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that is, C3H3+ + C2H2 → C5H5+, C5H5+→ C5H3+ H/2′, and C5H3++ C2H2 → C7H5+, have been studied based on chemi-ions withdrawn directly from a premixed methane-oxygen flame by supersonic molecular beam sampling.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 C5H5+ ion to proceed. The third reaction is reversible; however, in contrast to the initiating reaction, the C5H3+ ion formed from the [C7H5+]* 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