Solid-state NMR spectroscopy and ab initio calculations of some base-free organolithium compounds
This thesis is an investigation of the electronic structures for several base-free organolithium compounds using solid-state 6Li, 7Li, and 13C nuclear magnetic resonance spectroscopy and 'ab initio' calculations. NMR measurements of lithium chemical shift anisotropies ([Delta][sigma]) and nuclear quadrupolar coupling constants ([chi]) have been performed on solid samples of 't'-butyllithium (tBuLi), cyclopentadienyllithium (CpLi), penta-methylcyclopentadienyllithium (CpLi), 2,4,6-tri(isopropyl)phenyllithium (iPr3PhLi), and propynyllithium (PpLi). Of the various compounds examined, only in CpLi, CpLi, and iPr 3PhLi are the [Delta][sigma] values for lithium large enough to be measured. From spectral simulations, values of [Delta][sigma] = 42 ± 2 ppm, [Delta][sigma] = 45 ± 2 ppm, and [Delta][sigma] = 26 ± 2 ppm have been determined respectively for CpLi, CpLi, and iPr3PhLi. Moreover, 7 Li [chi] values of 100 ± 3 kHz, 11 ± 1 kHz, 18 ± 1 kHz, 323 ± 1 kHz, and 35 ± 2 kHz have been obtained for tBuLi, CpLi, CpLi, iPr3PhLi, and PpLi, respectively. It has been found that the 'ab initio' calculations of [Delta][sigma] and [chi] values do not agree with the measurements unless the solid-state structures are employed. Furthermore, computational studies have been carried out on MeLi and PhLi utilizing, correspondingly, their tetrameric and dimeric solid structures as reported in the literature. In the case of PpLi, whose crystal structure, to the best of our knowledge, has not been established, several models, having Td, C4h, and S4 symmetry, have been proposed. Unfortunately the NMR parameters calculated using these geometries do not agree with the experiments, and the solid PpLi structure remains unsolved. In addition to structural information, the dynamics of lithium atom exchange and Cp ring rotation in tBuLi and CpLi, respectively, have been studied. For lithium atom exchange in solid tBuLi, Ea = 23.8 ± 0.1 kJ/mol for the high-temperature phase and Ea = 23.0 ± 0.6 kJ/mol for the low-temperature phase. For Cp ring rotation in solid CpLi, Ea = 26.3 ± 0.3 kJ/mol.