The 1H-NMR spectra of 1,1',4,4'-tetra-tert-butyluranocene show a singlet for the tert-butyl protons and three singlets for the ring protons. Coalescence occurs at -65 to -85°C with splitting of each peak at lower temperatures, corresponding to ΔG* of rotation of 8.3 Kcal mol-1. Also, coalescence was also observed for 1,1',3,3'-tetra-tert-butylferrocene and a corresponding ΔG* = 13.1 Kcal mol-1 was obtained.
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Luke et al. (1981) studied this question.