Effective tunneling pathway length

A tunneling pathway can be described in terms of an effective covalent tunneling path comprised of n (nonintegral) covalent bonds, with a total length equal to relationship between o and the direct D-A distance (R) reflects... 

The internal ET reactions in cytochrome oxidase have recently been subjected to a detailed pathway analysis [45]. The pathway for the Cua —> a ET starts with a hydrogen bond between His204 and a carbonyl group in a loop of subunit I (Eigure 6). The total pathway comprises 14 covalent bonds and two H bonds, corresponding to an effective tunneling length of 25.2 A, and an activationless rate of... 

The elimination of a selenoxide is a common, mild method for the formation of a carbon-carbon double bond, occuring via a syn-elimination pathway as shown to the right. The activation energy difference between the H- and D-substituted compounds was found to be 2.52 kcal / mol, giving an isotope effect of 74. Other experimental parameters were also found to support tunneling. For example, the A l Aq is 0.092, considerably different from 1, Furthermore, a barrier width for the reaction was calculated to be 0.82 A, definitely less than the length of a common C-H bond (1.1 A). 

Figure 22 Scheme of tip-monolayer-substrate showing relevant distances in multiple conduction pathway model chain-to-chain distance dec, natural alkanethiol length dn, effective thickness as a function of tilt 0. Tunneling through chain (blue) has decay coefficienty c and through space (red) has coefficient yffg. 

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