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

Marcus V P Dos Santos,Ricardo L Longo,Erico S Teixeira

doi:10.1590/s0103-50532009000400009

Abstract

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.

Highlights

The dynamical aspects of each system are presented and compared to the intrinsic reaction coordinate (IRC) descriptions. These two systems have been chosen because they are prototypes for direct dynamics studies that start from the reactant(s) equilibrium structure(s) or from the transition state structure
The IRC path ends at the [(CH3)3C-CHCH3]+···OH2 intermediate complex, most trajectories ended at the dissociated secondary carbocation, [(CH3)3CCHCH3]+, and the water fragment
Unlike the IRC on the potential energy surfaces (PES) calculated by the RHF/6-31G and B3LYP/6-31G(d) methods that correspond to the concerted reaction path, this path is not observed in the AM1-PES, but some trajectories went through this highly non-IRC pathway

Summary

Introduction

Time-dependent methods,[1,2] direct dynamics classical trajectory ones,[3,4] have been more routinely used to study the dynamical aspects of chemical reactions.[5,6,7,8,9,10,11,12] As a result, several limitations of methodsVol 20, No 4, 2009 dos Santos et al.behavior of the CH2 group as it recoils from the expelledN .14,15 the formation of the CH OH···F– hydrogenbonded complex in the OH– + CH3F → CH3OH + F– SN2 reaction is avoided in 90% of the calculated trajectories, despite the complex being ca. 125 kJ mol–1 more stable than the dissociated product CH3OH + F–.12 Several other examples are related to borderline reactions[16] that are characterized by the convergence of two or more related mechanisms which leads to another combining features of each one. Dimethylbutan-2-ol or (R)-[(CH ) C-CH(OH )CH ]+, where a TST or IRC analysis predicts a concerted path for the water dissociation and CH3 migration leading to the tertiary cation product [(CH3)2C-CH(CH3)2]+,10 while a direct ab initio molecular dynamics trajectory calculations yield a reaction path through a secondary cation intermediate,

Results

Conclusion