The activation of carbon dioxide by first row transition metals (Sc-Zn)

The activation of CO2 by chloride-tagged first-row transition metal anions [ClM]- (M = Sc-Zn), was examined by mass spectrometry, quantum chemical calculations, and statistical analysis. The direct formation of [ClM(CO2)]- complexes was demonstrated in the reaction between [ClM]- and neutral CO2. In addition, the reverse reaction was investigated by energy-variable collisionally induced dissociation (CID) of the corresponding [ClM(CO2)]- anions generated in-source. Five different mono- and bi-dentate binding motifs present in the ion/CO2 complexes were identified by quantum chemical calculations and the relative stability of each of these isomers was established and analyzed for all first-row transition metals based on the experimental and theoretical ion/molecule binding energies. It was found that the early first row transition metals form strong covalent bonds with the neutral CO2 molecule, while the late ones and in particular copper and zinc are weakly bonded. Using simple valence bond Lewis diagrams, the different binding motifs and their relative stabilities across the first row were described using multi-configurational self consistent field (MSCSCF) wavefunctions in a quantitative manner based on the electronic structure of the individual metals. This analysis provides an explanation for the change of the most favorite bonding motif of the transition metals with CO2 along the 1st transition metal row. The nature of the activated CO2 complex and the relationship between its stability and other structural and spectral properties was also analyzed by Principal Component Analysis (PCA) and artificial neural networks.

Temperature dependence of the photodissociation of CO2 from high vibrational levels: 205-230 nm imaging studies of CO(X1Σ+) and O(3P, 1D) products

The 205-230 nm photodissociation of vibrationally excited CO₂ at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO₂ molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO₂. Photodissociation was observed from vibrationally excited CO₂ with internal energies up to about 20 000 cm^-1, and CO(X¹Σ⁺), O(³P), and O(¹D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO₂ vibrational excitation made this investigation feasible. The internal energies of heated CO₂ molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(X¹Σ⁺) products in v″ = 0. At 230 nm, CO₂ needs to have at least 4000 cm^-1 of rovibrational energy to absorb the UV radiation and produce CO(X¹Σ⁺) + O(³P). CO₂ internal energies in excess of 16 000 cm^-1 were confirmed by observing O(¹D) products. It is likely that initial absorption from levels with high bending excitation accesses both the A¹B₂ and B¹A₂ states, explaining the nearly isotropic angular distributions of the products. CO(X¹Σ⁺) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(X¹Σ⁺), O(³P), and O(¹D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO₂ temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO₂ (and the corresponding available energy) increases. The KER distributions of O(¹D) and O(³P) show broad internal energy distributions in the CO(X¹Σ⁺) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO₂ VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, a³B₂ and b³A₂, and the ground electronic state are involved in the dynamical pathways that lead to product formation.