Structures of [(CO2)n(H2O)m]− (n=1–4, m=1,2) cluster anions. I. Infrared photodissociation spectroscopy

Infrared Spectroscopy of Size-Selected Hydrated Carbon Dioxide Radical Anions CO2 .- (H2 O)n (n=2-61) in the C-O Stretch Region

Understanding the intrinsic properties of the hydrated carbon dioxide radical anions CO2 .- (H2 O)n is relevant for electrochemical carbon dioxide functionalization. CO2 .- (H2 O)n (n=2-61) is investigated by using infrared action spectroscopy in the 1150-2220 cm-1 region in an ICR (ion cyclotron resonance) cell cooled to T=80 K. The spectra show an absorption band around 1280 cm-1 , which is assigned to the symmetric C-O stretching vibration νs . It blueshifts with increasing cluster size, reaching the bulk value, within the experimental linewidth, for n=20. The antisymmetric C-O vibration νas is strongly coupled with the water bending mode ν2 , causing a broad feature at approximately 1650 cm-1 . For larger clusters, an additional broad and weak band appears above 1900 cm-1 similar to bulk water, which is assigned to a combination band of water bending and libration modes. Quantum chemical calculations provide insight into the interaction of CO2 .- with the hydrogen-bonding network.

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.

IR photodissociation spectroscopy of (OCS)n(+) and (OCS)n(-) cluster ions: Similarity and dissimilarity in the structure of CO2, OCS, and CS2 cluster ions

Infrared photodissociation (IRPD) spectra of (OCS)n(+) and (OCS)n(-) (n = 2-6) cluster ions are measured in the 1000-2300 cm(-1) region; these clusters show strong CO stretching vibrations in this region. For (OCS)2 +) and (OCS)2(-), we utilize the messenger technique by attaching an Ar atom to measure their IR spectra. The IRPD spectrum of (OCS)2 (+)Ar shows two bands at 2095 and 2120 cm(-1). On the basis of quantum chemical calculations, these bands are assigned to a C2 isomer of (OCS)2 (+), in which an intermolecular semi-covalent bond is formed between the sulfur ends of the two OCS components by the charge resonance interaction, and the positive charge is delocalized over the dimer. The (OCS)n(+) (n = 3-6) cluster ions show a few bands assignable to "solvent" OCS molecules in the 2000-2080 cm(-1) region, in addition to the bands due to the (OCS)2(+) ion core at ∼2090 and ∼2120 cm(-1), suggesting that the dimer ion core is kept in (OCS)3-6(+). For the (OCS)n(-) cluster anions, the IRPD spectra indicate the coexistence of a few isomers with an OCS(-) or (OCS)2(-) anion core over the cluster range of n = 2-6. The (OCS)2(-)Ar anion displays two strong bands at 1674 and 1994 cm(-1). These bands can be assigned to a Cs isomer with an OCS(-) anion core. For the n = 2-4 anions, this OCS(-) anion core form is dominant. In addition to the bands of the OCS(-) core isomer, we found another band at ∼1740 cm(-1), which can be assigned to isomers having an (OCS)2(-) ion core; this dimer core has C2 symmetry and (2)A electronic state. The IRPD spectra of the n = 3-6 anions show two IR bands at ∼1660 and ∼2020 cm(-1). The intensity of the latter component relative to that of the former one becomes stronger and stronger with increasing the size from n = 2 to 4, which corresponds to the increase of "solvent" OCS molecules attached to the OCS(-) ion core, but it suddenly decreases at n = 5 and 6. These IR spectral features of the n = 5 and 6 anions are ascribed to the formation of another (OCS)2(-) ion core having C2v symmetry with (2)B2 electronic state.

Structural evolution of the [(CO2)n(H2O)]- cluster anions: quantifying the effect of hydration on the excess charge accommodation motif

The [(CO2)n(H2O)]- cluster anions are studied using infrared photodissociation (IPD) spectroscopy in the 2800-3800 cm(-1) range. The observed IPD spectra display a drastic change in the vibrational band features at n = 4, indicating a sharp discontinuity in the structural evolution of the monohydrated cluster anions. The n = 2 and 3 spectra are composed of a series of sharp bands around 3600 cm(-1), which are assignable to the stretching vibrations of H2O bound to C2O4- in a double ionic hydrogen-bonding (DIHB) configuration, as was previously discussed (J. Chem. Phys. 2005, 122, 094303). In the n > or = 4 spectrum, a pair of intense bands additionally appears at approximately 3300 cm(-1). With the aid of ab initio calculations at the MP2/6-31+G* level, the 3300 cm(-1) bands are assigned to the bending overtone and the hydrogen-bonded OH vibration of H2O bound to CO2- via a single O-H...O linkage. Thus, the structures of [(CO2)n(H2O)]- evolve with cluster size such that DIHB to C2O4- is favored in the smaller clusters with n = 2 and 3 whereas CO2- is preferentially stabilized via the formation of a single ionic hydrogen-bonding (SIHB) configuration in the larger clusters with n > or = 4.

Photodetachment and photofragmentation pathways in the [(CO2)2(H2O)m]− cluster anions

The mass-selected [(CO(2))(2)(H(2)O)(m)](-) cluster anions are studied using a combination of photoelectron imaging and photofragment mass spectroscopy at 355 nm. Photoelectron imaging studies are carried out on the mass-selected parent cluster anions in the m=2-6 size range; photofragmentation results are presented for m=3-11. While the photoelectron images suggest possible coexistence of the CO(2) (-)(H(2)O)(m)CO(2) and (O(2)CCO(2))(-)(H(2)O)(m) parent cluster structures, particularly for m=2 and 3, only the CO(2) (-) based clusters are both required and sufficient to explain all fragmentation pathways for m>/=3. Three types of anionic photofragments are observed: CO(2) (-)(H(2)O)(k), O(-)(H(2)O)(k), and CO(3) (-)(H(2)O)(k), k</=m, with their yields varying depending on the parent cluster size. Of these, only CO(2) (-)(H(2)O)(k) can potentially result from (O(2)CCO(2))(-)(H(2)O)(m) parent structures, although an alternative mechanism, involving the dissociation and recombination of the CO(2) (-) cluster core, is possible as well. The O(-)(H(2)O)(k) and CO(3) (-)(H(2)O)(k) channels are believed to be triggered by the dissociation of the CO(2) (-) cluster core. In the CO(3) (-)(H(2)O)(k) channel, seen only in the range of m=3-6, the CO(2) (-) core dissociation is followed by an intracluster association of nascent O(-) with the solvent CO(2). This channel's absence in larger clusters (m>6) is attributed to hindrance from the H(2)O molecules.