DISCOVERY OF METHYL ACETATE AND GAUCHE ETHYL FORMATE IN ORION

We report on the discovery of methyl acetate, CH sub(3)COOCH sub(3), through the detection of a large number of rotational lines from each one of the spin states of the molecule: AA species (A sub(1) or A sub(2)), EA species (E sub(1)), AE species (E sub(2)), and EE species (E sub(3) or E sub(4)). We also report, for the first time in space, the detection of the gauche conformer of ethyl formate, CH sub(3)CH sub(2)OCOH, in the same source. The trans conformer is also detected for the first time outside the Galactic center source SgrB2. From the derived velocity of the emission of methyl acetate, we conclude that it arises mainly from the compact ridge region with a total column density of (4.2 + or - 0.5) x 10 super(15) cm super(-2). The derived rotational temperature is 150 K. The column density for each conformer of ethyl formate, trans and gauche, is (4.5 + or - 1.0) x 10 super(14) cm super(-2). Their abundance ratio indicates a kinetic temperature of 135 K for the emitting gas and suggests that gas-phase reactions could participate efficiently in the formation of both conformers in addition to cold ice mantle reactions on the surface of dust grains.