  (Abridged) The birth environment of the Sun will have influenced the
conditions in the pre-solar nebula, including the attainable chemical
complexity, important for prebiotic chemistry. The formation and distribution
of complex organic molecules (COMs) in a disk around a T Tauri star is
investigated for two scenarios: (i) an isolated disk, and (ii) a disk
irradiated externally by a nearby massive star. The chemistry is calculated
along the accretion flow from the outer disk inwards using a comprehensive
network. Two simulations are performed, one beginning with complex ices and one
with simple ices only. For the isolated disk, COMs are transported without
major alteration into the inner disk where they thermally desorb into the gas
reaching an abundance representative of the initial assumed ice abundance. For
simple ices, COMs efficiently form on grain surfaces under the conditions in
the outer disk. Gas-phase COMs are released into the molecular layer via
photodesorption. For the irradiated disk, complex ices are also transported
inwards; however, they undergo thermal processing caused by the warmer
conditions in the irradiated disk which tends to reduce their abundance along
the accretion flow. For simple ices, grain-surface chemistry cannot synthesise
COMs in the outer disk because the necessary grain-surface radicals, which tend
to be particularly volatile, are not sufficiently abundant on the grain
surfaces. Gas-phase COMs are formed in the inner region of the irradiated disk
via gas-phase chemistry induced by the desorption of strongly bound molecules
such as methanol; hence, the abundances are not representative of the initial
molecular abundances injected into the outer disk. These results suggest that
the composition of comets formed in isolated disks may differ from those formed
in externally irradiated disks with the latter composed of more simple ices.
