  The modified superexchange model is used to derive the expression for
nonresonant tunneling conductance mediated by localized and delocalized
molecular orbitals associated with the terminal and the interior molecular
units respectively. The model is shown to work as long as delocalization of
electron density in the chain's molecular orbitals is sustained during the
tunneling. The criteria for reduction of the superexchange model of charge
tunneling to the flat barrier model are formulated and the parameters of the
barrier model (energy gap and effective electron mass) are specified in the
terms of inter-site coupling and energy distance from the Fermi level to the
delocalized wire's HOMO level. Application of the theory tothe experiment shows
that the modified superexchange model is quite appropriate to explain the
experimental results in case of the nonresonance tunneling conductance in
--(CH$_2)$$_N$--NH$_2$ and HOOC--(CH$_2)$$_N$--COOH molecular wires.
