The [1,n] hydrogen shift reactions (n = 3, 5, 7, 9, and 11) in CH-(CH=CH)-CH=CH have been investigated by ab initio calculations, and the results are compared with the corresponding reaction in systems where the terminal double bond is substituted by an allne unit. Activation energies at the MP2/6-31G*//HF/3-21G level reproduce experimental results for the [1,5] and [1,7] reactions quite accurately. For the [1,3] and [1,5] reactions the introduction of an allene moiety lowers the activation energy by 10-12 kcal/mol, while the effect for the [1,7] and [1,9] reactions is only 1-2 kcal/mol. By considering the transition structures as two interacting radical fragments it is possible to rationalize both the variation in activation energy for the parent systems and the difference in activation energy between the parent and allene systems. The calculations indicate only a very weak dependence of primary kinetic isotope effects on transition state geometries (linearity and symmetry of the hydrogen transfer), and the calculated values are much smaller than experimental data, suggesting that tunneling is important.
