Bottom-Up View of Water Network-Mediated CO2 Reduction Using Cryogenic Cluster Ion Spectroscopy and Direct Dynamics Simulations

Mechanism of Aqueous Carbon Dioxide Reduction by the Solvated Electron

Aqueous solvated electron (e_aq^-), a key species in radiation and plasma chemistry, can efficiently reduce CO₂ in a potential green chemistry application. Here, the mechanism of this reaction is unravelled by condensed-phase molecular dynamics based on the correlated wave function and an accurate density functional theory (DFT) approximation. Here, we design and apply the holistic protocol for solvated electron's reactions encompassing all relevant reaction stages starting from diffusion. The carbon dioxide reduction proceeds via a cavity intermediate, which is separated from the product (CO₂^-) by an energy barrier due to the bending of CO₂ and the corresponding solvent reorganization energy. The formation of the intermediate is caused by solvated electron's diffusion, whereas the intermediate transformation to CO₂^- is triggered by hydrogen bond breaking in the second solvation shell of the solvated electron. This picture of an activation-controlled e_aq^- reaction is very different from both rapid barrierless electron transfer and proton-coupled electron transfer, where key transformations are caused by proton migration.

Probing the selectivity of Li+ and Na+ cations on noradrenaline at the molecular level

Although several mechanisms concerning the biological function of lithium salts, drugs having tranquilizing abilities, have been proposed so far, the key mechanism for its selectivity and subsequent interaction with neurotransmitters has not been established yet. We report ultraviolet (UV) and infrared (IR) spectra under ultra-cold conditions of Li+ and Na+ complexes of noradrenaline (NAd, norepinephrine), a neurotransmitter responsible for the body's response to stress or danger, in an effort to provide a molecular level understanding of the conformational changes of NAd due to its interactions with these two cations. A detailed analysis of the IR spectra, aided by quantum chemical calculations, reveals that the Li+-noradrenaline (NAd-Li+) complex forms only an extended structure, while the NAd-Na+ and protonated (NAd-H+) complexes form both folded and extended structures. This conformational preference of the NAd-Li+ complex is further explained by considering specific conformational distributions in solution. Our results clearly discern the unique structural motifs that NAd adopts when interacting with Li+ compared with other abundant cations in the human body (Na+) and can form the basis of a molecular level understanding of the selectivity of Li+ in biological systems.

Energetics of CO2- in Aqueous Solution

Gas-phase and aqueous solution properties of neutral and anionic clusters of CO₂ with 3, 4, and 8 explicit H₂O molecules are calculated at the coupled cluster (CCSD(T)) level plus a self-consistent reaction field. Anionic clusters with the radical electron density localized on the carbon of the CO₂ molecule rather than localized on the H₂O molecules are more favorable energetically by 10-20 kcal/mol in the gas phase (Δ H_gas(298 K)) and 20-30 kcal/mol in aqueous solution (Δ G_aq(298 K)). The most favorable structures are those with the largest number of strong hydrogen bonds between the CO₂^- and the explicit H₂O molecules. Adiabatic electron affinities were calculated in the gas phase and in aqueous solution for the microsolvated anion. The adiabatic electron affinity of aqueous CO₂^- is predicted to be 2.35 ± 0.08 eV and is converged with as few as 3 explicit H₂O molecules plus a self-consistent reaction field. The EA of aqueous CO₂ is significantly greater than the aqueous solvation free energy of the electron. The vertical attachment energies to CO₂ and the vertical detachment energies from CO₂^- were calculated. The solvated CO₂^- anion is substantially bent to 135°, which requires 1.52 eV. The large energy required for bending in combination with the vertical detachment and attachment energies shows that substantial local solvent reorganization occurs on detachment or attachment of an electron to solvated CO₂. The formation of aqueous C₂O₄^2- from CO₂^- was also explored, and dimerization is predicted to occur.

Carbon-carbon bond formation in the reaction of hydrated carbon dioxide radical anions with 3-butyn-1-ol

Electrochemical activation of carbon dioxide in aqueous solution is a promising way to use carbon dioxide as a C1 building block. Mechanistic studies in the gas phase play an important role to understand the inherent chemical reactivity of the carbon dioxide radical anion. Here, the reactivity of CO2 •-(H2O)n with 3-butyn-1-ol is investigated by Fourier transform ion cyclotron (FT-ICR) mass spectrometry and quantum chemical calculations. Carbon-carbon bond formation takes places, but is associated with a barrier. Therefore, bond formation may require uptake of several butynol molecules. The water molecules slowly evaporate from the cluster due to the absorption of room temperature black-body radiation. When all water molecules are lost, butynol evaporation sets in. In this late stage of the reaction, side reactions occur including H• atom transfer and elimination of HOCO•.

Vibrational Characterization of Radical Ion Adducts between Imidazole and CO2

We address the molecular level origins of the dramatic difference in the catalytic mechanisms of CO₂ activation by the seemingly similar molecules pyridine (Py) and imidazole (Im). This is accomplished by comparing the fundamental interactions of CO₂ radical anions with Py and Im in the isolated, gas phase PyCO₂^- and ImCO₂^- complexes. These species are prepared by condensation of the neutral compounds onto a (CO₂) _n^- cluster ion beam by entrainment in a supersonic jet ion source. The structures of the anionic complexes are determined by theoretical analysis of their vibrational spectra, obtained by IR photodissociation of weakly bound CO₂ molecules in a photofragmentation mass spectrometer. Although the radical PyCO₂^- system adopts a carbamate-like configuration corresponding to formation of an N-C covalent bond, the ImCO₂^- species is revealed to be best described as an ion-molecule complex in which an oxygen atom in the CO₂^- radical anion is H-bonded to the NH group. Species that feature a covalent N-C interaction in ImCO₂^- are calculated to be locally stable structures, but are much higher in energy than the largely electrostatically bound ion-molecule complex. These results support the suggestion from solution phase electrochemical studies (Bocarsly et al. ACS Catal. 2012, 2, 1684-1692) that the N atoms are not directly involved in the catalytic activation of CO₂ by Im.

The nature of the CO2 (-) radical anion in water

The reductive conversion of CO2 into industrial products (e.g., oxalic acid, formic acid, methanol) can occur via aqueous CO2 (-) as a transient intermediate. While the formation, structure, and reaction pathways of this radical anion have been modelled for decades using various spectroscopic and theoretical approaches, we present here, for the first time, a vibrational spectroscopic investigation in liquid water, using pulse radiolysis time-resolved resonance Raman spectroscopy for its preparation and observation. Excitation of the radical in resonance with its 235 nm absorption displays a transient Raman band at 1298 cm(-1), attributed to the symmetric CO stretch, which is at ∼45 cm(-1) higher frequency than in inert matrices. Isotopic substitution at C ((13)CO2 (-)) shifts the frequency downwards by 22 cm(-1), which confirms its origin and the assignment. A Raman band of moderate intensity compared to the stronger 1298 cm(-1) band also appears at 742 cm(-1) and is assignable to the OCO bending mode. A reasonable resonance enhancement of this mode is possible only in a bent CO2 (-)(C2v/Cs) geometry. These resonance Raman features suggest a strong solute-solvent interaction, the water molecules acting as constituents of the radical structure, rather than exerting a minor solvent perturbation. However, there is no evidence of the non-equivalence (Cs) of the two CO bonds. A surprising resonance Raman feature is the lack of overtones of the symmetric CO stretch, which we interpret due to the detachment of the electron from the CO2 (-) moiety towards the solvation shell. Electron detachment occurs at the energies of 0.28 ± 0.03 eV or higher with respect to the zero point energy of the ground electronic state. The issue of acid-base equilibrium of the radical, which has been in contention for decades, as reflected in a wide variation in the reported pKa (-0.2 to 3.9), has been resolved. A value of 3.4 ± 0.2 measured in this work is consistent with the vibrational properties, bond structure, and charge distribution in aqueous CO2 (-).

Water network-mediated, electron-induced proton transfer in [C5H5N ⋅ (H2O)n](-) clusters

The role of proton-assisted charge accommodation in electron capture by a heterocyclic electron scavenger is investigated through theoretical analysis of the vibrational spectra of cold, gas phase [Py ⋅ (H2O)n=3-5](-) clusters. These radical anions are formed when an excess electron is attached to water clusters containing a single pyridine (Py) molecule in a supersonic jet ion source. Under these conditions, the cluster ion distribution starts promptly at n = 3, and the photoelectron spectra, combined with vibrational predissociation spectra of the Ar-tagged anions, establish that for n > 3, these species are best described as hydrated hydroxide ions with the neutral pyridinium radical, PyH((0)), occupying one of the primary solvation sites of the OH(-). The n = 3 cluster appears to be a special case where charge localization on Py and hydroxide is nearly isoenergetic, and the nature of this species is explored with ab initio molecular dynamics calculations of the trajectories that start from metastable arrangements of the anion based on a diffuse, essentially dipole-bound electron. These calculations indicate that the reaction proceeds via a relatively slow rearrangement of the water network to create a favorable hydration configuration around the water molecule that eventually donates a proton to the Py nitrogen atom to yield the product hydroxide ion. The correlation between the degree of excess charge localization and the evolving shape of the water network revealed by this approach thus provides a microscopic picture of the "solvent coordinate" at the heart of a prototypical proton-coupled electron transfer reaction.

Ultrafast vibrational spectroscopy (2D-IR) of CO2 in ionic liquids: Carbon capture from carbon dioxide's point of view

The CO2ν3 asymmetric stretching mode is established as a vibrational chromophore for ultrafast two-dimensional infrared (2D-IR) spectroscopic studies of local structure and dynamics in ionic liquids, which are of interest for carbon capture applications. CO2 is dissolved in a series of 1-butyl-3-methylimidazolium-based ionic liquids ([C4C1im][X], where [X](-) is the anion from the series hexafluorophosphate (PF6 (-)), tetrafluoroborate (BF4 (-)), bis-(trifluoromethyl)sulfonylimide (Tf2N(-)), triflate (TfO(-)), trifluoroacetate (TFA(-)), dicyanamide (DCA(-)), and thiocyanate (SCN(-))). In the ionic liquids studied, the ν3 center frequency is sensitive to the local solvation environment and reports on the timescales for local structural relaxation. Density functional theory calculations predict charge transfer from the anion to the CO2 and from CO2 to the cation. The charge transfer drives geometrical distortion of CO2, which in turn changes the ν3 frequency. The observed structural relaxation timescales vary by up to an order of magnitude between ionic liquids. Shoulders in the 2D-IR spectra arise from anharmonic coupling of the ν2 and ν3 normal modes of CO2. Thermal fluctuations in the ν2 population stochastically modulate the ν3 frequency and generate dynamic cross-peaks. These timescales are attributed to the breakup of ion cages that create a well-defined local environment for CO2. The results suggest that the picosecond dynamics of CO2 are gated by local diffusion of anions and cations.

Reductions of oxygen, carbon dioxide, and acetonitrile by the magnesium(II)/magnesium(I) couple in aqueous media: theoretical insights from a nano-sized water droplet

Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + e(-) → Mg(+•)) confined in a nanosized water droplet ([Mg(H2O)16](•+)) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O2 requires an overall doublet spin state of the system. The reductions of CO2 and CH3CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O2(•-), CO2(•-), and CH3CN(•-), are initially formed on the cluster surface. O2(•-) and CO2(•-) can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[MgOCO](•+) core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO2)(H2O)16](•+) are -36 ± 4 kJ mol(-1) and -30 ± 9 kJ mol(-1), respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO2 and [Mg(H2O)16](•+). The values for the formation of ssip and cip of [Mg(O2)(H2O)16](•+) are estimated to be -112 ± 18 kJ mol(-1) and -128 ± 28 kJ mol(-1), respectively. CH3CN(•-) undergoes protonation spontaneously to form the hydrophobic [CH3CN, H](•). Both CH3CN and [CH3CN, H](•) cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol(-1) and ∼40 kJ mol(-1), respectively. These results provide fundamental insights into the solvation dynamics of the Mg(2+)/Mg(•+) couple on the molecular level.

Solvent-mediated reduction of carbon dioxide in anionic complexes with silver atoms

The development of efficient routes toward sustainable fuel sources by electrochemical reduction of CO2 is an important goal for catalysis research. While these processes usually occur in the presence of solvent, solvation effects in catalysis are largely not understood or even characterized. In this work, mass-selected clusters of silver anions with CO2 serve as a model system for reductive activation of CO2 by a catalyst in the presence of a well-controlled number of solvent molecules. Vibrational spectroscopy and electronic structure calculations are used to obtain molecular-level information on the interaction of solvent with the catalyst-CO2 complex and the effects of solvation on one-electron reductive activation of CO2. Charge transfer from the silver catalyst to CO2 increases with increasing cluster size. We observe the coexistence of catalyst-ligand complexes with CO2 monomer and dimer anions, indicating that CO2-based charge carriers can exist in the presence of a silver atom.

Vibrationally induced charge transfer in a bimolecular model complex in vacuo

We report vibrationally induced charge transfer from nitromethane anion to methyliodide in a molecular complex. Excitation of a CH stretching vibrational transition in either of the molecular constituents results in dissociative electron transfer to the CH3I molecule, resulting in I(-) product anions. Solvation of the pre-reactive complex with more than two Ar atoms leads to complete quenching of the reaction and can be used to estimate the barrier for this reaction. We discuss the results in the framework of electronic structure calculations and compare the intra-complex electron transfer with vibrationally mediated electron emission in bare nitromethane anion.