Doctor of Philosophy
Moore, David T.
Other advisers/committee members
Ferguson, Gregory S.; Roberts, James E.; Kiely, Christopher J.
This dissertation describes the development of a new deposition technique for matrix-isolation studies of metal ions codeposited with selected counter-ions. This method was developed to form and stabilize ionic complexes for spectroscopic characterization in a matrix under controlled conditions. Previous techniques have relied on inherently neutral sources such as plasmas, which use high energy transfer processes to create ionic species. The use of mass-selected cation beams has also been employed, which relies on deposition under high-energy to maintain charge balance. All of these former methods suffer from the fact that they rely on secondary processes to generate counter-ions, which are only coarsely controlled at best. One group before us has attempted to deposit mass-selected beams of both cations and anions selectively, but observed no bands assigned to an ionic species. We have shown for the first time that ions can be isolated in a matrix through the simultaneous deposition of selected cations and anions. As proof of concept, anionic copper atoms were codeposited with Ar+ or Kr+ into argon matrices along with varying concentrations of CO (0.02% up to 2%) at deposition temperatures of 10K or 20K. Both anionic and neutral copper carbonyl complexes Cu(CO)nq (n=1-3; q=0,-1) were observed in the spectra, with peak positions corresponding to previously reported assignments; new partially resolved bands appearing in the range 1830-1845 cm-1 are assigned to larger [Cu(CO)3*(CO)n]- aggregates, having additional CO ligands in the second solvation shell. Deposition in the absence of ambient light at 10 K affords "clean" distributions of matrix-isolated copper carbonyl anions, whereby only the anionic bands are present in the CO-stretching region of the vibrational spectrum. Furthermore, photodetachment by mild irradiation with visible light was used to initiate complete conversion of the anions into their corresponding neutral species. We demonstrated that the photodetached electrons initiate covalent chemistry in the van der Waals dimer of CO, which forms a C-C bond following electron capture to make trans-OCCO-. After deposition of anionic copper carbonyl precursors at 20 K, annealing led to many new sharp features in the anionic region. These peaks could then be converted into transient neutral bands upon photodetachment. Due to the high level of control afforded by this new deposition method, neutralization events occured sufficiently far from the copper centers such that the neutral transients formed were stabilized and could be traced back to their anionic precursors. Annealing the system after irradiation gets rid of the neutral transients that seemed to "relax" to previously assigned neutral copper carbonyl species. The phenomenology of these bands suggest that they may represent an unprecedented direct observation of vertical detachment products.Under high-energy deposition, cationic species form due to secondary charge-transfer interactions between Ar+ and dopant molecules. Using this method with CO-doped matrices, (CO)2+ has been identified for the first time in an argon matrix. Likewise, oxygen-doped matrices produce (O2)2+ species which display remarkable conformational thermal-equilibrium between 10-16 K. Quantitative analysis revealed a weakly endothermic reaction, driven by increased entropy of the matrix for the cyclic product state. Problems encountered with quantitative van't Hoff analysis of the temperature-dependence for the equilibrium constant reveal that the assumptions underlying this standard analysis technique may break down at the very low temperatures of these experiments. Furthermore, two other peaks display conformational photochromic-equilibria at 10 K. It is likely that one set of peaks arises due to the quartet electronic state, which has been the focus of all previous work, whereas the second set of peaks arises from the doublet electronic state, which is able to be stabilized in the matrix. The advantages of our new deposition technique made the study of this complicated system possible.Finally, in order to test whether some of the new behavior observed was unique to the copper carbonyl system or whether it was more general and extended to other metal carbonyl systems, nickel and silver anions were tested. Similar to the copper system, both anionic and neutral nickel carbonyl complexes Ni(CO)n- (n=1-3) and Ni(CO)n (n=1-4) were observed in the spectra, with peak positions corresponding to previously reported assignments. There were also many additional sharp bands that have not been previously observed. While the silver system did not produce any anionic carbonyl bands, it did form neutral complexes. Two of these absorbances can be assigned to the di- and tri-carbonyl complexes; the remaining bands do not match well with any previous assignments, which is puzzling for such a simple system. Possible explanations for this discrepancy are explored.Using selected counter-ions for the deposition of metal anions allows for the deposition and stabilization of ionic complexes in argon matrices. This method affords an unprecedented cleanliness and level of control for matrix isolation studies. The development of this technique has enabled the discoveries presented in this dissertation.
Ludwig, Ryan Michael, "Simultaneous Counter-Ion Codeposition: A Technique Enabling Matrix Isolation Spectroscopy Studies Using Low-Energy Beams of Mass-Selected Ions" (2015). Theses and Dissertations. 2697.