  The circumstellar envelopes of carbon-rich AGB stars show a chemical
complexity that is exemplified by the prototypical object IRC +10216, in which
about 60 different molecules have been detected to date. Most of these species
are carbon chains of the type CnH, CnH2, CnN, HCnN. We present the detection of
new species (CH2CHCN, CH2CN, H2CS, CH3CCH and C3O) achieved thanks to the
systematic observation of the full 3 mm window with the IRAM 30m telescope plus
some ARO 12m observations. All these species, known to exist in the
interstellar medium, are detected for the first time in a circumstellar
envelope around an AGB star. These five molecules are most likely formed in the
outer expanding envelope rather than in the stellar photosphere. A pure gas
phase chemical model of the circumstellar envelope is reasonably successful in
explaining the derived abundances, and additionally allows to elucidate the
chemical formation routes and to predict the spatial distribution of the
detected species.
