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The first set of theoretical cross sections for propylene oxide (CH3CHCH2O)colliding with cold He atoms has been obtained at the full quantum level usinga high-accuracy potential energy surface. By scaling the collision reducedmass, rotational rate coefficients for collisions with para-H2 are deduced inthe temperature range 5-30 K. These collisional coefficients are combined withradiative data in a non-LTE radiative transfer model in order to reproduceobservations of propylene oxide made towards the Sagittarius B2(N) molecularcloud with the Green Bank and Parkes radio telescopes. The three detectedabsorption lines are found to probe the cold (~ 10 K) and translucent (nH ~2000 cm-3) gas in the outer edges of the extended Sgr B2(N) envelope. Thederived column density for propylene oxide is Ntot ~ 3e12 cm-2, correspondingto a fractional abundance relative to total hydrogen of ~ 2.5e-11. The presentresults are expected to help our understanding of the chemistry of propyleneoxide, including a potential enantiomeric excess, in the cold interstellarmedium.

Collisional excitation and non-LTE modelling of interstellar chiral propylene oxide