  We report high-sensitivity millimetre observations of several molecular
species (13CO, HCN, HNC, CN, HCO+ and N2H+) in a sample of compact planetary
nebulae. Some species such as HCO+ and CN are particularly abundant compared to
envelopes around AGB stars or even interstellar clouds. We have estimated the
following average values for the column densities ratios: CN/HCN~2.6,
HCO+/HCN~0.5, and HNC/HCN~0.4. Thus, the chemical composition of the molecular
envelopes in these compact PNe appears somewhat intermediate between the
composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe
(such as the Ring, M4--9, or the Helix). From observations of the CO
isotopomers, we have estimated that the 12C/13C ratio is in the range 10 ~<
12C/13C ~< 40. These values are below those expected from standard asymptotic
giant branch models and suggest non-standard mixing processes. The observed
molecular abundances are compared to very recent modelling work, and we
conclude that the observations are well explained, in general terms, by
time-dependent gas-phase chemical models in which the ionization rate is
enhanced by several orders of magnitude with respect to the average
interstellar value. Thus, our observations confirm that the chemistry in the
neutral shells of PNe is essentially governed by the high energy radiation from
the hot central stars. The complexity of the chemical processes is increased by
numerous factors linked to the properties of the central star and the geometry
and degree of clumpiness of the envelope. Several aspects of the PN chemistry
that remains to be understood are discussed within the frame of the available
chemical models.
