Nanoscale rotational dynamics of four independent rotators confined in crowded crystalline layers

Abstract

We report a study where Car–Parrinello molecular dynamics simulations and variable-temperature (30–300 K) ¹H spin–lattice relaxation time experiments nicely complement each other to characterize the dynamics within a set of four crystalline 1,4-diethynylbicyclo[2.2.2]octane (BCO) rotors assembled in the metal–organic rotor, {Li⁺ ₄(⁻CO₂-Ph-BCO-py)₄(H₂O)₈}·2DMF. The remarkable finding of this work is that, despite the individual rotational barriers of four rotors being indiscernible and superimposed in a broad relaxation process, we were able to unravel a strongly interrelated series of rotational motions involving disrotatory and conrotatory motions in pairs as well as rotational steps of single rotators, all three processes with similar, sizeable rotational barriers of 6 kcal mol⁻¹. It is noteworthy that DFT molecular dynamics simulations and variable-temperature (30–300 K) proton spin–lattice relaxation time experiments deliver the same high value for the rotational barriers stressing the potential of the combined use of the two techniques in understanding rotational motion at the nanoscale.