Gas-phase reactions of V2O5+ and V2O6+ ions with CH3CF3 studied by density functional theory

摘要

The present work presents a computational study (B-P86 density functional, D(T)ZVP basis set) of structure and energetics of V2On+ cations (n=5-6) and their gas-phase reactivity toward CH3CF3. Both V2O5+ and V2O6+ are shown to have an asymmetric double-bridge-structure, (O)(2)-V-(O)(2)-V(O) and (O-2)(O)-V-(O)(2)-V(O) in their respective stablest configurations. The V2O6+ cation has a terminal peroxo ligand. The cations can form weakly bound V2OnF••• CF2CH3+ adducts with CH3CF3. The latter can rearrange to much stabler CH3CF2O(On-4)V(O-2)V(O)F alkoxy structures. The formation of V2On+• CH3CF3 from V2On+ and free CH3CF3 is very exothermic. The C-C bond cleavage in V2O6+• CH3CF3 to yield V2O6+• CF3 and the free CH3 radical is much more energy demanding that the analogous dissociation of V2O5+• CH3CF3, which accounts for dissimilarity in experimental behavior of V2O5+ and V2O6+ and agrees with the observation that V2O5+ but not V2O6+ renders the CH3-CF3 cracking. Another route of the cracking, with release of CF3, is not possible because the stablest configurations of V2On+• CH3CF3, contain an intact methyl group but no CF3.The abstraction of HF from CH3CF3 is computed to be roughly equally favorable, with &UDelta; E of about -80 kJ mol(-1) for both V2O5+• HF and V2O6+• HF.The direct dissociation of O-2 from V2O6+ requires 142 kJ mol(-1) only, while the alternative mechanisms are more exothermic. This confirms the experimentalists' assumption that dioxygen dissociation from V2O6+ is a collision-induced dissociation. © 2005 Elsevier B.V. All rights reserved.