Crankshaft Motion in a Highly Congested Bis(triarylmethyl)peroxide

Crankshaft motion has been proposed in the solid state for molecular fragments consisting of three or more rotors linked by single bonds, whereby the two terminal rotors are static and the internal rotors experience circular motion. Bis-[tri-(3,5-di-tert-butyl)phenylmethyl]-peroxide 2 was tested as a model in search of crankshaft motion at the molecular level. In the case of peroxide 2, the bulky trityl groups may be viewed as the external static rotors, while the two peroxide oxygens can undergo the sought after internal rotation. Evidence for this process in the case of peroxide 2 was obtained from conformational dynamics determined by variable-temperature 13C and 1H NMR between 190 and 375 K in toluene-d 8. Detailed spectral assignments for the interpretation of two coalescence processes were based on a correlation between NMR spectra obtained in solution at low temperature, in the solid state by 13C CPMAS NMR, and by GIAO calculations based on a B3LYP/6-31G** structure of 2 obtained from its X-ray coordinates as the input. Evidence supporting crankshaft rotation rather than slippage of the trityl groups was obtained from molecular mechanics calculations.