A Frontier Orbital Study with ab Initio Molecular Dynamics of the Effects of Solvation on Chemical Reactivity: Solvent-Induced Orbital Control in FeO-Activated Hydroxylation Reactions

Solvation effects on chemical reactivity are often rationalized using electrostatic considerations: the reduced stabilization of the transition state results in higher reaction barriers and lower reactivity in solution. We demonstrate that the effect of solvation on the relative energies of the frontier orbitals is equally important and may even reverse the trend expected from purely electrostatic arguments. We consider the H abstraction reaction from methane by quintet [EDTAH_<i>n</i>·FeO]^{(<i>n</i>−2)+}, (<i>n</i> = 0–4) complexes in the gas phase and in aqueous solution, which we examine using ab initio thermodynamic integration. The variation of the charge of the complex with the protonation of the EDTA ligand reveals that the free energy barrier in gas phase increases with the negative charge, varying from 16 kJ mol^–1 for [EDTAH₄·FeO]²⁺ to 57 kJ mol^–1 for [EDTAH_<i>n</i>·FeO]^2–. In aqueous solution, the barrier for the +2 complex (38 kJ mol^–1) is higher than in gas phase, as predicted by purely electrostatic arguments. For the negative complexes, however, the barrier is lower than in gas phase (e.g., 45 kJ mol^–1 for the −2 complex). We explain this increase in reactivity in terms of a stabilization of the virtual 3σ* orbital of FeO²⁺, which acts as the dominant electron acceptor in the H-atom transfer from CH₄. This stabilization originates from the dielectric screening caused by the reorientation of the water dipoles in the first solvation shell of the charged solute, which stabilizes the acceptor orbital energy for the −2 complex sufficiently to outweigh the unfavorable electrostatic destabilization of the transition-state relative to the reactants in solution

Ligand Field Effects and the High Spin–High Reactivity Correlation in the H Abstraction by Non-Heme Iron(IV)–Oxo Complexes: A DFT Frontier Orbital Perspective

The electronic structure explanation of H abstraction from aliphatic CH bonds by the ferryl ion, Fe^IVO²⁺, has received a great deal of attention. We review the insights that have been gained, in particular into the effect of the spin state. However, we emphasize that the spin state is dictated by the field of the ligands coordinated to the Fe ion and is but one of the effects of the ligand field. Using the model systems [FeO­(H₂O)₅]²⁺, representative of the weak field situation, and [FeO­(H₂O)_<i>ax</i>(NH₃)₄]²⁺, representative of a strong (equatorial) field, we distinguish the effect of spin state (high spin (quintet) versus low spin (triplet)) from other effects, notably the orbital interaction (pushing up) effect of the ligand donor orbitals and the electron-donating ability of the ligands, directly affecting the charge on the FeO group. We describe the changes in electronic structure during the reaction with the help of elementary orbital interaction diagrams involving the frontier orbitals. These give a straightforward electronic structure picture of the reaction but do not provide support for the description of the reactivity of FeO²⁺ as starting with oxyl radical formation

Is [FeO]²⁺ the Active Center Also in Iron Containing Zeolites? A Density Functional Theory Study of Methane Hydroxylation Catalysis by Fe-ZSM-5 Zeolite

Arguments are put forward that the active α-oxygen site in the Fe-ZSM-5 catalyst consists of the FeO2+ moiety. It is demonstrated that this zeolite site for FeO2+ indeed obeys the design principles for high reactivity of the FeO2+ moiety proposed earlier: a ligand environment consisting of weak equatorial donors (rather oxygen based than nitrogen based) and very weak or absent trans axial donor. The α-oxygen site would then owe its high reactivity to the same electronic structure features that lends FeO2+ its high activity in biological systems, as well as in the classical Fenton chemistry

Synergism of Porphyrin-Core Saddling and Twisting of<i> meso</i>-Aryl Substituents

The structural chemistry of meso-aryl-substituted porhyrins has uncovered a bewildering variety of macrocycle distortions. Saddling angles range up to 40°, while the plane of the phenyl groups at the meso positions may be anywhere between perpendicular to the porphyrin plane (θ = 90°) and tilted to quite acute angles (θ = 30° or even less). These two distortions appear to be correlated. This has naturally been explained by steric hindrance:  when the phenyls rotate toward the porphyrin plane, for instance, coerced by packing forces, the pyrrole rings can alleviate the steric hindrance by tilting away to a saddled conformation. We demonstrate, however, that the two motions are intrinsically coupled by electronic factors and are correlated even in the absence of external forces. A saddling motion makes it sterically possible for the phenyl rings to rotate toward the porphyrin plane, which will always happen because of increasingly favorable π-conjugation interaction with smaller angles θ. The considerable energy lowering due to π conjugation counteracts the energy cost of the saddling, making the concerted saddling/rotation motion very soft. Unsubstituted meso-aryl porphyrins just do not distort, but an additional driving force may tip the balance in favor of the combined distortion motion. Internal forces having this effect are repulsion of the four hydrogens that occupy the central hole of the ring in porphyrin diacids but also steric repulsion in peripherally crowded porphyrins. These findings lead to a clarification and systematization of the observed structural variety, which indeed shows a remarkable correlation between saddling and phenyl ring tilting

Cu(bipy)²⁺/TEMPO-Catalyzed Oxidation of Alcohols: Radical or Nonradical Mechanism?

In the oxidation of alcohols with TEMPO as catalyst, the substrate has alternatively been postulated to be oxidized but uncoordinated TEMPO⁺ (Semmelhack) or Cu-coordinated TEMPO^• radical (Sheldon). The reaction with the Cu­(bipy)²⁺/TEMPO cocatalyst system has recently been claimed, on the basis of DFT calculations, to not be a radical reaction but to be best viewed as electrophilic attack on the alcohol C–H_α bond by <i>coordinated</i> TEMPO⁺. This mechanism combines elements of the Semmelhack mechanism (oxidation of TEMPO to TEMPO⁺) and the Sheldon proposal (“in the coordination sphere of Cu”). The recent proposal has been challenged on the basis of DFT calculations with a different functional, which were reported to lead to a radical mechanism. We carefully examine the results for the two functionals and conclude from both the calculated energetics and from an electronic structure analysis that the results of the two DFT functionals are consistent and that both lead to the proposed mechanism with TEMPO not acting as radical but as (coordinated) positive ion

Nucleophilic or Electrophilic Phosphinidene Complexes ML<i>_n</i>PH; What Makes the Difference?

Density functional studies, based on the local density approximation including nonlocal corrections for correlation and exchange self-consistently, have been carried out for the equilibrium structures of the phosphinidene transition metal complexes MLnPH, with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir and L = CO, PH3, Cp. The chemical reactivity of the transition metal-stabilized phosphinidene P−R is influenced by its spectator ligands L. Ligands with strong σ-donor capabilities on the metal increase the electron density on the phosphorus atom, raise the π*-orbital energy, and enhance its nucleophilicity. Spectator ligands with strong π-acceptor capabilities lower the charge concentration on P and stabilize the π*-orbital, which results in a higher affinity for electron-rich species. The MLnPH bond is investigated using a bond energy analysis in terms of electrostatic interaction, Pauli repulsion, and orbital interaction. A symmetry decomposition scheme affords a quantitative estimate of the σ- and π-bond strengths. It is shown that the investigated phosphinidenes are strong π-acceptors and even stronger σ-donors. The metal−phosphinidene interaction increases on going from the first to the second- and third-row transition metals

On the Equivalence of Conformational and Enantiomeric Changes of Atomic Configuration for Vibrational Circular Dichroism Signs

We study systematically the vibrational circular dichroism (VCD) spectra of the conformers of a simple chiral molecule, with one chiral carbon and an “achiral” alkyl substituent of varying length. The vibrational modes can be divided into a group involving the chiral center and its direct neighbors and the modes of the achiral substituent. Conformational changes that consist of rotations around the bond from the next-nearest neighbor to the following carbon, and bond rotations further in the chain, do not affect the modes around the chiral center. However, conformational changes within the chiral fragment have dramatic effects, often reversing the sign of the rotational strength. The equivalence of the effect of enantiomeric change of the atomic configuration and conformational change on the VCD sign (rotational strength) is studied. It is explained as an effect of atomic characteristics, such as the nuclear amplitudes in some vibrational modes as well as the atomic polar and axial tensors, being to a high degree determined by the local topology of the atomic configuration. They reflect the local physics of the electron motions that generate the chemical bonds rather than the overall shape of the molecule

Understanding Solvent Effects in Vibrational Circular Dichroism Spectra: [1,1′-Binaphthalene]-2,2′-diol in Dichloromethane, Acetonitrile, and Dimethyl Sulfoxide Solvents

We present a combined experimental and computational investigation of the vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of [1,1′-binaphthalene]-2,2′-diol. First, the sensitive dependence of the experimental VA and VCD spectra on the solvent is demonstrated by comparing the experimental spectra measured in CH₂Cl₂, CD₃CN, and DMSO-<i>d</i>₆ solvents. Then, by comparing calculations performed for the isolated solute molecule to calculations performed for molecular complexes formed between solute and solvent molecules, we identify three main types of perturbations that affect the shape of the VA and VCD spectra when going from one solvent to another. These sources of perturbations are (1) perturbation of the Boltzmann populations, (2) perturbation of the electronic structure, and (3) perturbation of the normal modes

Relativistic DFT Calculations of the Paramagnetic Intermediates of [NiFe] Hydrogenase. Implications for the Enzymatic Mechanism

Relativistic DFT Calculations of the Paramagnetic Intermediates of [NiFe] Hydrogenase. Implications for the Enzymatic Mechanis

Secondary Kinetic Peak in the Kohn–Sham Potential and Its Connection to the Response Step

We consider a prototypical 1D model Hamiltonian for a stretched heteronuclear molecule and construct individual components of the corresponding KS potential, namely, the kinetic, the N – 1, and the conditional potentials. These components show very special features, such as peaks and steps, in regions where the density is drastically low. Some of these features are quite well-known, whereas others, such as a secondary peak in the kinetic potential or a second bump in the conditional potential, are less or not known at all. We discuss these features building on the analytical model treated in Giarrusso et al. J. Chem. Theory Comput. 2018, 14, 4151. In particular, we provide an explanation for the underlying mechanism which determines the appearance of both peaks in the kinetic potential and elucidate why these peaks delineate the region over which the plateau structure, due to the N – 1 potential, stretches. We assess the validity of the Heitler–London Ansatz at large but finite internuclear distance, showing that, if optimal orbitals are used, this model is an excellent approximation to the exact wave function. Notably, we find that the second natural orbital presents an extra node very far out on the side of the more electronegative atom