Conformations and Threshold Rotational Mechanisms of C5Ar5 and C5Ar4X Molecular Propellers: A Structure Correlation and Computational Study

Crystallographically independent structures possessing persubstituted cyclopentadiene and -dienyl moieties were retrieved from the Cambridge Structural Database, and the torsional angles of selected diaryl frames presented in the form of conformational plots. Semiempirical calculations of the corresponding potential energy surfaces reproduced the conformational trends observed in the solid state. By the structure correlation principle, the internal oscillations of nearest (1,2-) and next-nearest (1,3-) aryl rings in all pseudopropeller subunits were found to be only partially correlated. These solid-state data sets, in combination with energetic predictions of the molecules C(5)Ph(5)(-) (1), C(5)Ph(4)H(-) (2), C(4)Ph(4)C=O (3), and C(4)Ph(4)CH(2) (4) reveal that a delayed n-ring-flip (where n = 4 and 5 in C(n)()Ar(n)()(-)(m)()X(m)()), which is otherwise unobservable via NMR spectroscopic methods, is the threshold rotational mechanism.