Molecular modeling of the kinetic isotope effect for the [1,5]-sigmatropic rearrangement of cis-1,3-pentadiene

The primary kinetic isotope effect for the sigmatropic rearrangement reaction of cis-1,3-pentadiene is studied by the direct dynamics method. The calculations are carried out with the computer code MORATE, which combines the semiempirical molecular orbital package, MOPAC, and the polyatomic dynamics code, POLYRATE, developed previously by the authors' research group. Dynamics calculations are based on canonical variational transition-state theory including multidimensional tunneling corrections. The force field is obtained by molecular orbital theory with the AM1, PM3, and MINDO/3 parameterizations. The kinetic isotope effects calculated with the MINDO/3 and PM3 Hamiltonians agree with those calculated by AM1 within 13%, and the latter agree with experiment within 13%. The tunneling contributions to the kinetic isotope effects are analyzed, and the nature of the vibrationally assisted tunneling process is discussed. General features of the dynamics from all three parameterizations are similar, and the quantitative differences in the predictions of the three calculations can be understood in terms of global characteristics of the potential energy functions that they predict. 41 refs., 8 figs., 6 tabs.