Development of a low-temperature photoelectron spectroscopy instrument using an electrospray ion source and a cryogenically controlled ion trap

Deviation from the trans-Effect in Ligand-Exchange Reactions of Zeise's Ions PtCl3(C2H4)- with Heavier Halides (Br-, I-)

Four new Zeise's family ions with mixed-halide ligands, i.e., PtCl_nX_3-n(C₂H₄)^- (X = Br, I; n = 1, 2), were synthesized via ligand-exchange reactions of KX salts with KPtCl₃(C₂H₄) in aqueous solutions, and were detected in vacuum via electrospray ionization mass spectrometry. Their photoelectron spectra reveal a series of well-resolved spectral peaks with their electron binding energies (EBEs) decreasing with increasing halide size, with I having a much stronger effect than Br, i.e., 4.57 (-Cl₃) > 4.56 (-Cl₂Br) > 4.53 (-ClBr₂) > 4.34 (-Cl₂I) > 4.30 eV (-ClI₂). Ab initio electronic structure calculations including spin-orbit coupling (SOC) predict that the cis- and trans-isomers are nearly isoenergetic with the cis-isomer for -Cl₂X and the trans-isomer for -ClX₂ slightly favored, respectively. Excited-state spectra calculated with time-dependent density functional theory (TDDFT), and their comparison with the observed ones, suggest that for each species both the cis- and trans-configurations coexist in the experiments and contribute to the observed spectra, a fact that clearly deviates from the prediction of the widely accepted trans-effect, which suggests that only one isomer would have formed.

Infrared photodissociation spectroscopy of cold cationic trimethylamine complexes

Infrared spectroscopic studies reveal the general trends in the stepwise growth motif of trimethylamine (TMA)_n⁺ complexes.

A velocity map imaging mass spectrometer for photofragments of fast ion beams

We present the details of a fast ion velocity map imaging mass spectrometer that is capable of imaging the photofragments of trap-cooled (≥7 K) ions produced in a versatile ion source. The new instrument has been used to study the predissociation of N₂O⁺ produced by electric discharge and the direct dissociation of Al₂⁺ formed by laser ablation. The instrument's resolution is currently limited by the diameter of the collimating iris to a value of Δv/v = 7.6%. Photofragment images of N₂O⁺ show that when the predissociative state is changed from ²Σ⁺(200) to ²Σ⁺(300) the dominant product channel shifts from a spin-forbidden ground state, N (⁴S) + NO⁺(v = 5), to a spin-allowed pathway, N^*(²D) + NO⁺. The first photofragment images of Al₂⁺ confirm the existence of a directly dissociative parallel transition (²Σ⁺_u ← ²Σ⁺_g) that yields products with a large amount of kinetic energy. D₀ of ground state Al₂⁺ (²Σ⁺_g) measured from these images is 138 ± 5 kJ/mol, which is consistent with the published literature.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

Protonation sites can be controlled by the electrospray source as written in the figure.

Photoelectron spectroscopy of solvated dicarboxylate and alkali metal ion clusters, M+[O2C(CH2)2CO2]2− [H2O]n (M = Na, K; n = 1–6)

We present results of combined experimental photoelectron spectroscopy and theoretical modeling studies of solvated dicarboxylate species (⁻O₂C(CH₂)₂CO₂⁻) in complex with Na⁺ and K⁺ metal cations.

High-Resolution Photoelectron Imaging of Cryogenically-Cooled C59N- and (C59N)22- Azafullerene Anions

We report a photoelectron imaging study of cryogenically cooled C₅₉N^- and (C₅₉N)₂^2- anions produced from electrospray ionization. High-resolution photoelectron spectra are obtained for C₅₉N^- for the first time, allowing seven vibrational frequencies of the C₅₉N azafullerene to be measured. The electron affinity of C₅₉N is determined accurately to be 3.0150 ± 0.0007 eV. The observed vibrational features are understood on the basis of calculated frequencies and compared with those of C₆₀ and C₅₉HN. The photoelectron image of (C₅₉N)₂^2-, which has the same mass/charge ratio as C₅₉N^-, is also observed, allowing the second electron affinity of the (C₅₉N)₂ azafullerene dimer to be measured as 1.20 ± 0.05 eV. The intramolecular Coulomb repulsion of the (C₅₉N)₂^2- dianion is estimated to be 1.96 eV and is investigated theoretically using the electron density difference between (C₅₉N)₂^2- and (C₅₉N)₂.

Observation of Excited Quadrupole-Bound States in Cold Anions

We report the first observation of an excited quadrupole-bound state (QBS) in an anion. High-resolution photoelectron imaging of cryogenically cooled 4-cyanophenoxide (4CP^{-}) anions yields an electron detachment threshold of 24 927  cm^{-1}. The photodetachment spectrum reveals a resonant transition 20  cm^{-1} below the detachment threshold, which is attributed to an excited QBS of 4CP^{-} because neutral 4CP has a large quadrupole moment with a negligible dipole moment. The QBS is confirmed by observation of seventeen above-threshold resonances due to autodetachment from vibrational levels of the QBS.

Cluster Model Studies of Anion and Molecular Specificities via Electrospray Ionization Photoelectron Spectroscopy

Ion specificity, a widely observed macroscopic phenomenon in condensed phases and at interfaces, is a fundamental chemical physics issue. Herein we report our recent studies of such effects using cluster models in an "atom-by-atom" and "molecule-by-molecule" fashion not possible with the condensed-phase methods. We use electrospray ionization (ESI) to generate molecular and ionic clusters to simulate key molecular entities involved in local binding regions and characterize them by employing negative ion photoelectron spectroscopy (NIPES). Inter- and intramolecular interactions and binding configurations are directly obtained as functions of the cluster size and composition, providing molecular-level descriptions and characterization over the local active sites that play crucial roles in determining the solution chemistry and condensed-phase phenomena. The topics covered in this article are relevant to a wide range of research fields from ion specific effects in electrolyte solutions, ion selectivity/recognition in normal functioning of life, to molecular specificity in aerosol particle formation, as well as in rational material design and synthesis.

Vibronic spectra of protonated hydroxypyridines: contributions of prefulvenic and planar structures

The UV photofragmentation spectra of cold protonated hydroxypyridines display well resolved vibrational structures, interpreted with calculations at the CC2 level.

Structures and energetics of hydrated deprotonated cis-pinonic acid anion clusters and their atmospheric relevance

Pinonic acid, a C₁₀-monocarboxylic acid with a hydrophilic –CO₂H group and a hydrophobic hydrocarbon backbone, is a key intermediate oxidation product of α-pinene – an important monoterpene compound in biogenic emission processes that influences the atmosphere.

Probing microhydration effect on the electronic structure of the GFP chromophore anion: Photoelectron spectroscopy and theoretical investigations

The photophysics of the Green Fluorescent Protein (GFP) chromophore is critically dependent on its local structure and on its environment. Despite extensive experimental and computational studies, there remain many open questions regarding the key fundamental variables that govern this process. One outstanding problem is the role of autoionization as a possible relaxation pathway of the excited state under different environmental conditions. This issue is considered in our work through combined experimental and theoretical studies of microsolvated clusters of the deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI(-)), an analog of the GFP chromophore. Through selective generation of microsolvated structures of predetermined size and subsequent analysis of experimental photoelectron spectra by high level ab initio methods, we are able to precisely identify the structure of the system, establish the accuracy of theoretical data, and provide reliable description of auto-ionization process as a function of hydrogen-bonding environment. Our study clearly illustrates the first few water molecules progressively stabilize the excited state of the chromophore anion against the autodetached neutral state, which should be an important trait for crystallographic water molecules in GFPs that has not been fully explored to date.

Alkali Cation Chelation in Cold β-O-4 Tetralignol Complexes

We employ cold ion spectroscopy (UV action and IR-UV double resonance) in the gas phase to unravel the qualitative structural elements of G-type alkali metal cationized (X = Li(+), Na(+), K(+)) tetralignol complexes connected by β-O-4 linkages. The conformation-specific spectroscopy reveals a variety of conformers, each containing distinct infrared spectra in the OH stretching region, building on recent studies of the neutral and alkali metal cationized β-O-4 dimers. The alkali metal ion is discovered to bind in penta-coordinate pockets to ether and OH groups involving at least two of the three β-O-4 linkages. Different binding sites are distinguished from one another by the number of M(+)···OH···O interactions present in the binding pocket, leading to characteristic IR transitions appearing below 3550 cm(-1). This interaction is mitigated in the major conformer of the K(+) adduct, demonstrating a clear impact of the size of the charge center on the three-dimensional structure of the tetramer.

A Joint Experimental and Computational Study of the Negative Ion Photoelectron Spectroscopy of the 1-Phospha-2,3,4-triazolate Anion, HCPN3(.)

We report here the results of a combined experimental and computational study of the negative ion photoelectron spectroscopy (NIPES) of the recently synthesized, planar, aromatic, HCPN3(-) ion. The adiabatic electron detachment energy of HCPN3(-) (electron affinity of HCPN3(•)) was measured to be 3.555 ± 0.010 eV, a value that is intermediate between the electron detachment energies of the closely related (CH)2N3(-) and P2N3(-) ions. High level electronic structure calculations and Franck-Condon factor (FCF) simulations reveal that transitions from the ground state of the anion to two nearly degenerate, low-lying, electronic states, of the neutral HCPN3(•) radical are responsible for the congested peaks at low binding energies in the NIPE spectrum. The best fit of the simulated NIPE spectrum to the experimental spectrum indicates that the ground state of HCPN3(•) is a 5π-electron (2)A″ π radical state, with a 6π-electron, (2)A', σ radical state being at most 1.0 kcal/mol higher in energy.

Regioisomer-specific electron affinities and electronic structures of C70para-adducts at polar and equatorial positions with (bromo)benzyl radicals: photoelectron spectroscopy and theoretical study

Negative ion photoelectron spectroscopy shows interesting regioisomer-specific electron affinities (EAs) of 2,5- and 7,23-para-adducts of C70 [(ArCH2)2C70] (Ar = Ph, o-, m-, and p-BrC6H4). Their EA values are larger than that of C70 by 5-150 meV with the 2,5-polar adducts' EAs being higher than their corresponding 7,23-equatorial counterparts, exhibiting appreciable EA tunable ranges and regioisomeric specificity. Density functional theory (DFT) calculations reproduce both the experimental EA values and EA trends very well.

Negative ion photoelectron spectroscopy of P2N3-: electron affinity and electronic structures of P2N3˙

We report here a negative ion photoelectron spectroscopy (NIPES) and ab initio study of the recently synthesized planar aromatic inorganic ion P2N3-, to investigate the electronic structures of P2N3- and its neutral P2N3˙ radical. The adiabatic detachment energy of P2N3- (electron affinity of P2N3˙) was determined to be 3.765 ± 0.010 eV, indicating high stability for the P2N3- anion. Ab initio electronic structure calculations reveal the existence of five, low-lying, electronic states in the neutral P2N3˙ radical. Calculation of the Franck-Condon factors (FCFs) for each anion-to-neutral electronic transition and comparison of the resulting simulated NIPE spectrum with the vibrational structure in the observed spectrum allows the first four excited states of P2N3˙ to be determined to lie 6.2, 6.7, 11.5, and 22.8 kcal mol-1 above the ground state of the radical, which is found to be a 6π-electron, 2A1, σ state.

Making Mass Spectrometry See the Light: The Promises and Challenges of Cryogenic Infrared Ion Spectroscopy as a Bioanalytical Technique

The detailed chemical information contained in the vibrational spectrum of a cryogenically cooled analyte ion would, in principle, make infrared (IR) ion spectroscopy a gold standard technique for molecular identification in mass spectrometry. Despite this immense potential, there are considerable challenges in both instrumentation and methodology to overcome before the technique is analytically useful. Here, we discuss the promise of IR ion spectroscopy for small molecule analysis in the context of metabolite identification. Experimental strategies to address sensitivity constraints, poor overall duty cycle, and speed of the experiment are intimately tied to the development of a mass-selective cryogenic trap. Therefore, the most likely avenues for success, in the authors' opinion, are presented here, alongside alternative approaches and some thoughts on data interpretation.

Gas-Phase Femtosecond Particle Spectroscopy: A Bottom-Up Approach to Nucleotide Dynamics

We summarize how gas-phase ultrafast charged-particle spectroscopy has been used to provide an understanding of the photophysics of DNA building blocks. We focus on adenine and discuss how, following UV excitation, specific interactions determine the fates of its excited states. The dynamics can be probed using a systematic bottom-up approach that provides control over these interactions and that allows ever-larger complexes to be studied. Starting from a chromophore in adenine, the excited state decay mechanisms of adenine and chemically substituted or clustered adenine are considered and then extended to adenosine mono-, di-, and trinucleotides. We show that the gas-phase approach can offer exquisite insight into the dynamics observed in aqueous solution, but we also highlight stark differences. An outlook is provided that discusses some of the most promising developments in this bottom-up approach.

Incremental Tuning Up of Fluorous Phenazine Acceptors

In a simple, one-step direct trifluoromethylation of phenazine with CF3 I we prepared and characterized nine (poly)trifluoromethyl derivatives with up to six CF3 groups. The electrochemical reduction potentials and gas-phase electron affinities show a direct, strict linear relation to the number of CF3 groups, with phenazine(CF3)6 reaching a record-high electron affinity of 3.24 eV among perfluoroalkylated polyaromatics.

Examining the structural evolution of bicarbonate–water clusters: insights from photoelectron spectroscopy, basin-hopping structural search, and comparison with available IR spectral studies

Combining NIPES, theoretical calculations and available IR spectra allows us to identify the minimum energy structures that best fit the experiments.

Probing the early stages of solvation of cis-pinate dianions by water, acetonitrile, and methanol: a photoelectron spectroscopy and theoretical study

Photoelectron spectroscopy and theoretical studies indicate the coexistence of symmetric and asymmetric solvated clusters for cis-pinate dianions.

Negative ion photoelectron spectroscopy confirms the prediction that D3h carbon trioxide (CO3) has a singlet ground state

The CO₃ radical anion (CO₃˙^–) has been formed by electrospraying carbonate dianion (CO₃^2–) into the gas phase.

The CO₃ radical anion (CO₃˙^–) has been formed by electrospraying carbonate dianion (CO₃^2–) into the gas phase. The negative ion photoelectron (NIPE) spectrum of CO₃˙^– shows that, unlike the isoelectronic trimethylenemethane [C(CH₂)₃], D_3h carbon trioxide (CO₃) has a singlet ground state. From the NIPE spectrum, the electron affinity of D_3h singlet CO₃ was, for the first time, directly determined to be EA = 4.06 ± 0.03 eV, and the energy difference between the D_3h singlet and the lowest triplet was measured as ΔE_ST = – 17.8 ± 0.9 kcal mol^–1. B3LYP, CCSD(T), and CASPT2 calculations all find that the two lowest triplet states of CO₃ are very close in energy, a prediction that is confirmed by the relative intensities of the bands in the NIPE spectrum of CO₃˙^–. The 560 cm^–1 vibrational progression, seen in the low energy region of the triplet band, enables the identification of the lowest, Jahn–Teller-distorted, triplet state as ³A₁, in which both unpaired electrons reside in σ MOs, rather than ³A₂, in which one unpaired electron occupies the b₂ σ MO, and the other occupies the b₁ π MO.

How Anion Chaotrope Changes the Local Structure of Water: Insights from Photoelectron Spectroscopy and Theoretical Modeling of SCN(-) Water Clusters

The behavior of charged solute molecules in aqueous solutions is often classified using the concept of kosmotropes ("structure makers") and chaotropes ("structure breakers"). There is a growing consensus that the key to kosmotropic/chaotropic behaviors lies in the local solvent region, but the exact microscopic basis for such differentiation is not well-understood. This issue is examined in this work by analyzing size selective solvation of a well-known chaotrope, a negatively charged SCN(-) molecule. Combining experimental photoelectron spectroscopy measurements with theoretical modeling, we examine evolution of solvation structure up to eight waters. We observe that SCN(-) indeed fits the description of weakly hydrated ion, and its solvation is heavily driven by stabilization of water-water interaction network. However, the impact on water structure is more subtle than that associated with "structure breaker". In particular, we observe that the solvation structure of SCN(-) preserves the "packing" structure of the water network but changes local directionality of hydrogen bonds in the local solvent region. The resulting effect is closer to that of "structure weakener", where solute can be readily accommodated into the native water network, at the cost of compromising its stability due to constraints on hydrogen bonding directionality.