The electron-tunneling pathway between an electrode and the protein cytochrome c was studied on mixed C15-carboxylic acid/hydroxyl-terminated SAMs and C16-pyridine/alkane-terminated SAMs as a function of the surface concentration and the length of the diluent alkanethiol. It was found that by increasing the concentration of shorter-chain hydroxyl diluent thiol in the SAM the rate constant increased in a systematic way. When the diluent composition is high, the k 0 increases exponentially with decreasing chain length of the diluent molecules until the chain length of the diluent molecule reaches a length of about eight methylene groups. These results indicate that short-chain diluent molecules contribute to the electronic coupling between the protein and electrode, and that shortening of the diluent decreases the effective tunneling distance.

133 Cs NMR results are reported for liquid Cs-Au alloys ranging from pure Cs to 18% excess Au in the ionic liquid compound CsAu. As one approaches CsAu from the Cs-rich side, a rapid drop in the Knight shift is observed, accompanied by a sharp peak in the nuclear relaxation rate at about 5% excess Cs in CsAu. Analysis of the static and dynamic hyperfine fields shows that the conduction electrons abruptly localize, probably as Fcenter analogs, when the concentration of excess Cs falls below about 1%.

We describe a ' Cs NMR study of the ionic intermetallic semiconductor CsAu and the transition to its saltlike liquid state. Measurements of the resonance shift and nuclear relaxation rates extend from room temperature through the melting point to 650 and 700'C, respectively. A stronglytemperature-dependent spin-lattice relaxation rate and accompanying shift in the solid are attributed to extrinsic, nondegenerate, delocalized electrons which are excited from a donor band about 0.25 eV below the conduction band. A sharp 25-fold increase in the relaxation rate at the melting point is not accompanied by a similar jump in the shift, showing that the conduction electrons become localized immediately upon passing to the liquid state.