  We present observations of the carbon-rich protoplanetary nebula (PPN) CRL
2688 made with the Institut de Radioastronomie Millimetrique (IRAM) 30 m
telescope in the 3mm and 2mm bands. In total, 196 transition lines belonging to
38 molecular species and isotopologues are detected, among which, to our best
knowledge, 153 transition lines and 13 species are the first report for this
object. Additionally, in order to contribute to future research, we have
collected observational data on the molecular lines of CRL 2688 from the
literature and compiled them into a single unified catalog. We find that the
molecular abundance of CRL 2688 cannot be explained by the standard model of a
circumstellar envelope. The implications of metal-bearing molecules on
circumstellar chemistry are discussed.
