Stereoelectronic Effects on Molecular Geometries and State-Energy Splittings of Ligated Monocopper Dioxygen Complexes

Abstract

The relative energies of side-on versus end-on binding of molecular oxygen to a supported Cu(I) species, and the singlet versus triplet nature of the ground electronic state, are sensitive to the nature of the supporting ligands and, in particular, depend upon their geometric arrangement relative to the O 2 binding site. Highly correlated ab initio and density functional theory electronic structure calculations demonstrate that optimal overlap (and oxidative charge transfer) occurs for the side-on geometry, and this is promoted by ligands that raise the energy, thereby enhancing resonance, of the filled Cu d xz orbital that hybridizes with the in-plane π* orbital of O 2 . Conversely, ligands that raise the energy of the filled Cu d z 2 orbital foster a preference for end-on binding as this is the only mode that permits good overlap with the in-plane O 2 π*. Because the overlap of Cu d z 2 with O 2 π* is reduced as compared to the overlap of Cu d xz with the same O 2 orbital, the resonance is also reduced, leading to generally more stable triplet states relative to singlets in the end-on geometry as compared to the side-on geometry, where singlet ground states become more easily accessible once ligands are stronger donors. Biradical Cu(II)-O 2 superoxide character in the electronic structure of the supported complexes leads to significant challenges for accurate quantum chemical calculations that are best addressed by exploiting the spin-purified M06L local density functional, single-reference completely renormalized coupled-cluster theory, or multireference second-order perturbation theory, all of which provide predictions that are qualitatively and quantitatively consistent with one another.

Involvement of ferryl in the reaction between nitrite and the oxy forms of globins

“…The meta-generalized gradient approximation (GGA) M06-L functional [21] coupled with the double f 6-31G** basis set was employed for geometry optimizations. We point out that the M06-L functional offered the best agreement of several tested functionals compared to large multireference calculations and completely renormalized coupled cluster calculations and several benchmark studies that have confirmed its excellent accuracy including our recent study on oxyheme structures [22][23][24][25][26][27][28][29]. Additional single point energy calculations using the triple f 6-311?G** basis set were performed on all structures.…”

The restricted active space followed by second-order perturbation theory method: Theory and application to the study of CuO2 and Cu2O2 systems

“…In both of these cases, the resulting compounds with a variety of ligands can have singlet ground states that exhibit substantial biradical character because of the spin separation associated either with two d 9 Cu͑II͒ ions or one such ion and a superoxide radical anion. [40][41][42][43][44][45][46][47][48][49][50][51][52] Wave function theories restricted to a single determinant are poorly suited to the description of such species since singlet biradicals are intrinsically two determinantal. 53,54 Moreover, even when oxidation states more likely to be characterized as closed shell in nature are considered, e.g., ͓LCu͑III͔͒ 2 ͓O͑2−͔͒ 2 , computational studies have found that large contributions from dynamical correlation effects influence relative isomer energetics.…”

“…53,54 Moreover, even when oxidation states more likely to be characterized as closed shell in nature are considered, e.g., ͓LCu͑III͔͒ 2 ͓O͑2−͔͒ 2 , computational studies have found that large contributions from dynamical correlation effects influence relative isomer energetics. [40][41][42][47][48][49]51,52 In this section, we apply the RASPT2 method to two problems previously studied in considerable detail using a wide range of theoretical models ͑see Fig. 2͒.…”

“…The first case considers the singlet-triplet splitting in LCuO 2 , L =1,3-diketiminate 44,52 while the second examines the relative energies of the isomeric peroxo-and bis--oxo forms of ͕͓Cu͑NH 3 ͒ n ͔ 2 O 2 ͖ 2+ ͑n =0,1,2,3͒. In each case, our goal is primarily to compare the performance of different RASPT2 and CASPT2 protocols one to another, noting that in the latter case, i.e., the ͕͓Cu͑NH 3 ͒ n ͔ 2 O 2 ͖ 2+ system, prior experience has indicated that it is impossible to choose a CAS sufficiently large to provide converged, balanced results for the CASPT2 energetics of the isomeric equilibrium.…”

Diffusion of Atomic Oxygen on the Si(100) Surface

“…20,33 As illustrated below, the CR-CC(2,3) approach is capable of removing failures of CCSD(T) in regions of the potential energy surface (PES) that exhibit significant diradical character. [24][25][26][34][35][36][37][38][39][40][41] Some of the earlier studies of the reaction mechanisms for the oxidation and etching processes have been described 22, [42][43][44] using QM/MM and kinetic Monte Carlo (KMC) 45 simulation methods. The QM/MM method is described above.…”