Large Conformational Change in the Isomerization of Flexible Crown Ether Observed at Low Temperature

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Abstract
The dynamic processes of conformational changes of supramolecules are important to understand the motion in synthetic supramolecules. Although a host-guest complex is the most basic supramolecule, a detailed mechanism of its conformational changes has rarely been studied. Here, we observed the large conformational change of a dibenzo-24-crown-8 complex with four guest ions (Ag+, Na+, K+, and NH4+) at low temperature in the gas phase. The isomerization between the two types of conformers, which have different distances between the two benzene rings, proceeds even at 86 K. Using variable-temperature ion mobility-mass spectrometry (IM-MS) at 100-210 K, the activation energy for the isomerization is determined to be rather small (4.8-9.0 kJ mol-1). Reaction pathway calculations revealed that the isomerization is caused by the sequential rotation of two single bonds in the crown ether ring. The present cryogenic IM-MS study of the host-guest complexes at the molecular level opens an approach to detailed understanding of the motion in supramolecules.
Funding Information
  • Japan Society for the Promotion of Science (JP16K05641, JP21H05418)
  • Institute for Quantum Chemical Exploration
  • Salt Science Research Foundation (1916, 2022, 2116)

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