Volume 18, Issue 12, 01 December 1950
Index of content:
18(1950); http://dx.doi.org/10.1063/1.1747535View Description Hide Description
The infra‐red and Raman spectra of cyclopentane, cyclopentane‐d 1, and cyclopentane‐d 10 have been determined for the purpose of establishing the symmetry of cyclopentane. D 5h selection rules are found to hold very well. This does not constitute a rigorous criterion for structure in this case, however, because drastic alteration of the symmetry by substituting groups such as D, OH, CH3, or Cl for a hydrogen atom does not appreciably increase the complexity of the spectra. The data have provided two strong arguments against a D 5h structure. (1) An assignment could not be made which simultaneously satisfied the product rule and the expected band contours. (2) The entropy of the vapor demands a low frequency (near 140 cm−1 for the assignment in this paper) if the symmetry is D 5h . This is completely incompatible with the heat capacity of the solid.
It is concluded that cyclopentane definitely does not have D 5h symmetry. The actual geometry of the molecule is still not known. It is shown that a rigid structure of symmetry Cs , C 2, or C 1 is consistent with the available data. A decision between this and Kilpatrick, Pitzer, and Spitzer's model with a puckered ring and a pseudorotation in place of one of the genuine normal vibrations cannot be made at this time.
Analogous spectroscopic results for perfluorocyclopentane are mentioned briefly for comparison. This molecule is definitely non‐planar.
The method of preparation of the deuterium compounds is given. A Pd‐on‐charcoal catalyst is described which promises to be useful in other hydrogen‐deuterium exchange reactions.
The Quenching of Mercury Resonance Radiation. I. The Effective Cross Sections of Paraffin Hydrocarbons18(1950); http://dx.doi.org/10.1063/1.1747536View Description Hide Description
The effective cross sections of the paraffin hydrocarbons from ethane to n‐heptane in quenching mercury resonance radiation (λ2537) have been measured by the technique of Zemansky. The effective cross section of ethane is much smaller than the value previously found by Bates. Neopentane is reasonably efficient in quenching λ2537 although it is not decomposed readily by Hg(3P1) atoms. Increase in molecular weight and, up to a point, in branching of the carbon skeleton are accompanied by increases in the effective cross section. The quenching efficiency of the paraffins depends on the number and type of C–H bonds in the molecule and is not primarily a function of the number of carbon atoms or C–C bonds. The effective quenching diameters of the C–H bonds are: primary 0.05 to 0.10A, secondary 0.50 to 0.65A, and tertiary 1.3A; the actual values depending to some extent on the nature of the rest of the molecule. The number of collisions necessary for quenching to occur on primary, secondary, and tertiary C–H bonds is, therefore, approximately 3000, 30, and 5, respectively.
Rotation‐Vibration Spectra of Diatomic and Simple Polyatomic Molecules with Long Absorbing Paths IV. The Spectrum of Methyl Fluoroform (CH3CF3) from 19μ to 0.7μ18(1950); http://dx.doi.org/10.1063/1.1747537View Description Hide Description
The absorptionspectrum of methyl fluoroform has been investigated with path lengths up to 30 meters in the region 19 to 0.7μ. Three of the fundamentals have been assigned differently from Thompson and Temple. Since the molecule is nearly a spherical top, the ⊥ bands show the same typical PQR structure as do the ∥ bands except when the Coriolis interaction constant ζ is large. This seems to be the case for the degenerate C–F stretching vibration ν9(e) and for most binary combinations involving it. The CH3 rocking vibration ν10(e) is split into two close components (separation 6 cm−1) a splitting that is here ascribed to the presence of three potential minima separated by low potential maxima. A large number of infra‐red bands can be readily interpreted as binary combinations. The relative intensities of fundamentals and of overtone and combination bands are briefly discussed.
Rotation‐Vibration Spectra of Diatomic and Simple Polyatomic Molecules with Long Absorbing Paths V. The Spectrum of Isocyanic Acid18(1950); http://dx.doi.org/10.1063/1.1747538View Description Hide Description
The spectrum of isocyanic acid has been examined in the photographic infra‐red using a 21‐ft. gratingspectrograph and path lengths up to 500 m. The rotational structure of several bands has been analyzed and the energy levels and moments of inertia determined. With their help the coarse rotational structure of two of the fundamentals has also been analyzed. The molecular parameters fitting best our data and published electron diffraction results for the C–N and C–O bond distances are: r(H–N)=1.000A, ≰ H–N–C=125.5°.
Rotation‐Vibration Spectra of Diatomic and Simple Polyatomic Molecules with Long Absorbing Paths VI. The Spectrum of Nitrous Oxide (N2O) below 1.2μ18(1950); http://dx.doi.org/10.1063/1.1747539View Description Hide Description
The photographic infra‐red spectrum of N2O has been investigated with absorbing paths up to 4500 m atmos. Thirty‐four absorption bands have been found of which twenty‐three have been measured and analyzed in detail. Improved vibrational constants ω i 0 and Xik have been determined which give a fair representation of all the bands. The remaining deviations which are outside the present limits of error are due to the effect of Fermi resonance between ν1 and 2ν2.
Precise values of the rotational constantsB v1v2v3 have been determined for almost all vibrational levels involved in the observed bands. For the lowest vibrational level the six most accurately measured bands yield B 000=0.419125 cm−1. In this case also the rotational constantD 000 representing the influence of centrifugal stretching has been determined: D 000=0.193×10−6 cm−1. The value of B 000 is larger by 0.000091 than that derived from microwave data. From the B v1v2v3 values the following values for the rotational constants α i have been obtained,These values allow the determination of the rotational constant in the equilibrium position Be =0.42145 cm−1 from which the moment of inertia in the equilibrium position is found to be Ie =66.397×10−40 g cm2. The deviations of the individual B v1v2v3 values from those obtained from the formula are appreciably larger than would correspond to the errors of measurements and must be ascribed to the effect of the Fermi resonance.
Molecular Orbital Calculations of the Lower Excited Electronic Levels of Benzene, Configuration Interaction Included18(1950); http://dx.doi.org/10.1063/1.1747540View Description Hide Description
The lower excited π‐electron levels of benzene are calculated by the non‐empirical method of antisymmetrized products of molecular orbitals (in LCAO approximation) including configuration interaction. All configurations arising from excitation of one or two electrons from the most stable configuration are considered, and all many‐center integrals are retained. The results are in better agreement with experiment and valence‐bond calculations than those obtained previously by Craig in a calculation neglecting many‐center integrals. Configuration interaction is found to change the order of the 1 B 1u and 1 E 2g states but leave unchanged the order of the 3 B 1u and 3 B 2u states, in agreement with the assignments 1 A 1g —3 B 1u and 1 A 1g —1 E 2g for the experimental bands at 3.8 and 6.2 ev.
18(1950); http://dx.doi.org/10.1063/1.1747541View Description Hide Description
V‐centers are defined as stabilized positive holes. The peak wave‐lengths of three V‐bands of KCl, KBr, and NaCl are listed. Most of the paper deals with KCl, but the results are typical for KBr and NaCl as well. Experimental data are given concerning the conditions under which the various V‐bands are obtained and their unequal stability with regard to bleaching by light and temperature. Crystals irradiated at various temperatures and observed at room temperature show superpositions of the V 2‐ and V 3‐bands with relative intensities depending greatly on the temperature at which the crystals were irradiated and on the nature of the primary radiation. The V 1‐band can be observed only at temperatures below −100°C. These data are compared with those published by other investigators, and some remarks are made concerning the possible mechanisms which are responsible for the various phenomena.
18(1950); http://dx.doi.org/10.1063/1.1747542View Description Hide Description
The vapor pressure of SrO was measured by studying the product evaporated from platinum filaments coated with SrO. Most of the experiments employed radiactive isotopes.* The possibility of systematic error caused by chemical reduction of the oxide or by its thermal dissociation is discussed. A value of λ0, the heat of evaporation at 0°K computed from the results, is used to evaluate precision and to derive a vapor‐pressure equation.
18(1950); http://dx.doi.org/10.1063/1.1747543View Description Hide Description
It is shown that in the temperature range 1150–1550°K, SrO is reduced by tungsten in vacuum. Both the rate of the reaction and its equilibrium constant can be calculated, giving values in substantial agreement with the experiments, which were performed under conditions such that both could be measured. The use of radioactive isotopes simplified the experimental work.
18(1950); http://dx.doi.org/10.1063/1.1747544View Description Hide Description
Chromatographic processes with linear kinetics can be represented by a system admittance for the case where the influent concentration varies sinusoidally. The admittance has been obtained for a variety of assumed rate controlling mechanisms and is shown to depend strongly on the type of kinetics obeyed by the system. The results indicate experimental techniques for determining the kinetics of these processes. Representative results obtained with an electronic analogue computer are presented for a case of non‐linear kinetics.
18(1950); http://dx.doi.org/10.1063/1.1747545View Description Hide Description
Measurements of absorption coefficient were carried out in a binary mixture formed by two unassociated liquids having similar absorption coefficients. The curve absorption coefficientversus mole fraction has a minimum at an intermediate concentration. A theoretical interpretation of experimental results is presented.
18(1950); http://dx.doi.org/10.1063/1.1747546View Description Hide Description
The rate of the exchange reaction between carbon monoxide and carbon dioxide has been studied, using C14 as a tracer. The reaction, whose velocity in a quartz vessel is convenient for study between 800 and 900°C appears to follow the rate law: Rate = k(pCO)0.73(pCO2)0.85. The apparent activation energy is 77 kcal. The reaction is accelerated by added nitrogen but not argon. Packing increases the rate only slightly, giving no indication of a primarily heterogeneous reaction. However, this observation is considered questionable.
The present conclusions differ somewhat from those derived by Brandner and Urey from a similar investigation. Re‐examination of these authors' data seems to resolve the apparent inconsistencies. It is concluded that their proposed reaction mechanism is invalid; however, their conclusion of heterogeneity seems more acceptable.
A probable mechanism appears to be a bimolecular exchange between the two reactants adsorbed on the catalytic surface. The acceleration by nitrogen or, as observed by Brandner and Urey, by hydrogen or water is then possibly to be ascribed to ``promotion'' of the catalyst by surface adsorption of these substances.
18(1950); http://dx.doi.org/10.1063/1.1747547View Description Hide Description
For three types of linear polycondensing systems, equilibrium molecular size distributions, including rings as allowable species, are derived. Average molecular weights and amounts of ring and chain fractions are calculated therefrom. The fractions of rings are shown to increase with dilution, and with molecular weight. It is shown that beyond a critical dilution it is sometimes possible to obtain 100 percent yield of rings by driving the condensation to completion. Detailed calculations are made for two important cases corresponding to condensations of the decamethylene glycol‐adipic acid type: (1) for equimolar amounts of the two monomers, and (2) unequal amounts, with one monomer type completely reacted.
Intramolecular Reaction in Polycondensations. II. Ring‐Chain Equilibrium in Polydecamethylene Adipate18(1950); http://dx.doi.org/10.1063/1.1747548View Description Hide Description
Decamethylene adipate polymers prepared in the absence of solvent undergo a reversible decrease in solutionviscosity when heated for long times in chlorobenzene. This phenomenon is shown to be due to ester interchange reactions which establish equilibrium between ring and chain molecules. Quantitative agreement with the equilibrium theory of the previous paper is obtained if the effective length of a chain link is taken as 4.5±0.2A, which is of the order of magnitude to be expected from hindered rotation and steric effects. Measurements of ring‐chain equilibrium thus appear to offer a useful method for characterizing the stiffness of relatively short and flexible chains.
18(1950); http://dx.doi.org/10.1063/1.1747549View Description Hide Description
From measurements of the J=1→2 rotational transitions of methyl acetylene and some of its isotopic modifications, the moments of inertia, IB (in the units 10−40 gm cm2), were found for the ground vibrational state: for CH3CCH, 98.1723; for CH3C13CH, 98.2132; for C13H3CCH, 100.9193; for CH3CC13H, 101.1992; for CH3CCD, 107.7234; and for CD3CCD, 124.5806. The centrifugal distortion constants, DJK , in mc/sec. are 0.161, 0.16, 0.16, 0.13, 0.14, and 0.09, respectively. The J=2→3 rotational transition of CH3CCH was also measured, and the distortion constant DJ was found to be 0.005 mc/sec. Four lines of CH3CCH corresponding to the J=1→2 transition and six lines for the J=2→3 transition in the excited vibrational statev 10=1 and six lines for the J=2→3 transition in the excited statev 9=1 were measured and interpreted according to Nielsen's theory of l‐type doubling in symmetric tops. The calculated structure for the ground vibrational state is: C–H (acetylenic), 1.056A; C≡C, 1.207A; C–C, 1.460A; C–H (methyl), 1.097A; ∠HCH, 108°14′.
18(1950); http://dx.doi.org/10.1063/1.1747550View Description Hide Description
The absolute scattering powers of several strongly scattering solutions of polystyrene were measured by three independent methods, using transmission, integrated scattering, and scattering at right angles respectively. When a number of factors involving the geometry of the measuring system and the refraction of the light at interfaces are taken into account all the methods give results in excellent agreement. The absolute scattering power of benzene and carbon tetrachloride were then determined by comparison with the polystyrene solutions and were found to be in good agreement with Einstein's theoretical equation, though at variance with most of the previously reported experiments. The scattering from solutions of dibenzyl and sucrose octaactetate were also measured and found to give results in good agreement with those calculated theoretically from the molecular weights of these substances. In the course of the measurements it was also necessary to study the reflection properties of MgCO3 and MgO surfaces in more detail than has been published heretofore.
18(1950); http://dx.doi.org/10.1063/1.1747551View Description Hide Description
An experimental study has been made of the scattering of x‐rays by nitrogen at various pressures and temperatures in the general region of the critical point. Particular attention was given to the scattering at small angles. CopperKα‐radiation used with a single‐crystal spectrometer and narrow slits provided the means of attaining low values of sinθ/λ where 2θ is the scattering angle and λ the wave‐length of radiation. A qualitative demonstration of the effect of pressure and temperature upon the small‐angle scattering from a fixed volume of sample was made by observing at a fixed angle the amount of scattering from the sample as it was made to approach and then cross the liquid‐vapor transition curve. At 29 selected conditions of pressure and temperature, more complete patterns were taken in an attempt to make a quantitative application of theoretical expressions to obtain values of the isothermal compressibility and the sizes of regions of inhomogeneity which give rise to the small‐angle scattering. Values of compressibility from this work are roughly comparable to those obtained by other methods. The regions of inhomogeneity are found to be small at low temperatures, and they increase in size as the P—T conditions approach the critical region. Typical values for the radii of the regions of inhomogeneity are found to be 3.8A at 115°K and 18.7 atmospheres, and 8.1A at 128°K and 36.8 atmospheres.
18(1950); http://dx.doi.org/10.1063/1.1747552View Description Hide Description
A study has been made of the rate of recombination of atomic hydrogen in the presence and in the absence of the homogeneous catalyst, acetylene. The recombination in the absence of acetylene has a negative temperature coefficient and an order between one and two. The acetylene catalyzed recombination was found to be first order in atomic hydrogen and first order in acetylene, with a rate constant of 1.34×10−14 cc molecules−1 sec.−1 at 17°C. The effect of temperature on the reaction corresponded to a collision theoryactivation energy of 1.5 kcal./mole and a steric factor of 4×10−4.
18(1950); http://dx.doi.org/10.1063/1.1747553View Description Hide Description
By group theoretical arguments, it can be shown that a wave function, ψμ L , for a system of N particles corresponding to a total angular momentum quantum number, L, and a quantum number, μ, referring to the z component of angular momentum may be written as a sum of terms:The DL (R)μs are the representation coefficients for the Lth irreducible representation of the three‐dimensional rotation group, and are functions of the three coordinates specifying the orientation of the system of particles in space. The χμ L are functions of the 3N−6 coordinates specifying the relative configuration of the N‐particle system. The set of coupled differential equations for the functions, χμ L , is obtained explicitly. The special case of the three‐particle system is discussed in detail. The present treatment is more directly usable than the previous discussions since the basic equations do not involve implicit relationships between the variables.
18(1950); http://dx.doi.org/10.1063/1.1747554View Description Hide Description
An assignment of the vibrational frequencies of CH2BrCH2Br and CD2BrCD2Br has been made. The Raman spectra of these compounds have been redetermined using the new ``Toronto''‐type mercury arcs. It was possible, in the case of CH2BrCH2Br, to obtain a number of combination and overtone bands which were of assistance in the assignment. The liquid phase infra‐red spectra of both compounds have been redetermined with greater resolution and spectral range than had been previously reported. The solid infra‐red spectra of the same compounds were also determined in order to separate bands due to the trans isomer from those due to the skew isomer. Evidence for the C2 symmetry of the high energy form has been given.