bluuebunny/crossref_metadata_2025 huggingface dataset:
Semiempirical Extended Hartree–Fock LCAO MO Method for Molecules
Authors: Martino F. , Ladik J.
Published: February 2004

The equations for the extended Hartree–Fock method (spin projection before variation) are derived by means of the variation principle. Taking account of the pairing theorem with a single average overlap integral (λ̃), an LCAO MO formalism is developed to solve these equations. This formalism is applicable both in an ab initio form and in the semiempirical form used in the Pariser–Parr–Pople theory of π electrons. Finally the problems of eventual numerical applications are discussed.
Vibrational Modes in Ethyl Chloride Measured by Neutron Scattering
Authors: Strong K. A. , Brugger R. M. , Pugmire R. J.
Published: February 2004

Neutron molecular spectroscopy has been used to observe the C–C–Cl deformation and CH3 torsional modes of ethyl chloride in the liquid and solid states. The neutron data for the torsional mode are compared with the results obtained with infrared spectroscopy and Schottky-barrier tunneling in an attempt to resolve an ambiguity between the intensities of the 0 → 1 and 1 → 2 transitions measured by the latter two techniques.
Dipole Relaxation in a Liquid Crystal
Authors: Baessler H. , Beard R. B. , Labes M. M.
Published: February 2004

The real and imaginary parts of the dielectric constant of a 1.75:1.00 mixture of cholesteryl chloride and cholesteryl myristate have been measured in the frequency range 1 kHz–10MHz. Two modes of molecular rotation are found to contribute to dipole relaxation: rotation of cholesteryl chloride around a short molecular axis (process R1) and rotation of cholesteryl myristate around a long molecular axis (R2). The relaxation frequency associated with R1 changes by not more than a factor of 2 when going from the cholesteric phase to the isotropic melt or to a field-induced aligned nematic phase. This indicates that the potential barrier hindering molecular rotation is determined by short-range order rather than long-range order. From the magnitude of the dielectric increment, conclusions are drawn regarding the molecular arrangement in the various phases. The temperature dependence of the relaxation frequency is analyzed in terms of absolute rate theory.
Donor, Temperature, and Pressure Dependence of the Ionic Conductivity in Iodine
Authors: Ludwig Jan , Kommandeur Jan
Published: February 2004

In agreement with a previous analysis it has been shown that the low temperature ionic conductivity of iodine is independent of the donor used to introduce this conductivity. A comparison of the high temperature ionic conductivity data and the polarographic half-wave potentials shows that p-phenylenediamine (PPD), diaminodurene (DAD), and p-toluidine (PTD) ionize to the monopositive ion, while tetramethyl-p-phenylenediamine (TMPD) probably ionizes to the bipositive ion in solid iodine. From an analysis of the nonequilibrium conductivity a value (2.1 eV) could be derived for the activation energy of the motion of I2− in solid iodine, which compares well with values found for I3− and I− previously. The pressure dependence of the ionic conductivity has been measured. It was found that the free volume involved in either the transport or the generation process of the I3− ion corresponds to half the molar volume of iodine, which again lends further credence to the mechanisms proposed to explain this conductivity.
Phosphorescence and Zeeman Spectra of Anthracene in a Mixed Crystal
Authors: Clarke Richard H.
Published: February 2004

Phosphorescence has been observed at 4.2°K for anthracene-h10 and anthracene-d10 dissolved in a phenazine host crystal. The sharp-line spectra permit a detailed analysis of the anthracene vibrational frequencies and the observation of the Zeeman splitting of the emitting triplet level. Analysis of the relative intensities among the Zeeman components of the electronic origin of emission in a 40.6 kG magnetic field shows that a long-axis spin state provides 95% of the radiative spin-state activity in the anthracene phosphorescence.
Near-Resonant Vibration→Vibration Energy Transfer: CO2(υ3 = 1) + M → CO2(υ1 = 1) + M* + ΔE
Authors: Stephenson John C. , Moore C. Bradley
Published: February 2004

The laser-excited vibrational fluorescence technique has been used to determine the rate constants for deactivation of the asymmetric stretching vibration of CO2 in collisions with CO2, CH4, C2H4, CH3F, CH3Cl, CH3Br, CH3I, BCl3, and SF6. Rates were determined as a function of temperature in the range 300–800°K. The large deactivation cross sections σ for the latter seven molecules decreased as the temperature T increased. For the latter six collision partners σ∝1 / T. This result is interpreted as a near-resonant vibrational energy transfer process in which three vibrational quantum numbers change as the vibrational energy is shared between the collision partners.
Free-Ion Yields in Sundry Irradiated Liquids
Authors: Schmidt Werner F. , Allen A. O.
Published: February 2004

The yield of free ion pairs formed by 2-MeV x rays was determined in 38 pure liquids and in four mixed systems at room temperature, and in five liquids as a function of temperature. Marked effects of molecular constitution are found to be superimposed over the effects of dielectric constant and density of the liquids.
Exact Finite Method of Lattice Statistics. IV. Square Well Lattice Gas
Authors: Runnels L. K. , Salvant J. P. , Streiffer H. R.
Published: February 2004

The temperature-dependent phase equilibrium of simple classical molecules has been studied, using a model based on the two-dimensional square lattice. The intermolecular potential includes a hard core extending to the first-neighbor distance and a finite interaction (attractive or repulsive) at the second-neighbor distance. The transfer matrix method of calculation is used for lattices of infinite length and finite circumference up to 16 sites. At all temperatures studied there is a single phase transition, which is of first order for attractive interactions at sufficiently low temperatures. The model best describes the equilibrium between solid and gas at low temperatures or the equilibrium between solid and supercritical fluid at high temperatures. The deficiencies in the model which exclude a liquidlike phase are discussed.
Halogen Enhancement of Singlet–Triplet Transitions. II. Vibronic Paths in Phenanthrene
Authors: Ginsburg Jack L. , Goodman Lionel
Published: February 2004

Whereas almost all of the intensity in T1(ππ*) → S0 in phenanthrene and 1-methylphenanthrene arises from predominantly out-of-plane totally symmetric polarized vibronic bands (subspectrum I) in 9-bromo-and 3-chlorophenanthrene, an in-plane polarized nontotally symmetric vibronic subspectrum (subspectrum II) is approximately as prominent as the totally symmetric subspectrum. The nontotally symmetric subspectrum is predominantly x polarized and thus rules out predominantly z-polarized σ → σ* localized or nearly localized (C–X) states as providing the dominant source of intensity in subspectrum II of S0 → T1 in these compounds. In 4-bromophenanthrene the polarization is predominantly z. Through the evidence is not conclusive it is suggested that these vibronic orientation effects may be attributable to interaction with delocalized σσ* states.
Mass-Spectrometric Determination of the Dissociation Energies of the Gaseous CuNi, CuCo, and CuCr
Authors: Kant Arthur , Strauss Bernard , Lin Sin-Shong
Published: February 2004

The molecules CuX (X=Ni, Cu, and Cr) have been shown to exist in the vapor phase over liquid solutions of copper and the transition metals at high temperatures (1550–2000°K). Using information derived from a combination of effusion and mass-spectrometer techniques, the equilibrium constants and energetics of the reactions CuX+Cu=Cu2+X have been determined. The dissociation energies as obtained by the second-law method are 47.4 ± 5, 38.0 ± 5, and 36.0 ± 6 kcal mole−1 for CuNi, CuCo, and CuCr, respectively.
Nuclear Quadrupole Frequencies and Relaxation Times in Ferroelectric Hydrogen Chloride
Authors: O'Reilly D. E.
Published: February 2004

The anomalous temperature dependence of the chlorine nuclear quadrupole resonance frequency in the ferroelectric solid II of HCl is interpreted by means of an attenuated 90° flip process of the molecules. Correlation times τf are calculated for the 90° flip process from 35Cl spin–lattice relaxation time (T1) data and an activation energy is derived. The calculated values of τf are in fair agreement with those previously estimated from proton rotating frame relaxation times (T1ρ) in solid II. The internal local electric field Eloc at an HCl molecule in solid II is calculated by the fourier transformation summation method. Eloc due to the electric dipole and quadrupole moments of the HCl molecule deviates by about 30° from the direction of a molecular dipole. The shift of the deuteron and chlorine-35 nuclear quadrupole frequencies in the rigid lattice from the corresponding gas phase frequencies are explained as electrically induced quadrupole shifts by the field Eloc. Perturbation theory is used to calculate the magnitude of the electrically induced shifts to be expected which agree well with the shifts observed. It is proposed that most deuteron quadrupole frequency shifts from gas to solid may be explained by this mechanism. A mechanism for the ferroelectric phase transition in solid II of HCl is specified which involves the 90° as well as 180° flips of the molecules.
Internal Rotation and the Breakdown of the Adiabatic Approximation: Many-Phonon Radiationless Transitions
Authors: Gelbart William M. , Freed Karl F. , Rice Stuart A.
Published: February 2004

In this paper we consider the role of hindered internal rotation in the breakdown of the Born–Oppenheimer approximation. The intramolecular coupling matrix elements are derived in a form which allows the theory of many-phonon radiationless transitions to be extended to include torsional motion. It is shown that the strongly coupled degree of freedom (that mode whose equilibrium positions differ greatly in the initial and final electronic states) can be separated out from the remaining vibrations in the general rate expressions. We treat, in particular, a model of photoisomerism in which the adiabatic potential curves describing the zero-order states have certain simplifying features. We are able to derive in this case a simple rule governing the distribution of electronic energy amongst the internal modes in the radiationless deactivation of the lowest (twisted) triplet. In marked contrast to the usual situation encountered in the literature, this intersystem mixing with the ground state is predicted, under certain conditions, to show a negligible isotope effect.
Electronic Structure, Quadrupole Splitting, Chemical Shift, and Susceptibility of Ferrous Iron in Anhydrohemoglobin, Anhydromyoglobin, and Bispyridinehemin
Authors: Trautwein A. , Eicher H. , Mayer A.
Published: February 2004

Ferrous iron in anhydrohemoglobin (AHb) shows a high-spin and a low-spin quadrupole splitting. Ferrous iron in anhydromyoglobin (AMb) shows a high-spin quadrupole splitting. The high-spin quadrupole splittings and their temperature dependence on AHb and AMb are the same as observed for ferrous iron in deoxyhemoglobin (Hb) and deoxymyoglobin (Mb). The low-spin quadrupole splitting and its temperature dependence on AHb is the same as observed for bispyridinehemin (BPH). A one-to-one superposition of the high-spin quadrupole splitting of Mb and the low-spin quadrupole splitting of BPH yields the observed quadrupole splitting of AHb. The same situations hold also for the chemical shift and the molar susceptibility. From this finding it is concluded that upon dehydrating hemoglobin, one type of the subunits undergoes a conformational change which results in the coordination of a N of the distale histidine to the sixth position of the iron, while the conformation of the other type of subunits and therefore also the symmetry arrangement around the iron remains unchanged.
Molecular g Values, Magnetic Susceptibility Anisotropies, Diamagnetic and Paramagnetic Susceptibilities, Second Moment of the Charge Distribution, and Molecular Quadrupole Moments of H3CCN and H3CNC
Authors: Pochan J. M. , Shoemaker R. L. , Stone R. G. , Flygare W. H.
Published: February 2004

The high-field, linear and quadratic Zeeman effect has been observed in methyl cyanide (H3CCN) and methyl isocyanide (H3CNC). Both spectra are complicated by the presence of the 14N nuclear quadrupole coupling. The H3CNC system leads to uncoupled spectra in high fields, and the H3CCN system does not exhibit fully uncoupled spectra even at fields of 25 000–30 000 G. In the numbers given below, c = b is perpendicular to the C3 or a axis. The perpendicular g values in H3CC14N and D3CC14N are g⊥ = − 0.0338 ± 0.0008 and g⊥ = − 0.0315 ± 0.0008, respectively. The sign of the electric dipole moment is +H3CCN–. The average magnetic susceptibility anisotropy from the above isotopic molecules is χ⊥ − χ‖ = (10.2 ± 1.0) × 10−6erg/G2·mole. Assuming g‖ = 0.310 from other similar systems gives the molecular quadrupole moment of Q‖ = −(1.8 ± 1.2) × 10−26esu·cm2. Using the known molecular structure and above g values allows a determination of the paramagnetic susceptibilities which are χ⊥p = (145.3 ± 1.0) × 10−6erg/G2·mole and χ‖p = (9.6 ± 0.5) × 10−6erg/G2·mole. Combining the above experimental magnetic susceptibility anisotropy with the known bulk magnetic susceptibility gives the total and diamagnetic terms, χ⊥ = −(24.1 ± 0.4), χ⊥d = −(169.4 ± 1.1), χ‖ = −(34.6 ± 0.5), and χ‖d = −(44.2 ± 1.0), all in units of 10−6erg/G2·mole. The individual elements in the tensor describing the second moment of the electronic charge distribution are 〈c2〉 = 5.2 ± 0.3 and 〈a2〉 = 34.7 ± 0.4 in units of 10−16 cm2. The Zeeman spectra of H3CC15N was also observed to confirm the above results with the 14N isotope. The molecular g value in H3CC15N is g⊥ = − 0.0317 ± 0.0003 and (χ⊥ − χ‖) = (10.5 ± 0.5) × 10−6erg/G2·mole. The perpendicular g value and magnetic susceptibility anisotropy of H3C14NC are g⊥ = − 0.0546 ± 0.0015 and χ⊥ − χ‖ = (13.5 ± 0.7) × 10−6erg/G2·mole. The sign of g⊥ was not determined experimentally; however, an analysis of the molecular quadrupole moment indicates the proper choice of sign. Assuming g‖ = 0.310 from similar systems gives the molecular quadrupole moment of Q‖ = −(2.7 ± 1.6) × 10−26esu·cm2. Using techniques as outlined above give the paramagnetic susceptibilities of χ⊥p = 137.8 ± 0.9 and χ‖p = 9.6 ± 0.6 in units of 10−6 erg/G2·mole. The total and diamagnetic susceptibilities are also estimated. The anisotropy in the second moment of the electronic charge distribution is 〈a2〉 − 〈c2〉 = (27.0 ± 0.5) × 10−16cm2.
Excitation Energy Transfer in Alkanes. III. Radiation Chemistry of Alkane Polymers
Authors: Partridge Roger H.
Published: February 2004

Studies described in the preceding paper indicate that excitation energy transfer probably occurs in alkanes, particularly alkane polymers, with high efficiency. This raises many questions concerning primary radiation reactions in such materials and the part that excitation energy transfer may play in them, not the least of which is why such an effect was not clearly characterized long ago. The predictions made by exciton theory on such problems are compared with available experimental data, and as far as can be ascertained at present the two are almost completely consistent with one another.
ESR Studies of Heisenberg Spin Exchange. II. Effects of Radical Charge and Size
Authors: Eastman Michael P. , Bruno Gerald V. , Freed Jack H.
Published: February 2004

The Heisenberg spin exchange behavior of several radical–solvent systems has been studied. This work has shown that k, the second-order rate constant for bimolecular encounters, and | J | τ1 (where | J | is twice the exchange integral and τ1 is the lifetime of the collision pair) depend markedly on the ionic strength of the solution and upon the size and charge of the radical. For the peroxylamine disulfonate radical in a 5 × 10−2M solution of 1:2 electrolyte, our simple analysis yields | J | τ1 ∼ 0.95 and k = 2.0 × 10+9M−1·sec−1 at 24°C; increasing the electrolyte concentration to 6.1 × 10−1M gives | J | τ1 ∼ 1.3 and k = 3.2 × 109M−1·sec−1. These changes in k and | J | τ1 are attributed to screening by the electrolyte of the charge of the dianion radical; this is discussed in terms of the Debye theory. The di-tertiarybutyl nitroxide (DTBN) radical undergoes apparent strong exchange (| J | τ1 ≫ 1) in pure water (k = 2.4 × 109M−1·sec−1 at 24°C). The dependence of the exchange frequency (ωHE) on added 1:2 electrolyte is of the form ωHE = ωHE(0) exp(− CE / C′), where CE is the molar electrolyte concentration and ωHE(0) the exchange frequency in pure water; for a 2.4 × 10−2M DTBN solution C′ = 0.23 ± 0.02M. This reduction of exchange frequency with electrolyte concentration is attributed to the formation of DTBN aggregates. The durosemiquinone radical undergoes strong exchange in dimethoxyethane (k = 2.9 × 109M−1·sec−1at 15°C). The closeness of k to the diffusion-controlled limit indicates that ion pairing takes place in solution. The tetraphenyl arsonium peroxylamine disulfonate radical undergoes apparent weak exchange (| J | τ1 < 1) in dimethyl sulfoxide. However, rapid radical decay made quantitative study of exchange impossible for this radical.
Use of the Stimulated Echo in NMR Diffusion Studies
Authors: Tanner J. E.
Published: February 2004

The stimulated echo in a three-rf-pulse experiment is shown to be useful in extending the range of measurement of diffusion coefficients to more viscous substances or the measurement of barrier separations to wider spacings in systems where the diffusing substance has T1 > T2. The spin-echo attenuation due to self-diffusion is derived for the general case of a time-dependent field gradient, and the result is found experimentally to be correct for the special case of a field gradient applied in two equal, square pulses.
Calculation of NMR Relaxation Times for Quadrupolar Nuclei in the Presence of Chemical Exchange
Authors: Marshall Alan G.
Published: February 2004

The correlation function for a quadrupolar nucleus which may exchange its environment between two kinds of sites in solution has been calculated from an appropriate joint conditional probability for changes in site and in electric field gradient direction with time. In the extreme narrowing limit, the resultant NMR linewidth depends both on the exchange lifetimes and also on the correlation times for rotation at each site; the NMR linewidth may thus be used to obtain information about any of these quantities. The results bear directly on the use of quadrupolar nuclei in the presence of chemical exchange of nuclei between suitable sites to obtain motional information about macromolecules in dilute solution. In particular, the proposed correlation function may account for anaomalously small calculated rotational-correlation times for large molecules previously obtained from such studies. A precise criterion for applicability of McConnell-modified Bloch equations is proposed. Other applications of the correlation function derived in this paper are discussed.
Theory of Atom–Diatom Collisions. III. An Integral Equation Formalism for Level Widths and Shifts
Authors: Sams W. Neal , Kouri Donald J.
Published: February 2004

An integral equation formalism for calculating level widths and shifts for Feshbach-type compound state resonances is presented. As in previous studies, the resonances are initially defined by means of the distortion approximation eigenstates of the closed channel projected Hamiltonian. The level shift then contains corrections to the real part of the resonance energy arising due to coupling of the distortion state to other closed channels as well as to the open channels. The level width contains effects due to both open and closed channels and far resonances. Because the Feshbach formalism is employed, the level width and shift are dependent on the scattering energy, in contrast to the Breit–Wigner parameterization of the resonances. The method of Sams and Kouri for obtaining noniterative solutions to integral equations is proposed as a practical method for obtaining accurate level widths and shifts beginning with the distortion approximation to the resonances. It results, in this method, that the solution of the integral equation for the level width and shift is very similar to that for the nonresonant scattering integral equation.
Absorption Spectrum of H2 in the Vacuum-uv Region. I. Rydberg States and Ionization Energies
Authors: Takezawa Sanzo
Published: February 2004

The absorption spectrum of molecular hydrogen has been investigated in the 950–700-Å region with a 6.65-m normal-incidence spectrograph in the second order with the helium continuum as background. Seventeen Rydberg series are identified. Fourteen belong to npπ 1Πu ← X 1Σg+ transitions and consist of Q(K) lines which converge to levels υ = 0–5 of the ground state of H2+. The other three series belong to npσ 1Σu+ ← X 1Σg+ transitions and consist of P(1) lines for two of the series, and R(0) lines for the third. The lowest ionization energy of H2, obtained from the Q(K) line series analysis, is 124 417 ± 2 cm−1 less than the previously accepted value by 12 cm−1. The autoionization effects observed by Beutler and Jünger in the (6, 0) band of the D 1Πu ← X 1Σg+ system are discussed.
ESR Rigid-Lattice Line Shape in a System of Two Interacting Spins
Authors: Leigh J. S.
Published: February 2004

A theory is developed to describe the rigid-lattice line shape of an electron spin resonance signal which is influenced by dipolar coupling to a second spin. The polarization lifetime T1 of the dipolar field is assumed short. Computer generated plots are presented. Broadening, peak shifts, and amplitude changes are noted. Under certain conditions no observable broadening is predicted, merely an apparent decrease in spectrum amplitude. Experimental results for a nitroxide radical interacting with a paramagnetic ion are compared with theoretical curves. The results allow calculation of the distance between the two spins. Values for the directional angles of the second spin in the g tensor coordinate system of the observed spin are also calculated.
Infrared Spectrum of Crystalline CH2Cl2 and CD2Cl2. Polarization Measurements and Crystal Structure
Authors: Marzocchi M. P. , Manzelli P.
Published: February 2004

The infrared spectra of crystalline CH2Cl2 and CD2Cl2 were measured at temperatures ranging from 5 to 125°K and at 85°K, respectively, between 5000 and 200 cm−1. Splitting into two or more components was observed for several fundamentals. The mixed-crystal technique was used to prove the origin of the observed splittings, and the isotopic shifts for 37Cl and 13C as well as the factor-group splittings were identified. These data, together with polarization measurements on polycrystalline-oriented films, relative intensity measurements, and other independent data, such as crystal density and Raman spectrum, were used to determine the crystal structure. The data accumulated are consistent with an orthormbic D2h14(Pbcn) unit cell with four molecules located on C2 sites.
Magnetic Resonance of KCuCl3
Authors: Hynes J. E. , Garrett B. B. , Moulton W. G.
Published: February 2004

Chlorine nuclear resonance data for KCuCl3 have been used to show that no magnetic ordering occurs above 1.3°K, and that a low-lying excited state of the dimer Cu2Cl6= exists with ΔE / k ≈ 32°K if the ground state is a triplet or ΔE / k ≈ 48°K for a singlet ground state. The chlorine nuclear quadrupole coupling tensors have been determined at each of the three chemically inequivalent chlorine sites in Cu2Cl6=. A model which includes both ionic and covalent contributions to the quadrupole coupling tensor was used to explain the data and to estimate the charge distribution in the dimer. Observation of an EPR spectrum with g = 2.157 at 1.35°K and of the increase in the EPR intensity at 77°K compared to that at 300°K are evidence that the ground state of Cu2Cl6= is a triplet.
Optical Constants of Solid Xenon in the Vacuum-uv Region
Authors: Steinberger I. T. , Atluri C. , Schnepp O.
Published: February 2004

The reflectance spectrum in the range 1180–1600 Å solid xenon was measured near 4°K. The Kramers–Kronig inversion technique was used to evaluate the real part ε′ and the imaginary part ε″ of the dielectric constant. A new self-consistent approximation method was developed and used to evaluate the contribution of absorption bands lying beyond the range of measurements to the phase of the reflection. The ε″ vs photon energy E curve thus obtained exhibited all the details of the best thin-film absorption measurements, and it also showed two hitherto unknown peaks, at 8.76 and 8.93 eV, respectively. Resolution of the ε″ vs E curve into a sum of absorption bands was an inherent part of the new approximation method. From this resolution, the oscillator strengths f of the transitions were determined. The largest f value, namely 0.16, was found for the lowest-energy peak at 8.378 eV, as compared with f = 0.256 for the corresponding P31 resonance line in xenon gas. The f value for this band was also computed on the basis of simple theory, whereby the ground state was approximated by a Slater function and the excited state by the Wannier function derived from free-electron Bloch functions. The result thus calculated was in good agreement with the measured oscillator strength, assuming resonable values for the average dielectric constant. The conclusions of the present work are thus consistent with predictions of the intermediate exciton model.
Inelastic Neutron Scattering Spectra and Raman Spectra of CsHCl2 and CsDCl2
Authors: Stirling G. C. , Ludman C. J. , Waddington T. C.
Published: February 2004

This paper describes the use of neutron inelastic scattering data and Raman spectral data on CsHCl2 and CsDCl2, in conjunction with earlier infrared data, to confirm the noncentrosymmetric nature of the bichloride ion in these salts. The inelastic neutron scattering data eliminates D∞h as the point group of the ion and the Raman data shows that the ion is bent, so that the ion belongs either to the point group C2υ or Cs. Reinterpretation of the infrared spectra suggests that the bending vibration in the ion is split by dynamical tunneling between the two possible bent configurations.
Photolysis of Chloroform in the Presence of Ethane at 25°C
Authors: Yu W. H. S. , Wijnen M. H. J.
Published: February 2004

The photolysis of CCl3H in the presence of ethane yielded the following reaction products: CH2Cl2, and C2H5CHCl2, C2H5CCl3, (CHCl2)2, C2HCl5, C2Cl6, and C4H10. The formation of these products is explained by recombination and disporportionation reactions of the radicals produced in Reactions (1)–(3): CHCl3+hv→CHCl2+Cl, Cl+C2H6→C2H5+HCl, Cl+CHCl3→CCl3+HCl. Data regarding various ratios of rate constants for recombination and disproportionation reactions of these free radicals are given and discussed. An approximate value k2 / k3 ≃ 0.1 is derived for H atom abstractions by Cl atoms from ethane and chloroform.
Nuclear Resonance Determination of the Magnetic Space Group of MnCl2·2H2O
Authors: Spence R. D. , Rao K. V. S. Rama
Published: February 2004

The Cl and H nuclear magnetic resonance spectra have been used to study magnetic ordering in MnCl2·2H2O. It was found that the material orders antiferromagnetically with a Néel temperature of 6.8°K. The magnetic space group deduced from the Cl data was shown to be C2c2 / m. The proton resonance data were shown to substantiate the magnetic space group assignment. The compound is isostructural with CoCl2·2H2O. Both compounds contain linear chains of magnetic ions running along the c axis. However, in the Mn compound the chains are antiferromagnetically coupled while in the Co compound they are ferromagnetically coupled.
Model Calculations of Cooperative Motions in Chain Molecules
Authors: Anderson J. E.
Published: February 2004

This article describes model calculations of the so-called “perpendicular mode” of dielectric relaxation in chain molecules. Particular emphasis is given to cooperative interactions between adjacent chain elements. An extension of Glauber's model for the time-dependent statistics of the Ising chain is used for the purpose. A closed-form expression is given for the dielectric relaxation of an isolated dipolar element in an infinite ring. The influence of chain length and variable mobility along the chain upon dielectric relaxation is also described.
Photoelectron Spectroscopy of HCl and DCl Using Molecular Beams
Authors: Weiss M. J. , Lawrence G. M. , Young Robin A.
Published: February 2004

A focusing field electrostatic energy analyzer was used to examine photoelectrons from HCl and DCl using the 584 Å line of helium and the 736–744-Å doublet of neon. Ionization potentials for the X 2Πi and A 2Σ+ states of HCl+ and DCl+ were determined and found to be essentially in agreement with values obtained by spectroscopic methods. Relative transition probabilities to the molecular–ion states were measured and are in agreement with calculated values.
Proton Dynamics in KH2PO4 Type Ferroelectrics Studied by Infrared Absorption
Authors: Wiener E. , Levin S. , Pelah I.
Published: February 2004

The infrared absorption spectrum of ferroelectric crystals analogous to KH2PO4 was measured at several temperatures. Substantial changes in the intensity and width of the ir absorption peaks, especially in the low-energy spectral region, were observed upon crossing the Curie temperature. Assignments of the ir absorption peaks for both phases were made on the basis of a systematic comparison of the spectra of compounds isomorphic to KH2PO4 and containing isotopes of D and 18O. Correlation charts were constructed for both phases, permitting harmonic interpretation of the ir selection rules. The structure of the ir absorption spectrum of KH2PO4 at temperatures above Tc, and comparison of this spectrum with that of KD2PO4. suggests a dynamic hydrogen symmetrizing process along the H bonds. It also indicates the existence of strong coupling between certain PO4 modes and the soft longitudinal hydrogen modes, caused by the extremely anharmonic nature of the potential well along the H bond. The coupling results in a broadening of the PO4 lines involved. The partial broadening that remains in KH2PO4 below Tc indicates that the soft hydrogen modes persist to a small extent even in the ferroelectric phase of this crystal. The ir data are explained on the basis of the fluctuating double-minimum-potential-well theory for the hydrogens.
Magnetothermodynamics of α-MnCl2·4H2O. II. Heat Capacity, Entropy, Magnetic Moment, from 0.4 to 4.2°K with Fields to 90 kG along the b Crystallographic Axis
Authors: Giauque W. F. , Fisher R. A. , Brodale G. E. , Hornung E. W.
Published: February 2004

This work is a continuation of our previous magnetothermodynamic measurements on a 3.934-cm-diam spherical single crystal of MnCl2·4H2O in which case the field was directed along the c crystallographic axis. Here the heat capacity and magnetic moment have been measured between 0.3° and 4.2°K with stabilized solenoid fields of 0, 1, 5, 7.5, 10, 14, 18, 22, 25, 40, 65, and 90 kG, along the b crystallographic axis. At 0.716°K and fields of 90, 80, and 70 kG. the electron system was magnetically saturated. The derived temperature-independent magnetic susceptibility was found to be − 1.6 × 10−4 cm3 mole−1 and the saturation moment of the temperature-dependent system is 27 974 G·cm3 mole−1, equivalent to gb = 2.004. At 90 kG the nuclear spins were found to contribute a heat capacity term of (5.9 × 10−3 / T2) gibbs mole−1. The amount of enthalpy required to remove quanta of angular momentum from the saturated condition at 90 kG was found to be 20.63 cal mole−1 of angular momentum. This is less than gβH = 24.07 cal mole−1, the magnetic work, by 3.44 cal mole−1, which is the amount of stored internal energy contributed to the saturated condition for this limiting process, and indicates antiferromagnetic interactions. Extrapolation of the magnetic moment–temperature curves to absolute zero yields the expression M = 1.056H + 9.0 × 10−7H2G·cm3 mole−1 at 0°K. On this slightly curved line the saturation value, 27 974 G·cm3 mole−1, is reached at a field of 25 920 G, and the work of magnetization, ∫ 0HHdM, equals 8.73 cal mole−1 at 0°K. Temperature–field observations on 31 isentropes were used to correlate the entropies along 12 iso-Oerstedic heat capacity series. The zero of electronic and lattice entropy was located from the low-temperature values of the heat capacity series at 90 kG. The upper limit of the electronic entropy was found to be 3.559 gibbs mole−1, compared to R ln6 = 3.561 gibbs mole−1. Smoothed, correlated values of the heat capacity, entropy, enthalpy, internal energy, magnetic moment, iso-Oerstedic temperature coefficient of the magnetic moment (which equals the isothermal derivative of entropy with respect to field), and the magnetic work have been tabulated.
Investigations on the Unrestricted Hartree–Fock Method as a Tool for Computing Potential Surfaces
Authors: Salotto A. W. , Burnelle L.
Published: February 2004

The possible uses of the ab initio spin-unrestricted Hartree–Fock (UHF) method for computing potential energy surfaces have been investigated, expanding the molecular orbitals from a basis set of Gaussian functions. In order to approximate spin-pure states, an exact spin projection operator, as well as an operator annihilating only the state of next higher multiplicity, has been used. Preliminary studies undertaken on the H2 and LiH molecules indicate that among the various methods employed, the unprojected UHF approach gives the most satisfactory shape for potential curves. The HNO molecule was then investigated, and five electronic states of the system were considered, namely A1,A″3,A″1 symmetry. The ground state is A′1. Its UHF energy was minimized by varying the three geometrical parameters independently. A value of 〈Ŝ2〉 of 0.32 was obtained at the experimental geometry. This system with an even number of electrons thus has the unexpected property of being unstable with respect to a splitting of the α and β orbitals. Application of the spin projection operator lowers the energy by 0.0265 a.u. The UHF wavefunctions give an excitation energy A″1←A′1 of 3860 cm−1. When pure spin states are produced, it becomes 11 660 cm−1 (experimental value 13 150 cm−1). The A″3 state practically coincides with the ground state in the UHF approximation, but after projection an excitation energy of 5850 cm−1 is obtained. A slight maximum is observed in the UHF potential energy curve of the A″ states as a function of the N–H distance, which we associate with the predissociation observed in the spectrum of the compound. Based upon projection at the experimental minimum for the ground state, and at infinite N–H separation, a set of adjusted curves has been generated for the system H + NO. The features of these curves are discussed in relation with the mechanism of the recombination reaction between hydrogen atoms and nitric oxide.
Relative Electron Impact Ionization Cross Sections by a Double Crossed-Beam Technique
Authors: Rovner L. H. , Norman J. H.
Published: February 2004

Accurate values of electron impact ionization cross sections of high-temperature species are of importance in mass spectrometric investigations as well as a variety of other fields. However, the present experimental techniques are subject to many inherent difficulties or are limited in applicability and have yielded relatively few reliable values. Theoretical calculations are not sufficiently accurate. An experimental technique for accurate measurement of relative cross sections is described. The measurement depends only on one experimentally determined quantity, as well as the mass and temperature of the gaseous species, and is applicable to a wide variety of systems. The technique utilizes a mass spectrometer that is fitted with a Knudsen cell and employs electron impact ionization. A second, modulated electron beam intercepts the molecular beam, causing a slight modulation in the intensity of the molecular beam that is reflected in the output signal of the mass spectrometer. The relative attenuations of the various component species of the molecular beam are proportional to the relative total ionization probabilities of these species. The relative cross sections for Ca, Tl, Ag, and Pb have been measured and are compared with literature values obtained by other techniques. Preliminary application has also been made to the molecular species W2O6 and W3O9.
“Appearance Energy” and Energy Dependence of the Recoil Product Yield in Indium Ethylenediaminetetra-acetate by the In115(γ, γ′)115mIn Reaction
Authors: Yoshihara K. , Kudo H.
Published: February 2004

The concept of the “appearance energy” has been proposed by observing the recoil product yield as a function of recoil energy in a solid system. The yield of 115mIn3+ ion following the In115(γ, γ′)115mIn reaction in indium ethylenediaminetetraacetate (In–EDTA) was zero in the low-recoil-energy region and it increased from a threshold value of recoil energy, that was named the “appearance energy,” to a saturation yield in the higher energy region. The appearance energy of the 115mIn3+ ion in In–EDTA was about 60 eV with possible errors of factor 2. The meaning of the appearance energy has been discussed in comparison with the displacement energy in solid-state physics. From the energy dependence curve it was suggested that the reaction of the recoil atom might be divided into three categories corresponding to the three parts of the recoil range.
Perturbation Theory for Intermolecular Forces Including Exchange
Authors: Lekkerkerker H. N. W. , Laidlaw W. G.
Published: February 2004

Generalized solutions to the Eisenschitz and London perturbation equations are derived. It is pointed out that the results obtained in the formalisms proposed by Hirschfelder (HAV), by Hirschfelder and Silbey, by Murrell and Shaw, and by Musher and Amos are special cases of the generalized treatment.
Emission from Metastable States in a Nitrogen Ion Beam
Authors: Maier William B. , Holland Redus F.
Published: February 2004

Metastable nitrogen ions are observed to radiate in the wavelength range 1700–3200 Å. Two lifetimes, 4.4−1.4+2.0 and 44 ± 11 μsec, have been measured. The electron energy dependence of the excited radiation is similar to the energy dependence previously observed for the production of N3+ when N2 is bombarded by electrons.
Intermolecular Potentials for N2 Molecules and the Lattice Vibrations of Solid α-N2
Authors: Kuan Teh-Soong , Warshel Arieh , Schnepp O.
Published: February 2004

The lattice energy, crystal structure, and optical properties in the far infrared and Raman spectrum have been used as a basis to test various forms of an intermolecular potential for N2. The librational mode frequencies observed in the Raman spectrum are exclusively characteristic of the anisotropic part of the intermolecular potential. It was found that the quadrupole–quadrupole interaction term does not give an adequate description of the angle-dependent part of the intermolecular potential for nitrogen molecules at distances corresponding to the solid. On the other hand, an atom–atom or dumbbell intermolecular potential has been found to give very good agreement between calculated and experimental frequencies of all optical lattice modes. Potentials of the 6–12 type and 6-exponential types have been used. In order to obtain good quantitative fit to the librational frequencies an effective interatomic distance in the molecule had to be used which is 20% smaller than the actual bond length. Also intensity ratios in the infrared and Raman spectra were used as criteria for the quality of the potential models. In addition it was shown that the molecular orientations as observed in the solid are accounted for by the potential models.
Quantum Mechanical Analysis of the Elastic Scattering of Reactive Systems
Authors: Eu Byung Chan
Published: February 2004

A numerical method of calculating optical phase shifts for spherically symmetric complex potential is described. Computations of optical phase shifts are carried out for the purposes of analyzing phenomenologically the elastic scatterings of chemically reactive systems and examining the behavior of the so-called opacity function. It is shown that the calculated phase shifts may be concisely represented in terms of a set of reduced variables. The existence of the reduced variables is very useful and conveniently exploited in the analysis of the elastic scattering of K + HBr system at E = 1.49 kcal/mole. The calculated phase shifts for various potential models are presented in tables and figures.
Green's Functions in Atomic and Molecular Calculations
Authors: Laurenzi Bernard J.
Published: February 2004

The use of Green's functions in calculations of second-order properties is presented. In the case of a second-order energy calculation for the helium atom it is found that the “discrete” part of the energy correction converges very rapidly.
Configuration-Interaction Calculations for Small Pi Systems. II. The Benzyl and Linear Heptatriene Radicals
Authors: Amos A. T. , Burrows B. L.
Published: February 2004

The results of full CI calculation for the pi orbitals of benzyl and the linear polyene C7H9 are reported. The spin density on each carbon atom is calculated and compared with data available from other approximations. For benzyl the best agreement is with the empirical variant of the unrestricted Hartree–Fock method, while for the linear polyene our results show a definite trend of increasing positive spin densities towards the center of the polyene in agreement with valence bond calculations.
Effects of Traps on Migration and Annihilation of Triplet Excitation in Phenanthrene Crystals
Authors: Yee E. M. , El-Sayed M. A.
Published: February 2004

Delayed fluorescence originating from triplet–triplet annihilation is reported from a mixed-crystal system characterized by an added impurity in low concentrations and large triplet trap depth. Temperature and impurity (guest) concentration dependences are extensively investigated in the anthracene in phenanthrene system. It is observed that for concentrations below 10−4 moles/mole, and at temperatures below 50°K, host-delayed fluorescence is independent of added anthracene and is best described as originating from triplet–triplet annihilation between host triplet excitons and shallow triplet traps, whose phosphorescence emissions are found to correlate with the phenanthrene-delayed fluorescence as the temperature is changed in the range 4.2–60°K. Near 77°K, these shallow traps become ineffective acceptors of host triplet excitation and delayed fluorescence arises mainly from the interaction between the anthracene and phenanthrene triplet excitations. The temperature dependence of the delayed fluorescence strongly supports the trapping and the detrapping mechanism formulated by Siebrand [J. Chem. Phys. 42, 395 (1965)]. In this present system, two discernible shallow trap phosphorescences are observed. A kinetic model is proposed and supported by the observed dependence of the delayed fluorescence intensity on guest concentration, temperature, exciting light intensity, as well as the observed activation energy.
Evaluation of the Conformal Solution Theory of Mixing
Authors: Tan P. Y. , Luks K. D.
Published: February 2004

A rigorous derivation of excess mixing properties is performed for molecules whose potential is characterized by an energy parameter ε and a length parameter σ based on the concepts of the conformal solution theory. The integral coefficients of the resultant mixing rules for the excess thermodynamic functions are evaluated a priori, using pair correlation functions from the Percus–Yevick approximate theory. The results are compared with excess mixing properties predicted by the Percus–Yevick theory for mixtures in order to provide a self-consistent test of the conformal solution theory. The test calculations suggest that the conformal solution theory is inherently poor from a quantitative standpoint.
Flowing Afterglow Studies of Formation and Reactions of Cluster Ions of O2+, O2−, and O−
Authors: Adams N. G. , Bohme D. K. , Dunkin D. B. , Fehsenfeld F. C. , Ferguson E. E.
Published: February 2004

The temperature-controlled flowing afterglow system has been utilized at temperatures below 300°K to measure the rate constants for the association of H2, N2, O2, CO2, N2O, and SO2 to O2+ with helium third body. Rate constants for the association of N2, O2, and CO2 to O2− and for the association of N2 and CO2 to O− have been measured at 200°K. Some of the ion clusters formed in this way reacted via binary neutral interchange reactions of the type A±·B+C → A±·C+B. Rate constants for these processes were obtained at and below 300°K. The observation of these reactions allows an ordering of ion cluster bond energies. The increasing order of binding energies to O2+ is H2, N2, O2, N2O, SO2, and H2O. For the O2− ion clusters the increasing order is N2, O2, H2O, CO2, and NO. In some cases, equilibria are observed allowing a determination of binding energy differences. The three-body association rate constants increase with increasing binding energy of the ion cluster, in accord with theoretical expectation.
Fundamental Studies on the Bearman–Kirkwood Theory of Transport. I. The Diffusion Coefficient Ratio
Authors: Bearman Mirion Y. , Bearman Richard J.
Published: February 2004

Although the Bearman–Kirkwood theory of transport in binary mixtures was developed as a theory of the liquid state, it is formally possible to carry out calculations of the self-diffusion coefficient ratio, D1 / D2, at all densities at which radial distribution functions are available for a given intermolecular potential. These calculations can be used to check the internal consistency of the theory at any density. For, except at unit mole fraction of solvent, the theory provides three alternative expressions for D1 / D2, which should agree when the theory is consistent. For Lennard-Jones parameters of a model system corresponding to argon and krypton, explicit calculation shows that at each mole fraction and temperature, the three expressions disagree except at a density characteristic of a liquid or dense fluid. At the densities of consistency, D1 / D2 assumes a not unreasonable value and remains quite independent of temperature in agreement with available data on argon and krypton. At unit mole fraction, another consistency test is available which qualitatively justifies the application of the theory to so-called “regular solutions” and quantitatively gives densities of consistency smoothly connected to those at other mole fractions.
Matrix Elements of Spin-Dependent Operators over Total Molecular Wavefunctions
Authors: Bottcher C. , Browne J. C.
Published: February 2004

There is given a complete formulation of the evaluation of matrix elements of spin-dependent operators over total wavefunctions for diatomic molecules. General account is taken of the nonseparability of position and spin spaces for polyelectronic problems, but consideration is restricted to orbital product electronic wavefunctions. The analysis has two phases. The first phase makes a complete reduction (including transformation of the operators to the molecular axis fixed coordinate system) of the N-electron two-nuclei coupled angular-momentum problem using Racah tensor operator techniques. A full derivation is given for Hund's case b coupling scheme. Transformations to other coupling schemes are given in an appendix. The second phase extends the Slater-Löwdin matrix element analysis for antisymmetric functions to include spin-dependent operators.
Lattice Parameters of Cerous Magnesium Nitrate Hydrate at 4.2, 78, and 302°K
Authors: Schiferl D.
Published: February 2004

The lattice constants a and c (hexagonal cell) of cerous magnesium nitrate hydrate, Ce2Mg3(NO3)12·24H2O have been determined at 302 ± 1, 78, and 4.2°K. The Bond precision technique was used with CoKα1 radiation (λ = 1.788965) and MoKα1 radiation (λ = 0.709300) with several samples. Accuracy was limited by unavoidable damage done to the crystals in cooling to cryogenic temperatures. Values obtained for a and c are a = 10.9857 ± 0.0009, c = 34.068 ± 0.006 at 4.2°K; a = 10.9885 ± 0.0008, c = 34.106 ± 0.003 at 78°K; a = 11.0354 ± 0.0001, c = 34.640 ± 0.001 at 302 ± 1°K.