Density Functional Study of the Electronic Structure and Related Properties of Pt(NO)/Pt(NO2) Redox Couples

Abstract

Density functional calculations at the B3LYP level of theory, using the SDD basis set, provide satisfactory description of geometric, energetic, electronic and spectroscopic properties of the Pt(NO)/Pt(NO2) redox couple. The neutral Pt(NO) species adopts a bent 2A' ground state, while the cationic [Pt(NO)]+ species adopts a linear 1Σ+ ground state. The B3LYP/SDD- predicted Pt-N bond lengths are 2.016 and 1.777 Å for Pt(NO) (2A') and [Pt(NO)]+ (1Σ+), respectively, while the ∠Pt-N-O bent angle for [Pt(NO)] (2A') is 119.6°. On the other hand, the anionic [Pt(NO)]- species adopts the bent 1A' ground state with a Pt-N bond length of 1.867 Å and a ∠Pt-N-O bent angle of 122.5°. The computed binding energies of the NO, NO+ and NO- ligands with Pt(0) were found to be 29.9 (32.8), 69.9 (78.4) and 127.4 (128.7) kcal/mol at the B3LYP/SDD and CCSD(T)/SDD (numbers in parentheses) levels of theory, respectively. Moreover, the structure of the [Pt(NO2)]+ component of the Pt(NO)/Pt(NO2) redox couple and its transformation to [Pt(NO)]+ upon reaction with CO was analysed in the framework of the DFT theory. The coordination of the CO ligand to [Pt(NO2)]+ affords the cationic mixed-ligand [Pt(CO)(NO2)]+ complex, which is stabilized by 66.6 (60.5) kcal/mol, with respect to the separated [Pt(NO2)]+ and CO in their ground states. The O-transfer reaction from the coordinated NO2 to the coordinated CO ligands in the presence of the [Pt(NO2)]+ species corresponds to an exothermic process; the heat of the reaction (∆RH) is -85.2 (-80.5) kcal/mol and the activation barrier amounts to 27.7 (33.0) kcal/mol. Finally, the equilibrium structures of selected stationary points related to the transformation of NO to NO2 ligand located on the potential energy surfaces of the [Pt(NO),O2], [Pt(NO)+,O2], and [Pt(NO)-,O2] systems were analysed in the framework of the DFT theory. The computed interaction energies of O2 with Pt(NO), [Pt(NO)]+ and [Pt(NO)]- species were found to be 106.9 (105.3), 49.2 (48.4) and 26.9 (26.5) kcal/mol, respectively. The O2 ligand is coordinated to the Pt central atom in an end-on mode for [Pt(NO),O2] and [Pt(NO)-,O2] systems and in a side-on mode for the [Pt(NO)+,O2] system. The transformation of NO to NO2 in [Pt(NO)]- species upon reaction with dioxygen corresponds to an exothermic process; the heat of the reaction (∆RH) is -60.6 (-55.8) kcal/mol, while the activation barrier amounts to 35.5 (30.2) kcal/mol. Calculated structures, relative stability and bonding properties of all stationary points are discussed with respect to computed electronic and spectroscopic properties, such as charge density distribution and harmonic vibrational frequencies.