bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of the Ligand Field Spectra of Some Low Spin Pentacoordinate Co2+ Complexes
Authors: Caride A. O. , Panepucci H. , Zanette S. I.
Published: February 2004

The optical and EPR spectra of three low spin pentacoordinate cobalt complexes are studied using the strong crystal field approach. Assignments of the absorption bands are made which give excellent agreement with regard to the band positions and relative intensities. Calculations based on these assignments also give the correct g values assuming k≈ 0.6. The one-electron energy of the e orbital is found to be higher than that of the b2 orbital, and this result is discussed from a molecular orbital point of view.
High-Temperature Vaporization and Thermodynamics of the Titanium Oxides. VII. Mass Spectrometry and Dissociation Energies of TiO(g) and TiO2(g)
Authors: Hampson Peter J. , Gilles Paul W.
Published: February 2004

Two experiments of the successive vaporization type, five mass spectrometric vaporization experiments, and two isomolecular exchange reaction experiments were undertaken to resolve discrepancies in the thermodynamics of the titanium—oxygen system. The first two types of experiments confirmed that Ti3O5(s) is the congruently vaporizing phase. The second type of experiment showed that the principal reaction accounting for the congruent vaporization is Ti3O5(S)=TiO(g)+2TiO2(g). In the third type of experiment, measurements on the isomolecular exchange reaction TiO(g)+Sc(g)=Ti(g)+ScO(g) showed that the dissociation energy of TiO(g) is smaller than that for ScO(g) and on the basis that D0o(ScO) is 160.4±2.0 kcal/mole, D0o(TiO) becomes 156.9±2.2 kcal/mole, or 6.80±0.10 eV. This result may be combined with earlier vapor pressure measurements on Ti3O5(s) and the congruency condition to yield D0o(TiO2)=311.7±1.8 kcal/mole, 13.52±0.08 eV, or very nearly twice D0o(TiO). One of the two previously discussed discrepancies in the thermodynamics of the titanium oxides appears to have been resolved, and the likely resolution of the other is discussed.
Nonempirical LCAO MO SCF Calculation on Acetylene, Vinylidene Carbene, and the Vinyl Cation
Authors: Hopkinson A. C. , Yates K. , Csizmadia I. G.
Published: February 2004

Nonempirical LCAO MO SCF calculations using Gaussian-type functions have been used to predict the geometries of acetylene, vinylidene carbene, π-protonated acetylene, and the vinyl cation. The computed geometry and molecular ionization potential of acetylene give good agreement with experimental data. The vinyl cation is predicted to be more stable than π-protonated acetylene by about 18 kcal/mole.
Determination of Rate Constants for Reactions of O Atoms with C2H2, C2D2, C2H4, and C3H6 Using a Pulsed Vacuum-uv Photolysis—Chemiluminescent Method
Authors: Stuhl F. , Niki H.
Published: February 2004

Absolute rate constants for the primary reactions of O atoms with C2H2, C2D2, C2H4, and C3H6 were determined using vacuum-uv flash photolysis of NO to produce the O atoms. Decays of O atoms were monitored by NO2* chemiluminescent emission with initial O-atom concentrations in the range 1–5×1010 atoms cm−3. The rate constants obtained at 300°K are (in units of cubic centimeters molecule−1· second−1): k=1.31×10−13 (+5; −10%), for O+C2H2 and O+C2D2; k=6.3×10−13 (+5; −15%), for O+C2H4; k=3.6×10−12 (+5; −10%), for O+C3H6. The parameters involved in the assignment of the error limits are discussed in detail.
Gaussian Basis Functions for Use in Molecular Calculations. IV. The Representation of Polarization Functions for the First Row Atoms and Hydrogen
Authors: Dunning Thom. H.
Published: February 2004

The representation of polarization functions for the first row atoms and hydrogen is investigated by means of Hartree—Fock calculations on the water and nitrogen molecules. It is found that a two-term Gaussian expansion is required to adequately represent the 3d polarization functions of the first row atoms, whereas for the 2p polarization functions of hydrogen, a single Gaussian function is sufficient. The interaction of oxygen and hydrogen polarization functions in the water molecule is briefly discussed.
Estimation of Lennard-Jones (6,12) Pair Potential Parameters from Gas Solubility Data
Authors: Wilhelm Emmerich , Battino Rubin
Published: February 2004

The Lennard-Jones (6,12) pair potential parameters for solvents can be calculated from gas solubility data by using the scaled particle theory to calculate the work of cavity formation. Values of the hard sphere diameter σ1 and the energy parameter ε1/k at 298.15°K and 1 atm pressure are reported for 39 liquids, polar as well as nonpolar. These parameters are discussed in relation to values determined by other methods.
Theory of k-Independent Depolarized Rayleigh Wing Scattering in Liquids Composed of Anisotropic Molecules
Authors: Keyes T. , Kivelson D. , McTague J. P.
Published: February 2004

A discussion is presented of the k-independent factors that contribute to the depolarized Rayleigh wing scattering in liquids composed of anisotropic molecules. The spectrum consists of a sharp line (several tenths cm−1) and a broad line (several cm−1 or tens of cm−1). It is suggested that the spectrum arises from at least two separate processes, each of which induces fluctuations in off diagonal elements of the dielectric tensor: molecular reorientation, which is a relatively slow process, and some form of intermolecular interaction, which is a rapid process. The autocorrelation functions for the slow and rapid processes account in large part for the sharp and broad lines, respectively; however, the heretofore neglected cross correlation function between the slow and rapid processes may contribute significantly to the spectrum, particularly to the sharp line which may be appreciably reduced in intensity due to the cross correlation. This reduction in sharp line intensity suggests that the difference between the observed integrated sharp line intensity and that expected on the basis of single particle static orientational correlations, 〈D(1)D(1)〉, cannot be simply related to the presence of two particle static orientational correlations, 〈D(1)D(2)〉.
Properties of Self-Avoiding Walks Not Constrained to Lattices
Authors: Lax Melvin , Windwer Stanley
Published: February 2004

A study of self-avoiding walks not confined to a lattice was performed. By use of the method of direct enumeration, walks of 14 steps were generated. Two different models were studied. Configurational properties, such as the mean square end-to-end distance, the mean-square radius of gyration, etc., plus hydrodynamic as well as phase transition properties were obtained. They are discussed in terms of the theories incorporating the excluded volume effect. Some of the results were applied to the DNA molecule.
Shear Viscosity of 3-Methylpentane—Nitroethane near the Critical Point
Authors: Stein Arnold , Allegra Joseph C. , Allen Guy F.
Published: February 2004

Shear viscosity coefficients for a mixture of 3-methylpentane—nitroethane of critical composition were determined over the temperature range 6×10−6≤ ε ≤ 2×10−2 by an improved capillary viscometer. The data were analyzed by alternate procedures both of which give strong support to the view that the viscosity singularity is indeed very weak over a wide temperature range.
Nonradiative Transitions: A Unified Green's Function Formalism
Authors: Tric C.
Published: February 2004

A unified Green's function formalism is proposed which clarifies the different decay modes conceivable in a molecule, with and without external perturbers, within the simplified model of one radiative Born—Oppenheimer level | s 〉 coupled with very weakly radiative levels | l 〉. For an isolated molecule, it is well known that the ensemble | s 〉 and l levels) gives rise to a new set of ``molecular quasistationary states.'' We distinguish the case of ``weak'' and ``strong'' coupling, where the coupling of | s 〉 and | l 〉 levels is compared with the difference between radiative widths ( Γs−Γ l)/2. The total number of solutions is conserved. In addition to the molecular eigenstates, other decay modes are readily obtained when a time limit is introduced in the experiment. Certain levels {u}, characterized by a sufficiently high density of states and coupling, then act as a dissipative quasicontinuum, contributing to an additional linewidth Δu; by extension of the previous treatment, it is found that the weakly coupled levels falling within the over-all linewidth are slightly perturbed, whereas for the levels which do not fall within the linewidth, both the radiative and dissipative linewidth are reduced, in proportion to the content | α |2 of Born—Oppenheimer radiative s state in each quasistationary state, leading to the lifetime τ : ℏ/τ=| α |2(Γs+Δu).
Continuity at the Dissociation Threshold in Molecular Absorption
Authors: Allison A. C. , Dalgarno A.
Published: February 2004

A continuity relationship is established for molecules that relates discrete absorption oscillator strengths to photodissociation cross sections. The relationship is verified by explicit numerical calculations.
Calculated Properties of Metal Aggregates. I. Diatomic Molecules
Authors: Baetzold Roger C.
Published: February 2004

The extended Hückel and a modified CNDO method are described and applied to the calculation of electronic properties of Ag2, Au2, Cu2, Pd2, Na2, Ca2, and Cd2, molecules. The results indicate that these calculation procedures may be parameterized to describe the electronic properties of metallic diatomic molecules. The methods of calculation are compared and the origin of bonding in each molecule is described. The ordering of molecular energy levels calculated by EH and CNDO is the same, but their dependence on internuclear distance is different.
Force Fields for Crystalline BH4− and NH4+ Ions
Authors: Harvey K. B. , McQuaker N. R.
Published: February 2004

Symmetry force constants for crystalline BH4+ and NH4+ ions of Td symmetry have been calculated. These have been used to compute the spectra of crystalline BD4− and NH4+ ions. An attempt to relate the spectra and structure of crystalline BH4− and NH4+ ions of D2d symmetry is made.
Use of Pseudopotential for Crystal-Field Splitting of J Levels of TmCl3· 6H2O
Authors: Olsen David N. , Stöhr Joachim , Gruber John B.
Published: February 2004

A method of calculating the crystalline Stark field splitting of J levels for Tm3+ in TmCl3· 6H2O is described which uses the idea of a higher symmetry crystal field. The higher symmetry crystal-field Hamiltonian is selected by considering the Stark levels within a given J manifold having the same intensity as degenerate. It is found that the calculation is in a coordinate system with the higher symmetry axis perpendicular to the direction of the symmetry axis reported for this crystal. [D. N. Olsen and J. B. Gruber, J. Chem. Phys. 54, 2077 (1971)]. A rotation of the coordinate system is performed to correct for this condition. Finally, the thulium parameters obtained are compared with the parameters for HoCl3· 6H2O where polarization data were available for identifying individual Stark levels necessary in making a definitive crystal-field splitting calculation.
Boundary Effects of Orientation of Nematic Liquid Crystals
Authors: Lee James D. , Eringen A. Cemal
Published: February 2004

The governing partial differential equation for the distribution of orientations is obtained for nematic liquid crystals. The boundary condition for orientation at the surface of the container depends on the type of surface and the type of liquid crystal. Two extreme cases, orientation either parallel or perpendicular to the surface, have been solved and the orientational effects due to magnetic field is determined.
Calculation of Collisional Dissociation of Alkali Halide Molecules by Classical One-Dimensional Models
Authors: Fan Hsiang
Published: February 2004

Classical one-dimensional calculations have been carried out to estimate the rate of dissociation of CsBr by collisions with Xe, as a function of collision energy and vibrational energy. Three models have been used: a truncated harmonic oscillator for CsBr with impulsive collisions with Xe, a Rittner-type ionic potential for CsBr with impulsive collisions with Xe, and a Rittner-type potential for CsBr with a soft (12–4–6) potential representing collisions with Xe. Results are in qualitative agreement with the experimental findings of Tully et al., but poor quantitative agreement implies that the problem requires a three-dimensional treatment for proper description, and, near threshold, probably inclusion of charge exchange in the potential. The calculations show very clearly how kinematic constraints prevent collisions near threshold energy from dissociating molecules which have little or no vibrational energy. The dependence of the dissociation probability on the vibrational temperature is also discussed. It is shown that, in some cases, a negative temperature dependence may arise.
Theoretical Study of Open Chain Dimers and Trimers Containing CH3OH and H2O
Authors: Bene Janet E. Del
Published: February 2004

Ab initio minimal basis LCAOSCF molecular orbital calculations have been performed on open chain dimers and trimers containing methanol and water. The equilibrium structures and energies of the dimers are determined and compared. The rigidity of the hydrogen bond in each dimer is discussed in terms of calculated intermolecular force constants. The energies of seven open chain trimers are presented and analyzed. It is found that the energies of all trimers deviate from additivity, indicating the existence of a cooperative effect in hydrogen bonding in these systems.
Anisotropic proton hyperfine interaction in the electron spin resonance spectrum of the ethyl radical
Authors: McDowell C. A. , Raghunathan P. , Shimokoshi K.
Published: February 2004

The ESR spectrum of the ethyl radical isolated in an argon matrix at 4.2°K has been measured. The radical was produced by the photolysis of a mixture of C2H4 and HI with radiation from a high-pressure mercury lamp. From a line-shape analysis of the observed spectrum, the following anisotropic hyperfine interaction parameters for the α and β protons, and g values have been obtained: A⊥α=19.4± 0.5 G, A∥α=29.6± 0.5 G, A⊥β=25.9± 0.5 G, A∥β=28.7± 0.5 G, gav=2.0023± 0.0003 (Δ g= g∥ − g⊥ ≤ 0.0003). These ESR parameters provide the first experimental assessment of currently available theoretical results on the proton hyperfine anisotropy in C2H5.
Spin-lattice relaxation and molecular reorientations near Tc with application to NH4Cl
Authors: Michel K. H.
Published: February 2004

Reorientations of the ammonium tetrahedra around C2 axes in the neighborhood of the structural phase transition in a NH4Cl crystal are described by a master equation. Within each group of two sterically different tetrahedral configurations characterized by a pseudospin variable σ=± 1 distinction is made of a=12 sterically equivalent positions. Cooperative dynamical behavior is introduced by extending Glauber's theory of the dynamics of an Ising system to the tetrahedral motion in molecular crystals. The (approximate) solution of the master equation yields a critical slowing down behavior for the pseudospin correlation function. This explains quadrupole spin-lattice relaxation T1Q experiments. In addition the decay time τ for the probability P(σa;t) of finding a tetrahedron in a given position σ, α∈ 12 is found to depend on the equilibrium pair correlation function g=<σiσi+1>. By relating this quantity to the thermal expansion, the proton spin-lattice relaxation time T1P can be expressed in terms of the anomalous thermal expansion. The theory explains the anomalous behaviour found experimentally for T1P around the phase transition at low pressure and high pressure.
Magnetic susceptibilities of the compounds AFeCl3(A=Tl,Rb,NH4,Cs) with antiferromagnetic linear chains
Authors: Witteveen H. T. , Veen J. A. R. van
Published: February 2004

The magnetic susceptibilities of the compounds AFeCl3(A=Tl,Rb,NH4,Cs) were investigated in the temperature region 2–250°K. The results are interpreted in terms of an antiferromagnetic linear-chain model. The values of the effective anisotropic intrachain exchange interaction are calculated. The exchange interaction depends on the distance between neighboring Fe2+ ions in the chains.
Second-order effect of spin-orbit coupling on the angular dependence of the zero-field splitting in CH2
Authors: Roy Hall William , Hameka Hendrik F.
Published: February 2004

The theoretical zero-field splitting of a molecule is the sum of two contributions: (1) the first-order perturbation terms of the spin-spin coupling and (2) the second-order perturbation terms of the spin-orbit coupling. We have calculated contribution (2) for the methylene radical as a function of the H–C–H bond angle. We found that the magnitude of contribution (2) is about 0.07 cm−1 at the experimental bond angle 135°. For this bond angle the value of contribution (1), as calculated by Harrison is about 0.71 cm−1 and the sum of the two contributions is 0.78 cm−1 which compares satisfactorily with the experimental value 0.93± 0.30 cm−1. Even though the total value of contribution (2) is only 10% of the total effect it varies much more strongly with the bond angle than contribution (1). For example, the values for contribution (2) vary by as much as 0.14 cm−1 as a function of the bond angle whereas the values of contribution (1), as calculated by Harrison, vary only by an amount of 0.05 cm−1. We conclude therefore that the angular dependence of the zfs is mainly due to the second-order effect of the spin-orbit coupling.
Collinear hydrogen atom transfer probabilities, O+HBr→OH+Br
Authors: Shipsey Edward J.
Published: February 2004

Quantum mechanical calculations of a collinear model of the reaction O+HBr→OH+Br have been performed using a semiempirical potential energy surface. At thermal energies this simple model predicts OH to be produced in its ground state and first excited state with population ratios 0.4 to 0.6, giving a moderate inversion over a considerable range of reactant kinetic energy.
Approximate interaction potentials for benzene-halogen complexes
Authors: Schug John C. , Dyson Maynard C.
Published: February 2004

Approximate interaction potentials have been calculated for the interaction of benzene with Cl2, Br2, and I2 molecules for a variety of geometries. In the majority of configurations, we have found that the interactions are attractive. The depths of the potential minima are somewhat smaller than the experimental energies of complexation, and in most cases only a slight dependence on the relative molecular orientation is observed. Exceptions occur when the halogen approaches either in the plane of the benzene ring with its bond axis directed toward the benzene, or directly over a C–C bond with the halogen axis perpendicular to the bond; the interactions were found to be repulsive at all distances in those configurations.
Application of the equations-of-motion method to the excited states of N2, CO, and C2H4
Authors: Rose John , Shibuya Tai-ichi , McKoy Vincent
Published: February 2004

We have used the equations-of-motion method to study various states of N2, CO, and ethylene. In this approach one attempts to calculate excitation energies directly as opposed to solving Schrödinger's equation separately for the absolute energies and wavefunctions. We have found that by including both single particle-hole and two particle-hole components in the excitation operators we can predict the excitation frequencies of all the low-lying states of these three molecules to within about 10% of the observed values and the typical error is only half this. The calculated oscillator strengths are also in good agreement with experiment. The method is economical, requiring far less computation time than alternative procedures.
Predissociation and its inverse, using resonance absorption NO(C 2Π) ⇄ N+O
Authors: Mandelman Michael , Carrington Tucker , Young Robert A.
Published: February 2004

Radiative recombination of N+O has been investigated by using this radiation as a highly specific light source for dissociation of NO. This direct proof of predissociation of NO(C 2Π) supports previous indirect evidence. Measurements of the rate of N2 production in the photodissociation lead to a rate coefficient for the process N+O ⇄ NO(C 2Π, v′ = 0) → NO(X 2Π, v″ = 0) + hν, kδ0,0 = (2.9 ± 0.5) × 10−18 cm3 sec−1 at 300°K. The total rate coefficient summed over v″ is (1.5 ± 0.4) × 10−17 cm3 sec.−1 When N and O concentrations are measured by titration, the recombination emission can be used as an absolute intensity standard (±20%) at 1900 Å.
Irreversible statistical mechanics of polymer chains. III. A dynamic model of vinyl polymers
Authors: Iwata Kazuyoshi
Published: February 2004

Computational works are made of the dynamic rotational isomer model of vinyl polymers, of which theoretical foundation has been given in the first paper of this series. First, a concept of ``simple elementary mode'' is newly introduced, based on the assumption of short-range order of vinyl polymers, and 2317 simple elementary modes for ν ≤ 14 (where ν is the length of local chains) are found with the aid of a computer. Langevin equations for local chains rotating in a viscous medium are introduced to derive transition probabilities for these modes. They are given as functions of the monomer size (Rm), the free volume (νf) of the system, and a hydrodynamic parameter (ε). Two kinds of systems, dilute polymer solutions and bulk polymers (including concentrated polymer solutions), are discussed separately. Two dimensionless quantities, [Da] (intrinsic diffusion coefficient) and [Ta] (intrinsic relaxation time) for normal coordinates Qa, are newly defined and they are computed numerically for various ε, Rm, νf, and gauche state energy (Eg). The following are found in the present work. (1) [Dα] strongly depends on νf both in the range 0≤α̃≤ 0.3 and 0.8≤α̃≤ 1, but not on 0.4 ≤α̃≤ 0.75, where α̃=α/N. (2) [Dα] is almost independent of Rm and Eg in the whole range 0≤α̃≤ 1. (3) In dilute solutions (i.e., νf=∞) all the (2317) elementary modes participate in the Brownian motion of the polymer chains, but with the increase in polymer concentration (i.e., with the decrease in νf the larger elementary modes begin to freeze up and, near the glass-transition temperature, the only two smallest modes, (4,1)S and (6,2)S remain unfrozen. (4) Even in dilute solutions, the Brownian motion of the normal coordinate with a large α̃(0.3≤α̃≤ 1) is determined by several small elementary modes alone. (5) [τα̃] takes its minima [τα]min in the range 0.4≤ α ≤ 0.75. (6) [τα] strongly depends on both νf and Eg, but [τα]min depends only on Eg. The following subjects are also discussed: The superposition of time and temperature changes, behaviors of long elementary modes, and the glass-transition phenomena.
Excitation of atomic nitrogen by electron impact
Authors: Stone E. J. , Zipf E. C.
Published: February 2004

Absolute cross sections have been measured for the excitation of the NI(λλ1134 1164, 1168, 1200, 1243, and 1743 Å multiplets by electron impact on atomic nitrogen. The λ1134 and λ1200 Å cross sections are large, reaching 2.0× 10−16 and 2.5× 10−16 cm2 at their peaks, respectively. The magnitude of these cross sections is about a factor of two lower than the values reported previously by the authors, due to the discovery of a fault in the apparatus. The presence of vibrationally excited molecular nitrogen in the discharged gas is confirmed and its effect on the measurements is discussed. The ratio of the oscillator strengths of the λ 1200 and λ 1134 Å resonance transitions is measured to be 2.6 ± 0.3. The branching ratio for the NI(λ 1131Å/λ 1164Å multiplets is measured to be 2.8 ± 0.3. Striking differences in the distribution of intensity between the spectra of atomic nitrogen and molecular nitrogen excited by energetic electrons suggest an optical method for measuring the density of atomic nitrogen in the upper atmosphere.
Trajectory studies of atomic recombination reactions. IV. Recombination of iodine atoms
Authors: Wong W. H. , Burns George
Published: February 2004

The recombination of iodine atoms has been studied, between 300 and 1500°K, using classical 3D trajectory calculations and the Monte Carlo method of sampling. This approach, previously applied by A. G. Clarke and G. Burns [J. Chem. Phys. 56, 4636 (1972)] to bromine atom recombination, is presently extended to include the quasibound recombining atom-inert gas complexes. It. was found that such complexes provided an important recombination path. On the other hand, collisional complexes involving energy and angular momenta higher than those of quasidimers, rarely lead to recombination reactions. If the well depths for the IM interaction ε∞IM's are taken to be 0.7, 0.6, and 0.5 kcal/mole for M=Xe, Ar, and He, respectively, the trajectory calculations yield recombination rate constants in fair agreement with experiment. These well depths are 1.6, 1.9, and 5.3 times larger than the well depths between M and Xe, which is iodine's nearest neighbor in the periodic table. In the case of helium, the rate constants obtained by trajectory calculations are relatively insensitive to the variations in ε∞He value, which is thus less reliable than ε∞IXe and ε∞IAr. The redissociation of the nascent products was found to be substantial, ranging from 20% to 60%. The extent of redissociation was greater for lighter third bodies and at higher temperatures. Mass effect and pressure dependence in recombination reactions were also studied.
Infrared spectra of matrix-isolated uranium oxide species. I. The stretching region
Authors: Gabelnick S. D. , Reedy G. T. , Chasanov M. G.
Published: February 2004

Stretching modes of several uranium oxide species vaporized from tungsten and iridium Knudsen cells and deposited in argon and krypton matrices have been observed in the 700–-900 cm−1 region of the infrared. Studies of the spectra as a function of temperature and composition of the condensed urania together with normal coordinate analyses of the spectra of 16O and 18O-containing species have led to the following assignments for the 16O species in argon: U16O(ν) = 820.0 cm−1; U16O2(ν3) = 776.1 cm−1, U16O2(ν1) = 765.4 cm−1 (calc). The spectroscopic evidence strongly indicates a linear geometry for UO2. Tentative assignments for the stretching modes of UO3 are also given.
Far infrared measurements on the 19°C phase transition in polytetrafluoroethylene
Authors: Johnson K. W. , Rabolt J. F.
Published: February 2004

The far infrared spectra of crystalline polytetrafluoroethylene has been obtained at temperatures above and below the 19°C phase transition. These measurements have shown, for the first time, the appearance of four lattice modes below 19°C at 42, 46, 55, and 84 cm−1. Upon cooling the lines become more intense and shift to higher frequencies indicating an increasing intermolecular interaction between the two molecules per unit cell. The appearance of these lattice modes, along with previously reported Raman and x-ray data, supports the idea that the unit cell of polytetrafluoroethylene below 19°C belongs to the monoclinic crystal system with space group P21 containing two molecules per unit cell.
Resonance line shapes for semi-integer spins in liquids
Authors: Baram A. , Luz Z. , Alexander S.
Published: February 2004

An effective Liouville equation which describes resonance line shapes and is accurate to first order in the nonsecular term is derived. The equation has a quasisecular form but includes T1 contributions and the effect of the modulation of second order shifts by the molecular rotation. Simple explicit expressions for the linewidth and shift of the −1/2 → 1/2 transition are derived which are valid when the rotation is fast compared to the static second order shifts. The width has a maximum near the the T1 minimum and a minimum. The line shape for slow motion is also discussed and results of numerical calculation are presented. It is shown that the equations can be interpreted as a modulation of the second order shifts.
Rotating sample technique for measuring electrical resistivity of liquid metals
Authors: Taher S. M. A. , Gruber J. B. , Ayers R. D.
Published: February 2004

A rotating sample technique for measuring the electrical resistivity of liquid metals is developed in some detail and is checked for accuracy and dependability by making measurements of the resistivity of indium-mercury alloys of different compositions. End corrections have been made to the rotating sample method. This method has several advantages over the rotating magnetic field technique that is currently in use. Use of the null technique in this method has allowed us to measure the balancing current in a more precise and reliable way as compared to measuring the deflection of the sample in the rotating magnetic field method.
Low-energy study of O+ + N2 → NO+ + N
Authors: Neynaber R. H. , Magnuson G. D.
Published: February 2004

Laboratory-energy distrubitions of NO+ were measured for the reaction O+ + N2 → NO+ + N with a merging-beams apparatus over a range of interaction energy W from 0.05 to 15 eV. Absolute and relative cross sections as a function of internal energy of the reactants (N2 was always in the ground electronic state), information on the partition of available energy into translational and internal energy of the products, and some information on the angular scattering of NO+ in the center-of-mass system were obtained from the measured distributions. The results verify some previously observed flowing-afterglow data which indicate a large enhancement of the reaction cross section at thermal energies due to vibrational excitation of N2. It is shown that vibrational excitation of N2 is much more effective in accelerating the reaction rate at W = 0.05 eV than at W = 9 eV. It appears that the NO+ at the peaks of the distributions for 4 eV ≤ W ≤ 12 eV are associated with spectator stripping when O+ is in the ground state and N2 is in the ground vibrational state, i.e., v = 0. For W = 0.05 eV and vibrationally excited N2, the cross section for excited O+ is larger than for ground-state O+. For O+ in the 4S state, absolute and relative cross sections for vibrationally unexcited and vibrationally excited N2 are consistent with those obtained by other investigators.
Radial asymptotic expansions for singular potentials
Authors: O'Brien T. J. P.
Published: February 2004

Asymptotic expansions, which are valid for large values of r, are obtained for linearly independent solutions of the radial Schrödinger equation with a spherically symmetric potential of the form Σm=0∞ um+2/rm+2. These solutions are asymptotically equivalent to spherical Bessel functions and, in the case of vanishing potential, the formalism reduces to their representation in terms of elementary functions. It is shown that by comparing the results of numerical integration of the regular solution at large values of r to a linear combination of these asymptotic expansions, rather than the spherical Bessel functions, the phase shift can be determined accurately at much smaller values of r. Thus the range of r for which numerical integration is required can be reduced significantly.
Magnetic properties of the t2g4 configuration in low symmetry crystal fields
Authors: Oosterhuis W. T. , Lang G.
Published: February 2004

The actions of low symmetry crystalline electric fields and the spin-orbit coupling in the t2g4 electronic configuration have been considered for the interpretation of magnetic data (susceptibility. Mössbauer effect, etc.). The results of numerical calculations of the various spin Hamiltonian parameters (D.E.ḡ.Ā.efg) are tabulated and the applicable perturbation theory expressions have been worked out.
Three-step photoselection and the assignment of the electronic states of the benzyl radical
Authors: Friedrich Donald M. , Albrecht A. C.
Published: February 2004

A contradiction between assignments of the first excited state of the benzyl radical, originally made by three-step photoselection in rigid solutions, and later by gas-phase rotational analysis, has been resolved. The photochemically produced radical in a rigid solution can be subject to rotational diffusion over the time duration of the experiment. This weakens (or even confuses) the crucial correlation with reference axes of the parent molecule necessary for achieving absolute assignments. By increasing the viscosity of the medium, such randomization can be considerably reduced and photoselection polarization ratios greater than 3/1 (a characteristic of three-step photoselection) are obtained for p-methylbenzyl. The photoselection data for the p-methylbenzyl radical indicate that the first three observed excited states can be assigned 2A2, 2A2, 2B2, respectively. If the 2A2 assignment for the lowest excited state of p-methylbenzyl in a rigid solvent can be applied to the benzyl radical itself, then agreement is achieved with the gas-phase rotational analysis.
Infrared circular dichroism and linear dichroism of liquid crystals
Authors: Holzwarth G. , Chabay Ilan , Holzwarth N. A. W.
Published: February 2004

In order to test the validity of our recently reported theory of optical activity near absorption bands of cholesteric liquid crystals, we have measured the infrared linear dichroism (LD) of the nematic liquid crystal p-methoxy-p′-n-butylaxoxybenzene and the circular dichroism (CD) of the cholesteric phase of this liquid crystal obtained by doping with 0.75 mole-% cholesteryl-2-(2-ethoxyethoxy)ethyl carbonate. Using the measured LD data to define the optical properties of the individual layers of the cholesteric structure, we calculate the CD of the cholesteric material by means of our macroscopic theory. The calculated signs, amplitudes, positions, and shapes of the CD bands agree closely with observation. This demonstrates that the CD is quantitatively explained by introducing frequency-dependent complex terms into the spiralling dielectric tensor of the Oseen-de Vries model for cholesteric liquid crystals.
ENDOR and ESR studies of irradiated cytidine
Authors: Hampton Don A. , Alexander Chester
Published: February 2004

Single crystals of cytidine have been x-irradiated and studied by electron nuclear double double resonance, electron nuclear double resonance, and electron spin resonance techniques at room temperature. One radical species was studied in detail and the results show anisotropic couplings of an unpaired electron to three hydrogen nuclei with the following principal values: a1, −66.3 MHz, −43.5 MHz, −20.7 MHz; a2, −68.9 MHz, −43.4 MHz, −20.3 MHz; a3, −37.2 MHz, −27.0 MHz, −13.6 MHz. The g tensor principal values are 2.0031, 2.0025, and 2,0021. These data indicate a radical formed in the ribose sugar group with spin density localized on C4′ and C5′.
Order-disorder phenomena. VIII. Acoustic attenuation and dispersion in NH4 Cl
Authors: Garland Carl W. , Pollina Richard J.
Published: February 2004

The attenuation coefficient a of longitudinal ultrasonic waves propagating in the [100] direction of an NH4 Cl single crystal has been measured as a function of temperature at 1270, 1500, 1700, and 2656 bar. The elastic constant c11 corresponding to this wave has also been measured at 1500 and 2656 bar. By combining these data with a variety to other ultrasonic and hypersonic information about NH4 Cl it is possible to determine the dynamical behavior near the order-disorder transition in terms of the variation in the relaxation strength and the relaxation time. At a given value of the reduced temperature t=|T−Tλ|/Tλ the acoustic velocity and attenuation in the ordered phase are almost identical at all pressures, but the behavior is different in the disordered phase at 1 atm and at the ``tricritical'' pressure of ∼ 1500 bar. In particular, the relaxation rate 1/τ (in units of 108 sec−1) is 414 t0.60 at 1 atm and is 123 t0.88 at 1500 bar.
Temperature dependence of a vibrational exciton: Some methyl motions of durene
Authors: Prasad Paras N. , Kopelman Raoul
Published: February 2004

An intramolecular vibrational frequency in a molecular crystal has been found to show a significant temperature dependence. The low lying intramolecular mode at 272 cm−1 (room temperature value) of durene-h14 (and the corresponding durene-d14 frequency at 242 cm−1) shows an increase of [inverted lazy s] 9 cm−1 in frequency in going from room temperature to 100°K. A theoretical discussion is presented on the role of various interactions that may give rise to the temperature dependence of the intramolecular mode. The theoretical considerations suggest that, although the thermal expansion of the lattice may be contributing to this temperature dependence, the dominant contribution is due to the cubic anharmonic interactions involving methyl torsions. It is suggested that in complex molecular crystals, where there are some very low lying intramolecular modes, the distinction between lattice modes and intramolecular modes based on the criterion that only lattice modes show frequency shift with temperature may be misleading. In such cases our mixed crystal criterion that the phonons obey the amalgamation limit in isotopic (H–D) mixed crystals, whereas intramolecular vibrations are in the separated band limit, should be more reliable. This is clearly demonstrated in the case of durene.
Gaseous phosphorus compounds. IX. Mass spectrometric studies of equilibria in the carbon-phosphorus system
Authors: Kordis J. , Gingerich K. A.
Published: February 2004

Mass spectroscopic studies of the effusate from a graphite-lined tantalum Knudsen cell containing a mixture of graphite (excess) -TiP–Rh yielded the enthalpy changes (Δ H°298, kilocalories) for the following reactions: 2C(s) = C2 (199.0 ± 1.6), 3C(s) = C3 (188.2 ± 2.1), P + C(s) = CP (48.6 ± 1.6), P + 2C(s) = C2P(71.9 ± 2.0), P2 +C(s) = CP2 (54.0 ± 2.0), and P2 + 2C(s) = C2P2 (55.8 ± 4.0). From considerations of bond strengths and comparison of Second- and Third-Law thermodynamic data it appeared that the dominant isomeric species of the gaseous carbon-phosphorus compounds were CCP, CPP, and PCCP. With respect to CCP and PCCP, this observation agrees with previous suggestions, but CP2 had been considered to be a mixture of PCP and CPP [Smoes et al., Chem, Phys. Lett. 8, 10 (1971)]. The enthalpy changes of the above reactions and of other complementary homogeneous gas phase reactions permitted the calculation of atomization energies (D°o, kilocalories per mole) and standard heats of formation (Δ H°f,298,kilocalories per mole) respectively, of C2 (142.2 ± 2.5, 199.0 ± 1.6), C3 (322.0 ± 3.5, 188.2 ± 2.1), CP (121.3 ± 2.1, 128.4 ± 1.7), CCP (267.6 ± 4.9, 151.7 ± 4.1), CPP (231.9 ± 4.6, 96.7 ± 2.5), and PCCP (400.0 ± 5.0, 98.5 ± 4.5). Agreement with literature values [Smoes et al., Chem. Phys. Lett. 8,10,1971; Gingerich, Thermochim. Acta 2, 233 (1971); JANAF Thermochemical Tables, Dow Chemical Co., Midland, Mich. (1965)] is generally good.
Molecular dipole model of internal optic modes
Authors: Frech Roger
Published: February 2004

The internal optic modes of a crystal are discussed, first using the diagonalized dynamical interaction matrix formalism and then in terms of a molecular dipole model which is more familiar to molecular spectroscopists. The relationship between the two methods is discussed. The approximations that are made to derive the molecular dipole model result in a description that includes interactions between the optic branches. As a further result of these approximations the internal optic mode dispersion curves are wave vector dependent only through the dipolar coupling terms.
Multiconfiguration self-consistent field (MCSCF) theory for excited states
Authors: Das G.
Published: February 2004

An exact MCSCF theory is proposed for excited states belonging to the same symmetry species and is illustrated by actual application to some of the excited states of the CN radical.
Mass spectroscopic investigation of the equilibrium dissociation of gaseous Sb2, Sb3, Sb4, SbP, SbP3, and P2
Authors: Kordis J. , Gingerich K. A.
Published: February 2004

Knudsen effusion mass spectroscopic studies of the vapor above a gadolinium antimonide-molybdenum phosphide-graphite system led to the determination of the standard enthalpy changes, Δ H298°(kcal), for the following reactions: Sb4(g)=2Sb2(g) (71.3 ± 1.8), Sb4(g) =4Sb(g) (215.0 ± 3.0), Sb2(g) = 2Sb(g) (71.8 ± 1.0), Sb3(g)=3Sb(g)(129 ± 4), P2(g) =2P(g) (117.1 ± 1.1), Sb2(g) + P2(g) = 2SbP(g) (17.6 ± 0.4), SbP(g) = Sb(g)+P(g) (86.0 ± 1.2), and 2SbP3(g)=Sb2(g)+3P2(g) (149 ± 8). These reaction enthalpies have been combined with appropriate literature data to obtain the dissociation or atomization energies D°298 and standard heats of formation Δ H°f,298 of the molecules given. D298°kcal mol−1or kJ mol−1Δ Hf,298°kcal mol−1or kJ mol−1Sb271.8 ± 1.0300.4 ± 4.854.7 ± 2.2288.7 ± 9.2Sb3129.0 ± 4.0539.7 ± 16.760.7 ± 5.8254.0 ± 24.3Sb4215.0 ± 3.0899.6 ± 12.637.9 ± 5.4158.6 ± 22.6SbP85.7 ± 0.9358.6 ± 3.857.3 ± 1.6239.8 ± 6.7SbP3285.8 ± 5.21195.8 ± 21.816.8 ± 6.070.3 ± 25.1
Vibrational relaxation of CO by O atoms
Authors: Center R. E.
Published: February 2004

The vibrational relaxation of carbon monoxide by atomic oxygen has been measured behind incident shock waves in the temperature range of 1800–4000 °K. The atomic oxygen was produced by the rapid thermal decomposition of ozone. The experimentally derived relaxation times can be expressed in the form p τCO–O=exp(54 T−1/3−7.3) μsec·atm, which corresponds to a collisional probability of the order of 10−2 for the vibration-translation energy transfer. This is from two to three orders of magnitude larger than the probability for self-relaxation by CO over the same temperature range.
ESR spectrum and structure of the MgOH radical
Authors: Brom J. M. , Weltner W.
Published: February 2004

The MgOH molecule has been trapped in solid argon and neon at 4 °K, and its ESR spectrum measured. It has been isolated in these matrices by (1) code positing Mg atoms with a matrix gas containing 1% H2O2, (2) codepositing Mg atoms with a matrix gas and the products of a microwave discharge of H2O vapor, or (3) passing the metal atoms, along with H2O vapor, through a tantalum cell at 1700 °C before quenching in the matrix gas. g tensors and nuclear hyperfine components for interaction with 25Mg and H were determined. In both argon and neon these magnetic tensors appear to have axial symmetry, indicating that the molecule is probably linear (2Σ ground state). In neon g‖= 2.0017(1), g⊥= 2.0007(1), |A‖ (25Mg)|= 312.7(3) MHz, |A⊥ (25Mg)|= 300.7(3) MHz, |A‖(H)|= 14.8(3) MHz, |A⊥(H)|= 10.1(3) MHz. Spin density distribution derived from hyperfine splittings places a majority of the spin on Mg and implies that the molecule is highly ionic. A CNDO calculation slightly favors a linear structure and supports the observed spin distribution. Trapped Mg+ and probably MgO2H were also observed in the ESR spectra. The large excess of Mg atoms in the matrices dominated the optical absorption spectrum in the ultraviolet.
Adiabatic reference functions and chemical reactions
Authors: Shipsey Edward J.
Published: February 2004

An adiabatic differential equation describing chemical reaction is developed which allows convenient application of the reference function technique. The resulting equations are a generalization of Eu's equations for inelastic scattering. Some properties of these equations are discussed and a stationary principle derived for them.
Quantum scattering theory of rotational relaxation and spectral line shapes in H2–He gas mixtures
Authors: Shafer Richard , Gordon Roy G.
Published: February 2004

A systematic study is presented of the rotational relaxation and spectral line shape properties of dilute gas mixtures of H2 in He, in an effort to determine the radial and angular dependence of the H2–He intermolecular potential. The quantum mechanical theory of relaxation in gases is reviewed, and we express the results in terms of a matrix of cross sections that determines each correlation function, and thus the relaxation properties of the system. The cross section is calculated from binary collision transition amplitudes, or S matrix elements, for H2–He scattering. A Morse-spline-fitted-van der Waals potential of the form V0(R)+V2(R)P2(cosθ) is assumed and we treat the hydrogen molecule as a rigid rotor. The Schrödinger equation is solved in the close-coupling approximation using the method of Gordon. We apply the theory to sound absorption measurements of rotational relaxation, NMR spin-lattice relaxation times, and the S and Q branches of the pure rotational Raman spectrum of H2–He mixtures. Elastic phase shift and reorientation effects are examined, along with the effects of energetically inelastic collisions. We find asymptotically closed channels to be unimportant, but the rigid rotor approximation breaks down in calculating the Raman line shifts due to the vibration-rotation interaction. This effect is accounted for by assuming different radial potentials V o(R) for scattering in two spectroscopic (vibrational) states. We find a potential that can accurately fit all the experimental data as well as the theoretical calculations of the long- and short-range behavior of the intermolecular potential.
Pulsed NMR studies of fluorine relaxation in MPF6
Authors: Gutowsky H. S. , Albert Shmuel
Published: February 2004

The fluorine spin-lattice relaxation time T1 was studied in polycrystalline Na, K, Rb, and Cs hexafluorophosphates at temperatures between 77 and 476 °K by pulsed NMR methods. In each case a discontinuity occurs in the temperature dependence, at 276, 275, 210, and 82 °K, respectively; those for the K and Rb salts corresponding to previously reported phase transitions. The temperature dependence of T1 observed in the low-temperature phase of the Na, K, and Rb salts has a minimum and an asymmetric shape which are attributed to dipolar relaxation by thermally activated, random reorientations of the PF6− ions, the activation energy Ea being smaller on the low-temperature side of the T1 minimum than on the high side. The temperature of the T1 minimum and Ea decrease with increasing size of the cation, as does the transition temperature. The T1 's are very sensitive to the physical state of KPF6 and RbPF6 and to thermal cycling in RbPF6. Powering the sample broadens the T1 minimum, shifts it to lower temperatures by 2 to 8 °K, reduces its value by about 25%, and decreases the low-temperature Ea. These effects are analogous to those reported in earlier, line shape studies of MPF6 and are consistent with the two-phase model proposed in that connection. One phase consists of relatively perfect crystalline domains, with a high Ea, and the other of domains with defects which lower Ea. Spin exchange between the two types of domains is fast enough to average out the different T1 's in the two phases and produce exponential relaxation. However, in finely powdered RbPF6 at 77 °K, nonexponential relaxation was observed and is attributed to paramagnetic impurities. In the high-temperature phase of all four salts, T1 exhibits a broad frequency-independent maximum of 4–7 sec which is attributed to relaxation by spin-rotation interactions within the rapidly reorienting PF6− ions.
Monte Carlo estimation of communal entropy
Authors: Torrie G. , Valleau J. P. , Bain A.
Published: February 2004

The Helmholtz free energy difference between unconstrained and ``single occupancy'' classical fluids may be calculated in a Monte Carlo experiment at a single density by using special sampling techniques to measure the fraction of unconstrained configurations that show single occupancy. Results obtained by this method for the hard sphere fluid are in agreement with values obtained by Hoover and Ree by the numerical integration of machine generated pressure-density data. The relative merits of these two methods for locating the melting transitions of model systems are discussed.