Multiconfigurational Self-consistent Field Calculations Research Articles

Origin of the Distinctive Electronic Structure of Co- and Fe-Porphyrin-Nitrene and Its Effect on Their Nitrene Transfer Reactivity

Metal-bound nitrene species are the crucial intermediate in catalytic nitrene transfer reactions exhibited by engineered enzymes and molecular catalysts. The electronic structure of such species and its correlation with nitrene transfer reactivity have not been fully understood yet. This work presents an in-depth electronic structure analysis and nitrene transfer reactivity of two prototypical metal-nitrene species derived from CoII(TPP) and FeII(TPP) (TPP = meso-tetraphenylporphyrin) complexes and tosyl azide nitrene precursor. Parallel to the well-known "cobalt(III)-imidyl" electronic structure of the Co-porphyrin-nitrene species, the formation mechanism and electronic structure of the elusive Fe-porphyrin-nitrene have been established using density functional theory (DFT) and multiconfigurational complete active-space self-consistent field (CASSCF) calculations. Electronic structure evolution analysis for the metal-nitrene formation step and CASSCF-derived natural orbitals advocates that the electronic nature of the metal-nitrene (M-N) core of Fe(TPP) is strikingly different from that of the Co(TPP). Specifically, the "imidyl" nature of the Co-porphyrin-nitrene [(TPP)CoIII-•NTos] (Tos = tosyl) (I1Co) is contrasted by the "imido-like" character of the Fe-porphyrin-nitrene [(TPP)FeIV[Formula: see text]NTos] (I1Fe). This difference between Co- and Fe-nitrene has been attributed to the additional interactions between Fe-dπ and N-pπ orbitals in Fe-nitrene, which is further complemented by the shortened Fe-N bond length of 1.71 Å. This stronger M-N bond in Fe-nitrene compared to the Co-nitrene is also reflected in the higher exothermicity (ΔΔH = 16 kcal/mol) of the Fe-nitrene formation step. The "imido-like" character renders a relatively lower spin population on the nitrene nitrogen (+0.42) in the Fe-nitrene complex I1Fe, which undergoes the nitrene transfer to the C═C bond of styrene with a considerably higher enthalpy barrier (ΔH‡ = 10.0 kcal/mol) compared to the Co congener I1Co (ΔH‡ = 5.6 kcal/mol) possessing a higher nitrogen spin population (+0.88) and a relatively weaker M-N bond (Co-N = 1.80 Å).

Experimental and theoretical study of resonant core-hole spectroscopies of gas-phase free-base phthalocyanine.

We studied N 1s-1 inner-shell processes of the free base Phthalocyanine molecule, H2Pc, in the gas-phase. This complex organic molecule contains three different nitrogen sites defined by their covalent bonds. We identify the contribution of each site in ionized, core-shell excited or relaxed electronic states by the use of different theoretical methods. In particular, we present resonant Auger spectra along with a tentative new theoretical approach based on multiconfiguration self-consistent field calculations to simulate them. These calculations may pave the road towards resonant Auger spectroscopy in complex molecules.

Formation and relaxation of K-2 and K-2V double-core-hole states in n-butane.

Using a magnetic bottle multi-electron time-of-flight spectrometer in combination with synchrotron radiation, double-core-hole pre-edge and continuum states involving the K-shell of the carbon atoms in n-butane (n-C4H10) have been identified, where the ejected core electron(s) and the emitted Auger electrons from the decay of such states have been detected in coincidence. An assignment of the main observed spectral features is based on the results of multi-configurational self-consistent field (MCSCF) calculations for the excitation energies and static exchange (STEX) calculations for energies and intensities. MCSCF results have been analyzed in terms of static and dynamic electron relaxation as well as electron correlation contributions to double-core-hole state ionization potentials. The analysis of applicability of the STEX method, which implements the one-particle picture toward the complete basis set limit, is motivated by the fact that it scales well toward large species. We find that combining the MCSCF and STEX techniques is a viable approach to analyze double-core-hole spectra.

A combined first- and second-order optimization method for improving convergence of Hartree–Fock and Kohn–Sham calculations

In this work, we investigate the optimization of Hartree-Fock (HF) orbitals with our recently proposed combined first- and second-order (SO-SCI) method, which was originally developed for multi-configuration self-consistent field (MCSCF) and complete active space SCF (CASSCF) calculations. In MCSCF/CASSCF, it unites a second-order optimization of the active orbitals with a Fock-based first-order treatment of the remaining closed-virtual orbital rotations. In the case of the single-determinant wavefunctions, the active space is replaced by a preselected "second-order domain," and all rotations involving orbitals in this subspace are treated at second-order. The method has been implemented for spin-restricted and spin-unrestricted Hartree-Fock (RHF, UHF), configuration-averaged Hartree-Fock (CAHF), as well as Kohn-Sham (KS) density functional theory (RKS, UKS). For each of these cases, various choices of the second-order domain have been tested, and appropriate defaults are proposed. The performance of the method is demonstrated for several transition metal complexes. It is shown that the SO-SCI optimization provides faster and more robust convergence than the standard SCF procedure but requires, in many cases, even less computation time. In difficult cases, the SO-SCI method not only speeds up convergence but also avoids convergence to saddle-points. Furthermore, it helps to find spin-symmetry broken solutions in the cases of UHF or UKS. In the case of CAHF, convergence can also be significantly improved as compared to a previous SCF implementation. This is particularly important for multi-center cases with two or more equal heavy atoms. The performance is demonstrated for various two-center complexes with different lanthanide atoms.

Open Shells in Endohedral Clusters: Structure and Bonding in the [Fe2@Ge16]4- Anion and Comparison to Isostructural [Co2@Ge16]4.

The anionic cluster [Fe2@Ge16]4- has been characterized and shown to be isostructural to the known D2h-symmetric α isomer of the cobalt analogue [Co2@Ge16]4-. Together with the known pair of compounds [Co@Ge10]3- and [Fe@Ge10]3-, the title compound completes a set of four closely related germanium clusters that allow us to explore how the metal-metal and metal-cage interactions evolve as a function of size and of the identity of the metal. The results of spin-unrestricted density functional theory (DFT) and multiconfigurational self-consistent field (MC-SCF) calculations present a consistent picture of the electronic structure where transfer of electron density from the metal to the cage is significant, particularly in the Fe clusters where the exchange stabilization of unpaired spin density is an important driving force.

Cyclic (alkyl)(amino)carbenes in organic and organometallic methane C-H activation: a DFT and MCSCF study.

Density functional theory (DFT) and multi-configurational self-consistent field (MCSCF) calculations are used to investigate the electronic and steric properties of cyclic (alkyl)(amino)carbenes (CAACs). Calculations show CAACs' diverse electronic characteristics in terms of its donor and acceptor capabilities. Reactions of CAACs in methane C-H bond activation via insertion and also as a supporting ligand (L) for L(Cp)M (M = Co, Rh, Ir) motifs via C-H oxidative addition are studied. The binding energies and buried volumes are calculated for selected CAACs as ancillary ligands to the L(Cp)Rh motif. Overall, CAACs show highly tunable electronic and steric properties by adjusting their substituents and backbones. Although CAAC may not be viable in activating inert small molecules with low polarity like methane, this class of ligand has great potential as ancillary ligands for transition metal complexes in catalysis. Calculation of C-H activation by L(Cp)Ir and L(Cp)Rh with CAACs as supporting ligands show CAACs can render the reaction more amenable to catalysis by destabilizing the oxidative addition products while keeping the reaction mildly exergonic, and the barriers reasonable.

Full-dimensional three-state potential energy surfaces and state couplings for photodissociation of thiophenol.

An analytic full-dimensional diabatic potential energy matrix (DPEM) for the lowest three singlet states of thiophenol (C6H5SH) at geometries accessible during photodissociation is constructed using the anchor points reactive potential (APRP) scheme. The data set used for modeling is obtained from electronic structure calculations including dynamic correlation via excitations into the virtual space of a three-state multiconfiguration self-consistent field calculation. The resulting DPEM is a function of all the internal coordinates of thiophenol. The DPEM as a function of the S-H bond stretch and C-C-S-H torsion and the diabatic couplings along two in-plane bend modes and nine out-of-plane distortion modes are computed using extended multiconfigurational quasidegenerate perturbation theory followed by the fourfold way determination of diabatic molecular orbitals and model space diabatization by configurational uniformity, and this dependence of the DPEM is represented by general functional forms. Potentials along 31 tertiary internal degrees of freedom are modeled with system-dependent, primary-coordinate-dependent nonreactive molecular mechanics-type force fields that are parameterized by Cartesian Hessians calculated by generalized Kohn-Sham density functional theory. Adiabatic potential energy surfaces (PESs) and nonadiabatic couplings are obtained by a transformation of the DPEM. The topography of the APRP PESs is characterized by vertical excitation energies, equilibrium geometries, vibrational frequencies, and conical intersections, and we find good agreement with available reference data. This analytic DPEM is suitable for full-dimensional electronically nonadiabatic molecular dynamics calculations of the photodissociation of thiophenol with analytic gradients in either the adiabatic or diabatic representation.

Transient X-ray Absorption Spectral Fingerprints of the S1 Dark State in Uracil.

Low-lying dark nπ* states play an important role in many photophysical and photochemical processes of organic chromophores. Transient X-ray absorption spectroscopy (TXAS) provides a powerful technique for probing the dynamics of valence states by exciting the electrons into high-lying core excited states. We employ multiconfigurational self-consistent field calculations to investigate the TXAS of uracil along its nonradiative photodecay pathways. An open issue is whether dark nπ* state S1 (n is the lone pair localized on an oxygen atom) is accessible when bright ππ* state S2 is selectively excited. Vertical core excitations were calculated along the potential energy surfaces of the three lowest states, S0-S2, interpolated between two minima and two minimum-energy conical intersections. Computed TXAS data from the C, N, and O K edges show distinct spectral fingerprints of the dark state in all spectral regimes. At the O 1s edge, the nπ* state has a very strong absorption at 526-527 eV, while at the C (N) 1s edge, by contrast, there is almost zero (very weak) absorption at 279-282 eV (397-398 eV). All K-edge spectra can be used to sensitively detect the dark states. Our proposed O 1s feature has already been observed in a recent TXAS experiment with thymine. Natural transition orbital analysis is used to interpret all dominant features of the three lowest-valence states along the reaction coordinate and reveal some important valence fine-structure information from the core excitation.

The red bands of TaH: Identification of the ground and low-lying electronic states

Doppler-limited laser excitation spectra were recorded for 12 bands of the molecule tantalum hydride (TaH) in the red region, covering the wavelengths 605–665 nm. These excitation bands originate from three different lower electronic states. Dispersed fluorescence experiments revealed three additional low-lying electronic states of the molecule. The identities and energies of these six states are found to be in good agreement with the multiconfiguration self-consistent field calculations of Koseki et al. (2004). The ground state is X3Φ2, with the [0.1]0+ state lying only 76 ± 6 cm−1 above it. The upper states display energy shifts caused by numerous local perturbations with other dark states.

Active Space Selection Based on Natural Orbital Occupation Numbers from n-Electron Valence Perturbation Theory.

Efficient and robust approximations to the full configuration interaction (full-CI) method such as the density matrix renormalization group (DMRG) and the full-CI quantum Monte Carlo (FCIQMC) algorithm allow for multiconfigurational self-consistent field (MC-SCF) calculations of molecules with many strongly correlated electrons. This opens up the possibility to treat large and complex systems that were previously untractable, but at the same time it calls for an efficient and reliable active space selection as the choice of how many electrons and orbitals enter the active space is critical for any multireference calculation. In this work we propose an Active Space Selection based on 1st order perturbation theory (ASS1ST) that follows a "bottom-up" strategy and utilizes a set of quasi-natural orbitals together with sensible thresholds for their occupation numbers. The required quasi-natural orbitals are generated by diagonalizing the virtual and internal part of the one-electron reduced density matrix that is obtained from strongly contracted n-electron valence perturbation theory (SC-NEVPT) on top of a minimal active space calculation. Self-consistent results can be obtained when the proposed selection scheme is applied iteratively. Initial applications on four chemically relevant benchmark systems indicate the capabilities of ASS1ST. Eventually, the strengths and limitations are critically discussed.

Halide Perovskite-Derived Compounds Rb2TeX6 (X = Cl, Br, and I): Electronic Structure of the Ground and First Excited States.

Herein, we report a study of the electronic structure of the ground and first excited states of Rb2TeCl6, Rb2TeBr6, and Rb2TeI6 halide-perovskite-derived crystals. Using X-ray photoelectron spectroscopy (XPS) measurements and density functional theory and multiconfiguration self-consistent field (MCSCF) calculations, the experimental and theoretical XPS spectra of the valence region were obtained. In addition, the effects of the cations and halogen atoms on the electronic structure were determined, and the classification of the excited states in double point group representation was carried out. Furthermore, a possible reason for the luminescence quenching in an isostructural series of crystals containing the [TeI6]2- anions was determined.

Computational Analysis of Transition Metal-Terminal Boride Complexes.

A computational analysis of model transition-metal terminal boride [MB(PNPR)] complexes is reported. A combination of density functional theory methods, natural bond orbital analysis, and multiconfiguration self-consistent field calculations were employed to investigate the structure and bonding of terminal boride complexes, in particular, the extent of metal dπ-boron pπ bonding. Comparison of metal-boride, -borylene, and-boryl bond lengths confirms the presence of metal-boron π bonds, albeit the modest shortening (∼3%) of the metal-boron bond suggests that the π-bonding is very weak in terminal borides. Calculated free energies of H2 addition to the boride complexes to yield the corresponding boryl complexes indicate that metal-boride π-bond strengths are 22 kcal/mol or less as compared to 44 kcal/mol for an analogous nitride complex. It is concluded that, for the boride complexes studied, covering a range of different 4d and 5d metals, that the metal-boride bond consists of a reasonably covalent σ but two very polarized metal-boron π bonds. The high polarization of the boron-to-metal π bonds indicates that the terminal boride is an acceptor or Z-type ligand.

Pushing configuration-interaction to the limit: Towards massively parallel MCSCF calculations.

A new large-scale parallel multiconfigurational self-consistent field (MCSCF) implementation in the open-source NWChem computational chemistry code is presented. The generalized active space approach is used to partition large configuration interaction (CI) vectors and generate a sufficient number of batches that can be distributed to the available cores. Massively parallel CI calculations with large active spaces can be performed. The new parallel MCSCF implementation is tested for the chromium trimer and for an active space of 20 electrons in 20 orbitals, which can now routinely be performed. Unprecedented CI calculations with an active space of 22 electrons in 22 orbitals for the pentacene systems were performed and a single CI iteration calculation with an active space of 24 electrons in 24 orbitals for the chromium tetramer was possible. The chromium tetramer corresponds to a CI expansion of one trillion Slater determinants (914 058 513 424) and is the largest conventional CI calculation attempted up to date.

Cheap and Near Exact CASSCF with Large Active Spaces.

We use the recently developed Heat-bath Configuration Interaction (HCI) algorithm as an efficient active space solver to perform multiconfiguration self-consistent field calculations (HCISCF) with large active spaces. We give a detailed derivation of the theory and show that difficulties associated with non-variationality of the HCI procedure can be overcome by making use of the Lagrangian formulation to calculate the HCI relaxed two-body reduced density matrix. HCISCF is then used to study the electronic structure of butadiene, pentacene, and Fe-porphyrin. One of the most striking results of our work is that the converged active space orbitals obtained from HCISCF are relatively insensitive to the accuracy of the HCI calculation. This allows us to obtain nearly converged CASSCF energies with an estimated error of less than 1 mHa using the orbitals obtained from the HCISCF procedure in which the integral transformation is the dominant cost. For example, an HCISCF calculation on the Fe-porphyrin model complex with an active space of (44e, 44o) took only 412 s per iteration on a single node containing 28 cores, out of which 185 s was spent in the HCI calculation and the remaining 227 s was used mainly for integral transformation. Finally, we also show that active space orbitals can be optimized using HCISCF to substantially speed up the convergence of the HCI energy to the Full CI limit because HCI is not invariant to unitary transformations within the active space.

Is Photolytic Production a Viable Source of HCN and HNC in Astrophysical Environments? A Laboratory-based Feasibility Study of Methyl Cyanoformate

Motivated by the possibility that cyano-containing hydrocarbons may act as photolytic sources for HCN and HNC in astrophysical environments, we conducted a combined experimental and theoretical investigation of the 193 nm photolysis of the cyano-ester, methyl cyanoformate (MCF). Experimentally, nanosecond time-resolved infrared emission spectroscopy was used to detect the emission from nascent products generated in the photolysis reaction. The time-resolved spectra were analyzed using a recently developed spectral reconstruction analysis, which revealed spectral bands assignable to HCN and HNC. Fitting of the emission band shape and intensity allowed determination of the photolysis quantum yields of HCN, HNC, and CN (A22 Π1)and an HNC/HCN ratio of ∼0.076 ± 0.059. Additionally, multiconfiguration self-consistent field calculations were used to characterize photoexcitation-induced reactions in the ground and four lowest singlet excited states of MCF. At 193 nm excitation, dissociation is predicted to occur predominantly on the repulsive S 2 state, with minor pathways via internal conversion from S 2 to highly excited ground state. An automated transition-state search algorithm was employed to identify the corresponding ground-state dissociation channels, and Rice-Ramsperger-Kassel-Marcus and Kinetic Monte Carlo simulations were used to calculate the associated branching ratios. The proposed mechanisms were validated using the experimentally measured and quasi-classical trajectory-deduced nascent internal energy distributions of HCN and HNC. This work, along with previous studies, illustrates the propensity for cyano-containing hydrocarbons to act as photolytic sources for astrophysical HCN and HNC and may help explain the observed overabundance of HNC in astrophysical environments.

Fragmentation of Valence and Core-Shell (Cl 2p) Excited C2Cl4 Molecule.

The dynamics of the photofragmentation pathways of tetrachloroethylene with photon energies from 15 up to 250 eV encompassing the Cl 2p edge is presented. In order to distinguish the fragmentation channels, the ionic fragments were separated according to their mass-to-charge ratio, measured in coincidence with the photoelectrons, and collected as a function of the incident photon energy. Distinct minima or maxima are found in the partial ion yield in the region between 40 and 50 eV. These features are believed to be associated with the Cooper minimum which results from a molecular orbital with a strong atomic 3p subshell character. In the shallow core region, some fragmentation patterns are considered in terms of fast fragmentation of the C2Cl4 molecule, despite the heavy mass of its fragments. In the present case, the fast fragmentation is favored by the very strong antibonding character of the LUMO, understandable in the frame of the core equivalent model for halogen-containing molecules. In addition, ab initio calculations were performed to obtain states at the Cl 2p edge. Singlet and triplet states at the Cl 2p edge of the C2Cl4 molecule, corresponding to the Cl(2p → 9b1u*) and Cl(2p → 8b2u*) transitions, were calculated in order to form a basis set of molecular states from which the spin-orbit splitting can be inferred. Multiconfigurational self-consistent field (MCSCF) calculation followed by multireference configuration interaction (MRCI) was the method chosen to establish a set of singlet and triplet states at the 2p excitation edge in addition to the ground state.

A combined theoretical and experimental study of the ionic states of iodopentafluorobenzene

A new synchrotron radiation photoelectron spectral (PES) study of iodopentafluorobenzene, together with a theoretical analysis of the spectrum, where Franck-Condon factors are discussed, gives detailed insight into the ionization processes, and this exposes the need for a reinvestigation of the vacuum ultraviolet spectral (VUV) assignments. We have calculated adiabatic ionization energies (AIEs) for several ionic states, using the equation-of-motion coupled cluster method for ionic states combined with multi-configuration self-consistent field calculation study. The AIE sequence is: X2B1 < A2A2 < B2B2 < C22B1 < D2A1 < E32B1. This symmetry sequence has a major impact on previous VUV spectral assignments, which now appear to be to optically forbidden states. Changes in the equilibrium structures for these ionic states are relatively small, but a significant decrease and increase in the C-I bond length relative to the X1A1 structure occurs for the X2B1 and C2B1 states, respectively. The PES shows major vibrational overlaps between pairs of ionic states, X with A, and A with B. The result of these overlaps is the loss of vibrational structure and considerable broadening of the higher energy PES state. Although the baseline is nearly re-established between the A and B states, where the two bands are nearly separate, the B state is also broadened by the A state. Only the C ionic state, which shows the most highly developed vibrational structure, can be regarded as free from vibrational coupling to a neighbor state. The Franck-Condon analysis of the PES bands X, A, B, and C is described in detail; the apparent simplicity of some of these bands is illusory, since almost all the observed peaks arise from super-position of several calculated vibrational states. The experimental AIE of the A state, which is submerged under the X state envelope, has been determined by the subtraction of the calculated X state envelope from the observed PES spectrum. The overlap of these PES bands and the apparent closeness of the potential energy curves describing them have been investigated, using the state-averaged, complete active space self-consistent field method. We have identified two structures, one where the potential energy curves for the X and A states cross and another for the A and B states. At these two conical intersections (ConInts), there is zero-energy difference within each pair of states. Although similar in energy, the ConInt for the crossing of the X with A states, and that for the A with B states, shows that the open-shell occupancies correspond to the 4 lowest AIE states, and all four states that are quite different from each other.

Electron correlation within the relativistic no-pair approximation.

This paper addresses the definition of correlation energy within 4-component relativistic atomic and molecular calculations. In the nonrelativistic domain the correlation energy is defined as the difference between the exact eigenvalue of the electronic Hamiltonian and the Hartree-Fock energy. In practice, what is reported is the basis set correlation energy, where the "exact" value is provided by a full Configuration Interaction (CI) calculation with some specified one-particle basis. The extension of this definition to the relativistic domain is not straightforward since the corresponding electronic Hamiltonian, the Dirac-Coulomb Hamiltonian, has no bound solutions. Present-day relativistic calculations are carried out within the no-pair approximation, where the Dirac-Coulomb Hamiltonian is embedded by projectors eliminating the troublesome negative-energy solutions. Hartree-Fock calculations are carried out with the implicit use of such projectors and only positive-energy orbitals are retained at the correlated level, meaning that the Hartree-Fock projectors are frozen at the correlated level. We argue that the projection operators should be optimized also at the correlated level and that this is possible by full Multiconfigurational Self-Consistent Field (MCSCF) calculations, that is, MCSCF calculations using a no-pair full CI expansion, but including orbital relaxation from the negative-energy orbitals. We show by variational perturbation theory that the MCSCF correlation energy is a pure MP2-like correlation expression, whereas the corresponding CI correlation energy contains an additional relaxation term. We explore numerically our theoretical analysis by carrying out variational and perturbative calculations on the two-electron rare gas atoms with specially tailored basis sets. In particular, we show that the correlation energy obtained by the suggested MCSCF procedure is smaller than the no-pair full CI correlation energy, in accordance with the underlying minmax principle and our theoretical analysis. We also show that the relativistic correlation energy, obtained from no-pair full MCSCF calculations, scales at worst as X(-2) with respect to the cardinal number X of our correlation-consistent basis sets optimized for the two-electron atoms. This is better than the X(-1) scaling suggested by previous studies, but worse than the X(-3) scaling observed in the nonrelativistic domain. The well-known 1/Z- expansion in nonrelativistic atomic theory follows from coordinate scaling. We point out that coordinate scaling for consistency should be accompanied by velocity scaling. In the nonrelativistic domain this comes about automatically, whereas in the relativistic domain an explicit scaling of the speed of light is required. This in turn explains why the relativistic correlation energy to the lowest order is not independent of nuclear charge, in contrast to nonrelativistic theory.

Spin–orbit coupling analyses of phosphorescence: the effects of cyclometalated ligand replacement in fac-Ir(ppy)3with various bpy ligands on blue phosphorescence

Multi-configuration self-consistent field (MCSCF) calculations followed by second-order configuration interaction (SOCI) and spin–orbit coupling (SOC) calculations were performed using one of the effective core potentials and its associated basis set, the Stevens–Basch–Krauss–Jasien–Cundari (SBKJC) basis set referred to here as SBKJC+p, in order to examine the effects of replacement of a methine group in each phenylpyridine (ppy) ligand of the facial (fac) isomer of tris(2-phenylpyridinato)iridium(III)[fac-Ir(ppy)3] by a nitrogen atom for the purpose of obtaining deep blue phosphorescence. Only replacements of the methine group at the 3′ or 5′ site of each ppy ligand by nitrogen atoms were calculated to induce a spectral shift to the blue region. These results can be explained by the strong electronegativity of nitrogen atoms and by the magnitude of the electron density at the 2′ carbon atom of the ppy or bipyridine (bpy) ligands. Subsequently, the substituent effects in the fac isomer of tris(bipyridinato)iridium(III)[fac-Ir(bpy)3] were investigated and found to be similar to those in fac-Ir(ppy)3 as reported in our previous study (RSC Adv., 2015, 5, 35760–35772). Based on systematic calculations and theoretical consideration, the most effective modification for deep blue phosphorescence should be the introduction of a nitrogen atom into the 3′ or/and 5′ site of benzene rings of the ppy ligands in fac-Ir(ppy)3 together with the combination of the substituents X = F, NR2, or OR (R = alkyl group) and Y = CN or NO2.

Formation of reactive oxygen species by vanadium complexes

The role of the ligand in vanadium IV and V complexes to produce reactive oxygen species (OH and O2-) by Fenton and Haber–Weiss reactions is analyzed theoretically, in gas phase and considering solvated ions. By using reactivity indexes, we found that the presence of the oxo group induces the electron donating process over complexes that in first instance does not present it. Also, we show that the vanadyl ion promotes the production of OH radicals, which is a contrary behavior presented by others vanadium complexes that may act as O2- producers or scavengers. For Haber–Weiss reactions, the analysis of the Gibbs free energy predict the following ordering: H2O>NH3>Cl->OH->C2O42-, Cl->C2O42-, NH3>oxo, H2O>oxo, NH3>oxo, Cl-, for ligands with vanadium V and IV in the formation of reactive oxygen species (ROS); which allow for the first time a criteria to predict ROS activity in vanadium complexes. All the results were obtained by using the Density Functional Theory, in its Kohn–Sham version, with four different exchange–correlation functionals (PBE, PBE0, B3LYP and M06-2X) coupled with the aug-cc-pVDZ basis set. It is well-known that transition metals sometimes exhibit a multireference character, for that reason several tests were considered over the vanadium complexes reported here. In particular, T1 test, multiconfiguration self-consistent field and coupled-clusters calculations were performed in order to test the feasibility of our results. In general, we can say that the M06-2X exchange–correlation functional exhibits the best performance to study theoretically vanadium complexes.