The present project aims to investigate the presence of a pi-hole interaction stabilizing a non-covalent system in the gas phase, through the study of its structure and conformational landscape by ESI-CID-MS and ESI-MS-IRMPD experiments. The anion receptor chemistry plays a role of increasing importance in the supramolecular chemistry field for the unquestionable importance to have a selective system, working for instance as pollutant sensor, transmembrane channel aimed to transport anions essential for life itself, and finally as an anion extractant, able to selectively transfer ions from water to organic solutions. In the context of a supramolecular architecture stabilization, between the noncovalent interactions involved in the anion sensing, we are focusing on the anion-pi interaction, that for long has not been accounted since it is counterintuitive. The contribution of anion-pi interaction involved in the formation of complexes between electron-deficient arenes and halide ions has been underlined in crystal lattices, but investigations in solution and even more in the gas-phase are definitely rare. Our experimental setup includes two mass spectrometers: a triple quadrupole provided with a linear ion trap and a Paul trap modified with a hole that allows a laser beam to enter and activate the trapped ion cloud. Both instruments are equipped with an electrospray ion source, that very often works as reaction chamber where species not stable in solution can be formed, kinetically trapped and then isolated in the gas-phase. The experimental results need the further support by the most suitable theoretical approach (DFT, QM or MD).
Over the last twenty years our research group developed a consolidated experience in the gas-phase study of intra-and intermolecular interactions occurring in several classes of compounds. Naked ions, as for instance protonated vitamin[1], aminosugars[2], and nucleobases[3] have been selected on the ground of their archetypal importance from the structural standpoint, but even for their crucial importance in multiple biochemical processes. Furthermore, we constantly focused on the artificial receptors that can express a significant selectivity towards several classes of substances, by forming very stable supramolecular architectures. Gas-phase is a very ¿clean¿ environment where the ion chemistry can be investigated at a fundamental level, and is expected to be in close contact with other branches of knowledge. In more detail, the anion receptor chemistry is a field that has grown up over decades, for the unquestionable importance to have a selective system, working for instance as pollutant sensor, transmembrane channel aimed to transport anions essential for life itself, and even more anion extractants, able to transfer specific ions from water to organic solutions. In the context of a supramolecular architecture stabilization, between the non-covalent interactions involved in the anion sensing, we are focusing on the anion-pi interaction, that for long has not been accounted since it is counterintuitive. The solution and solid state studies that focused on the aggregates formed by neutral receptors and halide anions, revealed an intriguing scenario. In the crystal lattice the length of the methylene spacer connecting the electron deficient arene to a hydrogen-donor group (amino or amido group) has been modified to sterically tune the relative contributions of the H-bond and the anion-pi interactions. In solution, the formation constants of halide complexes with neutral receptors characterized by a pi-hole have been measured, and the result encouraged to design new multicentered receptor increasingly sensitive to the anion size and shape. The study of anion-pi interactions in the gas phase is very rare, since the absence of a solvent mediating multiple long range interactions could emphasize the hydrogen bond as the predominant binding motif. The collaboration of our group with Prof. Albrecht (organic chemistry institute of RWTH Aachen University), that has been working for years on the stabilizing effect of pi-hole in crystal aggregates, will promote both a stimulating discussion and a positive feedback between our gas-phase results and the design of new even more effective anion receptors.
Finally, the present proposal focused on the molecular recognition can provide a fundamental contribution to the deep comprehension of processes that cannot be really understood in condensed phase, where the environmental factors can drastically affect the development of the intrinsic forces involved. The isolation of a pi-hole interaction in the gas-phase could provide a revolutionary contribution to the comprehension of the cooperativity in the supramolecular self-assembly. It should clearly emerge how the importance of the molecular recognition crosses a large number of fields, some of them applicable, other more fundamental. Actually the innovation provided by the molecular recognition debate is always ¿recycling¿, since the new artificial receptors as well as the requirements arising from the biomaterials, analytical, and biochemical fields continuously stimulate the fundamental chemistry one.