Infrared Spectrum of the Pyrene Anion in the CH Stretching Region.

In this work, we report the infrared spectrum of the pyrene anion, measured using messenger tagging with up to three Ar atoms. We assign the spectrum using density functional theory and vibrational perturbation theory. We discuss our results in the context of computed and experimental spectra from the literature as well as recent observations from astronomical sources, addressing the question of whether polycyclic aromatic hydrocarbon anions could contribute to the strong infrared emission bands at 3.29 µm from carbon-rich regions of space.

Competition between Water-Water Hydrogen Bonds and Water-π Bonds in Pyrene-Water Cluster Anions.

We present infrared spectra and density functional theory calculations of hydrated pyrene anion clusters with up to four water molecules. The experimental spectra were acquired by using infrared Ar messenger photodissociation spectroscopy. Water molecules form clusters on the surface of the pyrene, forming hydrogen bonds with the π-system. The structures of the water clusters and their interaction with the π-system are encoded in OH stretching vibrational modes. We find that the interactions between water molecules are stronger than the interactions between water molecules and the π-system. While all clusters show multiple conformers, three- and four-membered rings are the lowest energy structures in the larger hydrates.

Probing Ion-Receptor Interactions in Halide Complexes of Octamethyl Calix[4]Pyrrole.

Ion receptors are molecular hosts that bind ionic guests, often with great selectivity. The interplay of solvation and ion binding in anion host-guest complexes in solution governs the binding efficiency and selectivity of such ion receptors. To gain molecular-level insight into the intrinsic binding properties of octamethyl calix[4]pyrrole (omC4P) host molecules with halide guest ions, we performed cryogenic ion vibrational spectroscopy (CIVS) of omC4P in complexes with fluoride, chloride, and bromide ions. We interpret the spectra using density functional theory, describing the infrared spectra of these complexes with both harmonic and anharmonic second-order vibrational perturbation theory (VPT2) calculations. The NH stretching modes of the pyrrole moieties serve as sensitive probes of the ion binding properties, as their frequencies encode the ion-receptor interactions. While scaled harmonic spectra reproduce the experimental NH stretching modes of the chloride and bromide complexes in broad strokes, the high proton affinity of fluoride introduces strong anharmonic effects. As a result, the spectrum of F-·omC4P is not even qualitatively captured by harmonic calculations, but it is recovered very well by VPT2 calculations. In addition, the VPT2 calculations recover the intricate coupling of the NH stretching modes with overtones and combination bands of CH stretching and NH bending modes and with low-frequency vibrations of the omC4P macrocycle, which are apparent for all of the halide ion complexes investigated here. A comparison of the CIVS spectra with infrared spectra of solutions of the same ion-receptor complexes in d3-acetonitrile and d6-acetone shows how ion solvation changes the ion-receptor interactions for the different halide ions.

Water-Hydrocarbon Interactions in Anionic Pyrene Monohydrate.

Interactions between water and polycyclic aromatic hydrocarbons are essential in many aspects of chemistry, from interstellar and atmospheric processes to interfacial hydrophobicity and wetting phenomena. Despite their growing importance, the intermolecular potentials of the water-hydrocarbon interactions are underdeveloped compared to the water-water potentials, and there are similarly few experimental probes that are sensitive to the details of the water-hydrocarbon potential. We present a combined experimental and computational study of anionic pyrene monohydrate, one of the simplest water/hydrocarbon clusters. The action spectrum in the OH region of the mass-selected cluster ion provides a rigorous benchmark for intermolecular potentials and computational methodologies. We identify missing intermolecular interactions and shortcomings in conventional dynamics calculations by comparing experimental data to density functional theory and classical molecular dynamics calculations. Kinetic trapping is prevalent, even for one water molecule and one pyrene molecule, leading to slow equilibration in conventional molecular dynamics calculations, even on nanosecond time scales and at low temperatures (50 K). At constant energy, temperature fluctuations for the pair of molecules are substantial. Immersing the system in a bath of soft spheres and employing parallel tempering alleviates kinetic trapping and dampens temperature fluctuations, bringing the system closer to the thermodynamic limit. With such augmented sampling, a simple, flexible water model reproduces the line width and the asymmetric broadening of the symmetric OH stretching mode, which we assign to spectral diffusion. In the OH stretching region, dynamics calculations predict a more intense antisymmetric peak than experiments observe but do not predict the bimodal split symmetric peak that the experiments show. Our work suggests that electronic polarization, missing in the empirical force field, is responsible for the first discrepancy and that quantum nuclear effects, captured neither in density functional theory nor in classical dynamics, may be responsible for the second.

Relation Between Bond Angle and Carbon-Oxygen Stretching Frequencies in CO2-Containing Compounds.

The symmetric (νs) and antisymmetric (νas) O-C-O stretching modes of CO2-containing compounds encode structural information that can be difficult to decipher, due to the sensitivity of these spectral features to small shifts in charge distribution and structure, as well as the anharmonicities of these two vibrational modes. In this work, we discuss the relation between the frequency of these modes and the geometry of the O-C-O group, showing that the splitting between νs and νas (Δνas-s = νas - νs) can be predicted based only on the O-C-O bond angle obtained from quantum chemical calculations with reasonable accuracy (±46 cm-1, R2 = 0.994). The relationship is shown to hold for the infrared spectra of a variety of CO2-containing molecules measured in vacuo. The origins of this model are discussed in the framework of elementary mode analysis.

Influence of Metal Identity in Bipyridine-Based Metal-4N Complexes With Formate.

We present the vibrational spectra of a series of dicationic, organometallic complexes consisting of a transition metal center (Co, Ni, or Cu) coordinated by 4,4'-di(tert-butyl)-2,2'-bipyridine (DTBbpy) ligands and a formate adduct. Spectral features are analyzed and assigned through comparison with density functional theory calculations, and structures are reported. Natural population analysis shows that the DTBbpy ligands serve as flexible charge reservoirs in each complex. Shifts in the vibrational signatures of the formate moiety reveal that the nature of the metal center plays a crucial role in the charge distribution and formate-metal binding motif in each complex, illustrating the impact of the metal center on the structural and electronic properties of these complexes.

Binding Pocket Response of EDTA Complexes with Alkaline Earth Dications to Stepwise Hydration─Structural Insight from Infrared Spectra.

We investigate the microhydration structures of complexes of alkaline earth dications and ethylenediaminetetraacetic acid (EDTA) for up to two water molecules, using cryogenic ion vibrational spectroscopy in concert with density functional theory (DFT). The interaction with water shows a clear dependence on the chemical identity of the bound ion. For Mg2+, microhydration mostly involves the carboxylate groups of EDTA and does not entail direct contact with the dication. In contrast, the larger ions (Ca2+, Sr2+, and Ba2+) interact electrostatically with the microhydration environment, and this interaction increases in importance with the size of the ion. This trend reflects the ion position in the EDTA binding pocket, which comes closer to the rim of the pocket with increasing ion size.

Influence of Transition Metal Electron Configuration on the Structure of Metal-EDTA Complexes.

The vibrational spectra of cold complexes of ethylenediaminetetraacetic acid (EDTA) with transition metal dications in vacuo show how the electronic structure of the metal provides a geometric template for interaction with the functional groups of the binding pocket. The OCO stretching modes of the carboxylate groups of EDTA serve as structural probes, informing on the spin state of the ion as well as the coordination number in the complex. The results highlight the flexibility of EDTA in accepting a large range of metal cations in its binding site.

Ion Binding Site Structure and the Role of Water in Alkaline Earth EDTA Complexes.

The interactions between molecular hosts and ionic guests and their dependence on the chemical environment are challenging to disentangle from solution data alone. The vibrational spectra of cold complexes of ethylenediaminetetraacetic acid (EDTA) chelating alkaline earth dications in vacuo encode structural characteristics of these complexes and their dependence on the size of the bound ion. The correlation between metal binding geometry and the relative intensities of vibrational bands of the carboxylate groups forming the binding pocket allows us to characterize water-induced changes in molecular geometry. The evolution of these structural markers from bare ions to water adducts to aqueous solution illustrates the role of water for the structure of ion binding sites in chelators. The binding pocket of EDTA opens up in aqueous solution, bringing the bound ion closer to the mouth of the binding site and leading to an increased exposure of the ion to the chemical environment.

Intrinsic Structure and Electronic Spectrum of Deprotonated Biliverdin: Cryogenic Ion Spectroscopy and Ion Mobility.

We investigated the structural and spectroscopic properties of singly deprotonated biliverdin anions in vacuo, using a combination of cryogenic ion spectroscopy, ion mobility spectrometry, and density functional theory. The ion mobility results show that at least two conformers are populated, with the dominant conformer at 75-90% relative abundance. The vibrational NH stretching signatures are sensitive to the tetrapyrrole structure, and they indicate that the tetrapyrrole system is in a helical conformation, consistent with simulated ion mobility collision cross sections. The vibrational spectrum in the fingerprint region of this singly deprotonated species shows that the two propionate groups share the remaining acidic proton. The S1 band of the electronic spectrum in vacuo is broad, despite ion trap temperatures of 20 K during ion preparation, with a congested Franck-Condon envelope showing partially resolved vibrational features. The vertical transition exhibits a small solvatochromic red shift (-320 cm-1) in aqueous solution.

Effects of Formate Binding to a Bipyridine-Based Cobalt-4N Complex.

We report the vibrational spectrum of a metal-organic complex consisting of a Co center surrounded by two bipyridine-based ligands and explore the change of the spectrum upon addition of a formate ligand to the complex. We assign the spectra using density functional theory. The infrared response encodes the binding motif of the formate to the metal, and the calculated charge distributions highlight the ability of the organic ligand framework to act as charge reservoirs modulating the redox properties of the metal center.

Tag-Free, Temperature Dependent Infrared Spectra of the GFP Chromophore: Revisiting the Question of Isomerism.

We report infrared spectra of a model chromophore of green fluorescent protein, prepared in an ion trap at temperatures ranging from 30 K to room temperature. We compare the changes in the infrared spectrum with predicted infrared spectra for the Z and E isomers of this molecule, and we confirm that the molecule exists as the Z isomer at low temperatures. We revisit the question whether or not it can thermally isomerize in the temperature range of this experiment, and we find no evidence for isomerization.

Intrinsic electronic spectra of cryogenically prepared protoporphyrin IX ions in vacuo - deprotonation-induced Stark shifts.

We present electronic spectra containing the Qx and Qy absorption bands of singly and doubly deprotonated protoporphyrin IX, prepared as mass selected ions in vacuo at cryogenic temperatures, revealing vibronic structure in both bands. We assign the vibronic progression of the Qx band using a Frank-Condon-Herzberg-Teller simulation based on time-dependent density functional theory, comparing the observed bands with those calculated for porphine. A comparison of the electronic spectra of the two charge states allows investigation of the electronic Stark effect with an electric field strength beyond the capabilities of typical laboratory setups. We analyze the differences in the electronic spectra of the two charge states using n-electron valence perturbation theory (NEVPT2) and simulated charge distributions.

Spectroscopy of Resonant Intermediate States for Triplet-Triplet Annihilation Upconversion in Crystalline Rubrene: Radical Ions as Sensitizers.

Photoluminescence upconversion in crystalline rubrene can proceed without an added sensitizer, but the mechanism for this process has not been well-understood. In particular, the species responsible for photon absorption has not been identified to date. To gain insight into the identity of the intermediate state, we measured the near-infrared (NIR) upconversion photoluminescence (UCPL) excitation spectrum of rubrene crystals and found three distinct spectral features. The UCPL yield has a quartic dependence on the laser intensity, implying a four-photon process. On the basis of electronic spectra of radical cations and anions of rubrene, we propose a mechanism in which photoexcited radical anions and cations undergo recombination, forming an excited neutral triplet while conserving spin. The triplets formed this way ultimately undergo triplet-triplet annihilation, resulting in the observed photoluminescence. This mechanism explains the origin of the NIR absorption as well as the four-photon nature of the UCPL process.

Probing the Microsolvation Environment of the Green Fluorescent Protein Chromophore In Vacuo.

We present vibrational and electronic photodissociation spectra of a model chromophore of the green fluorescent protein in complexes with up to two water molecules, prepared in a cryogenic ion trap at 160-180 K. We find the band origin of the singly hydrated chromophore at 20 985 cm-1 (476.5 nm) and observe partially resolved vibrational signatures. While a single water molecule induces only a small shift of the S1 electronic band of the chromophore, without significant change of the Franck-Condon envelope, the spectrum of the dihydrate shows significant broadening and a greater blue shift of the band edge. Comparison of the vibrational spectra with predicted infrared spectra from density functional theory indicates that water molecules can interact with the oxygen atom on the phenolate group or on the imidazole moiety, respectively.

Size-Dependent Pressure-Response of the Photoluminescence of CsPbBr3 Nanocrystals.

We report the size-dependent pressure response for CsPbBr3 perovskite nanocrystals in the size range 5.7-10.9 nm using photoluminescence spectroscopy in a diamond anvil cell. As the nanocrystal size decreases below ca. 7.5 nm, we observe a decrease in the transition pressure at which there is a change in the mode of deformation concomitant with an isostructural phase transition. We hypothesize that surface fluctuations regarding the tilt and distortion of surface PbBr6 octahedra facilitate the change in the mode of deformation and phase transition at lower pressures for smaller nanocrystals.

Cryogenic Ion Spectroscopy of the Green Fluorescent Protein Chromophore in Vacuo.

We present the spectrum of the S1 ← S0 transition of an anionic model for the chromophore of the green fluorescent protein in vacuo at cryogenic temperatures, showing previously unresolved vibrational features, and resolving the band origin at 20 930 cm-1 (477.8 nm) with unprecedented accuracy. The vibrational spectrum establishes that the molecule is in the Z isomer at low temperature. At increased temperature, the S1 ← S0 band shifts to the red, which we tentatively attribute to emergent population of the E isomer.

Vibrational Autodetachment from Anionic Nitroalkane Chains: From Molecular Signatures to Thermionic Emission.

We report the kinetic energy distributions in electron autodetachment from nitroethane, 1-nitropropane, and 1-nitrobutane anions upon laser excitation of CH stretching modes measured using velocity map electron imaging. In striking contrast to the case of nitromethane, the kinetic energy distributions show almost no distinct vibrational features, and they can be described by thermionic emission, relating the shape of the distributions to the electron capture cross section of the neutral molecule. The data suggest that a classical description is warranted above ca. 20 meV electron kinetic energy. At lower energies, quantum effects suppress the attachment cross section.

Intrinsic photophysics of nitrophenolate ions studied by cryogenic ion spectroscopy.

The intrinsic photophysics of nitrophenolate isomers (meta, para, and ortho) was studied at low temperature using photodissociation mass spectrometry in a cryogenic ion trap instrument. Each isomer has distinct photophysics that affects the excited state lifetimes, as observed experimentally in their spectroscopic linewidths. Visible-light-induced excitation of m-nitrophenolate gives rise to well-resolved vibronic features in the spectrum of the S1 state. The para and ortho isomers have broad spectra - even at cryogenic temperatures - due to their shorter excited state lifetimes and spectral congestion. We present computational evidence for mixing of the first and second excited states of o-nitrophenolate, leading to significant additional broadening in the experimental spectrum.

Interactions of Molecular Titanium Oxides TiO x ( x = 1-3) with Carbon Dioxide in Cluster Anions.

We study small titanium oxide-CO2 cluster anions in vacuo to understand the fundamental interactions between TiO x and CO2 in the presence of an excess electron. Infrared spectra of [TiO x(CO2) y]- ( x = 1-3, y > 1) were obtained using photodissociation spectroscopy and assigned through quantum chemistry calculations, identifying the formation of carbonato, oxalato, oxo, η2-(O,O), and carbonyl ligands in the core ions of these clusters, with carbonato ligands being the dominant ligand species.