Cryogenic "Iodide-Tagging" Photoelectron Spectroscopy: A Sensitive Probe for Specific Binding Sites of Amino Acids

Organic Molecules Mimic Alkali Metals Enabling Spontaneous Harpoon Reactions with Halogens

The harpoon mechanism has been a milestone in molecular reaction dynamics. Until now, the entity from which electron harpooning occurs has been either alkali metal atoms or non-metallic analogs in their excited states. In this work, we demonstrate that a common organic molecule, octamethylcalix[4] pyrrole (omC4P), behaves just like alkali metal atoms, enabling the formation of charge-separated ionic bonding complexes with halogens omC4P+ ⋅ X- (X=F-I, SCN) via the harpoon mechanism. Their electronic structures and chemical bonding were determined by cryogenic photoelectron spectroscopy of the corresponding anions and confirmed by theoretical analyses. The omC4P+ ⋅ X- could be visualized to form from the reactants omC4P+X via electron harpooning from omC4P to X at a distance defined by the energy difference between the ionization potential of omC4P and electron affinity of X.

Photogeneration of singlet oxygen catalyzed by hexafluoroisopropanol for selective degradation of dyes

Singlet oxygen (¹O₂) shows great potential for selective degradation of dyes in environmental remediation of wastewater. In this study, we showcased that ¹O₂ can be effectively generated from an anion complex composed of deprotonated hexafluoroisopropanol anion ([HFIP_-H]^‒) with hydroperoxyl radical (⋅HO₂) via ultraviolet (UV) photodetachment. Electronic structure calculations and cryogenic negative ion photoelectron spectroscopy unveil critical proton transfer upon complex formation and electron ejection, effectively photoconverting prevalent triplet ground state ³O₂ to long-lived excited ¹O₂, stabilized by nearby HFIP. Inspired by this spectroscopic study, a novel "photogeneration" strategy is proposed to produce ¹O₂ with the incorporation of atmospheric O₂ and HFIP, acting as a catalyst. Conceptually, the designed catalytic cycle upon UV irradiation and electron injection is able to achieve different degradations of dye molecules in a controllable fashion from decolorization to complete mineralization, shedding new light on potential water purification.

How generic is iodide-tagging photoelectron spectroscopy: An extended investigation on the Gly·X- (Gly = glycine, X = Cl or Br) complexes

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- 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.

The primary gas phase hydration shell of hydroxide

The number of water molecules in hydroxide's primary hydration shell has been long debated to be three from the interpretation of experimental data and four from theoretical studies. Here, we provide direct evidence for the presence of a fourth water molecule in hydroxide's primary hydration shell from a combined study based on high-resolution cryogenic experimental photoelectron spectroscopy and high-level quantum chemical computations. Well-defined spectra of OH^-(H₂O)_n clusters (n = 2 to 5) yield accurate electron binding energies, which are, in turn, used as key signatures of the underlying molecular conformations. Although the smaller OH^-(H₂O)₃ and OH^-(H₂O)₄ clusters adopt close-lying conformations with similar electron binding energies that are hard to distinguish, the OH^-(H₂O)₅ cluster clearly has a predominant conformation with a four-coordinated hydroxide binding motif, a finding that unambiguously determines the gas phase coordination number of hydroxide to be four.

Sodium Cationization Enables Exotic Deprotonation Sites on Gaseous Mononucleotides

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 the resultant mononucleotide dianions. The most stable isomer for the whole family is derived from detaching one proton from the phosphate and the other from the nucleobase (amino group for N = A, G, and C, but nitrogen atom for T), whereas high-lying isomers with protons detached separately from the phosphate and the hydroxy group 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 a general formulism to better model metal cation and nucleotide interactions.

Manifesting Direction-Specific Complexation in [HFIP-H·H2O2]-: Exclusive Formation of a High-Lying Conformation

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, that is, 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 hydrogen-bonding configuration that is preferably formed driven by influential direction-specific 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. Accompanied with electron detachment, a proton transfer occurs to form the neutral complex of [HFIP·HOO^•] structure. 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.

Structural Versatility and Energy Difference of Salt-Water Complex NaCl(H2O) Encoded in Cryogenic Photoelectron Spectroscopy

A weakly bound complex usually has multiple structural isomers with small energy differences. The sophisticated ab initio calculations are the main workhorse for providing theoretical results of different isomers. In contrast, the experimental determination of the energy difference is very rare. We report the energy-difference measurement of a model complex: salt-water complex NaCl(H₂O). We measured the energy difference among the structural isomers of the negatively charged NaCl(H₂O) complex and the neutral counterpart using cryogenic photoelectron spectroscopy. The temperature-dependent photoelectron spectra (15-300 K) revealed that the negatively charged NaCl(H₂O) and the neutral counterpart both have three isomers. The two higher-lying isomers are 186(22) and 481(48) cm^-1, respectively, above the most stable isomer for the negatively charged and 123(10) and 1821(24) cm^-1 for the neutral. These results provide a benchmark for the development of theoretic methods of weakly bound complexes. The experimental technique demonstrated here can be employed to investigate other weakly bound complexes with multiple isomers.

Comment on "Microhydration of Biomolecules: Revealing the Native Structures by Cold Ion IR Spectroscopy"

This Viewpoint presents a re-examination of the conclusions of a study reported in The Journal of Physical Chemistry Letters (Saparbaev, et al. 2021, 12, 907) that compared the structure of microsolvated ions formed by electrospray ionization to those formed in the gas-phase via a previously published cryogenic ion trap approach. We conducted additional experiments that clearly show that most of the observed differences in the IR spectra can be accounted for by considering the different spectroscopic action schemes used to obtain them. In particular, the presence of the D₂-tag induces shifts in some of the N-H and O-H peaks which need to be carefully considered before comparing spectra. Once these spectral effects are taken into account, we show that both clustering approaches yield similar cluster structures for the small GlyH⁺(H₂O)_n species. Using unimolecular reaction rate theory, we also show that for the small complexes considered here, only the gas-phase equilibrium distribution of conformers should be expected in both experimental approaches. In addition, the barrier heights necessary to kinetically trap high-energy conformers at 298 K is explored using a series of model polyalanine chains.

Observation and Exploitation of Spin-Orbit Excited Dipole-Bound States in Ion-Molecule Clusters

We report an observation of spin-orbit excited dipole-bound states (DBSs) in arginine-iodide complexes (Arg·I^-) by using temperature-dependent, wavelength-resolved "iodide-tagging" negative ion photoelectron spectroscopy. The observed DBSs are bound to the spin-orbit excited I(²P_1/2) level of the neutral Arg·I complex in zwitterionic conformations and identified based on the resonant enhancement due to spin-orbit electronic autodetachment from the I(²P_1/2) DBS to the I(²P_3/2) neutral ground state. The observed DBS binding energies are correlated to the dipole moments of neutral Arg·I isomers and tautomers. This work thus demonstrates a new and generic spectroscopic approach to identify ion-molecule cluster conformations based on their distinguishable dipole moments.

Observation of Conformational Simplification upon N-Methylation on Amino Acid Iodide Clusters

This Letter reports a counterintuitive observation that methylation of the glycine-iodide cluster leads to fewer conformations and spectroscopic simplicity. Cryogenic "iodide-tagging" negative ion photoelectron spectroscopy (NIPES) is used to probe specific binding sites of three N-methylated glycine derivatives, i.e., N-methylglycine (sarcosine), N,N-dimethylglycine, and N,N,N-trimethylglycine (glycine betaine). NIPES reveals a progressive spectral simplification of the iodide clusters with increasing methylation due to fewer contributing structures. Low energy conformers and tautomers of each cluster are computationally identified, and those observed in the experiments are assigned based on excellent agreement between the NIPE spectra and theoretical simulations. Zwitterionic cluster structures are found to be less stable than their canonical forms and do not contribute to the observed spectra. This work demonstrates the power of iodide-tagging NIPES in probing conformations of amino acid-iodide clusters and provides a molecular level understanding on the effect of methyl substitution on amino acid binding sites.

Hygroscopicity of Hofmeister Salts and Glycine Aerosols-Salt Specific Interactions

The Hofmeister effect of inorganic ions to precipitate proteins has been used to understand the coagulation phenomenon in colloid and protein science. Herein, for the first time, this effect is studied on the hygroscopicity of aerosols using ATR-FTIR spectroscopy. The representative Hofmeister salts (MgSO₄, KCl, NH₄NO₃) and amino acid (glycine) with different amino acid/salt molar ratios (ASRs) are mixed and atomized into micrometer-sized particles. For mixed kosmotrope (MgSO₄)/glycine and chaotrope (NH₄NO₃)/glycine with an ASR of 1:1, both ERHs (efflorescence relative humidities) and DRHs (deliquescence relative humidities) are absent. However, for the mixtures of glycine and neutral salt (KCl), no DRH is observed while 66.2 and 61.4% ERH of glycine is detected for mixtures with ASRs of 1:1 and 1:3, respectively, which is similar to pure glycine. For the mixture of NH₄NO₃/glycine with an ASR of 1:3, ERH and DRH are found to be 15.4 and 32.2% RH, less than that of pure NH₄NO₃. Further, interactions between glycine-salt and/or water is also studied in the mixtures during hydration and dehydration. Water-mediated ion-glycine interaction is detected based on the two glycine bands merging into one band. Glycine-SO₄^2- interaction is present for glycine/sulfate in all ASRs, while glycine-NO₃^- interaction is only seen for 1:3 glycine/NH₄NO₃ mixtures during hydration. This work opens a window to understand the Hofmeister effect on the hygroscopicity of atmospheric aerosols.

Electron Affinity and Electronic Structure of Hexafluoroacetone (HFA) Revealed by Photodetaching the [HFA]•- Radical Anion

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, we performed a joint study on the electron affinity and electronic structure of HFA employing negative ion photoelectron (NIPE) spectroscopy and quantum 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.

Microhydration of Biomolecules: Revealing the Native Structures by Cold Ion IR Spectroscopy

The native-like structures of protonated glycine and peptide Gly₃H⁺ were elucidated using cold ion IR spectroscopy of these biomolecules hydrated by a controlled number of water molecules. The complexes were generated directly from an aqueous solution using gentle electrospray ionization. Already with a single retained water molecule, GlyH⁺ exhibits the native-like structure characterized by a lack of intramolecular hydrogen bonds. We use our spectra to calibrate the available data for the same complexes, which are produced by cryogenic condensation of water onto the gas-phase glycine. In some conformers of these complexes, GlyH⁺ adopts the native-like structure, while in the others, it remains "kinetically" trapped in the intrinsic state. Upon condensation of 4-5 water molecules, the embedded amino acid fully adopts its native-like structure. Similarly, condensation of one water molecule onto the tripeptide is insufficient to fully eliminate its kinetically trapped intrinsic states.