Formation, structure, and properties of alkanethiolate monolayers
on micrometrically driven hanging mercury
drop electrodes were investigated electrochemically. Alkanethiols
with the chain length from C8 to C18 were
shown
to form densely packed (ca. 20.3 Å2/molecule for
C12SH), perpendicularly oriented monolayers on mercury
in a
process involving two electron oxidation of Hg to form mercuric
thiolate, in agreement with earlier literature reports
for a number of thiols. Electron tunneling rates across these
films (due to Ru(NH3)6
3+
electro-reduction in aqueous
0.50 M KCl) exhibit characteristic exponential increase with the
electrode potential (with transfer coefficient α =
0.25), and an exponential decay with the monolayer thickness (with a
through-bond decay constant, βtb = 1.14
per
methylene group or 0.91 Å-1). Slow stepwise
expansion of the mercury drop electrodes coated with
alkanethiolates
(C9−C14 only) results in an only small
increase of the tunneling current maintaining the pin-hole free
structure of the
monolayers. Capacitance measurements showed that the film
thickness changes inversely proportionally with the
electrode surface area. The increase of the tunneling current
recorded in the drop expansion experiments was accounted
for by postulating existence of an additional tunneling pathway
involving chain-to-chain coupling. Data analysis in
view of this parallel pathways model yielded a through-space decay
constant, βts = 1.31 Å-1. Ab
initio computations
of the electronic coupling matrix element (based on Koopmans' theorem
approximation) and its distance dependence
across a number of perpendicularly orientated n-alkanes yielded a decay
constant of 1.25 Å-1 in excellent agreement
with the measurements.
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