bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Verification, with Krypton, of the Theory of the Thermal-Diffusion Column for Multicomponent Systems
Authors: Roos W. J. , Rutherford W. M.
Published: February 2004

Thermal-diffusion column transport equation coefficients were measured for the multi-isotope gas krypton. Measurements were made in a column of the hot-wire type for both static and flow conditions, at hot-wire temperatures of 623°, 773°, and 1073°K. The agreement between the theoretical and experimental values of the coefficients was reasonably good in all cases. The average ratio of the experimentally determined values of the reduced initial transport coefficient to the theoretically calculated value was 1.09 ± 0.11; the similar average ratio for the convective remixing coefficient was 1.11 ± 0.12; the average ratio for the diffusive remixing coefficient was 1.04 ± 0.06. The theoretical prediction that the reduced initial transport coefficient should be independent of the choice of isotope pair was also verified.
Intensities of Eu3+ Fluorescence and Absorption Spectra
Authors: Gashurov G. , Sovers O. J.
Published: February 2004

Intensities of a number of electronic transitions of Eu3+ in solids are investigated within the framework of the usual theory of electric and magnetic dipole transition probabilities. It is found that variation in the “free-ion” part of the electronic structure of the ion is unlikely to have significant repercussions on the changes of transition probabilities among different host lattices. The theoretical explanation of intensity variations must therefore be sought in the odd-order crystal-field parameters.
Additional Quantum Effects in Atom–Atom Scattering: Higher-Order Glory Scattering
Authors: Helbing Reinhard K. B.
Published: February 2004

In atomic scattering theory the phase shift η has often been treated as a continuous function of angular momentum l, in order to replace summation over l by integration. However, because l is quantized, additional (higher-order) forward and backward glories may occur. The effect is most pronounced in like-particle scattering, where “hard-sphere” glories show up in total cross section. General formulas are derived and applied to like particles. For comparison, exact quantum calculations are presented. A criterion for the applicability of the continuous phase approximation is derived. Construction of phase-shift curves from experimental data only is discussed.
Intensity Perturbations Due to Configuration Interaction Observed in the Electron Energy-Loss Spectrum of N2
Authors: Geiger J. , Schröder B.
Published: February 2004

The electron energy-loss spectrum of 25-keV electrons in molecular nitrogen has been re-investigated in the 12.5–14.9 eV energy-loss range with improved energy resolution of 10 meV. The band intensities of the b and b′ progressions, which consist of the bands j, b, m, p, q, and b′, g, f, r, s, t as recently turned out, exhibit strong perturbations due to configuration interaction with the c and c′ state, respectively. The disturbed intensity in the progressions is quite analogous to the one in the two-electron excitation region of helium. Intensity perturbations in the o series, if any, should be very weak.
Inter- and Intramolecular Processes in Organic Solutions under Excitation in the Vacuum Ultraviolet
Authors: Laor U. , Weinreb A.
Published: February 2004

The efficiency of energy transfer from solvent to solute is investigated as a function of solute concentration and excitation wavelength for solutions of diphenyloxazole in benzene, toluene, cyclohexane, and hexane. The results for benzene and toluene are similar but different from those for cyclohexane and hexane. For the first pair of solvents energy transfer takes place almost exclusively from the first excited level for all wavelengths of excitation; only for higher acceptor concentrations does the direct transfer from higher levels become noticeable. The transfer efficiency from the first excited level depends, however, strongly on the electronic level which has been initially excited. The effect of dilution of the transferring solvent is studied. For cyclohexane and hexane two or more transfer mechanisms or transferring species seem to be significantly involved even for low acceptor concentrations. The quenching effect of carbon tetrachloride and of chloroform is investigated for benzene and toluene and that of oxygen for all four solvents. While quenching of benzene and toluene by oxygen is primarily “dynamic” the quenching effect in cyclohexane and hexane is “static.” The results are compared with available results for excitation with high-energy radiation.
Radiative Lifetime of the B 2Σ+ State of CO+
Authors: Fowler Richard G. , Skwerski Peter R. , Anderson Richard A. , Copeland Gary E. , Holzberlein Thomas M.
Published: February 2004

The lifetime of the B 2Σ+ state of CO+ was measured by noting the decay time of 11 band heads of the (0, 0), (0, 1), (1, 0), and (1, 2) electronic–vibrational transitions of the Baldet–Johnson system. The lifetime exhibited no pressure dependence and no variation of the lifetime was noted for different rotational or vibrational levels. The lifetimes of the υ′ = 0 and υ′ = 1 states are 51.0 ± 0.7 and 52.2 ± 1.8 nsec, respectively. These lifetimes are in agreement with other recent measurements.
Vibronic Coupling. VI. Vibronic Borrowing in Cyclic Polyenes and Porphyrin
Authors: Perrin Marilyn H. , Gouterman Martin , Perrin Charles L.
Published: February 2004

Vibronic borrowing in closed-shell aromatics is investigated by means of the cyclic-polyene model with the π electrons treated according to simple Hückel theory and the σ electrons treated as outlined by Longuet-Higgins and Salem. The basis set for the vibronic wavefunctions consists of single-product Born–Oppenheimer functions whose electronic factors are LCAO cyclic-polyene molecular orbitals. The coupling between these Born–Oppenheimer functions is treated by means of perturbation theory. The dependence of the vibronic coupling on the size of the aromatic is given. Intensities of the vibronic transitions in benzene, coronene, triphenylene, 18-annulene, and porphyrin are calculated. Porphyrin is approximated by a 16-membered cyclic polyene with 18 electrons, and the absorption spectra, fluorescence polarization, and Zeeman spectra are interpreted in terms of vibronic interactions. The Jahn–Teller distortion vanishes in these cases, a result shown to be a general phenomenon for one-electron terms in E states arising from two e electrons or holes.
Moments of Chain Vectors for Models of Polymer Chains
Authors: Jernigan R. L. , Flory P. J.
Published: February 2004

Four model chains, namely, the freely jointed chain (1), the freely rotating chain (2), the Porod–Kratky wormlike chain (3), and the model with independent bond rotations hindered by a symmetrical potential (4), are compared with a realistic isomeric-state representation (5) of the polymethylene chain through calculations of various properties of their distributions. Parameters used for Models 1–4 were adjusted to secure agreement with 5 at the infinite chain limit. Properties treated are the mean-square end-to-end distance, its temperature derivative, the mean-square radius of gyration, and higher even moments of the end-to-end distance. Convergences to asymptotic ratios with chain length are examined. None of the simpler models 1–4 adequately represents these properties of the realistic chain 5 at all chain lengths. The success of any one of the models 1–4 in accounting for the dependence of a given property on chain length varies considerably from one property to another.
Magnetothermodynamics of MnCl2·4H2O. I. Heat Capacity, Entropy, Magnetic Moment, from 0.4° to 4.2°K with Fields to 90 kG along the c Crystallographic Axis
Authors: Reichert T. A. , Giauque W. F.
Published: February 2004

The heat capacity and magnetic moment of a 3.934-cm-diam spherical single crystal of MnCl2·4H2O have been measured over the range 0.4°–4.2°K, with stabilized magnetic fields of 0, 5, 7.5, 10, 14, 18, 25, 40, 65, and 90 kG, along the c crystallographic axis. At the lower temperatures, magnetic saturation was attained at 90 and 65 kG. The temperature-dependent saturation value was found to be 28 076 G·cm3/mole, corresponding to a gc = 2.011. At 90 kG the several nuclear spins were found to contribute a heat-capacity term equal to (6.0 × 10−3) / T2gibbs/mole. The amount of energy required to remove a quantum of angular momentum from the saturated condition at 90 kG was found to be 22.9 cal/mole. This is less than gβH = 24.16 cal/mole, the equivalent required magnetic work at 90 kG, by 1.3 cal/mole, which is the amount of stored internal energy contributed by the saturated condition for this limiting process and indicates antiferromagnetic interactions. Using magnetic saturation at 90 kG to locate a zero entropy reference for the electronic and lattice systems, the entropy was evaluated over the above range of field and temperature. Temperature–field observations on a large number of isentropes were used to correlate the entropies along the various isoerstedic heat-capacity curves. The upper limit of the electronic entropy was found to be 3.559 gibbs/mole, compared to the amount R ln6 = 3.561 gibbs/mole expected for an S = 5 / 2 system. Smoothed values of the heat capacity, entropy, enthalpy, internal energy, magnetic moment, and the magnetic work have been tabulated.
Dissociation of Polyatomic Molecules and the Relation of Ensuing Geometrical Change to the Force Constants
Authors: Machida Katsunosuke , Overend John
Published: February 2004

As a triatomic molecule dissociates to give a diatomic molecule and an atom, the length of one bond must change as a function of the extension of the other, becoming eventually the length of the bond in the diatomic molecule. If this change is assumed to be monotonic, there follow two rules: Both bonds in the triatomic molecule must be longer, or both must be shorter, than the bonds in the diatomic molecules, and, if they are both longer, the quadratic interaction force constant is positive, otherwise it is negative. These rules are tested with a number of experimental data. The rules are also extended to more-than-three atomic molecules.
Investigation of Low-Frequency Motions of H2O Molecules in Ionic Solutions by Neutron Inelastic Scattering
Authors: Safford G. J. , Leung P. S. , Naumann A. W. , Schaffer P. C.
Published: February 2004

Intermolecular frequencies below 900 cm−1 of H2O molecules in water and aqueous solutions of LaCl3, CaCl2, MgCl2, CsI, CsCl, KSCN, KI, KBr, KCl, KF, NaCl, LiNO3, and LiCl have been measured by slow neutron inelastic scattering. The diffusive kinetics of H2O molecules in aqueous solutions of LaCl3, MgCl2, CsCl, KSCN, KCl, KF, NaCl, and LiCl have also been investigated. Changes were observed simultaneously in both the intermolecular frequencies and the diffusive motions of H2O in ionic solutions relative to water, and they were specific to the size and charge of the ions, the concentrations, and the temperature. Ionic solutions containing a small and/or highly charged ion (e.g., LaCl3, MgCl2, LiCl, and KF) showed vibrational maxima at frequencies similar to rocking, wagging, and twisting librational modes and to ion–water stretching modes of H2O molecules in the corresponding solid salt hydrates. These maxima intensify with increasing concentration. In general, these frequencies lose intensity and broaden with increasing temperature, but many persist to 75°C. Solutions containing large singly charged ions (e.g., CsCl, KCl) also show new frequencies at lower temperatures, but these are generally broader and weaker than those characteristic of smaller or highly charged ions. Many of these maxima appear better defined at higher temperatures. At lower temperatures (1° and 25°C) the diffusive motions of water molecules in most salt solutions are in accord with a delayed diffusion model. Small and/or highly charged ions decrease the self-diffusion coefficient D and increase the residence time τ0 relative to water. D increases and τ0 decreases with increasing temperature, but they remain smaller and larger, respectively, than for pure water. At higher temperatures the diffusion kinetics depart from delayed diffusion behavior. The neutron spectra indicate that ions of high charge-to-radius ratio disrupt the water structure and form complexes having local ordering and bonding similar to that of H2O molecules in solid salt hydrates. These strong ion–water interactions give rise to higher activation energies for the movement of H2O molecules relative to pure water; thus, these salts act as “positive hydrators.” In contrast, salts of ions of low charge-to-radius ratio increase D and decrease τ0 relative to water and act as “negative hydrators.” The values of the intermolecular frequencies, diffusion coefficients, and residence times obtained during this study are in agreement with those obtained by other techniques. A tentative explanation for the diffusion kinetics is given for cases where the delayed diffusion model is not valid.
On the Orientations of the Water Molecules in Rochelle Salt: A Proton Magnetic Resonance Study
Authors: El Saffar Z. M.
Published: February 2004

The angular dependence of the NMR spectrum of single crystals of NaKC4H4O6·4H2O has been studied in eight planes of rotation with a view to establishing the orientations and proton–proton separations of the water molecules. The proton–proton vectors corresponding to (H2O)7, (H2O)8, and (H2O)9 have been uniquely determined and the related H coordinates calculated, but the vector corresponding to (H2O)10 has not been determined as accurately. The experimental results are found to agree well with a previous two-dimensional neutron-diffraction study and with a recent deuteron magnetic resonance study of this compound in its orthorhombic modification.
Electronic Structure of LiH According to a Generalization of the Valence-Bond Method
Authors: Palke William E. , Goddard William A.
Published: February 2004

The wavefunction of LiH has been calculated according to a generalization of the valence-bond method, called the G1 method, which leads to significantly better energies than the Hartree–Fock method, yet retains an independent-particle interpretation. The total energy of the LiH G1 wavefunction is − 8.017 a.u., which accounts for 36% of the difference between the Hartree–Fock and experimental energies. The G1 molecular orbitals, which are discussed in detail, correspond closely to chemists' intuitive concepts of electron orbitals and display bonding properties more clearly than do the Hartree–Fock orbitals. In particular, the bonding orbitals are nodeless between the nuclei and, when compared to the corresponding atomic orbitals, show increased amplitude over essentially the entire internuclear region. Finally, several one-electron properties calculated from the G1 wavefunction are presented.
Nonexistence of 1sns Autoionizing States of H−
Authors: Wallis Anthony , McElwain D. L. S. , Pritchard H. O.
Published: February 2004

The convergence and coalescence properties of a two-electron wavefunction containing integral powers and exponential terms in r1, r2, and r12 have been investigated. Solutions were obtained for the singlet and triplet 1sns states for both integral and nonintegral values of the nuclear charge Z, and it is concluded that the 1sns manifold converges to a degenerate eigenvalue as Z→1 for all n≥2.
Isotherms of Nitrogen to 400°C and 10 000 bar
Authors: Robertson Stanley L. , Babb Stanley E.
Published: February 2004

Isothermal measurements of the density of nitrogen between pressures of 2000 and 10 000 bar have been carried out between 35° and 400°C. The results are presented and a comparison with other workers is discussed. Some coordinates for the curve (∂Z / ∂T)υ = 0 are given.
Distribution of Reaction Products. V. Reactions Forming an Ionic Bond, M+XC (3 d)
Authors: Kuntz P. J. , Mok M. H. , Polanyi J. C.
Published: February 2004

In the previous paper (Part IV) a preliminary investigation was made of the electron-jump model for reactions M+XC[→M++(XC)−]→products, in which an alkali-metal atom M reacts with a halogen molecule (XC≡XY) or with a halide, to give alkali halide. The method used was the classical trajectory method in two dimensions (2d). In the present work some of these early calculations have been extended into 3d, and comparison has been made between theory and recent molecular-beam experiments. The 12 principal potential-energy hypersurfaces used in this 3d work comprised six which permitted charge migration and six which, though identical in other respects, did not permit charge migration. The groups of six exemplified the effects of separate variation in electron-jump distance, r1x, in the magnitude of the B·C repulsion in (BC)−, Ri, and in the total energy released, QC. All but the last of these variables had important effects on the product energy and angular distributions. It was found that with a proper choice of r1x and Ri, the crude electron-jump model used here was able to account very satisfactorily for the major features of the experimental findings. Sharp forward scattering of the molecular product was one of the features of reactions M+XY, both experimentally and theoretically. The mechanics of this forward scattering, according to the present work consist of an initial M+- -X− attraction, followed by charge migration to give M+Y−+X. The rejected atom is not, therefore, a “spectator,” but is subject to strong forces which enhance its translational energy. A qualitatively similar correlation between forward scattering and increased relative translational energy of the products has been observed experimentally by Birely and Herschbach. In addition, the calculations predict enhanced rotational excitation in the forward-scattered portion of the molecular product; experimental evidence is lacking on this point. The experiments show a striking change from rebound to forward scattering in going from M+XR (where R is an alkyl radical) to M+XY. This change in behavior stems, according to the present work, from the occurrence of charge migration coupled with markedly diminished product repulsion in the M+XY as compared with the M+XR family. A simple analytical model termed the DIPR (Direct Interaction with Product Repulsion) model was shown to account quantitatively for product energy and angular distributions in direct trajectories without secondary encounters. The model provides insight into the transition (on nonmigration surfaces) from backward to sideways to forward scattering as product repulsion is decreased, and shows the utility of the concept of a “stripping threshold energy.”
Production of Hydrogen Atoms by Photolysis of H2S and the Rates of Their Addition to Olefins
Authors: Woolley G. R. , Cvetanović R. J.
Published: February 2004

Photolysis of H2S at 2490 Å has been used to generate H atoms and redetermine the rates of their reaction at room temperature with several selected olefins. The initially produced “hot” hydrogen atoms were thermalized by additions of a large excess of carbon dioxide. The relative rate constants obtained are compared with the earlier determinations and with another independent recent redetermination in this laboratory. The values obtained by the three independent techniques are in very good agreement.
Origin of Photocarriers in Phthalocyanines
Authors: Harrison S. E.
Published: February 2004

The generation of photocarriers in copper and metal-free phthalocyanines (CuPc and H2Pc, respectively) is shown to be an extrinsic process, i.e., the result of absorbed oxygen impurities. ir Photoconduction found in CuPc is absent in H2Pc. This suggests that the ir response may arise from the singlet–triplet absorption which is enhanced by the spin–orbit coupling of the Cu ion. The photocurrent is proportional to (light intensity)s, where s is the same constant for ir and visible (intrinsic) absorptions. This constancy supports the interpretation that both singlet and triplet excitons generate carriers via interaction with the same trapping centers.
Coupled-Channels Approach to the Quantum-Mechanical Treatment of Chemical Exchange Reactions
Authors: Diestler Dennis J.
Published: February 2004

A formalism for the quantum-mechanical treatment of chemical exchange reactions of the type A+BC→AB+C based on the coupled-channels technique of solving the Schrödinger equation is developed. Essentially, the method consists of expanding the stationary-state wavefunction describing the reaction as a linear combination of linearly independent functions satisfying the relevant Schrödinger equation and also initial conditions specified on a “surface” of a particular arrangement channel. These linearly independent functions are generated by integrating the close-coupled equations throughout the various arrangement channels successively for linearly independent sets of initial conditions. The formalism is explicitly derived for collinear, electronically adiabatic encounters below the three-particle threshold. It is then applied to a simplified model for symmetric exchange reactions in which the center atom is very massive. The calculated transition and reaction probabilities are in agreement with recently reported results for the same model.
Chemisorptive Luminescence
Authors: McCarroll Bruce
Published: February 2004

The chemisorption of each of the gases oxygen, nitric oxide, and carbon monoxide on a clean, polycrystalline tungsten ribbon is accompanied by a faint luminescence in the visible. This effect is not observed during the chemisorptions of carbon dioxide, hydrogen bromide, hydrogen, or nitrogen. The effect, when it does occur, is a small one, corresponding to photon emission for one out of every 109 CO or NO adsorptions and one out of every 107 O2 adsorptions. The decay of this self-excited luminescence does not follow first-order kinetics; it persists for many seconds with hyperbolic time dependence. No luminescent behavior is observed during electrochemical adsorption and desorption of oxygen.
Dynamical Langevin Equation for a Simple Polar Fluid
Authors: Harris S.
Published: February 2004

We consider a dilute system of heavy (Brownian) polar molecules dissolved in a solvent of lighter molecules. Using a mass ratio expansion technique, equations of motion for the Brownian particle are obtained which have the form of coupled Langevin equations for the translational and rotational motion. Expressions for the friction coefficients in terms of the molecular properties are also obtained.
Pulse-Radiolysis Study of the Formation of Benzyl Radicals from Benzyl Chloride in Cyclohexane in the Gas Phase
Authors: Johnson G. R. A. , Sauer Myran C. , Warman John M.
Published: February 2004

The yield of benzyl radical (Bz) formed by electron-beam pulse radiolysis of benzyl chloride (BzCl) in cyclohexane vapor (1.3 atm/120°) has been studied by optical absorption measurements. Electron scavengers lower the benzyl radical (Bz) absorption by competing with dissociative electron capture by BzCl. A study of this competition gave the relative rate constants for electron capture at 120°C (SF6 = 1) : BzCl = 1.6 × 10−3, CCl4 = 0.3, CH3Br = 5 × 10−5, C6H5NO2 ∼ 2 × 10−2. The maximum depression of the absorption, Δ(Bz)max, which was independent of the electron scavenger used and of the BzCl concentration (0.2 to at least 4 mole %), is believed to correspond to the yield of Bz from dissociative capture. With benzyl bromide, acetate, and formate in cyclohexane, Δ(Bz)max was the same as with benzyl chloride. Bz is also formed by processes other than dissociative capture which are not influenced by electron scavengers. The yield from these processes increases with increasing BzCl concentration. Using electron beam radiolysis, the decomposition of BzCl due to dissociative capture, measured by chemical analysis, G(− BzCl)e = 2.6 ± 0.4 molecules per 100 eV. 60Co γ-radiolysis gave G(− BzCl)e = 4.3 ± 0.1, which is close to the value expected if all of the electrons react by dissociative capture. Possible reasons for the low value with electron radiolysis are discussed.
Derivatives of the Classical Deflection Function
Authors: Ioup G. E. , Thomas B. S.
Published: February 2004

Integral expressions have been derived for the second and third derivatives with respect to the impact parameter of the classical deflection function. These are then given in a useful form for numerical quadrature and their evaluation is discussed. The expressions are investigated for their behavior as various quantities become zero, and in particular, formulas valid at zero impact parameter are obtained. Tables of results are presented for (12,6) and (12,4,6) potentials, the former given by V(r) = 4ε[(σ / r)12 − (σ / r)6], and the latter by V(r) = (ε / 2) [(1 + γ) (rm / r)12 − 4γ(rm / r)6 − 3(1 − γ) (rm /r )4], in which ε, σ, rm, and γ are adjustable parameters.
Canted Antiferromagnetism in Gadolinium Tribromide
Authors: Varsanyi F. , Andres K. , Marezio M.
Published: February 2004

Anhydrous GdBr3 has a monoclinic layer structure. The arrangement is pseudohexagonal with “honey-comb” layers of Gd ions separated by two hexagonal bromine layers. Canted antiferromagnetic ordering of the Gd moments is observed below 2°K with a net remanent moment normal to the c axis of the pseudohexagonal structure.
Molecular Schrödinger Equation. IX. Square and Rectangular States of H4 and the Molecular Ions H43+ and H42+
Authors: Conroy Harold , Malli Gulzari
Published: February 2004

Our method for the solution of the molecular Schrödinger equation has been further developed and extended to the case of the four-electron system. Ab initio calculations are presented which lead to the electronic wavefunctions and energies of square and rectangular forms of H4. As a preliminary to the four-electron studies, the energies and wavefunctions for a number of states of the ions H43+ and H42+ are calculated. Complete contour maps for the potential energy surfaces of these systems are reported and the H4 surface is discussed in connection with the bimolecular isotope exchange reaction H2+D2→2HD. Contour maps for the electron density distribution in square and rectangular H4 are presented and discussed.
ESR Study of the Kinetics of the Reaction of H Atoms with Methane
Authors: Kurylo Michael J. , Timmons Richard B.
Published: February 2004

A wide-temperature-range study using ESR atom detection has been employed to measure the rate of the reaction H + CH4→H2 + CH3. Over the temperature range 426°–747°K, we obtain a specific rate constant for the above reaction of k1 = 6.9 ± 0.6 × 1013exp[(−11 800 ± 200) / RT] expressed in units of cubic centimeters per mole per second. The value obtained in the present work is compared to a number of other results obtained by various workers. The activation energy we observe is considerably higher than previous values obtained in this temperature range. However, our results coupled with the heat of the reaction predict an activation energy for the reverse reaction which agrees well with experimental values. In addition, the pre-exponential factor we obtain agrees with absolute rate theory predictions as well as with entropy considerations. We do not agree with literature results which give very low pre-exponential factors suggesting steric factors of the order of 10−3–10−5 for this reaction.
Reactions of 2.8-eV Tritium Atoms with Methane
Authors: Chou C. C. , Rowland F. S.
Published: February 2004

Tritium atoms with 2.8-eV kinetic energy have been formed by 1849-Å photolysis of TBr, and have been reacted with CH4 in the presence of Br2. In addition to HT from the abstraction reaction known in thermal systems, the direct substitution of T for H to form CH3T has been observed. The ratio of HT/CH3T is 3.6 to 4.0 in low Br2 mole fraction. In Br2 mole fractions exceeding 0.5, this ratio is reduced to 3.0, and the absolute yields of both HT and CH3T are greatly reduced. The formation of HT and CH3T from hot reactions of T with CH4 competes with the hot reaction of T with BR2 to form TBr.
Interaction of Two Metastable Triplet Helium Atoms
Authors: Klein D. J.
Published: February 2004

Ab initio quantum-mechanical calculations have been performed for the Σg+5, Σu+3, and Σg+1 states arising from the interaction of two 1s2s3S helium atoms. A rigorous upper bound to the quintet potential was found by use of the variational principle. Crude estimates of the autoionizing singlet and triplet states were obtained by the “stabilization” method. The quintet-state potentials exhibit a van der Waals minimum, and the triplet and singlet potentials exhibit more substantial minima. The behavior of the wavefunctions in the separated-atom limit was studied, and the range of applicability of the effective Heisenberg spin Hamiltonian was tested on the calculated potentials.
Optical Properties of Polymers: Model Calculations for Dinucleoside Phosphates
Authors: Schneider Allan S. , Harris Robert A.
Published: February 2004

The optical properties (absorption, polarizability, circular dichroism, and optical rotatory dispersion) are calculated for models of dinucleoside phosphates using the time-dependent Hartree theory with a simple free-electron monomer model. Detailed results over the entire frequency spectrum have been obtained for a variety of dimer and polymer structures. The relation between the following conformational effects and the resulting polymer optical properties were determined: base stacking and unstacking, helical winding and unwinding, and changes in distance between monomer planes. In addition, several other optical effects were calculated: the relative contributions of nearest and nonnearest neighbors along an infinite helical polymer; the effect of band half-width on the observable splitting; and the relation between monomer and polymer line shape. The calculated results predict reasonable quantitative values as well as the main qualitative features of the experimental dinucleoside phosphate and polyribonucleotide spectra. The stacking–unstacking calculations are consistent with the experimental temperature dependencies of the hypochromism and optical rotation of ApA, and the right-handed-stacked conformation of this dimer is confirmed.
Configuration Interaction in the Simple Valence-Bond Wavefunction for the Potential-Energy Surfaces of Sigma-Bonded Four-Center Exchange-Reaction Complexes
Authors: Porter R. N. , Raff L. M.
Published: February 2004

The two closed-shell and nine open-shell symmetry-adapted MO electron configurations contained in the simple nonionic VB wavefunction for H4 in the limit of 2H2 with D2h symmetry and in a D4h complex are identified and their coefficients are obtained in terms of overlap integrals. Numerical comparison of the configuration coefficients obtained from the nonionic VB wavefunction with those obtained by the full CI calculation of Wilson and Goddard show that the ionic contribution to the D4h H4 complex of side 2.54a0 is small in the ground state but significant in the upper state. The implications of a VB or CI treatment of four-center potential-energy surfaces regarding a recent extension of the Woodward–Hoffmann rules (conservation of orbital symmetry) to the reactions H2 + D2→2HD and H2 + I2→2HI are discussed.
Direct Enumeration Study of Self-Avoiding Walks on the Tetrahedral Lattice
Authors: Kumbar Mahadevappa , Windwer Stanley
Published: February 2004

By application of a general procedure devised by Martin, we generated, up to N = 14, the number of self-avoiding open chains, and determined their mean-square end-to-end distance, their radius of gyration, the number of returns to the origin, and its corresponding mean-square end-to-end distances. The self-avoiding chain results were in excellent agreement with Monte Carlo calculations, and the mean-square radius of gyration of ring systems agreed with our previous Monte Carlo estimates. The number of returns to the origin was used to calculate the order of a phase transition for a tetrahedral model of the helix-to-random-coil system. The higher-order transition found is the same as that previously obtained by Fisher for other three-dimensional model systems.
Optical, EPR, and ENDOR Studies of CdF2:V3+, V2+
Authors: Borcherts R. H. , Lohr L. L.
Published: February 2004

Single crystals of CdF2 containing either V3+ or V2+ ions have been prepared and studied by optical, EPR, and ENDOR techniques. As expected, the properties of V3+ in the eight-coordinated (cubal) environment closely resemble those of Ni2+ in octahedral sites. The ENDOR data show that Ms = ± 1, confirming the identification of the ion as V3+. The V2+ ions, produced by heating samples containing V3+ in Cd vapor, exhibit evidence for a large trigonal Jahn–Teller distortion of the cubal site. On the basis of the EPR and optical data it is suggested that the environment about the V2+ ions may be more nearly octahedral than cubal, thus reflecting the presence of a static trigonal field comparable in magnitude to the (undistorted) cubic field. Some optical and EPR data are presented for isoelectronic Cr3+ ions, which also display a strong Jahn–Teller effect.
Molecular Charge Distributions and Chemical Binding. IV. The Second-Row Diatomic Hydrides AH
Authors: Cade Paul E. , Bader R. F. W. , Henneker W. H. , Keaveny I.
Published: February 2004

An interpretation of the binding in the second-row diatomic hydrides NaH, MgH, AlH, SiH, PH, SH, and HCl is presented based on the molecular charge distributions and the forces exerted on the nuclei. The total density distributions are discussed in relation to “molecular” size and the arbitrary partitioning of the total charge between different spatial regions. Density difference maps are employed to compare the Hartree–Fock molecular charge distribution with the appropriate Hartree–Fock separated-atom charge densities and also the corresponding Hartree–Fock united-atom charge density. In addition, for SiH, PH, SH, and HCl, two-center Hartree–Fock molecular charge distributions are compared with extensive one-center charge distributions. The molecular orbital charge densities are classified as binding, nonbinding, or antibinding on the basis of their partial contributions to the force acting on each nucleus. The orbital forces provide a quantitative assessment of the relative binding abilities of the orbital charge densities for a given molecule, through a complete series of molecules, or between homologous series of molecules. In terms of the total charge distributions, the various density difference maps, and the partial forces exerted on the nuclei, a qualitative and quantitative comparison is made between the bonding in the first- and second-row hydrides. The bonding in NaH is classified as ionic, that in SiH, PH, SH, and HCl as covalent, and for the second row both MgH and AlH appear transitional between the limiting classifications of ionic or covalent. Particular attention is paid to the role of the large and diffuse K L-shell core on A and the Increased role of the proton in determining the details of the molecular charge distribution in the second-row hydrides. The latter two features account for the major differences in AH bonds between the first- and second-row hydride congeners.
On the Consistency of the Yvon–Born–Green Hierarchy and Its Truncations
Authors: Raveche Harold J. , Green Melville S.
Published: February 2004

The assumption of truncation of the hierarchy of reduced distributions for fluids at thermodynamic equilibrium is discussed. We consider the question of whether a truncated hierarchy yields expressions for the mean force on a molecule, in the presence of its neighbors, which are conservative. It is concluded that in general it does not, although the Kirkwood superposition and the infinite hierarchy do. The conclusion is reached that, unless the closure satisfies a certain condition, the truncated hierarchy in general has no solutions.
Higher Asymmetry in Optical Rotation
Authors: Chiu Ying-Nan
Published: February 2004

This study shows that optical rotation could arise from higher-order interference effects of two-photon processes in quantum electrodynamics. In the lowest-order limit, applicable to electric- or magnetic-dipole allowed transitions in small molecules, the conventional expression of angle of rotation proportional to Im(0| R |b)·(b| M |0) is attributable to the interference of one photon being in an electric-dipole (R) mode which has odd parity and the other photon being in a magnetic-dipole (M) mode which has even parity. The asymmetry stems from the opposite parities of the transition operators R, M, both transforming as tensors of the first rank. For application to larger molecules and forbidden transitions, we dispense with the usual refractive index and medium polarization formalism and derive, using quantum electrodynamics, the higher-asymmetry contribution to optical activity due to the interference of higher electric and magnetic multipoles of any order. It is shown that the perpendicularity of the polarization directions of the incident and scattered photon in the forward direction requires that nonvanishing interference be between an electric-multipole and a magnetic-multipole transition. Furthermore, random molecular arrangement requires that these multipoles transform as tensors of the same rank. The asymmetry then stems from the opposite parities of the electric and magnetic multipoles of the same (L) rank, being ( − )L and ( − )L−1, respectively. The derivation is given in detail showing the properties of the radiation field of polarized photons and the transformation properties of the multipole operators. The multipole tensor expansion of the radiative interaction in emission is given in the Appendix, in anticipation of its future use in multiphoton processes. The angle of rotation is expressed in terms of the matrix element over the scalar contraction of the electric and magnetic tensor operators. Such contraction is shown to transform as a pseudoscalar regardless of L. In the lowest limit of L = 1, the result agrees with conventional work and has the correct dimension of radians per centimeter. Specific examples of optical rotation due to the interference of electric quadrupole (even)–magnetic quadrupole (odd) and the interference of electric octopole (odd)–magnetic octopole (even) are given. The symmetry of these quadrupole and octopole operators in the C2υ and C3υ point group is derived. As asymmetric potentials inducing optical activity the pseudoscalar representations of functions in these point groups are also derived up to spherical harmonics of L = 6. The relative magnitudes of these higher-order interference effects and the physical situations in which these effects may be useful are discussed.
Low-Frequency Infrared and Raman Spectra of Hydrogen-Bonded Pyridinium Halides
Authors: Foglizzo R. , Novak A.
Published: February 2004

The infrared spectra of crystalline C5H5NH+Cl−, C5H5NH+Br−, C5H5NH+I−, and their deuterated derivatives at room and liquid-nitrogen temperature have been investigated in the 4000–1500- and 300–20-cm−1 ranges. The Raman spectra of these compounds have been examined below 300 cm−1. All six infrared-active and four of the nine Raman-active lattice modes have been observed for pyridinium chloride. The lattice frequencies of pyridinium halides have been assigned in terms of translational and rotational as well as hydrogen-bond vibrations. The hydrogen-bond stretching frequencies have been identified at 192, 133, and 110 cm−1 for chloride, bromide, and iodide, respectively. The relationships between the NH and (NH)···X stretching frequencies and shapes of the corresponding absorption bands are discussed.
Kinetics of Hydrogen Halides in Shock Waves. IV. HBr
Authors: Giedt R. R. , Cohen N. , Jacobs T. A.
Published: February 2004

Shock-tube studies were made of the thermal dissociation rate of HBr in Ar over the 2100°–4200°K temperature range, using ir emission and uv absorption techniques. It was necessary to invoke a low activation energy (compared to bond energy) in order to explain the HBr dissociation data. The rate constant found for HBr dissociation by Ar was 1012.19 exp(− 50 000/RT cc/mole·sec. It was also found that HBr is about 15 times more efficient than Ar as a collision partner in dissociating HBr.
Microwave Spectrum, Rotational Isomerism, and Internal Barrier Functions in Propionyl Fluoride
Authors: Stiefvater Otto L. , Wilson E. Bright
Published: February 2004

The microwave spectrum of propionyl fluoride [CH3CH2COF] proves the existence of two stable rotameric conformations. For the cis form (methyl eclipsing oxygen) with a planar heavy-atom skeleton, rotational constants A = 10 042.53, B = 3762.19, C = 2832.69 MHz and a dipole moment of μtotal = 2.90, μa = 2.22, μb = 1.87 D have been determined. An approximate structure has been fitted to these data. Eight satellite spectra of the cis form have been analyzed yielding a torsional vibration frequency around the central carbon–carbon bond of 85 ± 3 cm−1 and an apparent barrier height V3 = 2400 ± 60 cal/mole hindering the internal rotation of the methyl group. The gauche conformation has rotational constants A = 8701.9, B = 3976.9, C = 3108.7 MHz and a total dipole moment of μtotal = 3.09 D with components μa = 2.93, μb = 0.32, and μc = 0.91 D. The dihedral angle between the CCC and COF plane is found to be 120° ± 2° and the torsional vibration frequency of the gauche form around the central carbon–carbon bond is 54 ± 5 cm−1. Energetically, the cis form is favored over the gauche form by 1290 ± 50 cal/mole. The data obtained on the two rotamers are used to derive the potential-energy function V(υ) for rotation around the central C–C bond. The first four Fourier coefficients are found to be V1 = 930, V2 = 720, V3 = 1150, and V4 = 130 cal/mole, resulting in a potential barrier of 2100 cal/mole separating the cis from the gauche form and 800 cal/mole separating the two equivalent gauche forms from each other. The modification of the υ-dependent intramolecular forces which accompanies the substitution of one hydrogen atom by a methyl group is obtained. With the assumption that these modifications are additive and from the observed approximate constancy of V3, it becomes possible to predict the potential functions of several related molecules, for later comparison with experiment.
Energy-Level Iterative NMR Method for Sets of Magnetically Nonequivalent, Chemical-Shift Equivalent Nuclei
Authors: Bowers M. T. , Chapman T. I. , Manatt S. L.
Published: February 2004

A technique is described which greatly facilitates the use of the Swalen and Reilly energy-level NMR spectral analysis method for spin systems having sets of magnetically nonequivalent, chemical-shift equivalent nuclei and systems in which it is not possible to assign sufficient transitions to link all energy levels within a particular symmetry group of energy levels. As a test of this scheme, results are described for five iterative analyses of the fluorine-19 spectrum of hexafluoro-1,3-butadiene for the cases where the problem is arbitrarily broken into 27, 16, 8, and 4 groups and the actual problem which involves two groups of energy levels. For the 27-group case it is demonstrated that an excellent set of NMR parameters could be derived based on iteration with only 37 experimental lines to determine the 64 energy levels. The problem of convergence in the cases involving many groups of energy levels is discussed.
Production of Excited Hydrogen Atoms by Impact of Fast Electrons on Some Simple Hydrocarbons
Authors: Vroom D. A. , de Heer F. J.
Published: February 2004

Absolute emission cross sections have been determined for Lyman and Balmer radiation from hydrogen atoms formed in dissociative excitation of methane, methane-d4, ethane, ethylene, and benzene by impact of electrons (0.5 → 6 keV). The excited atoms have been observed by optical detection of their fluorescence. The results are analyzed in terms of the Bethe–Born approximation. It is found that the relevant collision processes go mainly via optically forbidden transitions. An isotope effect of about 20% has been measured for the productions of excited atoms from CH4 and CD4, indicating the importance of superexcited states. The cross sections for formation of excited hydrogen atoms are almost the same for the different hydrocarbons and therefore seem to be rather insensitive to the number of H atoms in the molecule.
Vibrational Spectrum and Vibro-Rotational Analysis of Cyanopropyne
Authors: Tubino Riccardo , Dellepiane Giovanna , Zerbi Giuseppe
Published: February 2004

The interpretation of the vibrational spectrum of cyanopropyne (CH3–C≡C–C≡N) in the gas, liquid, and solid states has been carried out. The gas spectrum under medium resolution allows the determination of a few Coriolis constants. The rotational constants obtained from the infrared spectrum are in agreement with those from the microwave spectrum. Bandwidths in the liquid and solid states are accounted for by hindered rotations. Thermodynamic properties are evaluated taking into account a correction for the Coriolis ξ coefficients.
Accurate Foreign-Gas-Broadening Parameters of the 22-GHz H2O Line from Refraction Spectroscopy
Authors: Liebe Hans J. , Dillon Thomas A.
Published: February 2004

A differential microwave refractometer has been found to be an excellent tool for accurate study of foreign-gas broadening of isolated spectral lines. Many experimental and analytic difficulties disappear when foreign gas is admitted at the maximum of a self-broadened dispersion pressure profile. The new principle has been applied to determine the broadening efficiencies for helium, argon, oxygen, hydrogen, nitrogen, acetylene, carbon dioxide, and acetonitrile on the rotational transition Jτ:5−1 → 6−5 of water vapor at 22 GHz. The gases chosen represent a wide range in effective collision cross sections, as well as the main atmospheric constituents.
Ab Initio Calculation of Some Lower-Lying Excited States of H2O
Authors: Horsley J. A. , Fink W. H.
Published: February 2004

Portions of the potential surfaces of the B1A1 excited state of the H2O molecule and the corresponding triplet state have been calculated using the LCAO–MO–SCF method in order to investigate the origins of the OH (A2Σ+) fragment having an abnormal rotational distribution produced on photodissociation of H2O. The calculated wavefunctions indicate that these states are largely Rydberg states at O–H distances close to 2.0 a.u., but at greater O–H distances the antibonding configuration predominates. In both states, the molecule shows a strong preference for the linear geometry. The calculated equilibrium O–H distance for the symmetric stretch in the linear conformation is about 0.4 Å greater than the ground-state equilibrium distance. For the asymmetric dissociation of the bent molecule in the B1A1 state the cross section through the potential surface at an H–O–H angle of 135° is calculated to have a maximum as well as a minimum. The origin of the maximum is discussed in terms of the variation of the exchange integral between the singly occupied orbitals. The relevance of these results to the experimental problem is considered.
Quantum-Chemical Treatment of the Reaction of Hot Carbon Atoms with O2
Authors: Kaufman Joyce J. , Harkins Jerome J. , Koski Walter S.
Published: February 2004

Extended Hückel three-dimensional LCAO–MO calculations were performed for the axial attack of C on O2 with the objective of determining how the energies of the ground Σ2+ state and several excited states varied as a function of C–O(1)–O(2) distances. The calculated energies for the probable intermediate state (ground 1Σ+) indicate that as C moves in from a C–O(1) distance of 4 Å to 1.5 Å if the O(1)–O(2) distance relaxes to 1.7–1.9 Å, the C–O(1)–O(2) system exhibits an energy minimum indicative of a conditionally stable intermediate. However, when the C–O(1) distance becomes shorter than 1.5 Å the curves indicate clearly that there will be O(1)–O(2) repulsion leading to CO+O. Confirmation of the same mechanism is indicated by the result that when the C–O(1) distance is 1.5 Å or less, the entire system passes from resembling C+O2 to resembling CO+O. Population analysis results also lead to the same conclusion. Thus the LCAO–MO results aid one in gaining insight into the mechanism of the reaction as it is proceeding. The method is perfectly general and applicable to the reactions of any other hot atoms and, even more important, is applicable to the whole general problem of reactive collisions.