  We compute the transmission of an electron through an impurity in polyacene.
An analytical expression for the transmission coefficient is found. For
simplicity the disorder is confined to a single unit cell, but the
generalization to several unit cells is straightforward. Both on-site and
off-diagonal defects are considered. When the impurity preserves the inversion
symmetry around the central axis, the scattering problem can be reduced to that
of two independent chains with an alternating sequence of two types of atoms.
