bluuebunny/crossref_metadata_2025 huggingface dataset:
Third Virial Coefficients of Ar, Kr, and Xe
Authors: Chen Chimin T. , Present R. D.
Published: February 2004

In view of serious discrepancies in polynomial fitting of PVT measurements over different density ranges, we have reanalyzed the data of the Amsterdam group to obtain improved third virial coefficients Cexpt1(T) for Ar, Kr, and Xe at low temperatures. The third virial coefficients assuming additivity CA(T) and the nonadditivity corrections ΔC(T), assuming the triple-dipole potential to be valid at arbitrarily small distances rij, have been calculated for some of the better pair potentials, and in all cases it is found that CA(T)+Δ C(T) agrees well with the reanalyzed experimental value Cexpt1(T). We consider the values of ΔC when a cutoff at r* is made such that the three-body interaction vanishes when any rij<r*. This shows that the major contribution to the ΔC integral comes from triangles with one or more sides rij smaller than rm, the separation at the pair-potential minimum. Since one expects both higher multipole and exchange-overlap three-body interactions to be appreciable for these small triangles, it is difficult to understand the consistently good agreement between CA+Δ C and the experimental values. The large discrepancies between CA(T) and Cexpt1(T) cannot be reasonably attributed to uncertainties in the pair potential or in the revised polynomial fitting of the PVT data.
The Two-Phase Fluid Interface at Equilibrium: A Continuum Model
Authors: Morris J. W.
Published: February 2004

This investigation considers the thermodynamics of a continuous two-phase interface in a one-component fluid. The interface is assumed to involve a continuous change of state from one homogeneous phase to the other. The internal energy density of the fluid at equilibrium is assumed everywhere determined by the fields of entropy and mass according to a single fundamental equation. The first and second laws of thermodynamics are assumed to hold for every element of the fluid, and the thermodynamic behavior of the interface is required to be such that Gibbs' thermodynamics of surfaces at equilibrium follows when the Gibbs construction is used. It is argued that under these conditions the internal energy density cannot depend on local gradients of entropy but must depend on local gradients of the mass density, with the consequence that the fluid within the interface behaves as a multipolar continuum. The fluid may then support continuous transitions between homogeneous phases at equilibrium, and obeys conventional surface thermodynamics.
Quadrupole Coupling and Anisotropic Chemical Shift in Re2(CO)10, Mn2(CO)10, and ReMn(CO)10
Authors: Mooberry E. S. , Spiess H. W. , Sheline R. K.
Published: February 2004

Single crystals of the bimetallic carbonyls Mn2(CO)10, Re2(CO)10, and ReMn(CO)10 were studied at room temperature by broadline NMR. Earlier work on the orientation of the principal axis system of the electric field-gradient tensor in Re2(CO)10 and Mn2(CO)10 was corrected. The z axis of the tensor in Re2(CO)10 was found to lie within 6° of the Re–Re bond. In Mn2(CO)10 the y axis of the field gradient tensor was within 10° of the Mn–Mn bond. The coupling constant and asymmetry parameter for 55Mn in MnRe(CO)10 were determined as e2qQ/h=8.6 MHz and η=0.61. The z axis of the field gradient tensor at the 55Mn site was within 2° of the Mn–Re bond. The orientation of the principal axis system of the field gradient tensor in Mn2(CO)10 and Re2(CO)10 was determined at liquid nitrogen temperature. The angle between the z axis and the Re–Re bond in Re2(CO)10 was found to double by going from room temperature to liquid nitrogen temperature. The orientation of the principal axis system in Mn2(CO)10 was unchanged within experimental error by changing the temperature from 25°C to 77°K. A small anisotropy in the chemical shift was found for Re2(CO)10 while the shift in Mn2(CO)10 was essentially isotropic. An NMR probe for studies of air unstable compounds and constant temperature is described.
Study of the Proton Magnetic Resonance Spectrum of Ethylene Trithiocarbonate in a Thermotropic Nematic Solvent
Authors: Raza M. A. , Reeves L. W.
Published: February 2004

The proton magnetic resonance spectra of (a) the 13C satellites in the isotropic neat liquid ethylene trithiocarbonate at 40°C, and (b) solutions of 5 wt% ethylene trithiocarbonate in the nematic liquid N-(p-methoxybenzylidene)-p-butylaniline at 30.5°C, have been recorded and analyzed. From the dipole-dipole coupling constants the four protons are shown to be arranged in a effective plane with D2h symmetry. Interproton distance ratios are compared with corresponding values for ethylene carbonate and ethylene monothiocarbonate studied previously. It is clear that the 5 membered ring is not significantly distorted from a plane in this nematic solvent and that the H–C–H bond angle is smaller than in ethylene carbonate due to the presence of adjacent sulphur atoms in the ring. This result is consistent with conclusions reached in an earlier study with regard to the molecule ethylene monothiocarbonate. While interproton distance ratios obtained from studies of oriented molecules by NMR are not precise in absolute terms, comparison of a related series of molecules in the same and varied nematic solvents gives considerable comparative structural information.
Thermal Diffusion Factor and Quantum Collision Integrals for the H2–4He System: The Effect of Inelastic Collisions
Authors: Taylor W. L.
Published: February 2004

We have extended our measurements of the composition dependence of the thermal diffusion factor of the hydrogen-helium system to lower temperatures in search of the inelastic (rotational) collision effect reported by Ghosh et al. and Mathur and Watson. We measured αT as a function of composition in a seventube trennschaukel at an average temperature of 82°K and failed to find any maxima or minima greater than our average experimental scatter of approximately ± 10%. We have also determined the temperature dependence of the thermal diffusion factor for an equimolar mixture from 30 to 550°K which is characterized by a broad maximum at approximately 125°K. A sharp decrease in αT was observed on the low temperature side of the maximum while a gradual, almost linear decrease was obtained on the high temperature side. We have calculated quantum mechanical transport collision integrals for hydrogen-helium assuming an (exp-6) spherical interaction potential and have used them to evaluate [αT]3, the third order Chapman-Enskog approximation to the thermal diffusion factor. The theoretical calculations are in excellent agreement with the experimental determinations.
Electronic Properties of Sulphur Hexafluoride. II. Molecular Orbital Interpretation of Its X-Ray Absorption Spectra
Authors: Gianturco F. A. , Guidotti C. , Lamanna U.
Published: February 2004

The unusual behavior of the sulphur hexafluoride spectra in the x-ray region is discussed and interpreted within the framework of a MO-LCAO description of the molecular wavefunction. Single- and double-excitation processes from inner and outer orbitals are suggested to take place during the x-ray absorption and calculated energies are compared with experimentally observed peaks. A qualitative general agreement of the fitting with the measured data seems to indicate that such processes are very likely to happen in large, multielectron, molecular systems.
Surface Tension Calculations for the Hard Square Lattice Gas
Authors: Woodbury George W.
Published: February 2004

Calculations based on previously derived expressions of the surface tension are presented for the interface between the two solid phases occurring in the hard square lattice gas. (The phases are identical except for occupying different sublattices.) It is found that the formula d(σ/kT)/d(lnz)=ρ (where σ, z, and ρ are the surface tension, activity, and bulk density) is consistent with the numerical work. An expression for the boundary tension of a free surface is derived, and application is made also to the hard square lattice gas. This boundary tension is found to be negative and small in magnitude—with maximum magnitude slightly below the transition activity.
Method of Heavily Doped Isotopic Mixed Crystal for Determination of Exciton Splittings and Normal Modes: Raman Spectra of Naphthalene
Authors: Prasad Paras N. , Kopelman Raoul
Published: February 2004

Recent developments on concentrated mixed crystal exciton spectra enabled us to develop new qualitative and quantitative criteria for the identification of vibrational exciton splittings: Observation of an appropriately structured spectrum in the exciton k≠ 0 gap. This is especially useful for Raman Spectra where the ideal mixed crystal approach is not very useful due to the low intensity of very dilute solid solutions and where the classical pure crystal polarization criterion is difficult to use, especially at low temperatures. The conflicting literature assignments of Raman exciton splittings for naphthalene are resolved: the eight major splittings (≤ 4 cm−1) found here have been missed before, while all major ``Davydov'' splittings (≤ 4 cm−1) recently reported are invalidated. The clarification of exciton splittings leads to some clarification of the normal mode assignments in the naphthalene molecule. A helpful criterion is utilized here as well: isotopic substitution has mild and predictable effects on the exciton bandwidth and splitting.
Intermolecular Potentials from Crossed Beam Differential Elastic Scattering Measurements. VI. Atoms and Diatomic Molecules. Ar+N2,Ar+O2, Kr+N2, and Kr+O2
Authors: Tully F. P. , Lee Y. T.
Published: February 2004

Differential cross section measurements of atom-diatomic molecule scattering were made for the colliding pairs Ar+N2,Ar+O2,Kr+N2, and Kr+O2. Potential parameters were obtained for various Lennard-Jones (n, 6) potentials and it was found that the Lennard-Jones (20, 6) and (12, 6) potentials gave best agreement to the angular locations of the rainbow extrema.
Molecular Rotation in Liquids. Conditional Fast Rotational Diffusion
Authors: O'Reilly D. E.
Published: February 2004

The theory of finite step rotational Brownian motion is generalized to anisotropic rotators. It is assumed that fast rotational diffusion will occur with the event of a ``hard'' collision between molecules. An expression for the rotational correlation time τ2 is given which is anisotropic and depends on the orientation of the z principal axis of the quadrupolar or dipolar interaction with respect to the principal axes of the rotational diffusion tensor. A general expression for the product of the spin-rotational correlation time (τ1) and τ2 is given. The quasilattice random flight model is generalized with the aid of an expression for the collision frequency given by Einwohner and Alder. The microcanonical conditional rotation model of Atkins et al. is reformulated to the case of rotation upon a hard collision.
Vibrational Distribution of N2+B 2Σ Products from Ne+–N2 Collisions
Authors: Saban G. H. , Moran T. F.
Published: February 2004

A crossed ion beam-molecular beam apparatus has been used to study light emitted from radiative transitions of N2+B 2Σ products formed in reactions of Ne+ with N2. The cross section for this reaction has been measured for collisions of 12 to 450 eV Ne+ ions having well defined kinetic energies. Vibrational population distributions, determined from spectroscopic analysis of light emitted by the excited reaction products, were intermediate between those predicted by an application of the Franck-Condon principle and those computed using the statistical phase-space model for ion-molecule interactions.
Semiconductor Model of Electric Conductivity in Dense Metallic Vapors
Authors: Popielawski Jan
Published: February 2004

The detailed description of the semiconductor model of electric conductivity in dense alkali metal vapors is presented. The interaction of free electrons with the neutral alkali atoms is described by borrowing the ideas applied by Lekner to the description of free electrons in simple liquids. Explicit expressions for the number density of electrons in the conduction band of dense metallic vapors are given. The kinetic equation describing the behavior of the system under the influence of a weak steady external electric field is written down and solved by the standard expansion procedures. The explicit expression for the electric conductivity is obtained. The numerical calculations of electric conductivity for the model hard-sphere fluid of cesium atoms are described in some detail. The results of these calculations are discussed in comparison with the measurements of the electric conductivity in dense cesium vapors. The possibility of more exact calculations on the basis of the theory presented is also discussed and the possibility of the application of the theory to dense mercury vapors is briefly indicated.
Quantum Mechanical Scattering of Asymmetric Molecules. I. The Interaction Potential
Authors: van Montfort J. Th. , Heukels W. F. , van de Ree J.
Published: February 2004

Legendre polynomial expansions are derived for the interaction potential of molecules with a distance δ between the center of interaction and the center of mass. The scattering cross sections converge to a final value for a small number of terms of the expansion. The effect of the asymmetry parameter δ is to cause a quenching and a shift of the orbiting maxima in weakly coupled systems. The differential inelastic cross sections increase rapidly from zero with growing δ.
Statistical Model of Chemical Reactions in Nonisothermal Low Pressure Plasma
Authors: Vepřek S.
Published: February 2004

The chemical composition of steady state nonisothermal low pressure plasma in which a chemical reaction occurs is studied by the methods of statistical physics. A comparison of velocities of different processes of energy transformation within the plasma leads to the idealized model of the plasma for which the partition function is found. Then, the ``chemical equilibrium'' is described by an equilibrium constant defined by such partition functions. The applicability of this model is illustrated by the example of the reaction of carbon with nitrogen plasma.
SCF Dirac-Slater Calculations for an Ion in a Dielectric Medium
Authors: Waber James T. , Liberman David A.
Published: February 2004

The effect of a static dielectric medium on the energy levels and charge density of an ion have been determined by modifying the Dirac-Slater code. In this work the reduced form of Slater's exchange potential was used. It has been possible to form stable oxygen and sulfur anions with two negative charges. The effects of the dielectric constant and of the radius of approach of the dielectric medium have been examined. The heat of hydration of Na1+ and Cl1− were calculated with this model to be 102.4 and 42.0 kcal, which is in good agreement with the experimental value for Na+ but less accurate for Cl−.
High Resolution Magnetic Circular Dichroism Spectrum of Cs2ZrCl6:Ir4+
Authors: Piepho S. B. , Dickinson J. R. , Spencer J. A. , Schatz P. N.
Published: February 2004

High resolution, liquid helium temperature MCD, and absorption spectra over the range 17 000–24 500 cm−1 are presented for the Ir4+ ion doped into the cubic host Cs2ZrCl6 in which it exhibits extensive fine structure. The spectra have been interpreted as arising almost exclusively from ligand-to-metal charge-transfer transitions on the basis of intensity arguments and an analysis of the MCD spectrum. There is considerable evidence that the band at ∼23 000–24 500 cm−1 assigned as Eg″(2T2g)→ Eu″(2T2u)+Uu′(2T2u) shows a large excited state Ham effect (quenching of spin-orbit coupling by a dynamic Jahn-Teller effect) so that the no-phonon lines of the two spin—orbit components (Eu″ and Uu′), rather than being split by ∼(3/4) ζC1=440 cm−1, are actually separated by ≲ 6 cm−1. The other intense absorption region, ∼19 000–20 500 cm−1, is assigned to Eg″(2T2g)→ Uu′(2T1u)+Eu′(2T1u). The Eu′ component is orbitally forbidden but very probably borrows intensity from Uu′. There is also some possibility of a Ham effect in this region, but uncertainties in the location of band origins prevents any definite conclusions. To the red of the strong absorption, at ∼17 500–19 000 cm−1, are some broad, much weaker bands which are assigned to the parity-forbidden charge-transfer transitions, Eg″(2T2g)→ Ug′(2T1g)+Eg′(2T1g). The region 20 800–22 000 cm−1 shows a complex spectrum which we attribute primarily to the ligand-to-metal excitation t2g(π)6t2g5→ t2g(π)5t2g6. Again, a Ham effect is possible, but we are not able to offer a detailed analysis. A series of weak lines in the range 22 000–22 600 cm−1 are tentatively attributed to ligand-field transitions. Our interpretations differ in almost every particular from those offered previously by Douglas.
Molecular Motion in Liquids: Rotational and Vibrational Relaxation in Highly Polar and Strongly Associated Systems
Authors: Rothschild Walter G.
Published: February 2004

The characteristics of the rotational-vibrational relaxation of acetonitrile, benzonitrile, 2-propanol, and p-dioxane were observed by determining the correlation functions of several infrared and Raman rotation-vibration bands of these liquids. Benzonitrile and 2-propanol were also investigated as dilute solutions in CCl4 and in CS2, whereas p-dioxane was also observed in solution with a variety of electron-donating, electron-accepting (among them chloroform, water), and nonpolar molecules. The results show that the intermolecular forces need about 0.1 × 10−12–0.2 ×−12 sec to cause an observable effect on the molecular motion. Regardless of the polarity or association of the liquids or solutions investigated, rotational relaxation is the predominant mode of decay for times less than 0.5 × 10−12 sec after the onset of observation: The C≡ N stretch of benzonitrile and the investigated modes of p-dioxane are, probably, exceptions. It is also shown that the vibrational correlation functions do not decay purely exponentially but have Gaussian-Lorentzian shape-as the rotational correlation functions. As a consequence, vibrational lifetimes cannot predict how the initial rotational decay is affected by fast vibrational energy transfer. The results show that rotation of acetonitrile is strongly hindered in those motions which tend to tilt the permanent dipole moment axis but that the molecules undergo rotational jumps by about 0.5–1.2 rad around this axis. Rotational motion of benzonitrile is strongly hindered in all directions, but fast vibrational decay makes interpretation difficult. The long-time decay (beyond 0.8 × 10−12 sec) of the C–C stretch of 2-propanol is mainly governed by vibrational decay; for shorter times, rotational decay is predominant. Molecular relaxation in neat p-dioxane and its solutions seems to be influenced mainly by vibrational relaxation.
Binding Energies of Inert Gas Trimers
Authors: Bruch Ludwig W. , Stenschke Herbert
Published: February 2004

The ground state energy of inert gas trimers X3(X is Ne, Ar, Kr, Xe) is calculated for Lennard-Jones 12–6 pair potentials. The results of a variational calculation and an estimate of the zero-point vibrational energy are in good agreement. A lower bound on the ground state energy is also discussed.
Luminescence of Zinc Oxide Crystals Controlled by Electrode Potential and Electrochemical Reactions
Authors: Petermann Günter , Tributsch Helmut , Bogomolni Roberto
Published: February 2004

The voltage dependence of the green luminescence of ZnO crystals used as electrodes in an electrochemical cell was investigated and compared with the voltage dependence of the photocurrent. When the exciting light is of shorter wavelength than the band edge of ZnO, this luminescence can be varied from its maximal value to complete extinction by applying small positive voltages. The luminescence resulting from excitation within the long wavelength tail of absorption, however, cannot be influenced. The potential dependence of the luminescence has been investigated with differently doped crystals under various conditions. The addition of formic acid to the electrolyte causes an increase of luminescence; this is found to be a consequence of photoelectrochemically induced radical reaction by which electrons are injected into the conduction band of the electrode. The potential dependence of the green luminescence is explained by a mechanism in which the trapping of the holes in recombination centers competes with their extraction, which leads to the generation of the photocurrent. The average concentration of photogenerated holes in the crystal is therefore rate limiting for the intensity of luminescence, provided that an excess of electrons is available for recombination. At sufficiently large positive potentials, however, when electrons are being depleted in the space charge layer, their concentration also becomes limiting, and electron injection can stimulate luminescence. The experimentally observed potential dependence of luminescence and photocurrent, as well as their interrelation, is shown to be consistent with results which were derived from a theoretical treatment of these effects. It is suggested that this green luminescence may be used as a probe for the study of the space charge layer and of electrochemical reactions.
On the Lowest Triplet State of Substituted Benzenes. III. Optically Detected Magnetic Resonance Studies on the 3π π State of 1,2,4,5-Tetrabromobenzene and 1,2,4,5-Tetrachlorobenzene
Authors: Francis A. H. , Harris C. B.
Published: February 2004

The optically detected magnetic resonance (ODMR) in zero field and the phosphorescence microwave double resonance (PMDR) spectra of 1,2,4,5-tetrabromobenzene and 1,2,4,5-tetrachlorobenzene have been obtained monitoring the low temperature (1.3°K) phosphorescence from both trap emission in neat single crystals and emission from a durene host. From the PMDR the major vibronic features of the spectra are assigned. The orbital symmetries in the excited triplet state, the principal and secondary contributions from the individual triplet spin sublevels to the electronic and vibronic origins in phosphorescence, the spin sublevel populations, and lifetimes are determined. In addition the spin Hamiltonian parameters, including the zero-field splittings and Br and Cl excited state nuclear quadrupole coupling constants, are presented and interpreted. Finally, the relationship between spin sublevel activity in phosphorescence, Frank-Condon maxima in emission, and molecular distortions are discussed.
Molecular Weight Distribution of Polybenzimidazole. A Regularizable Ill-Posed Problem
Authors: Gehatia Matatiahu , Wiff D. R.
Published: February 2004

A purified and unfractionated sample of polybenzimidazole (PBI) in dimethylacetamide (DMAC) was subjected to equilibrium sedimentation in order to determine the molecular weight distribution (MWD). Since the solubility of PBI is very limited and Θ conditions could not be achieved, the experimental concentration (or concentration gradient) curves have been theoretically corrected to make application of equations derived for ideal conditions possible. It has been shown elsewhere that conventional equations relating MWD to the experimental data furnished by equilibrium sedimentation are an improperly posed problem in the Hadamard sense and cannot be used for a reliable MWD determination. Therefore, new expressions have been applied which include a regularization mechanism to stabilize this mathematical operation and to rule out unreliable fluctuations of the resulting MWD curves. The results reveal the existence of three separate unimodal fractions with weight average molecular weights: 5 000, 32 500, and 137 700. The MWD of the total sample can be characterized by the following average values: Mn = 5 100; Mw = 27 000; Mz = 79 300, and Mz+1 = 118 000.
Study of the Structure of Molecular Complexes. I. Energy Surface of a Water Molecule in the Field of a Lithium Positive Ion
Authors: Clementi Enrico , Popkie Herbert
Published: February 2004

In the Introduction we have outlined the technique adopted in this paper (and in others of this series to follow) in order to elucidate the structure and energetics of water molecules around positive ions. The raw data to be analyzed are obtained from the Hartree-Fock model. We have started by considering the H2O molecule making use of the bond energy analysis formalism. Next we report Hartree-Fock data for the Li+–H2O energy surface keeping the H2O geometry rigid. These data are analyzed in the same way as for the H2O molecule. In addition, the energy of rotation of the H2O molecule for a fixed Li–O distance has been studied in detail. Analytical expressions aimed at reproducing the energy surface data in a very simple and condensed form are presented. Finally, the effect of complete optimization of the geometry and use of a large basis set for Li+–H2O energy calculations is discussed. The most stable configuration for the Li+–H2O complex is planar and is predicted to have the following geometry: The Li–O and O–H bond lengths are 1.84 and 0.96 Å, respectively, the Li–O–H and H–O–H angles are 127.0° and 106.1°, respectively. The binding energy of the H2O molecule to the Li+ ion is 1.53 eV in the Hartree-Fock approximation.
Calculation of Davydov Splitting of the First Triplet State in Pyrene
Authors: Harrison S. W. , Fischer C. R. , Arnold S.
Published: February 2004

The Davydov splitting of the first triplet state of crystalline pyrene has been calculated for three different molecular orbital π-electron wavefunctions. It is found that the contribution to the splitting from charge transfer states is an order of magnitude larger than that due to neutral exciton states. With an estimated value of 0.25± 0.05 for the square of the (0, 0) vibrational overlap integral, the calculated Davydov splitting of the zero vibrational component of the first triplet state is between 1 and 2 cm−1. The Bu state is found to be primarily b polarized. These results compare favorably with the measured splitting of 2.4± 0.6 cm−1 with the b polarized state at higher energy.
Ultraviolet Photolysis of GeH3Br; Infrared Spectra of Matrix-Isolated GeH2Br and GeHBr
Authors: Isabel Roy J. , Guillory William A.
Published: February 2004

The ultraviolet photolyses of GeH3Br and GeD3Br in an argon matrix have been performed between 8 and 24°K. The major products resulting from photolysis appear to be GeH2Br and GeHBr and their deuterated counterparts. All the fundamental vibrational frequencies of these species are observed with the exceptions of one of the GeH stretches and the GeH2 and GeD2 valence bends (ν2) of GeH2Br and GeD2Br, respectively. The results of the normal coordinate and self-consistent extended Hückel calculations are consistent with the experimental observations and suggest that GeH2Br and GeHBr are pyramidal and bent, respectively. Structural and bonding properties of these species are also discussed.
Theoretical Potential Energy Curves for OH, HF+, HF, HF−, NeH+, and NeH
Authors: Bondybey Vladimir , Pearson Peter K. , Schaefer Henry F.
Published: February 2004

Ab initio calculations have been carried out on the ground states of OH, HF+, HF, HF−, NeH+, and NeH. Extended basis sets were used and electron correlation was included by way of first-order wavefunctions. Dissociation energies and other spectroscopic constants are in good agreement with available experimental data except for the bond distance of HF+. Electron detachment in collisions between H and F− is discussed on the basis of the calculated potential curves. Potential curves were also obtained ab initio for the three lowest excited states of NeH. These curves are qualitatively similar to those reported earlier by Slocomb, Miller, and Schaefer for HeH. The C 3Σ+ state of NeH is predicted to have a potential maximum of 0.87 eV at internuclear separation ∼ 4 bohr.
Monte Carlo Calculations on Polypeptide Chains. VII. Higher Even Moments of the End-to-End Distance and Radius of the Gyration for Hard-Sphere Models of Randomly Coiling Random Copolymers of Glycine and l-Alanine
Authors: Warvari Harold E. , Knaell Kenneth K. , Scott Roy A.
Published: February 2004

A Monte Carlo study of the higher even moments of the end-to-end distance and radius of gyration of randomly coiling random copolymers of glycine and l-alanine was carried out using the same hard sphere model used previously in this laboratory. Random copolymers containing on the average 0% (i.e., poly-l-alanine), 25%, 50%, 75%, and 100% glycine were studied. For each of these five polymers non-self-intersecting chains of various chain lengths were generated using the sample enrichment technique of Wall and Erpenbeck. These chains were then used to calculate the average dimensions. Unperturbed chains were also generated and used to calculate average unperturbed dimensions for comparison with those for non-self-intersecting chains. As was found previously for 〈 r2〉 and 〈 s2〉, the higher even moments for r and s up to the eighth moments could be represented by equations of the form 〈 r2p〉 =a2pNb2p and 〈 s2p〉 =a2p′Nb2p′ for long chain lengths and 〈 r2p〉 =a2pNb2p exp[(c1,2p/N)+(c2,2p/N2)+(c3,2p/N3)] and 〈 s2p〉 =a2p′Nb2p′ exp[(c1,2p′/N)+(c2,2p′/N2)+(c3,2p′/N3)] for all chain lengths. Similar equations were also valid for the unperturbed chains. Within the statistical reliability of the data, the parameters of the b2p and b2p′ type were consistent with the values 1.00, 2.00, 3.00, and 4.00 for, respectively, the second, fourth, sixth, and eighth moments for the unperturbed chains and with the values 1.20, 2.40, 3.60, and 4.80 for the non-self-intersecting chains, thus suggesting a linear dependence of b2p and b2p′ on p. The confidence limits on the parameters of the a2p and a2p′ type were so large that no general conclusions could be made.
Influence of Paramagnetic Species on the Internuclear Overhauser Effect
Authors: Gutowsky H. S. , Natusch D. F. S.
Published: February 2004

It is shown that internuclear Overhauser enhancements can be substantially reduced if a paramagnetic species is added to the sample. Such reduction or elimination of the Overhauser enhancement can be accomplished without significant broadening of a nuclear resonance, if the nuclei observed are embedded within a molecule and those decoupled are in the periphery. Thereby the intensity information and resolution can be retained in proton decoupled 13C spectra of molecules in solution. Also, the method should be applicable to decoupled spectra of species such as B, N, Si, and P. In cases where Overhauser enhancements are not reduced to zero, the actual magnitude of the enhancement can be calculated from a plot of signal intensity versus concentration of paramagnetic species. Characteristic spin-lattice relaxation times can also be inferred from such a plot. The theory is extended to multiple spin systems and to cases of complex formation, and it is shown that preferential reduction of Overhauser enhancements by a paramagnetic complexing species offers an extremely sensitive means for determining ligand binding sites and for inferring paramagnetic-diamagnetic spin separations. Observation of transient nuclear Overhauser effects in both the presence and absence of added paramagnetic material can also be used to determine the absolute magnitude of Overhauser enhancements and nuclear relaxation times.
Internal Rotation of Molecules with Two Inequivalent Methyl Groups: The Microwave Spectrum of N-Methylethylidenimine
Authors: Meier J. , Bauder A. , Günthard Hs. H.
Published: February 2004

The microwave spectra of two isotopic species of trans-N-methylethylidenimine (CH3CH=NCH3) were recorded between 8 and 40 GHz. The rotational transitions clearly demonstrate the coupling with the internal motions of the two methyl groups, which are inequivalent in this molecule. The theory for the over-all and internal rotation was extended to molecules with two inequivalent symmetric internal rotors. The internal axis method (IAM) was used to calculate the internal rotors' splittings of the rotational transitions. They were split into five components according to the different irreducible representations of the symmetry group. In addition to this, the spectra were further complicated by the nuclear quadrupole multiplets of the nitrogen. A least squares analysis of the measured transition frequencies yielded the following rotational and quadrupole coupling constants for CH3CH=NCH3: A=38258.627± 0.048 MHz, B=4078.1354± 0.0058 MHz, C=3862.7391± 0.0072 MHz, χaa=1.578± 0.039 MHz, χbb=−4.601± 0.037 MHz, χcc=3.023± 0.041 MHz; and for CH3CD=NCH3: A=32982.409± 0.050 MHz, B=4076.6142± 0.0066 MHz, C=3799.5528± 0.0079 MHz, χaa=1.551± 0.038 MHz, χbb=−4.595± 0.023 MHz, χcc=3.044± 0.036 MHz. The mean values for the barriers of the two methyl groups were found to be 1642± 19 cal/mole for the C–CH3 top and 2109± 84 cal/mole for the N–CH3 top. A dipole moment of 1.499± 0.007 D was determined from the measured Stark splittings.
Thermodynamic Properties of Europium and Ytterbium Ammoniates and the Gaseous Ammoniation Energies of Divalent Europium and Ytterbium Ions
Authors: Frisbee Robert H. , Senozan N. M.
Published: February 2004

The equilibrium pressure of ammonia was measured at fifteen temperatures extending from −30 to −10oC over europium ammoniate, Eu(NH3)6, and at nineteen temperatures extending from −40 to −22oC over ytterbium ammoniate, Yb(NH3)6. The standard enthalpy, free energy (at 0°C), and entropy of dissociation, M (NH3)6 (s) ⇄ M (s) + 6NH3 (g), per mole of ammonia were determined to be, respectively, 9.46 ± 0.10, 1.11 ± 0.02 kcal, and 30.6 ± 0.4 e.u. for Eu(NH3)6, and 8.92 ± 0.15, 0.62 ± 0.03 kcal, and 30.4 ± 0.6 e.u. for Yb(NH3)6. Combining the dissociation enthalpies with other thermodynamic data, we found the gaseous ammoniation energies, M2+ (g) + 6NH3 (g) → M (NH3)62+ (g), to be −260 kcal/mole for Eu2+ and −281 kcal/mole for Yb2+. These values show that divalent lanthanide ions, in so far as their complexation energies are concerned, behave very much like alkaline earth ions of comparable size.
Application of the Semicontinuum Model to Pressure Effects on Solvated Electron Spectra in Methanol and Ethanol
Authors: Feng Da-Fei , Fueki Kenji , Kevan Larry
Published: February 2004

The semicontinuum model for solvated electrons has been applied to the calculation of pressure effects on solvated electron spectra in methanol and ethanol at 298°K. The pressure dependence of the density, static, and optical dielectric constants and the quasifree electron energy state have all been included. The calculated results satisfactorily explain the experimentally observed blue shifts with pressure. Analysis shows that pressure affects short range interactions much more than long range interactions and results in a decrease in the electron cavity radius and an increase in the energy of the quasifree electron state.
Exact Dynamical Basis for a Fluctuating Cell Model
Authors: Hoover William G. , Ashurst William T. , Grover Richard
Published: February 2004

Cell-like models for many-body thermodynamic properties can be derived by considering the motion of a single very light particle in a classical system. Because the configuration probabilities are mass independent, the pressure and the energy calculated for such a light particle are identical with thermodynamic values. In the special case of hard spheres it is shown that the pressure from collisions is proportional to the surface-to-volume ratio of the hard sphere free volume.
Calculation of the Anisotropic Hyperfine Coupling in Cu(II) Bis(dithiocarbamate) and Cu(II) Bis(diselenocarbamate). A Formula for the Anisotropic Hyperfine Coupling Tensor
Authors: Keijzers C. P. , DE Boer E.
Published: February 2004

For a molecule with one unpaired electron a general formula is derived for the elements of the anisotropic hyperfine coupling tensor, taking into account all possible electron excitations. For a check of the formula the hyperfine coupling tensors are calculated for the copper atom in bis(N, N-diethyl dithiocarbamato) copper(II) and for the copper and selenium atoms in bis(N, N-diethyldiselenocarbamato) copper(II), using the iterative extended Hückel method. Both the calculated principal values of these tensors and the direction cosines of the principal axes are in good agreement with the experimentally observed ones.
Self-Diffusion in Viscous Liquids: Pulse NMR Measurements
Authors: Burnett L. J. , Harmon J. F.
Published: February 2004

It is shown that measurement of the nuclear magnetic resonance (NMR) relaxation times in viscous media can lead to a determination of the coefficient of self-diffusion, D. Of particular value is the measure of the spin-lattice relaxation time in the rotating frame T1p as a function of the applied rf field, H1. Depending upon the particular system under study, the method may be used to measure proton self-diffusion in the range 10−8 cm2/sec ≳ D ≳ 5 × 10−13 cm2/sec and is, therefore, applicable to the study of very viscous liquids. By employing nuclei other than protons, both limits of this range may be decreased by two orders of magnitude or more. In contrast to other NMR techniques which measure properties of slow molecular motion, this technique measures the coefficient of self-diffusion rather than the correlation time for diffusive motion. As an illustration of the method, measurement of the self-diffusion constant in glycerol has been extended to a value more than an order of magnitude smaller than that measured by conventional NMR techniques.
ESR Studies of Electron Irradiated K3IrIII(CN)6 in KCl Single Crystals. I. Ligand Hyperfine Structure in IrII(CN)5 Species
Authors: Vugman N. V. , Muniz R. P. A. , Danon J.
Published: February 2004

Electron irradiation of the diamagnetic [Ir(CN)6]3− ion in a KCl host lattice produces two paramagnetic species. ESR studies reveal that one of the species can be assigned to the pentacoordinated [Ir(CN)5]3−, of approximate C4ν symmetry, with 5d7 low spin configuration, arising from the trapping of an electron in a dz2 orbital followed by the loss of an axial cyanide. Ligand hyperfine structure with four equivalent and one nonequivalent nitrogens is observed and confirmed by isotopic substitution. The g values and the ligand hyperfine tensors are presented and discussed. Ligand fs and fp spins densities are calculated and discussed.
Electron Attachment and Excitation Processes in Selected Carbonyl Compounds
Authors: Naff W. T. , Compton R. N. , Cooper C. D.
Published: February 2004

Short-lived negative ions were observed in urea (2.2 eV), acetone (1.6 eV), acetaldehyde (1.2 eV), acetophenone (0.95 eV), benzophenone (0.75 eV), and benzaldehyde (0.72 eV), using the SF6 scavenger technique. The energy position of the peak of each resonance is given in parentheses. These resonances were assigned to the capture of an electron into the lowest vacant (π*) orbital of the carbonyl group. Hexafluoroacetone captures slow electrons (∼0 eV) into a long-lived temporary negative ion state with an autodetachment lifetime of 65± 10 μsec. Cl− was observed through dissociative attachment of 0.5-eV electrons to chloroacetone. Dissociative attachment to urea yielded CN− and NCO− around 2 eV and NH2−, NH−, OH−, and N2H3CO− around 6 eV. Peaks in the threshold excitation spectra were observed in acetone at 4.15, 6.13, 7.5, and 8.1 eV; in acetaldehyde at 3.8, 6.35, and 6.62 eV; and in urea 6.1 and 8.4 eV.
Polarizations of Low Energy Sn← S Transitions in Phthalaldehyde and 10-Methylene Anthrone
Authors: Smith J. C. , Dearman H. H.
Published: February 2004

The polarizations of the transitions to the lowest nπ* and the two lowest ππ* transitions of phthalaldehyde have been measured relative to the phosphorescence by the method of photoselection. The polarization of transitions to the lowest 10-methylene anthrone nπ* state and the 1La and 1Lb ππ* states, which are split by exciton interaction, has also been determined. Molecular orbital calculations, spin-orbit coupling, and considerations of the experimental results indicate that the T1← S0 transition of 10-methylene anthrone is polarized along the carbonyl carbon-oxygen bond (Z axis). Application of Platt's spectroscopic moment theory to the 10-methylene anthrone system, considered as two weakly interacting benzene units perturbed by formyl and vinyl substituents, does not account for the observed polarization in the ππ* region for a Z-polarized phosphorescence transition moment. The polarization curve in the ππ* region is explained on the basis of perturbed 1La and 1Lb benzene states split by inter-ring exciton interaction. The nπ* transition is believed to be polarized in the molecular plane and perpendicular to the carbonyl axis; that is, along the Y axis. An alternation in polarization in the nπ* region is observed. Indirect evidence is presented which indicates that the T1← S0 transition in phthalaldehyde is primarily polarized along the Z axis. If this transition is localized in the formyl groups, a small out-of-plane component of the transition moment may be present since the formyl groups are not expected to be in the molecular plane. The higher ππ* transition in phthalaldehyde is polarized parallel to that of the phosphorescence and is assigned to the La1←A1 benzene analog transition. The lower ππ* transition is perpendicular and is assigned to the Lb1←A1 transition on the basis of Platt's spectroscopic moment theory. The polarization in the phthalaldehyde nπ* region is uncertain but appears to be polarized in the molecular plane and approximately parallel to the carbonyl carbon-oxygen bond; that is, along the Z axis. The lifetime of the phthalaldehyde phosphorescence was found to be 3 msec.
Two Methods for the Evaluation of Exact Moments of Crystal Frequency Distribution Functions
Authors: Schmidt Ann Marti , Huckaby Dale A.
Published: February 2004

The moments of the frequency distribution function for the honeycomb lattice are determined by employing a change of variables. The moments for the triangular lattice are obtained from an analysis of the closed walks on the lattice, and a general formula is derived which sheds new light on this method. Further development of these methods, when used in conjunction with a computer, should reduce the difficulties incurred in the purely machine calculation of high order moments.
Raman Spectra of the Products of Na and K Atom Argon Matrix Reactions with O2 Molecules
Authors: Smardzewski Richard R. , Andrews Lester
Published: February 2004

Raman spectra of samples of sodium and potassium atoms codeposited at 16°K with oxygen molecules at high dilution in argon showed strong bands at 1094 and 1108 cm−1, respectively, which are assigned to the superoxide fundamentals in the Na+O2− and K+O2− molecules. These features were split into triplets using 16O2, 16O18O, 18O2 samples, which indicates isosceles triangular structures; absence of M+–O2− interionic modes confirms the ionic model for the alkali superoxide molecules. Infrared bands at 1079 and 1097–1104 cm−1 in Na and K atom −O2 reactions, respectively, are assigned to the dimeric species (M+O2−)2. The symmetric metal-oxygen mode for O2KO2 was observed in the Raman at 305 cm−1, inferring more covalent character in the metal oxygen bonding in this species than in the alkali superoxides.
Microwave Double Resonance Studies of Collision-Induced Transitions in C2H5OH, CH2DCH2OH, and C2H5OH–He
Authors: Seibt Peter J.
Published: February 2004

The technique of modulated microwave double resonance spectroscopy has been used to investigate rotational relaxation in ethyl alcohol. For both pure C2H5OH and pure CH2DCH2OH, b-dipole collision-induced transitions have been observed for the trans-OH configuration. Weak b-dipole collision-induced transitions have also been observed for a mixture of C2H5OH and He. Collision-induced transitions obeying a-dipole ``selection rules'' have been searched for, but not observed, in pure C2H5OH and in C2H5OH–He. From the data, it may be inferred that for pure C2H5OH, collision-induced transitions for which ΔJ=2 are much slower than b-dipole transitions. Collision-induced transitions for the gauche-OH configuration of C2H5OH have also been searched for but not observed. Based on the assumption of dipole-dipole interactions, approximate calculations of Rfi have been performed for one series of transitions in pure C2H5OH. These results are in fair agreement with the experimental results, and they support the conclusion that dipole-dipole interactions are the principle mechanism responsible for the collisional transfer of rotational energy in ethyl alcohol. In addition to the double resonance measurements, the linewidths have also been measured for one series of signal transitions in C2H5OH. These data are consistent with those obtained in the double resonance experiments.
Kinetics of the Reaction of O(3P) Atoms with Benzene
Authors: Bonanno Richard A. , Kim Poonzag , Lee Jai-Hoon , Timmons Richard B.
Published: February 2004

The kinetics of the gas phase reaction of O(3P) atoms with benzene was investigated using fast flow techniques. Specific rate constants were obtained using two separate systems, one with ESR detection to monitor the decrease in O(3P) atoms and the other with mass spectrometry to follow the change in benzene concentration. The rate constant computed with respect to O(3P) removal is much larger than that obtained in the mass spectrometer studies and this is believed to result from further reactions of oxygen atoms with radical species produced in the initial O(3P)+C6H6 interaction. On the basis of the mass spectrometric studies a specific rate constant for the reaction O(3P)+C6H6→ [C6H6O]of (3.8± 1.5) × 1013 exp(−4,400± 500/RT) cm3 mole−1· sec−1 was obtained over the temperature range of 255–305°K. A negligibly small kinetic isotope effect was observed in competitive experiments with C6H6 and C6D6. The results obtained are interpreted in terms of a rate determining step involving the addition of oxygen atoms to the benzene ring followed by subsequent isomerization and/or decomposition reactions.
Influence of Surface Charge and Surface Structure on the Sublimation of Ionic Crystals
Authors: Short D. W. , Rapp R. A. , Hirth J. P.
Published: February 2004

The formation of charged defects in ionic crystals is shown to be accompanied by the formation of surface defects: kinks. The resultant defect pairs contribute to a Debye-Hückel charge distribution at the surface. Surface charge, surface structure, and crystal defect equilibria are found to be interrelated and to influence sublimation rates. The theory is applied to the case of sublimation of II-VI compounds and compared with experiments.
Quantum Mechanical Scattering of Asymmetric Molecules. II. Comparison between the Systems He–D2 and He–HT
Authors: Heukels W. F. , van de Ree J.
Published: February 2004

Exact quantum mechanical coupled channel calculations have been performed in order to compare the rotating potential model for heteronuclear molecules with an asymmetric mass distribution, and the usual potential with anisotropy terms for homonuclear molecules. The scattering cross sections of He–D2 and He–HT show no difference in shape, apart from some quenching of the orbiting resonances. There is, however, a significant difference in absolute magnitude, due predominantly to inelastic processes. In the inelastic cross sections, the effect of the shift of the center of mass from the center of interaction in asymmetric molecules counteracts the effect of a change in moment of inertia.
Elastic Constants of the Nematic Liquid Crystalline Phase of p-Methoxybenzylidene-p-n-Butylaniline (MBBA)
Authors: Haller Ivan
Published: February 2004

The bend and splay elastic constants of MBBA were measured over its nematic temperature range. The ratio of the two constants is 1.25± 0.05. The temperature dependence of the elastic constants is found to be adequately described by the simple mean-field models. A comparison is made between viscosity coefficients obtained from the elastic constants and light scattering spectra and from classical viscosity measurements. A method proposed for the measurement of twist elastic constants is evaluated and an upper limit of this constant of MBBA as a function of temperature is reported.