Electron Tunneling Studies

Electron transfer in biological systems where the electron donor and acceptor are separated by a long molecular distance is encountered in very important processes such as photosynthesis and respiration [54]. As natural systems are not appropriate for such studies. Gray et al. have employed proteins chemically labeled with transition metal complexes to measure ET rates in metaUoproteins. In particular, they have shown that long-lived luminescent probes enabled a wider range of ET measurements than is possible with non-luminescent complexes [55]. The blue copper protein azurin is a convenient model for the study of ET in p-sheet proteins. 

In fact this metalloprotein has a P-barrel tertiary structure and the central Cu atom adopts a trigonal planar structure by coordination of Cysll2(S), Hisll7(N) and His46(N) [56]. 

Wild-type azurin including a single surface histidine at position 83 and Glnl07His/His83Gln azurin mutant were chemically modified by the luminescent rhenium(I) phenantroline complex 30. 

The X-ray crystal structure of [Re(CO)3(phen)(His83)] AzCu was solved at high resolution. The rate of oxidation of Cu by photoexcited Re was measured on the protein crystal by transient absorption spectroscopy. It was found to be in 

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B. Wang, X.D. Xiao, X.X. Huang, P. Sheng, J.G. Hou, Single-electron tunneling study of two-dimensional gold clusters. Appl. Phys. Lett. 77(8), 1179-1181 (2000)... 

Vibrational spectroscopic studies have been reported on clusters containing a ( M3-H) ligand and approximate metal-metal force constant calculations have been carried out on some small carbonyl clusters. A simple quantitative treatment of relative CO i.r. band intensities for [M3(CO)i2] (M=Ru and Os) has been presented together with a comparative i.r. and Raman study of the v(CO) region in [MaM 3 a (CO)i2] (M = Ru, M =Os, x=0, 1, 2, 3). It has been noted that the A 2 vibrational mode of the axial CO s in [Ru3(CO)i2] is identical to the collective CO stretching mode of CO adsorbed on Ru(001) and an inelastic electron-tunneling study of [Ru3(CO)i2] on aluminium oxide has been reported. ... 

The Creutz-Taube anion, [(NH3)5Ru- N(CH=CH)2N Ru(NH3)5] + displays more obvious redox properties, yielding both 4+ and 6- - species, and much interest has focused on the extent to which the pyrazine bridge facilitates electron transfer. A variety of spectroscopic studies supports the view that low-energy electron tunnelling across the bridge delocalizes the charge, making the 5- - ion symmetrical. Other complexes, such as the anion [(CN)5Ru (/z-CN)Ru (CN)5] , are asymmetric... 

The well-known tetrahedral [Co(NCS)4]2 ion has continued to attract attention from analytical chemists, physical chemists, and spectroscopists. The inelastic electron tunneling (IET) spectrum of (Me4N)2[Co(NCS)4] was compared with IR and Raman spectra of the same complex.359 The vibrational bands due to the Me4N+ were prominent in all three spectra, but Coligand stretches were absent from the IET spectra. The lowest 4 42 4T2 electronic transition was strong in the IET spectrum but absent from the IR spectrum. The electric dipole allowed 4A2 4TX electronic transition was observed in both the IET and IR spectra and no fine structure was observed. Complex formation equilibria between Co11 and SCN- were studied calorimetri-... 

For in situ investigations of electrode surfaces, that is, for the study of electrodes in an electrochemical environment and under potential control, the metal tip inevitably also becomes immersed into the electrolyte, commonly an aqueous solution. As a consequence, electrochemical processes will occur at the tip/solution interface as well, giving rise to an electric current at the tip that is superimposed on the tunnel current and hence will cause the feedback circuit and therefore the imaging process to malfunction. The STM tip nolens volens becomes a fourth electrode in our system that needs to be potential controlled like our sample by a bipotentiostat. A schematic diagram of such an electric circuit, employed to combine electrochemical studies with electron tunneling between tip and sample, is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution... 

Napper AM, Head NJ, Oliver AM et al (2002) Use of U-shaped donor-bridge-acceptor molecules to study electron tunneling through nonbonded contacts. J Am Chem Soc 124 10171-10181... 

Fig. 7 shows a schematic of the electric circuit, employed to combine electrochemical studies of the sample with electron tunneling between sample and tip. 

Section 5 is on one particular molecule, p-benzene dithiol. This is one of the most commonly studied molecules in molecular electronic transport junctions [7] (although it is also one of the most problematic). Section 6 discusses a separate measurement, inelastic electron tunneling spectroscopy [8, 9] (IETS). This can be quite accurate because it can be done on single molecules at low temperatures. It occurs because of small perturbations on the coherent transport, but it can be very indicative of such issues as the geometrical arrangement in the molecular transport junction, and pathways for electron transport through the molecular structure. 

The nanoscale world is exciting because it is governed by rules differing from those in the macroscopic, or even microscopic, realm. It is a world where quantum mechanics dominates the scene, and events on the single-molecule scale are critical. What we know about the behavior of material on our scale is no longer true on the nanometer scale, and our formularies must be re-written. In order to study this quantum world, a quantum-mechanical probe is essential. Electron tunneling provides that quantum-mechanical tool. 

Hipps KW, Mazur U (1980) An inelastic electron tunneling spectroscopy study of some iron cyanide complexes. J Phys Chem 84 3162-3172... 

Korman CS, Lau JC, Johnson AM, Coleman RV (1979) Studies of aromatic-ring compounds adsorbed on alumina and magnesia using inelastic electron tunneling. Phys Rev B 19 994-1002... 

Yamaguchi K (1995) Phonons of indium selenide single crystals studied by inelastic electron tunneling spectroscopy. Phys Status Solidi B 190 409 113... 

Rathore et al. (2006) studied the intramolecular single-electron transfer in anion-radicals formed from fluorenylidene derivatives. The derivatives used for the reduction were Me— Flu—CH2—Flu—CH2—Flu—CH2—Flu—Me and its deuterated analog, Me—Flu—CH2—(Flu-d8)— CH2—(Flu-dj)—CH2—Flu—Me. Each parent compound initially gave an anion-radical in which an unpaired electron was tunneled between the two internal Flu nuclei and then occured within the outer Flu nuclei. In the outer part, coordinative solvation of the anion-radical by HMPA proceeded much more effectively because of ready space accessibility. Such a solvation provides a driving force for electron tunneling. As the solution electron affinities of perdeuterated aromatic hydrocarbons are less than those of perprotiated hydrocarbons, the electron tunneling was found to be at least an order of magnitude faster only in the case of [Me—Flu—CH,—(Flu-do)—CH,— (Flu-d8)-CH2-Flu-Me]-. ... 

Specific conditions of the electron transfer reactions on Hg surfaces covered with sulfur compounds have been intensively investigated by Majda, Bilewicz, Slowihski, and coworkers [122-129]. The studies on electron tunneling involving Hg—Hg junction and mono- or bilayers of alka-nethiolate trapped between small mercury... 

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