High-Resolution Rotational Spectroscopy of a Cyclic Ether

Thermodynamics of Li+-Crown Ether Interactions in Aqueous Solvent

Lithium ion-based batteries are ubiquitous in modern technology due to applications in personal electronics and high-capacity storage for electric vehicles. Concerns about lithium supply and battery waste have prompted interest in lithium recycling methods. The crown ether 12-crown-4 has been studied for its abilities to form stable complexes with lithium ions (Li⁺). In this paper, molecular dynamics simulations are applied to examine the binding properties of a 12-crown-4-Li⁺ system in aqueous solution. It was found that 12-crown-4 did not form stable complexes with Li⁺ in aqueous solution due to the binding geometry which was prone to interference by surrounding water molecules. In addition, the binding properties of sodium ions (Na⁺) to 12-crown-4 are examined for comparison. Subsequently, calculations were performed with the crown ethers 15-crown-5 and 18-crown-6 to study their complexation with Li⁺ as well as Na⁺. It was determined that binding was unfavorable for both types of ions for all three crown ethers tested, though 15-crown-5 and 18-crown-6 showed a marginally greater affinity for Li⁺ than 12-crown-4. Metastable minima present in the potential of mean force for Na⁺ render binding marginally more likely there. We discuss these results in the context of membrane-based applications of crown ethers for Li⁺ separations.

Conformational Panorama of Cycloundecanone: A Rotational Spectroscopy Study

The conformational landscape of the medium-size cyclic ketone cycloundecanone has been investigated using chirped-pulse Fourier transform microwave spectroscopy and computational calculations. Nine conformations were observed in the rotational spectrum and identified from the comparison of experimental and theoretical rotational constants as well as the observed and predicted types of rotational transitions. All singly substituted ¹³C isotopologues were observed for the most abundant conformer, which allowed the determination of partial substitution and effective structures. The most abundant conformer dominates the rotational spectrum and is almost 40 times more abundant than the least abundant conformer. Conformational preferences are governed by the combination of transannular H···H and eclipsed HCCH interactions.

Proton in the ring: spectroscopy and dynamics of proton bonding in macrocycle cavities

The proton bond is a paradigmatic quantum molecular interaction and a major driving force of supramolecular chemistry. The ring cavities of crown ethers provide an intriguing environment, promoting competitive proton sharing with multiple coordination anchors. This study shows that protons confined in crown ether cavities form dynamic bonds that migrate to varying pairs of coordinating atoms when allowed by the flexibility of the macrocycle backbone. Prototypic native crown ethers (12-crown-4, 15-crown-5 and 18-crown-6) and aza-crown ethers (cyclen, 1-aza-18-crown-6 and hexacyclen) are investigated. For each system, Infrared action spectroscopy experiments and ab initio Molecular Dynamics computations are employed to elucidate the structural effects associated with proton diffusion and its entanglement with the conformational and vibrational dynamics of the protonated host.

Seven Conformations of the Macrocycle Cyclododecanone Unveiled by Microwave Spectroscopy

The physicochemical properties and reactivity of macrocycles are critically shaped by their conformations. In this work, we have identified seven conformations of the macrocyclic ketone cyclododecanone using chirped-pulse Fourier transform microwave spectroscopy in combination with ab initio and density functional theory calculations. Cyclododecanone is strongly biased towards adopting a square configuration of the heavy atom framework featuring three C-C bonds per side. The substitution and effective structures of this conformation have been determined through the observation of its ¹³C isotopologues. The minimisation of transannular interactions and, to a lesser extent, HCCH eclipsed configurations drive conformational preferences. Our results contribute to a better understanding of the intrinsic forces mediating structural choices in macrocycles.

Water induces the same crown shapes as Li+ or Na+ in 15-crown-5 ether: a broadband rotational study

Complexation with water reduces the structural landscape of 15-crown-5 ether to the two forms observed for Li⁺ or Na⁺ complexes.

Medium-sized rings: conformational preferences in cyclooctanone driven by transannular repulsive interactions

Three conformers of cyclooctanone have been identified by broadband rotational spectroscopy. Cyclooctanone shows a strong preference to adopt a boat-chair conformation, driven by minimisation of repulsive non-bonded transannular interactions.

Water-Induced Structural Changes in Crown Ethers from Broadband Rotational Spectroscopy

The complexes of 12-crown-4 ether (12C4) with water, generated in a supersonic jet, have been studied using broadband Fourier transform microwave spectroscopy. Three 1:1 and one 1:2 clusters have been observed and their structures unambiguously identified through the observation of isotopologue spectra. The structures of the clusters are based on networks of O-H···O and C-H···O hydrogen bonds. The most abundant 1:1 cluster is formed from the most stable S₄ symmetry conformer of 12C4, even though it is not the energetically favored water complex. Interestingly, the structures of the most stable water cluster and the other remaining observed 1:1 and 1:2 complexes are formed from the second or the fifth most abundant conformers of 12C4. This shows the existence of a mechanism that changes the conformation of 12C4 so that the host-guest interactions can be maximized, even for a "soft" ligand like water.