Volume 18, Issue 3, 01 March 1950
Index of content:
18(1950); http://dx.doi.org/10.1063/1.1747613View Description Hide Description
Magnetic susceptibilities per gram atomic weight of elements 92 to 95 in most of their oxidation states were measured at 20°C on 0.1 ml of solution which was 0.005 to 0.09M in heavy element. The values obtained (all paramagnetic) in units × 106 were: U(IV), 3690; Np(VI), 2060; Np(V), 4120; Np(IV), 4000; Pu(IV), 1610; Pu(III), 370; Am(III), 720. The results could be interpreted only on the basis of electronic configurations 5fn , even though susceptibilities were generally lower than the theoretical values and lower than experimental values for corresponding lanthanide 4fn cations. The lower values should be expected as a result of the Stark effect produced by electric fields of anions and of water dipoles. Failure of the Russell‐Saunders approximation to the coupling between electrons may account for some of the error in the theoretical calculations. That the susceptibilities of Pu(III) and Am(III) are manyfold lower than those of Sm(III) and Eu(III), respectively, is attributed to wider multiplet splitting in the actinide atoms.
18(1950); http://dx.doi.org/10.1063/1.1747614View Description Hide Description
Although Holt, McLane, and Oldenberg state that they were unable to observe a sharp cut in the absorptionspectrum of hydrogen peroxide vapor, it is shown that their measurements indicate a rather abrupt increase of the absorption coefficient at 1920A.
It is doubtful that the dissociation energy of hydrogen peroxide can be derived from its absorptionspectrum. However, it seems possible that the photo‐chemical decomposition of hydrogen peroxide gives an upper limit for its dissociation energy.
Our measurements of the absorption coefficient of hydrogen peroxide agree well with the results of Holt, McLane, and Oldenberg.
18(1950); http://dx.doi.org/10.1063/1.1747615View Description Hide Description
This paper is a generalization of Part V of this series. We show here that the usual solution thermodynamics can be recast into an equivalent but alternative form which is more natural and useful in adsorption work. Relations between the two systems of thermodynamics are pointed out. The extension to the case of sorption from a mixture of gases is included.
The Occurrence of Hypsochromic Shifts on Alkyl Substitution: Structure and Color of Methylated Derivatives of Azulene18(1950); http://dx.doi.org/10.1063/1.1747616View Description Hide Description
While two methylated derivatives of azulene show the usual bathochromic shift with respect to azulene itself, the remaining three isomers show an unusual displacement of the visible spectrum toward shorter wave‐lengths. No steric effects can be claimed to account for these last results which are considered as representing genuine hypsochromic shifts brought about by alkyl substitution and due to the particularities of the structure of azulene and its derivatives. It is then shown by molecular orbital method calculations that Mulliken's N→V theory of color accounts satisfactorily for the observed facts. In the methylated derivatives showing a bathochromic shift, the calculated excitation energy of the N→VI transition appears to be smaller than the corresponding energy in azulene itself, while the reverse is true for the compounds exhibiting a hypsochromic shift. The general conditions necessary for the occurence of hypsochromic shifts upon alkylation and a few predictions of such shifts in other hydrocarbons are given.
18(1950); http://dx.doi.org/10.1063/1.1747617View Description Hide Description
Gold smokes made by evaporation in an inert gas atmosphere are initially in a thermodynamicallyunstable state. A theory based on diffusion of vacant sites to the surface of the crystallites has been developed to describe the process by which the deposit approaches equilibrium. Measurement at different temperatures of the change of resistance associated with the loss of vacant sites gives a value of 23.5 kcal./g atom for the activation energy of this process.
When heated above 100°C the deposits sinter rapidly and undergo pronounced changes in optical properties. However, a gold smoke deposit can be stabilized, by heat treatment at 70°C, so that sintering proceeds at an appreciably slower rate.
Some Studies in Molecular Orbital Theory I. Resonance Structures and Molecular Orbitals in Unsaturated Hydrocarbons18(1950); http://dx.doi.org/10.1063/1.1747618View Description Hide Description
The predictions of qualitative resonance theory, as to the electronic structures of unsaturated hydrocarbons, are analyzed in terms of LCAO molecular orbital theory. The following results are proved for ``alternant'' hydrocarbons, that is, hydrocarbons containing no odd‐membered unsaturated rings:
(1) The number of unpaired electrons present in the ground state is at least as great as the number of carbon atoms having a deficiency of valence bonds in any principal resonancestructure.
(2) With a few special exceptions, these odd electrons are distributed over just those atoms which have a deficiency of valence bonds in one or more of the principal resonancestructures.
(3) In singly charged hydrocarbon anions or cations the ionic charge is located on just those atoms which bear charges in the various principal resonancestructures.
It follows that enumeration of the principal resonancestructures of a hydrocarbon molecule or ion gives much valuable information as to its electronic structure; and in particular, an alternant hydrocarbon possessing no normal Kekulé‐type structure must have a paramagneticground state, and behave as a radical or multiradical.
These results do not depend on the neglect of overlap, or the assumption of a constant carbon‐carbon resonance integral.
Some Studies in Molecular Orbital Theory II. Ionization Constants in Heteroaromatic Amines and Related Compounds18(1950); http://dx.doi.org/10.1063/1.1747619View Description Hide Description
The relative basicities of a series of mono‐aza‐aromatic amines are calculated by a very simple application of LCAO molecular orbital theory. Rough quantitative agreement between theory and experiment is obtained if the heterocyclic nitrogen atom is supposed to take up the proton; on the other hand, if the amine group is assumed to add the proton in the first ionization, the observed basicities are negatively correlated with theory.
It is also shown how the qualitative differences in reactivity between methyl‐aza‐hydrocarbons and between heteroaromatic phenols may be understood in similar very simple terms.
18(1950); http://dx.doi.org/10.1063/1.1747620View Description Hide Description
It is shown that LCAO molecular orbital theory, when combined with first‐order perturbation theory, provides a very simple procedure for estimating how far electronic effects will modify the transition state energies for substitution reactions in alternant molecules. This procedure is shown to lead to the same qualitative predictions as resonance theory, but has the advantage that it gives at the same time quantitative information as to the relative effects of substitution at different points in a complex system. The theoretical results agree well with experiment as far as can be ascertained at present.
The approximations involved, and their relation to those of resonance theory, are discussed in detail.
The Effect of the Electrostatic Polarization of the Solvent on Electronic Absorption Spectra in Solution18(1950); http://dx.doi.org/10.1063/1.1747621View Description Hide Description
Regarding the solvent as a continuous dielectric medium, it is shown that its effect on the Franck‐Condon absorption of light by solute molecules must be expressed in terms of the electronic polarization part of its dielectric constant,K = n 2. Using methods based both on quantum theory and on classical dispersiontheory, it is shown that the red shift of absorption in solution depends directly on f, the oscillator strength, and inversely either on a 3 (a is the radius of the spherical solute molecule) or the polarizability α. The expression Δν(cm−1)=const. (f/νa 3)[(n 2−1)/(2n 2+1)] with two possible values of the constant, and alternatively with the substitution of α for a 3, is tested on experimental data for isoprene, benzene, bromine, and iodine. Good quantitative agreement is obtained for the (V, N) transitions of isoprene and benzene. If the strong ultraviolet absorption of bromine and iodine solutions is regarded as the displaced (V, N) transition, the quantitative agreement is poor, although qualitatively in accordance with the theory. The weak λ2600 system of benzene, and the visible continua of bromine and iodine, show the expected smaller Δν with smaller f, although quantitative comparison with theory is prevented by the superposition of other solvent effects which become important in weak absorption bands.
The Vibrational Spectra of Molecules and Complex Ions in Crystals III. Ammonium Chloride and Deutero‐Ammonium Chloride18(1950); http://dx.doi.org/10.1063/1.1747622View Description Hide Description
The infra‐red spectra of thin non‐scattering films of NH4Cl and ND4Cl were obtained at 28°, −78° and −190°C. A convenient low temperature transmission type cell usable for such films is described. No indication of fine structure due to free rotation of the NH4 + ions was found. Instead, evidence is presented for the existence, both above and below the λ‐point, of a torsional lattice mode involving the NH4 + ions. The limiting frequencies of the torsional oscillations were observed at about 390 and 280 cm−1 for NH4Cl and ND4Cl, respectively. These values agree quite well with the frequencies calculated on the basis of a purely electrostatic potential function. The spectra of the low temperature modifications indicate strongly that the structures belong to the space group Td 1 in which the NH4 + ion symmetry is Td . Of the eight observed bands, two are assigned to the triply degenerate fundamentals ν3 and ν4, one to the overtone 2ν4, one to the combination ν2+ν4 which resonates strongly with ν3, one to the combination of the totally symmetric mode, ν1, with the limiting lattice frequency, ν5, and two to the combinations involving the lattice torsional mode, ν6, i.e., ν4+ν6 and ν2+ν6. The spectra of the room temperature modifications are consistent with a structure in which the NH4 + ion tetrahedra are randomly distributed between the two possible equilibrium orientations in each unit cell. The λ‐point transformations are probably simple order‐disorder transitions between the two modifications.
The Vibrational Spectra of Molecules and Complex Ions in Crystals IV. Ammonium Bromide and Deutero‐Ammonium Bromide18(1950); http://dx.doi.org/10.1063/1.1747623View Description Hide Description
The infra‐red spectra of thin non‐scattering films of NH4Br and ND4Br were obtained at 28°C, −78°C and −190°C. The spectra of both compounds in Phase III are in agreement with the x‐ray symmetry, D 4h 7. The torsional oscillation frequencies occur at 319 cm−1 and 230 cm−1 in NH4Br and ND4Br, respectively. The spectra of Phase II are similar to those of the chloride and indicate a structure in which the NH4 + ions are disordered in their equilibrium orientations. There is no evidence of free rotation at any temperature studied. The spectra support x‐ray evidence for a further transition at −100°C in ND4Br to Phase IV which has symmetry Td 1 as in NH4Cl.
18(1950); http://dx.doi.org/10.1063/1.1747624View Description Hide Description
Reflection coefficients are calculated for a matter wave in a parabolic channel and in a square channel, each channel closed at one end by a vertical plane tilted slightly from the normal to the axis of the channel. The number of collisions necessary to dissipate one quantum of vibrational energy is given in terms of λ=tanθ, where θ is the angular deviation of the plane end from the normal to the axis of the channel. Comparison with the data from measurements on sound dispersion shows good agreement for N2O and CO2. The temperature dependence of the average probability of transition per collision is compared with data for Cl2 and a mixture of Cl2 and CO.
18(1950); http://dx.doi.org/10.1063/1.1747625View Description Hide Description
The specific magnetic susceptibilities of manganese‐activated zinc fluoride phosphors of diverse activator concentration, and prepared by various crystallization procedures, have been measured from −60°C to 200°C. A trivial Weiss constant, except for phosphors of inordinately high manganese concentration, demonstrates the absence of appreciable exchange demagnetization of adjacent paramagnetic activator ions and suggests that there exists negligible departure from a random distribution of manganese ions over cation sites. Utilizing this distribution, and recognizing that only manganese ions not having other manganese ions at any of the nearest cation sites are capable of luminescence, an expression for the dependence of luminescent efficiency on activator concentration is derived and found to be in quantitative agreement with experiment.
18(1950); http://dx.doi.org/10.1063/1.1747626View Description Hide Description
The infra‐red absorption of CF2:CH2 gas has been investigated between 2 and 22.7μ with a prism spectrometer of high resolution. The Raman spectrum of the compound in the gaseous state has been photographed with a three‐prism glass spectrograph of linear dispersion 15A/mm at 4358A. The 12 fundamental vibration frequencies have been assigned and the observed spectra have been interpreted in detail.
18(1950); http://dx.doi.org/10.1063/1.1747627View Description Hide Description
The rotational transition J=0→J=1 of the isotopic species C13H3Cl, C12H2DCl, and C12HD2Cl have been investigated by microwave absorption. The data have been used to obtain the ground state dimensions of the methyl chloride molecule. The effects of zero‐point vibration are rather large, and do not permit very accurate results for the dimensions of the methyl group: C–H=1.103±0.006A, ≰ H–C–H=109.8°±0.5°.
18(1950); http://dx.doi.org/10.1063/1.1747628View Description Hide Description
A sample of n‐pentforane, C5F12, of high purity was made available by the operation of the electrochemical process for the production of fluorocarbons. The physical properties of this material were determined, and a study was made of the two‐component, liquid‐gas system of it with n‐pentane, C5H12. Deviations from Raoult's law were found which were much greater than could be predicted by existing theories for solutions of nonpolar substances. In order to correlate the experimentally determined properties of these mixtures, it was necessary to extend the existing theories. This was done by assuming a new concept of possible significance in solubility relationships. This concept is pictured as the interpenetration of the molecules in the condensed state. For the particular system herein studied the interpenetration between hydrocarbon molecules is much greater than between fluorocarbon molecules, and the interpenetration between the fluorocarbon and hydrocarbon molecules is as small or smaller than that between fluorocarbon molecules. This additional concept has provided a means for correlating the measurements.
Substituted Methanes. I. Raman and Infra‐Red Spectral Data, Assignments, and Force Constants for Some Tribromomethanes18(1950); http://dx.doi.org/10.1063/1.1747629View Description Hide Description
As a part of a systematic investigation of the vibrational spectra of substituted methanes and ethanes, Raman displacements, semiquantitative relative intensities, quantitative depolarization factors, and infra‐red absorption frequencies in the region 700–5000 cm−1 have been obtained for CBr3H, CBr3Cl, and CBr4. Details about the design of the infra‐red spectrograph are given. The Raman and infra‐red spectral data in the literature for CBr3H, CBr3D, CBr3F, CBr3Cl, and CBr4 have been collected, tabulated, and critically examined in comparison with the present results and a decision was made as to the probable values of the Raman and infra‐red data at the present time. Assignments of the observed Raman and infra‐red bands, consistent with the selection rules, were made for all of the molecules. Finally, force constants were calculated for the five molecules, using a potential energy function containing all possible second degree terms.
18(1950); http://dx.doi.org/10.1063/1.1747630View Description Hide Description
The infra‐red absorptionspectrum of freshly prepared dimethyldiacetylene has been obtained for the vapor existing above the solid or liquid at temperatures of 25°, 40°, 60°, 80°, 85°, and 110°C; for the gas at 130°C; and for CCl4 and cyclohexane solutions. A Beckman IR−2 spectrophotometer with rocksalt optics was used. The observed values of the Ā 2 frequencies were 940, 1369, 2157, and 2920, as compared with the calculated values 903, 1368, 2131, and 2946 cm−1 given previously by Meister and Cleveland. A frequency assignment based upon the D3h ′ structure is given: One of the 20 distinct fundamental frequencies (10 non‐degenerate, 10 doubly degenerate) corresponds to the internal free rotation; the other nine non‐degenerate and seven of the doubly degenerate fundamentals are rather certainly assigned and tentative values are given for the remaining three. All of the other observed Raman and infra‐red bands could be accounted for as binary combinations, except the 2225‐cm−1 Raman line and the 2228‐cm−1 infra‐red band which seem to originate from the coincidence of a large number−43 and 40, respectively—of ternary combination frequencies. The sample used melted at 64.95±0.03°C, corresponding to a purity of 99.77±0.05 mole percent. A description of the method of preparation is given.
18(1950); http://dx.doi.org/10.1063/1.1747631View Description Hide Description
The potential barrier to the internal rotation in methyl alcohol is recalculated from the entropy with the aid of new molecular dimensions generously provided by Burkhard and Dennison. The barrier calculation is examined for temperature dependence and checked for reliability by recalculation with the Clapeyron equation substituted for parts of the data. The result is 1600±700 cal./mole, which includes the new spectroscopic barrier of Burkhard and Dennison at 326 cm−1. Within the range covered by the result, a residual entropy up to 0.50 cal./mole deg. is possible, and the retention of 0.46 e.u. by the crystal is consistent with a reasonable simple assumption about the distribution of hydrogen bonds.
It is suggested, by analogy, that the barrier to the hydroxyl group rotation in ethyl alcohol should be about 1000 cal./mole.
On the Non‐Orthogonality Problem Connected with the Use of Atomic Wave Functions in the Theory of Molecules and Crystals18(1950); http://dx.doi.org/10.1063/1.1747632View Description Hide Description
The treatment of molecules and crystals by the Heitler‐London method or by the collective electron model can be based on the atomic orbitals φμ of the system. These orbitals are in general overlapping, and the corresponding overlap integrals S μν, given by (1), have almost universally been neglected in the literature as causing undesirable complications. Here we will take these overlap integrals into consideration and show that they, instead of being negligible, are of essential importance in molecules and in crystals. The problem is simply solved by considering the orthonormalized functions [open phi]μ, given by (21), as the real atomic orbitals. The solution is worked out in detail for (I) the molecular orbital method of treating molecules, (II) the Bloch orbital method of treating crystals, and (III) the Heitler‐London method of treating both these systems in some simple spin cases. Some numerical applications are given for ionic crystals, showing that the overlap effects are responsible for all the repulsive forces in these solids. It is also shown that the overlapping adds interesting new features to the properties of molecules and crystals, namely the ``many‐orbital‐effects,'' corresponding to certain ``many‐body‐forces'' in ionic solids.