bluuebunny/crossref_metadata_2025 huggingface dataset:
Studies in molecular dynamics. XVIII. The square-well phase diagram
Authors: Young David A. , Alder Berni J.
Published: February 2003

Accurate molecular dynamics calculations have been made for the thermodynamic properties of the square-well potential in the solid phase. Hard-sphere perturbation terms for fcc and hcp phases have been computed over the solid range, and a phase diagram including solid–solid and solid–liquid phase boundaries has been constructed. The discontinuous square-well potential gives rise to an fcc–fcc phase transition which ends in a critical point. Both fcc and hcp phases have regions of stability, but calculations show that bcc is unstable.
Phase relations of ice under pressure
Authors: Mishima O. , Endo S.
Published: February 2003

In order to clarify the phase relations of ice at low temperatures, the transitions between various ice polymorphs were observed with a manganin sensor under pressure. The stable region of ice V disappeared at low temperature and the direct transition from ice II to ice VI was observed. The triple point of ice II, V, and VI exists in the neighborhood of −60 °C and 6 kbar. The P–T phase diagram of H2O was compiled from the present data together with the literature data.
Mossbauer spectroscopy of the mixed-spin and high-spin states of C
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m ferricytochrome c′
Authors: Maltempo M. M. , Moss T. H. , Spartalian K.
Published: February 2003

Mossbauer spectra are reported for Chromatium ferricytochrome c′ in the pH range 1.0–10.5. Measurements were made in the 150 °–4.2 °K range in the absence of an external field, and at 4.2 °K in longitudinal and transverse applied fields. Samples at pH 1.0 and 10.5, 150 °K, show a strongly asymmetric, diffuse doublet, similar to hemin. Spectra of the protein at pH 1.0 and 10.5, 4.2 °K, exhibit the 6-line hyperfine pattern typical of high-spin ferric heme proteins, consistent with previous EPR data. Protein samples at neutral pH and 150 °K show a clear symmetric doublet with a quadrupole splitting of magnitude 2.31 mm/s (±0.02 mm/s) and an isomer shift of 0.35 mm/s relative to the center of an Fe metal absorption spectrum. At neutral pH and 4.2 °K, the spectrum consists of a 4-line hyperfine pattern which is substantially different from the high and low pH forms or published spectra of high-spin or low-spin ferric heme proteins. The spectral transitions are consistent with interpretations of previous EPR data. The neutral pH hyperfine pattern is characteristic of a species having a large A⊥, a small A∥, and a quadrupole interaction whose strength is comparable with the strength of the magnetic hyperfine interaction. The 4-line hyperfine spectrum was computer fitted to a quantum mixed-spin model, yielding values of 2.91 mm/s for the quadrupole splitting, 0.29 mm/s for the isomer shift, and 6.2 T per electronic spin for the Fermi contact term. Implications of this unusually low value are discussed.
OH (X 2Πi) product internal energy distribution formed in the reaction of O(1D2) with H2
Authors: Smith G. K. , Butler J. E.
Published: February 2003

OH(X 2Πi, v=0,1) vibrational and rotational internal energy distributions, produced by the reaction of O(1D2) with H2, have been measured by laser induced fluorescence. Highly excited, non-Boltzmann rotational distributions are reported. The most populated state observed is at N=25 for v=0 and N=17 for v=1. A nearly statistical vibrational ratio of v=1 to v=0, equal to 1.0±0.3, is observed. A dynamical interpretation of these results is given by comparison with recent trajectory calculations. This interpretation suggests that ’’microscopic branching’’ may occur between a simple hydrogen ’’abstraction’’ channel and ’’insertion’’ into the H–H bond to form a short lived vibrationally excited adduct, with the ’’insertion’’ channel dominating the formation of OH(v=0,1).
Structure, properties, and inversion of the Ar⋅SO2 van der Waals molecule
Authors: DeLeon Robert L. , Yokozeki Akimichi , Muenter J. S.
Published: February 2003

Microwave and radiofrequency spectra of the Ar⋅SO2 van der Waals molecule have been studied by molecular beam spectroscopy. The molecule has Cs point group symmetry and tunnels through a very low barrier at a C2v structure at a 980 MHz rate. The tunneling interchanges the two oxygen atoms which causes half of each inversion doublet to have zero statistical weight. The inversion frequency and assumptions about the bending vibration lead to a barrier height of ?10 cm−1. Stark effect measurements give μa=0.2674 D, μb=0.0 D, μc=1.465 D. These moments are interpreted in terms of a vibrational amplitude of 24° associated with the inversion coordinate. The geometry of the complex is essentially perpendicular with the Ar atom above the plane of SO2 and located almost directly over the SO2 center-of-mass at a 3.675 Å distance.
Measurement of the vibrational state dependence of the LiH A 1Σ+ dipole moment
Authors: Dagdigian Paul J.
Published: February 2003

Zero field molecular level crossing spectroscopy in the J′=2 rotational state has been employed to measure the electric dipole moment μv of several vibrational levels v′ of the LiH A 1Σ+ excited electronic state. The dipole moments are determined to be μ1=1.88±0.19 D, μ2=1.49±0.18 D, and μ5=0.29±0.05 D, where the quoted uncertainties represent 95% confidence intervals. These experimental values are found to agree rather well with vibrationally averaged dipole moments from the ab initio calculation by Docken and Hinze [J. Chem. Phys. 57, 4928, 4936 (1972)]. The extreme dependence of μv on vibrational level is striking and, on the basis of the ab initio results, can be ascribed to a change in the electronic structure of the LiH A state with internuclear separation.
Vacuum ultraviolet photoelectron emission spectroscopy of water and aqueous solutions
Authors: Watanabe Iwao , Flanagan James B. , Delahay Paul
Published: February 2003

Experimental methods are developed for the photoelectron emission spectroscopy of water and aqueous solution (halides, hydroxyl ion, phosphoric acid, and phosphates) up to 10.5 eV. The yield for emission of photoelectrons by the liquid into water vapor is measured as a function of photon energy. Threshold energies are determined by extrapolation from emission spectra. Photoelectron emission by liquid water obeys Urbach’s rule and has a threshold energy of 9.3±0.3 eV. This threshold is compared with the literature value of 8.7±0.1 eV for the threshold energy for amorphous ice. Threshold energies for halide and hydroxyl ions are obtained by application of the Brodsky–Tsarevsky theory of photoelectron emission by solutions: 8.7, 7.9, 7.2, 8.4 eV, respectively, for Cl−, Br−, I−, OH− ions. The emission spectrum for Br− is analyzed for spin–orbit coupling. The threshold energies of these ions are higher by 1.6 to 1.8 eV than the energies at the maxima of the corresponding charge-transfer-to-solvent absorption spectra. The difference of 1.6 to 1.8 eV agrees with a theoretical prediction. Preliminary results are reported on the effect of ionic charge on the emission of phosphoric acid and phosphate ions.
A study of methyl reorientation in solid nitromethane by neutron scattering
Authors: Trevino S. F. , Rymes W. H.
Published: February 2003

The geometrical and dynamical properties of the reorientation of the methyl group of nitromethane in the solid state have been studied by inelastic neutron scattering. Quasielastic scattering spectra have been obtained for several values of momentum transfer at five temperatures between 50° and 150 °K. The spectra are shown to be consistent with jumps of 120 ° about an axis coincident with the C–N bond. The temperature dependence of the residence time yields a barrier of 234 cal/mole. An inelastic neutron spectrum obtained at 4.2 °K suggests a tunnel splitting of the torsional ground state of 0.045±0.005 meV, which is consistent with the derived activation energy. Inelastic neutron scattering has also been used to determine the energy of the tunnel split first excited state which in CH3NO2 has a mean energy of 7.4 meV above the ground state and is split by 1.8 meV. In the deuterated compound, the first excited stated is at 5.3 meV above the ground state and its splitting is less than the instrumental resolution.
Photoselection in uniaxial liquid crystals: The effect of rotational Brownian motion on measurements of orientational distribution function
Authors: Naqvi K. Razi
Published: February 2003

The orientation distribution function of uniaxial liquid crystals derived by Chapoy and Dupré2,3, including the effects of rotational Brownian motion, is discussed in detail. Particularly, photoselection and partial orientation of molecules, and the resulting fluorescence anisotropy is discussed.(AIP)
Effect of phase and orbital wave parameter choices on CS and IOS degeneracy averaged differential cross sections
Authors: Khare V. , Fitz D. E. , Kouri D. J.
Published: February 2003

The effect of phase choice and partial wave parameter choice on CS and IOS inelastic degeneracy averaged differential cross sections is studied. An approximate simplified CS scattering amplitude for ?=1/2(l′+l) is derived and is shown to have a form which closely resembles the McGuire–Kouri scattering amplitude for odd Δj transitions and reduces to it for even Δj transitions. The choice of phase in the CS wave function is shown to result in different approximations which yield significantly different shapes for the degeneracy averaged differential cross section. Time reversal symmetry arguments are employed to select the proper phase choice. IOS calculations of the degeneracy averaged differential cross sections of He–CO, He–Cl and Ne–HD using ?=1/2(l+l′) and the phase choice which ensures proper time reversal symmetry are found to correct the phase disagreement which was previously noted for odd Δj transitions using ?=l or l′ and either the time reversal phase or other phase choices.
Mode damping in multiconfigurational Hartree–Fock procedures
Authors: Yeager Danny L. , Albertsen Preben , Jo/rgensen Poul
Published: February 2003

Mode damping techniques that allow a controlled ’’walk’’ on the energy hypersurface both within the orbital and the configuration space are introduced. The use of these techniques is particularly crucial in cases where there is strong coupling between the orbital and the configuration space, e.g., the second 1Σ+g state of C2. The mode damping technique allows us to use a set of single configuration Hartree–Fock orbitals as an initial guess of orbitals and then coverge in very few iterations.
On the structures of van der Waals molecules and solids
Authors: Burdett Jeremy K.
Published: February 2003

The structures of van der Waals molecules are viewed using simple molecular orbital ideas. In most cases studied the observed geometry is the one where the molecular orbital based repulsion of closed shells of electrons is minimized, i.e., the angular geometry is determined by covalent rather than van der Waals forces. In some, but not all, cases there is an attractive contribution to the energy arising via HOMO–LUMO interaction between orbitals located on the inert gas atom and the molecule to which it is coordinated as suggested by Klemperer. The following systems were studied: ArX2 (X2=Cl2, N2, O2, HCl, ClF), (X2)2, (X2=Cl2, N2, O2, CO2), and ArBF3.
Complex coordinate rotation of the electron propagator
Authors: Donnelly Robert A. , Simons Jack
Published: February 2003

It is now widely appreciated that the real poles of the electron propagator G(E) yield information on the ionization potentials and electron affinities of the stationary states of an atom or molecule. It is herein shown that application of the Aguilar–Balslev–Combes–Simon coordinate transformation, r→r exp(iΘ), to G(E) yields an analytically continued complex propagator G(Z, Θ) whose complex poles correspond to the complex electron affinities associated with nonstationary, resonance states of an atomic or molecular anion. As an initial application of the coordinate rotation technique we derive and discuss the working equations for a coordinate rotated propagator which is correct to second order in the electron–electron interaction. This is followed by use of the formalism in a model study of a 2P shape resonance in the Be atom. Our second-order results for this system are then compared to those obtained by previous authors employing static exchange, and static-exchange plus cutoff polarization methods.
An SCF and MCSCF description of the low-lying states of MgO
Authors: Bauschlicher C. W. , Silver D. M. , Yarkony D. R.
Published: February 2003

SCF and MCSCF wave functions for the low-lying 3,1Σ+ and 3,1Π states of MgO are presented and discussed. It is argued that a viable description of these states is possible if the 3Σ+ and 3,1Π states are treated at the SCF level while the 1- and 2- 1Σ+ states are treated with a limited MCSCF expansion.
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o model calculations for graphite: Bulk and basal plane electronic densities
Authors: Butkus Aldona M. , Fink William H.
Published: February 2003

Results of ab initio calculations on carbon atom clusters modeling the electronic structure of the bulk and basal plane of the graphite lattice are reported. Models consisting of four, six, and eight carbon atom clusters are considered for the bulk case and four and five carbon units are chosen for the basal plane representation. Reference SCF calculations are carried out for all of the clusters treated together with Mulliken population analyses for the resultant SCF and model wave functions. Comparison between SCF and model results indicate a more uniform electron density distribution for the model approach than for the SCF calculations. Comparison of two and three dimensional models for the graphite lattice predict weak interaction between atoms in adjacent stacking planes consistent with known properties of graphite.
The correlation length and lateral compressibility of phospholipid bilayers in the presence of thermodynamic density fluctuations
Authors: Zuckermann M. J. , Pink D.
Published: February 2003

Pink’s ten state statistical model for the gel–fluid (main) phase transitions of phospholipid bilayers is used in conjunction with linear response theory to obtain expressions for the coherence length, ξ and the lateral compressibility χL for wet lipid bilayers in the presence of thermodynamic density fluctuations near the temperature Tf of the main phase transition. The advantage of this method is that the parameters of Pink’s model are known for several lipid systems from previous fitting to experimental data. ξ and χL can therefore be calculated numerically and the relative importance of the static and fluctuation contributions to χL can be examined. One conclusion of the article is that the coherence length and Tf is short and therefore that the contribution of thermodynamic density fluctuations to χL is about 13%. This implies that the free energy barrier EB experienced by an ion as it penetrates the bilayer has a dominant contribution from the static lateral compressibility, provided that the interaction between the ion and the polar heads of the lipid molecules is a δ-force. However, it is further shown that the static and the density fluctuation effects contribute almost equally to EB at Tf when the range ξo of interaction between the ion and the polar heads is greater than half a molecule diameter. This remains true in the limit ξo→∞ as the coherence length ξ is quite short at any temperature.
A comment on the inversion of gas transport properties
Authors: Monchick L.
Published: February 2003

The Eikonal approximation to the principal part of the gas viscosity coefficient is shown to be exactly invertible for certain classes of potentials. The supplementary conditions that must be added to make it so for more general classes are similar to those introduced in all successful heuristic methods of deducing molecular interactions from gas transport properties.
An ESR study of molecular reorientation in liquids
Authors: Herring F. G. , Phillips P. S.
Published: February 2003

Electron spin resonance (ESR) has been used to study the reorientational correlation times (τ2) of a series of bis(dialkyldithiocarbamato)63 Cu(II) spin probes, in toluene, as a function of temperature. It has been found that the linear relationship τ2=C(η/T)+τ02 is obeyed over most of the temperature range studied, but at high temperatures some evidence of nonlinearity exists for the smaller spin probes. The value of C observed is intermediate between the value calculated using slip and stick boundary conditions of hydrodynamic theory. The magnitude of C is indicative of moderate non-hard-core interactions.
Energy splitting between the B and C states of xenon chloride
Authors: Bokor J. , Rhodes C. K.
Published: February 2003

The energy splitting between B(1/2) and C(3/2) excimer states in xenon chloride has been determined by comparing the thermal equilibrium intensity ratio of the broadband C(3/2)→A(3/2) fluorescence emission at 340 nm to that of the narrowband B(1/2)→X(1/2) fluorescence emission at 308 nm. The formation of the excimers is initiated via two-photon ionization of Xe atoms in the presence of Cl2 and an Ar buffer. The room temperature equilibrium intensity ratio IC–A/IB–X is found to be 0.15±0.01. Using the Einstein A coefficients calculated by Hay and Dunning for these transitions, the energy splitting EC−EB is determined as (5.4±25) cm−1. Strong emission from the triatomic species Xe2Cl was also observed.
CS+ (B 2Σ+–X 2Σ+) emission produced from Penning ionization of the CS (X1 Σ+) radical
Authors: Tsuji Masaharu , Obase Hiroshi , Nishimura Yukio
Published: February 2003

Emission spectrum resulting from the energy transfer reaction of He (2 3S) with transient CS radical produced through a microwave discharge of Ar/CS2 mixture was observed in a flowing afterglow apparatus. A new CS+ (B 2Σ+–X 2Σ+) band system due to He (2 3S)/CS Penning ionization was identified in the 220–340 nm region. The dominant bands were the (ν′,0) and (0, ν″) progressions, which was consistent with the prediction from calculated Morse potential Franck–Condon factors. Although the observed bandhead wavelengths of the major CS+ (B–X) emission were in good agreement with those predicted from the reported spectroscopic data, the (4,0) band was located at an abnormal position. It was interpreted as a consequence of vibrational perturbation in the B 2Σ+ state. The relative intensities of CS+(B–X), CS+(B–A), CI, and SI emission were estimated to represent that the dominant pathway of the He (2 3S)/CS reaction is Penning ionization.
Relaxation processes of some aromatic sulfides, sulfoxides, and sulfones in a polystyrene matrix
Authors: Desando M. A. , Walker S. , Baarschers W. H.
Published: February 2003

Dielectric absorption studies have been made of a number of aromatic sulfides, sulfoxides, and sulfones in a polystyrene matrix at a variety of temperatures in the frequency range of 102–105 Hz, and in three cases between 104–107 Hz. One compound, bis (4-pyridyl) sulfide, was also examined as a pure compressed solid in the frequency range of 104–107 Hz. The Eyring parameters have been determined and used to characterize the relaxation processes. Low enthalpies of activation (∼15 kJ mol−1) have been estimated for an intramolecular relaxation process of diphenyl, bis (4-tolyl) and bis (4-pyridyl) sulfide, and rotation about the C–S bond in these symmetrical aromatic sulfides would, thus, be expected to occur quite readily. These low barriers are to be contrasted with those for the molecular relaxation values of 60–78 kJ mol−1 for diphenyl, bis(4-tolyl) and bis(4-chlorophenyl) sulfoxides and sulfones. The enthalpies of activation (∼39 kJ mol−1) for the molecular relaxation of the disklike molecules, dibenzothiophene and its oxides, are also lower than those for the corresponding nonplanar diphenyl sulfur oxides.
Mechanisms of CO ejection from ion bombarded single crystal surfaces
Authors: Winograd N. , Garrison B. J. , Harrison D. E.
Published: February 2003

We have calculated the response of a model Ni(001) microcrystallite to 600 eV Ar+ ion bombardment when it is covered with 0.5 monolayer of CO. Calculations were performed using a standard molecular dynamics treatment which employs pair potentials fit to the elastic constants of the solid to evaluate Hamilton’s equations of motion. The model microcrystallite contained ∼240 atoms. The CO was adsorbed in a c(2×2) coverage in both an atop or linear bonded and a twofold bridge bonded position with a binding energy to the surface of 1.3 eV. The results showed that most of the CO molecules eject molecularly, although a few (∼10%–15%) eject dissociatively if they are hit directly with the primary ion or with other energetic solid atoms. We also found that NiCO as well as Ni2CO and Ni3CO formation probably occurs over the surface via interaction between Ni and CO species, and that the probability of NiC or NiO clusters ejected from a CO covered surface is extremely low.
Direct determination of rotational correlation time by electron-spin echoes
Authors: Stillman Arthur E. , Schwartz Leslie J. , Freed Jack H.
Published: February 2003

The measurement of rotational correlation time by determination of the outer ERS hyperfine extrema is demonstrated for tempone in a glycerol-water solution.(AIP)
On the solution of the nonuniform Percus–Yevick equation
Authors: Sokol/owski S.
Published: February 2003

In the present note we propose a method of solution of the nonuniform Percus–Yevick equation. It was showed that application of the Fourier transform technique to the three-dimensional problem with a planar symmetry leads to an essentially one-dimensional mathematical problem. In the case of hard spheres in contact with a hard wall the set of equations composed of the nonuniform Ornstein–Zernike relation, the Percus–Yevick approximation and the exact relation between the local density and the two-particle direct correlation function was solved numerically with respect to one- and two-particle distribution functions.(AIP)
Temperature dependence of the C2 (a 3Πu)+CH4 reaction from 337 to 605 K
Authors: Pasternack L. , Baronavski A. P. , McDonald J. R.
Published: February 2003

The reaction rate of C2(a3Πu)+CH4 is reported for temperatures of 337 to 605 K. The C2(a3Πu) is produced by multiphoton 193 nm laser photolysis of hexafluorobutyne-2 and monitored with a tunable dye laser. The data are fit to an Arrhenius equation of the form kII(T)?0.20)×10−11 exp {[−5.57±0.11 kcal/mol]/RT} cm3 molecule−1 s−1.
Proton spin–lattice relaxation in meta-carborane
Authors: Beckmann Peter , Wendel Armin
Published: February 2003

The proton spin lattice relaxation time, T1 has been measured at 30 MHz in meta-carborane between 171 and 303°K. The temperature dependence of T1 in meta-carborane is discussed.(AIP)
Rational fraction representation of diatomic vibrational potentials. Application of H2+ ground state
Authors: Beckel Charles L. , Findley Paul R.
Published: February 2003

Jordan, et al.10, have proposed use of Padé approximants for representing V(R); an extension of this procedure to te use of rational fractions [N/M] is proposed in the present paper, and applied to the ground state of H2+.(AIP)
Temperature effects on the infrared spectrum of chemisorbed CO
Authors: Antoniewicz P. R. , Cavanagh R. R. , Yates J. T.
Published: February 2003

The temperature dependence of the infrared spectrum of CO chemisorbed on Rh supported by Al2O3 has been measured. A shift in the CO stretching mode of 1.4×10−2 cm−1 K−1 to higher wave number is observed on cooling. An electrodynamic model of the surface interactions is presented. The model accounts for both the magnitude and direction of the observed shift.
Rotational distribution of N2 in Ar shock wave
Authors: Koura Katsuhisa
Published: February 2003

The rotational distribution of N2 in the Ar shock wave is quantitatively studied using the rotational transition cross sections of the exponential gap and power laws. The relaxation of the rotational distribution in the shock wave is described by the master equation, where the velocity distributions are taken as the local Maxwell distribution at a low Mach number and the Mott–Smith bimodal distribution at a high Mach number. The rotational distribution indicates a near Boltzmann distribution at a low Mach number. At a high Mach number a merging pattern of the Boltzmann distributions at the lower and higher rotational levels corresponding to temperatures before and after the shock wave, respectively, is obtained at the fore part of the shock wave, where the rapid increase in the populations of the higher levels is caused by the accumulation of the fluxes from the lower levels. The merging pattern is well established by the existence of hot molecules in the Mott–Smith bimodal velocity distribution. The rotational temperature profile precedes the density profile both at low and high Mach numbers. The fact that the pattern of the rotational distribution in the N2–Ar system is in qualitative agreement with the measured pattern in the pure N2 system may indicate that the rotation–rotation transition does not play an important role in the pattern of the rotational distribution of N2 in the shock wave.
Approximate solution of the inversion problem for atom–surface scattering
Authors: Gerber R. B. , Yinnon A. T.
Published: February 2003

It is shown that atom–surface interaction potentials can be recovered from the diffraction peak intensities measured in beam scattering experiments by a direct, simple inversion method. The potential is determined uniquely, apart from a term depending only on the coordinate perpendicular to the surface. The method does not assume any parametrized functional form for the potential. The results are obtained for a rigid surface, using the sudden approximation for the scattering dynamics. Application to a test case using simulated data for Ne/W (110) yielded an inverted potential of remarkable accuracy.
A possible role for triplet H2CN+ isomers in the formation of HCN and HNC in interstellar clouds
Authors: Allen Thomas L. , Goddard John D. , Schaefer Henry F.
Published: February 2003

The structures and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self-consistent field and configuration interaction calculations. When both hydrogen atoms are attached to the nitrogen atom, H2NC+, the molecule has its lowest triplet state energy, which is 97.2 kcal mol−1 above the energy of the linear singlet ground state. The structure has C2v symmetry, with an HCH bond angle of 116.8°, and bond lengths of 1.009 Å (H–N) and 1.268 Å (N–C). Other isomers investigated include the H2CN+ isomer at 104.7, the cis-HCNH+ isomer at 105.3, and the trans-HCNH+ isomer at 113.6 kcal mol−1. The H2CN+ isomer has an unusual ’’carbonium nitrene’’ structure, with a C–N bond length of 1.398 Å. It is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3. Specifically, a triplet path is postulated in which C+ and NH3 yield the triplet H2NC+ isomer, which then yields the singlet H2NC+ isomer by phosphorescent emission. Because this emission removes a large amount of energy, the singlet H2NC+ isomer may have insufficient energy to isomerize to the linear singlet ground state. Subsequent dissociative recombination would yield the HNC isomer exclusively.
Use of one-electron theory for the interpretation of near edge structure in K-shell x-ray absorption spectra of transition metal complexes
Authors: Kutzler Frank W. , Natoli C. R. , Misemer D. K. , Doniach S. , Hodgson Keith O.
Published: February 2003

We report the results of multiple scattered wave SCF X-alpha calculations of the one-electron cross section for K-shell photoabsorption in the molecular complexes MoO4−−, CrO4−−, and MoS4−−. We show that the method can successfully account for energy separations and relative cross sections of spectral features both below and above the K-shell ionization threshold. Furthermore, we show: (a) that the first fairly intense peak on the low energy side of the rising edge for molybdate and chromate is due to a dipole allowed transition to a bound antibonding state of mainly nd character on the metal ion; this transition is possible because of the mixing with the ligand p orbitals having the proper T2 symmetry induced by the tetrahedral molecular potential; (b) the shoulder on the rising absorption edge can be explained by the beginning of the steplike continuum absorption when convolved with a Lorentzian function of frequency to imitate lifetime and monochromator broadening: (c) the main absorption peak is due to the dipole allowed transition to a continuum state (positive energy) of T2 symmetry, and the existence of this peak is the result of a ’’shape resonance’’ of the outgoing continuum electron in the molecular potential due to the ligands bound to the transition metal atom and bears only a vague resemblance to the (n+1)p atomic state on the isolated transition metal ion.
Application of Xα multiple-scattering theory to planar organic molecules: One-electron properties and ionization potentials of benzene, pyridine, pyrazine, pyrrole, and imidazole
Authors: Case David A. , Cook Michael , Karplus Martin
Published: February 2003

Xα multiple scattering wave functions and molecular properties are reported for benzene, pyridine, pyrazine, pyrrole, and imidazole. The calculations demonstrate that useful results can be obtained for planar conjugated organic molecules with the Xα method. Values are obtained for ionization potentials and one electron properties, including dipole and quadrupole moments, diamagnetic susceptibilities and nuclear quadrupole coupling constants. In general the results are in better agreement with experiment than minimum basis set Hartree–Fock calculations; this generalization is not true for molecular properties that are dominated by hydrogen atoms, such as deuteron quadrupole coupling constants. The sensitivity of the results to changes in sphere overlap is studied in detail for pyridine and it is found that useful accuracy can be achieved over a fairly broad range of overlap parameters. Calculated ionization potentials agree well with recent assignments of the photoelectron spectra, except for the relative ordering of π and nitrogen lone-pair orbitals. This difference is analysed in terms of correlation effects in lone-pair ionizations. As part of the comparisons, detailed tabulations of one-electron properties and ionization potentials obtained from experiments and other calculations are provided.
Study of the water octamer using the polarization model of molecular interactions
Authors: Stillinger F. H. , David C. W.
Published: February 2003

A geometry search was carried out to locate the global energy minimum for (H2O)8, in its full 66 dimensional space of relative configurations. The ’’polarization model’’ was used to represent interactions. The optimal structure uncovered has nine nearly linear hydrogen bonds, and amounts to an aqueous analog of the saturated hydrocarbon bicyclo[2.2.2] octane. This structure is recognizable as a fragment of the hexagonal ice crystal, and thus may be important as a prenucleation cluster for that phase. The zwitterionic form of the octamer was found to have no region of local stability.
Interpretation of the unusual behavior of H2O and D2O at low temperatures: Tests of a percolation model
Authors: Stanley H. Eugene , Teixeira J.
Published: February 2003

The unusual low-temperature behavior of liquid water is interpreted using a simple model based upon connectivity concepts from correlated-site percolation theory. Emphasis is placed on examining the physical implications of the continuous hydrogen-bonded network (or ’’gel’’) formed by water molecules. Each water molecule A is assigned to one of five species based on the number of ’’intact bonds’’ (the number of other molecules whose interaction energy with A is stronger than some cutoff VHB). It is demonstrated that in the present model the spatial positions of the various species are not randomly distributed but rather are correlated. In particular, it is seen that the infinite hydrogen-bonded network contains tiny ’’patches’’ of four-bonded molecules. Well-defined predictions based upon the putative presence of these tiny patches are developed. In particular, we predict the detailed dependence upon (a) temperature, (b) dilution with the isotope D2O, (c) hydrostatic pressure greater than atmospheric, and (d) ’’patch-breaking impurities’’—for four separate response functions, (i) the isothermal compressibility KT, (ii) the constant-pressure specific heat CP, (iii) the constant-volume specific heat CV, and (iv) the thermal expansivity αP, as well as for dynamic properties such as (v) the transport coefficients self-diffusivity Ds and shear viscosity η, (vi) the characteristic rotational relaxation time τch, and (vii) the Angell singularity temperature Ts. The experimentally observed dependence of these seven quantities upon the four parameters (a)–(d) is found in all cases to agree with the predicted behavior. The paradoxical behavior associated with the absence of a glass transition in pure liquid water is also resolved. Finally, we propose certain experiments and computer simulations—some of which are underway—designed to put the proposed percolation model to better tests than presently possible using available information.
Hydrophobic solvation of nonspherical solutes
Authors: Pratt Lawrence R. , Chandler David
Published: February 2003

The theory of hydrophobic effects presented by Pratt and Chandler is generalized to include nonpolar solutes which are distinctly aspherical. The theory is used to study the solvation of simple aspherical hydrocarbon solutes in liquid water. The radial solvation of each component of diatomiclike solutes is studied as a function of their separation, or bond length. From these results it is found that when the bond length is large enough that one water molecule can fit between the apolar pair, the radial solvation of each is the same as that when the bond length approaches infinity. The solvation of the various sites of the homologous series methane, ethane, propane, and n-butane is also studied, and effects of the geometrical structure of the solutes on their solvation is discussed.
Absorption and emission spectra of Li atoms trapped in rare gas matrices
Authors: Wright J. J. , Balling L. C.
Published: February 2003

Pulsed-dye-laser excitation has been used to investigate the optical absorption and emission spectra of Li atoms trapped in Ar, Kr, and Xe matrices at 10 °K. Attempts to stabilize Li atoms in a Ne matrix at 2 °K were unsuccessful. Results for all three rare gases were qualitatively the same. White light absorption scans showed a single absorption with three peaks centered near the free-atom 2s→2p transition wavelength. The intensity of fluorescence produced by dye-laser excitation within this absorption band was measured as a function of emission wavelength. Excitation of the longest- and shortest-wavelength absorption peaks produced identical emission profiles, but no distinct fluorescence signal was detected when the laser was tuned to the central absorption peaks, indicating that the apparent absorption triplet is actually the superposition of a singlet and a doublet absorption originating from two different trapping sites. No additional absorption bands were detected.
Broadening by strains of lines in the g-parallel region of Cu2+ EPR spectra
Authors: Froncisz W. , Hyde James S.
Published: February 2003

Linewidths of the mI=−3/2 and −1/2 hyperfine lines in the g-parallel region of several square-planar copper complexes with oxygen ligands in frozen solutions have been measured at 2.6, 3.8, and 8.9 GHz. A theory has been developed in terms of distributions of molecular bonding parameters that satisfactorily accounts for the observed dependence of linewidths on mI and the microwave frequency. It has been found that distributions of the bonding parameters are strongly correlated, indicating that copper complexes when subjected to strains during freezing distort in a well-defined, predictable manner. It is found that the mI=−3/2 line generally is narrowest near 6 GHz and the mI=−1/2 line near 2 GHz. By proper selection of the microwave frequency, improved spectral resolution can be obtained. Complexes with nitrogen ligands also have been examined. Proper selection of microwave frequency often permits observation of superhyperfine structure, and even when it cannot be seen, analysis of the linewidth variation with microwave frequency yields information on nitrogen ligation. Octave bandwidth S-band EPR spectroscopy is emphasized as a preferred method for studying copper complexes.
Composite double oscillation in a modified version of the oregonator model of the Belousov–Zhabotinsky reaction
Authors: Janz Robert D. , Vanecek David J. , Field Richard J.
Published: February 2003

A number of nonmonotonic behaviors appear when the Belousov–Zhabotinsky reaction is run in a flow system (CSTR) which are not observed when the reaction is run in a closed system. Among these behaviors is composite double oscillation in which nearly identical bursts of oscillation are separated by regular periods of quiescence. Here we use a modified version of the oregonator model of the Belousov–Zhabotinsky reaction to simulate composite double oscillation. Our modification involves the addition of a new variable which is related to the amount of brominated organic material present in the system. This new variable changes slowly on the time scale of the oscillations and controls the value of f, the stoichiometric factor of step 5 in the oregonator. Thus the behavior of the modified oregonator in CSTR mode when flowrates are moderate can be rationalized in terms of the properties of the unmodified oregonator in a closed system. We show that composite double oscillation is a hysteresis phenomenon occurring over a small range of values of f where a locally stable steady state and a locally stable limit cycle coexist. Composite double oscillation occurs as the system is carried back-and-forth across the area of coexistence by the new, slowly moving variable whose concentration grows during the oscillatory phase, when the system is on the locally stable limit cycle, and decays during the quiescent phase, when the system is on the locally stable steady state.
Mobility of Cl− ions in Xe gas and the Cl−–Xe interaction potential
Authors: Thackston M. G. , Eisele F. L. , Pope W. M. , Ellis H. W. , McDaniel E. W. , Gatland I. R.
Published: February 2003

The mobility of Cl− ions in Xe gas at 300 °K has been measured in a drift tube mass spectrometer for a wide range of values of the ionic energy parameter E/N (the ratio of the electric field strength to the neutral gas number density). A Cl−–Xe interaction potential is assumed and a kinetic theory appropriate for the ion motion is used to derive the mobility from the potential. Then an iterative technique is used to modify the potential so as to fit the predicted mobility to the experimental data. This interaction potential is directly determined by the experimental data for a range of internuclear separation distances from about 4 to about 10 a.u. At distances greater than 10 a.u., the potential is the induced dipole polarization potential.
Temperature dependence of vibrational relaxation from the upper vibrational levels of HF and DF
Authors: Wilkins Roger L. , Kwok Munson A.
Published: February 2003

A kinetic model of infrared laser-induced fluorescence experiments has been used to simulate quenching coefficients between 300 and 2400 K for the vibrational relaxation of HF(v1) and DF(v1) by HF(v2=0) and DF(v2=0). This rotational nonequilibrium model is based on the predicted energy-transfer mechanisms in hydrogen–fluoride and deuterium–fluoride systems reported earlier by Wilkins. The deactivation rates for the V→R processes for HF(v1)+HF(v2=0) and their isotopic analogs are predicted to scale as vn with n varying from 2.3 to 1.6 as v varies from 2 to 6. These quenching coefficients for V→R processes from the upper vibrational levels are predicted to have a temperature dependence very similar to that for V→R relaxation from the v=1 level. The results are discussed in relation to V→V energy transfer and V→R intramolecular energy conversion.
On the relative stability of the metallic and the insulating states of a half-filled band
Authors: Whangbo Myung-Hwan
Published: February 2003

The electronic state of a half-filled band may become insulating rather than metallic so as to minimize electron–electron repulsion. The insulating state of a half-filled band can be magnetic as in ferromagnetic and antiferromagnetic states or nonmagnetic as in a charge density wave state. The relative stability of these metallic and insulating states was studied within the framework of Hartree–Fock band theory in order to examine the conditions that make the insulating state of a half-filled band magnetic or nonmagnetic.
The inversion barriers of NF3, NCl3, PF3, and PCl3. A theoretical study
Authors: Marynick Dennis S.
Published: February 2003

The inversion barriers of NF3, NCl3, PF3, and PCl3 have been calculated from ab initio molecular orbital theory using large basis sets including polarization functions on the central atom. The calculated inversion barriers (in kcal/mole) are 78.5(NF3), 22.9(NCl3), 121.5(PF3), and 90.8(PCl3). The NF3 inversion barrier is clearly larger than the experimental first bond dissociation energy (∼57 kcal/mole), indicating that planar NF3 is not bound relative to NF2+F. For PF3 and PCl3, bond dissociation is energetically competitive with inversion, since the experimental first bond dissociation energies are 130±14 and 81±5 kcal/mole, respectively. The lowest energy closed shell electronic configuration for planar PF3 is shown to have the phosphorus nonbonding (lone pair) electrons in a A1′ type orbital, rather than the pure p orbital of A2″ symmetry usually assumed for molecules of this type. Limiting the description of inner shells to a minimum basis set is shown to have relatively small (0–3 kcal/mole) effects on the calculated barrier.
Resonance CARS (coherent anti-Stokes Raman scattering) line shapes via Frank–Condon scattering: Cytochrome c and β-carotene
Authors: Dutta Prabir K. , Dallinger Richard , Spiro Thomas G.
Published: February 2003

The CARS (coherent anti-Stokes Raman scattering) line shapes of the 1362 cm−1 Raman transition of dilute aqueous FeII cytochrome c have been studied with excitation in the region of the Soret band. In contrast to previous results for flavin, for which a Lorentzian CARS peak was observed at the electronic origin, the Lorentzian peak for cytochrome c occurs on the low energy side of the absorption band. The relative positions of the dispersive and negative line shapes are also red-shifted compared to flavin. When the scattering model previously developed for flavin is extended to include higher vibrational levels of the excited state, the cytochrome c pattern emerges as a consequence of a small origin shift parameter for the 1362 cm−1 mode. A line shape analysis for β-carotene indicates large origin shift parameters, near unity, for the vibrational modes at 1525, 1155, and 1005 cm−1, in agreement with previous analyses of the spontaneous Raman and CARS excitation profiles. Inclusion of multimode terms in the scattering model, however, decreases the estimated origin shifts substantially.
Fermi resonance of Raman modes in the condensed phase: A theoretical investigation of the concentration dependent effects in molecular liquids
Authors: McHale J. L. , Wang C. H.
Published: February 2003

The effect of intermolecular interaction on the Raman spectrum of two modes which are in Fermi resonance has been investigated theoretically. The theory includes the resonance exchange of vibrational energy as well as the inter- and intramolecular Fermi coupling interaction and is expected to be a realistic model for the interpretation of the concentration dependence of the observed Raman spectral features of the Fermi resonance bands of liquid NH3 and other molecular liquids. Explicit microscopic expressions for the Fermi interaction parameter, the peak positions, and linewidths of the Fermi-resonance bands are provided. It is shown that the peak positions of the two coupled bands will be dependent on the concentration of the scattering molecules, and for the case of strong coupling the Fermi interaction parameter will have a nonlinear concentration dependence, as observed in NH3 dissolved in various solvents.
Electron impact study of the energy levels of t
r
a
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s-1,3-butadiene: II. Detailed analysis of valence and Rydberg transitions
Authors: Doering J. P. , McDiarmid Ruth
Published: February 2003

In this electron energy loss investigation of the electronic states of trans-1,3-butadiene, high energy resolution (25 meV) and a wide range of incident energies (9.5 to 49 eV) have been employed to locate and assign valence and Rydberg transitions in the typically congested 5–9 eV (2500 to 1400 Å) region of the spectrum of a polyatomic molecule. Comparisons of the observed relative differential scattering cross sections (DCS) of various features in the lower energy (5.0 to 6.8 eV) region with the unambiguous assignments of these features from optical absorption and multiphoton ionization spectra allowed certain conclusions to be drawn regarding the typical DCS of Rydberg and valence singlet–singlet transitions. These ’’rules’’ have been applied to the congested 7–9 eV energy loss region of the butadiene spectrum. A diffuse process near 7.3 eV has been assigned as a forbidden valence transition, probably 1Ag←1Ag, on the basis of its relative DCS. The identification of the 7.07 and 8.00 eV transitions as successive members of the same Rydberg series is supported, and both transitions are demonstrated to contain considerable valence component.
A search for non-Čerenkov UV emission from liquid water irradiated with pulsed electrons
Authors: Quickenden T. I. , Irvin J. A. , Sangster D. F.
Published: February 2003

Liquid water was irradiated with a nanosecond pulsed electron beam and the emitted light was examined for any non-?erenkov emission in the wavelength range 210–380 nm. In order to avoid interfacial effects associated with containment, a transient jet of liquid water was irradiated while it traversed an evacuated chamber having dimensions considerably larger than the jet. Analysis of the decay profiles and peak intensities of the emitted luminescence provided no evidence for the existence of a non-?erenkov component in the emission.
Cross-correlation trajectory study of V–V energy transfer in HF–HF and DF–DF
Authors: Coltrin M. E. , Koszykowski M. L. , Marcus R. A.
Published: February 2003

Results of a fully three-dimensional classical trajectory calculation of vibrational energy transfer are presented for the collision of HF(v=1) with HF(v=1) and its deuterium analog. A cross-correlation method, together with quasiclassical trajectories, is introduced to relate the changes in vibrational states of the two molecules to probabilities and rate constants. Multiple collisions are found to make an important contribution to the vibrational energy transfer cross-sections for the present potential surface. Vibrational anharmonicity is shown to decrease the energy transfer rate constant by a factor of ten, by causing the process to be further from exact resonance. Excellent agreement with experiment is obtained for the HF–HF and DF–DF systems.
Isotopic selectivity in the laser induced dissociation of molecules with overlapping absorption bands
Authors: Horsley J. A. , Cox D. M. , Hall R. B. , Kaldor A. , Maas E. T. , Priestley E. B. , Kramer G. M.
Published: February 2003

Measurement of the isotopic selectivity as a function of temperature are described for IR laser induced dissociation of UO2(hfacac)2⋅THF in a low density molecular beam. The measured selectivity was found to increase from 1.2 at 120 °C to 1.9 at 65 °C. This behavior is consistent with a model which takes into account both the initial thermal energy content of the molecule and the energy barrier to dissociation. Indeed, such behavior is expected for any complex molecule which has isotopically shifted, but overlapping absorption spectra as is the case for the 235U and 238U containing molecules used in this study.