We present molecular observations of several preplanetary nebulae (PPNe) that had been identified from their peculiar IR spectrum, regardless of their spatial shape, which was indeed unknown. We report the detection of (CO)-C-12 and (CO)-C-13 J = 2-1, 1-0 emission in four objects, IRAS 07134 + 1005 (not observed in (CO)-C-13 J = 1-0), IRAS 17436 + 5003, IRAS 19114 + 0002, and IRAS 19500 - 1709. SiO J = 3-2, 2-1 is detected in IRAS 19114 + 0002; HCN J = 1-0 in IRAS 07134 + 1005, IRAS 17436 + 5003 and IRAS 19500 - 1709; and probably HCO+ J = 1-0 in IRAS 07134 + 1005. (CO)-C-12 j = 1-0 emission is probably detected in IRAS 18095+2704. Maps of the (CO)-C-12 J = 2-1 lines were also performed in IRAS 07134 + 1005, IRAS 17436 + 5003, IRAS 19114 + 0002, and IRAS 19500 - 1709. In the two best studied objects, IRAS 19114 + 0002 and IRAS 19500 - 1709, evident signs of a bipolar structure are found. IRAS 19114 + 0002, the only object resolved by our mapping, presents a clearly elongated structure in (CO)-C-12 J = 2-1. In IRAS 19500 - 1709 we detect wide and relatively intense wings in (CO)-C-12 and (CO)-C-13, probably produced by a high-velocity bipolar outflow that involves a significant fraction of the total molecular envelope. Narrower wings are also found in IRAS 17436 + 5003. We conclude from our results and other data found in the literature that probably all preplanetary nebulae, even our relatively young objects, present evidences of significant departures from spherical symmetry. We find a significant correlation between the (CO)-C-12 integrated intensity and the far infrared fluxes, particularly for the 60-mu-m wavelength. This relation is probably valid for young preplanetaries, the CO emission being stronger in AGB envelopes (which would present a steeper regression line) and weaker in more evolved preplanetary nebulae and planetary nebulae. From the analysis of the CO intensity we also conclude that the molecular abundance is normal in IRAS 19114 + 0002, but very low in IRAS 18095 + 2704. Other general properties of the studied nebulae, particularly their distance and the galactic population to which they belong, are discussed.
