bluuebunny/crossref_metadata_2025_split huggingface dataset:
NMR Studies of Phase Transitions. I. NaSH
Authors: Coogan C. K. , Belford Geneva G. , Gutowsky H. S. 
Published: January 2005

NaSH is one of a group of compounds M+(QR)— whose high-temperature crystalline form might be called pseudo-NaCl cubic, but which crystallize in less symmetrical lattices at low temperatures. Two models have been proposed to account for the obviously linear ion QR— replacing a spherical ion in the cubic phase. Pauling suggested that the QR— ion tumbles rapidly and randomly enough to have an effective spherical form to the surrounding ions. Frenkel suggested that the QR— ions are static and point along one of the eight cube body diagonals (〈111〉 directions) at random so as to give long-range cubic symmetry. Previous attempts, based on x-ray diffraction experiments, to decide between these models have been unsuccessful. We have studied the H1 and Na23 nuclear magnetic resonances in polycrystalline samples of NaSH at temperatures between —65° and 200°C. Linewidth changes at 70°C and above reveal a 10° region of transition from the rhombohedral to the cubic phase. In this transition region, diffusion of the Na+ ions causes major narrowing of the Na23 resonance and partial narrowing of the H1. The narrowing of the proton resonance is continued at temperatures above 110°C by diffusion of the SH— ions.
The second moment of the proton absorption observed at ∼90°C, when compared with values calculated for several models, is consistent with rapid, random reorientations of the SH— ion among the eight 〈111〉 directions in the cubic phase. Similar calculations and comparisons provide evidence that the SH— ion reorients also in the rhombehodral phase. Here, however, the stable orientations appear to be in the six pseudo-〈110〉 directions perpendicular to the trigonal axis. Thus, the phase transition is a cooperative process resulting from the increasing population, at higher temperatures, of the pseudo-〈111〉 directions in the rhombohedral phase.
The Na23 resonance is observed in the rhombohedral phase, even though the Na23 quadrupole splitting is generally large enough in noncubic crystals to obliterate the NMR absorption in powders. Electric-field gradient calculations, assuming point charges, indicate that the Na23 quadrupole splitting can be negligible in the dynamic structure we propose.
Paramagnetic Resonance Measurements of the Phosphorescent State of Quinoxaline
Authors: Vincent James S. , Maki August H. 
Published: January 2005

Paramagnetic resonance absorption has been observed in a single-crystal solid solution of quinoxaline in durene when irradiated at 77°K with uv light from an AH-6 mercury-arc source. The fine structure of the resonance spectrum observed at 9.8 Gc/sec may be described by the spin Hamiltonian H=BH·g·S+DSz2+E(Sx2−Sy2),in which S = 1. D/hc=±0.1007±0.0003 cm−1E/hc=∓0.0182±0.0002 cm−1  gxx=2.0047±0.0005  gyy=2.0035±0.0005gzz=2.0019±0.0005.The fine structure is practically identical to that of the naphthalene triplet state observed by Hutchison and Mangum.
The observed hyperfine structure is assigned to a coupling with the 5, 8 protons, and the two N14 nuclei. The angular dependence is in qualitative agreement with the tensor for C–H proton hyperfine interaction for doublet-state aromatic radicals measured by Cole, Heller, and McConnell, as well as the tensor for aromatic N14 interaction derived from experiments by Weissman. With the assumption that hyperfine interaction in a triplet state may be described by the same tensors, normalized π-electron spin densities are approximately 0.28 for the 5, 8 carbon atoms, and 0.1 for the N14 atoms. Satellite lines which are split from the primary hyperfine lines by the proton resonance frequency are observed and interpreted as simultaneous electron-spin nuclear-spin transitions. The interactions leading to these transitions are assumed to be primarily due to hyperfine coupling with protons whose unresolved first-order interaction is much smaller than the proton-resonance frequency.
Structure and Rotational Isomerization of Free Hydrocarbon Chains
Authors: Bartell L. S. , Kohl D. A. 
Published: January 2005

An electron diffraction investigation has been carried out for the molecules n-butane, n-pentane, n-hexane, and n-heptane in the gaseous phase. Mean bond lengths and bond angles were reported in earlier papers. The present paper is concerned with information derivable from the observed spectrum of nonbonded distances. A scheme is presented for determining both trans and gauche bond angles despite the fact that the 2.5-Å nonbonded C···C trans and gauche radial distribution peaks are too badly overlapped to be resolved. Formulas are given for calculating torsional ``shrinkage effect'' corrections for nonbonded distances.
It was found that a given gauche conformation has a higher free energy than its corresponding trans counterpart by about 610 cal/mole. If the source of this free-energy difference is steric, arising from repulsions between hydrogens on gauche carbons, there should be structural as well as thermodynamic manifestations. The positions of a total of 21 independent nonbonded C···C radial distribution peaks were interpreted in terms of a model of steric deformations in gauche structures. The results, including estimates of standard errors, were φ(gauche)=61.0±3° dihedral angle, α(trans)=112.65±0.3° C–C–C angle, α(gauche)—α(trans)=0.1°±1°, and the distortion from planarity of trans conformations adjacent to gauche was η=7±4°. The C–C bond length and C–C–C bond angle, averaged over all conformations, were 1.5328±0.002 Å and 112.71±0.15°, respectively.
Time-Weighted-Average Approximation in the NMR Spectrum of CF2BrCFBrCl
Authors: Newmark R. A. , Sederholm C. H. 
Published: January 2005

The spectrum of CF2BrCFBrCl has been redetermined at 150°K in CFCl3 as a solvent. The coupling constants, chemical shifts, and relative energies of the three rotamers have been determined with considerable care. Assuming that high-temperature coupling constants and chemical shifts are time-weighted averages of the coupling constants and chemical shifts of the individual rotamers, these quantities have been calculated for several high temperatures, and compared with the experimental values, which have been determined in the same solvent.
All of the spectra involved are typical ABX spectra. The experimental value for ½(JAX+JBX)Av is 0.6 to 1.3 cps lower than the calculated value. Excellent agreement is obtained between the calculated and observed chemical shift between the A and B fluorine atoms. At 371°K there is a large difference between the calculated and observed chemical shifts between any other pair of fluorine nuclei, including the solvent nuclei. This difference decreases linearly with temperature and extrapolates to 0 at approximately the temperature of the low-temperature data. The cause of these discrepancies is discussed.
Molecular Weight Distribution of Linear Polymers from Stress Relaxation in Polymer Melts
Authors: Peticolas Warner L. 
Published: January 2005

The method of Menefee and Peticolas for obtaining the molecular weight distribution of linear polymers from stress relaxation after steady-state flow has been applied to data on melt samples: a linear polyethylene and polystyrene sample. The results offer considerable promise for a rapid estimation of the high molecular portion of the distribution curve.
The method is based on a rather straightforward extension of the theory of Roase to bulk molten polymers. A discussion of the validity of this process is given in the light of recent developments.
Effect of Isotopic Substitution on the Dipole Moment of Molecules of Symmetrical Configuration
Authors: Gangemi Francis A. 
Published: January 2005

As is well known, molecules of symmetrical configuration possess no permanent dipole moment. Upon isotopic substitution, however, a dipole is produced due to the changed zero-point motion of vibration and the asymmetry of the restoring forces.
This effect is calculated for two molecules: O16C12O18, and CH3D.
A normal coordinate treatment is carried out for these two molecules using a simplified form for the force fields as an approximation to the general quadratic force fields of both CO2 and CH3D. It is assumed therefore that the omitted terms play a negligible part in dipole moment calculations. The vibrational wavefunctions for the nuclear skeletons of the systems are calculated, including the effect of the cubic terms in the potential function as perturbations to the simple harmonic wavefunctions. The shift from the equilibrium separation of the atoms is then calculated by taking the quantum mechanical average values of the position coordinates.
Finally, the dipole moment is given by the product Δμ=∂μ/∂r〈Δr〉Av, where ∂μ/∂r is the change in bond moment with respect to internuclear distance which is determined from experiment, and 〈Δr〉Av is the shift or displacement from the equilibrium separation of the nuclei.
The calculations yielded the following results: for O16C12O18, a permanent dipole moment of the order of 10—3 D, and for CH3D, a permanent dipole moment of the order of 10—3 D, for the vibrational ground states.
Gaussian Expansion of Hydrogen-Atom Wavefunctions
Authors: Whitten Jerry L. 
Published: January 2005

The 1s, 2s, 2p, and the four equivalent 3d hydrogen-atom wavefunctions were expanded as linear combinations of functions of the form, exp ( —ar2), without the use of spherical harmonic functions to give angular dependence. The Gaussian functions were located at different points in space in order to give the proper orbital symmetry. Expansion parameters were determined by a variational calculation of the hydrogen-atom energy. Energy errors are 0.2, 0.45, 0.25, and 0.9%, respectively, for the 1s, 2s, 2p, and 3d functions.
It is suggested that the approximations are sufficiently accurate to find use in molecular-energy calculations, particularly since integrals involving several centers are more readily evaluated in terms of Gaussian rather than exponential radial functions.
Infrared Absorption Intensities in Mixed Crystals of Benzene and Deuterobenzene
Authors: Hollenberg J. Leland , Dows David A. 
Published: January 2005

Absolute absorption intensities of the infrared-active fundamentals in dilute crystalline solutions of benzene in deuterated benzene and vice versa have been measured. The intensities are, within experimental error, equal to those measured for the pure crystals.
The spectroscopic resolution was sufficient to show clearly the effects of isotopic dilution on the crystal multiplets.
Vaporization of Tungsten Studied by Flash Heating and Kinetic Spectroscopy. I. Near-Ultraviolet Region
Authors: Nelson L. S. , Kuebler N. A. 
Published: January 2005

Thin wires or strips of tungsten have been vaporized by heating with an intense pulse of light from a capacitor discharge lamp. The appearance of the metal as it vaporizes has been recorded by high-speed photography; the characteristics of the vapor were studied at various times by kinetic absorption spectroscopy. When flash heated, tungsten behaves grossly like an electrically exploded metal. Using an approximate spectroscopic method, we have estimated the electronic temperature of the vapor to be near 3000°K as it sublimes from the solid, and near 7000°K as it boils from the liquid. Thermal excitation of nearly 22000 cm—1 occurs under the latter conditions. Production of tungsten ions seems negligible. Some secondary photoexcitation of the atoms to states lying as high as 55000 cm—1 may occur.
Flash heating shows promise as a general method for producing vapors of even the most refractory elements for absorption spectroscopy.
Ion Radicals of Methyl-Substituted Benzenes: Temperature Dependence of Spin and Charge Densities
Authors: Brown Thomas H. , Karplus Martin 
Published: January 2005

The spin and charge densities for the monomethyl and dimethyl benzene negative ions are calculated with the valence-bond perturbation procedure developed previously. Because the difference in energy (ΔE) between the two lowest eigenvalues of these systems is found to be small, thermal averaging is included in the determination of the theoretical spin densities. Meta-xylene negative-ion data are used to evaluate the required valence-bond parameters. Comparison of the measured and calculated hyperfine constant values, including the available temperature-dependence data, shows good agreement in most cases. However, a number of discrepancies appear to exist and further investigation is required. The experimental and theoretical difficulties in obtaining good ΔE values are also discussed.
The formulation is used to predict spin densities for some unknown methyl-substituted benzene negative ions and for the methyl-substituted benzene positive ions. Also, values are obtained for the perturbation parameter (ΔCx—) for the other alkyl-substituted benzene negative ions which have been studied experimentally. Some discussion is given concerning the extension of the treatment to systems other than substituted benzene ion-radicals. The reliability of the method is considered, and comparisons are made with available molecule-orbital calculations.
Photochemical Studies in Flash Photolysis. III. Photolysis of Acetone in Different Wavelength Regions
Authors: Shilman A. , Marcus R. A. 
Published: January 2005

The flash photolysis of acetone was studied at wavelength regions centered around 260, 280, and 300 mμ, using absorbed intensities of the order of 1019 quanta/cc/sec for each wavelength region. The light source was an exploding wire, and the maximum temperature increase per flash was calculated to be not more than 5°C. The products, analyzed by gas chromatography using a sensitive electric discharge detector, consisted of C2H6, CO, biacetyl, and, in smaller amounts, CH4. A search was made for other products as well, and detection limits are given.
The C2H6/CO ratio decreased with increasing acetone pressure at all wavelength regions and was independent of light intensity in the range investigated. The RCO/pacet ratio, measured over a wide pressure range, was pressure-independent at 260 and 300 mμ but increased appreciably with pressure at 280 mμ. At low pressures, where the C2H6/CO ratio approached a limiting value, the ratio decreased with wavelength according to the order 280&gt;260&gt;300 mμ.
The lifetime of the excited acetone molecules at the respective wavelengths was estimated, from a steady-state treatment, to be 0.6×10—9, 1×10—9, and 4×10—9 sec at 260, 280, and 300 mμ, assuming a collision deactivation efficiency of unity. At 300 mμ there was a marked difference in value of C2H6/CO at flash and low intensities: certain second-order reactions involving excited states appear to occur completely at the relatively high concentrations prevailing under flash conditions. The transition region of intensity effects is described in subsequent papers of this series.
``Smoothed Potential'' Theory of Chemical Binding
Authors: Arnold James R. 
Published: January 2005

A simple model, similar to the ``free electron'' theory of metals and conjugated systems, is shown to yield good values for the energies of most single covalent bonds. The model can be extended to multiple bonds and charged species, but not in its present form to excited states and unstable species. It can be correlated with the free electron theory of metals.
The significance of the success of this and other simple models in predicting bond energies is discussed.
Ultraviolet and Infrared Absorption Spectra of Substituted Acetophenones and Benzoic Acids
Authors: Tanaka Jiro , Nagakura Saburo , Kobayashi Michio 
Published: January 2005

Influences of substituents on the ultraviolet and infrared absorptions of acetophenone and benzoic acid are investigated from theoretical and experimental points of view. In the present paper, all theoretical considerations are made by the molecular energy-level diagram method developed by the authors.
Ultraviolet absorption spectra of some substituted acetophenones are measured in n-heptane, alcohol, water, and sulfuric acid. It is found that most molecules studied here have absorption bands corresponding to the n→π* transition of the carbonyl group, A1g→B2u and A1g→B1u transition of the benzene ring, and intramolecular charge-transfer absorption. Especially concerning last absorption, a somewhat quantitative theoretical consideration is presented and satisfactory agreement is obtained between the theoretical and observed excitation energies.
Infrared absorption spectra of carbonyl groups in substituted acetophenones are measured in carbon tetrachloride. The observed carbonyl frequencies are compared with the bond orders of this bond calculated on the basis of the above method. Good correlation is also found between these two quantities. A similar consideration is also made with substituted benzoic acids.
Pressure Coefficient of the Dielectric Constant of Water
Authors: Mavroides John G. 
Published: January 2005

Theoretical calculations of the pressure coefficient of the dielectric constant of water have been made based on the following models: (a) water molecules considered as tetrahedrally packed (one-state model) and (b) water molecules considered as packed in two ways: tetrahedrally and simple close-packed (two-state model). These calculations are compared with experiment.
The pressure coefficient has been measured by a newly derived dynamic method in which the pressure is applied ultrasonically. The measurement compares favorably with previously published values which were obtained with static pressures.
Method for the Determination of the Surface Tension of Solids, from Their Melting Points in Thin Wedges
Authors: Sill Richard C. , Skapski Adam S. 
Published: January 2005

A method is proposed which allows one to measure the surface tension of solids whose liquids wet glass and steel and whose solid/liquid contact angle against those materials is zero.
A thin wedge, formed of optically flat steel and glass slabs, is filled with liquid substance and then allowed to solidify. It is then immersed in a large heat bath whose temperature is raised at a rate of about 1.5°C per 24 hours, and is known at any time within ±0.002°C. The solid substance starts melting from the thinnest part of the wedge, well below its ``normal'' melting point Tm. From the lowering of the melting point ΔT corresponding to a measured thickness h of the wedge in the solid/liquid interface, the surface tension of the solid σS is then arrived at from the equation: σS=QfρS2TmhΔT+σL,where Qf is the heat of fusion in erg/g, ρS the density of the solid, and σL the surface tension of the substance in liquid state.
The method was applied to stearic and myristic acids. For myristic acid four independent measurements gave an average value of σS=116±10 erg/cm2, all four samples showing similar orientation of the crystal lamellae (gross crystal orientation) with respect to the interface solid/liquid. For stearic acid three similar gross orientations gave an average σS=179±8 erg/cm2. One gross orientation, markedly different from the others, yielded the value of σS=135±6 erg/cm2.
Thickness h was determined from the interference lines (sodium light). The gross orientation was observed in polarized sodium light.
Nuclear Magnetic Relaxation of Nonuniform Systems
Authors: Broersma S. 
Published: January 2005

The effect of electric ions in water upon the proton magnetic relaxation is discussed. Formulas pertaining to solutions of paramagnetic and nonmagnetic ions are derived. The viscosity factor occurring in the theory for uniform systems has to be replaced by an average of inverse local viscosities weighted by the dimensions of the moving particles or by individual diffusion constants. The concept of mutual viscosity—describing the interaction between two unlike liquid layers—is helpful in correlating the numeric results.
The influence of temperature and concentration has been studied experimentally. The behavior of trivalent ions like aluminum can be understood by assuming hydration. The effect of monovalent ions can be explained on the basis of a slightly decreased diffusion constant of the free water molecules. Treated as a two-phase system, the experimental results enable the evaluation of the average time of adsorption. For aluminum, this is one-hundredth of a second.
Iodine Complexes in Inert Solvents. I. I2–I2 in Carbon Tetrachloride and in n-Heptane
Authors: de Maine P. A. D. 
Published: January 2005

The shape of the ultraviolet absorption curve for the I2–I2 complex in CCl4 and in n-heptane, has been obtained by analysis of the ultraviolet absorption of iodine dissolved in these pure solvents. In this analysis it has been necessary to assume that the assumed equilibrium constant, for the reaction I2+I2⇌I2—I2, does not change with the wavelength. In carbon tetrachloride and in n-heptane the I2–I2 complex absorption maximum occurs near 2935 A, which is in good agreement with the value predicted by the McConnell, Platt, and Ham relation. The heat of formation of this complex in CCl4, has been estimated as 1.96 kcal/mole (within 0.15 kcal) by assuming that its molar extinction coefficient curve does not change with the temperature.
The absorption spectrum of iodine dissolved in CCl4 has been extended to 2250 A, and it has been shown that iodine-carbon tetrachloride solutions obey Beer's law below 2500 A, a region in which there is strong absorption by CCl4 and no absorption by I2–I2.
Infrared Spectrum of Benzene-d6
Authors: Miller Foil A. 
Published: January 2005

Improved infrared data have been obtained for benzene-d6 from 300—3700 cm—1 for both vapor and liquid phases. The frequencies can be interpreted very completely and satisfactorily. Good evidence was found for assigning v14 and v15 to 1287 and 838 cm—1 in benzene-d6. This provides valuable confirmation for Mair and Hornig's choice of 1310 and 1152 in benzene itself. All the fundamental frequencies for both benzene and benzene-d6 now seem to be well established, aside from small uncertainties in a few cases where the values have been inferred from combination tones or from the product rule.
The infrared spectrum of benzene vapor for the range 300—700 cm—1 is also given.
Infrared Absorption Anisotropy of Crystalline Iodoform
Authors: Hexter R. M. , Cheung H. 
Published: January 2005

The polarized infrared spectrum of crystalline iodoform has been obtained using single crystals of thicknesses from 50 to 200 microns. Spectra were recorded with the crystals oriented with respect to the electric vector by an x-ray diffraction type goniometer, and the anisotropy of the spectrum was studied carefully as a function of angle of orientation.
A complete assignment of all fundamentals was possible, including at least two of the lattice frequencies of the crystal. The anisotropic behavior of v1, the C–H stretching fundamental, is such that it is possible to state that Pimentel's ``oriented gas model'' holds within well-defined limits. It is demonstrated that a much better procedure of locating transition moment directions in crystals than the use of ``dichroic ratios'' is the complete goniometric study of the absorption anisotropy. Finally the ``threefold degenerate'' first overtone of v4, the C–H bending mode, was split, and the intensity ratio of its members is reported.