Abstract

The pyrolysis mechanism of the strained ring quadricyclane is complex and produces the soot precursors, acetylene cyclopentadiene, and toluene (Fig. 1, top). Substitution of one of the quadricyclane hydrogen atoms (with OR, phenyl, and Cl) at the three position yields a substituted norbomadiene exclusively during the first pyrolysis step (Fig. 1, middle). The substituted norbomadiene isomerizes typically 10⁵ times faster than norbomadiene itself and an overall mechanism with less obvious soot producing potential results. Heterosubstituted quadricyclanes with oxygen at the three position do not form intermediate norbomadienes (Fig. 1, bottom) during thermal isomerization. In this case an oxepin intermediate is formed enroute to more stable products. The oxepin molecule has a broad uv absorption (Fig. 2), which is accessible with a frequency-doubled visible laser diode at 340 run. This represents a very uncommon situation wherein the pyrolysis of a hydrocarbon species produces an optically absorbing intermediate. This paper details the application of a diode-laser-based technique for real-time observation of the oxepin intermediate.

© 1996 Optical Society of America