Vibrational Spectroscopy of Small Hydrated CuOH⁺ Clusters

Coordinated copper hydroxide centers can play an important role in copper catalyzed water oxidation reactions. To have a better understanding of the interactions involved in these complexes, we studied the vibrational spectra of D₂ tagged CuOH⁺(H₂O)_<i>n</i> clusters in the OH stretch region. These clusters are generated by electrospray ionization and probed via cryogenic ion vibrational spectroscopy. The results show that the copper center in the <i>n</i> = 3 clusters has a distorted square planar geometry. The coordination in CuOH⁺(H₂O)_<i>n</i> is therefore more akin to Cu²⁺(H₂O)_<i>n</i> with four ligands in the first solvation shell than Cu⁺(H₂O)_<i>n</i> with two ligands in the first solvation shell. There is also no evidence of any strong axial ligand interactions. The well-resolved experimental spectra enabled us to point out some discrepancies in the calculated spectra, which were found to be highly dependent on the level of theory used

Probing the Hydrogen-Bonded Water Network at the Active Site of a Water Oxidation Catalyst: [Ru(bpy)(tpy)(H₂O)]²⁺·(H₂O)_0–4

The infrared spectra of gas-phase mass-selected [Ru­(bpy)­(tpy)­(H₂O)]²⁺·(H₂O)_0–4 clusters (bpy = 2,2′-bipyridine; tpy = 2,2′:6,2″-terpyridine) in the OH stretching region are reported. These species are formed by bringing the homogeneous water oxidation catalyst [Ru­(bpy)­(tpy)­(H₂O]²⁺ from solution into the gas phase via electrospray ionization (ESI) and reconstructing the water network at the active site by condensing additional water onto the complex in a cryogenic ion trap. Infrared predissociation spectroscopy is used to probe the structure of these clusters via their distinctive OH stretch frequencies, which are sensitive to the shape and strength of the local hydrogen-bonding network. The analysis of the spectra, aided by electronic structure calculations, highlights the formation of strong hydrogen bonds between the aqua ligand and the solvating water molecules in the first solvation shell. These interactions are found to propagate through the subsequent solvation shells and lead to the stabilization of asymmetric solvation motifs. Electronic structure calculations show that these strong hydrogen bonds are promoted by charge transfer from the H atom of the aqua ligand to the Ru–OH₂ bond

Accessing the Vibrational Signatures of Amino Acid Ions Embedded in Water Clusters

We present an infrared predissociation (IRPD) study of microsolvated GlyH⁺(H₂O)_<i>n</i> and GlyH⁺(D₂O)_<i>n</i> clusters, formed inside of a cryogenic ion trap via condensation of H₂O or D₂O 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⁺(H₂O)_<i>n</i> and GlyH⁺(D₂O)_<i>n</i> 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

Vibrational Characterization of Microsolvated Electrocatalytic Water Oxidation Intermediate: [Ru(tpy)(bpy)(OH)]²⁺(H₂O)_0–4

The infrared predissociation spectra of the mass-selected electrocatalytic water oxidation intermediate [Ru­(tpy)­(bpy)­(OH)]²⁺(H₂O)_0–4 are reported. The [Ru­(tpy)­(bpy)­(OH)]²⁺ species is generated by passing a solution of [Ru­(tpy)­(bpy)­(H₂O)]­(ClO₄)₂ through an electrochemical flow cell held at 1.2 V and is immediately introduced into the gas phase via electrospray ionization (ESI). The microsolvated clusters are formed by reconstructing the water network in a cryogenic ion trap. Details of the hydrogen bonding network in these clusters are revealed by the infrared predissociation spectra in the OH stretch region. This improved method for capturing microsolvated clusters yielded colder complexes with much better resolved IR features than previous studies. The analysis of these spectra, supported by electronic structure calculations and compared to previous results on [Ru­(tpy)­(bpy)­(H₂O)]²⁺(H₂O)_0–4 clusters, reveals the nature of the Ru–OH bond and the effect of hydrogen bonding on facilitating the subsequent oxidation to [Ru­(tpy)­(bpy)­(O)]²⁺ in the proposed catalytic cycle. Particularly, the hydrogen bonding interaction in [Ru­(tpy)­(bpy)­(OH)]²⁺(H₂O)₁ is much weaker than that in the corresponding [Ru­(tpy)­(bpy)­(H₂O)]²⁺(H₂O)₁ and thus is less effective at activating the hydroxyl ligand for further oxidation via proton coupled electron transfer (PCET). Furthermore, the results here reveal that the Ru–OH bond, though formally described as an Ru³⁺/OH^– interaction, has more covalent bond character than ionic bond character

Quantifying Intrinsic Ion-Driven Conformational Changes in Diphenylacetylene Supramolecular Switches with Cryogenic Ion Vibrational Spectroscopy

We report how two flexible diphenylacetylene (DPA) derivatives distort to accommodate both cationic and anionic partners in the binary X^±·DPA series with X = TMA⁺ (tetramethylammonium), Na⁺, Cl^–, Br^–, and I^–. This is accomplished through theoretical analysis of X^±·DPA·2D₂ vibrational spectra, acquired by predissociation of the weakly bound D₂ adducts formed in a 10 K ion trap. DPA binds the weakly coordinating TMA⁺ ion with an arrangement similar to that of the neutral compound, whereas the smaller Na⁺ ion breaks all intramolecular H-bonds yielding a structure akin to the transition state for interconversion of the two conformations in neutral DPA. Halides coordinate to the urea NH donors in a bidentate H-bonded configuration analogous to the single intramolecular H-bonded motif identified at high chloride concentrations in solution. Three positions of the “switch” are thus identified in the intrinsic ion accommodation profile that differ by the number of intramolecular H-bonds (0, 1, or 2) at play

Isomer-Specific IR–IR Double Resonance Spectroscopy of D₂-Tagged Protonated Dipeptides Prepared in a Cryogenic Ion Trap

Isomer-specific vibrational predissociation spectra are reported for the gas-phase GlySarH⁺ and SarSarH⁺ [Gly = glycine; Sar = sarcosine] ions prepared by electrospray ionization and tagged with weakly bound D₂ adducts using a cryogenic ion trap. The contributions of individual isomers to the overlapping vibrational band patterns are completely isolated using a pump–probe photochemical hole-burning scheme involving two tunable infrared lasers and two stages of mass selection (hence IR²MS²). These patterns are then assigned by comparison with harmonic (MP2/6-311+G­(d,p)) spectra for various possible conformers. Both systems occur in two conformations based on cis and trans configurations with respect to the amide bond. In addition to the usual single intramolecular hydrogen bond motif between the protonated amine and the nearby amide oxygen atom, <i>cis</i>-SarSarH⁺ adopts a previous unreported conformation in which both amino NH's act as H-bond donors. The correlated red shifts in the NH donor and CO acceptor components of the NH···OC linkage to the acid group are unambiguously assigned in the double H-bonded conformer