  Grain-surface reactions play an essential role in interstellar chemistry,
since dust grain catalyses reactions at its surface allowing for the formation
of molecules. We used a chemical model in which both gas-phase and
grain-surface reactions occur and studied particularly the diffusion mechanisms
on the surface of the grains. Surface reactions can occur via thermal hopping
when species cross over a potential barrier or via quantum tunneling when
species cross through this barrier. We show that the thermal diffusion
(hopping) can be much more efficient after a cosmic ray particle collides with
a dust grain, heating it to a peak temperature of 70K. We present here the
results of numerical simulations after including the quantum tunneling
mechanism for species H, H2 and O and considering the effect of cosmic ray
particle collision on the surface reactions. As a consequence, the gas-phase
and grain-surface abundances are affected and we show that more complex
molecules can be formed in molecular clouds.
