The dimeric steroid SMR-3, featuring a 1,4-phenyldiboronic
ester flanked by two pregnan-triol frameworks, was synthesized to
explore the intramolecular dynamics of its central component. The
structural data from single-crystal X-ray diffraction studies and
the Hirshfeld analyses indicate small steric effects around the aromatic
ring that should favor the intended motion. However, solid-state NMR
data obtained through VT 13C{1
H} CPMAS and 2H spin-echo experiments, using the deuterated
analogue SMR-3D4
, revealed that this component
is rigid even at temperatures where other reported steroidal molecular
rotors experience fast rotation (85 °C). A combination of classical
molecular dynamics, molecular mechanics, and correlated ab initio
calculations allowed us to distinguish the steric and electronic factors
that restrict the potential motion in this compound. The experimental
and computational data reveal that electronic components dominate
the behavior and are responsible for the high rotational barrier in
the SMR-3 crystal.