UV and IR Spectroscopy of Cold H2O+-Benzo-Crown Ether Complexes

The H2O+ radical ion, produced in an electrospray ion source via charge transfer from Eu3+, is encapsulated in benzo-15-crown-5 (B15C5) or benzo-18-crown-6 (B18C6). We measure UV photodissociation (UVPD) spectra of the (H2O·B15C5)+ and (H2O·B18C6)+ complexes in a cold, 22-pole ion trap. These complexes show sharp vibronic bands in the 35 700–37 600 cm–1 region, similar to the case of neutral B15C5 or B18C6. These results indicate that the positive charge in the complexes is localized on H2O, giving the forms H2O+·B15C5 and H2O+·B18C6, in spite of the fact that the ionization energy of B15C5 and B18C6 is lower than that of H2O. The formation of the H2O+ complexes and the suppression of the H3O+ production through the reaction of H2O+ and H2O can be attributed to the encapsulation of hydrated Eu3+ clusters by B15C5 and B18C6. On the contrary, the main fragment ions subsequent to the UV excitation of these complexes are B15C5+ and B18C6+ radical ions; the charge transfer occurs from H2O+ to B15C5 and B18C6 after the UV excitation. The position of the band origin for the H2O+·B18C6 complex (36323 cm–1) is almost the same as that for Rb+·B18C6 (36315 cm–1); the strength of the intermolecular interaction of H2O+ with B18C6 is similar to that of Rb+. The spectral features of the H2O+·B15C5 complex also resemble those of the Rb+·B15C5 ion. We measure IR–UV spectra of these complexes in the CH and OH stretching region. Four conformers are found for the H2O+·B15C5 complex, but there is one dominant form for the H2O+·B18C6 ion. This study demonstrates the production of radical ions by charge transfer from multivalent metal ions, their encapsulation by host molecules, and separate detection of their conformers by cold UV spectroscopy in the gas phase.

UV and IR Spectroscopy of Transition Metal–Crown Ether Complexes in the Gas Phase: Mn2+(benzo-15-crown-5)(H2O)0–2

Yoshiya Inokuchi, Thomas Rizzo

Ultraviolet photodissociation (UVPD) and IR–UV double-resonance spectroscopy are performed for bare and microhydrated complexes of Mn2+(benzo-15-crown-5), Mn2+(B15C5)(H2O)n (n = 0–2), under cold (∼10 K) gas-phase conditions. Density functional theory (DFT) calculations are also carried out to derive information on the geometric and electronic structures of the complexes from the experimental results. The n = 0 complex shows broad features in the UVPD spectrum, whereas the UV spectra of the n = 1 and 2 complexes exhibit sharp vibronic bands. The IR–UV and DFT results suggest that there is only one isomer each for the n = 1 and 2 complexes in which H2O molecules are directly attached to the Mn2+ ion through Mn2+···OH2 bonds with no intermolecular bond between the water molecules. Time-dependent DFT calculations suggest that the π–π* transition of the B15C5 part is highly mixed with the “ligand to metal charge transfer” transition in the n = 0 complex, which can result in broad features in the UVPD spectrum. In contrast, attachment of H2O molecules to Mn2+(B15C5) suppresses the mixing, providing sharp vibronic bands assignable to the π–π* transition for the n = 1 and 2 complexes. These results indicate that the electronic structure and transition of benzo-crown ether complexes with transition metals are strongly affected by solvation.

2019

Microhydration of Dibenzo-18-Crown-6 Complexes with K+, Rb+, and Cs+ Investigated by Cold UV and IR Spectroscopy in the Gas Phase

Yoshiya Inokuchi, Thomas Rizzo

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K+, Rb+, and Cs+ ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K+·DB18C6·(H2O)n, Rb+·DB18C6·(H2O)n, and Cs+·DB18C6·(H2O)n (n = 1–8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb+·DB18C6·(H2O)1–5 and the Cs+·DB18C6·(H2O)3 complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K+·DB18C6, Rb+·DB18C6, and Cs+·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K+ to Cs+. Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K+·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K+·DB18C6 conformer, while hydration occurs only on top of the Rb+·DB18C6 and Cs+·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb+·DB18C6 and Cs+·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K+·DB18C6 complex can accept H2O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K+·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K+ ion by DB18C6 in solution.

2018

Spectroscopic Probes of Conformational Isomerization of Biological Molecules in a Cold Ion Trap

Caroline Seaiby

The function of biologically active molecules depends both on their structure and on their conformational dynamics. In solution, intramolecular interaction with the solvent will influence different biological processes, i.e. protein folding. However secondary structure (helices, sheets) is heavily influenced by intramolecular forces and hence it is important to isolate biological molecules in order to study their intrinsic behavior. Moreover, gas phase studies on biomolecules provide critical information that can serve as benchmarks to test the accuracy of theoretical predictions. The main focus of this thesis is the investigation of the potential energy surfaces of biomolecules in the gas phase. Biomolecular ions are produced in the gas phase via nano-electrospray, mass-selected and guided into a cold, 22-pole ion trap where they are cooled via collisions with cold helium. A variety of double-resonance techniques based on photofragmentation detection are then applied to obtain information on the molecule stable conformations, the potential barriers separating stable conformations and their connectivity. We applied these techniques on molecules of increasing size, starting with protonated phenylalanine and proceeding to the 7amino acid peptides, Ac-Phe-(Ala)5-LysH+ and the 12-residue peptide Ac-Phe-(Ala)3-(Gly)4-(Ala)3-LysH +. As a first step, electronic spectra and conformation-specific IR-UV double resonance spectra are measured. These measurements, in combination with DFT calculations, allow the assignment of two stable conformers in the case of the protonated phenylalanine and four in the case of the seven residue peptide. In the second part of this work we focused on the conformational isomerization of these molecules by performing infrared and ultraviolet hole-filling spectroscopy in the ion trap. We demonstrated that we can induce isomerization between stable conformers of each molecule via vibrational excitation. After energy dissipation, the excited molecules are redistributed among the initially identified conformers, but no new minima were detected. In the last part of this thesis, the fractional populations and the isomerization quantum yields are determined through infrared induced population transfer spectroscopy. Protonated phenylalanine reveals conformational selectivity in its isomerization while the relaxation of the infrared excitation leads to the equilibrium distribution in the case of the 12-residue glycine-containing peptide. The steps that occur during the energy dissipation are discussed in this thesis.

EPFL2011

Spectroscopic Probes of Conformational Isomerization of Biological Molecules in a Cold Ion Trap

Caroline Seaiby

EPFL2011