Quantum Rotations and Chiral Polarization of Qubit Prototype Molecules in a Highly Porous Metal–Organic Framework:¹H NMR<i>T</i>₁Study

Abstract

Rotations of bistable diazabicyclooctane (dabco) molecules N2(C2H4)3 in a layered highly porous metal–organic framework [Zn2(C8H4O4)2·N2(C2H4)3] have been studied at 310–8 K by the 1H NMR spin–lattice relaxation method. Above 165 K, the relaxation is characterized by a single longitudinal relaxation time, T1, related to the hindered rotation of the dabco molecules. Below 165 K, the quantum tunneling mechanism was found to be responsible for dabco reorientation and spin relaxation. At 165–25 K, the system is characterized by two different spin–lattice relaxation times, T1_1 and T1_2, related to two separate states of dabco in the ratio of ∼1:2. The first state is related to the conformation of point symmetry D3h, and the second to the sum of right- and left-twisted D3 forms. At 25 K, a transition to a low-temperature phase occurs. The transition is characterized by three different spin–lattice relaxation times, T1_1, T1_2, and T1_3, related to three conformation states of dabco in the ratio of ∼2:3:5. The observed relationship of state populations indicates an inequality of right- and left-twisted forms and a chiral polarization of the system because of the break in its right/left symmetry.