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Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Understanding the non-covalent interaction between metals and small ligands such as methane and ammonia is of key importance because of their industrial and biological applications. However, these interactions are difficult to study and quantify in the bulk phase due to the interaction with neighboring molecules or atoms. Gas phase spectroscopy of mass-selected clusters is a powerful technique that overcomes this challenge by allowing clusters with known composition to be studied in the gas phase. In this thesis, we investigate the interaction between three types of small molecular ligands with metal and metal cluster ions, and answer questions about their geometries and bonding by employing photofragment spectroscopy and density functional theory (DFT).
The vibrational spectra of Fe2+(CH4)n (n=1-3) are dominated by a single intense peak about 100 cm-1 redshifted from the bare methane C-H stretch suggesting that the interaction is similar among the clusters. Comparison with calculations carried out using DFT methods indicate that the observed spectra are due to octet complexes with η3 hydrogen coordination between methane ligands and the Fe atoms and that the observed peak is due to the symmetric C-H stretch. Understanding the geometry of these complexes reveals the increased extent of covalency in bonding responsible for selective reactivity as the size of the metal core increases.
Vibrational spectra of Alx+(C2H6)n (x=1,2; n=1-3) have distinct complex features, and are very different from spectra of typical methane ligated metal ion complexes. The spectra show that the most intense peak is ~200 cm-1 red shifted from the bare ethane C-H stretches, enough to exhibit Fermi resonance between the bends and the stretches. Comparison with spectra of Al+(C2H6)1-6 reveals larger red shifts for the ethane complexes, indicating that ethane interacts more strongly with Alx+ than methane. We find that scaled harmonics using dispersion corrected DFT methods do not successfully predict the spectra, and we employ dressed local mode Hamiltonian method to predict the observed spectra and identify possible isomers. These results suggest that more rigorous DFT models are needed to predict metal-ion hydrocarbon ligand interaction. Calculations in best agreement with the observed spectra predict that ethane ligands tend to bind the same side of the Alx+ indicating dispersion interaction among the ligands, and that geometries are favored where the metal core is interacting with hydrogens from both methyl groups in the ethane. These findings are crucial in understanding how C-H bond strength and the size of hydrocarbon affect bonding in metal-ion hydrocarbon complexes.
The electronic spectrum of the Cr+(NH3) complex measured via loss of NH3 ligand exhibits spin orbit splitting suggesting a transition from the ground 6A1 state to the excited 6E (Π) state. The spectrum shows a thermodynamic onset of photodissociation at 14850 cm-1 providing a precise upper limit on the binding energy. Comparison with the A ← X transition in Cr+ reveals that the excited Cr+(NH3) binds roughly 3000 cm-1 less strongly than the ground state Cr+(NH3).
Ashraf, Muhammad Affawn, "SPECTROSCOPIC STUDIES OF GAS-PHASE METAL CATION COMPLEXES WITH ALKANES AND AMMONIA" (2018). Doctoral Dissertations. 1150.