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A fundamental understanding of cyclodextrin-closo-dodecaborate inclusion complexes is of great interest in supramolecular chemistry. Herein, we report a systematic investigation on the electronic structures and intramolecular interactions of perhalogenated closo-dodecaborate dianions B₁₂X₁₂^2– (X = F, Cl, Br and I) binding to α-, β-, and γ-cyclodextrins (CDs) in the gas phase using combined negative ion photoelectron spectroscopy (NIPES) and density functional theory (DFT) calculations. The vertical detachment energy (VDE) of each complex and electronic stabilization of each dianion due to the CD binding (ΔVDE, relative to the corresponding isolated B₁₂X₁₂^2–) are determined from the experiments along α-, β- and γ-CD inmore » the form of VDE (ΔVDE): 4.00 (2.10), 4.33 (2.43), and 4.30 (2.40) eV in X = F; 4.09 (1.14), 4.64 (1.69), and 4.69 (1.74) eV in X = Cl; 4.11 (0.91), 4.58 (1.38), and 4.70 (1.50) eV in X = Br; and 3.54 (0.74), 3.88 (1.08), and 4.05 (1.25) eV in X = I, respectively. All complexes have significantly higher VDEs than the corresponding isolated dodecaborate dianions with ΔVDE spanning from 0.74 eV at (α, I) to 2.43 eV at (β, F), sensitive to both host CD size and guest substituent X. DFT-optimized complex structures indicate that all B₁₂X₁₂^2– prefer binding to the wide openings of CDs with the insertion depth and binding motif strongly dependent on the CD size and halogen X. Dodecaborate anions with heavy halogens, i.e., X = Cl, Br, and I, are found outside of α-CD, while B₁₂F₁₂^2– is completely wrapped by γ-CD. Partial embedment of B₁₂X₁₂^2– into CDs is observed for the other complexes via multipronged B–X^…H–O/C interlocking patterns. The simulated spectra based on the density of states agree well with those of the experiments and the calculated VDEs well reproduce the experimental trends. Molecular orbital analyses suggest that the spectral features at low binding energies originated from electrons detached from the dodecaborate dianion, while those at higher binding energies are derived from electron detachment from CDs. Energy decomposition analyses reveal that the electrostatic interaction plays a dominating role in contributing to the host–guest interactions for the X = F series partially due to the formation of a O/C–H^…X–B hydrogen bonding network, and the dispersion forces gradually become important with the increase of halogen size.« less

A great deal of effort has been focused on developing a metal-free catalytic system for epoxidation of unreactive alkenes. Fluoroketones are thought as remarkably promising catalysts for epoxidation reactions. The combination of fluorinated alcohols and catalytic amounts of hexafluoroacetone (HFA) gives a versatile and effective medium for epoxidation of various olefins with hydrogen peroxide. However, the fundamental physicochemical properties of HFA remained largely unclear, although they were very important to understand the related interactions. Here, in this study, we performed a joint study on the electron affinity and electronic structure of HFA employing negative ion photoelectron (NIPE) spectroscopy and quantummore » chemistry calculations. Two distinct bands with complicated vibrational progressions were observed in the 193 nm NIPE spectrum. The adiabatic/vertical detachment energies (ADE/VDE) were derived to be 1.42/2.06 and 4.43/4.86 eV for the ground singlet state and excited triplet state, respectively. Using the optimized geometries and vibrational frequencies of the anion and the neutral, the Franck–Condon factors were calculated for electron detachments to produce HFA in its lowest singlet and triplet states. Good agreements are obtained hereby for both bands between the experimental and calculated NIPE spectra, when taking into account combination vibrational excitations, unequivocally revealing that HFA possesses a singlet ground state with a giant singlet–triplet energy difference (ΔE_ST). The electron affinity (EA) and ΔE_ST of HFA were therefore determined to be EA = 1.42 ± 0.02 eV and ΔE_ST = −3.01 eV.« less

Nitro-functionalized undecahalogenated closo-dodecaborates [B₁₂X₁₁(NO₂)]^2– were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X-diffraction, mass spectrometry, and gas-phase ion vibrational spectroscopy. The NO₂ substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B₁₂X₁₂]^2– (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum-chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO^.). The homolytic NO^. elimination from the dianion under very soft collisional excitation in gas-phase ion experiments results in the formation of the radicalmore » [B₁₂X₁₁O]^2–.. Theoretical investigations suggest that the loss of NO^. proceeds via the rearrangement product [B₁₂X₁₁(ONO)]^2–. The O-bonded nitrosooxy structure is thermodynamically more stable than the N-bonded nitro structure and its formation by radical recombination of [B₁₂X₁₁O]^2–. and NO^. is demonstrated.« less

In this work, the recently synthesized acetylide compound KSeCCH containing the main group element selenium within the novel and in crystalline form unprecedented [SeCCH]^– anion was successfully investigated in the gas phase by high-resolution velocity-map imaging (VMI) and magnetic-bottle (MB) photoelectron spectroscopy coupled with an electrospray ionization source. Both VMI and MB spectra exhibited identical electron affinities (EA, 2.517 ± 0.002 eV), spin–orbit coupling (SOC) splittings (1492 ± 20 cm^–1), and Se–C stretching frequencies (573 ± 20 cm^–1) of the corresponding neutral tetra-atomic radical [SeCCH]^• with the VMI spectrum possessing six times higher spectral resolution compared with the MB spectrum.more » These experimental values were well reproduced by calculations at the CCSD(T) level, in which both the isolated [SeCCH]^– anion and the [SeCCH]^• radical adopted linear geometries. The simulated spectra based on the calculated Franck–Condon factors, the SOC splitting, and the experimental line width matched well with the measured spectra. Furthermore, comparisons of the EA and SOC splitting values with the previously reported isolobal species [SeCN]^• are also made and discussed. The decrease in the EA and SOC splitting of [SeCCH]^• is ascribed to the differences in the electronegativities between C and N atoms as well as the electron density on the Se atom in its singly occupied molecular orbital (SOMO).« less

Here we show elusive carbonic acid being effectively stabilized in the gas phase by interacting with halide anions X^– (X = F, Cl, Br, and I). The formed H₂CO₃·X^– complexes, characterized by negative ion photoelectron spectroscopy and ab initio calculations, all contain intact trans–trans carbonic acid binding onto the respective halide via two identical strong ionic O–H...X^– hydrogen bonds. For X = Cl, Br, and I, the complex spectra exhibit the corresponding X^– signature by simply shifting to the higher binding energy side, while an extremely 2 eV wide broader band is observed for X = F. This spectroscopic evidencemore » indicates that an excess electron is removed from each halide in the former case, while a proton is transferred from carbonic acid to fluoride upon electron detachment for the latter. The above H₂CO₃·X^– structures as well as those of the previously studied H₂SO₄·X^– along the homologous halogen series cannot be explained using the proton affinity (PA) argument. Instead, a qualitative correlation is found between these structural motifs and the constituent acid pK_a values, strongly suggesting that pK_a is a more suitable factor to predict correct acid–base chemistry between these diprotic oxyacids and halides.« less

Photoelectron spectroscopy and theoretical investigations have been performed to systematically probe the intrinsic electronic properties of [Mo6X14]2− (X = halogen). All three PE spectra of gaseous [Mo6X14]2− (X = Cl, Br, I) dianions, which were generated by electrospray ionization, exhibit multiple resolved peaks in the recorded binding energy range. Theoretical investigations on the orbital structure and charge distribution were performed to support interpretation of the observed spectra and were further extended onto [Mo6F14]2−, a dianion that was not available for the experimental study. The measured adiabatic (ADE) and vertical detachment energies (VDE) for X = Cl–I were well reproduced bymore » density functional theory calculations (accuracy ∼0.1 eV). Corresponding ADE/VDE values for the dianions were found to be 1.48/2.13 (calc.) and 2.30/2.65, 2.30/2.62, and 2.20/2.42 eV (all expt.) for X = F, Cl, Br, and I, respectively, showing an interesting buckled trend of electron binding energy (EBE) along the halogen series, i.e., EBE (F) ≪ EBE (Cl) ∼ EBE (Br) > EBE (I). Molecular orbital analyses indicate different mixing of metal and halogen atomic orbitals, which is strongly dependent on the nature of X, and suggest that the most loosely bound electrons are detached mainly from the metal core for X = F and Cl, but from halide ligands for X = Br and I. The repulsive Coulomb barrier (RCB), estimated from the photon energy dependent spectra, decreases with increasing halogen size, from 1.8 eV for X = Cl to 1.6 eV for X = I. Electrostatic potential modeling confirms the experimental RCB values and predicts that the most favorable electron detaching pathway should lie via the face-bridging halide ligands.« less

This work showcases cryogenic and temperature-dependent “iodide-tagging” photoelectron spectroscopy to probe specific binding sites of amino acids using the glycine-iodide complex (Gly?I-) as a case study. Multiple Gly?I- isomers were generated from ambient electrospray ionization and kinetically isolated in a cryogenic ion trap. These structures were characterized with temperature-dependent “iodide-tagging” negative ion photoelectron spectroscopy (NIPES) where iodide was used as the “messenger” to interpret electronic energetics and structural information of various Gly?I- isomers. Accompanied by theoretical computations and Franck-Condon simulations, a total of five cluster structures have been identified along with their various binding motifs. This work demonstrates that “iodide-tagging”more » NIPES is a powerful general means for probing specific binding interactions in biological molecules of interest.« less

The electronic properties, fragmentation by collision-induced dissociation and bond properties of gaseous closo -[B ₆ X ₆ ] ²⁻ (X = Cl, Br, I) dianions are discussed and compared to their larger-sized [B ₁₂ X ₁₂ ] ²⁻ analogues.

We report a joint negative ion photoelectron spectroscopy and computational study on the electronic structures and noncovalent interactions of a series of cyclodextrin- closo -dodecaborate dianion complexes, χ-CD·B ₁₂ X ₁₂ ²⁻ (χ = α, β, γ; X = H, F).