Index of content:
Volume 19, Issue 6, 01 June 1951
19(1951); http://dx.doi.org/10.1063/1.1748328View Description Hide Description
The data on the thermal decomposition of nitrous oxide, which as they stand are anomalous in many respects, are examined. It is found at low concentrations of nitrous oxide that a heterogeneous first‐order reaction of low energy of activation becomes important. Correction of all data for this heterogeneous reaction removes most of the anomalous spread of values of the energy of activation which had varied from 48 to 65 kcal per mole. The energy of activation at the low concentration limit is 59 kcal per mole, which gives normal pre‐exponential factors and ``number of oscillators.'' The data, corrected for heterogeneity, are those for a straightforward unimolecular reaction, giving the low concentration limit and approaching the high concentration limit. By empirical modification of existing theories specific rate constants are found for the variously excited molecules which give a good description of the observed curve of log rate‐constant vs log‐concentration. The function of specific rate constants vs energy of the molecule is closely related to the continuous form proposed by Rice and Ramsperger, who assume four effective oscillators, which is the number of normal modes of vibration of the nitrous oxide molecule. More experimental work is needed on this kinetic system.
19(1951); http://dx.doi.org/10.1063/1.1748329View Description Hide Description
Free methyl radicals from thermally decomposing di‐t‐butyl peroxide were used to initiate the polymerization of gaseous butadiene. The rate of polymerization was found to follow the equation —d[M]/dt = kp(ki/kt) ½ [M][C] ½, where kp, ki , and kt refer to the rate constants for propagation, initiation, and termination, respectively; M and C refer to butadiene and peroxide. From the effect of temperature on the over‐all rate constant and the known values of Ei and Et , the value of Ep , the activation energy for chain propagation, was found to be 2.6 kcal/mole.
19(1951); http://dx.doi.org/10.1063/1.1748330View Description Hide Description
Molecules in their metastable triplet state are obtained by intense ultraviolet excitation, and the absorptionspectrum of the excited molecules is then measured. The conditions needed to obtain the required concentration of excited molecules are shown, and a method for determining this concentration is developed. This enables one to determine the extinction coefficient for the triplet‐triplet transition and is an aid in characterizing the new state. The methods are applied to a number of compounds having long‐lived triplet states.
19(1951); http://dx.doi.org/10.1063/1.1748331View Description Hide Description
The microwave spectra of C2 12H4O, C12C13H4O, C2 12D4O, C2 12H4S32, C2 12H4S34, and C2 12D4S32 have been determined. For each isotopic species three moments of inertia are obtained. From the 9 moments of inertia for each compound, a set of ``effective'' bond distances and angles (5 in number) have been selected. These ``effective'' bond distances are all self consistent to 0.002A or better. A brief discussion of the resulting structure is given. The dipole moments of C2H4O and C2H4S were determined from the magnitude of the Stark splitting.
19(1951); http://dx.doi.org/10.1063/1.1748332View Description Hide Description
Prasad and co‐workers have recently established, from the measurements of the susceptibilities of a number of hydrated and anhydrous salts, that hydrates do not generally obey the additivity rule. Further, they have come to the interesting conclusion that the deviation from the additivity rule is closely connected with the number of molecules of water in the hydrate; in the case of several hydrates of the same salt, the departure from the additivity rule per molecule of water is highest for the hydrate containing the least number of molecules of water of crystallization and is least for the most stable hydrate. Boron is known to form several hydrated compounds. The polyborates are particularly interesting, since they can be dehydrated stepwise at suitable temperatures, and several intermediate hydrates can thus be prepared. The present paper deals with the study of the susceptibilities of the hydrated and anhydrous compounds containing boron and the examination of the applicability of the additivity law to hydrates.
19(1951); http://dx.doi.org/10.1063/1.1748333View Description Hide Description
The application to molecules of the cellular method of Wigner, Seitz, and Slater has been tested by using it, in modified form, to calculate the electronic binding energy of the H2 molecule. The results are not better than those obtained by the Heitler‐London and molecular‐orbital methods, but they are good enough to suggest investigation of more complicated molecules, where the method may be more successful.
19(1951); http://dx.doi.org/10.1063/1.1748334View Description Hide Description
The velocity and absorption of ultrasonic waves in benzene vapor were measured by means of the sonic interferometer, at frequencies of 251.44 kc, 497.44 kc, and 1008.06 kc, within the pressure range 2 to 9.5 cm Hg, between the temperature limits 30.2°C to 37.6°C. Dispersion of the velocity, after being reduced to 30°C, ranged from 190.4 m/sec to 207.31 m/sec. The corresponding values of the specific heat Cv/R dropped from 9.45 to 3.0. Values of the molecular absorption coefficients, which are several times larger than the classical ones, are interpreted as due to the loss of vibrational degrees of freedom only. The theoretical values calculated from Kneser's method, show a close agreement with the dispersion curve, but not so well with the absorption values. An improvement of fit was made by the assumption of two relaxation times, but owing to the scattering of the experimental points, it is very hard to decide which theory fits better. The value of the single relaxation time is 5×10−8 sec, and those from the second assumption are 2×10−8 sec and 6.8×10−8 sec.
19(1951); http://dx.doi.org/10.1063/1.1748335View Description Hide Description
The kinetics of the thermal decomposition of di‐t‐butyl peroxide were studied in the presence of excess toluene using a static and a flow system, the temperature range being 120° to 280°C. The decomposition was represented by the following mechanism:The rate of reaction was measured by the rate of formation of CH4+C2H6. It was shown that the decomposition is a homogeneous gas reaction of the first order. The activation energy seems to be 36±1 kcal/mole, corresponding to the frequency factor of the order 4.1014−7.1014 sec−1. It was shown that the rate of reaction is not influenced by the pressure of toluene and by the toluene/peroxide ratio. It remains the same even in the extreme case when no toluene is present, or when benzene is used instead of toluene. The rates of decomposition obtained in this study are in fair agreement with the rates obtained for the same reaction by Rust and Vaughan. The following values have been obtained for dissociation energies:and the heat of formation of (CH3)3.CO· radical has been estimated at −23 kcal/mole.
19(1951); http://dx.doi.org/10.1063/1.1748336View Description Hide Description
Raman spectra for liquid methylhydrazine and sym‐dimethylhydrazine, and infrared spectra for both liquid and vapor methylhydrazine and sym‐dimethylhydrazine in the region 1600–650 cm−1 have been obtained. Frequency assignments are proposed for methylhydrazine and sym‐dimethylhydrazine. The strong polarization of the Rayleigh scattering from these two compounds is explained on the basis of associated structures of approximately spherical symmetry.
19(1951); http://dx.doi.org/10.1063/1.1748337View Description Hide Description
Raman spectra of water solutions of NaH2PO2, KH2PO2, HPO3, NaPO3, K4P2O7, K2Na2P2O7, K2H2P2O7, and Na2H2P2O7 were photographed using a Hilger constant‐deviation spectrograph. Illumination was provided by four Hanovia mercury lamps, the 4358A line being used as the exciting radiation. A system of liquid filters reduced the effect of the other mercury lines so that their presence was not objectionable. A Wollaston prism made possible the separation of the perpendicular and parallel components of the scattered radiation. Intensity and depolarization values were calculated from traces obtained by means of a Kipp and Zonen microphotometer. The Raman spectra of the NaH2PO2 and KH2PO2 solutions were identical, while those of the HPO3 and NaPO3 solutions differed markedly from each other. The spectra of the four pyrophosphate solutions were all similar in appearance, and their spectral lines showed similar relative intensities and depolarization values. Suggested models have been postulated.
19(1951); http://dx.doi.org/10.1063/1.1748338View Description Hide Description
The near ultraviolet spectra of the diazines and various derivatives have been obtained in several solvents. Intensities, contours, and positions of the observed bands are combined with a valence‐bond treatment of the diazine π‐electrons to assign the transitions and to evaluate the C–C and C–N exchange integrals. The diazine band near 30,000 cm−1 (3333A) is assigned to a nonbonding nitrogen electron transition, and the band near 40,000 cm−1 (2500A) to a π‐electron transition.
19(1951); http://dx.doi.org/10.1063/1.1748339View Description Hide Description
Third‐order perturbation theory is applied to the van der waals‐type interaction between neutral atoms. An interaction between triplets of atoms results. The derivation of the third‐order energy interactionW 0′′′, is outlined, the procedure being somewhat similar to that used by London in his application of second‐order perturbation theory to interatomic interaction. Since the perturbing potential is limited to the dipole‐dipole term, the energy W 0′′′ is called the triple‐dipole interaction. The latter depends not only on the interatomic distances but also on the shape of the triangle formed by the three atoms; W 0′′′ is positive for all acute and negative for most obtuse triangles.
19(1951); http://dx.doi.org/10.1063/1.1748340View Description Hide Description
The dependence of the triple‐interaction between three neutral atoms on the configuration of the latter suggests a possible explanation of the structure of the crystals of Ne, A, Kr, and Xe. The latter crystallize in the face‐centered cubic (f.c.c.) lattice, one of the two closest‐packed structures, the other being the hexagonal closest‐packed. The triple‐dipole interaction was directly summed in both the f.c.c. and h.c.p. lattices for a cylindrical region whose radius and semi‐altitude were about three times the nearest‐neighbor distance.
The calculations, which were made on IBM punched‐card machines, are outlined. The triple‐dipole interaction amounts to two to nine percent of the cohesive energy for the crystals of the above rare gases. The difference in the triple‐dipole interaction for the f.c.c. and h.c.p. lattice, although favoring the former structure, is less than 0.01 percent of the cohesive energy and hence cannot account for the structure of these rare gas solids.
19(1951); http://dx.doi.org/10.1063/1.1748342View Description Hide Description
It has long been known that the problem of random flights of a single molecule provides in principle a method for calculating the diffusion coefficient D 12 for the case in which the concentration of the diffusing component is small. This method has been, in effect, an elementary and rather crude one, because correlations of speed and direction were not taken into account and, accordingly, the rather ambiguous concept of mean free path had to be given an arbitrary definition. In this paper we overcome these difficulties and give an accurate treatment of gaseous diffusion based on the random flight method. The determination of the diffusion coefficient is made to depend on the solution of an integral equation; the unknown function of this equation can be interpreted as the unambiguously defined effective mean free path, for diffusion, of molecules of given speed. The resulting value of D 12 is the same as is obtained from the standard Enskog‐Chapman treatment based on Boltzmann's equation.
On the Hyperfine Structure of the Rotational Spectra of XYZ 3‐Type Molecule, Where Nuclei Z Have Electric Quadrupole Moments19(1951); http://dx.doi.org/10.1063/1.1748343View Description Hide Description
Hyperfine structure due to the electric quadrupole moments of nuclei is calculated for a special molecule of XYZ 3 type, where nucleus Z has a spin larger than 1/2, and nuclei X and Y have no electric quadrupole moments. The number of the hyperfine structural lines is calculated group‐theoretically for the cases where I 1=1, 3/2, 2, 5/2, I 1 being the spin of nucleus Z. The theoreticalspectra of the J=0→1 transition are given for these cases.
19(1951); http://dx.doi.org/10.1063/1.1748344View Description Hide Description
The vapor pressure of zinc has been measured using Knudsen's effusion method. The temperature‐vapor pressure relation was found to be log10 pmm =(−6688/T)+8.888 in the range 300°—360°C. These results are about 20 percent higher than those previously reported. It is suggested that this difference may be attributed principally to the omission of a probability factor by earlier workers.
19(1951); http://dx.doi.org/10.1063/1.1748345View Description Hide Description
Transference numbers have been determined by the moving boundary method for sodium chloride in anhydrous methanol at concentrations from 0.003 N to 0.01 N, and for potassium chloride at concentrations from 0.005 N to 0.02 N. Autogenic cells were used for cation boundaries and sheared cells (with paratoluenesulfonate, di‐ and triiodobenzoate as indicator ions) for anion boundaries. The results are comparable in precision with those for aqueous solutions and satisfy all criteria for transference data. The Longsworth function is linear in the concentration up to 0.01 N, thus making it possible for the first time to obtain precise limiting transference numbers in an anhydrous solvent.
19(1951); http://dx.doi.org/10.1063/1.1748346View Description Hide Description
The conductance of sodium and potassium chlorides has been determined in anhydrous methanol for concentrations from 0.0002 N to 0.01 N. For trace amounts of water, the decrease in equivalent conductance for a given electrolyte concentration is proportional to the water content, the proportionality factor being a function of the ionic strength. The results up to 0.002 N satisfy the Onsager‐Shedlovsky equation. The limiting conductances, when combined with the transference data of the preceding paper, give the first precise limiting ionic conductances in an anhydrous solvent. The limiting values for chloride ion are the same for the two salts within experimental precision; but with increasing concentration, there are deviations from the Kohlrausch rule of independent ionic mobilities which are considerably greater than is the case in aqueous solution. The equivalent conductance data are in definite disagreement with Frazer and Hartley's results for the two salts in this solvent.
Fractionation of the Carbon Isotopes in Decarboxylation Reactions. III. The Relative Rates of Decomposition of Carboxyl‐C12 and ‐C13 Mesitoic Acids19(1951); http://dx.doi.org/10.1063/1.1748347View Description Hide Description
The relative rates of decarboxylation of C12‐ and C13‐carboxyl mesitoic acids have been studied at 61.2 and 92.0°C. The ratios of the rate constants at these temperatures are 1.037±0.003 and 1.032±0.001, respectively. A comparison is made with similar ratios determined experimentally for other acids and with theoretical calculations of the ratios.
Fractionation of the Carbon Isotopes in Decarboxylation Reactions. IV. The Relative Rates of Decomposition of 1‐C12 and 1‐C13 Trichloracetate Ions19(1951); http://dx.doi.org/10.1063/1.1748348View Description Hide Description
A precise determination of the relative rates of decomposition of 1‐C12 and 1‐C13 trichloracetate ions has been made. The 1‐C12 ion has been found to decompose 1.0338±0.0007 times as fast as the 1‐C13 ion into chloroform and bicarbonate at 70.4°C.
Trichloracetate ion has been found to undergo a decomposition reaction which gives chloride ion but no CO2, OH−, nor H+. The ratio of the rate constants for the production of Cl− and bicarbonate is 0.078.
The errors in the determination of the effect of isotopic substitution on the rates of chemical reactions are discussed. In the study of the isotopes of carbon, experiments using mass spectrometric analyses of C13 at the natural abundance level are capable of a precision of one order of magnitude better than ones in which the specific activity of C14 is determined by counting.