bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of short-range repulsions
Authors: Böhm Hans-Joachim , Ahlrichs Reinhart
Published: February 2003

A first-order SCF theory is proposed to treat nonbonded interactions of atoms (subsystems) in arbitrary states in order to investigate intermolecular atom–atom repulsions between closed-shell molecules. Applications to 12 homonuclear and 15 heteronuclear pairs of atoms are reported. The repulsions so obtained can always be represented by exponentials V(R) = C exp(−η R). These results are used to establish the systematic dependence of C and η on ε (the highest orbital energy), and to test the validity of combination rules, of which an ’’energy dependent hard core type’’ rule [Eq.(32)] shows the best performance. Ideas emerging from these investigations are then applied to discuss van der Waals radii and to develop a simplified treatment of repulsive parts of intermolecular interactions. A new and generally applicable approach allows for an approximate calculation of repulsive contributions which lead for (N2)2 to errors corresponding to a shift of ≲0.1a0 in the corresponding curves.
Two-dimensional separation of dipolar and scaled isotropic chemical shift interactions in magic angle NMR spectra
Authors: Munowitz Michael , Aue Walter P. , Griffin Robert G.
Published: February 2003

Two-dimensional magic angle NMR methods produce high resolution dipolar-chemical shift spectra of polycrystalline solids which permit determination of the mutual orientation and principle values of the two coupling tensors. This technique is applied to the amide 15N–1H spin pair in the dipeptide GlyGly ⋅ HCl ⋅ H2O, and we find the nearly axially symmetric 15N shift tensor is tilted 25° away from the NH bond. In many situations, however, only the magnitude of the heteronuclear dipolar coupling is required, and simplification of the spectrum would be desirable. An experiment that projects the dipolar information onto a single rotational sideband pattern for each magnetically inequivalent spin is proposed. The approach involves scaling the isotropic chemical shifts with a multiple-pulse train, together with rotationally synchronized sampling. The method is demonstrated with spectra of the above mentioned dipeptide.
Optical emission spectra and crystal field analysis of Pr3+ in the cubic Cs2NaYCl6 host
Authors: Morley J. P. , Faulkner T. R. , Richardson F. S. , Schwartz R. W.
Published: February 2003

Unpolarized and magnetic-field-induced circularly polarized luminescence spectra are reported for Pr3+ doped into the cubic Cs2NaYCl6 host. These spectra were obtained at high resolution under variable temperature conditions, and they span the 3P0→3H4, 3H5, 3H6, 3F2, 3F3, 3F4, and 1D2→3H4 transition regions of Pr3+. The crystal field levels split out of the 3H4, 3H5, 3H6, 3F2, 3F3, and 3F4 terms are located and assigned, and an existing controversy regarding the relative locations of the 3P0 and 3P1 term levels is resolved. The spectra obtained on the Cs2NaYCl6:Pr3+ (25 mol %) system are dominated by magnetic dipole origin transitions and electric dipole (one phonon) vibronic transitions which obey octahedral (0h) selection rules. However, these spectra also include weak but persistent features which can only be accounted for in terms of Pr3+ sites with lower than 0h symmetry. A number of weak origin lines are assigned to crystal field transitions which are forbidden in 0h symmetry, and the vibronic intensity distributions observed within certain term-to-term transitions reflect the presence of some lowered site symmetries. The vibronic structure observed in the luminescence spectra is dominated by contributions from transitions involving the ν3(t1u), ν4(t1u), and ν6(t2u) PrCl3−6 cluster (moiety) modes, with lesser contributions being made by two nonmoiety modes with frequencies of ∼40 and ∼60 cm−1.
Analysis of EPR results on GdF3 and GdCl3 molecules in rare gas solids
Authors: den Hartog H. W.
Published: February 2003

In this paper, we give an analysis of EPR results on GdF3 and GdCl3 molecules in the rare gas solids Ne and Ar in terms of two different theoretical models. The electrostatic model, in which the long range electrostatic interactions are assumed to produce dominant contributions to the crystal field splitting of the 8S7/2 ground state of Gd3+. On the other hand, we have considered the superposition model, in which the short range interactions between Gd+3 4f7 electron system and the neighboring ligands are taken into account only. The results obtained with the electrostatic model turn out to be in far better agreement with experiment than those calculated with the superposition model.
Spinning-sideband-free and spinning-sideband-only NMR spectra in spinning samples
Authors: Dixon W. Thomas
Published: February 2003

NMR spectra are needlessly confusing when spinning sidebands and isotropic peaks lie on the same curve. Qualitative analysis is impossible if these spinning sidebands cannot be distinguished from isotropic peaks and quantitative measurement is frustrated whenever any sideband overlaps any other peak. Resolving a spectrum into several curves, one containing only isotropic peaks, another only first higher frequency sidebands, still another only first lower, etc., eliminates these problems. These extra curves, which can be generated by an echo technique, do not require extra acquisition time. An ordinary echo spectrum refocused after two revolutions has the same S/N ratio as each curve in a separated spectrum which is made with the same total number of acquisitions.
The temperature dependence of the molecular structure parameters in SF6
Authors: Kelley Michael H. , Fink M.
Published: February 2003

The importance of anharmonic vibrational effects in SF6 has been demonstrated by the dependence of the molecular structure on temperature. The apparent interatomic distances and mean vibrational amplitudes were measured in the temperature range between 200 and 500 K and compared with existing theories. Though the mean amplitudes were found to be well modeled by the usual normal coordinate analysis, the increase with temperature of the apparent interatomic distances was found to be significantly greater than predicted either by harmonic normal mode analysis or the Morse potential function. However, the combination of these approximations describes the experimental results quite well. Inclusion of stretching anharmonicity, nonbonded fluorine repulsions, and stretch–bend interactions in a modified Uray–Bradley potential also gives agreement for the bonded distance. However, the dependence on the temperature of the nonbonded distances is at present not adequately accounted for.
Formation and quenching kinetics of electron beam excited Xe2Br*
Authors: Wilson W. L. , Williams R. A. , Sauerbrey R. , Tittel F. K. , Marowsky G.
Published: February 2003

Formation and quenching processes, as well as the optical emission characteristics of Xe2Br*, centered at 440±30 nm were investigated for electron beam pumped mixtures of argon, xenon, and several different bromine donors. Three-body collisional quenching of XeBr* was identified as the primary formation mechanism for the triatomic species. Quenching rates for Xe2Br* and XeBr* were measured and the Xe2Br* radiative lifetime was determined to be 245±30 ns.
Temperature coefficient of the dipole moment of poly(4-chlorostyrene, 4-methylstyrene) copolymers in benzene solutions
Authors: Mashimo S. , Nozaki R. , Work R. N.
Published: February 2003

Mean square values of the dipole moments of poly(4-chlorostyrene) and copolymers of poly(4-chlorostyrene, 4-methylstyrene) have been determined at up to five different temperatures. There is a significant positive temperature coefficient of the mean square dipole moment. Curves of the dipole moments and of the slopes, normalized to unity at P4CS, have essentially the same shapes. The copolymers in benzene solutions lead to values of the mean square dipole moments that are about 20% larger than measurements in p-xylene.
Observation of a steric effect in the reaction of Xe* with photodissociation polarized IBr
Authors: de Vries Mattanjah S. , Srdanov Vojislav I. , Hanrahan Ciaran P. , Martin Richard M.
Published: February 2003

This communication reports the first use of photodissociation polarization to study reaction cross sections as a function of molecular orientation. A beam of metastable Xe* atoms was crossed with a beam of rotationally oriented IBr molecules. Excimer formation was studied as a function of IBr orientation, by observation of the excimer emission intensity. The reaction cross section was found to be largest when the Xe* approaches parallel to the plane of rotation of the IBr, and smallest when the Xe* approaches perpendicular to the plane of rotation.
Temperley’s model of gas condensation in the (T, P, N) ensemble
Authors: Datoussaïd H. , DeConinck J. , de Gottal Ph.
Published: February 2003

It is shown rigorously that there is no need for the Maxwell rule when Temperleys model in the pressure ensemble is studied. (AIP)
Remarks on the modified moment method and irreversible thermodynamics
Authors: Chen M. , Eu Byung Chan
Published: February 2003

A solution of the Boltzmann equation for irreversible thermodynamic processes is discussed. Some remarks are made on the modified moment method of the solution of Boltzmann equations. (AIP)
Comment on ‘‘A simple equilibrium statistical mechanical theory of dense hard-sphere fluid mixtures’’
Authors: Speedy Robin J.
Published: February 2003

Some comments are presented on the paper by Andrews and Ellerly1 on the hard-sphere model of fluid mixtures. (AIP)
Vibrational potential function and structure of nitrosyl hypofluorite by normal coordinate analysis
Authors: Sorenson Sterling A. , Noble Paul N.
Published: February 2003

While the nitrosyl hypofluorite molecule has previously been identified as a product in the reaction between fluorine and nitrogen dioxide in matrix isolation experiments, the bonding characteristics of the molecule were not investigated, and assignments of four infrared absorption bands were not clearly established. Normal coordinate analysis has now allowed determination of the bonding characteristics and definite assignment of the absorption bands. The fluorine atom in ONOF is loosely bonded to one oxygen of an NO2 radical. The central NO bond is strong, with a bond order of about 1.5 while the terminal NO bond is very strong (bond order about 2.0). The ONO bond angle is intermediate between those of NO2 and NO2+. The absorption band at 1716 cm−1 has been assigned to terminal NO bond stretch, that at 1200 cm−1 to central NO bond stretch, and that at 412 cm−1 to OF bond stretch. The band at 702 cm−1 is due to bending of the ONO group. Torsional and NOF bending frequencies are too low to be observed with conventional infrared spectrophotometers.
Theoretical study of the spin-orbit coupling constants of the c(2p)3Πu, d(3p)3Πu, k(4p)3Πu, i(3d)3Πg, r(4d)3Πg, j(3d)3Δg, and (4f)3Δu states of H2
Authors: Langhoff Stephen R. , Huo Winifred M. , Partridge Harry , Bauschlicher Charles W.
Published: February 2003

Theoretical spin-orbit coupling constants as a function of internuclear distance A(R), are reported for the c(2p)3Πu, d(3p)3Πu, k(4p)3Πu, i(3d)3Πg, r(4d)3Πg, j(3d)3Δg, and (4f)3Δu states of H2. Full configuration-interaction wave functions and large Slater-type basis (STO) sets were used. A 14σ14π8δ2φ basis set optimized for the c3Πu state was used in all 3Π state calculations and a 14σ12π8δ2φ basis set optimized for the j 3Δg state was used in the 3Δ calculations. The A(R) were vibrationally averaged using our theoretical potentials. In addition, we used the more accurate c3Πu and i3Πg theoretical potentials of Kolos and Rychlewski [J. Mol. Spectrosc. 66, 428 (1977)], and for the d3Πu state a potential derived from the experimental data of Dieke [J. Mol. Spectrosc. 2, 494 (1958)]. The resulting theoretical Av values (MHz) for the v = 0, N = 1 rovibrational level compare with the experimental values (in parenthesis) as follows: c(2p)3Πu: −3887.22(−3740.987); d(3p)3Πu: −863.85 (−814.5); k(4p)3Πu: −398.8 (−306.7); i(3d)3Πg: −144.59 (−146); j(3d)3Δg: −400.82 (−409). The corresponding theoretical values for the r(4d)3Πg and (4f)3Δu states, for which no experimental data exist, are −47.1 and −114.78 MHz, respectively. These values, as well as the calculated Av for many of the higher vibrational levels, should be useful in future experimental work.
Density functional theory: Non-Born–Oppenheimer Legendre transforms and Maxwell relations, equilibrium and stability conditions
Authors: Nalewajski Roman F. , Capitani Joseph F.
Published: February 2003

The Legendre-transformed representations of the non-Born–Oppenheimer (NBO) density functional theory are analyzed and the corresponding Maxwell relations are derived. These relations exhibit various couplings between parameters of the NBO ground-state energy hypersurfaces: ε[{Ni, Zi, Mi}] and εφ[{Ni, Zi, Mi},φ], where Ni, Zi, and Mi denote, respectively, the number of particles i, their charge and mass, while φ stands for an external electric field. The criteria for intrinsic equilibrium and stability of molecular systems are formulated and discussed within both the BO and NBO approximations. The physical content of stability criteria is interpreted in terms of the Le Châtelier and the Le Châtelier–Braun principles. The classical nature of these criteria is revealed through the introduction of internal partial scalar pressures of the system components (groups of identical particles) within the local formulation of the theory. It is then shown that the criteria of equilibrium and stability in isolated molecular systems become the classical criteria of the ’’mechanical’’–electrostatic equilibrium and stability.
Intermolecular transfer and quenching of electronic excitation energy in fluid solutions: Interpretation of experimental data with a diffusion model including distance dependent interaction
Authors: Heisel F. , Miehe J. A.
Published: February 2003

The efficiency for the transfer of the electronic excitation between moving molecules is analyzed by means of a diffusion equation taking into account long and/or short range interactions. The analysis of experimental data concerning donor–acceptor systems with large Förster radius in viscous and fluid solvents demonstrates that (i) in the low diffusion regime, energy transfer takes place by the resonant dipole–dipole mechanism; (ii) for high diffusivities, the efficient interaction is not that of the dipole–dipole type, and exchange interactions are responsible for the energy transfer. Experimental efficiencies of fluorescence quenching by an intermolecular charge transfer mechanism are interpreted in view of the diffusion model including a specular boundary condition and an exponential sink term of short range. It is apparent that the diffusion tends to increase the effective quenching rate for strongly coupled interacting donor–inhibitor species to a greater extent than for weakly coupled ones. Effective reaction radii larger than those predicted by diffusion-controlled reactions are interpreted.
Orientational order in the nematogenic homologous series t
r
a
n
s-4-alkyl (4-cyanophenyl)-cyclohexane
Authors: Abdoh M. M. M. , Srinivasa  , Shivaprakash N. C. , Prasad J. Shashidhara
Published: February 2003

The temperature variation of refractive indices (ne,no), birefringence (Δn), density (ρ), and order parameter of the homologous compounds trans-4-alkyl (4-cyanophenyl)-cyclohexane are reported in the nematic and isotropic phases. The principal molecular polarizabilities (αe,αo) and the order parameters S have been evaluated using the anisotropic internal field model (Neugebauer’s approach). The theoretical estimations of the mean and principal molecular polarizabilities made by two different approaches, viz., the additivity rule of bond polarizability and the modified Lippincott-δ-function potential model are in close agreement with the values evaluated from the experimental studies.
Rotational tunneling in CH4 under pressure (CH4 III)
Authors: Prager M. , Press W. , Heidemann A. , Vettier C.
Published: February 2003

High resolution neutron inelastic scattering techniques were used to measure tunneling spectra of methane (CH4) in its phase III. By increasing the pressure from 0.6 to 1.8 kbar the tunneling lines shift by about a factor of 1.5. The rather complicated spectra can be equally well described by two simple structural models. Both contain two inequivalent molecular sites of low symmetry. They already have provided a rather good fit of the spectrum of CD4 III. An exponent n = 8±2 of an effective intermolecular potential V∼r−n is obtained by analyzing the pressure dependence of the tunneling frequencies.
Interfacial density profiles of a binary van der Waals fluid
Authors: Sullivan Donald E.
Published: February 2003

The van der Waals theory of the interface between coexisting fluid phases of a binary mixture is studied. The theory can be solved analytically on making certain assumptions about the nature of the intermolecular attractive forces, most importantly, the assumption that the cross interaction parameter a12 appearing in van der Waals’ equation of state satisfies the geometric-mean combining rule. In this case, it is found that the interfacial tensions between pairs of phases coexisting under three-phase equilibrium conditions satisfy Antonow’s rule. The theory is not, however, a ’’one-density’’ theory in the sense of Widom, and consequently the interfacial density profile of the more volatile component can be nonmonotonic. At three-phase equilibrium, the location and magnitude of the nonmonotonicity is related to the proximity of two of the phases to their upper critical end point (UCEP). Which pair of phases becomes critical at the UCEP is in turn related to the type of fluid–fluid equilibrium exhibited by the system, according to the classification scheme of van Konynenburg and Scott.
Infrared spectra of silver atom/cluster complexes with pyridine and their relationship to the surface enhanced Raman effect
Authors: Tevault D. E. , Smardzewski R. R.
Published: February 2003

Argon matrix reactions between Ag atoms or very small Ag clusters and pyridine molecules have been observed by infrared absorption spectroscopy. Pyridine-d5 experiments indicate that the reaction product contains a pyridine molecule whose vibrations are slightly altered by the metal atom. Analogous Cu atom matrix reactions with pyridine produced larger frequency shifts than the Ag atom reactions. Increased silver atom concentrations produced additional new absorptions which are attributed to the reaction products of very small Ag clusters with pyridine. These latter frequencies approached those of pyridine on rough metal surfaces observed in surface enhanced Raman spectroscopy (SERS). The vibrational similarities between the matrix infrared spectra of the Ag–pyridine and Agx–pyridine molecules and the SERS spectra of pyridine on various metal surfaces are discussed.
Symmetry of the europium hexachloride complex in ethanolic solutions: An MCD investigation
Authors: Görller-Walrand C. , De Moitié-Neyt N. , Beyens Y. , Bünzli J. C.
Published: February 2003

Various spectroscopic data indicate that the symmetry of hexa(chloro) europiate(III) ion in ethanolic solutions is not quite octahedral. An MCD study is presented in which a formalism is developed for randomly oriented molecules, in the case of z absorption, in order to assess this symmetry. The MCD spectrum of the 5D2←7F0 transition displays two ’’B’’ terms of the same intensity, but of opposite sign. A detailed analysis of these features leads to the conclusion that the symmetry of the EuCl3−6 ion is D2.
The infrared laser photoisomerization of HONO in solid N2 and Ar
Authors: McDonald Paul A. , Shirk James S.
Published: February 2003

The IR induced photoisomerization of HONO in solid N2 and Ar is a single photon process. The isomerization is not first order but follows a rate law (d [HONO]/dt) = (kc/π r2l)[I0−I][HONO]. Extensive energy migration occurs in these solids. The reaction occurs at sites where the isomerization is least hindered by the cage. There is evidence for the importance of direct coupling of the excited mode and the reaction coordinate.
Low temperature free energy relations for electron transfer reactions
Authors: Webman Itzhak , Kestner Neil R.
Published: February 2003

Using the multiphonon theory of electron transfer reactions we present calculations and simple expressions for the rate of electron transfer between molecules in a medium as a function of the free energy changes, especially in the exothermic region. We include effects of the medium modes as well as higher frequency modes which are allowed to change during the reaction. We find that there are activated as well as nonactivated regions depending on the free energy change and the temperature. The rate in the so called ’’anomalous’’ (or nonactivated) region decreases much more slowly with free energy than it does in the activated region. In fact, the logorithm of the rate depends roughly linearly on the free energy change in that region. Fortunately the rate is most dependent on the frequency of the high energy modes and less on the coupling strength about which we know the least. In addition we have studied uses of frequency averaging to reproduce the results of a multimode case.
Rotational motion in plastic neopentane by Raman spectroscopy
Authors: Depondt Ph. , Debeau M. , Pick R. M.
Published: February 2003

High and low frequency Raman scattering experiments have been performed in the whole temperature range of plastic neopentane (140–257 K). High frequency spectra are well accounted for by Larsson’s isotropic two-step stochastic model when the vibrational contribution is not neglected. The discussion shows that the results are the best for the spectral part where the isotropic aspect of the model is less important. The low frequency spectra are explained by a transient dielectric anisotropy of the molecules induced by their deformation as they overcome a high potential barrier during reorientation movements. The analysis of both spectra provide consistent results for the temperature dependence of the fraction of molecules performing large amplitude rotations.
Bond polar parameters from integrated infrared intensities in ethylene, allene, and benzene
Authors: Galabov Boris , Lozanova Christina
Published: February 2003

The integrated band intensities in the infrared spectra of a series of sp2-hybridized hydrocarbons— ethylene, allene, and benzene— are interpreted in terms of C–H bond polar parameters using a recently proposed parametric model of IR intensities. The respective C–H bond polar parameters have identical signs in the entire series of molecules. Their magnitude varies within acceptable limits with more distinct differences for some parameters. These differences are attributed to the varying electronic structure of the C–H bonds in the three molecules, reflected in the intensities of the corresponding vibrational bands.
The far-IR spectra of kappa-casein
Authors: Varaprasad A. M.
Published: February 2003

The far-IR spectrum of kappa-casein, a giant molecular diode, has been obtained in the region 50–250 cm−1 and its electronic conduction properties have been determined. The conductivity is found to be four orders of magnitude higher in the ℱ-helix form than in a globule.
Effects of structural disorder on two-particle exciton–phonon excitations in organic materials
Authors: Klafter Joseph , Jortner Joshua
Published: February 2003

In this paper we explore the effects of diagonal structural disorder on the optical absorption line shapes for two-particle exciton–vibron bound states and for exciton–vibron continua. ’’Local-type’’ two-particle bound states are drastically broadened by disorder scattering, while ’’band-type’’ two-particle continua are only slightly affected by disorder scattering. These results are utilized as a diagnostic tool to distinguish between local-type and band-type many-particle excitations, establishing that the two-particle singlet exciton–vibron excitation in crystalline and in amorphous films of tetracene and pentacene corresponds to a two-particle continuum.
Effects of collisional relaxation on spectral functions in the quasiperiodic and chaotic regimes
Authors: Koszykowski M. L. , Liu Wing-Ki , Noid D. W.
Published: February 2003

A line shape formalism is derived classically which considers molecules undergoing both collisional and intramolecular relaxation. Results for a model system demonstrate a significant effect on the line shapes when chaos sets in and the intramolecular relaxation becomes important. An experiment on polyatomic molecules is proposed and should isolate the two effects.
Energy transfer in LaF3: R3+, Pr3+ (where R = Nd, Dy)
Authors: Reddy B. R. , Venkateswarlu Putcha
Published: February 2003

Fluorescence is observed for 1D2 levels of Pr3+ on exciting the higher lying level 3P0 in doubly doped systems LaF3: R3+, Pr3+ (R = Nd or Dy) but not in LaF3:Pr3+. From the recorded excitation spectra, and the measured decay times, it has been found that the drain mechanism of population to 1D2 levels is caused in doubly doped systems by ion-pair relaxation between Pr3+ (3P0–1D2) and Nd3+ (Z-X) or Dy3+ (Z-Y) ions.
Effect of pulse intensity distributions on fragment internal energy in the infrared multiphoton dissociation of vinyl cyanide
Authors: McKillop John S. , Gordon Robert J. , Zare Richard N.
Published: February 2003

A plasma shutter has been used to remove the less intense tail of a CO2 laser pulse without otherwise altering the pulse characteristics. It is found that variations in the pulse intensity distribution control the rotational distribution of CN fragments formed in the IR photolysis of vinyl cyanide H2C = CHCN.
The mechanism for photofragmentation of H2S revealed by multiphoton ionization photoelectron spectroscopy
Authors: Achiba Yohji , Sato Kenji , Shobatake Kosuke , Kimura Katsumi
Published: February 2003

Multiphoton ionization photoelectron measurements for H2S were carried out at several laser wavelengths in the 422–475 nm region, in order to obtain a direct evidence for the mechanism of ionic fragmentation which takes place by resonantly enhanced multiphoton ionization (REMPI). An ion current spectrum of multiphoton ioniztion was also measured for H2S in this wavelength region, indicating that the ionization takes place via three-photon resonant Rydberg states. From photoelectron spectra obtained here, it has been found that the main peaks are attributed to the H2S+ ion in the ground state with v = 0. Other photoelectron bands due to v = 1 have also been observed together with some satellite bands. It should be mentioned that no photoelectron bands above 1.3 eV have been found. These experimental evidences directly support the parent ion fragmentation mechanism that the formation of HS+ and S+ ions mainly results from the ground state H2S+ ion with v = 0 and v = 1, respectively, by additional photon absorption. This conclusion differs from that recently suggested by Carney and Baer from their mass-resolved REMPI experiments.
Proton-polarization transfer enhancement of a heteronuclear spin multiplet with preservation of phase coherency and relative component intensities
Authors: Pegg David T. , Doddrell David M. , Bendall M. Robin
Published: February 2003

The DEPT pulse sequence (π/2)(H,y)−(2J)−1−π(H), (π/2)(C,x)−(2J)−1 −ϑ(H,x)π(C)−(2J)−1−(acquire 13C) is analyzed theoretically for a variable ϑ pulse for three spin systems: CH, CH2, and CH3. It is shown that the pulse train produces an enhanced distortion-free 13C signal which has the following characteristics: (a) there is phase coherency within and between the components of the 13C multiplets; (b) the enhancements vary with ϑ as (γH/γC)sin ϑ for CH, (γH/γC)sin 2ϑ for CH2, and (3γH/4γC) (sin ϑ+sin 3ϑ) for CH3. Experimental evidence is provided for these predictions. An important application of the DEPT pulse train is for the generation of both individual proton-coupled and proton-decoupled 13C methine (CH), methylene (CH2), and methyl (CH3) subspectra. This can be readily achieved by forming suitable combinations of DEPT spectra determined at ϑ = (π/4), (π/2), and (3π/4). Such spectral editing is less sensitive to variations in J values than the INEPT pulse sequence. Signal enhancement for 195Pt and 29Si NMR signals are also demonstrated using the DEPT sequence. The only disadvantage of this pulse train compared with the INEPT sequence appears to be its greater sensitivity to spin relaxation, a consequence of its time span.
Structure of an organic charge-transfer salt derived from dibenzotetrathiafulvalene and tetrafluorotetracyanoquinodimethane (DBTTF–TCNQF4). Observation of a high-temperature phase transition
Authors: Emge Thomas J. , Bryden Wayne A. , Wiygul F. Mitchell , Cowan Dwaine O. , Kistenmacher Thomas J. , Bloch Aaron N.
Published: February 2003

The crystal structure of the organic charge-transfer salt DBTTF–TCNQF4 is reported. The structure has been determined by single-crystal x-ray diffraction techniques and has been shown to consist of segregated stacks of donor and acceptor molecules. Within both the donor and acceptor stacks, significant dimerization is observed at room temperature. From the geometries of the DBTTF and TCNQF4 molecules, it is concluded that the charge transfer is complete. The electrostatic contribution to the crystal cohesion for DBTTF–TCNQF4 has been calculated and compares well to those for similar salts of unit charge transfer. It is suggested that the room-temperature structure may well be representative of the low-temperature phase of a system affected by a Peierls instability. Based on diffraction data, a phase transition near 390 K is reported. The driving force for the transition is likely a spin–phonon instability. Crystal data for DBTTF–TCNQF4 are triclinic, space group P1̄; a = 13.159(3) Å, b = 13.703(4) Å, c = 7.533(2) Å, α = 102.89(2)°, β = 105.98(2)°, γ = 63.47(2)°, V = 1160 Å3; with a final R value for the refinement at 298 K of 0.063 based on the 2797 data with F0≳3σ(Fo).
Interparticle velocity correlations in simple liquids
Authors: Balucani U. , Vallauri R. , Murthy C. S.
Published: February 2003

The momentum transfer of a test particle to a cluster of atoms initially in the first shell of neighbors is investigated at liquid densities by molecular dynamics methods. The comparison of the results obtained for Lennard-Jones and soft spheres models is discussed. In the short time regime the interpretation of the data is shown to parallel that of the single particle velocity autocorrelation function. At longer times, a simple physical model can account for the decay of the cross correlation in terms of the increasing number of atoms involved in the process.
NMR relaxation study of water molecules in concentrated zinc chloride solutions
Authors: Nakamura Yoshio , Shimokawa Shigezo , Futamata Katsuya , Shimoji Mitsuo
Published: February 2003

The proton spin-lattice relaxation time and self-diffusion coefficient of water in concentrated aqueous ZnCl2 solutions have been measured as a function of salt concentration and temperatures. The intermolecular and intramolecular contributions of the dipolar interactions to the relaxation rate were determined separately with the aid of the self-diffusion data. The present data indicate that the translational motion of water molecules in ZnCl2 solutions is retarded more significantly than their rotational motion with decreasing water content.
Time scale and product energy for the IRMPD of CF2HCl at steady state
Authors: Stephenson John C.
Published: February 2003

The energy yield is studied for the CO2 laser induced reaction CF2HCl→CF2+HCl. (AIP)
EPR of tetradentate ligand coordinated ethylenebisguanidine copper(II) chloride monohydrate
Authors: Sthanapati J. , Pal A. K.
Published: February 2003

The Jahn–Teller resistant character in ethylenebisguanidine copper (II) chloride monohydrate has been revealed x-band EPR studies in the temperature range 77–400 K.
Comments on Ascarelli’s papers on electron trapping in liquid Ar, Kr, and Xe
Authors: Shibamura E. , Takahashi T. , Kubota S. , Doke T. , Mozumder A.
Published: February 2003

The criticisons of Ascarelli1,2 regarding Shibamura et al5’s measurements of electron drift velocity in liquid Ar, Kr and Xe are answered. (AIP)
Reply to the previous comments by Shibamura e
t a
l.
Authors: Ascarelli G.
Published: February 2003

It is pointed out that in the case of high fields the Townsend method discussed in the previous letter does not give a direct measure of the averege carrier because the electron distribution is anisotropic. An estimate of the rms depth of traps originating from density fluctuations is given based on the recent calculation of the electron mobility by Basak and Cohen. This value is in excellent agreement with those of the present author based on a completely different type of reasoning.
Triplet exciton dynamics: Exciton phonon scattering at low temperature
Authors: Jackson Bret , Silbey R.
Published: February 2003

In this paper, we calculate the k→k′ scattering rates of excitons by acoustic phonons. We consider two-phonon processes only, having considered one-phonon processes in an earlier paper. We treat one-phonon events and a single two-phonon event in both pure and impure crystals. We apply these results to the recent experimental work on tetrachlorobenzene and discuss the dependence of the rates on energy mismatch.
Spectroscopy and kinetics of the optically excited mercury–thallium excimer
Authors: Pfaff J. , Stock M.
Published: February 2003

Pulsed laser excitation at 380 nm is used to study the kinetics of the B 2Σ1/2 state of HgTl. Results are obtained qualitatively by time-resolved spectroscopy of the bands around 459 and 656 nm and quantitatively for the following processes by decay time measurements: (i) three-body association, Tl*+2Hg→HgTl*+Hg, k3 = 1×10−31 cm6 s−1; (ii) spontaneous decay of 7 2S1/2 state of Tl, τ0 = 7.4 ns; (iii) decay of high-lying vibrational levels of the B state: spontaneous decay, τ0 = 9.1 ns, collisional relaxation and dissociation, k = 2.9×10−10 cm3 s−1; (iv) decay of low-lying vibrational levels of the B state: spontaneous decay, τ0 = 38.7 ns, collisional deactivation cross section σ≊2 Å2; (v) energy transfer from metastable Hg*, Hg*2 to Tl*, HgTl*, cross section σ = 165 Å2. Cw laser excitation is used to study the spectroscopy by analysis of the temperature dependence of the band shapes. The following dissociation energies are determined: B 2Σ1/2: 4400 cm−1, X 3/2: 2750 cm−1 and X 1/2: 800 cm−1. A gain measurement is performed, pumping the B state with a pulsed laser at 380 nm with 10 mJ pulse energy and probing in the fluorescence bands using a dye laser. Absorption is measured for all wavelengths of the bands.
Thermal desorption of argon and neon from solid xenon. I. Transition state theory rate constants
Authors: Adams John E. , Doll Jimmie D.
Published: February 2003

Rate constants for thermal desorption of physisorbed argon and neon from a xenon(111) crystal face have been calculated using a generalization of Slater’s unimolecular reaction rate theory, with a particular emphasis being placed on an investigation of the dependence of these rates upon the degree of surface coverage and upon the lattice motion of the solid. For argon our Monte Carlo results show a non-negligible coupling between adatom dynamics and the thermal vibration of the crystal surface which leads roughly to a 20% enhancement of the desorption rate. The increase is, however, only half as great for neon, with a static-solid description becoming appropriate as the system temperature is decreased. The adatom concentration effect on the kinetics also appears to be more dramatic for adsorbed argon than for neon, presumably as a consequence of the longer range and greater strength of the argon–argon potential.
Kinetics of the Mg(3P,1P)+N2O reaction
Authors: Bourguignon B. , Rostas J. , Taieb G.
Published: February 2003

The reaction of Mg atoms with N2O has been studied in steady-state conditions, by the laser induced fluorescence technique and the chemiluminescence spectrum analysis. Mg atoms are produced in a Broida-type apparatus. Experimental evidence is presented which shows that both Mg(3s3p3P) and Mg(3s3p1P) excited states reacts with N2O whereas the ground Mg(3s2 1S) state does not. Mg(3s3p3P) atoms produced in the oven are also responsible for the formation at Mg(3s3p1P) in the afterglow, through triplet–triplet annihilation. Chemiluminescence is partly due to the direct reaction of Mg 1P with N2O and partly due to secondary reactions. The kinetic behavior of reactants and products is studied and modeled. Absolute cross sections for quenching of Mg(3s3p3P) and Mg(3s3p1P) by N2O, and for the state-to-state reactions of Mg(3P)+Mg(3P)→Mg(1P)+Mg(1S) and Mg(1P)+N2O→MgO(B 1Σ+)+N2 have been measured. Rotational and vibrational temperatures of the B 1Σ+ state of MgO have been calculated via simulation of the chemiluminescence spectra.
Upper bounds to the impact parameter and cross section for atom–diatom exchange reactions
Authors: Chesnavich Walter J.
Published: February 2003

We establish upper bounds to the impact parameter and cross section for any A+BC exchange reaction in which the reagents are in a given initial state. The approach we take is to use the centrifugal forces generated in a collision to place a bound on the values of the total angular momentum for which reaction can occur. The bounds on the impact parameter and cross section then follow directly from the restrictions imposed by energy and angular momentum conservation. Our approach is related to theories based on the properties of periodic trajectories in that the system configuration which determines the angular momentum bound is also that of a quasibound ABC rigid rotor periodic trajectory. The equation which defines the configuration of this trajectory is similar in form to a generating function recently derived by Child and Pollak. Furthermore, an analysis of the symmetric stretch periodic trajectories in the H+H2 reaction suggests that the rigid rotor trajectory is the maximum angular momentum member of a family of periodic trajectories which exist at energies below and above the dissociation threshold. Our approach is also related to variational transition state theory. However, rather than vary the location of a diving surface, we keep the surface fixed in the reagents’ region of the system phase space and vary instead its boundary. We compare the bounds we place on the impact parameter and cross section to the quasiclassical trajectory data of Karplus, Porter, and Sharma for the H+H2 exchange reaction and to that of Persky for the reactions of Cl with H2, D2, and HD. The cross section ratios show a near-linear dependence on the fraction of the total system energy which is partitioned initially into relative translational energy of the reagents, whereas the impact parameter ratios smoothly increase from ∼0.2 near threshold to a maximum of ∼0.9.
Algebraic approach to molecular rotation-vibration spectra. I. Diatomic molecules
Authors: Iachello F. , Levine R. D.
Published: February 2003

An algebraic approach to molecular rotation-vibration spectra, similar to that used in the analysis of nuclear rotation-vibration spectra, is suggested. The spectrum generating algebra appropriate to diatomic molecules U(4) is constructed. Its two dynamical symmetries O(4) and U(3) are discussed and their relation to rigid and nonrigid structures is investigated. It is shown how other properties, such as intensities of dipole radiation and state-to-state excitation probabilities, can be evaluated.
Stable methods for achieving MCSCF convergence
Authors: Camp R. Nicholas , King Harry F.
Published: February 2003

A composite strategy is presented for systematically achieving convergence of complete active space MCSCF calculations. The method efficiently combines first- and second-order steps. Three aspects are discussed. (1) An approximate matrix of second derivatives with respect to orbital rotations is projected onto a Krylov subspace and diagonalized. If there are small or negative eigenvalues, then a crude search is performed along the damped, steepest descent path (the curved direction of water flowing downhill). One or more of these ’’opening game’’ moves overcomes problems associated with poor starting orbitals. (2) For the final approach to convergence, a preconditioned conjugate gradient (cg) search is performed in which the approximate second derivative matrix plays the role of the preconditioning transformation. The cg algorithm corrects for the neglect of CI mixing and any other approximations introduced into the second derivative matrix. (3) Localized and associated oscillator orbitals of Foster and Boys provide reasonable core and active space starting orbitals generated from a preliminary closed-shell SCF calculation. Explicit formulas for all derivatives are given. Tables of computational effort for a wide variety of complete active space calculations are presented.
High-spin hydrocarbons
Authors: Klein D. J. , Nelin C. J. , Alexander S. , Matsen F. A.
Published: February 2003

The existence of a class of (very) high-spin hydrocarbons is predicted on the basis of (a) new theorems for valence-bond models, (b) classical structure theory, (c) standard theorems for molecular orbital theory, (d) several small-system calculations with Hubbard and PPP Hamiltonians, and (e) a large-system cluster expansion calculation applied to the polyallyl high-spin candidates. Some properties, especially the magnetic ones, for these conjugated alternants are briefly discussed.
CI calculations on metallic and molecular hydrogen rings
Authors: Seel M. , Bagus P. S. , Ladik J.
Published: February 2003

To evaluate the importance of correlation effects in metallic and insulating polymers self-consistent field (SCF) and configuration interaction (CI) calculations were performed for equidistant (metallic) and alternant (insulating) hydrogen rings. Two main effects–increasing the number of electrons and changing the distance between the hydrogen atoms–were studied. Minimal basis set calculations were performed for 6-, 10-, 14-, 22-, and 30-membered rings, potential curves for the metallic and insulating H14 system have been computed with a double-zeta basis supplemented by polarization functions. The correlation energy of one-dimensional metallic hydrogen is estimated to be about 0.84 eV/atom compared with 0.54 eV in H2. For molecular hydrogen a van der Waals minimum was found for separation of two H2 molecules in the ring of about 5.5 a.u. The calculated value for the energy minimum suggests nonpairwise-additive contributions to the van der Waals interaction energy of seven H2 molecules. Finally, the pressure to induce a transition from the molecular to the metallic phase was calculated to be about 1.8 Mbar both on the SCF and CI level.
A discontinuous energy–density functional
Authors: Levy Mel , Tal Yoram , Clement Stephen C.
Published: February 2003

The properties of the integrand in the Foldy–Wilson formula E(Z̄) = −∫0Z̄ R(Z)dZ are studied. Here, E(Z̄) denotes the total, nonrelativistic, electronic energy of an atom with nuclear charge Z̄, and R(Z) = ∫dr r−1ρ(r,Z), where ρ(r,Z) is the electron density corresponding to nuclear charge Z. From studies of two-electron atoms and an analysis of the Z−1 perturbation expansion, it is postulated that, in small atoms at least, R(Z) generally resembles a series of discontinuous straight lines in the range 0⩽Z⩽Z̄, at constant number of electron N. The discontinuities in R(Z) occur at critical points in Z, which are close to intergral values. Each critical point corresponds to the ejection of an electron from the confines of the nucleus. For two-electron atoms, the discontinuity is compatible with Stillinger’s observation of a two-electron localized bound state in the continuum at Z<1. The properties of R(Z) are then used to construct equations to generate total energies that incorporate electron–electron correlation from Hartree–Fock densities by means of ’’trapezoid recursion relations.’’ Use is made of sums of the R(Z) and sums of chemical potentials (electronegativities) as defined by Parr et al. Near-exact total nonrelativistic energies are obtained for all neutral atoms through Ar.
Resonance of electronic and electron–exciton complex modes in molecular crystals
Authors: Bader G. , Caron L. G.
Published: February 2003

New modes are predicted to exist in molecular crystals. These are the result of a resonance coupling between an electron–exciton complex, bound by the additional polarizability of an excited molecule (Frenkel exciton), and a free conduction band electron. The dispersion relation for the complexlike branch can show a minimum at nonzero momentum. A case is made for benzene. These modes, however, are extremely fragile with respect to electron–phonon interactions. They are expected to be stable only for times short in comparison to their thermal lifetime. This restricts their detection to experiments utilizing fast nonequilibrium probes, such as electron injection in thin films.
Distribution of charged particles near a charged hard wall in a uniform background: Comparison with exact results
Authors: Totsuji Hiroo
Published: February 2003

The results of a simple approximation for the short-range part of the direct correlation function are compared and shown to be in good agreement with exact values for the two-dimensional one-component Coulomb system, when the wall charges are of the same sign as particles. (AIP)
Remark on the use of nonsecond-quantized radiation
Authors: Davis Jon P.
Published: February 2003

A brief discussion is presented of a case of chemical interest where it appears that quantized and unquantized radiation fields give extremely different results in calculations of interactions between matter and radiation. (AIP)
Elastic scattering of electrons from SO2
Authors: Orient O. J. , Iga I. , Srivastava S. K.
Published: February 2003

Using a crossed electron beam–molecular beam scattering geometry and the relative-flow technique, ratios of elastic differential cross sections of SO2 to those of He have been measured at electron impact energies of 12, 20, 50, 100, 150, and 200 eV. At each energy, an angular range of 15° to 150° has been covered. These ratios have been multiplied by previously known He elastic differential cross sections to obtain elastic differential cross sections for SO2. From a knowledge of differential cross sections, integral and momentum transfer cross sections have been determined. Using the two-potential theory of e-molecule scattering, calculations were also performed and compared with the measurements.
Inclusion of core-valence correlation effects in pseudopotential calculations. I. Alkali atoms and diatoms
Authors: Jeung G. H. , Malrieu J. P. , Daudey J. P.
Published: February 2003

A simple procedure is developed to include the core-valence polarization and correlation effects in small metallic clusters of alkali atoms. The fluctuation of the electric field on the polarizable core, as calculated from optimized limited valence basis sets, plays the dominant role. The atomic ionization potentials and the ground state spectroscopic characteristics of Na2, K2, Cs2, and of their positive ions are significantly improved. The method is applicable to polar compounds as well, as illustrated through a study of CsH. The relationship between the present procedure and Bardsley’s −αD/2 (r2+d2)2 potentials is discussed.
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o molecular orbital study of the first two bands in the photoelectron spectrum of hydroxylamine (NH2OH)
Authors: Curtiss L. A.
Published: February 2003

The geometries and energies of the cationic states of NH2OH corresponding to electron loss from the nitrogen lone pair and from the oxygen lone pair have been investigated with an extended basis set. The results confirm previously assigned orbital characters for the first two bands in the photoelectron spectrum of NH2OH, help to explain the width and vibrational structure of the bands, and provide information on the symmetries of the ionic states.