  The Diels-Alder cycloaddition, in which a diene reacts with a dienophile to
form a cyclic compound, counts among the most important tools in organic
synthesis. Achieving a precise understanding of its mechanistic details on the
quantum level requires new experimental and theoretical methods. Here, we
present an experimental approach that separates different diene conformers in a
molecular beam as a prerequisite for the investigation of their individual
cycloaddition reaction kinetics and dynamics under single-collision conditions
in the gas phase. A low- and high-level quantum-chemistry-based screening of
more than one hundred dienes identified 2,3-dibromobutadiene (DBB) as an
optimal candidate for efficient separation of its gauche and s-trans conformers
by electrostatic deflection. A preparation method for DBB was developed which
enabled the generation of dense molecular beams of this compound. The
theoretical predictions of the molecular properties of DBB were validated by
the successful separation of the conformers in the molecular beam. A marked
difference in photofragment ion yields of the two conformers upon
femtosecond-laser pulse ionization was observed, pointing at a pronounced
conformer-specific fragmentation dynamics of ionized DBB. Our work sets the
stage for a rigorous examination of mechanistic models of cycloaddition
reactions under controlled conditions in the gas phase.
