  The explicit expressions for a nonresonant tunneling current mediated by the
bridging units of the molecular wire embedded between the metallic electrodes,
are derived. The specific regimes of the charge transmission controlled by
nonresonant and resonant participation of terminal's energy levels are studied,
and the role of energy position of delocalized orbitals in formation of a
distant superexchange electrode-electrode coupling is clarified. 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 (LUMO) level. Special attention is payed to derivation of explicit
analytic expressions for the tunneling current using different approximations.
It is shown that if terminal units play barrier's role in the charge
transmission process, the best correspondence with the observed current-voltage
characteristics is achieved with the simplest Gauss and the mean-value explicit
forms. This is supported by comparison of the theory with experimental data
concerning the current-voltage characteristics of $N-$alkanedithiol chain.
