bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarizability, Dielectric Constant, Pressure, and Density of Solid Parahydrogen on the Melting Line
Authors: Younglove B. A.
Published: February 2004

Polarizabilities, calculated from measurements of the dielectric constant, are reported for solid parahydrogen on the melting line between 18 and 320 atm (14.4°–22.2°K). Also new melting pressures below 17°K are presented. Previously reported solid densities are adjusted slightly, using the new resulting pressure data. Liquid parahydrogen polarizabilities were determined over part of the density range of the solid and are seen to be slightly less than the solid polarizabilities.
Electric-Field-Induced Spectra: Theory and Experimental Study of Formaldehyde
Authors: Bridge N. J. , Haner D. A. , Dows David A.
Published: February 2004

An experiment is described which measures the modulation, produced by an applied electric field, of the absorption coefficient of the rovibronic spectrum of a gaseous sample. Phase-sensitive detection provides for measurement of 0.01% modulation at a spectral resolution of 0.1 cm− 1 in the near ultraviolet. A theoretical development of the electric-field-induced spectral line shape and intensity as an expansion in powers of the applied field is presented. The important molecular parameters are the ground-state and excited-state dipole moments and rotational constants; measurement of the excited-state moment is discussed. The dependence of line intensity on quantum numbers is predicted to be such as to emphasize transitions to states of low angular momentum; observation of this emphasis is of importance in assigning spectra. The correspondence between field-induced and absorption spectra in a single experiment lends great confidence to assignments of specific absorption features. Experimental study of formaldehyde confirms many of the predictions of the theory. Assignments are obtained for the low-K subbands in the 3390-, 3430-, and 3520- Å bands, and the asymmetry parameters are presented. Effects of asymmetry on the EFS are shown. A method of determination of the excited-state dipole moment based on measurement of ratios of intensities of EFS in different polarizations is presented. The dipole moment of HCHO in the A 1A″ state is determined to be 1.4 ± 0.1 D.
Radial Distribution Function and the Direct Correlation Function for One-Dimensional Gas with Square-Well Potential
Authors: Katsura Shigetoshi , Tago Yoshio
Published: February 2004

The properties of the one-dimensional gas with square-well potential in classical statistical mechanics are examined. The radial distribution function and the direct correlation function are calculated exactly. The direct correlation function in the range where the potential vanishes is shown to be almost zero. This means that, for the one-dimensional gas with square-well potential, the Percus–Yevick approximation, although fairly good, does not hold exactly (while it does for the hard-core gas in one dimension).
Vibrational Spectra and Structure of Organophosphorus Compounds. VI. Infrared and Raman Spectra of CH3OPSF2 and CD3OPSF2
Authors: Durig James R. , Clark James W.
Published: February 2004

The infrared spectra of both CH3OPSF2 and CD3OPSF2 in the vapor phase have been recorded from 4000 to 33 cm−1. Carbon disulfide solution spectra were also investigated to 200 cm−1. The Raman spectra of the liquids and solids have been recorded and depolarization values measured. At least two isomeric configurations are indicated from the low-temperature Raman spectra, and from the lack of the variation of band intensity, it is concluded they have nearly equal energy. Only one torsional vibration was observed at 192 cm−1, and it is concluded that it is composed of the CH3 and P–O–C torsional vibrations and possibly mixed with other low-frequency skeletal modes. The doublet due to the P=S stretching vibration has a separation of only 31 cm−1, whereas 100–150 cm−1 has been proposed as the separation for this normal mode in this type of compound. The other fundamental vibrations are assigned based on band positions, depolarization values, and isotopic shifts. The assignments are also compared to those previously given for the CH3OPCl2 and CH3OPSCl2 molecules.
Diffusion Coefficients of Vapors Measured with a Moving Boundary
Authors: Katan Theodore
Published: February 2004

A method for the measurement of diffusion coefficients of vapors in gases is described in which the motion of a bead of volatile liquid is observed in a tube with a microporous membrane at one end. Results are given for air with ethyl acetate, benzene, ethanol, and methanol at 22.3° ± 0.3°C and 1 atm total pressure. The determined behavior of the diffusion-tube system may be considered in studies and measurements of diffusion coefficients of vapors.
Vibronic Effects in Photochemistry—Competition between Internal Conversion and Photochemistry
Authors: Becker Ralph S. , Dolan Edward , Balke David E.
Published: February 2004

An emission spectroscopic investigation was carried out on molecules capable of undergoing photochemistry as well as on those that cannot. Where photochemistry does occur, the quantum yield of fluorescence (φF) depends upon the particular vibrational level excited within a vibrational manifold of an excited electronic state. That is, φF generally decreases with excitation into successively higher vibrational levels. The mechanism responsible for these results involves competition between internal conversion and photochemistry at each vibrational level within a vibrational manifold. Based on this mechanism, corresponding equations have been developed from which the ratio of the rate constant of internal conversion to that of photochemistry, kIC / kPC, can be evaluated. The value of this ratio is dependent upon the particular vibration involved as well as the particular excited electronic singlet state involved. It appears as if internal conversion may proceed only within a particular vibrational manifold.
Proton Hyperfine Splittings in the Electron Spin Resonance Spectra of Hydrocarbon Ions: A Comparison of the Colpa-Bolton and Giacometti–Nordio–Pavan Relations
Authors: Moss Richard E. , Fraenkel George K.
Published: February 2004

The Giacometti–Nordio–Pavan modification of McConnell's relation between proton hyperfine splittings and spin densities has been shown to reduce to a simpler form than that originally proposed. It has been demonstrated further that the Giacometti–Nordio–Pavan modification should not be regarded as arising from a nearest-neighbor effect. A statistical analysis of the available experimental proton hyperfine splittings from even-alternant hydrocarbon ions indicates, however, that the Colpa–Bolton relation is superior to the Giacometti–Nordio–Pavan relation in explaining the differences between proton splittings in corresponding cation and anionx radicals. These results are valid within the framework of Hückel molecular-orbital theory.
Thermodynamic Investigation of Solid Argon–Nitrogen Mixtures
Authors: Pace E. L. , Smith J. H. , Jepson B. E.
Published: February 2004

The heat capacity from 13° to 77°K, the enthalpies of transition and fusion have been measured for argon–nitrogen mixtures containing 70.193, 83.072, 90.033, and 93.702 mole % nitrogen. The entropy of vaporization could not be measured, and therefore its value was estimated by calculation from other data. The zero-point entropy of the mixtures with 90.033% and 93.702% nitrogen consists entirely of the entropy of ideal mixing, and of the 70.193% and 83.072% mixtures consists largely, or even entirely, of the entropy of mixing of the two compounds. In the case of the 70.193% mixture, the presence of excess heat capacity at 13°K indicates that the quadrupole ordering of nitrogen molecules in the solid is not complete at the lowest experimental temperature. However, for the other compositions which are richer in nitrogen, it appears quite likely that the ordering is complete at the lowest experimental temperatures. Correlation of the measured thermodynamic properties with the theoretical values predicted by the theory of Pople and Karasz has been attempted and the results appear reasonable for the temperatures and entropies of transition and fusion.
Chemical Kinetics of the Shock-Initiated Combustion of Hydrogen at High Pressure and Low Temperatures
Authors: Wakefield C. B. , Ripley D. L. , Gardiner W. C.
Published: February 2004

The ignition mechanism of the hydrogen–oxygen explosion at temperatures near 1000°K and pressures greater than 1 atm was investigated theoretically using the complete analytic solution to the kinetic equations of an abbreviated, linearized mechanism and numerical integration of the full conventional mechanism for these conditions. It was found that the analytic solution of the simplified mechanism is capable of only a qualitative description of the second limit effect observed in reflected shock experiments on ignition delays, and cannot be forced to yield quantitative agreement. The numerical integration can be forced to give quantitative agreement, but only at the cost of accepting rate-constant expressions which do not allow quantitative description of the simpler ignition phenomena at higher temperatures and lower pressures. An exception to these conclusions is the use of analytic expressions for interpretation of branching rates early in the induction zone, which appears to be valid within wide limits.
EPR of Spin-52 Mn(II) Complexes with Very Large Ligand-Field Splittings
Authors: Van Heuvelen Alan , Lundeen M. D. , Hamilton H. G. , Alexander M. Dale
Published: February 2004

An unusual manganese complex has been constructed in which the Mn(II) is in a spin-52 state with large ligand-field splitting. The angular dependence of the EPR spectrum indicates significant mixing of |M〉 states (i.e., those differing in electronic quantization along the z axis) by terms such as 12(− 3 sinθ cosθ)b20 × [(S+′ + S−′)Sz′ + Sz(S+′ + S−′(] and B(S+I− + S−I+). This mixing causes an unusual angular dependence of EPR transitions that occur near magnetic fields where two levels cross. When water is removed from the crystal, the Mn(II) ligand-field splitting becomes greater such that only one EPR transition is observed for most orientations of the crystal in the magnetic field. Magnetic susceptibility measurements indicate that the Mn(II) is still in a spin-52 state. This is the only Mn(II) complex known by us in which the Mn(II) has a single-line EPR spectrum while in a spin-52 state. The g factor of the dehydrated complexes is larger than the free-spin value indicating strong covalent coupling of the Mn(II) d states with low-lying ligand states.
Nuclear Spin–Lattice Relaxation in Phenylacetylene
Authors: Jonas J. , DiGennaro T. M.
Published: February 2004

The temperature dependence of the proton and deuteron spin–lattice relaxation times T1 in phenylacetylene and phenylacetylene-d1 has been measured by the adiabatic fast-passage method. The intramolecular and intermolecular contributions to relaxation have been separated by the T1 measurement of ring protons at different concentrations of phenylacetylene in CS2 and C6D6. The deuteron electric field gradients in the R–C ≡ C–D compounds are relatively insensitive to the nature of the substituent R, and therefore the –C ≡ C–D grouping can be employed as a probe to study rotational motions in molecular liquids of the R–C ≡ C–D type. The reorientational correlation time, τq = 5.5 × 10−12sec in neat phenylacetylene-d1, and τq = 3.3 × 10−12sec at infinite dilution in CS2 at 26°C, has been calculated from the deuteron T1 and found in good agreement with those calculated from viscosity using both the microviscosity model and the Hill model. Analogous comparison of the experimental translational correlation time τt = 1.7 × 10−10sec for neat phenylacetylene at 26°C, and the calculated values has also been carried out.
Fluorescence Self-Quenching through Hydrogen Bonding
Authors: Horrocks Donald L.
Published: February 2004

A new process of self-quenching has been observed which seems to involve the dissipation of excitation energy through the formation of hydrogen bonds or possibly a proton-transfer mechanism. Special derivatives of the basic chromophore 2-phenylindole which could not self-quench by normal processes were shown to have decreasing fluorescence and scintillation yields with increasing concentration in toluene solutions. All of these compounds had in common a hydrogen atom on the N atom of the indole part of the molecule. By replacement of that H atom with a methyl group all self-quenching was completely eliminated.
Infrared Spectrum, Structure, Vibrational Potential Function, and Bonding in the Lithium Superoxide Molecule LiO2
Authors: Andrews Lester
Published: February 2004

Lithium superoxide has been synthesized by reacting lithium atoms and oxygen molecules at high dilution in inert and oxygen matrices. Isotopic substitution at all atomic positions and use of isotopic mixtures verify the molecular identity and indicate an isosceles triangular structure. Eighteen frequencies from six isotopic molecules determine the potential constants FO–O = 5.59 ± 0.05, FLi–O = 1.18 ± 0.02, and FLi–O,Li–O = − 0.21 ± 0.03 mdyn / Å. The oxygen–oxygen force constant for LiO2 coincides with that calculated for O2−, which suggests that the LiO2 molecule is highly ionic and may be considered as a lithium cation bonded to a superoxide anion by Coulombic forces.
Improved Variational Bounds on Some Bulk Properties of a Two-Phase Random Medium
Authors: Prager Stephen
Published: February 2004

The Hashin–Shtrikman bounds on the bulk magnetic permeability (or dielectric constant, thermal and electric conductivity, solute diffusion coefficient) for a random two-phase material can be considerably improved by the inclusion of experimentally accessible information, such as the bulk permeability of the material at a different temperature [the main results are expressed in the inequalities (27), (45), and (51)]. These inequalities can also be written as bounds on the rate at which the bulk permeability changes with changing permeabilities of the individual phases [(29) and (30)]. In particular, application of (30) yields the upper and lower bounds (59) on the observed energy of activation for diffusion through a two-phase medium. The inequalities (27), (45), and (51) can be expressed in a mixed form to bound, for example, the bulk permeability through use of data on the thermal conductivities of the mixed material and the individual phases. Bounds on the viscosity and elastic moduli of a two-phase medium are also discussed briefly.
Paramagnetic Resonance of Cr3+ in EuAlO3
Authors: van der Ziel J. P. , Merritt F. R. , Van Uitert L. G.
Published: February 2004

The paramagnetic resonance spectrum of Cr3+ in EuAlO3 at the Ci symmetry Al3+ sites has been measured at 77°K and 24 GHz. With g = 1.98 we find D = − 0.235 cm−1 and E = 0.0522 cm−1.
Knight Shifts in Potassium–Ammonia Solutions Doped with Potassium Iodide
Authors: O'Reilly D. E.
Published: February 2004

A relatively large Knight shift of 127I in solutions of potassium in liquid ammonia with added potassium iodide has been observed. The effect was investigated from M = 0.12 mole liter−1 of potassium to M = 0.95 at 300°K and a concentration of KI equal to 0.03 mole liter−1. At M = 0.95 the shift k = 380 ppm to lower magnetic field; k increases monotonically with increase in M at fixed concentration of KI. At M(K) = 0.44 and M(KI) = 0.14, k increases with increase in temperature from 9 ppm at 211°K to 176 ppm at 300°K similar to that of 23Na in sodium–ammonia solutions. An approximate fit of the data is made in which it is proposed that the species KI− is formed. Self-consistent field calculations were performed for the system electron, iodide ion, and solvent. A weakly bound state is obtained but this species is deemed to be of minor importance. Spin-density calculations are performed for the species KI−. The lifetime of KI− is calculated to be less than 10−11 sec.
Ferroelectric Tungsten Bronze-Type Crystal Structures. II. Barium Sodium Niobate Ba(4 + x)Na(2 − 2x)Nb10O30
Authors: Jamieson P. B. , Abrahams S. C. , Bernstein J. L.
Published: February 2004

Ferroelectric Ba(4+x)Na(2 − 2x)Nb10O30, with a Curie temperature of 833°K, has a tungsten bronze-type structure and crystallizes in the orthorhombic system, with subcell lattice constants a = 17.59182 ± 0.00001, b = 17.62560 ± 0.00005, and c = 3.994915 ± 0.000004 Å at 298°K. The space group is Cmm2, and there are two formulas in the subcell. The c axis of the true cell is double that of the subcell. The integrated intensities of 6911 reflections within a reciprocal hemisphere of radius (sinθ)/λ = 1.02 Å−1 were measured with PEXRAD, 1873 symmetry-independent structure factors being significantly above background. The metal atom positions were determined from the three-dimensional Patterson function and the oxygen atoms from metal-phased Fourier series. The final agreement index between measured and calculated structure factors is 0.0579. The structure differs only in detail from previously determined tetragonal tungsten bronze structures. In the general formula (A1)2(A2)4C4(B1)2(B2)8O30, the B1 and B2 sites are fully occupied by Nb, the A2 sites by Ba and the A1 site by 87.0% Na and 6.5% Ba. Evidence from chemical analysis, x-ray density calculations and the present determination suggests that the best approximation to the formula of the crystal studied is Ba4.13Na1.74Nb10O30. The Ba and O atoms at z ≃ 12 are disordered in a manner similar to the O atoms in the Ba layer in barium strontium niobate. The four crystallographically independent Nb atoms, each in octahedral coordination, are linked to O atoms by distances ranging from 1.765 ± 0.021 to 2.270 ± 0.021 Å, with a mean value of 1.989 Å. Ba is 10 coordinated, with Ba–O distances ranging upward from 2.671 ± 0.013 Å. Na is 12 coordinated, with Na–O distances ranging from 2.660 ± 0.014 to 2.990 ± 0.015 Å, with a mean of 2.788 Å. The Nb-atom displacements from the mean oxygen planes lie in the range 0.171–0.205 Å; the parabolic relation with Curie temperature predicts a displacement of 0.204 Å. The measured value of Ps at room temperature is 40 μC cm−2: the linear relation between displacement and polarization predicts a saturation value of 44–53 μC cm−2. All metal atoms are displaced from the oxygen planes in the sense given by the macroscopic positive polarity.
Correlation Effects of π Electrons. II. Low-Lying Excitations of Polycyclic Hydrocarbons
Authors: Gutfreund H. , Little W. A.
Published: February 2004

We show how the Coulomb interaction between π electrons in a conjugated molecule is screened by the presence of the other π electrons. We describe how the resulting effective interaction can be calculated and used in a configuration-interaction calculation to determine the low-lying excitations of such molecules. The methods are applied to the study of a series of hydrocarbons from benzene to hexabenzocoronene and good agreement with the experimental spectra is obtained. The inclusion of these correlation effects predicts a characteristic nonuniform distribution of charge in alternant hydrocarbons.
Ligand ENDOR of Cu-8-Hydroxyquinolinate Substituted into Organic Single Crystals
Authors: Rist Günther H. , Hyde James S.
Published: February 2004

The technique of electron–nuclear double resonance (ENDOR) has been applied to copper-8-hydroxyquinolinate substituted into single crystals of phthalimide and of 8-hydroxyquinoline. This method has permitted determination of the nitrogen hyperfine tensor, with sufficient accuracy to detect small orthorhombic deviations from axial symmetry, and the nitrogen quadrupole tensor. These tensors have the same principal axes and the coupling constants of the two interactions have opposite sign. The axis corresponding to the largest quadrupole splitting is oriented along the nitrogen lone pair in the plane of the metal complex. One g axis of the slightly monoclinic copper-8-hydroxyquinolinate complex is oriented approximately along the oxygen–copper axis, and the other in-plane g axis is at a small angle with respect to the nitrogen–copper direction. The delocalization of the unpaired electron density to the ligand protons depends on the host lattice. The quadrupole tensor and its dependence on the host lattice are discussed. It is shown that the unpaired electron in the aromatic rings of 8-hydroxyquinoline resides mainly in the σ system, in accordance with the symmetry of the ground state. The isotropic and anisotropic coupling constants of the proton adjacent to the nitrogen and some coupling components of other protons in the aromatic ligands and in the neighbouring molecules of the host lattice have been determined. The spin densities in the aromatic rings are positive.
Certain Integrals Useful in Studies of Electronic Structure. II
Authors: Messmer R. P. , Lin C. S. , Birss F. W.
Published: February 2004

The three-electron, one-center integral over Slater basis functions 〈χ(1)χI(1) | 1 / r1c | χII(c)χIII(c) | 1 / r2c | χIV(2)χV(2)〉, found in applications of the generalized variational method, is evaluated.
Preferential Scattering of Ar from LiF: Correlation with Lattice Properties
Authors: Smith Joe N. , O'Keefe D. R. , Saltsburg Howard , Palmer R. L.
Published: February 2004

A study of the scattering of thermal energy Ar atoms from the (001) plane of LiF is reported. Multiple peaks in the scattered flux distribution are observed. The major peak in the distribution is the analog of the single lobe directed scattering distributions observed previously, using metal targets, in terms of its behavior as a function of angle of incidence and beam temperature. However, this major peak in the Ar/LiF data also displays a dependence on the azimuthal angle of the target which may be associated with the anisotropy in the elastic properties of LiF. Other peaks in the Ar distribution are observed to lie in fixed directions along rows of second-nearest-neighbor unlike ions in the LiF lattice. It is suggested that dipole–induced-dipole attractive forces serve to enhance the coupling of the Ar atoms to the longitudinal optical modes of vibration in these directions, giving rise to preferential re-emission.
Relative Rates of Reaction of Hydrogen Atoms with Olefins
Authors: Cvetanović R. J. , Doyle L. C.
Published: February 2004

A new technique has been developed and used to redetermine the rates of H atom reactions at room temperature with a number of simple olefins. Hydrogen atoms are generated by the mercury photosensitized decomposition of H2 in the presence of small amounts of ethylene or its mixtures with an added olefin. The relative rate constants are determined from a comparison of the yields of n-butane in the two cases. The values obtained are compared with the earlier determinations and with another independent recent redetermination in this laboratory. The values obtained by the three independent techniques are in very good agreement.
Rotational Isomerism in meta-Halobenzaldehydes
Authors: Crowder G. A. , Northam Fred
Published: February 2004

Infrared spectra have been obtained for m-chlorobenzaldehyde, m-bromobenzaldehyde, and m-iodobenzaldehyde. Each of these compounds exists as a mixture of two rotational isomers in the neat liquid. The cis-O form of m-chloro- and m-bromobenzaldehyde is the low-energy form, and is the only form present in the solid state. The trans-O form of m-iodobenzaldehyde is the only form present in the solid state of that compound, but the difference in energy between rotamers in the neat liquid is small. The stability of the cis-O form of m-bromo- and m-chlorobenzaldehyde relative to the trans-O form is attributed to the inductive effect of the halogen.
Thermal-Diffusion Factors for the Neon–Xenon System
Authors: Taylor W. L. , Weissman S. , Haubach W. J. , Pickett P. T.
Published: February 2004

The thermal-diffusion factor, αT, for the neon–xenon system has been measured from 328°–873°K at seven compositions ranging from 5%–95% neon. These measurements were made with a 20-tube trennschaukel where the pumping rate, the quantity of gas pumped, the duration of the experiments, and the temperature gradient could be varied over a wide range. This flexibility allowed the operating conditions to be chosen so that the theoretical corrections for trennschaukel operation were small and therefore could be applied to the measured separations with confidence. The present corrected data have been compared with theoretical calculations based on the Lennard-Jones (12–6) potential and the modified Buckingham (exp-6) potential. Both the experimental data and the theory indicate that the reciprocal of αT is a linear function of the composition. The present data agree well with the results of Grew and of Heymann and Kistemaker, but do not agree with that of Saxena and co-workers nor with the results of Atkins, Bastick, and Ibbs. The mutual diffusion coefficient for neon–xenon, which was derived from the measured composition dependence of αT, agrees well with experimental diffusion coefficients reported in the literature and also with values calculated from experimental viscosity and thermal conductivity data. Although the values of αT are in disagreement with the present work, the composition dependence of αT reported by Saxena, Nain, and Saxena also appears to be consistent with the values of the diffusion coefficient; however, that reported by Atkins et al. and Mathur and Saxena are not.
Vibrational Spectra and Structure of Four-Membered Ring Molecules. XIV. Vibrational Analysis and Ring Puckering Vibration of Trimethylene Selenide and Trimethylene Selenide-d4
Authors: Harvey A. B. , Durig J. R. , Morrissey A. C.
Published: February 2004

The infrared and Raman spectra of trimethylene selenide (TMSe) and trimethylene selenide-2,2,4,4-d4 (TMSe-d4) have been recorded from 4000 to 15 cm−1 in gaseous, liquid, and solid states. A vibrational assignment of the fundamentals is consistent with the expected Cs equilibrium configuration. A series of sharp bands associated with the ring puckering vibration have been observed in the mid- and far-infrared regions. The frequencies of the transitions have been employed in determining the following double-minimum potential, barrier-to-ring inversion, and the equilibrium separation of the ring diagonal, 2x in angstroms, for TMSe, respectively: V(cm−1 = 4.258 (± 0.02) × 105x4 − 2.5391 (± 0.007) × 104x2; 378.1 ± 4 cm−1; 0.35 Å. For TMSe-d4 these quantities are: V(cm−1) = 3.242(± 0.1) × 105x4 − 2.208 (± 0.033) × 104x2; 375 ± 10 cm−1; 0.37 Å, respectively. The dihedral angle of both molecules is calculated to be 32.5 ± 2°.
Excimer Emission from Derivatives of 2-Phenyl-5-(4-Biphenylyl)-1 ,3 ,4-Oxadiazole (PBD)
Authors: Horrocks Donald L.
Published: February 2004

The formation of excimers of 2-phenyl-5-(4-biphenylyl)-1 ,3 ,4-oxadiazole (PBD) was shown by the examination of the fluorescence spectra of concentrated solutions of derivatives of PBD in toluene. A slight increase in the amount of emission in the longer-wavelength region of the spectra was observed with increasing concentration of the PBD derivatives. At high solute concentrations the amount of the excimer emission was altered by variation of the temperature of the solution. Lower temperatures increased the relative amount of longer-wavelength emission (favored excimer formation) and higher temperatures decreased the relative amount of longer-wavelength emission (favored disproportionation of the excimer).
Proton Relaxation in Hexamethylbenzene
Authors: Bernard H. W. , Tanner J. E. , Aston J. G.
Published: February 2004

Proton T1 and T1ρ of solid hexamethylbenzene have been measured at 30 MHz from 4.2°K to above the melting point. The T1 results are in satisfactory agreement with similar measurements of Allen and Cowking, and of Anderson and Slichter. The T1ρ results confirm their conclusions concerning molecular motion over most of the solid-phase temperature range, but reveal a previously undetected motion just below the melting point. Mention is made of a method for measuring long T1's in solids.
Local Permutational Symmetry and the Separated-Atom Limit
Authors: Klein D. J.
Published: February 2004

Local permutational symmetries are shown to supply approximate quantum numbers for a system composed of approximately localized subsystems. Local permutational symmetries associated with simple valence-bond wavefunctions in the separated-atom limit are studied and in certain cases are found to yield a nonphysical combination of local permutational symmetries. This nonphysical behavior can be avoided by use of a certain suitable form of configuration interaction or by a projection with either an appropriate chemically canonical matric basis element or a generalized structure projector.
ReO3 Band Structure in the Tight-Binding Approximation
Authors: Honig J. M. , Dimmock J. O. , Kleiner W. H.
Published: February 2004

Using tight-binding, LCAO methodology, a formal band structure scheme has been specified for the ReO3 structure based on the use of 2s and 2p orbitals for oxygen and of 5d, 6s, and 6p orbitals for rhenium. The scheme should also be useful in the characterization of band structures for perovskites in general. Two sets of sufficient conditions have been found under which the secular determinant reduces to two special cases: one corresponding to a model previously considered by Goodenough and the other to a model considered by Kahn and Leyendecker. The nature of the approximation schemes is discussed in some detail.
Mass-Spectrometric Measurement of the Dissociation Energies of BeCl and BeCl2
Authors: Hildenbrand Donald L. , Theard Lowell P.
Published: February 2004

A mass spectrometer was used to study gaseous equilibria in the Be–Al–Cl system by passing HCl(g) over a Be–Al mixture in a Knudsen cell molecular source. For the reactions Be(g) + AlCl(g) = BeCl(g) + Al(g) and Be(g) + BeCl2(g) = 2 BeCl(g), ΔH298 values of 26.2 ± 2 and 36.1 ± 2 kcal, respectively, are derived. Also, the sublimation of BeCl2 was studied by torsion-effusion and mass-spectrometric techniques, from which the heat of sublimation ΔH298 = 32.5 ± 0.5 kcal/mole is derived for β-BeCl2. These data yield the dissociation energies D0°(BeCl) = 91.9 kcal (3.98 eV) and D0°(BeCl2) = 219.4 kcal (9.51 eV). The results are discussed briefly in terms of the chemical bonding.
Experimental Verification of the Thermal Diffusion Column Theory as Applied to the Separation of Isotopically Substituted Nitrogen and Isotopically Substituted Oxygen
Authors: Rutherford W. M. , Roos W. J. , Kaminski K. J.
Published: February 2004

By means of techniques previously developed, column transport coefficients were measured for the 14N14N–14N15N and 16O16O–16O18O separations in a 7.32-m hot-wire thermal diffusion column at wire temperatures of 500° and 800°C. These data and several sets of column data from the recent literature were compared with predicted values calculated from the Jones and Furry theory with integrations based on carefully selected tables of smoothed transport properties. On the basis of measurements for nitrogen reported in the literature it was assumed that the isotopic thermal diffusion factors for both gases were 0.7 times the value given by the Lennard-Jones potential. Good agreement was obtained between theory and experiment. The average ratios of the measured values of the column coefficients to the corresponding predicted values were found to be unity within essentially twice the standard deviation of the experimental determinations. There were negligible differences between column coefficients calculated from tabulated transport properties and those calculated from Lennard-Jones shape factors.
Vacancy Calculation of the Viscosity of Potassium–Ammonia Solutions
Authors: O'Reilly D. E.
Published: February 2004

The theory of random flights and the Stokes–Einstein expression for translational diffusion are combined to calculate the viscosity of a liquid. The viscosity is inversely proportional to pυ, the probability of a molecular vacancy. pυ is evaluated from a quasilattice model for salt solutions as well as alkali-metal solutions in liquid ammonia and the contribution to the viscosity η which varies linearly with concentration M is calculated. The calculated values of η0−1(dη / dM)0 of salt solutions are 20%–35% larger than the observed values. Calculated values of | η0−1(dη / dM)0 | for alkali-metal solutions are somewhat larger than the observed values but the coefficients have the correct sign and temperature dependence.
Average-Excitation-Energy Approximation in the Calculation of Carbon-13 Chemical Shifts of Nitrogen Heterocyclics
Authors: Adam Waldemar , Grimison Alec , Rodríguez Gladys
Published: February 2004

The carbon-13 chemical shifts of the six-membered-ring heterocyclics pyridine, pyrazine, pyrimidine, pyridazine, s-triazine, s-tetrazine, and their cations and dications, and of the five-membered-ring heterocyclics pyrrole, imidazole, pyrazole, and their anions and cations have been calculated from various approximate perturbation theory expressions. The diamagnetic contribution to the screening tensor was calculated exactly. The paramagnetic contribution was calculated using (i) individual transition energies in the full perturbation theory expression, (ii) the Karplus–Pople approximation, (iii) the Karplus–Das approximation, and (iv) from π-electron densities. Valence-electron wavefunctions computed by the extended Huckel theory (EHT) were used, in nonorthogonal and in orthogonal form. The detailed perturbation expansion fails badly; this failure can be linked to the limited basis set used in EHT calculations. The neglect of orbitals on vicinal atoms, or the use of actual geometries does not improve the results significantly. Good agreement with experiment is obtained by the judicious use of the average excitation energy approximation and orthogonal wavefunctions. A normalized 〈ΔE〉Av parameter calculated from EHT energy values reproduces (a) the displacement of the chemical shift to higher field in the isoelectronic series C7H7+, C6H6, C5H5− (b) the displacement of the chemical shift to higher field on protonation of the nitrogen lone pairs. The use of the π-electron approximation does not permit the latter correlation to be made with experimental trends.
Photoelectron Spectra of Molecules. I. Ionization Potentials of Some Organic Molecules and Their Interpretation
Authors: Dewar Michael J. S. , Worley S. D.
Published: February 2004

The photoelectron spectra of 67 organic compounds, including paraffins, cycloparaffins, olefines, dienes, aromatic hydrocarbons, alcohols, ethers, aldehydes, ketones, and diene iron tricarbonyl complexes, have been measured, using a simple retarding-potential grid-type spectrometer, and the results interpreted in terms of a recently developed semiempirical SCF MO treatment (MINDO approximation). The observed ionization potentials show a remarkably close relationship to the calculated Hartree–Fock MO energies over the whole range of ionization potentials (<20 eV) studied. An estimate is given for the first ionization potential of cyclobutadiene.
Atomic Multiconfiguration Self-Consistent-Field Wavefunctions
Authors: Sabelli Nora , Hinze Jürgen
Published: February 2004

The ground-state 1S wavefunctions of the atomic isoelectronic series with two and four electrons (Z = 1 – 8, and 3–8, respectively), were calculated using the MC–SCF method restricted to pair replacements. These calculations were to test and demonstrate how rapidly the configuration expansions of the MC–SCF wavefunctions converge. In the two-electron series, nine configurations were required to yield 97% of the correlation energy, while ten configurations accounted for 86% of the correlation energy in the four-electron series. The quality of the reported wavefunctions has been tested by computing the expectation values of several one- and two-electron operators. The results obtained agree in general satisfactorily (deviations of ∼1%) with values computed from more complex wavefunctions, indicating that the compact MC–SCF functions reported are indeed of high quality.
Far-Infrared Spectra and the Ring-Puckering Potential Function of Silacyclopent-3-ene and Silacyclopent-3-ene-1,1-d2
Authors: Laane Jaan
Published: February 2004

A series of a far-infrared absorption bands have been observed between 35 and 100 cm−1 in the spectra of silacyclopent-3-ene, CH2CH=CHCH2SiH2, and silacyclopent-3-ene-1,1-d2. These absorptions result from the transitions between the various energy levels of the ring-puckering vibration. The frequencies of the bands can be calculated quite accurately using a pure quartic potential, V = 14.34Z4 for the hydride and V = 13.20Z4 for the deuteride where Z is the ring-puckering coordinate in reduced form. A slightly better fit for each spectrum is obtained by adding a small positive quadratic term to the potential function. The ring-puckering potential for this molecule shows that the structure of the five-membered ring is a planar one and that it has no barrier to inversion like that in cyclopentene. This lack of a potential barrier is attributed to a smaller barrier to internal rotation in the –CH2–SiH2– bond as compared to –CH2–CH2–. Also there is evidence from the isotope shift of the frequencies (which is related to the reduced masses of the 2 isotopic species) that the distance between the silicon atom and the double bond is much smaller than expected. This suggests that a weak bond between the Si d orbitals and the π system is partly responsible for the planarity of the compound.
Molecular Force Field for Trimethylamine
Authors: Gayles J. N.
Published: February 2004

Molecular potential functions are obtained which reproduce the observed frequencies for trimethylamine and three of its deutero-methyl isomers. The fitting procedure employed was a linearized least-square analysis with potential constant regression employed to damp convergence oscillation. It was found that anharmonicity corrections become important in the case of the skeletal motions in almost all cases, and the physical significance of the skeletal stretch–bend interactions derived was checked by independent means using an orbital rehybridization model.
Electronic States of Mixed Molecular Crystals: Singlet Excited States of Isotopic Impurities in Crystalline Benzene and Naphthalene
Authors: Sommer Bat-Sheva , Jortner Joshua
Published: February 2004

In this paper the one-particle Green's-function method has been applied for the study of singlet excited states of isotopically mixed crystals of benzene and naphthalene. Calculations of energy levels, of excitation amplitudes, and of line-shape functions were performed, considering both bound and virtual impurity energy levels. The present treatment relates experimentally observable properties for the dilute mixed crystals with the exciton density-of-states function of the pure crystal. The available experimental data for mixed crystals can be adequately accounted for using empirical data for the exciton density-of-states function derived from hot-band spectroscopy. The theoretical density of states for naphthalene based on the transition-octupole coupling terms provide only qualitative agreement with the experimental mixed-crystal data. We consider the applicability of simple perturbation expressions by using a moments-expansion method of the Green's function. Finally, we discuss the information on the intermolecular interactions derived from the moments of the density-of-states functions.
Deactivation of Bending Mode of CO2 by Hydrogen and Deuterium
Authors: Sharma R. D.
Published: February 2004

The rate of deactivation of bending mode of CO2 by hydrogen and by deuterium is calculated using long-range forces. It is shown that orthohydrogen is an order of magnitude more effective than parahydrogen at room temperature, the deactivation being affected by J = 1 → J = 3 rotational transition of the ground vibrational level. The agreement with the available data is good up to ∼500°K. The calculation for deactivation by deuterium due to rotational transition J = 4 → J = 6 yields results which are in reasonable agreement with the results of Cottrell and Day.
NMR Investigation of Relaxation and “Chemical-Exchange Spin Decoupling” in Dilute Solutions of Co2+ with Various “P–O” Containing Solvents
Authors: Frankel Lawrence S.
Published: February 2004

The concentration and temperature dependence of the proton relaxation times of complexes of Co2+ with dimethyl methylphosphonate, dimethyl hydrogen phosphite, and methyl phosphate have been examined. As the concentration of the paramagnetic ion Co2+ is increased, the phosphorus nucleus may be spin decoupled from the proton. This phenomenon has been called “Chemical-exchange spin decoupling.” The equations of Swift and Connick are employed to describe the solvent exchange process. The approach of McConnell is shown to apply to the two nuclear spin orientations of phosphorus as a kinetic process in a double resonance experiment. Combining these equations accounts for the dependence of the observed relaxation times on the concentration of paramagnetic ion.
Use of Relativistic Electron Scattering Factors in Electron-Diffraction Analysis
Authors: Yates A. C. , Bonham R. A.
Published: February 2004

Relativistic partial-wave scattering factors are employed in an attempt to explain recently observed discrepancies between theoretical and experimental results for molecules containing one heavy and several light atoms in gas electron diffraction. It is shown that relativistic effects may make slight corrections to some observed amplitudes of vibrations but that they apparently do not account for the observed experimental deviations from theory.
Diffusion and Relaxation Phenomena in Ice
Authors: Onsager Lars , Runnels L. K.
Published: February 2004

The underlying mechanisms of several rate processes in ice are examined through cross comparisons of the processes with each other and with experimental observations. The assumption that the migration of orientational defects (Bjerrum faults) is the common origin of dielectric and elastic relaxation leads to a predicted ratio of dielectric-to-elastic relaxation time of 32, in close agreement with experiment. The conclusion that a separate process is responsible for diffusion is based on a comparison of diffusion and dielectric relaxation data. The faster diffusive motion controls the thermal equilibration of the proton spins as well as the magnetic resonance linewidths; an interstitial migration appears to be the mostlikely diffusion mechanism.
Applications of Diatomics-in-Molecules Theory. III. The Li4 System
Authors: Companion Audrey L.
Published: February 2004

Some simple diatomics-in-molecules relations for homonuclear four-atom systems with s-electron bonding are presented and applied to Li4 clusters of D4h, Td, and D2d symmetry. Predictions include a square Li4 molecule (with side 5.47 Bohr units) and an atomization energy 63.94 kcal/mole at 0°K and a disphenoidal (Li2)2 cluster (two diatomic Li2 species with perpendicular axes separated by 5.89 units) with atomization energy 60.52 kcal/mole at 0°K, both species stable with respect to isolated diatomics. The optimum energy of tetrahedral Li4 (side 5.8 Bohr units) lies about 11 kcal/mole above that of the square; however the tetrahedron exhibits Jahn–Teller instability with respect to the square and disphenoid conformations. Fundamental vibrational frequencies are calculated for both D4h and D2d species.
Anisotropic Molecular Rotation in Liquid N,N-Dimethylformamide by Nuclear Magnetic Resonance
Authors: Wallach Daniel , Huntress Wesley T.
Published: February 2004

The anisotropy in the molecular rotation of N,N-dimethylformamide (DMF) in the liquid is investigated by considering the NMR relaxation times of the 14N, 17O, and formyl deuteron nuclei in DMF, DMF17O, and DMF-d1, respectively. Assuming that the molecular rotation is diffusional and that one principal diffusion axis (the z axis) is perpendicular to the molecular plane, a range of possible orientations for the diffusion axes in the plane and a corresponding range of values for each of the three diffusion constants are calculated. The range of possible orientations of the x diffusion axis, which varies little with temperature, extends over about 26°, from the direction of the x principal inertial axis toward and past the direction of the molecular dipole moment. At 280°K the diffusion constants are Dx = (2.0 ± 0.4) × 1011sec−1, Dy = (6 ± 3) × 109sec−1, and Dz = (2.9 ± 0.7) × 1010sec−1. At 390°K, Dx = (4.0 ± 0.7) × 1011sec−1, Dy = (10 ± 8) × 109sec−1 and Dz = (2.1 ± 0.7) × 1011sec−1. The ratio of the rotation rate about the ith axis in the free gas to Di is a direct measure of the extent to which rotation about the ith axis is hindered in the liquid by intermolecular interactions. Defining this ratio as χi (where larger values of χi indicate a more hindered rotation), it is found that at low temperatures χy, χz ≫ χx and at high temperatures χy ≫ χz, χx. Once the rotation of the molecular framework is known, the relaxation times of the methyl deuterons in DMF-d7 can be used to calculate the internal rotation rates of the deuterated methyl groups. These rotation rates are shown to be comparable to the rate of the molecular rotation.
Exposure-Dependent Surface Recombination Efficiencies of Atomic Oxygen
Authors: Myerson Albert L.
Published: February 2004

Surface recombination efficiencies (γ) on metals have been measured as a function of exposure time in steady Oseen flows which provide a step-function increase, or pulse, in oxygen-atom concentration. Cylindrical, circumferentially coated, thin-film resistance thermometers were used to determine the rate of heat transfer. The relationship between the surface and free stream concentration of oxygen atoms was obtained by means of the mathematical analogy between heat and mass transfer. It was possible to use relatively high partial pressures of atomic oxygen and carrier gas (1% atomic oxygen in a flow pressure of 4–6 mm Hg of an Ar–O2 mixture) which are important to the accuracy of the measurements of many of the parameters involved. Under these conditions, the dependence of γ on exposure time was obtained as a function of time for periods up to several seconds. Each of the metals investigated exhibits a markedly different time history of γ during the period of initial gas–solid interaction, and the γ of each metal, after reaching an initial peak value, eventually reaches a final but different stable-state value. The final values are 0.12, 0.03, 0.017, 0.0085, 0.007, and 0.005 for Ag, Cu, Fe, Ni, Al, and Au, respectively, which values are generally lower than those previously reported. Reasons for these differences include the elimination of excess heating due to excited species from the discharge which generates the atom flow.
Energy Transfer by Benzene in the Cadmium-Photosensitized Isomerization of Butene-2
Authors: Hunziker Heinrich E.
Published: February 2004

Quantum yields at 590°K of the cadmium-photosensitized cis–trans isomerization of butene-2 have been determined, with butene pressures ranging from 1.2–13.5 torr, in the presence of 0 to 35.9 torr of benzene. Without benzene, quantum yields close to 0.5 were found for both isomers, and there was no significant difference in the quenching cross sections. With benzene, the yield was larger than one-half the fraction of Cd(53P1) atoms quenched by butene. This result is interpreted by a mechanism in which the 3.8-eV Cd(3P1) atom excites the 3.6-eV (3B1u) level of benzene, the triplet benzene molecule then causing the excess isomerization. The rate of reaction of the triplet species with benzene is 0.03 ±0.04 times that with butene-2, and the ratio of its unimolecular decay rate vs rate of reaction with butene-2 is (5.0 ± 0.7) torr. From the latter figure a lifetime of 0.3 × 10−6 sec and an upper limit of 6 kcal/mole for the activation energy of unimolecular decay are estimated.