a b s t r a c tThe Fourier transform microwave spectrum of ethyl acetate has been measured under molecular beam conditions. The trans conformer, where all heavy atoms are located within a mirror plane, was identified after analyzing the spectrum by comparison with theoretical calculations. The barrier to internal rotation of the acetate methyl group was found to be only 99.57(11) cm À1 whereas for methyl torsion in the ethyl group a barrier of 1112.3(37) cm À1 was determined. A comparison between two theoretical approaches treating the internal rotation, the so-called RAM (Rho Axis Method) and CAM (Combined Axis Method), is also performed.

We report on the discovery of methyl acetate, CH 3 COOCH 3 , through the detection of a large number of rotational lines from each one of the spin states of the molecule: AA species (A 1 or A 2 ), EA species (E 1 ), AE species (E 2 ), EE species (E 3 or E 4 ). We also report the detection, for the first time in space, of the gauche conformer of ethyl formate, CH 3 CH 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±0.5)×10 15 cm −2 . The derived rotational temperature is 150 K. The column density for each conformer of ethyl formate, trans and gauche, is (4.5±1.0)×10 14 cm −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.

The rotational spectra of two conformers of pentan-2-one (also known as methyl propyl ketone) were recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. Fine splittings due to the internal rotations of the acetyl methyl group CHCO- and the butyryl methyl group -COCHCHCH were resolved and analyzed with high accuracy. The torsional barriers of the acetyl and the butyryl methyl groups are 238.14 and 979 cm, respectively, for the lowest energy conformer, as well as 188.384 and 1032 cm, respectively, for the other one. From the results obtained for the acetyl methyl group, a general rule to predict its torsional barrier in ketones based on the molecular symmetry is proposed.