Carbon tunneling

Subsequent calculations by other workers,utilizing more sophisticated theoretical methods, have confirmed the notion that carbon tunneling dominates the au-tomerization of cyclobutadiene. Interestingly, the more recent calculations suggest that the tunneling isomerization is even more rapid than Carpenter s estimates. 

Carbon Tunneling in Singlet Carbenes As described earlier in... 

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

Fused-ring cyclopropylchlorocarbenes have been shown to ring expand to fused-ring chlorocyclobutenes with activation energies of 3-4 kcal mol-1 and activation entropies of ca —20 e.u., values that are similar to those observed for the parent cyclopropy-lchlorocarbene (Scheme 66).102 Carbon tunnelling is considered to be unimportant at low temperatures. 

Current Issues in Enzymatic Hydrogen Transfer from Carbon Tunneling and Coupled Motion from Kinetic Isotope Effect Studies... 

Zuev, P.S., R.S. Sheridan, TV Albu, D.G. Truhlar, D.A. Hrovat, and W.T. Borden (2003), Carbon tunneling from a single quantum state. Science 299, 867-870. 

When the central barrier in a double energy minimum decreases below the lowest zero point vibrational level, the situation is equivalent to a true single minimum case (Forsen et al., 1978). A carbon tunnelling mechanism suggested to explain the dynamic behaviour in 2-norbornyl cations, (Fong, 1974 Dewar and Merz, 1986) has been shown to be unimportant (Yannoni etaL 1985). 

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