Index of content:
Volume 20, Issue 4, 01 April 1952
Hindered Intermolecular Rotation in the Solid State: Thermal and Dielectric Phenomena in Long‐Chain Compounds20(1952); http://dx.doi.org/10.1063/1.1700488View Description Hide Description
It has been assumed that the potential energy barrier hindering rotation of chain molecules about their long axes in the solid state has one deep minimum and Ω other minima with a potential energyV above that of the deeper well. Molecular models suggest Ω should be between 8 and 18. Application of the Bragg‐Williams cooperative approximation to this system leads to first‐order thermal and dielectric transitions. Dielectric and thermal data on n‐C22H45Br, n‐C30H61Br, and n‐C28H58 show that the transitions are first‐order. The static dielectric constant of the polar compounds calculated from Onsager's equation on the basis Ω=12 agree closely with experiment. The model suggests the existence of two Debye‐type loss regions below the transition temperature. Loss data confirm this and further indicate Ω is close to 12. There is no evidence of resonance absorption. Ω may also be found from the entropy of transition at constant volume, R lnΩ. Thermal data on all three compounds are consistent with a value of Ω of 12. The theory indicates that the transition temperatures of a homologous series should arise almost linearly with increasing chain length if the chains do not twist appreciably. Data from the literature confirm this.
20(1952); http://dx.doi.org/10.1063/1.1700489View Description Hide Description
The dielectric properties of a series of long‐chain esters have been investigated in the solid state. Like other long‐chain molecules containing 20 or more carbon atoms, most of the ester molecules showed evidence of hindered molecular rotation about their long axes in the crystal lattice. It was also found that higher dielectric constants resulted when the samples were rapidly frozen than when they were allowed to solidify gradually, the effect being more pronounced as the chain length was increased. In order to explain this behavior, it has been assumed that the esters may crystallize in an ordered lattice in which the dipoles are in a plane, or in a longitudinally disordered lattice in which some of the chains are reversed. In the latter case, the dipoles may or may not form two planes depending upon the position of the polar group in the chain. It has also been assumed that the degree of longitudinal disorder increases with increase in freezing rate. Once frozen in, however, it remains fixed until the sample is remelted.
The potential barrier hindering rotation has been assumed to be much higher in the ordered state than in the disordered state because separation of the dipoles should lower steric and dipole interaction in the latter. Since the rotational transition temperature is proportional to a potential energy difference V 0 related to the barrier against rotation, the transition temperature should increase with increase in longitudinal order. In a completely ordered ester or a nearly symmetrical disordered ester, the barrier may be sufficiently high to raise the transition point to a temperature near or above that of fusion. The experiments agree with these assumptions.
20(1952); http://dx.doi.org/10.1063/1.1700490View Description Hide Description
Under the assumption of V([open phi])=V 0[1—cos([open phi]—[open phi]0)]/2 for the potential hindering internal rotation, visual interpretation of the electron‐diffraction pattern leads to the results (best values and estimated limits of error): C–Cl, 1.79±0.035A; C···Cl, 2.715±0.025A; C–C, 1.56A (range: 1.49–1.60A); ∠C–C–Cl, 108±4°; |[open phi]0| ≤30° (i.e., essentially trans in the chlorine atoms), with a small positive value of [open phi]0 (increasing CH3···Cl primary ) most likely; V 0≥4 kcal/mole. The best value for V 0 ranges from 10 kcal/mole for [open phi]0=0° to 30–40 kcal/mole for [open phi]0=25°. The diffraction pattern is almost indifferently compatible with fractions ranging up to 50 percent of a skew‐isomer with [open phi]0∼120° and a sufficiently low value of V 0, but consideration of dipole moment data provides some further evidence on its structure while showing the fraction present to be about 20 percent with an upper limit of about 30 percent.
20(1952); http://dx.doi.org/10.1063/1.1700491View Description Hide Description
The problem of the absolute configuration of optically active molecules is investigated with the aid of the Kirkwood theory of optical rotatory power. Absolute configurations are assigned to the enantiomorphs of 2,3‐epoxybutane and 1,2‐dichloropropane. The assignments are consistent with the established experimental configurational relationships between these compounds. The Fischer convention is confirmed as a structurally correct representation of absolute configuration. The magnitudes of the calculated rotations of the compounds are in reasonably good agreement with experiment. The theory accounts satisfactorily for the effect of temperature and solvent on the optical rotation of 1,2‐dichloropropane.
On the Distribution Function in Phase Space, with Application to the Stationary State of an Ideal Gas20(1952); http://dx.doi.org/10.1063/1.1700492View Description Hide Description
An infinite set of partial differential equations which are to be satisfied by the coefficients in a Bruns' series expansion of the distribution function, as a consequence of the principle of ``conservation of density‐in‐phase,'' are obtained. By means of these equations there are derived meteorologically useful expressions for the density and temperature gradients in an ideal gas.
20(1952); http://dx.doi.org/10.1063/1.1700493View Description Hide Description
20(1952); http://dx.doi.org/10.1063/1.1700495View Description Hide Description
The kinetics of the gas‐phase decomposition of diethyl peroxide have been studied in a flow system, using excess of toluene. The main reaction products were found to be ethane and formaldehyde, and there were smaller amounts of methane and dibenzyl. The frequency factor of the reaction is 2.1×1013, and the activation energy is 31.7 kcal per mole. The mechanism of the reaction is discussed, and it is shown that the measured rate must refer to the initial dissociation C2H5OOC2H5→2C2H5O. With a new value of 47.8 kcal for the heat of formation of gaseous diethyl peroxide, the heat of formation of the C2H5O radical is calculated to be 8.1 kcal, which agrees satisfactorily with estimates from other kinetic data. Thermochemical values for other reactions involving this radical are calculated.
20(1952); http://dx.doi.org/10.1063/1.1700496View Description Hide Description
The reactions of methyl radicals formed by the photolysis of acetone with several halogenated methanes have been investigated between 110°C and 220°C. The activation energies of these reactions have been determined relative to the activation energy of the combination of methyl radicals and are found to be (in kcal): methyl fluoride 8.7, methylene fluoride 6.2, methyl chloride 9.4, methylene chloride 7.2, chloroform 5.8, methyl bromide 10.1, methylene bromide 8.7. The steric factors of the reactions are in the range 10−2−10−4.
20(1952); http://dx.doi.org/10.1063/1.1700497View Description Hide Description
Radioactive telluriummirrors have been used to investigate the rate of reaction of methyl radicals with nitric oxide in a flow system. The collision efficiency of this reaction has been found to be 1.5×10−4, which is higher by a factor of 10 than the previous value obtained by Forsyth. This value when applied to results obtained by Miller and Steacie for the ratio of the two reactions leads to a collision efficiency or steric factor, assuming the activation energy to be zero, of 0.01 for methyl combination. A comparison of this value with those obtained by other authors from photochemical studies suggests the value for the collision efficiency of methyl combination is between 0.5 and 0.05.
20(1952); http://dx.doi.org/10.1063/1.1700498View Description Hide Description
The evaluation of the thermodynamic functionsCp 0, S 0, —(F 0—E 0 0)/T, and (H 0—E 0 0)/T of the fluorine atom and molecule and of hydrogen fluoride from 100°K to 5000°K have been made by statistical calculations from available spectral constants for the atom and for hydrogen fluoride; for fluorine the constants have been derived or estimated from the Raman shift in gaseous fluorine, the equilibrium internuclear distance from electron diffraction measurements, and the heat of dissociation obtained recently by R. N. Doescher at this Laboratory. These results have been combined with the thermodynamic functions of hydrogen to evaluate the heats and free energies of dissociation and dissociation constants for the equilibria F2⇌2F, HF⇌½H2+½F2, and HF⇌H+F.
20(1952); http://dx.doi.org/10.1063/1.1700499View Description Hide Description
The pure rotational spectra of trifluoromethyl bromide, iodide, and cyanide have been studied in the microwave region. The B 0 values in Mc/sec were determined as follows: 2098.06 for CF3Br79; 2078.50 for CF3Br81; 1523.23 for CF3I; 2945.535 for CF3CN14; 2855.859 for CF3CN15. The nuclear quadrupole couplings, eQq, are in Mc/sec: 619 for Br79; 517 for Br81; −2150 for I127; −4.70 for N14. With any reasonable assumption of bond angles, d CF is found to be 1.33±0.015A in each molecule. The other distances depend more strongly on the assumption of the bond angles. With ∠FCF assumed to be 108°±1°, as suggested from other similar molecules, the values, d CBr = 1.91±0.02A, d CI = 2.14±0.02A, d CC = 1.46±0.02A, are obtained, where the additional assumptions d CF = 1.332A for CF3I and d CN = 1.158A for CF3CN are made.
20(1952); http://dx.doi.org/10.1063/1.1700500View Description Hide Description
This paper combines earlier preliminary reports on F2CO, FClCO and Cl2CO into a complete report on the Raman and infrared spectra of these molecules. Details of measurement and handling of the samples are given. The fundamental frequencies observed in both Raman effect (liquid) and infrared (gas) and a large number of overtone and combination frequencies observed in the infrared have been identified and are collected in tables. The observed infrared band envelopes are described and compared with the band envelopes predicted on the basis of the molecular models. Resonance between ν2 and 2ν1 in F2CO and ν2 and ν1+ν4 in FClCO is discussed.
20(1952); http://dx.doi.org/10.1063/1.1700501View Description Hide Description
Sufficient isotopic combinations have been studied to make complete structural determinations of fluoroform and chloroform from microwave rotational spectra. The dimensions of CHF3 are d CH = 1.098A, d CF = 1.332A, and ∠FCF = 108°48′; those of CHCl3 are d CH = 1.073A, d CCl = 1.767A, and ∠ClCCl = 110°24′. Measurements were made on only the most abundant isotopic species of CH3CCl3. If the CCl3 configuration as in chloroform and the CH3 configuration as in methane are assumed, the CC distance in CH3CCl3 is found to be 1.55A.
Spectroscopic Studies in the Near Ultraviolet of the Three Isomeric Dimethylbenzene Vapors. I. Absorption and Fluorescence Spectra of Para‐Dimethylbenzene20(1952); http://dx.doi.org/10.1063/1.1700502View Description Hide Description
The absorptionspectrum of para‐xylene between 2850 and 2350A was photographed using a 3‐meter gratingspectrograph, and the corresponding fluorescencespectrum between 3125 and 2720A was photographed with a Bausch and Lomb medium quartzspectrograph. The electronic transition involved is interpreted as 1 A 1g —1 B 3u with the 0, 0 band at 36733 cm−1. Progressions in the fluorescencespectrum involve the totally symmetric vibrations 829 and 1208 cm−1, while progressions of the corresponding frequencies 775 and 1185 cm−1 appear in the absorptionspectrum. A weak portion of the spectrum is identified with a so‐called forbidden transition involving the β1g ‐component of the 606ε g + benzene vibration. This β1g ‐vibration has a frequency of 648 cm−1 in the ground state and of 552 cm−1 in the upper state. The α1g ‐component of the ε g + benzene vibration in para‐xylene has a lower state frequency of 458 cm−1 and the upper state value of 367 cm−1. The Tesla luminescencespectrum is interpreted in relation to the fluorescence and absorption spectra.
20(1952); http://dx.doi.org/10.1063/1.1700503View Description Hide Description
Hydrogen overpotential at nickelcathodes has been measured under very pure conditions in aqueous solutions of hydrogen chloride (0.001N − 1.0N) and in aqueous sodium hydroxide (0.001N − 0.2N). The measurements have been made in the current density range 10−8 − 10−1 amp/apparent cm2, and in the temperature range 0°—50°C. Observations were also made of the buildup and rate of decay of overpotential and of the capacity of the electrode/electrolyte interface. Direct measurements were made of the number of acts of the rate‐determining step associated with one act of the over‐all hydrogen evolution reaction (i.e., the stoichiometric number μ). The application of μ has been extended to hydrogen overpotentials greater than about − 20 millivolts. The experimental data were treated statistically and show that the most probable mechanism of hydrogen overpotential at nickelcathodes is that of a rate‐determining discharge step followed by a recombination of hydrogen atoms. The discharge probably takes place from hydroxonium ions in acid solution. In alkaline solution the observed pH effect on overpotential is best explained by assuming that the discharge occurs from water molecules whose activity depends on the electrode field. Consideration of the role of chemisorption in the hydrogen electrode process indicates that a rate‐determining discharge step can take place from a nickel surface substantially occupied with hydrogen atoms.
20(1952); http://dx.doi.org/10.1063/1.1700504View Description Hide Description
Experiments on the interchange of the fission products 2.4‐hr Br83 and 55.6‐sec Br87 with carrier bromine have been performed. Essentially the same degree of interchange is effected by the following chemical procedures: carrier added as BrO3 − and reduced by H2S, carrier added as BrO3 − and reduced by excess carrier Br−, and carrier added as Br− to HCl and irradiated uranium metal dissolved.
20(1952); http://dx.doi.org/10.1063/1.1700505View Description Hide Description
It is shown in this paper that above the second‐order transition the partition function of a rubber can be represented to a reasonably good approximation by the product of the partition function of a liquid formed of molecules similar to the chain elements and the partition function of the noninteracting chains divided by the partition function of a perfect gas. In terms of the free energy this means that the free energy of rubber will be equal to the sum of the free energy of the chain network without interactions and that of a liquid formed of the chain elements alone minus the free energy of a perfect gas having the same number of molecules as the rubber has chain elements.
This same procedure has permitted the calculation of the free energy and partial pressure of a rubber solution. This is, however, by no means the only possible application. All the thermodynamic properties of elastomersolvent systems can be obtained.
This is a generalization of the methods previously used in the statistical studies of rubber and rubbersolvent mixtures.
The Influence of First‐Order Rotational Resonance Interaction on Certain Bands in the Infrared Spectrum of the Allene Molecule20(1952); http://dx.doi.org/10.1063/1.1700506View Description Hide Description
The Coriolis coupling between two degenerate vibrations in allene has been discussed. This theoretical treatment gives a satisfactory explanation of the main features of two perpendicular bands, which have been observed in the infrared absorptionspectrum of this molecule between 8 and 14μ. An analysis of the nuclear spin statistics has been carried out in order to account for the intensity alternations and to help in identifying the Q branches. A derivation of the frequency formula has been given on the basis of ordinary first‐order perturbation theory. Some emphasis is laid on the ambiguity in notation and the relation of this formula to others, reported in the literature. After making some reasonable assumptions with regard to the geometry of the molecule, we have obtained numerical values for the band centers and the various Coriolis coupling coefficients. A tentative suggestion has been made in partial explanation for the appearance of satellites between the main Q branches in the spectrum.
20(1952); http://dx.doi.org/10.1063/1.1700507View Description Hide Description
Calculations are performed on a simple model to show that dilute solutions of asymmetric particles possess a frequency dependent intrinsic viscosity and a rigidity. The model used is that of a solution containing elastic dumbbells, that is, particles consisting of two rigid bodies held together by elastic forces. Part of the rigidity would also arise from inelastic dumbbells and is caused by a force, usually neglected, as a result of Brownian motion. An additional rigidity is caused by the elastic nature of the dumbbells. At sufficiently low or high frequencies the results approximate a form similar to the ordinary dispersion formula for the dielectric constants of solutions and involve two relaxation times, one at low frequencies and one at high frequencies. The results for inelastic dumbbells are described by one relaxation time and follow the usual dispersion curve for all frequencies.