1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Counterion-Trapped-Molecules: From High Polarity and Enriched IR Spectra to Induced Isomerization Fedor Y. Naumkin* [a] and David J. Wales [b] We report extensive computational studies of some novel intermolecular systems and their properties. Recombination of alkali-halide counterions separated by a noncovalently trapped hydrocarbon molecule is prevented by significant potential energy barriers, resulting in unusual metastable insertion complexes. These systems are extremely polar, while the inserted molecule is strongly counter-polarized, leading to significant cooperative nonadditivity effects. The compression and electric field produced by the counterions favours isomer- ization of the trapped molecule via a significant reduction of the barriers to bond rearrangement, in a field-induced mechanochemical process. The predicted IR intensity spectra clearly reflect (1) formation of the insertion complex, rather than simple attachment of alkali halide, and (2) isomerization of the trapped molecule, thus allowing experimental access to these events. 1. Introduction Molecular systems with high polarity have a wide range of potential applications. These possibilities include applications based on (1) light-matter interactions, such as in lasers and solar cells, [1,2] nonlinear optics [3] and ferroelectrics, [4] as well as (2) interactions between molecules, in particular in complexation, self-assembly, and facilitated chemical reactions. Very large dipole moments, up to a few dozen Debye, have recently been predicted for intermolecular complexes M-mol-X with nonpolar or polar molecules inserted inside a pair of counterions, such as alkali halides. The molecules include saturated species with concave electron densities, such as hexafluoroethane, [5] cyclic hydrocarbons with 3-, 4- and 6- membered carbon rings, [6–8] and their fluorinated derivatives. [9–12] A related family of systems with ion-pi inter- actions is represented by the complexes of (unsaturated) benzene and its derivatives (see, e.g., Refs. [13-15] and recent review [16] ). Most of the above systems can be viewed as smaller counterparts of macrocycles hosting, in particular, inorganic ion-pairs, as reviewed very recently. [17] For instance, calix[4] pyrrole and hexacyclen as receptors of both alkali and halogen ion pairs [18,19] represent structural extensions of the above systems. Both fluoroethanes with axial C À C bonds and small hydro- carbon cycles perpendicular to the M À X axis have relatively low barriers (up to about 0.2 eV) to recombination of the separated counterions. In particular, the carbon ring is part-folded in free cyclobutane, but can be flattened inside M À C 4 H 8 À X complexes, [7] which destabilizes the system in terms of dissoci- ation into MX + C 4 H 8 , and may explain, in spite of the larger cycle, the stability similar to that for M À C 3 H 6 À X with a more rigid molecule. The highly polar fluorinated cycles, such as all- cis C n H n F n (n = 4, 6), provide greater stabilization, with barriers up to 1 eV, [10,12] but are not easy to produce. [20] To increase the polarity of such complexes, thicker insertion molecules could be considered, as long as they still preserve the ion-pair character of the alkali-halide. For instance, the characteristic avoided crossing of the ionic and covalent potential energy curves of, e.g., LiF occurs at around 7 Å, [21] while the Li À F distance in, e.g., Li À C 3 H 6 À F is less than 4.5 Å, [6] so there is a room for larger inserts. In particular, adamantane complexes have been studied recently, [8] although from a different viewpoint. On the other hand, for greater stability, more rigid components would be preferred. The aim of the present work is to attempt to combine the three above factors using cubane as a structural extension of cyclobutane, with a relatively rigid “double” carbon core. A related aim is to investigate the feasibility of inducing, or at least favouring, intra-complex isomerization of the inserted molecule under pressure of the mutually attracting counterions in combination with their electric field. In particular, the electrostatic-field induced reactions are an active area of research, and some bond rearrangement (e.g. Diels-Alder) processes can be accelerated by an order of magnitude. [22,23] In our case, however, the internal field generated by the constituent ions of the system is involved. Here, the cubane-to- tricyclooctadiene transformation is considered along with the smallest alkali halide LiF, allowing the closest approach of the counterions. Computational Methods Calculations were carried out at the MP2/aug-cc-pvdz level of theory for geometry optimization, followed by a single-point calculations with aug-cc-pvtz basis sets, as implemented in the Gaussian 09 package. [24] This level is a reasonable compromise [a] Dr. F. Y. Naumkin On sabbatical leave from Faculty of Science, UOIT/Ontario Tech University, Oshawa, L1G 0C5, Canada E-mail: F.Y.Naumkin@uoit.ca [b] Prof. D. J. Wales Department of Chemistry, University of Cambridge, Cambridge, CB2 1EW, United Kingdom Supporting information for this article is available on the WWW under https://doi.org/10.1002/cphc.201901112 Articles DOI: 10.1002/cphc.201901112 1 ChemPhysChem 2020, 21,1–9 © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ��