IRMPD action spectroscopy of metallated amino acids and hydrated metal cations have been examined. Completed projects include a collaborative project with Prof. M. T. Rodgers to study M+(Ser) [1] and M+(Thr) [2] where M+ = Li+, Na+, K+, Rb+, and Cs+. Extensions of these studies have now been made to M⁺(Asn) [3], M⁺(Met) [4], M⁺(Cys) [5], and M⁺(His) [9,10] where M⁺ = Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ and, in some cases, H⁺ and Ba²⁺ as well. For these amino acids, comparison with theoretical predictions indicate that the only conformations accessed for the complexes to the smaller alkali metal cations, Li⁺ and Na⁺, are charge solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the functionalized side chain, [CO, N, X]. For the cesiated complexes, bands corresponding to a zwitterionic structure [CO₂?] are clearly visible. The potassium and rubidium complexes exhibit evidence of the charge solvated analogue of the zwitterions, in which the metal cation binds to the carboxylic acid group, [COOH]. Calculations indicate that the relative stability of the [COOH] and zwitterionic structures are very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. Interestingly, no obvious evidence of zwitterionic structures are evident for the Ba²⁺(Asn) complex, making it the first barium-amino acid complex examined that is not primarily zwitterionic.

These studies have recently been extended to include complexes of the dications of M = Zn and Cd with histidine, His [8]. In these systems, halogenated complexes, CdCl⁺(His), deprotonated complexes, M⁺(His-H), and dimers, M²⁺(His)2, were investigated and definitively show the presence of the same tridentate conformers favored for the smaller alkali cations. For the dimeric complexes, the binding motif adopted by the second ligand is less clear, with evidence for both ligands having tridentate conformations and for one ligand having a zwitterionic binding motif. These experiments have now been extended to other amino acids, in particular, cysteine (Cys), which is also known to be a favorable binding site for Zn in many metalloproteins. A primary motivation of examining these particular metals is their environmental and biological implications. For that reason, complexes of these ions with crown ethers, a potential sequestering agent for environmental remediation purposes, were also studied. Here, the likely conformations were readily identified using IRMPD spectroscopy compared to theoretical calculations for several crown sizes, 12c4, 15c5, and 18c6.

In a collaborative effort with E. R. Williams, IRMPD spectra have also been acquired for Zn²⁺(H₂O)_n (n = 6 – 12) complexes using an OPO laser system in the OH stretch region of the spectrum [6]. These spectra suggest that the Zn²⁺(H₂O)_n complexes have a preference for a five-coordinate inner shell (as predicted using MP2 theory, but in contrast to DFT methods that predict an inner shell size of four).

In a completely different application, we have interacted with Joost Bakker and Vivike Lepoutre on a molecular beam apparatus to examine the structures of the products formed by reactions of transition metal cations with hydrocarbons using the Free Electron Laser for IntraCavity Experiments (FELICE). In the earliest work, transition metal carbene cations, MCH₂⁺, were generated from the reactions of laser ablated metal cations and methane for M = Ta, W, Ir, and Pt and examined over a spectral range running from about 400 to 3000 cm^-1. The work has now been published [11] and demonstrates that the structures of these simple species varies considerably across the periodic table. We have also examined the reactions of Ta+ and Pt+ with ethane and ethene over a spectral range running from about 200 to 3500 cm^-1. Species observed included TaC₂H₄⁺, TaC₂H₆⁺, TaC₄H₈⁺, andTaC₄H₁₂⁺, all of which lose H₂ or multiple H₂ molecules. For reactions of Pt⁺ with ethane, similar spectral ranges were covered and molecules interrogated included PtC₂H₂⁺, PtC₂H₄⁺ PtC₄H₆⁺, and PtC₄H₈⁺ which either lose H₂ or an intact C₂H₂ ligand. These spectra are presently undergoing evaluation and comparison to spectra predicted for different isomers by quantum chemical calculations.