Colin MacKay

Free Carbon Atom Chemistry: The elusive carbon atom, simplest of organic species, can now be studied with new techniques

The studies of the chemistry of free carbon atoms described here represent only a start toward understanding the properties of this intriguing species. Although many important aspects of its chemistry, such as the influence of its electronic-spin state, are still poorly understood, some generalizations may be made.

Phase Independence of Major Reaction Mechanisms of Recoil Carbon Atoms1

Summary The reactions of recoil carbon with ethylene, ethylene oxide, and isobutane are compared in the gaseous and condensed states. Remarkably little phase dependence of the products of these reactions is found. The results show that there is little if any contribution from mechanisms involving reactions of the carbon atom with radical debris produced in its recoil track. Instead the reactions by which recoil carbon enters combination seem quite analogous to those of similar reactive species (such as CH2) produced by more conventional means. The detailed reaction mechanisms previously proposed for reactions of atomic carbon with gaseous hydrocarbons, C-Η and C=C bond insertions, also appear dominant in the condensed phases. The small changes with phase that are observed are consistent with the expected action of a solvent cage in promoting the de-excitation or secondary reaction of unstable intermediates produced by the insertion reactions

The reactions of atomic carbon with butene

Abstract C11 resulting from nuclear transformations has been used to investigate the reactions of free carbon atoms with both cis and trans 2-butene. Studies were made in both gaseous and condensed systems, and moderator techniques were used to distinguish between hot and thermal processes. As in the case of other hydrocarbons, primary reactions appear to be insertion of the carbon into CH and CC bonds. The adducts formed, if not stabilized by rapid removal of their excess internal energy, will then fragment to give either small molecules, or radicals which add to another molecule of 2-butene. Such decomposition is particularly favoured when hot carbon atoms react in the gas phase where collisional de-excitation is slow. Stabilisation of the adduct is maximized when the 2-butene reagent is in a dilute dispersion in a xenon matrix in which thermalization of the carbon atom and de-excitation of the adduct are both efficient. The relative complexity of the 2-butene molecule, which enables it to distribute excitation energy internally, also diminishes fragmentation and promotes stabilization of the adduct. Thus the yield of the fragmentation product normally most prominent, acetylene, becomes very small with 2-butene dispersed in a xenon matrix. Under these conditions, there is also evidence that some cyclic adducts survive sufficiently long to react with a second 2-butene molecule to form spiro compounds.

The Fate of Trapped Recoil Atoms in Heterogeneous Reaction Systems

A tempting method of studying the reaction of thermalized recoil atoms would seem to be to produce them in an inert matrix and then to mix this matrix with the other reagent. For instance, one could let the hot species recoil into and be trapped in a solid noble gas matrix. Subsequently the matrix would be warmed in the presence of that substance whose reactions with the trapped atoms are to be studied. The reactions thus observed would be those of fully thermalized species and would moreover be accompanied by much less radiation damage. Alternatively, the recoil atoms could be introduced into a stream of flowing inert gas which is then mixed with a stream of the other reagent.