59th Annual Report on Research 2014

For the first test case, we studied the structure of [Cu(II)OH]⁺(H₂O)_n clusters, with n=1-3.   This is a relatively simple coordinated metal complex, with OH^- and H₂O serving as ligands.  It is a good model system for the coordinated Pt(II) catalysts of interest.  The experimental results, shown in Figure 1, indicated that we can acquire well-resolved vibrational spectra of the metal-center complexes, which allowed for a detailed analysis and assignment.  Notably, the spectra showed that the copper center in the CuOH⁺(H₂O)₃ cluster has a distorted square planar geometry.  Therefore, the coordination in CuOH⁺(H₂O)_n is more akin to Cu²⁺(H₂O)_n with four ligands in the first solvation shell than Cu⁺(H₂O)_n with two ligands in the first solvation shell.  This study also highlighted the dependence of the calculated hydroxide frequency on the theoretical method used, and illustrated potential inaccuracies in theoretical treatment of open-shell coordinated metal complexes.      Next, we studied the structures of deprotonated glycine peptides of different lengths.  For these peptides, we observed very well-resolved vibrational features from 1200 cm^-1 to 3500 cm^-1, in the amide A, amide I, amide II, and N-H stretch regions, allowing for unambiguous assignment of the observed features. We can clearly see the evolution of the hydrogen bonding network as a function of peptide length, and observed the effect of such interactions on the CO and NH vibrational frequencies. The results from these studies show that the instrument is fully capable of spectroscopically characterizing the platinum catalytic complexes.  To generate and isolate the complexes of interest, we are currently modifying the source region of our instrument.  Particularly, we installed a gas phase reaction trap between the first and second ion guide, which allows us to utilize gas-phase chemistry to controllably form and isolate specific complexes of interest.  Specifically, a suitable precursor can be trapped in the reaction cell, and via collisions, desired species can form either by fragmentation or by reaction with a gaseous reactant. An example of such process is shown in the Figure 2, in which fragmentation and reaction have been used to produce the important [Ru(tpy)(bpy)O₂]²⁺ complex, the presumed last intermediate in the [Ru(tpy)(bpy)]²⁺ catalyzed water oxidation cycle.

We are currently modifying the reaction trap to enable liquid nitrogen cooling and precise temperature regulation, necessary for optimal formation of complexes with varying binding energies. We will utilize this reaction cell to isolate the s-alkane Pt(II) complex.  Previous work has shown that electrospray ionization can produce the reactive [(N-N)Pt(CH₃)(solvent)]⁺ species.  Collision induced dissociation (CID) yields the [(N-N)Pt(CH₃)]⁺ reactive species with a vacant ligand position, which can further react in the gas-phase with benzene and produce [(N-N)Pt(CH₃)(C₆H₆)]⁺.  This is analogous to the reactions shown in Figure 2.  Therefore, we fully expect to be able to isolate the Pt(II)-CH₄ complex using our gas phase reaction trap.  We will start with generating [(NH₃)₂Pt(Cl)(H₂O)]⁺ using our ESI, and trap the ions in the temperature-regulated reaction cell filled with a buffer gas containing some percentage of methane gas. Shown in Figure 3 is the schematic potential energy surface, calculated at the cam-B3LYP/SDD/6-311+g(d,p) level, for the gas-phase reaction of [(NH₃)₂PtCl]⁺ with methane.

 It is clear from these energies that starting from [(NH₃)₂PtCl]⁺ makes it thermodynamically favorable to capture the s-CH complex, which is the lowest energy structure and thus the most likely outcome of the gas-phase reaction. Once isolated, the [(NH₃)₂Pt(Cl)(CH₄)]⁺ complex will be probed via infrared predissociation spectroscopy in the 1000-4000 cm^-1 range to clearly identify its structure.  We will pay special attention to the 2000-3000 cm^-1 region where the s-CH stretch mode is expected to appear. The grant has been used to support one graduate student and part of the PI’s summer salary.  The graduate student participated in the construction of the instrument and the initial experiments outlined above.  In the process, he has gained new knowledge and experiences in instrumentation and data analysis, and an overall deeper understanding of fundamental molecular properties and interactions.  He is currently continuing the experiments while teaching new graduate students in the lab.  The grant was also used for the graduate student’s travel to conferences, where he interacted closely with other scientists in similar fields.  This grant has also help the PI by yielding preliminary data for federally-funded grant proposals.