Carbon dioxide-methanol intermolecular complexes in interstellar grain mantles

We present new laboratory data to interpret the Infrared Space Observatory (ISO) spectra of protostellar objects, and particularly RAFGL7009S. Our exp erimental results show that solid methanol and carbon dioxide exhibit speci fic intermolecular interactions. We propose the formation of a Lewis acid-b ase complex between carbon dioxide and methanol molecules to explain specif ic substructure of the 15.2 mu m CO2 bending mode observed in different obj ects. The various CO2 bending mode patterns observed in many lines of sight can b e interpreted as a combination of both this complex formation and the tempe rature evolution of the ices. The temperature induced segregation of ice ma ntles containing CO2 can be monitored by the (CO2)-C-13 stretching mode shi ft toward the pure CO2 ice position. The large width observed for this mode towards interstellar sources partly results from the different temperature s sampled along the line of sight. Given the amount of methanol involved in RAFGL7009S, on the basis of ground based observations, we derive that about half of the so called "6.85 mu m" band and a quarter of the 4.9 mu m bands can be accounted for by the defor mation modes and 2v(8) transitions of CH3OH.