bluuebunny/crossref_metadata_2025 huggingface dataset:
A quantitative comparison of multiple-photon absorption in polyatomic molecules
Authors: Judd O. P.
Published: February 2003

A quantitative comparison of multiple-photon absorption for a number of polyatomic molecules has been performed. A basis for this comparison has been developed that takes into account the different experimental conditions and molecular parameters, and provides suitable normalization of the fluence and absorption parameters. The normalization, and consequently the generalized interaction process, can be specified in terms of the spectroscopic absorption cross section of the molecule σ0(ν) and effective fraction of molecules that interact with the radiation field 〈f〉. The results of the study indicate that in terms of the normalized absorption variables, multiple-photon absorption is a general phenomenon that is qualitatively the same for all molecules; quantitative differences can be related to differences in σ0 and 〈f〉. The absorption of all polyatomic molecules can be described in terms of the number of photons absorbed per molecule η (Φ) at a fluence Φ. The data indicate that in spectral regions where σ0(ν) ≳0.1 (max), η (Φ) can be represented by the function Φγ where γ=1 for σ0Φ/〈f〉<1 and γ?2/3 for σΦ/〈f〉≳1. Quantitative values of 〈f〉 and the functional dependence of η (Φ) on the exerimental parameters such as gas pressure, optical bandwidth, and optical pulse duration are derived and shown to be in good agreement with experimental data for SF6. These results are also compared with the anharmonic oscillator model for multiple-photon excitation of polyatomic molecules.
A simple model for describing the density dependence of the vibrational relaxation in a polyatomic nonassociated fluid
Authors: Sedlacek M. , Asenbaum A.
Published: February 2003

A model is presented which allows the description of the density dependence of the vibrational relaxation in a fluid consisting of polyatomic molecules over a wide range of densities. In order to calculate the number of intermolecular collisions a molecule undergoes per unit time, use is made of the radial distribution function for a system of Lennard-Jones molecules as follows from molecular dynamics computer experiments. A density dependent energy exchange probability is introduced which partially compensates the influence of the steep increase in the intermolecular collision rate on the vibrational relaxation frequency above 350 amagats. Thus satisfactory agreement between theory and experiment can for instance be obtained between 0 and 600 amagats for CO2 at 25 and 50 °C.
Gaseous NH4Cl revisited: A computational investigation of the potential surface and properties
Authors: Raffenetti Richard C. , Phillips Donald H.
Published: February 2003

An investigation of the potential surface and properties of the 1A1 ground state of gaseous NH4Cl is presented. The calculations, which utilize a ’’good’’ basis set and include investigation of the valence correlation effects, result in a description of gaseous NH4Cl as a weakly bound complex with NH3 and HCl structures essentially equivalent to those of the isolated fragments. The charge distributions within the fragments are modified in the complex, however, and there is a small amount of charge transfer. The results are in reasonable agreement with estimate of D00 from high temperature mass spectroscopic investigations and vibrational frequencies from matrix experiments. The equilibrium constant for NH4Cl?NH3+HCl indicates that the homogeneous formation of NH4Cl does not represent a significant chemical sink for HCl in the upper atmosphere.
Combined multiple pulse NMR and sample spinning: Recovery of 1H chemical shift tensors
Authors: Taylor R. E. , Pembleton R. G. , Ryan L. M. , Gerstein B. C.
Published: February 2003

Combined multiple pulse NMR and sample spinning techniques have been used to obtain the carboxylic acid 1H chemical shift tensor in 2,6-dimethylbenzoic acid. The values obtained were σ⊥=−18.5 ppm and σ∥=−4.2 ppm relative to TMS. A discussion of the changes in line shape of chemical shift tensors under sample rotation is given. This leads to a quantitative expression for the sample rotation speed, ωr≫σzz−1/3Trσ, to define ’’rapid’’ sample rotation.
The total scattering cross sections for H2+H2, D2+D2, and HD+HD for relative collision energies below 10 meV
Authors: Johnson Dean L. , Grace R. S. , Skofronick J. G.
Published: February 2003

Relative total scattering cross sections for normal H2+normal H2, para-H2+para-H2, normal D2+normal D2, ortho-D2+ortho-D2, and HD+HD were measured at energy below 10 meV by colliding particles from two inclined nozzle beams. Cross sections for the H2+H2 pairs were measured over a relative velocity range of 200–1450 m/s; the normal H2+normal H2 results suggest a l=3 orbiting resonance near 375 m/s while the para-H2+para-H2 results have a l=4 orbiting resonance near 585 m/s. This latter resonance has a peak energy of 1.79 meV and a FWHM of 1.05 meV. The D2+D2 cross sections were measured over the velocity range of 190 to 1000 m/s and a minimum between the l=4 and l=5 orbiting resonances was observed. Some preliminary measurements on HD+HD over the range of 250–1250 m/s indicate a possible l=4 orbiting resonance near 300 m/s. The experimental work compares favorably to cross sections calculated from a theoretical potential.
A molecular dynamic calculation of the structure of sodium silicate glasses
Authors: Soules T. F.
Published: February 2003

Molecular dynamic calculations of the structure of a series of sodium silicate glasses, a common soda–lime glass, a sodium borosilicate glass and a sodium–potassium silicate glass were made. Calculated radial distribution functions are in good agreement with existing x-ray diffraction data and a previous MD calculation on silica glass. During the dynamical runs alkali and alkaline earth cations cluster around nonbridging oxygen atoms. In the borosilicate glass studied, the boron atoms enter the silica network and are tetrahedrally coordinated by oxygen. Other features of the simulated glasses include the coordination number of cations, the degree of inhomogeneity, the distribution of bridging and nonbridging oxygen atoms, the Si–O–Si bond angles, the ring structure, etc. Some preliminary diffusion studies will be presented.
Radiation damage studies by x-ray photoelectron spectroscopy. III. Electron irradiated halates and perhalates
Authors: Sasaki T. , Williams R. S. , Wong J. S. , Shirley D. A.
Published: February 2003

X-ray photoelectron spectroscopy (XPS) was employed to identify radiolytic products in NaClO3, LiClO3, LiBrO3, and LiIO4 which were irradiated in situ with 1.4 keV electrons. While LiIO4 was exceedingly resistant to radiation damage, the remaining compounds decomposed readily by an apparent stepwise release of oxygen from the parent oxyanion to form XO−3, XO−2, X− (X=Cl or Br) and O2. Heavy irradiation produced IO−3 in LiIO4 and alkali halide, and alkali-metal oxide in the remaining compounds. These final products were cathodochromic, with both trapped electron (F or anion vacancy) and hole (Vk or X−2) centers being formed. Photobleaching of these centers also induced a partial reoxidation of the halogen species that had been formed by radiation-induced reduction. Comparison of the production rate of Li2O with that of X−2+X− suggests that Li2O is not a primary product,but may be produced via oxidation of colloidal particles of metallic lithium in the alkali halide layer of the heavily irradiated samples.
The (2p2)3P state of H− and the convergence of the CI series
Authors: Jáuregui Rocío , Bunge Carlos F.
Published: February 2003

A sequence of increasingly accurate configuration interaction (CI) wave functions is used to discuss binding and the convergence of the CI series for the (2p2)3P state of H−. We get an upper bound energy Eu=−0.1 253 547 166 a.u.(H), the lowest yet obtained, lying within an estimated 0.4 μhartree from the exact eigenvalue of Schrödinger’s nonrelativistic equation. The angular energy limits define angular energy increments ΔEl which follows the formula ΔEl?0.18 (l+1/2)−7a.u. for l?4. Five or eighty per cent of binding is obtained with a three-term Ψ=c1(2p)2+c2(2p′)2+c3 (3d)2 depending on whether the 2p,2p′, and 3d orbitals are energy optimized STO’s or accurate natural orbitals, respectively.
Many-body perturbation theoretic calculation of the nuclear spin-coupling constant in the hydrogen molecule
Authors: Itagaki T. , Saika A.
Published: February 2003

The Fermi contact term of the nuclear spin-coupling constant in the HD molecule is calculated by diagrammatic many-body perturbation theory (MBPT) employing extended Gaussian basis sets in the standard Hartree–Fock potential. The calculation through second order in electron correlation gives a value for the coupling constant of 28.1 Hz, which is only 65% of the experimental value. However, Hartree–Fock exclusion principle violating diagrams are found to contribute significantly. By summing these dominant diagrams selectively by the geometric approximation, we obtain a value of 37.0 Hz. The convergence is enhanced with shifted denominators, which finally yield 38.0 Hz within ∼10% of the probable Fermi contact value. Advantages of MBPT in calculating nuclear spin-coupling constants are discussed.
Local mode combination bands and local mode mixing
Authors: Burberry Mitchell S. , Albrecht A. C.
Published: February 2003

Side bands appear in the local-mode (LM) overtone vibrational spectra of many substances, including the aromatic and aliphatic hydrocarbons. Many of these side bands are interpretable as being LM–LM combination bands of various C–H oscillators in these molecules. The simultaneous excitation of two LM’s represents the most commonly observed transition of the many LM–LM combinations possible. However, such transitions are strictly forbidden as long as the dipole moment and Hamiltonian operators are separable within the LM subspace. The consequences of the breakdown of each of these separabilities is investigated. Application is made to detailed new spectra for benzene (containing the prototypical aromatic C–H oscillator) and tetramethylsilane (containing a prototype of the aliphatic primary C–H oscillator). Multisite terms in the dipole operator are treated empirically and are able to rationalize the intensities of binary LM combination bands. Inter-LM coupling terms in the Hamiltonian are taken from previous ab initio calculations and normal mode analysis. The Hamiltonian is diagonalized within a truncated basis of LM (anharmonic) eigenfunctions. The new eigenfunctions explain not only the appearance of the side bands correctly, but also account for energy splittings in the spectra, particularly conspicuous at low quantum numbers. Separability for both operators is found to be a good zeroth-order approximation for both molecules, but somewhat less so for tetramethylsilane whose methyl substituents contain three LM oscillators sharing a common atom.
Optical studies of liquid formamide in the vacuum ultraviolet
Authors: Heller J. M. , Hubbell H. H. , Painter L. R. , Birkhoff R. D.
Published: February 2003

Optical and dielectric functions of liquid formamide have been measured between 2 and 24.5 eV. Below 7.8 eV a semicylinder technique was used, whereas above 10.4 eV a new ionization method was employed. Peaks in the absorption spectrum at approximately 6.8 and 16.0 eV are attributed to excitation of π and σ electrons in the formamide molecule. The latter electrons participate in a collective oscillation as evidenced by a shift in the peak in the energy loss function from the corresponding peak in the absorption spectrum. Sum rule calculations give 11.8 and 4.3 of the eighteen formamide valence electrons participating in optical absorption and energy loss processes, respectively, out to 24.5 eV.
An open shell polarization propagator: Doublet–doublet transitions
Authors: Swanstro/m Peter , Jo/rgensen Poul
Published: February 2003

The system density operator is expanded in powers of an arbitrary perturbation which may be electronic correlation and an efficient formalism is obtained for evaluating the matrix elements required to perform molecular property calculations. A generalized Brillouin condition is derived and used to define a Fock potential. The problem of assigning the poles of the polarization propagator to transitions of pure space and spin symmetry is discussed and the polarization propagator is expanded through second order in correlation for a spatially nondegenerate doublet state. Preliminary results for the Li(2S) and BeH(2Σ) systems indicate that agreement with experimental excitation energies is as favorable for open shell systems as for closed shells.
The microwave spectrum of argon methyl chloride
Authors: Steed Joseph M. , Bernstein Lawrence S. , Dixon Thomas A. , Janda Kenneth C. , Klemperer William
Published: February 2003

Molecular beam electric resonance spectroscopy of the complex ArCH3Cl is reported. The spectra, only observable at low resolution (3 MHz), are complex and not analyzed. No evidence was found for a symmetric rotor spectrum comprising part of the observed features. It is concluded that the molecular structure of the complex consists of Ar binding off the C–Cl axis.
Vibrational energy redistribution in glyoxal following internal conversion
Authors: Naaman R. , Lubman D. M. , Zare R. N.
Published: February 2003

The vibrational redistribution of energy following internal conversion in glyoxal has been studied in a molecular beam using the ’’pump-and-probe’’ technique. A beam of glyoxal is initially pumped from So to a selected vibronic level of S1. After internal conversion occurs, a tunable dye laser is used to probe the hot band absorption of the molecules by re-excitation into S1. The total fluorescence as a function of probe laser wavelength is observed. The excitation spectrum is broad and featureless in the red but shows sharp structure in the vicinity of the pump laser wavelength. However, the sharp structure depends on the portion of the rotational contour excited. The experimental results show that the redistribution of energy is nonstatistical among the isoenergetic levels. This is explained by assuming that combination bands of low quanta of vibrations are the accepting levels in the internal conversion process and that these accepting levels undergo slow energy redistribution.
Dipole moments of poly(propylene sulfide) chains
Authors: Riande E. , Boileau S. , Hemery P. , Mark J. E.
Published: February 2003

Dielectric constant measurements in carbon tetrachloride and in benzene were used to obtain dipole moments of atactic poly(propylene sulfide) chains of sufficiently low molecular weight that excluded volume effects were absent. The results are in very good agreement with rotational isomeric state calculations. Comparisons with previously reported experimental results on the high molecular weight polymer confirm the existence of a significant specific solvent effect on the dipole moments.
Pressure broadening of the O2 microwave spectrum
Authors: Smith Earl W. , Giraud Michel
Published: February 2003

The pressure broadened half-widths of the 60 GHz microwave spectrum in O2 have been calculated for low pressures where the lines do not overlap. Both self-broadening and foreign gas broadening by noble gases have been calculated using various semiempirical potential surfaces. Agreement with experimental results is quite good. Differences with various other theoretical calculations are discussed.
Some relations between electronic distribution and electronegativity
Authors: Politzer Peter , Weinstein Harel
Published: February 2003

Some important electronegativity concepts, which have recently been given a precise and rigorous basis by Parr and his collaborators in terms of density functional theory, are here deduced from two different physical models of a molecule, without specifying any particular theoretical framework. The principle of electronegativity (or chemical potential) equalization is reaffirmed as is the dependence of the direction of electronic charge migration upon electronegativity difference. These concepts are then generalized to any arbitrary portions of the total number of electrons in a system.
Experimental detection of macromolecular flexibility by observation of time-dependent diffusion coefficients
Authors: Harvey Stephen C.
Published: February 2003

Large intramolecular motions with a single degree of freedom, such as bending, stretching, or torsion, are often hydrodynamically coupled to the translational or rotational motions of the molecule. For such a molecule, the diffusion coefficient along the free coordinate is time dependent, and the form of that time dependence is obtained here from the solution to the Smoluchowski equation. In the case of rotational diffusion, these results have application for the examination of macromolecular flexibility by magnetic resonance, transient birefringence, depolarized light scattering, and fluorescence depolarization. The time dependence of the translational diffusion coefficient means that in dynamic light scattering experiments the power spectrum of the scattered intensity will not be a single Lorentzian, and the intensity autocorrelation function will not be described by a single exponential decay. These results hold even when the molecule is much smaller than the wavelength of light, offering a new experimental approach to the detection of molecular flexibility.
Chemiluminescent ion–molecule reactions: Rotational–vibrational population distributions of CH+(A 1Π) and CD+(A 1Π) from C++H2 and C++D2 collisions
Authors: Kusunoki I. , Ottinger Ch.
Published: February 2003

Optical emission in the 3000–5000 Å range was observed as a result of low-energy (3–8 eVCM) ion–molecule reactions of C+ ions with H2 and D2 molecules. Most of the emission is due to (A 1Π→X 1Σ+) transitions of CH+ and CD+, which were each studied with 6 Å and 20 Å FWHM optical resolution. Rotational lines of much higher J were observed than are present in spectra from conventional CH+ light sources. The observed spectra were simulated on a computer, taking into account the strong dependence of the electronic transition moment on the internuclear distance, as given by ab initio calculations. Relative band oscillator strengths fv′v″(J) were calculated for each rotational line J and were found to depend strongly on J as result of centrifugal stretching. These calculations were done using the R centroid approximation as well as the exact band oscillator strength formula. From the spectrum simulations, accurate rotational and vibrational population distributions were derived. A simple model is proposed which explains the isotope effect on the rotational distributions and gives directly the distribution of impact parameters leading to reaction.
Molecular dynamics studies of depolarized light scattering from argon at various fluid densities
Authors: Ladd A. J. C. , Litovitz T. A. , Montrose C. J.
Published: February 2003

Molecular dynamics simulations have been used to calculate the two-, three-, and four-particle contributions to depolarized Rayleigh scattering from argon at various fluid densities. This decomposition is helpful in understanding how the dynamical processes that cause depolarized light scattering vary with density. The two-, three-, and four-particle time correlation functions decay very slowly in the dense fluid, their relaxation times being an order of magnitude greater than that associated with depolarized Rayleigh scattering. These long-time tails and their cancellation are explained by a simple diffusional model. The correlation functions obtained from this theory are in agreement with the results of the computer simulation. The high frequency slope of the dense fluid spectrum was found to be similar to that of the pair contribution, but it cannot be unambiguously concluded from this that the high frequency regime of the dense fluid spectrum is dominated by binary-like dynamics.
Elastic electron scattering by CO2, OCS, and CS2 from 0 to 100 eV
Authors: Lynch Maureen G. , Dill Dan , Siegel Jon , Dehmer J. L.
Published: February 2003

The integrated elastic electron scattering cross sections for CO2, OCS, and CS2 were calculated from 0 to 100 eV using the continuum multiple-scattering model with the Hara exchange approximation. For each molecule, a strong π-type shape resonance occurs between ∼1–4 eV, followed by multiple, weakly resonant features at higher kinetic energy. The latter are only marginally observable in the integrated cross section, but, in some cases, should be clearly observable in vibrational excitation spectra. Agreement with available experimental information is good for all three cases, except for a gross underestimation of the background (nonresonant) scattering cross section in OCS resulting from our omission of dipole scattering in this work.
Vibrational dynamics of hydrogen bonds. I. FHF− system
Authors: Barton S. A. , Thorson W. R.
Published: February 2003

A general treatment of nonharmonic vibrational dynamics in an isolated hydrogen bond is presented, based on an adiabatic mode separation of hydrogenic motions from those of the heavier particles. The validity of the Stepanov (adiabatic) approximation is examined by including correct nonadiabatic couplings and solving the resultant coupled eigenvalue problem for a model system (FHF−) in which the couplings have maximal intrinsic strength. Convergence is demonstrated by including up to three coupled protonic states. Comparison with exact results thus obtained shows that the adiabatic approximation gives transition frequencies within 1%, and infrared relative intensities to about 10%—in cases where the coupled protonic levels are not degenerate. Cases of isolated strong coupling, arising from accidental or systematic degeneracies of proton levels, can be accommodated in the theory in a direct way, but do not arise for FHF−. The theory may similarly be extended as the basis for the dynamical analysis of systems involving two or more hydrogen bonds. Calculations on the FHF− system are based entirely on the ab initio potential energy surface of Almlo/f, but employ a model form with correct dissociation properties. The close agreement of computed and observed transition frequencies and IR relative intensities leads to a convincing assignment (in the adiabatic mode description) for nearly all transitions in the bifluoride spectrum (e.g., in KHF2(s)), on the basis of an isolated ion model.
Interpretation of ES, CS, and IOS approximations within a translational–internal coupling scheme. I. Atom–diatom collisions
Authors: Coombe D. A. , Snider R. F.
Published: February 2003

Rotational invariance is applied to the description of atom–diatom collisions in a translational–internal coupling scheme, to obtain energy sudden (ES), centrifugal sudden (CS), and infinite order sudden (IOS) approximations to the reduced scattering S matrix S (??;L;jλ). The method of presentation emphasizes that the translational–internal coupling scheme is actually the more natural description of collision processes in which one or more directions are assumed to be conserved.
Nanosecond time-resolved EPR in pulse radiolysis via the spin echo method
Authors: Trifunac A. D. , Norris J. R. , Lawler R. G.
Published: February 2003

The design and operation of a time-resolved electron spin echo spectrometer suitable for detecting transient radicals produced by 3 MeV electron radiolysis is described. Two modes of operation are available: Field swept mode which generates a normal EPR spectrum and kinetic mode in which the time dependence of a single EPR line is monitored. Techniques which may be used to minimize the effects of nonideal microwave pulses and overlapping sample tube signals are described. The principal advantages of the spin echo method over other time-resolved EPR methods are: (1) Improved time resolution (presently ∼30–50 nsec) allows monitoring of fast changes in EPR signals of transient radicals, (2) Lower susceptibility to interference between the EPR signal and the electron beam pulse at short times, and (3) Lack of dependence of transient signals on microwave field amplitude or static field inhomogeneity at short times. The performance of the instrument is illustrated using CIDEP from acetate radical formed in pulsed radiolysis of aqueous solutions of potassium acetate. The relaxation time and CIDEP enhancement factor obtained for this radical using the spin echo method compare favorably with previous determinations using direct detection EPR. Radical decay rates yield estimates of initial radical concentrations of 10−410−3M per electron pulse. The Bloch equations are solved to give an expression for the echo signal for samples exhibiting CIDEP using arbitrary microwave pulse widths and distributions of Larmor frequencies. Conditions are discussed under which the time-dependent signal would be distorted by deviations from an ideal nonselective 90°–τ–180° pulse sequence.
Spin decoupling in two-dimensional J-resolved NMR spectroscopy
Authors: Nagayama Kuniaki
Published: February 2003

Spin decoupling techniques are described which are suitable to enhance the utility of two-dimensional J-resolved NMR spectroscopy with regard to (a) simplification of 2D spectra, and (b) assignment of resonance peaks. The theoretical foundations are developed to analyze three different types of double resonance experiments, which differ by the time period during which decoupling is applied. The theoretical predictions are confirmed by experiments on the AX spin system of 2,3,4-trichlorophenol. An extension of this technique permits the recording of pure phase 2D J-resolved NMR spectra.
Nonlinear shear viscosity of a gas
Authors: Zwanzig Robert
Published: February 2003

This article contains a derivation of the shear rate dependence of the viscosity of a gas in steady Couette flow. The viscosity is found from a solution of the Bhatnagar-Gross-Krook model kinetic equation, taking into account the viscous heating of the gas. The results are in reasonably good agreement with computer molecular dynamic experiments by Naitoh and Ono.
Nuclear magnetic resonance study of ionic motions in ammonium bifluoride
Authors: Reynhardt E. C. , Watton A. , Petch H. E.
Published: February 2003

Proton and fluorine second moments in powdered NH4HF2 have been measured over the temperature range 77 <T<395 K. Detailed second moment calculations have been made for various combinations of HF2− and NH4+ reorientations. Comparison of these values with the experimentally observed plateau’s shows that the lattice is rigid at 77 K and that the first motion affecting the second moments is the isotropic reorientation of the NH4+ ion. At higher temperatures one of the nonequivalent HF2− ions starts executing 180° flips followed by the other HF2− ion with a similar reorientation. Proton line shapes for powdered NH4HF2 and KHF2 have been calculated by substituting for each fine structure component of an isolated HF2− ion a Gaussian line weighed by isotropic angular distribution functions. Good agreement between experimental and calculated line shapes have been obtained. Proton and fluorine spin-lattice relaxation times in the laboratory frame, T1, and the rotating frame, T1ρ, have been measured over the temperature range 167<T<395 K. At lower temperatures the isotropic reorientation of the NH4+ ion is the dominant relaxation mechanism. The activation energy is 6.0±0.2 kcal/mole. At higher temperatures the unixial 180° reorientations of the two nonequivalent HF2− ions modulate the interionic H–F and F–F dipole–dipole interactions. Activation energies of 5.9±1 and 15.5±2 kcal/mole have been obtained for the reorientations of the HF2− ions. These values are discussed in terms of the environments of the nonequivalent HF2− ions. The influence of the reorientations on the strength of the hydrogen bonds is discussed and in terms thereof an explanation is offered for the absence of the phase transition observed in other bifluoride compounds.
Isotope chemistry and molecular structure. The WINIMAX weighting factor
Authors: Lee Myung W. , Bigeleisen Jacob
Published: February 2003

The modulating coefficients for the finite polynomial expansion of the logarithm of the reduced partition function, lnb (u), of a harmonic oscillator have been obtained for the range of 0⩽u⩽umax with a weighting function g (u) =ua by the method of least squares. The coefficients obtained for a=o are almost identical with the MINIMAX coefficients which correspond to an unweighted expansion; the least square coefficients obtained with a=1 are almost identical with the finite orthogonal polynomial coefficients derived by Ishida, Spindel, and Bigeleisen with the assumption g (u) ∼u. Comparison of the least square modulating coefficients derived as a function of u shows that the WINIMAX coefficients for the reduced vibrational partition function derived by Lee and Bigeleisen corresponds to a weighting function u6. It is shown that this weighting function is near optimum to insure minimum amplitudes of oscillation in the expansion of lnb (u) as a function of the order of the expansion and to include most of the important molecular structure information contained in the moments of the eigenvalues. Beyond Σui6, there is little new structural information.
Electron pair interactions in H2O, H2S, NH3, and CH4
Authors: Palke William E.
Published: February 2003

A simple method is presented for determining the force between two pairs of electrons that arises from the Pauli principle’s requirement that doubly occupied orbitals be orthogonal. The method does not involve wave functions that violate the Pauli principle. Strengths of the Coulomb and exchange energy interactions between electron pairs as well as the effects of the orthogonality constraints are determine in H2O, H2S, NH3, and CH4, and a comparison is made with results of a different approach for H2O and H2S. For all but one example, bond–bond repulsions are found to exert a greater effect on the internuclear angle than bond–lone pair or lone pair–lone pair forces.
EPR study of temperature dependent rotations of polytetrafluoroethylene chains labeled with peroxy radicals
Authors: Olivier D. , Marachi C. , Che M.
Published: February 2003

The temperature dependence of the EPR spectra of peroxy chain radicals (R–CF2–CFOO ̇ –CF2–R′) and peroxy propagating radicals (R″CF2–CF2OO ̇) trapped in polytetrafluorothylene (PTFE) is studied in the range −196 °C, +300 °C, and analyzed in terms of the rotation freedom in the polymer. The peroxy propagating radical undergoes rotation along the polymer chain axis above −120 °C, whereas the peroxy chain radical has the same rotation but only above −20 °C. This difference is explained in terms of a particular mobility of the extremity of the carbon chain. Above −40 °C, the peroxy propagating radical rotates about the C–O bond in addition to the rotation about the chain axis. The additional rotation about the C–O bond is not observed below the decomposition temperature of the peroxy chain radical.
Variable angle photoelectron spectroscopy of the fluoroethylenes
Authors: Sell Jeffrey A. , Kuppermann Aron
Published: February 2003

He I photoelectron spectra of fluoroethylene, 1,1-difluoroethylene, cis-1,2-difluoroethylene, trans-1,2-difluoroethylene, trifluoroethylene, and perfluoroethylene were obtained over the scattering angle range of 45° to 120° and compared with those of ethylene. Vibrational frequencies of the ionic states were measured and their symmetry modes assigned. The asymmetry parameter β as a function of the ionization potential was measured for each molecule. The value of β for the first ionization potential band of these molecules was found to decrease monotonically with increasing fluorine substitution. This variation was interpreted as being due to resonance mixing of the lone pair F π orbitals with C–C π orbitals. The data obtained were used to assign some of the spectral bands observed.
Infrared intensities: A MO study of some properties of bond polar parameters in fluorinated ethylenes
Authors: Galabov B. , Orville-Thomas W. J.
Published: February 2003

Infrared intensities depend upon the polar nature of valence bonds. Model molecular orbital studies show that to a great extent contributions assumed to arise from vibrational motions associated with individual bonds are additive. It is shown that in general it is not possible to obtain identical bond polar parameters from the consideration of vibrations belonging to different symmetry classes.
A comparison of classical and quantum mechanical calculations of vibrational energy transfer. III. Use of the forced quantum oscillator and moment methods for quantization of product state distributions
Authors: Gordon Robert J.
Published: February 2003

Vibrational transition probabilities calculated using the semiclassical forced quantum oscillator model and the information theoretic moment method were compared with exact quantal results in two model systems. In both cases the moment method was generally more accurate. Under conditions leading to a large distortion of the molecular force constant, the forced oscillator model was found to be in significant error. At low collision energies with two or three open states the moment method was inaccurate, and the forced oscillator model is preferred.
Dispersion relations and sum rules for the normal reflectance of conductors and insulators
Authors: King Frederick W.
Published: February 2003

A systematic derivation of sum rules satisfied by the normal reflectivity and phase of conductors and insulators is presented. The derivation of the dispersion relations for the generalized reflectivity is investigated, and some special features of these relationships are noted. The reflectivity at complex frequencies is examined, and this leads to a simple sum rule for testing theoretical models of reflectance data.
Intramolecular dynamics of some van der Waals dimers
Authors: Beswick J. A. , Jortner Joshua
Published: February 2003

In this paper we consider the problem of intramolecular vibrational energy flow from a conventional bond to a van der Waals bond in a linear AB–CD van der Waals molecule, where AB and CD are conventional diatomics. A model for collinear vibrational predissociation of AB–CD on a single ground state potential surface was developed. The vibrational predissociation of a van der Waals heterodimer, consisting of a pair of ddistinc diatomics, can be described in terms of a simple exponential decay of a single discrete zero order state which corresponds to excited bond modes, into a dissociative continuum. The dependence on the vibrational predissociation rate of the heterodimer on the parameters of the molecular bonds and of the van der Waals bond is elucidated. The details of the intramolecular dynamics of a homodimer, consisting of a pair of identical diatomics, are determined by the excitation conditions. Optical infrared short time excitation is expected to result in a coherent, in phase, superposition of degenerate bond modes, which will subsequently exhibit a simple exponential time evolution, the decay rate being determined by the resonance width. Collisional excitation of the dimer is expected to result in an incoherent initial superposition of degenerate bond modes, the subsequent time evolution being determined by the discrete–discrete coupling and by the widths of the metastable states. Model calculations were performed for the nuclear dynamics of collisionally excited linear halogen dimers A2⋅⋅⋅A2(A=F,Cl,Br,I). The linear (Cl2)2, (Br2)2, and (I2)2 dimers are characterized by negligibly small discrete–discrete coupling terms whereupon the nuclear dynamics of these van der Waals molecules is determined by exponential decay due to vibrational predissociation, without direct energy exchange between the bond modes. For the (F2)2 linear dimer the discrete–discrete coupling terms overwhelm the widths of the resonances and the system will exhibit efficient direct energy exchange between the molecular bond modes before the occurrence of vibrational predissociation. Model calculations for the vibrational predissociation lifetimes for the halogen dimers reveal remarkably long lifetimes, which can be accounted for in terms of the energy gap law for vibrational predissociation. Our results account for the ineffective process of intramolecular vibrational energy flow discovered by Dixon and Herschbach in the (Cl2)2 dimer.
EPR study of Mn2+ and Cu2+ in single crystals of strontium acetate hemihydrate [Sr(CH3COO)2⋅1/2H2O]
Authors: Jain Akhilesh Kumar , Upreti G. C.
Published: February 2003

EPR of Mn2+ doped in single crystals of strontium acetate hemihydrate [Sr(CH3COO)2⋅1/2H2O] has been studied in detail at ∼300 K and the spin Hamiltonian analysis is presented. The angular variation of the EPR spectrum of Cu2+ in strontium acetate hemihydrate crystals in relation to the principal axes of the D-tensor of Mn2+ has also been studied. From this study the unit cell symmetry of strontium acetate hemihydrate crystal is determined to be triclinic, contrary to the previously reported monoclinic symmetry from a preliminary x-ray structure analysis. It is further concluded that Cu2+ and Mn2+ get incorporated at different sites in strontium acetate hemihydrate crystals.
Molecular dynamics simulation of polymers. I. Structure
Authors: Weber Thomas A. , Helfand Eugene
Published: February 2003

Three-dimensional stereo pictures of polyethylene are generated.(AIP)
Linear differential equations in chemical equilibrium calculations
Authors: Strandberg M. W. P.
Published: February 2003

It is pointed out that the non-linear equations of chemical equilibrium have simple, useful solutions in terms of differentials of the particle reaction saturation parameters. (AIP)
Raman scattering in NH4Br and NH4I under hydrostatic pressure
Authors: Hochheimer H. D. , Shand M. L. , Walker C. T. , Hüller A.
Published: February 2003

Raman spectra are reported for NH4Br and NH4I from 86 to 295 K under hydrostatic pressures from 1 bar to 7 kbar. Phases II, III, and IV were studied. The pressure dependence is reported for the lattice modes, for the librational mode and its harmonics, and for certain combination bands. Several theoretical models for the librational potential are discussed and compared with the data.
Laser excitation study of the second excited singlet state photophysics of ClFCS
Authors: Clouthier D. J. , Knight A. R. , Steer R. P. , Hackett P. A.
Published: February 2003

A tunable dye laser has been used to excite selected single vibronic levels of the ? (1A′) state of ClFCS in the gas phase. Fluorescence (?→?) excitation spectra, quantum yields, and excited state lifetimes have been measured. Radiative and nonradiative decay constants and bimolecular self-quenching constants have been calculated for each selected state. A model in which a single dissociative vibrational mode provides the sole nonradiative decay mechanism is proposed.
An intermolecular potential function for the methanol dimer from a
b i
n
i
t
i
o calculations
Authors: Jorgensen William L.
Published: February 2003

An intermolecular potential function for the methanol dimer has been derived from ab initio molecular orbital calculations with a minimal basis set (STO-3G). Dimerization energies for 270 orientations of the dimer chosen using the energy distributed random geometries method were fit to a 12-6-1 potential function. A five particle model was adopted for each monomer consisting of the oxygen atom with two pseudotone pairs, the hydroxyl hydrogen and the methyl group. The fit for bound geometries is excellent, while repulsive points are less well represented owing to the simple model for the methyl group. By analyzing accurate potential functions for the methane and water dimers, corrections for the methyl–methyl and oxygen–hydrogen dispersion interactions are estimated. The modified potential has proven successful in simulations of liquid methanol.
Electronic structure of the noble gas dimer ions. II. Theoretical absorption spectrum for the A 2Σ+1/2u→D 2Σ 1/2g+ system
Authors: Michels H. H. , Hobbs R. H. , Wright L. A.
Published: February 2003

A systematic study of the UV photoabsorption characteristics of the dimer ion sequence Ne2+, Ar2+, Kr2+, and Xe2+ has been carried out. Spin–orbit coupling effects, which become progressively more important in the sequence Ne2+→ Xe2+, result in a mixing of the strong Σ–Σ and weak Σ–Π electronic transition moments. By combining these calculated transition moments with the potential energy curves given in paper I, an analysis of the bound-free absorption spectra of these ions has been carried out for the strong component of the transition moments. Detailed calculations of the photodissociation cross sections corresponding to the A 2Σ+1/2u→D 2Σ+1/2g system of these ions are presented as a function of individual ion vibrational levels and as a Boltzmann average for temperatures of 150, 300, and 600 °C. The importance of strong UV absorption by noble gas dimer ions in analyzing photon loss processes in excimer laser systems is discussed.
The electro-optic coefficients of urea
Authors: Morrell James A. , Albrecht A. C. , Levin Kenneth H. , Tang C. L.
Published: February 2003

The two independent electro-optic coefficients of crystalline urea, r63 and r41, are determined experimentally using a voltage modulation technique. The results are compared with theoretical values obtained by direct summation over CNDO/S generated electronic states in a perturbation theory based expression for nonlinear optical coefficients. A further comparison is made with the predicted results of an anharmonic oscillator model for nonlinear optical processes. The results obtained by all three methods, after proper internal field corrections, agree within the limitations of the theoretical calculations and the margins of experimental error.
Raman spectra of n-alkanes. I. Raman intensities of longitudinal acoustic modes
Authors: Mazur J. , Fanconi B.
Published: February 2003

Raman intensities are calculated for longitudinal acoustic modes of n-alkanes which couple through methyl interactions with transverse acoustical modes. Calculated intensities from normal mode analysis and perturbation theory compare favorably with observed intensities. The results lend support to the method of calculating intensities of longitudinal acoustic modes of chains with structural defects in which the intensity is proportional to the square of the dot product of the atomic displacements of the LAM of the all trans conformation chain and the displacement of modes calculated for defect conformations.
Phase fluorometry as a probe of diffusion-controlled molecular encounters in dense fluids
Authors: Baird J. K. , Arakawa E. T. , Noid D. W. , Petty H. R.
Published: February 2003

The Wilemski–Fixman model for diffusion-controlled molecular encounters in dense fluids is included as one step in a mechanism governing fluorescence quenching. The mechanism is analyzed by mathematics appropriate to a phase fluorometry experiment. As customary, time varying parts of the excitation and fluorescence are found to differ from one another by an amplitude and a phase. Expressions for this amplitude and phase are given in terms of the frequency of modulation of the excitation, the lifetime of the excited state, the diffusion coefficients and radii of the fluorophore and quencher, respectively, and the specific rate of collisions between fluorophore and quencher. There is obtained as a by-product of this analysis an expression for the Stern–Volmer coefficient in terms of the same set of variables. We analyze in some detail a combination of Stern–Volmer and phase fluorometry experiments which can be used to test the Wilemski–Fixman model.
Electron–molecule scattering at intermediate energy. Centrifugal-dominant channel decoupling and the INDOX polarized SCF model applied to N2 at 50 eV
Authors: Onda Kunizo , Truhlar Donald G.
Published: February 2003

We have calculated cross sections for vibrationally–electronically elastic electron−N2 scattering at 50 eV impact energy. The model interaction potential includes static, exchange, and polarization interactions calculated by the INDOX/1s method and the semiclassical exchange approximation with adiabatic polarization at large electron–molecule distances. The scattering is treated by converged rotational close coupling using a new channel decoupling scheme.
A
b i
n
i
t
i
o molecular orbital study of the catalytic exchange reaction between hydrogen and sodium amide
Authors: Pineda J. , Novaro O. , Daudey J. P. , Blaisten-Barojas E.
Published: February 2003

An ab initio MO–SCF study of the reaction pathway of the catalytic exchange between NaNH2 and HD is presented. The results compare quite well with previous studies of the kinetics of the ammonia–deuterium exchange reaction with homogeneous alkali amide catalysts, which is a process used for the production of heavy water. The capacity of the amide to effectively lower the activation barrier for the HD scission is evidenced from this study. A parallel study of the same reaction using the nonempirical pseudopotential method is also carried out in order to show the value and deficiencies of this alternative approach vis-à-vis the all-electron results.
Grain-boundary enhanced interdiffusion in polycrystalline CaO-stabilized zirconia system
Authors: Oishi Y. , Ichimura H.
Published: February 2003

A solution of the flux density equation is presented for grain−boundary enhanced diffusion in a polycrystalline system when the penetration depth is large relative to the grain size. The solution is applied to interdiffusion between 13 and 19 mol% CaO-stabilized zirconia polycrystals from 1375–1650 °C. Lattice interdiffusion coefficients calculated from the concentration distributions are slightly larger than the previously reported self-diffusion coefficients of the individual cations.
On the field dependence of the optical phase difference and capacitance of nematic layers
Authors: Schad Hp. , Scheuble B. , Nehring J.
Published: February 2003

The field dependence of the optical phase difference and capacitance of a nematic layer with homogeneously planar surface alignment is investigated for magnetic as well as electric fields. For certain combinations of material parameters a rounding-off in the low field portions of the optical phase and capacitance curves is found which is not due to surface or field misalignment and which may lead to considerable errors when elastic constants are determined using the low-field approximation.
Thermodynamics of solutions of hydrogen and deuterium in palladium–gold alloys at 555 and 700 K
Authors: Shamsuddin M. , Kleppa O. J.
Published: February 2003

The thermodynamic properties of solutions of hydrogen and deuterium in palladium–gold alloys with gold contents of 10, 25, and 40 at.% have been determined by a calorimetric–equilibrium method at 555 K (H2 and D2) and 700 K (H2). The results are compared with corresponding data recently reported from this laboratory for hydrogen and deuterium in palladium–silver and palladium–copper, and with earlier equilibrium data for hydrogen in palladium–gold measured at room temperature. (1) Comparisons between the solutions of hydrogen and deuterium in the same alloy and at the same temperature allow calculations to be made of the vibrational frequencies of hydrogen. The calculated values of ?H increase significantly with increasing gold content. In this respect these alloys are similar to palladium–silver but differ from palladium–copper. (2) The partial excess entropies of hydrogen in dilute solutions in palladium–gold alloys are reduced very significantly compared to the values in pure palladium. These reductions, in large measure, are caused by a nonrandom distribution of the hydrogen atoms among the available interstitial sites.