bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Spin Resonance of the Mono-tert-butyltropenyl and Tri-tert-butyltropenyl Radicals: A Study of Vibronic Near Degeneracy
Authors: Vincow Gershon , Morrell M. Lee , Hunter Frank R. , Dauben Hyp J.
Published: February 2004

The mono-tert-butyltropenyl and tri-tert-butyltropenyl radicals have been prepared by thermal and photochemical cleavage of di-tert-butylbitropenyl and hexa-tert-butylbitropenyl. The syntheses of these dimers are reported. Electron spin resonance spectra of the tert-butyltropenyls (tert-butyl-2,4,6-cycloheptatrien-1-yls) have been investigated. Proton hyperfine splittings of both radicals are markedly temperature dependent. In addition, for tri-tert-butyltropenyl, the g value and the linewidth are temperature dependent and the proton splittings vary slightly with solvent. The experimental temperature variations are ascribed to vibronic near degeneracy in the tert-butyltropenyls, and a model is proposed in order to calculate these effects. Approximate spin densities for the two nearly degenerate states are given by ρkkπ ≈ (1 / 7)[1 ∓ P cos(8πk / 7)], where the minus sign corresponds to the ground state, ρkkπ is the diagonal element of the pi-spin-density matrix for carbon atom k, and P is a parameter which combines the effects of electron correlation and vibronic coupling. Splitting calculations using these values of ρkkπ and McConnell's relationship lead to good agreement with experiment. For tri-tert-butyltropenyl it is found that P = 1.0 ± 0.1, and the energy splitting between nearly degenerate states (ΔE) is approximately 200 ≤ ΔE ≤ 300 cm− 1. In the case of mono-tert-butyltropenyl a range of 100 ≤ ΔE ≤ 180 cm− 1 corresponds to P = 1. Consideration of the Colpa–Bolton and Giacometti–Nordio–Pavan extensions of McConnell's relationship leads to negligible modifications in the hyperfine splittings. On the basis of Hückel MO calculations in which the alkyl-group perturbation is treated as an inductive effect, it is concluded that the tert-butyl group does not appreciably affect the pi-electron wavefunction but does modify the Coulomb integral on the adjacent carbon by approximately − 0.02 β. It is found that the temperature dependence of the unresolved tert-butyl proton splittings gives rise to a major contribution to the temperature variation of the linewidth in tri-tert-butyltropenyl. Analysis of the temperature dependence of the g value of this radical on the basis of a simple two-level scheme leads to approximate values for (gE–gG) and gG, where gG and gE designate the g values in the ground and excited vibronic levels.
Afterglow Studies of the Reactions He+, He(23S), and O+ with Vibrationally Excited N2
Authors: Schmeltekopf A. L. , Ferguson E. E. , Fehsenfeld F. C.
Published: February 2004

The following reactions have been measured as a function of N2 vibrational temperature from 300°K to 6000°K: He++N2→N++N+He →N2+ + He, He(23S) + N2→N2 + + He + e, O+ + N2→NO+ + N. Reactions (1) and (3) are the first ion–molecule reactions to be measured as a function of vibrational temperature of the neutral. The rate constants for Reactions (1) and (2) are found to be insensitive to the N2 vibrational temperature. Reaction (3) increases its rate constant by about a factor of 40 with increased N2 vibrational temperature. The branching ratio (1a) to (1b) increases measurably with N2 vibrational temperature.
Defect Interactions and Their Effect upon the Intrinsic Defect Concentrations for AgCl
Authors: Sevenich R. A. , Kliewer K. L.
Published: February 2004

The effect of defect interactions upon the equilibrium concentration of intrinsic Frenkel defects in the silver halides was investigated employing the formalism developed by Allnatt and Cohen. Using the value of the Frenkel pair free formation energy obtained by Abbink and Martin, the intrinsic defect concentrations were calculated for temperatures between 400°K and the melting temperature. It was found that the results using this theory were in reasonable agreement with results obtained from the well-known Debye–Hückel-type theory of Lidiard for temperatures below ∼620°K, significant differences appearing above this temperature. Defect concentrations were also evaluated using the free formation energy determined by Müller. In all calculations, significant effects were associated with the temperature dependence of the dielectric constant.
Optical Activity of Saturated Ketones
Authors: Bouman Thomas D. , Moscowitz Albert
Published: February 2004

Previous theoretical treatments of the octant and quadrant rules for predicting the sign of the rotational strength of the n → π* transition in optically active saturated ketones are re-examined and combined to determine which of the two rules is better able to account for experimental results. The basis set which leads to the octant rule is found to reproduce the experimental rotational strengths fairly well. The anomalous effect of a fluorine atom is examined within the framework of a static, dissymmetric perturbation, and it is found that in this case, the method is inadequate to account for the experimental results.
Phonon Modes of an Ionic Crystal Slab
Authors: Lucas A. A.
Published: February 2004

The vibrational problem of a slab-shape diatomic ionic crystal is studied within the harmonic approximation and neglecting retardation effects. The interest is centered on a proper treatment of the physical boundary conditions existing at the free surfaces of the slab. By taking advantage of the translational symmetry of the slab for lattice translations parallel to the free surfaces, the problem is first reduced to finding the normal modes of a linear model consisting of two parallel diatomic chains coupled together. Using the so-called planewise summation method developed by deWette and Schacher for computing the Coulomb interaction coefficients, it is shown that the consideration of nearest and n ext-nearest-neighbor interactions between the ions of this double-chain problem is largely sufficient to determine the eigenmodes accurately. This remarkable feature arises from the fact that the Coulomb field created by a plane array of dipoles decreases exponentially with increasing distance from the plane. The double-chain dynamics is then developed following the general method recently given by Gazis and Wallis. In this paper, we restrict the computation to this class of phonons which propagate perpendicularly to the boundary planes. We found that (i) surface modes of vibrations (with exponentially decreasing amplitude from the surfaces) exist, resulting from the use of physical boundary conditions; (ii) when the slab thickness increases, the “bulk” modes (with a wavelike character) rapidly converge to the solutions obtained with cyclic boundary conditions. It is concluded that, for sufficiently thick ionic slab-shape crystals (a few tens of layers), the use of periodic boundary conditions does not significantly affect the statistical properties of the crystal (as shown by Ledermann's theorem in the case of short-range force model) and also accurately provides the true individual eigenfrequencies with the exception, however, of this class of modes involving localized vibrations at the surfaces of the slab.
Electronic Factors in the Paramagnetic Relaxation of High-Spin Fe2 +
Authors: Clark Michael G.
Published: February 2004

The theory of the electronic matrix elements of the orbit–lattice interaction is developed with a view to its application to the case of high-spin ferrous iron. Spin–lattice transitions are classified according to their electronic matrix elements, and the necessary selection rules are determined. The ideas developed are used to discuss Fe2 + in octahedral, tetragonally distorted octahedral, and square-planar environments. It is concluded that partial quenching of the orbit–lattice interaction will occur in a square-planar environment. The line broadening observed in the Mössbauer spectrum of gillespite (BaFeSi4O10), in which iron is square planar coordinated by oxygens, is attributed to this effect. Spin–orbit coupling coefficients for the terms arising from 5D in a ligand field of D4 symmetry are tabulated.
Scavenging of Ions in Irradiated Liquids
Authors: Hummel A.
Published: February 2004

Monchick's theory of the probability of reaction between two particles in each other's potential field while a scavenger is present, was used to study the scavenging of ions formed in dielectric liquids by high-energy radiation. The ions originating in a track of a fast electron are formed in groups. Only reaction of the last pair in each group, after recombination of the others, has been considered. The distribution of separations of the last pairs in a track of a 1-MeV electron in hexane has been estimated from conductivity measurements, by applying Onsager's theory of the probability of recombination of an ion pair. This distribution was used to calculate the concentration dependence of the yield of reaction of the ions with a scavenger. The calculated concentration dependence was compared with several experimental results from the literature.
Electron Spin Resonance Studies of Carbon-13 Splittings in a Series of Phenoxy Radicals
Authors: Gilbert Leonard , Kreilick Robert
Published: February 2004

We have observed carbon-13 splittings in the electron spin resonance spectra of a series of phenoxy radicals. Through use of the Karplus–Fraenkel equation we were able to calculate π-electron spin densities in the phenolic aromatic ring. Values for Qc−o and Qo−c were determined for the carbon–oxygen bond.
Electron Correlation and Augmented Separated-Pair Expansion in Berylliumlike Atomic Systems
Authors: Miller Kenneth J. , Ruedenberg Klaus
Published: February 2004

The method of the augmented separated-pair expansion, developed in the preceding paper, is applied to the berylliumlike atomic systems from Li− to Ne6 +. The variational calculation recovers 94% of the total correlation energy, corresponding to an absolute error of about 2.5 kcal in beryllium. The wavefunctions are given explicitly and contain 28 separated-pair configurations, the only ones out of several hundred investigated which contribute more than 0.01 kcal. The intergeminal correlation energy is analyzed in terms of contributions from various separated-pair configurations and the latter are ordered according to their importance. The variation of the intergeminal correlation with nuclear change is discussed and the relation to second-order perturbation theory is investigated.
Pure Nuclear Quadrupole Resonance in Triethylene Diamine
Authors: Zussman A. , Alexander S.
Published: February 2004

A detailed study of the pure nuclear quadrupole resonance of 14N in solid triethylene diamine is described. The resonance frequency, spin–lattice relaxation time (T1), and the free-induction decay time (T2) were measured as a function of temperature from 80°K to the plastic transition at 351°K. From 200°–300°K T1 is dominated by hindered molecular rotations about the trigonal axis of the molecule modulating the dipolar interaction with the protons. The activation for this reorientation was found to be 8.17 kcal/mole, in reasonable agreement with NMR results of Smith. The linewidth is discussed in detail and indicates an acentric structure of the molecule. Effects of the approach to the plastic transition are evident in all the three quantities measured but could not be interpreted in detail.
Optical Model Calculation of the Electronic States of Mixed Disordered Systems
Authors: Nicolis Gregoire , Rice Stuart A. , Jortner Joshua
Published: February 2004

This paper reports a study of the normal modes of simple disordered systems using a dispersion oscillator model with dipolar coupling for the unperturbed electronic states. An equation is derived for the normal-mode frequencies and for the excitation amplitudes of the system. This equation displays the local structure of the fluid in an explicit way. The nature of the resonance and localized impurity modes is discussed in the particular case of an isotopically disordered uniform system. Finally, the contribution of the thermal motion of the atoms to the damping of the normal modes is discussed. The significance of the results and the relationship to other formal studies of disordered systems is briefly considered.
Perylene Crystal Spectra under Pressure
Authors: Offen H. W. , Beardslee R. A.
Published: February 2004

The absorption and fluorescence shifts of perylene at high pressures are compared for the two crystalline forms. The red shift − Δν at 20 kbar is greater for β-perylene than for α-perylene. Also, it is observed that − Δνf < − Δνa for α-perylene and − Δνf ∼ − Δνa for β-perylene. The smaller shift of the excimer fluorescence compared to the pressure shift in absorption of α-perylene is indicative of large specific interactions in the excited state. The condition − Δνf < − Δνa has been previously observed in charge-transfer complexes so that charge-transfer forces could be responsible for the stabilization of the excimeric state although the present data cannot exclude other explanations for the specific interactions. Intensity changes in the crystal absorption band are not easily related to vibronic coupling mechanisms. Fluorescence intensities are reduced for both crystals under compression.
Evidence of the Existence of an He3+ Ion
Authors: Patterson P. L.
Published: February 2004

An ion-mobility apparatus was used to study the chemistry of helium ions in helium at gas pressures of 1–30 torr and temperatures of 300°–76°K. At temperatures below 200°K, the He2+ ion was observed to convert to an ion believed to be He3+. Identification of the He3+ was based upon observations of its behavior under varied experimental conditions. At 76°K, the rate of formation of He3+ from He2+ was faster than the three-body formation of He2+ from He+. The rate coefficient of the latter reaction was found to be 1.7 ± 0.4 × 10− 31 cm6/sec at 76°K. Between the temperatures of 135° and 200°K, He2+ and He3+ were present in equilibrium. A He3+ dissociation energy of 0.17 ± 0.03 was determined from measurements of the equilibrium constant as a function of temperature.
Isotope Effects of Iodine Reactions Activated by the Radiative Neutron Capture and Isomeric-Transition Processes in Liquid Hydrocarbons
Authors: Lambrecht Richard M. , Rack Edward P.
Published: February 2004

The organic yields of 130I + 130mI and 128I activated by radiative neutron capture are compared to those of 130I activated by isomeric transition. The organic yields of (n, γ)-activated 130mI + 130I are consistently lower than the (IT) induced organic yields of 130I and (n, γ)-activated 128I. Radiogas-chromatography analysis has indicated the similarity of products, but differences in abundance of the relative product yields of 128I, 130I, and 82Br. Similarities in yields and product distributions between iodine and bromine isomeric-transition induced reactions may indicate that radiohalogens mainly trace their chemical environment. The autoradiation hypothesis appears to be the most satisfactory model to explain the chemical effects of nuclear transformations of halogen atoms in liquid hydrocarbon media.
Linewidth Effects in the ESR Spectra of 2,2′-Dinitrodiphenyl Radical Anion Obtained by Alkali Metal Reduction
Authors: Subramanian J. , Narasimhan P. T.
Published: February 2004

The ESR spectra of anion radicals, obtained by reduction of 2,2′dinitrodiphenyl with lithium, sodium, and potassium in dimethoxyethane (DME) and tetrahydrofuran (THF), in the temperature range +50°– − 80°C, are discussed. The two nitrogen couplings are not equivalent in the anion radical obtained by lithium reduction, while they are equal in the case of sodium and potassium reduction in both DME and THF. Anomalous linewidth behavior is observed in all these spectra which depend on temperature and solvent, as well as the cation. Freed and Fraenkel's relaxation-matrix theory is used to explain the observed linewidth behavior, taking into account the metal-ion exchange between the two nitro groups. Such pronounced variation in linewidth, which has not been hitherto reported in the metal reduction of dinitro compounds, is attributed to the favorable spatial disposition of the nitro groups in 2,2′-dinitrodiphenyl.
Vibration Spectra of Vanadium Hydride in Three Crystal Phases by Inelastic Neutron Scattering
Authors: Rush J. J. , Flotow H. E.
Published: February 2004

The vibration spectra of vanadium hydride in three crystal phases have been investigated by the energy-gain scattering of cold neutrons. The measured spectra are generally split into two broad bands above and below about 300 cm− 1, which are primarily associated with optical hydrogen vibrations and metal-atom vibrations, respectively. Pseudofrequency distributions for the hydrogen vibrations were derived from the measured neutron spectra. These indicate broad optical vibration bands peaked at 970 ± 50 and ∼1400 cm− 1 in the α (bcc) phase, 440 ± 20, 970 and ∼1400 cm− 1 in the β (bct) phase, and ∼1300 cm− 1 in the γ (fcc) phase. The width of the 440-cm− 1 band increases with hydrogen concentration, possibly due to hydrogen–hydrogen interactions. The “metal-atom” vibration spectra also show changes in peak positions and widths as the distribution of phases is changed. The changes in the neutron spectra with temperature and as a function of composition between VH0.20 and VH1.71 correlate quite well with the previously measured phase diagram. A comparison of the present results with neutron spectra for hydrides with known hydrogen positions suggests that in the α phase hydrogens only occupy tetrahedral-type sites, while in the β phase both octahedral and tetrahedral sites are occupied. The results for VH1.49 and VH1.71 indicate that in the γ phase the hydrogens occupy regular tetrahedral sites corresponding to the fluroine positions in a CaF2-type structure.
Asymptotic Solution of the Excluded-Volume Problem in a Linear Polymer Chain
Authors: Yamakawa Hiromi
Published: February 2004

An asymptotic form of the expansion factor α for the end-to-end distance of a linear polymer chain is derived, based on the self-consistent-field approach developed recently by Reiss. Reiss' integral equation for the distribution function is corrected, since some factor in the transition probability is dropped. The self-consistent potential is evaluated in the perturbed state by introducing the uniform-expansion approximation. In addition, the present potential is appropriate for use near the peak of the distribution function, whereas his potential is valid at extremely large distances. The corrected integral equation is equivalent to that of James except that the present potential is evaluated in the perturbed state. The present theory predicts that at large z, α5 = 1.45z, where z is the excluded-volume parameter. An approximation that leads to an equation for α of the fifth-power type, as suggested first by Flory, is discussed in detail.
Electron-Impact Ionization Cross Sections. V. Classical Calculations and Comparison
Authors: Lin Sin-Shong , Stafford Fred E.
Published: February 2004

Absolute ionization cross sections for single ionization by electron impact on gaseous atoms have been calculated for elements 41–98 using the equations derived by Gryzinski based on classical theory. The results are compared with those given by Mann and with available experimental data. The discrepancies are discussed and analyzed.
Radiolysis of Methylcyclopentane. I. Nonhomogeneous Kinetics of Charge Scavenging in the Liquid Phase
Authors: Stover E. Diane , Freeman Gordon R.
Published: February 2004

Methylcyclopentane (MCP) was irradiated in the presence of a number of solutes. The hydrogen yield from pure MCP, G(H2) = 4.2, was decreased to 2.2 by the addition of N2O or SF6, whereas the addition of ND3 increased the total hydrogen yield above the value in pure MCP. In the ND3 solutions the yield of HD was twice the decrease in H2 yield, so not all positive ions formed in the absence of ND3 lead to H2 formation. In pure MCP, PH+ leads to hydrogen formation and QH+ does not. Both types of positive ions react with ammonia by proton transfer: G(PH+) = 2.0 and G(QH+) ≈ 1.2. By using a nonhomogeneous kinetics model of charge scavenging, it was determined that electrons are an order of magnitude more readily scavengable than are positive ions in the radiolysis of methylcyclopentane. Determination of the value of the kinetics parameter β in various radiolysis scavenging systems will help to determine the nature of the scavenging reactions that occur in them.
Measurement of Threshold Electrons in the Photoionization of H2 and D2*
Authors: Villarejo Don
Published: February 2004

Measurements of electrons produced in the photoionization of H2 and D2 are reported. The experimental technique employs a differential electron-energy analyzer to detect photoelectrons produced with nearly zero initial kinetic energy (≲0.1 eV). By varying the photon energy a spectrum of “threshold” photoelectrons is recorded exhibiting the vibrational structure of the H2+ and D2+ ions. From these spectra values of I(H2), I(D2), ωe(H2+), ωe(D2+), ωexe(H2+), and ωexe(D2+) are derived. In favorable cases the relative vibrational transition probabilities are also reported and compared with theoretical predictions as well as with other experiments. Some disagreements are noted in comparing the results obtained by this method with theoretical Franck–Condon factors based on potential curves appropriate to nonrotating molecules. It is suggested that the vibration–rotation interaction should be taken into account in calculations pertaining to electronic transitions in light molecules.
Vibrational Spectra of Organophosphorus Compounds. IV. Infrared and Raman Spectra of CH3OPCl2 and CD3OPCl2
Authors: Durig J. R. , DiYorio J. S.
Published: February 2004

The infrared spectra of gaseous and liquid CH3OPCl2 and CD3OPCl2 have been recorded from 4000 to 33 cm− 1. The Raman spectra of the liquids have been recorded and depolarization values measured. No indication of a second conformer could be found in either the low-temperature Raman or infrared spectra. The interesting “methyl” torsion was observed at 127 cm− 1 for the hydrogen compound and 119 cm− 1 for the deuterium-containing molecule. Considerable mixing among the low-frequency skeletal modes and the torsional vibration was apparent. Assignment of the other fundamentals based on position, depolarization values, and isotopic shifts is given and discussed. It is shown that the band in the region of 1190-1170 cm− 1 which is characteristic of the CH3OP moiety results from a rocking motion of the methyl group rather than to a stretching motion of the COP group to which it is frequently assigned. The vibrations resulting from the COP group are better characterized as COP symmetric and COP antisymmetric stretching modes rather than C–O and P–O motions.
Studies of Ge and Si Tetraiodide Crystals by Means of the Mössbauer Effect in 129I
Authors: Bukshpan S.
Published: February 2004

The Mössbauer effect of the 27.7-keV 129I transition has been studied in GeI4 and SiI4 crystals. From the temperature dependence of the recoilless fraction, the characteristic Mössbauer temperatures have been derived. Using these temperatures and a simple model for the Mössbauer factor in a molecular crystal, the Debye temperatures of the crystals have been calculated. From the intensities of the peaks in the spectra, the absolute recoilless fractions have been calculated. The intensities of various transitions in the spectra show the existence of the Goldanski effect. From the quadrupole coupling and isomer shift measurements, the character of the Ge–I and Si–I bond is derived.
Electric Field Effect on Critical Opalescence of Dipolar Binary Mixture
Authors: Cooper Martin J.
Published: February 2004

A modified van der Waals model is used to describe the effect of a time-varying external electric field on the critical mixing-point properties of a dipolar binary mixture. Assuming that the behavior of the concentration fluctuations about equilibrium is governed by linearized hydrodynamic equations and that the diffusion coefficient is determined by the instantaneous equilibrium state, the decay of the fluctuations is described. The relaxation time characterizing the electric field effect upon the light scattering by critical opalescence is related to the free relaxation of the concentration fluctuations.
Structures of the Alkyl Ions Produced in the Reactions between H3+ and Propane or Butanes
Authors: Aquilanti V. , Galli A. , Giardini-Guidoni A. , Volpi G. G.
Published: February 2004

The structures of the alkyl ions produced in the reactions of H3+ ions with propane or the butanes, H3+ + C2nH2n + 2→C2nH2n + 1+ + 2H2 and H3+ + C2nH2n + 2→CmH2m + 1+ + Cn − mH2(n − m) + 2 + H2, have been determined by high-pressure mass spectrometry, using deuterium-labeled hydrocarbons. Reactions (1) and (2) take place by exothermic rupture of a C–H and a C–C bond, respectively, without reshufflings of hydrogen atoms or rearrangements in the carbon skeleton. Information is also obtained about the alkyl ions involved in the reactions CmH2m + 1+ + CnH2n + 2→CnH2n + 1+ + +CmH2m + 2.
Configuration-Interaction Wavefunctions for Small Pi Systems
Authors: Amos Terry , Woodward Martin
Published: February 2004

Complete CI wavefunctions for the pi electrons in allyl, pentadienyl, butadiene+, and butadiene− are obtained in the context of the Pariser–Parr–Pople approximation. Because the basic set of Slater determinants is built up from atomic orbitals rather than molecular orbitals, it becomes trivially easy to set up the secular equation, and indeed many of the matrix elements required are zero. A comparison of the results obtained here with those found using more approximate methods indicates that the most serious discrepancies arise in the calculation of pi spin densities. The best of the approximate methods for calculating spin densities turns out to be a variant of the unrestricted Hartree–Fock (UHF) method.
Electronic States of Mixed Molecular Crystals
Authors: Sommer Bat-Sheva , Jortner Joshua
Published: February 2004

In this paper we consider the energy levels of dilute mixed crystals in the weak vibronic coupling limit using the one-particle Green's-function method. The perturbation Hamiltonian is treated explicitly with reference to impurity molecules differing from the host molecules by isotopic substitution. The general equations derived for the energy levels, for excitation amplitudes, and for the optical properties of the mixed crystal are expressed in terms of the weighted density-of-states function of the pure crystal. This treatment emphasizes the relation between the density of exciton states in the pure crystal and the optical properties of mixed crystals. The nature of bound and virtual impurity states are considered, and new experiments using the impurity-induced electronic-absorption method are suggested.
Kinetics of Triplet–Triplet Annihilation in the Vapor Phase of Naphthalene, Anthracene, and Phenanthrene
Authors: Finger Gerhard , Zahlan A. B.
Published: February 2004

The triplet–triplet annihilation process has been investigated in naphthalene, anthracene, and phenanthrene vapor. The observation of spontaneous and delayed fluorescence produced by flash excitation when combined with steady-state fluorescence measurements permits the determination of the temperature and pressure dependence of cSTΦ (absolute) and γ / c (in arbitrary units) (ΦST = S1 → T intersystem-crossing yield; γ, triplet–triplet annihilation rate constant; c, probability of a triplet–triplet annihilation reaction leading to an excited singlet A*1). An attempt is made to separate the observed temperature and pressure dependence of cΦST and γ / c into the individual contributions of c, γ, and ΦST. The behavior of c with respect to temperature is interpreted in terms of the Stevens–Parker mechanism, where excimers are involved as an intermediate product in the annihilation process of two triplets: in molecules where ΔE = 2E(T0) − E(S1) is smaller than the excimer binding energy Ebinding, c is temperature dependent (anthracene). In molecules where ΔE > Ebinding, c has a constant “saturation value” near unity (napthalene, phenanthrene). In anthracene the temperature dependence of c shows a marked parallelism to that of fluorescence self-quenching. Napthalene is found to exhibit a temperature-dependent γ in contrast to phenanthrene and anthracene.
Electron Resonance Studies of Perchlorodiphenylmethyl and Perchlorotriphenylmethyl Free Radicals
Authors: Falle H. R. , Luckhurst G. R. , Horsfield A. , Ballester M.
Published: February 2004

The stable free radicals perchlorodiphenylmethyl, (C6Cl5)2ĊCl, and perchlorotriphenylmethyl, (C6Cl5)3C, have been examined by electron resonance. In solution, the isotropic α-chlorine hyperfine splitting in PDM was found to be 6.08 Mc/sec. Splittings from 13C in natural abundance at the methyl carbon (about 100 Mc/sec) and at additional phenyl–carbon sites were measured for both radicals. By incorporating PDM in the liquid crystal p-azoxyanisole, the principal values of the anisotropic hyperfine coupling tensor of the α chlorine were derived and found to be A11 = − 9.2, A22 = − 18.9, and A33 = 47.2 Mc/sec. The variation with temperature of the degree of ordering of the radicals in the nematic mesophase was determined.
Vacuum-Ultraviolet Photolysis of N2O. III. Reaction Rates of O(1S)
Authors: Young R. A. , Black G. , Slanger T. G.
Published: February 2004

Photolyses of N2O at 1470 Å (Xe resonance lamp) and CO2 at 1048 + 1067 Å (argon resonance lamp) have been used as sources of O(1S) atoms. Quenching rate coefficients for this species, relative to those for N2O or CO2, were determined for a wide variety of gases.
Spin Relaxation: The Multiple-Time-Scale Point of View
Authors: Cukier R. I. , Deutch J. M.
Published: February 2004

The linear (Redfield) form of the equation of motion for the density matrix of a spin system immersed in a heat bath is derived. The usual derivations employ perturbation theory which suffers from the presence of secular terms familiar from nonlinear mechanics. We employ the multiple-time-scale technique as developed by Frieman and Sandri to eliminate the secular terms and render the expansions valid for all time. The method introduces explicit time variables to exploit the multiplicity of time scales inherent to the problem.
Low-Temperature Heat Capacities of Concentrated Lithium–Ammonia Systems
Authors: Mammano N. , Coulter L. V.
Published: February 2004

The heat capacities of Li/NH3 systems having the molecular compositions 12.00%, 14.14%, 20.40%, and 22.25% were measured in an adiabatic calorimeter from 60° to 110°K. Measurements on the 20.40% and 22.25% solution extended to about 200°K. First-order transitions were observed at 82.2° and 88.8°K and the associated heats of transition at each transition temperature were measured for the several compositions. Maximum enthalpy change is observed at a composition corresponding to NH3/Li = 4.15 and is consistent with the proposal that a solid compound, lithium tetrammine, is formed at low temperatures. The transition at 82.2°K is associated with a solid–solid phase transformation in Li(NH3)4 [or Li(NH3)4.15]; the transition at 88.8°K is discussed in terms of the equilibria which would have to occur in order to account for the formation of lithium tetrammine.
Microwave Spectra, Barriers to Internal Rotation, Quadrupole Coupling Constants, and Dipole Moments of cis- and trans-Acetaldoxime
Authors: Rogowski R. S. , Schwendeman R. H.
Published: February 2004

The microwave spectra of cis-and trans-acetaldoxime (CH3CHNOH) have been investigated in the region 8–37 GHz. Several a-type transitions have been identified for both species and the rotational constants in MHz are as follows: cis-acetaldoxime, A = 17364, B = 6658.92, C = 4921.04; trans-acetaldoxime, A = 44613, B = 4237.73, C = 3973.81. The transitions of cis- and trans-acetaldoxime are split by tunneling of the methyl group through the potential barrier hindering the internal rotation. From these separations the height of the barrier has been determined to be 375 cal/mole for cis and 1835 cal/mole for trans-acetaldoxime. The quadrupole coupling constants are as follows: cis acetaldoxime, χaa = − 2.8 MHz, χbb − χcc = 0.6 MHz; trans-acetaldoxime, χaa = 4.07 MHz, χbb − χcc = − 5.0 MHz. The dipole moments for the cis and trans species are 0.850 and 0.938 D, respectively.
Electron Spin Resonance Absorption Spectrum of the AsO22− Molecule–Ion in γ-Irradiated Single-Crystal Calcite
Authors: Marshall S. A. , Serway R. A.
Published: February 2004

An electron spin resonance absorption spectrum corresponding to S = 12, I = 32, and exhibiting orthorhombic symmetry has been observed in γ-irradiated single-crystal calcite. At 4.2°K, the spin-Hamiltonian parameters are gzz = 2.0150, gxx = 1.9991, gyy = 1.9910, Azz = −152 MHz, Axx = 614 MHz, Ayy = −173 MHz, (gnβn/h) = −7.24 × 10−4MHz/Oe, e2qQ = 16.2 MHz, and η ≃ 0. The paramagnetic defect associated with this spectrum is identified as the AsO22− molecule–ion which is located at carbonate ion sites with its plane coincident with that of the carbonate ions. From the hyperfine structure splittings and the g-tensor anisotropy, it is deduced that the unpaired electron has approximately 1% 4s, 96% 4p, and 3% 4d character and that it is almost completely localized on the arsenic atom.
Fluorescence and Induced Phosphorescence of Formaldehyde in Solid Low-Temperature Solutions
Authors: Smith J. J. , Meyer B.
Published: February 2004

The absorption and emission spectrum of formaldehyde was studied in xenon, krypton, and sulfur hexafluoride and without matrix between 20° and 200°K in the region of 2600–6000 Å. Matrix isolation of formaldehyde, even at M/R 1000, is dependent on solvent. The SF6 yields more than 99% isolation, xenon 90%, and krypton only 50%. The S1←S0 absorption is broad in all cases, and the system T1←S0 is too weak to be studied in detail. Strong emission in the 3800–5400-Å region is observed. The emission spectrum is independent of the exciting frequency between 2500 and 3500 Å. In pure formaldehyde it is due to fluorescence of S1→S0 only, and in matrices it is due to simultaneous and overlapping fluorescence and phosphorescence. The fluorescence resembles the gas-phase spectrum and is analyzed accordingly. Phosphorescence is caused by intersystem crossing. The phosphorescence yield increases with solvent polarizability and decreases reversibly with increasing temperature until diffusion occurs. Phosphorescence yields are always smaller than 5% of fluorescence. The lifetime of phosphorescence is 0.23 sec in xenon, 1.1 sec in krypton, and 0.63 sec in SF6. The phosphorescence spectrum is deduced from the intensity of the long lifetime component, and also from the reversible temperature-dependent component of the emission intensity.
Pressure and Temperature Dependence of the Nuclear Quadrupole Frequency of 23Na in NaBrO3
Authors: Whidden C. J. , Williams C. D. , Tipsword R. F.
Published: February 2004

The temperature and pressure dependences of the nuclear quadrupole coupling frequency of 23Na in NaBrO3 have been measured. The quadrupole frequency is found to range from 886 kHz at 4.1°K to 849 kHz at 300°K at atmospheric pressure. The pressure dependences were determined at 196°, 273°, and 298°K. From the pressure dependences and the compressibility the frequency was found to vary with the volume about as V−2.5 at all three temperatures. The rate of change in frequency with temperature at a constant volume, ν−1(∂ν / ∂T)v, was found to have a positive value and to decrease in magnitude with increasing temperature above 196°K.
Zeeman Effect of the Nuclear Quadrupole Resonance of 81Br in para-Bromophenol and 35Cl in para-Chlorophenol
Authors: Bucci P. , Cecchi P. , Colligiani A.
Published: February 2004

The Zeeman effect of the 81Br and 35Cl nuclear quadrupole resonance has been studied with high precision on single crystals of para-bromophenol and para-chlorophenol, respectively. The results substantially agree with previous x-ray and NQR studies of para-chlorophenol (x-ray studies are known only for this substance). In p-bromophenol, four physically inequivalent sites were found, instead of two, as reported by other authors. Values of the asymmetry parameter are, respectively, ηBr = 0.0534 ± 0.001, ηCl = 0.071 ± 0.001 (34.12 MHz), and ηCl = 0.096 ± 0.001 (34.35 MHz).
Studies of Chemical Shift Anisotropy in Liquid-Crystal Solvents. II. Theoretical Calculations for the Methyl Halides
Authors: Ceasar Gerald P. , Dailey Benjamin P.
Published: February 2004

Experimentally determined values of the proton chemical-shift anisotropy for the methyl halides have been used to test simple phenomenological theories which have been proposed to account for proton magnetic shielding. It is shown that the frequently used magnetic dipole model predicts that the long-range contribution to the proton shielding anisotropy should largely depend on the average neighbor magnetic susceptibility. Anisotropies calculated from this model are much larger than those measured. Expressions have also been derived relating the proton shielding anisotropy to the electric dipole moment and charge densities of polar substituents. Results of these calculations are not as conclusive as in the magnetic dipole case.
Mechanisms of Energy Transport in Ruby
Authors: Birgeneau R. J.
Published: February 2004

The experimental results of Imbusch on energy transfer between single ions and pairs in 0.003%–1% Cr3+:Al2O3 are re-examined. It is shown that the energy transfer can only be plausibly explained on the basis of orbitally dependent exchange rather than the electric quadrupole–quadrupole interaction as has been postulated. This result, besides being of intrinsic interest, also has important consequences for other energy-transfer experiments. In particular, the experimental concentration dependence of the intensities seems to imply an Rmean5 law for the interaction operative in the energy transfer rather than the much more rapid falloff characteristic of exchange. It is suggested that this discrepancy arises from the propagating nature of the single ion excitation which tends to enhance the importance of short-range interactions. This same anomaly should be present in many other systems in which energy transfer has been studied.
Electron Spin Resonance Studies of Radical Pairs Formed in Hydroxyurea by X Irradiation at Low Temperatures
Authors: Reiss Keith , Shields Howard
Published: February 2004

Radical pairs have been identified in single crystals of hydroxyurea x irradiated at 5°, 30°, and 77°K. There are two chemically identical but magnetically nonequivalent pairs present and at least one other weak pair. The main pairs have an average separation between the unpaired spins of 6.38 Å, and the radicals of the pairs are located in adjacent molecular layers. A separation of R ≤ 5.3 Å was measured for the weak pair. The magnetically nonequivalent pairs were identified as NH2CONHO···OHNOCH2N. Each radical has the same structure as the stable room-temperature radical. Decay of the main radical pairs with R = 6.38 Å into isolated radicals is exponential and follows a first-order rate law giving an activation energy of 0.4 eV per molecule.
Dynamic Nuclear Polarization: Collision Mechanics in Fluorocarbon Solutions
Authors: Webb R. H. , Nghia N. v. , Pearlman M. R. , Poindexter E. H. , Caplan P. J. , Potenza J. A.
Published: February 2004

Dynamic polarization measurements at three widely different magnetic field strengths are presented for an array of chemically different free radicals and fluorocarbons. Observed fluorine NMR signal enhancements ranged from − 225 to + 430. The scalar hyperfine correlation time, which is shown to be the major factor governing the enhancement, is systematically longer for systems showing higher positive enhancement at low field. A generalized pulse-transform collision model is applicable to all cases, and it is found that the slope of the scalar spectral cutoff becomes progressively less steep as low-field scalar coupling increases. In extreme cases, the spectrum may be interpreted as a superposition of at least two component spectra, each corresponding to a pulse from a stereospecific collision attitude. The observed behavior is consistent with molecular orbital calculations. The range of computed hyperfine coupling energies is also in good qualitative agreement with observed differences between fluorocarbons of diverse structure and chemical nature. Over all, the results demonstrate the usefulness of measurements of this sort for understanding the microscopic character of molecular collisions.
Mössbauer Effect in the System Co1−xFexS2
Authors: Gallagher P. K. , MacChesney J. B. , Sherwood R. C.
Published: February 2004

X-ray diffraction analysis and magnetic-susceptibility measurements of samples in the system, Co1−xFexS2, where x = 0.01, 0.25, 0.50, and 0.75, indicate formation of a solid-solution series having the pyrite structure. These compounds are ferromagnetic with Curie temperatures in the range of 0°–125°K. In spite in the appearance of long-range magnetic order, Mössbauer effect measurements of 57Fe at 300°, 100°, and 5°K do not exhibit magnetic hyperfine splitting. This is interpreted as evidence that the divalent iron ions remain in the low spin state throughout the system and that the magnetic ordering process involves electrons localized on the cobalt ions. Trends in isomer shift and quadrupole splitting are discussed relative to changes in the lattice constant.
Errors in the Measurement of Electron-Transfer Rates by Means of Electron Spin Resonance
Authors: Johnson Charles S. , Holz John B.
Published: February 2004

Rates of electron-transfer reactions can be conveniently determined from linewidths of electron spin resonance spectra in the limit of fast exchange by using an equation derived by Piette and Anderson (PA). Here the range of validity of this equation is investigated by a comparison of its predictions with exact linewidths obtained by numerical calculations. A correction term, depending on the second and fourth moments of the uncollapsed spectrum, is also derived which extends the range of validity of the PA fast-exchange equation. The ratio of the experimental linewidth to the square root of the second moment is a good indicator of the fast-exchange limit. This ratio must be less than 0.2 for the uncorrected fast-exchange equation to hold within 5%.
Anharmonic Stretch–Stretch Interaction Force Constants of Triatomic Molecules
Authors: Machida Katsunosuke , Overend John
Published: February 2004

A model is proposed which, using the generally accepted form of the potential-energy surface in the two bond stretching coordinates, allows the prediction of the anharmonic stretch–stretch interaction force constants from the quadratic interaction force constant. The model is applied to CO2 and N2O. Hydrogen cyanide, OCS, and H2O are also discussed.
Correlation Effects of π Electrons. III. Oscillator Strength Distribution
Authors: Gutfreund H. , Little W. A.
Published: February 2004

The response of the π electrons in a large organic molecule to a scalar field is discussed. The case of coronene (24 π electrons) is treated numerically. The results show an enhancement of the oscillator strength near the upper edge of the single-particle excitation band. The same qualitative behavior is found in inelastic electron scattering experiments on several polycyclic hydrocarbons.
Spin Degeneracy and the Stability of Hartree–Fock States
Authors: Lunell Sten , Lindner Peter
Published: February 2004

The stability of a Hartree–Fock determinant under a splitting of the doubly occupied orbitals with simultaneous symmetry projection is studied. As a consequence of the spin degeneracy, the energy is given by the lowest root to a secular equation. It is found that even for an infinitesimal splitting of the orbitals there will be a discontinuous, finite lowering of the energy for both closed-shell and open-shell states. Numerical illustrations are given by the Li and Be atoms, the H4 molecule, and the benzene molecule.
Averaged Potentials and the Viscosity of Dilute Polar Gases
Authors: Danon F. , Amdur I.
Published: February 2004

The collision integral Ω(2,2)* for the viscosity of a single dilute polar gas has been obtained for a central pair potential which is an average of the Stockmayer potential over all orientation of the dipoles. Comparison is made between the canonical average potential and the free-energy average potential up to a second-order approximation. The collision integrals from the two types of average potential and the average, over all orientations, of the collision integrals calculated directly from the Stockmayer potential on the basis of the fixed orientation approximation are also compared. It is found that both average potentials lead to collision integrals that differ significantly from those based on the more reasonable model of fixed orientations during collisions. However, the collision integrals from the free-energy average potential show better agreement than those obtained from the canonical average potential. A similar trend is observed with respect to the second virial coefficients of several polar gases.
Unpaired Electron Spin Correlation Times in Metal–Ammonia Solutions
Authors: O'Reilly D. E.
Published: February 2004

Electron spin resonance linewidths have been measured at 7 MHz for Cs, Rb, K, and Li solutions from 0.03 mole liter−1 to about 1 mole liter−1. Linewidths increase in the order Li, K<Rb<Cs; the widths of Li solutions are essentially the same as those found for K. Correlation times of the hyperfine and spin–orbit interactions for the unpaired electron spins are calculated for K from very dilute solutions to 1.0 mole liter−1 and are compared with a theoretical expression previously proposed. From this comparison it is found that the mean-square flight distance associated with the conductivity increases by a factor of 3 from very dilute solutions to 0.6 mole liter−1 and is indicative of the onset of exchange processes in the conductivity. The magnitude of the spin–orbit relaxation is discussed and the much larger observed linewidths for Cs and Rb compared to K are interpreted.
Calculation of the Zero-Field-Splitting Parameters of the Triplet Dianion of Triphenylene and its Ion Triple
Authors: Sommerdijk J. L. , de Boer E.
Published: February 2004

The results are reported of calculations of the zero-field-splitting parameter D(E = 0) for the free triplet dianion of triphenylene and of D and E for its ion triple with two counterions in 2-methyltetrahydrofuran. For the free dianion good agreement with experiment is obtained if triplet SCF MO's are used and if configuration interaction and σ–π interaction are included according to a semiempirical method proposed by van der Waals and ter Maten. The ESR spectrum of the triplet ion triple does not reflect the trigonal symmetry of triphenylene. To account for this, three different models for the ion triple are considered. The electric fields of the counterions and solvent molecules are represented as the fields of two point charges. Their effect is introduced in the SCF calculations after which D and E are calculated with allowance for configuration interaction and σ–π interaction. A comparison with the experimental results shows that the polarizing influence of the counterions can largely account for the observed D and E values.