Heavy-atom tunneling

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

Thus, these results indicated the involvement of heavy atom tunneling in the localized biradicals. The rates of decay for 19,20, and 9 could be fitted with Bell s simple model of tunneling through a parabolic barrier. Assuming log A (s ) = 8.0, and... 

M. Sponsler, R. Jain, F. Corns, and D. A. Dougherty, Matrix-isolation decay kinetics of triplet cyclobutanediyls. Ohservation of both Arrhenius behavior and heavy-atom tunneling in C-C bond-forming reactions, J. Am. Chem. Soc. 1989, 111, 2240. 

Heavy atom tunneling of entire alkyl groups between the different sides of the hydrogen bonded ring plane is much less likely even for methanol, in contrast to the analogous but fighter water case [108]. These motions correspond to hindered rotation of the monomer and are slowed down considerably in the complex due to the librational constraints. 

With = 1 and mP = 2, the low-temperature rate constant is then determined mainly by for a given value of E. The low-temperature and temperature independent kinetic isotope effect k /k is, therefore, determined by which is obtained experimentally at high temperatures. In other words, kg /kg and the high-temperature kinetic isotope effects cannot be varied independently of each other, which is not in agreement with experimental data. This effect can be associated with heavy atom tunneling during the H-transfer. The tunneling mass is increased and the low-temperature H/D isotope effect decreased. 

In order to take heavy atom tunneling into account, the expansion... 

Note that, at low temperatures, a very small minimum energy for tunneling to occur is found, which refers to the reactive complex. This value could, therefore, be determined in addition to the values of AH and AS of the pre-equilibrium. In other cases, as discussed later, only the sum of AH -i- can be determined. The barrier for the transfer is similar to that found for TTAA. The difference between the barriers for H and D is substantially large, of the order of that found for porphyrin. In addition, a contribution for heavy atom tunneling is observed. 

Week, P.F. and Balakrishnan, N., Heavy atom tunneling in chemical reactions Study of H + LiF collisions, J. Chem. Phys., 122, 234310, 2005. 

Sponsler, M. B., Jain, R., Corns, F. D., and Dougherty, D. A. "Matrix-Isolation Decay Kinetics of Triplet Cydobutanediyls. Observation of Both Arrhenius Behavior and Heavy-Atom Tunneling in Carbon-Carbon Bond-Forming Reactions." j. Am. Chem. Soc., Ill, 2240-2252 (1989). 

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