CHEMICAL-MODELS OF HOT MOLECULAR CORES

We present the results of observational studies of a number of molecul ar clouds associated with ultracompact HII regions in regions of massi ve star formation. We derive molecular abundances and show that gas-ph ase-only chemical models of these regions are incapable of reproducing the observations. It appears that a rich chemistry occurs in the ice matrices frozen onto dust grains at high densities in the collapse of molecular clouds to form stars. The switch-on of a protostar can evapo rate the ice matrices returning processed material to the gas phase. W e present evidence that the surface chemistry can fractionate molecule s in deuterium as well as form complex organic molecules.