Cyclobutadiene tunneling

Thus far the only known example of tunneling exchange of heavy particles is automerization of cyclobutadiene [Dewar et al. 1984 Carsky et al. 1988],... 

Cyclobutadiene—Heavy Atom Tunneling Without Intersystem Crossing 431... 

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. 

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