Volume 21, Issue 5, 01 May 1953
Index of content:
A Semi‐Empirical Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules. II21(1953); http://dx.doi.org/10.1063/1.1699030View Description Hide Description
The theory of electronic spectra and electronic structure, the elucidation of which was begun in the first paper of this series, is further developed and applied to ethylene, butadiene, benzene, pyridine, pyrimidine, pyrazine, and s‐triazine.
A realistic and consistent LCAO‐MO π‐electron theory should allow the σ‐electrons to adjust themselves to the instantaneous positions of the mobile π‐electrons. This is accomplished in the theory by assignment of empirical values to the Coulomb electronic repulsion integrals and Coulomb penetration integrals which enter the formulas, these values being obtained in a prescribed way from valence state ionization potentials and electron affinities of atoms. Use of the empirical values in the molecular orbital theory reduces the magnitude of computed singlet‐triplet splittings and the effects of configuration interaction without complicating the mathematics. From the valence‐bond point of view, ionic structures may be said to be enhanced.
The applications to hydrocarbons and heteromolecules which are considered show that the theory can correlate known π‐electron spectral wavelengths and intensities very successfully, which, together with the simple structure of the theory, signals that manifold applications of the theory are in order elsewhere.
21(1953); http://dx.doi.org/10.1063/1.1699031View Description Hide Description
By use of an icecalorimeter precision measurements of the enthalpies of pure samples of KBr and KI between 300°C and their melting points were made. Values for the specific heats at constant pressure of these substances through the same temperature range were calculated and tabulated.
21(1953); http://dx.doi.org/10.1063/1.1699032View Description Hide Description
The critical thickness of a gas film that is just capable of withstanding a given negative pressure PN in a liquid column has been studied. The film was generated electrolytically on the surface of a spherical platinumelectrode of known area. The number of gas molecules corresponding to the measured charge required to release the negative column was then computed from the relation between the Faraday constant and Avogadro's number. The number of gas molecules required to form a monolayer on the electrode was next computed from the molecular diameter, based on van der Waals' equation. The ratio of these numbers gives the ``thickness'' t of the film in fractions of a monolayer.
Both O2 and H2 gas films were measured under conditions such that the same number of gas molecules were deposited on the same electrode and the corresponding critical negative pressures observed. Assuming the effectiveness of the film to be proportional to the area covered, the ratio of the diameters of the O2 and H2 molecules is inversely proportional to the square root of the ratio of their critical negative pressures. This gave O2/H2=1.11 as compared with 1.06 from measurements based on van der Waals' equation.
Measurements of oxygen films were made at various negative pressures, ranging from PN =90 cm of water, t=0.55 monolayer, to PN =4 cm, t=0.87 monolayer. The results lie on a smooth curve down to PN =10 cm, t increasing steadily as PN is reduced. Below PN =10 cm, the thickness of the film no longer increases, indicating that the zone of influence of the gas molecule extends beyond its projected area.
21(1953); http://dx.doi.org/10.1063/1.1699033View Description Hide Description
In connection with the theory of solutions proposed by Prigogine and Mathot [J. Chem. Phys. 20, 49 (1952)], total vapor pressures and volume changes on mixing have been measured for the systems: Volume changes only for the systems:
For the first three systems I, II, and III, one observes a positive excess free energy (positive deviations to the Raoult law) together with a negative excess volume (contraction on mixing), whereas for the last four IV, V, VI, and VII there is a gradual passage from negative (IV) to positive (VII) excess volume. These results are discussed in terms of the molecular interactions AA, BB, and AB. Reasonable agreement is observed between Prigogine and Mathot's theory and experiment.
21(1953); http://dx.doi.org/10.1063/1.1699034View Description Hide Description
With 85‐cm and 21‐foot normal‐incidence vacuum spectrographs the NO emission spectrum in the far‐ultraviolet region was investigated. The existence of the ε‐system is confirmed. The β′ bands are reinvestigated, and a curious appearance was observed in the (4,0) band. New bands which have an appearance similar to the β′ bands are observed in the region of 1500A, and tentative analysis is made.
21(1953); http://dx.doi.org/10.1063/1.1699035View Description Hide Description
The paper deals with a number of phenomena observed in lithium fluoride after formation of color centers by exposure to x‐rays, Van de Graaff electrons, or neutrons and not observed in other alkali halides under similar conditions. The most important of these phenomena are: (1) The appearance of a new band at 222 mμ, when the crystal is bleached with light of wavelength 254 mμ absorbed in the F band, and the inverse effect by subsequent irradiation with light of shorter wavelengths. (2) The production of F centers in a crystal heated to temperatures up to at least 450°C. The higher the temperature at which the F centers were produced, the greater is the intensity of the band at 222 mμ obtained by subsequent optical bleaching of the F band at room temperature. (3) The appearance of fine structure in a relatively weak band with peak at 380 mμ and of a very weak line at 523 mμ when the spectrum is recorded at the temperature of liquid nitrogen. (4) The very strong increase in the intensity of the band at 380 mμ and the line at 523 mμ by irradiation with short wavelength ultraviolet. (5) The sharpness at low temperatures of the first line of the band at 380 mμ and the line at 523mμ. At the temperature of liquid helium their half‐widths are of the order of 1A.
21(1953); http://dx.doi.org/10.1063/1.1699036View Description Hide Description
The compounds Mg2TiO4, MgTiO3, MgTi2O5, CaTiO3, SrTiO3, and BaTiO3 were prepared in crystalline form by reactions in the solid state. The rod‐shaped samples were sintered in oxygen at 1450°C and reduced in hydrogen at temperatures from 560°C to 1400°C producing n type semiconductors.Resistivity values from 10−1 to 108Ω·cm at 25°C were obtained. Reductions of about 3 percent by weight could be given the most reduced magnesiumtitanates while the largest reduction obtainable in CaTiO3, SrTiO3, or BaTiO3 was less than 0.25 percent.
The logarithm of resistivity was plotted versus reciprocal absolute temperature and showed a linear dependence in most cases over a wide range of temperature and resistivity. The curves for BaTiO3 changed the slope abruptly at approximately 115°C, probably attributable to the crystal structure change at the Curie temperature, about 120°C.
The dependences of impurity activation energy and Fermi level on percent reduction were determined for the case of Mg2TiO4. The Biltz‐Meyer rule, stating a linear relationship of impurity activation energy with resistivity at a reference temperature, was not obeyed over the large range of reductions obtained.
21(1953); http://dx.doi.org/10.1063/1.1699037View Description Hide Description
The thermal diffusion data on argon have been examined on the basis of the Lennard‐Jones 12:6 model by reducing the experimental data through a more appropriate variation of RT with temperature than has been done hitherto. The value of the force constant ε, thus determined from the observed variation of RT with temperature, is found to increase slightly with temperature. This variation of ε shows the inadequacy of the model in the entire range of temperatures but gives a good idea of its effective range of usefulness. These values of ε have been utilized to give r 0 from the viscosity and self‐diffusion data. From these the force constants μ and λ have also been evaluated and are likewise found to vary with temperature. These variations of ε, r 0, μ, and λ have been explained theoretically and are shown to be in accord with experimental data on the second virial coefficients of argon. The force constants for O2, N2, CH4, and CO have also been similarly determined and are found to agree with other determinations.
21(1953); http://dx.doi.org/10.1063/1.1699038View Description Hide Description
Raman spectra of chloroacetone were measured in the liquid and solid states and in solutions. Infrared absorption spectra of this substance were measured in the gaseous, liquid, and solid states and in solutions. The temperature dependence of the intensity of the Raman lines and of the absorption bands were measured in the liquid state. From the experimental results together with the dipole data it was concluded that chloroacetone exists in two molecular forms in the liquid and gaseous states and in one form in the solid state. The less stable form in the liquid state becomes much more abundant than the other form in the gaseous state. Some considerations on the nature of hindering potential of internal rotation have been made.
21(1953); http://dx.doi.org/10.1063/1.1699039View Description Hide Description
The velocity of compressional waves in liquid water (H2O and D2O) has been measured over a range of temperature extending to some six degrees below the nominal freezing point, using a fixed‐frequency variable‐path sonic interferometer. The density of heavy water in the supercooled state has been determined and values of the adiabatic compressibility calculated. The curves representing the variation of density with temperature are symmetrical and congruent for the two isotopic liquids. It is confirmed that the compressibility of D2O is greater than that for ordinary water. The new measurements of the three properties yield values which fall on smooth extrapolations of the previously known data, indicating that there is no partial ordering of the liquid structure into the solid structure at temperatures below the nominal freezing point, at least none at times removed from the start of observed freezing.
21(1953); http://dx.doi.org/10.1063/1.1699040View Description Hide Description
Pure siliconcarbide was prepared by decomposition of the vapors of silicon tetrachloride and toluene on a hot carbon filament. Various n‐ and p‐type impurity atoms were introduced by the addition of volatile metallic chlorides during the preparation. For the pure substance either n‐ or p‐type crystals could be obtained by varying the ratio of the reaction constituents. The results show that semiconductivity in siliconcarbide may be due either to nonstoichiometric proportions of silicon and carbon, or to the presence of impurity atoms, or (as probably occurs in the commercial product) to both circumstances simultaneously.
A theoretical model for electronic conduction in siliconcarbide is proposed, which satisfactorily explains the magnitude of the low and high temperature activation energies observed, their different ratios in n‐ and p‐type crystals, the occurrence of conduction at one of the disturbance energy levels, and the anomalous low Hall constant at low temperatures found in black but not in green siliconcarbide. The model may be applicable to other semiconductors in which disturbance centers can give rise to electrons in unlocalized excited states.
21(1953); http://dx.doi.org/10.1063/1.1699042View Description Hide Description
The microwave spectrum of AsF3 is re‐examined under high resolution permitting the evaluation of small interactions due to centrifugal distortion and I·J effects, as well as a more precise determination of the rotational constant and quadrupole interaction. The constants determined are: B = 5878.971±0.002 Mc, (eqQ)As = −236.23±0.05 Mc, DJK = −9.±2. kc, a = −6.±7. kc, b = 6.±2. kc, α2 = −5.±1. Mc, and α4 = −0.16±0.04 Mc. In addition, the bond angle is estimated from the quadrupole interaction by means of a comparison with AsCl3, yielding ∠F–As–F = 102°±2°.
Rotation‐Vibration Spectra of Diatomic and Simple Polyatomic Molecules with Long Absorbing Paths. X. The Spectrum of Tri‐Deuteromethane in the Photographic Infrared21(1953); http://dx.doi.org/10.1063/1.1699043View Description Hide Description
The absorptionspectrum of CD3H has been investigated under high resolution in the photographic infrared with absorbing paths of up to 7 m atmos obtained by multiple reflection. Two simple parallel bands were observed at 8870A and 11590A. The K structure in the former was clearly resolved. The rotational structure of the two bands was analyzed yieldingOn the assumption of tetrahedral symmetry for the CD3H molecule this value leads to a C–H distanceThis value is compared with the corresponding values obtained by other authors from studies of CH4 and CH3D.
21(1953); http://dx.doi.org/10.1063/1.1699044View Description Hide Description
The term ``sensitized luminescence'' in crystalline phosphors refers to the phenomenon whereby an impurity (activator, or emitter) is enabled to luminesce upon the absorption of light in a different type of center (sensitizer, or absorber) and upon the subsequent radiationless transfer of energy from the sensitizer to the activator. The resonance theory of Förster, which involves only allowed transitions, is extended to include transfer by means of forbidden transitions which, it is concluded, are responsible for the transfer in all inorganic systems yet investigated. The transfer mechanisms of importance are, in order of decreasing strength, the overlapping of the electric dipole fields of the sensitizer and the activator, the overlapping of the dipole field of the sensitizer with the quadrupole field of the activator, and exchange effects. These mechanisms will give rise to ``sensitization'' of about 103−104, 102, and 30 lattice sites surrounding each sensitizer in typical systems. The dependence of transfer efficiency upon sensitizer and activator concentrations and on temperature are discussed. Application is made of the theory to experimental results on inorganic phosphors, and further experiments are suggested.
21(1953); http://dx.doi.org/10.1063/1.1699045View Description Hide Description
Four low‐J rotational transitions of 0316 have been measured in the region from 42 to 118 kmc. The rotational spectroscopic constants found are A = 106,530.0±1.1 Mc, B = 13,349.06±0.06 Mc, C = 11,834.3±1.1 Mc. The oxygen nuclei form an isosceles triangle with an apex angle of 116°49′±30′, and the two equal internuclear distances are 1.278±0.003A. From the Stark splitting of the 20,2→11,1 transition the dipole moment is 0.53±0.02 debye. Zeeman splittings of this transition show that ozone is not significantly paramagnetic.
21(1953); http://dx.doi.org/10.1063/1.1699046View Description Hide Description
The extension of the Teller‐Redlich product rule to the substitution of different atoms or groups, which was previously applied to the methyl benzenes, is here applied to the halogenated methanes. The rule is found to hold accurately enough to be useful in assigning fundamental frequencies or in computing unobserved fundamentals.
21(1953); http://dx.doi.org/10.1063/1.1699047View Description Hide Description
The energy of the π—π part of the double bond is calculated by (1) the perturbation method using the ``united atom'' model, and (2) the MO method where the wave function used is constructed as a linear combination of atomic orbitals, as a function of internuclear distance. In each case the wave function includes the 3p atomic orbital. The results of the investigation indicate (1) that the wave function ordinarily used in the MO method is not appreciably improved by the inclusion of the 3p atomic orbital, and (2) that the absolute value of the energy calculated by the MO method using p functions centered at the nuclei, is at least 20 percent too low at the distances occurring in the C–C bond.
21(1953); http://dx.doi.org/10.1063/1.1699048View Description Hide Description
Symmetry coordinates are set up from the standpoint of group theory for describing the normal modes of oscillation of the XY 5 molecular model in such a manner that maximum factorization of the secular determinant is accomplished. The cubic and quartic portions of the anharmonic potential function are derived, and the components of vibrational angular momentum are set down. The complete valence‐type potential function is discussed. Explicit relations are derived between the force constants occurring in the secular determinant and the physical valence force constants.
21(1953); http://dx.doi.org/10.1063/1.1699049View Description Hide Description
The quantum mechanics is applied to the calculation of the intrinsic probability of excitation of vibration and rotation of an idealized F2O molecule by a neon atom. Comparisons are made of the excitation of the three normal vibrations from the zeroth to the first states. The probabilities of excitation of rotation accompanying vibrational excitation are explicitly shown and discussed. Further, excitations involving a change of two and four vibrational quantum numbers are approximated.
21(1953); http://dx.doi.org/10.1063/1.1699050View Description Hide Description
Specific heat data on diamond at temperatures between 20° and 300°K are reported. Comparisons of the data have been made with the predictions of the Debye theory. The deviation of the specific heat from the value given by the three‐dimensional continuum theory in the low temperature region can be qualitatively described in terms of a superposition of a simple Einstein frequency. It has been found that variations that occur in expressing reduced characteristic temperatures as a function of reduced temperatures for diamond are qualitatively similar to those of the face‐centered cubic metals Al, Cu, and Ag.
The values of entropy,enthalpy, and free energy have been determined and tabulated at integral values of temperature from 25° to 300°K. The entropy of diamond at 298.16°K is 0.568±0.005 cal/g‐atom/deg.