Abstract
Thermal properties of Zn2(C8H4O4)2•C6H12N2 metal-organic framework compound at 8–300 K suggest the possibility of subbarrier tunnelling transitions between left-twisted (S) and right-twisted (R) forms of C6H12N2 dabco molecules with D3 point symmetry. The data agree with those obtained for the temperature behavior of nuclear spin-lattice relaxation times. It is shown that there is a temperature range where the transitions are stopped. Therefore, Zn2(C8H4O4)2•C6H12N2 and related compounds are interesting objects to study the effect of spontaneous mirror-symmetry breaking and stabilization of chiral isomeric molecules in solids at low temperatures.
Highlights
Chirality-related interactions are demonstrated by chiral molecules, i.e. those that can exist in both left- and right-handed forms
energy barrier (Ea) values are assumed to correspond to activation energies only of hydrogen and carbon atoms of the dabco molecule, and nitrogen atoms are not involved in the reorientation
The temperature dependence of 1Н NMR T1 in Zn2(C8H4O4)2C6H12N2 at 310–165 K obeys the classical theory of nuclear spin-lattice relaxation and is characterized by a single-exponential recovery of the free induction decay (FID)[18, 19]
Summary
Chirality-related interactions are demonstrated by chiral molecules, i.e. those that can exist in both left- and right-handed forms. The expected fundamental conclusions are still not supported by detailed analysis of the interactions within crystal structures where the stabilization effect is not suppressed by other impacts[8, 9] In this respect, of high interest are metal-organic framework compounds with large pores, open internal channels, and large internal surface areas[10,11,12]. The untwisted D3h state of the dabco molecule corresponds to its transition state and its energy is equal to the value of the energy barrier Ea (Fig. 1)[13] This state can be stable in crystals, and the untwisted molecule can move using both the activation and the tunnelling mechanisms
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