Absorption against the cosmic 2.7 K background

Two interstellar lines, 1(10) --> 1(11) of formaldehyde at 4.831 GHz, and 2 (20) --> 2(11) of cyclopropenylidene at 21.590 GHz, have so far been observ ed in absorption against the cosmic 2.7 K background. Observation of an int erstellar line in absorption against the cosmic 2.7 K background is an unus ual phenomenon, and can only be possible under rather peculiar conditions d eveloped in the molecule, generating: the line. We predict that two more li nes, 3(30) --> 3(21) at 27.100 GHz, and 3(31) --> 3(22) at 59.500 GHz of cy clopropenylidene, and three lines, 2(20) --> 2(11) at 15.600 GHz, 3(30) --> 3(21) at 23.100 GHz, and 3(31) --> 3(22) at 39.700 GHz of ethylene oxide, may show absorption against the cosmic 2.7 K background. We speculate that such peculiar conditions are characteristic for b-type asymmetrical top mol ecules. The observation of these lines may be used to place upper bounds to the density in the absorbing region.