Steric effects in the proton magnetic resonance spectra of fluorenes and bifluorenyls

High-resolution PMR spectra, measured at 60 MHz over temperatures between −80° and +160° in CS 2, CDCl 3 and (CD 3) 2SO solution, are described for bifluorenyl (IIa), 9-methyl-9,9′-bifluorenyl (IIc), 9-t-butyl-9,9′-bifluorenyl (IIb), five 9-substituted fluorenes, and two 9,9-disubstituted fluorenes. Both 9-(α-naphthyl)-fluorene (Ib) and IIb show coexistence of topographical variants at low temperatures, and comparison of 9-methyl-9-t-butyl- (Ig) and 9-ethyl-9-t-butylfluorene (Ih) spectra with that of 9-t-butylfluorene (If) indicates a reduction in primary proximity deshielding as the size of the second substituent increases. In IIb, the t-butyl resonance is broad at low temperatures, narrows with increasing temperature, and then passes through a remarkable maximum in width of 30 Hz at room temperature before sharpening at high temperatures, when rotation appears to be free about both the 9,9′ and 9-t-bntyl bonds.