Volume 28, Issue 2, 01 February 1958

Electron Diffraction Study on the Rotational Isomerism in n‐Propyl Chloride
View Description Hide DescriptionThe molecular structure and the rotational isomerism of n‐propyl chloride were investigated by means of the sector‐microphotometer method of electron diffraction. The following structural parameters were obtained: C–Cl=1.77_{5}±0.02 A, C–C=1.53_{5}±0.02 A, and ∠C–C–Cl=111°±2°. It was confirmed that there are two isomers in the vapor phase; one is the trans‐form, and the other is the gauche‐form. From the analysis of the scattering intensity of the small‐angle region (q=5∼15), the angle of ∠C–C–C, the azimuthal angle of the gauche‐form, and the fraction of the trans‐form were found to be 109.5°±2.5°, 59°±5°, and 19±5%, respectively. It was also found through the calculation of the partition functions that the gauche‐form is only a few hundred calories/mole more stable than the trans‐form, in fairly good agreement with the spectroscopic results. The procedure used in the analysis of the small‐angle region is described in detail.

Molecular Structure and Internal Rotation of Hexachloroethane, Hexachlorodisilane, and Trichloromethyl‐Trichlorosilane
View Description Hide DescriptionThe molecular structure and internal rotation of C_{2}Cl_{6}, SiCCl_{6}, and Si_{2}Cl_{6} 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 SiCCl_{6} 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 C_{2}Cl_{6}, SiCCl_{6}, and Si_{2}Cl_{6}, respectively.

Energy of Interaction between Two Excited Hydrogen Atoms in either 2s or 2p States
View Description Hide DescriptionA complete set of potential energy curves arising from the interaction between two excited hydrogen atoms in either 2s or 2p states is given for intermediate and large internuclear separations. These include first‐ and second‐order perturbation energies. The first‐order perturbation energies are obtained by using exact integrals and properly symmetrized wave functions with an effective nuclear charge of unity. The possibility of occurrence of ionic states, which become important at small internuclear separations, is ignored. The first‐order energies are essentially Coulombic in the long‐range region and can be described in terms of simple electrostatic dipole‐dipole, dipole‐quadrupole and quadrupole‐quadrupole interactions. Approximate values of the second‐order perturbation energies are obtained by using nonsymmetrized wave functions and omitting in the expansion of the perturbation potential all terms beyond the leading one. For most states, the second‐order perturbation energies are small compared to the first‐order energies throughout the long‐range region.

Absorption and Ionization Coefficients of Ethylene Oxide
View Description Hide DescriptionTotal absorption coefficients of ethylene oxide vapor were obtained in the region from 1050 to 1850 A by a photoelectric method, and photoionization yields were measured in the region from 1050 to 1190 A. A reassignment of the absorption bands in the region from 1180 to 1480 A yielded a Rydberg series which converged to 10.565 ev, a value in agreement with the ionization potential obtained by photoionization measurements. A second Rydberg series was found to converge to a vibrationally excited ionic ground state.

Separation of Boron Isotopes. I. Boron Trihalide Addition Compounds
View Description Hide DescriptionIsotopic separation factors for several gas‐liquid systems for concentrating boronisotopes have been determined. Separation factors at 25°C are:Separation factors for a number of BCl_{3}‐organic systems have also been studied, and a new compound (C_{6}H_{5})_{2}O·BCl_{3} has been prepared and characterized.

Separation of Boron Isotopes. II. The BF_{3} Anisole System
View Description Hide DescriptionA new gas‐liquid countercurrent system has been developed for the separation of boronisotopes. This system utilizes the exchange reaction between BF_{3} gas and the BF_{3}·methyl phenyl ether (anisole) complex. Quantitative dissociation of the complex is attained by heating, and recombination by cooling. Half‐time for the isotopic exchange has been shown to be less than three seconds. The single‐stage separation factorvaries from 1.039 at 0°C to 1.028 at 30°C. Physical properties of the complex have been determined and solvent decomposition has been studied.

Stress‐Enhanced Diffusion in Glass. I. Glass under Tension and Compression
View Description Hide DescriptionThis paper describes a new technique for the determination of diffusion constants of gases in gas‐solid systems. The method demands a careful analysis of the transient quantity of gas flowing after the gas pressure at the boundary is discontinuously changed. The exact quantity of gas flowing is recorded by a mass spectrometer. The method was used to study the effect of high tensile stress upon the diffusion constant of helium and other gases in glass.
The diffusion constant of helium was found to be a true constant with dilation of the glass until the stress became about one‐half the breaking stress of the glass. Beyond this point the diffusion constant increased, and under very high stress was a factor of ten larger than its original value. No apparent change was produced in the glass, and reduced stress gave the same diffusion constant as previously, indicating reversibility of the effect with stress. It was further determined that equal and even larger compressional stresses on the same glass specimen had little or no effect upon the diffusion constant. The effect of shear stress upon the diffusion constant is dealt with in the following paper.
The magnitude of the observed increase in diffusion constant is larger by a considerable factor than can be explained by a straight‐forward extension of prevalent theories of diffusion in glass. If it is assumed that the glass sample dilates on an atomic scale, i.e., each atom being isotropically displaced by the stress, then calculations show that the diffusion constant should not be strongly increased under stress. Observation of the large increase then points toward the opening of flaws or voids within the glass.
Diffusion of hydrogen, heavy water, oxygen, and nitrogen is discussed and upper limits for diffusion of the latter three under high stress are set.

Stress‐Enhanced Diffusion in Glass. II. Glass under Shear
View Description Hide DescriptionGlass has been subjected to large shear strain to determine its effect upon the diffusion rate of gases through the solid. It was found that shear strain, γ, up to 4·10^{—3} does not alter the diffusion constant appreciably in the glass. Additional evidence is presented in support of the conclusion of the previous paper that the large increase in diffusion constant in glass results from the effect of tension. Arguments are expanded which support the interpretation that stress enhancement is caused by the reversible opening of submicroscopic fissures or channels in the random glass network.

Configuration Interaction Study of the Electronic Structure of the OH Radical by the Atomic and Molecular Orbital Methods
View Description Hide DescriptionThe role played in configuration interaction calculations by the particular form of the orbitals selected as basis is explored by a direct application on the electronic structure of the ground state of the OH molecule. Two configuration interaction studies have been carried out using different sets of one‐particle functions: (1) the set natural to the isolated atoms (called AO's) and (2) the orbitals belonging to the molecule (MO's).
In (1), using the Hartree‐Fock 1s, 2s, and 2p functions for atomic oxygen and the 1s state for hydrogen, and keeping the 1s oxygen state doubly occupied, all Slater determinants of the appropriate ground state symmetry (^{2}π) were found. The dissociation energy and the expansion coefficients of the determinants were determined by a variational procedure which minimized the energy of the system. The nonorthogonality of the basic AO's was treated without approximation by the method of Löwdin. The necessary integrals were calculated on Whirlwind I, the M.I.T. high‐speed digital computer; all two‐center integrals were computed by expanding the hydrogen orbital about the oxygen center (analogously to the methods of Barnett and Coulson, Coolidge, Löwdin, etc.).
In (2), the molecular orbitals (MO's) were found as linear combinations of the above AO's, with coefficients determined by a modified self‐consistent field (Roothaan) procedure, in which the energy of a single Slater determinant was made stationary at three values of the internuclear separation. A configuration interaction calculation was then made using these ``best'' MO's as the basic set of one‐particle functions. Also included are a calculation of the dipole moment and Mulliken's electron distribution analysis.
The results of these studies are discussed and comparison is made to experiment and other theoretical calculations.

Influence of the Crystal‐Field Potential on Interionic Separation in Salts of Divalent Iron‐Group Ions
View Description Hide DescriptionThe crystal energies of ionic lattices containing iron group ions with nonspherically symmetric electronic ground states are appreciably lowered as a result of crystal field splitting of the lowest energy levels. This gives rise to a characteristic periodic variation of the equilibrium cation‐anion separation as the 3d shell is progressively filled. It is possible to estimate the contractions of interionic separation resulting from the crystal field effect by comparison with the distances in lattices containing only ions with Sground states. In the divalent oxides and halides, these contractions are of the order of 0.04–0.22 A. By treating the crystal‐field stabilization as a small perturbation, the contractions can be calculated for simple lattice structures. Good agreement between theory and experiment is found for oxides crystallizing in the sodium chloride lattice, with the exception of VO. The lattice contractions of D _{3d 3 } iodides and D _{4h 14 } fluorides can also be consistently correlated with stabilization energy in these salts.

Transference Experiments on Lead and Thallous Ions in Potassium Chloride Crystals
View Description Hide DescriptionThe electrolytic migration of lead and thallium in potassium chloride crystals to which small additions of lead chloride and thallous chloride have been made has been studied in a transference experiment. Lead was found to migrate toward the anode, indicating that it is bound in a negatively charged complex. Thallium did not migrate appreciably at temperatures below 600°; at higher temperatures it moved toward the cathode.Thallium clearly may exist as the simple cation at elevated temperatures; however, the results at lower temperatures do not permit a conclusion to be drawn with certainty regarding the existence of complex ions.

Interaction Energy and Scattering Cross Sections of H^{—} Ions in Helium
View Description Hide DescriptionAn approximate quantum‐mechanical calculation is made of the energy of interaction of H^{—} and He. The results are not of high accuracy, but serve to indicate the general nature of the interaction. This knowledge makes it possible to calculate more accurate values of the interaction from measuredelasticscattering cross sections. The resultant interaction is combined with the previously known interaction of H and He to give an approximation to the inelastic (electron detachment) cross section, which is shown to be in reasonable agreement with experiment. The calculations emphasize the importance of the repulsion forces rather than the attraction forces. The accuracy of a correction for the finite width of the ion beam is also examined in some detail.

Free Energy of a Nonuniform System. I. Interfacial Free Energy
View Description Hide DescriptionIt is shown that the free energy of a volume V of an isotropic system of nonuniform composition or density is given by : N_{V}∫_{V} [f _{0}(c)+κ(▿c)^{2}]dV, where N_{V} is the number of molecules per unit volume, ▿c the composition or density gradient, f _{0} the free energy per molecule of a homogeneous system, and κ a parameter which, in general, may be dependent on c and temperature, but for a regular solution is a constant which can be evaluated. This expression is used to determine the properties of a flat interface between two coexisting phases. In particular, we find that the thickness of the interface increases with increasing temperature and becomes infinite at the critical temperature T_{c} , and that at a temperature T just below T_{c} the interfacial free energy σ is proportional to .
The predicted interfacial free energy and its temperature dependence are found to be in agreement with existing experimental data. The possibility of using optical measurements of the interface thickness to provide an additional check of our treatment is briefly discussed.

Exchange of Radiocobalt between the Hexammines of Cobalt(II) and Cobalt(III) in Liquid Ammonia
View Description Hide DescriptionExchange of radiocobalt between Co(NH_{3})_{6} ^{+2} and Co(NH_{3})_{6} ^{+3} nitrates in liquid ammonia has been studied at 25° and 45°C. Reactants were separated by distilling off the solvent ammonia and extracting the Co(II) as the thiocyanate complex into a mixture of ether and isoamyl alcohol. Runs were not accurately reproducible, probably because of catalytic effects from traces of oxygen in the system. Rate constants, calculated for an assumed bimolecular rate law, are 6×10^{—5} and 7×10^{—4} l·mole^{—1}·sec^{—1} at 25° and 45°C, respectively. The rate in liquid ammonia is compared with published rates in aqueous ammonia solution and with the rate of exchange of labeled NH_{3} between solvent liquid ammonia and ligand ammonia in Co(NH_{3})_{6} ^{+3}.

Reactions of Alkoxy Radicals. III. Photolysis of Methyl‐d _{3} Acetate below 100°C
View Description Hide DescriptionThe photolysis of CH_{3}COOCD_{3} has been investigated at 30 and at 90°C at varying initial pressures and light intensities. The following primary steps were shown to occur:The following reactions regarding methoxy and methyl radicals have been investigated:A value of about 1.4 was obtained for k _{7}/k _{6} 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.

Exchange Broadening in Nuclear Magnetic Resonance
View Description Hide DescriptionA general theory of the nonsaturated NMRspectra of chemically exchanging molecules is formulated. As an example the case of a molecule with two nonequivalent protons exchanging with a molecule with one proton is worked out in detail. Connection is made with previous theories by taking limiting expressions of the general results. In the appendix it is shown how the method can be extended to include saturation and hindered rotation.

Drude‐Model Calculation of Dispersion Forces. III. The Fourth‐Order Contribution
View Description Hide DescriptionThe results of part I are applied in calculating the fourth‐order dipole‐dipole contribution to the dispersion interaction energy of a system containing N identical spherical molecules. This contribution to the energy consists of pair terms proportional to R_{ij} ^{‐12}, triplet terms proportional to (R_{ij}R_{jk} )^{‐6}, and quadruplet terms proportional to (R_{ij}R_{jk}R_{kl}R_{li} )^{‐3}, where R_{ij} is the separation of the ith and jth molecules. The expression for the fourth‐order energy is summed approximately over the face‐centered‐cubic lattice. The result is E ^{(4)} ≃—100NV(α/R _{0} ^{3})^{4}, where α is the polarizability and V the characteristic dispersion energy of the molecules, R _{0} is the nearest‐neighbor distance in the lattice, and N is the number of molecules in the crystal. The triplet terms are responsible for the main contribution to this sum. The fourth‐order dipole‐dipole dispersion energy of the lattice is negative and comparable in magnitude with the positive triple‐dipole energy computed by Axilrod and others, in cases where the latter contribution is substantial.

Vibrational Spectra and Thermodynamic Properties of ClF_{3} and BrF_{3}
View Description Hide DescriptionThe Raman and infrared spectra of ClF_{3} and BrF_{3} have been studied. The spectra of ClF_{3} give strong support for a planar T‐shaped molecular model. The spectra of BrF_{3} are less complete but are sufficiently similar to that of ClF_{3} to confirm a like shape for this molecule. The fundamental vibrational frequencies of ClF_{3} observed for the vapor are 326(a _{1}), 364(b _{2}), 434(b _{1}), 528(a _{1}), 703(b _{1}), and 752(a _{1}) cm^{‐1}. S ^{0} for ClF_{3}, calculated statistically at the boiling point, 11.75°C, is 66.60 cal mole^{‐1} deg^{‐1} compared to the value of 67.04 obtained from a revised calculation of this quantity from available thermal data. For BrF_{3} only two fundamental frequencies were observed in the vapor, 613(b _{1}) and 674(a _{1}) cm^{‐1}; the others were estimated by a normal coordinate calculation. S ^{0} for BrF_{3} calculated statistically is 70.86 cal mole^{‐1} deg^{‐1} at 43.11°C compared to a value of 71.90 calculated from available thermal data. Tables of the thermodynamic functions of ClF_{3} and BrF_{3} from 250 to 1000°K are given.

Diffusion of Cobalt and Chromium in Chromite Spinel
View Description Hide DescriptionThe diffusion of cobalt and chromium in sintered samples of CoCr_{2}O_{4} has been measured in the temperature region 1400°C—1600°C where it is believed that bulk diffusion predominates. The diffusion can be represented as D _{Co}=(10^{—3}) exp(—51 000/RT), D _{Cr}=(2) exp(—70 000/RT).

Heat Capacity of Sodium Tungsten Bronzes from 1.8 to 4.2°K
View Description Hide DescriptionThe heat capacity of sodiumtungsten bronze (Na_{ x }WO_{3}) has been measured over the temperature range 1.8–4.2°K for various concentrations of sodium. An approximation to the density of states curve for the conduction band was obtained from the experimental data. This may be correlated with the previously described maximum in the electrical conductivity. It was found that the lattice contribution to the heat capacity was appreciably affected by the thermal history of the sample while the electronic contribution was relatively unaffected.