The vaporization of the nine compounds of Zn, Cd and Hg with S, Se and Te has been investigated by mass spectrometry. The typical process is the decomposition into gaseous metal atoms and diatomic group 6B molecules. Small amounts of polymeric species of sulphur were observed in the vapour above HgS and considerable amounts of polymeric selenium molecules above HgSe ; HgTe yields Hg(g) and solid Te. No gaseous group 2B-6B molecules were observed ; their concentration is below 1 part in 103-105. Estimates of dissociation energies are made and the possibility of observing these molecules is discussed. Decomposition pressures of all nine compounds were measured by the Knudsen method and corresponding enthalpy and entropy data deduced and compared with literature data. The heat of vaporization of lead has been remeasured. The heats of atomization of the group 2B-6B compounds are compared with those of isosteric compounds and elements, and the trends are discussed.
High-temperature mass spectrometry (hitherto at temperatures between I000 and 2500 " K and at pressures between 10-3 and 10-12 atm) has already made possible the identiJication of many previously unknown and unpredicted diatomic and polyatomic molecular species. The present paper deals with the investigation of many oxides, of halides, and of carbon molecules and intermetallic molecules. An apparatus for the production and investigation of molecular beams is described. Knudsen cells permit the production of molecular beams under equilibrium conditions; in special cases the "doubleoven" type of Knudsen cell is used. Thermodynamic and kinetic quantities such as d H; values, dissociation energies, and ionization cross sections of the species investigated are given.
The relative rates of hydrogen and deuterium abstraction by chlorine atoms from H2–D2, CH4–CD4, and C2H6–C2D6 in direct competition have been studied by a mass spectrometer technique permitting direct determinations of rates. The pair CHCl3–CDCl3 was investigated by a substitution—addition competition with C2Cl4. Data were obtained between 300° and 475°K, and values of kH/kD ranged between 3.64 and 10.9 for CH4/CD4, 2.37 to 2.69 for C2H6/C2D6, 1.58 to 2.26 for CHCl3/CDCl3, and 4.7 to 8.9 for H2/D2. For the family of reactions involving C–H–Cl, an extensive series of model calculations were made by the ``bond-energy—bond-order'' method. The expected variation of activation energy with heat of reaction was computed and compared with experiment. The expected variation of kinetic isotope effect with both temperature and heat of reaction was computed for a range of conditions much wider than these experiments. Simple activated complex theory predicts several strong trends of kinetic isotope effect with heat of reaction. The data, in a general semiquantitative way, follow these predicted trends, but in terms of fine details the data do not fit the theoretical lines. In view of the known simplifications in activated complex theory, this agreement with broad trends is all that anyone should expect.
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