  Carbon-rich evolved stars from the asymptotic giant branch to the planetary
nebula phase are characterized by a rich and complex carbon chemistry in their
circumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC
11, whose Spitzer-IRS spectrum shows remarkable and diverse molecular
absorption bands. To study how the molecular composition in this object
compares to our current understanding of circumstellar carbon chemistry, we
modeled this molecular absorption. We find high abundances for a number of
molecules, perhaps most notably benzene. We also confirm the presence of
propyne (CH3C2H) in this spectrum. Of all the cyanopolyynes, only HC3N is
evident; we can detect at best a marginal presence of HCN. From comparisons to
various chemical models, we can conclude that SMP LMC 11 must have an unusual
circumstellar environment (a torus rather than an outflow).
