bluuebunny/crossref_metadata_2025 huggingface dataset:
Free-Electron Energy Levels for Symmetric Three-Dimensional Frameworks
Authors: Duffey George H. , Houtman Joel A. , Riashy Joseph
Published: February 2004

The methods previously applied to the tetrahedron and cube have been extended to cover frameworks in which the number of edges leaving a corner is greater than the number of valence orbitals. The other regular polyhedrons have been treated, together with the trigonal bipyramid, the trigonal prism, the tetragonal antiprism, the cube-octahedron, and certain bicapped structures.
Hydrogen Iodide Reaction. A Paradox for Chemical Kinetics
Authors: Noyes Richard M.
Published: February 2004

Sullivan has measured the second-order rate constant for the thermal reaction of hydrogen with iodine and also the third-order rate constant for the reaction in a photostationary system obeying the kinetics for H2+I+I→2HI. If the photostationary rate constant is extrapolated to the temperature of the thermal measurements, all of the thermal rate can be accounted for as due to the iodine atoms known to be present in equilibrium with the iodine molecules. If iodine molecules and pairs of iodine atoms can both react with hydrogen molecules and make independent contributions to the over-all rate, the observations seem to require that the molecular reaction is of negligible importance. However, the transition-state structure of lowest free energy should be accessible either from an iodine molecule or from a pair of atoms; absolute reaction rate theory then requires that the rate in the photostationary system be less than that observed experimentally. The paradox can be resolved if the transition-state structure of the lowest free energy is not accessible from iodine molecules or if the rate in the thermal system is determined by a pseudothermodynamic iodine activity rather than by independent contributions from kinetically equivalent mechanisms, but reasons are given for rejecting both of these resolutions. It is proposed that the paradox be resolved by rejecting the customary description of the reaction by absolute reaction rate theory; such a description employs partition functions based on molecular states defined by energies in randomly phased normal modes of motion and assumes a transmission coefficient of unity. An alternative description is developed based on trajectories uniquely defined by the initial coordinates and momenta of the reactant atoms. Failure to observe the molecular reaction is then ascribed to the large mass ratio of iodine to hydrogen; because of this mass ratio, the H–H bond will not be split during a collision and the centers of mass of the H2 and I2 systems will follow trajectories corresponding to elastic collisions. The conclusions of this paper could be tested further by repeating the thermal and photostationary measurements in solution, by studying the reaction in crossed molecular beams, and by measuring the photostationary reaction of iodine with deuterium.
Mercury-Photosensitized Decomposition of Ketene as a Source of Triplet Methylene
Authors: Avery H. E. , Cvetanović R. J.
Published: February 2004

Mercury [Hg 6(3P1)]-photosensitized decomposition of ketene has been investigated at room temperature and at various pressures of ketene from the point of view of its utilization as a convenient source of triplet methylene. The relative quenching cross section of ketene has been determined and an analysis of the likely extent of direct absorption of light by ketene at 2537 Å in competition with the absorption by mercury atoms has been made. Some aspects of the reaction mechanism are discussed.
Gas-Phase Photolysis and Radiolysis of Isobutane
Authors: Lias S. G. , Ausloos P.
Published: February 2004

The photolysis of (CH3)3CH, (CH3)3CD, and of (CH3)3CH–(CD3)3CD and (CD3)3CD–H2S mixtures has been investigated at 1470 Å (8.4 eV), 1236 Å (10.0 eV), and 1048–1067 Å (11.6–11.8 eV). Since the irradiating photons in these experiments have energies both higher and lower than the ionization energy of isobutane (10.6 eV), the modes of decomposition of the “superexcited” molecule can be compared with those of the molecule excited to states below the ionization potential. On the basis of isotopic analyses of the major products, it was determined that the following primary modes of decomposition of the excited isobutane molecule occur in the photolysis at all three energies: i-C4H10*→CH4+C3H6; i-C4H10*→C3H8+CH2; i-C4H10*→CH3+ sec-C3H7; i-C4H10*→H2+i-C4H8. Most of the propyl radicals decompose further: C3H7→H+C3H6;C3H7→CH3+C2H4. An increase in photon energy increases the relative importance of C–C cleavage processes. The parent ions formed in the photolysis with 11.6–11.8-eV photons decompose to form C3H6+ and sec-C3H7+. The C3H7+ ions react with isobutane to form sec-C3H7 radicals or propane: C3H6++i-C4H10→CH3CHCH3+C4H9+;C3H6++i- C4H10→C3H8+C4H8+. The probability of the former reaction is 1.5 times that of the latter, in agreement with a value for this ratio derived from the radiolysis of C4D10–H2S and C4H10–C4D10 mixtures. The role of superexcited molecules in the radiolysis is discussed and it is estimated that the number of dissociating neutral excited molecules per ion pair is no greater than 0.46.
Far-Infrared Absorption Spectra of Aqueous Solutions of Strong Electrolytes
Authors: Draegert David A. , Williams Dudley
Published: February 2004

The infrared absorption of solutions of LiCl, KF, KCl, KBr, KI, NaCl, NaBr, NaI, NaNO3, NaClO3, DCl, and NaOD in NaClO4 has been studied in the spectral region 75 to 650 D2O; the absorption of HCl and NaOH solutions in H2O in the region 75 to 1000 cm−1 has also been investigated. Even for the highest concentrations, the solution spectra have a striking resemblance to the spectrum of the solvent. A broad, intense band attributable to the hindered rotation of water molecules and a much less intense band attributable to the hindered translation of water molecules appear in the spectrum of each solution. The frequencies, widths, and shapes of these bands are compared in some detail with the corresponding bands in the spectrum of pure water. Small but significant differences between the spectra of the solutions and the spectrum of water are noted.
Separation of Gas Mixtures in Chemical Equilibrium in a Temperature Gradient. I
Authors: Paul R. , de Vries A. E.
Published: February 2004

Theoretical and experimental results for the separation in different systems of chemically reacting species are given. A comparison between theory and experiment for the mixtures He/NO2/N2O4 and Ar/NO2/n2O4 in which the separation between the inert gas and the N2O4⇋2NO2 is studied shows quantitative agreement. In the first system both the ordinary thermal diffusion factor and the contribution due to the chemical equilibrium, the thermochemical effect, has to be taken into account. On the other hand in the Ar/NO2/N2O4 system the physical part of the separation is so small that it can be neglected. The theory is general and always gives the total effect. It is also applied to the separation of two inert gases in the presence of a dissociating component and to the separation in a system of the type ν1A⇋ν2B + ν3C.
Transformations and Averaging Processes for Differential Cross-Section Measurements at Thermal Energies
Authors: Helbing Reinhard K. B.
Published: February 2004

Theoretical predictions on differential cross sections apply to the center-of-mass system of the colliding particles but not to the laboratory system. In order to compare theoretical results with measurements it is necessary either (a) to approximate the center-of-mass system by a suitable choice of the colliding particles, and the region of energy, or (b) to transform the differential cross section from the center-of-mass system into the laboratory system. If the target particles are at rest before collision, this transformation is simple and well known. If the target particles are not stationary before collision, the problem of transformation becomes more difficult, especially if the target particles have a distribution of velocities. In this paper, formulas are derived which allow the transformation from the center-of-mass system to the laboratory system (and vice versa). Both elastic and inelastic collisions are treated. In addition, the averaging process over all initial relative velocity vectors is described, i.e., for a given differential cross section in the center-of-mass system, the intensity distribution as measured in the laboratory system is calculated.
Energy Dependence for Single and Double Ionization by Electron Impact Between 250 and 2200 eV
Authors: Fiquet-fayard Florence , Chiari Jean , Muller Françoise , Ziesel Jean-Pierre
Published: February 2004

We have completed a preceding work by studying Rb, Zn, Mg, NH3, NO, N2, CO, COS, CS2, and H2S. All the results prove that the direct double-ionization cross section (simultaneous ejection of two electrons) can be written as σ ∝ E−1, where E is the energy of incident ions. When doubly charged ions are formed, not by direct ionization, but by the Auger effect, we observed departure from an E−1 dependence. Experiment shows that molecular ions COS++, CS2++, H2S++, and NH3++ can be formed by the Auger effect. Discussion suggests that some of these ions are stable only in excited states. Furthermore, our experiments show that for all studied species, excepting magnesium, the single-ionization cross section satisfies the usual dependence σ ∝ E−1log B E, where B has a reasonable value. The magnesium cross section decreases anomalously rapidly and we were unable to find a satisfactory interpretation of this phenomena.
Electron Spin Resonance Spectra of Gamma-Ray-Irradiated Phosphate Glasses and Compounds: Oxygen Vacancies
Authors: Weeks R. A. , Bray P. J.
Published: February 2004

The electron spin resonance spectra of γ-ray-irradiated glassy and polycrystalline phosphorus pentoxide and alkali phosphates with molar ratios of phosphorus pentoxide to alkali oxide ranging from 3.33 to 0.625 have been observed at 10 and 35 Gcps and at 300° and 78°K. Well-resolved hyperfine doublets were observed in each glass or polycrystalline powder. Additional resonances were also present in all the specimens but these are not discussed. The widths and shapes of the doublets were independent of temperature and only small alterations in the shapes of each component of a doublet were found as a function of frequency. One set of doublets has been discussed elsewhere and is not considered. Those doublets which are discussed are attributed to an electron trapped at a nonbridging oxygen vacancy. Interaction of the electron with the nearest-neighbor 31P nucleus (100% natural abundance, I = 12) produces the doublet structure. These doublets exhibited isotropic hyperfine interactions ranging from 1880 to 4490 Mcps. The smaller interactions, 1880-2196 Mcps, were found in glasses and polycrystalline powders with molar ratios ≤ 1 and the larger interactions, 2630-4490 Mcps, were found in those with molar ratios > 1. The anisotropic hyperfine interaction, (A‖ − A⊥) / h, of all the doublets was ≤ 540 Mcps. The defect electron state is assumed to be the nonbonding phosphorus sp hybrid orbital. On the basis of this assumption, the different isotropic hyperfine interactions are attributed to different geometric configurations of the phosphorus ion and the oxygens to which it is bonded. The assumed sp hybridization accounts for most of the electron density. An approximate g value is calculated for the doublets and is found to range from 2.005 to 2.040.
Enthalpy Changes and Heat-Capacity Changes in the Transformations from High-Surface-Area Amorphous Ice to Stable Hexagonal Ice
Authors: Ghormley J. A.
Published: February 2004

Amorphous ice prepared by slow condensation of water vapor onto a surface cooled with liquid nitrogen or liquid hydrogen undergoes several exothermal transformations on warming. Thermal analysis by means of warming curves indicates that the heat liberated on warming ice prepared at − 253°C is 8 cal/g from − 253° to − 196°C and 22 cal/g from − 196° to − 120°C.Ice prepared at − 196°C gives up 14 cal/g between − 185° and − 125°C. These enthalpy changes are attributed in part to decreases in the surface area of the amorphous ice. Beginning at − 120°C, 24 cal/g heat is evolved on crystallization to the cubic form (ice Ic). Between − 80° and − 50° an additional 3 cal/g is attributed to recrystallization of the cubic phase to larger crystals. In the cubic to hexagonal (ice Ih) transition between − 50° and − 5°, ΔH is less than ±0.3 cal/g, but a decrease occurs in the low-temperature heat capacity. The heat capacity of amorphous ice or ice Ic in the range − 170° to − 120°C exceeds that of ice Ih by 27% and the heat capacity of ice Ic between − 120° and − 70°C exceeds that of ice Ih by 12%. The higher heat capacity of ice Ic is attributed to a greater degree of order at low temperature. Heat evolved in the hypothetical transition from amorphous ice at − 253°C to ice Ic at the same temperature is 55 ± 5 cal/g. No glass transition was observed in amprophous ice. Within experimental error, the enthalpy and heat-capacity changes are the same in D2O as in H2O.
Applications of the Quantum-Mechanical Green's Functions to the Study of Chemical Reactions. II
Authors: Paul R. , Fowler G. N. , Laidlaw W. G.
Published: February 2004

Starting from a field theoretical representation of chemical reactions, the rate equation is developed in terms of the quantum mechanical Green's functions. The equation is valid for non-Markowian behavior and the approximations that are necessary in order to obtain the conventional (Markowian) rate equation are investigated. The effect of finite collision times is also discussed.
Simple Formulas for the Vibrational and Rotational Eigenvalues of the Lennard-Jones 12-6 Potential
Authors: Cashion J. K.
Published: February 2004

Dunham expressed the term values of a generalized rotating vibrator as a double power series in the vibrational and rotational quantum numbers. Eighteen coefficients in this series have been evaluated for the Lennard-Jones (12-6) potential. The accuracy of the resulting term-value formula has been tested by direct numerical solution of the radial equation and was found to be very good. The addition of a few empirical corrections for states lying close to the dissociation limit provides an eigenvalue predictor which is accurate to better than a part per million in the most favorable cases and never worse than a part per thousand. The range covered is from systems with only a single bound state to ones with more than ten thousand, e.g., the iodine dimer. When used to provide trial eigenvalues for Cooley's procedure for solving the radial equation, the predictor successfully discriminates between states separated from each other and from the dissociation limit by less than one part in 105. Simple formulas have also been derived for the zero-point energy, the number of pure vibrational states, and the total number of bound states. Finally, it is shown that even fourth-order WKB terms can make significant contributions for very light systems and that there is no indication that Dunham's series for this potential model fails to converge even for term values lying arbitrarily close to the dissociation limit.
Generalized Theory of Nucleation Kinetics. II. Athermal Nucleation Involving Spherical Clusters
Authors: Ziabicki Andrzej
Published: February 2004

Based on the general formulation of the previous paper1 the effects of athermal nucleation involving spherical, isotropic clusters are discussed. The formulas describing absolute and relative athermal nucleation rates, concentration of athermal nuclei after rapid cooling, and time-lag change due to the presence of such nuclei are derived and analyzed. Some athermal nucleation characteristics for sodium disilicate are computed.
Atom–Molecule Kinetics using ESR Detection. IV. Results for Cl + H2 ⇄ HCl + H in Both Directions
Authors: Westenberg A. A. , Haas N. de
Published: February 2004

The technique described in previous papers of this series has been used to measure separately the rate coefficients k1 for the reaction Cl + H2 → HCl + H over the temperature range 251°–456°K, and k−1 for the reverse reaction over 195°–497°K. Both sets of data are Arrhenius linear and obey (cm3 mole − 1·sec − 1) k1 = 1.2 × 1013exp ( − 4300 / RT); k − 1 = 2.3 × 1013exp ( − 3500 / RT). Interpretation of the individual rate coefficients is made in terms of absolute rate theory with no tunneling, using a potential surface constructed by the semiempirical Sato method for a linear transition complex. The properties of the complex give a theoretical pre-exponential factor for the forward reaction about 2.5 times higher than experiment, while that for the reverse reaction is in fair agreement with experiment. It is shown that the ratio k1 / k − 1 is a factor of 2–3 below the thermodynamic equilibrium constant over the experimental temperature range. A possible qualitative explanation for this deviation from the strict prediction of microscopic reversibility is offered in terms of the greater probability of obtaining rotationally excited product HCl in the forward reaction than excited product H2 in the reverse.
Mössbauer Effect in 129I Derivatives of Methane
Authors: Bukshpan S. , Sonnino T.
Published: February 2004

The Mössbauer effect in the 27.8-keV transition of 129I was used to investigate the electronic structure of solid CH3I, CHI3, and CI4. From the quadrupole split spectra observed at 85°K the isomer shift and quadrupole coupling constants were derived. The asymmetry parameter η was found to be 0. The structure of the C–I bond was derived for the three cases.
Numerical Single-Center Self-Consistent-Field Function for the Hydrogen Molecule
Authors: Wilkins Roger L. , Taylor Howard S.
Published: February 2004

A Hartree–Fock single-determinant wavefunction is calculated numerically for the normal state Σg+1 of the hydrogen molecule. The one-electron molecular orbitals in this determinant are expanded up to fourth-order terms as a product of a series of spherical harmonics and radial wavefunctions about the molecular midpoint of the molecule. The potential field due to the nuclei is expanded in spherical harmonics up to eighth-order terms and the complete Hartree–Fock equations are set up and solved numerically for the three radial functions occurring in one-electron molecular orbitals. The numerical technique used to solve the three coupled second-order differential–integral equations differs from that usually used in atomic calculations. The total numerical energy is −1.13236 a.u. The more accurate Kolos and Roothaan analytic total energy result is −1.13364 a.u.
cis–trans Isomerization of 2-Butenes in the Gas Phase: Benzene Photosensitization at 2537 Å
Authors: Lee Edward K. C. , Denschlag H. O. , Haninger G. A.
Published: February 2004

cis–trans Isomerization of 2-butenes in the gas phase has been studied by the benzene photosensitization method, and the detailed experimental results are given. The ratio of the isomerization rate (Rc→t / Rt→c) and the photostationary-state ratio [(trans-2-butene)/(cis-2-butene)]PSS, have been determined to be 1.02 ± 0.02 and 0.92 ± 0.02, respectively. trans-2-Butene has a slightly larger cross section for quenching the triplet benzene than cis-2-butene, and its cross section may be as much as 10% greater than that of cis-2-butene. Quenching cross sections of various olefins (πσ2) vary by two orders of magnitude: ethylene has the smallest value of 0.8 Å2 and 1,3-butadiene has the largest value of 50 Å2 found so far. The energy transfer to 1,3-butadiene appears to occur on almost every collision. The decay rate of the triplet benzene at the benzene pressure range of 0.5–5.7 mm is comparable to the rate of the triplet energy transfer to cis-2-butene at 0.028 ± 0.004 mm of cis-2-butene. The above rate data are in serious disagreement with the data reported by Cundall et al.
Electron Spin Resonance Study of γ-Irradiated Single Crystals of Semicarbazide Hydrochloride
Authors: Shrivastava Keshav N. , Anderson Roy S.
Published: February 2004

Electron spin resonance studies of γ-irradiated single crystals of semicarbazide hydrochloride and the deuterated form, ND2CONHND3+·Cl−, revealed the presence of the stably trapped ·N(α)H(α) − N(β)H(β)3+ radical obtained by C–N bond rupture of the parent molecule. The principal g values were gaa = 2.0041 ± 0.0003, gbb = 2.0023, and gcc = 2.0044. The principal elements of the hyperfine couplings were found to be N(α) = 4.8 ± 4.0, 34.2 ± 1.0, and 11.8 ± 4.0 G; H(α) = − 10.5 ± 4.0, −27.6 ± 1.0, and −36.7 ± 4.0 G; N(β) = − 4 G; H(β) = 15 ± 0.5 G. The N(β) coupling was estimated from the angular dependence of the linewidths. The direction cosines of the N(α)and H(α) couplings were evaluated with reference to the orthorhombic crystallographic axes (a, b, c). The direction cosines of the N–H bond calculated from the ESR spectra were found to agree with those obtained by x-ray diffraction. The spin densities calculated for this radical indicate that the unpaired electron is highly localized on the α nitrogen, there being a spin density of 0.51 in the 2pπ orbital of the α nitrogen.
Temperature Dependences of the Hyperfine Interactions, Linewidths, and Line Asymmetries in Methyl Radicals Stabilized on Porous Glass Surfaces
Authors: Garbutt G. B. , Gesser H. D. , Fujimoto M.
Published: February 2004

Methyl radicals were stabilized on the surface of porous Vycor glass at low temperatures by the uv photolysis of methyl iodide, deuterium-enriched methyl iodide, and carbon-13-enriched methyl iodide. The proton, deuterium, and carbon-13 hyperfine splittings were measured as a function of temperature in order to make comparisons with the dependences calculated by Schrader. In addition, the linewidths, line asymmetries, and relative line intensities in the methyl radical ESR spectra were measured as a function of temperature. The absolute values of the temperature coefficients of the proton and deuterium splittings were found to be slightly larger than those predicted by theory. The carbon-13 hyperfine splittings were found to agree with theory above 200°K but a large deviation was observed below 200°K. Below 275°K the proton lines become increasingly asymmetric. The relative line intensities approach but do not attain the binomial values as the temperature is increased. The proton linewidths were found to have different temperature dependences below 200°K, while having the same dependences above 200°K. The observed results may be explained by a physical model based on increased surface interaction at low temperatures.
Ternary Hydrides Possessing the Cubic Perovskite Structure. I. A Neutron-Diffraction Study of BaLiH3 and BaLiD3
Authors: Maeland A. J. , Andresen A. F.
Published: February 2004

Neutron-diffraction studies on powered samples of BaLiH3 and BaLiD3 have shown that the structure is of the inverted perovskite type with the atomic positions of space group Pm3m: Ba in (0, 0, 0), Li in (12, 12, 12), and H or D in (12, 12, 0; 0, 12, 12; 12, 0, 12). The SrLiH3 and EuLiH3 are presumably isostructural. The lattice parameter for BaLiH3 is 4.023 ± 0.001 Å and slightly smaller for BaLiD3. The total neutronscattering cross section for hydrogen is 30 ± 2 b, a rather low value compared to other hydrogen-containing compounds, but comparable to that observed in other hydrides.
Asymmetry in Magnetic Second-Rank Tensor Quantities
Authors: Schneider R. F.
Published: February 2004

The possibility of asymmetry in a variety of second-rank tensor quantities of importance in magnetic resonance phenomena is discussed. A classical model demonstrating this effect in the case of shielding is described. The present methods for the measurement of anisotropy of tensor quantities in crystals are discussed, and the contributions of the antisymmetric part of such a tensor are shown to occur in terms quadratic in the shielding (or g) tensor components.
Nonradiative Decay Processes of the Triplet State of Aromatic Compounds: Benzene
Authors: Martin T. E. , Kalantar A. H.
Published: February 2004

The observed phosphorescence decay times of C6H6, 13CC5H6, C6H5D, C6HD5 and C6D6 have been measured in reproducibly treated rigid media of cyclohexane, 3-methylpentane, and isopropanol at 77°K. The lifetimes are in the approximate ratio 1:1:1.06:1.5:2, respectively. The precision is ∼1% and the results show solvent dependence. The CH motion is clearly responsible for a large increase in the nonradiative transition probability on passing from C6D6 to C6HD5. The relative effect of further substituent hydrogens is less, on the average, as the H/D ratio increases. It appears that some nontotally symmetric CH stretching normal mode(s) contribute less to the radiationless depopulation of the B1u3 state than does the CH stretch corresponding to ν2(a1g) in C6H6. The discussion is based upon Franck–Condon considerations. The theory proposed by Siebrand [J. Chem. Phys. 47, 2411 (1967)] is considered in light of the results.
High-Resolution Study of Electron-Impact Spectra at Kinetic Energies between 33 and 100 eV and Scattering Angles to 16°
Authors: Lassettre Edwin N. , Skerbele Ausma , Dillon Michael A. , Ross Kevin J.
Published: February 2004

An electrostatic lens system which compensates for chromatic aberration has been tested in an electron spectrometer. The results indicate that this lens is suitable for comparisons of peak intensities in electron-impact spectra. Relative intensities in vibrational progressions that belong to a single electronic transition have been studied in N2, CO, and NH3 and found to be nearly independent of the scattering angle. Electron-impact spectra have been reported for helium, nitrogen, oxygen, argon, nitric oxide, nitrous oxide, ammonia, water vapor, carbon dioxide, ethylene, acetylene, and benzene at electron kinetic energies between 33 and 100 eV. Spectral regions of special interest are encountered in CO2 and C6H6. At excitation energies of 7–10 eV in CO2 a change in intensity distribution, attributed to transition from an electric-quadrupole to an electric-dipole spectrum, is observed as the kinetic energy is raised. In the case of C6H6 a change in the spectrum with angle is encountered which strongly suggests that two electronic transitions occur in a spectral region which was thought previously to contain only one.
Melting and Polymorphism of Potassium Cyanide and Thiocyanate to 44 kbar
Authors: Pistorius Carl W. F. T. , Clark J. B. , Rapoport Eliezer
Published: February 2004

The phase diagrams of KCN and KSCN were studied by means of differential thermal analysis. Bridgman's KCN II appears to be metastable. The transition line between cubic KCN I and orthorhombic KCN V runs from −60°C at 1 bar to the I/IV/V triple point at 20.0 kbar, 41°C. The KCN III/IV transition line lies at slightly higher temperatures than found by Bridgman. The melting curve of KCN is similar to that of KCl, with the III/I/liquid triple point at 22.2 kbar, 823°C. The phase behavior and the probable structures of the high-pressure phases of KCN and also of KNO2 can be simply explained as a combination of coordinational polymorphism and disordering. Three new solid phases of KSCN were found, and the melting curves and transition lines were followed to 44 kbar. There was no evidence of premelting in KSCN.
Spin-Extended PPP SCF Spin Densities in Some Hydrocarbon Radicals
Authors: Harriman John E. , Sando Kenneth M.
Published: February 2004

Spin-extended SCF calculations in the Pariser–Parr–Pople approximation have been done for the pi electrons of the radicals allyl, pentadienyl, and benzyl, and for the radical anions and cations of naphthalene, anthracene, and azulene. The spin-extended wavefunctions show several interesting properties. Calculated spin densities are compared with the results of other calculations. They are found, frequently, but not always, to lie between the unrestricted and the projected-unrestricted values, generally closer to the unprojected values. Calculated hyperfine coupling constants are compared with experimental values. The problem of the ratio of coupling constants in Positions 1 and 2 in anthracene and naphthalene has not been resolved, and the discrepancy between MO and VB results for the azulene cation remains.
Enthalpies of Mixing in Ternary Fused Nitrates
Authors: Meschel S. V. , Kleppa O. J.
Published: February 2004

The ternary enthalpies of mixing have been determined in three ternary alkali nitrate mixtures and in seven mixtures of silver nitrate plus two alkali nitrates. We find that in the three systems which contain only alkali nitrates, the ternary enthalpies of mixing can be represented to a very good approximation by the contributions calculated from the component binaries. However, this is not the case for the silver nitrate-containing systems, which all exhibit more negative enthalpies of mixing than calculated from the binaries. These ternary excess enthalpies increase in magnitude in a regular fashion with increasing difference in size between the two alkali-metal cations. An interpretation is suggested in terms of the influence of these cations on the covalency contribution to the Ag+–NO3− bonding.
Evaporation Rates and Lifetimes of Clouds and Sprays in Air. III. The Effects of Cloud Expansion
Authors: Zung Joseph T.
Published: February 2004

A simple method is given for determining theoretically the effects of cloud expansion upon the rate of evaporation of droplets in assemblages. The expansion of clouds is examined in terms of the Brownian motion, electric charge, and turbulent diffusion and dispersion. Results obtained by using both the cellular and the continuum models for cloud evaporation show that the Brownian diffusion slightly speeds up the evaporation rates and the electric charge has only a negligible effect, but the turbulent diffusion with a relative turbulent velocity of 1 cm/sec can shorten the lifetimes of a cloud by several hundred times.
Vibrational Energy Transfer from a Homologous Series of Chemically Activated Radicals
Authors: Tardy D. C. , Rabinovitch B. S.
Published: February 2004

The relative collisional efficiencies β of several inert bath molecules for the collisional deactivation of a homologous series of highly vibrationally excited species have been measured. The inert gases were H2, and N2 CH4, and CF4, The excited species were 2-butyl, 2-methyl-2-butyl,2-pentyl, 2-hexyl, and 2-octyl radicals. The relative efficiency of a particular bath gas was roughly constant for all members of the series. This finding is discussed and compared with theoretical results on a stochastic model. The approximate values of β are: 0.22, 0.53, 0.78, and 1.0 for H2, N2, CH4, and CF4, respectively. These correspond to average values of the mean energy decrement per collision, 〈ΔE〉 = 1.3, 2.3, 4.3, and >9 kcal mole−1, respectively, for a stepladder model of transition probabilities, and to somewhat larger average values for an exponential distribution of the sizes of energy decrements.
Helix–Coil Transition in Heterogeneous Chains. I. Protein Model
Authors: Fixman M. , Zeroka D.
Published: February 2004

Helix–coil transition curves are calculated for heterogeneous chains with random and correlated sequences of residue types by an approximate method. The distinction is made between the a priori probability of a residue sequence, and an effective probability which must be used when the sequence is treated as if it were in open chemical equilibrium with a reservoir. It is assumed that the effective probability does not correlate residue types in different sections of unbroken helix or coil. The required cumulants of the effective probability can then be determined from those of the a priori probability. Agreement with recent exact calculations is tested and found to be especially good when the cumulant expansion of the effective probability is terminated with three cumulants. The method is applicable to models of DNA which include entropy of ring formation. In this approximation the partition function for the heterogeneous chain has the same form as for the homogeneous chain, but an effective energy parameter is determined by the scheme just outlined. In the next approximation, which allows correlations between adjacent sections of helix and coil in the effective probability, the partition function still has the same form but an effective cooperative parameter would be required. The next approximation has not been studied in detail since no tractable scheme to evaluate the effective cooperative parameter has come to light.
Mössbauer-Effect Study of Some Iron(III) N,N-Dialkyldithiocarbamates
Authors: Rickards R. , Johnson C. E. , Hill H. A. O.
Published: February 2004

A Mössbauer-effect study has been carried out for 57Fe in a series of iron (III) N,N–dialkyldithiocarbamates, in which a ferric ion is octahedrally coordinated to six sulfur atoms. All the compounds showed a single spectrum at 300°K so that the rate of exchange between the high-spin and low-spin states must be greater than 107 sec− 1, and there was a large increase in resonant absorption on cooling to 77°K. The isomer shifts were large for covalent ferric ions (∼ + 0.4 mm sec− 1 with respect to iron at room temperature) and suggest that the metal s electrons are screened from the nucleus by electron donation from the ligands. Iron (III) pyrrolidyldithiocarbamate (S = 52) has long electron spin relaxation times at 4.2°K and its spectrumshows a hyperfine field of 430 kG and a small quadrupole splitting [12(e2Qq) = 0.20 mm sec−1] which results from an elongated distortion. The other compounds studied show relaxation effects though with shorter times, and have larger and more temperature-dependent quadrupole splittings (∼0.78 mm sec− 1 at 4.2°K). The decrease in quadrupole splitting observed as the temperature increases is partly due to the increasing population of the A16 state. Spectra in magnetic fields show the electric field gradient to be positive and perpendicular to the axis of magnetization.
Lattice Dynamics of Imperfect Alkali Halides
Authors: Benedek Giorgio , Nardelli G. F.
Published: February 2004

In the framework of the T-matrix formalism the generalized phonon dispersion relations for a crystal containing a finite concentration of randomly distributed defects are introduced. The Green functions and the resonance conditions are given for the nearest-neighbor perturbation model of defects in NaCl-type lattices. Systematic calculations concerning monovalent positive and negative defects in Na, K, and Rb chlorides, bromides, and iodides are then performed in the framework of Hardy's deformation dipole (dd) model for the host lattice dynamics, and applications to the U center are given. In the same framework the nearest-neighbor effective force constant is defined as a function of the interionic distance and calculated for all the family of alkali halides. The results for several defects in NaCl and KCl are compared and found to be in good agreement with thermal conductivity data. Predictions concerning the offcenter systems are presented. Finally, theoretical acoustic dispersion curves, phonon relaxation rates, and infrared absorption spectra for copper- and silver-doped NaCl crystals are also reported.
Optical Spectrum of Cr3+ Ions in Spinels
Authors: Wood D. L. , Imbusch G. F. , Macfarlane R. M. , Kisliuk P. , Larkin D. M.
Published: February 2004

The optical absorption and fluorescence spectra of two spinels, ZnAl2O4:Cr3+ and MgAl2O4:Cr3+ have been measured at various temperatures between 2°K and room temperature. The data have been fitted to a crystal-field theory which includes spin-orbit coupling and the trigonal field of the D3d site. The parameters for a best fit for both crystals were: Δ = 18 250 cm−1, B = 700 cm−1, C = 3200 cm−1, ζ = 250 cm−1, ν = −200 cm−1, ν′ = −1700 cm−1, and k = 0.7. The E2 splitting was directly observed optically for both spinels, and in the case of the ZnAl2O4:Cr3+, the A24 splitting was also observed. In addition to fitting the energies and zero-field splittings, the theory gave values of absolute R-line intensities and lifetimes which agree well with observed values. Vibrational sidebands and pair lines observed in fluorescence were identified.
Improved Quantum Theory of Many-Electron Systems. IV. Properties of GF Wavefunctions
Authors: Goddard William A.
Published: February 2004

The GF method for obtaining accurate many-electron wavefunctions was described in a previous paper. In this paper, some of the properties of this method are explored, and it is shown that the Hellmann–Feynman, Koopmans', and Brillouin's theorems apply to GF wavefunctions. Calculations are reported on Li2, CH4, and CH3 in order to demonstrate some aspects of the method.
Interaction of Nitrogen with Mo and the Reaction 28N2 + 30N2⇆229N2 Catalyzed by Mo and Observed by Mass Spectrometer
Authors: Moore George E. , Unterwald F. C.
Published: February 2004

This paper describes interactions between N2 and a Mo catalyst, believed significant for synthesis of ammonia. These interactions were sorption, desorption, and isotopic exchange using the same ribbon filament catalyst S as for exchange in hydrogen and for ammonia synthesis; it had previously been used for thermal dissociation of H2. Nitrogen dissolves in S at all temperatures significant for NH3 synthesis. Below 1000°K it also forms the nitride. 30N2 was used where possible to avoid mass ambiguity with CO. 14N and 15N compete in sorption, both on the surface and interior. During the exchange, the three molecular nitrogens desorb from S in equilibrated abundances at all accessible temperatures (T's) and covers. However, accessible T is limited on the low side because: (a) Nitrogen binds so tightly to Mo that puffs adequate for measurement will not desorb below 1000°K. (b) CO, which was not resolved from 28N2, is less tightly bound; it causes significant error in measuring convenient sized 28N2 puffs, unless T during sorption is sufficiently high to eliminate CO. Even more clearly than for exchange in hydrogen, nitrogen exchange by Mo brings into serious question tacit assumptions in much recent published work on adsorption and catalysis. These assumptions consider that all interactions occur on the surface of the catalyst (or adsorbent). For N2 exchange by Mo, diffusion into S was fully as decisive as for hydrogen exchange. Most published experiments employ an ionization gauge to measure pressure changes and also assume, without chemical identification, that pressure changes observed are caused entirely by gas intentionally admitted. Neglect of chemical identification may be responsible for failure to observe volume diffusion effects in recent literature. The measured probability for sorption per collision of N2 on Mo was 0.14 ± 0.04, which is considerably below values previously published.
Excimer Emission of Naphthalene, Anthracene, and Phenanthrene Crystals Produced by Very High Pressures
Authors: Jones Peter F. , Nicol Malcolm
Published: February 2004

Fluorescence spectra of ultrapure crystals of naphthalene, anthracene, and phenanthrene have been studied at pressures as great as 50 kbar. These spectra exhibit complex irreversible effects superimposed upon the expected reversible shifts in the energies of the normal fluorescence spectra. The irreversible effects include a loss of the intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The crystals continue to exhibit this excimer fluorescence after the pressure is reduced to atmospheric pressure, but the irreversible effects can be removed by thermal annealing of the crystal at atmospheric pressure. The irreversibility is attributed to trapping of pairs of molecules in an excimer-like orientation as a crystal defect after they lose the excitation energy of the excimer. These defects then act as traps for excitation energy in the crystal. Other possible interpretations of the irreversible effects are discussed.
Spectral Shifts and Broadening of the Fluorescence of Anthracene and Tetracene in Several Host Crystals at High Pressures
Authors: Jones Peter F.
Published: February 2004

The fluorescence spectra of anthracene in biphenyl, p-terphenyl, naphthalene, and phenanthrene and those of tetracene in p-terphenyl, naphthalene, phenanthrene, and anthracene were measured at pressures as great as 50 kbar. The pressure effects on the spectra of the guests are generally analogous to the pressure effects in solution or the gas phase. Due to the increased dispersion interaction between these nonpolar molecules upon compression, the energies of the electronic transitions are shifted toward the red (lower energy), and the spectral bands are broadened, while the integrated intensity remains approximately constant. As the pressure is released, the fluorescence spectra return to those obtained before compression. The spectral shifts with pressure are satisfactorily interpreted in terms of a second-order perturbation theory using a dipole–dipole approximation. For three of the eight guest–host combinations, the fluorescence spectra above 35 kbar were considerably broader than expected. The similar broadening observed for the pure host crystals suggests that mixed excimers are produced at high pressures. On the basis of the present data, however, it is not possible to give an unambiguous explanation of the extensive broadening.
Kinetics of Excited Molecules. VI. Energy Transfer from Hexafluoroacetone to Hexafluorobiacetyl
Authors: McIntosh John S. E. , Porter Gerald B.
Published: February 2004

In mixtures of hexafluoroacetone and hexafluorobiacetyl, both singlet–singlet and triplet–triplet energy transfer occur. Low pressures of hexafluorobiacetyl quench the phosphorescence of hexafluoroacetone and sensitized phosphorescence of hexafluorobiacetyl appears. The same phenomenon is observed with high pressures of hexafluorobiacetyl with regard to fluorescence quenching and sensitization. The bimolecular rate constants for energy transfer evaluated from these data and from the fluorescence and phosphorescence lifetimes are: 8.5 × 108 liters/mole·sec (triplet–triplet) and 6.0×109 liters/mole·sec (singlet–singlet).
Euler-Angle Reduction of the Schrödinger Equation for Three-Electron Atoms
Authors: Kalotas T. , Lloyd M. H. , Delves L. M.
Published: February 2004

A correction is given to a recent Euler-angle classification of the states of three-electron atoms. The non-relativistic Schrödinger equation for these states is written in terms of the Euler-angle coordinates and the spin–Euler-angle integrations carried out explicitly for a state of arbitrary (J, π).
Low-Frequency Vibrations of Water Molecules in Beryl
Authors: Safford G. J. , Naumann A. W. , Hallowell A. L.
Published: February 2004

The spectra of the water molecules in beryl samples of different origins and having differing types and amounts of alkali or alkali earth impurities were measured by neutron inelastic scattering and by infrared spectroscopy. Broad maxima were observed at 300–345 cm− 1 and at 170–195 cm− 1 and were associated with the hindered vibrations of two types of H2O molecules weakly bound at two sites in the cavities of the beryl structure. The relative population of the two sites is determined by the total alkali impurities present and not by the types of impurities. Additional frequencies were observed which were attributed to the water molecules vibrating at frequencies characteristic of the beryl lattice. Spectra for a sample which was successively heated to a series of temperatures in the range 500°–800°C indicated that the two types of H2O molecules were lost with essentially equal probability. The release rate of the water is probably determined by impurity cations blocking the openings of the channels.
Jahn–Teller Distortion: Magnetic Studies of Vanadium Tetrachloride
Authors: Johannesen Rolf B. , Candela George A. , Tsang Tung
Published: February 2004

Paramagnetic resonance of polycrystalline VCl4 diluted with TiCl4 has been observed near 9,24, and 36 GHz below 9°K. Both g values and hyperfine A tensors are axially symmetric and temperature independent with the same principal axes: g‖ = 1.920, g⊥ = 1.899, | A‖ | = 72 G, | A⊥ | = 120 G. The average g value obtained from susceptibility data is in agreement with the resonance value. The paramagnetic relaxation time T1 varies with temperature as shown by the relation T1 = 2 × 10− 9exp(25/T) sec, indicating the presence of a potential barrier approximately 18 cm− 1 in height. The results indicate a static Jahn–Teller distortion, but with large zero-point vibrations. This situation is in between Ballhausen–de Heer's ligand field and Ball-hausen–Liehr's crystal-field calculations.
Two-Exciton States of the Rigid-Lattice Model of a Molecular Crystal
Authors: Philpott M. R.
Published: February 2004

A theory is developed which can treat Frenkel and Wannier excitons, weak vibronic coupling, and exciton–exciton reactions for the rigid-lattice model of a molecular crystal. The treatment assumes tight-binding wavefunctions but allows for electron exchange contributions to interaction matrix elements. States belonging to different two-particle channels are assumed to couple only through single-exciton states so that direct interaction between channels is ignored. By two-particle states we mean states containing an electron and hole, or a vibronic and vibrational exciton, or two vibronic excitons. The coupling of Frenkel and Wannier excitons to each other and to the conduction band is briefly discussed. A previously derived theory of weak vibronic coupling is extended to include phonon propagation and states of the isolated molecules which are not Born–Oppenheimer separable. Finally, for the two-exciton system a Green's function expression for the energy levels is given.
Re-examination of the Theoretical Interpretations of the Spectra of Crystalline Benzene and Naphthalene
Authors: Greer William L. , Rice Stuart A. , Jortner Joshua , Silbey Robert
Published: February 2004

In order to explain the crystal spectra of naphthalene and benzene, a theory is presented in which the combined effects of charge delocalization and interactions via transition octupole moment coupling are important. Charge delocalization, achieved by means of configuration interaction of the lowest crystal charge-transfer states with the lowest singlet neutral exciton state, is predicted to explain most of the environmental shift and to contribute approximately one-third of the Davydov splitting. The transition-octupole–transition-octupole interactions, on the other hand, are predicted to induce the remaining two-thirds of the splitting and to determine the polarization ratios for the different Davydov components. Either of the two interactions alone shows drawbacks if it is used to explain the spectra, but when combined, the two interactions lead to a theory in good agreement with experiment.
Thermal Diffusion Flux Equation for Zirconium Hydride at High Hydrogen Contents
Authors: Barsell A. W.
Published: February 2004

The thermal diffusion mechanism for hydrogen in zirconium is studied in terms of a simple kinetics model appropriate for nondilute as well as dilute solutions. The equations resulting from this model are correlated with available experimental data for zirconium hydride at high hydrogen contents. The results demonstrate that the thermal diffusion flux term for a concentrated solution must contain a factor to account for the nonavailability of interstitial sites. This factor is essentially proportional to the probability of an individual atom finding a vacant site on an adjacent lattice in which to jump. For δ- or ε-phase zirconium hydride, this factor is given with reasonable accuracy by (2–H/Zr)/3.25.