Investigation of the initial reactions of the Calcote mechanism for soot formation

The first three reactions of the Calcote mechanism for soot formation, that is, C3H 3 (+) +C2H2→C5H 5 (+) , C5H 5 (+) →C5H 3 (+) H2, and C5H 3 (+) +C2H2→C7H 5 (+) , have been studied based on chemi-ions withdrawn directly from a premixed methane-oxygen flame by supersonic molecular beam sampling. The first reaction is reversible and involves the formation of a specific encounter complex sensitive to pressure and ion kinetic energy. The second reaction appears to require large amounts of internal energy in the C5H 5 (+) ion to proceed. The third reaction is reversible; however, in contrast to the initiating reaction, the C5H 3 (+) ion formed from the [C7H 5 (+) ]* complex exhibits a much lower reactivity. The conclusions are based on ion-molecule reactions as well as collision activation mass spectrometry of isolated chemi-ions. In addition, the product distributions as functions of pressure and ion kinetic energy were studied.