bluuebunny/crossref_metadata_2025 huggingface dataset:
Overtone spectra of C–H bonds and vibrational a
b i
n
i
t
i
o study of methoxy boranes
Authors: Manzanares I Carlos , Walla Dalat , Seburg Randal , Wedlock Michael R. , Gonzalez Carlos , Schlegel H. Bernhard
Published: July 2002

The C–H overtone absorption spectra of (CH3O)3B in liquid and gas phases are reported. The observed energies of the C–H stretching overtones corresponding to Δv=3, 4, 5, and 6 (liquid) are obtained by conventional spectroscopy. The C–H overtones Δv=5 and 6 (gas) are obtained by laser intracavity photoacoustic spectroscopy. Computer deconvolution of the gas-phase absorptions shows two bands at each overtone which are assigned as the overtones of nonequivalent C–H bonds. In general, for molecules in which a methyl group is directly attached to an oxygen atom, nonequivalent C–H bonds are produced due to the ‘‘trans effect.’’ A different mechanism seems to occur in the case of the methyl groups in trimethylborate even though they are adjacent to an oxygen atom. In order to interpret the experimental results, ab initio molecular-orbital calculations were performed on (CH3O)BH2, (CH3O)2BH, and (CH3O)3B. Equilibrium geometries, vibrational frequencies, and infrared intensities were calculated at the Hartree–Fock level using the 3-21G split valence basis set. In the three molecules studied, the equilibrium conformation is such that the methyl groups have one C–H bond (C–Ha ) in the plane of the molecule and cis to a B–O bond, and two equivalent C–H bonds located symmetrically above and below the plane of the molecule (C–Hb ). In (CH3O)BH2, the in-plane C–H bond C–Ha is longer than the out-of-plane C–H bonds C–Hb (1.082 Å vs 1.0795 Å). This trend is reversed in the case of (CH3O)3B, where the C–Ha and C–Hb bond lengths are 1.0792 and 1.0807 Å, respectively.

The situation is more complicated in the case of the molecule (CH3O)2BH, where in one methyl group the C–Ha bond is shorter than the C–Hb bond (1.0794 Å vs 1.0802 Å), while in the other methyl group the corresponding C–Ha bond is longer than the C–Hb bond (1.0829 Å vs 1.0802 Å). The nature of the difference in the C–H bond lengths is studied and discussed in terms of group orbital interactions in both, the σ and π systems. In addition, a correlation between the C–H bond length, the corresponding C–H stretching force constant, and the vibrational frequency is discussed based on vibrational frequency calculations performed on the deuterated species (CHD2O)nBH3−n for n=1, 2, 3 in cases where the isolated C–H was considered to be the in-plane C–Ha or the out-of-plane C–Hb.
Disorder dynamics in solid 9-hydroxyphenalenone
Authors: Tegenfeldt J. , Ojamäe L. , Svensson C.
Published: July 2002

1 H nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation data as well as 1H decoupled 13C spectra have been utilized to study the dynamics of the molecular disorder in solid 9-hydroxyphenalenone. In the room-temperature phase—stable between 255 and 380 K—the proton spectrum is narrowed compared to the spectrum of a static structure. This is consistent with a dynamically disordered structure where one of the two nonequivalent molecules reorients rapidly between its two possible equilibrium orientations. The proton spin-lattice relaxation has a maximum of 7.2 s−1 in the same phase at about 355 K, in close agreement with a value of 7.6 s−1 calculated from the dynamical disorder model. The 1H decoupled 13C powder spectrum in the room-temperature phase is also consistent with this picture. Above the 385 K phase transition, the 13C powder spectrum is approaching axial symmetry, proving that all molecules reorient rapidly in that phase.
Bound state contributions to transport coefficients
Authors: Garland Gregory E. , Dufty James W.
Published: July 2002

Green–Kubo expressions for transport coefficients have three distinct terms arising from correlation of the kinetic and potential parts of appropriate fluxes. Bound state effects on transport coefficients at finite density are studied here by an exact calculation of the first density coefficient for the kinetic-potential cross terms. Attention is restricted to the special case of a square well potential. Results for the shear viscosity and thermal conductivity are reported. A strong dependence on temperature is found for both scattering and bound state contributions. Effects of bound states dominate those from scattering states for temperatures approaching the critical temperature. Agreement with available computer simulation results is obtained.
An observable consequence of molecular rotational inertia in an alternating current Kerr effect
Authors: Déjardin Jean-Louis , Debiais Georges
Published: July 2002

A study of phase angles of the electric birefringence including inertia is proposed in the case of the simultaneous action of two electric fields, namely a direct current (dc) bias field added to an alternating current (ac) field of angular frequency ω. The solution of the Fokker–Planck–Kramers (FPK) equation applied to the orientational motion of a polar (permanent moment μ) and anisotropically polarizable (induced moment Δα) molecule in two-dimensional (2D) space (disk model) leads to distinct phase angles θ1(ω) and θ2(ω), both of them corresponding to harmonic components in ω and 2ω of the birefringence. The evolution of the plots θj(ω), where j=1,2, for different values of the inertial parameter a2=τI/τ and given a value of P=(Δα/μ2)kT is presented. It is shown that in the high-frequency region, the phase angles may take up to twice the values they would have with zero inertia. τI and τ are the friction time and the Debye relaxation time, respectively, while kT stands for the usual thermal energy. Such variations must therefore be measurable by experiment and allow a new way of research in Kerr effect relaxation when inertia is taken into account.
The spectroscopic and photophysical effects of the position of methyl substitution. I. 4- and 5-methylpyrimidine
Authors: Bandy Ralph E. , Nash John , Zwier Timothy S.
Published: July 2002

Laser-induced fluorescence excitation and dispersed fluorescence spectra of the first n–π* transition of jet-cooled 4- and 5-methylpyrimidine (4-mp and 5-mp) have been recorded and analyzed. In 5-mp, methyl substitution preserves many of the spectroscopic signatures of the unsubstituted pyrimidine molecule. Dispersed fluorescence spectra are used to assign most of the major features in the first 1000 cm−1 of the excitation spectrum. High resolution scans at the origin reveal a 0.20 cm−1 splitting of the origin arising from the 0a′1–0a1 and 1e″–1e″ internal rotor transitions. From this small splitting we deduce that the nearly free internal rotation of the methyl group in the ground state is carried over to the S1 state as well. In 4-methylpyrimidine, the reduction in symmetry accompanying methyl substitution (G12 to G6 ) results in allowed transitions to all in-plane fundamentals. The methyl group is seen to participate in the electronic transition to a greater degree in 4-mp than in 5-mp.

We observe clear activity in both the C–CH3 stretch and C–CH3 in-plane bend in the dispersed fluorescence from the origin of 4-mp. 4-mp also differs remarkably from 5-mp in the magnitude of the barrier to methyl internal rotation in S0 and S1. By fitting the positions and intensities of internal rotor structure in ground and excited states we deduce a ground state barrier to internal rotation of V″3=95±5 cm−1 and a best-fit excited state barrier of V3=745 cm−1, V′6=−100 cm−1. Ab initio calculations on 4-mp which reproduce both the magnitude and shape of the experimental barrier to internal rotation in the ground state. The lowest energy methyl conformation places a hydrogen atom in the plane of the ring pointing away from the nitrogen lone pair. Finally, in both molecules we observe spectroscopic signatures of vibrational state mixing in the S1 state. Density-of-states calculations on both molecules using the experimentally determined internal rotor energy levels predict a similar density of same-symmetry states in the two molecules at a given energy. Experimental evidence is presented that the small V6 barrier in 5-mp leads to modest vibration/internal rotation coupling matrix elements of ∼1 cm−1. The high V3 barrier in 4-mp is observed to give strong vibration/internal rotation coupling in the case of the 6a10(0a1) level which shifts its position by 17 cm−1 from its companion 6a10(1e) level due to interaction with an X1(3a2) vibration/internal rotation combination band.
Piezomodulation study of Langmuir–Blodgett films of copper phthalocyanine derivatives
Authors: Tanaka Masatoshi , Tsuyoshi Hiroaki , Sakata Kazumasa , Yudasaka Masako , Kurita Susumu , Ogawa Kimiya , Yonehara Hisatomo , Kinoshita Sho-ichi , Nakahara Hiroo , Fukuda Kiyoshige
Published: July 2002

A method for quantitative analysis of piezomodulation spectra of a thin film on a substrate, including the internal reflection in the system, is presented. This method is applied to Langmuir–Blodgett (LB) films of copper phthalocyanine (CuPc) derivatives on glass substrates. A uniaxial stress was applied along the symmetry axis of the film. The spectra were measured in the region from 1.5 to 2.4 eV in three configurations; transmission, reflection from the film side, and reflection from the substrate side. The optimum Lorentzian parameters and their changes due to the stress are determined by the least-mean-square fit. The microstructures and electronic states of the LB film are discussed from the stress effects. The very small feature, which is hardly identified in the absorption spectra because of the large Frenkel exciton band, is attributed to a charge transfer (CT) excitation.
Electronic states of Ru2
Authors: Das Kalyan K. , Balasubramanian K.
Published: July 2002

Complete active space multiconfiguration–self-consistent-field (CAS–MCSCF) calculations are carried out on 18 electronic states of ruthenium dimer (Ru2). In addition, multireference singles+doubles configuration interaction calculations which included up to 600 000 configurations are made on the lowest state of Ru2. We find that the 7Δu state is the lowest with re=2.36 Å, ωe=273 cm−1, and De=2.0 eV at the highest level of theory. We find 18 electronic states within ∼2000 cm−1 of the ground state.
Raman spectroscopic investigation of the structure of silicate glasses. IV. Alkali–silico–germanate glasses
Authors: Furukawa Toshiharu , White William B.
Published: July 2002

Raman spectra were measured on bulk Na2O⋅(2−x)SiO2⋅xGeO2 glasses, where x varied from 0 to 2, and on Na2O⋅SiO2⋅xGeO2 glasses, where x varied from 0 to 1. Raman spectra were measured also on Na2O⋅(1−x)SiO2⋅xGeO2 glasses crystallized at 650 °C, mostly a solid solution with the β-Na2Si2O5 structure at x between 0 and 1.5. The spectra of the glasses as well as the crystalline solid solutions can be interpreted in terms of random substitution of Ge for Si. The symmetric stretching vibrations of the nonbridging bonds Si–O−, Ge–O−, −O–Si–O−, and −O–Ge–O− are localized and exhibit two-mode behavior. On the other hand, the symmetric stretchings of the bridging bonds Si–O–Si, Ge–O–Ge, and Si–O–Ge are coupled together to give only one Raman band as the Si/Ge ratio is varied.
Pressure effects on hole-burning spectra in glasses: Calculation beyond the Gaussian approximation
Authors: Kador L.
Published: July 2002

In a recent publication, Laird and Skinner [J. Chem. Phys. 90, 3274 (1990)] proposed a microscopic statistical theory describing the effects of external hydrostatic pressure on hole-burning spectra of impurity molecules in amorphous solids. Using the so-called Gaussian approximation, which is valid in the limit that the density of the solvent molecules is high, the theory predicts the pressure kernel of a hole spectrum as well as the shape of the inhomogeneous band to be characterized by Gaussian profiles. Whereas the maximum position of the kernel increases from lower to higher solvent shift values in the inhomogeneous distribution, its width is constant. Numerical calculations performed without this approximation, however, show that for the data of poly(ethylene) and poly(styrene) doped with free-base phthalocyanine, not only the pressure shift but also the pressure broadening of hole-burning spectra increases from the blue to the red edge of the absorption band. Moreover, the hole spectra are predicted to become asymmetric when the sample is exposed to hydrostatic pressure. These deviations from the results of the Gaussian approximation are distinctly more pronounced than the deviations of the inhomogeneous band shapes from Gaussian profiles.
Quantum relaxation in high density fermion and boson tunneling dynamics: Implications for low temperature anomalous surface diffusion
Authors: Zhang Qing , Whaley K. Birgitta
Published: July 2002

Quantum relaxation behavior for many-particle tunneling systems of interacting identical particles is studied for both fermions and bosons within the infinite U Hubbard model, using a numerically exact checkerboard time propagation. Dependence of relaxation times upon vacancy concentration is obtained, and conclusions for nuclear statistical effects in hydrogen surface diffusion presented.
One-electron diatomics in momentum space. VII. Nonvariational approach to ground and excited states of heteronuclear systems
Authors: Koga Toshikatsu , Horiguchi Takehide , Ishikawa Yasuyuki
Published: July 2002

The nonvariational method proposed previously for the solution of the momentum-space Schrödinger equation is applied to the ground and various excited states of one-electron heteronuclear diatomic systems. Detailed numerical results are reported for 34 electronic states of the HeH2+ system. The method is demonstrated to give highly accurate electronic energies for heteronuclear systems as for the case of the homonuclear H+2 system.
A
b i
n
i
t
i
o study of the electronic states of O−2 i
n v
a
c
u
o and in simulated ionic solids
Authors: Ewig Carl S. , Tellinghuisen Joel
Published: July 2002

The ground electronic state and several low-lying excited states of the superoxide ion O−2 have been studied by ab initio computations. For comparison the ground state of O2 is also included. Parallel computations are carried out for the species in vacuo and in a simulated KCl crystal lattice. The in vacuo results indicate that even though the X 2Πg and A 2Πu states of O−2 are resonance states whose energies are above that of O2, they are electronically stable within the domain of the calculation. The a 4Σ−u state exhibits an energy minimum lying below O2(X) where it should be a stable species. All other states with energies less than 4 eV above the O−2 ground state are found to be electronically unstable in the region of internuclear distances studied (1–2 Å). By contrast, in the ionic crystal lattice all low-lying electronic states are rendered electronically stable by the Madelung potential of the lattice. Computed spectroscopic parameters are in good agreement with experiment for the X and A states of O−2 in vacuo. There is also substantial agreement between the computed energy curves for both the X and A states in the point-charge lattice and those measured in alkali halide lattices, including the prediction of appreciable crystal-field splitting in the 〈110〉 orientation. Further, the spectroscopic parameters of the electron-scattering resonance states in vacuo agree well with those of the analogous lattice-stabilized excited electronic states in the solid.
Electrostriction: A density functional theory
Authors: Vijayadamodar G. V. , Bagchi B.
Published: July 2002

A study of electrostriction in a dipolar liquid is carried out by using the density functional theory for inhomogeneous systems. A general expression for electrostriction due to a position-dependent electric field is derived. The expression for the electrostriction due to a constant macroscopic electric field is derived in terms of both orientational structure factors and dielectric function. This expression is found to reduce exactly to the expression derived earlier by Rasaiah, Isbister, and Stell when the proper identifications are made. A study of the change in density profile around a point ion in the dipolar liquid is carried out. It is found that the density profile is oscillatory, which is expected. However, our expression which considers only the quadratic nonlinearity in the electric field, breaks down at a very short distance from the ion. The microscopic structure of the liquid is shown to play an important role at the molecular length scales.
Collision energy dependence of the cross sections for production of CN(B 2Σ+) in the reactions of Ar(3P0,2) and Kr(3P0,2) with BrCN
Authors: Nagata Takashi , Kondow Tamotsu , Kuchitsu Kozo , Tabayashi Kiyohiko , Shobatake Kosuke
Published: July 2002

Cross sections for the formation of CN(B 2Σ+) in collisions of Ar(3P0,2) and Kr(3P0,2) with BrCN were measured in a crossed beam geometry as a function of the collision energy. An arc-heated metastable rare-gas beam was velocity selected by a time-of-flight method, and the collision energy Ec was varied in the range of 0.7–5.2 eV for Ar(3P0,2) and 1.0–5.2 eV for Kr(3P0,2). The cross sections were found to be proportional to E−mc, with m=0.355±0.008 for Ar(3P0,2) and 0.124±0.010 for Kr(3P0,2). The relative cross sections were normalized against the known absolute cross section for collisional excitation transfer from Ar(3P0,2) to N2. The absolute cross sections thus determined were, for example, 16.0±4.9 Å2 for the Ar(3P0,2)+BrCN system and 42.3±14.8 Å2 for the Kr(3P0,2)+BrCN system at the collision energy of 1.0 eV. The results were explained in terms of the curve-crossing mechanism that an ion-pair [Rg+BrCN−] intermediate state intervenes between the entrance and the exit covalent states.
Electronic tunneling and exchange energy in the D-dimensional hydrogen-molecule ion
Authors: Kais S. , Morgan J. D. , Herschbach D. R.
Published: July 2002

Dimensional scaling generates an effective potential for the electronic structure of atoms and molecules, but this potential may acquire multiple minima for certain ranges of nuclear charges or geometries that produce symmetry breaking. Tunneling among such minima is akin to resonance among valence bond structures. Here we treat the D-dimensional H+2 molecule ion as a prototype test case. In spheroidal coordinates it offers a separable double-minimum potential and tunneling occurs in only one coordinate; in cylindrical coordinates the potential is nonseparable and tunneling occurs in two coordinates. We determine for both cases the ground state energy splitting ΔED as a function of the internuclear distance R. By virtue of exact interdimensional degeneracies, this yields the exchange energy for all pairs of g, u states of the D=3 molecule that stem from separated atom states with m=l=n−1, for n=1→∞. We evaluate ΔED by two semiclassical techniques, the asymptotic and instanton methods, and obtain good agreement with exact numerical calculations over a wide range of R. We find that for cylindrical coordinates the instanton path for the tunneling trajectory differs substantially from either a straightline or adiabatic path, but is nearly parabolic. Path integral techniques provide relatively simple means to determine the exact instanton path and contributions from fluctuations around it. Generalizing this approach to treat multielectron tunneling in several degrees of freedom will be feasible if the fluctuation calculations can be made tractable.
Interactions of charged Langmuir monolayers with dissolved ions
Authors: Ahn Dong June , Franses Elias I.
Published: July 2002

Dissociation of protons from fatty acid Langmuir monolayers causes competitive adsorption of other counterions and partial charge neutralization. A new equilibrium model incorporating surface electrochemical and thermodynamic features is presented. The Stern–Gouy–Chapman equations are used, along with a modified Langmuir isotherm in which ion activities in the mixed surface monolayer instead of concentrations are employed. A two-dimensional lattice model with Flory–Huggins interaction parameters χij is invoked to obtain the activities of the ions in the monolayer. The competitive adsorption of ions to fatty acid monolayers is either ‘‘ideal’’ (χij=0), or nonideal, depending on the ions mixing behavior with protons. Previous electrochemical models apply only for ideal adsorption, which appears to occur for Cd2+ and Ca2+. The nonideal adsorption is characterized successfully by the use of the above interaction parameters. Ba2+ ions show negative deviations from ideal adsorption, i.e., they adsorb less than expected for ideal adsorption in the presence of H+ ions. In contrast, Pb2+ ions show positive deviations from ideal adsorption. The influence of ions on surface electrochemical properties of the monolayer is also calculated from the theory.
Laser guided chemistry: Control of the 26Li(g) = 6Li2(g) equilibrium and ortho/para enrichment
Authors: He Chun , Bernheim R. A.
Published: July 2002

The shift in the equilibrium distribution between reactants and products in the vapor phase chemical reaction 26Li■6Li2 that is caused by the optical orientation of atomic Li spin angular momenta is shown to be controlled by the degree of atomic orientation. The resolved rotational structure of the 6Li2 electronic spectrum observed by laser induced fluorescence reveals that the major part of the shift occurs for the odd molecular rotational levels associated with the para-nuclear spin variety of the homonuclear 6Li2 molecules. In this respect the results are the same as those previously found for 7Li2 where the nuclei are fermions instead of bosons and the para-nuclear spin symmetry is associated with even-J molecular rotational levels. For He buffer gas pressures of between 50 and 200 Torr and vapor temperatures between 550 and 600 K, over half of the para-molecules could be destroyed using only a few milliwatts of optical pumping power at the lithium D1 wavelength.
A Monte Carlo study of surface diffusion coefficients in the presence of adsorbate–adsorbate interactions. II. Attractive interactions
Authors: Uebing C. , Gomer R.
Published: July 2002

Chemical, jump, and tracer diffusion coefficients D, Dj, and D*, respectively, have been determined by Monte Carlo simulations on a square lattice for the case of nearest-neighbor attractive interactions. D was determined by the fluctuation autocorrelation method and by the Kubo–Green equation, using values of 〈(δN)2〉/〈N〉 determined from isotherms. 〈(δN)2〉/〈N〉 can only be determined above Tc and, for this situation, the two methods yield identical D values within the error of the simulations. Distinct high- and low-T regimes for D are observed, because of the changes in 〈(δN)2〉/〈N〉 with T, with much higher activation energies and prefactors for T/Tc<1.4 than for T/Tc>1.4. For D* and Dj much smaller effects are seen. The numerical values of activation energies suggest that diffusing atoms are either isolated atoms in the dilute lattice gas or those which become detached from corners of close-packed (1×1) islands below Tc, but have a higher nearest-neighbor complement in the dense lattice gas above Tc.
The electronic structure of NiH: The {Ni+3d 9 2D} supermultiplet
Authors: Gray Jeffrey A. , Li Mingguang , Nelis Thomas , Field Robert W.
Published: July 2002

A global deperturbation is presented for all electronic states of NiH with T0 values below 0.3 eV. These states form an isolated group and are treated as components of a molecular ‘‘supermultiplet’’ which is derived from a nickel-centered 3d9 electron configuration such as that found in the 2D term of Ni+. Observed term energies for all low-lying states, including some vibrationally excited levels, are used in a least squares fit to the supermultiplet model. A crucial feature of the supermultiplet model is its employment, wherever possible, of atomic angular (e.g., L±‖LΛ〉=[L(L+1)−Λ(Λ±1)]1/2‖LΛ±1〉) and radial (e.g., spin–orbit coupling constants) matrix elements to define and constrain the molecular effective Hamiltonian. A relatively small number of adjustable parameters are required to represent the v=0 and 1, J=0.5–11.5 term values in the supermultiplet picture and accurately describe a variety of observations, which include large Ω doublings, unusually large and J, Ω, e/f-dependent Zeeman g values, and a symmetry-forbidden (ΔΛ=2) rotational pertubation. The number of independently adjustable parameters required by the supermultiplet model is significantly smaller than a standard 2Δ, 2Π, 2Σ+ deperturbation model. In addition, the fitted deperturbed (i.e., nonrelativistic and nonrotating) molecular constants for the 2Δ, 2Π, 2Σ+ components of the NiH supermultiplet are in better agreement with theoretical descriptions than previous empirical constants taken directly, without deperturbation, from spectra. The fit model also yields an empirical value of the (3d9)σ∼3d10 configuration mixing coefficient, which is relevant to a global understanding of the d9 and d10 states in the homologous NiH, PdH, PtH series of molecules.
A dynamic simulation method suppressing uninteresting degrees of freedom. II. Mechanism of π flips in a lattice of benzene rings
Authors: Romiszowski Piotr , Yaris Robert
Published: July 2002

Our method [P. Romiszowski and R. Yaris, J. Chem. Phys. 94, 6751, (1991)] of simulating infrequent motions by using an equation of motion with the uninteresting degrees of freedom suppressed—the generalized Langevin equation—has been extended to enable us to obtain the mechanism for the gated transition. The model problem is a two-dimensional array of benzene rings interacting with nearest neighbor potentials and we are looking for the mechanism of the π-flip transition of the central benzene ring. Thus we retain only the angular coordinates of the central ring and its four nearest neighbors in the generalized Langevin equation. The mechanism obtained for the gating by the nearest neighbor rings is the same both qualitatively and quantitatively in the simulation with a reduced number of degrees of freedom and in the full simulation.
Experimental determination of the S1 torsional potential of 9,9′-bianthryl in 2-methyl-butane by simultaneous Franck–Condon and band shape analysis of temperature dependent optical fluorescence spectra
Authors: Wortmann Rüdiger , Elich Karsten , Lebus Sonja , Liptay Wolfgang
Published: July 2002

Optical fluorescence spectra of anthracene (AN) and of 9,9′-bianthryl (BA) have been measured in the apolar solvent 2-methyl-butane (2MB) at seven different temperatures in the range 123 K to 298 K. The AN spectra are strongly structured and consist of simple Franck–Condon (FC) progressions. The BA spectra on the other hand are broad and show unusual temperature dependencies. These effects are interpreted within a two state model with five vibrational and one torsional degree of freedom. A FC analysis is carried out using vibronic band shape functions which are convolutions of two functions. The first one is a Gaussian representing the spectral broadening due to librations and intermolecular interactions. The second one results from a semiclassical treatment of the intramolecular torsional mode (large amplitude motion, LAM). Optical transitions are assumed to occur from ensembles which are thermally equilibrated with respect to their nuclear degrees of freedom. The Boltzmann distribution with respect to the LAM potentials in combination with a change of the transition frequency as a function of the torsional angle φ is identified as the predominant broadening mechanism. The LAM potentials in the S0 and S1 state are represented by a three parameter cosine series. A nonlinear least-squares fit of the experimental spectra leads to a single consistent set of model parameters valid for all temperatures. The S1 double minimum LAM potential of BA in 2MB is found to have minima at φeq=62° (and 118°) and a local barrier at φ=90° of 255 cm−1.
Splay mode behavior in a nematic mixture of DDA9+PAA
Authors: Devanand K.
Published: July 2002

Quasielastic light scattering has been used to study the splay mode of p-azoxyanisole (PAA) and mixtures of a thermotropic main chain polymer liquid crystal, DDA9, with PAA in the nematic phase. The splay viscosity of the mixtures is substantially enhanced with respect to that of PAA. Of the two mixtures studied, the temperature dependence of the splay mode decay rate for the mixture with the higher polymer concentration shows behavior atypical of conventional low molecular weight liquid crystals.
Competitive adsorption of two species: Doublet closure approximation and simulation
Authors: Lemos M. C. , Córdoba A.
Published: July 2002

Starting from a master equation, competitive adsorption of two kinds of interacting particles on a linear chain is considered. The transition probabilities are chosen in the Arrhenius form, and the activation energy is split into two additive terms, corresponding, respectively, to the action of the substratum and to interactions between nearest neighbor adatoms. The kinetic equations are obtained by using a doublet closure approximation, writing triplet densities in terms of doublet and singlet densities. In this approximation, for the range of parameters being considered, only one stable steady state results, unlike in the case of the mean field approximation (where up to three stable steady states can exist). In view of the disagreement between the results of both approximations, a Monte Carlo simulation is carried out and results similar to those of doublet closure approximation are obtained. In neither of these two models interactions between nearest adatoms produce multistability. Thus, one may conclude that multistability resulting from the mean field model is spurious and caused by the approximations used. Therefore, the mean field approximation can be unsuitable for studying multistability in these kinds of problems.
A generalized Heitler–London theory of the chemical bond in H+2
Authors: Tang K. T. , Toennies J. Peter
Published: July 2002

The perturbation theory for the total energies of the 1sσg and 2pσu states of H+2 is reformulated in a manner which avoids problems resulting from symmetrization. First a new set of localized wave functions are defined and calculated from polarization perturbation theory. After symmetrization of these wave functions, a generalized Heitler–London energy, which can be expanded to all orders, is obtained. In second order the result agrees with the Murrell–Shaw Musher–Amos expression. An expansion of the expectation value of the energy is also carried out and provides similar results. Many symmetry-adapted perturbation results are obtained simply and directly from the present localized wave-function approach. The present theory suggests a novel interpretation of the terms contributing to the total potential energy and helps to rearrange them to give improved results.
Improved techniques for outgoing wave variational principle calculations of converged state-to-state transition probabilities for chemical reactions
Authors: Mielke Steven L. , Truhlar Donald G. , Schwenke David W.
Published: July 2002

We present improved techniques and well-optimized basis sets for application of the outgoing wave variational principle to calculate converged quantum mechanical reaction probabilities, and we illustrate them with calculations for the reactions D+H2→HD+H with total angular momentum J=3 and F+H2→HF+H with J=0 and 3. The optimization involves the choice of distortion potential, the grid for calculating half-integrated Green’s functions, the placement, width, and number of primitive distributed Gaussians, and the computationally most efficient partition between dynamically adapted and primitive basis functions. We present benchmark calculations with 224–1064 channels.
Observation of propagating collective excitations in liquid SO2
Authors: Bermejo F. J. , Martínez J. L. , Martín D. , García-Hernández M. , Mompeán F. J. , Alonso J.
Published: July 2002

The spectrum of collective excitations in liquid sulphur dioxide at two thermodynamic states has been measured by neutron triple-axis spectroscopy for momentum transfer ranges of 0.35≤Q≤2.0 Å−1. Well-resolved inelastic peaks indicating the presence of propagating modes are found for Q values below 0.5 Å−1 at T=266 K and below 0.7 Å−1 at T=210 K. A precise value for the hydrodynamic limit has been found in both cases and the measured ‘‘dispersion relations’’ are discussed in relation to predictions made by recent theoretical approaches.
Calculated properties of solid N2O at various temperatures and pressures
Authors: Kuchta Bogdan , Etters R. D.
Published: July 2002

Properties of solid N2O at pressures P≤15 GPa and at temperatures T=0 and 300 K have been calculated using energy optimization, Monte Carlo methods in an (N,P,T) ensemble with periodic, deformable boundary conditions, and lattice dynamics. A stable Pa3 structure is predicted at zero pressure and a phase transition into an orthorhombic Cmca phase is predicted at P=4.75 GPa, with a volume change ΔV≂0.25 cm3/mole. These results are in complete agreement with experiments conducted in conjunction with this work. Calculated libron frequencies are in agreement with recent Raman measurements. In addition, second virial coefficients, lattice parameters, internal energies, and pressure-volume relations have been calculated.
Model for microemulsions: An exactly solvable case
Authors: Renlie Lasse , Ho/ye Johan S. , Skaf Munir S. , Stell George
Published: July 2002

The microscopic model for microemulsions, introduced earlier by Ciach, Ho/ye, and Stell [J. Chem. Phys. 90, 1214 (1989)] is here specialized to a one-dimensional lattice and solved exactly by the transfer matrix method. The microemulsion phase is identified by the formation of thermally distributed surfactant-bounded domains of oil. For this phase we find scattering functions and characteristic lengths that have some of the same features found in experimental data for microemulsions. Mean-field interactions beyond nearest-neighbor sites are introduced in order to study the phase diagram for the nonperiodic phases we encounter.
Time and energy resolved processes in molecular dynamics
Authors: Jolicard Georges , Austin Elizabeth
Published: July 2002

The validity of the time-dependent Bloch’s wave operator theory compared to widely used time propagation algorithms such as the split operator method and the short iterative Lanczos propagation is tested on a collinear collision system, which exhibits energy transfer probabilities with pronounced structure and on the two color ir multiphoton dissociation of a diatom. Both energy- and time-resolved processes are investigated by analyzing the spectra of the outgoing flux in the excited and/or in the dissociative channels.
Thermophysical properties of gaseous refrigerants from speed-of-sound measurements. II. Results for 1,1-dichloro-1-fluoroethane (CCl2FCH3)
Authors: Goodwin A. R. H. , Moldover M. R.
Published: July 2002

The speed of sound in gaseous 1,1-dichloro-1-fluoroethane (CCl2FCH3, commonly known as R141b) has been measured between 260 and 315 K. Perfect-gas heat capacities and second acoustic virial coefficients have been calculated from the results. The second acoustic virial coefficients are used to estimate the density virial coefficients B(T) and an effective square-well potential. Our estimates of B(T) are consistent with B(T) deduced from high-quality p(Vm, T) results. We provided estimates of the vibrational relaxation time for R141b.
A transition-rate investigation by molecular dynamics with the Langevin/implicit-Euler scheme
Authors: Nyberg Anna M. , Schlick Tamar
Published: July 2002

We report results from molecular dynamics simulations for a bistable piecewise-harmonic potential. A new method for molecular dynamics—the Langevin/implicit-Euler scheme—is investigated here and compared to the common Verlet integration algorithm. The implicit scheme introduces new computational and physical features since it (1) does not restrict integration time step to a very small value, and (2) effectively damps vibrational modes ω≫ωc, where ωc is a chosen cutoff frequency. The main issue we explore in this study is how different choices of time steps and cutoff frequencies affect computed transition rates. The one-dimensional, double-well model offers a simple visual and computational opportunity for observing the two different damping forces introduced by the scheme—frictional and intrinsic—and for characterizing the dominating force at a given parameter combination. Another question we examine here is the choice of time step below which the Langevin/implicit-Euler scheme produces ‘‘correct’’ transition rates for a model potential whose energy distribution is ‘‘well-described’’ classically.
Molecular beams of polyenes: Retinals and β-carotene
Authors: Dey M. , Moritz F. , Atkinson G. H. , Grotemeyer J. , Schlag E.
Published: July 2002

Low temperature, molecular beams of polyenes (retinol, retinal, a Schiff base retinal, and β-carotene) are formed in jet expansions by CO2 laser-induced desorption from a deposited solid surface. Mass spectra of these intact polyenes are detected by resonantly-enhanced ionization mass spectrometry. Laser-desorption conditions which produce essentially no polyene fragmentation are obtained by controlling the focusing conditions of the CO2 laser on the deposited sample.
Accurate closed-form approximations to partition functions
Authors: Fernández F. M. , López Piñeiro A.
Published: July 2002

We develop a method for obtaining accurate closed-form approximations to partition functions for all values of the temperature. The method is based on an alternative application of the Euler–Maclaurin summation formula and is therefore restricted to those partition functions which can be treated by such an approach. As an illustrative example we discuss the rigid rotor and show analytic expressions for the partition functions obtained by summing over all states, even states, and odd states.
Multiplicity dependence of matrix-induced frequency shifts for atomic transitions of the group 12 metals in rare gas solids
Authors: Laursen Sandra L. , Cartland Harry E.
Published: July 2002

Atomic resonances of the group 12 metal atoms, Hg, Cd, and Zn, undergo frequency shifts from the gas phase atomic line when trapped in rare gas matrices of Ar, Kr, and Xe at 12 K. As expected, the shifts are approximately linear in polarizability of the rare gas, but the slope of this line depends on whether the transition in question is 1P1←1S0 or 3P1←1S0. Thus the matrix-induced frequency shift is dependent on the singlet or triplet nature of the excited state as well as on the matrix material. This dependence on multiplicity is discussed in terms of interactions between the excited-state atomic orbitals and the matrix. The results are compared to matrix studies of other metals and to related gas-phase work on diatomic van der Waals complexes of group 12 metals with rare gases.
Recursion relations for vibrational–rotational matrix elements involving two displaced Morse potentials
Authors: Moreno B. , López-Piñeiro A.
Published: July 2002

Using a technique based on the hypervirial theorem along with a second quantization formalism, recursion relations for matrix elements and Franck–Condon factors of Morse potentials are obtained. These expressions can be used to calculate recursively matrix elements of the operators qk=(r−re)k, xs=exp[−as(r−re)], dl/dr l, or any combination of them, corresponding to vibrational–rotational states belonging to two distinctly separated Morse oscillators of arbitrary De. The proposed equations give accurate numerical results even when the traditional methods (numerical integration, asymptotic expansions, semiclassical, etc.) present serious inconveniences. The results are valid for any value of the power of the operator and of the quantum numbers v and J of both oscillators.
The numerical solution of the linearized and exponential hypervertex approximations for electrolytes
Authors: Ho/ye Johan S. , Lomba Enrique
Published: July 2002

In this paper we present numerical solution of the linearized and exponential hypervertex approximations for the restricted primitive model for electrolytes. The method of solution is based on a Newton–Raphson scheme previously applied to simple fluids. Thermodynamic results computed from both approximations agree remarkably well with previously published Monte Carlo data even to a greater extent than well-known approximations such as the hypernetted-chain equation.
Product energy disposal in the nonadiabatic reaction S(1D)+CS2→S2 (X 3Σ−g)+CS (X 1Σ+)
Authors: Sapers Steven P. , Andraos Nancy , Donaldson D. J.
Published: July 2002

The product energy disposal in the reaction S(1D)+CS2→S2+CS is measured via laser-induced fluorescence. Molecular sulphur is produced exclusively in its electronic ground state (3Σ−g) with up to 3 quanta of vibrational excitation and rotational excitation that roughly approximates a 1000 K Boltzmann distribution. The CS produced from the reaction is formed predominantly in its vibrationless state. The total internal energy content of the product molecular sulphur is only about 12% of the total available energy of the reaction; the CS fragment also has minimal internal energy, implying that most of the reaction energy resides in product translation. These results are different from those observed in the isovalent S+OCS reaction and suggest that a different detailed mechanism is operative.
A theoretical investigation of the ground and low-lying excited states of butadiene radical cation
Authors: Cave Robert J. , Perrott Michael G.
Published: July 2002

Results are presented from ab initio calculations on the ground and several low-lying excited states of the butadiene radical cation. In particular, we have calculated geometries for the ground and several excited states at the multiconfiguration self-consistent field level and characterized the planar stationary points. The vertical ionization potentials from the neutral molecule ground state and vertical excitation energies at the computed equilibrium geometry of the cation were also examined. A variety of methods were tested for the calculation of ionization potentials and excitation energies, including multiconfiguration self-consistent field, multireference singles and doubles configuration interaction (with and without size-consistency correction), and multireference averaged coupled pair functional theory. It is found that several of the excited states are strong mixtures of so-called Koopmans’ and non-Koopmans’ configurations. In most cases, good agreement is found with experiment, but the excitation energy for the second excited π state, a state which is multiconfigurational at zeroth order, is apparently overestimated at all levels of theory employed here.
On the existence of exact conditions in the theory of electrical double layers
Authors: Attard Phil , Wei Dongqing , Patey G. N.
Published: July 2002

It has long been thought that the total potential drop V across an isolated electrical double layer must be a monotonically increasing function of the surface charge density σ (i.e., ∂V/∂σ≥0). This result has been ‘‘established’’ by thermodynamic arguments of Landau and Lifshitz [Electrodynamics of Continuous Media (Pergamon, Oxford, 1960)] and by a more recent statistical mechanical method of Blum et al. [J. Chem. Phys. 72, 1902 (1981)]. Here we describe statistical mechanical analyses for both constant and fluctuating charge models. It is shown that the derivation of Blum et al. is in error and that correct statistical mechanical treatments do not determine the sign of ∂V/∂σ. However, some rigorous bounds for related quantities are found. We also point out a mathematical problem in the method of Landau and Lifshitz which appears to invalidate their argument. We conclude that at present there is no rigorous proof that ∂V/∂σ must be positive and that the existence of negative values cannot be ruled out.
Composition fluctuations and phase diagrams in ternary micellar solutions
Authors: Teubner Max
Published: July 2002

The composition fluctuations and the fluctuation ellipses are calculated for ternary micellar solutions. This permits, in conjunction with earlier papers on the relation between composition fluctuations and phase diagrams, information on the width of the size distribution of the micelles to be extracted from the geometry of the phase diagram. The slope of the phase boundary above the cmc is shown to provide information on the composition of the micelles.
Dissociation energies of BaI and CaI from equilibrium studies
Authors: Hildenbrand D. L. , Lau K. H.
Published: July 2002

Effusion-beam mass spectrometry was used to study the gaseous dissociation equilibrium BaI=Ba+I and the isomolecular exchange reactions of BaI and CaI with ScI over wide temperature ranges, and thermochemical data were derived from second law analysis. The results yielded the dissociation energies D○0(BaI) =76.2±1.5 kcal mol−1 and D○0(CaI) =67.0±2 kcal mol−1. Both of these values are about 5 kcal mol−1 larger than previous results obtained in this laboratory from third law analysis of other reactions. Reasons for the disparities are discussed. Our new second result for BaI is in good agreement with a recent value obtained from crossed molecular beam studies.
Improved rate theories of chemical reactions
Authors: Morita Akio
Published: July 2002

A mathematical difficulty in Kramers’ theory on reaction rate is pointed out and his treatment is improved by considering the Brownian motion near the bottom of potential minimum explicitly. We used the Fokker–Planck–Kramers equation (FPK) and derived a modified Smoluchowski’s equation by eliminating the velocity dependence. Moreover, we assumed the stationary state and defined the reaction coefficient by the flux divided by the concentration of the reactant. Finally, we solved the modified Smoluchowski by using computer assisted symbolic manipulation programs and obtained the expression for the rate coefficient by a single theory. It is found that there exists a lower limit for the friction value which is corresponding to the critical damping motion for an ordinary harmonic damped oscillator. It is also shown that we can have Kramers’ turnover for the case of the steep decline of the potential after passing the maximum. The Boltzmann transport equation for the strong collision case with the collision term by Bhatnager, Gross, and Krook (BBGK) is also solved exactly by the symbolic manipulation to find the rate coefficient which does not lead to the Kramers turnover.
Vibrational analysis of the X2(3/2)–X1(1/2) infrared system of BiO
Authors: Linton C. , Bacis R. , Martin F. , Rosenwaks S. , Vergès J.
Published: July 2002

The vibrational analysis of the X2(3/2)–X1(1/2) system of BiO in the near infrared region, 6400–7700 cm−1, has been extended. Transitions in the Δv=−1, 0, and +1 sequences have been observed and assigned up to v=6 in the upper state and no transitions have been observed with v′>6. The Δv=−1 sequence is shown to form a head of heads. The upper state appears to be strongly perturbed, especially at v=6.
The NH stretch in pyrrole: A study of the fundamental (Δv=1) and third overtone (Δv=4) bands in the bulk gas and in a molecular beam
Authors: Douketis Constantine , Reilly James P.
Published: July 2002

We have examined the NH stretching vibration in pyrrole both in the infrared and visible regions of the spectrum. Three techniques were used—direct absorption spectroscopy, intracavity photoacoustic spectroscopy, and optothermal molecular beam spectroscopy. In the bulk gas, the Δv=1 transition is composed of a well-resolved manifold of subbands. Molecular beam spectra of a number of those subbands reveal detailed structure due to single state-to-state transitions. An asymmetric rotor model was found to provide an adequate description of the spectra and a complete set of rotational parameters has been determined. Little evidence of rotational perturbations was observed in the Δv=1 spectrum. The bulk gas Δv=4 overtone band is composed of three vibrational transitions, whose rotational contours overlap. These transitions involve Fermi resonance among assignable NH and CH stretching excited states. Attempts to detect Δv=4 overtone transitions on the molecular beam failed, in spite of the large anticipated signal-to-noise ratio calculated by extrapolation from the Δv=1 data. This negative result is a likely consequence of extensive level mixing in the Δv=4 region.