Infrared Multiple Photon Dissociation Research Articles

Unravelling the structure of one-electron oxidation products of model peptide backbones containing methionine by IRMPD spectroscopy: the effect of the neighbouring groups.

A series of Methionine (Met) derivatives, where either the amino group and/or the carboxylic acid group is blocked by acetyl and/or methyl ester functionalities, has been investigated by Collision Induced Dissociation-tandem mass spectrometry (CID-MS2) and Infrared Multiple Photon Dissociation (IRMPD) spectroscopy. The CID-MS2 experiments were performed using a Fourier-transform ion-cyclotron-resonance (FT-ICR) mass spectrometer equipped with an electrospray ionization (ESI) source. The IRMPD spectra were recorded employing a Paul type ion-trap coupled with the free-electron laser (FEL) FELIX in the fingerprint region from 600 to 2000 cm-1. We show that the oxidation of the methionine residue with protected terminal groups induces the formation of a sulfoxide group. However, compared to the IRMPD spectrum of protonated methionine and methionine sulfoxide, significant spectral differences are observed in the spectra of model peptide backbones containing methionine. DFT calculations show that protonation occurs on the sulfoxide group in the gas phase for these model peptide backbones containing methionine, shifting the diagnostic signature of the sulfoxide group from 1000 to 862 cm-1.

Structure Determination of Zinc and Cadmium Dication Complexes with Intact and Deprotonated Histidyl Glycine and Glycyl Histidine Dipeptides.

Metalated intact and deprotonated histidyl glycine and glycyl histidine dipeptides were investigated in the gas phase by using infrared multiple photon dissociation (IRMPD) spectroscopy with light from a free-electron laser (FEL). The dipeptides M2+(GlyHis), M2+(HisGly), [M(GlyHis-H)]+, and [M(HisGly-H)]+, where M = Zn and Cd, were probed to elucidate how the His position along the peptide chain and ligand charge state might influence the structures observed in the gas phase. Simulated annealing calculations were performed to determine energetically low-lying conformers and isomers of these structures. Quantum chemical calculations were used to optimize the structures at the B3LYP level of theory using the 6-311+G(d,p) and def2-TZVP basis sets for zinc and cadmium complexes, respectively. IRMPD and calculated linear absorption spectra were compared to evaluate which structures are present. Relative energies of the various species were evaluated using single-point energy calculations for low-lying structures at the B3LYP, B3LYP-GD3BJ, ωB97XD, and MP2(full) levels using the 6-311+G(2d,2p) and def2-TZVPP basis sets. For all species, structures for both metals mirror each other, and those that reproduce the experimental spectrum were determined to be iminol structures for the intact ligands or iminol-like structures for the deprotonated ligands. Additionally, when the spectra of the deprotonated dipeptides are compared to the intact dipeptides, the change in the spectra is correlated to the group that is deprotonated.

Rearrangement of Proline Complexes with Zn2+: An Infrared Multiple Photon Dissociation and Theoretical Investigation.

Complexes of proline (Pro) cationized with Zn2+ and Cd2+ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. Complexes of intact Pro with CdCl+, CdCl+(Pro), a complex of (Zn+Pro-H)+ where a proton has been lost, as well as Zn+(Pro-H)(Pro) were formed by electrospray ionization. In order to identify the structures formed experimentally, the IRMPD spectra were compared to those calculated from optimized structures at the B3LYP/6-311+G(d,p) level for zinc complexes and B3LYP/def2-TZVP level with an effective core potential on cadmium for the CdCl+(Pro) system. For the latter complex, the main binding motif observed has a zwitterionic proline ligand structure, [CO2-]cc, where the metal binds to the two carboxylate oxygens. In contrast, for Zn+(Pro-H)(Pro), both ligands interact with zinc via a [N,CO-][N,CO] binding motif, where binding is observed at the carbonyl oxygens and nitrogens for both ligands, consistent with previous work. In both cases, contributions from different puckers of the proline ring may contribute. For (Zn+Pro-H)+, we identify that the structure is actually ZnH+(Pro-2H), in which the proline has been dehydrogenated and one of the hydrogens has migrated to form a covalent bond with Zn, which verifies a previous report relying on a single OH stretch band.

Protonation-Induced Tautomerization Lowers the Activation Barriers for N-Glycosidic Bond Cleavage of Thymidine and 5-Methyluridine

Synergistic infrared multiple photon dissociation (IRMPD) action spectroscopy experiments and electronic structure calculations revealed that both 2,4-dihydroxy tautomers and O2 protonated conformers of protonated thymidine, [dThd+H]+, and protonated 5-methyluridine, [Thd+H]+ coexist in the gas phase with the 2,4-dihydroxy tautomers dominating the population. In the current study, the kinetic energy dependence of the collision-induced dissociation behavior of [dThd+H]+ and [Thd+H]+ are examined in a guided ion beam tandem mass spectrometer. The primary dissociation pathways observed involve N-glycosidic bond cleavage leading to competitive elimination of either protonated or neutral thymine. The potential energy surfaces (PESs) for these N-glycosidic bond cleavage pathways are mapped via electronic structure calculations for the mixture of 2,4-dihydroxy tautomers and O2 protonated conformers of [dThd+H]+ and [Thd+H]+ populated in the IRMPD experiments. The activation energies and heats of reaction predicted for N-glycosidic bond cleavage at the B3LYP and MP2(full) levels of theory are compared to the measured values. The agreement between experiment and theory indicates that B3LYP provides better estimates of the energetics of the species along the PESs for N-glycosidic bond cleavage of [dThd+H]+ and [Thd+H]+ than MP2, and that the 2,4-dihydroxy tautomers, which are stabilized by strong hydrogen-bonding interactions, control the threshold dissociation behavior of [dThd+H]+ and [Thd+H]+.

A trapped ion mobility enabled Fourier transform ion cyclotron resonance mass spectrometer for infrared ion spectroscopy at FELIX

We report the installation of a new infrared ion spectroscopy platform at the free-electron laser facility FELIX, based on a Bruker SolariX Fourier Transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with a trapped ion mobility spectrometry (TIMS) stage. The instrument allows one to record infrared multiple-photon dissociation (IRMPD) spectra for mass and mobility selected ions, produced by a range of ion sources. We describe two strategies to achieve consistent overlap between the laser beam and the ion packet. Removing the original ECD cathode significantly enhanced the IR transmission and the overlap with the ion cloud, resulting in improved IRMPD yield per pulse. Enhancement of the photodissociation yield is observed when multiple pulses are used, as there is negligible collisional deactivation in the ultra-high vacuum trapping region of the ICR cell. We compare IRMPD spectra recorded on the new platform with IRMPD spectra of the same species recorded on one of our 3D-quadrupole ion trap platforms. We demonstrate the instrument’s performance using a sample containing a mixture of two trisaccharides. Mobility selection allows us to record individual IR spectra for the two isomeric species. This multi-modal platform, encompassing liquid chromatography, ion mobility spectrometry, ultra-high resolution (tandem) mass spectrometry, infrared ion spectroscopy (and soon also mass spectrometry imaging), is available to users at the FELIX facility.

IRMPD spectroscopy of deprotonated selenocysteine - The 21st proteinogenic amino acid

The effect of deprotonation on the structure and stability of the 21st proteinogenic amino acid selenocysteine, generated as bare deprotonated species by electrospray ionization, has been investigated by infrared multiple photon dissociation (IRMPD) spectroscopy over an extended frequency range (700-3600 cm−1), encompassing both the fingerprint and X–H (X = C, N, O) stretching ranges. IRMPD spectra, interpreted by anharmonic DFT calculations, provide evidence of a thermally averaged ion population of two types of low-lying canonical conformers deprotonated at the selenol (Se–H) group and involved in different H-bonding motifs.The broadened and diffuse band structures observed in the H-stretching range are well interpreted by Born-Oppenheimer molecular dynamics computations that provide a valuable description of the flexible backbone arrangements of Sec and of the proton sharing dynamics in the Se–HO and Se–HN moieties.Other prototropic isomers, deprotonated at the carboxylic group or with a zwitterionic structure, should not be significantly populated, according to their higher free energy and calculated IR spectra inconsistent with experimental evidence.

Infrared Spectroscopic Characterization of Early 4d Transition Metal Carbene Cations, ZrCH2+ and NbCH2.

IR multiple-photon dissociation (IRMPD) action spectroscopy is combined with quantum chemical calculations to examine the [M,C,2H]+ species for the early 4d metals, M = Zr and Nb. These ions were formed by reacting laser ablated M+ ions with cyclopropane (c-C3H6) in a molecular beam apparatus. Both IRMPD spectra exhibit one major band near 700 cm-1 and a second weaker band at about twice that wavenumber, more evident when irradiated in focus. The [Nb,C,2H]+ species also has a sharp band at 800 cm-1. Comparison with B3LYP calculations allow assignment of the [M,C,2H]+ structures to agostic carbenes, which is similar to the structures found for the 5d analogues, WCH2+ and TaCH2+. A molecular orbital analysis traces the reasons for the agostic deformation from a classic C2v symmetric carbene.

Protonation Effects on the Benzoxazine Formation Pathways and Products Distribution.

The effect of acidic media on the formation of the 3,4-dihydro-2H-3-phenyl-1,3-benzoxazine Bz is evaluated, focusing on the differentiation of intermediates and products formed by the distinct pathways observed in the presence and absence of acid. The use of real-time mass spectrometry (PSI-ESI-MS) coupled to tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) allowed the differentiation of the species observed during the synthesis of benzoxazines in these different conditions. The results suggest that formic acid promotes the formation of aniline and phenol condensation products (IC and IIC) by protecting the aniline amino group and enhancing the formaldehyde electrophilicity. The results also suggest that although the presence of acid allow a more efficient potential energy landscape to be accessed, the last cyclization step for the formation of benzoxazines cannot be mediated by the protonation route intermediate (ROP Bz). Overall, the conclusions presented here provide important information about the synthesis of benzoxazines under acidic conditions, allowing the development of optimal reaction conditions.

Structure and Conformation Determine Gas-Phase Infrared Spectra of Detergents.

Native mass spectrometry of membrane proteins relies on non-ionic detergents which protect the protein during transfer from solution into the gas phase. Once in the gas phase, the detergent micelle must be efficiently removed, which is usually achieved by collision-induced dissociation (CID). Recently, infrared multiple photon dissociation (IRMPD) has emerged as an alternative activation method for the analysis of membrane proteins, which has led to a growing interest in detergents that efficiently absorb infrared light. Here we investigate whether the absorption properties of synthetic detergents can be tailored by merging structural motifs of existing detergents into new hybrid detergents. We combine gas-phase infrared ion spectroscopy with density functional theory to investigate and rationalize the absorption properties of three established detergents and two hybrid detergents with fused headgroups. We show that, although the basic intramolecular interactions in the parent and hybrid detergents are similar, the three-dimensional structures differ significantly and so do the infrared spectra. Our results outline a roadmap for guiding the synthesis of tailored detergents with computational chemistry for future mass spectrometry applications.

Structural characterizations of histidine-containing tripeptides complexed with zinc and cadmium dications using IRMPD spectroscopy and theoretical calculations

Metalated gas-phase complexes, M2+(HisAlaAla), M2+(AlaHisAla), and M2+(AlaAlaHis), where M = Zn and Cd, were examined using infrared multiple photon dissociation (IRMPD) spectroscopy with light from a free-electron laser (FEL). These complexes were chosen because they provide model systems for metal binding to proteins. Complementary simulated annealing calculations were performed to determine energetically low-lying conformers and isomers of these structures. Quantum chemical calculations were used to optimize the structures at the B3LYP level of theory using 6-311+G(d,p) and def2-TZVP basis sets for zinc and cadmium complexes, respectively. IRMPD and calculated linear absorption spectra were compared to evaluate which structures are present. Relative energies of the various species were evaluated using single-point energy calculations for low-lying structures at the B3LYP, B3P86, and MP2(full) levels using 6-311+G(2d,2p) and def2-TZVPP basis sets. For species with histidine at a terminal position (AAH or HAA), the conformations that best reproduce the IRMPD spectra are charge-solvated (CS) conformers, where the metal dication binds to the amine and carbonyl groups of the peptide backbone and to the nitrogen of the histidine side chain, along with contributions from an iminol structure for AAH. The species with the histidine in the center position (AHA) adopt an iminol structure, where the metal dication binds to the backbone iminol nitrogens, the α-amine, π-imine, and the carbonyl of the C-terminus.

Molecular Hydrogen Acts as a Hydrogen Bond Proton Acceptor: From Protonated Betaine Tagging to the Weakest Hydrogen Bond.

In an attempt to gain further insights into the intermolecular interactions implied by Rizzo's group's cautionary tale related to molecular tagging in infrared multiple photon dissociation (IRMPD) spectroscopy with molecular messengers [Masson, A. . J. Chem. Phys. 2015, 143, 104313], in the present study, we provide an in-depth analysis of the noncovalent interaction between the molecular hydrogen and protonated betaine molecule in the gas phase. We aim to shed some new light on the fundamental issues concerning the wide diapason of hydrogen-bonding-type intermolecular interactions, with a wide variety of proton acceptors. We demonstrate that in the course of tagging the protonated betaine with molecular hydrogen from the OH group side, it is the σ bond of molecular hydrogen that plays the role of hydrogen-bonding proton acceptor. The tagging thus induces a small yet significant red shift of the protonated betaine O-H stretching mode. We investigate the performance of a wide range of density functional theory (DFT) functionals for the calculation of anharmonic vibrational frequency shifts of the studied system, which are essential for the correct interpretation of the experimental IRMPD data. For an accurate prediction of the OH stretching frequency shifts, specifically designed functionals such as Handy's group HCTH/407 should be applied. The empirical dispersion correction enhances the systematic overestimation of the anharmonic frequency shift, characteristic of the most widely used DFT functionals. Combining the full-wave function approach with the charge field perturbation and natural bond orbital (NBO) deletion analyses, we demonstrate that the frequency shift in the OH-tagged structure is governed by the σHH → σ*OH intermolecular charge transfer. This interaction stabilizes the OH-tagged dimer as well, in contrast to the dipole-quadrupole electrostatic interaction energy term. Topological analysis of the electron density reveals the presence of an intermolecular bond critical point with a positive value of the density Laplacian very close to the lower limit for hydrogen bonds. NCI analyses demonstrate that the OH···H2 interaction is weaker than the intramolecular CH···O one within the protonated betaine molecule, with the through of reduced density gradient appearing at less negative sign(λ2)·ρ values. Analyzing the O-H stretching vibrational potential with the second-generation absolutely localized molecular orbitals energy decomposition analysis (ALMO-EDA 2) revealed that in the case of betaineH(+) tagged from the OH group side, the permanent electrostatics (ΔEelec), polarization (ΔEpol), and charge-transfer (ΔEct) contributions to the total intermolecular interaction energy contribute favorably to the weak hydrogen bond formation and to the red shift of the fundamental O-H stretching frequency, the ΔEct contribution being the most significant in the last context. The Pauli repulsion term, on the other hand, favors an O-H stretching frequency blue shift as a consequence of the vibrational confinement effects.

Taming Conformational Heterogeneity on Ion Racetrack to Unveil Principles that Drive Membrane Permeation of Cyclosporines.

Our study reveals the underlying principles governing the passive membrane permeability in three large N-methylated macrocyclic peptides (N-MeMPs): cyclosporine A (CycA), Alisporivir (ALI), and cyclosporine H (CycH). We determine a series of conformers required for robust passive membrane diffusion and those relevant to other functions, such as binding to protein targets or intermediates, in the presence of solvent additives. We investigate the conformational interconversions and establish correlations with the membrane permeability. Nuclear magnetic resonance (NMR) and cyclic ion-mobility spectrometry-mass spectrometry (cIMS-MS) are employed to characterize conformational heterogeneity and identify cis-amides relevant for good membrane permeability. In addition, ion mobility selected cIMS-MS and infrared (IR) multiple-photon dissociation (IRMPD) spectroscopy experiments are conducted to evaluate the energy barriers between conformations. We observe that CycA and ALI, both cyclosporines with favorable membrane permeabilities, display multiple stable and well-defined conformers. In contrast, CycH, an epimer of CycA with limited permeability, exhibits fewer and fewer stable conformers. We demonstrate the essential role of the conformational shift from the aqueous cis MeVal11-MeBmt1 state (A1) to the closed conformation featuring cis MeLeu9-MeLeu10 (C1) in facilitating membrane permeation. Additionally, we highlight that the transition from A1 to the all-trans open conformation (O1) is specifically triggered by the presence of CaCl2. We also capture a set of conformers with cis Sar3-MeLeu4, MeLeu9-MeLeu10, denoted as I. Conformationally selected cIMS-MS and IRMPD data of [CycA+Ca]2+ show immediate repopulation of the original population distribution, suggesting that CaCl2 smooths out the energy barriers. Finally, our work presents an improved sampling molecular dynamics approach based on a refined force field that not only consistently and accurately captures established conformers of cyclosporines but also exhibits strong predictive capabilities for novel conformers.

Fluorinated Propionic Acids Unmasked: Puzzling Fragmentation Phenomena of the Deprotonated Species.

Environmental contamination by per- and polyfluorinated substances (PFAS) is an emerging concern for the public. In this study, short-chain PFAS such as deprotonated per- and polyfluorinated propionic acids are investigated using a combination of infrared multiple-photon dissociation (IRMPD) spectroscopy, collision-induced dissociation (CID), and density functional theory calculations. IRMPD and CID proceed via multiple competing pathways: (1) production of fluoroformate (FCO2-) and the associated ethylene derivative, (2) production of HF and the associated carbanion, or (3) loss of CO2 and the associated carbanion. Fluorinated propionic acids with at least one fluorine atom bound to the terminal carbon yield FCO2-, whereas loss of HF is observed in polyfluorinated species with at least one fluorine atom bound to the α-carbon. To explore the reaction pathways of the various fluorinated propionic acids, the nudged elastic band method is employed. The relative energy of the four-membered ring transition state leading to FCO2- dictates which product channel is observed in dissociation.

CO2 activation by copper oxide clusters: size, composition, and charge state dependence.

The interaction of CO2 with copper oxide clusters of different size, composition, and charge is investigated via infrared multiple-photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. Laser ablation of a copper target in the presence of an O2/He mixture leads to the preferred formation of oxygen-rich copper oxide cluster cations, CuxOy+ (y > x; x ≤ 8), while the anionic cluster distribution is dominated by stoichiometric (x = y) and oxygen-deficient (y < x; x ≤ 8) species. Subsequent reaction of the clusters with CO2 in a flow tube reactor results in the preferred formation of near-stoichiometric CuxOy(CO2)+/- complexes. IR-MPD spectroscopy of the formed complexes reveals the non-activated binding of CO2 to all cations while CO2 is activated by all anions. The great resemblance of spectra for all sizes investigated demonstrates that CO2 activation is largely independent of cluster size and Cu/O ratio but mainly determined by the cluster charge state. Comparison of the IR-MPD spectra with DFT calculations of the model systems Cu2O4(CO2)- and Cu3O4(CO2)- shows that CO2 activation exclusively results in the formation of a CO3 unit. Subsequent CO2 dissociation to CO appears to be unfavorable due to the instability of CO on the copper oxide clusters indicating that potential hydrogenation reactions will most likely proceed via formate or bicarbonate intermediates.

Structure and fragmentation chemistry of the peptide radical cations of glycylphenylalanylglycine (GFG).

Herein, we explore the generation and characterization of the radical cations of glycylphenylalanylglycine, or [GFG]•+, formed via dissociative electron-transfer reaction from the tripeptide to copper(II) within a ternary complex. A comprehensive investigation employing isotopic labeling, infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations elucidated the details and energetics in formation of the peptide radical cations as well as their dissociation products. Unlike conventional aromatic-containing peptide radical cations that primarily form canonical π-radicals, our findings reveal that 75% of the population of the experimentally produced [GFG]•+ precursors are [GFα•G]+, where the radical resides on the middle α-carbon of the phenylalanyl residue. This unexpected isomeric ion has an enthalpy of 6.8 kcal/mol above the global minimum, which has an N-terminal captodative structure, [Gα•FG]+, comprising 25% of the population. The [b₂-H]•+ product ions are also present in a ratio of 75/25 from [GFα•G]+/ [Gα•FG]+, the results of which are obtained from matches between the IRMPD action spectrum and predicted IR absorption spectra of the [b₂-H]•+ candidate structures, as well as from IRMPD isomer population analyses.

Symmetry reduction induced by argon tagging gives access to low-lying excited states of FeH+ in the overtone region of the Fe-H stretching mode.

Iron is the most abundant transition metal in the interstellar medium (ISM), and is thought to be involved in a variety of astrochemical processes. Here, we present the infrared multiple photon dissociation (IRMPD) spectra of Ar1,2FeH+ and their deuterated isotopologues in the region of 2240-14 000 cm-1. The Fe-H overtone stretching mode in ArFeH+ and Ar2FeH+ is observed at 3636 ± 28 cm-1 and 3659 ± 13 cm-1, respectively. Deuteration shifts these bands to 2618 ± 31 cm-1 and 2650 ± 14 cm-1 in ArFeD+ and Ar2FeD+, respectively. Additionally, the spectra of Ar2FeH+ and Ar2FeD+ feature broad transitions at ∼2200-4000 cm-1 and ∼4500-6500 cm-1. We assign these bands to electronic transitions from the thermally populated X5A2/X'5A1 ground state manifold into the A'5B2 and B5A1 states, which we model with multi-reference quantum chemical calculations including spin-orbit coupling. The calculations show that these transitions are symmetry forbidden in FeH+ and in the equilibrium geometry of ArFeH+/ArFeD+, while the zero-point oscillation of the bending mode of the triatomic molecule leads to some oscillator strength. Upon addition of the second argon atom, the transitions become weakly allowed in the equilibrium geometry of Ar2FeH+/Ar2FeD+ due to symmetry reduction from C∞v to C2v.

IR spectroscopic characterization of [M,C,2H]+ (M = Ru and Rh) products formed by reacting 4d transition metal cations with oxirane: Spectroscopic evidence for multireference character in RhCH2.

A combination of infrared multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Ru and Rh) species. These ions were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room-temperature ion trap. IRMPD spectra for the Ru species exhibit one major band and two side bands, whereas spectra for the Rh species contain more distinct bands. Comparison with density functional theory (DFT), coupled-cluster (CCSD), and equation-of-motion spin-flip CCSD (EOM-SF-CCSD) calculations allows assignment of the [M,C,2H]+ structures. For the spectrum of [Ru,C,2H]+, a combination of HRuCH+ and RuCH2+ structures reproduces the observed spectrum at all levels of theory. The well-resolved spectrum of [Rh,C,2H]+ could not be assigned unambiguously to any calculated structure using DFT approaches. The EOM-SF-CCSD calculations showed that the ground-state surface has multireference electronic character, and symmetric carbenes in both the 1A1 and 3A2 states are needed to reproduce the observed spectrum.

IR spectroscopic characterization of 3d transition metal carbene cations, FeCH2+ and CoCH2+: periodic trends and a challenge for DFT approaches.

A combination of IR multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H]+ (M = Fe and Co) species. These were formed by reacting laser ablated M+ ions with oxirane (ethylene oxide, c-C2H4O) in a room temperature ion trap. IRMPD spectra for the Fe and Co species are very similar and exhibit one major band. Comparison with density functional theory (DFT) and coupled cluster with single and double excitations (CCSD) calculations allows assignment of the spectra to MCH2+ carbene structures. For these 3d transition metal systems, experimental IRMPD spectra compare relatively poorly with DFT calculated IR spectra, but CCSD calculated spectra are a much better match primarily because the M-C stretch gains significant intensity. The origins of this behavior are explored in some detail. The present results are also compared to previous results for the 4d and 5d congeners and the periodic trends in these structures are evaluated.

Pathways of cluster growth: infra-red multi-photon dissociation spectroscopy of a series of Re-Si clusters, [ReSin]+, n = 3-9.

Infra-red multiple-photon dissociation spectroscopy on Xe-tagged Re/Si clusters, [ReSin]+, n = 3-9, reveals intense absorption features around 400 cm-1, along with, in some cases, additional bands in the 250-350 cm-1 window. A survey of the potential energy surface using density functional theory in conjunction with particle swarm optimisation indicates a growth pattern based on a growing network of Si atoms wrapped around the Re centre: the Sin units can be viewed as fragments of a putative 16-vertex Frank-Kasper polyhedron. The structural evolution for the [ReSin]+ series differs significantly from the iso-electronic Mn series studied previously, where the metal ion is typically bound externally to the surface of a growing 3-dimensional Sin cluster, the differences reflecting the greater accessibility of 5d vs. 3d electron density.

Microstructural Analysis of Benzoxazine Cationic Ring-Opening Polymerization Pathways.

Herein, an evaluation of the initial step of benzoxazine polymerization is presented by mass spectrometry, with a focus on differentiating the phenoxy and phenolic products formed by distinct pathways of the cationic ring opening polymerization (ROP) mechanism of polybenzoxazine formation. The use of infrared multiple photon dissociation (IRMPD) and ion mobility spectrometry (IMS) techniques allows for differentiation of the two pathways and provides valuable insights into the ROP mechanism. The results suggest that type I pathway is favored in the initial stages of the reaction yielding the phenoxy product, while type II product should be observed at later stages when the phenoxy product would interconvert to the most stable type II phenolic product. Overall, the findings presented here provide important information on the initial step of the benzoxazine polymerization, allowing the development of optimal polymerization conditions and represents a way to evaluate other multifunctional polymerization processes.