The comparison between
electrical transport in C
n
H
2
n
+2
S
2
alkane and C
n
H
n
+2
S
2
alkene (
n
= 4, 6, 8, 10) is studied by using a
generalized Breit-Wigner approach and considering coherent transport
mechanisms and eventual changes in the state of charge (i.e., cotunneling
processes) for both molecules. In general, the conductance of alkanes
tends to be smaller than that of similar-sized alkenes. However, cotunneling
processes have an important participation in the overall transport
in the case of alkanes but not for the alkene family. The progressive
changes in both the eigenenergies of the relevant frontier molecular
orbitals of the charged species and their spatial localization play
decisive roles in the observed differences. While the molecular orbitals
of the charged species of the conjugated molecules are hardly affected
by the applied voltage, their saturated counterparts are quite sensitive
to the external field. With this, successive avoided-crossing events
between the molecular orbitals of the single-charged alkane molecules
can lead to the appearance of nonballistic conduction channels that
make no negligible contributions to the molecular transport.