Dissociation Energies

Gaydon, A. G.

doi:10.1038/161731a0

Cited by 47 publications

(59 citation statements)

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“…for C-H in acetylene and of 102 ltcal. for HC1 and even lower for HBr (8). CONCLUSION The mechanism presented in this communicatioil explains the greater part of the effects reported below 500" C., and particularly that of surfaces.…”

Section: G the Si-iape Of The Triplet Statementioning

confidence: 74%

Excited States of Acetylene and Their Role in Pyrolysis

Minkoff¹

1958

Can. J. Chem.

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Previous theories of acetylene pyrolysis are reviewed in the light of recent work by Minkoff, Newitt, and Rutledge. I t is shown that the relatively large rates observed a t the beginning of the induction period do not agree with mechanisms involving the intervention of comparatively stable dimers. The required kinetic form is obtained, however, if a triplet diradical is produced on the surface in a bimolecular process, followed by gas phase polymerization, with some chain ending on the surface. The detailed mechanism closely resembles Flory's scheme for liquid phase vinyl polymerization. The shape of the radical is discussed, and it is suggested that the trans-configuration leads to polymerization, while the cis facilitates dehydrogenation.There has recently been some interest in the reactions of atoms or molecules in excited states (12,14), and particularly of unsaturated molecules in triplet states. The purpose of this communication is to examine the role played by acetylene, and its polymers, in triplet states, during the pyrolysis of acetylene a t temperatures below 500" C., particularly in view of the failure of mechanisms based on ground state molecules and radicals (21). In fact, there has been considerable controversy not only over the proposed mechanisms, but also over the experimental results. As a consequence of some recent work a t temperatures below 525" C. (15), it has been possible to select those results which are most likely to be free of unexpected complications, and to propose a simple mechanism to account for most of the observations. REVIEW O F PREVIOUS WORKThe maill difficulties indicated in the literature are threefold; the first are coi~cerned -/ , -with the nature of the products, the second with the effect of surfaces, and the third is related to the effect of nitric oxide and the role of radicals. These will be considered in turn.Confusion has often arisen regarding the products, since it is frequently stated, for example, that benzene is the main product of pyrolyzing acetylene. The latter result is only obtained under special conditions; the main reaction a t low temperatures (i.e. below 500" C.) is polymerization to polycyclic and substituted aromatic coinpounds (see Ref.15 for detailed references). Above 7-800" C., this is almost entirely superseded b y decomposition to carbon and hydrogen (2). Products in the intervening region vary continuously from hydrogen and methane down to lower aromatics, including benzene, depending mainly on the temperature, but also on other factors. In particular iVIinkoff, Newitt, and Rutledge have show11 that even a t low temperatures, polymerization call be replaced by decomposition to carbon and hydrogen when halogen compounds or some metals are added to acetylene.The elucidation of the mechanism has also been hindered by the differing reports of the effect of packing the reaction vessel with glass or other supposedly inert materials. Schlapfer and Bruniler (19), and Frank-Kamenetsky (7), found the rate to be unaffected by increasing the su...

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Section: G the Si-iape Of The Triplet Statementioning

confidence: 74%

Excited States of Acetylene and Their Role in Pyrolysis

Minkoff¹

1958

Can. J. Chem.

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“…Also listed in Table I1 are the excitation energies E of the emission lines (15) and the dissociation energies D of the halides (16,17). For lead halides the energies for dissociation into atoms a t 400' C were calculated from a thermodynamic cycle using data from Lewis, Randall, Pitzer, and Brewer (18).…”

Section: Resultsmentioning

confidence: 99%

Intensities of Emission Lines in Flames of Metal Halides With Active Nitrogen

Phillips¹

1963

Can. J. Chem.

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The intensities of spectral lines emitted by flames of a number of metal halides with active nitrogen have been found to vary as the square of the nitrogen atom concentration. \\Then the total energy required for simultaneous dissociation of the halide and excitation of the metal atom is less than about 200 kcal/mole the energy transfer process is too efficient to be attributed to the termolecular reaction of a halide molecule with a pair of nitrogen atoms. The observations are consistent with the hypothesis that in this case energy is transferred to the halide molecule during collision with a nitrogen molecule in the 2,+ state. Possible excitation mechanisms are discussed for less intense lines which would require up to 276 kcal/mole for simultaneous dissociation and excitation.

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“…'l%e relations are (1,2). In molecular orbital theory, the bond order, which is eters; e.g., dipole moment and oscillator strength.…”

Section: Introductionmentioning

confidence: 99%