¹H NMR spectra, structure, and conformational exchange of S-n-alkyl-tetrahydrothiophenium cations of some ionic liquids

Abstract

S-n-alkyl-tetrahydrothiophenium cations [R-THT]⁺ in some iodide or tetraphenylborate ion-pairs [R-THT][X] (R = methyl to n-decyl, X⁻= I⁻, [BPh₄]⁻, [NTf₂]⁻) are characterized in the solid state by puckered THT rings in twist forms. In contrast, solution ¹H NMR spectra of compounds [R-THT][X] indicate a rapid conformational exchange leading to dynamic [ABCD]₂ spin systems due to eight protons of the THT ring. These strongly coupled spectra obtained at high magnetic fields were analyzed by iterative methods and simulated successfully. In addition, molecular modeling and gauge-including atomic orbitals (GIAOs) based calculations of proton NMR chemical shifts for two such ion-pair compounds in solution were performed at the density functional theory (DFT) level . As a result, conformational equilibrium between two different pairs of isoenergetic twist forms of the THT ring in [Me-THT][I] (global and local energy minima) was found to exist. These theoretical results are discussed in view of experimental ¹H NMR spectral data.

Graphical Abstract

Keywords:

• n-Alkyl-tetrahydrothiophenium cations
• ¹H NMR
• iterative spectral analysis
• [ABCD]₂ spin system
• DFT-GIAO prediction of NMR parameters
• molecular structure
• conformations

Acknowledgments

Thanks are due to Mrs. Beate Rau for running the NMR experiments. S. X. acknowledges support from the Hoffmann Institute of Advanced Materials (HIAM), Shenzhen Polytechnic (China), Deutsche Forschungsgemeinschaft and Infrastruktura PL-Grid. The authors also thank two anonymous reviewers for their constructive comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work has been supported by the German Research Foundation (DFG) in the framework of the priority program SPP 1708 “Material Synthesis Near Room Temperature” JA 466/31-2. The part of the research (DFT calculations for 6a and 6b) was supported by PLGrid Infrastructure at the Academic Computer Center CYFRONET (Prometheus supercomputer; AGH – University of Science and Technology, Kraków, Poland) − grant to R. B. N.