bluuebunny/crossref_metadata_2025 huggingface dataset:
Phosphorescence Mechanisms in Naphthalene N-Heterocyclics
Authors: Ziegler Stanley M. , El-Sayed M. A.
Published: February 2004

The polarization of the phosphorescence spectra of quinoline, isoquinoline, and quinoxaline is measured at 4.2 and 77°K in biphenyl and durene hosts. The results for quinoline and isoquinoline resemble those obtained previously for naphthalene and quinoxaline. The apparent anamolous polarization in the durene is also observed for these molecules. A discussion of the “apparent” change is briefly given. As in quinoxaline, a new spectrum not observed for naphthalene is found to originate from mechanism(s) involving vibronic interaction. A tentative assignment of the vibrations responsible for the appearance of the new spectra is made. A theoretical discussion is given of the possible mechanisms responsible for the appearance of the vibronically induced part of the spectrum. A comparison of theoretical predictions with observed results leads to the following conclusions:

(1) There is some indication that, as in pyrazine, the contamination of the ground state with triplet character of the (n−3, π*) state in quinoxaline is not negligible. But unlike pyrazine, the effect of this mixing appears only in the vibronic second-order spectrum (and not in the first-order spectrum as it does in pyrazine). This is due to the fact that the emitting state in quinoxaline is of the (π, π*) type.

(2) In the second-order schemes, as well as the first order, the important spin–orbit interaction takes place between the symmetric and antisymmetric states with respect to reflection in the molecular planes.

(3) Qualitative estimates show that the presence of low energy n, π* states can account for: (a) the difference between the total radiative power of N-heterocyclics and their parent hydrocarbons; and (b) the preferential enhancement of the vibronic part of the spectrum in N-heterocyclics over that existing in aromatic hydrocarbons.
Numerical Solutions of the Percus–Yevick Equation for the Lennard-Jones (6–12) and Hard-Sphere Potentials
Authors: Mandel Frederic , Bearman Richard J. , Bearman Mirion Y.
Published: February 2004

Numerical solutions of the Percus–Yevick equation for the Lennard-Jones (6–12) potential are presented largely for high densities and the liquid region in a form particularly useful to workers who have access only to small computers which cannot be used for solving the PY equation. The solutions, given in a form suitable for calculating either the radial distribution functions or direct correlation functions, are deposited with the National Auxiliary Publications Service (Document No. NAPS-00720). A table of densities and temperatures at which the PY solutions are available is given. Energies and pressures, including the compressibility pressure for temperatures below the critical, are also tabulated and comparisons are made with molecular dynamics results. Solutions for the hard-sphere potential are also presented completing the tables of Throop and Bearman [J. Chem. Phys. 43, 2408 (1965)] for large intermolecular separations.
Atomic and Molecular Calculations with the Pseudopotential Method. VI. Two-Valence-Electron Energy Values for Doubly Excited States
Authors: McGinn George
Published: February 2004

The pseudopotential method is applied to a number of npms 1,3P doubly excited states of Be, Mg, Al+, Si2+, Ca, and Zn. Most of these excited states are in the autoionizing region. Besides the pseudopotential approximation, the further approximation of “freezing” one of the two valence electrons is used. The fixed valence electron is selected to be the state of the one-valence-electron system which is the limit for the particular series of states in the two-valence-electron system, e.g., for the Be 1s22pns 1,3P series the fixed 2p valence electron is set at the value of the Be+ 1s22p 2P state. Comparison is made with the observed values and other calculated values with good agreement.
Study of the H+ and OH− Hydrated Ions by the CNDO/2 Method
Authors: De Paz M. , Ehrenson S. , Friedman L.
Published: February 2004

The ions obtained by hydration of H+ and OH−, e.g., H3O+, H5O2, H7O3, H9O4+, H3O2−, H5O3−, and H7O4− have been studied theoretically using the CNDO/2 method. The results indicate chain structures are generally preferred albiet weakly over cyclic structures by the total energy criterion and in the case of positive ions over proton centered structures. Binding energies are found to be in reasonable agreement with experimental data. Charge distributions and individual bond energy effects have been examined in detail and are generally consistent with the notion that protons serve as bridging centers for delocalization which remain strongly positive throughout both series to shield the fragment oxygens from each other.
Heat Capacity of Silver Iodide. V. Heat Content of Nonstoichiometric Material
Authors: Perrott C. M. , Fletcher N. H.
Published: February 2004

A program of study of the cation disorder in α-AgI is completed by a series of heat content experiments on quenched samples which contain small excesses of silver. The results presented are analyzed in terms of lattice disorder, and a model of the behavior of doped material is developed. This model shows that despite significant differences in thermal properties, the cooperative disordering of the cations is very similar in stoichiometric and doped material. It is proposed that deviation from stoichiometry causes an elevation of the critical temperature for disordering.
High-Resolution He i and He ii Photoelectron Spectra of Xenon Difluoride
Authors: Brundle C. R. , Robin M. B. , Jones G. R.
Published: February 2004

The vertical I.P.'s of the first eight ionizations in the XeF2 molecule have been determined, and they compare favorably with the results of Gaussian-type orbital calculations. The first two ionic states of XeF2 are the 2Π3/2 (12.42 eV) and 2Π1/2 (12.89 eV) spin–orbit components formed by ionization from the highest-filled π orbital (5πu). As judged by the vibronic structure of the transitions, the two upper states are linear, with Xe–F distances very much like those of the ground state. The fifth (15.60 eV) and sixth (16.00 eV) I.P.'s similarly involve the ejection of an electron from the 4πu orbitals, and thus they are separated by the action of spin–orbit coupling. Substitution of He ii α excitation (40.82 eV) for the He i α excitation (21.21 eV) results in both the additional ionization of electrons from the 9σg molecular orbital (22.5 eV), and a drastic change in the relative intensities of certain peaks in the lower I.P. region. Rydberg bands in the optical spectrum of XeF2 can be correlated with those of the photoelectron spectrum, yielding assignments in essential agreement with those given by earlier workers. The work with the He ii resonance line is described in some detail because a description of its use in photoelectron spectroscopy has not previously appeared in the literature.
Phosphorescence of Phenylcarboxylic Acids and Their Salts
Authors: Maria H. J. , McGlynn S. P.
Published: February 2004

The phosphorescence spectra and lifetimes of a series of carboxylic acids, phenyl–(CH2)n–COOH, and their salts have been measured. It is shown, contrary to accepted attitudes, that the lowest triplet state of benzoic acid is not an excitation localized on the benzene ring but involves considerable delocalization onto the carboxyl group. The lowest triplet energy of the carboxyl group and its radiative lifetime are estimated as 29.7 kK and 4 sec, respectively.
Nuclear Overhauser Effects and 13C Relaxation Times in 13C–{H} Double Resonance Spectra
Authors: Kuhlmann Karl F. , Grant David M. , Harris Robin K.
Published: February 2004

The effect of proton decoupling upon carbon-13 magnetic resonance spectra is treated in detail with the Solomon formulation for multiply irradiated spin systems. The factors affecting the nuclear Overhauser enhancement are discussed in terms of competition between the dipole–dipole relaxation mechanism essential to the Overhauser effect and other relaxation processes. Using a density matrix formulation, it is exhibited that the maximum Overhauser effect (achieved when the dipolar mechanism dominates) is independent of the number of hydrogen atoms interacting with a relaxing carbon-13 nucleus. On the other hand, if the molecular tumbling motion is isotropic and the dipole–dipole mechanism dominates the relaxation process, then to a first approximation T1 can be shown to depend inversely upon the number of directly bonded hydrogen atoms. Expressions for treating an AMX three-spin system are given also, and the effect of the third spin is discussed. The highly symmetric adamantane molecule provides an excellent test case for the validity of the theoretical conclusions as it is an isotropic tumbler because of the rigidity and its tetrahedral symmetry. Furthermore, it has two nonequivalent carbon positions having different numbers of hydrogen atoms. The experimental Overhauser enhancement was found to be the same for both carbons and the T1's for these two carbons, measured with the adiabatic rapid passage technique, reflected the two-to-one factor expected for CH and CH2 groups, respectively. Minor deviations are explained with small contributions from vicinal hydrogens. Having exhibited the validity of the theoretical treatment for adamantane, it becomes evident that exceptions may be interpreted in terms of deviations from isotropic tumbling motion. As most molecules lack the symmetry to be isotropic in rotational diffusional processes, this experimental technique offers an important way for studying anisotropic molecular motion in liquids.
Semiempirical VB Calculation of the (H2I2) Interaction Potential
Authors: Raff L. M. , Stivers Lewis , Porter R. N. , Thompson D. L. , Sims L. B.
Published: February 2004

A simple, nonionic valence-bond formalism is employed to obtain the interaction potential energy for the (H2I2) system. The system is treated as one consisting of four electrons moving in the potential of two nonpolarizable I+ cores and two protons. All three- and four-center integrals are omitted to obtain the London expression for the system potential. All remaining Coulomb and exchange integrals are evaluated semiempirically in terms of ground state Σ1 and lowest triplet Σ3 state potentials by utilizing the Heitler–London expression for the energy of the diatomic systems. The result is a closed-form analytical expression for the system potential energy as a function of the six interparticle distances. Potential-energy contour maps are presented for the C2υ and distorted planar trapezoidal conformations, symmetric and nonsymmetric linear arrangements, and a planar “staggered” conformation. The three-body (H2I) potential-energy surface is also investigated and contour maps for the 90° and linear surfaces are given.
ESR Study of the Behavior of H Atoms Trapped in Frozen Perchloric Acid Solutions
Authors: Ultee C. J. , Kepford C. R.
Published: February 2004

Aqueous solutions of perchloric acid were irradiated at 77°K with Co-60 γ rays and then examined by ESR spectroscopy. The H atoms formed in these systems are stable at 77°K and decay only on warming the samples. The kinetics of the H-atom decay were examined in detail for 0.13 mole fraction HClO4 solutions over the temperature range 100–130°K. There are two types of trapped H atoms, both of which decay with first-order kinetics. The least stable type has a decay rate constant given by k = 1.84 × 103exp(− 3.24 × 103 / RT) sec−1 and the second-type decays with a rate constant given by k = 9.43 × 1010exp(− 8.02 × 103 / RT) sec−1. The first-order decays are interpreted as resulting from a mechanism which has an activated release of H atoms from their traps as the rate-determining step. The resulting mobile or free H atoms then react rapidly with other radicals to complete the reaction. Based on this mechanism, the trapping energies for the two types of sites are 3.24 and 8.02 kcal/g atom, respectively.
Equilibrium of Large Atoms on Any K × ∞ Square Lattice and the Occupation Ratio for K = 3
Authors: Verhagen A. M. W.
Published: February 2004

Under repeated random desorption and readsorption the patterns of atoms which may arise on a square lattice depend on the size of the atoms in relation to the cell size, but irrespective of size all possible patterns become equally likely. The dynamic equilibrium of large atoms thus defined (Fermi–Dirac equilibrium) is in this paper characterized by a probabilistic process, which generates all possible patterns with equal probabilities for any specified K × ∞ lattice and adsorption density ρ. The general formulas developed are used to find and evaluate exact mathematical relations between occupation ratio and adsorption density on the 3 × ∞ lattice. An approximate polynomial expression for the ∞ × ∞ lattice taken from the literature is evaluated for comparison with the exact solutions for K = 2 and 3.
Theory of Magnetic Circular Dichroism
Authors: Stephens P. J.
Published: February 2004

The magnetic circular dichroism of systems of localized absorbing centers is treated on the basis of the semiclassical theory of radiation absorption. The extreme cases of simple line spectra and smooth absorption bands are discussed in detail. Moment analysis is employed for bands and general expressions for their zeroth and first moments are derived. These are applied to electronic absorption bands, with emphasis on allowed transitions. The particular case of molecular solutions is considered explicitly. The relationship of the results to earlier theoretical treatments is discussed, and their relevance to the interpretation of experimental data indicated.
Infrared and Raman Spectra of ClF3 and BrF3
Authors: Selig H. , Claassen H. H. , Holloway J. H.
Published: February 2004

The infrared and Raman spectra of the interhalogens ClF3 and BrF3 have been studied in the vapor phase. Frequencies for the six fundamental vibrations of each molecule have been assigned based on C2ν symmetry. For ClF3 the frequencies are 752.1, 529.3, and 328 (a1), 702 and 442 (b1), and 328 (b2) cm−1. For BrF3 they are 675, 552, and 242 (a1), 612 and 350 (b1), and 242 (b2) cm−1.
LCAO–MO–SCF Calculations Using Gaussian Basis Functions. IX. Possible Rare Gas Adducts with Beryllium Species, HeBeH2 and BeHe
Authors: Kaufman Joyce J. , Sachs Lester M.
Published: February 2004

As a result of the extreme electron deficiency of the Be in BeH2 (only 3.3 electrons instead of the 4.0 in the neutral Be atom), it should be possible to prepare novel complexes of beryllium hydride such as its rare gas adducts. As an initial study we performed ab initio LCAO–MO–SCF investigations using Gaussian basis functions of the helium adduct of beryllium hydride, HeBeH2. The calculated energy results showed this species to be marginally unstable with respect to He+BeH2 (in many different geometrical configurations). However, the population analysis results do indicate a slight propensity to binding between the He and the Be. The gross atomic populations indicate a slight shift of electron density from the He to the Be and the He–Be total overlap population is positive, again indicative of an incipient bond. For comparison the BeHe system itself was subjected to a similar theoretical treatment. The energy results show there to be considerably less repulsion energy between He and BeH2 than between He and the isolated Be atom. (This behavior would be expected commensurate with the greater electron deficiency of Be in BeH2 than in the isolated Be atom.) The same phenomenon is reflected in the positive He–Be total overlap population as a function of distance for HeBeH2 as contrasted with a negative He–Be total overlap population in BeHe at the same distances. (The repulsive curve for BeHe itself also implies that species such as BeHe, BeHe2, or BeHe3 will not be preparable, contrary to recent proposals by experimentalists.) Thus while HeBeH2 appears not to be a promising species it is postulated that the higher rare gas complexes will be more stable. We conjecture that it should be possible to make and perhaps even to isolate the xenon complex of BeH2. It should be emphasized that any scattering data of rare gases, even He, should be analyzed taking into account the short range chemical type forces which our LCAO–MO–SCF calculations have demonstrated to exist between rare gases and molecules. Our results imply that even He is not inert and any experiments using carrier gases should be considered very carefully in this context.
Spectroscopic Study of Perchloric Acid Dihydrate H5O2+ClO4−
Authors: Pavia André C. , Giguère Paul A.
Published: February 2004

The infrared and Raman spectra of the H5O2+ ion have been observed in the crystalline dihydrate of perchloric acid at − 180°C. Existence of H5O2+ ions in that crystal, first detected by x-ray diffraction, is further confirmed by the absence of certain characteristic bands of the H3O+ and H2O species. The spectra are consistent with a centrosymmetric H2O–H–OH2 structure of trans configuration approximating the C2h point group. In the deuterated compound the infrared spectra indicate the same structure for the D5O2+ ion. Interference from the strong bands of ClO4− prevented identification of all the fundamentals of H5O2+.
Triplet-State Zero-Field Splitting Calculations in Aromatic Hydrocarbons and Heterocycles
Authors: Zuclich Joseph
Published: February 2004

Zero-field splitting in the lowest triplet states of aromatic hydrocarbons and heterocycles is calculated using several molecular orbital schemes. The most reliable results are obtained using closed-shell SCF molecular orbitals with configuration interaction between all triplet states singly excited relative to the ground state. Slater-type atomic orbitals are used and the two-center Coulomb integrals involving these orbitals are evaluated exactly. Results calculated using best Slater-type orbitals are consistently higher than experimental values, but good agreement with experiment is obtained when two-zeta of four-zeta atomic orbitals are used.
Integral Hellmann–Feynman Analysis of the Binding Energy of H2 and LiH Using Atomic Reference States
Authors: Marron Michael T.
Published: February 2004

The integral Hellmann–Feynman formula is applied to study the energy differences involved in the processes He→H2H− and Be→LiH→Li−. The energy differences are computed using single determinant SCF wavefunctions to represent the states. The transition density is reduced to diagonal form by means of a corresponding orbital transformation and each term in the energy expressions for the Be→LiH→Li− process is given a physical interpretation in terms of classical electrostatic concepts. A qualitative discussion is given which indicates a simple model might readily account for all factors contributing to the energy difference.
Homogeneous Exchange Reaction: CD4+CH4→CD3H+CH3D. Single Pulse Shock Tube Studies
Authors: Burcat Alexander , Lifshitz Assa
Published: February 2004

The homogeneous exchange reaction between CH4 and CD4 was investigated using the single pulse shock tube technique. The temperature range covered was 1340–1745°, and concentrations up to 8% total methane in argon were used. The over-all rate of exchange is proportional to the 32 power of the methane concentration and is independent of the argon density. The rate of exchange in mixtures containing 4% CH4 + 1% CD4 and 4% CD4 + 1% CH4 was found to be the same, indicating a negligible isotope effect at the temperature range covered in this study. For mixtures in which [CH4] = [CD4] the rate of exchange is given by d[CD3H] / dt = kr[CH4]0.75[CD4]0.75, where kr = 4.3 × 1014exp[−(70 ± 2) × 103 / RT]cc0.5mole−0.5·sec−1. It was shown that the exchange proceeds via a chain reaction mechanism carried by CH3 and CD3 free radicals. The evaluated rate constants for the chain, CH3+CD4→ lim k3CD3+CH3D, CD3+CH4→ lim k4CH3+CD3H, are by a factor of 10 higher than the ones extrapolated from the low temperature studies on the basis of an Arrhenius plot.
Theory of a Novel Odd-Parity Raman Scattering Mechanism: Depolarization Ratios and Reversal Coefficients for Random Molecular Systems
Authors: Chiu Ying-Nan
Published: February 2004

Raman scattering is treated as a two-photon process. A scattering mechanism that involves one-photon in an electric dipole mode of radiation and one photon in a magnetic dipole mode (or an electric quadrupole mode) has been worked out. The scattering tensors are of odd parity and may be of higher than the second rank. Therefore, this mechanism may be used to study the “silent” modes forbidden in the ir and the conventional Raman effect which is of even parity. When the molecule has inversion or reflection symmetry, this mechanism gives rise to an odd-parity electronic Raman effect between two (different) electronic states of opposite parities. This is in contrast to the recently discovered electronic Raman effect due to antisymmetric tensors which connect states of the same parity. When the molecule is optically active and has no inversion or reflection symmetry, this mechanism gives rise to novel vibrational and rotational Raman effects with symmetry and selection rules which are different from the conventional (even-parity) Raman scattering. Intensities due to scattering by random molecular systems are computed. These will permit the calculation of the depolarization ratios for this odd-parity Raman scattering of linearly polarized incident light into perpendicular directions, and the reversal coefficients for the scattering of circularly polarized incident light into forward as well as into arbitrary directions. Some useful angular factors for this odd-parity scattering have been tabulated (Tables I and II). To average over all random orientations, use is made of the coupling of irreducible spherical (vs Cartesian) tensors, the properties of the rotation matrices, and the symmetry of Clebsch–Gordan vector coupling coefficients, instead of the method of direction cosines used by early workers. Such angular momentum techniques are capable of being extended to multiphoton (hyper) Raman effects and to scattering by quantized rotors. A few examples of the spherical scattering tensors of odd parity and their symmetry in the D2 point group are presented.
Electret Behavior at Low Pressure
Authors: Palaia Frank L. , Catlin Avery
Published: February 2004

Electrets prepared from polystyrene and Mylar were exposed to low pressure. The measured surface charge of the electrode decreased as the pressure was reduced from 760 torr to 10−2 torr. An interpretation of this charge behavior is given in terms of a spark breakdown mechanism.
Augmented van der Waals Theory for Mixtures
Authors: Rigby M. , Alder B. J. , Sapse A. M. , Hecht C. E.
Published: February 2004

The excess mixing properties and the critical line for mixtures of square-well molecules of equal core sizes and equal range of the forces are exactly evaluated in the high temperature limit. This model, when used at any temperature, can predict all the qualitative features, including immiscibility, observed in simple gas mixtures. This confirms the conclusion of the van der Waals theory of mixtures that differences in core size are not essential for these phenomena. On the other hand, such differences have a large quantitative effect, and if they are too great immiscibility disappears, as shown here for a mixture of square-well and perfect gas particles.
Pi-Electron Theory of Acetylene
Authors: Falicov L. M. , Harris R. A. , Visscher P. B.
Published: February 2004

A Pariser–Parr–Pople pi-electron Hamiltonian for acetylene is constructed and diagonalized exactly. Electron correlation in the ground state is examined as a function of the parameters of the Hamiltonian. Correlation effects are found to be sizeable. The Franck–Condon allowed transitions are studied. These include the single photon (1Σg+ → 1Σu+), double photon (1Σg+ → 1Σu+ → 1Σg+), and triplet–triplet (3Σu+ → 3Σg+) transitions. Transition amplitudes are calculated for the various parameter sets, including those which give the best least squares fit to the observed excitation energies.
Exciton States and Optical Spectra in Linear Molecular Crystals
Authors: Pollans Warren L. , Choi Sang-il
Published: February 2004

Configuration interaction of many Frenkel exciton bands and charge-transfer states is carried out exactly. Based on the exact solution of the problem, the optical spectra is discussed. If the monomolecular optical transition is allowed, the intensities of exact exciton states are borrowed from the Frenkel excitons. Within the borrowed intensity approximation, resonance peaks and antiresonance dips in the continuum band are expected to appear depending on the magnitude of parameters. Although locations of resonances are independent of the orientation of the light polarization vector, positions and numbers of antiresonances do depend on it. Total number of antiresonances is at most one less than the number of Frenkel exciton bands. In the absence of resonance the continuum intensity depends very strongly on the nearest-neighbor charge pair interaction energy with larger intensity for smaller interaction energy. For the discrete spectra, the intensity is mainly distributed to the number of states equal to or one more than the number of Frenkel exciton bands. The relative intensities of these lines depend on the orientation of the light polarization vector. The intensity and position of the extra line varies with the nearest-neighbor charge pair interaction energy.
Exciton States and Optical Rotatory Dispersion in Helical Polymers
Authors: Deutsche Craig W.
Published: February 2004

A general formula for the optical rotatory dispersion (ORD) of oriented solute molecules is used to discuss the optical properties of helical polymers. When periodic boundary conditions are applied to both the electromagnetic field and to the excitation of the polymer the results previously obtained by Moffitt, Fitts, and Kirkwood are recovered. The procedure used, although different in appearance, is essentially that recently employed by Ando. It is shown that these results are rigorously correct in the limit of infinite polymer length. Furthermore, it may be seen that the absorption band with polarization perpendicular to the helix axis is, in fact, the sum of two bands at very slightly different frequencies having rotational strengths of opposite sign and nearly equal magnitudes. It is this splitting which was overlooked in Moffitt's original treatment of the problem and which was implicitly included, although not explicitly recognized, by Moffitt, Fitts, and Kirkwood. Finally, when the results for an infinite, arbitrarily oriented polymer are averaged over all angles the conditions under which familiar formulas for ORD of polymer solutions are valid may be seen.
Use of Thallium(I), Lead(II), and Bismuth(III) as Spectroscopic Probes for Ionic–Covalent Interaction in Glasses
Authors: Duffy J. A. , Ingram M. D.
Published: February 2004

Ultraviolet spectra of thallium(I), lead(II), and bismuth(III) in chloride–sulfate and all-sulfate glasses have been studied, and it is observed that the P13 ← 1S0 transition undergoes a blue shift as the sulfate: chloride ratio increases. This is interpreted in terms of increased ionicity of the probe metal ions in the all-sulfate glass. Spectral data indicate comparable ionicity of the probe metal ions in phosphate glass, but more covalent interactions in silicate glass. It is suggested that the study of these spectral bands may be applied to the problems of oxide ion activity in glasses and of covalency in molten salt chemistry.
Devitrification Ordering and Vitrification Disordering Processes: EPR Study of ZnCl2:Mn2+
Authors: Chatelain A. , Weeks R. A.
Published: February 2004

The electron paramagnetic resonance of Mn2+ in both the crystal and glass form of ZnCl2 has been studied at 9 and 35 GHz. The spectra are found to fit the same spin Hamiltonian but with different values of D and E. Glass to crystal and crystal to glass transformations were induced by heat treatment and neutron irradiation, respectively. The changes of the crystal field parameters and of their distributions show that both transformations can be described in terms of short-range and long-range processes which take place successively. A short-range ordering process is observed at the beginning of devitrification before any change in the x-ray diffraction pattern is detected. Irradiation of the crystal shows that a long-range disordering process begins after only 5 × 1016 fast neutrons/cm2.
Molecular and Crystal Structure of Hexamethylenetetramine Triphenol
Authors: Jordan Truman H. , Mak Thomas C. W.
Published: February 2004

The crystal structure of a 1:3 adduct of hexamethylenetetramine (HMT) and phenol has been determined using three-dimensional x-ray data. The crystals are trigonal, with a = 14.88 Å, c = 6.007 Å, and space group P3̄. Three phenol molecules are linked to each HMT molecule through O–H···N hydrogen bonds in the form of a three-leaved propeller, and the crystal structure is based on a packing of such HMT triphenol units. A notable structural feature is the hollow channel parallel to c through the origin and surrounded by the free ends of the phenyl rings. Although the channel is too narrow in places for the enclosure of guest molecules, reorientation of the phenol groups might allow sufficient expansion for the accommodation of normal aliphatic hydrocarbons and their straight-chain derivatives.
Porphyrins. XIX. Tripdoublet and Quartet Luminescence in Cu and VO Complexes
Authors: Gouterman Martin , Mathies Richard A. , Smith Barry E. , Caughey Winslow S.
Published: February 2004

Luminescence spectra, excitation spectra, and lifetimes taken on Cu and VO porphyrins in polymethylmethacrylate on a Cryo-Tip between 80 and 10°K confirm the prediction that the unpaired metal d electron couples with the porphyrin triplet to give “tripdoublet” and quartet states. Four Cu porphyrins show loss of the main short-wavelength luminescence peak and a lifetime increase from ∼ 100 to ∼ 1000 μsec over the temperature range 80–10°K that is consistent with a change from tripdoublet to quartet luminescence. CuTPP and VOEtio show no spectral change, but similar lifetime increases for CuTPP around 80°K and for VOEtio around 15°K suggest that luminescence from the former is quartet at all temperatures studied and from the latter tripdoublet. VOTPP gives anomalously short lifetimes between 8 and 16 μsec. Nonexponential decay is observed in all cases but VOTPP, and is attributed to slow spin–lattice relaxation among the quartet sublevels.
Microwave Spectrum of Benzaldehyde
Authors: Kakar Ramesh K. , Rinehart Edgar A. , Quade C. R. , Kojima Takeshi
Published: February 2004

The microwave absorption spectrum of C6H5CHO has been investigated in the region 17.5–40.0 GHz and that for C6H5CDO in the region 26.5–40.0 GHz. Rotational transitions in the ground and first three excited torsional states in both molecular species have been identified. There is evidence for interaction of the second torsional state of C6H5CHO with another mode of vibration of the molecule. Accurate relative intensity measurements give the first torsional frequency of 113.8 ± 5.0 cm−1 for C6H5CHO and of 108.4 ± 4.25 cm−1 for C6H5CDO. The barriers to internal rotation of the aldehyde group calculated from these frequencies are 4.90 ± 0.43 and 5.28 ± 0.42 kcal/mole for C6H5CHO and C6H5CDO, respectively. The discrepancy between the observed and calculated values of inertial coefficients suggests the importance of vibration–rotation interaction in the theory of internal rotors with twofold potential barriers.
Manganese Disulfide (Hauerite) and Manganese Ditelluride. Thermal Properties from 5 to 350°K and Antiferromagnetic Transitions
Authors: Westrum Edgar F. , Gro/nvold Fredrik
Published: February 2004

The heat capacities of manganese disulfide and manganese ditelluride were determined by adiabatic calorimetry in the range 5–350°K. Lambda-type transitions are present in both compounds with maxima at 47.93°K for MnS2 and at 83.0°K for MnTe2. Entropies, enthalpies, and Gibbs energy function values are calculated and tabulated. At 298.15°K they are: S° = 23.88 cal/mole·°K, H° − H0° = 3384 cal/mole, − [(G° − H0°) / T] = 12.258 cal/mole·°K for MnS2 and 34.66, 4416, and 19.847 for MnTe2. The clearly cooperative entropy increments are only 0.71 cal/mole·°K for MnS2 and 0.80 for MnTe2. Available magnetic susceptibility data are interpreted in terms of zero-field splitting of the S5/26 state of the manganese 3d5 electrons. The resulting contributions to the heat capacity are evaluated. At 298°K the combined λ-transitional and Schottky contributions to the entropy are 2.6 and 2.4 cal/mole·°K for MnS2 and MnTe2, respectively.
Kinetics of Chemically Activated Isobutane and Neopentane from the 4358- and 3660-Å Photolyses of Diazomethane with Propane and Isobutane
Authors: Johnson R. L. , Hase W. L. , Simons J. W.
Published: February 2004

The results of a study of chemically activated isobutane and neopentane produced by excited singlet-state methylene radical insertion into the secondary and tertiary C–H bonds of propane and isobutane, respectively, are reported. The methylene radicals were produced at two energies by the 4358- and 3660-Å photolyses of diazomethane. The measured decomposition rates for the energized isobutane are 1.9 × 107 and 3.6 × 107 sec−1 in the 4358- and 3660-Å photolysis systems, respectively, and for the energized neopentane are 4.4 × 106 and 6.8 × 106 sec−1 in the 4358- and 3660-Å photolysis systems, respectively. These rates correlate well via RRKM theoretical calculations with thermal A factors in the range of 1016.3–1016.9 sec−1 for isobutane and 1016.5–1016.9 sec−1 for neopentane.
Reactions of Ar, Kr, and Xe Metastables with Simple NH-Containing Compounds
Authors: Stedman D. H.
Published: February 2004

The reactions of Ar*, Kr*, and Xe* with HN3, NH3, and HNCO were studied. Emission spectra from NH(c 1Π), (A 3Π), CO(a 3Π), and N2(B 3Πg) were observed. The data are consistent with the recent value of 91 kcal mole−1 for ΔHf°(NH), but not with standard values in some tables, and support a lower value for the NH(X 3Σ−–a 1Δ) separation than is currently accepted. Ar* and Xe* were used in further work on the bond energies of CN-containing compounds leading to ΔHf°(CN) ≤ 103 kcal mole−1. The validity of spin conservation in metastable atom reactions and comparison of metastable impact excitation with vacuum uv photolysis are discussed.
Kinetics of Electron Scavenging and Ion Recombination in the Radiolysis of Hydrocarbon Solutions
Authors: Rzad Stefan J. , Infelta Pierre P. , Warman John M. , Schuler Robert H.
Published: February 2004

A functional description of the lifetimes of the ion pairs produced in the radiolysis of pure hydrocarbons is derived from the concentration dependence observed for ion scavenging. A number of consequences which follow from this description are explored. In particular it is shown that in the absence of scavenger the decay of the geminate ions must be very closely described by F(t) = eλterfc(λt)1/2, where λ is a constant and F(t) is the fraction of ions present at time t. Appropriate descriptions for the growth and decay of secondary ions are given, and the results are applied to real systems of finite pulses. Comparison is made between the predictions given here for the time dependence of the population of secondary negative ions and the experimental observations of Thomas and co-workers on the decay of diphenylide negative ion in pulse irradiated cyclohexane solutions of diphenyl. From this comparison it is concluded that the rate constant for electron scavenging by the diphenyl is ∼ 3 × 1011M−1·sec−1 and that the preponderant fraction of ion recombination occurs in the time region of 10−11–10−9 sec.
Theory of Diffusion in Simple Liquid Mixtures
Authors: Raveche Harold J. , Mayer Joseph E.
Published: February 2004

A method is presented for obtaining the diffusion coefficient in terms of functions of molecular properties for a liquid system of monatomic molecules near equilibrium. The approach considers the reduced probability density functions Wn(r1···pn) for n molecules (n = 1, 2, and 3) in the limit of a steady state. By operating on the complete probability density function WN with the conventional Liouville operator and by integrating over the phase space of N − n molecules, we write the time dependence of Wn in a form not explicitly involving Wm for m > n. The reduced probability densities and the average vector forces are developed as a power series in a small parameter λ. Starting with singlet functions corresponding to a gradient in the density of a binary isothermal solution at constant pressure and the corresponding flux of molecules, we determine a singlet momentum-dependent force which maintains the flux and gradient stationary in time. The force can only be consistent with a certain class of pair probability functions, and we write the required general forms. Repeating the operation, we compute the average pair force which maintains the fluid in a steady state. The operation may, in principle, be carried to Wn+1 for any n. From symmetry requirements we obtain a correction to the Kirkwood closure for the nonequilibrium factors in the triplet probability density. The closure fixes the ratio between the assumed density gradient and flux, which leads to the diffusion coefficient in terms of functions of pairs and triples of molecules and the intermolecular potential energy.
Chemically Activated Tetramethylsilane from the Reaction of Singlet Methylene Radicals with Trimethylsilane
Authors: Hase W. L. , Simons J. W.
Published: February 2004

An experimental study of the photolysis of trimethylsilane–diazomethane–oxygen mixtures and tetramethylsilane–trimethylsilane–diazomethane–oxygen mixtures at 4358 and 3660 Å is reported. Experimental rate constants for the decomposition of chemically activated ethyltrimethylsilane, ethyldimethylsilane, and tetramethylsilane are determined. These specific rates range from 3 × 104 sec−1 to 6.5 × 105 sec−1. A comparison of the 4358- and 3660-Å rate constants for tetramethylsilane decomposition with RRKM theory calculations suggest that a thermal A factor of 1015.0±0.5 sec−1 is correct for primary decomposition by Si–C bond rupture. The uncertainty in this A factor reflects the uncertainties in the E* and E0 values for chemically activated tetramethylsilane. A discussion of this A factor relative to that found previously for neopentane decomposition is given.
Positron Annihilation in Helium and the System of e+ He Bound State
Authors: Wang Cherng-Maw , Ache Hans J.
Published: February 2004

Within the framework of the valence bond method, the system of e+ He bound state was treated with a two-center problem approximation. The e+ He bound state was found to have a binding energy of 5.03 eV at its equilibrium-positron–helium-nucleus distance, Re = 0.693 Å. The ratio of the effective nuclear charges for the positron and for the helium nucleus ε was found to have a value of 0.63 at Re. An attempt was made to compare the results of the present work with those of others.
Shock-Tube Study of the Acetylene–Oxygen Reaction. I. CH(A 2Δ→X 2π) Chemiluminescence and CO Production during the Induction Period
Authors: Gutman David , Matsuda Shimpei
Published: February 2004

The growths of CO infrared and CH(A 2Δ → X 2π) visible emission were monitored simultaneously during the induction period of shock-heated C2H2–O2–Ar gas mixtures. For both emissions, regions of exponential growth were observed, and the growth constants for the two phenomena were found to be the same in each experiment from 1320 to 2320°K using a gas mixture with the C2H2:O2:Ar proportions 5:10:985. Similar results were obtained with other gas mixtures. The results indicate that CH chemiluminescent emission observed during the induction period of the C2H2–O2 reaction is produced in a reaction first order in reaction intermediates. The use of this emission and its origin are discussed. Evidence is presented indicating that CO is produced with a “hot” nonthermal distribution of vibrational populations. Estimates of the degree of vibrational excitation of the CO produced during the induction period are made using measured vibrational relaxation times for CO and the exponential growth constants for CO production.
Electronic Structure and Inversion Barrier of Ammonia
Authors: Rauk Arvi , Allen Leland C. , Clementi Enrico
Published: February 2004

Ab initio molecular orbital wavefunctions have been constructed for the planar and pyramidal conformations of ammonia in its ground electronic state. These solutions are very close to the Hartree–Fock limit and exhibit a lower total energy (ETequil = − 56.22191 hartree) than any other such calculations which have been carried out. The computed inversion barrier is 5.08 kcal/mole (exptl = 5.8 kcal/mole) and results almost solely from the differential mixing of d-type polarization functions in the planar and pyramidal geometries. In contrast to the conclusions of much earlier work, the inversion barrier may be quantitatively obtained within the framework of the molecular orbital (Hartree–Fock) approximation. Other properties of ammonia that have been determined and discussed are: binding energy, heat of formation, correlation energy, dipole moment, population and energy component analysis, force constants, and the Walsh diagram.
Hameka Theory of Linewidths
Authors: Lawetz V. , Hutchinson D. A.
Published: February 2004

A method is presented in which linewidth parameters are introduced into a stationary state description of scattering theory. A Green operator formalism is used to introduce the linewidth parameters from first principles. For both off-resonance and resonance situations, the results agree with well known results. A straightforward calculation shows that the linewidth parameters of this paper are identical with those of resonance scattering theories. A slight modification of the Heitler–Ma perturbation method is used to show that the stationary state linewidth parameters are the same as those of time dependent methods.
Concentration Dependence of the Sedimentation Coefficient at the Theta State
Authors: Imai Satoshi
Published: February 2004

The sedimentation constant at the theta state at a finite concentration was calculated on the basis of Yamakawa's formulation, taking more rigorous account of hydrodynamical interaction. The results show that even at the theta state the concentration effect still remains and that ks, the coefficient of the first power of weight concentration, is proportional to M1/2.
Exact Solution of the One-Band π-Electron Theory of Benzene
Authors: Visscher P. B. , Falicov L. M.
Published: February 2004

The Pariser–Parr Hamiltonian for benzene is set up and solved exactly (complete configuration interaction). The Hamiltonia parameters are determined so as to fit the experimental values of the observed excited states. The exact solutions of the model Hamiltonian are used to determine correlation functions, oscillator strengths, and two-photon transition probabilities.
Nonequilibrium Contributions to the Rate of Reaction. I. Perturbation of the Velocity Distribution Function
Authors: Shizgal B. , Karplus M.
Published: February 2004

The correction to the equilibrium rate of reaction in a one-component reactive system is calculated from the perturbation of the velocity distribution function obtained by solving the Chapman–Enskog and Burnett equations. A method of solution of the Chapman–Enskog equation with a Sonine polynomial expansion is described that is not limited by the number of terms retained and is applicable to realistic systems characterized by elastic and reactive cross sections which may be available only in tabulated form. The convergence of the Sonine polynomial expansion is demonstrated for a variety of model reactions with and without activation energy and for a set of cross sections obtained with a semiempirical potential for the reaction H2(i) + H2(j) → (products), where i and j denote the vibrational quantum numbers. The Sonine polynomial method is compared with the recent variational solutions of Present and Morris. It is shown that the extent of the departure from equilibrium is due to the deviation of the reactive collision number R(0)(c) from a special form R̄(0)(c). It was found that for the realistic systems considered here the decrease in the equilibrium rate of reaction due to a perturbation of the velocity distribution function by reaction is very small (≲1%) and that the Burnett correction to the distribution function can be neglected. For the set of (H2, H2) reactions, the greatest decrease in the equilibrium rate of reaction is 0.66% at 6400°K for (i, j) = (3, 2). A formal transport theory description of reaction rates analogous to the description of heat conduction and viscous flow by Chapman and Cowling is also presented.
Conjecture Concerning the Width of the Lowest Singlet–Singlet Transition in Crystalline Anthracene
Authors: Rice Stuart A. , Morris Graeme C. , Greer William L.
Published: February 2004

The width of the lowest singlet–singlet transition in crystalline anthracene is at least 10 times the width of (a) the corresponding free molecule transition, (b) the same transition in a diluted condensed phase. Further, in fluorescence crystalline anthracene emits lines as narrow as those in an absorption in a diluted condensed phase. In contrast, for benzene and naphthalene, the absorption lines and emission lines of the crystal, free molecule and matrix-isolated molecule have about the same widths. In this paper, we consider possible reasons for the breadth of the lowest optical absorption band of crystalline anthracene. These are (a) thermal deformation, (b) radiationless processes, (c) exciton phonon coupling, (d) exciton defect coupling, (e) pathological behavior of the exciton dispersion curve near k = 0, (f) localized absorption at the crystal surface, (g) breakdown of the selection rule k = Q near the crystal surface. We postulate that the observed linewidth arises from localized absorption by disordered molecules in the vicinity of the crystal surface.
Flow Light Scattering. I. Theoretical Principles of the Effect and Apparatus for Its Measurement
Authors: Heller W. , Tabibian R. , Nakagaki M. , Papazian L.
Published: February 2004

A brief introduction is given into the theory of radiation scattering by rigid spheroids and deformable random coils dispersed or dissolved in a liquid medium subjected to flow at a well-defined velocity gradient. The theory is limited, in the former instance, to spheroids whose largest dimension does not exceed 13 of the wavelength of the incident radiation. It is shown that the radiation scattering increment produced by flow makes it possible to determine not only the axial ratio, as obtainable by streaming birefringence, but also the numerical values of semimajor and semiminor axis of a spheroid provided two orthogonal components of incident linearly polarized light are used. Essentials of the construction and operation of an apparatus designed for the study of hydrodynamic radiation scattering are described.