions. Electromotive-force measurements of H2, NH3 gas concentration cells with NH4Cl as a solid electrolyte give the transference numbers of ionic, protonic, and electronic conduction in NH4Cl as f(PH2, PNH3). Measurements of the conductivity in controlled atmospheres of H2 and NH3 as f(T) show a dependence on the gas composition in one case only. It is attempted to interpret the results on the basis of simple defect models.
Infrared Spectra of the Methylammonium Halides: Effects of Orientational Disorder about One Axis
Authors: Whalley E.
Published: February 2004

The spectroscopic effects of orientational disorder of molecules about one axis only have been identified in the spectra of the α-solid phases of the methylammonium halides published by others. The C3υ methylammonium ion occupies a crystallographic tetragonal axis, and so must be either disordered or freely rotating. The entropy of transition from the ordered phase is nearly R ln4, and there is no drop of heat capacity that can be associated with the onset of free rotation. It seems likely that the anion is disordered among four equivalent positions. The A1 vibrations of the methylammonium ion are little affected by the disorder, and most of the intensity resides in the zero-wave vector vibrations The coupling of the E vibrations, on the other hand, is greatly affected by the disorder, and all coupled vibrations are active. The A1 vibrations therefore cause sharp bands whereas the E vibrations cause broad bands. The expected effects occur clearly in the NH3 and CH3 bending vibrations.
Anisotropic Thermal Motion of Trigonal Bipyramidal Molecules from Spectroscopic Data. Pentacoordinated Molecules. XIII
Authors: Holmes Robert R. , Deiters Rose Mary
Published: February 2004

A systematic vibrational analysis has been carried out on trigonal bipyramidal molecules for which adequate spectroscopic data exists. From the resulting description of the normal modes based on the harmonic approximation, the mean-square amplitudes of thermal motion are computed as a function of temperature. Considerable anisotropy is evident in the displayed thermal–vibrational ellipsoids. The thermal parameters are listed in internal coordinates of interest to the electron diffractionist as well as Cartesian coordinates for comparison with crystallographic studies. In dealing with molecular crystals containing light ligands or having low-frequency internal modes, the importance of correcting bond distances for the effects of thermal motion due to internal vibrations is emphasized in connection with the problem of increasing the precision of structural parameters obtained from diffraction studies. The use of thermal ellipsoids representing internal motion in determining missing fundamentals and in aiding frequency assignments is discussed.
High-Pressure Mass Spectra and Ion Chemistry of Carbon Dioxide
Authors: Schildcrout Steven M. , Franklin J. L.
Published: February 2004

A mass-spectrometric study of ion–neutral reactions occurring in CO2 subjected to electron impact yields rates in agreement with previous studies for reaction of C+ and O+ with CO2. Charge transfer from O+ to CO2 is believed to compete with formation of O2+ in these experiments. Reaction of CO+ with CO2 is found to be somewhat faster than previously reported. An approximate rate has been obtained for loss of CO22+. The great abundance of C2O4+ in this system at pressures near 1 torr results from reaction of CO2+ with several CO2 molecules. This reaction is not well understood but a mechanism involving stepwise ion polymerization followed by collision-induced decomposition of the complex to C2O4+ is suggested.
Quantum-Mechanical Variational Theory of Bimolecular Gas-Phase Reaction Rates
Authors: Snider Neil S.
Published: February 2004

Chemical reactions in the gas phase can often be regarded as resulting from collisions which can be idealized as motion on a potential-energy surface. An upper bound to the rate constant of such a reaction was derived by Wigner and generalized by Horiuti. Both of these workers made the additional assumption that the motion of the system on its potential-energy surface is classical. In the present paper a quantum-mechanical analog of this upper bound expression is derived for the particular case of a bimolecular reaction.
States of Helium Atoms Adsorbed on Krypton and Xenon Crystals
Authors: Ricca Franco , Pisani Cesare , Garrone Edoardo
Published: February 2004

The quantum-mechanical state of a He atom absorbed on the (100) face of Kr and Xe crystals has been studied. The variational function for the adsorbed atom has been constructed using a linear combination of the Bloch sums of local functions corresponding to suitable three-dimensional anharmonic oscillators. The probability distribution for the coordinates of the He atom has been described by a set of isodensity curves similar to those normally employed when studying atomic or molecular orbitals. The gap between the energy bands corresponding to the fundamental and first excited state implies that, at the lowest experimental temperatures, the surface mobility must be interpreted in terms of the fundamental state. For the fundamental state, the total energy is higher than the potential barrier to surface migration in the case of Kr and slightly lower in the case of Xe.
Conduction-Induced Alignment of Nematic Liquid Crystals: Basic Model and Stability Considerations
Authors: Helfrich W.
Published: February 2004

It is shown by way of model orientation patterns how conduction in nematic liquid crystals may cause a torque per unit volume that more than offsets the dielectric torque felt in the insulating state. This conduction-induced torque arises from an anisotropy of the conductivity or of the dielectric constant. It is partly dielectric but its more important part is shear-induced, the shear flow being a consequence of space charge which is generated by and interacts with the applied electric field. The stability of a uniform orientation pattern in electric fields parallel or perpendicular to the preferred axis is investigated, with respect to a complete set of sinusoidal perturbations and including distortional and (stabilizing) magnetic torques. The analysis shows that the shear-induced torque cannot be treated like its dielectric counterpart. It depends in a peculiar way on the direction of the perturbational wave vector and may contain a nonconservative contribution. Formulas for the instability threshold, i.e., the electric field strength at which the orientation pattern becomes unstable, are derived. Friction is taken into account, in particular the strong coupling between rotational and translational motion in liquid crystals. The formulas are applied to p-azoxyanisole. The threshold field of anomalous alignment (as opposed to dielectric alignment in the insulating state) in voluminous samples and the threshold voltage of domain formation in sandwich cells are calculated. Good agreement with experiment is found.
Fluorescence Spectra of Some Retinyl Polyenes
Authors: Thomson A. J.
Published: February 2004

The fluorescence and absorption spectra of two series of retinyl1 polyenes have been measured. One series consists of the retinyl–carbon moiety with different end groups, these being the alcohol, acetate, acid, oxime, and methyl-amine. The second group comprises the trans, 11-cis, 13-cis, and 9-cis stereoisomers of retinol (vitamin A). The spectra of the first series have been measured at room temperature and 77°K in both hydrocarbon and methanol solvents. The acetate and acid have also been studied as crystals. The relative shapes and the degree of overlap of the absorption and fluorescence spectra are discussed in terms of simple potential-energy diagrams. It is shown that the bands are severely Franck–Condon forbidden on two counts; first, by virtue of the bond alternation in the ground state that leads to a gross change in geometry of the nuclear framework on excitation, and second, from steric hindrance that occurs at various points in the retinyl–carbon framework. The spectra reported here show how an end group or a bend in the chain can affect the nature of the vibrational envelope. The effect of an environment is shown to be particularly pronounced upon Franck–Condon forbidden bands. The quantum yields of fluorescence at room temperature have been determined for the trans, 11-cis, 13-cis, and 9-cis retinols, and trans-retinyl acetate. The yields at room temperature are given in both hydrocarbon and polar solvents. The intensity of the fluorescence as a function of temperature has been estimated in some cases. Fluorescence could not be detected from retinyl acid and retinylidene methylamine at room temperature. The emission yield is shown to be correlated with the relative shapes of the ground- and excited-state potential-energy surfaces. It is suggested that vibrational relaxation in the excited state is the major process deactivating excited polyenes in fluid solution. The efficiency of this deactivating process is much reduced by increasing the viscosity of the environment. This process may be of great significance when polyenes are employed in photobiology. The radiative lifetime of trans-retinol in methanol calculated from the data is 2.35 nsec.
Absolute Differential Cross Sections of Elastically Scattered Electrons. II. A Polarization Scattering Potential for Hg at 500, 400, and 300 eV
Authors: Bromberg J. Philip
Published: February 2004

Absolute differential cross sections of electrons elastically scattered by Hg have been measured at 500, 400, and 300 eV. We find that over this energy range, for angles between 2.5° and 20°, the measured cross sections can be accurately described using the Born approximation with a long-range effective potential of the form V(r) = 4Cb / π(r2 + b2)2, where C and b are constants arising from the intercept and slope, respectively, of plots of lnσ vs ΔP, and where 4Cb / π = α / 2, where α is the polarizability of the Hg atom.
Interpretation of the Crystal Field for V2+, Cr3+, and Mn4+ in Corundum
Authors: Stedman G. E.
Published: February 2004

The observed crystal field for substituted corundum is re-expressed using the superposition model, by analogy with previous work on ruby and MgO. The assumption that overlap-dependent contributions from the coordinated neighbors are dominant gives an adequate qualitative explanation of the trends in both the static and the trigonal stress crystal-field data for the isoelectronic series V2+, Cr3+, Mn4+, contrary to some previous suggestions.
Nanosecond Pulse Radiolysis of Liquid Naphthalene and Biphenyl
Authors: Theard L. M. , Peterson F. C. , Holroyd R. A.
Published: February 2004

The pulse radiolysis of liquid naphthalene and biphenyl was investigated with nanosecond time resolution. In naphthalene, excited species in both singlet and triplet states were observed. The fluorescence spectrum showed that the excited singlet is present to a large extent as the excimer. A short-lived (τ1 / 2 < 100 nsec) transient absorbing broadly in the 450–575-mμ spectral region was observed, the decay of which corresponded to the decay of the fluorescence. This short-lived species is suggested to be the naphthalene excimer. The formation of triplet states by crossover from the first excited singlet state (ISC) is of minor importance in the radiolysis of molten naphthalene. When 1,2-benzanthracene was added to the naphthalene, absorption by the first excited singlet state of 1,2-benzanthracene was observed. In the pulse radiolysis of molten biphenyl at 100°C most of the ions recombine in less than 10 nsec; thus triplet excited states formed by ion recombination are expected to be present 10 nsec after the pulse. The fluorescence indicated that two excited singlet states of biphenyl are present.