Conformer-dependent reactivity

Most organic molecules are flexible. Their chemical bonds can turn and twist, shifting the molecule into several distinct shapes called conformers. At room temperature, conformers often rapidly interconvert. But in a molecular beam, internal molecular motions are cooled and the interconversion is suppressed. Five years ago Jochen Küpper and colleagues showed that they could sort a molecular beam of 3-aminophenol into its two conformers, trans-3AP (on the left in the figure; carbon atoms are shown in yellow, oxygen in red, nitrogen in blue, and hydrogen in gray) and cis-3AP (on the right). (See Physics Today, June 2008, page 17 .) Because cis-3AP has an electric dipole moment three times that of trans-3AP, it’s more strongly deflected by an electric field gradient. Now Küpper and colleagues have teamed up with Stefan Willitsch to show that the conformers have different chemical reactivities. The researchers confined laser-cooled calcium ions in a cigar-shaped trap. Then they directed a beam of 3AP through an inhomogeneous electric field and toward the trap. Gradually tilting the angle of the beam allowed them to control which of the 3AP conformers interacted with the Ca⁺ ions. By monitoring the Ca⁺ population in the trap, they were able to derive the reaction rate constants for both conformers; the Ca⁺ + cis-3AP reaction, they found, was twice as fast as the Ca⁺ + trans-3AP reaction. The researchers are working to extend their method to other molecules and more complicated reactions. (Y.-P. Chang et al., Science 342, 98, 2013, doi:10.1126/science.1242271 .)