Chemical Models of Hot Molecular Cores

We present the results of observational studies of a number of molecular clouds associated with ultracompact HII regions in regions of massive star formation. We derive molecular abundances and show that gas-phase-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 evaporate the ice matrices returning processed material to the gas phase. We present evidence that the surface chemistry can fractionate molecules in deuterium as well as form complex organic molecules.