  Two bipolar proto-planetary nebulae, IRAS16594-4656 and IRAS17150-3224, have
been detected in various molecular lines, namely CO, ^{13}CO, HCN and CN, and
remain undetected in several other species. CO(J=2-1) and CO(J=3-2) line
profiles are compared to new spectra of similar PPN candidates, previously
undetected in CO(J=2-1): CPD-53^{o}5736, IRAS17106-3046, IRAS17245-3951 and
IRAS17441-2411. CO(J=2-1) maps of IRAS16594-4656 and IRAS17150-3224 show that
both PPNe have little separation between blue, centre and red-shifted emission,
and also that the CO(J=2-1) emission is of a similar size to the telescope
beam. Fractional abundances of all detected molecules (except CO) are
calculated using the results of CO line modelling and a simple
photodissociation model. For those species not detected, upper limits are
derived. Comparisons between these fractional abundances and those of other
PPNe show that IRAS16594-4656 and IRAS17150-3224 are quite under-abundant when
compared to molecule-rich sources like CRL618, CRL2688 and OH231.8+4.2. As a
reason for this deficit, the difference in circumstellar envelope/torus density
between the molecule-rich sources and the molecule-poor sources is proposed,
and supported by a chemical model which follows the transition of a
circumstellar envelope through the AGB phase and into the PPN phase of
evolution. The model includes the effects of UV radiation, cosmic rays and also
X-rays. Finally, the post-AGB ages of these two objects (200-400 yr) are
estimated using CN/HCN and HCN/CO ratios and both ages are found to be in
agreement with previous figures cited in the literature, IRAS17150-3224 being
the younger of the two PPNe.
