Date of Award
Doctor of Philosophy (PhD)
Leslie G. Butler
The goal of this study is to develop a method of characterizing the aluminum sites in solid aluminum containing systems that are difficult to determine by normal nuclear magnetic resonance spectroscopy, NMR, or X-ray crystallography. Specifically, the aim is to characterize the aluminum sites in the industrial cocatalyst methylaluminoxane, MAO. The structure of MAO, found in both solid and gel form, has not been well determined. First, an automated 27Al (spin 5/2) single-crystal NMR experiment was used to determine the experimental electric field gradient, EFG, of andalusite, a well-studied mineral with large quadrupolar interactions. The experimental results were compared to ab initio calculated values. All aluminum sites in the unit cell could be assigned, even the crystallographically equivalent but magnetically inequivalent sites. Next, a variable-temperature frequency-stepped NMR experiment was developed for use with powder samples. The method was tested on a niobium (spin 9/2) complex and validated with a spinning side-band analysis and X-ray crystal determination. From the frequency-stepped NMR spectra of four imidoalanes, containing ring or cage structures, the 27Al Cq and eta values were determined. The experimental results compared well to ab initio calculations and connections could be made between the corresponding ring angles or distances and values for Cq and eta. Finally, variable-temperature solid-state 27Al NMR spectra of MAO were acquired from -80° to 130°C. An analysis shows that pristine MAO has a highly rigid structure. Slight exposure to air causes the MAO molecules to become more mobile with no detectible -OCH3 groups in the 13C[1H] CPMAS spectrum. The 27Al frequency-stepped NMR spectrum was used to determine a four-coordinate aluminum resonance with a Cq of ∼26 MHz and eta ∼ 1. This study thus proposes a highly rigid polymeric structure of MAO containing groups of two six-membered ring dimers connected by agostic Al-H-CH2 bonding all with four-coordinate aluminum sites. On slight exposure to air the agostic bond is replaced with oxygen and individual cage structures are formed which have more mobility than the polymeric structure and potential for four-, five- and six-coordinate aluminum sites.
Bryant, Pamela Louise, "Solid -State NMR of High Spin Nuclei With Large Quadrupolar Coupling Constants." (2000). LSU Historical Dissertations and Theses. 7144.