bluuebunny/crossref_metadata_2025 huggingface dataset:
The infrared photodissociation spectra and the internal mobility of SF6, SiF4, and SiH4 dimers
Authors: van Bladel J. W. I. , van der Avoird A.
Published: July 2002

We present an analysis of the couplings originating from different intermolecular interactions (electrostatic, exchange, dispersion, induction) which split and shift the frequencies of the vibrational transitions in van der Waals dimers, and determine their intensities. Model potential calculations illustrate the importance of the various contributions in (SF6)2, (SiF4)2, and (SiH4)2 and their dependence on the monomer orientations. The results, in conjunction with calculated equilibrium structures, barriers to internal rotation and (harmonic) van der Waals vibrational frequencies, lead to several observations which are relevant for the interpretation of the infrared photodissociation spectra of these complexes. We confirm that in (SF6)2 and (SiF4)2 (orientation independent) resonant dipole–dipole coupling dominates the appearance of the spectra. For (SiH4)2 we conclude, however, that other than electrostatic terms are not negligible and, moreover, that the electrostatic coupling leads to orientation-dependent vibrational frequencies and intensities. This orientational dependence is related to the large displacements of the hydrogen atoms in the ν4 mode of SiH4. We also find that the internal rotations in (SF6)2 and (SiF4)2 are more strongly locked than those in (SiH4)2. Especially the geared internal rotations in the latter dimer could easily occur at the experimental molecular beam temperatures.
O
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a transitions in reactive H++H2 collisions
Authors: Gerlich D.
Published: July 2002

An experimentally well-proven dynamically biased statistical theory is used to calculate state specific cross sections and product state distributions for the proton–hydrogen scrambling reaction. Conservation of parity and nuclear spin and the anisotropy of the interaction potential are explicitly taken into account. The influence of the permutation symmetry is explained with a simple statistical model. The results are used to calculate rate coefficients for state to state transitions, and for conversion of ortho-H2 into para-H2. At low energies (<43 meV), the decay of a H+⋅H2( j=1) complex into the exothermic H++H2( j=0) channel is strongly hindered, the probability is smaller than 1/12. Therefore, this j=1 to j=0 transition occurs with a rate coefficient of only 2.2×10−10 cm3 /s at temperatures of interstellar space. A van’t Hoff plot of the results shows that the ortho–para coupling via proton exchange leads to an equilibrium abundance ratio n(ortho)/n(para)=9.35⋅exp(−169.4 K/T). The concept of separate conservation of nuclear spin during the lifetime of the intermediate complex was tested experimentally in a high resolution molecular beam experiment. The partially resolved rovibrational distributions are in good agreement with populations calculated with the statistical model.
An a
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n
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t
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o investigation of the spectroscopic properties of ClF, ArF+, SF−, and ClO− in their ground electronic states
Authors: Peterson Kirk A. , Woods R. Claude
Published: July 2002

The potential energy and electric dipole moment functions of the 26-electron species ClF, ArF+, SF−, and ClO− in their X 1∑+ electronic states have been calculated by the coupled electron pair approximation (CEPA) and singles and doubles configuration interaction (CI-SD) using a basis set of 101 contracted Gaussian-type orbitals. Potential energy functions have also been calculated by Mo/ller–Plesset fourth-order perturbation theory with single, double, and quadruple excitations (MP4SDQ) for all four species. Values of re and ωe calculated for the ClF molecule agree with experiment to within 0.004 Å and 16 cm−1, respectively, at the CEPA and MP4SDQ levels of approximation, and also when a size consistency correction was applied to the CI-SD energies [designated CI-SD(s)]. Predictions of rotational and vibrational spectra have been made for the isoelectronic ions, ArF+, SF−, and ClO−. The MP4SDTQ (MP4SDQ with the addition of triple substitutions) level of approximation was used to calculate dissociation energies for all four species and electron affinities for the radical precursors of the negative ions. From the CEPA potential energy and electric dipole moment functions, dipole moment matrix elements and infrared radiative transition probabilities have been predicted. Molar IR integrated absorption intensities for the fundamental transitions have been predicted to be 3065 cm2 mol−1 (ClF), 15 500 cm2 mol−1 (SF−), 835 cm2 mol−1 (ClO−), and 22.3 cm2 mol−1 (ArF+).
The effect of polarization energy on the free energy perturbation calculations
Authors: Ramnarayan K. , Rao B. G. , Singh U. C.
Published: July 2002

A detailed implementation of the polarization energy and its derivatives into a molecular dynamics program is described. In order to examine the effect of the polarization energy on the calculated free energy differences, we have computed the free energy of solvation, with coordinate coupling, of normal alkanes, tetraalkylmethane, tetraalkylammonium ions and some closed shell ions in water. The pattern of the computed free energy change is compared with the results of our earlier simulations where the polarization energy was not included in the calculation. It is found that in the majority of the cases the polarization energy contribution to the free energy change is additive. The results of the simulations are also compared with the available experimental data.
The dynamics of binary mixtures of nonpolymeric viscoelastic liquids as studied by quasielastic light scattering
Authors: Gerharz B. , Meier G. , Fischer E. W.
Published: July 2002

In this paper we report quasielastic light-scattering experiments of the compatible binary mixture of simple viscoelastic liquids made up of o-terphenyl as one component and newly synthesized model materials, 1,1-di(paramethoxyphenyl)-cyclo-hexan and 1,1-di(paramethoxyphenyl, metamethyl)-cyclohexan, as the other components. The measurements were done in a wide temperature range above Tg (≂Tg+100 K) of the mixtures and in a volume fraction range up to φ=0, 5 of bis-cresol-cyclohexane-dimethylether and bis-phenol-cyclohexane-dimethylether. We have found that the measured dynamics is governed by three effects: At short times we observe the dynamics of the density fluctuations of the system characterized by a broad distribution of relaxation times, which can be scaled to a masterplot taking the Tg of the mixtures into account. At longer times we observe two translational diffusion processes whose decay is single exponential with a characteristic time proportional to q−2, q being the scattering vector. The faster one is attributed to the interdiffusion coefficient of the system. The second, slower mode is related to the motion of dynamic clusters which have their origin in a spatial inhomogeneity of the samples. The mean cluster diameter is about 1000 Å, thus in any case the occurrence of the slow mode is accompanied by a strong increase of the elastically scattered intensity towards small scattering angles. This mode also appears in pure components. We relate its intensity to the fluctuations of an order parameter, usually introduced to describe the glassy state.
Ground and excited states of CaCH3, CaNH2, CaOH, and CaF through electron propagator calculations
Authors: Ortiz J. V.
Published: July 2002

Ground and excited state energies of CaCH3, CaNH2, CaOH, and CaF are obtained from electron propagator calculations of electron affinities of the corresponding cations at the optimized geometries of the neutral ground states. Feynman–Dyson amplitudes (FDA’s) describe the distribution of the least bound electron in various states. The ground states differ from the cations by the occupation of a one-electron state dominated by Ca s functions. The next two FDA’s have π pseudosymmetry and the corresponding states have unpaired electrons in orbitals which display mixing between Ca p and d functions. Above these lies a σ pseudosymmetry FDA with nearly equal contributions from Ca d and p functions. Next one finds two FDA’s with δ pseudosymmetry. Higher excited states are also discussed.
Near-critical coexistence curve and critical exponent of an ionic fluid
Authors: Singh Rajiv R. , Pitzer Kenneth S.
Published: July 2002

The coexistence curve of an ionice liquid–liquid system has been obtained very close to the critical point by measuring the refractive indices of the two phases in equilibrium. The effective exponent βe remains close to the classical value of 0.5 to t=0.0005 or closer to Tc. The data are, however, not inconsistent with a limiting exponent of 0.326 provided the Wegner correction terms are very large. This behavior is qualitatively consistent with available theory and other experimental knowledge, but further theory and experiments are needed before quantitative comparisons can be made.
Collective excitations in a dense dipolar liquid: How important are dipolarons in the polarization relaxation of common dipolar liquids?
Authors: Chandra A. , Bagchi B.
Published: July 2002

It was recently proposed that collective polarization excitations, called dipolarons in analogy with the plasmons in the Coulomb systems, can exist for a long duration in a dense dipolar liquid. In this article, we present a microscopic analysis of the properties of such collective excitations, both at small and at intermediate wave vectors. The theory predicts the existence of dipolarons at values of the relevant parameters which are in good agreement with the computer simulation of Pollock and Alder. However, the predicted range of the parameter values in which the ‘‘true’’ dipolaronic behavior is significant are rather too small to be important in common dipolar liquids like water, acetonitrile or methyl iodide. We find that the microscopic structure, especially the local orientational correlations and also the translational modes of the liquid, play important roles in determining the nature and the lifetime of the dipolaronic modes. Especially, these modes are more likely to occur at intermediate wave numbers if the translational contribution to the polarization relaxation is significant. The consequences of these theoretical predictions in experimental studies are discussed.
Interdimensional degeneracy and symmetry breaking in D-dimensional H+2
Authors: Frantz D. D. , Herschbach D. R.
Published: July 2002

An interdimensional degeneracy linking the orbital angular momentum projection ‖m‖ and spatial dimension D gives D-dimensional eigenstates for H+2 by simple correspondence with suitably scaled D=3 excited states. The wave equation for fixed nuclei is separable in D-dimensional spheroidal coordinates, giving generalized two-center differential equations with parametric dependence on the internuclear distance R. By incorporating‖m‖ into D, the resulting eigenstates can be classified by the two dimension-independent ‘‘radial’’ quantum numbers denoted in united atom notation by k and l−‖m‖, corresponding, respectively, to the number of ellipsoidal and hyperboloidal nodal surfaces in the wave function. The two eigenparameters, the energy ED(R), and a separation constant AD(R) related to the total orbital angular momentum and the Runge–Lenz vector, have been determined numerically for the ground state and several low lying excited states for selected dimensions from D=2 to D=100.

The system simplifies greatly in the limit D→∞, where the electronic structure reduces to a classical electrostatic form with the electrons in a fixed geometrical configuration relative to the nuclei, akin to the traditional Lewis electron-dot structure. For a given R, the energy E∞ reduces to the minimum of an effective potential surface and the separation constant A∞ reduces to a simple function of the energy. The surfaces are separable in spheroidal coordinates resulting in analytical expressions for the energy in terms of the coordinates. The surfaces exhibit a characteristic symmetry breaking as functions of R, changing from a single minimum surface in the united atom limit (R→0) to a double minimum surface in the separated atom limit (R→∞). Effects of this symmetry breaking are found at finite D as well. Analysis of excited state D-dimensional energies reveals that bonding in H+2 is determined primarily by k, contrary to the standard scheme of bonding and antibonding molecular orbitals, which in the case of H+2 correspond to even and odd l−‖m‖, respectively. When the D-dimensional energies are examined as functions of 1/D, the resulting curves resemble typical perturbation diagrams with 1/D as the perturbation parameter.
Electron- and nuclear-spin relaxation in an integer spin system, t
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s-(acetylacetonato)Mn(iii) in solution
Authors: Bayburt Timothy , Sharp Robert R.
Published: July 2002

Expressions are derived for the intermolecular contribution to the nuclear-spin relaxation rate in solutions containing dissolved paramagnetic ions with spin S≥1. The calculation assumes that the electron-spin Hamiltonian is dominated by a large axial zero-field splitting, and it accounts for effects of Zeeman interactions to first order. The expressions are used to analyze proton-spin relaxation of the acetone solvent in solutions of tris-(acetylacetonato)Mn(iii)/ acetone. The main objective was to measure electron-spin relaxation times of Mn(iii), which in this complex is a high-spin, d4 ion with integer spin S=2. Spin-lattice relaxation measurements were conducted over a range of magnetic field strengths (0.28–1.1 T) where the zero-field splitting is large compared to the Zeeman energy. Electron-spin relaxation times of Mn(iii) were found to be 8±2 ps, with little dependence on temperature over the range 215–303 K and on magnetic field strength up to 1.1 T. Use of the assumption that Zeeman splittings dominate zero-field splittings (Solomon–Bloembergen–Morgan theory) resulted in computed electron-spin relaxation times that are too short by a factor of 3–4.
PF3, PF2, and PF on Ni(111): Theoretical aspects of their chemisorption
Authors: Chan A. W. Edith , Hoffmann Roald
Published: July 2002

The chemisorption of PF3, PF2, and PF on Ni(111) is examined, using tight binding extended Hückel calculations on a three-layer model slab. We have studied various adsorption sites. Calculations indicate that PF3 should be preferentially bound to the onefold on-top Ni sites, with P–F bonds azimuthally oriented over neighboring Ni atoms. PF2 is indicated to bind in a ‘‘perpendicular’’ manner at a twofold bridging site or possibly at a threefold site; and PF to the threefold hollow sites. These calculations are in agreement with experimental findings. PF3 remains a σ donor and a π acceptor (through backbonding) on the surface; in the bonding of PF2 and PF the π interaction is more important.
Nonequilibrium computer simulation of a salt solution
Authors: Zhu S.-B. , Lee J. , Zhu J.-B , Robinson G. W.
Published: July 2002

A nonequilibrium computer simulation is performed to investigate the relaxation of a realistic polar solvent near a rapidly dissociating ion pair. The time evolution of the reaction coordinate, the ultrashort time scale changes in solvation energy and solvent forces, the local density response, the heating of certain librational degrees of freedom, and the time-dependent polarization are studied during the first 125 fs of the reaction. It is found that the relaxation behaviors in the anionic and cationic shells are very different. On average, the solvation process under study takes about 30-40 fs to break the original cage. After another 50 fs, the solvated ion pair reforms a new metastable structure, which feeds energy back into the reacting system to break the cage further. This procedure is apparently repeated many times until dissociation is complete. The results obtained in this work provide a graphic picture of some of the features of ultrashort dynamics of ionic photodissociation reactions in a polar medium.
A comparison of models for depolarized light scattering in supercritical CO2
Authors: Geiger Lynn C. , Ladanyi Branka M. , Chapin Mark E.
Published: July 2002

Depolarized Rayleigh scattering (DRS) in supercritical CO2 is studied using molecular dynamics computer simulation. Results are presented for three thermodynamic states at approximately 313 K and at the following densities; ρ/ρc=1.38, 1.95, and 2.35, where ρc=0.468 g/cm3 is the critical density. We study the effects of intermolecular potential and interaction-induced polarizability models on the DRS spectra. Results for three different atom–atom intermolecular potentials are compared: Two potentials with Lennard-Jones and Coulomb terms, one developed by Murthy, Singer, and McDonald (MSM) and the other by Steele and Posch, as well as a third potential with MSM Lennard-Jones parameters, but without Coulombic interactions. The interaction-induced polarizability is calculated using center–center and site–site dipole–induced dipole (DID) interaction models. First order perturbation theory results are obtained for both models. The exact solution of the center–center DID model is also obtained. We find that both polarizability delocalization and higher-order DID interactions have a significant impact on the DRS intensity, but a more modest one on the related time-dependent properties. The results for the three intermolecular potentials differ significantly, with those for the MSM potential being in closest agreement with the experimental data. The collision-induced contribution to the DRS spectrum is found to decrease with increasing density and to exhibit cancellation effects due to contributions from 3 and 4 molecule correlations.
The ligand field spectrum of the hexafluorochromate (III) anion: An a
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o study including correlation effects
Authors: Pierloot K. , Vanquickenborne L. G.
Published: July 2002

The ground and ligand field excited states of CrF3−6 have been studied, using different Gaussian basis sets of atomic natural orbitals. Each state was first optimized separately in a complete active space self-consistent field (CASSCF) calculation, including three active electrons in the 2t2g and 4eg shells. Further correlation was then added by using either a singles and doubles configuration interaction approach (SDCI) or by the average coupled pair functional method (ACPF) with the CASSCF configuration space as the reference space. Thereby the number of correlated electrons was raised up to 15. It is shown that the quartet–quartet transitions, corresponding to a 2t2g→4eg excitation, are described already very accurately at the CASSCF level. Further improvement of the 4A2g→4T2g transition was obtained by extending the CI treatment so as to include the F 2p electrons from the 1t2g,3eg, and finally also from the 6a1g shell. For the intraconfigurational t32g quartet–doublet transitions on the other hand, the best results were obtained by an 11 electron CI treatment, including the Cr(III) 3s and 3p electrons.
Reply to the comment on: ‘‘Electron bombardment fragmentation of size selected NH3 clusters’’
Authors: Buck Udo , Krohne Reinhard , Linnartz Harold
Published: July 2002

The experiments reported in Ref.1 were repeated with size−selected clusters of ammonia. It was concluded from the results of these experiments that the missing ion intensity maxima in ammonia cluster spectra is a cosequence of the cluster size and not, as was stated in Ref.3, of the beam conditions.(AIP)
Threshold and cage effect for dissociation of H2O and D2O in Ar and Kr matrices
Authors: Schriever R. , Chergui M. , Ünal Ö. , Schwentner N. , Stepanenko V.
Published: July 2002

A threshold near 6.8 eV is observed for the dissociation of H2O (and D2O) via the repulsive Ã(1B1) potential in rare gas matrices. The threshold varies within 0.2 eV for exchange of Kr with Ar matrix, upon deuteration and with temperature. The observations are consistent with a prompt dissociation across matrix induced barriers of about 1.8 eV caused by the repulsive hydrogen–rare gas potentials. The increase of dissociation efficiency above threshold and its variation with matrix and isotope substitution seen in the experiments is rationalized by the mass dependence of energy exchange between fragment and matrix and the dependence of the accessible exit cone on kinetic energy. A strong increase in the dissociation efficiency with temperature is observed which is different for Ar and Kr.
A Monte Carlo estimation of surface diffusion by simulating laser-induced thermal desorption
Authors: Ray Lawrence A. , Baetzold Roger C.
Published: July 2002

A current method of experimentally estimating surface diffusion is laser-induced thermal desorption (LITD). We consider the behavior of adsorbed species such as hydrogen on a (111) face-centered-cubic surface of platinum or rhodium. The diffusion coefficients for a variety of systems and surface coverages are estimated by the simulation of a laser-induced thermal desorption experiment. Novel Monte Carlo methods are used that eliminate the time conversion difficulties that arise when using a standard Metropolis algorithm. In particular, the process of adsorbate hopping is determined strictly on the basis of local configurations of adsorbed particles. The results from the simulation are favorably compared to experimental data and provide estimates of the diffusion parameters for the studied systems.
Laboratory observation of hot bands of H+3
Authors: Bawendi M. G. , Rehfuss B. D. , Oka T.
Published: July 2002

The (2ν2,l=2←ν2), (2ν2,l=0←ν2), and (ν1+ν2←ν1) hot bands of H+3 were observed. The vibrationally hot ions were produced in a liquid nitrogen cooled 6 kHz ac discharge using gas mixtures of H2 and He. The spectra were detected in direct absorption using a newly extended tunable difference frequency spectrometer using both LiNbO3 and LiIO3 crystals as nonlinear optical elements. The range of this spectrometer is now ∼5300–∼1900 cm−1. The positions of the rovibrational transitions compare extremely well with the theoretical predictions of Miller and Tennyson. A vibrational temperature study of the discharge indicates a significant population inversion between the ν1 and ν2 levels.
A movable basis method to calculate vibrational energies of molecules
Authors: Bowman Joel M. , Gazdy Bela
Published: July 2002

A new basis-set representation of the vibration/rotation eigenfunctions of triatomic molecules in mass-scaled Jacobi coordinates is presented. The basis is a nondirect product, consisting of radial basis functions in which the centers and ‘‘shapes’’ are functions of the angular variable. The functional dependence of these parameters is arbitrary, thus giving the method the ability to move the radial basis anywhere in the angular space. This results in a basis with the potential to describe considerable coordinate–coordinate correlation. The advantage of this is noted in the context of a new formulation of self-consistent field theory, in which a single product function of the above type is variationally optimized. A simple version of the theory, in which only one basis is movable, is applied to two model potentials representing isomerization. The convergence properties are shown to be dramatically better than those using a conventional direct-product basis, especially for delocalized states, and for the model potential with large curvature.
Semiclassical model for analysis of dissociative electron transfer reactions involving Rydberg atoms
Authors: Ling X. , Durham M. A. , Kalamarides A. , Marawar R. W. , Lindsay B. G. , Smith K. A. , Dunning F. B.
Published: July 2002

Collisions between atoms in high Rydberg states and molecules that dissociatively attach free low-energy electrons can lead to ionization through capture of the excited electron by the target molecule. A Monte Carlo code is described that models the detailed kinematics of such dissociative electron transfer reactions. The model takes into account the velocity distributions of the reactants, the lifetime and decay energetics of the transient intermediate negative ion, and the electrostatic interaction between the product positive and negative ions. Data for CF3I are presented that illustrate how detailed comparisons between model predictions and experimental data can lead to a better understanding of the dynamics of dissociative electron attachment reactions. In particular, such comparisons can provide estimates of the lifetime of the intermediate negative ions and show how the excess energy of reaction is partitioned between translational and internal degrees of freedom in the dissociation process.
Quantum chemical and vibronic analysis of the 1 2B2↔1 2A2, 2 2B2 transition in benzyl-h7 and benzyl-d7 radicals
Authors: Negri Fabrizia , Orlandi Giorgio , Zerbetto Francesco , Zgierski Marek Z.
Published: July 2002

The QCFF/PI+CI method and the CNDO/S+CI method in the floating atomic orbital basis set are used to study vibronic structure of the 1 2B2↔1 2A2, 2 2B2 transition in benzyl-h7 and benzyl-d7 radicals, arising from the activity of a1 and b1 in-plane modes. It is shown that the 6a10 line has its intensity almost entirely due to the borrowing from the 1 2A2→1 2A2 transition via the 1 2A2–1 2A2 coupling. We demonstrate tha the 1 2A2 and 2 2B2 states are strongly mixed by ν8b and ν6b modes. The unperturbed 1 2A2 state lies ∼850 cm−1 below the unperturbed 2 2B2 state. The coupling depresses strongly the position of the 6b10 level mixing it markedly with the level arising from C–CH2 rock (ν18b in other assignments) fundamental. Three-mode vibronic analysis of the absorption and emission spectrum produces vibronic structure that agrees well with the observed spectra. The adiabatic frequency of the ν14 (b1) mode in the 1 2B2 state is shown to be depressed markedly by vibronically induced mode mixing with ν8b mode due to the 1 2B2–n 2A2 interactions.
Effect of pressure and temperature on the Raman spectra of solid N2O
Authors: Olijnyk H. , Däufer H. , Rubly M. , Jodl H.-J. , Hochheimer H. D.
Published: July 2002

Raman studies as a function of pressure (P<40 GPa) and temperature (T=20–300 K) have been made of the external and internal modes of solid N2O The occurrence of orientational disorder manifests itself in the line shape of the lattice and intramolecular modes. A new high pressure phase N2O–II was found above 5.5 GPa. The pressure dependence of external and internal modes is determined. The temperature shift of the external modes is found to decrease drastically with increasing pressure. The available data from the experiments are used to obtain some information about the contributions of the explicit and implicit terms to the total temperature shift. Analysis of the Fermi resonance shows that pressure strongly weakens Fermi resonance which is not influenced by temperature.
Investigation of the F+H2 transition state region via photoelectron spectroscopy of the FH−2 anion
Authors: Weaver A. , Metz R. B. , Bradforth S. E. , Neumark D. M.
Published: July 2002

The photoelectron spectrum of the FH−2 anion is reported. The spectrum provides a probe of the transition state region for the F+H2 reaction. The experimental spectrum is compared to the recent simulation by Zhang and Miller which assumes the T5a potential energy surface for the F+H2 reaction. The experimental spectrum is substantially broader. While this may be due to inaccuracies in the T5a surface, the possibility of additional transitions to low-lying excited electronic surfaces not included in the simulation must also be considered.
Inversion potentials in the ground and excited electronic states of 9,10-dihydroanthracene as probed by the absorption and fluorescence spectra of jet-cooled molecules
Authors: Shin Young-Dong , Saigusa Hiroyuki , Zgierski Marek Z. , Zerbetto Francesco , Lim Edward C.
Published: July 2002

The inversion potentials in the ground and the lowest excited singlet states of 9,10-dihydroanthracene were deduced from the fit of the absorption and fluorescence spectra of jet-cooled molecules to model potentials. The barrier height in the ground state (615 cm−1 ) is substantially larger than in the excited state (80 cm−1 ). Correspondingly, the folding angle in the ground state is smaller (144.6° vs 164.1°). Comparison of the calculated line intensities with the experimental absorption spectrum indicates that the transition dipole moment is dependent on the nuclear coordinate of the inversion mode. The measured quantum yield of the single vibronic level fluorescence is essentially independent of the quantum number and the symmetry of the inversion level excited.
An ab initio study of Fe(CO)n, n=1,5, and Cr(CO)6
Authors: Barnes Leslie A. , Rosi Marzio , Bauschlicher Charles W.
Published: July 2002

Ab initio calculations have been performed for Cr(CO)6 and Fe(CO)n, n=1,5. Basis sets of better than double zeta quality are used and correlation is included using the modified coupled-pair functional method. The computed geometries and force constants are in reasonable agreement with experiment. The sequential bond dissociation energies of CO from Fe(CO)5 are estimated to be: 39, 31, 25, 22, and ≳5 kcal/mol. We note that the first bond dissociation energy is relative to the singlet ground state of Fe(CO)5 and the lowest singlet state of Fe(CO)4, whereas the second is relative to the ground triplet states of Fe(CO)4 and Fe(CO)3. In addition, the binding energy for Fe–CO would be modified to 18 kcal/mol if dissociation occurred to the Fe(5F) excited state asymptote. The CO binding energies for Fe and Cr are found to be in poorer agreement with experiment than those found in a previous study on Ni(CO)4. The origins of this difference are discussed.
Difference-frequency laser spectroscopy of gas phase C2D in the 2800 cm−1 region
Authors: Yan Wen-Bin , Warner H. E. , Amano T.
Published: July 2002

The gas phase C2D absorption spectrum has been studied in the 2800 cm−1 region using a high resolution difference-frequency laser spectrometer. The C2D molecules were produced in a hollow cathode discharge in a mixture of C2H2 or C2D2 (∼20 mTorr) and D2 (∼400 mTorr). Two bands were identified and rotationally analyzed. One band at 2796 cm−1 was found to be a 2Π–2Σ+ transition with the lower state being the ground state. The upper level of this band is assigned to the (110) state in the ground electronic state, X̃ 2Σ+. The other band at 2851 cm−1 is a 2Σ+–2Σ+ transition from the (001) state in X̃ 2Σ+ to a vibronic 2Σ+ state which was found to be perturbed. Accurate molecular constants for the upper levels involved were determined from least-squares fit. These constants as well as the observed transition frequencies are reported.
Dissociation dynamics of C3O2 excited at 157.6 nm
Authors: Strauss C. E. M. , Kable S. H. , Chawla G. K. , Houston P. L. , Burak I. R.
Published: July 2002

The dissociation of carbon suboxide by single photon absorption at 157.6 nm has been studied under the collisionless environment of a molecular beam. The primary products are 2CO+C [3P(97%) or 1D(3%) ]. The spin–orbit levels of the 3P carbon are statistically distributed. The CO rotational populations in the first four vibrational levels are found to be well described by Boltzmann distributions with temperatures 3430, 4120, 4670, and 2340 K for υ=0,1,2,3, respectively. A second low temperature component in the υ=0 rotational distribution is attributed to CO produced in coincidence with C(1D). Significant population is found in the first four vibrational levels with less than 3% estimated in the higher levels; a vibrational temperature of 3700 K fits the distribution. Analysis of the Doppler profiles of the CO and carbon suggest that the dissociation is stepwise; the first dissociation appears to be described by an anisotropy parameter near β=2, while the second appears to be isotropic. The mean CO fragment speeds were nearly constant for all rotational levels, though slightly faster for υ=1 than υ=0. From the translational energetics of the CO at least a small amount of stable C2O is inferred to exist. The overall energetics place the stable C2O quantum yield under 2% assuming that excited C2O is not radiatively stabilized. We were unable to detect C2O directly in any electronic state. The dissociation of C3O2 into C(3P)+2CO appears to be best described as a stepwise reaction that produces a nearly statistical partitioning into all fragment degrees of freedom. The best agreement is obtained for an intermediate C2O electronic state in the vicinity of the b̃ state (e.g., b̃, ã or Ã); a ground state C2O intermediate is unlikely. The singlet to triplet crossing most likely occurs in the C2O system on a time scale longer than a rotation (a few picoseconds).
Theoretical studies of DNA during orthogonal field alternating gel electrophoresis
Authors: Madden Thomas L. , Deutsch Joshua M.
Published: July 2002

We perform numerical simulations on a model of DNA molecules during gel electrophoresis in which the effect of periodically changing the applied field by 90° is examined. A random gel is used in this simulation. We find a scaling relationship between mobility and the ratio of the chain length to pulse time for 100 and 200 link chains. Also presented are results for the orientation, with respect to the new field direction, of chains being subjected to periodically varying orthogonal fields. We find no overshoot in this orientation. Results supporting the existence of persistence in the orientation of a chain over several field changes is presented for 400 link chains.
All electron versus pseudopotentials in a
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o chemisorption cluster model calculations
Authors: Illas Francesc , Bagus Paul S.
Published: July 2002

The interaction of atomic oxygen with a Cu5 cluster model simulating chemisorption on the fourfold site is studied at the ab initio SCF level. Three different levels of approximation were tested. In the first one all the electrons were explicitly included while in the other two only the 3d104s1 or the 4s1 electrons were explicitly included, the corresponding Cu cores were treated by means of a nonempirical pseudopotential. We have obtained Cu5O wave functions where only the Cu 4sp or the 4sp plus the 3d electrons are varried and deeper core electrons are frozen as well as wave functions where all the Cu electrons are varied. This allows us to separately examine the effect of the pseudopotentials on the Cu 4sp and Cu 3d contributions to the Cu–O bond. It is found that the pseudopotential errors for these individual contributions are about 0.25–0.50 eV. These uncertainties are sufficiently large so that they limit the use of pseudopotentials to providing qualitative rather than quantitative information about the bond. Finally, a proposal is made for the proper way to obtain the Cu–O binding energy with SCF wave functions for clusters.
Molecular dynamics simulation of nanoscale thermal conduction and vibrational cooling in a crystalline naphthalene cluster
Authors: Kim Hackjin , Dlott Dana D.
Published: July 2002

A molecular dynamics simulation of crystalline naphthalene is used to study nanometer scale thermal transport in solids. One molecule in a cluster of 75 is heated to a large initial temperature and then allowed to cool. Stochastic boundary conditions which preserve the time averaged volume of the cluster are used. The excess translational and librational energy of the hot molecule is lost within 1 ps. The excess vibrational energy is lost on the 100 ps time scale. Translational and librational energy propagates rapidly throughout the cluster at velocities which are comparable to the speed of sound. Despite the far slower rate of vibrational energy loss from the hot molecule, the growth of vibrational energy occurs uniformly on the other molecules in the cluster. Therefore intermolecular vibrational energy transfer occurs primarily via an indirect mechanism. Vibrational excitations are first converted into translational and librational excitations, which propagate throughout the cluster and then excite vibrations on distant molecules via multiphonon up pumping. Examination of the molecular neighbors shows that intermolecular transfer of mechanical energy can be anisotropic, since the hot molecule can only transfer energy where it contacts atoms on adjacent molecules. Energy transfer along the b- and c-crystallographic axes is more efficient than along the a axis. The most efficient energy transfer is in the direction of two of the four nearest neighbors. Transient hot spots are produced on these neighboring molecules. The implications of this anisotropic conduction for the propagation of thermal reactions, e.g., the decomposition of high explosives, are discussed briefly.
Composition dependent χ and microphase transitions of diblock copolymers
Authors: Tang Hai , Freed Karl F.
Published: July 2002

The known composition dependence of the effective χ interaction parameter is shown to produce several qualitatively different phase diagrams for microphase separation in diblock copolymers. This alone suffices to explain observations of nonuniversal phase behavior in these systems.
Capture cross sections on adiabatic vibronic potential curves—The reaction of Ar+(2P
J)+H2
Authors: Gislason Eric A. , Parlant Gérard
Published: July 2002

Capture cross sections and rate constants have been calculated for collisions of state-selected Ar+(2PJ) ions with H2 (v=0). The calculations were done using the orientation-averaged adiabatic vibronic (AV) potential energy curves for this system. After correcting for the fact that half of all Ar+(2P3/2)+H2 collisions are completely unreactive, the capture cross sections are assumed to equal the reactive cross sections for the process Ar+(2PJ)+H2(v=0)→ArH++H. The calculations agree well with the experimental data for this system at collision energies below 0.5 eV. In particular, the increase in the rate constant with collision energy is shown to be due to the strongly attractive nature of the potential curves at small distances.
Infrared diode laser study of the 248 nm photodissociation of CH3I
Authors: Suzuki Toshinori , Kanamori Hideto , Hirota Eizi
Published: July 2002

The vibrational distribution of CH3 radicals produced by the 248 nm photodissociation of CH3I has been investigated by infrared diode laser kinetic spectroscopy. The ν2 bands of CH3, v2=1←0 up to 4←3, were observed as functions of time for the sample pressure ranging from 9 to 100 mTorr. The observed Doppler line shape showed a clear correlation between the transition dipole moment and the recoil velocity (μ−v) in the photodissociation, and the observed linewidth indicated that CH3 fragments were predominantly generated through the I(2P1/2) adiabatic dissociation channel. The relative population in each vibrational level was determined to be 0.66:0.26:0.08:0.004 for v=0, 1, 2, and 3, in sharp contrast to the inverted vibrational distribution reported previously. The present result suggests that the geometry of the CH3 moiety in the precursor CH3I is relaxed gradually to its equilibrium planar structure before the cleavage of C–I bond takes place.
Subpicosecond relaxation of solvent perturbations of nonpolar electronic states measured by transient hole burning
Authors: Yu Jongwan , Kang Tai Jong , Berg Mark
Published: July 2002

The results of transient hole burning of the nonpolar solute dimethyl-s-tetrazine (DMST) in glycerol, glycerol triacetate, sorbitol hexaacetate, squalane, and hexane solutions are reported. Despite the high viscosity and diversity of chemical types of the solvents, the solvent-induced perturbations of the electronic states of DMST relax in less than 1 ps in all cases. This contrasts strongly with the relaxation of polar electronic states, which are typically slow in viscous solvents. A new type of solvent-interaction mechanism appears to be important. We suggest that short-range repulsions modulated by phonon-like solvent modes are responsible.
Hyperpolarizabilities of molecules with frequency dependence and electron correlation
Authors: Sekino Hideo , Bartlett Rodney J.
Published: July 2002

Frequency dependent second hyperpolarizabilities of N2 and the prototype organic molecule trans-butadiene are reported using generalized time dependent Hartree–Fock (TDHF) theory for several frequencies of applied fields. A monotonic increase of the values (positive dispersion) is observed for every nonlinear optical process in a range of frequencies for the applied field. Correlation effects are estimated using a second-order many body perturbation theory and coupled cluster singles and doubles relaxed density method for the analytical determination of the induced dipole moment. Such hybrid results for DC-induced second harmonic generation provide reasonable values in comparison with experiment for N2. However, dispersion and correlation effects in trans-butadiene are both found to be large and could be nonadditive.
Reaction dynamics on barrierless reaction surfaces: A model for isoergic gas-phase proton-transfer reactions
Authors: Lim Kieran F. , Brauman John I.
Published: July 2002

The kinetics of some gas-phase alcohol/alkoxide proton-transfer reactions is slower than predicted by simple Rice–Ramsberger–Kassel–Marcus (RRKM) rate theory modeling on the near-barrierless reaction surfaces. Reaction dynamics can be investigated in isolation from nonequilibrium and/or thermodynamic considerations through the study of a generic isoergic ion–molecule system X−+X−H. Monte Carlo quasiclassical trajectory simulations on barrierless reaction surfaces show that the slow experimental kinetics is consistent with both (i) locking of the external rotations of the reactants and (ii) passage over the (orbital angular momentum) centrifugal barrier being the rate-determining steps in bimolecular association, rather than only the latter process. In addition, there may be non-RRKM product selectivity.
Kinetics of intramolecular carbon atom exchange in ketene
Authors: Lovejoy Edward R. , Kim Sang Kyu , Alvarez Ramón A. , Moore C. Bradley
Published: July 2002

Intramolecular carbon atom exchange in highly vibrationally excited ketene was studied by monitoring the carbon monoxide fragments (12CO and 13CO ) from the photodissociation of 12CH213CO and 13CH212CO. Two experimental techniques were employed. In one set of experiments the IR transient absorptions of 12CO and 13CO were measured following pulsed excimer excitation of ketene 13C isotopomers, giving carbon atom exchange yields at 351 and 308 nm in a low pressure gas cell. In the other set of experiments, jet-cooled ketene 13C isotopomers were excited with tunable near-UV radiation, and the CO products were detected by monitoring their VUV laser-induced fluorescence. Carbon atom exchange yields were measured for energies extending from below the triplet decomposition threshold (CH2CO→CH2(X 3B1)+CO(X 1Σ+)) to about 4000 cm−1 above the singlet threshold (CH2CO→CH2(a 1A1)+CO(X 1Σ+)).

The exchange yields range from 4 to 19%, and the energy dependence of the yield exhibits pronounced structure, with maxima at the triplet and singlet decomposition thresholds. Kinetic measurements of the appearance of the CO products were also performed. The time constant for the appearance of the exchanged CO (e.g., 13CO from 13CH212CO ) is significantly longer than that for the direct CO fragment (e.g., 12CO from 13CH212CO ). All the experimental observations are consistent with a simple reaction mechanism involving ketene isomerization, 13CH212CO⇄12CH213CO, and dissociation, 13CH212CO→13CH2+12CO and 12CH213CO→12CH2+13CO. The isomerization rate constant was determined by analyzing the CO kinetics and the carbon atom exchange yields in terms of the simple isomerization mechanism. A fit of the energy dependence of the isomerization rate constant to the results of tunneling-corrected Rice–Ramsberger–Kassel–Marcus (RRKM) calculations gave the threshold (28360±60 cm−1 ) for the isomerization process.
The generalized symmetry adapted cluster theory
Authors: Hirao K.
Published: July 2002

The generalized version of the symmetry adapted cluster theory is developed for general classes of nonclosed-shell cases. We started with the restricted Hartree-Fock complete active sapce self-consistent-field wave function but redefined a reference function which includes the state-specific correlation effects. The transferable dynamical correlation effects are expressed through an exponential operator and a linear operator is used to represent the nontransferable state-specific correlation effects such as quasidegeneracies, internal and semi-internal correlation effects, the spin-polarization effect, etc. The present theory is exact and does not involve any noncommutative algebra. There is a very close parallel between the standard single-reference cluster theory and the present multireference version. We have also discussed some aspects of practical applications of the generalized symmetry adapted cluster theory.
Elucidation of electronic structure by the analysis of hyperfine interactions: The MnH A 7Π–X 7Σ+ (0,0) band
Authors: Varberg Thomas D. , Field Robert W. , Merer Anthony J.
Published: July 2002

We present a complete analysis of the hyperfine structure of the MnH A 7Π–X 7Σ+ (0,0) band near 5680 Å, studied with sub-Doppler resolution by intermodulated fluorescence spectroscopy. Magnetic hyperfine interactions involving both the 55Mn (I=5/2) and 1H (I=1/2) nuclear spins are observed as well as 55Mn electric quadrupole effects. The manganese Fermi contact interaction in the X 7Σ+ state is the dominant contributor to the observed hyperfine splittings; the ΔF=0, ΔN=0, ΔJ=±1 matrix elements of this interaction mix the electron spin components of the ground state quite strongly at low N, destroying the ‘‘goodness’’ of J as a quantum number and inducing rotationally forbidden, ΔJ=±2 and ±3 transitions. The hyperfine splittings of over 50 rotational transitions covering all 7 spin components of both states were analyzed and fitted by least squares, allowing the accurate determination of 14 different hyperfine parameters. Using single electronic configurations to describe the A 7Π and X 7Σ+ states and Herman–Skillman atomic radial wave functions to represent the molecular orbitals, we calculated a priori values for the 55Mn and 1H hyperfine parameters which agree closely with experiment. We show that the five high-spin coupled Mn 3d electrons do not contribute to the manganese hyperfine structure but are responsible for the observed proton magnetic dipolar couplings. Furthermore, the results suggest that the Mn 3d electrons are not significantly involved in bonding and demonstrate that the molecular hyperfine interactions may be quantitatively understood using simple physical interpretations.
Absence of photoelectron spectroscopic evidence for ‘‘proton tunneling’’ in the cation ground state of jet-cooled tropolone
Authors: Ozeki H. , Takahashi M. , Okuyama K. , Kimura K.
Published: July 2002

From our high resolution REMPI threshold photoelectron measurements of jet-cooled tropolone, it has been found that no spectral doublet showing ‘‘proton tunneling’’ in the cation ground state (D0) is observed within an experimental accuracy of 2 cm−1. The adiabatic ionization potential was obtained as Ia=68 365±5 cm−1.
Correlation field, structure, and phase transition in the monolayer CO adsorbed on NaCl(100) as revealed from polarization Fourier-transform infrared spectroscopy
Authors: Heidberg J. , Kampshoff E. , Suhren M.
Published: July 2002

The polarized infrared spectra of the monolayer of natural and isotopic CO adsorbed on NaCl(100) were measured and calculated. Above 35 K the monolayer forms a p(1×1) structure with one CO molecule, aligned normal to the surface, per unit cell. Below 35 K a phase transition occurs. The CO molecules tilt, a correlation field splitting appears and a p(2×1) structure is formed with two equivalent molecules per unit cell. These are related by glide reflection.