27 Search Results

The oxidation of phenolic compounds is one of the most important reactions prevalent in various biological processes, often explicitly coupled with proton transfers (PTs). Quantitative descriptions and molecular-level understanding of these proton-coupled electron transfer (PCET) reactions have been challenging. This work reports a direct observation of PCET in photodetachment (PD) photoelectron spectroscopy (PES) of hydrogen-bonded phenolic (ArOH) nitrate (NO₃^-) complexes, in which a much slower rising edge provides a spectroscopic signature to evidence PCET. Electronic structure calculations unveil the PCET processes to be isomer-specific, occurred only in those with their HOMOs localized on ArOH, leading to charge-separated transient states ArOH^•+·NO₃^-more » triggered by ionizing phenols while simultaneously promoting PT from ArOH^•+ to NO₃^-. Importantly, this study showcases that gas-phase PD-PES is a generic means enabling to identify PCET reactions with explicit structural and binding information.« less

We report a joint negative ion photoelectron spectroscopy (NIPES) and quantum chemical computational study on glycine-chloride/bromide complexes (denoted Gly·X^-, X = Cl/Br) in close comparison to the previously studied Gly·I^- cluster ion. Combining experimental NIPE spectra and theoretical calculations, various Gly·X^- complexes were found to adopt the same types of low-lying isomers, albeit with different relative energies. Despite more congested spectral profiles for Gly·Cl^- and Gly·Br^-, spectral assignments were accomplished with the guidance of the knowledge learned from Gly·I^-, where a larger spin–orbit splitting of iodine afforded well-resolved, recognizable spectral peaks. Three canonical plus one zwitterionic isomer for Gly·Cl^-more » and four canonical conformers for Gly·Br^- were experimentally identified and characterized in contrast to the five canonical ones observed for Gly·I^- under similar experimental conditions. Taken together, this study investigates both genericity and variations in binding patterns for the complexes composed of glycine and various halides, demonstrating that iodide-tagging is an effective spectroscopic means to unravel diverse ion-molecule binding motifs for cluster anions with congested spectral bands by substituting the respective ion with iodide.« less

In this work, we report observation and photoelectron spectroscopic characterization of sodium cationization on four doubly deprotonated mononucleotide dianions Na⁺·[dNMP-2H]^2- (N = A, G, C, or T) in the gas phase. Multiple tautomers with distinct deprotonated sites are identified, in which Na+ enables novel double deprotonation patterns and folds resultant mononucleotide dianions. The most stable isomer for the whole family is derived from detaching one proton from phosphate and the other from nucleobase (amino group for N = A, G and C, but nitrogen atom for T), while a high-lying isomer with protons detached separately from phosphate and hydroxy groupmore » of sugar coexist. Particularly, an exotic deprotomer with both protons deprived from guanosine is populated as well. This work thus displays a remarkably diverse binding landscape enabled by sodium cationization, a potentially critical element in developing general formulism to better model metal cation and nucleotide interactions.« less

Here, we report a combined photoelectron spectroscopy and theoretical investigation of a series of size-selected hydrated closo-dodecaborate clusters B₁₂X₁₂^2–·nH₂O (X = H, F, or I; n = 1–6). Distinct structural arrangements of water clusters from monomer to hexamer can be achieved by using different B₁₂X₁₂^2– bases, illustrating the evident solute specificity. Because B–H···H–O dihydrogen bonds are stronger than O···H–O hydrogen bonds in water, the added water molecules are arranged in a unified binding mode by forming highly structured water networks manipulated by B₁₂H₁₂^2–. As a comparison, the hydrated B₁₂F₁₂^2– clusters display similar water evolution for n values of 1 andmore » 2 but different binding modes for larger clusters, while water networks in B₁₂I₁₂^2– share similarities with the free water clusters. This finding provides a consistent picture of the structural diversity of hydrogen bonding networks in microhydrated dodecaborates and a molecular-level understanding of microsolvation dynamics in aqueous borate chemistry.« less

Size-selective, negative ion photoelectron spectroscopy in conjunction with quantum chemical calculations is employed to investigate the geometric and electronic structures of a protype system in catalytic olefin epoxidation research, i.e., deprotonated hexafluoroisopropanol ([HFIP_-H]^-) complexed with hydrogen peroxide (H₂O₂). Spectral assignments and molecular electrostatic surface analyses unveil a surprising prevalent existence of a high-lying isomer with asymmetric dual hydrogenbonding configuration that is preferably formed driven by influential directionspecific electrostatic interactions upon H₂O₂ approaching [HFIP_-H]^- anion. Subsequent inspections of molecular orbitals, charge and spin density distributions indicate the occurrence of partial charge transfer from [HFIP_-H]^- to H₂O₂ upon hydrogen bonding interactions. Accompanyingmore » with electron detachment, a proton transfer occurs to form the neutral complex of [HFIP•HOO^•] structure. In conclusion, this work conspicuously illustrates the importance of directionality encoded in intermolecular interactions involving asymmetric and complex molecules, while the produced hydroperoxyl radical HOO^• offers a possible new pathway in olefin epoxidation chemistry.« less

Intermolecular interactions as those present in molecule…water complexes may profoundly influence the physico-chemical properties of molecules. Here, we carried out an experimentally-computational study on the doubly-deprotonated guanosine monophosphate…water clusters, [dGMP-2H]^2-•nH₂O (n=1–4), using a combination of negative anion photoelectron spectroscopy (NIPES) with molecular dynamics (MD) and quantum chemical (QM) calculations. Successive addition of water molecules to [dGMP-2H]^2- increases the experimental adiabatic detachment (ADE) and vertical detachment energy (VDE) by 0.5 to 0.1 eV, depending on the cluster size. In order to choose the representative conformations, we combined MD simulations with a clustering procedure to identify low energy geometries for which ADEsmore » and VDEs were computed at the CAM-B3LYP/6-31++G(d,p) level. Our results demonstrate that the assumed approach lead to the sound geometries and energetics of the studied micro-solvates since the calculated ADEs and VDEs are in pretty good agreement with the experimental characteristics. The evolution of hydrogen bonding with cluster size indicates possibility of the occurrence of proton transfer for clusters comprising a larger number of water molecules.« less

A joint gas-phase ion spectroscopic and multiscale theoretical study reveals unequivocally the predominance of the hydridic-to-protonic dihydrogen bond over the prototypical strong hydrogen bond in monohydrated dodecaborate clusters.

Functionalization of electrodes with clusters of hydrophobic molecules may improve the energy efficiency and selectivity of electrochemical separations by modulating the desolvation process occurring at the interface. Ionic liquids (IL), which have a broad range of compositions and properties, are potential candidates for controlling the transport, desolvation, and adsorption of target ions at electrochemical interfaces. We report a joint experimental and theoretical investigation of the structure, stability, and selective adsorption properties of the IL clusters 1-ethyl-3-methylimidazolium chloride [EMIM]_x[Cl]_x+1^- (x = 1 – 10) and demonstrate their ability to adsorb and separate ions from solution. The structure and stability of themore » IL clusters are determined experimentally using high-mass-resolution electrospray ionization mass spectrometry, collision-induced dissociation, and negative ion photoelectron spectroscopy. Global optimization theory and ab initio molecular dynamics simulations provide molecular-level insight into the bonding and structural fluxionality of these species. Ion soft landing is used to selectively functionalize the surface of highly oriented pyrolytic graphite (HOPG) working electrodes with [EMIM]₁[Cl]₂^-, [EMIM]³[Cl]₄^-, and [EMIM]₅[Cl]₆^- clusters. Kelvin probe microscopy provides insight into the relative stability of the clusters on HOPG and their effect on the work function of IL-functionalized electrodes. Cyclic voltammetry measurements reveal irreversible adsorption of Fe(CN)₆^4-/3- anions during redox cycling, while electrochemical impedance spectroscopy indicates a substantial decrease in the electron transfer resistance of the IL-functionalized electrodes due to adsorption of Fe(CN)₆^4-/3-. Overall, our findings demonstrate that IL clusters with different size and stoichiometry may be used to increase the efficiency of electrochemical separations, opening new horizons in selective electrode functionalization.« less

Here, we report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B₂(CN)₆]^2– in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) —a remarkably high value of electronic stability and a ~2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)₃]^•– is energetically favorable. Quantum chemical calculations demonstrate a $$D_{3d}$$ staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match themore » experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances« less