A direct dynamics study of protonated alcohol dehydration and the Diels-Alder reaction

the dynamics of dehydration of the protonated (r)-3,3-dimethylbutan-2-ol (pinacolyl alcohol), [(ch3)3c-ch(oh2)ch3]+, and of ethene + 1,3-butadiene cycloaddition were studied with the born-oppenheimer molecular dynamics (bomd) technique for direct dynamics using the am1 method. more than 10,000 trajectories were generated, most of them related to the unexplored simulated annealing/fragmentation approach. the am1 potential energy surface (pes) for the protonated pinacolyl alcohol presents two transition states related to the [(ch3)3c-chch3]+hhhoh2 intermediate complex and to ch3 migration leading to the [(ch3)2c-ch(ch)3]2+hhhoh2 product complex. direct dynamics yielded negligible trajectories involving these complexes, since the momentum acquired by the h2o fragment led to a complete dissociation. thus, rearrangement of the secondary carbocation [(ch3)3c-chch3]+ was practically inexistent during the dynamics. despite the concerted path (h2o dissociation and ch3 migration) not being an irc (intrinsic reaction coordinate) path in am1-pes, a statistically significant number of trajectories involved this path. as for the diels-alder reaction, even when started from a symmetric transition state using the spin restricted am1 wavefunction, the dynamics yielded a significant number of trajectories that followed asymmetric, i.e.non-irc, paths toward cyclohexene, independent of the initialization approach. it is noteworthy that all these asymmetric path trajectories led to a concerted reaction mechanism.