An Infrared Investigation of the (CO₂)_n^- Clusters:  Core Ion Switching from Both the Ion and Solvent Perspectives

Abstract

The (CO2)n- clusters are thought to accommodate the excess electron by forming a localized molecular anion, or "core ion", solvated by the remaining, largely neutral CO2 molecules. Earlier studies interpreted discontinuities in the (CO2)n- photoelectron spectra to indicate that both the CO2- and C2O4- species were present in a size-dependent fashion. Here we use vibrational predissociation spectroscopy to unambiguously establish the molecular structures of the core ions in the 2 < or = n < or = 17 size range. Spectra are reported in the 2300-3800 cm(-1) region, which allows us to independently monitor the contribution of each ion through its characteristic overtone and combination bands. These signature bands are observed to be essentially intact in the larger clusters, establishing that the CO2- and C2O4- molecular ions are indeed the only electron accommodation modes at play. The size dependence of the core ion suggested in earlier analyses of the photoelectron spectra is largely confirmed, although both species are present over a range of clusters near the expected critical cluster sizes, as opposed to the prompt changes inferred earlier. Perturbations in the bands associated with the nominally neutral CO2 "solvent" molecules are correlated with the changes in the molecular structure of the core ion. These observations are discussed in the context of a diabatic model for electron delocalization over the CO2 dimer. In this picture, the driving force leading to the transient formation of the monomer ion is traced to the solvent asymmetry inherent in an incomplete coordination shell.

Further Evidence for Resonant Photoelectron−Solvent Scattering in Nitrous Oxide Cluster Anions

“…In addition, the results peripherally reveal the signature of another core anion for n ) 1-4, assigned to the ONNNO -structure first proposed by Hiraoka et al 16 and characterized spectroscopically by Continetti and co-workers. 17 We examine the coexistence of the NO -and ONNNO -cluster cores under stepwise solvation by N 2 O and interpret the findings in terms of gradual core-shifting (rather than abrupt core-switching [18][19][20][21] attributed to the interplay between the relative stabilities of the NO -and ONNNO -anions and the preferential solvation of the smaller NO -core.…”

The interaction of negative charge with carbon dioxide – insight into solvation, speciation and reductive activation from cluster studies

“…The groups of Johnson and of Nagata found that the VDEs of CO 2 cluster anions show pronounced discontinuities [104,105] at cluster sizes n ¼ 6 and 14 (see Figure 6). The infrared spectra of (CO 2 ) À n cluster ions reported jointly by the Johnson and Weber groups show similarly abrupt changes for the same cluster sizes [107], although the changes around n ¼ 6 are more gradual (see Figure 6). These two independent observations show unambiguously that the charge carrying species changes as a function of cluster size, and they allow us to draw structural conclusions on the nature of the core ions as well.…”

“…This peak Figure 6. Size evolution of the vertical detachment energies (left, adapted from [105] with permission from Elsevier) and vibrational spectra (right, reprinted with permission from [107], copyright (2005) American Chemical Society) for (CO 2 ) À n anions. The dashed line is meant to guide the eye.…”

“…The [(0 2 1)||(1 0 1)] Fermi resonance is also present in these spectra with peaks around 3600 cm À1 and 3720 cm À1 , which are also signatures of solvent CO 2 molecules. In addition to these solvent signatures, the spectra of the small clusters (n = 3-6) show a set of peaks between 3000 and 3200 cm À1 that have been identified as signatures of the D 2d form of the (CO 2 ) À 2 dimer anion [33,107]. Here, the most intense, lowest energy peak is assigned to the combination band of the m 5 ðb 2 Þ and the m 7 ðeÞ stretching fundamentals of the dimer, which are the out-of-phase motions of the symmetric and antisymmetric stretching modes of the two CO 2 moieties [107].…”

Structure and spectroscopy of water‐fluoride microclusters: A combined genetic algorithm and DFT‐based study

“…Infrared(IR) spectroscopic study of stretching frequency vibrations with alteration in sizes have been done. [16][17][18] Theoreticians have tried to do quantum chemical calculations at various levels of theory like DFT, MP2, etc. [19][20][21][22] to arrive at the stretching frequency values that experimentalists have obtained.…”