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The Diels-Alder cycloaddition, in which a diene reacts with a dienophile toform a cyclic compound, counts among the most important tools in organicsynthesis. Achieving a precise understanding of its mechanistic details on thequantum level requires new experimental and theoretical methods. Here, wepresent an experimental approach that separates different diene conformers in amolecular beam as a prerequisite for the investigation of their individualcycloaddition reaction kinetics and dynamics under single-collision conditionsin the gas phase. A low- and high-level quantum-chemistry-based screening ofmore than one hundred dienes identified 2,3-dibromobutadiene (DBB) as anoptimal candidate for efficient separation of its gauche and s-trans conformersby electrostatic deflection. A preparation method for DBB was developed whichenabled the generation of dense molecular beams of this compound. Thetheoretical predictions of the molecular properties of DBB were validated bythe successful separation of the conformers in the molecular beam. A markeddifference in photofragment ion yields of the two conformers uponfemtosecond-laser pulse ionization was observed, pointing at a pronouncedconformer-specific fragmentation dynamics of ionized DBB. Our work sets thestage for a rigorous examination of mechanistic models of cycloadditionreactions under controlled conditions in the gas phase.

physical chemistry chemical physics/pccp. physical chemistry chemical physics

Quantum-chemistry-aided identification, synthesis and experimental  validation of model systems for conformationally controlled reaction studies:  Separation of the conformers of 2,3-dibromobuta-1,3-diene in the gas phase