Spectroscopic Characterization of a Cryo-cooled Gas-phase Anion Molecular Cage: Amide-Cryptand Complexed with F^-, Cl^- and Br^- Anions

Cryptands are three-dimensional molecular cages that are designed to selectively capture ions from aqueous solution. By pre-organizing the binding pocket, the cryptand cages efficiently capture particular ions based on the shape, size, and charge state of the ion. While in its early manifestations the cryptand cages were designed to capture cations, a growing number of cryptand-like anion receptors have also been developed. We report on the IR spectroscopy and structural analysis of an amide-based cryptand (AmCrypt) cage synthesized by one of us (KBJ) to selectively capture fluoride ion out of aqueous solution. The amide cryptand cage incorporates an isophthalamide moiety, with its two amide groups, on each of the three linking chains. The three chains are linked together by tertiary amines at either end of the cage. When the six amide NH groups point in towards the cage center, they form a binding pocket for anions, particular the halides. We show that all three halide anions are efficiently captured by the amide cryptand cage in the gas phase. The IR spectra in the NH and CH stretch regions are remarkably simple, reflecting a highly symmetric structure. We will compare the spectra with first-principles predictions of the infrared spectra in the NH, CH, and fingerprint regions. The CH stretch region is modeled using a local mode anharmonic model. We assign all three X^-…AmCrypt complexes to a structure in which the six NH groups take up an octahedral geometry, forming six H-bonds with the central halide anion. We will report on the spectroscopic signatures and what they reveal about the mode of binding of each halide as a function of anion size.