bluuebunny/crossref_metadata_2025 huggingface dataset:
Monte Carlo Calculations on Polypeptide Chains. V. A Hard-Sphere Model for Randomly Coiling Random Copolymers of Glycine and l-Alanine
Authors: Warvari Harold E. , Scott Roy A.
Published: February 2004

A Monte Carlo study of the average dimensions of randomly coiling random copolymers of glycine and l-alanine was carried out using a hard-sphere model similar to that used previously in this laboratory for polypeptide chains. In addition to polyglycine and poly-L-alanine, random copolymers containing on the average 25%, 50%, and 75% glycyl residues 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, and these chains were then used to calculate average dimensions. Unperturbed chains were also generated and used to calculate average unperturbed dimensions for comparison with exact theory and with those for non-self-intersecting chains. As in previous work from this laboratory, chain attrition for all five polymers could be represented by the empirical equation λN=λ∞[(N–c)/(N+d)]6. It was found that the attrition constant λ∞ was greater for the copolymers than for either polyglycine or poly-L-alanine and was greatest for the copolymers containing 25% and 50% glycine. The mean square end-to-end distance and mean square radius of gyration for both non-self-intersecting and unperturbed chains could be represented by equations of the form 〈 r2〉 =a2Nb2 and 〈 s2〉 =a2′Nb2′ for long chain lengths and 〈 r2〉 =a2Nb2 exp[(c12/N)+(c22/N2)+(c32/N3)] and 〈 s2〉 =a2′Nb2′ exp[(c12′/N)+(c22′/N2)+(c32′/N3)] for all chain lengths. Within the statistical reliability of the data, the parameters of the b and b′ type were consistent with the theoretical value of 1.00 for unperturbed chains and, except for the polymer with 25% glycyl residues, with the value 1.20 for non-self-intersecting chains. The 25% glycine containing copolymer had a b value less than 1.20 which is thought to be due to the small size of the data sample. Also as in previous work the ratio 〈 r2〉 /〈 s2〉 for all the chains in this study passes through a maximum as a function of increasing chain length 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 data for 〈 r2〉 /〈 s2〉 vs chain length N could be well represented by an equation of the form 〈 r2〉 /〈 s2〉 =a2″Nb2″ exp[(c12″/N)+(c22″/N2)+(c32″/N3)] with b2″≈ 0.
Monte Carlo Calculations on Polypeptide Chains. VI. Average Dipole Moments for Hard-Sphere Models of Randomly Coiling Polypeptide Chains
Authors: Warvari Harold E. , Scott Roy A.
Published: February 2004

A Monte Carlo study of the average dipole moments of randomly coiling polypeptide chains was carried out using the same hard sphere models used previously in this laboratory for polyglycine, poly-L-alanine, and random copolymers of glycine and alanine. For each polymer 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 mean square dipole moments. Unperturbed chains were also generated and used to calculate unperturbed mean square dipole moments for comparison with those for non-self-intersecting chains. The total chain dipole moment was separated into two parts, the backbone dipole moment, which is due to the sum of the individual dipole moments in the peptide group of each amino acid residue, and the dipole moment due to the unit positive and negative charges at, respectively, the N terminus and C terminus of the chain. It was found that the chain length dependence of these dipole moments for both non-self-intersecting chains and unperturbed chains could be represented a long chain lengths by an equation of the form 〈 p2 〉 =aNb and for all chain lengths by an equation of the form 〈 p2〉 =aNb exp× [(c1/N)+(c2/N2)+(c3/N3)]. Within the statistical reliability of the data, the parameter b for unperturbed chains was consistent with the value 1.00 for the total dipole moment, the backbone dipole moment, and the dipole moment due to the charges at the N- and C-terminal ends of the chain. For non-self-intersecting chains, this parameter was consistent with a value of 1.20 for the total dipole moment and the dipole due to the charges at the chain ends and with a value of 1.00 for the backgone dipole moment. An explanation for this latter result is given which attributes the value of 1.00 to the fact that the individual dipole moments in the peptide groups of each amino acid residue are oriented approximately perpendicular to the chain backbone. A brief comparison is given between the results of this study and that of Flory and Schimmel for short peptides of glycine and L-alanine.
Many-Body Green's Functions for Finite, Nonuniform Systems: Applications to Closed Shell Atoms
Authors: Doll Jimmie D. , Reinhardt William P.
Published: February 2004

Ab initio calculations of natural orbitals, ionization potentials, and total ground state energies for the closed shell systems helium and beryllium are given using the technique of many-body Green's functions. The necessary formalism for application of the Green's function theory to finite, nonuniform, many-body systems is developed following the work of Layzer; connections with standard perturbation theory are made. The natural orbitals obtained were of high quality. Analysis of the diagrammatic expansion of the Green's function led to the surprising result that the main effect of the infinite-order summations implicit in the solution of Dyson's equation was to ``renormalize'' the second-order perturbation corrections.
Electron Spectroscopy of Open-Shell Systems: Spectra of Ni(C5H5)2, Fe(C5H5)2, Mn(C5H5)2, and Cr(C5H5)2
Authors: Rabalais J. W. , Werme L. O. , Bergmark T. , Karlsson L. , Hussain M. , Siegbahn K.
Published: February 2004

The high resolution HeI electron spectra of Ni(C5H5)2, Fe(C5H5)2, Mn(C5H5)2, and Cr(C5H5)2 have been recorded and analyzed in terms of a molecular orbital description of the electronic structure. The ground state electronic configurations have been assigned by considering the feasible ground state configurations, determining the number and type of ionic states obtained from ionization of these configurations, and then comparing the predicted transitions with those observed experimentally. The ground state configuration and adiabatic first ionization potential of these molecules are: Cr(C5H5)2, ··· (e2g)3(a1g)1, 3E2g, I.P.=5.50 eV; Mn(C5H5)2, ··· (e2g)4 (a1g)1, 2A1g, I.P.=6.55 eV; Fe(C5H5)2, ··· (a1g)2 (e2g)4, 1A1g, I.P.=6.72; Ni(C5H5)2, ··· (a1g)2 (e2g)4 (e1g)2, 3A2g, I.P.=6.2 eV. Vibrational structure has been observed in the spectrum of ferrocene and is assigned to progressions in ν4, the symmetric ring-metal stretching mode.
Mechanism of Shear-Induced Structural Changes in Liquid Crystals-Cholesteric Mixtures
Authors: Pochan J. M. , Marsh D. G.
Published: February 2004

The reflection spectrum of the cholesteric Grandjean texture has been obtained as a function of mechanical shear. Data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal-conic texture for shear rates less than 103 sec−1. The correlated helical structure is shown to tilt under applied shear with little distortion. At shear rates greater than 103 sec−1, homeotropic texture is generated. A theoretical model is proposed which explains observed spectral intensity and wavelength data.
Electron Spin Resonance of Alkylaromatic Radical Cations
Authors: Dessau R. M. , Shih S.
Published: February 2004

The electron spin resonance spectra of several previously unobserved alkyl- and cycloalkyl-aromatic radical cations were obtained in a flow system, and the ratio of the methylene hyperfine splittings to the corresponding methyl hydrogen splittings was measured. These ratios, which ranged from 0.46 to 1.79, were rationalized on the basis of the preferred geometrical conformations of these molecules.
Raman Scattering in Wurtzite-Type AgI Crystals
Authors: Bottger G. L. , Damsgard C. V.
Published: February 2004

The polarized laser-Raman scattering of oriented single crystals of β-AgI has been investigated at 300°, 80°, and 4.2°K. The fundamental optic lattice modes predicted by group theory were observed and assigned on the basis of scattering tensors. Further support for the present assignments of the β-AgI optic lattice modes comes from the approximately linear relations found between the squares of the optic-mode frequencies of a number of wurtzite-type crystals and their respective reciprocal reduced masses. The frequencies of the two silent optic lattice modes were determined by a normal coordinate treatment.
Molecular Dynamics Study of Temperature Effects on Water Structure and Kinetics
Authors: Stillinger Frank H. , Rahman Aneesur
Published: February 2004

For a system of 216 water molecules, molecular dynamics calculations have been carried out at two temperatures in addition to the one studied and reported previously. As before, the Ben-Naim and Stillinger effective pair potential was used for these calculations. The results document the breakdown of hydrogen-bond order and the rapid increase in the freedom of molecular motions that accompany temperature rise in real water. We find no evidence at any temperature to support those water models which partition molecules into two classes (bonded framework or cluster molecules vs unbonded molecules). Nevertheless the pair potential distribution function changes with temperature in such a way as to suggest a basic hydrogen-bond rupture mechanism characterized by an excitation energy of about 2.5 kcal/mole. Some results indicate that the potential utilized is a bit too tetrahedrally directional to represent real water faithfully, so a possible modification is mentioned.
Laser Induced Combination Band Fluorescence in Ethylene Gas
Authors: Yuan Raymond C. L. , Flynn George W.
Published: February 2004

Laser induced vibrational fluorescence decay measurements have been made for the (v7+v8) and (v6+v10) combination bands of C2H4 gas. The rate of vibration-translation/rotation energy transfer in ethylene was found to be 7.23± 0.3 msec−1·torr−1. This result is in agreement with one sound absorption study but disagrees with several ultrasonic dispersion studies. The rate constants for deactivation of C2H4 during collisions with CH4 and C2H6 were also determined as 8.0 msec−1·torr−1 and 92.0 msec−1·torr−1, respectively. The efficiency of ethane as a collision partner is almost certainly an indication of near resonant vibrational energy transfer between the C2H4 and C2H6 molecules. The dependence of the 5-μ and 3-μ fluorescence intensity on laser power suggests that excitation of the fluorescing states is achieved through a combination of multiphoton absorption and collision processes.
Studies of the Triplet State of Carbonyl Compounds. I. Phosphorescence of β-Diketones
Authors: Gacoin P.
Published: February 2004

The phosphorescence intensity of the diketone form and of one of the two chelated enol forms of β-diketones in solution at 77°K depends largely on the solvent used. The phosphorescence of the diketone form (seen principally in polar solvent) is similar to that of aromatic monoketones and arises from a Tnπ* state. The phosphorescence of one of the two chelated enol forms in nonpolar solvent is emitted from a Tπ π* state as suggested by the external and internal heavy atom effects. However the shortness of the phosphorescence lifetime shows that the lowest triplet state is probably mixed with some higher energy triplet state of nπ* type. The diketone form seems much more luminescent than the chelated enol form. This result is consistent with the suggestion of Lamola [A. A. Lamola and L. J. Sharp, J. Phys. Chem. 70, 2634 (1966)] that a very fast radiationless decay takes place in molecules possessing an intramolecular hydrogen bond between the carbonyl oxygen and the hydroxyl hydrogen. The comparison of chelated enol spectra with those of crotonophenone and benzalacetone indicates that phosphorescent chelated enol has structure II, the other form being nonradiatively deactivated or nonexistent. Benzoyltrifluoroacetone gives a spectrum with a long lifetime which seems to originate from a nonchelated enol form.
Kinetic Theory of Droplet Growth in Nucleation
Authors: Kelly G. E. , Sengers J. V.
Published: February 2004

A kinetic theory is presented for the mass flux to a liquid droplet surrounded by its pure vapor. When the mass flux Γ is expanded in terms of a parameter α which is the ratio of the droplet size to the mean free path (inverse Knudsen number), one obtains a series of the form Γ=Γ(0)+Γ(1)α +Γ̃(2)α2 lnα +···. The coefficients of the first three terms of this expansion are derived by solving the Boltzmann equation using a modified Knudsen number iteration procedure. It is shown that the coefficients are determined by integrals associated with sequences of successive collisions among a number of vapor molecules and the droplet. These collision integrals bear a close similarity to the collision integrals derived earlier from the generalized Boltzmann equation for the density dependence of the transport properties of gases.
Reactions of NO2− and NO3− with HCl and HBr
Authors: Ferguson E. E. , Dunkin D. B. , Fehsenfeld F. C.
Published: February 2004

Reactions of NO2− and NO3− with HCl and HBr were carried out in a flowing afterglow system at room temperature and the reaction rate constants were measured. The reaction NO3−+HBr→ Br−+HNO3 is found to be nearly thermoneutral. The results lead to these values of electron affinities: E.A. (NO2)< 2.6 eV, E.A.(NO3)=3.9± 0.2 eV. In addition, rate constants for the reactions of NO− and O2− with HCl are reported. A number of cluster ions were observed: NO3−(HCl)1,2, NO3−(HNO3)1,2, NO3−· HNO3(HCl)1,2, and Br− (HNO3)1,2.
Optical Model for Vibrational Relaxation in Reactive Systems
Authors: Poulsen Lise Lotte , Ross John , Steinfeld Jeffrey I.
Published: February 2004

The probabilities for vibrational transitions in the presence of open reactive channels are calculated with an optical model. A linear collision of an atom and a diatomic molecule is considered. The distorted wave Born series is derived and an analytic expression is given for the first order result for an exponential repulsive potential. A semiclassical analysis is also presented. The results are applied to an electronically excited I2 molecule colliding with an inert atom. The vibrational transition probabilities are found to be appreciably affected only in the few cases where the predissociation probability is very high, that is larger than about 0.3.
Phase and Temperature Dependence of Positron Annihilation in Liquid Crystals
Authors: Nicholas J. Blair , Ache Hans J.
Published: February 2004

Positron lifetime measurements were carried out in a series of liquid crystalline compounds. The resulting lifetime spectra were decomposed into two components. The short-lived component (lifetime τ1 and intensity I1) was attributed to free-positron and parapositronium annihilation whereas the long-lived component τ2, I2 was considered to be the result of orthopositronium annihilation. Changes of τ2 and I2 during mesomorphic phase transitions are generally very small and can be related to the transition entropies involved, supporting the contention that the positron annihilation process is affected by variations in the structural regularities of the environment. Various mechanisms which could account for the observed changes of the positron lifetimes and their intensities at the phase-transition points, are discussed.
Raman Spectrophotometric Study of Ion Pairing in Aqueous Zinc Nitrate Systems
Authors: Lemley Ann T. Garahan , Plane Robert A.
Published: February 2004

Raman spectrophotometric measurements of concentrated aqueous zinc nitrate solutions at 50°C, 70°C, and 88°C are explained in terms of a model consisting of three nitrate ion species: contact ion paired nitrate, solvent separated nitrate, and ``free'' nitrate. Integrated intensity measurements are related to concentrations of each type of nitrate ion, and concentration quotients for the equilibrium reactions between species are calculated. For the free nitrate to solvent separated nitrate reaction, the apparent ΔG° is 0.89 kcal/mole at 50°C and 1.31 kcal/mole at 88°C. The apparent ΔS° is −14.5 e.u.. For the solvent separated nitrate to contact ion paired nitrate reaction the apparent ΔG° is −0.92 kcal/mole at 50°C and −2.4 kcal/mole at 88°C. The apparent ΔS° is +40 e.u..
Photolysis of Benzyl Chloride Vapor in the Presence of Ethane or n-Butane
Authors: Ichimura Teijiro , Mori Yuji
Published: February 2004

Benzyl chloride vapor has been photolyzed in the presence of ethane or n-butane at 184.9, 206.2, and 253.7 nm. Pressure, wavelength, and temperature effects on the quantum yields of products were examined. Benzyl chloride was found to photodecompose predominantly to a benzyl radical and a chlorine atom. The chlorine atoms abstract the hydrogen atom from ethane or n-butane to form ethyl or butyl radicals. The reaction of these radicals with benzyl radicals results in the formation of various products. The quantum yield of the initial photodecomposition of the C–Cl bond in benzyl chloride at 184.9 nm (at zero n-butane pressure) was obtained to be 0.9. The initial photodecomposition rate constant of benzyl chloride at 184.9 nm has been estimated. The ratio between disproportionation and recombination reaction rate constants of radicals has also been obtained.
Effective Potentials in Molecular Collisions
Authors: Rabitz Herschel
Published: February 2004

Procedures are developed for obtaining effective potentials Veff between two linear molecules. The potential Veff couples the internal rotational states of the molecules, regardless of the spatial effects associated with the projection quantum numbers M. Since just Veff is not sufficient for treating the dynamics, an effective Hamiltonian Heff = H0eff + Veff is obtained from the original Hamiltonian H = H0 + V, where H0 describes the unperturbed motion of the molecules. A consistent procedure is presented for finding both effective operators H0eff and Veff, where the matrix elements of an effective operator Qeff are written as 〈 J1J2  |  Qeff  |  J1′J2′ 〉. The effective states |  J1J2〉 are orthonormal eigenstates of the rotational portion of H0eff with the same level spacings as for H0. The effective potential is shown to have the form of a radial function times an internal state coupling matrix, or in general a sum of such terms. The effective close-coupled equations are developed from Heff, and it is demonstrated that there is a decoupling of the orbital and internal rotational angular momenta. The significantly reduced dimensionality of the effective equations is shown by comparison with the corresponding equations derived from H. A numerical example using Veff in semiclassical time-dependent perturbation theory is presented.
Phonons, Librons, and the Rotational State J = 1 in HCP and FCC Solid Hydrogen by Neutron Spectroscopy
Authors: Stein H. , Stiller H. , Stockmeyer R.
Published: February 2004

Transitions between the rotational states J = 1 and J = 0 and the densities of phonon and libron states are investigated by incoherent inelastic neutron scattering at 4.2 and at 1.15°K for several concentrations c of the ortho component. By extrapolation of the observed c-dependent energy of the J = 1 state to c = 0 and c = 1 the effective quadrupole-quadrupole coupling constant is determined. The observations on the densities of phonon and libron states are compared to theoretical predictions.
Hypervirial and Hellmann-Feynman Theorems Applied to Anharmonic Oscillators
Authors: Swenson Robert J. , Danforth Scott H.
Published: February 2004

A set of hypervirial theorems plus the Hellman-Feynman theorem are applied to a general anharmonic oscillator. The exact energy and expection values of powers of the position coordinate are expanded in a power series of the anharmonic coupling constant and it is shown that use of the above theorems enables one to express each term in these expansions solely in terms of the unperturbed energy and known constants. This procedure thus eliminates the usual tedious calculations of sums over intermediate states of products of matrix elements which arise in nth order Rayleigh-Schrödinger perturbation theory.
On the Application of Perturbation Theory for the Calculation of Molecular Constants due to Small Mass Changes and on the Determination of Force Constants from Very Heavy Isotope Substitution
Authors: Müller A. , Schmidt K. H. , Mohan N.
Published: February 2004

The first order perturbation theory has been used to develop the expressions for studying the mass effect on Coriolis coupling constants in isotopically substituted molecules. The corresponding treatment reported by Wilson, Decius, and Cross for the calculation of isotopic frequency shifts has been applied to a large number of molecules with heavy and very heavy atom isotopic substitution, for which the experimental data have been published in recent years. The use of a simple method (L matrix approximation) for the calculation of isotope shifts in molecules with small mass coupling, from the knowledge of the geometrical parameters only, is suggested. The results obtained using the perturbation theory for the calculation of Δv and Δζ are found to be quite encouraging. An outline of the utility of data on very heavy isotopic substitution for the determination of force constants is given with illustrative examples.
Photoelectron Spectra of Small Rings. IV. The Unsaturated Three-Membered Rings
Authors: Robin M. B. , Brundle C. R. , Kuebler N. A. , Ellison G. B. , Wiberg K. B.
Published: February 2004

The HeI and HeII photoelectron spectra of cyclopropene, diazirine, and difluorodiazirine are compared with the results of Gaussian orbital calculations of double-zeta quality. The generally excellent agreement between calculation and experiment strongly supports the theoretically derived inversions of many of the levels. For example, on going from cyclopropane to cyclopropene, there is a crossover of the uppermost pi and sigma MO's, which again recross to place sigma above pi in diazirine and difluorodiazirine. In the diazirines, the lone pair splitting is 3.5 eV, with the uppermost pi MO located between them. Several accidental degeneracies occur in difluorodiazirine.
Phosphorescence of Benzophenone Crystals
Authors: Laporte J. L. , Nouchi G. , Rousset Y.
Published: February 2004

The phosphorescence from benzophenone crystals has been investigated as a function of temperature between 7 and 77°K. Three types of T → S emission are detected and the importance of vitreous defects in crystalline benzophenone is examined.
Density Expansions for Effective Pair Potentials
Authors: Stillinger Frank H.
Published: February 2004

The leading density dependence for the variationally defined effective pair potential v is derived, using van Kampen's product technique to evaluate the multidimensional configuration integrals that arise. This function v is compared in that density order to two other effective pair potentials that have been proposed, and it is shown to be the only one that simultaneously reduces both free energy and pair correlation errors to quadratic order in the magnitude of triplet nonadditivity. The signs of density shifts in v at low temperature are analyzed for noble gases and for water in various configurations, and a reasonable arbitrary-density approximation to v is suggested that may have utility in simulation of liquids by rapid electronic computer.
Temperature Dependence of the ESR Spectrum from a Tunneling Methyl Group
Authors: Davidson R. B. , Miyagawa Ichiro
Published: February 2004

The electron spin resonance spectrum for the unstable radical CH3ĊHR in irradiated L-alanine was observed from 4.2–80°K. The hyperfine lines for MI = ± 1/2 showed relative maxima in peak intensity at 11°K when partially saturated. The presence of the maxima is interpreted in terms of spin-diffusion and Brownian mixing.
Approach to Partially Predetermining Molecular Electronic Structure. The Li He Interaction Potential
Authors: Fink William H.
Published: February 2004

The utility of the simplest possible formulation of the group function method of McWeeney for electronic structure calculations of systems involving identifiable molecular subunits is discussed. The method is applied to the calculation of the Li He interaction potential in order to provide a means of identifying the interaction energy as an electrostatic interaction, a backbonding interaction and a full molecular orbital SCF interaction. Some interesting results for methods of integrated charge analysis are also discussed.
Crystal Coulomb Energies. I. The Electrostatic Binding Energy of Wurster's Blue Perchlorate by Ewald's Method
Authors: Metzger Robert Melville
Published: February 2004

Wurster's blue perchlorate, an ion-radical salt which is a Curie-Weiss paramagnet above 188°K and exhibits spin pairing below 188°K, was found by Ewald's method to have an electrostatic binding energy of −4.15 ± 0.06 eV/molecule. In the isomorphous Wurster's blue iodide the crystal binding energy is very slightly greater. Nearest-neighbor linear-chain cation-cation repulsions play a minor role in the overall stability of both lattices. From room-temperature crystallographic data, no conclusions could be drawn about the changes in perchlorate geometry at the first-order phase transition in Wurster's blue perchlorate.
Calculations on Alkali and Halide Ion Hydration
Authors: Eliezer I. , Krindel P.
Published: February 2004

Potential energy surfaces of ion-water molecule interactions have been calculated using a classical model. The ions considered were the alkali and halide ions. The interaction energy was represented as a sum of electrostatic, polarization, dispersion, and repulsion energy terms. The calculation yielded the energy of the ``hydration'' reaction: H2O+ion→ion(H2O) as well as information on the configuration and the flexibility of the ion-water bond. Possible consequences of this calculation for certain hydration properties (especially ``structure making'' and ``structure breaking'') are discussed.
Spectra and Structure of Small Ring Compounds. XXV. Microwave Spectrum of Methylenecyclopentane
Authors: Durig J. R. , Li Y. S. , Carreira L. A.
Published: February 2004

The microwave spectrum of methylenecyclopentane has been recorded from 18.0 to 26.5 GHz. Both Q-branch and R-branch assignments have been made for the ground state. Five vibrationally excited states of the ring-puckering mode and one vibrationally excited state of the ring-twisting mode have been assigned. From the relative intensity measurements, it is concluded that the first excited state of the ring-puckering vibration has a frequency of 63 ± 10 cm−1 and that the vibration is nearly harmonic with a barrier of pseudorotation of 1.8 kcal/mole. The two out-of-plane ring vibrations are essentially independent and the methylenecyclopentane ring is not significantly different in rigidity than that of cyclopentanone. The components of the dipole moment were determined by the Stark effect to be μa=0.60 ± 0.01 D and μc=0.00 ± 0.06 D. All the observed data are consistent with a permanently twisted ring of C2 symmetry.
Molecular Orbital Theory of the Hydrogen Bond. III. Dimers Containing NH2OH, H2O, HOF, and H2O
Authors: Del Bene Janet E.
Published: February 2004

Ab initio calculations using a minimal STO-3G basis set have been performed on the pairs of mixed dimers NH2OH–H2O and HOF–H2O, and on (NH2OH)2 and (HOF)2. Equilibrium structures and energies are reported and analyzed for each. It is found that all dimers have equilibrium open chain structures except (NH2OH)2, which exists as a cyclic dimer. Two stable dimer structures of comparable energies are found for the mixed dimer HOF–H2O which has HOF as the proton donor molecule, and for (HOF)2. In the entire series of dimers ROH–H2O and (ROH)2, calculated dimer properties are analyzed as a function of the substituent R, and correlations are made where possible.
Reaction Rates of Uranium Ions and Atoms with O2 and N2
Authors: Johnsen R. , Biondi Manfred A.
Published: February 2004

A drift mobility-tube-mass-spectrometer apparatus has been used to study the ion-molecule reaction, U++O2→UO++O, from thermal (300°K) energy to ∼5 eV ion mean energy. At 300°K, the reaction rate is (8.5, −1, +4) × 10−10 cm3/sec and falls slowly with increasing ion energy to ∼ 7 × 10−10 cm3/sec at 5 eV. The corresponding reaction of U++N2 is endothermic and not detected to occur up to ∼5 eV ion energy. Chemi-ionization of uranium by the association reaction U+O2 → UO2++e is detected. An estimate of the total reaction rate for U+O2, including the oxidation branch to produce UO+O, yields a value ∼ 2 × 10−12 cm3/sec at 300°K. Implications of these studies for ion cloud generation in the ionosphere are discussed.
Ion-Molecule Reactions in Silane-Methane Mixtures
Authors: Stewart G. W. , Henis J. M. S. , Gaspar P. P.
Published: February 2004

The ion-molecule reactions leading to the formation of SiCH2+, SiCH3+, SiCH4+, and SiCH5+ in the silane-methane system are reported along with relative and absolute rate constants for the observed reactions. CH3+, CH4+, and CH5+ are observed to undergo H− transfer reactions with large rate constants. The primary ions Si+ and CH3+ did not react to form products containing silicon-carbon bonds. It is therefore concluded that methylisilanes produced in activated silane-methane systems (discharges, nuclear recoil experiments, etc.) do not result from reactions of Si+ and/or CH3+ at thermal kinetic energies.
Selection of the Proper Canonical Roothaan-Hartree-Fock Orbitals for Particular Applications. I. Theory
Authors: Davidson Ernest R.
Published: February 2004

The traditional Roothaan-Hartree-Fock orbitals are not invariant under changes in the partitioning of the Hamiltonian into one and two body operators. This arbitrariness may be exploited to derive canonical orbitals adapted to specific application in spectroscopy or in correlation energy calculations.
Selection of Proper Canonical Orbitals. II. Water
Authors: Elbert Stephen T. , Langhoff Stephen R. , Davidson Ernest R.
Published: February 2004

Numerical results of a 13 basis function calculation for the water molecule are presented to illustrate the theory developed in Paper I. As expected, the internally consistent self-consistent-field results are better for describing spectra than the traditional results. Orbitals specifically adapted to each transition are still better. No choice of orbitals tried here was really satisfactory for a configuration interaction or perturbation calculation of the correlation energy.
X-Ray Diffraction Studies on Liquids at Very High Pressures along the Melting Curve. I. Methods and Techniques
Authors: Brown Keith H. , Barnett J. Dean
Published: February 2004

A technique using the tetrahedral anvil high-pressure x-ray system has been developed to extract coherent x-ray intensity patterns of liquids near melting from measured scattering curves in the presence of high-background counts. Radial distribution functions for liquids along the melting curve to pressures of at least 50 kbar can be obtained. At each pressure scattering curves are recorded for the liquid and solid on opposite sides of the melting curve and a subtraction is made to eliminate background and incoherent scattering. Theoretical estimates of the thermal diffuse scattering from the solid are added to the appropriately corrected difference patterns to yield the coherent intensity patterns for the liquid. The technique is directed to the measurement of pressure-induced changes rather than absolute measurement of the intensity patterns and the radial distribution functions.
X-Ray Diffraction Studies on Liquids at Very High Pressures along the Melting Curve II. Sodium
Authors: Brown Keith H. , Barnett J. Dean
Published: February 2004

Coherent x-ray diffraction patterns for liquid Na have been extracted from measured patterns at 0.5, 7.5, 17, and 43 kbar along the melting curve. The experimentally determined liquid structure factor and resulting radial distribution function are given at each pressure. The position of the principal x-ray intensity peak is reproducible to within 0.5% and shows shifts of the order of 12%. The resulting nearest neighbor distance in the radial distribution function also shifts 12% but is reproducible only to approximately 2%. The distribution functions obtained at points along the melting curve suggest a simple dimensional compression or scaling; however, a sharpening of the distribution peaks slightly in excess of that predicted by simple scaling is indicated by the measurements.
Computer-Graphics Studies of Dipole-Dipole Collisions: Evidence for Neutral Collision Complexes
Authors: Dugan John V. , Palmer Raymond W.
Published: February 2004

Three-dimensional computer-graphics studies of orientation-dependent forces are extended to multiple reflection behavior in dipole-dipole collisions. The collision pairs DCl–DCl and HCl–HCl are studied for rotational temperatures of 25, 77, and 300°K and translational temperatures of 77 and 300°K. The probability of a multiple reflection collision is a very sensitive inverse function of hard-core reflection distance Rc for fixed temperatures. A brief comparison of the results is made with two-dimensional results of Clarke and Smith for CHF3–CHF3.
Exciton Theory of the Excited Electronic States of Coupled Molecular Aggregates
Authors: Philpott Michael R. , Lee Jo W.
Published: February 2004

The properties of the excited electronic states of coupled molecular aggregates, which include ligand-polymer complexes and crystals with interstitial impurities, are considered using two versions of second quantized exciton theory. The first version makes use of the dipole approximation and assumes that one aggregate is not disturbed or distorted by the presence of the other. The second version is not restricted to dipole interactions or undistorted host aggregates. Illustrative calculations are presented for the exciton interactions between dyes externally bound to a helical polymer.
New Method for Generating Density Expansions
Authors: Ramshaw John D.
Published: February 2004

The calculus of finite differences is used to develop a new method for expressing the thermodynamic limit of a reasonably arbitrary statistical-mechanical average as a power series in the number density ρ. The method is simple, straightforward, and purely analytic: it involves no intermediate expansion in powers of the activity and it avoids the use of graph theory. Moreover, the method is developed independently of the prescription for computing the statistical average, a fact which lends to the results an especially wide range of applicability. In particular, these results may be used in classical or quantum statistical mechanics, for intermolecular potentials which are not spherically symmetric or pairwise additive, for molecules of arbitrary internal structure and complexity, and for polar molecules. A general formula is obtained for the coefficient of ρk in the series; as usual, the most difficult problem one need solve in order to compute this coefficient is the evaluation of a k-molecule average. It is shown that if all the coefficients exist and if the density is less than a certain well-defined critical density, then the series converges to the thermodynamic limit of the average in question. The practical use of the method is clarified by examples.
Ground and Low-Lying Excited Electronic States of FeH
Authors: Walker J. H. , Walker T. E. H. , Kelly H. P.
Published: February 2004

Many-body perturbation theory is used to carry out an approximate Hartree-Fock calculation on electronic states of FeH with a single-center basis set. Approximate potential curves are obtained and used to predict the main features of the spectrum. They are also employed to interpret the known spectra of the ``isoelectronic'' FeCl.
Lattice Dynamics of Polyglycine I
Authors: Fanconi Bruno
Published: February 2004

The two-dimensional lattice vibrations of the extended form of polyglycine have been calculated from a five mass model of the glycine chemical repeat unit and with interchain hydrogen bond stretching. The effects of hydrogen bond bending and torsion on the lattice modes are determined using a model in which the entire chemical repeat unit is treated as a single dynamical unit. The valence force field for the latter model is found by fitting the optical frequencies determined from the five mass calculation. The density of phonon states for the two-dimensional lattice is determined for the single mass model.
Scattering of Fast Potassium Ions by Helium, Neon, and Argon
Authors: Amdur I. , Jordan J. E. , Chien K. -R. , Fung L. W. -M. , Hance R. L. , Hulpke E. , Johnson S. E.
Published: February 2004

A new apparatus has been designed and built, and measurements have been made of the effective total cross sections for the scattering of potassium ions with energies between 150 and 2350 eV by room temperature helium, neon, and argon. The repulsive interaction potentials were deduced from the measured cross sections with two different representations of the potential, the inverse power, and the exponential. The results are as follows (energies are in electron volts and distances are in angstroms): K+–HeV(r)=27.0/r6.22,V(r)=2200 exp(−4.56r),1.18 ≤ r ≤ 1.83,K+–NeV(r)=122/r7.64,V(r)=7850 exp(−4.79r),1.42 ≤ r ≤ 2.02,K+–ArV(r)=667/r7.78,V(r)=8900 exp(−3.96r),1.76 ≤ r ≤ 2.47.These results compare very favorably with other experimental potentials reported recently but are in sharp disagreement with some earlier results. They also disagree with reported potentials which were calculated theoretically, except for the case of K+–Ar where the agreement between theory and experiment is remarkably good. The potentials are also compared to those determined from measurements of the scattering of argon by the same three gases, and the effect on the potential of the additional nuclear charge is discussed.
Magnetic Structure of HoZn2
Authors: Debray D. , Sougi M.
Published: February 2004

Neutron powder diffraction measurements reveal that HoZn2, having the body-centered orthorhombic crystal structure (CeCu2-type), becomes an antiferromagnet below the Néel temperature of 13.5°K. The moment alignment is represented by the composite of two sinusoidal transverse waves both polarized in the b axis direction and propagating along the c axis but with the phase difference of 60.7 ± 1.2°. The propagation wavelength and amplitude at 4.2°K are, respectively, 2.27 in units of c and 9.4 ± 0.1 Bohr magnetons. Below 4°K, there are indications of the onset of another magnetic transition.
Proton Magnetic Susceptibility of Solid CH3D
Authors: Code R. F. , Noble J. D.
Published: February 2004

The temperature dependence of the proton magnetic susceptibility of solid CH3D (commerical purity) has been measured between 4.2 and 1.2°K by a NMR absorption experiment. The susceptibility was found to vary as T−1 between 4.2 and 1.5°K within an experimental error of ± 3%. At 1.15°K the observed susceptibility was approximately 5% greater than predicted from a T−1 law, indicating that some conversion from E to A type nuclear spin symmetry may have occurred.
Determination of Reaction Rates by the Use of Cubic Spline Interpolation
Authors: Dunfield Lawrence G. , Read John F.
Published: February 2004

The differential method of van't Hoff is a powerful method for kinetic data analysis if accurate rates of reaction can be found. Differentiation methods reported in the literature are inadequate, particularly when applied to data which is unequally spaced or which is subject to random error. A need is therefore expressed for a general numerical method of differentiation which can be readily applied in kinetic analysis. This paper suggests the use of the cubic spline, an interpolation method which overcomes the difficulties of other methods and gives exceptionally accurate derivatives. The theory of the cubic spline, its properties, and uses are discussed. Its application to experimental data is demonstrated and its performance is quantitatively compared with better known but less accurate methods.
ESR Study of MnO42− in γ-Irradiated KMnO4
Authors: Subramanian S. , Rogers Max T.
Published: February 2004

When KMnO4 is γ irradiated at 77°K a single paramagnetic species is detected which is shown to be basically the manganate ion, Mn(VI)O42−, however, the single odd electron interacts at the same time with a second manganese nucleus of a neighboring diamagnetic species. Although the species giving rise to the weaker, superhyperfine 55Mn interaction could not be identified with certainty, it is probably a neighboring MnO4− ion or MnO3+ lattice defect so that the center produced might be written [Mn(VI)O42−··· Mn(VII)On7−2n]. The g tensor and both 55Mn hyperfine splitting tensors have been obtained, and the structure of the radical is discussed. On warming to room temperature a radical not containing manganese, probably O3−, is found.
NMR Relaxation of Adsorbed Gases: Methane on Graphite
Authors: Riehl J. W. , Koch K.
Published: February 2004

Spin-lattice (T1) and spin-spin (T2) relaxation times have been measured for methane (CH4) adsorbed on a graphitized carbon black, using pulse techniques at 30 MHz. Temperatures ranged from 4–100°K, fractional coverages from 0.02 to 0.9 of a statistical monolayer. The data indicate an adsorbed gas phase at high temperatures and low coverages and an adsorbed ``solid'' phase at low temperatures and low coverages. The data are discussed in terms of the molecular dynamics in the two phases and the barrier to rotation with respect to the surface.
Photoequilibrium and the Decay Mechanism in Dinitrobenzyl Pyridine Photochromism
Authors: Ben-Hur David , Hardwick R.
Published: February 2004

The maximum intensity of photochromic color change in 2- and 4-(2′, 4′-dinitrobenzyl) pyridine solutions is controlled by a photon equilibrium. The equilibrium has been approached from both sides by means of irradiation. Solutions of the compound almost entirely in the form of its colored modification have been prepared. In the absence of photon activated decay, which is controlling in experiments where the preparation is made photolytically, these solutions decay thermally to the ordinary form by two mechanisms, the relative importance of which depends on temperature and pH.
Hydride Ion Transfer and Isotope Effects in CD4-Silane Ion Molecule Reactions
Authors: Stewart G. W. , Henis J. M. S. , Gaspar P. P.
Published: February 2004

Hydride ion transfer is identified as an important mechanism in the reactions of methane and deuterated methane primary ions with silane. The pathway for these reactions is shown not to involve the production of a long lived intermediate. However, it is shown that addition reactions proceeding through a long lived intermediate show relatively little isotopic scrambling. Furthermore, it appears that the probability of losing a H atom is very much higher than that for losing a D atom from such an intermediate.
Torsion-Vibration-Rotation Interactions in Methanol. IV. Microwave Spectrum of CH3OH in the Excited CO Stretching State
Authors: Lees R. M.
Published: February 2004

The K=2← 1 E1 Q branch and the JK=10← 00 A-E doublet have been observed for the first excited CO stretching mode in the microwave spectrum of normal methanol. A value for the excited state barrier to internal rotation of 397.9± 3 cm−1 is required to reproduce the Q-branch origin frequency of 47 929.9 MHz, with reasonable limits on geometry changes assumed. This value is consistent with previous infrared results.
Another Hard-Sphere Equation of State
Authors: Hall Kenneth R.
Published: February 2004

A basic form for a Pade approximant of the hard-sphere equation of state is proposed. This form incorporates both high and low density limiting behavior and can easily be extended to include any further virial coefficients as they are calculated. Furthermore, addition of a simple term converts the approximant into an accurate equation for hard-sphere fluids. Hard-sphere solid behavior can also be estimated using a modification of the approximant, although the results are not as satisfactory as those of the high density hard-sphere equation of state.
Photophysical Processes in Polymers. V. Triplet Depletion in Solid Vinylnaphthalene Polymers and Copolymers
Authors: Fox R. B. , Price T. R. , Cozzens R. F. , McDonald J. R.
Published: February 2004

The results of a study of triplet energy trapping at 77°K are presented for vinylnaphthalene polymers and copolymers with styrene and methyl methacrylate. Delayed emission spectra originating from traps both intrinsic and extrinsic to the polymer chains are identified. In the solid polymers both intramolecular and interchain triplet energy migration occur, with eventual transfer to a trap at collisional distances of about 15 Å. Extrinsic traps lead to phosphorescence and delayed fluorescence, while traps that are part of the polymer chain lead to delayed exciplex fluorescence. In the model compound 1-ethylnaphthalene, only extrinsic trapping phenomena are observed. The influence of concentration on trapping phenomena in both polymers and the model compound is considered from the standpoint of chain or molecular aggregation.
Microwave Spectrum and Ring-Bending Vibration of 3-Oxetanone
Authors: Gibson James S. , Harris David O.
Published: February 2004

The microwave rotational spectrum of 3-oxetanone, O–CH2–C=O||,–CH2–has been observed and assigned for the ground vibrational state, the first five excited states of the ring-bending vibrational mode, and the two carbon-13 isotopically substituted species. The features of the microwave spectrum indicate that the molecule has a single-minimum ring-bending vibrational potential and consequently a planar equilibrium configuration. Approximate models for the ring-bending vibrational mode were set up and the vibrational potential function interpreted in terms of contributions from ring-angle deformations and torsional interactions. The 13C data were used to derive a partial substitution structure. With the carbonyl carbon being labeled Cβ, the structural parameters determined in this way were r(CαCβ)=1.522±0.002 Å,∠CαCβCα′=88.06°±0.15°.The moment equations for the hypothetically planar molecule were solved to give some indication of the remaining parameters. Stark effect measurements yielded the dipole moment as 0.887± 0.005 D for the common isotopic species in the ground vibrational state.
Paramagnetic Resonance of Divalent Manganese in Thallous Azide
Authors: Owens F. J.
Published: February 2004

The electron spin resonance spectrum of Mn2+ doped in single crystals of TIN3 has been investigated. The spectrum is described by the spin Hamiltonian H· g· S + S· D· S + AI· S having I=52 and S=52 and at room temperature |D| = 70 G., |A∥| = |A⊥|= 88 G. and g∥=2.0029± 0.0003. The z axis of the axially symmetric zero field tensor is parallel to the c axis. The zero field tensor is strongly dependent on temperature, increasing linearly as the temperature is decreased from +80° to −30°C. The individual hyperfine lines of the spectra show an unusual broadening as the temperature is decreased. The parameters of the spin Hamiltonian for Mn2+ in TIN3 are compared with those of Mn2+ in KN3 and RbN3 which have the same tetragonal structure but different electronic properties.