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.

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.

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.

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 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.

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.

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.

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.

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.

Characterization of Intermediate Oxidation States in CO2 Activation.

Redox chemistry during the activation of carbon dioxide involves changing the charge state in a CO2 molecular unit. However, such changes are usually not well described by integer formal charges, and one can think of COO functional units as being in intermediate oxidation states. In this article, we discuss the properties of CO2 and CO2-based functional units in various charge states. Besides covering isolated CO2 and its ions, we describe the CO2-based ionic species formate, oxalate, and carbonate. Finally, we provide an overview of CO2-based functional groups and ligands in clusters and metal-organic complexes.

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.

Infrared Photodissociation Spectra of [Sn(CO2) n]- Cluster Ions.

We present infrared spectra and density functional theory calculations of mass selected [Sn(CO2) n]- cluster anions ( n = 2-6). The spectra and structures of these clusters exhibit less structural diversity than those of analogous clusters with first-row transition metals, but are more complex than those for the heavy coinage metals or for the related [Bi(CO2) n]- clusters. The most favorable core ion structure for all cluster sizes can be characterized as a Sn-oxalate complex, Sn[C2O4]-. Higher energy isomers based on a bidentate η2-(C,O) CO2 ligand tightly bound to the metal atom in SnCO2- complexes are also observed, even for the largest cluster sizes studied here. For n = 2, another high energy isomer is found, featuring a CO2 ligand weakly bound to the metal atom in a SnCO2- ion.

Titanium Insertion into CO Bonds in Anionic Ti-CO2 Complexes.

We explore the structures of [Ti(CO2) y]- cluster anions using infrared photodissociation spectroscopy and quantum chemistry calculations. The existence of spectral signatures of metal carbonyl CO stretching modes shows that insertion of titanium atoms into C-O bonds represents an important reaction during the formation of these clusters. In addition to carbonyl groups, the infrared spectra show that the titanium center is coordinated to oxalato, carbonato, and oxo ligands, which form along with the metal carbonyls. The presence of a metal oxalato ligand promotes C-O bond insertion in these systems. These results highlight the affinity of titanium for C-O bond insertion processes.

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.

Electronic Spectra of Tris(2,2'-bipyridine)-M(II) Complex Ions in Vacuo (M = Fe and Os).

We measured the electronic spectra of mass-selected [M(bpy)3]2+ (M = Fe and Os, bpy = 2,2'-bipyridine) ions in vacuo by photodissociation spectroscopy of their N2 adducts, [M(bpy)3]2+·N2. Extensive band systems in the visible (predominantly charge transfer) and near-ultraviolet (ππ*) spectral regions are reported. The [M(bpy)3]2+·N2 target ions were prepared by condensing N2 onto electrosprayed ions in a cryogenic ion trap at ca. 25 K and then mass-selected by time-of-flight mass spectrometry. The electronic photodissociation spectra of the cold, gas-phase ions closely reflect their intrinsic properties, i.e., without perturbation by solvent effects. The spectra are interpreted using time-dependent density functional theory calculations both with and without accounting for relativistic effects.

Interaction of CO2 with Atomic Manganese in the Presence of an Excess Negative Charge Probed by Infrared Spectroscopy of [Mn(CO2)n]- Clusters.

We report infrared photodissociation spectra of manganese-CO2 cluster anions, [Mn(CO2)n]- (n = 2-10) to probe structural motifs characterizing the interaction between Mn and CO2 in the presence of an excess electron. We interpret the experimental spectra through comparison with infrared spectra predicted from density functional theory calculations. The cluster anions consist of core ions combining a Mn atom with a variety of ligands, solvated by additional CO2 molecules. Structural motifs of ligands evolve with increasing cluster size from simple monodentate and bidentate CO2 ligands to oxalate ligands and combinations of these structural themes.

Structural Motifs of [Fe(CO2)n]- Clusters (n = 3-7).

We present IR spectra and quantum chemical calculations for anionic iron-CO2 clusters of the form [Fe(CO2)n]- (n = 3-7). All observed clusters have at least two CO2 units strongly bound to the metal atom. These strongly bound iron-CO2 complexes form the core ions of the clusters and are solvated by additional, weakly bound CO2 molecules. Larger clusters show clear infrared signatures of core ion isomers with three CO2 moieties as well. Dominant structural motifs are based on bidentate CO2 ligands with Fe-O/Fe-C bonds, oxalate ligands, and metal insertion into a CO bond.