Index of content:
Volume 24, Issue 4, 01 April 1956
24(1956); http://dx.doi.org/10.1063/1.1742588View Description Hide Description
Empirical correlations between X–H≡Y distance R and frequency shift of the X–H stretching motion Δv are presented for O–H≡O, N–H≡O, and N–H≡N hydrogen bonds. A linear correlation is possible provided a distinct straight line is assumed for each hydrogen bond type. The equations of the best straight lines and standard deviations are:
24(1956); http://dx.doi.org/10.1063/1.1742589View Description Hide Description
The entropies of 17 oxy‐anions have been calculated in the (hypothetical) gas state. The gas entropies for these ions have been combined with their molal entropies in aqueous solution, S̄ 0, where available, to calculate the entropies of hydration of 14 of these ions. The entropies of hydration of these ions having the formula, XOn —m , can be represented by the empirical equation ΔS hyd=5+(29—4.5n)Z 2.
Method for the Determination of the Surface Tension of Solids, from Their Melting Points in Thin Wedges24(1956); http://dx.doi.org/10.1063/1.1742590View Description Hide Description
A method is proposed which allows one to measure the surface tension of solids whose liquids wet glass and steel and whose solid/liquid contact angle against those materials is zero.
A thin wedge, formed of optically flat steel and glass slabs, is filled with liquid substance and then allowed to solidify. It is then immersed in a large heat bath whose temperature is raised at a rate of about 1.5°C per 24 hours, and is known at any time within ±0.002°C. The solid substance starts melting from the thinnest part of the wedge, well below its ``normal'' melting point Tm . From the lowering of the melting point ΔT corresponding to a measured thickness h of the wedge in the solid/liquid interface, the surface tension of the solid σ S is then arrived at from the equation:where Qf is the heat of fusion in erg/g, ρ S the density of the solid, and σ L the surface tension of the substance in liquid state.
The method was applied to stearic and myristic acids. For myristic acid four independent measurements gave an average value of σ S =116±10 erg/cm2, all four samples showing similar orientation of the crystal lamellae (gross crystal orientation) with respect to the interface solid/liquid. For stearic acid three similar gross orientations gave an average σ S =179±8 erg/cm2. One gross orientation, markedly different from the others, yielded the value of σ S =135±6 erg/cm2.
Thickness h was determined from the interference lines (sodium light). The gross orientation was observed in polarized sodium light.
24(1956); http://dx.doi.org/10.1063/1.1742591View Description Hide Description
A model is proposed for monolayers composed of linear macromolecules, particularly at an oil/water interface. If the polymer at the interface is sufficiently flexible, the layer will consist of anchored portions connected by bridges of segments lying out of the plane in which the anchored segments are deposited. An equation of state for the monolayer is derived and its limiting form is compared with those due to Singer and Bull. The limiting form of our equation of state seems to be in satisfactory agreement with some published experimental data. These results suggest the possibility of obtaining rough indications of the shape and mean dimensions of the macromolecule at the interface from film studies at both the air/water and oil/water interface. Finally the application of this model to explain the apparent dissociation of proteinmonolayers is indicated.
24(1956); http://dx.doi.org/10.1063/1.1742592View Description Hide Description
The infrared spectra of gaseous ethylene oxide and ethylene oxide‐d 4 in the range 375—3800 cm—1 and the Raman spectra of the liquids have been determined. With the help of the infrared band contours and the product rule, a complete vibrational assignment has been made for both molecules. The assignments correlate well with those of cyclopropane and cyclopropane‐d 6.
24(1956); http://dx.doi.org/10.1063/1.1742593View Description Hide Description
The effect of electric ions in water upon the proton magnetic relaxation is discussed. Formulas pertaining to solutions of paramagnetic and nonmagnetic ions are derived. The viscosity factor occurring in the theory for uniform systems has to be replaced by an average of inverse local viscosities weighted by the dimensions of the moving particles or by individual diffusion constants. The concept of mutual viscosity—describing the interaction between two unlike liquid layers—is helpful in correlating the numeric results.
The influence of temperature and concentration has been studied experimentally. The behavior of trivalent ions like aluminum can be understood by assuming hydration. The effect of monovalent ions can be explained on the basis of a slightly decreased diffusion constant of the free water molecules. Treated as a two‐phase system, the experimental results enable the evaluation of the average time of adsorption. For aluminum, this is one‐hundredth of a second.
24(1956); http://dx.doi.org/10.1063/1.1742594View Description Hide Description
The theory of Kirkwood and Auer is extended to include the dependence of the viscoelasticmoduli of solutions of rod‐like macromolecules on rate of shear. Power series expansions in the rate of shear of the intrinsic viscosity and intrinsic rigidity are constructed. In the stationary case, the intrinsic viscosity is presented as a function of rate of shear with use of coefficients of the pertinent spherical harmonics kindly supplied by H. H. Scheraga.
24(1956); http://dx.doi.org/10.1063/1.1742595View Description Hide Description
The scintillation counting behavior of a group of fifty‐five pure crystalline organic compounds has been extensively studied. The data obtained have been analyzed with the goal of developing a better understanding of the scintillation process and of scintillator behavior.
The materials were carefully purified, and massive crystals were grown. Relative scintillation average‐pulse‐height efficiencies at 30°C and —70°C for cobalt‐60 gamma‐ray excitation, gamma‐ray excited scintillation decay times, and 2537 A ultraviolet‐excited reflection and transmission photofluorescence spectra have been determined. A few solutions were also studied for comparison purposes. The purification and properties of the different materials are discussed in detail.
The experimental data have been analyzed on the basis of Birks' photon cascade theory of the scintillation process. The ratio of the scintillation efficiency to the integrated photofluorescence intensity is shown to be a measure of Birks' primary photon production efficiency. The primary photon efficiencies vary over a relatively narrow range, so that there is a rough general correlation between the scintillation efficiencies and the integrated photofluorescence intensities. The best phosphors are those with rigid molecular and crystal bonding so that both the primary photon production and photofluorescence efficiencies are reasonably high. There is a tendency for the best scintillators to have the simplest and most sharply defined photofluorescence spectra.
The scintillation efficiencies have been correlated with molecular structures and the mobility of the π electrons (or with the resonance interactions) within the molecules. In the simpler cases, consideration of singly charged quinoid structures permits the calculation of parameters which can be directly correlated with the scintillation efficiencies. In more complex cases, steric hindrance, hyperconjugation, ``unshared'' and ``nonbonding'' electron pairs, five‐membered aromatic rings, heavy atom (triplet‐state) effects, and quinoid (quenching) ground states must be considered. In all cases, the effects of thermal (vibrational) perturbations are present. ``Bond density'' and ``bound valence'' values have proved to be useful in correlating scintillation phenomena and molecular structure effects.
Strongly colored compounds are usually poor scintillators. Earlier predictions of high scintillation efficiency for quinquephenyl and sexiphenyl are reaffirmed, and coronene, benzo[ghi]‐perylene, and 1,2,5,6‐dibenzanthracene are also predicted to be significantly better than the presently known organic scintillators. The further study of compounds containing five‐membered aromatic rings appears promising.
A generally good correlation has been found between the scintillation efficiencies and molecular diamagnetic anisotropy values. Some relationships between the scintillation decay times and the molecular structures can also be seen.
The best crystalline organic scintillators are colorless substances of high melting point possessing molecules of simple structure and low atomic number in which there is extensive resonance conjugation of rings, ethylenic double bonds, and other groups to give extended, rigidly interlocked systems.
24(1956); http://dx.doi.org/10.1063/1.1742596View Description Hide Description
The electrification produced by the freezing of dilute aqueous solutions was studied in an attempt to determine (1) the factors governing which ion is rejected by the ice phase, and (2) the interrelationship of concentration, freezing rate, observed potential, and ionic segregation between the phases for sodium chloride, which appears to be typical. It was shown that ionic rejection is not on the basis of ion size alone, though this is doubtless important. Ion segregation increases with freezing rate over the range studied. Observed interphase potential goes through a maximum as concentration is increased; at a given concentration, the potential also goes through a maximum as freezing rate increases. The freezing rate corresponding to this maximum potential is lowest in the vicinity of the concentration yielding the highest potential. The effects of concentration and freezing rate on the potential‐time curves were also noted. Chemical analyses of the phases was complicated by self‐electrolysis of the solution. Physical interpretations of some of these phenomena are offered.
24(1956); http://dx.doi.org/10.1063/1.1742597View Description Hide Description
A number of deuterated pyrroles have been prepared and their microwave spectra recorded. For each isotopic species the two 1→2 transitions observable around 20 000 MHz were localized together with a number of Q‐lines in the 17 400—26 000 MHz region scanned. For all isotopic molecules rotational constants were calculated. In this way seven mutually independent numerical data were obtained which is insufficient for a direct calculation of the nine geometrical parameters of pyrrole. Additional assumptions as to the length of the two C, H bonds lead to definite models for pyrrole.
24(1956); http://dx.doi.org/10.1063/1.1742598View Description Hide Description
The relative rates of isomerization of cyclopropane and monotritiated cyclopropane have been determined in the temperature range of 447° to 555°C. The ratio of the rate constants for the two reactions can be fitted by the equation:
The marked increase in the temperature‐independent term upon tritium substitution is discussed in terms of Slater's unimolecular reaction rate theory and Eyring's absolute reaction rate theory. The possibility of classical vibration coupling causing the observed effect is suggested.
24(1956); http://dx.doi.org/10.1063/1.1742599View Description Hide Description
The theory of the cell model is developed in its most general form, allowing an arbitrary volume and a variable number of occupants to each cell. Kirkwood's principle of minimizing the free energy is used to determine the best cell distribution functions and thermodynamic functions. It is shown that the isotherms computed by Lennard‐Jones and Devonshire are subject to two large but almost compensating corrections. It is proposed to eliminate the inconsistencies of the cell model by introducing de Boer's concept of cell clusters, which in principle allows an exact evaluation of the partition function. Numerical corrections for clusters of two cells are calculated and shown to be small in effect.
24(1956); http://dx.doi.org/10.1063/1.1742600View Description Hide Description
The mechanism of the hyperfine splitting observed in paramagnetic resonance spectra of certain aromatic free radicals containing protons adjacent to the aromatic ring is examined. It is shown that the magnitude of the interaction between an unpaired electron in a π orbital and a vibrating hydrogen atom is insufficient to account for the observed splitting. Such a vibrational mechanism is considered to be untenable: (i) on the basis of quantitative calculations, (ii) by the failure to observe lines corresponding to the first excited vibrational state, and (iii) by a comparison of the splitting caused by protons and deuterons. It is suggested that the unpaired electron is not in a purely π state and that the splitting may be accounted for by configuration interaction between π and δ states.
24(1956); http://dx.doi.org/10.1063/1.1742601View Description Hide Description
The homogeneous gas phase reaction between diborane and phosphine at 0°C, yielding a solid product of composition BH3PH3, has been observed to give a linear relation between the initial rate of reaction and the pressure of either reactant, provided a certain pressure of each reactant, determined by the equilibrium constant and the pressure of the other gas, is exceeded. The reaction rate is independent of the amount of surface of the solid product or of the wall. When either reagent is in appreciable excess, the rate‐time data obey a pseudo first‐order equation.
These facts are consistent with a rate‐controlling reaction of diborane with phosphine to give BH3PH3 and BH3, the borine reacting further with phosphine, and the BH3PH3 condensing rapidly to a solid. This mechanism leads to a rate equation which is in good agreement with the experimental evidence. The activation energy of the rate‐controlling step is 11.4±2.0 kcal mole—1.
24(1956); http://dx.doi.org/10.1063/1.1742602View Description Hide Description
Self‐diffusion in liquidmercury was determined by a shear cell method. At atmospheric pressure,D=8.5 × 10—5 exp(—1005/RT) cm2 sec—1 from 0° to 98°. At 30° for pressures ranging up to 8366 kg cm—2, log10 D = —0.572Tm/T—4.348, or log10 D=—4.7889—9.637×10—6 P (kg cm—2). The activation volume is 0.587 cm3. The pressure dependences of viscosity and diffusivity are identical within experimental error, but the temperature variations are not equal. The results are discussed in terms of an imperfect crystal lattice model of a liquid.
24(1956); http://dx.doi.org/10.1063/1.1742603View Description Hide Description
The quadrupolar hyperfine structure of the Al27 and Be9nuclear magnetic resonance peaks in beryl crystal at 300°K have been examined. From the observed spectra, the following values were obtained for the nuclear spinsI, the effective nuclear magnetic moments μ, the nuclear quadrupole coupling constants eQ[open phi] zz , and the electric fieldasymmetry parameters η:
24(1956); http://dx.doi.org/10.1063/1.1742604View Description Hide Description
The absorption and emission spectra of azulene have been studied in a substitutional solid solution in napthalene at 20°K. The spectra are polarized and are very sharp, and from the polarizationproperties of the absorption transitions it has been determined that the first excited state at 14 652 cm—1 is Lb , 1 B 1 [we will designate an electronic state by giving first the Platt symbol (J. R. Platt, J. Chem. Phys. 17, 484 (1949) and then the C2v group symbol, Herzberg, Infrared and Raman Spectra of Polyatomic Molecules (D. Van Nostrand Company, Inc., New York, 1945)] (the transition with the ground state is transversally polarized), whereas the second excited state at 28 050 cm—1 is La , 1 A 1 (the transition with the ground state is long‐axis polarized), in agreement with the theoretical predictions of Platt and Moffitt. Vibrational analyses have been performed for the two absorption transitions, and the second transition appears to be perturbed by vibrational‐electronic interaction.
A sensitized emission spectrum has also been found, in which excitation of the napthalene leads to emission from the second excited state of azulene to the ground state. A vibrational analysis of the emission transition yields ground state vibrational frequencies which are in excellent agreement with the vibrations deduced from the infrared absorptionspectrum.
Environmental effects are discussed, and an attempt is made to correlate the vibrations in the different electronic states.
24(1956); http://dx.doi.org/10.1063/1.1742605View Description Hide Description
It is shown that an electron spin polarization associated with an odd electron in a π‐molecular orbital in an aromatic free radical can, through an atomic exchange coupling mechanism, produce an appreciable resultant electron spin polarization in s‐atomic orbitals at the aromatic protons. This resultant spin polarization gives rise to hyperfineinteractions with aromatic protons which are of the same order of magnitude as those observed in the paramagnetic resonance spectra of aromatic free radicals. An equation is developed which relates (proton‐spin)‐(electron‐spin) hyperfine coupling constants to the π distribution of the odd electron in the aromatic free radicals.
24(1956); http://dx.doi.org/10.1063/1.1742606View Description Hide Description
The thickness of shock fronts in carbon dioxide and nitrous oxide gases has been measured by the light reflectivity method. The thickness of these fronts are in agreement with thicknesses predicted theoretically when only shear viscosity and heat conductioneffects are considered. They demonstrate that the contribution of any real bulk viscosity is small. The molecular vibrations are not excited in the shock front and hence the large sound absorption in these gases in excess of that calculated using shear viscosity can only come from the vibrational excitation process. Real bulk viscosity in ideal gases is not a manifestation of vibrational excitation but only of the rotational excitation process.
24(1956); http://dx.doi.org/10.1063/1.1742607View Description Hide Description
The electronic transition energies of cyclo‐octatetraene, tetraphenylene and s‐dibenzcyclo‐octatetraene have been calculated by treating each molecule as four separate π‐electron subsystems, the excited configurations of which interact under the influence of the Hamiltonian for the complete system.
The calculated transition energies for cyclo‐octatetraene and tetraphenylene are in good agreement with experimental data, while the calculations for s‐dibenzcyclo‐octatetraene suggest that the absorption data for this compound is in error.