Volume 36, Issue 11, 01 June 1962
Index of content:
36(1962); http://dx.doi.org/10.1063/1.1732382View Description Hide Description
The observation of V bands in the absorption spectra of pressed disks prepared from KBr, KCl, and NaCl are recorded. The V centers were produced by two methods: (a) by heating a powdered mixture of the alkali halide and cupric chloride, (b) by heating the alkali halide powder previously soaked in halogen vapor at room temperature. In both cases the powders were heated to about 100°C under reduced pressure; the presence of air interfered with the reaction.
36(1962); http://dx.doi.org/10.1063/1.1732383View Description Hide Description
The thermal conductivity of dissociating N2O4 involving both the reactions N2O4⇌2NO2 and 2NO2⇌2NO+O2 has been measured between 100° and 200° at a pressure below 1 atm. The absolute hot‐wire method and an all‐glass apparatus with a glass diaphragm manometer has been employed. The results are, however, not in good agreement with the values calculated on the assumption of local chemical equilibrium for both the reactions. Reasons for this discrepancy have been discussed.
36(1962); http://dx.doi.org/10.1063/1.1732384View Description Hide Description
An estimate is made of the enthalpy of formation of oxygen vacancies in barium oxide. This estimate is derived from known thermochemical data together with published data on defect equilibria and on the electronic work function of barium oxide. The result provides confirmation of previously published calculations based on the method of Mott and Littleton.
36(1962); http://dx.doi.org/10.1063/1.1732385View Description Hide Description
Mulliken's atomic gross charge and overlap populations are computed for the C3, C4, and C2N2 molecules. These populations are then compared with those of related molecules. The comparison is given in terms of s, p, and σ, π‐atomic populations and σ, π‐intramolecular charge transfer. The σ and π charge transfers are found to reinforce one another (one‐way charge transfer) in homonuclear molecules and oppose one another (two‐way charge transfer) in heteronuclear molecules. In addition, for the one‐way case, the σ, π charge transfer is not as large as for the two‐way case. It is pointed out that there are correlations between population analysis and orbital energies. This analysis of the electronic distribution is based on recent computations of simple and preliminary wave functions in the self‐consistent field one‐electron approximation.
36(1962); http://dx.doi.org/10.1063/1.1732386View Description Hide Description
High‐resolution infrared spectra of crystalline ethylene and fully deuterated ethylene have been obtained. Accurate frequencies of all band components observed at 65°K are presented. Appearance of one gas‐phase inactive fundamental in the crystal spectrum, and the structures of certain multiplets, shows that the presently accepted crystal structure is not correct. This conclusion is in agreement with that from a previous study under lower resolution.
Intermolecular Coupling of Vibrations in Molecular Crystals. III. Intermolecular Potential and Crystal Structure of Ethylene36(1962); http://dx.doi.org/10.1063/1.1732387View Description Hide Description
Intermolecular force constants and vibrational frequency splittings are calculated for ethylene and deuteroethylene on the assumption of certain types of intermolecular potentials. The results of a calculation based on intermolecular hydrogen repulsions as the mechanism of vibrational coupling are in reasonable agreement with the observed band splittings. To a limited extent electrostatic (dipole) interactions are of importance.
In the calculations detailed consideration of the as yet uncertain crystal structure were necessary, and the results lend support to the P21/n space group for ethylene at 65°K.
The hydrogen repulsion parameter obtained from analysis of the data was found to be consistent with a theoretically obtained result and with values obtained from independent experimental studies. A certain amount of information about the variation of repulsive potential with distance is now available from crystal studies.
36(1962); http://dx.doi.org/10.1063/1.1732388View Description Hide Description
The spinless first‐order transition density between two atomic states with quantum numbers L′,ML′, and L″,ML″ is expanded in spherical harmonicsIt is shown that the dependence of the expansion coefficients on ML′ and ML″ for fixed L is completely determined by the use of the Wigner‐Eckart theorem, so that all (2L′+1) (2L″+1) expansion coefficients of a given order L are known in terms of one of them. The same is true for the expansion coefficients of the first‐order transition spin density. These quantities are calculated for all terms of the electron configurations snpm and the results collected in tables.
36(1962); http://dx.doi.org/10.1063/1.1732389View Description Hide Description
Radiationless transitions of a solute molecule embedded in a crystalline solvent are treated in close analogy to radiative transitions. A semiclassical interaction Hamiltonian employing empirically determined coupling constants connects solute particles to a force field amplitude set up by the phonons of the solvent. By relating this amplitude to phonon energy density an Einstein B coefficient is derived. Debye's formula for the phonon energy density combined with thermodynamic arguments yield Einstein A coefficients. It is shown that the theory (1) explains the fast rates of radiationless transitions, (2) provides for temperature dependence, (3) has a cutoff frequency for radiationless transition, (4) provides for the Franck—Condon principle, and (5) explains the lack of selection rules. A value is derived for the empirical coupling constants.
36(1962); http://dx.doi.org/10.1063/1.1732390View Description Hide Description
New spectrographic observations in the vacuum‐ultraviolet region of the pink nitrogen afterglow are reported. Nitrogen bands representing 13 ev of energy, N I lines of 10.6 ev energy, and, upon the addition of Hg,Hg I lines of 10.1 ev and Hg II lines of 7.5 ev were observed with the 1st positive and 2nd positive N2 bands and 1st negative N2 + bands characteristic of the pink glow. The excitation of the nitrogen bands is discussed. Quenching of the pink glow by an acelectric field is related to the excitation mechanisms producing this glow.
Organic Liquid Scintillators. III. On the Quantum Yield of Fluorescence and the Quenching of Fluorescence by Oxygen36(1962); http://dx.doi.org/10.1063/1.1732391View Description Hide Description
Expressions for the quantum yield of fluorescence of aromatic hydrocarbons in benzene and alkylbenzenes have been derived assuming zero internal quenching. These expressions indicate that the molecular quantum yield of fluorescence increases with (a) the lifetime of the first excited singlet state of the solvent, (b) the transition probabilities from the levels of the solute that correspond to the levels involved in the excitation of the solvent [A. Heller, J. Chem. Phys. 35, 1980 (1961)], and (c) the increase in the degree of degeneracy in the solute levels corresponding to those involved in the excitation of the solvent.
Minimum values of the molecular quantum yield of fluorescence of a group of aromatic hydrocarbons have been computed at different concentrations of the solutes. The compounds with the highest values are p‐divinylanthracene, perylene, p‐quaterphenyl and p‐terphenyl.
The dimensions of the excited aggregates that are formed by the exchange interaction between the molecules in liquid or plastic scintillators are estimated from Basile's data for solutions of PBD in polyvinyltoluene [L. S. Basile, J. Chem. Phys. 27, 803 (1957)]. The number of monomer units per aggregate is about 1.2×103 and the radius of interaction about 18 A.
The quenching of fluorescence by oxygen is explained by the existence of π‐electron level pairs of almost similar energy difference in the solvent and the solvent—oxygen complex [H. Tsubormura and R. S. Mulliken, J. Am. Chem. Soc. 82, 5966 (1960)], and by the short lifetime of the complex compared with that of the solvent. The latter results from the lower order of symmetry of the complex in comparison with that of the π‐electron system in benzene.
36(1962); http://dx.doi.org/10.1063/1.1732392View Description Hide Description
The 3–0 band of the CO ``triplet'' system has been photographed under high resolution and a rotational analysis carried out. The upper state is shown to be 3Δ in agreement with electron‐configuration theory and not 3Π as indicated by previous work. The spin structure in the 3Δ state has been studied and the intensity distribution in the branches is discussed. Revised constants for the d 3Δ state are given.
36(1962); http://dx.doi.org/10.1063/1.1732393View Description Hide Description
Crystalline powders of the aromatic amino acids and the enzyme trypsin were exposed at 77°K to Co‐60 γ rays, and the spectral distribution (between 3000 and 6500 A) of the thermoluminescence emitted upon subsequent heating was determined using band‐pass filters. At a temperature corresponding to a particular peak in a plot of emission intensity vs T, a large fraction of the emission (except from phenylalanine) consisted of one characteristicspectral component. In general, the proportion of emission at the longer wavelengths increased with increasing temperature. The thermal‐activation energies of thermoluminescence, estimated from plots of lnIT vs 1/T (about 0.2 ev for the amino acids and 0.1 ev for trypsin), are lower than similar determinations at comparable temperatures for alkali halide crystals. The γ‐ray induced thermoluminescence from amino acid crystals is composed of longer wavelengths than the uv‐excited fluorescence (at room temperature) or long‐lived phosphorescence (at 77°K). The possible limitations within which lattice imperfections might account for these effects are discussed briefly. The spectral results for trypsin provide additional evidence that, in accord with previous predictions, at least some of the electronic rearrangements arising from γ‐ray interaction become preferentially localized in only a restricted number of configurations. The metastable species which persist at 77°K likely depend upon the native conformation of the protein, and the available evidence suggests that the decay of an excited triplet state of tryptophan accounts for the majority of the trypsin emission. Even so, the activation energies and spectra indicate that the initial trapping and untrapping does not occur at the aromatic amino acid residues. Accordingly, mechanisms are considered whereby electronic rearrangements at other sites in trypsin could lead to the excitation of an aromatic residue followed by phosphorescence from a tryptophan residue.
36(1962); http://dx.doi.org/10.1063/1.1732394View Description Hide Description
The infrared spectra of gaseous MnCl2, FeCl2, CoCl2, NiCl2, CuCl2, and CoBr2 have been measured in absorption between 220 and 4000 cm—1. With the exceptions of MnCl2 and CuCl2, the emission spectra of these molecules have also been obtained in this region. The emission from unsaturated vapor indicates that one of the two absorptions observed for each of the dihalides, other than manganous chloride, is attributable to a vibration of a polymeric species. Assuming that the monomeric dihalide molecules have D∞h symmetry, the monomeric absorption is assigned to ν3, the antisymmetric stretching vibration. The M—X bond stretching force constants calculated using the linear model and a simple valence force field are quite similar to those of the corresponding diatomic monohalides. Both sets of constants show a maximum between manganese and zinc.
36(1962); http://dx.doi.org/10.1063/1.1732395View Description Hide Description
The charged fragments resulting from the decay of CH3I131 have been measured by a specially designed mass spectrometer. In 70% of the decays, the parent ion CH3Xe+ remained intact in direct contrast to the decay of CH3T in which hardly any CH3He+ was found [A. H. Snell and F. Pleasonton, J. Phys. Chem. 62, 1377 (1958)].
More than 20 other ions were also detected including the doubly charged methyl xenon (CH2Xe)+2. Their relative abundances are discussed in terms of excitation resulting from the sudden change in nuclear charge, and from multiple Auger processes in cases where internal conversion accompanies the decay.
36(1962); http://dx.doi.org/10.1063/1.1732396View Description Hide Description
Infrared spectra of gaseous MoF6 and TcF6 from 6–43 μ, the Raman spectrum of MoF6 gas, and an incomplete Raman spectrum of liquid TcF6 are reported. These are interpreted in terms of the octahedral point group 0 h . The fundamental frequencies for MoF6 are 741(ν1), 643(ν2), 741(ν3), 264(ν4), 306(ν5), and 190(ν6). For TcF6 they are 705(ν1), 551(ν2), 748(ν3), 265(ν4), 255(ν5) and ν6 has not been determined.
Differences between the spectra of MoF6 and TcF6 are explained as a consequence of orbital degeneracy in the ground state of the latter, in analogy with similar differences in the 5d series.
36(1962); http://dx.doi.org/10.1063/1.1732397View Description Hide Description
The infrared‐absorption spectrum from 6–43 μ and the Raman spectrum of ReF6 gas are reported. They are interpreted in terms of the octahedral point group 0 h . The six fundamental frequencies are 755(ν1), 596(ν2), 715(ν3), 257(ν4), 246(ν5), and 193(ν6). Thermodynamic functions are calculated for the ideal gaseous state.
36(1962); http://dx.doi.org/10.1063/1.1732398View Description Hide Description
A study has been made of the effect of applied electric fields upon the absorptionspectrum of a dye molecule (methyl red) dissolved in a relatively inert, transparent, matrix of high dielectric strength (polystyrene). Theory indicates that the existence of a large dipole moment in either ground or excited state (or in both, if the moment differs in the two states) should lead to a broadening of the absorption band. In contrast, excited states of appropriate type lying close to that involved in the transition should produce a shift to the red. In the present instance the second shift dominates, and one observes, with a field of about one million v/cm, a shift of around 10 Å of the principal band which is centered near 5000 Å. Possible uses of this technique are in evaluating the local field at an absorbing molecule and in estimating dipole moments of various states.
36(1962); http://dx.doi.org/10.1063/1.1732399View Description Hide Description
Partial molal enthalpies,entropies, and internal energies are reported for hydrogen in liquid‐benzene solutions at temperatures up to 250°C, pressures up to 150 atm, and hydrogen concentrations up to 13 mole %. Both the cohesive energy density ratio a 12/a 11 and the pressure at which the entropy of mixing becomes ideal, turn out to be roughly independent of temperature and hydrocarbon type.
36(1962); http://dx.doi.org/10.1063/1.1732400View Description Hide Description
The radiolysis of CH3CD3 has been investigated with particular emphasis on the effect of pressure, temperature, and addition of xenon on the molecular hydrogen formation. The hydrogen formed by molecular detachment was determined by irradiating CH3CD3 in the presence of C2H4, NO, O2, or I2. The formation of molecular hydrogen in the unimolecular decomposition of the parent ion and in the decomposition of the neutral excited ethane molecule are considered. Methane formed in the presence of scavengers may be attributed to an ion‐molecule reaction or to an intramolecular process similar to that proposed in the direct photolysis of ethane. Evidence is presented for occurrence of a reaction producing ethylene in addition to the one in which molecular hydrogen accompanies the formation of ethylene.
36(1962); http://dx.doi.org/10.1063/1.1732401View Description Hide Description
The photolysis of CH2N2 and of CD2N2 in CO2 at temperatures near 50°K has been studied using infrared techniques. Evidence has been obtained for the production of one or more intermediates by the reaction of CH2 with CO2. These intermediates are precursors of H2CO+CO. Possible structures for such species are proposed and discussed.