bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal dissociation and recombination of hydrogen according to the reactions H2+H⇄ H+H+H
Authors: Shui Ven H.
Published: February 2004

Three-body recombination and dissociation rate coefficients (kr and kd, respectively) for the reactions H2+H⇄ H+H+H have been evaluated for temperatures ranging between 300 and 10 000°K on the basis of the modified phase-space theory of reaction rates and Monte Carlo trajectory calculations. The semiempirical Porter-Karplus surface for H3 was used in the calculations. Good agreement was obtained between the theoretical estimates of kd and the bulk of the dissociation rate measurements. However, in contradistinction to some of the recent experimental investigations, neither the extremely steep temperature decay ([inverted lazy s] T−6) of kr at high temperatures nor the existence of a pronounced maximum in kr at moderate temperatures are supported by this theoretical work. Possible reasons for these differences are suggested. A simple ratio was deduced which relates kr(H) and the corresponding rate coefficients of the isotopes of hydrogen. The result is in good agreement with available experimental measurements and enables one to predict the latter rate coefficients without additional trajectory calculations. An example is given to illustrate the capability of the present Monte Carlo method in obtaining other useful information about atomic and molecular collision processes.
Dipolar coupling and molecular vibrations in crystals. V. Correlation of microscopic and macroscopic theory
Authors: Carlson R. E. , Decius J. C.
Published: February 2004

The application of dipolar coupling theory to the computation of dipole derivatives is reviewed from both a microscopic and macroscopic basis. It is shown that the two methods are essentially equivalent. In addition, two frequently used approximations to the complete dipolar coupling formalism are discussed. The results indicate that the assumption of a Lorentz field is not unreasonable for a number of crystals.
Diffraction pattern and structure of aqueous lithium chloride solutions
Authors: Narten A. H. , Vaslow F. , Levy H. A.
Published: February 2004

We present the first extensive study of an ionic solution by both neutron and x-ray diffraction. Pure water and aqueous LiCl at concentrations to saturation (mole ratio D2O:LiCl=3) have been examined at room temperature by both methods. This extensive study reduces greatly the nonuniqueness of interpretation usually associated with diffraction data from liquids. All data are shown to be consistent with a simple model involving only nearest neighbor interactions. In the pure liquid and in dilute solutions, the molecular structure is observed to be close to that found for the gas molecule. The tetrahedral coordination of oxygen atoms around each water is clearly shown, with deuterium atoms located near the O–O lines. This basic water structure gradually diminishes with increasing LiCl concentration, and is unobserved at mole ratios of 10 or lower. The coordination of oxygen atoms about Cl− appears to be octahedral on the average, with a deuterium atom near each connecting line. The coordination around Li+ appears to be tetrahedral; the orientation of the coordinated water molecule is such that Li and 2D occupy three tetrahedral positions about oxygen.
Brueckner-Hartree-Fock method for finite electronic systems
Authors: Larsson Sven
Published: February 2004

A Hartree-Fock-like model where the electrons in their motions are surrounded by exchange-correlation holes is described. Information about the correlation holes may be provided by Bethe-Goldstone type pair functions. An interpretation of the new one-particle equations is given. The theory for Bethe-Goldstone pair functions is shortly reviewed and a simplification is suggested for the case of a Hylleraas basis set. Finally the new method is illustrated for the Li ground state and for diatomic molecules at large internuclear distance.
Photochemistry of fluorotrifluoromethylbenzenes. III Photoisomerization of 1-fluoro-2, 1-fluoro-3, and 1-fluoro-4-trifluoromethylbenzenes in the gas phase
Authors: Al-Ani Khalid E.
Published: February 2004

The photolysis of 1-fluoro-2-, 1-fluoro-3-, and 1-ffluoro-4-trifluoromethylbenzene has been studied in the gas phase at excitation wavelengths of 248, 253.7, and 265 nm, respectively. At 248 nm, the quantum yield of the 1,2 isomer to give the 1,3 isomer was about 0.29 × 10−3; that of the 1,3 isomer to give the 1,2 isomer was 0.03 × 10−3; and to give the 1,4 isomer was 0.09 × 10−3. At the same wavelength, the yield of the 1,3 isomer from the 1,4 isomer was 0.36 × 10−3. Increase of the excitation wavelength, in general caused decrease in isomerization quantum yields. The quantum yield of isomerization of the 1,2 to the 1,3 isomer increased with increase in pressure, and a small increase was noticed for the 1,3 isomer. No pressure effect was noticed for isomerization of the 1,4 isomer.
Rydberg states in H2O
Authors: Boring Michael , Wood J. H. , Moskowitz J. W. , Connolly J. W. D.
Published: February 2004

Ionization and excitation energies for the Rydberg states (3,1B1, 3,1A2, 3,1A1 lying above the ground state 1A1) of the water molecule have been calculated via the multiple-scattering X a (MSX a) method. Both singlet and triplet states have been computed self-consistently. The first ionization energy of the molecule was also computed. In general, the results are in good agreement with experiment, and a few levels were found that have not been observed. The results were also compared with Hunt and Goddard's recent improved virtual orbital (IVO) calculation on H2O.
Cation self-diffusion in single-crystal MgO
Authors: Wuensch B. J. , Steele W. C. , Vasilos T.
Published: February 2004

Cation self-diffusion coefficients have been obtained for MgO in an argon atomosphere over a temperature range 1000–2400°C from concentration gradients determined with the aid of mass spectrometry. The problems associated with use of the short-lived radioisotope 28Mg were avoided by employing the stable isotope 26Mg as a tracer. Samples were prepared utilizing vapor-exchange, thick-film, and semi-infinite source boundary conditions at high, intermediate, and low temperatures, respectively, and were protected from contamination and volatilization by encapsulation in cylinders of pressed MgO powder. No significant difference in transport behavior was noted between crystals of moderate quality and those of the highest purity presently available. The temperature dependence of the diffusion coefficients may be represented by an activation energy of 2.76±0.08 eV and D0 of 4.19 × 10−4 cm2/sec. These parameters are comparable to those previously reported for impurity ion diffusion. The activation energy obtained is interpreted as that for cation migration. Previous measurements of cation self-diffusion coefficients obtained with the short-lived radisotope 28Mg are an order of magnitude larger and had been interpreted as intrinsic diffusion. The discrepency is attributed to doping introduced by the 28Si decay product of 28Mg.
Polyhedral clathrate hydrates. XVII. Structure of the low melting hydrate of n-propylamine: A novel clathration framework
Authors: Brickenkamp C. S. , Panke D.
Published: February 2004

The crystal structure of the monoclinic modification of n-propylamine clathrate hydrate has been determined at −100°C. The space group is P21/n with cell dimensions, a= 12.43, b= 20.73, c= 17.28 Å. and β=89.3°. The water framework is described as a packing of small unoccupied 11 hedra and large concave polyhedra. The disordered amine molecules occupy convex regions of the concave polyhedra with the amine nitrogens hydrogen bonding to the framework.
Endpoint distribution of self-avoiding walks
Authors: Grishman Ralph
Published: February 2004

The dimerization algorithm for the generation of self-avoiding walks was modified to generate a large number of walks with a relatively large endpoint separation. Walks of up to 512 steps were generated on the square and cubic lattices using this modified algorithm. From these walks the distribution of endpoint x coordinates of self-avoiding walks was accurately determined over a much wider range than would be possible using earlier techniques. The distributions obtained were found to be in general agreement with those proposed earlier for shorter walks by Domb, Gillis, and Wilmers.
Significance of both polarizability and polarizing power of cations in nitrate vibrational spectra
Authors: Brooker Murray H. , Bredig Max. A.
Published: February 2004

Vibrational frequencies of the NO3− ion in matrix-isolated monomers of several nitrates MNO3 are shown to correlate with the polarizing power of the cation M+, P≡ (z/r)/Seff, properly defined as the quotient, rather than the product, of the ionic potential, z/r, and a factor Seff= (5z1.27)/(r(1/2)I) (Ahrens, 1954) representing the efficiency with which the electron shell of the cation screens its nuclear charge. For molten nitrates MNO3, this correlation breaks down: in the condensed state, the polarizability of the cation, a, becomes important as the result of the mutual enhancement of the polarization of a given cation by several anions. A correlation of ν1 with the product of cation polarizing power and polarizability, P a, is presented.
Simplest halogen atom plus alkali dimer potential surface: F+Li2→LiF+Li
Authors: Pearson Peter K. , Hunt William J. , Bender Charles F. , Schaefer Henry F.
Published: February 2004

Ab initio electronic structure calculations have been carried out to investigate some features of the potential energy surface for the chemical reaction F+Li2→LiF+Li. The basis set of contracted Gaussian functions was of ``double zeta plus polarization'' quality, with an additional set of p functions on F added to describe F−. Single-configuration and two-configuration self-consistent-field calculations are reported here. A minimum energy path was obtained for the collinear reaction, but the most important feature determined was the nature of the potential minimum due to the FLi2 complex. For linear F–Li–Li, this complex is bound by 4 kcal/mole relative to separated LiF + Li. The attraction is much stronger, 34 kcal, for C2v geometry, and this species is predicted to have a bond angle of 99° and Li–F bond distance of 1.79 Å. Several excited electronic states of FLi2 are discussed briefly.
Thermochemical and kinetic information from metastable decompositions of ions
Authors: Klots Cornelius E.
Published: February 2004

Formulations are examined for connecting average kinetic energies of the products of a unimolecular decomposition with the total available energy. A straightforward method for the calculation of kinetic shifts is then described. Existing data suggest, nevertheless, that the effective number of internal degrees-of-freedom for energy disposal may be greater than the physical number, a result inaccessible to existing theory. A formal reconciliation is indicated here.
Many-body interaction free energy to infinite order. I. General susceptibility formulation
Authors: MacRury T. B. , Linder B.
Published: February 2004

A unified treatment is presented for the van der Waals interaction between N nonoverlapping molecular systems of arbitrary sizes and electron delocalizations. The theory takes account of many-body effects and multipolar interactions to all orders, and is valid for any finite temperature. By neglecting correlations between multiple excitations within each system, a closed-form expression is obtained for the interaction free energy in terms of the charge density susceptibilities of the isolated molecular systems and the Coulomb potentials between their charge distributions. Then an approximation is introduced which allows the free energy shift to be written in simple diagrammatic form. Finally, the general theory is specialized to the case of small molecular systems (or molecular units), and the dipolar interaction is worked out in detail.
Study of the structure of molecular complexes. III. Energy surface of a water molecule in the field of a fluorine or chlorine anion
Authors: Kistenmacher H. , Popkie H. , Clementi E.
Published: February 2004

The interaction between F− or the Cl− anion and water has been computed in the Hartree-Fock approximation with a large basis set of Gaussian functions: The obtained energies and wavefunctions are therefore close to the Hartree-Fock limit. We find that the geometrical configuration of maximal stability corresponds to a hydrogen-bonded configuration between the water and the anion; however, the hydrogen bond is somewhat bent with a deviation from linearity of about 4.5° for F−–H2O and about 14.6° for Cl−–H2O. At the most stable configuration the oxygen-anion distance is 4.75 a.u. for F− and 6.26 a.u. for Cl−. At the equilibrium configuration the Hartree-Fock binding energy is 23.54 and 11.86 kcal/mole for the F−–H2O and the Cl−–H2O complex, respectively. The energy surfaces are analyzed in terms of an energy partitioning consisting of the energy of the water molecule in the field of the anion, the anion in the field of the molecule of water, and of the interaction energy between the water molecule and the anion.
Vibrational spectrum of crystalline iodoform
Authors: Neto N. , Oehler O. , Hexter R. M.
Published: February 2004

The vibrational spectra of iodoform and deutero iodoform single crystals have been obtained at room temperature and liquid hydrogen temperature. The experimental data included Raman and far infrared spectra of the compound. The vibrational assignment of iodoform is discussed and the observed frequencies are compared with values calculated for k=0. Several dispersion curves along lines of symmetry in the Brillouin zone were calculated and their symmetry properties are discussed in terms of irreducible representations of space groups. Selection rules for two-phonon transitions at critical points on the boundary of the Brillouin zone are obtained. On this basis, side bands with multiplet structure in the C–H stretching region are interpreted as due to combinations of the C–H stretching mode with lattice modes for k≠ 0.
Phase waves in oscillatory chemical reactions
Authors: Ortoleva Peter , Ross John
Published: February 2004

A theory is presented for the effect of heterogeneity on an oscillatory chemically reactive system in a stable limit cycle. We develop a perturbation technique free of secular behavior for the solution of the nonlinear partial differential equations. The solutions are obtained in terms of a phase function which obeys a diffusion equation. Due to this diffusion there exist phase waves which propagate down the phase gradient. We derive equations for the position of the phase front as a function of time, the departure (arrival) times of waves, the wave pattern and the lifetime of each wave emitted from a center of heterogeneous catalysis, the interference effects between neighboring heterogeneities (pacemakers), and local renormalization effects (frequency changes) from heterogeneities extending over regions of nonzero extent. The theory is shown to break down for systems with relaxation oscillations but is confirmed quantitatively for more smoothly varying limit cycles. The spatial range of dynamic control of heterogeneity grows with time in an oscillatory reacting system, but is limited by the chemical relaxation time for a monotonically reacting system.
Intermolecular nuclear Overhauser studies of coupled spin systems in high resolution NMR
Authors: Krishna N. Rama , Gordon Sidney L.
Published: February 2004

A density matrix description of intermolecular nuclear Overhauser experiments in high resolution NMR systems is developed. The formalism is specific for a binary solution of solute A in solvent B. The solute and solvent have homonuclear spin systems with magnetogyric ratios γI and γS, respectively. The steady state intermolecular nuclear Overhauser effect (INOE) experiment, where the solvent spins are saturated and the frequency sweep NMR spectrum of the solute is examined, offers a technique complementary to intramolecular double resonance for the analysis of relaxation in high resolution NMR systems. For the INOE experiment, the density matrix equations may be expressed completely in the spin space of a single solute molecule. The resulting INOE equations are straightforward to use, and explicit solutions are given for the Overhauser enhancement of each line in an AX spin system with internal dipolar relaxation and different intermolecular dipolar interactions at the sites of the A and X spins. Theory is compared with experiment for a dilute sample where the solute is the AX2 spin system, 1,1,2-trichloroethane, and the solvent is tetramethylsilane. Saturation of the tetramethylsilane signal led to 37% enhancement in each of the transitions of the A proton and to 16% enhancement in each of the transitions of the X protons. An analysis of these enhancements allowed the accurate determination of the fraction of internal dipolar interaction in the solute. It is suggested that the INOE technique can be useful for molecular conformation analysis.
Raman spectra and crystal structure of solid CHCl3 and CDCl3
Authors: Shurvell H. F.
Published: February 2004

Raman spectra of polycrystalline films of CHCl3 and CDCl3 have been recorded at 77°K. Splittings due to the four chlorine siotropic molecules have been resolved for the ν3(A1) bands of both CHCl3 and CDCl3. After taking isotopic effects into account, further splittings of the fundamentals in the Raman spectra together with those previously recorded in the infrared spectra are essentially in agreement with predictions based on the factor group D 2h, a site symmetry C s, and four molecules per unit cell.
Hypersonic velocities in aqueous alkali chloride solutions
Authors: Gross Miron , Azoulay J. , Gerlich D.
Published: February 2004

Measurements of Brillouin scattering in aqueous solutions of LiCl, NaCl, KCl, RbCl, and CsCl are presented, and the values of the hypersonic velocity as a function of molal concentration are determined. The velocity change with concentration depends markedly on the type of solute ion, it increases monotonically above the value for pure water for the light ions, while in the case of the heavy ions, the velocity first dips below the value of pure water, and then increases with larger concentration of solute. In all cases, the measured hypersonic velocity thus obtained was found to be lower than the corresponding ultrasonic one.
Rotational energy transfer in pure HCN and in HCN-rare gas mixtures by microwave double resonance and pressure broadening
Authors: Cohen Jonathan B. , Wilson E. Bright
Published: February 2004

Microwave double resonance studies have been made of collision-induced rotational transitions occurring in pure HCN and in HCN in the presence of He, Ar, or Xe. Double resonance intensities and phase shifts have been determined as a function of pressure for HCN l doublets up to Δ J= +3 from the pumped transition. The effects of Ar and Xe are similar and quite distinct from those of He. In the latter case the Δ J= 2, ± ↔ ± transition is strongly preferred while in the former case no preferences are observed. Collision broadening linewidth parameters have been determined for the J=6, 7, 8, and 10 HCN l doublets for the pure gas and for the three rare gas collision partners. Experimental evidence is presented that indicates that quantum coherence effects are being observed in both Stark and double resonance absorption spectroscopy. An analysis is presented that utilizes the experimental total cross sections (linewidths) in a linear kinetic model of the energy transfer processes to determine partial cross sections (rates of collision induced transitions) that reproduce the double resonance data. General consideration of the atom-dipole collision dynamics allows a preferred set of rates to be selected for each collision partner from several sets that are found to reproduce the data.
Microwave double resonance studies of rotational relaxation in polar gases
Authors: Cohen Jonathan B. , Wilson E. Bright
Published: February 2004

Four-level, modulated microwave double resonance has been used to study rotational energy transfer in some dilute polar gases. A survey has been made of double resonance signals observable in four asymmetric rotors: trimethylene oxide, β-propiolactone, cyclobutanone, and cis-difluoroethylene. Double resonance intensities have been measured in vibrational ground and excited states. Also, angular momentum reorientation caused by intermolecular collisions has been studied by microwave double resonance. Four-level experiments have been performed on individual |JM〉 components of ethylene oxide and 3,3,3-trifluoropropyne and on |JF〉 components of ICN. These experiments provide evidence that the energy transfer and momentum reorientation occurring during collisions are determined by the long range dipole-dipole interaction. Qualitatively, a first-order perturbation theory description of the collision process accounts for the selection rules of the observed collision-induced transitions.
Autoionization in photoelectron emission by solutions: Anthracene anion radicals
Authors: Aulich Hubert , Baron Bill , Delahay Paul
Published: February 2004

Photoelectron emission (PEE) spectra (up to 5 eV) of anthracene anion radicals A− and A2− in tetrahydrofuran (THF) and hexamethyl phosphoric triamide (HMPA) exhibit three bands with maxima at photon energies (≈ 3.35, 3.8, and 4.8 eV) nearly coinciding with the energies at the maxima of absorption bands. PEE is interpreted as arising primarily from autoionization of excited states of the 3.35, 3.8, and 4.8 eV bands into the continuum located above the 1.7 eV absorption band. Direct transition to the continuum appears minor except possibly at the foot of the first PEE band. Quantum yields for A− in THF are lower than in HMPA because stronger ion pairing in THF favors geminate recombination of parents with emitted quasifree electrons. The single PEE band of A2− in THF at ≈ 3.7 eV, which nearly coincides with an absorption band, is also explained by autoionization into the continuum above the 2.1 eV absorption band. Quantum yields for A2− in THF are higher than for A− because of lower probability of geminate recombination and/or higher autoionization efficiency. Results for A2− in HMPA were not conclusive, probably because of the presence of some A−. Anthracene in THF or HMPA and the cation A+ in dichloromethane and fluorosulfonic acid do not exhibit any PEE (≤ 5 eV) because the continuum is too high to allow autoionization or direct bound-continuum transitions. These are the first reported examples, to our knowledge, of autoionization in solutions.
Calculation of the crystal field splittings of Sm3+ levels in LaCl3 with inclusion of J mixing
Authors: Rajnak K. , Mehlhorn R. , Edelstein N.
Published: February 2004

A complete diagonalization of the matrices for the electrostatic, spin-orbit, and crystal field interactions for Sm3+ in LaCl3 has been carried out using a truncated set of basis vectors containing 42 of the 73 LS states. This diagonalization allowed many new assignments to be made. Corrections of some of the centers of gravity previously used in a fit of free ion parameters was also possible. The newly obtained free ion parameters are more in accord with the trends for the other lanthanide ions in LaCl3. They reduce the rms deviation from 119 to 54 cm−1. The parameters (in cm−1)used in the final diagonalization are E1=5465.27 ± 17.21, E2=25.40 ± 0.39, E3=547.62 ± 1.65, ζ=1153.8 ± 8.9, a=24.29 ± 1.98, β=−795.72 ± 98.4, γ=800, B02=161.70, B04=−182.00, B06=−710.24, B66=448.14.
Orientational averaging of rotational transition probabilities computed using the sudden approximation
Authors: Scott Paul B.
Published: February 2004

Rotational group methods are used to average the results of a semiclassical sudden approximation calculation of rotational transition probabilities over all molecular orientations. The orientation average removes the even Δm selection rule while not affecting that for j. Calculations presented are done for intermolecular potentials selected to represent the argon-nitrogen system. These show the transition probability for Δj=0, ±2 to be slightly reduced compared to the v ∥ z case. For b*>1.2, the angular averaged Pj′j are nearly identical to the parallel case.
Quantum path integrals and reduced fermion density matrices: One-dimensional noninteracting systems
Authors: Light J. C. , Yuan J. M.
Published: February 2004

A new approach has been used to determine directly the reduced single particle density matrix for noninteracting fermions in a one-dimensional potential. A time evolution operator closely related to the classical limit of the Feynman path integral propagator is derived which has superior behavior in turning point and nonclassical regions. This is used to determine the reduced density matrix in terms of the phase and time of oscillation at the Fermi energy. The density so obtained has the correct quantum oscillations and nonclassical tails and is a distinct improvement over the Thomas-Fermi density. Two simple examples are presented and implications for electronic structure calculations are discussed.
Photolysis of ketene in the presence of propylene: Multistep collisional deactivation of methylcyclopropane and the excess energy of singlet methylene
Authors: Topor Michael G. , Carr Robert W.
Published: February 2004

The photolysis of ketene in the presence of propylene-5% oxygen mixtures was investigated at average wavelengths of 3550, 3340, 3130, and 2680 Å, and ketene photolysis in the presence of propylene-n-pentane-5% O2 mixtures was investigated at average wavelengths of 3130 and 2680 Å. Reaction pressures ranged approximately between 10 and 760 torr. Chemically activated methylcyclopropane, arising from singlet methylene addition to propylene, isomerized to the four butene isomers at each wavelength. The rate of isomerization increased monotonically with decreasing wavelength, demonstrating that the methylcyclopropane average energy increased with increasing photon energy. Of the chemically activated butenes formed either by methylcyclopropane isomerization or by insertion of singlet methylene into the carbon-hydrogen bonds of propylene, butene-1 decomposed significantly at the lowest pressures. A multistep process was required to describe the collisional deactivation of chemically activated methylcyclopropane. Theoretical analysis of data at 3550 Å, where methylcyclopropane was considered to have an average energy of 93 kcal mole−1, gave a deactivation step size of 1.5 kcal mole−1 per collision for a stepladder model. Extension of the theoretical treatment to shorter wavelengths predicts that singlet methylene retains only 3 kcal mole−1 of the approximately 25 kcal mole−1 of excess photon energy available at 2680 Å when it adds to the propylene double bond.
PNO–CI Studies of electron correlation effects. I. Configuration expansion by means of nonorthogonal orbitals, and application to the ground state and ionized states of methane
Authors: Meyer Wilfried
Published: February 2004

It is shown that the convergence of the configuration expansion of a many-electron wavefunction may be drastically improved without significantly complicating the energy matrix elements by using partially non-orthogonal orbitals. The wavefunction thus obtained is the many-electron analog to the natural orbital expansion of a two-electron wavefunction. The orbitals involved may be approximated by pseudonatural orbitals (``PNO–CI''), and an efficient way of calculating these orbitals is presented. An approximate treatment of multiple substitutions is discussed yielding a coupled electron pair theory formulated within the CI scheme. These methods are applied to the ground state and three ionized states of methane. Including only double substitutions, the computed upper bound to the energy of the ground state is −40.4584 hartree, i.e. 0.057 above the experimental nonrelativistic energy. About 83% of the total correlation energy is accounted for variationally, whereas the coupled-pair approximation yields 89%. The ratio between intraorbital and interorbital valence shell correlation energy is calculated as 1.56 in the case of localized electrons. The equilibrium distance in CH4 is found to be shifted from 2.046 bohr to 2.061 bohr due to correlation, and the harmonic frequency ν1 is reduced from 3149 cm−1 to 3037 cm−1. These results indicate that the anharmonicity corrections to the observed values have probably been over estimated. The correlation contributions to the ionization potentials are analyzed. The theoretical potentials differ from the observed values by 1%–2%. The energy surface of CH4+ is investigated and is used to discuss the photoelectron spectrum of CH4. Contrary to previous work, the lowest minimum of CH4+ is found to have C2v symmetry. In all applications the variational results are compared with those of the independent-pair and coupled-pair approaches, and it is concluded that the latter represents a definite improvement over the first two.
Coulomb holes and expectation values. I. Explicitly correlated wavefunctions
Authors: Banyard K. E. , Seddon G. J.
Published: February 2004

We have examined, in depth, several wavefunctions for the two-electron systems H−, He, Li+, and Be++ already reported in the literature: three noncorrelated wavefunctions and ten explicitly correlated wavefunctions. For each wavefunction we calculated the two-particle distribution function f(r12) and from it obtained the Coulomb hole. The expectation values 〈 rn〉, Δr, ρ(0), 〈 δ3(r1)〉, 〈 δ3(r12)〉, 〈 r12n〉, Δr12, 〈cosγ12〉, 〈r1·r2〉, 〈 r1(r1·r2)〉, and α, the electric dipole polarizability, have also been evaluated. By comparing the results for different wavefunctions some insight was obtained into the roles played by different types of correlation factor.
Extravalence molecular excitations in inert matrices
Authors: Gedanken Aharon , Raz Baruch , Jortner Joshua
Published: February 2004

In this paper we present the results of an experimental study of extravalence excitations of molecular impurities in solid rare gases and in molecular host matrices. We report the absorption spectra of methyl iodide, benzene, ethylene, and acetylene in solid Ne, Ar, Kr, Xe, N2, and CF4 in the spectral region 2000–1150 Å. Spectroscopic evidence has been obtained for the observation of high (n ≥ 2) Wannier impurity states originating from a molecular positive ion. These molecular Wannier series yield spectroscopic information concerning the energetics of molecular photoionization in a dense medium. The large blue spectral shifts of the lowest extravalence molecular excitations in dense media can be rationalized in terms of central cell corrections to the n=1 Wannier state, which are determined by the exciton binding energy.
ENDOR of trapped hydrogen atoms in gamma-irradiated glassy acid matrices and in polycrystalline oxyanion matrices
Authors: Helbert John , Kevan Larry
Published: February 2004

Trapped H atoms are produced in H2SO4/H2O and H3PO4/H2O glasses and in KHSO4/H2O and KH2PO4/H2O polycrystalline matrices by γ-irradiation at 77°K. High power proton ENDOR spectra of the H atoms have been obtained and analyzed. Prominent matrix ENDOR lines at the free proton frequency together with weak sidebands are observed. Two methods have been developed for increasing the resolution of weak sidebands due to isotropic hyperfine interaction. The two methods involve high irradiation dose and the addition of paramagnetic transition metal ions. The ENDOR line shapes are analyzed in terms of modulation of the electron-nuclear dipolar interaction by the relaxing electron. The isotropic hyperfine constant is about 0.75 G in all matrices and is assigned to the nearest neighbor matrix protons 1.8 Å distant. The center matrix ENDOR lines have been analyzed by computer simulation of the line shape to arrive at the spatial extent of the H atom wavefunction. In glassy matrices the wavefunction is nearly unperturbed from that of the free atom, while in polycrystalline matrices the extreme tail of the wavefunction shows significant distortion.
Polymorphism in phenol under pressure: The volume compressibilities of phenol I and II at 10°C, and the phase diagram below 0°C
Authors: Bertie J. E. , Tremaine Peter
Published: February 2004

The volume compression and compressibility of polycrystalline phenol I and II at 10°C are presented up to 3000 bar. The phenol I-II transition below 0°C has been studied for thin samples in a parallel-plate dielectric cell. Phenol II cannot be prepared from phase I below about 1400 bar, and at 1 bar phase II reverts to phase I at −49°C.
Low frequency modes in molecular crystals. XIX. Methyl torsions and barriers to internal rotation of some three top molecules with C3 symmetry
Authors: Durig J. R. , Craven S. M. , Mulligan J. H. , Hawley C. W. , Bragin J.
Published: February 2004

The low frequency infrared spectra of (CH3)3COH, (CD3)3COH, (CH3)3CSH, (CH3)3CNCO, (CH3)3−CNCS, (CH3)3CCHO, (CH3)3CNH2, (CH3)3COCH3 and (CH3)3CODC3 have been recorded for these molecules in both the gaseous and solid states. Harmonic barriers have been calculated for t-butyl methyl group rotation from frequencies measured for the compounds in their solid state. The values are 4.13, 4.52, 4.41, 4.67, 4.86, 3.96, 4.28, 4.71, and 4.55 kcal/mole, respectively. Estimates of the periodic barriers to rotation of the tertiary butyl groups were also determined for (CH3)3COH(∼ 1 kcal/mole), (CH3)3CSH (1.5 kcal/mole), (CH3)3CNCO (0.3 kcal/mole), (CH3)3CNCS (1.3 kcal/mole), (CH3)3CCHO (0.7 kcal/mole), and (CH3)3COCH3 (3.57 kcal/mole) from the observed fundamentals in the infrared spectra of the gases.
Laser fluorescence study of the equilibration of excited vibrational states of SF6 during SF6-rare gas collisions
Authors: Knudtson J. Thomas , Flynn George W.
Published: February 2004

Laser-induced infrared fluorescence was used to measure the vibration to vibration energy transfer process in SF6 during collisions with rare gas partners. The probability of transfer for these collision partners was found to be ¼ to ½ that of pure SF6. Results are compared to Schwartz, Slawsky, Herzfeld-type calculations and the effects of a long range dipole-induced dipole mechanism are considered. The probability of energy transfer for the lighter rare gas collision partners almost certainly contains contributions from several energy transfer paths making comparison between theory and experiment difficult.
Application of bonding molecular orbitals to the valence bands of diamond
Authors: Kervin Paul W. , Lafon Earl E.
Published: February 2004

The valence bands of diamond are determined using a linear combination of bonding molecular orbitals formed from s-p hybridization of the 2s, 2p Hartree-Fock atomic orbitals of the isolated carbon atom. The valence bands obtained in this manner are compared directly with those of more customary methods, and the effect of omitting the antibonding orbitals is found to be extremely small—rarely more than 1% of the width of the valence band. The hybridization parameter is varied and a point-by-point investigation of the valence band is used to verify that the optimum hybridization is identical to that of the usual sp3 orbitals of quantum chemistry.
Polymer conformational statistics. I. Probability distribution
Authors: Fixman Marshall , Alben Richard
Published: February 2004

The probability distribution P(R) of the end-to-end distance is studied for the rotational-isomeric model of polymer chains. A Monte Carlo investigation provided reliable numerical data for P(R), which was then compared with results from two relatively analytic studies. The first of these, and by far the more complicated, was a steepest descent inversion of the characteristic function. This method was found to be more satisfactory, particularly for large R, than Hermite function expansions that have been used in the past. However for chains of N bonds the method fails, for N<40, to show the stiffness evident for small R. A second quite simple method was to maximize the entropy functional of P(R) subject to assigned 〈 R2〉 and 〈 R4〉. The results for P(R) were in good agreement with Monte Carlo results down to N=12, and for all R.
Vibrational spectra of solid methylzinc borohydride, CH3Zn+··· BH4−
Authors: Nibler Joseph W. , Cook Thomas H.
Published: February 2004

Infrared and Raman spectra are presented for solid CH3ZnBH4 and CH3ZnBD4, and these results indicate that the solid is best viewed as being composed of coupled CH3Zn+··· BH4− ions. Least squares valence force constant calculations based on an assumed D2d symmetry for the BH4− ion reproduce the observed frequencies quite well but unfortunately shed no light on the extent of distortion because of correlation between the distortion parameters and the force constants. Reasonable force constants are obtained for distortions of ± 10° from the tetrahedral angle, and these are only slightly different from those of ``free'' BH4− in NH3. Comparison of the BH4− ions in CH3ZnBH4 and BeB2H8 suggests that the BH4− ion is more strongly coupled to the cation in the latter compound.
Rotational spectrum and structure of thioborine: HBS
Authors: Pearson Edwin F. , McCormick Rodney V.
Published: February 2004

The millimeter-wave spectra of eight isotopic species of thioborine have been observed. This unstable molecule displays the rotational spectrum of a linear rotor. For the most abundant isotopic species, H11B32S, the rotational and centrifugal distortion constants are B0=19,083.006 MHz and D0=21.89 kHz. Complete substitution structures have been evaluated eight different ways from the variety of isotopic species observed. The resulting mean values for the B-H and S-B distances (in angstrom units) are 1.1692 and 1.5994, respectively.
Improved classical path approximation for the Boltzmann density matrix
Authors: Miller William H.
Published: February 2004

A classical path approximation for diagonal matrix elements of the Boltzmann density operator, i.e., the equilibrium particle density, is derived and its properties analyzed, which is not only more accurate than an earlier result, but considerably simpler to apply to systems with many degrees of freedom. The most important simplifying feature is that it is not necessary to deal with classical trajectories with double-ended boundary conditions. The partition function, for example, is given by a phase space average over initial conditions of an exponential function of a classical action integral along the trajectory with these initial conditions.
On the Mott-Cabrera oxidation rate equation and the inverse-logarithmic law
Authors: Ghez Richard
Published: February 2004

It is shown that the often cited inverse-logarithmic oxidation law, X−1 ∝ −logt, where X is the oxide thickness and t the observation time, is not an asymptotic solution of the rate equation derived by Mott and Cabrera in their theory of low temperature oxidation. Thus whether or not the inverse-logarithmic law is experimentally verified has no bearing on the validity of this theory. A correct solution is presented, and it is shown that for thin oxide films a plot of X−1 vs log(t/X2) should yield straight lines. Vermilyea's data on Ta is reexamined according to this analysis and yields reasonable results for the activation energy of defect solution (1.59 eV) and for the potential across the oxide (1.79 V).
Vibrational intensities of F2CS and Cl2CS
Authors: Bruns Roy E.
Published: February 2004

The CNDO approximate molecular wavefunctions for F2CS and Cl2CS have been applied to the calculation of the derivatives of the dipole moment with respect to the symmetry coordinates. Unique sets of experimental derivatives in the A1 symmetry species of these molecules are definitely preferred according to these calculations. Two experimental sets of derivatives in the B1 symmetry species of each molecule are consistent with the calculated results. All the preferred experimental derivatives for the stretching coordinates, ∂ p/∂ rCF and ∂ p/∂ rCCl, and the in-plane bending coordinate, ∂ p/∂α, have negative signs. The CF out-of-plane bending derivative in F2CS appears to be negative although the corresponding CCl derivative is probably positive. The theory qualitatively reflects trends in the magnitudes of these derivatives in the different X2CY molecules and in different symmetry species of the same molecule. Empirical equations of the type (∂ p/∂ rCCl)Cl2CS−(∂ p/∂ rCF)F2CS=(∂ p/∂ rCCl)Cl2CO−(∂ p/∂ rCF)F2CO, relating corresponding derivatives for F2CO, Cl2CO, F2CS, and Cl2CS, are discussed.
Trajectory studies of atomic recombination reactions. III. Energy transfer in bromine-inert gas systems
Authors: Clarke A. G. , Burns G.
Published: February 2004

Energy transfer in collisions of helium, argon, and xenon atoms with highly vibrationally excited bromine molecules is studied by computing 3D classical trajectories. Dissociation probabilities Pdiss are obtained as a function of the energy barrier Δ E/kT to be overcome. They vary approximately as Pdiss=P0(1+Δ E/kT)−4.5, and P0∝μ0.6 where μ is the reduced collision mass. Mean vibrational and rotational energy changes are calculated for systems in which the bromine molecules are initially in thermal rotational equilibrium but have varying degrees of vibrational excitation. The relative importance of vibration-rotation coupling at different vibrational energies is demonstrated. The order of collision numbers for rotational relaxation is He>Ar>Xe. The relative efficiencies of He, Ar, and Xe atoms in producing vibrational relaxation of Br2 vary with the initial degree of vibrational excitation.
Collision amplitudes and molecular electron distributions
Authors: Lassettre Edwin N.
Published: February 2004

It is demonstrated that the Hartree-Fock integral equations in momentum space can be put into a form where the kernel function is the collision amplitude in Born approximation (with exchange) for the collision of an additional incident electron with the system under consideration. The possibility of an experimental determination of the kernel function is discussed.
Single crystal neutron diffraction study of sodium cyanide
Authors: Rowe J. M. , Hinks D. G. , Price D. L. , Susman S. , Rush J. J.
Published: February 2004

The neutron diffraction pattern of a single crystal of cubic NaCN has been measured at 295°K. These data and earlier results for KCN at 180 and 295°K have been analyzed in terms of a model which expands the angular distribution of CN− orientations in symmetry-adapted spherical harmonics. From this analysis, the distributions of CN− in these crystals have been derived, and these indicate a monotonic trend in the series KCN (295°K) thru KCN (180°K) to NaCN (295°K) involving a shift in preferential orientation from the [111] toward the [100] directions in the crystal. Unusually large translational Debye-Waller factors have also been derived for NaCN and KCN from the fitting procedure, in agreement with earlier results and analysis on KCN. In addition, an average C≡N bond distance of 1.18 Å is obtained.
Photoelectron and optical spectra of TiCl4 and VCl4
Authors: Parameswaran T. , Ellis D. E.
Published: February 2004

The photoelectron and optical spectra of TiCl4 and VCl4 are investigated within the framework of the Hartree-Fock-Slater model. The sensitivity of these spectra to choice of atomic configuration (in a non-self-consistent model) and exchange scaling parameter α is explored, making use of a discrete variational method previously reported. It is shown that values of α can be found to match both ionization potentials and optical transitions fairly well in the simplified one electron model based on superimposed atomic charge densities. Results of self-consistent field models are presented, and the relationship to more rigorous theories is discussed.
Argon matrix Raman spectrum of LiO2. Bonding in the M+O2− molecules and the ionic model
Authors: Andrews Lester , Smardzewski Richard R.
Published: February 2004

The Raman spectrum of LiO2 in argon matrices has been observed using 4880 Å excitation. The intraionic (O–O)− mode was observed as a sharp intense band at 1097 cm−1; the symmetric interionic (Li+–O2−) mode was recorded as a weaker broad feature at 694 cm−1. Oxygen-18 and lithium-6 substitution confirmed these vibrational assignments. The trend of increasing O2− vibrational frequency with increasing alkali atomic weight in the M+O2− molecules is rationalized using the Rittner ionic model of polarizable ion pairs.
Variational approach to the quantum theory of scattering. II. The Kohn method and Kato eigenvalue problem
Authors: Kolker H. J.
Published: February 2004

Certain similarities between the equations arising in scattering theory and the variational approach to time dependent perturbation theory are used to show that the Kohn variational technique for solving scattering problems is equivalent to a variational method for calculating a special set of eigenfunctions, in terms of which the solution to the scattering problem is expanded. Contrary to the results of previous studies, the error term in the scattering problem is bound from below, and the Kohn method is equivalent to a minimum principle. Levinson's theorem is rederived, and extended to higher energies. On the basis of the theory presented in this study, a new method for calculating upper and lower bounds on the energies of resonance and Ramsauer-Townsend states is suggested.
Mobility, diffusion, and clustering of K+ ions in gases
Authors: Thomson G. M. , Schummers J. H. , James D. R. , Graham E. , Gatland I. R. , Flannery M. R. , McDaniel E. W.
Published: February 2004

We have measured, with a drift tube mass spectrometer, the mobilities and longitudinal diffusion coefficients of K+ ions in nitrogen and carbon monoxide at 300°K. The measurements were made over a range of E/N extending from thermal values up to 636 × 10−17 V·cm2. Here E is the drift field intensity and N is the gas number density. The zero-field reduced mobilities of K+ ions in N2 and CO were determined to be (2.54 ± 0.05) and (2.30 ± 0.04) cm2/V · sec, respectively. The low-field diffusion coefficients are in excellent agreement with the values calculated by the Einstein equation from the experimental zero-field mobilities. The experimental diffusion coefficients are compared with the predictions of an equation developed by Wannier on the assumption that the ion-molecule interaction consists of only the attractive polarization force, of which a constant mean free time between collisions is a consequence. Comparison is also made with a modified version of this equation which contains the ionic drift velocity rather than the mean free time. The agreement between the experimental data and the modified Wannier equation is very close up to average ionic energies of about 0.6 eV. We also observed clustering of single molecules of CO2, NO, CO, N2, O2, Ar, D2, Ne, and He to K+ ions when the drift tube was filled successively with the indicated gases. At room temperature, clustering is likely to be of importance in studies of the transport properties of K+ ions in all of these gases except D2, Ne, and He.
On the electronic spectrum of protonated adenine: A single crystal study of adenine hydrochloride
Authors: Chen Holly Ho , Clark Leigh B.
Published: February 2004

The polarized reflection spectra of four faces of adenine·HCl single crystals have been measured between 6000 and 1830 Å. The faces which were examined included the naturally occurring (100), (001), and (102̄) planes plus the artificially prepared (104̄) face. Absorption spectra are derived from the corresponding reflectance curves via the Kramers-Kronig transformation. Clear evidence for the presence of four transitions which are polarized in the molecular plane and are presumably π → π* in origin is observed. The effects of intermolecular interactions are considered within the dipole approximation, and a procedure is given for extracting free molecule transition moments from the crystal spectra. This treatment leads to a satisfactory accounting of the observed dichroic ratios and the observed k̂ dependence of the factor group splittings and band shapes. Transition moment directions (measured contraclockwise from the C4–C5 axis) and oscillator strengths are evaluated to be as follows: I (λ ≃ 2730 Å)−28°, f=0.08; II (λ ≃ 2570 Å) + 100°, f=0.20; III (λ ≃ 2060 Å)+15°, f=0.16; IV (λ ≃ 1960 Å)+120°, f=0.51. No evidence for perpendicularly polarized n→ π* transitions was found.