Charge Accommodation in Organic Motifs: Using Vibrational Predissociation Spectroscopy of Cold Gas Phase Ions to Unravel Structures of Reactive Intermediates

Infrared spectroscopy, one of the most important structural diagnostics in chemistry, allows the "fingerprinting" of a molecule based on its characteristic vibrational transitions. Often, it is challenging to obtain information beyond functional group identity by using conventional methods of vibrational spectroscopy because spectra typically are broadened at room temperature and are complicated by intermolecular interactions in the condensed phase. To circumvent these issues, a recent approach has been to remove solute ions from solutions by electrospray ionization and to cool these ions to near 10 K using a cryogenic ion trap. Alternatively, cold isolated ions can be prepared in an ionized free-jet expansion of a volatile precursor. In this thesis, both of these approaches are used to address charge accommodation in organic motifs, which often include strong ionic hydrogen bonds. Cryogenic-ion vibrational predissociation (CIVP) is employed, in which the ions are interrogated by one-photon resonant infrared photodissociation of weakly bound van der Waals adducts (i.e., analyte bound to H2, N 2, or Ar). This method is a useful tool for isolating and obtaining infrared spectra of cold reactive intermediates that are transient and difficult to study in situ. The specific systems that will be the focus of this document include silyl cations, deprotonated cysteine, disubstituted naphthalene bases, protonated diglycine, and hydrated pyridinium radicals. In each of these examples, the inherent spectroscopic signatures of the isolated ions are revealed, which are in some cases quite simple (i.e., silyl cations) and in other cases intrinsically complicated through Fermi Resonance interactions and floppy motions of the molecular framework in its zero-point vibrational level. In the latter cases, much of the complexity has been elucidated by theoretical studies that incorporate third-order terms into the vibrational Hamiltonian.