Theor Chem Acc (2006) 115: 434–440 DOI 10.1007/s00214-006-0124-2 REGULAR ARTICLE F. Rondinelli · N. Russo · M. Toscano CO 2 activation by Zr + and ZrO + in gas phase Received: 30 November 2005 / Accepted: 27 January 2006 / Published online: 10 March 2006 © Springer-Verlag 2006 Abstract The gas-phase reduction of carbon dioxide to carbon monoxide, induced by Zr + and ZrO + catalysts, was investi- gated at density functional level of theory. Calculations were carried out using both hybrid and pure exchange-correlation functionals in order to reproduce adequately the energetic gap between the Zr + 4 F and 2 D electronic states and experi- mental reaction heats. In agreement with a guided ion beam tandem mass spectrometer study, we have found that carbon dioxide activation by Zr + presents a spin-forbidden mech- anism because of a spin inversion process occurring during reaction in the rate- determining step. ZrO + interacts with CO 2 through two possible pathways both endothermic: for- mation of ZrO + 2 and CO products is less unfavourable. Infor- mation about ground and excited states of ZrO + and ZrO + 2 oxides and bond dissociation energies of species present on the reaction paths was also given. Keywords Carbon dioxide activation · Zirconium ions · Electronic states · Density functional theory 1 Introduction Continuous emission of carbon dioxide into the atmosphere represents the main cause of greenhouse effect because of the stratospheric ozone depletion. Since the first structural compounds containing CO 2 [1] were synthesized and char- acterized, many studies were performed to introduce new reaction mechanisms that allow the elimination of this gas in mild conditions [2–5]. However, till now, carbon dioxide chemistry is not developed much. CO 2 effective activation is still an unresolved problem because it is thermodynami- cally stable and kinetically inert. The possibility to convert F. Rondinelli · N. Russo · M. Toscano (B ) Dipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite-Centro d’Eccellenza MIUR, Universita’ della Calabria, 87030 Arcavacata di Rende (CS), Italy E-mail: m.toscano@unical.it Fax: +39-0984-493390 carbon dioxide into a non-dangerous species not only could limit the greenhouse environmental damages, but also con- stitutes a carbon source alternative to petroleum, natural gas and coal, that are all energetic resources destined to exhaust themselves. Besides, because of its large-scale availability at low cost, carbon dioxide could represent a precursor com- pound for the synthesis of useful chemical products, such as methanol, urea and salicylic acid. CO 2 natural mechanisms of activation have received much attention in recent years, through the simulation of photosynthesis processes and the study of metal-enzyme catalysed reaction paths on the model of carbonic anhydrase [6–8]. A great deal of experimental [9– 15] and theoretical [16–20]works was based on the coordina- tion of CO 2 to one or more metals. As far as the coordination modes are concerned, carbon dioxide gives rise to insertion mechanisms in bonds between a metal and elements such as H, O, N, P, Si, C, in intermetallic bonds and can interact with unsaturated substrates coordinated to transition metals. To give insight into the thermodynamic and the work mechanisms of some zirconium compounds used as hydro- genation catalysts for CO and CO 2 , Sievers and Arment- rout [21] studied the gas-phase reactions of Zr + , ZrO + and ZrO + 2 cations with carbon mono- and dioxide. This investi- gation, performed at experimental level using the guided ion beam mass spectrometry, allowed to characterize all stable species involved in the reactions. Emphasis was put on the importance to consider the electronic states of the metal cat- ion and its oxides. In fact, for some of these species, there is no information, either at experimental or at theoretical level, on both ground (i.e. for ZrO + 2 ) and excited states (i.e. for both ZrO + and ZrO + 2 ). Only speculative determinations of electronic excitation energies for two excited states of ZrO + were possible in this study. For several species involved in the examined reactions, these approximate assignments allowed an estimate of binding energies. With the aim to corroborate the literature data on zirco- nium catalysts previously studied experimentally [21] and to check the utility of density functional theory in determin- ing some missing information, we have retained interest in performing calculations on the following processes: