Showing papers in "Journal of the American Chemical Society in 1959"

Large molecules in carbon vapor

Abstract: The molecular orbital theory is used in appropriate semi-empirical forms to predict the properties of carbon vapor. The results indicate that linear polyatomic molecules: C = C = C - - - C = C: are the important species. Experimental results from the literature for C/sub 3/ are combined with the calculated conjugation or resonance energies and with the heats of formation of allene and ethylene to predict heats of formation for all larger carbon molecules. It is found that the odd species have closed shell structures and lower energies than the even species but that the even species should show greater electron affinity. Both of these results are consistent with the mass spectrometric results of Honig and of Chupka and Inghram. Molecular spectroscopic data on C/sub 3/O/sub 2/ are used to estimate the free energy function increments for the species above C/sub 3/. The calculated partial vapor pressures predict C/sub 5/ to be the most abundant species in the saturated vapor even at 2000 deg K with C/sub 7/T becoming comparably abundant in the 2500 to 3000 deg K range. At higher temperatures even larger molecules should become important. The results are shown to be generally consistent with allmore » reliable vaporization data provided the evaporation coefficients decrease rapidly for increasing molecular size and vary for different crystal surfaces of graphite. The calculated electronic energy levels for C/sub 2/ and C/sub 3/ agree satisfactorily with the observed spectra and trends are predicted for both even and odd larger species. It is proposed that liquid carbon consists of essentially infinite linear chains of this type. Both entropy and energy considerations lead to predicted heats of fusion of about 10 kcal/gm atom at at least the absence of any serious inconsistency. (auth)« less