bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotational diffusion studied by passage saturation transfer electron paramagnetic resonance
Authors: Thomas David D. , Dalton Larry R. , Hyde James S.
Published: February 2003

A comprehensive description is given of instrumental and theoretical methods employed to make accurate measurements of rotational correlation times using passage saturation transfer electron paramagnetic resonance (ST–EPR). Saturation transfer methods extend by several orders of magnitude the sensitivity of EPR to very slow motion; for example, for nitroxide spin labels, correlation times as long as 10−3 sec become accessible to measurement. Two ST–EPR detection schemes are discussed in detail: dispersion, detected 90° out of phase with respect to the 100 kHz field modulation, and absorption, detected 90° out of phase with respect to the second harmonic of the 50 kHz field modulation. The sensitivities of these configurations are illustrated with experimental spectra obtained from a system obeying isotropic Brownian rotational diffusion; namely, maleimide spin labeled human oxyhemoglobin in aqueous glycerol solutions. Two theoretical approaches, one employing coupled Bloch equations and the other utilizing the stochastic Liouville equation for the density matrix with the orientation variables treated by transition rate matrix or orthogonal eigenfunction expansion methods, are in excellent agreement with each other and with model system spectra. Both experimental and theoretical spectra depend on a number of relaxation processes other than rotational diffusion; consequently, considerable care must be taken to ensure the accuracy of measured rotational correlation times. Although the absorption method is currently the more sensitive and convenient one to apply with most conventional (commercial) spectrometers, the dispersion ST–EPR method is potentially more powerful, providing strong motivation for future technological efforts to decrease noise levels in dispersion experiments.
On the coupling of the hole–particle and particle–particle (or hole–hole) Green’s functions
Authors: Deguchi Kisaburo
Published: February 2003

It is shown that a ladder vertex function can be related with an approximate particle–particle (or hole–hole) Green’s function in the same way as a screened potential is represented by a hole–particle Green’s function. Then, a screened potential and a ladder vertex function are constructed of solutions of the RPA and the PVA equation, respectivly, and using the ’’static’’ forms of these, we propose the coupled scheme of the RPA and the PVA.
Three-body effects in the exchange and dissociation encounters for Ar+Ar2
Authors: Howard Robert E. , Roberts Robert E. , DelleDonne Michael J.
Published: February 2003

Many-body effects in reactive scattering are studied by investigating Ar+Ar2(v=0, j=0) reactions through the use of quasiclassical dynamics. The Axilrod–Teller triple dipole term is used to describe nonadditive contributions, while Morse and MSV III expressions are employed as pair potentials. Inclusion of the three-body term is found to modify polar differential cross section, opacity, and total cross section for exchange and dissociation at initial kinetic energies below 0.5 kcal/mole (250°K), although all differences calculated with the Morse lie within statistical uncertainties. The dissociation total cross section calculated with the more accurate MSV III pair potential is shifted beyond statistical uncertainty at an initial kinetic energy of 0.3 kcal/mole (150°K). This suggests that low energy reactive scattering in the Ar3 system may be measurably influenced by many-body forces.
Numerical methods for solving the chemical mass action equilibrium problem
Authors: Park David J. M.
Published: February 2003

New numerical methods for the solution of the mass action chemical equilibrium problem are presented. They are based upon a manipulation of the stoichiometry to obtain favorable iteration conditions. The methods are suitable for all mass action equilibrium problems.
Nonresonant energy transfer interactions between randomly distributed guest molecules in host lattices
Authors: Wassam W. A. , Fong F. K.
Published: February 2003

In this paper, we derive an expression for the rate constant of energy transfer processes in which the energy mismatch between the donor and acceptor excited states are dissipated through the phonon modes of the host lattice. The adiabatic states are written as products of the donor and acceptor molecular wavefunctions, and nonadiabatic couplings between the stationary states result from Coulomb interactions between the valence electrons of the donor and acceptor molecules. The Boltzmann-averaged rate constant is calculated in first-order perturbation theory using the linear-response time-correlation function. The resulting expression, given in the weak-coupling limit, relates the rate constants to the off-resonance energy gap, the frequencies of the accepting modes, and the potential surface displacement parameters. The theoretical results are applied to a numerical analysis of available experimental data on nonresonant interactions between rare earth ions in crystalline hosts. The present formulation is compared with related theories of multiphonon electronic relaxation and of vibrational relaxation of molecules in host lattices.
Theoretical studies of magnetic shielding in H2O and (H2O)2
Authors: Ditchfield Robert
Published: February 2003

Magnetic shielding constants are calculated for the nuclei in H2O and (H2O)2 using a slightly extended$set of atomic functions modified by gauge factors. Calculated shielding results for the theoretical equilibrium structures of H2O and (H2O)2 lead to a value for the hydrogen bond shift of −2.95 ppm, which is in reasonable agreement with the available experimental data. The anisotropy of the magnetic shielding tensor associated with the hydrogen bonded proton is found to increase by 12 ppm on dimer fromation. This suggestes that proton magnetic shielding anisotropies may be more sensitive to features of hydrogen bonding than are isotropic values. An analysis of the theoretical results reveals that the downfield hydrogen bond shift arises (a) from a decrease of the electronic charge on the hydrogen bonded proton, and (b) from deshielding effects of the currents induced on the oxygen atom of the porton acceptor by the external magnetic field. For (H2O)2 these two types of contribution are of equal importance. The latter type of contribution (b) is found to be almost tottally responsible for the anisotropy changes produced on dimer formation. The sensitivity of the shielding tensor associated with the hydrogen bonded proton to changes in the intermolecular geometrical parameters of (H2O)2 is examined. The calculatted isotropic and anisotropic values of thius tensor show large changes of the O⋅⋅⋅O distance%is reduced below 3.5 Å. In contrast, the sensitivity of this shielding tensor to the relative orientation of the water monomers in (H2O)2 is quite small. Finally, the results of relaxing the geometry of the proton donor are reported. The isotropic value of the shielding at the hydrogen bonding proton is found to decrease markedly as the O−H distance of O−H⋅⋅⋅O is increased slightly from its equilibrium value.
Solute structuring in aqueous [Cr(H2O)6]Cl3
Authors: Caminiti R. , Licheri G. , Piccaluga G. , Pinna G.
Published: February 2003

A [Cr(H2O)6]Cl3 aqueous solution ∼1M at T=18 °C is investigated by x-ray diffraction. The experimental correlation function shows three main peaks, centered at about 2.00, 2.85, and 3.25 Å, which are ascribed, respectively, to Cr3+–H2O, H2O–H2O, and Cl−–H2O nearest neighbor interactions. A simple model describing the coordination around the ions only in terms of nearest neighbor interactions is shown to be inconsistent with experimental data. A satisfactory agreement with experiments is reached with a model in which the hydrated [Cr(H2O)6]3+ ions are considered as interacting with a second shell of water molecules. This fact demonstrates that in the experimental conditions the Cr3+ ions hexa-aquo must be considered as real kinetic units.
Mössbauer study of transferred hyperfine interactions and magnetic ordering in Fe(III) bis(dithiocarbamato) iodides
Authors: Petridis D. , Simopoulos A. , Kostikas A. , Pasternak M.
Published: February 2003

Mössbauer spectra of 57Fe and 129I have been measured in three members of the group of Fe(III) bis(dithiocarbamato) iodides in the temperature region 1.4 to 4.2°K. All three complexes order magnetically below 4.2°K. Transition temperatures have been determined from the abrupt appearance of magnetic hyperfine splitting. The values of isomer shift, quadrupole coupling, and the magnitude and direction of the effective field in the EFG principal axis system were determined by computer fits of the spectra. The temperature dependence of the hyperfine magnetic field is essentially the same for all three compounds and differs considerably from the calculated dependence from a molecular field model for a spin S=3/2 system. Spin and charge transfer coefficients for the iron–iodine bond have been determined by an analysis of the hyperfine parameters for 129I. It has been found that about 30% of one electronic charge is transferred to iron 3d orbitals and that the bond has a predominant σ character. The 57Fe data have been analyzed in terms of an electronic spin Hamiltonian of rhombic symmetry. It has been concluded that the expectation value of the spin and the molecular exchange magnetic field are essentially perpendicular to the direction of the iron–iodine bond.
Statistical mechanics of chemical equilibria and intramolecular structures of nonrigid molecules in condensed phases
Authors: Chandler David , Pratt Lawrence R.
Published: February 2003

An exact classical statistical mechanical theory is developed which describes how intermolecular forces alter the average intramolecular structures of nonrigid molecules and how these forces affect the equilibrium constant of chemically reacting species. The theory lends itself to computationally convenient approximations. Three illustrative applications of the theory are given. First, the equilibrium constant for the reaction N2O2?2NO2 is studied. Second, the shift in the chemical bond lengths of N2 and O2 from their gas phase values to those in the liquid are calculated from the theory. Third, the average conformational structure of n-butane in various dense fluid solvents is predicted. The basic methods used in the derivation of the theory are the techniques of physical cluster series (as opposed to the usual Mayer ’’mathematical’’ cluster expansions), and topological reductions. The formalism gives rise to a renormalization of chemical bonding Boltzmann factors so that the theoretical expressions are not swamped by essentially infinite and unnormalized cluster functions. Along with the results on chemical equilibria and intramolecular structures, the interaction site cluster series for intermolecular correlation functions is rederived in a more general manner than the derivation originally presented by Ladanyi and Chandler.
An ab initio potential-energy surface study of several electronic states of NO2
Authors: Jackels Charles F. , Davidson Ernest R.
Published: February 2003

The results of ab initio potential-energy surface calculations are presented for 12 doublet and four quartet states of NO2. In several cases Cs as well as C2v conformations have been studied. The predicted equilibrium conformations and the character of the wavefunctions are discussed. Vertical excitation energies are given for 40 doublet and 12 quartet states. The electronic spectrum of NO2 is discussed, and barriers to photodissociation are estimated for several electronic states. Whether states will be strongly Jahn–Teller coupled by a symmetry-reducing vibrational interaction depends on their potential surfaces and the character of their wavefunctions. This vibronic interaction in some cases is expected to produce non-C2v equilibrium conformations.
Vibrational resonance energy transfer and dephasing in liquid nitrogen near its boiling point: Molecular computations
Authors: Rothschild Walter G.
Published: February 2003

On the basis of current models of oscillator–oscillator and oscillator–anharmonicity–lattice coupling, the band broadening increments leading to the spontaneous width of the (near-isotropic) Raman stretching fundamental of liquid nitrogen near its boiling point are predicted. Using known constants of the atom–atom Lennard-Jones potential and known dynamic autocorrelation times of the system, the observed spontaneous bandwidth of 0.067 cm−1 and its Lorentzian band contour are satisfactorily reproduced (0.07 cm−1) by a vibrational dephasing mechanism if it is assumed that motional narrowing takes place through a modulation process decaying on the same fast time scale as that of the velocity correlation between two nitrogen molecules. On the other hand, resonance energy transfer contributes less than 1% to the band broadening.
The density dependence of bulk viscosity in a simple dense gas
Authors: Collings A. F. , Hain D. L.
Published: February 2003

Radial distribution functions for a hard-sphere modified Lennard-Jones (12,6) fluid have been calculated for six reduced densities at a reduced temperature of 2.0 by a standard Monte Carlo procedure. These functions are used to calculate the bulk viscosity of dense gaseous argon according to the Enskog and Rice–Allnatt theories, and the results are compared with the experimental data of Madigosky.
Apparent and partial molal volumes in mixed salt solutions. I. The ternary systems LiCl–LiNO3–H2O, NaCl–NaNO3–H2O and KCl–KNO3–H2O
Authors: Padova J. , Lewkowicz I.
Published: February 2003

The densities of aqueous mixtures of LiCl–LiNO3, NaCl–NaNO3, and KCl–KNO3 were determined at 25 °C at constant chloride-nitrate molality ratios. The mean apparent molal volumes were calculated and found to obey the limiting law modified by additional terms in ionic strength. The partial molal volumes for the electrolytes in each mixture were calculated and the conditions under which Young and Smith’s mixtures rule held were examined. The results obtained are discussed and interpreted in terms of ion–ion interactions and v0 (Friedman’s parameter).
Orientation autocorrelation functions of a spherical Brownian particle
Authors: Pomeau Y. , Weber J.
Published: February 2003

The cumulant expansion method is used to compute quantities like 〈n(t) ⋅n(0) 〉, where n is a unit vector fixed on a Brownian particle immersed in a classical fluid, with an angular velocity obeying a generalized Langevin equation. The results are valid both in the limits of a free and of a completely hindered rotation. In the long time limit corrections to the Debye equation of diffusion on the unit sphere are found.
A Gaussian and Slater orbital study of fine structure in the X 3Σ− and A 3Π states of NH, OH+, PH, and SH+
Authors: Palmiere P. , Sink M. L.
Published: February 2003

The fine structure constants λ and C0 for the X 3Σ− and A 3Π states of the NH, OH+, PH, and SH+ molecules have been calculated with double-zeta Slater and Gaussian basis functions using single configuration wavefunctions. The spin–spin and spin–orbit matrix elements computed with the different basis functions are found to agree better than 5% in most cases. Effects of increased nuclear charge on the various fine structure parameters are compared between the neutral and positive ion, and between the first- and second-row molecules.
Fine structure in the pure quadrupole resonance of oxygen-17 in Ba(ClO3)2⋅H2 17O by nuclear double resonance
Authors: Shporer Mordechai , Achlama Abraham M.
Published: February 2003

A technique of double resonance in the laboratory frame is applied for the measurement of the pure quadrupole resonances of oxygen-17 in polycrystalline Ba(ClO3)2⋅H2 17O at 77°K. The spectrum comprises three frequency bands displaying fine structure that is interpreted in terms of the intramolecular dipolar interactions within the hydration water molecule. The experimental results are compared with theoretical spectra calculated for various geometries of the water molecule and relative orientations of the quadrupolar and dipolar interactions. It is not possible to obtain a complete agreement between the calculated and experimental spectra, since apparently not all predicted lines show experimentally. It is suggested that the transitions involving triplet eigenstates of the proton–proton dipolar interaction are not observed, presumably due to excessive line broadening. Based on this assumption a good fit with the experimental spectrum is obtained with the following parameters: e2qQ/h (17O) = (−7.61±0.01)MHz, η=0.94±0.01, proton–oxygen distance ROH= (0.99±0.02) Å, and the HOH angle 2ϑ=110°±2°. These results are discussed in terms of the electronic and geometric structure of the hydration water molecule.
Collisional relaxation of CO2(X 1Σ+g)(001) and CO(X 1Σ+)(v=1) by molecular bromine (X 1Σ+g)
Authors: Davis Christopher C. , McFarlane Ross A.
Published: February 2003

The rate constants for the collisional relaxation of vibrationally excited CO2(0001) and CO(v=1) molecules by molecular bromine have been determined by the laser induced fluorescence technique. The measured rates were kCO2–Br2 =246±23 s−1 torr−1 and kCO–Br2 =6.34±0.5 s−1 torr−1. The relevance of these rates to E→V transfer lasers is discussed.
On the interpretation of differential scattering in rare gas excitation transfer systems: He* (2 1S)+Ne and Ar* (4s 3P2,0)+Kr, Xe
Authors: Martin D. W. , Fukuyama T. , Gregor R. W. , Jordan R. M. , Siska P. E.
Published: February 2003

Crossed beam time-of-flight measurements on scattered excited species in the title systems are used to identify some of the unusual structure at wide scattering angles as being due to reactively scattered excited products, and to correct two previous misinterpretations. Unexplained structure remains in the elastic scattering, while the reaction products are scattered sharply forward in every case, giving rise to resolved wide-angle peaks corresponding to each state or group of states of the excited atom product. The time-of-flight diagnostic thus enables ’’reactive scattering spectroscopy’’ for these and other systems, in which absolute state-to-state reaction cross sections, branching ratios, and reaction thresholds may be obtained from analysis of the angular distribution.
Theoretical study of energy transfer in molecule–solid collisions: H2, D2+Ag(111)
Authors: Lin Ying-Wei , Wolken George
Published: February 2003

A quantum mechanical calculation of the angular distribution of H2 and D2 scattered from a Ag(111) surface is presented. Energy transfer between the gas and the solid is included using an isotropic Debye model of the solid. The coupling of the rotational levels of H2, D2 with the phonon modes of the solid is investigated treating the molecules as rigid rotors. The results indicate that the mass difference between H2 and D2 is more important than the different rotational energy spacing in determining the angular scattering pattern. Differences between the present results and classical ’’cubes’’ models are observed and compared with experimental results.
Generalized Langevin theory for gas/solid processes: Dynamical solid models
Authors: Adelman S. A. , Garrison B. J.
Published: February 2003

A new class of smooth and structured solid models is developed from the generalized Langevin theory of gas/solid processes [S. A. Adelman and J. D. Doll, J. Chem. Phys. 64, 2375 (1976)], and numerical results for scattering off the simplest of these model solids are presented. The models, which may be refined to arbitrary precision, allow one to treat the many-body or lattice effect in gas/solid dynamics in a qualitatively correct but computationally simple manner. Scattering calculations based on the models may be carried out using standard classical trajectory methodology; the many-body dynamics modifies the usual classical equations of motion through noise terms and auxiliary variables. Collisional studies based on the simplest of the new models reveal the importance of many-body dynamics on energy transfer and trapping thresholds. The percentage of energy transfer due to many-body effects is found to be a rapidly increasing function of solid Debye temperature ΘD; at ΘD≳225°K the many-body contribution to energy transfer often exceeds the uncoupled oscillator contribution. The threshold energy for trapping on the simplest model solid is often more than doubled due to many-body influence. Finally, helium scattering from silver is simulated and the results are compared with the measurements of Sau and Merrill.
Lattice dynamics and hyperfine interactions of intercalation compounds: A single crystal study of TaS2⋅Sn
Authors: Herber Rolfe H. , Davis Richard F.
Published: February 2003

An oriented single crystal sample of the dichalcogen layer intercalation compound, TaS2⋅Sn, has been examined by 119Sn Mössbauer effect spectroscopy both at room temperature and over the temperature range 4.2⩽T⩽295 °K. From the angular dependence of the intensity ratio of the two components of the doublet spectrum at room temperature, as well as from the insensitivity of this ratio as a function of temperature at a fixed observational angle, it is concluded that there is a single lattice site for the tin atom in the layer structure. Comparison of the single crystal Mössbauer data with that obtained for a polycrystalline, (nearly) randomly oriented sample shows that even in the latter, there is a small residual orientation effect which must be corrected for in determining the anisotropy of the tin atom motion. From the isomer shift data it is clear that the tin atom is formally Sn2+ and acts as an electron donor to the TaS2 lattice. From the temperature dependence of the isomer shift parameter, it is inferred that the motion as probed by the Mössbauer resonance technique is that of a bare tin atom, in contrast to comparable data for SnS, in which appreciable participation of the nearest neighbor atoms in the motional behavior of the tin atom may be deduced. The high temperature region dependence of the recoil-free fraction leads to 119Sn Mössbauer temperatures of ϑm∼176±5 and 190±5 °K for TaS2⋅Sn and SnS, respectively.
ESR of trapped centers in γ irradiated silver chlorates single crystals
Authors: da Silva E. C. , Gualberto G. M. , Rettori C. , Vargas H. , Foglio M. E. , Barberis G. E.
Published: February 2003

The paramagnetic centers O−3, ClO2, and (ClO2–Cl)− have been identified in irradiated AgClO3 single crystals by X and Q band ESR experiments at room temperatures and by the optical absorption bands at 480 and 360 nm, characteristic of the first two centers, respectively. The ESR spectrum and its angular dependence show that the first two centers have two magnetically inequivalent sites per unit cell. The third center was clearly observed only at certain orientations. The data for ClO2 show that the principal directions of the g and A tensors do not coincide; the two magnetically inequivalent sites have similar principal values of the g and A tensors related by a 90° rotation about the tetragonal crystal axis of symmetry, and can therefore be assumed to be chemically equivalent. It is found that the ClO2 center is formed substitutionally in the ClO−3 ion position, but that the ozonide O−3 is not. The thermal annealing of the γ irradiated AgClO3 has revealed an increase in the concentration of the O−3 radical that seems to be connected with the simultaneous destruction of the ClO2 center. 149
Spectroscopic constants and radiative lifetimes for valence-excited bound states in SiO
Authors: Oddershede Jens , Elander Nils
Published: February 2003

Using the self-consistent polarization propagator approximation we have calculated the spectroscopic constants for the following bound states in silicon monoxide: a 3Σ+, b3Π, d3Δ, e 3Σ-, C 1Σ+, D 1Δ, A 1Π, E 1Σ+, 3Σ+(II), 3Σ+(III), 3Π (III), F 1Σ+, H 1Σ+, and G 1Π. The molecular parameters for the seven lowest states agree very well with experiments, whereas the seven remaining levels were determined less accurately. The electronic transition moments were calculated for the dipole allowed transitions. The calculated radiative lifetime for the A 1Π state was found to be 49.5 nsec. With the experimental potential energy curves the corresponding lifetime was 31.6 nsec.
Proton-magnetic resonance lines in liquid crystalline p[(p-methoxybenzylidene) -amino]benzonitrile, p-[(p-ethoxybenzylidene) -amino]benzonitrile, and p-[(p-ethoxybenzylidene)amino]phenyl acetate
Authors: Gerofsky S. E. , Moses H. A. , Fink J. J. , Grover A. , Ross J.
Published: February 2003

Experimental linewidth (2.4 to 3.4 G) vs temperature measurements are given for the broad NMR signals of p-[(p-methoxybenzylidene) -amino]benzonitrile (PMBAB) and linewidth (2.7 to 3.7 G) vs temperature measurements for the broad NMR signal of p-[(p-ethoxybenzylidene) -amino]benzonitrile (PEBAB). For the very broad five peak line of p-[(p-ethoxybenzylidene)amino]phenyl acetate (PEBAP) both inner linewidth peak separation (3.1 to 3.7 G) and outer peak separation (5.8 to 7.3 G) are given as a function of temperature. All three of the above compounds have the same central chemical structure yet the first two give a three peak line and the last a five peak line. Explanation of these outer lines is given. Also, for each substance graphs of order parameters vs temperature ranging from 0.57 to 0.68 for PEBAB and 0.45 to 0.62 for PMBAB and 0.56 to 0.67 for PEBAP are given.
A variational theory for high Rydberg Stark ionization threshold scaling laws in atomic hydrogen
Authors: Herrick David R.
Published: February 2003

We examine hydrogen atom Stark energies calculated with nonlinear variational theory using square-integrable wavefunctions. The trial function for each state has a characteristic critical field (F*) above which the variational energy is complex. F* approximates the experimentally important Stark ionization threshold for Rydberg states and typifies critical fields encountered in mathematical ’’catastrophe theory.’’ Zero-field wavefunctions yield analytic formulas for both threshold fields and energies in terms of parabolic quantum numbers. Aspects of the model suggest a ’’law of corresponding states’’ for Stark ionization near the critical field. The threshold fields scale as n−4 for all high Rydberg Stark states, while threshold energies scale as n−2 for the most unstable Stark components, and as n−8/3 for the most stable components. This variationally determined threshold behavior is compared with existing classical ionization criteria, perturbation theory, and semiclassical threshold orderings for different Stark components.
Double-quantum vibrational energy transfer in the O3–CO and O3–OCS systems
Authors: Hui Kin-Kwok , Cool Terrill A.
Published: February 2003

Rapid rates for vibrational energy transfer at room temperature have been observed for energy transfer from the CO(v=1) to the coupled O3(200), O3(101), and O3(002) levels. Very rapid transfer also occurs from the coupled O3(001) and O3(100) levels to the OCS(020) level. The measured rates are compared with results calculated with the first order perturbation theory of dipole transition–moment interactions. A measurement of the V→T deactivation of the bending mode of OCS gives a rate constant of (3.4±0.4) ×103 sec−1 Torr−1 at 298 °K. Values are also given for the deactivation of OCS(001) by N2 and for the V→T deactivation of OCS by Ar.
H3 +: A
b i
n
i
t
i
o calculation of the vibration spectrum
Authors: Carney G. D. , Porter R. N.
Published: February 2003

The vibration spectrum of H3 + is calculated from the representation of a previously reported [J. Chem Phys. 60, 4251 (1974)] ab initio potential-energy surface in a fifth degree Simons–Parr–Finlan (SPF) expansion. Morse- and harmonic-oscillator basis functions are used to describe the motions of the three oscillators and the Harris–Engerholm–Gwinn quadrature technique is used to obtain matrix elements of the Hamiltonian in the basis of vibrational configurations. Our variational method is thus analogous to configuration– interaction calculations for electronic states. The ground state is found to have a zero-point energy of 4345 cm−1 and a vibrationally averaged geometry of R1=R2=0.91396 Å, ϑ=60.0012°, where ϑ is the angle between the two equivalent bonds. The transition frequencies for the E and A1 fundamentals are ?E=2516 cm−1 and ?A=3185 cm−1 and those for the corresponding first overtones of the bending mode are 2?E=5004±4 cm−1 and 2?A=4799 cm−1. The first overtone of the breathing mode is 6264 cm−1. The first-excited A1 vibration state is metastable with a dipole–radiation lifetime of 3 sec. Transition frequencies, Einstein coefficients, and lifetimes are reported for a total of 21 transitions. Analysis of results for Dunham number and normal-coordinate expansions in comparison with those for SPF expansion show the latter to be superior for ab initio vibrational calculations. A scheme for possible direct measurement of the fundamental A1 and E vibrational bands is suggested.
A variational equations approach to the onset of statistical intramolecular energy transfer
Authors: Brumer Paul , Duff James W.
Published: February 2003

Recent developments in mathematics and statistical mechanics indicate that efficient intramolecular energy transfer in coupled anharmonic oscillator systems can only occur above a critical energy Ec. A simple method for computing Ec is proposed and is applied to the Barbanis, Henon–Heiles, Tredgold, and Thiele–Wilson systems. The critical energy for these systems is shown to be easily and accurately predicted by this technique.
Optical spectra of slab molecular crystals
Authors: Philpott Michael R.
Published: February 2003

A continuum theory is given for the effect of spatial dispersion on the s- and p-polarized absorption, transmission and reflection spectra of a molecular crystal. Polariton effects in crystal spectra are discussed and the connection with the microscopic theory of optical properties established in the limit of weak spatial dispersion.
Electron capture cross sections for multiply charged argon ions passing through nitrogen and krypton targets at energies from 10 to 110 keV
Authors: Klinger H. , Müller A. , Salzborn E.
Published: February 2003

Cross sections σif for electron capture processes of up to four electrons in single ion–atom encounters of multiply charged argon ions (up to Ar7+) in nitrogen and krypton targets are measured. The cross sections show no significant energy dependence in the investigated energy range from 10 to 110 keV. A plot of the cross sections for nitrogen and krypton target gases vs the projectiles initial charge state number i reveals a structure which is very similar to the structure previously reported for argon.
Interferometric studies of carbon dioxide dissociation in a free-piston shock tube
Authors: Ebrahim N. A. , Sandeman R. J.
Published: February 2003

Time-resolved interferometry is used to measure the dissociation rate of carbon dioxide in a free-piston shock tube. Over the temperature range of 2500–7000 K the dissociation rate is kd=7.5×1012 T1/2 exp[−(104 000±2500)/RT] cc/mole-s. This gives an activation energy of (104±2.5 kcal/mole+(1/2) RT) in agreement with recent theoretical predictions. The present data are also fitted to the RRK expression and gives a threshold energy of 132 kcal/mole. The effect of impurities was minimized by maintaining low background pressures in the shock tube and low leak rates. The present results are compared with previously published data. Using the values of the equilibrium constant, and the measured forward rate, the recombination rate has been deduced. The recombination rate is compared with some of the experimental measurements reported.
Ionic conduction bands at zeolite interfaces
Authors: Vuc̆elić D.
Published: February 2003

The electrical conductivity and NMR relaxation of dehydrated synthetic zeolites of the types A and X with various cations have been investigated. Also the effect of sorbed inert molecules and occlusion of salts on the electrical conductivity of zeolites has been investigated. In all the cases, the sorption of inert molecules and occlusion of salts leads to an increase in the ionic conductivity and to a decrease in the activation energy. On the basis of the results obtained a model of electrical conduction with a ’’cationic conduction band’’ is proposed.
Sign inversion in the MCD spectrum of the Eu3+ ion in aqueous solution
Authors: Görller-Walrand C. , Godemont J.
Published: February 2003

The MCD spectrum of the Eu3+ ion in aqueous solution exhibits A terms for the transitions 5D1←7F0, 5D2←7F0, 5D0←7F1. The transition 5D2←7F0, which is a forced electric dipole one, shows a sign inversion with respect to the two others, which are magnetic dipole allowed. By considering the expression for an A term in the light of the mechanisms which explain the intensity of rare earth spectra, it is shown that only a trigonal symmetry can account for this sign inversion.
Quantum interpretation of fully state-selected rotationally inelastic collision experiments
Authors: Alexander Millard H. , Dagdigian Paul J. , DePristo Andrew E.
Published: February 2003

We develop an expression for the cross section corresponding to fully oriented rotationally inelastic atom–diatomic molecule collisions in the general case where both the initial and final axes of quantization are arbitrarily oriented with respect to the initial relative velocity vector. The cross section appears as an inner product of a tensor cross section and various rotation matrix elements. Considerable simplification occurs if one averages over the final orientation of the rotational angular momentum. The thermal averaging appropriate to a beam–gas configuration can be reduced to a one-dimensional integration. A detailed discussion is devoted to the potential use of laser induced fluorescence detection to determine experimentally the orientation dependence of rotationally inelastic collisions.
Singlet–triplet fusion in metal-free phthalocyanine
Authors: Menzel E. Roland , Sharp James H.
Published: February 2003

Fluorescence quenching has been observed in 10−4 to 10−7M solutions of metal-free phthalocyanine in 1-chloronaphthalene at 77°K. This quenching is attributed to singlet–triplet fusion. A triplet lifetime of 200 μsec was measured at 77°K. A dipole–dipole interaction formulation for singlet excitation migration was adapted to the singlet–triplet fusion process. A critical distance of about 200 Å was computed for the system under study. Unlike in singlet excitation migration, this critical distance depends on the experimental illumination conditions. If singlet–triplet fusion plays a role in photogeneration of charge carriers in metal-free phthalocyanine thin films, a photocurrent linear in light intensity, in the absence of recombination and trapping effects, can be understood.
Shear viscosity in ternary liquid systems near a plait point
Authors: D’Arrigo G. , Mistura L. , Tartaglia P.
Published: February 2003

Shear viscosity measurements in the ternary liquid mixtures water–benzene–ethanol and water–diethylether–ethanol near their plait points at 25°C and 1 atm are reported. Various paths of approach to the plait point have been followed. A new method of plotting the data is introduced to show that the viscosity anomaly appears with the same qualitative features as in a binary mixture or in a pure fluid.
The kinetics and mechanisms of the reactions of O(3P) atoms with CH3CN and CF3CN
Authors: Bonanno R. J. , Timmons Richard B. , Stief Louis J. , Klemm R. Bruce
Published: February 2003

Absolute rate constants for the reactions of O(3P) atoms with CH3CN and CF3CN were measured using the flash photolysis–resonance fluorescence method under conditions which minimized complications from secondary reactions. These reactions were studied over the temperature range of 383–500 K. The Arrhenius expression obtained for O(3P)+CH3CN was (7.27±1.75) ×10−13 exp(−4770±200/RT) cm3 molecule−1⋅sec−1, while that for the O(3P)+CF3CN reaction was (14.2±5.2) ×10−13 exp(−6130±310/RT) cm3 molecule−1⋅sec−1. Mechanistic information concerning these reactions was obtained from kinetic isotope effect studies using CD3CN in place of CH3CN and from fast flow tube studies of both the CH3CN and CF3CN reactions using direct mass spectrometric and ESR detection methods. The lack of an isotope effect in the CD3CN/CH3CN experiments coupled with the reaction products identified mass spectrometrically leads to the conclusion that the major reaction channel in both systems involves displacement of CH3 and CF3 by O(3P). In addition, there may be some contribution to the overall reaction from a direct molecular rearrangement for both O(3P)+CH3CN and O(3P)+CF3CN and from abstraction of H atom for O(3P)+CH3CN, the latter being more significant at higher temperatures. The kinetic parameters obtained in this work are compared with values from O(3P) reactions with other nitriles, alkynes, and olefins. In all three reaction classes, the major initial step is assumed to be interaction of the electrophilic O(3P) atom with the Π bonds of the substrate molecules.
Stochastic theory of molecular collisions. II. Application to atom–vibrotor collisions
Authors: Augustin Stuart D. , Rabitz Herschel
Published: February 2003

In this work stochastic theory is applied to the treatment of atom–vibrotor collisions. This is an extension of a previous paper which described molecular collisions by a Pauli master equation or a Fokker–Planck equation. In this framework an energy conserving classical path model is explored, and methods for solving the equations numerically are discussed. The coefficients of the Fokker–Planck equation are shown to be expressible as simple functions of the interaction potential. Estimates of the computational labor are also discussed. Finally as a followup on the initial work, numerical solutions of the master equation for the collinear vibrational excitation problem of Secrest and Johnson are presented in an Appendix.
Proton NMR study of the structural phase transitions in perovskite layer compounds: (CnH2n+1NH3)2CdCl4 and (NH3–(CH2)n–NH3) CdCl4
Authors: Blinc R. , Burgar M. , Loz̆ar B. , Seliger J. , Slak J. , Rutar V. , Arend H. , Kind R.
Published: February 2003

The temperature dependences of the second moments M2 of the proton magnetic resonance absorption spectra and of the proton spin–lattice relaxation times T1 of perovskite layer compounds (CnH2n+1NH3)2CdCl4 with n=1–3 and (NH3– (CH2)n–NH3) CdCl4 with n=2−5 have been studied together with the frequency dispersion of T1. The structural phase transitions in these compounds were found to be connected with a change in the state of motion of the alkyl or alkylene groups, i.e., by an interplay of transitions between different N–H−−−Cl hydrogen bonding schemes and the excitation of hindered rotations of the hydrocarbon chains.
Sudden rotation calculations of atom–molecule scattering
Authors: Bowman Joel M. , Leasure Steven C.
Published: February 2003

Rotational inelastic cross sections for two atom–rigid rotor systems, 4He+H2(j) and 4He+I2* (j), have been calculated using a quantal sudden approximation that we term the sudden rotation approximation. Vibrational inelastic cross sections have also been calculated for 4He+H2(ν,j) using a two and three vibration basis set for total energies between 1.2 and 4.2 eV. The present results for 4He+H2 are compared with the coupled channel and coupled states results of McGuire and Kouri and with the two vibration results of McGuire. The 4He+I2* results are compared with quasiclassical trajectory calculations and the experiments of Steinfeld and co-workers.
Sudden rotation total rotational inelastic cross sections for 4He+I2*
Authors: Bowman Joel M.
Published: February 2003

An expression to calculate the total rotational inelastic cross section, based on the sudden rotation approximation, is derived. It is tested against the usual direct summation expression for the 4He+I2* system. The dependence of this cross section on the initial I2* rotational state is investigated.
An a
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i
o study of the rearrangement of carbonyl compounds to oxacarbenes
Authors: Altmann Julianna A. , Csizmadia I. G. , Yates Keith , Yates Peter
Published: February 2003

As the first part of a systematic theoretical study of oxacarbene rearrangement, ab initio SCF MO calculations employing a split valence shell basis set have been carried out for the model conversion of formaldehyde to the corresponding oxacarbene. The various cross-sections of the reaction hypersurface were obtained by complete geometry optimization. The results suggest that, at least for the present model system, the reaction takes place preferentially in the first triplet (T1) state via a concerted pathway.
G+K 1Σ+g double-minimum excited state of H2
Authors: Glover R. M. , Weinhold F.
Published: February 2003

We have obtained a Born–Oppenheimer potential curve for the previously uncharacterized third 1Σ+g state of H2, using a correlated 20-term wavefunction of generalized James–Coolidge type. We find this potential curve to have a double-minimum character, with the inner (Rydberg-like) and outer (’’ionic’’) wells having minima at about 1.99 and 3.30 bohr, respectively, and an intervening maximum at 2.76 bohr. Unlike the extensively studied E+F double-minimum state, the outer well here appears to be the deeper, by some 450 cm−1 in our calculation. The inner and outer minima can apparently be associated with spectral lines that in experimental tables have previously been attributed to distinct G and K electronic states. The appropriate spectroscopic term symbol of this combined state is accordingly G+K 1Σ+g (1sσ3dσ+2pπ2).
Vibrational spectra and intermolecular forces of 1,3,5,-trioxane and 1,3,5,7-tetroxocane crystals
Authors: Kobayashi Masamichi
Published: February 2003

Polarized infrared and Raman spectra of 1,3,5-trioxane (CH2O)3 and 1,3,5,7-tetroxocane (CH2O)4 crystals have been measured. The correlation field splittings for the E vibrations of the trigonal cell of trioxane have been detected by measuring polarized Raman spectra on a single-crystal specimen. The observed polarization properties of the infrared and Raman bands of tetroxocane single crystal have been interpreted by the crystal structure. The optically active lattice vibrations of the two crystals have been detected in the far-infrared and/or Raman spectra. The normal mode calculations have been performed for the three-dimensional crystal lattices by using the valence-force-field intramolecular forces and the van der Waals intermolecular interactions for the atomic pairs of H⋅⋅⋅H, O⋅⋅⋅H, C⋅⋅⋅H, O⋅⋅⋅O, C⋅⋅⋅O, and C⋅⋅⋅C. The parameters of the Lennard-Jones type potential functions have been adjusted so as to reproduce the observed lattice mode frequencies by the least-squares method and compared with the values permitted empirically by previous workers.
Thermal behavior of the near ir absorption of H2O and NaClO4 aqueous solutions
Authors: Andaloro G. , Chiricò P. , Guzzio G. , Leone M. , Palma-Vittorelli M. B.
Published: February 2003

The IR absorption spectrum of water and NaClO4 solutions in the 1.45 μ region was studied as a function of temperature and salt concentration. Band profile synthesis with Gaussian components shows the existence of three such components. The temperature and concentration dependence of their intensities, peak frequencies, and widths is analyzed and discussed.
Low frequency vibrations of 1,2,3-trichlorobenzene
Authors: Burgos E. , D’Alessio E. , Bonadeo H.
Published: February 2003

The optically active low frequency vibrations of 1,2,3-trichlorobenzene have been investigated. Unpolarized infrared and Raman spectra at room and liquid nitrogen temperature have been obtained, and the internal and lattice vibrations have been analyzed and compared with calculations based on the atom–atom model. The calculation method for the case of crystals containing two sets of symmetry unrelated molecules is discussed.
High resolution NMR in randomly oriented solids with homonuclear dipolar broadening: Combined multiple pulse NMR and magic angle spinning
Authors: Gerstein B. C. , Pembleton R. G. , Wilson R. C. , Ryan L. M.
Published: February 2003

The first combined multiple pulse-magic angle experiment is reported in which both homonuclear dipole interactions and chemical shift anisotropies are removed to resolve chemically shifted fluorines. The sample selected was KEL-F.(AIP)
Effect of density on excess electron localization in ethane and propane
Authors: Kimura Toyoaki , Fueki Kenji
Published: February 2003

A theoretical model for localized excess electrons in alkanes is described. (AIP).
Production of vibrationally excited H2(X 1Σ+g,υ″) in charge transfer reactions
Authors: Moran T. F. , Flannery M. R.
Published: February 2003

Charge transfer cross sections involving reactant ions in given vibrational states are reported for hydrogen. (AIP)
An approximate relation between orbital SCF energies and total SCF energy in molecules
Authors: Ruedenberg Klaus
Published: February 2003

The relation between the orbital energies and total SCF energy of a molecule is discussed.(AIP)
Vibration–rotation relaxation in bimolecular collisions with application to para-hydrogen
Authors: Ramaswamy Ramakrishna , Rabitz Herschel
Published: February 2003

Three-dimensional quantum mechanical calculations in the effective potential approximation have been made on the para-hydrogen system. At low temperatures, vib–rotationally inelastic collisions were examined while breathing sphere calculations were used to probe the high-temperature regime. It was found that simultaneous vibrational and rotational processes contribute to the overall mechanism of vibrational relaxation. Collisionally induced intra- and intermolecular energy transfer is possible in the present calculation, and the corresponding cross sections are examined in detail. Rates are calculated from the cross sections and compared with experiment.
Dynamic polarizabilities of metastable 2 1,3S excited states of He and Li+, with rigorous upper and lower bounds
Authors: Glover R. M. , Weinhold F.
Published: February 2003

Methods previously described for calculating rigorous upper and lower bounds to dynamic dipole polarizabilities are applied to the metastable 2 1S and 2 3S excited states of He and Li+, using highly correlated variational trial functions. For each of these species, the bounds rigorously establish the values of α (ω) to within about 1% and appear to represent the first determination of this property at frequencies above the first excitation threshold.
Imaginary-frequency polarizability and van der Waals force constants of two-electron atoms, with rigorous bounds
Authors: Glover R. M. , Weinhold F.
Published: February 2003

Variational functionals of Braun and Rebane for the imaginary-frequency polarizability α (iω) have been generalized by the method of Gramian inequalities to give rigorous upper and lower bounds, valid even when the true (but unknown) unperturbed wavefunction must be represented by a variational approximation. Using these formulas in conjunction with flexible variational trial functions we have computed tight error bounds for α (iω) and the associated two- and three-body van der Waals interaction constants of the ground 1 1S and metastable 2 1,3S states of He and Li+. These bounds generally establish the ground-state properties to within a fraction of a percent and metastable properties to within a few percent, permitting a comparative assessment of competing theoretical methods at this level of accuracy. Unlike previous ’’error bounds’’ for these properties, the present results have a completely a priori theoretical character, with no empirical input data.
Quantum dynamics for pseudorotation of XPF4 molecules
Authors: Russegger Peter , Brickmann Jürgen
Published: February 2003

The dynamics of pseudorotation of CH3PF4 in the gas phase was investigated on the basis of a semiempirical model Hamiltonian for the intramolecular nuclear motion. The time dependence of physically reasonable initial states results from a superposition of phase correlated eigenstates of this Hamiltonian. Two pseudorotation rates result from the analysis of the time dependent probability to find one of the conformers. The long range term is due to conformational changes while the short time rate concerns vibrational relaxation. An energy limit for ergodic and nonergodic behavior of the initial states is obtained.