What is it about?
The novelty of our approach is an atypical use of the QST2 method by starting with asymmetric saddle-point structures of an apparently symmetric IL 1a of type A/B consisting of two enantiomeric forms of type A and B, derived from ionic units Ai and Bj coming from its different pairs A/B, and then applying standard IRC analysis in the second step, which allows finding a set of new distinct energy minima Ak and Bl as reactants and products. It should be emphasized that the Bj unit should differ significantly in terms of structure from the Ai unit. Thanks to this non-automatic procedure, 34 pairs of A/B of the studied ionic aggregate 1a were found, which should be treated as its multicomponent molecular structure at the applied DFT level. In these numerous structurally diverse enantiomeric units A and B, an iodine ion (I−) occupies various spatial positions, stabilized by weak C−H···I− hydrogen bonds fully confirmed by AIM calculations performed within the framework of QTAIM analysis.
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Why is it important?
The system 1a under investigation exists as binary sets of enantiomeric forms of type A and B, which, according to time-averaged NMR spectra and calculated Gibbs free energy (ΔG) data, exist in a fast (on the NMR time scale) symmetrical two-site conformational exchange A ⇌ B; see the figure below. Its constituent I− ion is located in very different spatial places, usually stabilized by one or two weak and little-known unconventional directional HB interactions of the type C−H···I− fully confirmed by the QTAIM topological analysis, in which the C−H bond acts as a weak HB donor and the iodide acts as a weak acceptor site. The results of molecular modeling of system 1a performed at the DFT level were analyzed by comparing the calculated NMR spectroscopic parameters (deltaH, deltaC, nJHH) with their experimentally observed values. In addition to the standard separate DFT NMR predictions of deltaX values, a modified and significantly simplified joint two-nucleus H/C approach with a multiplier mH = 6 was also used.
Perspectives
Our approach can generally be treated as a kind of guide for all future similar studies, including, in addition to stable structures being local energy minima, first- and higher-order saddle-point species with the number of imaginary frequencies (NImag) ≥ 1. We believe that such an approach is worth using (or at least trying) in all challenging cases involving structurally similar ILs, as well as other different types of conformationally flexible multicomponent chemical systems.
Associate Professor Ryszard Bolesław Nazarski
University of Lodz
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This page is a summary of: Molecular Structure of a Model
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-Alkylthiolanium Ionic Liquid Based on NMR Measurements and DFT Predictions for Its Enantiomeric C–H···I
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Hydrogen Bonde..., The Journal of Physical Chemistry A, October 2025, American Chemical Society (ACS),
DOI: 10.1021/acs.jpca.5c05146.
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