Volume 44, Issue 6, 15 March 1966
Index of content:
44(1966); http://dx.doi.org/10.1063/1.1727026View Description Hide Description
Paramagnetic Fe3(PO4)2·4H2O is monoclinic, with lattice constantsa=10.541±0.005, b=4.646±0.004, c=9.324±0.005 Å and β=100°25.8′±1.0′ at 298°K, with two formula weights in the unit cell. The space group is P21/a. The positions of the iron, phosphorus, and oxygen atoms, determined by Ito and Mori in 1951, together with those of the hydrogen atoms, have been refined by the method of least squares. The integrated intensities of 6212 reflections in a reciprocal lattice hemisphere of radius (sinθ)/λ=1.02 Å−1 were measured with PEXRAD; 1919 symmetry‐independent F(hkl) were significantly above background. The final agreement factor between measured and calculated structure factors was 0.0336. There are two crystallographically unrelated iron atoms per unit cell, each occupying a distorted oxygen octahedron; the iron—oxygen distances range from 2.016±0.002 through 2.362±0.002 Å. The average distance is 2.156 Å. The phosphate group is very nearly a regular tetrahedron, the range in P–O distance being from 1.536±0.002 through 1.549±0.002 Å. The average O–H distance is 0.87 Å. The shortest O–H···O bond is 2.541±0.003 Å. One phosphate—oxygen atom forms no hydrogen bond, one forms two, and the remaining two oxygen atoms form one hydrogen bond each. The thermal vibrations are significantly anisotropic.
44(1966); http://dx.doi.org/10.1063/1.1727027View Description Hide Description
A neutron‐diffraction study of Fe3(PO4)2·4H2O below the Curie temperature has determined both the crystal and the magnetic structure. At 4.2°K, the crystal remains monoclinic, with lattice constantsa=10.541±0.010, b=4.638±0.008, c=9.285±0.010 Å, β=100°43.7′±4.0′ and two formula weights in the unit cell. The nuclear and Shubnikov space groups are both P21/a. The iron—oxygen octahedral distances range from 2.019±0.010 through 2.317±0.010 Å, with average distance 2.154 Å. The phosphate group is very nearly a regular tetrahedron, with the phosphorus—oxygen distance ranging from 1.521±0.008 through 1.553±0.007 Å. The average O–H distance is 1.00 Å, and the shortest O–H···O bond is 2.546±0.005 Å. The magnetic unit cell coincides with the nuclear cell. The spins on each linear triad of iron atoms, separated by 3.267±0.004 Å, are ferromagnetically coupled and are essentially parallel. Triad spin components in the ac plane, related by the screw‐axis or glide‐plane operators, are antiferromagnetically coupled. Spin components parallel to the b axis are ferromagnetically coupled. The spins are very nearly normal to the a axis and are inclined 10° out of the ac plane, always in the same sense, forming a canted spin array. The magnetic moment per iron atom is 4.5±0.2 μ B . The effective ferromagnetic moment, based on this spin array, is 0.81 μ B in excellent agreement with Mays' NMR measurement.
44(1966); http://dx.doi.org/10.1063/1.1727028View Description Hide Description
The characteristic CH3 stretching frequencies in a number of selected compounds in carbon tetrachloride solution have been measured and compared with the nuclear 13C–H scalar coupling constant, J. The correlation provides a basis for expressing the C–H stretching force constant Kr in terms of J. Using a modified valence‐force‐field potential function for the CH3 group, Kr =4.74+8.8×10−3 (J−125) mdyn/Å.
The orbital contributions to Kr have been examined theoretically. Variation in fractional s character in the carbon orbital to hydrogen does not, of itself, lead to a significant variation in Kr . The force constant increases with increasing ionic character in the C–H bond, rather than as a result of hybridization changes.
44(1966); http://dx.doi.org/10.1063/1.1727029View Description Hide Description
The effect of volume exclusion on the mean‐square radius of gyration s 2 is treated by use of the empirical expression for the statistical distribution of s for a system of linear chain molecules unperturbed by volume exclusion. Segment interactions within the perturbed molecule are treated in the smoothed‐density approximation. The result is α5—α3=Azh(z/α3), where α2=〈s 2〉/〈s 2〉0, A is a numerical constant, z is the usual parameter proportional to the excluded volume for a segment pair and to the square root of the chain length, and h(ζ) is a function which decreases by almost one‐half as ζ increases from 0 to ∞. Asymptotic dependence of α2 on the ⅕th power of the chain length is thus confirmed. Extremely long chains, exceeding 106 bonds, would be required however for realization of this limiting behavior.
44(1966); http://dx.doi.org/10.1063/1.1727030View Description Hide Description
Mixtures with interactions consisting of a strong repulsion of short range and a weak long‐range attraction are studied. Results to the first order in an expansion parameter defined essentially as the ratio between the volume of repulsion and the volume of attraction are given for the correlation function and the equation of state. The basis for a numerical treatment of the results is outlined.
44(1966); http://dx.doi.org/10.1063/1.1727031View Description Hide Description
The ions SiH3 +, Si2H+, Si2H2 +, Si2H3 +, Si2H4 +, and Si2H5 + appear as products in ionized SiH4. All are formed from the attack of SiH2 + ions on SiH4 molecules. The specific reaction rates for the formation of these ions are, in units of cubic centimeters per moleule·second×1010, respectively, 1.7, 0.13, 0.67, 0.39, 1.1, and 0.068.
44(1966); http://dx.doi.org/10.1063/1.1727032View Description Hide Description
Vitreous sodium silicate has two quite distinct thermoluminescence maxima between 20° and 250°C, each having its own spectral composition (about 420 and 515 mμ, respectively). The peaks have equal activation energies (about 0.8 eV). It seems likely that, between 20° and 250°C, the thermoluminescence emission model for vitreous sodium silicate is a recombination of electrons trapped in a single energy level, with centers that probably consist of positive holes having different capture cross sections.
For the low‐temperature thermoluminescence, Kikuchi's suggestion appears to have been confirmed.
44(1966); http://dx.doi.org/10.1063/1.1727033View Description Hide Description
Measurements are reported of the optical properties of polystyrene in the wavelength region from 400 to 3000 Å. The real (n) and imaginary (k) parts of the complex index of refraction were obtained from reflectance measured at 20° and 70° incidence. The complex dielectric constant and the energy‐loss function were determined from these values of n and k. The energy‐loss function Im (1/ε), was found to exhibit maxima at 4.6, 5.2, 6.5, 8.5, and 19.2 eV. Sum rule calculations of the effective number of electrons per monomeric unit participating in these losses have been carried out.
44(1966); http://dx.doi.org/10.1063/1.1727034View Description Hide Description
The thermodynamic properties of a system with given intermolecular potential, including first quantum corrections, are shown to be identical to those for a classical system with an effective intermolecular potential. This technique, together with a statistical‐thermodynamic derivation of a molecular expression for the surface tension, enables one to calculate the classical and the first‐quantum‐corrected surface tension. The first quantum corrections are utilized to discuss the difference in the thermodynamic properties of isotopes. The agreement between the experimental and theoretical difference in surface tension of H2 and D2 is excellent. It has been found that the major difference is due to the difference in density along the vapor—liquid equilibrium line. The effect is similar though not as pronounced in the case of the energy of vaporization.
Observation of Conformational Isomers of the Tetraisopropylnitrobenzene Anion Radical by Electron Spin Resonance Spectroscopy44(1966); http://dx.doi.org/10.1063/1.1727035View Description Hide Description
The electron spin resonance spectrum of the anion radical obtained by electroreduction of 2,3,5,6‐tetraisopropylnitrobenzene has been studied extensively. The spectrum is interpreted in terms of two discrete conformational isomers existing in thermodynamic equilibrium. The isomeric difference arises from concerted interaction of the ring substituents which allows the nitro group to assume two different angular orientations with respect to the plane of the ring. These orientations are manifested as two different 14N isotropic coupling constants. In acetonitrile solution these values are 23.6 and 22.0 G for the more‐ and less‐twisted conformers, respectively. An apparent equilibrium constant for interconversion is defined and evaluated under a wide variety of solvent and temperature conditions. The free energy of conversion from the more‐ to the less‐twisted conformation is estimated as ΔG 298° = −5±1×102 cal mole−1 in acetonitrile. A necessary condition for the proposed equilibrium is that both species decay at the same rate and this condition is observed experimentally.
2,4,6‐Tri‐isopropylnitrobenzene anion radical was also studied. The nitrogen coupling constant was 20.6 G in acetonitrile, but no conformational isomerism was observed presumably because of the absence of buttressing 3,5‐substituents.
44(1966); http://dx.doi.org/10.1063/1.1727036View Description Hide Description
An extended analytical solution to the rate equations for the branching‐chain reactions of the H2–O2reaction under shock‐tube conditions is presented. Together with an experimental determination of the detection threshold for OH of the Bi (3067) line‐absorption method used in earlier experiments, this solution allows quantitative comparison of calculated and experimental ignition delays. It is shown that calculation and experiment can be brought into agreement with slight modification of chain‐reaction rate coefficients deduced from the results of other studies of the H2–O2reaction. The essential role of the rate of chain initiation by a path other than diatom dissociation in determining the ignition delays is brought to light. Rate‐coefficient expressions for the chain reactions are suggested and supported.
Absorption Spectra of the Solvated Electron in Polar Liquids: Dependence on Temperature and Composition of Mixtures44(1966); http://dx.doi.org/10.1063/1.1727037View Description Hide Description
The effect of temperature on the absorption spectra of the solvated electron in various liquid alcohols is reported. From 25° to −78°C, the energy corresponding to the absorption maximum increases by 0.23, 0.35, 0.36, and 0.46 eV for methanol, ethanol, isopropanol, and n‐butanol, respectively. Within experimental error there is no change in the width (in electron volts) of the absorption at half‐maximum.
The absorption spectra of the solvated electron in several binary mixtures at room temperature are reported. In all mixtures examined the spectrum has only one peak at wavelengths intermediate to the absorption maxima of the pure components, and the exact position depends on the mixture composition. In 50:50 mixtures of many of the materials used, the absorption maxima and half‐widths are closer to those of one pure component than of the other.
The fact that there is no evidence of two peaks in mixtures indicates that the solvation of the electron depends on the macroscopic properties of the solution in the sense that a particular electron interacts significantly with a large number of solvent molecules. However, the results indicate that at least in some mixtures the electron is associated on a microscopic scale preferentially with one component.
Partial spectra, at low temperature, of the solvated electron in the liquids diethyl ether, monomethylamine, and monoethylamine are also reported.
Operator Form for the Linear and Quadratic Jahn—Teller Coupling in Octahedrally Coordinated Eg Electronic States44(1966); http://dx.doi.org/10.1063/1.1727038View Description Hide Description
The operator form for the linear and quadratic Jahn—Teller effect in octahedrally coordinated Cu+ + is derived by consideration of the simultaneous effect of three tetragonal distortions. This leads to the same result as found by Liehr and Ballhausen, but in fewer steps. The final form of the operator is independent of the choice of phase of symmetrized distortion coordinates.
44(1966); http://dx.doi.org/10.1063/1.1727039View Description Hide Description
The ion—molecule reactions of H3 + and D3 + with methane, ethane, propane, isobutane, and n‐butane have been studied by high‐pressure mass spectrometry, using an ion source which employs the beta rays of tritium as the ionizing medium.
The rate constants have been measured, and proton transfer has been shown to be the major mode of interaction. If not stabilized by collision, protonated methane and protonated ethane decompose by elimination of hydrogen, with a lifetime of the order of 1 and 10−1 μsec, respectively. Protonated propane and protonated butanes, which have not been detected as stable species at pressures as high as 0.3 torr, decompose by elimination of hydrogen or a smaller saturated hydrocarbon to give an alkyl carbonium ion. Some results concerning the reactivity of these alkyl ions with deuterium and the hydrocarbons are also reported.
44(1966); http://dx.doi.org/10.1063/1.1727040View Description Hide Description
The adsorption and surface reaction of carbon monoxide and oxygen on clean films of palladium and nickel at room temperature at pressures in the range of 10−9−10−2 torr were studied by means of mass spectrometer and by measuring simultaneously electrical resistance.
CO preadsorbed on clean films of palladium and nickel reacted readily with gaseous O2 and no induction period was observed. Oxygen preadsorbed on palladium film immediately reacted with CO, but on nickel never reacted.
Some discrepancies between the results and those in the literature were discussed.
44(1966); http://dx.doi.org/10.1063/1.1727041View Description Hide Description
The polarized crystal spectra of cobalt‐doped MnF2 have been measured down to near 4.2°K. Three broad, anisotropic bands near 7200, 14 000, and 20 000 cm−1 are seen that are presumably due to the spin‐allowed 4 T 1g (F)→4 T 2g (F), 4 A 2g (F), and 4 T 1g (P) transitions of substitutional Co2+, 3d 7 ions. The D 2h symmetry provided by the rutile structure of the MnF2 lattice produced marked orthorhombic splitting of about 3000 cm−1 in the 4 T 1g (P) band. Several weak spin‐forbidden bands are also seen. The observed anisotropy indicates that the 4 T 2g (F) band is largely magnetic dipole while the other two are electric dipole. These latter parity‐forbidden transitions are aided by coupling with odd lattice vibrations. Application of the appropriate selection rules allows tentative identification of several of the states involved.
44(1966); http://dx.doi.org/10.1063/1.1727042View Description Hide Description
Measurements of the proton spin—lattice relaxation timeT 1 were carried out in samples of CH4 from 110°—90°K having an oxygen concentration of 1.08% and 2.54%, respectively. The results verify the inverse proportionality of T 1 and oxygen concentration.
44(1966); http://dx.doi.org/10.1063/1.1727043View Description Hide Description
This article deals with a remarkable phenomenon in chemical‐phase theory, i.e., the limited mutual solubility of gases, which was predicted by van der Waals and first demonstrated by Krichevskii in the nitrogen—ammonia system. The phase behavior in question is characterized by a peculiar course of the critical locus in some systems for which the components have a considerable difference in volatility. Under special conditions such a system showing limited miscibility may also exhibit the barotropic phenomenon, i.e., the equilibrium phases change position in the gravitational field.
Using the conditions Temkin derived from van der Waals' applications of Korteweg's fold theory, we could predict that the helium—xenon system probably exhibits gas—gas equilibria. We have confirmed the predicted behavior by visual observations of phase transitions in helium—xenon up to 2000 atm and 65°C.
44(1966); http://dx.doi.org/10.1063/1.1727044View Description Hide Description
From the Rouse—Zimm theory for the viscoelastic properties of dilute polymer solutions,stress relaxation curves were computed for vanishing and for dominant hydrodynamic interaction. The results were converted numerically to creep data by the method of Hopkins and Hamming. Since the steady‐state compliances and steady‐flow viscosities can be calculated separately, computation of the creep curves afforded a check on the accuracy obtainable with the Hopkins—Hamming method. The primary interest of this work lies in its relevance to the stress relaxation and creep behavior of undiluted bulk polymers.
44(1966); http://dx.doi.org/10.1063/1.1727045View Description Hide Description
The structure of chloropicrin, an unstable compound, has been studied by electron diffraction, using the sector‐microphotometer method. Final parameters have been obtained by means of a least‐squares fit of a structural model to the experimental intensity curve. The structure is similar to those found previously for fluoropicrin and bromopicrin. The −CCl3 group and the N–O bond have the expected values for bond lengths and angles. However, the C–N distance is very long, 1.594 Å; the ONO angle is 131.7°; the −CNO2 group is not planar; and the molecule assumes a staggered conformation similar to that found in ethane compounds with one position vacant. On the basis of a potential function of the form , where 0.75≤A≤1.0 and A+B=1, a barrier of 2.70–3.35 kcal/mole hindering free rotation about the C–N bond has been found.