bluuebunny/crossref_metadata_2025 huggingface dataset:
Spin–Lattice Relaxation in Dilute Gases. III. 1H Relaxation in NH3 and 19F Relaxation in BF3
Authors: Armstrong R. L. , Hanrahan T. A. J.
Published: February 2004

Spin–lattice relaxation times have been measured for the protons in gaseous NH3 and for the fluorine nuclei in gaseous BF3 as a function of temperature and pressure in the dilute gas region. The measured temperature dependences are as predicted for relaxation resulting from a spin–rotation interaction. Cross sections for the relaxation are calculated and from them it is deduced that the long-range forces between the molecular electric dipole moments in NH3 contribute significantly to the randomization of the molecular rotational magnetic fields at the proton sites.
Potential Curves from Molecular Continuum–Continuum Emission. II. The Dissociation Energy of He2 A1Σu+
Authors: Smith Allan Laslett
Published: February 2004

The 600-Å bands of 4He2 were reinvestigated photoelectrically, and the pressure, temperature, and wavelength dependence of the oscillatory structure in the region of 602 < λ < 710 Å were determined. The analysis of the previous paper was applied to yield the potential curve of He2 A 1Σu+ in the range 1.5 < R < 2.4 Å. Combination of these data with published visible spectroscopic results on this state lead to a dissociation energy De (A 1Σu+) = 2.55 ± 0.17 eV. The position and height of the maximum in the potential at large R have been determined as 3.0 ± 0.3 Å and 0.03 ± 0.03 eV, respectively. The A–X electronic transition moment is shown to depend only weakly on R in the range 1.7 < R < 2.15 Å.
Photolysis of Hexafluoroacetone in the Presence of H2, D2, and HD. Kinetic Isotope Effects in the Reaction of CF3 with Molecular Hydrogen
Authors: Kibby Charles L. , Weston Ralph E.
Published: February 2004

The photolysis of hexafluoroacetone has been used as a source of CF3 radicals which either react with hydrogen to produce fluoroform or recombine to form hexafluoroethane. Relative rates of these reactions approximately obey the Arrhenius expressions over the temperature range from 333° to 870°K, kH2 / 2k21 / 2 = 9.3 × 103exp[( − 10 660 ± 140) / RT] liter1 / 2mole−1 / 2·sec−1/2, kHD / k21 / 2 = 7.4 × 103exp[(− 10 770 ± 150) / RT], kDH / k21 / 2 = 9.6 × 103exp[(− 11 630 ± 130) / RT], kD2 / 2k21 / 2 = 6.9 × 103exp[(− 11 660 ± 110) / RT]. Direct measurements of isotope effects were obtained by mass spectrometric analysis of the fluoroform produced during photolysis with added H2/D2 mixtures or with HD. Rate constant ratios are kH2 / kD2 = 1.30 exp(1050 / RT), kHD / kDH = 0.78 exp(860 / RT). The observed isotope effects are compared with those calculated on the basis of three- and six-atom LEPS and BEBO models for the activated complex.
Lifetime of Charged Species in Irradiated Dielectric Liquids
Authors: Hummel A.
Published: February 2004

The distribution of lifetimes of the charged species formed in cyclohexane by high-energy radiation is calculated using the experimental results on the number of these species reacting with solutes as a function of the concentration of the solutes. The time in which half of the ions recombine, when no reaction with a solute takes place, is 6 × 10−10 sec. Due to the fact that the diffusion coefficient of the negative entity in cyclohexane at room temperature is very large (2.5 × 10−4 cm2 sec−), scavenging of this entity increases the lifetime of the ions considerably. The effect of scavenging of the negative entity on the lifetime distribution is calculated. At large concentrations of α-chlorotoluene the lifetimes of the ions are increased by a factor of 16, approximately. The rate constant for the reaction of the negative entity with α-chlorotoluene at room temperature has been estimated at 4 × 10−11 cm3 sec−1.
Toward Understanding Vibrations of Polyatomic Molecules
Authors: Parr Robert G. , Brown Joyce E.
Published: February 2004

For a polyatomic molecule in the Born–Oppenheimer approximation, the quantum-mechanical virial theorem takes the form V = 2W + Σα < βRαβ(∂W / ∂Rαβ) or T = − W − Σα < β(∂W / ∂Rαβ), where W is the vibrational potential-energy function, expressed as a function of the internuclear distances Rαβ, and V and T are the electronic potential energy and electronic kinetic energy, as functions of Rαβ. If W for a particular case has the form W = W(0) + W(− 1) + W(− 2) + ···, where W(P) is a function of the Rαβ homogeneous of degree P, then V = 2W(0) + W(− 1) + ··· and T = − W(0) + W(− 2) + ···. Also, (B) would imply (A) and (C), and (C) would imply (A) and (B). These facts suggest use of empirical vibrational potential functions of the form (A), for then the terms W (− 1) will represent the Coulomb-like conformation-dependent part of the electronic potential energy, and the terms W (− 2) will be the particle-in-a-box-like conformation-dependent part of the electronic kinetic energy. To illustrate the method, the vibrations of the CO2 molecule are treated. With R1 and R2 the two CO distances, R3 the OO distance, and γ the OCO angle, the vibrational potential is written as W = W0 + W1(R1−1 + R2−1) + W11(R1−2 + R2−2) + W111(R1−3 + R2−3) + W3(R3−1) + W12(R1 + R2)−2 + Wγ(R1R2)−1tan2[12(π − γ)]. Values of the six (five independent) parameters W1 to Wγ are found which approximate 12 constants in the Overend–Suzuki valence-force potential function. Values of force constants determined from (D) are as follows (empirical values in parentheses): quadratic constants, 10.3 (10.3), 1.7 (1.7), 0.4 (0.4); cubic constants, − 29.6 (− 29.6), − 2.9 ( − 2.9), − 1.0 (− 0.7); quartic constants, 57.2 (47.3), 4.6 (4.3), 3.1 (6.0), 1.6 (1.1), 0.4 (3.5), 0.1 (0.0).
Rigid Disks and Spheres at High Densities. III
Authors: Rudd W. G. , Salsburg Z. W. , Yu A. P. , Stillinger F. H.
Published: February 2004

A formal derivation of an asymptotic expansion for the Helmholtz free energy FN of a system of N Λ-dimensional rigid spheres of diameter Σ is given, which has the form, FN / NkBT ∼ lim τ → 1 ν ln(λ / σ) − ν ln(τ − 1) + C + D(τ − 1) + E(τ − 1)2 + ···, where kB is Boltzmann's constant, λ is the mean thermal de Broglie wavelength, and θ = V / V0 is the reduced volume of a system of volume V, close-packed volume V0, at temperature T. Formal expressions for the constants C, D, and E are derived, and the cell-cluster technique is applied to the calculation of C and D for the hexagonal-close-packed and face-centered-cubic lattices. The results are CHCP = 1.7786846···, CFCC = 1.7795003···, DHCP = 0.54187···, DFCC = 0.557994···. A value for D for the two-dimensional triangular lattice is found to be DT = 0.098993···. Agreement with computer experiments is generally good.
Thermal Diffusion in Polyatomic Gases: Isotopic 15N2–14N2
Authors: Raman S. , Mathur B. P. , Howard A. J. , Champlin J. W. , Watson W. W.
Published: February 2004

The thermal diffusion factor, αT, for the isotopic gas mixture 15N2–14N2 has been measured as a function of temperature in the range 100°–740°K, using a ten-tube, stainless-steel trennschaukel. αT values are negative below 130°K and at higher temperatures have the practically constant value 0.36 within the experimental uncertainties. Monatomic gas theory as developed by Chapman and Enskog is unable to explain the results for any realistic potential model. The dimensional treatment of Waldman is also ineffective. Internal consistency and the agreement with the data of other workers suggest a rather dominant effect of the internal degrees of freedom and inelastic collisions on thermal diffusion in polyatomic gases.
ESR and ENDOR Spectra of Gd3+ in CdF2
Authors: Borcherts R. H. , Cole T. , Horn T.
Published: February 2004

ENDOR and ESR have been used to measure the spin-Hamiltonian parameters for Gd3+ ions in single crystals of CdF2. Measurements were made in insulating and semiconducting crystals. In both cases the predominant ESR spectrum arose from substitutional Gd3+ in cubic sites. Hyperfine splittings for the 155Gd and 157Gd were 12.8414 and 16.856 MHz, respectively. Parallel and perpendicular components of the hyperfine tensors of surrounding 19F nuclei were; A| = 8.693 MHz and A⊥ = − 6.952 MHz for the nearest-neighbor fluorines. More distant fluorine hyperfine splittings out to the fourth-nearest-neighbor shell have been measured and are found to fit closely to a point-dipole model. No differences in ESR and ENDOR Hamiltonian parameters, outside experimental errors have been found for Gd3+ between insulating and semiconducting CdF2 at low temperatures. Specifically, no charge-compensating F− ion was found within 7 Å of the Gd3+. The ENDOR measurements give values for the magnetic moments of 155Gd and 157Gd of − 0.2549 and − 0.3314 nuclear magnetons. A hyperfine anomaly of 1% was found for the Gd3+.
Biphotonic Ionization: A Flash Photoconductivity Study of TMPD in 3-Methylpentane Solutions
Authors: Pilloff Herschel S. , Albrecht A. C.
Published: February 2004

Flash photoconductivity studies are presented for a liquid organic solution. It was found that a dilute solution of N,N,N′,N′-tetramethylpharaphenylenediamine in 3-methylpentane at room temperature could be photoionized by light quanta with energies of the order of one-half those required for gas-phase ionization. This is fully reminiscent of the low-energy photoionizations seen when this and similar solutions are cooled to a very rigid state at 77°K. These ionizations are now known to be biphotonic, and the possibility of a similar mechanism holding for the liquid, room-temperature system is explored. A detailed kinetic analysis of one class of flash-biphotonic phenomena is examined. The general category treated consists of all cases in which a one-photon-produced intermediate state is subsequently ionized by a second photon. The charge-carrier density (and peak photocurrent) is proportional either to [ ∫ 0T I(t)dt]2or to ∫ 0T I2(t)dt, depending on whether the intermediate-state lifetime is either long or short compared to the flash duration T, respectively. Experimental studies verify proportionality of the peak photocurrent to both of these expressions, which, it happens, are equivalent under the conditions employed. The biphotonic character of the ionization is confirmed. A molecular-weight study of the solute molecule verifies the monomer nature of the photosensitive species, and a linear concentration dependence of the photocurrent at low concentrations eliminates a bimolecular (solute–solute) mechanism in this instance. The sublinear dependence of photocurrent on concentration at higher concentrations is analyzed as a bimolecular quenching of the intermediate state. The range of possible intermediate-state lifetimes obtained suggests (but does not prove) that, just as in the rigid solution at 77°K, the triplet state serves as the intermediate state in this fluid solution as well. The physical basis of the peak photocurrent as a meaningful parameter is discussed in an appendix.
SCF Wavefunctions for the NH3 Molecule. Potential-Energy Surface and Vibrational Force Constants
Authors: Body R. G. , McClure Donald S. , Clementi Enrico
Published: February 2004

SCF wavefunctions have been calculated for the NH3 molecule in a number of geometrical configurations, using a basis set of Gaussian s- and p-type functions. The equilibrium geometry was RNH° = 0.996 Å and αHNH° = 114.6 deg, the total energy was − 56.18517 hartree and the inversion barrier was 0.00127 hartree. Energies for configurations without C3υ symmetry were fitted to an assumed potential function which included all second- and third-degree terms, and the vibrational force constants were determined. The addition of d-type functions to the basis set gave much better results for the equilibrium geometry and inversion barrier. With this basis set, the equilibrium geometry was RNH° = 1.102 Å and αHNH° = 106.2 deg, the total energy was − 56.19821 and the inversion barrier was 0.0118 hartree.
Vibrational Exciton Structure in Crystals of Isotopic Benzenes
Authors: Bernstein Elliot R. , Robinson G. Wilse
Published: February 2004

New high-resolution infrared spectra of crystalline isotopic benzenes C6H6, C6H5D, p-, m-C6H4D2, sym-C6H3D3, and C6D6 are presented. Using these data in conjunction with isotopic mixed-crystal data and the polarized single-crystal spectra (C6H6) of Zwerdling and Halford, the exciton band structures and coupling constants for nondegenerate vibrations of C6H6 and C6D6 are obtained. One vibration of sym-C6H3D3 has also been analyzed. It is assumed that when the band shapes and splittings are nearly identical, the polarizations can be carried over from the C6H6 crystal to C6D6 and sym-C6H3D3 crystals. It is not possible to analyze completely the degenerate bands of these isotopes since experimental data do not yield values of all the independent matrix elements appearing in the expression for the energy of the exciton components. In the isotopic modifications of lower symmetry, exciton structure is not easily interpreted due to an orientational effect, crystal induced Fermi resonance, and the inevitable presence of isotopic impurities.
Excluded-Volume Effect of Polymethyl Methacrylate Solutions
Authors: Kuwahara N. , Oikawa T. , Kaneko M.
Published: February 2004

Osmotic pressures of polymethyl methacrylate solutions in acetone and in benzene are examined over a wide range of concentration (0 < υ2 < 0.17). The data are used to evaluate the thermodynamic parameter B appearing in the excluded-volume parameter z. The behavior of αη3 = [η] / [η]0 over a wide range of z(0 < z < 8) are compared to the theoretical predictions. The dependence of αη3 on z seems to be approximately described by the modified Flory–Fox relation αη5 − αη3 = 1 03z. The observed dependence of h(z̄) = 2A2/NAB on z̄ for polymethyl methacrylate solutions is very close to the result for polystyrene solutions. It is found that logh(z̄) is not a linear function of logz2 over a sufficiently wide range of z2(0 < z < 8).
Dissipative Structures in Chemical Systems
Authors: Lefever R.
Published: February 2004

A detailed analysis is reported of the behavior of a very simple mechanism that provides an example of chemical instability with respect to free diffusion. The appearance in time of a dissipative structure may be followed, and it is verified that the new steady state which is reached is completely stable.
ESR of VO in Argon Matrix at 4°K; Establishment of Its Electronic Ground State
Authors: Kasai Paul H.
Published: February 2004

ESR spectrum of VO isolated in argon matrix at 4°K was obtained. The analysis of the spectrum led to the unequivocal assignment of its electronic ground state as Σ4 arising from the configuration σ2π4σ1δ2. Due to the extremely large zero-field fine-structure splitting, the entire spectrum was found to be compatible with the effective spin Hamiltonian of the form H = g‖βHzSz + 2g⊥β(HxSx + HySy) + A‖IzSz + 2A⊥(IxSx + IySy), with the following parameters: g‖ = 2.0023 ± 0.001, g⊥ = 1.9804 ± 0.001, A‖ = 714.7 ± 0.5 MHz, and A⊥ = 837.1 ± 0.5 MHz.
Electron Paramagnetic Resonance Studies of VO2+ Doped in KAl(SO4)2·12H2O and NH4Al(SO4)2·24H2O Single Crystals
Authors: Rao K. V. S. , Sastry M. Dattatreya , Venkateswarlu Putcha
Published: February 2004

The electron paramagnetic resonance spectra of VO2+ ions doped in KAl(SO4)2·12H2O and NH4AL(SO4)2·24H2O single crystals are studied. In both the crystals it is observed that VO2+ ion substitutes for an Al3+ ion and forms a [VO(H2O)5]2+ complex, forming a distorted octahedron with an H2O vacancy along the VO bond direction. The octahedron axes are found to coincide with the crystallographic 〈100〉 axes, indicating that the presence of VO rotates the octahedron axes so as to coincide with the crystallographic axes. The formation of the vanadyl pentahydrate complex has been further confirmed by taking an optical-absorption spectrum. By correlating the EPR and the optical-absorption data, the covalency rates of σ bonding between the vanadium atom and the equitorial ligands and of the π bonding between the vanadium atom and the vanadyl oxygen have been estimated. All the spectra are analyzed by using the usual spin-Hamiltonian method.
Theoretical Study of Several Electronic States of the Hydrogen Fluoride Molecule
Authors: Bender Charles F. , Davidson Ernest R.
Published: February 2004

The low-lying electronic states of the hydrogen fluoride molecule have been investigated using extensive configuration interaction with a limited basis set. Potential curves, dipole moments, and oscillator strengths are given for selected states. In agreement with experiment, the results indicate that it would be difficult to observe any discrete absorption spectrum for HF. The dipole moment of the ground state behaves very differently from the SCF result.
Negative-Ion–Molecule Reactions in N2O
Authors: Moruzzi J. L. , Dakin J. T.
Published: February 2004

The relative concentrations of various negative ions present following electron attachment to N2O have been measured as a function of pressure. The ions were identified using a quadrupole mass spectrometer coupled to a drift tube operating at pressures between 0.3 and 8 torr. The dependence on E / p of the total negative-ion current is consistent with the change from a three-body to a two-body attachment process at E / p values near 1 V cm−1·torr−1. Using purified N2O at E / p = 10 V cm−1·torr−1, the O− ions formed by attachment undergo a series of ion–molecule reactions with the N2O to form ions with masses corresponding to NO−, N2O2−, NO2−, and N3O2−. Several reaction schemes are considered, and a fit to the observed pressure dependence is obtained by postulating the presence of intermediate excited states of N2O2− and N3O2− with lifetimes of 4 × 10−7 and 10−7 sec.
Electronic Spectra of Isoelectronic Amides, Acids, and Acyl Fluorides
Authors: Basch Harold , Robin M. B. , Kuebler N. A.
Published: February 2004

The first four bands in the gas-phase spectra of amides, carboxylic acids, and acyl fluorides are thought to be n → π*, n → 3s Rydberg, π → π*, and n → 3p Rydberg excitations. That the second and fourth bands are Rydberg, whereas the first and third are valence shell is demonstrated in a comparison of gas-phase and condensed-phase absorption and circular dichroism spectra. All-electron, SCF Gaussian orbital calculations are also presented which qualitatively explain the trends in the spectra of HCOX molecules, and predict several quantities of interest, such as upper-state dipole moments and magnetic transition moments, which have not been measured as yet.
Formulas for the Derivatives of Molecular Integrals Based on Hermite–Gaussian Functions
Authors: Živković T.
Published: February 2004

Explicit formulas for derivatives of all orders for matrix elements between HG functions, with respect to the parameters involved, are given. One- and two-electron operators, including nuclear-attraction and electron-repulsion energy operators, are considered. The derivatives can be conveniently applied to the calculation of matrix elements of new operators as well as for the determination of molecular geometry and force constants.
Thermogravitational Thermal Diffusion. V. Temperature Difference Effects. Thermal Diffusion Factors for the System Carbon Tetrachloride–Cyclohexane and Carbon Tetrachloride–Ethyl Alcohol
Authors: Beyerlein Adolph , Bearman Richard J.
Published: February 2004

Thermal diffusion factors calculated according to the revised theory of Horne and Bearman are shown to vary with temperature difference, contrary to theoretical expectations. On the assumption that the variation is due to reservoir effects, it is argued that the results should be extrapolated to zero temperature difference. When this is done, the present data for the carbon tetrachloride–cyclohexane system come into excellent agreement with those of Turner, Butler, and Story, who used a flow-cell technique. Data for the carbon tetrachloride–ethyl alcohol system agree within experimental uncertainties with those of Whitaker and Pigford, who used the pure thermal diffusion technique. It thus appears that three independent techniques will yield the same thermal diffusion factors. Hence, from an empirical standpoint, it would seem that the problem of their measurement has been solved for nonelectrolyte liquid solutions.
Dissociation of Hydrogen on Tantalum Using a Modulated Molecular Beam Technique
Authors: Krakowski R. A. , Olander D. R.
Published: February 2004

A molecular beam–mass spectrometric apparatus has been developed to study the kinetics of gas–solid reactions at low pressures. The system has been used in an investigation of the dissociation of hydrogen on tantalum. A modulated beam of molecular hydrogen, formed by effusion from a multichannel source, impinged upon the tantalum target. The reflected H2 and H atoms emitted after surface dissociation were monitored by a mass spectrometer in the vacuum system. The beams were modulated and the products measured by a phase-sensitive detector to improve the signal-to-noise characteristics of the detection system. The degree of dissociation was studied as a function of the surface temperature, beam temperature, and beam intensity. For a fixed target temperature, dissociation increased with beam temperature with an apparent activation energy of 1.4 kcal/mole. The rate of dissociation varied linearly with beam intensity. Mixed hydrogen–deuterium beams were employed to investigate the isotope exchange reaction, which, within the sensitivity of the detection system, did not take place. The results are consistent with a model in which the surface concentration of hydrogen is controlled by competition between atom evaporation and surface diffusion. The presence of the latter loss mechanism reduces the surface concentration to the point where second-order recombination is negligible. The atom-evaporation and surface-diffusion processes are satisfactorily described by the two-dimensional ideal-gas model. Collisions occur between migrating adatoms and scattering centers on the surface. The mean free path for this interaction was 0.02 cm for all temperatures investigated.
SCF Determination of the Ground and Excited States of NO2
Authors: Fink William H.
Published: February 2004

LCAO–MO–SCF wavefunctions have been determined for the ground and five excited states of NO2 at five different ONO angles. Molecular electronic configurations for the X̃, Ã, and B̃ states have been assigned on the basis of the calculated vertical excitation energies and speculations for those of the C̃, D, G, Ĩ, and J̃ states are made. The G̃ state is predicted to be B12 with a smaller equilibrium angle and considerably different value of R from the ground state. Estimates of correlation energy changes suggest that they are probably unimportant for most of the NO2 spectrum. The qualitative changes in orbital one-electron energies with angle agree with those reported by others for AB2 systems. Two closely related interpretations of geometry changes upon excitation are given, one based upon population analyses, the other on the AB2 correlation diagram.
Semiempirical Theory of Excimer Luminescence. II. Comparison with Previous Theories and Consideration of the Transition Probability and the Stability of the Excimer Triplet State
Authors: Chandra A. K. , Lim E. C.
Published: February 2004

The semiempirical theory of excimer luminescence in the Pariser–Parr–Pople scheme [Paper I, J. Chem. Phys. 48, 2589 (1968)] is shown to be formally equivalent to, but much simpler than, the theories based on the molecular exciton–charge resonance concept. On the basis of an assumption that excimers possess D2h equilibrium conformation, the question of the stability of the excimer triplet state and the thermal distortion of the excimer as the possible source of dipole-allowed character of excimer fluorescence is examined. It is shown that excimer triplet states of many polycyclic hydrocarbons are very likely unstable with respect to dissociation into monomer triplet and monomer ground state, and that thermal excitation of torsional distortion can account for the observed radiative lifetime of excimer fluorescence in naphthalene.
14N Nuclear Quadrupole Resonance in Aminopyridines
Authors: Marino R. A. , Guibé Lucien , Bray P. J.
Published: February 2004

The nitrogen-14 nuclear quadrupole resonance (NQR) spectra of the three isomeric aminopyrides and the five isomeric 2-aminomethylpyrides have been observed. The data is interpreted in terms of the electronic populations of the ring-nitrogen pπ and σ orbitals. The results show that the amino group is a strong π-electron donor and, in addition, behaves as a weak σ-electron donor. A model for the amino group which is consistent with the NQR data is presented.
Consistent Force Field for Calculations of Conformations, Vibrational Spectra, and Enthalpies of Cycloalkane and n-Alkane Molecules
Authors: Lifson S. , Warshel A.
Published: February 2004

An inductive method for a systematic selection of energy functions of interatomic interactions in large families of molecules is suggested and is applied to the family of cycloalkane and n-alkane molecules. Equilibrium conformations, vibrational frequencies, and excess enthalpies, including strain energies and vibrational enthalpies, are all derived from the same set of energy functions. The energy-function parameters are optimized by a least-squares algorithm to give the best possible agreement with a large amount and variety of observed data. Analytical derivatives of the various calculated quantities with respect to the energy parameters help to facilitate the computational procedures. The resulting agreement with experiment is used as a measure of success of the energy functions with optimized parameters, referred to as “consistent force field” (CFF). Different CFF's are compared and selected according to their relative success. Energy functions commonly used in conformational analysis are examined in preliminary consistent-force-field calculations and are found inadequate. Considerable imporvement is obtained when Coulomb interactions between residual atomic charges are included, and when interactions between atoms bonded to a common atom are represented by a Urey–Bradley-type force field, while Lennard-Jones potentials are used for nonbonded interactions between atoms separated by more than one atom. Vibrational frequencies as well as vibrational molecular enthalpies are derived for all cycloalkanes C5 through C12. The spectroscopic force constants are derived from the CFF as functions of molecular conformations. Agreement with available spectroscopic assignments of frequencies is reasonably good. A few improvements are suggested for cyclopentane assignments. The path and moment of inertia of pseudorotation in cyclopentane are calculated. Cyclodecane frequency assignments, based on the derived conformational C2h symmetry, are suggested for the first time. Calculated excess enthalpies of C5H10 and medium cycloalkanes agree well with observed values. Contributions of vibration–translation–rotation are shown to amount to a significant part of the excess enthalpy.
Mass-Spectrometric Studies of the Gaseous Systems Au–Ni, Au–Co, and Au–Fe, and Dissociation Energies of AuNi, AuCo, and AuFe
Authors: Kant Arthur
Published: February 2004

The molecules AuX (X=Ni, Co, and Fe) have been shown to exist in the vapor phase over liquid solutions of gold and transition metals at temperatures above 1800°K. Using information derived from a combination of effusion and mass-spectrometric techniques, the equilibrium constants and energetics of the reactions AuX+Au=Au2+X have been determined. The dissociation energies of AuX, as obtained by the second-law method, are 58.0 ± 5, 50.4 ± 3, and 43.8 ± 4 kcal/mole for X equal to Ni, Co, and Fe, respectively.
Pressure Broadening of Symmetric-Top Molecules in the Millimeter-Wavelength Region
Authors: Birnbaum George , Cohen E. R. , Rusk J. R.
Published: February 2004

Self-broadened and foreign-gas-broadened widths of a number of millimeter lines of the symmetric-top molecules CH3F and CHF3 have been measured and compared with values calculated by Anderson's and Murphy and Boggs' theories of pressure broadening. The lines studied are the J = 2 → 3, K = 0, 1, 2 lines of CH3F, and the J = 6 → 7, K = 3, 4, 5, 6 lines of CHF3 at 300°K and the K = 5 and 6 lines at 200°K. The broadening of the J = 2 → 3, K = 2 line of CH3F by CO2 at 300°K was also studied. With one exception where Anderson's theory and experiment are in good agreement, viz., the J = 2 → 3, K = 2 line of CH3F (300°K), this theory gives results greater than the experimental values. Although the theory of Murphy and Boggs is over all in better agreement with experiment, here too a number of discrepancies appear. Neither theory gives the observed dependence of the linewidth on the rotational quantum number K.
Absorption Spectrum of Silver Atoms in Solid Argon, Krypton, and Xenon
Authors: Brewer L. , King B. A. , Wang J. L. , Meyer B. , Moore G. F.
Published: February 2004

The uv absorption spectrum of trapped silver atoms has been studied in Ar, Kr, and Xe matrices at 20°K and above. With respect to the gas phase, transitions corresponding to P2 ← 2S were observed to be shifted to the blue by an amount characteristic of the particular matrix. In each matrix, four absorption bands were observed with half-widths ranging from 170–300 cm−1. With increasing temperature the absorption maxima exhibit further shifts together with increasing half-widths. For Xe and Kr these changes were found to be reversible between 20°K and 13 of the melting point of the matrix.
Accurate “Effective” Intermolecular Pair Potentials in Gaseous Argon
Authors: Copeland David A. , Kestner Neil R.
Published: February 2004

The “effective” intermolecular potential in gaseous argon has been determined, accurate through first order in density, for various assumed argon–argon potentials and the triple-dipole interaction. This potential should lead to the correct radial-distribution function. The calculations do not agree with the 1965 analysis of experimental data by Mikolaj and Pings [Phys. Rev. Letters 15, 849 (1965) ]. Our results suggest a density dependence of the minimum of the effective potential of only about 17 ρ̄ (in degrees Kelvin), where ρ̄ is the density in grams per cubic centimeter. This constant has a sizeable temperature dependence but only a small variation proportional to density. The contribution of other nonadditive effects is briefly examined as is the small contribution of nonadditive effects to the x-ray structure factor of the liquid.
Aromatic Carbonyl Spectra. I. The Polarized Absorption Spectrum of Single-Crystal 4,4′-Dichlorobenzophenone
Authors: Vala Martin , Tanaka Jiro
Published: February 2004

The polarized spin-allowed electronic absorption spectrum of single-crystal 4,4′-dichlorobenzophenone has been measured at room temperature and liquid-helium temperature. Three absorption bands have been observed and characterized: the nπ* band in the 25 000–32 000-cm−1 region and the B2u1 analog and intramolecular charge-transfer bands in the 32 000–41 000-cm−1 range. In the nπ* band, we have been able (1) to determine the polarization of the nπ* transition moment as exclusively parallel to the carbonyl axis; (2) to observe the 0–0 transition at 25 552 cm−1 indicating the electronic-allowed character of the transition; (3) to definitely exclude the possibility of a magnetic dipole transition or vibronic coupling via non-totally symmetric b-type vibrations as sources of intensity; (4) to observe and assign the torsional oscillations of the substituent phenyl rings; and, further (5) to observe and analyze the torsional doubling of the totally symmetric (in C2) vibration, ν3′, due to the nonbonded interaction between the phenyl rings. We have found no evidence to indicate that the excited state is nonplanar (i.e., pyramidal carbonyl group). The most probable source of intensity gain of the normally symmetry-forbidden nπ* transition is shown to be the breakdown of local carbonyl symmetry (i.e., a delocalization of the oxygen nonbonding orbital) such that there is significant mixing between the nonbonding orbital and the phenyl-ring π orbitals.
Thermochemistry of UOS; Evaporation of US–UO2 Mixtures; On the Attainment of Equilibrium in Knudsen Cells
Authors: Cater E. David , Rauh E. G. , Thorn R. J.
Published: February 2004

Evaporation of mixtures of US and UO2 and of UOS has been studied by molecular effusion, and the gaseous species have been investigated mass spectrometrically. A eutectic occurs in the system US–UO2 at 2240°K and at approximately 45% dioxide. In the vapor the gaseous species in order of decreasing partial pressures are: UO, UO2, UOS, US, S, U, and O. Because of the formation of the gaseous monoxide, the total pressure is about twice the sum of the vapor pressures for US and UO2 along. Calculated as though the vapor had the molecular weight of the monoxide, the total “effective” vapor pressure is given by logPE(atm) = − (3.496 ± 0.064)104 / T + (10.870 ± 0.298) below the eutectic temperature and logPE(atm) = − (2.450 ± 0.181)104 / T + (6.277 ± 0.755) above this temperature. The energy of atomization of gaseous UOS is 300 ± 7 kcal/mole at 0°K. In general, the mass-spectrometric measurements were much less precise and reproducible than those in previous studies of simpler systems with the same equipment. This was believed to be due to nonequilibrium conditions in the condensed phases in the Knudsen cell. It was concluded that one must study the condensed phases as well as the vapor phase before assuming that mass spectrometrically studied isomolecular reactions will yield equilibrium thermodynamic data.
General Theory of the Long-Range Pair-Correlation Function
Authors: Brunet J. , Gubbins K. E.
Published: February 2004

A general theory is presented for the long-range behavior of the pair-correlation function in a multi-component mixture, based on the thermodynamics of inhomogeneous systems. A set of nonlinear, coupled differential equations are obtained for the correlation function, and these reduced to linear equations under well-defined approximations. When applied to a one-component system, it is found that previously proposed equations of Ornstein and Zernike, and Hart are included as special cases of the linear theory. The assumptions implicit in these equations are clarified. The theory has also been used to obtain an asymptotic solution for the pair-correlation functions of a binary mixture. The conditions under which these equations reduce to expressions similar to those for pure fluids are investigated.
Molecular Reorientation in Collisions between an Atom and a Diatomic Molecule
Authors: Kinsey James L. , Riehl John W. , Waugh John S.
Published: February 2004

Collisions between atoms and diatomic molecules that result in reorientation of the molecular rotation relative to a laboratory-fixed frame are treated theoretically. An expression for the cross section in terms of elements of the scattering matrix is derived, and approximate numerical methods for calculation in a specific example, the low-energy scattering of H2 by He, are discussed.
Elastic Constants of Ammonium Bromide. II. High-Pressure Ultrasonic Investigation of the Phase Transitions
Authors: Garland Carl W. , Young Robert A.
Published: February 2004

The abiabatic elastic constants of single-crystal ammonium bromide have been measured at 20 MHz as functions of temperature and pressure in the region from 180°–240°K and from 0–6 kbar. A new high-pressure ordered phase, denoted as OII, has been discovered in this temperature range. The acoustic properties of the new OII phase were investigated with emphasis on the regions of the first-order phase transition from the OII phase to the ordered tetragonal phase and the lambda transition from the OII phase to the disordered cubic phase. The region of the lambda transition from the ordered tetragonal phase to the disordered cubic phase was also studied. A detailed comparison is made with the behavior of ammonium chloride near its order–disorder phase transition.
Viscosity-Dependent Ion Recombination Luminescence in Organic Liquids and Solids from Electron and Gamma Irradiation
Authors: Leone James A. , Hamill William H.
Published: February 2004

Recombination luminescence of triphenylamine (TPA), both fluorescence and phosphorescence, has been used to investigate ionic processes in (a) γ-irradiated 3-methylpentane (3MP) at 77°K, (b) thermoluminescence (TL) in γ-irradiated 3MP, and (c) in 1-MeV electron-pulse-irradiated paraffin oil and squalane from 215°–298°K. The shape of the time-dependent decay of luminescent intensity, I(t), in (a) was independent of a 500-fold change in irradiation dose, providing evidence for substantially complete correlation of geminate charge pairs. Photoionization of TPA under otherwise similar conditions led to the same I(t), which provides evidence that the distribution, P(R), of charge separation produced by γ irradiation is determined by very low-energy electrons. TL curves in part (b) showed two well-defined maxima of intensity I′ attributed to recombination of TPA+− e− at 83°K and I″ attributed to TPA+− TPA−  at ∼95°K. The ratio I″ / I′ increased with the concentration of TPA, I′ decreased with ir bleaching of solvent-trapped e− or addition of CO2 while both I′ and I″ decreased with addition of methyltetrahydrofuran, a hole trap. In part (c) intensity I was measured from 10−6–10−3 sec as appropriate at various temperatures and solvent viscosities which ranged from 1– ∼ 103 P. Excepting Cerenkov radiation, the luminescence spectrum coincides with TPA fluorescence, the phosphorescence intensity being negligible since its half-life is ∼ 2 sec. Linear plots of lnI vs t can be resolved into two exponential decays, with t1/2 of the slow component proportional to (viscosity)1/2, and attributed to recombination of TPA+ with TPA−. The faster component of I is attributed to recombination of TPA+ with e−, but t1/2 was too short for reliable measurement over most of the range. The dependence t1/2 ∝ η1/2 demonstrates the inapplicability of Stokes' law (which requires t1/2 ∝ R3) to recombination of isolated ion pairs, the high field reducing the randomness of relative ion displacements R. If the probability per ion jump mean free path λ against the central field at R is P = exp(− e2λ / εR2kT), the net displacement for dN jumps is dR = [1 − exp(− e2λ / εR2kT)]λdN. This leads to t1/2 ∝ Rm with 1 ≤ m ≤ 3. This relation combined with dn± / dt = (dn± / dR) (dR / dt) gives the ion pair distribution dn± / dR. From this expression and an assumed dependence Rn ∝ N for free charge one obtains the free-charge distribution function.
Infrared Spectra of Acetonitrile and Acetonitrile-d3
Authors: Pace E. L. , Noe Lewis J.
Published: February 2004

The infrared spectra of acetonitrile and acetonitrile-d3 have been studied at − 50°C for the high-temperature (β) crystalline phase and at − 65°, − 115° and − 192°C for the low-temperature (α) crystalline phase. A powder x-ray pattern obtained at liquid-nitrogen temperature indicates that the unit cell is primitive orthorhombic and that it contains eight molecules. It is shown that the consideration of all available structural data together with the site group and factor group splittings for the α crystal tends to favor D2h1 (Pmmm) and D2h9 (Pbam) as possible space groups.
Interpretation of Nuclear Quadrupole Coupling in Nitrogen Containing Molecules with Ab Initio Molecular-Orbital Wavefunctions
Authors: O'Konski Chester T. , Ha Tae-Kyu
Published: February 2004

Electric field gradients were computed at the nitrogen nuclei in a number of small molecules, using LCAO MO SCF wavefunctions constructed with Gaussian-lobe basis sets. Comparisons with molecular-quadrupole-constant data leads to a number of independent values for the quadrupole moment of 14N, which was poorly known. From this work, and other current studies, the best value of Q(14N) is 1.56 × 10−26 cm2. The contributions to the electric field gradient from various molecular orbitals are analyzed in detail for several cases. Procedures for estimating field gradients with the aid of overlap integrals are critically examined.
Molecular Motion and Raman Band Shapes in Liquid Nitrogen and Oxygen
Authors: Scotto Michael
Published: February 2004

Spontaneous Raman band shapes have been measured in liquid nitrogen and oxygen. Depolarization ratios have also been measured and compared with those in the gas. Correlation functions are obtained from Q-branch spectra, and the time dependence of rotational motion is discussed. Motional narrowing and rotational quenching are considered as possible mechanisms which could give rise to the observed spectra.
Photoionization Study of Ion–Molecule Reactions in Mixtures of Hydrogen and Rare Gases
Authors: Chupka W. A. , Russell Morley E.
Published: February 2004

The reactions producing HeH+, NeH+, and ArH+ in mixtures of hydrogen with rare gases were studied by photoionization mass spectrometry. The HeH+ and NeH+ ions are produced by vibrationally excited H2+ ions; the thresholds for reaction were found to be very near the υ = 3 and υ = 2 states of H2+, respectively. Above the vibrational threshold the reaction cross section increases with vibrational quantum number and there is no evidence for a kinetic-energy threshold for these states. When accelerated to sufficient kinetic energy, H2+ ions in vibrational states below threshold react with a cross section which increases as the deficit in vibrational energy decreases. The rate constant for reaction of those vibrational states that are near or about the reaction threshold and that contribute most to the observed reaction becomes nearly constant at low repeller voltages. The threshold for the reaction producing HeH+ can be made consistent with a theoretical value for the dissociation energy D0(HeH+) = 1.835 eV. The difference D0(NeH+)–D0(HeH+) is found to be 0.25 ± 0.03 eV. This yields D0(NeH+) = 2.085 eV. In argon–hydrogen mixtures, the ArH+ is formed by an exothermic reaction of H2+ in all vibrational states with Ar and also by reaction of Ar+ ions in the P3/22 ground state and (with ∼ 30% larger cross section) of Ar+ in the P1/22 excited state with H2. The cross section has little or no dependence on vibrational energy. Chemi-ionization processes leading to the formation of ArH+ by excited H* and Ar* atoms were observed. The dissociation energy of ArH+ was found to be certainly greater than 2.647 eV and, from a tentative interpretation of the chemi-ionization processes, probably greater than 3.397 eV.
Spin–Other-Orbit Matrix Elements for f3 Configurations
Authors: Barnes J. A. , Carroll B. L. , Flores L. M.
Published: February 2004

The spin–other-orbit matrix elements for f3 electron configurations are computed and tabulated for general usage.
NMR Studies of Aqueous Electrolyte Solutions. III. Hydration of Sodium Perchlorate, Hydrochloric Acid, and Perchloric Acid from Temperature Effects on Proton Shifts
Authors: Knapp Paul S. , Waite Ronald O. , Malinowski Edmund R.
Published: February 2004

Proton resonance shifts of aqueous solutions of NaClO4 and HCl are measured as a function of temperature and concentration. In each case the proton spectrum consists of a single resonance line which is sensitive to both temperature and concentration. The shift is considered to be an average of the proton in its various environments; namely, bulk water and ionic hydrates. At low concentrations the total hydration numbers of NaClO4, HCl, and HClO4 are determined to be 3.0 ± 0.2, 3.4 ± 0.4, and 2.6 ± 0.4, respectively. Assignments of individual hydration numbers to H+, Na+, Cl−, and ClO4− ions are discussed.
Angular Dependence of Low-Energy Electron-Impact Excitation Cross Section of the Lowest Triplet States of H2
Authors: Trajmar S. , Cartwright D. C. , Rice J. K. , Brinkmann R. T. , Kuppermann A.
Published: February 2004

The differential cross sections for the electron-impact excitation of the lowest triplet states of molecular hydrogen (b 3Σu+, a 3Σg+) have been calculated from threshold to 85 eV impact energy using the Ochkur–Rudge theory. For the X 1Σg+ → b 3Σu+ transition, the relative differential cross sections were measured with a low-energy, high-resolution electron-impact spectrometer from 10° to 80° scattering angle and impact energies of 25, 35, 40, 50, and 60 eV. Theory and experiment are in good agreement for the shape of the differential cross section for energies of 35 eV and above. However, at 25 eV, the theory continues to predict a rather well-developed maximum in the cross section at around 40° while the experimental cross sections are more isotropic. An appreciable contribution to the inelastic scattering in the energy loss region from 11 to 14 eV due to excitation to the a 3Σg+ and/or c 3Πu states is definitely established from the observed angular distributions. A quantitative evaluation of the individual angular behavior of the excitations in this region, however, would require a resolution higher than the presently available one of 0.030 eV.
Shock-Tube Study of Vibrational Exchange in N2–O2 Mixtures
Authors: White D. R.
Published: February 2004

Vibrational excitation in gas mixtures can occur both by translation-to-vibration (T–V) energy conversion in simple collisions and by the exchange of vibrational energy (V–V) between components. Nitrogen–oxygen mixtures containing 5%, 10%, 21%, 33%, and 50% O2 have been studied in a shock tube using both optical interferometry to measure the density profile through the relaxation zone and infrared emission of CO added in small quantities to serve as an indicator of the degree of vibrational excitation of N2. The density increase has been resolved into the sum of short and long time-constant exponentials. The former is approximately that expected from excitation of the O2 component. The longer time constant of the increasing density is approximately that observed for the infrared emission of CO and is thus indentified with the N2 excitation. This latter is found to be more rapid than can be accounted for by a T–V process and therefore vibrational exchange with excited O2 must contribute signficantly. Data have been analyzed to obtain a transition probability per collision for the exchange process which varies from 10−6 to 2.3 × 10−5 over the range 1000°–3000°K.
Measurements of Hyperfine Structure in NH2D
Authors: Kukolich Stephen G.
Published: February 2004

Hyperfine structure on the 414 – 404 rotation–inversion transition in NH2D was measured with a two-cavity maser spectrometer with 350-Hz linewidth. A reasonably good fit to the observed transitions is obtained by including the nitrogen quadrupole interaction, the nitrogen spin–rotation interaction, the deuteron quadrupole interaction, the deuteron spin–rotation interaction, and the nitrogen–deuteron spin–spin interaction in the Hamiltonian. The values of the coupling constants obtained are: for the 414 state (eqJQ)N = − 1895.65 ± 0.7 kHz, (eqJQ)D = − 75.78 ± 0.7 kHz, and for the 404 state (eqJQ)N = − 1811.55 ± 0.7 kHz, (eqJQ)D = − 80.13 ± 0.7 kHz, and for the values of the other parameters averaged over these states CN = 5.027 ± 0.18 kHz, CD = − 2.33 ± 0.18 kHz,and DND = 0.307 ± 0.70 kHz. The rotation–inversion frequency for this transition is 25 023 794.706 ± 0.07 kHz.
Composition and Temperature Dependence of the Thermal Diffusion Factor in H2–He Gas Mixtures
Authors: Mathur B. P. , Watson W. W.
Published: February 2004

A Swing separator has been used to measure the composition and temperature dependence of the thermal-diffusion factor αT in the H2–He system. This work was initiated following reports of abnormal behavior of this gas mixture in thermal conductivity measurements. We have measured αT values as a function of composition at three average temperatures: 129.3°, 239.8°, and 320.5°K. At each of these temperatures both a minimum and a maximum occur in the αT-composition curve. The position of the minimum shifts towards the higher percentages of H2 in the mixture as the temperature decreases, and the minimum tends to smooth out at higher temperatures. We have also measured αT as a function of temperature in the range 130°–600°K for H2 percentages 4.84, 14.5, 49.59, and 62.89 in the H2–He mixtures. As the temperature increases αT passes through a broad maximum and then slowly decreases. An approximate semitheoretical explanation is presented.