bluuebunny/crossref_metadata_2025 huggingface dataset:
The force between two planar electrical double layers
Authors: Lozada-Cassou Marcelo
Published: February 2003

An expression to calculate density profiles of ions between two charged plates is obtained through the hypernetted chain theory for pure electrolytes. In deriving this expression no superposition approximation is assumed. Separately, a contact theorem is derived from the Born–Green–Yvon equation for pure electrolytes. The combination of these two results allows the evaluation of the pressure which drives two charged plates apart, without assuming a superposition approximation. It is shown that for point ions, this theory reduces to the Deryaguin–Landau–Verwey–Overbeek theory.
Nonsteady state analysis for the kinetics of anionic polymerization of polar monomers
Authors: De-Yue Yan
Published: February 2003

In this work, the molecular weight distribution function and some molecular parameters of the polymers generated in the anionic polymerization with initiation, propagation, monomer termination, and primary termination of initiator are studied theoretically by means of nonsteady state method. During the process of solving the kinetic differential equations, Laplace transformation and the graphical theory are employed. The theoretical results obtained are applicable to the anionic polymerization of the polar monomers such as methyl methacrylate in polar solvents.
Trajectory studies of vibrational energy transfer in gas–surface collisions
Authors: Lucchese R. R. , Tully John C.
Published: February 2003

We have applied the stochastic trajectory method to the scattering of NO from the (001) surface of an LiF crystal. The surface was represented by a 32 atom primary zone and the effects of the rest of the surface atoms were included using the Generalized Langevin Equation method. The forces between the surface atoms were treated using a shell model and the gas–surface interactions included short-range repulsions, attractive dispersion forces, and the interaction of the dipole moment of NO and the surface ions. The dependence of the dipole moment and the dependence of the molecular polarizability on the internuclear separation of NO were also included. The calculated vibrational energy accommodation coefficient was less than 1% when trapping of the gas diatom was negligible. By modifying the vibrational frequency of the scattered diatom we have examined the dependence of vibrational energy transfer on frequency. An exponential dependence on the ratio of the molecular frequency to the surface Debye frequency was found, similar to behavior observed for vibrational relaxation in condensed phases. We have also examined the relative efficiency of the coupling of the vibrational mode to the translational and rotational modes of the gas diatom and to the phonon modes of the surface. We found that the vibrational mode is most strongly coupled to the surface phonons with a much weaker coupling to the rotational and translational modes.
Adsorption and decomposition of hydrocarbons on platinum black: Vibrational modes from NIS
Authors: Cavanagh R. R. , Rush J. J. , Kelley R. D. , Udovic T. J.
Published: February 2003

The adsorption and decomposition of acetylene and ethylene on platinum black have been investigated by neutron inelastic scattering (NIS) studies of the vibrations of chemisorbed species in the energy range 30–200 meV. Results are compared in detail with electron energy loss spectroscopy (EELS) data and with spectra of model inorganic complexes. The NIS spectra for C2H2 and C2H4 chemisorbed at 120–150 K exhibit a number of spectral features (from both the internal modes and modes associated with vibrations against the surface metal atoms) which are in general agreement with EELS results for Pt(111). Detailed comparison of the spectral peak intensities and positions with various models for the adsorbed molecules rule out linear (C2H2) or planar (C2H4) species and are consistent with bent molecular configurations on the surface. Bond angles and force constants are derived from the model fits to the neutron data in each case. The NIS spectra of a ‘‘saturated’’C2H2 monolayer warmed to 300 K indicates a rearrangement to more fully hydrogenated species. Subsequent introduction of H2 at low pressure shows little evidence of chemical change while spectra measured at high H2 pressure show coexistence of chemisorbed H and saturated hydrocarbons on the surface. Warming the saturated C2H4 monolayer to 300 K produces a discrete NIS spectrum consistent with rearrangement to methylated species, possibly ethylidyne, as suggested from previous EELS studies.
Quantum theory of inelastic collisions of a diatomic molecule in a 2Π electronic state with an uncorrugated surface: Λ-doublet, spin-orbit, and polarization effects in NO (X 2Π)–Ag (111) scattering
Authors: Alexander Millard H.
Published: February 2003

The formalism is developed for scattering of a molecule in a 2Π electronic state by an infinitely massive, uncorrugated surface. A general development of the electrostatic interaction potential is used to obtain matrix elements of the potential between molecular rotational and fine-structure states, described in an intermediate Hund’s coupling case. Explicit expressions for the inelastic S-matrix elements are derived in the energy sudden limit. Semiclassical (high J) expressions for the potential matrix elements and sudden S-matrix elements are obtained. This formal work is then used to make several general predictions for collisions of NO (X 2Π) with Ag(111), which has been recently studied experimentally. In particular, we predict: (1) for moderate to high values of the final rotational quantum number, the molecules will be scattered so that J lies in the plane of the surface, (2) in the energy sudden limit the F1 and F2 rotational manifolds will be equally populated by collisions, (3) and, again in the energy sudden limit, preferential population will occur in either the two Λ-doublet levels which are symmetric with respect to reflection of the NO electronic wave function in the plane of rotation or in the two levels which are antisymmetric with respect to this operation.
Kinetic energy dependence of cross sections for state-to-state vibrational excitation in collisions of S aniline with He
Authors: Liu Kopin , Hall Gregory , McAuliffe Michael J. , Giese Clayton F. , Gentry W. R.
Published: February 2003

The kinetic energy dependence of the v=0→1 excitation cross sections have been measured for the two lowest-frequency modes of aniline, induced by collisions with He in crossed, pulsed molecular beams. Mode 11, an out-of-plane bend of the -NH2 group, shows a linear onset at threshold, while the -NH2 hydrogen-inversion mode has an excitation cross section which is a strongly decreasing function of collision energy in the 150–2000 cm−1 energy range. The total vibrationally inelastic cross section is large—at least 15% of the total rotational excitation cross section. Excitations of several in-plane modes were not observed.
On the spherical representation of the polarizability tensor of an ensemble of interacting molecules within the DID model. Comparison with previous results
Authors: Kaźmierczak M. , Bancewicz T.
Published: February 2003

In some recent papers, erroneous expressions have been given for the spherical polarizability tensor of an ensemble of interacting molecules within the DID model. The sources of these discrepancies are discussed in detail with reference to the correct formulas.
Where to look for the electronic spectrum of hydrogen isocyanide, HNC
Authors: Laidig William D. , Yamaguchi Yukio , Schaefer Henry F.
Published: February 2003

The geometrical structures and vibrational frequencies of the ground and first excited electronic states of HNC have been predicted by a priori theoretical methods. Using a standard double zeta plus polarization basis set, both the self-consistent field (SCF) and configuration interaction with all single and double excitations (CISD) levels of theory have been employed. To allow a reasonable assessment of the reliability of the HNC theoretical predictions, analogous studies of the experimentally characterized HCN molecule are also reported. It is hoped that the HNC theoretical predictions will be of assistance in the identification of its electronic spectrum. The HNC electronic spectrum should be distinguishable from that observed by Herzberg and Innes for HCN by: (a) the prediction that the X̃ 1Σ+–Ã 1A″ energy difference is ∼5000 cm−1 less for HNC than for HCN; (b) for HNC the upper state vibrational frequencies ν2 and ν3 are nearly equal (to within 100 cm−1), while for HCN, the C ≡ N stretch occurs more than 50% higher in frequency than the bending mode.
A molecular model for dielectric shifts in infrared spectra of pure liquid phases and vapor pressure isotope effects
Authors: Wolfsberg Max
Published: February 2003

It is demonstrated that the same red shift of infrared fundamentals in pure liquid phases which follows from a model of a molecule imbedded in a dielectric also occurs in a quantum mechanical molecular model of a system of three-dimensional isotropic harmonic oscillators. The shift arises from dipole–dipole interactions which lead to a band of excited states; transitions from the ground state are permitted only to one state in the lower part of this band. The relationship of these findings to vapor pressure isotope effects is discussed.
Intramolecular distributions. II. Recurrence solutions
Authors: Kupka H. , Olbrich G.
Published: February 2003

The general theory developed in the first paper (hereafter referred to as I) of this series is applied directly to the problem of finding the values of the intramolecular distributions (ID). This is achieved by deriving four term recurrence equations. On the basis of these solutions, the dependence of the ID on the normal mode rotation is investigated; this gives qualitative information concerning the complexity of the mode mixing (rotation). We have also demonstrated the influence of mode mixing on the spectral line shape showing that for special values of the rotation angle the scrambling effect disappears and the spectrum exhibits single mode progressions. In this connection it is pointed out that the widely used parallel mode approximation is inadequate to explain the complex phenomena which occur in spectroscopy and related fields.
Optical dephasing by uncorrelated phonon scattering to librations. An optical and picosecond photon echo study of a photosite of pentacene in benzoic acid
Authors: Molenkamp Laurens W. , Wiersma Douwe A.
Published: February 2003

We report results of an optical and picosecond photon echo study on the zero-phonon line of photosite I of pentacene in benzoic acid. The results show that optical dephasing in this system proceeds via uncorrelated phonon scattering processes from the ground and optically excited state to singly excited librational levels. It is also shown that the data can not be consistently interpreted using coherence exchange theory. The librational frequencies and lifetimes at low temperature are determined to be 12 cm−1 and 18 ps in the ground state and 16.7 cm−1 and 2.5 ps in the singlet excited state. It is argued that population decay of librational overtones proceeds in single quantum steps down the librational ladder which excludes these modes from playing a role in optical dephasing. From a temperature-dependent line shape study of the libronic transition in absorption, it is concluded that the low-temperature librational lifetime is primarily due to resonant decay into acoustic phonons. At elevated temperature, cubic libration-phonon anharmonicity dominates the librational lifetime. These lifetime shortening processes are reflected in the temperature dependent dephasing of the origin. Finally, with the dephasing model presented in this paper for photosite I, existing controversies concerning optical dephasing in the unburned origin of pentacene in benzoic acid can be resolved.
Excited state absorption spectra and intersystem crossing kinetics in the naphthaldehydes
Authors: Boldridge David W. , Justus Brian L. , Scott Gary W.
Published: February 2003

Picosecond time resolved, excited state absorption spectra following excitation at 355 nm are reported for 1-naphthaldehyde (1-NA) and 2-naphthaldehyde (2-NA) in room temperature solutions. The kinetics of the lowest excited singlet state (S1) decay are determined for each. The S1 lifetimes in cyclohexane are ∼26 ps for 1-NA and ∼55 ps for 2-NA. Both Sn←S1 and Tn←T1 spectra are reported. Molecular orbital model calculations based on the INDO formalism predict excited state singlet–singlet spectra and spin-orbit coupling matrix elements that are consistent with the observed spectra and kinetics.
An improved simple model for the van der Waals potential based on universal damping functions for the dispersion coefficients
Authors: Tang K. T. , Toennies J. Peter
Published: February 2003

Starting from our earlier model [J. Chem. Phys. 66, 1496 (1977)] a simple expression is derived for the radial dependent damping functions for the individual dispersion coefficients C2n for arbitrary even orders 2n. The damping functions are only a function of the Born–Mayer range parameter b and thus can be applied to all systems for which this is known or can be estimated. For H(1S)–H(1S) the results are in almost perfect agreement with the very accurate recent ab initio damping functions of Koide, Meath, and Allnatt. Comparisons with less accurate previous calculations for other systems also show a satisfactory agreement. By adding a Born–Mayer repulsive term [A exp(−bR)] to the damped dispersion potential, a simple universal expression is obtained for the well region of the atom–atom van der Waals potential with only five essential parameters A, b, C6, C8, and C10. The model has been tested for the following representative systems: H2 3Σ, He2, and Ar2 as well as NaK 3Σ and LiHg, which include four chemically different types of van der Waals interactions for which either very precise theoretical or experimental data is available. For each system the ab initio dispersion coefficients together with the well-known parameters ε and Rm were used to determine A and b from the model potential. With these values the reduced potentials were calculated and found to agree with the experimental potentials to better than 1% and always less than the experimental uncertainties. Some of the implications of the new model are discussed.
On the ordering of modified virtual orbitals in doublet state radicals
Authors: Bird Stephen , Claxton Thomas A.
Published: February 2003

The orbitally ordered CI method of Cooper and Pounder is applied to open-shell molecules. The modified virtual orbitals (MVO’s) are calculated using an equivalent operator to the closed-shell operator but the ordering process needed to neglect some MVO’s is much more complicated for open-shell systems. It is shown that 80% of the (S+D)CI energy can be recovered using only one-tenth of the configurations.
Thermodynamics of dilute solutions of hydrogen in PdNi, PdCo, and PdFe alloys
Authors: Shamsuddin M. , Kleppa O. J.
Published: February 2003

We report the results of a calorimetric-equilibrium investigation of the dilute solutions of hydrogen and deuterium in the solid solutions Pd0.9Ni0.1, Pd0.75Ni0.25, Pd0.9Co0.1, and Pd0.9Fe0.1 at 555 and 700 K. The results are compared with corresponding data for hydrogen and deuterium in pure palladium and in the alloys of palladium with silver, gold, copper, and manganese. Alloying palladium with nickel, cobalt, and iron gives rise to more positive enthalpies of solution and more negative entropies of solution than in pure palladium. The reductions in entropy probably are largely caused by a highly nonrandom distribution of the hydrogen atoms among the available interstitial sites. Among the palladium solid solutions studied so far this effect is most pronounced for the palladium–iron alloys.
Low frequency Raman spectra of v-B2O3 as a function of temperature and pressure
Authors: Guha Soumyendu , Walrafen George E.
Published: February 2003

Raman studies of vitreous B2O3 have been conducted between 8 and 700 K, and as a function of hydrostatic pressure (∼8 kbar) at room temperature. The low temperature Raman spectra exhibit two broad bands at 50 and 137 cm−1. From the temperature dependence of the 50 cm−1 band, it has been concluded that the vibrational density of states in the low frequency region (30<ω<300 cm−1) is better described when the Raman coupling coefficient Cb varies as ω. A shift in frequency of the Raman spectra as a function of temperature is attributed to a structural change near the glass transition. It is postulated that at room temperature, the structure of v-B2O3 is comprised of an equal proportion of boroxol (B3O6) rings and BO3 triangles. The low frequency vibrational band at 50 cm−1 arises from a cooperative motion of random distributions of mass of various shapes and sizes in the B2O3 glass. Near the glass transition temperature, the boroxol rings break up thereby leading to a more open structure. Above the melting point, i.e., when the viscosity is low, a regrouping of atoms occurs which results in a low density random network structure of BO3 triangles. The mode Gruneisen constants obtained from the high pressure Raman data are found to be +3.42 and −2.05 for the two bands. The observed positive and negative values of γ lend support to our proposed model in which we assign the 137 cm−1 band to the librational motion of boroxol rings.
Statistical thermodynamics of aerosols and the gas–solid Joule–Thomson effect
Authors: Pierotti Robert A. , Rybolt Thomas R.
Published: February 2003

Due to the adsorption of a gas by a solid, it is expected that an aerosol created by dispersing a fine powder in a gas would have unique thermodynamic properties not found in pure or mixed gases. The virial equation of state associated with an aerosol dusty gas is obtained from statistical thermodynamic considerations. In the theoretical model presented here, the aerosol is considered to be a two component fluid made up of solid particles and gas molecules. The aerosol virial equation of state is used to derive an expression for the Joule–Thomson effect associated with a gas–solid dispersion. The magnitude of the gas–solid Joule–Thomson effect is expressed in terms of gas and gas–solid virial coefficients. Previous adsorption data for an argon–porous carbon system is used to obtain gas–solid virial coefficients and to predict the magnitude of the gas–solid Joule–Thomson effect. A significant enhancement of the Joule–Thomson effect is predicted for gas–solid systems which display a strong interaction. For example, at a temperature of 300 K an argon–Saran 746 porous carbon aerosol system at a concentration of (0.4 g of powder/ℓ of gas) is predicted to have a gas–solid Joule–Thomson coefficient of 3.6 K/atm which is ten times greater than the effect for pure argon.
Computer simulation of end-linked elastomers. I. Trifunctional networks cured in the bulk
Authors: Leung Yu-Kwan , Eichinger B. E.
Published: February 2003

The end linking of difunctional poly(dimethylsiloxane) with trifunctional cross linkers in the bulk has been simulated with a computer. A random array of primary chains with a Gaussian end-to-end distribution is generated in an image container on the computer. By joining chain ends at nearby junctions, inter- and intramolecular reactions occur in all species, allowing formation of cyclic structures of any size. At high extents of reaction, the sol components extracted from the spanning forest thus constructed consist of a substantial amount of cycles such as cyclic dimers. The effect of intramolecular reaction increases the extent of reaction by several percent for molecular weights in the range of thousands. Structural statistics of various types of network imperfections and the cycle ranks of the networks are reported. The gels obtained at complete conversions contain loop defects, their populations increasing with lower molecular weights of the prepolymers. The elastic activities of nonstoichiometric systems are compared and discussed.
Elucidation of the principal hyperfine tensor components for the methyl group interaction in semiquinone anion radicals utilizing powder ENDOR spectroscopy
Authors: O’Malley Pádraig J. , Babcock Gerald T.
Published: February 2003

The feasibility of obtaining the principal values of the hyperfine tensor for a rotating methyl group from an organic radical’s powder ENDOR spectrum is illustrated. For both the 2,5-dimethylsemiquinone and the 1,2,4,5 tetramethylsemiquinone anion radicals, the ENDOR methyl group band for the powder sample is characterized by a buildup of intensity at the perpendicular and parallel principal components. The presence of sharp, well resolved bands near the free proton frequency is in striking contrast to the broad matrix band usually observed in this region indicating that more detailed information concerning radical–matrix interactions can be extracted from the powder spectrum.
Temperature dependency of the affinity decay rate
Authors: Garfinkle M.
Published: February 2003

The temperature dependency of the affinity decay rate of the reacting system was found to be analogous to the temperature dependency of the Helmholtz function of the equilibrium system. Consequently, standard-state internal energies could be computed from the kinetic data and were found to be comparable to experimental internal energies.
Cluster expansion of the wave function. Valence and Rydberg excitations and ionizations of ethylene
Authors: Nakatsuji Hiroshi
Published: February 2003

The symmetry-adapted-cluster (SAC) and SAC-CI theories have been applied successfully to the calculations of the valence and Rydberg excitations and the outer- and inner-valence ionizations of ethylene. Among all, the result for the singlet (π,π*) excited state is very encouraging, showing the effectiveness of the SAC-CI theory for the descriptions of electron correlations in the excited state. A systematic assignment of the singlet and triplet Rydberg excited states is given. The outer-valence ionization spectra are well reproduced by the SAC-CI theory, though in the inner-valence region, the intensities of the calculated ionization peaks are rather weak in comparison with the experiment.
Internal coordinate formulation for the vibration-rotation energies of polyatomic molecules. II. Symmetric top molecules with a threefold axis
Authors: Clodius William B. , Quade C. Richard
Published: February 2003

The theory of vibration-rotation interactions in symmetric top molecules with a threefold axis has been developed using curvilinear internal coordinates for the vibrational degrees of freedom. General expressions for the inertial coefficients, Coriolis coupling coefficients, and vibrational kinetic energy anharmonic coefficients have been derived for planar and nonplanar XY3 molecules. A Van Vleck perturbation scheme is used to obtain the theoretical vibration-rotation and vibration anharmonic spectroscopic coefficients. Applications are made in the analyses of the empirical spectroscopic coefficients for BF3 and NH3 in the determination of incomplete sets of improved anharmonic potential energy coefficients. In many respects it is found that the curvilinear internal coordinate approach to vibration-rotation interactions has advantages over the Cartesian displacement coordinate approach for angle bending modes in the symmetric top molecules.
Visible spectroscopy of jet-cooled SiC2: Geometry and electronic structure
Authors: Michalopoulos D. L. , Geusic M. E. , Langridge-Smith P. R. R. , Smalley R. E.
Published: February 2003

SiC2 has been prepared in a supersonic molecular beam by laser vaporization of a silicon carbide rod within a pulsed supersonic nozzle. Rotational analysis of the 0-0 band of the well-known 4980 Å band system of this molecule reveals that, contrary to previous assumptions, the molecule is triangular in both the ground and excited electronic states. In both states the molecule is of C2V symmetry with a C–Si–C angle between 40° and 41°. The correct assignment of the spectrum is Ã ′B2←x̃ ′A1. The carbon–carbon bond length is 1.25 Å in the ground state, suggesting that the molecule is best understood as a silicon atom bound to the side of a triply bonded C2 fragment. The optical transition moment is polarized along the b axis of the molecule which is parallel to the carbon–carbon bond axis. In the Ã 1B2 excited state the carbon–carbon bond opens up to 1.30 Å consistent with a π*←π excitation of the carbon–carbon triple bond. The silicon–cargon distance is measured to be 1.81 Å in the x̃ 1A1 state, lengthening to 1.88 Å in the Ã ′B2 excited state. In addition, the ionization potential of SiC2 was determined to lie between 8.91 and 10.38 eV.
The NV2 valence transition of 2,3-dimethylbutadiene
Authors: McDiarmid Ruth , Sabljić A. , Doering J. P.
Published: February 2003

The NV2 valence transition of 2,3-dimethylbutadiene is demonstrated, by electron energy loss spectroscopy, to occur around 7 eV. The slight NV2 intensity difference between this molecule and butadiene is shown to arise from a ground state vibrational excitation dependent enhancement of the NV2 transition in DMB.
Equilibrium properties of charged hard spheres of different diameters in the electrolyte solution regime: Monte Carlo and integral equation results
Authors: Abramo Maria C. , Caccamo C. , Malescio G. , Pizzimenti G. , Rogde S. A.
Published: February 2003

Extensive Monte Carlo (MC) and integral equation calculations of thermodynamic and structural properties for the 1-1 aqueous electrolyte regime of the primitive model are reported. The salt concentration covers the range 0.1–2 M. The ratio of the radii of the two ionic species α covers the range 0.1–0.8. The MC results are compared to the results of the HNC, MSA, and EXP approximations. The excess internal energies calculated in the HNC are found to be in good agreement with the MC values. In the MSA the energy turns out to be too high for large α and too small for small α. For α=0.4 the MSA value is accurate as compared to the simulation. Similar remarks hold for the osmotic coefficient. The radial distribution functions (RDF) calculated in the EXP approximation agree fairly well with the MC results at all molarities and radius ratios investigated, except for α=0.1, where the agreement is only qualitative. The MSA yields RDF which are in qualitative agreement with the MC only at high concentrations and for not too small α. Some preliminary MC results are also reported for the 2-2 aqueous electrolyte solutions at 2 M with α=0.4. An enhancement occurs in the peak of g++(r) and g−−(r) with respect to the peak of the corresponding glike(r) of the 2-2 symmetrical case (α=1). This peak is commonly associated with ion clustering. It appears that this effect is more pronounced for ions of different diameters.
Theoretical study of miscibility and glass-forming trends in mixtures of polystyrene spheres
Authors: Shih Wei-Heng , Stroud D.
Published: February 2003

We present a theoretical study of glass-forming trends and miscibility in mixtures of polystyrene spheres (‘‘polyballs’’) of different diameters, suspended in an aqueous solution. The polyballs are assumed to be charged and to interact via a Debye–Hückel screened Coulomb potential. The Helmholtz free energy is calculated from a variational principle based on the Gibbs–Bogolyubov inequality, in which a mixture of hard spheres of different diameters is chosen as the reference system. It is found that when the charges of the two types of polyballs are sufficiently different, the variationally determined ratio of hard-sphere diameters differs substantially, leading to packing difficulties characteristic of glass formation. The experimentally observed range of glass formation corresponds to a ratio of hard-sphere diameters of 0.8 or less. Calculations of the free energy as a function of concentration indicate that the liquid polyball mixture is stable against the phase separation, even for widely different polyball charges.
Thermodynamic study of the gaseous dilanthanum carbides, La2C2, La2C3, La2C4, La2C5, La2C6, and La2C8 by high temperature mass spectrometry
Authors: Pelino Mario , Gingerich Karl A. , Nappi Bianca , Haque Reza
Published: February 2003

The high temperature Knudsen effusion mass spectrometric technique has been used to identify and characterize the molecules La2C2, La2C3, La2C4, La2C5, La2C6 and La2C8 in the equilibrium vapor above the La–Ir–graphite system. The enthalpies ΔH°0 of the reaction 2La(g) +n C(graph.)=La2Cn (g) (n=2–6, 8) were evaluated using the third law method. For La2C3(g) and La2C4(g) a second law evaluation was also performed, indicating the linear symmetric structure as the most probable. The selected values for the reaction enthalpies were combined with ancillary literature data to yield the atomization energies ΔH°a,0, and the standard heats of formation ΔH°f,298.15 in kJ mol−1 of the gaseous dilanthanum carbides: La2C2 La2C3 La2C4 La2C5 La2C6 La2C8 UFRULE1 ΔH°a,0 1670±40 2339±27 3060±21 3703±50 4395±50 5658±60 ΔH°f,298.15  621±40  658±27  648±21  717±50  737±50  898±60
Structure of liquid metals
Authors: Reddy M. Rami
Published: February 2003

The Percus–Yevick (PY) and mean spherical model (MSM) approximation integral equations are used to calculate the radial distribution functions of liquid sodium, rubidium, and aluminium. The resulting radial distribution functions are compared with molecular dynamics (MD) simulation results. The agreement is satisfactory.
Infrared photodissociation of ammonia dimers
Authors: Howard Mark J. , Burdenski Siegfried , Giese Clayton F. , Gentry W. R.
Published: February 2003

Ammonia dimers, formed by the pulsed supersonic expansion of a dilute mixture of NH3 in He, can be photodissociated under collision-free conditions with CO2 laser radiation in the frequency range 900–1100 cm−1. The dimers are detected with a mass spectrometer as NH+4 ions. The attenuation of the NH+4 signal by the laser is a measure of the frequency-dependent absorption cross section, which displays a single, broad, non-Lorentzian peak in this frequency range. The attenuation as a function of laser fluence shows saturation behavior which depends on the molecular beam source conditions. Conditions which produce colder beams result in a larger fraction of dimers which are not dissociated even at high laser fluence. These observations suggest that the only dimers which are photodissociated under our experimental conditions are those having enough internal excitation before absorption to be excited above the dissociation limit with a single laser photon. No evidence was found for the occurrence of two-photon processes.
H+D2 reaction dynamics. Determination of the product state distributions at a collision energy of 1.3 eV
Authors: Marinero Ernesto E. , Rettner Charles T. , Zare R. N.
Published: February 2003

Two-photon resonance, three-photon ionization has been used to determine the HD product internal state distribution formed by the reaction of fast H atoms with thermal D2 molecules. A mixture of HI and D2 is irradiated by a 266 nm laser pulse to dissociate the former, giving a center-of-mass collision energy of about 1.30±0.04 eV for H+D2. After a sufficiently short delay to ensure essentially collision-free conditions, a second laser is fired which causes multiphoton ionization of individual HD quantum states as well as D atoms, depending upon the choice of wavelength. Reaction occurs in a well-defined effusive flow which emerges from a glass orifice placed between the acceleration plates of a differentially pumped time-of-flight mass spectrometer. Ion signals are referenced to those obtained from HD or D produced in an auxiliary microwave discharge. Relative formation rates are reported for HD(v=1, J=0–6) and HD(v=2, J=0–6). Nascent D atoms are also observed and an upper limit is placed on the production of HD(v=3). Rotational surprisal plots are found to be linear for the HD product state distribution yielding a slope of θR=5.1 for HD(v=1) and θR=4.7 for HD(v=2). These are extrapolated to provide full distributions for HD(v=0–2, J=0–6). The present product state distributions are compared with the recent experimental data of Gerrity and Valentini as well as with the quasiclassical trajectory calculations of Blais and Truhlar.
The coupling of density and internal energy fluctuations by isothermal chemical reactions
Authors: Keizer Joel
Published: February 2003

Under nonequilibrium conditions chemical reactions can modify the coupling of fluctuations in number densities to fluctuations in the internal energy. At a nonequilibrium steady state these fluctuations renormalize the intensive thermodynamic variables, including the temperature and chemical potentials, so that they no longer have their local equilibrium form. Using the mechanistic statistical theory of nonequilibrium thermodynamics, we show at an isothermal steady state that the chemical reaction X+Y⇄2X provides a nonequilibrium coupling of the density and internal energy fluctuations. If thermal exchange with the reservoir is rapid, this coupling can be small, but only under specific conditions such as occur in dilute solution. Under such conditions the steady state temperature is identical to the Kelvin temperature and the density fluctuations are those calculated by treating the temperature as a fixed parameter. The steady state thermodynamics of the dilute, isothermal reactive system is discussed.
Reaction and relaxation of vibrationally excited formyl radicals
Authors: Langford Andrew O. , Moore C. Bradley
Published: February 2003

Ground state and vibrationally excited formyl radicals were produced by excimer laser (308 nm) photolysis of formaldehyde and glyoxal at 295 K. The subsequent evolution of the (0,1,0) and (0,0,0) vibrational levels was monitored through time-resolved laser resonance absorption. Rate constants of (6.4±1.0)×10−12, (3.0±0.5)×10−12, (2.5±0.3)×10−13, (2.5±0.8)×10−13, (3.4±0.4)×10−11, and (9.4±1)×10−12 cm3 molecule−1 s−1 were determined for removal of HCO(0,1,0) by (HCO)2, H2CO, He, N2, NO, and O2, respectively. Upper limits of 8.0 ×10−12 and 4.4×10−12 cm3 molecule−1 s−1 were established for reactive removal of HCO(0,1,0) by NO and O2. The rate constant for relaxation of an unidentified higher vibrational level of HCO to (0,1,0) by (HCO)2 was measured to be (2.4±0.4)×10−11 cm3 molecule−1 s−1 and rate constants of (7±2)×10−13 and (2.6±0.6)×10−11 cm3  molecule+1 s−1, respectively, were determined for removal of DCO(0,1,0) by D2CO and NO. The relaxation and reaction rates with NO and O2 are understood in terms of the formation and decomposition of an HCONO and HCOO2 collision complex.
Local spin density theory of free radicals: Nitroxides
Authors: Delley B. , Becker P. , Gillon B.
Published: February 2003

Local spin density theory is applied to the nitroxides H2NO, (CH3)2NO, (CH3)2NO⋅⋅⋅HOCH3. Spin density is localized on the NO group. In dimethylnitroxide the spin density is partitioned one to one between N and O. Substitution of the hydrogen atoms in dihydrogen nitroxide by methyl groups as well as addition of a hydrogen bridge result in an enhancement of the spin density on nitrogen. The hydrogen bridged compound models the spin density distribution in tanol (2,2,6,6-tetrametyl-4-piperidinol-1-oxyl) which has been investigated by polarized neutron diffraction.
Chemical application of the saddle point variational method. I. Oxygen 1s vacancy ions of formaldehyde
Authors: Luken William L. , Culberson John C. , Leonard Joseph M.
Published: February 2003

The unrestricted Hartree–Fock (UHF) self-consistent field (SCF) procedure is shown to be a special case of the saddle point variational method for an atom or molecule containing a vacancy in the most tightly bound orbital. Numerical results based on UHF-SCF wave functions are presented for the oxygen 1s hole state of formaldehyde. These calculations employ double zeta basis sets, with Slater-type orbitals being used for certain one-center integrals. The equilibrium geometries and charge distributions obtained for these innershell vacancy ions are compared to those of the corresponding equivalent cores ion.
Ab initio computation of radiative lifetimes of the H2 molecule in the nonadiabatically coupled J=1 states derived from {E
F+G
K+H
H̄} 1Σ+g+I 1Πg
Authors: Glass-Maujean M. , Quadrelli P. , Dressler K.
Published: February 2003

The vibrational matrix elements of the electronic dipole moments for the transitions (EF, GK, HH̄, I)→(B, C, B′) have been computed in the adiabatic approximation and they have been transformed according to a nonadiabatic representation of the radially and angularly coupled J=1 states of the initial 1∑+g and 1Πg states. The corresponding probabilities for spontaneous emission and the radiative lifetimes are computed and compared with experimental data.
Analyses of the electron-spin-resonance (ESR) hyperfine powder spectra of nitroxide radicals enriched with 15N (I=1/2)
Authors: Lee Sook , Sandreczki T. C. , Brown I. M.
Published: February 2003

A detailed investigation has been performed to analyze the electron-spin-resonance (ESR) properties of a nitroxide radical (TANOL) isotopically enriched with 15N with a nuclear spin I=1/2 in amorphous samples of a polycarbonate polymer. Unlike the case of 14N with I=1, the 15N ESR hyperfine powder spectra consist of two components that are well resolved at an X-band microwave frequency and each hyperfine component displays resolved powder-pattern structures so that the values of all the principal Zeeman and hyperfine coupling constants can be unambiguously determined as gx= 2.0096, gy= 2.0062, gz =2.0022, Ax/(gx μB)=−1.00 mT, Ay/(gy μB) =−1.00 mT, Az/(gz μB) =−4.89 mT, with an accuracy of ±0.0004 for the gi values and ±0.0015 mT for the Ai/(gi μB) values.
Effects of an external magnetic field on laser-induced fluorescence and photodissociation of NaK
Authors: Katô Hajime , Baba Masaaki , Hanazaki Ichiro
Published: February 2003

Each line of the fluorescence to the a 3Σ+ state following the excitation D 1Π (v′=12, J′=14) ← X 1Σ+(v″=0, J″=14) of the NaK molecule is found to split into three lines when an external magnetic field is applied. The changes of the intensity and the energy splitting are measured as a function of the field strength. Formulas for the intensities of the Zeeman lines of the resonance fluorescence to the 3Σ+ state following the Q excitation 1Π(v, J M±1) ← 1Σ+ (v0,J M) are presented. The contamination of the lower triplet state a 3Σ+ by a singlet perturber X 1Σ+ is shown to be negligible from the observed Zeeman splitting and a theoretical calculation. Intensity asymmetries of the three lines split by the Zeeman effect are observed and are explained by M dependent excitation to the Zeeman components of the D 1Π state. The intensities of emission from the dissociated Na(3 2P3/2), Na(3 2P1/2), K(42P3/2), and K(4 2P1/2) are found to change remarkably when a magnetic field is applied, and each line is found to change differently from others depending on the field strength.
A high resolution study of the ν3 band of the ammonium ion (NH+4) by velocity modulation laser absorption spectroscopy
Authors: Schäfer Eckhard , Saykally Richard J. , Robiette Alan G.
Published: February 2003

Over 200 transitions in the ν3 band of the ammonium ion (NH+4 ) have been measured near 3 μm with a color center laser spectrometer to an accuracy of 0.005 cm−1 using the technique of velocity modulation laser absorption spectroscopy. These have been assigned to strongly allowed Q0, P+ and R− branches in the triply degenerate asymmetric stretching mode of a spherical top molecule, and analyzed by nonlinear least–squares methods with an effective sixth order Hamiltonian. Ground state constants were fixed at values obtained with a recent ab initio molecular potential surface. Twenty-six molecular parameters were determined in the fit. The fact that terms in the Hamiltonian beyond fourth order are required to fit transitions with J as low as 12 indicates the existence of a perturbation involving the ν3=1 levels. It is suggested that a Fermi resonance between ν3 and ν2+ν4 is an important source of this perturbation, as in the case of CH4. The frequencies of ‘‘forbidden’’ transitions, required in order to determine the ground state parameters, are predicted to high accuracy with these new constants.
Asymmetry of the ESR linewidth angular dependence for antisymmetric exchange chains
Authors: Zaspel Craig E.
Published: February 2003

The effect of antisymmetric exchange on the electron spin resonance linewidth is investigated. For a certain class of D vector in the Hamiltonian it is shown that the angular dependence of the linewidth has a period of 360 instead of 180. This effect was first seen experimentally in bis-piperidinium tetrachlorocuprate.
Singlet→triplet transitions in C≡N containing molecules by electron impact
Authors: Rianda Ronald , Frueholz Robert P. , Kuppermann Aron
Published: February 2003

The electron-impact excitation spectra of hydrogen cyanide (HCN), acetonitrile (CH3CN), malononitrile [CH2(CN)2], propionitrile (C2H5CN), and butyronitrile (C3H7CN) have been studied experimentally at impact energies of 25, 50, and 75 eV and at scattering angles from 5° to 80°. Results for hydrogen cyanide are in excellent agreement with previous work. Previously unobserved singlet→triplet transitions of acetonitrile, propionitrile, and butyronitrile are reported. Also, the first study of the electronic spectrum of malononitrile is reported. Tentative assignments for transitions observed are reported.
Analysis of nuclear quadrupole interaction in three pulse electron spin echo modulation spectra
Authors: Romanelli M. , Narayana M. , Kevan L.
Published: February 2003

The introduction of nuclear quadrupole interaction in electron spin echo modulation by perturbation methods as proposed by Shubin and Dikanov [J. Magn. Reson. 52, 1 (1983)] is extended to the analysis of three pulse electron spin echo envelope modulation spectra. It is shown that an additional frequency difference occurs apart from the amplitude differences discussed by Shubin and Dikanov, because of an oversimplification of the expressions for the ENDOR levels. The effects of such inclusion of quadrupole interaction on the individual components contributing to the final modulation but with different time dependences are critically examined along with the effect of possible nonalignment of the quadrupole and dipolar tensors. The validity of the expressions obtained by the perturbation approach seems to be limited to relatively long electron-nuclear distances (r>0.35 nm for nuclei with I=1 and greater for nuclei with I>1). An independent determination of the magnitude of the quadrupole interaction may be needed to obtain unambiguous fits to experimental echo modulation spectra since the nonalignment of the hyperfine and quadrupole tensors reduces the influence of the quadrupole interaction on the echo modulation amplitudes.
Vibrational relaxation of HF(v=3, 4, and 5) by H2, D2, CH4, CD4, and CO2
Authors: Jursich G. M. , Ritter D. R. , Crim F. F.
Published: February 2003

Laser excited vibrational overtone fluorescence experiments provide a direct measurement of the collisional relaxation rate constants for HF(v=3, 4, and 5) with the collision partners H2, D2, CH4, CD4, and CO2. Vibration-to-vibration energy transfer is apparently the dominant relaxation path except for H2 where the higher levels of HF(v≥4) relax by vibration-to-translation, rotation energy transfer processes. Methane and carbon dioxide are the most efficient quenchers of highly vibrationally excited HF and remove a quantum of energy from HF(v=5) on roughly every other collision. Combining these direct excitation measurements with recent chemiluminescence results provides data which span a large range of initially excited vibrational states of HF.
MRD-CI potential surfaces using balanced basis sets. II. O+H2 and F+H2
Authors: Wright James S. , Donaldson D. J. , Williams Richard J.
Published: February 2003

Multireference configuration interaction (MRD-CI) methods are used to calculate ground state potential surfaces for three triatomic systems at frozen bond angles: H2O→OH+H at 104.5°, O(3P)+H2→OH+H at 180°, and F(2P)+H2→HF+H at 180°. The ‘‘balanced basis sets’’ are of double-zeta plus polarization quality, augmented by (s, p) bond functions optimized for the diatomic hydrides. The CI data points are computed along rays of constant θ and the data on each ray are fitted by generalized Morse functions. The fits are excellent over a wide range of the potential. Cubic splines interconnect each ray, forming a smooth, continuous surface. Reaction energetics in each case are accurate to 2 kcal/mol or better, and force constants are reasonable. Barrier heights are 12.8 kcal/mol for 0+H2 and 0.1 kcal/mol for F+H2. The OH2 barrier lies near the best theoretical estimate. The FH2 barrier disagrees with previous estimates, but may resolve a conflict between theory and experiment for this reaction.
Multireference CI gradients and MCSCF second derivatives
Authors: Page Michael , Saxe Paul , Adams George F. , Lengsfield Byron H.
Published: February 2003

A set of simple and efficient formulas for the calculation of multireference CI gradients and MCSCF second derivatives is presented. The CI gradient formalism is extended to include a general class of references in the CI. This extension is necessary for the calculation of gradients for a commonly employed class of CI wave functions for which the reference configurations are selected from a generalized CAS MCSCF wave function. In addition, we report the first general multireference CI gradient calculations. The calculations are for the reaction Be+H2→BeH2 constrained to C2v symmetry. Structures of the reactant and transition state and the activation energy calculated at the selected reference CI level compare favorably to the full second order CI results. MCSCF second derivatives are found to be useful for the optimization of the CI structures.
A theoretical analysis of photoactivated unimolecular dissociation: The overtone dissociation of t-butyl hydroperoxide
Authors: Chandler David W. , Miller James A.
Published: February 2003

We have used the master equation formalism to analyze the photoactivated unimolecular dissociation of a large polyatomic molecule, t-butyl hydroperoxide (t-BuOOH). Our results are compared to the experimental results of Chandler, Farneth, and Zare and of Chuang et al. We find that the curvature in their Stern-Volmer plots cannot be explained by a collisional energy transfer mechanism. At high pressure, the theoretical Stern–Volmer plots are linear, independent of the collisional energy transfer properties. This linearity is related to the existence of a limiting high pressure ‘‘reactive distribution.’’ We identify three different pressure regimes that exist in photoactivated unimolecular reaction experiments, and we relate these regimes to the properties of the reactive distribution. In particular, we find that, when a Stern-Volmer analysis is used, curvature at very low pressure can result in systematic errors in determining the unimolecular rate constants. In addition, we have successfully analyzed the real time experiments of Rizzo and Crim using the same molecular model and parameters necessary to predict the experiments of Chandler et al.
A statistical model for solvation shifts of core electron binding energies of atomic ions
Authors: Ågren Hans , Siegbahn Hans
Published: February 2003

A statistical model for solvation shifts of core ionization energies in photoelectron spectra of atomic ions is presented. It is numerically evaluated for positive and negative ions in aqueous solution. Trends and magnitudes of computed shifts are compared with available experimental data.