Theoretical Study of the Structure of ZCu(NO2)(NO). A Proposed Intermediate in the NO x Decomposition by Cu-ZSM-5

The geometries and harmonic vibrational frequencies for eight ZCu(NO2)(NO) and two ZCu(N2O3) structures have been determined using the B3LYP density functional approach. For ZCu(NO2)(NO) both the singlet and triplet states have been considered. The most stable structure (T1) is a triplet state in which Cu presents square-pyramidal five-coordination with NO in the apical position. The lowest singlet structure (S1) shows square-planar four-coordination and lies only 0.7 kcal mol-1 above T1. In T1 the ZCu(NO2)-NO interaction is mainly electrostatic, whereas in S1 the bonding is covalent. In ZCu(N2O3) species, the N2O3 ligand interacts with ZCu through one oxygen, the bonding being mainly electrostatic. These ZCu(N2O3) structures are less stable (∼11 kcal mol-1) than the lowest singlet ZCu(NO2)(NO) one. All structures are stable with respect to the ground-state ZCuO + 2NO asymptote, the corresponding ΔG° at 298 K for S1 being -30.5 kcal mol-1.