bluuebunny/crossref_metadata_2025_split huggingface dataset:
Isenthalpic Solidification and the Specific Heat of Supercooled Liquid Phosphorus
Authors: Glicksman M. E. , Schaefer R. J. 
Published: January 2005

Isenthalpic solidification of a unary (one-component) supercooled liquid results in either two-phase invariant equilibrium or in single-phase univariant equilibrium, depending on the degree of supercooling prior to solidification.
When postsolidification univariant equilibrium obtains, a determination of the specific heat at constant pressure, Cpl, of the supercooled phase relative to the stable, solid phase can be made by measuring the adiabatic temperature rise during recalescence.
Values of Cpl for highly supercooled phosphorus over the temperature range −1° to +18°C have been determined with this method. No evidence for the changes in molecular association deduced from earlier viscosity data on supercooled phosphorus was found in the temperature variation of the specific-heat function. The enthalpy and specific-heat values determined for supercooled liquid phosphorus agree with published values for the stable liquid above the normal melting point, Te=44°C.
19F Spin—Rotation and Spin—Spin Interactions, 19F Magnetic Shielding, and Molecular Magnetic Moments for OCF2
Authors: Lo Mei-Kuo , Weiss V. W. , Flygare W. H. 
Published: January 2005

The microwave spectrum of OCF2 is examined under high resolution using an L-band waveguide and 1 and 5-kc/sec modulation, where half-widths at half-height of 5–7 kc/sec are obtained. The 19F nuclear-spin-nuclear-spin and the 19F spin—rotation interactions are observed. The experimental 19F spin—rotation constants along the principal inertial axes are Maa = | 19±3 | kc/sec, Mbb = | 13±3 | kc/sec, and Mcc = | 5±3 | kc/sec. The calculated magnitude of the nuclear contribution to the 19F spin—rotation constants show that the signs of the above values are all negative, which then leads to the calculated 19F paramagnetic shielding in OCF2 of σp = −324×10−6.
The 19F chemical shift in OCF2 is measured relative to Cl3CF giving 23×10−6. Relating the 19F shielding back to F2 as a reliable standard gives the total shielding at 19F in OCF2 as 236×10−6. Combining this with the paramagnetic shielding obtained from the 19F spin—rotation constants gives the diamagnetic shielding as σd = 560×10−6, which is larger than the corresponding 19F diamagnetic shielding in F2 of 530×10−6.
The first-order molecular Zeeman effect was also observed in OCF2 using a zigzag cell in a magnetic field up to 11 kG giving the diagonal elements of the g-value tensor along the principal inertial axes of gaa = | 0.074±0.002 |, gbb = | 0.038±0.003 |, and gcc = | 0.028±0.003 |. Arguments are given to suggest that the signs of the g values are all negative, which yields the paramagnetic susceptibility of χp = 127×10−6. The g values are combined with an estimate of the molar susceptibility to yield the average diamagnetic susceptibility in OCF2 giving a most probable value of 〈ψ0 | ∑ iri2 |ψ0〉=47.4×10−16cm2.
A theory is presented which allows an estimation of either the molecular g values or the 19F spin—rotation interaction if one of the two quantities is known. The 19F parameters obtained in this study may be transferable to other systems.
Several low-J transitions in OF2 are examined in a 4-ft liquid-N2 absorption cell, and the magnetic properties of 19F in this molecule are re-examined.
Atomic-Nitrogen Recombination
Authors: Evenson K. M. , Burch D. S. 
Published: January 2005

Three-body and wall recombination coefficients of atomic nitrogen were measured using an electron spin resonance spectrometer to determine the atomic nitrogen concentrations. Studies were made of nitrogen gas at pressures from 3 to 35 torr flowing in a cylindrical tube and nonflowing nitrogen gas at pressures from 0.3 to 7 torr. The three-body recombination coefficient k1 with molecular nitrogen as the third body was found to be (2.25±0.2)×10−32 cm6 sec−1. The wall recombination probability, λ, of quartz was found to range from 7×10−6 to 5×10−4, decreasing with increasing oxygen impurity; the variation for Teflon was measured to be from 2×10−7 to 2.5×10−5, increasing with increasing oxygen content. The sum of the two-body radiative recombination and two-body wall recombination was a significant part of the total atom recombination; the sum of these coefficients ranged from 0.35×10−15 cm3 sec−1 to 2.8×10−15 cm3 sec−1.
For intervals up to 2 sec following atomic-nitrogen formation, the data indicate a slower decrease of atom concentration than can be accounted for by the measured mechanisms of recombination. The possible existence of an atom-forming mechanism in the afterglow is discussed.
Liquid Structure and Self-Diffusion
Authors: Rahman Aneesur 
Published: January 2005

The method of molecular dynamics has been used to study the structure of a liquid and its relation with the process of self-diffusion.
The particles around a given particle are separated into shells of primary, secondary, etc., neighbors by a precise geometrical procedure. Each shell of neighbors of a particle generates a closed convex polyhedron surrounding the polyhedron of the previous shell. The faces of the polyhedra are the bisector planes of lines joining the particle to its neighbors. The primary polyhedron is the smallest closed convex polyhedron around the particle.
The primary polyhedra of a random system of points, of a liquid, and of a solid are compared; the average number f̄ of faces are found to be 15.67, 14.45, and 14.26, respectively. The average number p̄ of sides to the polygons forming the faces is obtained through the relation p̄=6—(12/f̄).
The pair correlation function g(r) is decomposed into a sum Σig(i)(r), where g(i)(r), arises out of neighbors forming the ith-order polyhedra. The distributions up to i=5 have been calculated. g(1) and g(2) give the first two maxima in g(r). For higher values of i the g(i)(r) do not correspond to maxima in g(r). For the liquid and for the random set of points the average number of faces f̄i of the ith-order polyhedra increases linearly with i; the departure from the linear law is significant only for i=1 and 2 in the case of the liquid.
It is found that the shape of the primary polyhedra is related to the direction of displacement, in time τ, of the central particle; and the correlation is maximum when τ=0.5×10−12 sec.
The positions of the neighbors are analyzed with respect to this displacement direction and expressed as a distribution g(r, μ; τ), which, in addition to the dependence on the distance, contains the angular correlation between the position of a neighbor and the displacement direction in time τ of the central particle. The nearest neighbors are found to be preferably situated in a direction away from the displacement direction. As the distance increases, the forward direction becomes more favorable. In the region of the secondary neighbors the same behavior is repeated but is less in magnitude. This local fluctuation in the distribution of neighbors disappears beyond the secondary neighbors.
The velocity autocorrelation function and its spectrum f(ω) are found to have characteristics related to these local fluctuations. f(ω) is found to have a solidlike part having the same Debye ω as that in the solid before melting. The diffusive part of the velocity autocorrelation function arises out of the particle's taking advantage of the local fluctuation in the configuration of its neighbors to move in the ``easy'' direction afforded to it by the fluctuation.
Photogeneration of Charge Carriers in Tetracene
Authors: Geacintov Nicholas , Pope Martin , Kallmann Harmut 
Published: January 2005

The photocurrent and photovoltage of tetracene single crystals with aqueous electrodes were studied as a function of the excitation wavelength in the 220–560-mμ region. The fluorescence efficiency of these crystals was studied in the same wavelength region. At wavelengths longer than 410 mμ, the photocurrent is due to injection of holes at the illuminated electrode. A bulk-generated (electron) photocurrent is shown to be produced with excitation energies in excess of 3 eV, i.e., at wavelengths less than 410 mμ. The relative fluorescence efficiency Φ(λ) begins to decrease with decreasing wavelengths at 410 mμ. The drop in Φ(λ) at excitation energies greater than 3 eV is attributed to the appearance of a nonradiative competitive process. This process involves mostly the formation of nonconducting and nonfluorescent (or weakly fluorescent) charge-transfer states. Formation of separated charges with low quantum efficiency (φi≲5.10−3) also occurs which gives rise to the observed bulk-generated negative current. Since this current appears only for excitation energies ≳3 eV, we associate this value with the smallest band gap for conductivity in tetracene.
The wavelength dependence of the bulk-generated negative photocurrent is related to varying probabilities of transition from the different excited electronic states of the crystal to ionized states. The positive hole current is mostly due to the diffusion of the lowest-energy singlet excitons to the surface and their dissociation at the electrode. The wavelength dependence of this current can be adequately described by a diffusion equation and an exciton diffusion length of 2000 Å is calculated from the experimental data and this equation. The wavelength dependence of the photovoltage is strongly dependent on the negative charge density and exhibits minima in wavelength regions where this density is highest.
Effect of Zero-Point Motion on the Mössbauer Spectra of K4Fe(CN)6 and K4Fe(CN)6·3H2O
Authors: Hazony Y. 
Published: January 2005

A detailed calculation of the second order Doppler (SOD) shift of the Mössbauer line for some molecular solids demonstrates that the magnitude of this effect is significant in comparison with the full scale of the known isomeric shifts in iron compounds. The main purpose of this paper is to emphasize the importance of the SOD shift and, in particular, of the temperature-independent portion, which is due to the zero-point motion of the Mössbauer atom.
A simple model for calculation of the SOD shift for some molecular solids is presented, followed by a detailed analysis of the experimental results for K4Fe(CN)6 and K4Fe(CN)6·3H2O. An SOD contribution of 0.275 and 0.285 mm/sec to the measured shifts for the two compounds at room temperature is suggested on the basis of the analysis; an isomeric shift of +0.345 mm/sec, with respect to a 310 stainless-steel absorber at room temperature, is proposed for both compounds. An extrapolated difference of about 0.02 mm/sec at 0°K in the measured thermal-shift curves of the two compounds is attributed to the change in the zero-point motion due to modifications in the normal mode distributions that are associated with the water of crystallization.
Comparison with results on other compounds shows that differences as high as 0.15 mm/sec may be expected in the contribution of the zero-point motion to the Mössbauer line positions.
Thermal Diffusion and Self-Diffusion in Ammonia
Authors: Paul R. , Watson W. W. 
Published: January 2005

The temperature dependence of the isotopic thermal-diffusion factor α0 for ammonia has been investigated with the help of an equimolar mixture of 14NH3 and 15NH3. A 10-tube metal swing-separator as well as a two-bulb glass apparatus were used for these measurements. The value of α0 increases with temperature, its value being 0.00±0.02 at 242°K to 0.09±0.03 at 496°K, in disagreement with the values obtained by Watson and Woernley, who observed a definite inversion in the sign of α0 at temperature ∼290°K. The present set of observed values of α0 also disagrees with the theoretical calculation of Monchick and Mason. Possible causes of this disagreement are: (1) inadequacy of the potential model, (2) neglect of inelastic collisions, and (3) formation of dimers.
The self-diffusion coefficient of ammonia has also been studied in the temperature range of 233° to 353°K with a two-bulb apparatus. There is marked disagreement between observed and calculated values at low temperatures. This is also explained in terms of dimerization.
13C Spin—Rotation Interaction and Magnetic Shielding at the Carbon and Oxygen Nuclei in Formaldehyde
Authors: Flygare W. H. , Weiss V. W. 
Published: January 2005

A large-sized L-band microwave absorbtion cell was used to observe the 13C spin—rotation interaction in formaldehyde. The results yield | 32Maa+34(Mbb+Mcc)|=207.5±2.5 kc/sec. By making sign assumptions, the paramagnetic shielding at 13C is estimated to be σp=−215×10−6 compared to σp=−730×10−6 at the oxygen nucleus.
Hybrid orbitals and the symmetric orthogonalization method are used to compute 10 one-electron properties in formaldehyde to compare with the LCAO SCF results. The hybrid orbitals chosen yield slightly better agreement with experiment than the SCF functions. However, the minimal basis set used here and in the SCF functions does not allow enough flexibility to fit all of the observables within the experimental error. Reasonable values of the carbon and oxygen diamagnetic shielding are chosen on the basis of the calculations and are combined with the 13C spin—rotation constants in this paper and the 17O spin—rotation constants in a previous work to yield the absolute shielding at the carbon and oxygen nuclei in formaldehyde. The values for the total magnetic shielding in formaldehyde are σ(carbon)=125×10−6 and σ(oxygen)=−268×10−6.
The 17O–12C spectrum in the literature is reinterpreted to yield the 17O spin—rotation interaction in that molecule. Our interpretation for 17O–12C yields χaa=4.43±0.10 Mc/sec and Mbb=29±10 kc/sec. The 17O spin—rotation information is combined with a recent calculation of the oxygen diamagnetic shielding in CO to yield the absolute shielding at the oxygen nucleus which is σAv=−14×10−6.
Near-Infrared Spectra of H2O—D2O Solutions
Authors: Worley John D. , Klotz Irving M. 
Published: January 2005

Near-infrared spectra have been obtained for HOD in D2O solutions containing a variety of solutes. A sharp band occurs at 1.416 μ (7062 cm−1) and other bands at 1.525 (6557 cm−1), 1.556 (6427 cm−1), and 1.666 μ (6002 cm−1). The absorbances at 1.416 and 1.556 μ increase and decrease, respectively, with increasing temperature. From this variation, a ΔH° of formation of −2.4 kcal/mole has been estimated for the O–H···O bond in these solutions.
Nineteen electrolytes and one polymer, polyvinylpyrrolidone, have been examined for their effects on the spectra of water in this overtone region. No frequency shifts have been observed with added solute, only changes in absorbance. Sodium perchlorate shows the strongest structure-breaking effect. Tetrabutylammonium bromide and polyvinylpyrrolidone exhibit structure-making character. A definite correlation has been found to exist between the effects of salts on the aqueous solvent and their ability to affect the conformation of a macromolecule.
Electron Spin Resonance of the Excited Triplet States of Pyrimidines and Purines
Authors: Shulman R. G. , Rahn R. O. 
Published: January 2005

The electron spin resonances of the metastable triplet states of various pyrimidines and purines have been observed in ethylene glycol: H2O glasses at 77°K. Generally the Δm=2 transitions were observed and the parameter of spin—spin interaction determined was (D2+3E2)½. However for orotic acid, guanylic acid, and adenylic acid it was possible to observe the Δm=1 transitions and to determine D and E separately. From the values of D and E and the decay times, which were all &gt;0.2 sec, it was possible to characterize the lowest triplet states as π—π* states in all cases. For thymine, 5-methylcytosine, and orotic acid the values of D (and E) agreed quite well with those calculated approximately for π—π* states from Hückel molecular orbitals.
In addition to resonances attributed to neutral molecules, additional resonances were observed for the ionized states of azathymine, thymine, 5-methylcytosine, and 5-hydroxymethylcytosine. From the pH dependence of azathymine and thymine it was possible to show that one resonance appeared when the molecule lost its first proton (T−) and the second resonance replaced it when the molecules had lost two protons. The T− resonance was also observed in TMP and thymidine at pH values above 9.8 where they each lose the N3 proton. However, these compounds did not show the resonance characteristic of the doubly ionized state, presumably because they contain only one ionizable proton. In 5-methylcytosine and 5-hydroxymethylcytosine, one resonance was observed in the neutral molecules. Above pH 12.4, which corresponded to the pK value for the loss of a proton, a different resonance was observed. In 5-methylcytidine, in which the ionizable proton has been replaced by a ribose, only one resonance, characteristic of the neutral molecule, is observed. It is suggested that these results on the lowest triplet state can all be explained by dominant resonance structures in which double bonds switch from the carbonyl group to the ring.
Paramagnetic Resonance Spectrum of the Fluoroferrate Ion, FeF6———
Authors: Helmholz Lindsay 
Published: January 2005

The fluorine hyperfine structure in the paramagnetic resonance spectrum of the fluoro-ferrate ion, FeF6———, has been observed in a solid solution of the ion in K2NaGaF6. From the isotropic and anisotropic components of the spectrum the magnitudes of the coefficients of the fluorine atomic obitals in the molecular orbitals occupied by the unpaired electrons are estimated. The results are compared with results on Mn++ dissolved in other fluorides.
The antibonding eg orbitals are found to contain about 2.5% fluorine 2s orbital and approximately 10% fluorine 2p orbital.
Chemical Bond. III. A One-Dimensional Theory of the Energetics and Ionic Character of the Hydrogen Bond
Authors: McKinney Paul C. , Barrow Gordon M. 
Published: January 2005

The bond properties of a first row element bonded to a hydrogen atom are calculated from one-dimensional counterparts of the atomic orbitals. The potential energy function and the ionic character are found to be correctly predicted.
The hydrogen bond system O–H···O is described by means of the same atomic wave functions. The potential energy of the system is obtained for various positions of the hydrogen atom. Predicted properties are in good agreement with experiment. The existence of a second potential minimum is indicated. The contributions of the resonating structures of the system are obtained and are shown to lead to the observed value of dμ/dr for the hydrogen bonded OH system.
Theoretical Depolymerization Kinetics in Polymers Having an Initial ``Most Probable'' Molecular Weight Distribution
Authors: Boyd Richard H. 
Published: January 2005

The kinetic equations describing a general chain mechanism of depolymerization are solved at steady state for a polymer having an initial ``most probable'' molecular weight distribution. Chain scission initiation, end group initiation, depropagation, intermolecular chain transfer, and several types of termination are considered.
In the past, solutions have been limited to special cases and approximations. Now general expressions for molecular weight change and rate of weight loss are available for polymers with an initial molecular weight distribution of wide interest. The relationships to the previously considered special cases are discussed.
Electron Spin Resonance of Ultraviolet Irradiated Compounds. I. Unsaturated Hydrocarbons
Authors: Poole Charles P. , Anderson Roy S. 
Published: January 2005

Trapped radicals, formed by ultraviolet irradiation of pure, unsaturated aliphatic hydrocarbons, maintained at 77°K, have been observed with a 3-cm electron spin resonance spectrometer at a g factor of two. Straight-chain alkene radicals were characterized by well-resolved hyperfine structure of four or more components. Branched-chain alkene radicals showed more but poorly resolved components. Only those hydrogen atoms attached to carbon atoms at or adjacent to the location of the unpaired electron are found to contribute appreciably to the observed hyperfine structure. Their hyperfine coupling constants are approximately equal.
Radical formation and stabilization is attributed to the separation from the parent molecule of highly mobile hydrogen and methyl radicals which diffuse away from the remaining radical fragments, and eventually react with their environment or recombine. Stabilization of remaining fragments then occurs because of immobility and structural resonance. The number of radical spins detected was of the order of 1016 to 1018 in 1-ml samples. Most radicals were stable for weeks at 77°K. Consideration is given to the existence of radicals resulting from secondary hydrogen addition reactions.
Electronic Structure of LiH Ion
Authors: Platas O. , Hurst R. P. , Matsen F. A. 
Published: January 2005

A valence-bond calculation has been carried out for (LiH)+ with all twenty structures possible from Slater orbitals 1s, 2s, 2p on lithium and 1s(=1h) orbital on hydrogen. The total electronic energy computed for the ion at the equilibrium distance for the LiH molecule is — 209.50 ev with only the 1s21h structure making a significant contribution to the ground state. When this energy is combined with the corresponding calculated energy for the molecule, a vertical ionization energy of 7.52 ev is obtained. No experimental value has as yet been reported.
A technique is developed and is used to obtain the optimum orbital exponents for 1s21h over a wide range of internuclear distance. This calculated energy vs distance curve has no minimum suggesting that (LiH)+ is unstable with respect to dissociation.
At R=3.08 au the wave function is ``degenerate,'' that is, two different extremum sets of exponents yield the same energy. The principal difference is that the orbital exponent of the 1h function is 0.49 in one set and 1.04 in the other. At R&gt;3.08 au the second set gives the lowest energy while the reverse is true for R&lt;3.08 au. This is interpreted as representing the promotion 1s→2s as is required in approaching the united atom.
Molecular Structure of B10H12(CH3CN)2
Authors: van der Maas Reddy Josina , Lipscomb William N. 
Published: January 2005

An x-ray diffraction study of single crystals of B10H12 (CH3CN)2 has yielded the positions of all atoms, including hydrogen atoms. The structure is regarded as a substitution derivative of the B10H14—2 structure having C2v symmetry, two bridge hydrogens and two BH2 groups. A B–N single bond replaces a B–H of each BH2 group and the acetonitrile residue is linear with normal bond distances, except for C–CH3=1.45 A in agreement with C–CH3=1.46 in acetonitrile itself.
The unit cell is monoclinic, of symmetry I2/c, and has parameters a=7.81, b=11.31, c=14.18, and β=96°52′. Final values of R=0.15 and r=0.12 were obtained, as described in the text.