A Crystalline Dimeric Steroidal Diboronate with Electronically Impeded Rotation

Abstract

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.

Influence of Internal Molecular Motions in the Photothermal Conversion Effect of Charge-Transfer Cocrystals

“…First, we explored if the central phenylene could experience some motion by carrying out deuterium spin– echo experiments using deuterium-enriched phenylene versions of the cocrystals ( CP d 4 -TCNQ and CP d 4 -TFBQ ). Due to its quadrupolar nature ( I = 1), this nucleus is sensitive toward changes in the orientation of its charge distribution with respect to the applied magnetic field, which makes it a widely used isotope for the detection of molecular motions in solids. , The 2 H magic-angle-spinning spectra of both cocrystals show a characteristic pattern of rotational sidebands with a Pake-like envelope corresponding to quadrupolar coupling ∼150 kHz (Figure S31). This pattern does not change with temperature, which confirms the lack of phenylene motions even at temperatures as high as 385 K for CP d 4 -TCNQ or 343 K for CP d 4 -TFBQ .…”