On the nature of spin-and orbital-resolved Cu +-NO charge transfer in the gas phase and at Cu(I) sites in zeolites

Abstract

Electronic factors essential for NO activation by Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels (natural orbitals for chemical valence). NOCV analysis is performed for three DFT-optimized models of Cu(I)-NO site in ZSM-5: [CuNO] +, (T1)CuNO, and (M7)CuNO. NO as a non-innocent, openshell ligand reveals significant differences between independent deformation density components for a and b spins. Four distinct components are identified: (i) unpaired electron donation from NO π||* antibonding orbital to Cus,d; (ii) backdonation from copper d yz to π* antibonding orbital; (iii) donation from occupied pk and Cu dxz to bonding region, and (iv) donation from nitrogen lone-pair to Cus,d. Channel (i), corresponding to one-electron bond, shows-up solely for spin majority and is effective only in the interaction of NO with naked Cu +. Channel (ii) dominates for models b and c: it strongly activates NO bond by populating antibonding π* orbital and weakens the N-O bond in contrast to channel (i), depopulating the antibonding orbital and strengthening N-O bond. This picture perfectly agrees with IR experiment: interaction with naked Cu + imposes small blue-shift of NO stretching frequency while it becomes strongly red-shifted for Cu(I) site in ZSM-5 due to enhanced backdonation. © 2012 Springer Science+Business Media, LLC.