On the catalytic mechanism of Pt 4 +/- in the oxygen transport activation of N2O by CO

The two-state reaction mechanism of the Pt (4) (+/-) with N2O (CO) on the quartet and doublet potential energy surfaces has been investigated at the B3LYP level. The effect of Pt-4 (-) anion assistance is analyzed using the activation strain model in which the activation energy (Delta E-not equal) is decomposed into the distortion energies (Delta E-dist(not equal)) and the stabilizing transition state ( TS) interaction energies (Delta E-dist(not equal)), namely Delta E-not equal = Delta E-dist(not equal) + Delta E-int(not equal). The lowering of activation barriers through Pt4 - anion assistance is caused by the TS interaction DE6(-90.7 to -95.6 kcal/mol) becoming more stabilizing. This is attributed to the N2O pi*-LUMO and Pt d HOMO back-donation interactions. However, the strength of the back-donation interactions has significantly impact on the reaction mechanism. For the Pt-4 (-) anion system, it has very significant back-bonding interaction (N2O negative charge of 0.79e), HOMO has 81.5% pi* LUMO(N2O) character, with 3d orbital contributions of 10.7% from Pt-(3) and 7.7% from Pt-(7) near the (4)TS4 transition state. This facilitates the bending of the N2O molecule, the N-O bond weakening, and an O-(P-2) dissociation without surface crossing. For the Pt-4 (+) cation system, the strength of the charge transfer is weaker, which leads to the diabatic (spin conserving) dissociation of N2O: N2O((1)a(+)) -> N-2((1)a (g) (+) ) + O(D-1). The quartet to doublet state transition should occur efficiently near the (4)TS1 due to the larger SOC value calculated of 677.9 cm(-1). Not only will the reaction overcome spin-change-induced barrier (ca. 7 kcal/mol) but also overcome adiabatic barrier (ca. 40.1 kcal/mol).Therefore, the lack of a thermodynamic driving force is an important factor contributing to the low efficiency of the reaction system.