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, open-shell ligand reveals significant differences between independent deformation density components for α and β 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 π‖ and Cu d xz 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.
Highlights
Cationic sites in mesoporous structures, in particular Cu(I) sites in zeolites, have long-attracted attention and stimulated vigorous research due to their high-catalytic activity and selectivity
Electronic factors essential for nitric oxide (NO) activation by Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels
This seemingly unexpected behavior of spin density confirms suggestions formulated for other transition metal—NO systems [56] that the spin on NO moiety does not directly correlate with apparent oxidation state while total charge is rather an effect of the balance between opposing electron transfer processes that is spin-resolved donation and backdonation channels
Summary
Cationic sites in mesoporous structures, in particular Cu(I) sites in zeolites, have long-attracted attention and stimulated vigorous research due to their high-catalytic activity and selectivity. As already discussed by other authors, [22, 55] angle deviation from linearity prompts the transfer of electron density from the singly occupied NO p* orbital into the copper 4s orbital and the formation of one-electron bond, which might be rationalized by a positive charge on the ligand but not by unitary spin density.
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