Anisochronous dynamics in a crystalline array of steroidal molecular rotors: evidence of correlated motion within 1D
Braulio Rodríguez-Molina1, Norberto Farfán, Margarita Romero
1Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, México D.F. Apdo. Postal 14-740, 07000 México.
Abstract:
We describe the solid-state dynamics of a molecular rotator (2) consisting of a p-phenylene rotor flanked by two ethynyl steroidal moieties that act as a stator. Single-crystal X-ray diffraction analysis of polymorph I revealed a packing motif containing 1D columns of nested rotors arranged in helical arrays (space group P3(2)) with the central phenylenes disordered over two sites related by an 85° rotation about their 1,4-axes. Unexpected line shapes in quadrupolar-echo (2)H NMR measurements between 155 and 296 K for the same polymorph with a deuterated phenylene isotopologue (2-d(4)) were simulated by trajectories involving fast (>10(8) s(-1)) 180° rotation (twofold flips) in each of the two rotationally disordered sites and slower exchange (2 × 10(4) to 1.5 × 10(6) s(-1)) between them. A negative activation entropy and a low enthalpic barrier for the slower 85° exchange are interpreted in terms of highly correlated processes within the 1D helical domains.
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