A Study of Hypercoordination in Organogermanes using 73Ge NMR Spectroscopy
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Introduction Section I. Goals of the research 73 Ge NMR spectroscopy has received minimal attention in previous literature due to the difficulty in observing the resonance of the nucleus, which has a low resonance frequency due to its low magnetogyric ratio and large quadrupole moment. The anisotropic interaction betweenthe non-zero quadrupole moment of the germanium nucleus and the electric field surrounding thenucleus results in a significant reduction in relaxation time. According to the Heisenberg uncertainty principle (AEAt = h/2it), the product of AE (the energy difference of the spin states) and At (the lifetime of a spin state, as measured by the relaxation time) must be constant. If the lifetime of an excited state is very short (as in the case of germanium), the uncertainty of energy must be very large. An uncertainty in energy is associated with an uncertainty in the frequency of a resonance, which leads to line broadening (an increase in half-height width). The anisotropic interaction is minimized when the electric field surrounding the germanium nucleus is symmetrical. For example, the electron density in a compound with tetrahedral symmetry is evenly distributed around the quadrupolar nucleus, thereby increasing the relaxation time of the germanium nuclei. A main group atom is said to be hypervalent if it contains more than 8 valence electrons. Hypervalency is a fundamental principle that applies to many areas of chemistry. For example,many crucial intermediates of reactions, such as phosphoranes, phosphonates, and sulfonium ylides, are hypervalent.l In this study NMR spectroscopy was used as a tool to probe hypervalency in main group elements-specifically the research examined if NMR spectroscopy is a valuable method in observing hypervalency in germanes. NMR spectroscopy studies of other group IV elements such as silicon, tin, and lead (elements with I = 1 / 2) show that an increase in electron density around the nucleus due to hypercoordination produces an upfield shift in the NMR resonance of these elements?"null In the case of germanium, hypervalency should cause an upfield shift in resonance and an increase in half height width due to a decrease in symmetry around the germanium nucleus. However, the literature reports conflicting results about hypervalency effects on germanium resonance. In one study by Takeuchi et aZ, , it is stated that only an increase in half-height width of germanium resonance is a good indication of hypercoordination. However, Kupce et aZ. reports the ~e NMR spectra of numerous hypervalent germanium structures in which only an upfield shift is observed. The present study was conducted in an attempt to resolve the discrepancies that appear in the literature. Two main goals have been identified: to study the hypervalency of organogermanes and to determine whether 73Ge NMR spectroscopy can be used to study hypervalency in organogermanes. These goals were accomplished by looking at both the intermolecular coordination of GeC14 and the intramolecular coordination of tetrakis organogermanes. In studying intermolecular coordination, the Lewis acidity of GeC14 can be compared to that of other group IV tetrachlorides. The series of tetrakis organogermanium compounds provides an ideal model for the intramolecular study because the apparent symmetry of the electric field surrounding the nucleus should produce sharp 73Ge resonances. Observationof a change in half height width due to a decrease in symmetry around the germanium nucleus and/or an upfield chemical shift due to an increase in electron density around the germanium nucleus would provide evidence of intramolecular hypercoordination. Molecular modeling wasused to confirm the presence of intramolecular hypercoordination in tetrakis organogermanium compounds.
Franklin and Marshall College Archives, Undergraduate Honors Thesis 2008
- F&M Theses Collection