bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical Study of SO2 Molecular Properties
Authors: Rothenberg Stephen , Schaefer Henry F.
Published: February 2004

Using two contracted Gaussian functions for each atomic orbital, plus d functions on both sulfur and oxygen to describe molecular charge distortion, a self-consistent-field wavefunction was computed for the A11 ground state of SO2. It is estimated that the calculated SCF energy, − 547.2089 hartree, lies no more than 0.1 hartree above the Hartree–Fock energy for SO2. An additional calculation without d functions and population analyses indicate that (a) d functions on sulfur are much more important in SO2 than was the case in previous work on H2S and (b) in SO2, d functions on sulfur are much more important than those on oxygen. Calculated molecular properties are in good agreement with available experimental data. Of particular importance is the fact that the calculated elements of the molecular quadrupole moment tensor are 31%, 30%, and 29% greater than those recently determined experimentally by Pochan, Stone, and Flygare. Values of the third moments, octupole moment tensor, diamagnetic shielding tensor, electric field gradient tensor, and 17O quadrupole coupling constants are predicted. On the basis of ab initio electric field gradients and experimental 33S quadrupole coupling constants for OCS, H2S, and SO2, we deduce nuclear electric quadrupole moments: Q(33S) = − 0.062 b and Q(35S) = 0.043 b.
Statistical-Mechanical Study of Stability of Folded-Chain Polymer Crystals with Irregular Fold Surfaces
Authors: Roe Ryong-Joon
Published: February 2004

A model of folded-chain crystals of polymers is considered, in which the length of crystalline sequences between folds is assumed to be uniform, while the number of segments comprising the folds is allowed to vary. The partition function is formulated by the generating function technique, with the fixed crystalline layer thickness introduced as an additional parameter of the system. The random-walk problem in the presence of a barrier, as the representation of folds, is solved for simple and body-centered cubic lattices. As variants of the basic model, the two ends of the fold are either assumed constrained in adjacent positions in the adjacent re-entry model, or are allowed at an arbitrary separation in the random re-entry model. The free energy of the system as a function of crystalline layer thickness exhibits no minimum, confirming the extend-chain crystals as the thermodynamically most stable. As the temperature is raised toward the melting point, the number of segments in folds increases beyond bound in the random re-entry model; while they remain about 10 in the adjacent re-entry model. In both models the size of the amorphous chain ends grows much faster than that of folds with increasing temperature. The frequency distribution of fold sizes is very broad. Even when the majority of folds consists of tight loops of a small number of segments, the fraction of very large folds is small but still significant. The entropy term of the surface free energy, calculated for the random and adjacent re-entry models, differs by as much as 50 erg/cm2.
Calculation of Cartesian Coordinates and Their Derivatives from Internal Molecular Coordinates. II. Second and Higher Derivatives and Derivatives of Vectors
Authors: Thompson H. Bradford
Published: February 2004

A process is given to extend a previously described algorithm so as to calculate analytically second and higher partial derivatives of Cartesian with respect to internal molecular coordinates within molecules. Each derivative calculation requires at most a single vector multiplication per order and is thus much faster than calculation of the original coordinates. An algorithm is also presented to calculate derivatives of a variety of vectors within molecules with respect to internal coordinates.
ESR and Electronic Relaxation of Cr3+ and Fe3+ in Water and Water–Glycerol Mixtures
Authors: Levanon H. , Charbinsky S. , Luz Z.
Published: February 2004

ESR intensities and linewidths at X band and linewidths at Q band of [Cr(H2O)6]3+ and [Fe(H2O)6]3+ in water-glycerol mixtures are reported. At X band the linewidth increases with glycerol concentration and there is a concommitant decrease in the integrated intensity of the ESR line. The linewidth at Q band is independent of the glycerol content of the solvent. The results are interpreted in terms of the general theory of electronic relaxation. It is shown that the dominant electronic relaxation in solutions of Cr3+ and Fe3+ hydrates is due to modulation of the zero-field splitting interaction. This interaction and its time dependence result from collisions of the hydrated complex with solvent molecules. Using equations derived for relaxation by rotational tumbling, a correlation time of about 5 × 10−12 sec in water is found for both complexes. The decrease in the integrated intensity with increasing viscosity at X band is due to the smearing out of all but the 12 → − 12 transitions. At Q band the Cr3+ resonance is essentially the 12 → − 12 transition even in the aqueous solution. Linewidths for a number of other Cr3+ complexes are reported and discussed.
Exclusion of Disallowed States from State Density and Its Effect on Unimolecular Rate
Authors: Forst W. , Prášil Z.
Published: February 2004

Statistical rate theory requires the knowledge of the density of allowed states of the decomposing molecule. States may be disallowed if they are unbound, or if they are excluded by angular-momentum conservation requirements. To investigate such exclusion, the generalized method of steepest descents, as recently formulated by the authors [J. Chem. Phys. 51, 3006 (1969)], is used to compute the density and sum of states for a molecular species represented by a system of independent quantum oscillators and independent classical free rotors under the restriction that (1) all oscillators (harmonic or anharmonic) are subject to vibrational cutoff; or (2) one anharmonic oscillator is coupled with a two-dimensional rotor whose rotational energy is restricted by angular-momentum conservation, while its vibrational energy is not allowed to exceed its effective dissociation energy, all other oscillators being subject to Restriction (1). The molecular model used is a “small molecule” where the two restrictions produce an easily noticeable effect. It turns out that Restriction (1) has an appreciable effect on the sum of states and on the unimolecular rate constant when the oscillators are assumed to be harmonic, but the sum of states and rate constant calculated for Morse oscillators does not differ from the strictly harmonic (no cutoff) case by more than an order of magnitude at energies below the total dissociation energy. Restriction (2), applied to the weakest oscillator (bond) in the molecule, has an effect on the rate constant that depends on symmetry. If molecule and complex are of the same symmetry (angular momentum strictly conserved), the rotations are “adiabatic” and the unimolecular rate constant increases with increasing rotational quantum number J. If molecule and complex are of different symmetries (angular momentum not strictly conserved), the rotations are “inactive” and the unimolecular rate constant decreases at first with J and then goes through a minimum, at constant excitation energy in the complex. Experimental data on this point would be most desirable. Such checks as were feasible show the method of calculation to be accurate within better than 10%.
Magnetic Susceptibility of Molten Sodium–Sodium Chloride Solutions
Authors: Arendt Ronald H. , Nachtrieb Norman H.
Published: February 2004

The magnetic properties of dilute solutions of sodium metal in molten sodium chloride may be described by a free-electron gas model, whose spin degeneracy conforms to Fermi–Dirac statistics. Measurements are reported for the magnetic susceptibility in the temperature interval, 800–970°C, for solutions containing 0.00813–0.05347 mole fraction sodium. Correction of the observed susceptibility due to sodium and chloride ions leaves a residual paramagnetism which is ascribed to conduction electrons having rest mass and Landau diamagnetism. Exclusion of the cations and anions from the volume accessible to electrons leads to good agreement between the free-electron theory and experiment.
Radiative Lifetime of the B 1Πu State of K2
Authors: Tango William J. , Zare Richard N.
Published: February 2004

Using the phase-shift method, a radiative lifetime study has been made of the K2 B 1Πu molecule excited by the 632.8-nm line of an He–Ne laser. The radiative lifetime extrapolated to zero pressure is found to be τ = 12.4 ± 0.3 nsec where the error represents three standard deviations of the statistical uncertainty. A simple valence-bond model for the molecular transition is proposed which predicts that the radiative lifetime of the B 1Πu state of the dialkalis will be shorter, by about a factor of 2 or more, than the lifetime of the parent atomic P state into which the excited alkali dimer separates. The results of these calculations are compared with the experimentally observed lifetimes for several vibrational states of Na2 and K2.
Nonradiative Decay of Individual Vibronic Levels in Large Molecules
Authors: Brailsford A. D. , Chang Tai Yup
Published: February 2004

The nonradiative decay of particular vibronic levels in large molecules has been investigated by use of a simple model system involving identical surfaces shifted in energy and normal coordinates. A general expression for the nonradiative decay probability of individual vibronic levels is derived in the weak coupling statistical limit and the energy gap law is obtained. Our expression for the nonradiative transition reveals the explicit dependence on the vibrational quantum numbers of the decaying vibronic level, in contrast with previous treatments.
Equation of State of Dense Fluids. VIII. Comparison of the Internal Pressure from the PY Theory and the Lennard-Jones (6–12) Potential with Experiment
Authors: Theeuwes Felix , Bearman Richard J.
Published: February 2004

The internal pressures are calculated from the PY theory with the Lennard-Jones (6–12) potential in a density range from ρ* = 0.25 to ρ* = 0.75 and from temperatures T* = 0.805 to T* = 1.450. In the region of overlap with the krypton and xenon PVT data, a comparison is made with the experimental internal pressure and suitable scaling constants for this comparison are obtained. A comparison is made with the molecular dynamics calculations of Verlet in a limited range.
EPR and Mössbauer Spectra of Iron (III) N, N-Dimethyldithiocarbamate
Authors: Rickards R. , Johnson C. E. , Hill H. A. O.
Published: February 2004

The EPR and Mössbauer spectra have been measured at 4.2°K for samples of iron (III) N, N-dimethyldithiocarbamate diluted in cobalt (III) N, N-dimethyldithiocarbamate. g values close to 2 and an isotropic hyperfine interaction were observed. The results indicate that the ferric atom occupies a site of almost cubic symmetry. The orbital ground state consists of approximately equal amounts of | xz〉 and | yz〉 holes with slightly less of |xy〉. The value of the observed magnetic hyperfine field at the nucleus (300 ± 5 kG) is in agreement with that predicted.
Hard-Sphere Model Applied to the Solubility of Gases in Low-Boiling Liquids
Authors: Staveley L. A. K.
Published: February 2004

The hard-sphere model for fluids has been applied by Snider and Herrington to the calculation of the excess thermodynamic functions of binary liquid mixtures and also to the problem of the solubility of gases in liquids. While remarkably successful in the first case, it appeared to fail rather badly when applied to gas solubility. It is shown that this apparent failure was due to Snider and Herrington's choice of experimental data, and that in fact the approach is much more effective than they concluded in correlating and predicting the thermodynamic parameters for solutions of gases in low-boiling liquids. Results of calculations are presented for the systems argon–neon, nitrogen–hydrogen, methane–helium, and hydrogen–helium, where the first named component is the liquid solvent.
Effect of Vibrational Anharmonicity on the Isotopic Self-Exchange Equilibria H2X+D2X=2HDX
Authors: Wolfsberg Max , Massa Audrey A. , Pyper J. W.
Published: February 2004

The anharmonicity correction to the zero-point energy change in the triatomic dihydride isotopic self-exchange equilibria, H2X+D2X=2HDX, is discussed. It is shown that the inclusion of the usually ignored term G0 in the expression for the anharmonic correction to the zero-point energy (G0 + 14Σxij) causes the anharmonicity correction to the zero-point energy change to be quite small for the cases considered (X is O, S, Se), but when this term is ignored the correction is found to be nonnegligible. Equilibrium constants are calculated, and it is found that the agreement between theory and experiment is considerably improved by including the G0 factor.
Effects of High Rotational Quantum Numbers on Rydberg–Klein–Rees Franck–Condon Factors: The Nitric Oxide (NO) Beta Band System
Authors: Generosa John I. , Harris Richard A.
Published: February 2004

Rydberg–Klein–Rees (RKR) Franck–Condon factors (qυ′υ″) have been calculated for the nitric oxide (NO) beta (B 2Πr−X 2Πr) band system incorporating high vibrational and rotational quantum numbers and accounting for spin–orbit couplings and centrifugal distortions. Franck–Condon factors (FCF's) calculated using the rotationless potential indicate that some of these may differ appreciably (in some cases by more than a factor of 2) when compared with factors calculated with J = 60. Franck–Condon factors calculated for specific bands, but for separate fine structure components (B 2Π1/2,3/2−X 2Π1/2,3/2) were often found to be quite different. The results also indicate that adding the rotational contribution directly to the rotationless potential to form an effective potential as is usually done within the framework of the Born–Oppenheimer approximation may not be appropriate for some systems.
Single-Center Configuration-Interaction Calculations on the Ground State of H3+
Authors: Borkman Raymond F.
Published: February 2004

Results of several single-center expansion configuration-interaction calculations are reported for the ground state of the H3+ molecule. A basis set of D3h symmetry-adapted Slater-type orbitals is located at the center of the molecule, which forms an equilateral triangle, and the nonlinear parameters are extensively optimized. Using an 85-configuration wavefunction we obtain an energy of − 1.3392 hartree, vibration frequencies of ν1 = 3450 cm−1, ν2 = 2850 cm−1, and a bond length of Re = 1.64 bohr. The calculated energy is in error by about 4 kcal/mole compared to the estimated exact energy. The computed geometry and vibration frequencies agree to within a few percent with values reported in the literature. An analysis of the convergence of our single-center wavefunction is given. The Joy-Handler l−4 rule appears to be approximately obeyed, and our results indicate that convergence is at least as rapid for triangular H3+ as for linear H3+. The energy associated with the Hartree–Fock part of our wavefunction is estimated to be − 1.298 hartree (99.8% of exact). This, together with a correlation energy of about 0.041 hartree (91% of exact), gives our best total energy of − 1.339 hartree.
Electron Spin Resonance of Azulene Radical Cations
Authors: Dessau R. M. , Shih S.
Published: February 2004

Electron spin resonance spectra were obtained for the radical cations of the nonalternant hydrocarbons, azulene and 4,6,8-trimethylazulene. Coupling constants were determined and compared with those predicted by valence bond and various molecular orbital calculations. The experimental results were clearly more consistent with the molecular orbital approaches which satisfactorily predicted the correct symmetries and relative spin densities, whereas the valence bond method did not. Splitting constants calculated directly by the INDO method agreed quite well with the observed values.
Existence and Analytic Properties of Solutions to the Percus–Yevick and Hypernetted Chain Equations
Authors: Groeneveld J.
Published: February 2004

The PY equation is shown to have a solution which is analytic in the density ρ near ρ = 0. Analytic properties of G(k), the Fourier transform of the pair correlation function, as a function of k, are discussed. Corresponding results for the HNC equation are reported also.
Hydrazinelike Defect, N2H4+, in Irradiated Ammonium Halides. I Production and Characterization
Authors: Marquardt Charles L.
Published: February 2004

The hydrazinelike defect in irradiated ammonium halides has been studied using electron paramagnetic resonance. The results suggest that this defect is an N2H4+ molecular ion at a normal ammonium ion lattice site. In NH4Cl and NH4Br it freezes in the lattice in such a way that the p orbitals in which the unpaired spin is localized are aligned along a 〈110〉 direction. The defect appears to be produced as a result of chemical decomposition of NH3X centers (X=halogen) through the competetive processes: NH3X→NH2++X−+H0 and NH3X→NH20+X−+H+. Products of these reactions then combine to form N2H4+ in the following way: NH20+NH2+→N2H4+. The N2H4+ subsequently undergoes decomposition at higher temperatures, its stability at any given temperature being also influenced by impurities.
Hydrazinelike Defect, N2H4+, in Irradiated Ammonium Halides. II. Theoretical Analysis
Authors: Reilly M. H. , Marquardt C. L.
Published: February 2004

An LCAO–MO energy level diagram and bond length estimates are used to describe a planar ethylenic molecule model for the N2H4+ defect at an NH4+ site in ammonium halides. The unpaired spin orbital is found to be an antibonding combination of 2p orbitals on each nitrogen perpendicular to the molecular plane. The model is then further specified and used to analyze the low-temperature EPR data for NH4Cl in detail, from which hyperfine, bond length, and bond angle parameters are extracted and compared with LCAO–MO calculations. Thereby, more information about the defect and confirmation of the model are obtained. Satellite intensity and linewidth angular variations in the high-temperature spectra are explained, and the relation of high- to low-temperature spectra is discussed.
Microwave Studies of Collision-Induced Transitions between Rotational Levels. VII Collisions between NH3 and Nonpolar Molecules
Authors: Daly P. W. , Oka T.
Published: February 2004

The technique of high-power microwave double resonance has been used to study rotational transitions induced in NH3 by collisions with various nonpolar gases. The nonpolar gases used were H2, para-H2, HD, D2, O2, N2, CH4, and SF6. Most of the observed collision-induced transitions were found to be those obeying dipole selection rules (ΔJ = 0, ± 1, ΔK = 0, parity + ↔ −) but several weaker transitions with ΔJ = /mp/ 2, ΔK = 0, parity + ↔ +, − ↔ − were also observed. Thus the “selection rules” for the rotational transitions in NH3 induced by collisions with nonpolar gases are similar to those for NH3–NH3 collisions, but very different from those for NH3–rare-gas collisions. Collision-induced transitions with ΔK ≠ 0 or Δ ≥ 3 which are observed clearly in NH3–rare-gas collisions have not been observed here. The “selection rules” give us information concerning the symmetry of the dominant intermolecular interaction. The observed values of relative variations of the signal, ΔI / I, give us relations among rate constants for various transitions. The dependence of ΔI / I on spin species (H2 and para-H2) and isotopic species (H2, HD, and D2) are discussed, and several simple unanswered questions raised by the observations are summarized.
Ferroelectric Paraelastic Paramagnetic Barium Cobalt Fluoride, BaCoF4, Crystal Structure
Authors: Keve E. T. , Abrahams S. C. , Bernstein J. L.
Published: February 2004

Barium cobalt fluoride, BaCoF4, ferroelectric and paramagnetic at room temperature, crystallizes in the orthorhombic system with lattice constants a = 5.8519 ± 0.0003, b = 14.628 ± 0.002, and c = 4.2102 ± 0.0003 Å at 298°K and space group A21am. The integrated intensities of 1526 reflections within a hemisphere of reciprocal space of radius (sinθ) / λ = 1.02 Å−1 were measured with one crystal, and 2986 reflections with a second crystal of BaCoF4, using PEXRAD. The crystal structure was refined by the method of least squares, using 386 symmetry-independent Fmeas from the first crystal and 875 from the second. The combined set of 1261 Fmeas, together with anisotropic thermal coefficients for all atoms, resulted in a final agreement factor R = 0.0679. The structure is isomorphous with BaMnF4 and contains CoF6 octahedra sharing corners to form puckered sheets parallel to (010). These sheets are linked by Ba2+ ions only. The octahedra are distorted, with Co–F distances ranging from 1.972 to 2.109 Å, with an average of 2.040 Å. The crystal structure is consistent with the previously presented model for ferroelectric reversal in the BaMF4 compounds. Both the sense and magnitude (8.0 ± 0.3 μC cm−2) of the observed absolute polarization is in good agreement with that calculated from a point-charge model (8.6 ± 0.6 μC cm−2).
Electron Attachment and Compound Formation in Flames. III. Negative Ion and Compound Formation in Flames Containing Tungsten and Potassium
Authors: Jensen D. E. , Miller W. J.
Published: February 2004

Application of several different experimental techniques (flame-ion mass spectrometry, microwave cavity resonance, electrostatic probe, and absorption spectrophotometry) reveals that addition of tungsten to fuel-rich atmospheric-pressure H2/O2/N2 flames seeded with potassium results in reductions in concentrations of free potassium atoms and in formation of stable W-containing negative ions. The following reaction scheme and enthalpy changes provide a satisfactory quantitative interpretation of the experimental results: K+H2WO4⇆KHWO4+H; ΔH0° = − 40 ± 40 kJ mol−1, H2WO4+e−⇆HWO4−+H; ΔH0° = + 40 ± 40 kJ mol−1, HWO4−+H⇆WO3−+H2O; ΔH0° = − 22 ± 40 kJ mol−1. The enthalpy changes imply electron affinities for WO3 and HWO4 of approximately 351 and 420 kJ mol−1, respectively.
Energy-Transfer Processes in Monochromatically Excited Iodine Molecules. III Quenching and Multiquantum Transfer from υ′ = 43
Authors: Kurzel R. B. , Steinfeld J. I.
Published: February 2004

The fluorescence arising from absorption of the 5145-Å laser line by molecular iodine vapor has been analyzed. Quenching and vibrational and rotational energy-transfer efficiencies for collision between the excited iodine and other gases (ground-state I2, He, Ne, Ar, Kr, Xe, H2, and CO2) have been determined, using a numerical solution of the master equation for relaxation. Vibrationally inelastic processes with Δυ′ ranging from + 5 to − 8 have been observed; their relative magnitudes appear to be well predicted from the oscillator matrix elements of the intermolecular potential coupling terms, properly weighted by a detailed balancing factor.
Application of a Higher RPA to a Model π-Electron System
Authors: Shibuya Tai-ichi , McKoy Vincent
Published: February 2004

We have applied a proposed higher-order random phase approximation (RPA) to the simple model system of two π electrons in the double bond of ethylene. The higher-order RPA removes some difficulties involved in the usual RPA, but retains the form of the RPA equations. To derive the higher RPA we retain all terms linear in the ground state correlation coefficients in the equations of motion for the excitation energy and in the equation determining the ground state. These equations are solved self-consistently and are simpler to handle than a configuration interaction solution in a realistic example. We do not use the quasiboson approximation.
Effect of Pressure on the Anisotropic Reorientation of Acetonitrile-d3 in the Liquid State
Authors: Bull T. E. , Jonas J.
Published: February 2004

The deuteron and nitrogen spin–lattice relaxation times of acetonitrile-d3 have been measured as a function of pressure up to 2 kbar at 23°C using the NMR pulse method. Since the quadrupole coupling constants for nitrogen and deuterons in acetonitrile-d3 are known, the experimental T1 data were interpretable in terms of the rotational diffusion constants for motion perpendicular and parallel to the symmetry axis, D⊥ and D‖, respectively. In agreement with our previous results the calculated activation volumes show that ΔV‡(D⊥) is much larger than ΔV‡(D‖).
Comparison of High- and Low-Temperature Data for the Cl2⇋2Cl Reaction
Authors: Burns George , Browne R. J.
Published: February 2004

The radical–molecule mechanism for the three-body recombination reaction was applied to the reaction 2Cl+Ar→Cl2+Ar and 2Cl+Cl2→2Cl2. It was found that the interaction potentials between Cl atom and either Cl2 or Ar are approximately 2 kcal/mol. The interaction potential in the Cl–Ar complex is 8–10 times deeper than the potential between the two argon atoms. Using the same radical–molecule mechanism, it was possible to extrapolate the recombination rate constant data up to shock wave temperature. The extrapolation yielded a plausible comparison of the low-temperature (below 500°K) Cl recombination data, and the high-temperature shock-tube Cl2 dissociation data above 1500°K. The available experimental Cl2 dissociation rate constants show a scatter among data of various investigations by a factor of 20. Our extrapolation strongly favors the newer shock wave data which are therefore thought to be more reliable. On the other hand, the extrapolation is in distinct disagreement with the results of some earlier investigators.
Interaction Potentials Involving Br Atoms
Authors: Blake J. A. , Browne R. J. , Burns George
Published: February 2004

Three-body Br-atom recombination rate constants, obtained recently over a relatively wide temperature range, were used to estimate the interaction potentials between recombining atoms and an inert third body, M. It was found that the most reasonable set of assumptions yields εLJNe = 0.6 kcal/mol, εLJAr = 1.4 cal/mol, εLJKr = 2.0 kcal/mol, and εLJN2 = 1.5 kcal/mol, where εLJM is the depth of the interaction potential between Br and M. These values of εLJM are several times larger than εLJM between M and Kr, which is bromine's nearest neighbor in the periodic table. The calculations show that use of a more realistic (Sutherland, rather than hard-sphere) potential between the recombining Br and BrM complex reduces, in some cases by as much as 50%, the value of εLJM. The corresponding three-body reaction probabilities, P's, vary from, roughly, 0.01 to unity, depending upon the details of the assumed recombination mechanism, and the nature of the third body. For M as an inert gas, P decreases with the increasing of molecular weight.
Vibrational Anisotropy and Gol'Danskii–Karyagin Effect in Trimethyltin Cyanide
Authors: Herber Rolfe H. , Chandra Subhas , Hazony Yehonathan
Published: February 2004

Mössbauer effect measurements have been carried out on (CH3)3SnCN in the temperature range 9°K < T < 204°K. From the asymmetry of the intensity of the two components of the quadrupole split spectrum it is possible to calculate the anisotropy in the mean-square amplitude of vibration of the metal atom parallel and perpendicular to the molecular symmetry axis. Extrapolation of the low-temperature data to 300°K gives a value of this anisotropy which is in good agreement with the value derived from x-ray diffraction data. The latter can be used to normalize the temperature-dependent recoil-free fraction data which in turn permit an estimate to be made of the anharmonicity of the heavy atom vibration in the low-and high-temperature limits. The present data on (CH3)3SnCN are in agreement with an oblate electrostatic field (Vzz>0) around the metal atom and show that the vibrational amplitudes involving this atom approach an isotropic behavior in the low-temperature limit.
Irradiation of Cyclohexene with 8.4, 10.0, and 11.6–11.8-eV Photons. Dissociation of Neutral Excited Cyclohexene and Reactions of C6H10+
Authors: Lesclaux R. , Searles S. , Sieck L. W. , Ausloos P.
Published: February 2004

The photolysis of cyclohexene has been investigated at photon energies above (10.0, 10.6–11.8 eV) and below (8.4 eV) its ionization potential (8.9 eV). The ionic processes were studied in a mass spectrometer equipped with a photon source. The parent C6H10+ ion was seen to undergo the following reactions with cyclohexene: C6H10++cyclo-C6H10→cyclo-C6H12+C6H8+, k = 0.7 × 10−10cm3/molecule·sec →C12H20+, k = 4.1 × 10−10cm3/molecule·sec. Analysis of the yields of cyclohexane formed in the first of these reactions in the photolysis at 10.0 eV over the pressure range 1–10 torr indicates that in this pressure range the relative importance of the two reactions is the same as in the ion source of the mass spectrometer at a pressure of 10−2 torr. At 11.6–11.8 eV the parent ion also dissociates, mainly to form C5H7+ and CH3. Evidence is presented which indicates that at these energies, the majority of the C6H10+ ions retain a cyclic structure. The neutral, electronically excited cyclohexene molecule dissociates mainly as follows: C6H10*→1,3-C4H6+C2H4, at 8.4 eV, Φ = 0.82 →C5H7+CH3, at 8.4 eV, Φ = 0.08. The photolysis of 3,3,6,6-d4-cyclohexene demonstrated that the first of these processes occurs predominantly by the “symmetrical” cleavage of the 3–4 and 5–6 C–C bonds, at all energies.
Ion–Ion Recombination Total Cross Sections—Atomic Species
Authors: Olson Ronald E. , Peterson James R. , Moseley John
Published: February 2004

A multistate Landau–Zener method is set up for the calculation of atomic ion–ion mutual neutralization total cross sections. The results of the calculations are compared with experimental results for O++O−, N++O−, He++D−, He++H−, and H++H−. The energy range scanned depends on the system but varies between about 0.1 and 10 000 eV. The agreement between theory and experiment is usually within a factor of 2.
van der Waals Interactions in Multilayer Systems
Authors: Ninham B. W. , Parsegian V. A.
Published: February 2004

The principles of the Lifshitz theory of van der Waals interaction are used to formulate and compute energies in multilayer systems. The full analysis including relativistic, temperature, and many-body effects reveals a nonadditive long-range interaction that is not predicted by earlier theories.
Microwave Spectrum and Determination of the Ring Bending Potential Function of Trimethylene Selenide
Authors: Petit Michael G. , Gibson James S. , Harris David O.
Published: February 2004

The microwave spectrum of trimethylene selenide (TMSe) was measured in the region 7.3–32 GHz. The rotational constants for seven vibrational states of the ring bending mode for the most abundant isotopic species were determined. From the variation of the rotational constants with vibrational state a potential function was obtained for the ring bending vibration, V(Q) = 8.7056[Q4 − 13.2669Q2]. This potential function is in good agreement with that obtained from an analysis of the far ir spectrum, V(Q) = 8.7056[Q4 − 13.19Q2], and predicts a barrier height to the planar configuration of 383.1 ± 4 cm−1 as compared with 378.1 ± 4 cm−1 obtained from the far infrared. The equilibrium dihedral angle was calculated to be 29.5 ± 1°.
Fluorescence and Phosphorescence Produced by Electron–Ion Recombination in 3-Methylpentane at 77°K
Authors: Merkel Paul B. , Hamill William H.
Published: February 2004

Spontaneous luminescence and photoinduced luminescence from γ-irradiated 3-methylpentane (3MP) at 77°K have been investigated. In addition to the previously reported band at 415 nm, a second band is observed in the emission spectrum at 230 nm and is assigned to 3 MP fluorescence. The presence of the latter band confirms the existence of 3MP+ ion centers in neat 3MP. Measurements of the decay of recombination luminescence induced by a brief pulse of light indicate lifetimes of ≤ 2 msec for the 230-nm band and 4.4 sec for the 415-nm band. The decay of the fluorescence sets an upper limit on the rate of the electron–positive hole recombination in 3MP and establishes that the long decay time of the 415-nm band is intrinsic to the emission process itself. This phosphorescence is assigned to emission from the lowest triplet state of 3 MP
Interfacial Energies of Monomolecular Films of Fatty Acids Deposited on Mica in Aqueous and Nonaqueous Media—the Strength of Hydrophobic Interactions
Authors: Bailey Anita I. , Price Andrea G.
Published: February 2004

An investigation into the energy per unit area of interface formed between monomolecular layers of fatty acids adsorbed on a mica surface and polar and nonpolar liquids is described. The results are compared with values predicted from Young's relation, γsl = γsv − γlvcosθ, where the γ's represent interfacial energies between solid, liquid, and vapor phases, and θ is the contact angle. This relation is known to hold for systems in which the phases remain homogeneous right up to the interface. It is shown that for nonpolar liquids the predicted and measured values are the same, while for water an additional term amounting to 7.5 erg/cm2 of interface must be introduced in Young's relation to obtain equality. This additional energy has been associated with hydrophobic interactions and from model building a value of 213 cal/mol of water ordered at the interface is obtained. The results are in good agreement with values obtained by Frank and Evans. The experiments shed additional light on the action of urea on hydrophobic interactions. In a concentrated solution of urea (> 8M), the predicted and measured values of the solid/liquid interfacial energy are again equal. This leveling effect can only be interpreted as showing that the liquid phase is homogeneous and suggests that urea causes a breakdown of the ordered structure of the water in the neighborhood of the hydrophobic interface. This conclusion is consistent with current views of the denaturant action of urea on proteins and other biomolecules.
Temperature Dependence of the Li NMR Spectrum and Atomic Motion in LiNbO3
Authors: Halstead T. K.
Published: February 2004

Electric quadrupole coupling constants and linewidths of the Li NMR spectrum of a single crystal of LiNbO3 were measured at temperatures between 24 and 680°C and are interpreted in terms of the motion of Li. The linear increase of the quadrupole coupling constant is unusual and is assumed to be due to the anisotropic vibration of Li. A simple expression for the temperature variation is derived which depends only on 〈z2〉 − (x2〉 and ∂4V / ∂4z, where 〈z2〉 is a mean square amplitude of vibration of Li, and V is the crystal electric potential at a Li site. ∂4V / ∂z4 was computed by an ionic lattice summation. It is suggested that a linear increase of the quadrupole coupling constant is characteristic of a light atom that vibrates anisotropically. Analysis of the linewidth data, assuming that Li atoms diffuse, gives D0 = 1.8 × 10−5m2sec−1 and ε = 1.62 eV.
Direct Observation of Electron Disappearance in Pulse Irradiated Fluorocarbon Gases
Authors: Fessenden Richard W. , Bansal Krishan M.
Published: February 2004

A microwave conductivity detector has been used to follow electron disappearance in pulse irradiated fluorocarbon gases. A discussion of the application of the method to compounds of widely differing electron attachment rate constants and a number of experimental details is given. Eleven simple fluorocarbons, CH2Cl2, and SF6 were studied. Because (except for SF6) these compounds were relatively unreactive, considerable effort was expended upon purification. In many cases the rate of electron attachment found for the purified sample was orders of magnitude less than for the unpurified one. In the homologous series from CF4 to n-C4F10 only the latter was found to attach electrons at an appreciable rate (k = 9.6 × 10−12cm3molecule−1·sec−1). Significantly, CF4 and C2F6 were found to have rate constants < 10−16 cm3 molecule−1·sec−1. The attachment rates of the compounds CH2F2, CHF3, CClF3, and CH3CF3 are all comparable; those of the last three are nearly identical (4 − 5 × 10−14 cm3 molecule−1·sec−1) suggesting the involvement of the CF3 group. The electron attachment rate constant for SF6 was determined to be 2.2 × 10−7 cm3 molecule−1·sec−1.
Collisional De-excitation of the Thallium 62 P3/2 State
Authors: Bellisio J. A. , Davidovits P.
Published: February 2004

The collisional de-excitation cross sections of the 62 P3/2 metastable state of thallium have been measured for the following atoms and molecules: He, Ne, Ar, TlI, TlBr, TlCl, O2, NO, H2, CO, CH4, C2H4, C2H6, C3H6, C3H8, CO2, and N2. Metastable thallium was produced in a quartz cell by photodissociating thallium halide vapor with a pulse of ultraviolet radiation. The density of the metastable thallium decays due to collisions with the gas added to the cell. The decay is exponential with a time constant governed by the density of the added gas. The lifetime of the metastable state was measured by monitoring the absorption of the thallium 5350 Å line from a beam of resonance radiation passing through the cell. Since the density of the added gas is known, the cross section can be computed from the measured lifetime. Our results are accurate to about 15%.
dc and Pulsed Electroluminescence in Anthracene and Doped Anthracene Crystals
Authors: Williams Digby F. , Schadt M.
Published: February 2004

The current–voltage characteristics of anthracene crystals have been studied under dc and pulsed conditions in the temperature range 350–100°K. The crystals are provided with contact combinations consisting of an electron injecting contact together with either a hole injecting contact or an individually blocking contact. Intense electroluminescence is observed from all contact combinations, and at high crystal currents, ∼ 10−6 A cm−2, each combination shows similar steady-state characteristics. Pulsed measurements illustrate that different mechanisms for hole injection are involved, at high and low current densities, and a possible mechanism for enhanced hole injection under high electron current densities is discussed. The spectral distribution of the electroluminescence is found to be independent of contact material, but does depend upon crystal preparation.
Photodissociation of HNCO in the Vacuum Ultraviolet; Production of NCO A 2Σ and NH(A 3π, πc1)
Authors: Okabe H.
Published: February 2004

Strong emission bands originating from NCO A2Σ and NH (A 3π, c 1π) were found in the photodissociation of HNCO in the vacuum ultraviolet. In addition, weak bands coming from NCO B 2Π were observed. Threshold energies of incident photons to produce these bands were measured, from which the following bond dissociation energies and heats of formation were obtained: D(H–NCO) = 4.90 ± 0.01 eV, D(HN–CO) = 3.38 ± 0.1 eV, D(N–CO) = 2.14 ± 0.15 eV, ΔHf(HNCO) = − 1.0 ± 0.14 eV, ΔHf(NCO) = 1.6 ± 0.15 eV. The emission spectrum, NCO A 2Σ → X 2π, produced from the photolysis at the Xe and Kr lines shows that NCO A 2Σ is highly excited in bending vibration, indicating that the upper state of HNCO from which NCO A 2Σ predissociates has a bent NCO configuration. NH c1Π formed is highly rotationally excited. Experimental evidence is presented to show that NH A 3π is produced primarily by a secondary process, CO a 3π + NH X 3Σ → CO X 1Σ + NH A 3π. Primary processes in the near-ultraviolet photolysis of HNCO are discussed on the basis of bond dissociation energies obtained in the present work.
Formation of Very Thin Oxide Films on Metals: Contact Potential Measurements during the Oxidation of (100) Cu
Authors: Boggio Joseph E.
Published: February 2004

The coupled currents approach of Fromhold and Cook as applied to the formation of very thin oxide films on metals is modified to allow for the reflection of electrons which have tunneled to the gas–oxide interface. Reaction of the tunneling electrons with adsorbed oxygen atoms (present at low concentration) rather than with adsorbed oxygen molecules is suggested as a possible, although not exclusive, physical basis for the low capture probability. This modification leads to the hypothesis that neither a virtual electronic or ionic equilibrium may prevail across the oxide during low-temperature oxidation studies. Experimental measurements of the change in the contact potential difference between a (100) Cu single crystal and a high purity Au polycrystal during exposure to oxygen are compared with the predicted electric potential difference cross the oxide; the agreement between theory and experiment is good. A similar comparison between Rhodin's thickness measurements and predictions of the above model for the oxidation of (100) Cu shows poor quantitative agreement.
Classical S Matrix: Numerical Application to Inelastic Collisions
Authors: Miller William H.
Published: February 2004

A previously developed semiclassical theory of molecular collisions based on exact classical mechanics is applied to the linear atom–diatom collision (vibrational excitation). Classical, semiclassical, and uniform semiclassical results for individual vibrational transition probabilities corresponding to the H2+He system are presented and compared to the exact quantum mechanical results of Secrest and Johnson. The purely classical results (the classical limit of the exact quantum mechanical transition probability) are seen to be accurate only in an average sense; the semiclassical and uniform semiclassical results, which contain interference effects omitted by the classical treatment, are in excellent agreement (within a few percent) with the exact quantum transition probabilities. An integral representation for the S-matrix elements is also developed which, although it involves only classical quantities, appears to have a region of validity beyond that of the semiclassical or uniform semiclassical expressions themselves. The general conclusion seems to be that the dynamics of these inelastic collisions is basically classical, with all quantum mechanical structure being of a rather simple interference nature.
Proton Magnetic Relaxation Study of the Manganese–Transfer-RNA Complex
Authors: Danchin A. , Guéron M.
Published: February 2004

At 20°C, transfer ribonucleic acid (tRNA) binds manganese ions cooperatively. The binding sites are studied by spin relaxation of the water protons in a solution of the Mn2+–tRNA complex, as a function of temperature and proton NMR frequency. Below room temperature the frequency dependence of the proton relaxation shows that the correlation time τc of the ion–proton dipolar interaction is limited by electron spin relaxation. Hence τc is not in these conditions an indicator of molecular motion. This explains why the relaxation effects of Mn2+ bound to different polynucleotides were previously observed to be quite similar. This also shows that the rotational correlation time is long (> 2 × 10−9 sec) and hence there is rigidity in the binding of the ion to tRNA. The “effective” number, pw, of water molecules in the first hydration shell, taking into account possible anisotropies of the molecular movements, is equal to two. This may indicate that the first hydration shell contains only two water molecules, the other ligands being groups from tRNA. PolyA also has pw = 2, whereas polyU has five water molecules in the first hydration shell. Above room temperature, the rotational correlation time decreases with an enthalpy of ∼ 15 kcal/mol, which is similar to that observed in optical absorption. Therefore the large-scale movements observed by proton relaxation correlate with the near-neighbor geometry which governs hypochromism. There seems to remain some structure up to 100°C. These results reinforce the proposition that strongly bound divalent cations play a role in the macromolecule's tertiary structure. They are discussed with reference to recent models of tRNA.
Phosphorescent Decay Time of Matrix-Isolated GeO, GeS, SnO, and SnS, and the Lifetime of the Cameron Bands of CO-Type Diatomics
Authors: Meyer B. , Smith J. J. , Spitzer K.
Published: February 2004

The lifetime for the Cameron bands, a 3Π → X 1Σ, of GeO, GeS, SnO, and SnS has been measured in argon, krypton, and xenon at 20 and 33°K, and in SF6 at 20, 33, and 77°K. The values range from 100 μsec to 3 msec, depending on molecule, matrix, and temperature. The external heavy atom effect on lifetime is strongest for light solutes. The internal heavy atom effect, expressed as the ratio of lifetimes, is in excellent agreement with predictions by first-order perturbation theory. Therefore, we used this theory to estimate the lifetime of the Cameron bands of CO, CS, SiO, and SiS in argon. The matrix observations are then used to deduce values for the lower limit of the corresponding gas-phase lifetimes. It is demonstrated that, for forbidden transitions, lifetimes of matrix-isolated molecules are a useful tool to estimate unknown gas-phase values.
Atomic and Molecular Calculations with the Pseudopotential Method. VII One-Valence-Electron Photoionization Cross Sections
Authors: McGinn George
Published: February 2004

The pseudopotential method is applied to evaluate p continuum wavefunctions for a number of atomic one-valence-electron systems. Combining the continuum p results with the ns2S ground states previously calculated, the photoionization cross sections for the ns–εp transitions of each system are evaluated. The following atoms and ions are investigated with the indicated continuum energy range above the ionization threshold: Li, Be+, B2+, C3+, Na, Mg+, Al2+, and Si3+ from 0–2.0 Ry; K, Ca+, and Rb from 0–0.6 Ry; and Cu, Zn+, and Ag from 0–0.8 Ry. Comparison is made to other calculated values with good agreement and to the observed values of Li, Na, K, and Rb with reasonable agreement.
Time Correlation Functions in a Binary Gas Mixture
Authors: Kapral Raymond , Desai Rashmi C.
Published: February 2004

The low density forms of the correlation functions which yield the coefficients of mutual and thermal diffusion in a binary gas mixture are derived and examined from two points of view. One approach uses the binary collision expansion of the existing correlation function expressions, another uses the physical arguments given by Mori to extract these correlation functions from the Boltzmann equation by considering the evolution of the system from an arbitrary initial state. Both approaches indicate that the time correlation functions can be expressed in terms of single momentum averages of two dynamical functions which obey coupled integrodifferential equations of the Boltzmann type. These equations are solved by Sonine polynomial expansion and calculations are performed for a hard-sphere gas mixture in order to illustrate the results. The correlation functions which characterize the thermal diffusion coefficient are discussed in some detail and several interesting aspects of the dynamics are pointed out.
Polarizabilities and Shielding Constants of Two-Electron Ions
Authors: Schulman Jerome M. , Tobin Frank L.
Published: February 2004

Atomic dipole polarizabilities and shielding factors of two-electron ions (Z = 1 to 8) have been computed through first order in correlation. The Hartree polarizabilities converge somewhat faster than Hartree–Fock and a geometric approximation in the Hartree series is shown to hold. The shielding factors are derived in a triple-perturbation expansion and the orbitals perturbed by the operator z / r3 are shown to be of simple form. A geometric approximation is also shown to hold for β.
Vibrational Exciton Density of States in Solid Benzene
Authors: Laufer J. C. , Kopelman R.
Published: February 2004

Computer-aided calculations, based on experimentally-fitted pairwise interaction terms, give the complete exciton density-of-states profile for the entire Brillouin zone. The restricted Frenkel model, with short-range interactions, is the key assumption. Results are given and discussed for the out-of-plane a2u normal mode ν11(C6H6 and C6D6), for ν12(b1u), and for ν15(b2u). The wide range of parameters used makes this investigation pertinent to other vibrational and electronic exciton bands of benzene and any other molecular crystal with the same interchange symmetry. Also, Van Hove singularities are found to be more important for symmetry-based critical points than for “accidental” critical points. Present-day experimental and theoretical intermolecular excitation exchange interaction terms are compared.