Theoretical study of the structure of ZCu(NO2)(NO). A proposed intermediate in the NOx 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 f unctional approach. For ZCu(NO2)(NO) both the singlet and triplet states ha ve been considered. The most stable structure (T1) is a triplet state in wh ich Cu presents square-pyramidal five-coordination with NO in the apical po sition. The lowest singlet structure (S1) shows square-planar four-coordina tion and lies only 0.7 kcal mol(-1) above T1. In T1 the ZCu(NO2)-NO interac tion 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(N203) structures are less sta ble (similar to 11 kcal mol(-1)) than the lowest singlet ZCu(NO2)(NO) one. All structures are stable with respect to the ground-state ZCuO + 2NO asymp tote, the corresponding Delta G degrees at 298 K for S1 being -30.5 kcal mo l(-1).