bluuebunny/crossref_metadata_2025_split huggingface dataset:
Theory of Isotropic Hyperfine Interactions in π-Electron Radicals
Authors: McConnell Harden M. , Chesnut Donald B. 
Published: January 2005

Indirect proton hyperfine interactions in π-electron radicals are first discussed in terms of a hypothetical CH fragment which holds one unpaired π electron and two σ-CH bonding electrons. Molecular orbital theory and valence bond theory yield almost identical results for the unpaired electron density at the proton due to exchange coupling between the π electron and the σ electrons. The unrestricted Hartree-Fock approximation leads to qualitatively similar results. The unpaired electron spin density at the proton tends to be antiparallel to the average spin of the π electron, and this leads to a negative proton hyperfine coupling constant.
The theory of indirect proton hyperfine interaction in the CH fragment is generalized to the case of polyatomic π-electron radical systems; e.g., large planar aromatic radicals. In making this generalization there is introduced an unpaired π-electron spin density operator, ρN, where N refers to carbon atom N. Expectation values of the spin density operator ρN are called ``spin densities,'' ρN, and can be positive or negative. In the simple one-electron molecular orbital approximation a π-electron radical always has a positive or zero spin density at carbon atom N, 0≤ρN≤1. In certain π-electron radical systems; e.g., odd-alternate hydrocarbon radicals, the spin densities at certain (unstarred) carbon atoms are negative when the effects of π—π configuration interaction are included in the π-electron wave function.
The previously proposed linear relation between the hyperfine splitting due to proton N, aN, and the unpaired spin density on carbon atom N, ρN,aN=QρNis derived under very general conditions. Two basic approximations are necessary in the derivation of this linear relation. First, it is necessary that σ—π exchange interaction can be treated as a first-order perturbation in π-electron systems. Second, it is necessary that the energy of the triplet antibonding state of the C–H bond be much larger than the excitation energies of certain doublet and quartet states of the π electrons. This derivation of the above linear relation makes no restrictive assumptions regarding the degree of π—π or σ—σ configuration interaction. The validity of the above approximations is discussed and illustrated by highly simplified calculations of the proton hyperfine splittings in the allyl radical, assuming the π—π configuration interaction—and hence the negative spin density on the central carbon atom—to be small.
Isotropic hyperfine interactions in molecules in liquid solution can also arise from spin-orbital interaction effects, and it is shown that these effects are negligible for proton hyperfine interactions in aromatic radicals.
Application of the Method of Deformed Atoms in Molecules to the Li2 Molecule
Authors: Arai Tadashi , Sakamoto Michiko 
Published: January 2005

The method of deformed atoms in molecules, proposed previously to remedy deficiencies in the method of Moffitt, is employed to calculate energies of the ground state 1Σg+ and the lower excited states 3Σu+, 1Σu+, and 1IIu of the Li2 molecule. As basis for the molecular calculation, modified atomic functions are introduced to take deformation of atoms in molecules into account. These modified functions are constructed from atomic eigenfunctions in a manner which fully preserves correlation between electrons. Exact atomic energies are used to calculate the atomic energy parts of the matrix elements of the molecular calculation, while the energy defects due to deformation of atoms are determined by using approximate functions.
The modified atomic functions are made up so that their charge distributions are well described by the use of Slater orbitals with equal orbital exponents for all atomic states. Interatomic interaction operators, overlap integrals, and energy defects due to deformation of atoms are calculated using Slater orbitals. For the 1s)22s)2p) 3P and 1P states of the Li— ion, the exchange correction due to binding a free electron into the 2p orbital is considered. The energy defects of Slater orbitals from Hartree-Fock orbitals are found to have small effect on the dissociation energy of the ground state.
The calculations are performed for three sets of values for the orbital exponents. The dissociation energy obtained for one set of values is 0.96 ev, in good agreement with the observed value of 1.05 ev. The excitation energies obtained of 1.63 ev for 3Σu+—1Σg+, 2.05 ev for 1Σu+—1Σg+ and 2.59 ev for 1IIu—1Σg+ also agree reasonably well with the corresponding observed values of (1.35 ev), 1.76 ev, and 2.54 ev.
The calculation is discussed and compared with the orbital approach and Moffitt's method. Conclusions are stated regarding the nature of bonds and orbital exponents in Li2.
Molecular Structure and Internal Rotation of Hexachloroethane, Hexachlorodisilane, and Trichloromethyl-Trichlorosilane
Authors: Morino Yonezo , Hirota Eizi 
Published: January 2005

The molecular structure and internal rotation of C2Cl6, SiCCl6, and Si2Cl6 molecules were investigated by means of the sector-microphotometer method of electron diffraction. An unmodified radial distribution function was proposed together with a reliability factor for the analysis of intensity curves obtained by the sector method.
Two methods were used for estimating the height of potential barrier; one, which is applicable in case of high potential barrier, is to compute the height from the observed mean square amplitude of the gauche chlorine pair, the contribution of the skeleton vibrations being evaluated by a calculation based upon the force field compatible with the observed normal frequencies. The other method, which is available in case of low barrier, is to calculate the scattering intensity of a model having a certain potential barrier by taking a weighted average over-all values of internal angle. For SiCCl6 both approximate methods resulted in almost the same value for the barrier height. The heights of the potential barrier obtained were 10.8, 4.3, and 1 kcal/mole for C2Cl6, SiCCl6, and Si2Cl6, respectively.
Reactions of Alkoxy Radicals. III. Photolysis of Methyl-d3 Acetate below 100°C
Authors: Wijnen M. H. J. 
Published: January 2005

The photolysis of CH3COOCD3 has been investigated at 30 and at 90°C at varying initial pressures and light intensities. The following primary steps were shown to occur: (1a)CH3COOCD3+hv  →CH3CO+CD3O(1b)→CH3+CO2+CD3(1c)→CH3+CO+CD3O.The following reactions regarding methoxy and methyl radicals have been investigated: (3)CH3               →C2H6(6)CH3+CD3O        →CH3OCD3(7)→CH3D+CD2O(11)CD3O              →CD3OD+CD2O(16)CD3O+CH3COOCD3→CD3OH+CH2COOCD3(17)→CD3OD+CH3COOCD2.A value of about 1.4 was obtained for k7/k6 at 30 and 90°C, indicating the activation energy of reaction (7) to be zero kcal. Data obtained for the abstraction reactions (16) and (17) are discussed.
Quantitative data have been obtained and are discussed for the disproportionation and recombination reactions of methyl and acetyl radicals.
Effect of Pressure on Cyanide and Carbonyl Spectra in Solution
Authors: Wiederkehr R. R. , Drickamer H. G. 
Published: January 2005

The C–N vibration of CH3CN has been studied in a series of solvents, mostly substituted methanes, to 10 000 atmospheres pressure. Shapes of the curves of frequency versus pressure can be predicted reasonably well from a model which considers the interaction of each bond of the solvent molecule with the CN bonds. It is possible to estimate the relative contributions of dispersion, orientation, induction, and repulsion forces to the net shift.
Similar measurements and calculations have been made for the C–O vibration in a series of compounds, using CS2 as the solvent. In this case, induction forces account for 65% of the observed interaction.
Proton Magnetic Resonance of the CH3 Group. IV. Calculation of the Tunneling Frequency and of T1 in Solids
Authors: Stejskal E. O. , Gutowsky H. S. 
Published: January 2005

The thirty lowest eigenvalues of the Mathieu equation have been calculated numerically to nine places for a hindered rotor with threefold symmetry attached to a rigid framework. The results are tabulated for eight values of IV0/2ℏ2 ranging from 56.25 to 115.3125. The splittings of the torsional states of the CH3 group have been used to calculate the average tunneling frequency, νt, as a function of temperature for barrier heights of 825 cm—1 to 2770 cm—1. Some useful relations are developed between the eigenvalues and torsional splittings for the 2- and 3-nodal potential functions.
The BPP theory is used to calculate the proton spin-lattice relaxation time, T1, for CH3 groups undergoing hindered reorientations in solids and also over-all molecular tumblings, assuming Debye spectral density functions for both motions. The manner in which the tunneling affects T1 is considered at some length and two ways of introducing it are described. The coupling among the three protons within a given CH3 group is discussed briefly in relation to T1.
Role of Atomic Valence States in Molecular Energy Calculations
Authors: Hurley A. C. 
Published: January 2005

Ab initio calculations of molecular binding energies are examined critically in a basis of antisymmetrized products of approximate atomic state functions (the approximate composite functions of Moffitt). It is shown that much more accurate results may be expected if the binding energy is calculated relative to a suitably defined generalized valence state V of the dissociation products. The energy of this state V relative to the ground state of the dissociation products is estimated from atomic spectral data. This procedure leads directly to the intra-atomic correlation correction previously proposed by the author as a necessary modification of Moffitt's method of atoms in molecules.
Detailed calculations are carried out for the ground states of the molecules LiH and BH, and for the ground and excited states of the benzene molecule (in the usual π-electron approximation). In each case the intra-atomic correlation correction leads to a substantial improvement in the accuracy of the results.
Resonance in Sterically Hindered Aromatic Molecules
Authors: Adrian F. J. 
Published: January 2005

Many aromatic molecules, typical of which are the triphenylmethyl radical and cis-stilbene, cannot have the planar structure which would maximize the resonance energy, because of steric hindrance. A semiempirical method of treating such molecules has been developed. It is assumed that the primary effect of the steric strain is to rotate the phenyl groups out of the plane of the molecule. The resulting reduction in resonance energy is estimated using the valence-bond method. The steric interaction energies are calculated using a method of Eyring, based on the ``perfect pairing approximation'' of valence bond theory. A plot of the sum of these two energies vs twist angle gives the equilibrium configuration, and the effective resonance energy of the molecule.
The triphenylmethyl radical has a predicted twist angle of 32°, and is stabilized partly by its resonance energy and partly by steric strain in hexaphenylethane. The theoretical twist angle for the free radical di-p-anisyl nitric oxide is 25°, as compared with an experimental result of 33°. A close balance between steric strain and resonance in biphenyl leads to virtually free rotation about the central bond. Finally, it is shown that steric hindrance accounts for the energy difference between the cis- and trans-isomers of stilbene and azobenzene.
Absorption and Fluorescence Spectra of Crystalline Pyrene
Authors: Ferguson J. 
Published: January 2005

The absorption and fluorescence spectra of crystalline pyrene have been measured at room temperature and 77°K. Only the first absorption region could be studied because of crystal thickness but a Davydov splitting of about 18 cm—1 was detected with the a polarization at higher energy than the b polarization. Calculations of the splitting (dipole-dipole approximation for the intermolecular potential) give 23 and 13 cm—1, respectively, for the B2u and B3u levels with the observed sense, and so it is not possible to decide between two possible assignments from theory. The polarization ratio shows that mixing with a higher level of opposite polarization is present and this is consistent with the only theoretical prediction of energy levels in the molecule.
The fluorescence is displaced to long wavelengths from the absorption spectrum and is a broad band with a maximum which is further displaced on cooling the crystal. The band has been interpreted as a charge-transfer transition involving the molecules grouped in pairs in the crystal.
Paramagnetic Resonance Spectra of Some Semiquinone Free Radicals
Authors: Adams M. , Blois M. S. , Sands R. H. 
Published: January 2005

The paramagnetic resonance spectra of a number of semiquinone free radicals have been observed using a microwave ESR spectrometer.
The character of the hyperfine structure, splitting constants, and g values for these radicals are tabulated.
Chemical Effects of Photonuclear Reactions in the Propyl Bromides
Authors: Richardson A. E. , Voigt A. F. 
Published: January 2005

Two new photonuclear reactions are reported, Br79(γ,2n)Br77 and Br79(γ,3n)Br76.
The organic retention following the Br81(γ,n)Br80m reaction in liquid n- and isopropyl bromides was found to be 47 and 36%, respectively, appreciably higher than reported values following the Br79(n,γ)Br80m reaction. Higher retentions were also noted for irradiations of the solid bromides. The relative yields of individual organic products were determined for irradiations of both liquid and solid and were in many respects in striking agreement with data from activation by (n,γ) reaction and isomeric transition. In solid n-propyl bromide the (γ,n) and (γ,3n) reactions gave the same ratio of parent compound to total organic yield.
Data on the scavenger effect show that thermal recombination reactions occur in both liquid and solid. Marked increases are shown in the thermal organic yield with only slight increases in the nonthermal retention for both bromides for (γ,n) compared to (n,γ) activation. The reasons for these increases are discussed.
In general, the data indicate that elastic collision mechanisms are unimportant compared to random fragmentation as an explanation of the results.
Hydrogen Conversion and Exchange Reactions in Aqueous Solutions Induced by Gamma Rays
Authors: Friedman Harold L. , Zeltmann Alfred H. 
Published: January 2005

Experimental studies have been made of several γ-ray-induced reactions of hydrogen in aqueous solutions in which no net chemical change occurs. The most complete results were obtained at pH 2, where radiation-induced chain reactions are found in the conversion of parahydrogen and in the isotope exchange reactions of HD and of H2—D2 mixtures, but where there is only a nonchain exchange of D2 with the solvent. The exchange data are in agreement with the earlier work of Gordon and Hart, but are more extensive in some respects.
It is assumed that the chain conversion and exchange are carried by steps such as H+H2→H2+H,H+D2→HD+D,and isotopically related reactions, and furthermore that these reactions have the same specific rate constants in solution as in the gas phase at the same temperature. All the data at pH 2 can then be correlated by a single mechanism and the following conclusions are reached. At the lower dose rates in solutions of H2 the effective stationary-state concentration of H atoms is proportional to the dose rate, reaching 2×10—8 M at 1018 ev/l sec. The effective mean life of H atoms under these conditions is 0.2 sec. The rate constant of the reaction H+H→H2,in water at pH 2 is at most 1×108 l/mole sec. The pseudo firstorder rate constant of the reaction D+H2O→DOH+H,in water at pH=2 is 2 sec—1.
The last conclusion leads to an important restriction on the importance of H2+ in irradiated solutions, namely, H2+ cannot be in rapid equilibrium with H and H+ or this would be a path for a fast exchange of D with H2O.
The investigation of other pH ranges is less complete than that at pH 2, but the conclusion of Gordon and Hart, that the stationary-state H atom concentration produced by γ rays is much lower in basic than in acid solution, has been confirmed.
Hydrogen Fluoride Polymer Spectrum, Hexamer and Tetramer
Authors: Smith D. F. 
Published: January 2005

The experimental techniques employed to investigate the polymer absorption are presented.
Experimental results at various pressures at temperatures from —70°C to +73°C are presented. The more important qualitative features noted are: (1) the absorption is strongly dependent upon both the temperature and pressure; (2) in some cases the shape of the bands is dependent upon the pressure; (3) at low temperature only very low pressures of vapor (hence of polymer) are necessary to observe some absorption. The application of Dalton's law, the law of mass action, Lambert's law, and Beer's law, as well as Van't Hoff's equation to the hydrogen fluoride monomer-polymer system is discussed.
It is then shown that under some special circumstances the absorption varies simply as the fourth power of the pressure, under others as the sixth power of the pressure, but more generally where the absorption is strong as the sum of two terms, one containing the fourth power of the pressure the other the sixth power. The absorption thus corresponds to the overlapping spectra of a tetramer and of a hexamer, which can be separated on the basis of this pressure dependence. From the temperature variation of the absorption, the heats of polymerization are estimated as ΔH4 = —19 kcal/mole and ΔH6 = —40 kcal/mole. In general, the absorption outside the range of monomer absorption is as consistent with a tetramer-hexamer interpretation for deuterium fluoride as for hydrogen fluoride.
Nonstoichiometry in Uranium Dioxide
Authors: Aronson S. , Belle J. 
Published: January 2005

A thermodynamic study of the uranium-oxygen system, primarily in the single phase region UO2–UO2.20, has been made at temperatures of 1150° to 1350°K. An electrochemical technique was used to obtain emf measurements on uranium oxide half-cells. The cells were of the type Fe, FeO|(ZrO2+CaO)|UO2+x, Pt where x varied from 0.01 to 0.20.
Partial molar free energies, entropies, and enthalpies of solution of oxygen in UO2+x were calculated. The partial molar free energy is a linear function of temperature at constant composition. The partial molar entropy increases negatively with increasing oxygen content.
Equations relating the thermodynamic quantities to the degree of nonstoichiometry have been developed on the assumption that the UO2+x region can be treated as an interstitial solution of oxygen ions in the UO2 fluorite lattice. Semiquantitative agreement with the data was obtained.
Dielectric Properties of Aqueous and Alcoholic Electrolytic Solutions
Authors: Hasted J. B. , Roderick G. W. 
Published: January 2005

Results of dielectric constant and loss measurements on a range of aqueous and alcoholic electrolytes at wavelengths of 1.25, 3.2, 9.9, 17, and 51.5 cm from 3°C to 25°C are analyzed in terms of dielectric constant depression, relaxation time depression and spread, and conductivity elevation. The small relaxation time spread (α∼0.02) is interpreted as being due to hydrogen bond bending, the relaxation time depression as bond breaking, and the dielectric constant depression as restriction of dipole rotation by the ions. The conductivity elevation is believed to be an effect of the same type as that observed by Little and Smith.
It is shown that the alcohol dielectric constant depression is too large to be interpreted in terms of dipole restriction by solvation so that the ion must have a marked effect on the alcohol structure.
Neighbor Interactions and Internal Rotations in Polymer Molecules. I. Stereospecific Structure and Average Dimensions
Authors: Lifson Shneior 
Published: January 2005

The skeletal bonds of vinylic polymer molecules are classified as (+) bonds and (—) bonds according to the arrangement of the side groups relative to the bonds. The detailed structure of isotactic and syndiotactic polymers is characterized by the different regularities of arrangement of these two bond types along the molecular chain. Symmetry properties of (+) and (—) bonds, and of (+) (+), (—) (—), and (—) (+) bond pairs, are discussed. These lead to relations between the average values of functions of the internal-rotation angles of the various bond-pairs.
The end-to-end distance of isotactic and syndiotactic molecules is expressed in terms of bond lengths, bond angles, and internal rotation angles of the skeletal bonds, using methods of matrix algebra. The mean-square end-to-end distance 〈h2〉 of these molecules is determined by certain averages of trigonometric functions of the internal rotation angles enclosed between neighboring skeletal bond pairs. The averages depend on the energy of interaction of neighboring side groups as well as on the internal interactions within the skeletal bonds, and are different in the isotactic and syndiotactic molecules.
Relative Intensities of Microwave Absorption Lines
Authors: Verdier Peter H. , Wilson E. Bright 
Published: January 2005

A method is described for measuring the relative intensities of microwave absorption lines up to 80 mc. apart to within a few percent, using a Stark-modulated resonant cavity absorption cell of novel design. Barriers to internal rotation in CH3CHO and CH3CH2F have been obtained by measuring the relative intensities of torsional satellites. The resulting barrier heights are: 1103±60 cal for CH3CHO, 3310±210 cal. for CH3CH2F, in good agreement with the values 1162±30 cal for CH3CHO, and 3306±100 cal for CH3CH2F obtained from splitting measurements by Kilb, Lin, and Wilson, and by Herschbach, respectively.
Correction factors are given for errors in intensity and splitting measurements due to small doublet splittings.
Approximate Formulas for the Viscosity and Thermal Conductivity of Gas Mixtures
Authors: Brokaw Richard S. 
Published: January 2005

Approximate expressions for the viscosity and thermal conductivity of gas mixtures have been derived from the rigorous kinetic theory formulas. Three levels of approximation are distinguished, with the third approximation rigorous for binary mixtures.
The first approximation for mixture viscosity is found to be very nearly equivalent to empirical expressions known heretofore. The first and second approximations for mixture conductivity have been compared with rigorous calculations for binary mixtures of nonpolar gas pairs; the first approximation has an average error of 2.6% while the second approximation reduces this error to 0.5%.
The second approximation accounts for the thermal conductivity of binary mixtures of polar and nonpolar gases. (It is assumed that the interchange of translational energy is abnormal for the polar-polar interaction.)
Pseudo-Energy Transfer in Some Unimolecular Decompositions
Authors: Szwarc M. , Herk L. 
Published: January 2005

The unimolecular decomposition of molecules involving rupture of a bond and formation of unstable radicals (i.e., ones which decompose exothermally) is discussed. It is shown, by using contour maps, that two cases should be considered. If the decomposition of the unstable radical does not involve activation energy, then the process starts as expansion of one bond, but eventually the reaction path turns toward the center and the process proceeds as a simultaneous decomposition of the molecule into 3 or more fragments. The transition point is represented by a saddle point and the activation energy is lower than the expected bond dissociation energy. In a loose language, the exothermicity of the radical decomposition helps in the primary dissociation process.
On the other hand, if the decomposition of the unstable radical does involve activation energy, then the unimolecular rupture of the first bond proceeds normally.