Infrared spectroscopy of ionized corannulene in the gas phase

The gas-phase infrared spectra of radical cationic and protonated corannulene were recorded by infrared multiple-photon dissociation (IRMPD) spectroscopy using the IR free electron laser for infrared experiments. Electrospray ionization was used to generate protonated corannulene and an IRMPD spectrum was recorded in a Fourier-transform ion cyclotron resonance mass spectrometer monitoring H-loss as a function of IR frequency. The radical cation was produced by 193-nm UV photoionization of the vapor of corannulene in a 3D quadrupole trap and IR irradiation produces H, H2, and C2Hx losses. Summing the spectral response of the three fragmentation channels yields the IRMPD spectrum of the radical cation. The spectra were analyzed with the aid of quantum-chemical calculations carried out at various levels of theory. The good agreement of theoretical and experimental spectra for protonated corannulene indicates that protonation occurs on one of the peripheral C-atoms, forming an sp3 hybridized carbon. The spectrum of the radical cation was examined taking into account distortions of the C5v geometry induced by the Jahn-Teller effect as a consequence of the degenerate 2E1 ground electronic state. As indicated by the calculations, the five equivalent Cs minima are separated by marginal barriers, giving rise to a dynamically distorted system. Although in general the character of the various computed vibrational bands appears to be in order, only a qualitative match to the experimental spectrum is found. Along with a general redshift of the calculated frequencies, the IR intensities of modes in the 1000-1250 cm−1 region show the largest discrepancy with the harmonic predictions. In addition to CH "in-plane" bending vibrations, these modes also exhibit substantial deformation of the pentagonal inner ring, which may relate directly to the vibronic interaction in the radical cation

Direct evidence for the ring opening of monosaccharide anions in the gas phase: photodissociation of aldohexoses and aldohexoses derived from disaccharides using variable-wavelength infrared irradiation in the carbonyl stretch region

a b s t r a c t All eight D-aldohexoses and aldohexoses derived from the non-reducing end of disaccharides were investigated by variable-wavelength infrared multiple-photon dissociation (IRMPD) as anions in the negative-ion mode. Spectroscopic evidence supports the existence of a relatively abundant open-chain configuration of the anions in the gas phase, based on the observation of a significant carbonyl absorption band near 1710 cm À1 . The abundance of the open-chain configuration of the aldohexose anions was approximately 1000-fold or greater than that of the neutral sugars in aqueous solution. This provides an explanation as to why it has not been possible to discriminate the anomeric configuration of aldohexose anions in the gas phase when derived from the non-reducing sugar of a disaccharide. Evidence from photodissociation spectra also indicates that the different aldohexoses yield product ions with maximal abundances at different wavelengths, and that the carbonyl stretch region is useful for differentiation of sugar stereochemistries. Quantum-chemical calculations indicate relatively low energy barriers to intramolecular proton transfer between hydroxyl groups and adjacent alkoxy sites located on open-chain sugar anions, suggesting that an ensemble of alkoxy charge locations contributes to their observed photodissociation spectra. Ring opening of monosaccharide anions and interconversion among configurations is an inherent property of the ions themselves and occurs in vacuo independent of solvent participation

Metal Cation Binding to Gas-Phase Pentaalanine: Divalent Ions Restructure the Complex

Ion-neutral complexes of pentaalalanine with several singly- and doubly charged metal ions are examined using conformation analysis by infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) computations. The infrared spectroscopy in the 1500–1800 cm^–1 region is found to be conformationally informative; in particular, the frequency of the CO stretching mode of the terminal carboxyl group is diagnostic for hydrogen bonding of the terminal hydroxyl. The doubly charged alkaline earth metal ions (Ca²⁺ and Ba²⁺) enforce a highly structured chelation shell around the metal ion, with six strongly bound Lewis-basic chelation sites, and no hydroxyl hydrogen bonding. With the more weakly binding alkali metal ions (Na⁺, K⁺, and Cs⁺), structures with intramolecular hydrogen bonds are more favorable, leading to dominance of conformations with lower degrees of metal ion chelation. The favored coordination mode correlates with ionic charge and binding strength but is not related to the ionic radius of the metal ion

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 (18c6H4) 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 18c6H4 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 18c6H4 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 18c6H4 substrate are found to lie higher in energy and are ruled out by the experimen

Infrared Multiple Photon Dissociation Spectroscopy of Cationized Histidine: Effects of Metal Cation Size on Gas-Phase Conformation

The gas phase structures of cationized histidine (His), including complexes with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser, in conjunction with quantum chemical calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G­(d,p) (Li⁺, Na⁺, and K⁺ complexes) and B3LYP/HW*/6-311+G­(d,p) (Rb⁺ and Cs⁺ complexes) levels of theory, where HW* indicates that the Hay–Wadt effective core potential with additional polarization functions was used on the metals. Single point energy calculations were carried out at the B3LYP, B3P86, and MP2­(full) levels using the 6-311+G­(2d,2p) basis set. On the basis of these experiments and calculations, the only conformation that reproduces the IRMPD action spectra for the complexes of the smaller alkali metal cations, Li⁺(His) and Na⁺(His), is a charge-solvated, tridentate structure where the metal cation binds to the backbone carbonyl oxygen, backbone amino nitrogen, and nitrogen atom of the imidazole side chain, [CO,N_α,N₁], in agreement with the predicted ground states of these complexes. Spectra of the larger alkali metal cation complexes, K⁺(His), Rb⁺(His), and Cs⁺(His), have very similar spectral features that are considerably more complex than the IRMPD spectra of Li⁺(His) and Na⁺(His). For these complexes, the bidentate [CO,N₁] conformer in which the metal cation binds to the backbone carbonyl oxygen and nitrogen atom of the imidazole side chain is a dominant contributor, although features associated with the tridentate [CO,N_α,N₁] conformer remain, and those for the [COOH] conformer are also clearly present. Theoretical results for Rb⁺(His) and Cs⁺(His) indicate that both [CO,N₁] and [COOH] conformers are low-energy structures, with different levels of theory predicting different ground conformers

Stability of Gas-Phase Tartaric Acid Anions Investigated by Quantum Chemistry, Mass Spectrometry, and Infrared Spectroscopy

In an effort to understand the chemical factors that stabilize dianions, experimental and theoretical studies on the stability of the tartrate dianion were performed. Quantum chemical calculations at the coupled cluster level reveal only a metastable state with a possible decomposition pathway (O₂C–CH­(OH)–CH­(OH)–CO₂)^2– → (O₂C–CH­(OH)–CH­(OH))^•– + CO₂ + e^– explaining the observed gas-phase instability of this dianion. Further theoretical data were collected for the bare dianion, this molecule complexed to water, sodium, and a proton, in both the meso and l forms as well as for the uncomplexed radical anion and neutral diradical. The calculations suggest that the l-tartrate dianion is more thermodynamically stable than the dianion of the meso stereoisomer and that either dianion can be further stabilized by association with a separate species that can help to balance the charge of the molecular complex. Mass spectrometry was then used to measure the energy needed to initiate collisionally induced dissociation of the racemic tartrate dianion and for the proton and sodium adducts of both the racemic and meso form of this molecule. Infrared action spectra of the dianion stereoisomers complexed with sodium were also acquired to determine the influence of the metal ion on the vibrations of the dianions and validate the computationally predicted structures. These experimental data support the theoretical conclusions and highlight the instability of the bare tartrate dianion. From the experimental work, it could also be concluded that the pathway leading to dissociation is under kinetic control because the sodium adduct of the racemic stereoisomer dissociated at lower collisional energy, although it was calculated to be more stable, and that decomposition proceeded via C–C bond dissociation as computationally predicted. Taken together, these data provide insight into the gas-phase stability of the tartrate dianion and highlight the role of adducts in stabilizing this species

a₂ Ion Derived from Triglycine: An N₁-Protonated 4-Imidazolidinone

Fragmentation of protonated peptides in the gas phase constitutes the basis for gas-phase sequencing of peptides using tandem mass spectrometry. Several mechanistic studies have indicated possible loss of b_<i>n</i> ion sequence information as a consequence of macrocycle formation from internal nucleophilic attacks. Here, we show by infrared multiple-photon dissociation spectroscopy and density functional theory that the prototypical a₂ ion generated from protonated triglycine is predominantly a cyclic N₁-protonated 4-imidazolidinone. Cyclization resulting from internal nucleophilic attacks therefore may be a more general phenomenon than anticipated