Computational insights into reactions of controlled molecules

The regio- and stereoselective Diels-Alder reaction in which a diene reacts with a dienophile to form a cyclic product is widely used in synthetic organic chemistry. Many studies have aimed to unveil this mechanism and its dependence on the geometrical and electronic characteristics of the reactants.
Recent advances in molecular-beam experiments allow now conformational separation of isomers by electrostatic deflection of a molecular beam based on their different dipole moments. Hence, the separation of the conformers of a diene as well as the separation of different rotational states of small molecules is possible as long as the experimental requirements are met.
Here, the Diels-Alder reaction between conformationally selected 2,3-dibromo-1,3-butadiene (DBB) and supersonically cooled maleic anhydride (MA) is computationally studied both in its neutral and cationic variant as the first step of a combined experimental and theoretical study. Density functional theory (DFT) calculations show that the neutral reaction is concerted while the cationic reaction can be concerted or stepwise. RRKM calculations suggest that, under typical single-collision conditions, the neutral Diels-Alder product may reform the reactants and the cationic product will most likely eliminate CO2 . Reactive atomistic simulations on the neutral reaction indicate that rotational energy is crucial to drive the system towards the transition state in addition to collision energy. Comparison with the reaction of butadiene and MA shows that the presence of bromine substituents in the diene accentuates the importance of rotational excitation to promote the reaction. At the high total energies at which reactive events are recorded, the reaction is found to be direct and mostly synchronous. Reactive dynamics on the cationic reaction show that rotational energy promotes the reaction and that the recorded events are direct and mostly asynchronous.
The Diels-Alder reaction between DBB and propene has also been studied as an alternative to the reaction with MA. The neutral reaction is predicted to be concerted but with higher activation energies than the reaction of DBB with MA. The cationic variant can be either barrierless, concerted or stepwise with submerged barriers.
The proton-transfer reaction between N2H+ and H2O has been studied and found to be barrierless which supports experimental findings.