A new experimental approach is presented in which two separate cryogenic ion traps are used to reproducibly form weakly bound solvent clusters around electrosprayed ions and messenger-tag them for single-photon infrared photodissociation spectroscopy. This approach thus enables the vibrational characterization of ionic clusters comprised of a solvent network around large and non-volatile ions. We demonstrate the capabilities of the instrument by clustering water, methanol, and acetone around a protonated glycylglycine peptide. For water, cluster sizes with greater than twenty solvent molecules around a single ion are readily formed. We further demonstrate that similar water clusters can be formed around ions having a shielded charge center or those that do not readily form hydrogen bonds. Finally, infrared photodissociation spectra of D2-tagged GlyGlyH(+)⋅(H2O)1-4 are presented. They display well-resolved spectral features and comparisons with calculations reveal detailed information on the solvation structures of this prototypical peptide.

We report an IR-IR double resonance study of the structural landscape present in the Na(glucose) complex. Our experimental approach involves minimal modifications to a typical IR predissociation setup, and can be carried out via ion-dip or isomer-burning methods, providing additional flexibility to suit different experimental needs. In the current study, the single-laser IR predissociation spectrum of Na(glucose), which clearly indicates contributions from multiple structures, was experimentally disentangled to reveal the presence of three α-conformers and five β-conformers. Comparisons with calculations show that these eight conformations correspond to the lowest energy gas-phase structures with distinctive Na coordination. Graphical Abstract ᅟ.

The IR predissociation spectra of microsolvated glycine and l-alanine, GlyH+(H2O) n and AlaH+(H2O) n , n = 1–6, are presented. The assignments of the solvation structures are aided by H2O/D2O substitution, IR-IR double resonance spectroscopy, and computational efforts. The analysis reveals the water–amino acid as well as the water–water interactions, and the subtle effects of the methyl side chain in l-alanine on the solvation motif are also highlighted. The bare amino acids exhibit an intramolecular hydrogen bond between the protonated amine and carboxyl terminals. In the n = 1–2 clusters, the water molecules preferentially solvate the protonated amine group, and we observed differences in the relative isomer stabilities in the two amino acids due to electron donation from the methyl weakening the intramolecular hydrogen bond. The structures in the n = 3 clusters show a further preference for solvation of the carboxyl group in l-alanine. For n = 4–6 clusters, the solvation structure of the two amino acids is remarkably similar, with one dominant isomer present in each cluster size. The first solvation shell is completed at n = 4, evidenced by a lack of free NH and OH stretches on the amino acid, as well as the first observation of H2O–H2O interactions in the spectra of n = 5. Finally, we note that calculations at the density functional theory (DFT) level show excellent agreement with the experiment for the smaller clusters. However, when water–water interactions compete with water–amino acid interactions in the larger clusters, DFT results show greater disagreement with experiment when compared to MP2 results.

We present an infrared predissociation (IRPD) study of microsolvated GlyH(HO) and GlyH(DO) clusters, formed inside of a cryogenic ion trap via condensation of HO or DO onto the protonated glycine ions. The resulting IRPD spectra, showing characteristic O-H and O-D stretches, indicate that H/D exchange reactions are quenched when the ion trap is held at 80 K, minimizing the presence of isotopomers. Comparisons of GlyH(HO) and GlyH(DO) spectra clearly highlight and distinguish the vibrational signatures of the water solvent molecules from those of the core GlyH ion, allowing for quick assessment of solvation structures. Without the aid of calculations, we can already infer solvation motifs and the presence of multiple conformations. The use of a cryogenic ion trap to cluster solvent molecules around ions of interest and control H/D exchange reactions is broadly applicable and should be extendable to studies of more complex peptidic ions in large solvated clusters.

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IR predissociation spectroscopy of the Gly 3 H + (H 2 O) complex formed inside of a cryogenic ion trap reveals how the flexible model peptide structurally responds to solvation by a single water molecule. The resulting one-laser spectrum is quite congested, and the spectral analyses were assisted by both H 2 O/D 2 O substitution and IR−IR double resonance spectroscopy, revealing the presence of two contributing isomers and extensive anharmonic features. Comparisons to structures found via a systematic computational search identified the geometries of these two isomers. The major isomer, with all trans amide bonds and protonation on the terminal amine, represents ∼90% of the overall population. It noticeably differs from the unsolvated Gly 3 H + , which exists in two isomeric forms: one with a cis amide bond and the other with protonation on an amide CO. These results indicate that interactions with just one water molecule can induce significant structural changes, i.e., cis−trans amide bond rotation and proton migration, even as the clustering occurs within an 80 K cryogenic ion trap. Calculations of the isomerization pathways further reveal that the binding energy of the water molecule provides sufficient internal energy to overcome the barriers for the observed structural changes, and the minor solvation isomer results from a small fraction of the ions being kinetically trapped along one of the pathways.

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The infrared predissociation spectra of [bmim](+)·(H2O)n, n = 1-8, in the 2800-3800 cm(-1) region are presented and analyzed with the help of electronic structure calculations. The results show that the water molecules solvate [bmim](+) by predominately interacting with the imidazolium C2-H moiety for the small n = 1 and 2 clusters. This is characterized by a redshifted and relatively intense C2-H stretch. For n≥ 4 clusters, hydrogen-bond interactions between the water molecules drive the formation of ring isomers which interact on top of the imidazolium ring without any direct interaction with the C2-H. The water arrangement in [bmim](+)·(H2O)n is similar to the low energy isomers of neutral water clusters up to the n = 6 cluster. This is not the case for the n = 8 cluster, which has the imidazolium ring disrupting the otherwise preferred cubic water structure. The evolution of the solvation network around [bmim](+) illustrates the competing [bmim](+)-water and water-water interactions.