Paola Hurtado

Emergence of Symmetry and Chirality in Crown Ether Complexes with Alkali Metal Cations

Crown ethers provide a valuable benchmark for the comprehension of molecular recognition mediated by inclusion complexes. One of the most relevant crown ethers, 18-crown-6 (18c6), features a flexible six-oxygen cyclic backbone that is well-known for its selective cation binding. This study employs infrared spectroscopy and quantum mechanical calculations to elucidate the structure of the gas-phase complexes formed by the 18c6 ether with the alkali metal cations. It is shown that symmetric and chiral arrangements play a dominant role in the conformational landscape of the 18c6-alkali system. Most stable 18c6-M(+) conformers are found to have symmetries C(3v) and C(2) for Cs(+), D(3d) for K(+), C(1) and D(3d) for Na(+), and D(2) for Li(+). Remarkably, whereas the bare 18c6 ether is achiral, chirality emerges in the C(2) and D(2) 18c6-M(+) conformations, both of which involve pairs of stable atropoisomers capable of acting as enantiomeric selective substrates.

Crown Ether Complexes with H3O+ and NH4+: Proton Localization and Proton Bridge Formation

The complexes formed by crown ethers with hydronium and ammonium cations are of key relevance for the understanding of their supramolecular behavior in protic solvents. In this work, the complexes of the 15-crown-5 (15c5) and 18-crown-6 (18c6) ethers with H₃O⁺ and NH₄⁺ and their deuterated variants are investigated under isolated conditions. The study employs infrared multiple photon dissociation (IRMPD) vibrational spectroscopy and DFT B3LYP/6-31++G(d,p) calculations for conformational assignment. The 18c6 ether provides two energetically nearby C(3v) conformations with commensurate linear O-H···O and N-H···O bonds. The 15c5 ether ring adopts partially folded asymmetric pyramidal geometries, yielding one shorter linear H bond and two longer non-linear H bonds. Remarkably, an appreciable broadening of the IRMPD vibrational bands is observed for the 15c5-H₃O⁺/D₃O⁺ complexes. This can be interpreted as a signature for partial sharing of the proton (or deuteron) between the water and the crown ether along the linear O-H···O intermolecular H bond, which is indeed particularly short for this complex.

Spectroscopic Investigation of the Gas-Phase Conformations of 15-Crown-5 Ether Complexes with K +

Infrared multiple photon dissociation action spectra of the binary and ternary gas-phase complexes formed by 15-crown-5 ether with potassium cations (15c5-K(+) and 15c5-K(+)-15c5) are reported. The spectra span the 800-1500 cm(-1) infrared range. Particularly significant differences are found in the position and structure of the CO-stretching band of the two types of complexes. The computational prediction at the DFT-B3LYP/6-31G* level of theory agrees well with the experimental observations, and provides a correlation between the spectral differences and the structural changes associated with the coordination of the ether oxygens with the alkali cation. The 15c5-K(+) complex adopts a pyramidal structure, with the cation lying above the center of mass of the ether ring at a distance similar to its ionic radius. The ternary 15c5-K(+)-15c5 complex features a less tight coordination of the cation and a relative rotation between the backbones of the two crown ethers, which minimizes the intermolecular repulsions between the oxygens.

Solvent-Free MALDI Investigation of the Cationization of Linear Polyethers with Alkali Metals

The MALDI technique with solvent-free sample preparation has been applied to evaluate relative gas-phase affinities of polyether chain polymers with alkali metal cations. The study is performed on poly(ethylene glycol) and poly(propylene glycol) polymers of different lengths (PEG600, PEG1000, PPG425, PPG750) and the alkali metal cations Li(+), Na(+), K(+), and Cs(+). The experiments show that the lattice energy of the alkali metal salts employed as cation precursors can have a strong influence on the outcome of conventional MALDI measurements. With the solvent-free method, these crystal binding effects can be made negligible by combining in the same sample alkali metal salts with different counterions. The recorded MALDI spectra show that the polyether-cation aggregation efficiencies decrease systematically with growing cation size. This cation size selectivity is considerably enhanced for the polymers with the shorter chains, which can be attributed to the reduced ability of the polymer to build a coordination shell around the larger cations. The steric effects introduced by the side CH3 group of propylene glycol with respect to ethylene glycol also enhance the preference for cationization of the polymer by the smaller cations. These observations correct some qualitative trends derived from previous studies, which did not account for lattice energy effects of the cation precursors.

Cations in a Molecular Funnel: Vibrational Spectroscopy of Isolated Cyclodextrin Complexes with Alkali Metals

The benchmark inclusion complexes formed by α-cyclodextrin (αCD) with alkali-metal cations are investigated under isolated conditions in the gas phase. The relative αCD-M(+) (M=Li(+), Na(+), K(+), Cs(+)) binding affinities and the structure of the complexes are determined from a combination of mass spectrometry, infrared action spectroscopy and quantum chemical computations. Solvent-free laser desorption measurements reveal a trend of decreasing stability of the isolated complexes with increasing size of the cation guest. The experimental infrared spectra are qualitatively similar for the complexes with the four cations investigated, and are consistent with the binding of the cation within the primary face of the cyclodextrin, as predicted by the quantum computations (B3LYP/6-31+G*). The inclusion of the quantum-chemical cation disrupts the C(6) symmetry of the free cyclodextrin to provide the optimum coordination of the cations with the -CH(2)OH groups in C(1), C(2) or C(3) symmetry arrangements that are determined by the size of the cation.

Multipodal coordination of a tetracarboxylic crown ether with NH4+: a vibrational spectroscopy and computational study

The elucidation of the structural requirements for molecular recognition by the crown ether (18-crown-6)-2,3,11,12-tetracarboxylic acid (18c6H(4)) and its cationic complexes constitutes a topic of current fundamental and practical interest in catalysis and analytical sciences. The flexibility of the central ether ring and its four carboxyl side arms poses important challenges to experimental and theoretical approaches. In this study, infrared action vibrational spectroscopy and quantum mechanical computations are employed to characterize the conformational structure of the isolated gas phase complex formed by the 18c6H(4) host with NH(4)(+) as guest. The results show that the most stable gas-phase structure is a barrel-like conformation sustained by tetrapodal H-bonding of the ammonia cation with two C=O side groups and with four oxygen atoms of the ether ring in a bifurcated arrangement. Interestingly, a similar structure had been proposed in previous crystallographic studies. The experiment also provides evidence for a significant contribution of a higher energy bowl-like conformer with features resembling those adopted by 18c6H(4) in the analogous complexes with secondary amines. Such a conformation displays H-bonding between confronted side carboxyl groups and tetrapodal binding of the NH(4)(+) with the ether ring and with one C=O group. Structures involving even more extensive intramolecular H-bonding in the 18c6H(4) substrate are found to lie higher in energy and are ruled out by the experiment.

Binding Selectivity of Macrocycle Ionophores in Ionic Liquids versus Aqueous Solution and Solvent‐free Conditions

The understanding of supramolecular recognition in room-temperature ionic liquids (RTILs) is key to develop the full potential of these materials. In this work, we provide insights into the selectivity of the binding of alkali metal cations by standard cyclodextrin and calixarene macrocycles in RTILs. A direct laser desorption/ionization mass spectrometry approach is employed to determine the relative abundances of the inclusion complexes formed through competitive binding in RTIL solutions. The results are compared with the binding selectivities measured under solvent-free conditions and in water/methanol solutions. Cyclodextrins and calixarenes in which the peripheral OH groups are substituted by bulkier side groups preferentially bind to Cs(+) . Such specific ionophoric behavior is substantially enhanced by solvation effects in the RTIL. This finding is rationalized with the aid of quantum mechanical calculations, in terms of the conformational features and steric interactions that drive the solvation of the inclusion complexes by the bulky RTIL counterions.

Fragmentation and gas phase aggregation processes in the laser desorption ionization of chlorodiaminotriazines.

Fragmentation and supramolecular aggregation induced during the laser desorption/ionization (LDI) of four chlorodiaminotriazines (simazine, atrazine, terbutylazine and propazine) have been investigated. The laser wavelength employed (266 nm) lies within the first absorption band of the four triazines. The main fragmentation channel observed involves the prompt cleavage of the Cl atom, followed by partial or total fragmentation of the side alkyl chains. Breakage of the triazinic ring becomes efficient at moderate laser powers; however, the deamination of the triazine is not observed to take place. In addition, the formation of both covalent and non-covalent triazinic aggregates in the desorption plume is found to be particularly efficient. Aggregates as large as heptamers are neatly detected, with the observation that those with the most intense signal involve the dechlorinated triazinic fragment. Both aggregation and fragmentation are largely suppressed upon dilution of the triazine under matrix-assisted laser desorption/ionization conditions.