Index of content:
Volume 19, Issue 10, 01 October 1951
The Compressibility of Gaseous Xenon. I. An Equation of State for Xenon and the Weight of a Liter of Xenon19(1951); http://dx.doi.org/10.1063/1.1747999View Description Hide Description
The compressibility of xenon containing 0.14 mole percent of krypton has been measured from 16.65° (the critical temperature) to 300°C and over the density range 1 to 10 mole per liter. The constants of the Beattie‐Bridgeman equation of state for the sample used and for pure xenon have been determined from these measurements. The constants for pure xenon are R=0.08206, A 0=4.6715, a=0.03311, B 0=0.07503, b=0, c=30.02×104 in units of normal atmos, liter per mole, and °K (T°K=t°C+273.13). The weight of one liter of Xe at a pressure of one standard atmosphere is calculated from its molecular weight (131.3) and the above parameters to be 5.897 g per liter at 0°C and 5.467 g per liter at 70°F.
19(1951); http://dx.doi.org/10.1063/1.1748000View Description Hide Description
The second and third virial coefficients of xenon have been determined from our compressibility measurements over the temperature range 16.65° (the critical temperature) to 300°C. They are listed in Table II. The parameters of the Lennard‐Jones (6, 12) potential for xenon are μ=5.583×10−58 erg‐cm6, λ=2.514×10−102 erg‐cm12, ε=309.9×10−16 erg, b 0=84.65 cm3 per mole, θ=224.5°K, σ=4.064×10−8 cm, r 0=4.561×10−8 cm. The representation of the second virial coefficient is quite satisfactory. The third virial coefficients agree moderately well with the theoretical values for the Lennard‐Jones (6, 12) potential tabulated by Bird, Spotz, and Hirschfelder for (T/θ)>1.5 but diverge rapidly as the temperature approaches the critical. The observed second virial coefficients agree moderately well with the values tabulated by Buckingham and Corner for an inverse sixth power attractive and an exponential repulsive potential when the calculated values are read from the table for the parameters given by Kane from data on the solid state of xenon and from Born and Mayer's value of the exponent derived for neon.
19(1951); http://dx.doi.org/10.1063/1.1748001View Description Hide Description
Apparatus and techniques are described for obtaining reliable absolute dielectric constants by the Wyman resonance cell method.
Calcium hydride has been shown to be a drying agent for benzene far more satisfactory than sodium wire, reducing drying time from the six‐month period demonstrated in the literature to an interval approximating one day.
At 25°C the values for benzene's dielectric constant were 2.2747±0.0003, but the higher limit is presently preferred. These results fall within the range of the best recent values obtained in other laboratories. The reasons for the discrepancies appear explicable in view of the findings of this investigation.
19(1951); http://dx.doi.org/10.1063/1.1748002View Description Hide Description
A method of obtaining the equation of state of a classical assembly through the solid, liquid, and gas phases based on the order‐disorder concept is explained. At first, the method is illustrated on an imaginary two‐dimensional model. Next, the general procedure to increase the number of interstitial sublattices to infinity is explained for a one‐, two‐, and three‐dimensional lattice. In the one‐dimensional case, the present method proves to be identical with Gürsey's rigorous solution. An approximate treatment for the solid state is added.
19(1951); http://dx.doi.org/10.1063/1.1748004View Description Hide Description
Light transmission measurements have been made on ethane, and on a mixture of methane and propane near the critical point, using the 4050, 4360, and 5460A lines from a mercury lamp. Isothermal compressibilities have been calculated. This appears to be a considerably more accurate method than the use of p‐v‐t data for the prediction of compressibilities near the critical point.
19(1951); http://dx.doi.org/10.1063/1.1748005View Description Hide Description
Ethylene, acetylene, and butadiene are the predominant gaseous products of the reaction of sodium vapor with vinyl iodide in the ``diffusionflame.'' From the experimental evidence it is likely that these are formed by the disproportionation and recombination of vinyl radicals. In the presence of hydrogen, ethylene is also formed by the reaction C2H3+H2=C2H4+H. More vinyl iodide than sodium is consumed in the reaction and a nonvolatile organic iodide is formed. This has an important bearing on the calculation of the rate constant of the primary process.
19(1951); http://dx.doi.org/10.1063/1.1748006View Description Hide Description
The photolysis of acetic anhydride probably by 1849A radiation has been studied in the temperature range 80° to 300°C at pressures of 10–60 mm dibutyl phthalate. From 80° to 224°C the rate is first order during a run but the specific rate decreases with increasing pressure. The gaseous products are CO, CO2 and C2H6 in approximately equivalent amounts. Biacetyl and methyl acetate have been identified among the liquid products. At 300°C the reaction is more complex, yields considerable amounts of H2 and CH4 and is not first order. Comparison with formic acidphotolysis leads to an approximate quantum yield of 2. A mechanism is suggested which accounts qualitatively for the observations. Deactivation of activated reactant molecules by acetic anhydride and by the gaseous products accounts for the low quantum yield. Added C2H6 and CO2 are shown to increase the rate of photolysis, the more so the lower the pressure of acetic anhydride. Preliminary experiments have shown that the reaction can also be mercury sensitized at a much faster rate at comparable temperatures but yielding the same products in similar amounts.
19(1951); http://dx.doi.org/10.1063/1.1748007View Description Hide Description
The ionization potentials of methane, and of the four deuterated methanes, have been determined by electron impact using two 90° mass spectrometers with different types of sources. In addition, the ionization potentials of acetylene, deuterated acetylene, ethylene, and deuterated ethylene have been measured. A simple empirical method is used to obtain values which are reproducible to ±0.02 volt. For the methanes the ionization potential is found to increase with the number of deuterium atoms in the molecule, with a difference of 0.18 volt between the value for CH4 and that for CD4. Although this difference is found to be the same on each of the mass spectrometers, there is a difference of 0.11 volt between the absolute values of the ionization potential as determined for CH4 on the two instruments. In order to explain the difference between the ionization potentials of CH4 and CD4 on the basis of zero point energy differences, it is estimated that the force constants in the ionic state would need to have only about ¼ of their value in the ground state. Since this seems unreasonably low, it is suggested that the difference between the ionization potentials of the methanes, as measured by electron impact, may be caused, in part, by excess energy required for a ``vertical'' transition.
19(1951); http://dx.doi.org/10.1063/1.1748008View Description Hide Description
Measurements of the nuclear magnetic shielding of F19 and H1 are reported for a number of their respective binary covalent compounds. The applied fields required for the F19 and H1 nuclear magnetic resonances, at fixed frequency, vary in these compounds over a range of 3.98 and 0.122 gauss, respectively, at fields of about 6365 gauss. These ranges are of the same order of magnitude as the theoretical internal diamagnetic corrections for the unbound fluorine and hydrogen atoms. The dependence of nuclear magnetic shielding on the electronic structures of the molecules is discussed qualitatively and some general correlations indicated. One such correlation is that the magnetic shielding of the F19 nucleus decreases with increasing electro‐negativity of the atom to which the fluorine is bonded. This and other correlations differ in degree for the proton. The differences may arise from the relatively greater importance of the diamagnetic shielding term for the proton. Specific information regarding molecular and electronic structure is provided in several instances. The tetragonal pyramid structure for IF5 is confirmed and a similar structure demonstrated for BrF5. The appearance of double F19resonance lines in PF5, IF5, and BrF5 indicates nonequivalent electronic distributions in the structurally distinguishable bonds. The measurements were facilitated by a large permanent magnet, the design and field homogenization of which are described.
19(1951); http://dx.doi.org/10.1063/1.1748010View Description Hide Description
A three‐dimensional mobile electron model for unsaturation electrons in aromatic molecules is described which allows leakage of electrons across rings and takes aromatic atoms explicitly into account. The model contains the free electron model as a limiting case. Applications to the spectra of cata‐condensed hydrocarbons are discussed.
LCAO Molecular Orbital Computation of Resonance Energies of Benzene and Butadiene, with General Analysis of Theoretical Versus Thermochemical Resonance Energies19(1951); http://dx.doi.org/10.1063/1.1748011View Description Hide Description
The decrease in π x ‐electron energy for the change from a Kekulé to a proper benzene structure is computed purely theoretically by the method of antisymmetrized products of MO's (molecular orbitals), in LCAO approximation, using Slater 2pπ x AO's (atomic orbitals) of effective charge 3.18, and assuming a carbon‐carbon distance of 1.39A. The result (73.1 kcal/mole) is a theoretical value for the gross (vertical) resonance energy of benzene taken for constant C–C distances of 1.39A. In order to make a comparison with the net or ordinary empirical resonance energy, several corrections to the latter are required. The principal one is for the ``compression energy'' required to compress the single and stretch the double bonds of the Kekulé structure from normal single and double‐bond distances to 1.39A. The others (not hitherto clearly recognized) involve hyperconjugation and related effects. The corrections are discussed and their magnitudes estimated, but a reliable value can be obtained only for the compression energy. Allowing for this alone, the computed net resonance energy is 36.5 kcal. This agrees, within the uncertainties due to the omitted correction terms, with the value (41.8 kcal) of the ``observed'' resonance energy Δ based on thermochemical data.
Here Δ is the departure of the actual heat of formation ΔHf 0 of benzene from the value given by a standard formula for nonresonating hydrocarbons. A new standard formula containing corrections for the mutual effects of neighboring carbon‐carbon bonds is given; this is of interest also for itself, and its significance is briefly discussed.
The analysis given in the paper serves to clarify hitherto existing obscurities in what is meant by ``resonance energy.'' An analysis of the nature and significance of ``nonresonating'' structures (like for example Kekulé benzene) is also included, using He2, 1,3 butadiene, and benzene as examples. Repulsion terms in the LCAO MO theory occur for nonresonating structures, which appear to be the counterpart of exchange repulsions of the valence‐bond theory. When resonance occurs, it more or less overcomes these repulsions.
The gross resonance energies of cis‐ and trans−1,3‐butadiene are computed by the same method. For transbutadiene, the computed value corrected for compression is 3.7 kcal/mole, while the ``observed'' thermochemical value Δ is 6.5.
19(1951); http://dx.doi.org/10.1063/1.1748012View Description Hide Description
The band system between 3100 and 3900A which has been attributed both to HNO2 and enhanced NO2 is shown to belong to the former molecule by means of the isotope effect. Measurements of the spectrum of DNO2, and a partial interpretation of the band structure and isotope shift are given.
19(1951); http://dx.doi.org/10.1063/1.1748015View Description Hide Description
Scintillations in two series of structurally related organic compounds were investigated. One series was composed of the symmetrical diphenyl derivatives of ethane, ethylene, and acetylene. The other series consisted of the diphenyl polyenes from diphenyl to diphenyl octatetraene. In the first series it was found that the wavelength of the emission spectra ranged from 3550 to 4030A and the counting efficiency increased with the number of π‐electrons in the system. In the second series the emission spectra wavelengths ranged from 3400A to 6000A and increased as one went up the series, whereas the counting efficiency went through a maximum at diphenyl butadiene. The results were interpreted in terms of the ``metallic'' model for conjugated compounds.
19(1951); http://dx.doi.org/10.1063/1.1748016View Description Hide Description
In amorphous and hexagonal selenium optical absorption and photoconductivity were studied in their dependence on temperature. The absorption edge shifts for amorphous selenium toward shorter wavelengths with decreasing temperature. For a temperature change from 300° to 90°K and a layer thickness of 98μ, for example, a shift from 6600 to 6100A is observed at an optical density of 3; for higher densities (shorter wavelengths) the shift is smaller. It can be shown that this shift is related to the thermal excitation of vibrational levels. At higher temperatures the population of the higher vibration levels increases and the separation of these from the excited state diminishes.
At low temperature (ca 90°K) photoconductivity is observed only after the light quanta exceed 2.5 ev and the absorption coefficient has reached 105 cm−1. Increase of temperature brings about a shift of the photoconductive threshold toward longer wavelengths, paralleling the absorption‐edge shift. The absorption coefficient has been followed over the wavelength range 6900 to 2100A; its values at the end points of this range are 1.6×102 and 5.2×105 cm−1, respectively; there is a slight maximum at 2600A where the absorption coefficient reaches 5.6×105 cm−1.
The transformation of amorphous into hexagonal selenium by heat brings about an increase of the optical absorption in the 5000 to 7200A region and the appearance of a distinct hump in the absorption edge at 5200A. Decrease of temperature sharpens the hump and causes a decrease of absorption at all wavelengths in the 4000 to 7200A range except in the vicinity of the hump. In hexagonal selenium photoconductivity is observed at wavelengths as long as 7500A for films 0.5μ thick. A peak in the photoresponse is found where the optical density of these films reaches ca 1 (10 percent transmission). It can be shown that this behavior is to be expected for a bimolecular law of recombination of electrons and holes when their inhomogeneous distribution is taken into account. With decreasing temperature the peak response shifts consequently toward shorter wavelengths with the absorption edge.
19(1951); http://dx.doi.org/10.1063/1.1748017View Description Hide Description
Molecular orbital calculations of the quadrupole moments of N2 and O2 have been made. Models assuming 20 percent s—p hybridization for N2 and 5–10 percent s—p hybridization for O2 give calculated quadrupole moments agreeing with those deduced from microwave line broadening data (0.27e×10−16 esu for N2 and <0.09e×10−16 esu for O2).
19(1951); http://dx.doi.org/10.1063/1.1748018View Description Hide Description
The spectra of the 1:1 complexes between benzene and iodine and between benzene and bromine were examined in the wavelength range 2350–3500A using a 64‐micron cell. In both cases a new absorption band was found at 2600A. This band is of considerable theoretical interest in connection with previous discussions of the spectra and structures of the benzene‐halogen complexes. Also, the solution spectrum of iodine has been extended to 1820A and the f value compared with predictions.
Studies of Ionization Efficiency. I. The Determination of Molecular Appearance Potentials Using the Mass Spectrometer19(1951); http://dx.doi.org/10.1063/1.1748019View Description Hide Description
The factors affecting the shape of ionization efficiency curves can be separated into two categories: those due to instrument design, and those depending on the molecular structure of the substance studied. By taking precautions to eliminate the former, details of individual fine structure can be detected and measured in the curves. The methods for the determination of appearance potentials in use at the present time are discussed, and the results for a number of substances measured by the principal methods are compared with spectroscopicionization potentials. It is found that the critical slope method gives values in all cases within 0.3 volt of the spectroscopic figure, whereas the results of the other methods may be in error.