Abstract
Herein we report two crystalline molecular rotors 1 and 4 that show extremely narrow signals in deuterium solid-state NMR spectroscopy. Although this line shape is typically associated with fast-moving molecular components, our VT 2H NMR experiments, along with X-ray diffraction analyses and periodic DFT computations show that this spectroscopic feature can also be originated from low-frequency intramolecular rotations of the central phenylene with a cone angle of 54.7° that is attained by the cooperative motion of the entire structure that distorts the molecular axis to rotation. In contrast, two isomeric structures (2 and 3) do not show a noticeable intramolecular rotation, because their crystallographic arrays showed very restricting close contacts. Our findings clearly indicate that the multiple components and phase transitions in crystalline molecular machines can work in concert to achieve the desired motion.
Highlights
Understanding the fundamental mechanisms of motion in arti cial molecular machines is a challenging task and the focus of intense research around the globe.[1]
Drawing from these strategies, we recently reported a crystalline molecular rotor with halogen bonds, which exhibits a very fast and unusual rotation characterized by an extremely sharp deuterium signal,[12] a spectroscopic feature that is typically found in proton conducting solid materials,[13] or solvent molecules freely reorienting within crystal lattices.[14]
Taking the spectroscopic and computational data together, we propose that these minima and the distortion of the molecular axis at high temperatures are both structural requirements for the narrow deuterium signals to occur, irrespective of the rotational frequency
Summary
Understanding the fundamental mechanisms of motion in arti cial molecular machines is a challenging task and the focus of intense research around the globe.[1]. The X-ray analyses of these compounds revealed that three of them adopted a planar conformation, but contrary to the reported tetrabrominated counterpart, the solid-state 2H NMR experiments showed a broad line 2H shape at room temperature that is characteristic of static molecular components.
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