G. B. Kistiakowsky

Structural Investigations by Means of Nuclear Magnetism. I. Rigid Crystal Lattices

Experimental absorption line shapes at nuclear magnetic resonance are given for several simple assemblies of nuclei, with spin ½, in the solid state at temperatures between 90° and 95°K. Analysis of these data is facilitated by the theory of Van Vleck which relates the second moment of the absorption line in a rigid lattice to nuclear spins, gyromagnetic ratios, and internuclear distances, and which provides, therefore, an objective and general method for determining structural parameters from the experimental line shapes.1,2‐dichloroethane exhibits a proton resonance with line structure characteristic of nuclear magnetic moments interacting in pairs to produce the broadening of the absorption line. Consideration of both fine structure and second moment leads to an inferred H–H distance of 1.71±0.02A in the —CH2Cl group. In conjunction with the expected C–H bond distance of from 1.09A to 1.10A, this implies that the H–C–H bond angle is between 4° 30′ and 9° less than tetrahedral. Absorption line shapes fo...

The Rate Constant of Ethane Formation from Methyl Radicals

The rotating sector technique has been applied to a determination of the rate constant of methyl radical recombination in the photo‐decompositions of acetone and mercury dimethyl, using the small but accurately measurable rate of methane formation as a measure of methyl steady‐state concentration. The resulting value of the recombination rate constant is 4.5×1013 (moles/cc)−1 sec−1, with an activation energy of E=0±700 cal regardless of radical source. The determination of this constant permits the evaluation of the rate constants for several other reactions of methyl radicals. It is found that hydrogen abstractions have steric factors of the order of 10−4.

Bromination of Hydrocarbons. I. Photochemical and Thermal Bromination of Methane and Methyl Bromine. Carbon‐Hydrogen Bond Strength in Methane

The photochemical bromination of methane was studied in the temperature range 423–503°K and found to proceed through the following chain mechanism: (1)u2009u2009u2009u2009Br2+hν=Br+Br(2)u2009u2009u2009u2009Br+CH4=CH3+HBr(3)u2009u2009u2009CH3+Br2=CH3Br+Br(4)u2009u2009CH3+HBr=CH4+Br(5)u2009Br+Br+M=Br2+M. Bromination of methyl bromide is analogous and from 7.5 to 10 times more rapid in this temperature range. Hydrogen bromide inhibits bromination of methane but not of methyl bromide. Thermal bromination was studied at 570°K and found to follow the same mechanism as photochemical reaction, except that bromine atoms are produced thermally. The activation energy of photochemical bromination of methane is 17.8 kcal./mole and that of methyl bromide is 15.6 kcal./mole. Varying efficiencies of different molecules as third bodies in the homogeneous recombination of bromine atoms are discussed. Configurations of activated complexes have been assigned and by statistical mechanical calculations shown to be reasonable. Activation energies and other data have been combined to...

The Low Temperature Gaseous Heat Capacities of C2H6

The heat capacities of gaseous C2H6 and C2D6 have been measured down to 93°K with the low pressure thermal conductivity apparatus previously described. An improved cryostat and better designed conductivity cells were used. A new method of obtaining the heat capacities from the thermal conductivity data is described which does not require a knowledge of the accommodation coefficient α. It is based on the assumption that when the accommodating efficiency of the wire is changed, α changes in the same direction for the gases being compared and further that the percentage change in α is greater the more its absolute value deviates from unity. It is then possible to show that the unknown α for the gas being studied can be bracketed within narrow limits by the αs of comparison gases. The heat capacity results for light and heavy ethane indicate a potential barrier restricting internal rotation of essentially sinusoidal shape and with a depth of 2750 cal./mole.

The Polymerization of Gaseous Butadiene

Dimerization of butadiene has been studied in the temperature range from 446 to 660°K. The rate constant is expressed approximately by the equation: k2=9.20·109u2009expu2009(−23,690/RT)u2009ccu2009mole−1sec.−1. Deviations from this relation suggest an increase of activation energy with temperature. 3‐vinyl cyclohexene formed in the dimerization reaction undergoes further addition with butadiene to give Δ3,3′‐octahydro diphenyl. The rate constant of this reaction is given by the equation: k3=1.3·1014u2009expu2009(−38,000/RT)u2009ccu2009mole−1sec.−1, but the data on which it is based are not very accurate. Statistical calculation of the entropy of an activated complex which has the resonating structure H2C lim ⋮...−CH...−C lim ⋮H–CH2–CH2–C lim ⋮H...−CH...−C lim ⋮H2 gives agreement with experimental data to within a factor of ten whereas the assumption that the reaction goes in a single step to the cyclic product gives a rate differing by more than a thousand‐fold from the observed one. The role of resonating free radicals in other reactio...

Gaseous Heat Capacities. III

The present paper presents a continuation of measurements on the gaseous heat capacities by the adiabatic expansion method. The apparatus and the experimental procedure are exactly the same as described in the previous papers denoted herein as Part I and Part II. The compounds with which the present paper deals are dimethyl ether, ethylene oxide, dimethyl acetylene, cis-butane-2 and trans-butene-2. For the correction of the experimental data to the ideal gas state several procedures had to be used, as discussed in Part II.

The Activation Energy of the Reaction CH3+HBr = CH4+Br and the Carbon‐Hydrogen Bond Strength in Methane

Mixtures of hydrogen bromide, methyl iodide, and iodine have been illuminated with ultraviolet light, and the rate of formation of methane as a function of the relative concentrations has been determined. With iodine absent from the system, a maximum amount of methane is formed, due to the absence of recombining reactions of methyl radicals and iodine. A comparison of the amount of methane produced with iodine present in various amounts with that produced when iodine is absent enables one to calculate the relative rates of the reactions CH3+HBr=CH4+Br,CH3+I2=CH3I+I, when the relative steric factors of the two reactions are assumed to be approximately the same as for the reactions H+HBr=H2+Br,H+Br2=HBr+Br. This leads to an activation energy of 1.5 kcal. for the methyl‐hydrogen bromide reaction, which, combined with the activation energy of the reverse reaction as determined by Van Artsdalen, yields a binding energy of 102 kcal. for CH3–H.

The Thermal Dissociation of Cyanogen into Cyanide Radicals

The thermal dissociation of cyanogen into cyanide radicals around 1200°C has been studied. Determining the variation of CN concentration with temperature by the intensity of absorption bands, the heat of dissociation 77±4 kg cal. has been obtained. With this quantity the heat of dissociation of hydrogen cyanide into hydrogen atoms and cyanide radicals is calculated as 94.5±4 kg cal. By the use of existing thermochemical data these quantities are shown to be the most probable values for the energies of the C–C and C–H bonds, respectively. The heat of sublimation of carbon is calculated as 154±3 kg cal.

The Ultraviolet Absorption Spectrum of Benzene

The variation of the ultraviolet absorption spectrum of benzene with temperature has been studied. From the Boltzmann distribution function two fundamental frequencies of the normal state are deduced. One is best identified with the Raman frequency of 404 cm—1, whereas the other is possibly the 605 cm—1 vibration, but may be somewhat lower. The significance of these findings for the interpretation of the ultraviolet spectrum of benzene is discussed.

Gaseous Heat Capacities I. The Method and the Heat Capacities of C2H6 and C2D6

An apparatus is described for the adiabatic expansion method of Lummer and Pringsheim, new features of which are a device for accurate and rapid pressure measurements and an effective design of the expansion vessel. The Wollaston wires, serving as a resistance thermometer, are subjected to extensive heat treatment which stabilizes their electrical characteristics so well that resistance changes accompanying gas expansion can be translated into temperature changes by means of separately determined temperature coefficients of resistance. The method here described is thus an absolute one and is not dependent on comparison with other gases of known heat capacity. Causes of imperfect temperature constancy after expansion are quantitatively discussed and traced to two processes: the heat conduction by the gas and the absorption of thermal radiation from the walls by the gas. Experimental data on air and carbon dioxide are presented and are compared with the theoretical values. Heat capacities of C2H6 and C2D6 a...