Gas Phase Reaction Mechanism Of ZrO⁺ With CS₂,NbS⁺ With H₂O And N₂O With H₂ Cyclically Catalyzed By Ir⁺

Nowadays, the transition-metal-ion chemistry has been an active area for both experimental and theoretical studies. In industry, transition-metal oxides are used as versatile catalysts in many applications, however, for some processes their reactivity is too high and non-specific product formation occurs. In contrast, transition-metal sulfides are less reactive and susceptible to poisoning and can show higher selectivity. Transition metal sulfides also play a particular role in biochemistry in that heterometallic sulfur complexes form the active sites in several metalloenzymes. Besides, theoretical approaches to gas-phase transition-metal chemistry which involve two-state reactivity along reaction path have also been studied by many chemists. In this paper, on the basis of the molecular orbital theory, the transition state theory as well as quantum chemistry theory, three systems choosed have been studied using DFT method and coupled cluster CCSD (T) methods.For the ²â–³ground state of ZrO⁺ with CS₂ system, the mechanisms for three products ZrS⁺, ZrS₂⁺ and ZrOS⁺ have been studied by using B3LYP/6-311+G* and CCSD(T)/SDD+6-311+G* methods. It is found that both ZrS⁺ and ZrS₂⁺ formations involve the same O/S exchange process via a four-center transition state TS12 to form an intermediate IM2. Exception of that IM2 can dissociate into the ZrS⁺ product, a favorable intramolecular rearrangement mechanism associated with the ZrS₂⁺ formation has been identified, which explains why ZrS⁺ was excluded as a precusor for the ZrS₂⁺ formation and why the lower efficiency of the ZrS⁺ formation was observed in experiment. For the formation of ZrOS⁺, two parallel channels (path A and B) yielding their corresponding product isomer have been identified. Path B involving an insertion-elimination mechanism with a calculated barrier underestimated by ca. 25.0 kJ/mol should be attributed to the threshold of 114.8±12.5 kJ/mol assigned in the experiment. But path A is a S-shift reaction (the barrier is 174.5 kJ/mol )and should make some contributions to the formation of ZrOS⁺ at elevated energy.For the NbS⁺ (³âˆ‘-, ¹Î“) with H₂O system, two possible reaction mechanisms have been studied by using B3LYP/6-311++G** method: the S/O exchange reaction which involves two hydrogen atoms migration from the O atom to the S atom (NbS⁺+H₂O→NbO⁺+H₂S) and the dehydrogenation reaction which involve the elimination of molecular hydrogen from the transition metal niobium center (NbS⁺+H₂O→NbOS⁺+H₂). According to the identified reaction mechanisms, a triplet–singlet surface crossing for the dehydrogenation is suggested. The triplet–single intersystem crossing is shown to play a crucial role for the reaction. The crossing point (CP) has been localized with the approach suggested by Yoshizawa et al. The spin-forbidden reaction 3NbS⁺ (³âˆ‘^- ) + H₂O→1NbOS⁺-2 (1A') + H₂ was found to be energetically much more favorable than the spin-allowed reaction ³NbS⁺ (³âˆ‘^- ) + H₂O→³NbO⁺ (³âˆ‘^- ) + H₂S. Besides, two possible H₂-elimination pathways and one possible H-elimination pathway have been identified on the two different surfaces. All theoretical results are in reasonable agreement with the experimental observations.The reaction mechanism of N₂O + H₂→N₂ + H₂O cyclically catalyzed by the late third-row transition metal cation Ir⁺ has been investigated on quintet and triplet potential energy surfaces ( PES ) at the CCSD(T)/ [ SDD + 6-311+G** // B3LYP / [ SDD + TZVP ] level of theory. The calculated potential energy surfaces indicate that the activation energy of the first oxidation reaction step of Ir⁺ by N₂O is 42.8kJ/mol, which is the rate-determining step. However, the second reduction reaction step of IrO⁺ by H₂ on the two surfaces are both kinetically and thermochemically barrierless. The identified reaction mechanisms and the potential energy surfaces indicate that the crossings between the quintet and triplet surfaces are unlikely to occur. Furthermore, both steps of the reaction are exothermic. The experimental observations are well explained.