Spectroscopy of isolated and microsolvated ion pairs – IONPAIRS

IR and UV laser spectroscopy of isolated ion pairs has recently been enabled by laser desorption of saline samples. On the one hand, these experiments provide detailed knowledge of the ion pair structures, but the latter can be significantly different from those formed in solution. On the other hand, data on isolated species make it possible to test, validate and precisely calibrate theoretical methods and then apply them to solvated species. In particular, gas phase data can help to focus the efforts of calculations to make them sufficiently quantitative for a comparison with the IR or X-ray spectroscopy experiments carried out in solution.

In particular, this project made it possible to identify the pairings between carboxylate anions and alkaline cations which play a major role in shaping proteins. Unexpectedly, the study of isolated ion pairs revealed a strong correlation between the observed spectral shifts and the calculated electric fields, thus significantly expanding the experience-theory interface, which is essential for the interpretation of IR and UV spectroscopy for molecular
structure elucidation. The construction of an instrument dedicated to the study of isolated ion pairs is also one of the noticeable achievements of the project.

The project made it possible to establish new interdisciplinary collaborations between gas-phase chemical physics and solution chemistry, which constitute a basis for the pursuit of the research axes initiated. The new instrument dedicated to the spectroscopic study of isolated ion pairs has yet to be fully exploited on a larger set of systems, thus making it possible to consider many subsequent results. Finally, the new field of research opened up by the exploitation of intramolecular Stark effects to elucidate structures suggests the possibility of investigating larger molecular systems, and still needs to be further explored.

At the formal end of the project, 6 communications were published, and 3 others are in preparation. The results were disseminated in several scientific events, mainly through 13 oral communications including 8 invitations. In addition, 2 PhD theses were defended, including one directly funded by this project.

Ion pairs are ubiquitous in Nature, from sea water and aerosols, to living organisms. Being the very first step of crystallisation of ionic species and influencing the properties of ion-concentrated solutions or ionic liquids, they also play a key role in countless applications. Although they are met in many areas of Physics, Chemistry and Biology, their characterisation is complicated by the co-existence of several types of pairs and their elusive nature in solution. Research in this field gathers, so far, condensed phase experiments and theoretical studies, leaving the description of ion pairs at the microscopic level with rare experimental counterparts.

This project aims at documenting net neutral ion pairs at the atomic scale by investigating them in the gas phase using IR and UV laser spectroscopy combined with quantum chemistry calculations. For this purpose, we will develop a dedicated instrumental setup in order to reveal the intrinsic optical properties of ion pairs of each kind, which will lead, with the assistance of high level theoretical methods, to the characterisation of both their tridimensional structure and the interactions within the pair. We will apply this approach to a large variety of systems, from isolated to microsolvated, in a series of experiments where each ion pair or solvent molecule is added in a controlled way. We will target several basic questions from three different fields, demonstrating the potential interdisciplinary benefits of such a novel approach: the influence of nearby counterions on the structure of charged biomolecules, the ability of solvent molecules to dissociate an ion pair, and the early stages of ionic crystallisation will all be investigated at the microscopic level in an unprecedented way. It is expected that the advances resulting from such an original approach will advertise cold ion pairs as a great playground for gas phase physical-chemists, and will promote the construction of new bridges between scientific communities.