Volume 27, Issue 5, 01 November 1957
Index of content:
27(1957); http://dx.doi.org/10.1063/1.1743982View Description Hide Description
The unusual features of the infrared spectra of molecules with hydrogen bonds are discussed. It is shown that the broad hydrogen stretching bands associated with strong hydrogen bonds are not single bands, but groups of bands which are very sensitive to intermolecular influences. This is due to the anharmonicity of some parts of the (effective) molecular potential energy surface. This surface occasionally has two minima which implies typical double minimum phenomena. Variations in the intensity of the hydrogen stretching bands in going from the nonbonded to the bonded molecules is discussed and an explanation attributing this variation to the charge transfer associated with the hydrogen bondformation is offered.
27(1957); http://dx.doi.org/10.1063/1.1743983View Description Hide Description
The accuracies of the Poeschl‐Teller and Morse functions as vibrational potential energies are compared for the ground electronic states of sixty‐seven diatomic molecules. For each molecule both functions are fitted to the experimental values of Y 10, Y 20, and Y 01. The comparison of the functions is then made through examination of —Y 11. For fifty‐four of the molecules the P.T. function is more accurate than the Morse function. The superiority of the P.T. function seems more pronounced for ionized and for predominantly‐ionic molecules than for normal covalent molecules. Parameters of Poeschl‐Teller curves fitted to empirical values of Y 10, Y 20, Y 01 and Y 11 are tabulated for the sixty‐seven molecules.
Operation of a Center‐Fed Continuous‐Flow Thermogravitational Column with Unequal Flow Rates in the Two Column Sections. Comparison of Theory and Experiment27(1957); http://dx.doi.org/10.1063/1.1743922View Description Hide Description
Equations of Jones and Furry are shown to agree well with experimental data over a range of ratios of rectifying section flow to stripping section flow from 0.3 to 5. On the assumption of an effective feed location which yields maximum separation a tentative modification of the Jones‐Furry equation is developed which appears preferable at smaller values of the flow rate.
27(1957); http://dx.doi.org/10.1063/1.1743923View Description Hide Description
The theory of the second virial coefficient, A 2(M 2,T), for dilute polymer solutions is investigated from the point of view, first developed by Zimm, of the Ursell‐Mayer‐like development in terms which successively represent increasing numbers of contacts between molecular subunits of flexible chain molecules. The methods used by Fixman in treating excluded volume problems in the random flight model are here employed. Evaluation is carried out to the triple contact approximation. A formal analysis of all higher terms is presented and the contribution of one class of terms is evaluated exactly. The remaining terms appear to be inaccessible to numerical evaluation by ordinary methods so that a rigorous discussion of the asymptotic behavior of A 2 is precluded. Semiquantitative arguments, however, indicate favorable agreement with experiment, provided intramolecular excluded volume effects are considered.
27(1957); http://dx.doi.org/10.1063/1.1743924View Description Hide Description
The effects of intermolecular correlation on light scattered from flexible chain molecules are considered. The random flight model is treated correctly to the double contact approximation. Two smoothed density models are carried to the same approximation and comparisons are made. In every case a decrease of ``apparent'' A 2 with increasing angle of observation is predicted in qualitative agreement with experiment.
27(1957); http://dx.doi.org/10.1063/1.1743925View Description Hide Description
The infrared spectra of H2S in phases III, II, and I were studied at 66, 112, and 140°K, respectively. That of D2S was studied in phase III at 66°K. The spectra of phase III consist of extremely sharp lines and the structure must be ordered, probably with eight molecules in a primitive unit cell of symmetry D 4h . The stretching bands in the spectra of phases II and I are broader and indicate a disordered arrangement of the hydrogen atoms. The structure of the crystal in phase III is discussed. The crystal spectra indicate that the fundamental frequencies in gaseous D2S must be very nearly ν2=855 cm—1 and ν3=1910 cm—1.
27(1957); http://dx.doi.org/10.1063/1.1743926View Description Hide Description
The lattice constant for highly enriched (>99%) samples of Li6 and Li7 metal was determined at 20°C by x‐ray diffraction methods. The constant for Li6 was found to be fractionally larger than that for Li7 by about 4×10—4. These results are not inconsistent with a rough theoretical estimate for the fractional difference of 3×10—4. It is likely that the constant for Li7 does not differ from that for natural lithium by more than 1×10—4. The fact that the lighter isotope has the larger lattice constant constitutes experimental evidence for the reality of zero‐point energy.
27(1957); http://dx.doi.org/10.1063/1.1743927View Description Hide Description
The bulk density of extruded specimens of natural lithium and of enriched isotopes (>99%) was measured by the flotation‐equilibrium and by the hydrostatic‐balance methods. Upon correction for the slight chemical and isotopic impurities and upon reduction to 20°C, the following results were obtained for the bulk density in g/cm3: lithium‐6, 0.460±0.002; lithium‐7, 0.537±0.003; lithium‐natural, 0.531±0.001. The crystal densities, calculated from the lattice constants determined by x‐ray diffraction, are respectively 0.462, 0.539, 0.533, in each case about half a percent higher than the bulk density.
27(1957); http://dx.doi.org/10.1063/1.1743928View Description Hide Description
The thermal exchange of iodine with CH3I and with HI has been studied in the gas phase between 60° and 140°C. The exchange is strongly surface sensitive and follows the rate law: . The exchange proceeds by means of two competing mechanisms, one of which is independent of iodine concentration and possibly involves the formation of CH3· free radicals on the surface which then react with I2. This heterogeneous reaction may be eliminated by preheating the reaction vessel overnight at 550°C, which reduces the reaction to a simple bimolecular exchange following the rate law:which was obtained specifically for a surface/volume ratio of 1.19 cm—1.
The formation of a complex between CH3I and I2 was studied spectrophotometrically in dry n‐heptane. The heat of formation of the complex was found to be —4.5 kcal mole—1, and the entropy of formation was found to be —25.8 cal deg—1 mole—1 for a standard state of 1 mole‐liter—1. The formation of a complex between HI and I2 also was established in dry n‐heptane.
The gaseous exchange of HI with I2 was found to be so fast that kinetic data could not be obtained. A theoretical estimate of the exchange half‐life assuming an atomic iodine mechanism shows that the reaction would be rapid; however, experimental evidence is presented which suggests that a bimolecular mechanism is more likely.
Raman and Infrared Intensities in the Vibrational Spectra of Hydrocarbons. II. The CH Vibrations of Straight n‐Paraffins27(1957); http://dx.doi.org/10.1063/1.1743929View Description Hide Description
The Raman and infrared intensities of the CH vibrations of solid n‐paraffins are analyzed theoretically. The dependence of the relative intensity I of bands on the number N of C atoms is investigated experimentally. An empirical relation of the form I = ANp+BF(N), where A»BF(N), p=0 for factor group modes, p=—1 for terminal modes, and —3<p<—1 for ``inactive'' modes, is found and confirmed by the theory. The intensities of 5 infrared‐active factor group modes are calculated from 3 independent parameters only, in agreement with experiment. One of the parameters is the permanent CH‐bond moment whose magnitude is found to be 0.085±0.02 debye. The relative intensities of the Raman‐active twisting, wagging, and rocking modes are determined theoretically; one finds approximately: Itw :Iw :Ir =6:3:1. This supports previous assignments.
27(1957); http://dx.doi.org/10.1063/1.1743930View Description Hide Description
The diffusion of radioargon, A37, and of radioxenon, Xe131m , through inert xenon was measured at —78, 0, 57, and 105°C. The apparatus, a Loschmidt cell in which saturation ionization currents were used as a measure of tracer concentrations, yielded diffusion coefficients which are estimated to have an accuracy of about one percent.
The intermolecular potentials of Xe–Xe, derived from the present results, agree with those obtained from thermal conductivity data in the same temperature range, but do not agree with potentials derived from second virial coefficients and crystal data. Nor do they agree with potentials derived from viscosity and thermal conductivity data at higher temperatures. It is shown that this lack of agreement is due to the inadequacy of the potential form. The potential well for the rare gases should be narrower than that given by the Lennard‐Jones 12–6 potential.
27(1957); http://dx.doi.org/10.1063/1.1743931View Description Hide Description
The following effects of external treatments upon electrets have been observed. (1) When the surface of an electret which exhibited a homocharge was planed in a lathe, the sign of the equivalent surface charge changed to that of the heterocharge. This heterocharge then decayed and the homocharge finally became dominant again. The magnitude of the final homocharge was smaller than that of a control electret which was not planed. (2) Repeated planing of an electret, with time allowed for recovery between successive planings, resulted in repetition of the cycle described in (1). (3) Wax dummies (not subjected to an electric field during manufacture) which were given a frictional charge and then planed, showed no reversal of sign; they did exhibit a small recovery after being planed. (4) Electrets subjected to 100% relative humidity exhibited an abrupt decrease in equivalent surface charge. Removal of the moisture resulted in a gradual, slight recovery. (5) The curve relating the equivalent surface charge and the logarithm of the pressure of the air surrounding an electret, has the same form as the curve of the sparking potential for air versus the logarithm of the pressure. Thus, all the treatments described resulted in a decay of the homocharge, followed by a partial recovery upon removal of the disturbing influence. The observed behavior can be explained in terms of the electret mechanism proposed by Gross.
27(1957); http://dx.doi.org/10.1063/1.1743932View Description Hide Description
Chlorine resonance frequencies are reported for the following at 298°, 194°, and 77°K: CsICl2, KICl2, KICl2·H2O, CsICl4, RbICl4, KICl4, KICl4·H2O, NaICl4·2H2O. Variations due to crystal effects in these substances amount in most cases to less than a megacycle. The average frequencies at room temperature, 19.2 and 22.3 Mc/sec, are taken as representative of ICl2 — and ICl4 — ions, respectively, in a crystalline environment. These values are in the neighborhood of half those found for ICl, and suggest a bonding scheme in which the electron distribution is shifted outward toward the chlorine atoms to such an extent that the electron population of iodine orbitals is not so large as to require the use of d orbitals. The conclusion is that d orbitals are rather less important to bonding in the polyhalides than has generally been supposed. The problem of arriving at a practical and meaningful separation of crystal and chemical effects is considered qualitatively, and marked crystal effects evident in the spectra of KICl4·H2O and NaICl4·2H2O are briefly discussed.
Rates and Mechanisms of Protolysis of Di‐ and Trimethylammonium Ions Studied by Proton Magnetic Resonance27(1957); http://dx.doi.org/10.1063/1.1743933View Description Hide Description
A kinetic analysis of the protolysis reactions of di‐ and trimethylammonium ions in aqueous solution has been carried out using the nuclear magnetic resonance technique. The results complement those of a similar study of methylammonium ion reported on in an earlier paper. In the pH range studied (between 3 and 5) the main mechanism is: RNH++RN⇆RN+RNH+. The reaction involves water in nearly every exchange. The measured rate constants are: 6.0×108 and 3.1×108 sec—1M—1 for di‐ and trimethylammonium, respectively.
27(1957); http://dx.doi.org/10.1063/1.1743934View Description Hide Description
Transient methods have been used to measure the Cl35 nuclear relaxation times at room temperature in solid solutions of p‐dichlorobenzene and p‐dibromobenzene. The spin‐lattice relaxation timeT 1 and the phase memory time T 2 were found to be virtually independent of sample composition. However, the inverse line width T 2 * decreases very sharply from 3.1×10—4 sec in pure p‐dichlorobenzene to 0.15×10—4 sec in a sample with 20 mole percent p‐dibromobenzene. Thus, the main effect of the solute molecules is to broaden the absorption line by producing a distribution in the electric field gradients at the chlorine nuclei. Mechanisms for the T 1 and T 2 results are discussed.
27(1957); http://dx.doi.org/10.1063/1.1743935View Description Hide Description
The heat capacity and the heats and temperatures of fusion of 1,2,4‐trimethylbenzene have been determined experimentally. This substance exhibits two melting points: at 229.33±0.03°K and at about 224.2°K. The only solid phase having reproducible heat capacities was found to be the one having the highest melting point. The entropy of the ideal gas at 25°C and one atmos pressure has been found to be 99.59±0.15 cal/deg mole.
27(1957); http://dx.doi.org/10.1063/1.1743936View Description Hide Description
The spin‐lattice relaxation time (T 1) and the transverse relaxation time (T 2) are determined, by the spinecho nuclear magnetic resonance method, for the isotropic‐liquid state of the liquid‐crystal compounds p‐azoxyanisole (PAA), cholesteryl benzoate, cholesteryl n‐butyrate, and cholesteryl propionate. The T 1 values are 1.2 sec for PAA, and 0.06 to 0.15 sec for the cholesteryl compounds. The T 2 values are 0.02 to 0.03 sec for the cholesteryl compounds, and at least 0.05 sec for PAA. The results are consistent with those for other isotropic liquids. As the temperature decreases toward the transition to the anisotropic phase, a pretransitional decrease of T 2 is observed for the cholesteryl compounds, and a similar decrease of T 1 for PAA. An explanation in terms of Frenkel's theory of heterophase fluctuations is discussed. Thermal expansion curves and interphase surface tension for PAA are also considered, and hysteresis‐like behavior of the phase transition.
Theory of the Effects of Exchange on the Nuclear Fine Structure in the Paramagnetic Resonance Spectra of Liquids27(1957); http://dx.doi.org/10.1063/1.1743937View Description Hide Description
A theory of the influence of exchange forces in liquids upon the nuclear hyperfine structure in paramagnetic ``spin resonance'' spectra is presented. Formulas are derived for two limiting cases: for the exchange J much larger than the hyperfine separation A, and for A≪J. It is found that as J increases the hyperfine lines broaden (exchange broadening) and start to shift towards the ``unsplit'' Zeeman frequency. For J≈A, the hyperfine lines have coalesced into a single broad line and for J>A the line narrows (exchange narrowing) about the ``unsplit'' Zeeman frequency. These calculations are discussed in considerable detail. The effect upon the spectrum of electronic‐electronic and electronic‐nuclear dipolar interactions and the effect of the rapid molecular motions in liquids have also been considered, as well as their interactions with the exchange forces.
27(1957); http://dx.doi.org/10.1063/1.1743938View Description Hide Description
The surface diffusion and adsorption of hydrogen on Nifield emitters has been investigated by a technique previously used for similar studies of H and O diffusion on W. It was found that diffusion occurs with an activation energy of 7±1 kcal in the chemisorbed layer, without the precipitation boundaries observed on W. This is believed to result from the inherent homogeneity of f.c.c. substrates, on which most adsorption sites seem equivalent. This is supported by the marked lack of emission anisotropy of hydrogen‐covered Nifield emitters.
A field dependent transformation in the adsorbed film was found between 2° and 4°K and interpreted as a shift in the equilibrium between adsorbed H2 molecules and the terminal fraction of adsorbed H atoms. The implications of this result for the mechanism of H2–D2 exchange are discussed.
The activation energy of desorption at very low coverage is determined to be 46±4 kcal from the temperature dependence of desorption rates.
27(1957); http://dx.doi.org/10.1063/1.1743939View Description Hide Description
Expressions are developed which relate the rotational constants of the rigid asymmetric rotator to experimental data through moments of the energy levels. The procedure in many cases permits the computation of rotational constants with much less effort than is usually met in either numerical approximation to roots of secular equations or perturbation methods.