Synthesis, Characterization, and Solid State Dynamic Studies of a Hydrogen Bond-Hindered Steroidal Molecular Rotor with a Flexible Axis

A novel steroid molecular
rotor was obtained in four steps from
the naturally occurring spirostane sapogenin diosgenin. The structural
and dynamic characterization was carried out by solution NMR, VT X-ray
diffraction, solid state 13C CPMAS, and solid state 2H NMR experiments. They allowed the identification of a fast
dynamic process with a frequency of 14 MHz at room temperature, featuring
a barrier to rotation Ea = 7.87 kcal mol–1. The
gathered experimental evidence indicated the presence of a hydrogen
bond that becomes stronger as the temperature lowers. This interaction
was characterized using theoretical calculations, based on topological
analyses of the electronic density and energies. In addition, combining
theoretical calculations with experimental measurements, it was possible
to propose a partition to Ea (∼8 kcal/mol) into three contributions,
that are the cost of the intrinsic rotation (∼2 kcal/mol),
the hydrogen bond interaction (∼2 kcal/mol), and the packing
effects (∼2–3 kcal/mol). The findings from the present
work highlight the relevance of the individual components in the function
of molecular machines in the solid state.