bluuebunny/crossref_metadata_2025 huggingface dataset:
Reduced Hamiltonian of a Near-Symmetric Rotor
Authors: Choi Jong H. , Birss Fraser W.
Published: February 2004

The relation between a reduced Hamiltonian suitable for fitting the observed spectra and the level-shift transformation is discussed. A reduced Hamiltonian of an approximately symmetric rotor molecule in the absence of Coriolis interaction is obtained. Three types of terms are discussed: (i) those diagonal with respect to the zero-order Hamiltonian, (ii) those which have both diagonal and off-diagonal constituents and (iii) those which are purely off-diagonal. It is shown that in the jth order, j ≥ 2, j + 1 of first type, 2j—1 of the second type and none of the third type remain in a reduced Hamiltonian.
Zeeman Effect, Magnetic Properties, and the Electric Moments of Diketene and β-Propiolactone
Authors: Tigelaar H. L. , Gierke T. D. , Flygare W. H.
Published: February 2004

The molecular Zeeman effect is reported for diketene and β-propiolactone at fields near 20 000 G. The results for diketene are gaa=− 0.1091 ± 0.0004, gbb=− 0.0324 ± 0.0004, gcc=− 0.0169 ± 0.0004, 2χ aa −χ bb −χ cc=2.3 ± 0.9 × 10−6 erg/G2 · mole, 2χ bb −χ aa −χ cc=12.1 ± 0.4 × 10−6 erg/G2 · mole, Qaa − 2.5 ± 0.9 × 10−26 esu · cm2, Qbb=1.5 ± 0.8 × 10−26 esu · cm2, and Qcc=1.0 ± 1.3 × 10−26 esu · cm2. The results for β-propiolactone are gaa=− 0.0758 ± 0.0005, gbb=− 0.0356 ± 0.0004, gcc=− 0.0319 ± 0.0004, 2χ aa −χ bb −χ cc=− 0.5 ± 0.3 × 10−6 erg/G2 · mole, 2χ bb −χ aa −χ cc=1.7 ± 0.4 × 10−6 erg/G2 · mole, Qaa=− 3.1 ± 0.4 × 10−26 esu · cm2, Qbb=− 1.9 ± 0.5 × 10−26 esu · cm2, and Qcc=5.0 × 10−26 esu · cm2. The out-of-plane minus average in-plane magnetic-susceptibility anisotropies in four-membered rings show larger paramagnetism than predicted on the basis of localized susceptibilities. This increased paramagnetism is probably due to ring strain in the four-membered rings.
Exact Frequency Distribution Function for the Honeycomb Lattice
Authors: Huckaby Dale A.
Published: February 2004

The frequency distribution function for the honeycomb lattice with first-neighbor harmonic interactions is obtained in closed form in terms of the complete elliptic integral of the second kind.
Molecular Dynamics: The Dissociation of H2 by He
Authors: Brown Nancy Jeanne , Munn Robert J.
Published: February 2004

The dissociation of molecular hydrogen by helium was studied at 5500°K using the quasiclassical approximation. The temperature-dependent reaction cross sections were very dependent on the initial quantum states of the molecule. A rate constant was estimated for the dissociation reaction, and agreement between theory and experiment was satisfactory.
Computation of Dense Random Packings of Hard Spheres
Authors: Adams D. J. , Matheson A. J.
Published: February 2004

A computer based method of generating a random close packing of hard spheres is described. The largest assembly that has been produced contains 5402 spheres. The packing density is approximately 62.8% for large assemblies, though the density falls slightly as the size of the assembly increases. The pair distribution has been determined for a spherical assembly of 3900 spheres. The computed assembly is compared with the ball-bearing assembly of Scott which is about 0.8% more dense. The computer method gives the sphere positions with great precision so that the first peak of the pair distribution is much sharper for the computed assembly than for the ball-bearing assembly. The split in the second peak of the pair distribution of the ball-bearing assembly is absent from the computed assemblies.
Kinetics and Mechanism of the Reaction of O(3P) with Carbon Monoxide
Authors: Simonaitis R. , Heicklen Julian
Published: February 2004

The mercury-photosensitized decomposition of N2O [to produce O(3P)] was studied in the presence of mixtures of CO and 2-trifluoromethylpropene (TMP) at 298, 383, and 472°K to determine the relative rate constants for the reaction of O(3P) with the two gases. The absolute value of the rate constant with TMP was found to be 9.1 × 109 exp(−2,200/RT)M−1.sec−1 from a similar competitive study at the same temperatures between TMP and butene-1, whose rate constant with O(3P) is relatively well known. The absolute value for the O(3P)+CO reaction was thus determined, and it was found to depend on pressure. The reaction was intermediate between second and third order. For any pressure, the reaction became more nearly second order as the temperature was raised. Expressions were obtained for the limiting low-and high-pressure rate constants: k0= 5.9 × 109 exp(−4100/RT)M−2.sec−1 with N2O as a chaperone and k∞= 1.6 × 107 exp(−2900/RT)M−1.sec−1. These results were interpreted in terms of the mechanism: O(3P)+CO+M⇄ lim −aaCO2(3B2)+MCO2(3B2)⇄ lim −bbCO2(1B2),CO2(1B2)+M→ lim cCO2(1Σg+)+M,with k0=ka and k∞=kakb/k−a. The results are compared and correlated with the findings of other investigations.
Anomalous Behavior of 18O-Labeled Compounds. VI. The Vibrational Spectra of Isotopic Metaborates
Authors: Pinchas S. , Shamir J.
Published: February 2004

The infrared absorption spectrum of an aqueous solution of 18O-labeled sodium metaborate shows the antisymmetric B–18O stretching ν3 fundamental of its B(OH)4− ions at 982 cm−1, as compared with the frequency of 958 cm−1 observed for the respective normal borate vibration. The Raman spectrum of aqueous solutions of these borates exhibits their symmetric B–O stretching ν1 fundamental in both cases at 744 cm−1. The fact that the ν1 and ν3 frequencies of the hydrated B(18OH)4− ions are not decreased by the 16O–18O exchange is taken to indicate that the hydrogen bonding between the B(OH)4− ions and their solvating water molecules is appreciably lower in the case of the 18O-labeled ions than in the case of the B(16OH)4− ions.
EPR Studies of Ti3+–H+ Centers in X-Irradiated α-Quartz
Authors: Rinneberg H. , Weil J. A.
Published: February 2004

Two titanium centers (A–Ti–H and B–Ti–H) are formed in equal concentrations by reaction of hydrogen atoms with Ti4+ in the α-quartz lattice at 120°K. Above 150°K, B–Ti–H transforms into A–Ti–H. The spin Hamiltonians, including 47,49Ti hyperfine and nuclear quadrupole terms, have been determined by 9.850-GHz EPR at 20°K, for both centers. These data, discussed from a crystal-field viewpoint and compared with results for analogous lithium-compensated Ti3+ centers, lead to a model for the centers involving a substitutional Ti3+ ion with a nearby interstitial proton charge compensator located in either of two different sites.
One-Electron Properties of Ammonia Computed from Near-Hartree—Fock Wavefunctions
Authors: Laws Edward A. , Stevens Richard M. , Lipscomb W. N.
Published: February 2004

One-electron properties of ammonia are computed using a near-Hartree—Fock basis set of Slater orbitals. The properties considered include diamagnetic shielding and susceptibility constants, dipole moment, and nuclear quadrupole coupling constants. The basis set dependence of these properties and the effects of vibrational distortions due to the NH3 inversion motion are investigated. All computed properties are found to lie within 5% of experimental values.
Theory of Depolarized Light Scattering from Molecular Liquids: Existence of the Central Diffuse Line and the Broad Background
Authors: Ailawadi Narinder K.
Published: February 2004

In order to account for the simultaneous existence of the central diffuse (sharp) line and the broad background, the angular momentum theory of Ailawadi, Berne, and Forster is reconsidered. In particular, the polarizability tensor is coupled to the linear and angular momentum for the VH scattering, corresponding to the case when the plane of polarization of the incident light is perpendicular and that of the scattered light parallel to the plane of scattering. It is concluded that this theory cannot give rise to the broad background, thus substantiating a conclusion of Keyes and Kivelson.
Spin Relaxation in Nematic Solvents
Authors: Nordio Pier Luigi , Rigatti Giorgio , Segre Ulderico
Published: February 2004

The electron spin relaxation processes for oblate and prolate symmetric top paramagnetic probes dissolved in liquid crystals are studied by solving the diffusion equation for molecules subjected to an orienting potential, expressed as a series expansion of Legendre polynomials. The line shape variations upon ordering have been calculated for vanadyl complexes and nitroxide radicals, and the effect of the contributions of quadratic and quartic terms of the potential energy function, and of the diffusion tensor anisotropy have been examined.
Raman Scattering Study of the Disorder—Order Phase Transition in NH4Cl
Authors: Wang C. H. , Wright R. B.
Published: February 2004

The Raman scattering of the 93 cm−1 anomalous mode in NH4Cl above and below the λ-transition temperature (Tλ) was studied in detail. The peak and integrated intensities of this mode decrease rapidly to zero below Tλ. In contrast to the temperature behavior of a similar mode in NH4Br, the intensity data does not follow the anomalous specific heat accompanying the λ transition, suggesting that below Tλ the short-range order decays much more rapidly in NH4Cl than in NH4Br. A theory which considers the first order disordering effect (neglecting short-range order) on the Raman scattering was worked out. The result predicts that the temperature reduced integrated intensity has the same temperature dependence as Hc (the additional configurational enthalpy due to the λ transition) provided that the short-range order can be neglected. The result of experimental tests of the theory indicates that while for temperatures below 230°K the agreement is excellent, significant deviations occur between 230 and 250°K, therefore indicating that short-range order must also be included in the quantitative interpretation of the Raman scattering from the anomalous mode.
Microwave Spectrum and Intramolecular Hydrogen Bonding in Trifluoroethylamine
Authors: Warren Ira D. , Wilson E. Bright
Published: February 2004

The microwave spectra of three isotopic species of trifluoroethylamine (CF3CH2NH2, CF3CH2NHD, and CF3CH2ND2) have been investigated to obtain some information about a proposed intramolecular hydrogen bond. The only stable rotamer observed is the trans, in which the two amino protons are believed to be facing two fluorine atoms. In this form there are two potential hydrogen bonds. The NH bond length is 1.01 ± 0.02 Å, the CNH angle is 110° ± 2°, the CCN angle is 113° 50′ ± 1°, and the HNH angle is 110°30′ ± 1°. The dipole-moment components are μa=1.13 ± 0.03 D, μb=0.28 ± 0.03 D, and μc=0.0 ± 0.05 D; these are consistent with bond moment calculations. The H···F-distance is 2.65 ± 0.02 Å, which is 0.10 Å greater than the sum of the van der Waals radii for the two atoms. This seems to rule out any covalent character in the proposed hydrogen bond. Indeed, the NH and CF bonds are nearly parallel, which suggests that the stabilization of the trans form may be due to a dipole—dipole interaction.
Multiple-Perturbation Expansion of Polyatomic Vibrational Potential Functions
Authors: Chang S. Y.
Published: February 2004

A general multiple-perturbation theory is developed for the study of polyatomic vibrational potential functions. Using the quantities λi ≡ 1 − Rie/Ri as perturbation parameters for each pair of atoms at Ri near Ric, the polyatomic vibrational potential function is generated as a function of the several 1 / Ri n. By employing partial confocal elliptic coordinates, the Schrödinger equation is transformed into a form in which the kinetic energy, electronic—nuclear attractions, and nuclear—nuclear repulsions are the perturbations. There results a new approach to the perturbation treatment of polyatomic vibrational force constants. Because of their simple forms the perturbations are much easier to handle this way than in the usual Δ Ri ≡ Ri − Rie expansions, where the perturbations involve first and second derivatives of the Hamiltonian. The formulation covers stretching as well as bending force constants. As a specific example, the application to triatomic molecules is discussed.
Observation of a Singlet—Triplet Transition in Gas Phase Pyridine by Ion and Electron Impact
Authors: Doering John P. , Moore John H.
Published: February 2004

The excited states of pyridine have been investigated in the gas phase at low pressure using the techniques of ion- and electron-impact energy-loss spectroscopy. In addition to known singlet—singlet transitions and optically allowed Rydberg transitions above 4.3 eV, a transition peaking in intensity at 4.1 eV has been observed. This transition is induced by impact of He+ at several kiloelectron volts, but not by proton impact. Using electrons with incident energy of 13.0 eV, the 4.1 eV peak is also observed in the large-angle scattered electron spectrum but not in the small-angle spectrum. These observations confirm the assignment of this energy loss process to a singlet—triplet transition. The onset of the transition to the triplet state is estimated to be 3.8 ± 0.1 eV above the ground state of pyridine.
Application of the BBGKY Hierarchy to the Study of Self-Diffusion in Dense Fluids
Authors: Jordan Peter C. , Greenberg Allan D.
Published: February 2004

We present an internally consistent, iterative solution to the systematically truncated BBGKY hierarchy for a specific transport process, self-diffusion. Our method is an elaboration of one proposed by Stillinger and Suplinskas. We consider the decay of a particular nonequilibrium displacement—a single Fourier component corresponding to self-diffusion. Then, using the hierarchy, we obtain particularly simple forms for the equations which govern the behavior of the nonequilibrium contributions to the low order reduced distribution functions. We show that closure, by means of the physically plausible ``dynamical superposition'' approximation, is consistent with the general form of the equations. We develop truncation procedures which lead to less complicated equations which are also internally consistent. We then develop the complete, formal, systematic solution to the most highly truncated problem which yields an expression for the self-diffusion coefficient. The method we use seems applicable to the untruncated problem as well. We find that a direct momentum expansion of the perturbation functions which describe the nonequilibrium correlations is inconsistent with our form of the closed, truncated hierarchy equations. Instead, in a first approximation, we find that f(1)(Pz, r; t)=exp(−12Pz2)[1+σ (t)exp(−12α Pz2)sinkz]/[(2π)1/2V]describes the one-particle reduced distribution function averaged over the x and y momenta. The parameter α varies with the density. We outline some approaches which seem fruitful for further study.
Statistical Mechanics of Phase Diagrams. I. Inverse Power Potentials and the Close-Packed to Body-Centered Cubic Transition
Authors: Hoover William G. , Young David A. , Grover Richard
Published: February 2004

Most of the softer metals exhibit a low-temperature close-packed phase, an intermediate-temperature body-centered phase, and a high-temperature fluid phase. Here we relate this behavior to that of theoretical model systems in which particles interact with the inverse power potential φ (r)=ε ( σ / r)n. We show that the same three-phase behavior occurs for the models provided that the interparticle repulsion is sufficiently soft (n ≤ 7). For the model systems the phase boundary between the close-packed and the body-centered phases is located using lattice dynamics. The fluid—solid melting line is deduced from Monte Carlo computer experiments.
Exact and Approximate Quantum Mechanical Reaction Probabilities and Rate Constants for the Collinear H + H2 Reaction
Authors: Truhlar Donald G. , Kuppermann Aron
Published: February 2004

We present numerical quantum mechanical scattering calculations for the collinear H+H2 reaction on a realistic potential energy surface with an 0.424 eV (9.8 kcal) potential energy barrier. The reaction probabilities and rate constants are believed to be accurate to within 2% or better. The calculations are used to test the approximate theories of chemical dynamics. The reaction probabilities for ground vibrational state reagents agree well with the vibrationally adiabatic theory for energies below the lowest threshold for vibrational excitation, except when the reaction probability is less than about 0.1. For these low reaction probabilities no simple one-mathematical dimensional theory gives accurate results. These low reaction probabilities occur at low energy and are important for thermal reactions at low temperatures. Thus, transition state theory is very inaccurate at these low temperatures. However, it is accurate within 40% in the higher temperature range 450–1250°K. The reaction probabilities for hot atom collisions of ground vibrational state reagents with translational energies in the range 0.58 to 0.95 eV agree qualitatively with the predictions of the statistical phase space theory. For vibrationally excited reagents the vibrational adiabatic theory is not accurate as for ground vibrational state reagents. The lowest translational energy of vibrationally excited reagents above which statistical behavior manifests itself is less than 1.0 eV.
Self-Diffusion Coefficients and Rotational Correlation Times in Polar Liquids. III. Toluene
Authors: O'Reilly D. E. , Peterson E. M.
Published: February 2004

The self-diffusion coefficient of liquid toluene has been measured from 175 to 380°K and is represented by (4.5± 0.6) × 10−3 × Exp[−(3.12± 0.10) / Rt] Cm2Sec−1. Relaxation time data of Woessner and Snowden and the pressure dependence of the ring proton relaxation rates of Parkhurst, Lee, and Jonas are interpreted by means of the quasilattice random flight model of liquids. Root-mean-square angles of reorientation are calculated for the ring and methyl protons from a hindered rotation model of liquid state reorientation. The pre-exponential factor for self-diffusion is calculated to be 2.9 × 10−3 cm2sec−1 from the quasilattice model for the self-diffusion coefficient. Volumes of activation for self-diffusion and the intramolecular relaxation rate are compared with experimental values. It is demonstrated that the microviscosity model of Gierer and Wirtz, which is often invoked to explain the magnitude of rotational correlation times, does not apply to most liquids.
K α Emission Spectrum of Gaseous N2
Authors: LaVilla Robert E.
Published: February 2004

The nitrogen Kα emission spectrum excited by electron bombardment of nitrogen gas was obtained on a single planar crystal spectrometer with photon counting. The experimental profile is in fair agreement with a calculated profile based on molecular orbital theory and the single vacancy states measured by x-ray photoelectron spectroscopy. Evidence is presented to suggest that some of the emission intensity on the high-energy side of the main peak is due to ``resonance radiation.'' To fix the energy scale of the spectrum, the effective 2d spacing of the analyzing lead myristate crystal was experimentally determined.
Mössbauer Spectroscopy as a Probe for Polymer Dynamics in the Glass Transition Region
Authors: Reich S. , Michaeli I.
Published: February 2004

57Fe incorporated in polyacrylonitrile (PAN) was used as a probe for polymer dynamics in the glass transition region using the Mössbauer effect. Glass transition was followed by monitoring recoil free fraction (rff) as a function of temperature in PAN polymerized by Fe(ClO4)2·6H2O. Spectra could be obtained both in the glassy and in the rubbery states. The glass transition temperature, determined as a break in the slope of rff versus temperature, was found to be in good agreement with dilatometric measurements. The dependence of rff on temperature is discussed. It is found that the polymer in the glassy state can be characterized by an equivalent Debye θ temperature.
Intramolecular Multiple Scattering in Gas Phase Electron Diffraction: Some Model Calculations
Authors: Bonham R. A. , Peixoto E. M. A.
Published: February 2004

Several exactly soluble models are presented for rotationally inelastic electron scattering from rigid arrays of scatterers. It is shown that in the high energy limit the models can be expanded in the usual Born series. The first order correction for intramolecular multiple scattering obtained from this expansion is shown to possess a characteristic geometrical factor for various potential models which is in agreement with experimental observation.
Near-Degeneracy Correlation Effects in the Mg I Isoelectronic Sequence
Authors: Alper Joseph S. , Skutnik Bolesh J. , Wilson Michael
Published: February 2004

Nondynamical pair-correlation energies have been calculated for various configurations in seven members of the Mg I isoelectronic sequence. The energies were calculated using a configuration interaction approach within the framework of the ``Many Electron Theory'' of Sinanoğlu. The influence of the 3d2 configuration was investigated, and the dependence on Z, the nuclear charge, was examined. It is found that the nondynamical correlation energy of the 3s2 configuration, ε(3s2), is almost perfectly linear in Z.
Spectroscopy of Dimers
Authors: Levine Howard B.
Published: February 2004

The method of Stogryn and Hirschfelder, which divides two-particle phase space into bound, metastable, and free particle parts, is applied to the statistical averages which determine the first three spectroscopic moments of ``collision-induced'' far-infrared and Raman spectra. Numerical results are presented for a number of models of noble gas systems. It is shown that for some systems, but not all, dimers make important, although not necessarily well-separated, contributions to the spectrum.
Exact Series Solution of Classical Deflection Angle for Repulsive Inverse Power Potential
Authors: Gislason E. A.
Published: February 2004

An exact series solution of the classical deflection angle is developed for the potential V=C/r8 (C > 0). The series converges at all impact parameters. In addition, upper and lower bounds for θ are constructed from the series as an aid to numerical computation. Finally, the classical differential cross section is evaluated from the deflection angle.
Effects of an Attractive Well Potential on the Atom–Diatomic Molecule Collinear Collision
Authors: Secrest Don , Eastes Walter
Published: February 2004

The effect on the vibrational transition probability of an attractive well in the interaction potential for the collinear collision of an atom with a diatomic oscillator is investigated. Two forms of the potential are considered. For a square barrier potential, a square well introduces sharp resonances in the transition probability. It is shown that the position of these resonances may be predicted by a simple formula. For a ``soft'' Morse interaction potential no resonances are observed. It is shown in this case that the transition probability is determined more by the slope of the potential at the classical turning point than by the presence or absence of an attractive well.
Theory of Inelastic Collisions: Uniform Asymptotic (WKB) Solutions and Semiclassical Scattering Matrix Elements for Multichannel Problems
Authors: Eu Byung Chan
Published: February 2004

The method developed for the semiclassical solutions of two-channel problems is generalized for a semiclassical solution of multichannel problems. The semiclassical scattering matrix elements are constructed from the solution, which are given in terms of well defined quadratures involving the Airy functions. The results obtained are valid without curve crossings and do not require the usual assumption of a sufficiently wide separation of the turning points and crossing points. It is shown that an approximate solution of an N-channel problem can be constructed from the solutions of local two-channel problems, if the curves cross pairwise in sequence.
Formation of HS− and DS− by Dissociative Attachment in H2S, HDS, and D2S
Authors: Fiquet-Fayard F. , Ziesel J. P. , Azria R. , Tronc M. , Chiari J.
Published: February 2004

We have studied the formation of HS− by dissociative attachment in a total ionization chamber, with a RPD electron gun. The onset of the ionization efficiency curve (1.45 ± 0.1 eV) is equal to the minimum energy necessary for the formation of HS− (1.55 ± 0.03 eV). Nevertheless the curve does not exhibit the usual shape characteristic of a vertical onset process. The cross section at the maximum is σ (HS−/H2S)=1.7 × 10−18 cm2. We measured the isotope effects separately, in a mass spectrometer, and we found that the cross section in D2S is much smaller: σ (HS−/H2S) /σ (DS−/D2S)=25 ± 3. We tried to reproduce the experimental results by a semiempirical calculation, using the expression of O'Malley for the cross section and parameterizing the final state potential curve of the negative ion. We have shown that the experimental results are consistent with the assumption of a very shallow bound potential in the Frank–Condon region. Quantitative calculations are impossible because the formula of O'Malley is incorrect for that sort of potential curve.
Partial Charge-Transfer Reactions at Thermal Energies
Authors: Spears K. G. , Fehsenfeld G. C. , McFarland M. , Ferguson E. E.
Published: February 2004

Thermal energy reaction rate constants are reported for the partial charge-transfer reactions Ca2+ + NO → Ca+ + NO+ and Mg2+ + X → Mg+ + X+ with X as Xe, NO, O2, N2O, CO2, CO, SO2, NH3, and NO2. The rate constants for single electron transfer of polyatomic neutrals are often three orders of magnitude larger than the rate of electron transfer from Xe. Within a curve crossing model the molecular data yield approximate efficiencies of electron transfer from 2.5 to 14.4 Å. The difficulties of using a curve crossing model for polyatomic molecules and a simple Landau—Zener calculation at thermal energies are briefly discussed.
Computational Approach to the Construction of the Optical Potential
Authors: Wolken George
Published: February 2004

Atom—diatomic molecule scattering is considered in the close coupling approximation. It is shown how a local optical potential matrix can be constructed such that scattering from this optical potential re-produces exactly a specified submatrix of a given multichannel (and arbitrarily accurate) transition matrix (T). However, these two requirements on the optical potential (that it be local and reproduce a given submatrix of T) do not define it uniquely. For the special case of open channels a T (R) is defined such that T(∞) =T. Requiring the optical potential to be local and reproduce a specified submatrix of T(R) at every R does lead to a unique (and quite simple) result capable of trivial generalization to closed as well as open channels. H+H2 rotationally inelastic scattering, neglecting exchange, is considered as an example.
Measurement of Low Energy Scattering Cross Sections for the He–N2 System
Authors: Collins Frank G. , Hurlbut Franklin C.
Published: February 2004

Total elastic scattering cross sections for the He–N2 system were measured for relative velocities from 1.68× 104 to 1× 105 cm/sec utilizing the crossed beam technique. An orbiting resonance below the j=0 to 2 transition was observed. Two collimated, aerodynamically intensified molecular beams intersecting at 10° were used. Each beam had a speed distribution comparable to that obtained from mechanical velocity selectors. The scattering center density was determined from a calibration of the detector utilizing a thermal beam source, carefully considering the geometry of the system. Attenuation of the scattering signal due to the background gas was determined without recourse to pressure-gauge readings. One beam was modulated and integrating techniques were used to recover the signal. The time-of-flight technique was used to determine the initial conditions in each beam. The measurements were compared with quantum mechanical calculations performed at the University of Bonn using a LJ (12,6) potential. Generally, the measurements were larger than these calculations, but one resonance was partially resolved.
ESR Study of the Chlorofluoroacetamide Radical in Irradiated Dichlorofluoroacetamide Single Crystals
Authors: Kispert Lowell D. , Myers Fred
Published: February 2004

The electron spin resonance spectra of single crystals of dichlorofluoroacetamide and N, N-dideutero-dichlorofluoroacetamide irradiated with x-rays at 77°K and later warmed to 170°K, have been assigned to · CClFCONH2 and · CClFCOND2, respectively. A tensor Hamiltonian fit to the spectra have yielded either complete or partial hyperfine splitting tensors for fluorine-19, chlorine-35, carbon-13, and nitrogen-14 in addition to the chlorine-35 quadrupole coupling and the g tensor. These results along with a CNDO/2 molecular orbital calculation indicates that the · CClFCONH2 radical is a planar π radical. An INDO molecular orbital calculation was carried out for the · CH2CONH2, · CFHCONH2, and · CF2CONH2 radicals in order to interpret the experimentally observed amide nitrogen and proton splitting variations.
Effect of Temperature on Radiationless Transitions
Authors: Lin S. H.
Published: February 2004

The temperature dependence of the transition probability of radiationless transitions has been discussed by using the golden rule expression for the radiationless transition probability derived from the view-point of the nonstationary character of the Born—Oppenheimer adiabatic approximation. It is shown that the rate constant of radiationless processes depends on temperature as knr/knr 0=1 + ∑ k Ak exp(− ℏ ωk ′/kT).Ak's have been shown to be related to the changes in normal coordinates and frequencies in the equilibrium configuration of the two electronic states in which the radiationless transition takes place.
High Resolution Magnetic Circular Dichroism and Absorption Spectra of Cs2ZrBr6:Ir4+
Authors: Dickinson J. R. , Piepho S. B. , Spencer J. A. , Schatz P. N.
Published: February 2004

The high resolution absorption and MCD spectra of Ir4+ doped into the cubic host Cs2ZrBr6 are reported over the range ∼ 11 000–20 000 cm−1 at liquid helium temperature and as a function of temperature. This host has not been used previously for optical studies and yields an extraordinarily well resolved Ir4+ spectrum with individual vibronic linewidths in the range ∼ 3–15 cm−1. A study of the detailed vibronic structure of three of the bands including the associated ``hot'' absorption proves very helpful in making definitive assignments. It is shown clearly that the strong bands at ∼ 17 000 and 19 000 cm−1 correspond to the allowed ligand-to-metal charge-transfer transitions, Eg ′′ (2T2g) → Eu ′′ (2 T2u ) and Eg ′′ (2T2g) → Uu ′ (2 T2u ), and that in each case the extensive fine structure is associated with a single allowed electronic transition. The third band which shows sharp vibronic structure, at ∼ 14 500 cm−1, is very likely the parity-forbidden charge-transfer transition Eg ′′ → Eg ′ (2 T1g ) whose lower spin—orbit component, Eg ′′ → Ug ′ (2 T1g ), corresponds to a pair of weak absorptions ∼ 12 000 cm−1. The present results are consistent with our previous assignment of the intense band system ∼ 13 000 cm−1 to the ligand-to-metal charge-transfer transition Eg ′′ → Uu ′ (2 T1u ) split by Jahn—Teller interaction. The other component of this transition, Eg ′′ → Eu ′ (2 T1u ), is orbitally forbidden but is nominally assigned to an intense band overlapping the high energy side of the Eg ′′ → Eu ′′ (2 T2u ) transition with which it is strongly mixed. Aside from some very weak absorption in the 15 000 cm−1 region, there seems to be no basis for assigning any of the spectral features observed to ligand field transitions.
Ab Initio Effective Potentials for Use in Molecular Calculations
Authors: Kahn Luis R. , Goddard William A.
Published: February 2004

We have investigated the efficacy of ab initio effective potentials in replacing the core electrons of atoms for use in molecular calculations. The effective potentials are obtained from ab initio GI calculations on atoms and are unique and local. We find that the use of these effective potentials to replace the core orbitals of such molecules as LiH, Li2, BH, or LiH2, leads to wavefunctions in excellent agreement with all-electron ab initio results. The use of such effective potentials should allow ab initio quality wave-functions to be obtained for systems too large for the ab initio consideration of all the electrons.
Ab Initio Investigation on the σ-;π Interactions in the Electronic States of s-trans-1,3-Butadiene
Authors: Clark P. A. , Csizmadia I. G.
Published: February 2004

Interactions among sigma and pi electrons in the excited electronic states of the conjugated organic molecule s-trans-1,3-butadiene have been investigated by means of ab initio CI calculations. The results are compared with ab initio CI calculations including π electrons only and also with various semiempirical CI calculations. It is found that the inclusion of σ electron excitations has an appreciable effect on the lowering of the energies of those excited states which are predominantly π electronic states.
Collision Integrals for Isotopic Hydrogen Molecules
Authors: Brown Nancy Jeanne , Munn Robert J.
Published: February 2004

Elastic collisions involving o−H2+p−H2 and p−H2+HD were studied. The molecules interacted with a Lennard-Jones type potential modified to include angular terms. A set of cross sections, Q(l), and collision integrals, Ω(l, 8) (T), were obtained for each system. Collision integrals and thermal diffusion factors were calculated for p−H2+HD both with and without the angle dependent terms in the potential. The cross sections showed a small degree of sensitivity to the angular terms in the potential and to differences in the reduced mass of the systems. The collision integrals were quite insensitive to the angle dependent terms of the potential. The calculated thermal diffusion factor for each H2+HD case differed from the experimental value by approximately a factor of 2.
Electronic Processes in Ultraviolet and Pulse Irradiated Halide Ion Systems. The Effect of Heavy Atoms
Authors: Khorana Shamsher , Merkel Paul B. , Hamill William H.
Published: February 2004

Air-free, concentrated, aqueous solutions of alkali halides at ∼ 20°C containing small concentrations of heavy metal ions or I2 have been uv irradiated in the induced absorption bands. There was no evidence of chemical change unless an electron acceptor, e.g., N2O was also present when halide oxidation occurred. Luminescence from 1M KCl with 10−4M Tl+ was quenched by N2O and by I−, both systems obeying Stern-Volmer kinetics. In 8M LiCl glass saturated with TlCl, there were three absorption bands at 235, 217, and 187 nm at 77°K. They resemble, both in position and relative intensity, the A, B, and C bands of KCl:Tl+. Analogous results were observed for Tl+-doped aqueous glasses of LiBr and LiI, and all may involve the known halocomplexes of Tl+. The absorption bands in the systems examined are attributed to charge transfer from halide ion to heavy metal cation or to iodine. The relevance of these results to Tl+-doped alkali halide crystals is also considered. The absorption spectra of the diatomic halides MX (GaX, InX, and TlX) with 1Σ+ ground states have in common Π30+, 3π1, and 1π1 excited states. These may correlate with A1, A2, and C bands in ionic lattices. If MX is nearly substitutional in the alkali halide lattice, there are three distinguishable metal-halogen internuclear separations which may account for structure in the C band through Franck-Condon effects. γ-Irradiated LiCl glass at 77°K shows absorption by Cl2− and by trapped electrons and recombination luminescence, induced by optical bleaching of trapped electrons, has both 1 and 9 sec lifetimes. The luminescence and chemical reactivity are attributed to excited halide ions. Pulse radiolysis of neutral chloride ion aqueous solutions is known to yield Cl2−. It has been found that aeration affects G(Cl2−) and, in solutions saturated with N2O or CO2 instead of N2, yields of Cl2− are considerably enhanced.
Phonon Sidebands of Electronic Transitions in Molecular Crystals and Mixed Crystals
Authors: Hochstrasser Robin M. , Prasad Paras N.
Published: February 2004

An approximate theoretical description of phonon participation in the electronic transitions of molecular crystals and of mixed crystals is presented, and it is suggested that the ΔD dispersion shift term is a determining factor of the phonon-sideband spectra in triplet and weak singlet exciton spectra. We show that the phonon sidebands map the density of phonon states and do not correspond to discrete transitions to Raman-active states of the crystal lattice. We also show that the spectra of guest molecules should be accompanied by mainly resonance mode phonon sidebands and that the coupling and the selection rules are different for neat and for mixed crystals. Spectra are presented to illustrate some of the main points of the discussion: in particular, the differences between neat crystal and mixed crystal phonon sidebands; the effect of exciton bandwidth on phonon sidebands; the appearance of local modes; and the differences between phonon sidebands in absorption and luminescence spectra.
NMR Study of Molecular Reorientation in Solid Hexafluorobenzene (C6F6)
Authors: Albert Shmuel , Gutowsky H. S. , Ripmeester John A.
Published: February 2004

Continuous wave and pulsed NMR studies were carried out on solid hexafluorobenzene (C6F6) from 270 to 77°K. The temperature dependence of the second moment exhibits two regions of motional narrowing and that of the spin-lattice relaxation time shows minima at 142.5 and 260°K. The spin-lattice relaxation in the rotating frame has a minimum at 172°K and decreases toward a second minimum at low temperature. Although the line shape is asymmetric in the entire temperature range, it differs between 100 and 145°K from that at higher and lower temperatures. This intermediate region line shape is interpreted as a superposition of two components from motionally inequivalent hexafluorobenzene molecules. The second moment and relaxation time data are consistent with a model in which two different sets of C6F6 molecules reorient around their sixfold axis at different frequencies. Using BPP expressions, we obtain from our T1 and T1ρ data the following parameters for the low- and high-temperature motions, respectively, Ea=2.85± 0.15 kcal/mole, τ0=(1.60± 0.1)× 10−13sec,Ea′=7.00± 0.40 kcal/mole, τ0′=(5.00± 0.3)× 10−15 sec.
Partitioning of HCl in the Water-Ice System
Authors: Seidensticker R. G.
Published: February 2004

Ice single crystals grown from HCl solutions indicate a concentration-dependent partition coefficient. This departure from ideality may be tentatively attributed to bond defects generated by substitutional HCl molecules in the ice lattice.
Absorption Spectra of Dianthracene Anion Radical and Anthracene Dimer Anion
Authors: Shida Tadamasa , Iwata Suehiro
Published: February 2004

Dianthracene in γ-irradiated rigid ethereal solutions at 77°K reacts with mobile electrons produced by ionization to give a pair of anthracene molecule and anion which interact weakly to constitute a dimer anion. The dimer anion exhibits spectral features of (i) an extra intermolecular charge resonance band in the near infrared region, (ii) hypochromism in comparison to the free anthracene molecule and anion. The observed spectrum of the dimer can be well accounted for by molecular orbital calculations with the assumption of a sandwich structure for the dimer. In contrast to the mobile electron the matrix-trapped electron which has less kinetic energy reacts with dianthracene only to give dianthracene anion radicals which show doublet charge resonance bands in the near-infrared region. The anion is unstable both thermally and optically, and splits to an anthracene molecule and an anion via the dimer anion.
Many-Body Processes in Nonradiative Energy Transfer between Ions in Crystals
Authors: Fong Francis K. , Diestler Dennis J.
Published: February 2004

It is shown that many-body interactions can play a dominant role in nonradiative energy transfer processes between ions in crystals. These interactions formally arise from the Coulomb and exchange interactions between the electrons of the donor ion and two or more acceptor ions. In particular, three-body transfer processes arising from the dipole-dipole perturbation Hamiltonian through virtual transitions are considered in some detail. Such processes are manifested by the quadratic concentration dependence of the per-ion nonradiative energy transfer rate observed in concentration quenching and sensitized luminescence experiments. Many-body interactions are important in rare-earth ions because of the narrow widths of the 4fn crystal states and because of the large moments of the virtual transitions involving the opposite parity 4fn−1 5d states.
Monte Carlo Calculations on Polypeptide Chains. IV. Hard-Sphere Models for Randomly Coiling Polysarcosine and Poly-N-Methyl-L-Alanine
Authors: Warvari Harold E. , Knaell Kenneth K. , Scott Roy A.
Published: February 2004

Monte Carlo studies of the average dimensions of randomly coiling polysarcosine and poly-N-methyl-L-alanine were carried out using hard-sphere models for these polypeptide chains similar to those used previously in this laboratory for polyglycine and poly-L-alanine. Non-self-intersecting chains of various chain-lengths up to 272 amino acid residues for polyscarcosine and 320 for poly-N-methyl-L-alanine were generated, using the sample enrichment technique of Wall and Erpenbeck, and then used to calculate average chain dimensions. Unperturbed chains for both polypeptides up to 300 amino acid residues in length were also generated by ignoring hard-sphere conflicts and were then used to calculate average unperturbed dimensions. As in previous work reported from this laboratory chain attrition could be represented by the empirical equation λN=λ∞[ (N−c)/(N+d) ]e, with the attrition constants λ∞ having the values 0.0453 and 0.0195 for, respectively, polysarcosine and poly-N-methyl-L-alanine. These values are lower than those obtained previously for polyglycine and poly-L-alanine. The mean square end-to-end distance and mean square radius of gyration for both the non-self-intersecting and unperturbed chains could be represented at long chain lengths by the equations 〈 r2〉=aNb and 〈 s2〉=a′ Nb{′} and over the entire range of chainlengths by the equations 〈 r2〉=aNb exp[ (c1/N)+(c2/N2)+(c3/N3) ] and 〈 s2〉=a′Nb{′} exp[ (c1′/N)+(c2′/N2)+(c3′/N3) ]. The latter equations were also used to reanalyze the data from earlier calculations on multistate per residue models of polyglycine and poly-L-alanine. The parameters b and b′ for unperturbed chains in all cases were found to be consistent, within the statistical reliability of the data, with the theoretical value of 1.00 and for non-self-interesting chains with the theoretical value of 1.20, except for poly-N-methyl-L-alanine which had lower values. It is proposed that these low values are valid and not just statistical deviations and that b and b′ should not in all cases have the value 1.20 although values near 1.20 are found for all previous polypeptide models studied to date. It is further proposed that there should be a continuum of possible values between 1.00, characteristic of unperturbed chains, and 1.20, often found for non-self-intersecting chains, and including values above 1.20. As found in earlier studies the ratio 〈 r2〉/〈 s2〉 for the models in this study passes through a maximum as a function of increasing chain length for both non-self-intersecting chains and unperturbed chains and then approaches at long chain lengths, within the statistical reliability of the data, the theoretical value of 6.00 for unperturbed chains and a slightly larger value for non-self-intersecting chains. The entire range of the data for 〈 r2〉/〈 s2〉 could also be well represented by an equation of the form 〈 r2〉/〈 s2〉=a′ ′Nb{′ ′} exp[ (c1′ ′/N)+(c2′ ′/N2)+(c3′ ′/N3) ] with b′ ′≈ 0.
Cation Diffusion Study in NH4NO3 by Proton Spin Relaxation
Authors: Riggin M. T. , Knispel R. R. , Pintar M. M.
Published: February 2004

Measurements of the proton spin—lattice relaxation times (T1, T1ρ) have been made to study the translational and reorientational motions of the ammonium ions in ammonium nitrate in several solid phases. At the plastic-to-brittle phase transition at 398°K, the correlation time for translational diffusion of the NH4+ ion increases by more than three orders of magnitude: the increase is from 2 × 10−14exp(12.1 kcal mole−1 / RT) to 2 × 10−16exp (24.3 kcal mole−1/RT) sec. The self-diffusion constants for NH4+ in phases II and I obtained from these correlation times are in close agreement with the diffusion constants reported from electrical conductivity measurements in phases II and I.
35Cl NQR of 11 Organic and 3 Inorganic Molecules
Authors: Hart R. M. , Whitehead M. A. , Krause L.
Published: February 2004

The 35Cl NQR frequencies from 11 organic molecules and 3 inorganic molecules are given for 77°K. The frequencies are discussed in terms of σ and π inductive and conjugative effects of groups relative to the chlorine and compared to frequencies in analogous molecules. In [PCl4]+[BCl4]−, frequencies from both ions have been detected, and in [SCl3]+[SbCl6]− the first frequencies to date from [SCl3]+ have been measured.
BEBO Calculations. I. Activation Energies and Kinetic Isotope Effects for the Reactions of CH3 and CF3 Radicals with HCl and H2S
Authors: Arthur N. L. , McDonell J. A.
Published: February 2004

The BEBO method has been used to calculate activation energies, tunneling corrections, and kinetic isotope effects for the reactions of CH3 and CF3 radicals with HCl and H2S on the basis of a three-atom model for the activated complex. For comparison with previous work, calculations were also made for the reactions of CH3 and CF3 with H2 and SiH4. The theoretical values are compared with the corresponding experimental results, and the role of tunneling, the variation of input data, and the effect of the polarity of the substrate are discussed.
Theory for the Forces between Closed-Shell Atoms and Molecules
Authors: Gordon Roy G. , Kim Yung Sik
Published: February 2004

A simple model is presented for calculating the forces between closed-shell atoms and molecules in the regions both of the attractive well and of the repulsive wall at shorter distances. Account is taken of both the overlap of the separate atomic densities and of electron correlation. Applications to pairs of rare gas atoms and to alkali halide molecules demonstrate quantitative agreement with empirically determined intermolecular potentials for these systems over the whole range of separations inside and including the potential minimum.
Vibrational Frequencies and Entropies of Small Clusters of Atoms
Authors: Burton J. J.
Published: February 2004

The frequencies of the normal vibrations have been calculated in the harmonic approximation for small spherical clusters having 13 to 87 atoms in both the cubic and hexagonal-close-packed arrangements. The frequency distributions are rather complicated, but exhibit some regularities. The vibrational entropies were calculated from the frequency distributions, The entropy per atom is not a monotonic function of the cluster size but shows a maximum below 40 atoms per cluster and a minimum between 50 and 80 atoms. This behavior of the entropy function is explained by the competition of surface and edge vibrational modes which tend to increase the entropy and size effects which tend to lower it.
Electronic State of Iron in Hemoglobin, Myoglobin, and Derivatives, as Inferred from X-Ray Fluorescence Spectra
Authors: Koster A. S.
Published: February 2004

The Fe Kβ fluorescence emission spectrum was used to study the coordination of iron in some heme proteins. The spectrum was found to be dependent only on the direct environment of the iron. The iron atom in oxyhemoglobin can be regarded as trivalent, with a considerable negative charge on the outer oxygen atom of the O2 ligand. Carbonmonoxide hemoglobin contains divalent, zero spin iron. Methemoglobin is a low spin compound.