Electronic Processes in NO Dimerization on Ag and Cu Clusters: DFT and MRMP2 Studies

Experimentally observed NO dimerization on Cu and Ag surfaces is surprising because binding energy of NO dimer is very small in gas phase. MRMP2, MP2 to MP4, CCSD(T), and DFT studies of NO dimerization on Ag-2 and Cu-2 clusters disclosed that the CCSD(T) method could be applied to this reaction on Ag-2 and Cu-2 unlike NO dimerization in gas phase which exhibits significantly large nondynamical electron correlation effect. Charge-transfer (CT) from Ag-2 and Cu-2 to NO moieties plays important role in N-N bond formation between two NO molecules. This CT considerably decreases nondynamical correlation effect. Also, the DFT method could be applied to this NO dimerization, if appropriate DFT functional is used; all pure functionals examined here and most of the hybrid functionals underestimated the activation barrier (E-a), while only omega B97X provided E-a similar to CCSD(T)-calculated value. NO dimerization on similar Cu-2 and Cu-5 needs moderately larger E-a than those on Ag-2 and Ag-5, because frontier orbital participating in the CT exists at lower energy in Cu-2 and Cu-5 than in Ag-2 and Ag-5. The E-a decreases in the order Ag-2 >> Ag-38 > Ag-7 similar to Ag-5 and the reaction energy (Delta E) is positive (endothermic) in Ag-2 but significantly negative in Ag-38, Ag-7, and Ag-5, indicating that various Ag clusters could be effective for NO dimerization except for Ag-2. The decreasing order of E-a and increasing order of exothermicity are attributed to increasing order of the frontier orbital energy of Ag-2 < Ag-38 < Ag-7 similar to Ag-5. (C) 2018 Wiley Periodicals, Inc.