Radical-Pairing Interactions in a Molecular Switch Evidenced by Ion Mobility Spectrometry and Infrared Ion Spectroscopy

Isomeric Speciation of Bisbenzoxazine Intermediates by Ion Spectroscopy and Ion Mobility Mass Spectrometry

Bisbenzoxazines (BisBz) are a relevant model for the diverse bifunctional benzoxazines that are used to increase the polybenzoxazines cross-linking extensions and modulate the final resin properties for various usages. The presence of side products and intermediates during monomer formation can influence the resin characteristics by inducing chain termination and ramifications, affecting the polymerization and cure processes. This work investigated the diverse isomeric intermediates and side products that are present during the BisBz formation from bisphenol A, aniline, and formaldehyde by ion mobility coupled to tandem mass spectrometry (MS/MS) and ion spectroscopy techniques. The species detected in this work suggest that these multifunctional phenols open diverse concurrent reaction pathways based on two main reactive steps: (i) the imine/iminium phenol attack to form a phenylamino intermediate and (ii) the formaldehyde attack followed by dehydration to form the oxazine ring. The species observed also support previous studies of the benzoxazine formation mechanism and showcase the application of advanced analytical techniques in studying complex chemical systems.

Structural studies of supramolecular complexes and assemblies by ion mobility mass spectrometry

Recent advances in instrumentation and development of computational strategies for ion mobility mass spectrometry (IM-MS) studies have contributed to an extensive growth in the application of this analytical technique to comprehensive structural description of supramolecular systems. Apart from the benefits of IM-MS for interrogation of intrinsic properties of noncovalent aggregates in the experimental gas-phase environment, its merits for the description of native structural aspects, under the premises of having maintained the noncovalent interactions innate upon the ionization process, have attracted even more attention and gained increasing interest in the scientific community. Thus, various types of supramolecular complexes and assemblies relevant for biological, medical, material, and environmental sciences have been characterized so far by IM-MS supported by computational chemistry. This review covers the state-of-the-art in this field and discusses experimental methods and accompanying computational approaches for assessing the reliable three-dimensional structural elucidation of supramolecular complexes and assemblies by IM-MS.

Ion Mobility Mass Spectrometry for Large Synthetic Molecules: Expanding the Analytical Toolbox

Understanding the composition, structure and stability of larger synthetic molecules is crucial for their design, yet currently the analytical tools commonly used do not always provide this information. In this perspective, we show how ion mobility mass spectrometry (IM-MS), in combination with tandem mass spectrometry, complementary techniques and computational methods, can be used to structurally characterize synthetic molecules, make and predict new complexes, monitor disassembly processes and determine stability. Using IM-MS, we present an experimental and computational framework for the analysis and design of complex molecular architectures such as (metallo)supramolecular cages, nanoclusters, interlocked molecules, rotaxanes, dendrimers, polymers and host-guest complexes.

Encapsulation in Charged Droplets Generates Distorted Host-Guest Complexes

The ability of various hydrogen-bonded resorcinarene-based capsules to bind α,ω-alkylbisDABCOnium (DnD) guests of different lengths was investigated in solution and in the gas-phase. While no host-guest interactions were detected in solution, encapsulation could be achieved in the charged droplets formed during electrospray ionisation (ESI). This included guests, which are far too long in their most stable conformation to fit inside the cavity of the capsules. A combination of three mass spectrometric techniques, namely, collision-induced dissociation, hydrogen/deuterium exchange, and ion-mobility mass spectrometry, together with computational modelling allow us to determine the binding mode of the DnD guests inside the cavity of the capsules. Significant distortions of the guest into horseshoe-like arrangements are required to optimise cation-π interactions with the host, which also adopt distorted geometries with partially open hydrogen-bonding seams when binding longer guests. Such quasi "spring-loaded" capsules can form in the charged droplets during the ESI process as there is no competition between guest encapsulation and ion pair formation with the counterions that preclude encapsulation in solution. The encapsulation complexes are sufficiently stable in the gas-phase - even when strained - because non-covalent interactions significantly strengthen in the absence of solvent.

Stable Crystalline Nanohoop Radical and Its Self-Association Promoted by van der Waals Interactions

A stable nanohoop radical (OR3) combining the structures of cycloparaphenylene and an olympicenyl radical is synthesized and isolated in the crystalline state. X-ray crystallographic analysis reveals that OR3 forms a unique head-to-tail dimer that further aggregates into a one-dimensional chain in the solid state. Variable-temperature NMR and concentration-dependent absorption measurements indicate that the π-dimer is not formed in solution. An energy decomposition analysis indicates that van der Waals interactions are the driving force for the self-association process, in contrast with other olympicenyl derivatives that favor π-dimerization. The physical properties in solution phase have been studied, and the stable cationic species obtained by one-electron chemical oxidation. This study offers a new molecular design to modulate the self-association of organic radicals for overcoming the spin-Peierls transition, and to prepare novel nanohoop compounds with spin-related properties.

Exploring the Gas-Phase Formation and Chemical Reactivity of Highly Reduced M8 L6 Coordination Cages

Coordination cages with well-defined cavities show great promise in the field of catalysis on account of their unique combination of molecular confinement effects and transition-metal redox chemistry. Here, three coordination cages are reduced from their native 16+ oxidation state to the 2+ state in the gas phase without observable structural degradation. Using this method, the reaction rate constants for each reduction step were determined, with no noticeable differences arising following either the incorporation of a C60 -fullerene guest or alteration of the cage chemical structure. The reactivity of highly reduced cage species toward molecular oxygen is "switched-on" after a threshold number of reduction steps, which is influenced by guest molecules and the structure of cage components. These new experimental approaches provide a unique window to explore the chemistry of highly-reduced cage species that can be modulated by altering their structures and encapsulated guest species.

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine)

The structure of an isolated Ag+ (benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation-N and cation-π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800-1800 cm-1 fingerprint range using the IR free electron laser beamline coupled with an FT-ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N-Ag+ -π) involving both the amino nitrogen atom and the aromatic π-system of the phenyl ring. The gas-phase reactivity of Ag+ (benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N- and π-binding affinity and suggests an augmented silver coordination in the product adduct ionAg + ( benzylamine ) ( L ) .

[1]Rotaxanes based on phosphorylated pillar[5]arenes

[1]Rotaxanes based on monosubstituted phosphorus-containing pillar[5]arenes have been synthesized by the Kabachnik–Fields reaction for the first time in good yields.

Unveiling anion-induced folding in tripodal imidazolium receptors by ion-mobility mass spectrometry

The anion-induced folding of tripodal imidazolium receptors has been investigated by NMR spectroscopy, electrospray ionization ion mobility mass spectrometry and DFT calculations. Such folding can be switched by anion release upon collision induced dissociation.

Radical-Pairing Interactions in a Molecular Switch Evidenced by Ion Mobility Spectrometry and Infrared Ion Spectroscopy

The digital revolution sets a milestone in the progressive miniaturization of working devices and in the underlying advent of molecular machines. Foldamers involving mechanically entangled components with modular secondary structures are among the most promising designs for molecular switch‐based applications. Characterizing the nature and dynamics of their intramolecular network following the application of a stimulus is the key to their performance. Here, we use non‐dissociative electron transfer as a reductive stimulus in the gas phase and probe the consecutive co‐conformational transitions of a donor‐acceptor oligorotaxane foldamer using electrospray mass spectrometry interfaced with ion mobility and infrared ion spectroscopy. A comparison of collision cross section distributions for analogous closed‐shell and radical molecular ions sheds light on their respective formation energetics, while variations in their respective infrared absorption bands evidence changes in intramolecular organization as the foldamer becomes more compact. These differences are compatible with the advent of radical‐pairing interactions.