Structure, Bonding, and Relative Stability of the Ground and Low-Lying Electronic States of CuO2. The Role of Exact Exchange

Benchmarking Density Functional Theory Methods for Metalloenzyme Reactions: The Introduction of the MME55 Set

We present a new benchmark set of metalloenzyme model reaction energies and barrier heights that we call MME55. The set contains 10 different enzymes, representing eight transition metals, both open and closed shell systems, and system sizes of up to 116 atoms. We use four DLPNO-CCSD(T)-based approaches to calculate reference values against which we then benchmark the performance of a range of density functional approximations with and without dispersion corrections. Dispersion corrections improve the results across the board, and triple-ζ basis sets provide the best balance of efficiency and accuracy. Jacob's ladder is reproduced for the whole set based on averaged mean absolute (percent) deviations, with the double hybrids SOS0-PBE0-2-D3(BJ) and revDOD-PBEP86-D4 standing out as the most accurate methods for the MME55 set. The range-separated hybrids ωB97M-V and ωB97X-V also perform well here and can be recommended as a reliable compromise between accuracy and efficiency; they have already been shown to be robust across many other types of chemical problems, as well. Despite the popularity of B3LYP in computational enzymology, it is not a strong performer on our benchmark set, and we discourage its use for enzyme energetics.

Photoelectron spectra of copper oxide cluster anions from first principles methods

We present results and analyses for the photoelectron spectra of small copper oxide cluster anions (CuO^-, Cu O2- , Cu O3- , and Cu₂O^-). The spectra are computed using various techniques, including density functional theory (DFT) with semi-local, global hybrid, and optimally tuned range-separated hybrid functionals, as well as many-body perturbation theory within the GW approximation based on various DFT starting points. The results are compared with each other and with the available experimental data. We conclude that as in many metal-organic systems, self-interaction errors are a major issue that is mitigated by hybrid functionals. However, these need to be balanced against a strong role of non-dynamical correlation-especially in smaller, more symmetric systems-where errors are alleviated by semi-local functionals. The relative importance of the two phenomena, including practical ways of balancing the two constraints, is discussed in detail.

Phenol-Induced O-O Bond Cleavage in a Low-Spin Heme-Peroxo-Copper Complex: Implications for O2 Reduction in Heme-Copper Oxidases

This study evaluates the reaction of a biomimetic heme-peroxo-copper complex, {[(DCHIm)(F8)FeIII]-(O22-)-[CuII(AN)]}+ (1), with a phenolic substrate, involving a net H-atom abstraction to cleave the bridging peroxo O-O bond that produces FeIV═O, CuII-OH, and phenoxyl radical moieties, analogous to the chemistry carried out in heme-copper oxidases (HCOs). A 3D potential energy surface generated for this reaction reveals two possible reaction pathways: one involves nearly complete proton transfer (PT) from the phenol to the peroxo ligand before the barrier; the other involves O-O homolysis, where the phenol remains H-bonding to the peroxo OCu in the transition state (TS) and transfers the H+ after the barrier. In both mechanisms, electron transfer (ET) from phenol occurs after the PT (and after the barrier); therefore, only the interaction with the H+ is involved in lowering the O-O cleavage barrier. The relative barriers depend on covalency (which governs ET from Fe), and therefore vary with DFT functional. However, as these mechanisms differ by the amount of PT at the TS, kinetic isotope experiments were conducted to determine which mechanism is active. It is found that the phenolic proton exhibits a secondary kinetic isotope effect, consistent with the calculations for the H-bonded O-O homolysis mechanism. The consequences of these findings are discussed in relation to O-O cleavage in HCOs, supporting a model in which a peroxo intermediate serves as the active H+ acceptor, and both the H+ and e- required for O-O cleavage derive from the cross-linked Tyr residue present at the active site.

Density functional calculation of the structure and electronic properties of Cu(n)O(n) (n = 1-8) clusters

Ab initio simulations and calculations were used to study the structures and stabilities of copper oxide clusters, Cu(n)O(n) (n = 1-8). The lowest energy structures of neutral and charged copper oxide clusters were determined using primarily the B3LYP/LANL2DZ model chemistry. For n ≥ 4, the clusters are nonplanar. Selected electronic properties including atomization energies, ionization energies, electron affinities, and Bader charges were calculated and examined as a function of n.

Optoelectronic and Excitonic Properties of Oligoacenes: Substantial Improvements from Range-Separated Time-Dependent Density Functional Theory

The optoelectronic and excitonic properties in a series of linear acenes (naphthalene up to heptacene) are investigated using range-separated methods within time-dependent density functional theory (TDDFT). In these rather simple systems, it is well-known that TDDFT methods using conventional hybrid functionals surprisingly fail in describing the low-lying L(a) and L(b) valence states, resulting in large, growing errors for the L(a) state and an incorrect energetic ordering as a function of molecular size. In this work, we demonstrate that the range-separated formalism largely eliminates both of these errors and also provides a consistent description of excitonic properties in these systems. We further demonstrate that reoptimizing the percentage of Hartree-Fock exchange in conventional hybrids to match wave function-based benchmark calculations still yields serious errors, and a full 100% Hartree-Fock range separation is essential for simultaneously describing both of the L(a) and L(b) transitions. From an analysis of electron-hole transition density matrices, we finally show that conventional hybrid functionals over-delocalize excitons and underestimate quasiparticle energy gaps in the acene systems. The results of our present study emphasize the importance of both a range-separated and asymptotically correct contribution of exchange in TDDFT for investigating optoelectronic and excitonic properties, even for these simple valence excitations.

Density functional theory for transition metals and transition metal chemistry

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Probing the mechanism of O2 activation by a copper(I) biomimetic complex of a C-H hydroxylating copper monooxygenase

In this paper, we report, for the first time, a plausible full reaction pathway for the activation of O(2) by a tetraazamacrocyclic monocopper(I) complex and for the subsequent intramolecular alkylic hydroxylation to yield the alkoxide product. This theoretical insight offers remarkable support to the fundamental hypothesis in the field that a hydroperoxo complex of the type Cu(II)OOH intermediate is the key intermediate in this class of reactions. Overall, we give insight into an intramolecular alkylic C-H bond activation due to the O(2) binding to copper(I) with an end-on eta(1)-O(2) ligation. The loss of a water molecule involves the final substrate oxygenation. The complex we consider is a biomimetic of several systems of biological relevance, such as amine oxidases, peptidylglycine-alpha-hydroxylating monooxygenase, and dopamine-beta monooxygenases.