THROUGH-BOND AND CHAIN-TO-CHAIN COUPLING - 2 PATHWAYS IN ELECTRON-TUNNELING THROUGH LIQUID ALKANETHIOL MONOLAYERS ON MERCURY-ELECTRODES

Formation, structure, and properties of alkanethiolate monolayers on m icrometrically driven hanging mercury drop electrodes were investigate d electrochemically. Alkanethiols with the chain length from C-8 to C- 18 were shown to form densely packed (ca. 20.3 Angstrom(2)/molecule fo r C12SH), perpendicularly oriented monolayers on mercury in a process involving two electron oxidation of Hg to form mercuric thiolate, in a greement with earlier literature reports for a number of thiols. Elect ron tunneling rates across these films (due to RU(NH3)(6)(3+) electro- reduction in aqueous 0.50 M KCl) exhibit characteristic exponential in crease with the electrode potential (with transfer coefficient a 0.25) , and an exponential decay with the monolayer thickness (with a throug h-bend decay constant, beta(tb) = 1.14 per methylene group or 0.91 Ang strom(-1)). Slow stepwise expansion of the mercury drop electrodes coa ted with alkanethiolates (C-9-C-14 only) results in an only small incr ease of the tunneling current maintaining the pin-hole free structure of the monolayers. Capacitance measurements showed that the film thick ness 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 additiona l tunneling pathway involving chain-to-chain coupling. Data analysis i n view of this parallel pathways model yielded a through-space decay c onstant, beta(ts) = 1.31 Angstrom(-1). Ab initio computations of the e lectronic coupling matrix element (based on Koopmans' theorem approxim ation) and its distance dependence across a number of perpendicularly orientated n-alkanes yielded a decay constant of 1.25 Angstrom(-1) in excellent agreement with the measurements.