  Context. Time dependent gas-grain chemistry can help us understand the
layered structure of species deposited onto the surface of grains during the
lifetime of a protoplanetary disk. The history of trapping important quantities
of carbon- and oxygen-bearing molecules onto the grains is of special
significance for the formation of more complex (organic) molecules on the
surface of grains. Aims. Among other processes, cosmic ray-induced UV
photo-processes can lead to the efficient formation of OH. Using a more
accurate treatment of cosmic ray-gas interactions for disks, we obtain an
increased cosmic ray-induced UV photon flux of 3.8x10^5 photons cm^-2s^-1 for a
cosmic-ray ionization rate of H2 value of 5x10^-17 s^-1 (compared to previous
estimates of 10^4 photons cm^-2s^-1 based on ISM dust properties). We explore
the role of the enhanced OH abundance on the gas-grain chemistry in the
midplane of the disk at 10 AU, which is a plausible location for comet
formation. We focus on studying the formation/destruction pathways and
timescales of the dominant chemical species. Methods. We solve the chemical
rate equations based on a gas-grain chemical network and correcting for the
enhanced cosmic rayinduced UV field. This field is estimated from an
appropriate treatment of dust properties in a protoplanetary disk, as opposed
to previous estimates that assume an ISM-like grain size distribution. We also
explore the chemical effects of photo-desorption of water ice into OH+H.
Results. Near the end of the disk's lifetime our chemical model yields H2O, CO,
CO2 and CH4 ice abundances at 10 AU (consistent with a midplane density of
10^10 cm^-3 and a temperature of 20 K) that are compatible with measurements of
the chemical composition of cometary bodies for a [C/O] ratio of 0.16. Such
comparison provides constraints on the physical conditions in which comets were
formed.
