Transient Negative Species in Supercritical Carbon Dioxide:  Electronic Spectra and Reactions of CO₂ Anion Clusters

Abstract

Transient absorption spectra following ionization of supercritical CO2 have been investigated using the pulse radiolysis technique. Absorption spectra measured from 400 to 800 nm suggest that at least two transient species absorb. We have previously reported that one species is (CO2)2 +. In the near UV region, we observed a transient species of which the lifetime and reactivity are different from the dimer cation. We assign this species to a dimer anion, (CO2)2 -, or an anion−molecule complex, (CO2 -)(CO2) x . Comparison with the photobleaching of CO2 anion clusters in solid rare gas matrixes and their reactivity with H2 and O2 confirm the assignment. Theoretical calculations, in which solvation is taken into account, are consistent with these assignments. It is well-established that the adiabatic electron affinity of CO2 is negative, but the adiabatic electron affinity of CO2 dimer has been calculated to be 0.89 eV for D 2 d symmetry (CO2)2 - in the gas phase. The calculations predict that CO2 - in a model continuum solvent is stable to autodetachment.

Reaction rates of the hydrated electron with N2O in high temperature water and potential surface of the N2O− anion

“…The differences between gas and condensed phase are not confined to N 2 O solutions. For e.g., we have demonstrated for CO 2 molecules that the CO À 2 anion is unstable in the gas phase while the CO À 2 anion is stable in water and CO 2 dimer anion is stable in supercritical CO 2 [14,15]. The reactivity of CO 2 and the CO À 2 anion in a high-density supercritical CO 2 is significantly different from that in gas phase [15].…”

Solvation and Evolution Dynamics of an Excess Electron in SupercriticalCO2

“…Supercritical CO 2 (scCO 2 ), a nontoxic, nonflammable, easily recyclable, and environmentally friendly solvent, appears to be a promising medium for charge-transfer reactions, and for reprocessing nuclear waste [18][19][20][21][22][23][24][25][26][27][28][29][30][31]. Unlike polar water, ammonia, alcohols, and apolar or weakly polar organic solvents, a CO 2 molecule is bond polar but overall apolar and has a large negative gas phase vertical electron affinity (À 0:9 eV) [26,27], indicating that it can not directly bind an EE.…”

“…Unlike polar water, ammonia, alcohols, and apolar or weakly polar organic solvents, a CO 2 molecule is bond polar but overall apolar and has a large negative gas phase vertical electron affinity (À 0:9 eV) [26,27], indicating that it can not directly bind an EE. However, bending vibrations can lead to a transient dipole for each CO 2 , and thus to its potential to bind an EE.…”

Efficient trapping of silver cations in a rare gas matrix: Ag3+ in argon

“…While the electron affinity of CO 2 has been measured to be negative, equal to −0.6 eV in the gas phase, 24 Takahashi et al 25 have shown that a solvent with a dielectric constant between 1.5 and 2 is sufficient to stabilize the additional electron. CO 2 − is thus stable in an argon matrix that has a dielectric constant of 1.7, as it has been observed in several experimental studies in argon 21,26 and in neon.…”