Résumé
Almost 200 molecules have been detected so far in the interstellar medium and circumstellar shells among which 60 are organic compounds with more than 6 atoms with a straight-chain backbone. Understanding the formation and destruction paths of these molecules both in the gas-phase and in/on solids (cosmic grains, ices) is crucial to diagnose the physical condition of the environments where this chemistry take place. There is therefore a strong demand from the astrophysical community for its study and theoretical chemistry had become an important tool for such a task specially when experiments are not possible because of the unstable species involved (radicals, ions) or because of the extreme conditions of temperatures (10-100K) and densities (102-108 particles/cm3) to be considered and not always reproducible in the laboratory. To illustrate the role of theoretical chemistry in understanding space chemistry I will present few theoretical studies that have elucidated astrochemical problems