title: Anisochronous dynamics in a crystalline array of steroidal molecular rotors: evidence of correlated motion within 1D helical domains.
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.
authors.affiliation: Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, México D.F. Apdo. Postal 14-740, 07000 México.
authors.forename: Braulio
authors.initials: B
authors.lastname: Rodríguez-Molina
authors.forename: Norberto
authors.initials: N
authors.lastname: Farfán
authors.forename: Margarita
authors.initials: M
authors.lastname: Romero
authors.forename: J Manuel
authors.initials: JM
authors.lastname: Méndez-Stivalet
authors.forename: Rosa
authors.initials: R
authors.lastname: Santillan
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Garcia-Garibay