Mechanisms of the reactions of W and W⁺ with NO_x (x = 1, 2): A computational study

The mechanisms of the reactions of W and W⁺ with NO_x (x = 1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the insertion pathway of the reaction W( ⁷S) + NO₂(²A₁) is a multistate process, which involves several lower lying electronic states of numerous intermediates and transition states, and leads to oxidation, WO( ³Σ) + NO(²Π), and/or nitration, WN( ⁴Σ) + O₂(³Σ_g ^-), of the W-center. Oxidation products WO(³Σ) + NO(²Π) lie 87.6 kcal/mol below the reactants, while the nitration channel is only 31.0 kcal/mol exothermic. Furthermore, it was shown that nitration of W with NO₂ is kinetically less favorable than its oxidation. The addition-dissociation pathway of the reaction W(⁷S) + NO₂(²A₁) proceeds via the octet (ground) state potential energy surface of the reaction, requires 3.3 kcal/mol barrier, and leads exclusively to oxidation products. Calculations show that oxidation of the W⁺ cation by NO₂ is a barrierless process in the gas phase, proceeds exclusively via the insertion pathway, and is exothermic by 82.9 kcal/mol. The nitration of W⁺ by NO₂ is only 14.1 kcal/mol exothermic and could be accessible only under high-temperature conditions. Reactions of M = W/W⁺ with NO are also barrierless processes in the gas phase and lead to the N-O insertion product NMO, which are 105.4 and 77.4 kcal/mol lower than the reactants for W and W⁺, respectively.