Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2 (H2 O)n - , n=1-10, in the C-O Stretch Region.

We investigate anionic [Co,CO2 ,nH2 O]- clusters as model systems for the electrochemical activation of CO2 by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250-2234 cm-1 using an FT-ICR mass spectrometer. We show that both CO2 and H2 O are activated in a significant fraction of the [Co,CO2 ,H2 O]- clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C-O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C-O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)2 - . However, calculations find Co(HCOO)(OH)- as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590-1730 cm-1 are most likely due to higher lying isomers of the composition Co(HOCO)(OH)- . Upon additional hydration, all species [Co,CO2 ,nH2 O]- , n≥2, undergo IRMPD through loss of H2 O molecules as a relatively weakly bound messenger. The main spectral features are the C-O stretching mode of the CO ligand around 1900 cm-1 , the water bending mode mixed with the antisymmetric C-O stretching mode of the HCOO- ligand around 1580-1730 cm-1 , and the symmetric C-O stretching mode of the HCOO- ligand around 1300 cm-1 . A weak feature above 2000 cm-1 is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.

Photochemical activation of carbon dioxide in Mg+(CO2)(H2O)0,1.

We combine multi-reference ab initio calculations with UV-VIS action spectroscopy to study photochemical activation of CO2 on a singly charged magnesium ion, [MgCO2(H2O)0,1]+, as a model system for the metal/ligand interactions relevant in CO2 photochemistry. For the non-hydrated species, two separated Mg+ 3s-3p bands are observed within 5.0 eV. The low-energy band splits upon hydration with one water molecule. [Mg(CO2)]+ decomposes highly state-selectively, predominantly via multiphoton processes. Within the low-energy band, CO2 is exclusively lost within the excited state manifold. For the high-energy band, an additional pathway becomes accessible: the CO2 ligand is activated via a charge transfer, with photochemistry taking place on the CO2 - moiety eventually leading to a loss of CO after absorption of a second photon. Upon hydration, already excitation into the first and second excited state leads to CO2 activation in the excited state minimum; however, CO2 predominantly evaporates upon fluorescence or absorption of another photon.

Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO2 in [MgCO2 (H2 O)n ].+ , n=0-8.

We investigate activation of carbon dioxide by singly charged hydrated magnesium cations Mg .+ (H2 O)n , through infrared multiple photon dissociation (IRMPD) spectroscopy combined with quantum chemical calculations. The spectra of [MgCO2 (H2 O)n ].+ in the 1250-4000 cm-1 region show a sharp transition from n=2 to n=3 for the position of the CO2 antisymmetric stretching mode. This is evidence for the activation of CO2 via charge transfer from Mg .+ to CO2 for n≥3, while smaller clusters feature linear CO2 coordinated end-on to the metal center. Starting with n=5, we see a further conformational change, with CO2 .- coordination to Mg2+ gradually shifting from bidentate to monodentate, consistent with preferential hexa-coordination of Mg2+ . Our results reveal in detail how hydration promotes CO2 activation by charge transfer at metal centers.

Probing the Structural Evolution of the Hydrated Electron in Water Cluster Anions (H2O)n-, n ≤ 200, by Electronic Absorption Spectroscopy.

Electronic absorption spectra of water cluster anions (H2O)n-, n ≤ 200, at T = 80 K are obtained by photodissociation spectroscopy and compared with simulations from literature and experimental data for bulk hydrated electrons. Two almost isoenergetic electron binding motifs are seen for cluster sizes 20 ≤ n ≤ 40, which are assigned to surface and partially embedded isomers. With increasing cluster size, the surface isomer becomes less populated, and for n ≥ 50, the partially embedded isomer prevails. The absorption shifts to the blue, reaching a plateau at n ≈ 100. In this size range, the absorption spectrum is similar to that of the bulk hydrated electron but is slightly red-shifted; spectral moment analysis indicates that these clusters are reasonable model systems for hydrated electrons near the liquid-vacuum interface.

Electronic spectroscopy and nanocalorimetry of hydrated magnesium ions [Mg(H2O)n]+, n = 20-70: spontaneous formation of a hydrated electron?

Hydrated singly charged magnesium ions [Mg(H2O)n]+ are thought to consist of an Mg2+ ion and a hydrated electron for n > 15. This idea is based on mass spectra, which exhibit a transition from [MgOH(H2O)n-1]+ to [Mg(H2O)n]+ around n = 15-22, black-body infrared radiative dissociation, and quantum chemical calculations. Here, we present photodissociation spectra of size-selected [Mg(H2O)n]+ in the range of n = 20-70 measured for photon energies of 1.0-5.0 eV. The spectra exhibit a broad absorption from 1.4 to 3.2 eV, with two local maxima around 1.7-1.8 eV and 2.1-2.5 eV, depending on cluster size. The spectra shift slowly from n = 20 to n = 50, but no significant change is observed for n = 50-70. Quantum chemical modeling of the spectra yields several candidates for the observed absorptions, including five- and six-fold coordinated Mg2+ with a hydrated electron in its immediate vicinity, as well as a solvent-separated Mg2+/e- pair. The photochemical behavior resembles that of the hydrated electron, with barrierless interconversion into the ground state following the excitation.

CO2/O2 Exchange in Magnesium-Water Clusters Mg+(H2O) n.

Hydrated singly charged metal ions doped with carbon dioxide, Mg2+(CO2)-(H2O) n, in the gas phase are valuable model systems for the electrochemical activation of CO2. Here, we study these systems by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry combined with ab initio calculations. We show that the exchange reaction of CO2 with O2 proceeds fast with bare Mg+(CO2), with a rate coefficient kabs = 1.2 × 10-10 cm3 s-1, while hydrated species exhibit a lower rate in the range of kabs = (1.2-2.4) × 10-11 cm3 s-1 for this strongly exothermic reaction. Water makes the exchange reaction more exothermic but, at the same time, considerably slower. The results are rationalized with a need for proper orientation of the reactants in the hydrated system, with formation of a Mg2+(CO4)-(H2O) n intermediate while the activation energy is negligible. According to our nanocalorimetric analysis, the exchange reaction of the hydrated ion is exothermic by -1.7 ± 0.5 eV, in agreement with quantum chemical calculations.

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•.

Photochemistry and spectroscopy of small hydrated magnesium clusters Mg+(H2O)n, n = 1-5.

Hydrated singly charged magnesium ions Mg+(H2O)n, n ≤ 5, in the gas phase are ideal model systems to study photochemical hydrogen evolution since atomic hydrogen is formed over a wide range of wavelengths, with a strong cluster size dependence. Mass selected clusters are stored in the cell of an Fourier transform ion cyclotron resonance mass spectrometer at a temperature of 130 K for several seconds, which allows thermal equilibration via blackbody radiation. Tunable laser light is used for photodissociation. Strong transitions to D1-3 states (correlating with the 3s-3px,y,z transitions of Mg+) are observed for all cluster sizes, as well as a second absorption band at 4-5 eV for n = 3-5. Due to the lifted degeneracy of the 3px,y,z energy levels of Mg+, the absorptions are broad and red shifted with increasing coordination number of the Mg+ center, from 4.5 eV for n = 1 to 1.8 eV for n = 5. In all cases, H atom formation is the dominant photochemical reaction channel. Quantum chemical calculations using the full range of methods for excited state calculations reproduce the experimental spectra and explain all observed features. In particular, they show that H atom formation occurs in excited states, where the potential energy surface becomes repulsive along the O⋯H coordinate at relatively small distances. The loss of H2O, although thermochemically favorable, is a minor channel because, at least for the clusters n = 1-3, the conical intersection through which the system could relax to the electronic ground state is too high in energy. In some absorption bands, sequential absorption of multiple photons is required for photodissociation. For n = 1, these multiphoton spectra can be modeled on the basis of quantum chemical calculations.

Resonant electron attachment to mixed hydrogen/oxygen and deuterium/oxygen clusters.

Low energy electron attachment to mixed (H2)x/(O2)y clusters and their deuterated analogs has been investigated for the first time. These experiments were carried out using liquid helium nanodroplets to form the clusters, and the effect of the added electron was then monitored via mass spectrometry. There are some important differences between electron attachment to the pure clusters and to the mixed clusters. A particularly notable feature is the formation of HO2- and H2O- ions from an electron-induced chemical reaction between the two dopants. The chemistry leading to these anions appears to be driven by electron resonances associated with H2 rather than O2. The electron resonances for H2 can lead to dissociative electron attachment (DEA), just as for the free H2 molecule. However, there is evidence that the resonance in H2 can also lead to rapid electron transfer to O2, which then induces DEA of the O2. This kind of excitation transfer has not, as far as we are aware, been reported previously.

Observation of stable HO4(+) and DO4(+) ions from ion-molecule reactions in helium nanodroplets.

Ion-molecule reactions between clusters of H2/D2 and O2 in liquid helium nanodroplets were initiated by electron-induced ionization (at 70 eV). Reaction products were detected by mass spectrometry and can be explained by a primary reaction channel involving proton transfer from H3(+) or H3(+)(H2)n clusters and their deuterated equivalents. Very little HO2(+) is seen from the reaction of H3(+) with O2, which is attributed to an efficient secondary reaction between HO2(+) and H2. On the other hand HO4(+) is the most abundant product from the reaction of H3(+) with oxygen dimer, (O2)2. The experimental data suggest that HO4(+) is a particularly stable ion and this is consistent with recent theoretical studies of this ion.