bluuebunny/crossref_metadata_2025 huggingface dataset:
Environmental Effects on f–f Transitions. II. “Hypersensitivity” in Some Complexes of Trivalent Neodymium
Authors: Henrie D. E. , Choppin G. R.
Published: February 2004

The variations in the oscillator strengths of the “hypersensitive” I9/24 → 2G7/2, 4G5/2 transition of some complexes of Nd3+ are discussed. It is concluded that the order of magnitude of these oscillator strengths can be accounted for by means of vibronic mechanism. The variation in the oscillator strengths as a function of increasing “covalency,” as measured by the nephelauxetic effect, is also considered, and it is concluded that the phenomenon of “hypersensitivity” may be explained by the inclusion of covalency within the framework of a vibronic mechanism.
Mass-Spectrometric Study of Photoionization. XI. Hydrogen Sulfide and Sulfur Dioxide
Authors: Dibeler Vernon H. , Liston Susan K.
Published: February 2004

Photoionization-yield curves are obtained for the molecule and fragment ions of H2S and SO2 from onsets of ionization to 600 Å. Structural features of the curves are discussed. Ionization thresholds are tabulated and heats of formation of ions and radicals are calculated. Derived quantities (in kilocalories per mole) include D0(HS–H) = 89.3, D0 (SH) = 83.2, ΔHf0°(S) = 65.0, D0(S2) = 99.4, and D0(OS–O) = 129.1. The ionization energy, I(SO) = 10.21 eV, is obtained by indirect means.
Approximate LCAO–MO Treatment of Conjugated Polymers
Authors: Dewar Michael J. S. , Hashmall Joseph A.
Published: February 2004

An approximate LCAO–MO treatment is described of the π electrons in conjugated polymers. The method can be applied to any system which can be treated as an alternating sequence ··· ABAB ··· of two units A and B, the orbitals of the polymer being expressed in terms of the MO's of A and B. While the HMO method is used for interactions between units, basic set MO's of any degree of sophistication can be used. Examples of the application of this technique are given.
Shock-Tube Study of Vibrational Relaxation in Nitrous Oxide
Authors: Simpson C. J. S. M. , Bridgman K. B. , Chandler T. R. D.
Published: February 2004

The vibrational relaxation of nitrous oxide has been investigated by following the density changes occurring behind incident shock waves using a Mach–Zehnder interferometer. Measurements made up to an equilibrium temperature of 1700°K (Mach 7) show that neither the symmetric stretching mode nor the asymmetric stretching mode has a significantly longer relaxation time than the bending mode. The rate of relaxation has been measured as the gas relaxes towards equilibrium. During this process, the translational temperature of the gas falls, and it has been found that this fall in temperature will account for the observed increase of the relaxation time. Relaxation times have been measured from 320° to 850°K.
Theory of the Proton Hyperfine Splittings of Pi-Electron Free Radicals. I. The CH Fragment
Authors: Chang S. Y. , Davidson E. R. , Vincow G.
Published: February 2004

Using first-order perturbation theory, a general equation is derived for the Fermi contact hyperfine splittings of any free radical. This equation is essentially the same as that obtained previously by other workers, but restrictions concerning (1) planarity of the radical, (2) sigma–pi separability, (3) choice of spin eigenfunctions, and (4) choice of orbitals used to form excited configurations have been removed. The results of calculations of the proton hyperfine splitting (aH) of the prototype pi-electron hydrocarbon radical, the ·CH fragment, are presented. A seven-electron wavefunction with a minimum basis set of Slater atomic orbitals is employed. The CH bonding orbital incorporates sp2 hybridization and an electronegativity parameter which is optimized by variational minimization of the ground configuration energy. Admixture of three configurations is considered; two represent the single excitation from CH bonding to antibonding orbital and the third the corresponding double excitation. The splitting is computed using first- and second-order perturbation theories, invariant-perturbation theory, and the variation method. Sensitivity of aH to variations in (1) values of the orbital exponents (2) choice of spin eigenfunctions, (3) value of the electronegativity parameters, and (4) CH bond angle is investigated. The calculated splitting varies from about −25 to −50 G and depends most critically on the hydrogen 1s orbital exponent. A 10% maximum variation in aH is found in first order depending on the choice of spin eigenfunctions. Admixture of the doubly excited configuration increases | aH | by ≈ 10%. The optimum value of the CH bond-electronegativity parameter is quite close to that previously chosen on intuitive grounds; aH is not very sensitive to this parameter. The above calculations constitute an improvement in rigor over previous work on the CH fragment in that atomic orbital overlap is not neglected, the hydrogen 1s and carbon 2s orbitals are orthogonalized to the carbon 1s orbital, and arbitrary assumptions regarding CH bond polarity have been eliminated.
Magnetic Dipole Optical Absorption Spectrum of MnF2
Authors: Meltzer R. S. , Lohr L. L.
Published: February 2004

We present quantitative intensity measurements on six magnetic dipole absorption transitions in MnF2 as well as some high-magnetic-field experiments with components of the field along both the c axis and in the basal plane. We interpret these results and those of previous investigators on the T14(I), A14E(I)4, T24(II), and T14(II) states, at first using a crystal-field calculation which ignores the vibronic aspects of the problem. With this calculation we can phenomenologically obtain the low-symmetry-field energy-level scheme and explain the effects of high magnetic fields on the spectrum, but we cannot obtain a consistent set of one-electron low-symmetry-field parameters for all the excited states. By considering the vibronic problem, we find that we can understand all the excited states in a consistent manner, and explain the small spin–orbit coupling in the T14(I) state. In addition, we show that for the nearly degenerate A14E(I)4 states, the A14 state does not lie low.
11B NMR in Polycrystalline Calcium Metaborate
Authors: Kriz H. M. , Bishop S. G. , Bray P. J.
Published: February 2004

Boron-11 nuclear magnetic resonance has been observed in polycrystalline calcium metaborate doped with paramagnetic Mn2+. The spectrum arises from the m = 12↔m= − 12 transition broadened in second order by a quadrupole interaction having a nonzero asymmetry parameter. The quadrupole coupling constant of 2.58 ± 0.02 MHz is consistent with previously published values for three-coordinated boron. However, the electric field gradient asymmetry parameter of 0.55 ± 0.01 is several times greater than expected, and this large value is attributed to the presence of a nonbridging oxygen in the boron coordination shell. The possibility of using this result for the study of noncrystalline solids is discussed.
Theory of Radiationless Transitions in an Isolated Molecule
Authors: Chock David P. , Jortner Joshua , Rice Stuart A.
Published: February 2004

A treatment of radiationless transitions in an isolated molecule is considered using a Green's-function formalism. Molecular eigenstates are constructed by superposition of quasidegenerate Born–Oppenheimer states, considering the mixing of one and two discrete zero-order BO states(s) with a manifold of quasicontinuum BO states, which correspond to another electronic configuration. The nature of inhomogeneous line broadening and the time evolution of the excited states are considered. The radiative decay following a coherent excitation process is treated in detail. The various features and anomalies observed in the decay times of medium-size and large molecules can be properly accounted for.
Phosphorescence Spectrum of Pure Crystalline Naphthalene
Authors: Priestley E. B. , Haug A.
Published: February 2004

Phosphorescence spectra of pure crystalline naphthalene at room temperature and at 77°K are presented. In addition to the 0, 0 transition at 4715 Å, the spectra show several intense vibronic bands that correlate well with the most intense vibronic transitions observed in isotopic mixed-crystal phosphorescence spectra at 4.2°K. The width of the 0, 0 band at half-intensity decreases from 270 cm−1 at room temperature to about 75 cm−1 at 77°K, indicating the presence of exciton–phonon interactions in the crystal. Similar narrowing of the vibronic bands is also observed. Using incident light of sufficient intensity to permit the weak phosphorescence to be observed, the intensity ratio of delayed fluorescence to phosphorescence is found to be approximately 100:1 both at room temperature and at 77°K. The triplet state lifetime defined by the first-order component of the phosphorescence and delayed fluorescence decays is 130 msec and shows almost no temperature dependence. The lifetime is about six times longer than previously reported values. Impurities appear to have a significant influence on the exciton behavior even at the relatively high temperatures at which the experiments were carried out.
Far-Infrared Spectra and Structure of Small Ring Compounds. VIII. Pseudorotation in 1,3-Dioxolane
Authors: Durig J. R. , Wertz D. W.
Published: February 2004

The infrared and Raman spectra of 1,3-dioxolane have been examined from 800–33 cm−1. A series of Q branches in the spectral region of 33–100 cm−1 has been treated on the basis of pseudorotation. A series of Q branches was also observed in the 200–400 cm−1 region and has been assigned to hot band transitions arising from excited states of the pseudorotational levels. The mid-infrared also shows considerable structure, but the complexity due to overlapping fundamentals precludes their unique assignment. The spectroscopically derived parameter mq02 was found to be 7.4 × 10−40g·cm2 which agrees well with the value found for the corresponding quantity for the tetrahydrofuran molecule.
Pseudopotential Theory of Atoms and Molecules. I. A New Method for the Calculation of Correlated Pair Functions
Authors: Szasz Levente
Published: February 2004

It is shown that in all previously developed methods for the calculation of correlated pair functions the mathematical difficulties arise from the fact that the pair functions must be orthogonalized to the one-electron spin orbitals by the orthogonality projection operator (1–Ω12). Since this is an integral operator, if the pair functions are of Hylleraas type, containing r12, the orthogonalization will introduce integrals which are very time consuming to calculate. It is shown that the orthogonality projection operator can be transformed in good approximation into pseudopotentials, for which explicit expressions are derived. On the basis of this transformation a new method is developed for the calculation of correlated pair functions.
Properties of the Specular Low-Energy Electron Beam Scattered by Face-Centered Cubic Metal Single-Crystal Surfaces
Authors: Goodman R. M. , Farrell H. H. , Somorjai G. A.
Published: February 2004

The intensity, I00l, of the specularly reflected low-energy (10–500 eV) electron beam was monitored for the different low-index crystal faces of palladium, platinum, and lead as a function of electron energy (eV). The properties of the I00l (eV) curves were investigated as a function of temperature, crystal orientation, scattering angle, and the appearance of different ordered surface structures. For clean surfaces, the positions of the intensity peaks were insensitive to variation of temperature. They were found to change markedly, however, with variation of the scattering angle and the formation of new surface structures on the Pd (100) face. A “reduced” electron-energy scale was used to compare the intensity curves obtained for the different face-centered cubic metal surfaces. The same crystal face of the different metals yields similar intensity curves. Correlations between intensity curves from different surfaces of the same metal have also been pointed out. The occurrences of maxima and minima in the intensity of the specularly scattered low-energy electron beam are primarily determined by the periodicity of the crystal lattice rather than the nature of the crystal potential. However, the relative intensities of the diffraction features are sensitive to variations in the crystal potential.
Elementary Excitations in a Benzene Model
Authors: Linderberg Jan , Thulstrup Erik W.
Published: February 2004

A two-parameter model for the π electrons in the benzene molecule is examined in detail by the method of complete configuration interaction and the results are used for a comparison with an approximate propagator or Green's function technique applied to the model. It is demonstrated that the approximate method gives a qualitatively nearly correct description of the electronic structure for a range of parameter values which simulate the limits of separated atoms and of small electronic interaction. The quantitative comparison is generally better than what is possible with the molecular orbital method.
Derivation and Analysis of the Pariser–Parr–Pople Model
Authors: Linderberg Jan , öhrn Yngve
Published: February 2004

Second quantization formalism is used to derive the Pariser–Parr–Pople model for planar unsaturated molecules. The model is analyzed in detail in the atomic limit and the equivalence with an Heisenberg antiferromagnet is demonstrated by perturbation theory. An approximate Green's-function method is developed and its properties in the same limit are derived and shown to be correct. The case of a linear chain is calculated explicitly and compared with other results. Limitations and difficulties in the establishment of the results are discussed.
Theoretical Study of the BeH Molecule
Authors: Chan Arthur C. H. , Davidson Ernest R.
Published: February 2004

An iterative procedure, involving the use of an elliptical basis set, natural orbitals, “pair equations,” and consecutive use of the perturbation and variation methods, was used to calculate the ground-state energy of the BeH molecule—the Σ2 state. The energy with the best 50-configuration wavefunction, at the experimental internuclear separation of 2.5380 a.u., was found to be −15.221 a.u. (experimental = −15.254 a.u.), corresponding to 65% of the experimental dissociation energy. A dipole moment of −0.07 a.u. (−0.18 D) was obtained. Population analysis of the molecular wavefunctions was done to investigate the nature of the chemical bond involved. Less complex calculations involving configuration interaction were also done for the low-lying excited states at nine different internuclear separations. Potential energy curves were plotted. The transition energy obtained (2Π→2Σ) was found to be within 1.3% of the experimental value. Transition probabilities were also considered.
Mobility of CO+, CO2+ and C2O2+ Ions in Carbon Monoxide Gas
Authors: Saporoschenko M.
Published: February 2004

The mobilities of CO+, CO2+, and C2O2+ ions in carbon monoxide gas have been measured as a function of E / p0 (E is the electric field intensity, p0 is the normalized pressure) using the “four-gauze” electrical shutter device with a glow discharge ion source. Measurements were made at 29°–32°C gas temperature over a range of E / p0 from 15 to 180 V/cm·torr and in a pressure range of from 0.5 to 1.4 torr. The ions have been identified mass spectrometrically simultaneously with the measurement of their mobility. At p0 = 0.8 torr and E / p0 below 85 the predominant ion was found to be C2O2+ and at E / p0 above 85 the predominant ions were CO2+ and CO+. In addition, O2+, C3O2+, C4O2+ and C5O2+ ions in considerably smaller abundance have been observed. The mobility, extrapolated to zero field and atmospheric pressure, is 1.93 cm2/V·sec for both CO+ and CO2+ ions and 1.61 cm2/V·sec for C2O2+ ions. The binding energy of the C2O2+ ion against dissociation into CO+ and CO was found to be about 0.8 eV. Formation of the ions is discussed.
Far-Infrared Spectra of Ice II, V, and IX
Authors: Bertie J. E. , Labbé H. J. , Whalley E.
Published: February 2004

The far-infrared spectra of ice II, V, and IX as recovered at liquid-nitrogen temperature and atmospheric pressure have been obtained in the range 360–20 cm−1 to complement an earlier investigation in the range 4000–350 cm−1. The bands of ice II and IX are sharp, as is characteristic of ordered crystals, whereas those of ice V are broad, as is characteristic of orientationally disordered crystals. These observations agree with previous conclusions about these phases.
Ionic Processes in High-Energy-Induced Luminescence of Liquid Organic Systems
Authors: Ludwig P. K. , Huque M. M.
Published: February 2004

Luminescence of dilute organic scintillators in liquid aliphatic hydrocarbon solution excited by very short pulses of x rays decays nonexponentially. In contrast, excitation by pulsed uv gives luminescence the decay law of which can be described by an exponential function. A satisfactory explanation is that under high-energy radiation, the mechanism of excitation of the scintillator involves ion recombination, while direct excitation and energy transfer play a minor role. Additives (nitrous oxide, carbon tetrachloride, piperidine) which do not affect the excited scintillator but react with charged species such as ions and electrons show a marked effect on the luminescence of the high-energy-excited systems only, a result which supports the ionic mechanism. An attempt is made, on the basis of simplified assumptions, to derive from the luminescence intensity–time curves a distribution function of the secondary electrons released by the interaction of the x rays with the system.
Vibrational Spectra and Conformational Behavior of Cyclopropylcarbonyl Chloride
Authors: Katon J. E. , Feairheller W. R. , Miller J. T.
Published: February 2004

The vibrational spectra of cyclopropylcarbonyl chloride have been recorded and analyzed with the aid of the infrared spectrum of the corresponding bromide. The compound exists as a mixture of two discrete rotational isomers in the vapor and liquid states, but not in the solid state. The relative stabilities of the two isomers were determined by measuring the intensity–temperature dependence of their carbon–chlorine stretching vibrations in both the vapor and liquid states. The S-cis form is shown to be the more stable conformer in both the vapor and liquid states, but the S-trans form is the more stable in the solid state. Highly oriented polycrystalline masses are obtained on freezing, further indicating that intermolecular forces are unusual in this compound.
Charge-Transfer Spectra of Iodine with Hydrogen Sulfide and Benzene in Low-Temperature Matrices
Authors: Voigt E. M. , Meyer Beat
Published: February 2004

The absorption spectrum of benzene–iodine and hydrogen sulfide–iodine was studied at 20°K in argon, krypton, xenon, methane, nitrogen, and sulfur hexafluoride in the spectral range of 2300 to 4500 Å. In addition to the component spectra, strong bands were observed around 2800 Å in all solvents. They are assigned to the electronic charge-transfer transitions of the electron donor-acceptor systems benzene–iodine and hydrogen sulfide–iodine, respectively. The bands are broad and exhibit coarse as well as fine structure. The latter corresponds closely to the ground-state vibrational frequency of iodine. While the solvent shifts in the benzene–iodine system follow the predictions for the heavy-atom effect, the shifts of the hydrogen sulfide–iodine bands appear reversed. The half-width of the bands increases, for benzene–iodine, with increasing polarizability, but decreases for the hydrogen sulfide–iodine system. The absorbance of the charge-transfer bands of the hydrogen sulfide system are stronger than those of the benzene system, and both are greater than that of the a–X system of benzene. The electron-donor spectra seem little perturbed by the presence of the charge-transfer complex. However, a great increase of transition energy and absorbance is noted for the visible transition of iodine. The fine structure of the 3600-Å band of iodine in matrices containing hydrogen sulfide–iodine is 130 cm−1, corresponding well to the vibrational frequency of the B state of iodine.
Absorption Coefficients of Ozone in the Ultraviolet and Visible Regions
Authors: Griggs M.
Published: February 2004

The absorption coefficients of ozone have been measured in the ultraviolet and visible regions using essentially 100% pure ozone. The results confirm those of Inn and Tanaka in the ultraviolet, and those of Vigroux in the visible region.
Dissociation Behavior of Uranium Monocarbide
Authors: Vozzella P. A. , Miller A. D. , DeCrescente M. A.
Published: February 2004

Uranium monocarbide was found to lose uranium preferentially in the temperature range 2250°–2525°K until a constant vaporizing, nonstoichiometric composition approaching UC1.1 was formed. Rates of evaporation of this composition, measured from 2190° to 2525° by the Langmuir method, are described by log10m(UC1.1(s)) = 9.148 − 3.574 × 104T−1(g cm−2·sec−1). Based on the decomposition process, UC1.1(s) → U(g) + n1C1(g) + n2C2(g) + n3C3(g), the partial pressure of uranium is given by log10PU(g)(atm) = 8.191 − 3.624 × 104T−1. The partial pressure of carbon, expressed as effective monatomic pressure, is log10Pc = 8.341 − 3.608 × 104T−1. These data yield a heat of vaporization at 298°K of 235.1 ± 2.7 kcal/mole for the above vaporization process. The resultant standard heat of formation of UC1.1(s) at 298°K is −21.2 ± 4.1 kcal/mole if 125 ± 3 kcal/mole is taken as the heat of sublimation of uranium.
Inelastic Neutron Scattering by Gaseous O2
Authors: Summerfield G. C. , Lurie N. A.
Published: February 2004

The magnetic contributions to neutron diffraction by molecular oxygen have been studied both experimentally and theoretically. We extend the theoretical studies by performing a calculation of the magnetic interaction in the inelastic neutron cross section for gaseous O2. The magnetic inelastic cross section for O2 is significantly different from the nuclear inelastic cross section due to the lack of spherical symmetry in the magnetic-moment distribution about each oxygen nucleus. It is expected from these results that the inelastic cross section for O2 will provide more information about the magnetic-moment distribution than the angular distribution. We present representative calculations to indicate the size and nature of the magnetic contribution to the inelastic cross section.
Mechanism of the Acceleration of the Electrodic Dissolution of Metals during Yielding under Stress
Authors: Despic A. R. , Raicheff R. G. , Bockris J. O'M.
Published: February 2004

The effect of strain, in particular the effect of varying the rate of its application, both in the elastic and the plastic region, on the anodic dissolution kinetics of several metals has been studied. It has been found in the case of iron in acid solution that the over-all effect of elastic strain is small (i.e., the shift of corrosion potential is about +0.1 mV/1000 kg cm−2) and is mostly due to the change in the rate of hydrogen evolution. The effect of yielding on the rate of the electrodic dissolution of metals (Fe, Mo, Ni, and Cu) has been found to be much more pronounced than that of the elastic deformation. The dissolution rate undergoes a marked rise at the beginning of the plastic region, when straining is carried out at a constant rate, and then shows an asymptotic tendency with increasing strain. The asymptotic values of the anodic current dissolution (c.d.) reached at high values of strain increase linearly with the rate of straining. The slope of this linear relationship increases with increasing anodic potential. Metals which develop high-index planes (h.i.p.) as surface steps upon slipping [e.g., Fe—(112) and (123), Mo—(112)], should give much larger mechano-electrochemical effect in comparison with those which develop only low-index planes (l.i.p.) [e.g., Ni and Cu—(111)],and this should depend on the rate of straining. Some effect found in the l.i.p.-developing metals could be ascribed to increased dissolution rate at slip edges and to increasing surface roughness upon slipping. The rate of metal dissolution on h.i.p. is 50 times larger than that at l.i.p., while the rate of the dissolution at slip edges is about 10 times larger than that at the unstrained metal.
Spectral Distribution of Light Scattered from Flexible-Coil Macromolecules
Authors: Pecora R.
Published: February 2004

The spectral distribution of light scattered from monodisperse, infinitely dilute solutions of optically isotropic, flexible-coil macromolecules in the free-draining approximation of the pearl-necklace model is related to the macromolecular translational diffusion coefficient D and the set of intramolecular relaxation times, τk. The spectral distribution for scattering parameter x < 1 (small scattering angle and/or small molecules) is shown to consist essentially of a single Lorentzian line with half-width proportional to D. For x > 3 (large scattering angle and molecules with end-to-end distance ≳1000 Å), additional Lorentzian terms each with half-width dependent on D and some of the τk become important in the equations for the scattered spectral density. At x = 3, these terms are 15% of the total integrated scattered intensity. They rise to 50% at x = 7. Although the resultant line shape is complex, it is shown that the dominant contribution to this “intramolecular” spectral density comes from terms containing the longest relaxation time τ1.
Low-Frequency Vibrations in Polyethylene at 300°, 77°, and 4.2°K
Authors: Myers W. R. , Randolph P. D.
Published: February 2004

The absolute neutron scattering cross sections of oriented polyethylene have been measured at temperatures of 300°, 77°, and 4.2°K using the MTR phased chopper velocity selector. Low incident energies were used to optimize the energy resolution. The target was oriented such that the longitudinal and transverse cross sections could be observed simultaneously in different scattering angles. Data are presented in the form of absolute scattering cross sections and are also shown converted to the amplitude weighted directional frequency spectra. The conversion to frequency spectra includes use of measured Debye–Waller coefficients. Strong singularities are observed at phonon energies of 65, 25, 15.5, 12, and 5.6 meV.
Hydrogen-Bond Studies. XXIX. Crystal Structure of Perchloric Acid Dihydrate, H5O2+ClO4−
Authors: Olovsson Ivar
Published: February 2004

The crystal structure of HClO4·2H2O has been determined from single-crystal x-ray data obtained at −190°C. The crystals are orthorhombic, space group Pnma, with four formula units in a cell of dimensions: a = 5.819, b = 10.598, c = 7.369 Å. The water molecules are bonded to each other into pairs by a very short hydrogen bond (2.424 Å), forming H5O2+ ions. With the space group chosen there is a center of symmetry in the middle of this hydrogen bond; the H5O2+ ion is perfectly staggered. The H5O2+ ions are hydrogen bonded to the ClO4− ions into layers, which are held together by van der Waals forces. The average Cl–O distance in the perchlorate ion is 1.438 Å.
Relaxation by Vibration–Vibration Exchange Processes. Part II. Binary Mixtures
Authors: Fisher Edward R. , Kummler Ralph H.
Published: February 2004

A previous paper on vibration processes in a pure gas is extended to the case of binary mixtures of gases. Through numerical integration of the master equation for vibrational relaxation, three distinct time scales to the relaxation processes are identified. The first time scale characterizes the pure-gas vibrational exchange reactions, the second relaxation time describes the coupling between species, while vibration–translation processes are characterized by a third time scale. Depending on the relative vibrational spacing and vibration–translation rates of the components in the mixture, vibrational distributions consistent with each of these time scales may be evident during the relaxation process. Through a steady-state analysis to the vibrational exchange terms in the master equation, it is demonstrated that large differences can exist between population factors (or vibrational temperatures in the harmonic model) for each species at steady state. Only at high kinetic temperatures (i.e., >1000°K) or for equilibrium conditions does vibrational coupling between the species in a mixture give rise to equal vibrational temperatures. In a binary mixture with T(vib) > T(trans), the species having the smallest vibrational spacing is described by higher vibrational temperatures than the second species, and this difference in vibrational temperatures increases with both decreasing kinetic temperature and mole fraction of the component with the smaller spacing. Non-Boltzmann distributions are predicted in the anharmonic model at steady state. However, it is shown that the harmonic treatment to the mixture coupling gives a reasonable zeroth-order solution. Comparison of the results with recent discharge flow data verify the mixture analysis and are consistent with the presence of non-Boltzmann distributions at steady state. More complete agreement must await better kinetic temperature measurements in the discharge flow environment.
Infrared Study of Water Structure in Salt Hydrates
Authors: Hester R. E. , Krishnan K. , Scaife C. W. J.
Published: February 2004

Infrared spectra are reported for hydrates of zinc nitrate, of potassium thiocyanate, and of mixtures of potassium thiocyanate with thiocyanates of mercury and zinc. These systems were studied over the range of temperatures 90° to −163°C, and with variable water content. Most of these systems were liquid above room temperature, but were studied as solids below room temperature. The broad water absorption bands were seen to split into a number of distinct components on cooling, these components sharpening, changing in frequencies and in intensities as temperature was decreased. The ice structure, shown by systems of high water content, was not present in the low-temperature spectra of systems of low water content containing divalent metal ions. Instead, peaks characteristic of coordinated and variously hydrogen-bonded water molecules were distinguishable. Assignment of these peaks and their temperature dependence is discussed.
Vibrational Energy Transfer in Methane
Authors: Yardley James T. , Moore C. Bradley
Published: February 2004

The asymmetric stretching vibration ν3 of methane has been excited by a chopped He–Ne laser. The phase shift of infrared emission from the asymmetric stretch and from the infrared-active bend ν4 has been measured as a function of chopping frequency and gas pressure. Vibrational relaxation times have been found for the removal of energy from the asymmetric stretch (pτ = 7.0 ± 1 nsec·atm), for the appearance of this energy in the ν4 bend (pτ = 5 + 3, −2 nsec·atm), and for the relaxation of vibrational energy into translational and rotational energy (pτ = 1.90 ± 0.10 μsec·atm). The mechanism and rates of V → V energy transfer are compared to calculations for a repulsive intermolecular potential. The general applicability of the laser-excited vibrational fluorescence method and the interpretation of phase-shift data are discussed.
Calculation of Nitrogen Chemical Shifts
Authors: Wu Ting Kai
Published: February 2004

The Pople theory of chemical shift has been employed to present a theoretical interpretation for observed nitrogen chemical shifts. In the absence of low-lying n→π* transitions which give rise to a large paramagnetic shift, the shielding of nitrogen, as predicted by theory, arises primarily from: (1) the electron density on nitrogen; (2) the mean-excitation energy; and (3) the effect of multiple bonds of nitrogen. A theoretical expression was also derived for calculations of nitrogen chemical shifts in heteroaromatic molecules. It was found that the chemical shifts of the diazines depend on both the pi- and sigma-electronic distributions on the nitrogen atoms. The variations in the nitrogen sigma-electronic densities estimated from chemical shift data are in reasonable agreement with those observed in nuclear quadrupole resonance studies.
Photochemical Reaction of Bromine with Hydrogen at Room Temperature
Authors: Sullivan John H.
Published: February 2004

The rate of the photochemical production of hydrogen bromide was measured in a hydrogen–bromine system irradiated with 5780-Å light. The rate was found to be proportional to the square of the concentration of bromine atoms. Absolute light intensities were determined and third-order rate constants for the equation d(HBr) / d t =2k[Br]2[H2] were determined at 0° and 25°C. The rate constant is given by k = [ (4.8 ± 4) × 105exp(0∓800/RT) liter2mole−2·sec−1. The data of previous photochemical experiments at room temperature, usually but erroneously considered to be consistent with the chain mechanism, are shown to be consistent with the third-order reaction.
Theoretical Investigations of Gas–Solid Interaction Phenomena. II. Three-Dimensional Treatment
Authors: Lorenzen J. , Raff L. M.
Published: February 2004

A three-dimensional, classical study of gas–solid surface interaction phenomena is reported. An independent-oscillator lattice model is assumed to represent the crystal surface. Nine movable lattice sites are connected to fixed points by harmonic springs in a geometry chosen to represent either the (100) or (111) planes of a Ni crystal. The interaction potential is constructed from nine pairwise Morse potentials operating between the incident gaseous atom and the nine movable lattice points. Energy-transfer coefficients (ETC) and spatial distributions are calculated as a function of incidence angle, gaseous beam velocity and temperature, surface temperature, gaseous atom mass, lattice force constant, and attractive well depth and curvature by numerical solution of the 60 differential-motion equations. The results indicate that the ETC should decrease with increasing incidence angle, decreasing attractive well depth and curvature, increasing lattice force constant, and decreasing isotopic mass of the incident gaseous atom. As the surface temperature approaches and exceeds the gaseous beam temperature, the ETC is also predicted to decrease. The rate of decrease of the ETC with incidence angle seems sufficient to justify the normal component model in contrast to previous two-dimensional results. Energy transfer is found to be favored for collisions near lattice sites. The calculated ETC values are virtually independent of the crystal plane being attacked. The calculated three-dimensional spatial distributions exhibit peak maxima and half-widths characteristic of experiment. The distributions are strongly dependent upon incidence angle and, under certain conditions, exhibit a “bimodal” structure. This structure is observed to decrease with increasing beam and surface temperatures, decreasing incidence angle, and decreasing attractive well. Subspecular shifts of calculated peak maxima are predicted for increasing surface temperature. The angular distributions in planes parallel to the surface are peaked at the in-plane angle and decrease rapidly on either side of it. This effect becomes more pronounced as the incidence angle and the beam temperature are increased and as the attractive well depth is decreased. Insofar as comparison with experiment is possible, virtually all results are in qualitative to semi-quantitative agreement with available experimental data.
Thermal Diffusion in Polyatomic Gases: Nonspherical Interactions
Authors: Monchick L. , Sandler Stanley I. , Mason E. A.
Published: February 2004

The thermal-diffusion factor αT, of polyatomic gases is reconsidered and the assumption is dropped that the differential scattering cross section is independent of the entrance and exit channels in inelastic collisions. In addition to the linear relation previously found, linking αT to the partial translational thermal conductivities of the constituents, a new term arises relating αT to the partial internal thermal conductivities. Estimates of this new coupling coefficient were made using the loaded sphere model and an approximate analysis of the thermal separation of ortho- and parahydrogen. They both indicate that this coefficient is very small and ordinarily does not contribute much to the total thermal-diffusion factor. It is shown, however, that there are some situations in which this term becomes dominant. To first order in the eccentricity of isotopic series of diatomic molecules, αT turns out to be proportional to the difference of the moments of inertia, an empirical relation observed by many investigators.
ESR Study of the Dimer Cation of Coronene
Authors: van Willigen H. , de Boer E. , Cooper J. T. , Forbes W. F.
Published: February 2004

Treatment of coronene (c), dissolved in SO2, with BF3 affords a variety of ESR spectra depending on the BF3/c concentration ratio and the temperature. To account for the spectra, the formation of monomer and dimer cation species (c+ and c2+) is postulated. Possibly, a trimer cation (c3+) is also formed at relatively high concentrations of coronene. The spectral features are discussed in terms of current theories of ESR spectroscopy.
Kinetic Isotope Effects in the Unimolecular Reactions of Chemically Activated Chloroethane-d and -d5 and 1,2-Dichloroethane-d and -d4 Molecules
Authors: Dees Kenneth , Setser D. W.
Published: February 2004

The nonequilibrium kinetic isotope effects for H–Cl and D–Cl elimination from chemically activated (average energy of ∼91 kcal mole−1) chloroethane-d0 and-d5 and 1,2-dichloroethane-d0 and -d4 have been experimentally measured as 3.3 ± 0.4 and 3.5 ± 0.1, respectively. The chemically activated molecules were generated by the combination reactions of methyl and chloromethyl radicals or their deuterated equivalents at room temperature. These isotope effects and also some recently measured thermal activation isotope rate data were interpreted using RRKM computations based upon a four-centered model for the reaction. Rather good agreement between experimental results and the model calculations was obtained. Even all the kinetic data for chloroethane including the new isotope results were not sufficient to permit the definition of a unique model; however, both the thermal and chemical activation isotope measurements were best fitted by models with the hydrogen weakly bound in the four-centered ring. Suggestions are made for the relative contributions of the in-ring deuterium atom and of out-of-ring deuterium atoms to the over-all nonequilibrium isotope effect for C2D5Cl. Calculated isotope effects are presented for chloroethane d1, chloroethane-d4, and some tritiated chloroethanes on the basis of the optimized models.
Rhenium Pure Quadrupole Resonance Spectrum of Potassium Perrhenate
Authors: Rogers Max T. , Rao K. V. S. Rama
Published: February 2004

The rhenium pure quadrupole resonance spectrum of polycrystalline potassium perrhenate has been observed. The frequencies of both the ±52 ↔ ±32 and ±32 ↔ ±12 transitions for 185Re and 187Re were found and have the values 56.600, 28.312, 53.626, and 26.825 MHz, respectively, at room temperature. These values are compared with a “theoretical” value of the quadrupole coupling obtained by adding the calculated point-charge field-gradient tensor to that arising from the p character of the bonds, both based on an assumed electronic structure for the ReO4− ion. It is shown that a reasonable value of the polarization factor will provide agreement with experiment.
High-Temperature Defect Structure and Electrical Properties of NiO
Authors: Bransky I. , Tallan N. M.
Published: February 2004

The electrical conductivity and the Seebeck coefficient of single-crystal and polycrystalline NiO were measured as a function of temperature and of oxygen partial pressure over a large part of the phase field of stable NiO. In the temperature range 1000°–1600°C the electrical conductivity was found to be proportional to the fourth root of the oxygen partial pressure. This pressure dependence is that which one would expect for pure, nonstoichiometric NiO containing singly ionized metal vacancies as the predominant point defects. The activation energy for conduction, ΔHc, computed from σ = σ0exp (−ΔHc / kT), was found to be 0.92 ± 0.02 eV. The Seebeck coefficient of NiO was found to be p type over the entire range of temperatures and oxygen partial pressures measured. From the temperature dependence of the Seebeck coefficient, the enthalpy of formation of defects in NiO was calculated to be 0.66 ± 0.03 eV. From the measured temperature dependence of the electrical conductivity and the Seebeck coefficient it was found that the charge-carrier mobility increases with the temperature. Considering the drift mobility to be of the form μp∝T−1exp(−ΔHm / kT) a value of 0.37 eV could be attributed to the activation energy for motion of the charge carrier, ΔHm. The existence of such an activation energy supports the suggestion of a hopping process for the charge-carrier transport mechanism in NiO above about 1000°C.
Linear- and Angular-Momentum Autocorrelation Functions in Diatomic Liquids
Authors: Harp G. D. , Berne Bruce J.
Published: February 2004

The linear- and angular-momentum autocorrelation functions in liquid CO have been studied using computer-simulated molecular dynamics. The structure of these functions appears to be determined by the corresponding autocorrelation functions of the direction of the linear and angular momentum. The structure of the angular-momentum autocorrelation function is also shown to depend on the strength of the noncentral part of the pair potential used in the dynamics calculations. The assumptions that the linear and angular momenta are Gaussian random variables are examined and shown to be valid for the data presented.
Theoretical Analysis of the Electronic Structure and Molecular Properties of the Alkali Halides. IV. Potassium Fluoride
Authors: Matcha Robert L.
Published: February 2004

Results of a recent theoretical investigation of the electronic structure of KF within the SCF–LCAO–MO framework are discussed, giving special emphasis to the evaluation of various molecular properties. Computed values (followed by experimental values in parentheses) for the total energy E, equilibrium separation Re, spectroscopic constants ωe, ωexe, and Be, and dipole moment μ, are E = −698.68501 a.u., Re = 4.188 a.u. (4.1035), ωe = 448.5 cm−1 (426.0), ωexe = 2.484 cm−1 (2.430), Be = 0.2686 cm−1 (0.2799), μυ = 8.6886 + 0.0634(υ + 12) − 0.0001(υ+12)2, and μυ(exptl) = 8.5583 + 0.0684(υ + 12) + 0.0002(υ + 12)2. The computed dissociation energy (DeHF = 0.112 a.u.) has been coupled with estimated correlation and relativisitic contributions (Decorr = 0.065 a.u., Derel = −0.001 a.u.), producing a lower limit value, Decalc = 0.176 a.u., as compared to Deexptl = 0.187. This discrepancy is due to small corrections discussed in the paper. Computed values of the field gradients measured at the potassium nucleus (qK0 = − 0.6001 a.u.) have been combined with an experimentally determined value of the nuclear quadrupole coupling constant (eqK0QK = − 7933.87 kc/sec) to yield for the potassium nuclear quadrupole moments, Q39K = 0.0563 × 10−24cm2 and Q41K = 0.0685 × 10−24cm2.
Energy Expectation Values and the Integral Hellmann–Feynman Theorem: H2+ Molecule
Authors: Rothstein Stuart M. , Blinder S. M.
Published: February 2004

It is by now well known that the integral Hellmann–Feynman (IHF) theorem has little quantitative utility for chemically interesting problems, although the formalism potentially affords a ready physical interpretation of changes in molecular conformation. In this paper, the IHF theorem is applied to variational and simple LCAO wavefunctions for the H2+ ground state, which range in quality from crude to essentially exact. The IHF results improve quite dramatically with the quality of the wavefunctions. This suggests that errors in the IHF formula may be of the same order as those in the wavefunction. (In contrast, errors in variationally determined energies are of second order.) Our results suggest a convenient test which can be applied to any revised IHF formalism developed in the future.
Ground States, Configurations, and Truncated Orbital Bases of the Iron-Series Atoms
Authors: Claydon Charles R. , Carlson K. Douglas
Published: February 2004

This article presents an analysis of several related topics of importance to the orbital theory of electronic states of transition-metal atoms in the framework of the Hartree–Fock approximation. First, it is shown that the principle experimental features of the term separations for these atoms are determined primarily by the Hartree–Fock wavefunction. Within the framework of this one-particle description, it is then demonstrated that subtle competing electrostatic effects are responsible for the well-known inversions of ground-state configurations. Beyond the one-particle description, electron correlation contributions, determined as differences between experimental and Hartree–Fock term separations, are demonstrated to have remarkably systematic trends that enable predictions to be made of term values of stable but unobserved excited states of the ions. Finally, useful small bases that maintain the principal features of the Hartree–Fock description for reliable exploratory SCF calculations on these atoms and their molecules are presented and discussed.
Intermediate State in Monomolecular Films: A Re-evaluation
Authors: Cadenhead D. A. , Demchak R. J.
Published: February 2004

The monomolecular film characteristics of 4-amino-p-terphenyl have been determined over the temperature range 0°–31°C on a variety of aqueous substrates: alkaline (pH 10), alkaline (pH 10) with 2M NaCl and with 30 wt.% glycerol. No indication of an intermediate state was found. From these results and others already reported, it was concluded that the Kirkwood theory concerning this state is not correct, and that the exhibition of such a transition region is an inherent property of the internal motions of the paraffinic chains possessed by many amphipathic molecules.
Solvation of Co(II) in Methanol and Water Enriched in Oxygen-17
Authors: Fiat D. , Luz Z. , Silver B. L.
Published: February 2004

The solvation sphere of cobaltous ion in oxygen-17-enriched methanol and water solutions was observed by oxygen-17 magnetic resonance. The chemical shift between the coordinated methanol molecules and the bulk methanol and the transverse relaxation times of oxygen-17 in the two environments were measured in the temperature range of −27.5° to +21°C. The hyperfine coupling constant between the cobaltous ion unpaired electrons and the oxygen-17 nuclei was found to be temperature independent and its value −(1.70 ± 0.03) × 107 Hz. The relaxation time of the unpaired electrons of the cobaltous ion was calculated and found to be 7 × 10−13 sec at 25°C, assuming T1e = T2e. The dipolar contribution to the relaxation rate of the oxygen-17 was calculated and found to be about 1% of the hyperfine contribution. The oxygen-17 chemical shift between the coordinate water molecules and the noncoordinated water molecules and their transverse relaxation times were measured at temperatures of −15°, −10°, and −5°C. The hyperfine coupling constant and the unpaired electron relaxation time were found to be A / h = − (1.74 ± 0.03) × 107Hz and τe = 1.7 × 10−12sec at −10°C assuming T1e = T2e.
Dipolar Coupling and Molecular Vibration in Crystals. I. General Theory
Authors: Decius J. C.
Published: February 2004

Coupling of the transition dipoles associated with infrared-active vibrations produces one source of frequency shift and splitting of transverse and lingitudinal modes in solids. The intrinsic part of the dipole is distinguished from the induced part, the latter being related to the electronic polarizabilities of the molecular or atomic units in the structure. Although approximate treatments neglecting the induced part of the dipole have been given previously and, also, certain estimates of the effective field (related to the induced dipole) have appeared earlier in the literature, we here take both effects fully into account for a number of noncubic crystal structures.