DFT Methodology Research Articles

Mechanistic Investigation of Iridium-Catalyzed C–H Borylation of Methyl Benzoate: Ligand Effects in Regioselectivity and Activity

The Ir-catalyzed C–H borylation of methyl benzoate has been studied with DFT methodology in order to understand the experimentally observed ligand-induced regioselectivity and activity when different [(ligand)Ir(Bpin)3] catalysts are employed. While bidentate ligands such as 4,4′-di-tBu-2,2′-bipyridine (dtbpy) completely inhibit ortho-borylation, the use of selected triphenylphosphine derivatives enables the reaction on that position, avoiding the meta- and para-regioisomers. The analysis of the catalytic cycles for the borylation reactions with dtbpy, PPh3, P(p-CF3C6H4)3, and P(m,m-(CF3)2C6H3)3 allows the interpretation of the observed ligand effects. The different reactivity observed for the different monodentate phosphine ligands can also be rationalized in terms of catalyst stability.

Probing Oxide-Ion Mobility in the Mixed Ionic-Electronic Conductor La2NiO4+δ by Solid-State (17)O MAS NMR Spectroscopy.

While solid-state NMR spectroscopic techniques have helped clarify the local structure and dynamics of ionic conductors, similar studies of mixed ionic–electronic conductors (MIECs) have been hampered by the paramagnetic behavior of these systems. Here we report high-resolution 17O (I = 5/2) solid-state NMR spectra of the mixed-conducting solid oxide fuel cell (SOFC) cathode material La2NiO4+δ, a paramagnetic transition-metal oxide. Three distinct oxygen environments (equatorial, axial, and interstitial) can be assigned on the basis of hyperfine (Fermi contact) shifts and quadrupolar nutation behavior, aided by results from periodic DFT calculations. Distinct structural distortions among the axial sites, arising from the nonstoichiometric incorporation of interstitial oxygen, can be resolved by advanced magic angle turning and phase-adjusted sideband separation (MATPASS) NMR experiments. Finally, variable-temperature spectra reveal the onset of rapid interstitial oxide motion and exchange with axial sites at ∼130 °C, associated with the reported orthorhombic-to-tetragonal phase transition of La2NiO4+δ. From the variable-temperature spectra, we develop a model of oxide-ion dynamics on the spectral time scale that accounts for motional differences of all distinct oxygen sites. Though we treat La2NiO4+δ as a model system for a combined paramagnetic 17O NMR and DFT methodology, the approach presented herein should prove applicable to MIECs and other functionally important paramagnetic oxides.

A new DFT methodology for k-CNOT reversible circuits and its implementation using quantum-dot cellular automata

Testable design of reversible circuits leads to a large increment in operating costs from their original circuits. This paper presents a new cost efficient methodology of converting k-CNOT gates based circuits into respective parity preserving circuits. The testability of these circuits can be achieved simply by checking the input and output parity by means of CNOT gates. The design process requires only a single wire for its formulation without an increase in number of garbage outputs and provides full fault coverage under single bit fault detection with lesser design complexity. Experiments were performed on a set of benchmark circuits and the results show a reduction in operating costs up to 45% as compared to prior work. The work is also justified by implementing k-CNOT gates in quantum-dot cellular automata (QCA) to prove the efficiency of our work in physical foreground.

A Comprehensive Study of Copper Guanidine Quinoline Complexes: Predicting the Activity of Catalysts in ATRP with DFT.

Copper complexes of the hybrid guanidine ligands 1,3-dimethyl-N-(quinolin-8-yl)-imidazolidin-2-imine (DMEGqu) and 1,1,3,3-tetramethyl-2-(quinolin-8-yl)-guanidine (TMGqu) have been studied comprehensively with regard to their structural and electrochemical properties and their activity in atom transfer radical polymerization (ATRP). A simple analysis of the molecular structures of the complexes gives no indication about their activity in ATRP; however, with the help of DFT and NBO analysis the influence of particular coordinating donors on the electrochemical properties could be fully elucidated. With an adequate DFT methodology and newly applied theoretical isodesmic reactions it was possible to predict the relative position of the redox potentials of copper complexes containing DMEGqu and TMGqu ligands. In addition, predictions could be made as to whether the complexes of DMEGqu or TMGqu are more active in ATRP. Four new Cu(I) complexes were tested in standard ATRP reactions and kinetically investigated both in bulk and in solution. It could be proven that complexes featuring DMEGqu possess a lower redox potential and are more active in ATRP, although the tetramethylguanidine moiety represents the stronger donor.

Ab initio proposition of novel BCN3 hard material based on the intergrowth of wurtzite and pyrite-like motifs

A novel hard material, orthorhombic BCN3 based on the intergrowth of non-centrosymmetric wurtzite CN+ and centrosymmetric pyrite BN2− motifs alike in fully characterized AlSiP3 ternary is proposed from calculations within DFT methodology. BCN3 is identified as a “nitride-dinitride” related with Zintl phases. The ground state structure exhibits significantly short interatomic distances: d(C-N) = 1.47 Å and d(N-N) = 1.43 Å as compared to d(Si-P) = 2.24 Å and d(P-P) = 2.18 Å in the phosphide, resulting into high magnitude bulk modulus obtained from E,V equation of state (EOS) with B0 = 371 GPa versus B0 = 97 GPa in the phosphide. The complete set of elastic constants complies with the mechanical stability conditions of the new system and equally produces high hardness magnitude (384 GPa). Ratio of bulk/shear modules of 0.85 characterizes a brittle material. Electronic band structures identify small indirect gap semi-conducting AlSiP3 and large band gap insulator BCN3 with direct gap ΓV-ΓC ∼ 4 eV.

Using DFT methodology for more reliable predictive models: Design of inhibitors of Golgi α-Mannosidase II

Human Golgi α-mannosidase II (GMII), a zinc ion co-factor dependent glycoside hydrolase (E.C.3.2.1.114), is a pharmaceutical target for the design of inhibitors with anti-cancer activity. The discovery of an effective inhibitor is complicated by the fact that all known potent inhibitors of GMII are involved in unwanted co-inhibition with lysosomal α-mannosidase (LMan, E.C.3.2.1.24), a relative to GMII. Routine empirical QSAR models for both GMII and LMan did not work with a required accuracy. Therefore, we have developed a fast computational protocol to build predictive models combining interaction energy descriptors from an empirical docking scoring function (Glide-Schrödinger), Linear Interaction Energy (LIE) method, and quantum mechanical density functional theory (QM-DFT) calculations. The QSAR models were built and validated with a library of structurally diverse GMII and LMan inhibitors and non-active compounds. A critical role of QM-DFT descriptors for the more accurate prediction abilities of the models is demonstrated. The predictive ability of the models was significantly improved when going from the empirical docking scoring function to mixed empirical-QM-DFT QSAR models (Q2=0.78–0.86 when cross-validation procedures were carried out; and R2=0.81–0.83 for a testing set). The average error for the predicted ΔGbind decreased to 0.8–1.1kcalmol−1. Also, 76–80% of non-active compounds were successfully filtered out from GMII and LMan inhibitors. The QSAR models with the fragmented QM-DFT descriptors may find a useful application in structure-based drug design where pure empirical and force field methods reached their limits and where quantum mechanics effects are critical for ligand-receptor interactions. The optimized models will apply in lead optimization processes for GMII drug developments.

Substituents role in zinc phthalocyanine derivatives used as dye-sensitized solar cells. A theoretical study using Density Functional Theory

A series of zinc phthalocyanine dyes with different electron-donating and electron-withdrawing substituents and using the COOH as the anchor group that interacts with the semiconductor (TiO2) surface, were studied employing DFT and TDDFT methodologies. Their HOMO and LUMO orbital energies and, the redox mediator position would facilitate the injection process toward the conduction band of TiO2. We tested the effect of some electron-donating or electron-withdrawing substituents over zinc phthalocyanine dyes in terms of the Hammett parameters (Σσ). We obtained a linear correlation between the substituents properties versus the free energy change of the electron-injection (ΔGinject) process.

Dicationic derivatives of dinaphthotetraaza[14]annulene: synthesis, crystal structures and the preliminary evaluation of their DNA binding properties

Four new water-soluble, fluorescent derivatives of dinaphthotetraaza[14]annulene (DNTAA) have been synthesized varying in the structure, dimensions and spatial arrangements of their meso side groups. The products have been carefully characterized by elemental analyses, spectroscopy, crystal structures and quantum-chemical calculations employing DFT methodology. One representative product of the DNTAA series was tested for DNA binding using UV–vis titrations, CD and thermal denaturation experiments. Intercalation of ds-DNA was recognized as a dominant binding mode. In contrast to non-fluorescent phenylene analogues (DBTAA), reported previously, DNTAA derivative showed threefold fluorescence increase upon DNA binding, and offered intriguing new applications as a fluorescent DNA/RNA dye.

Computational mechanistic elucidation of the intramolecular aminoalkene hydroamination catalysed by iminoanilide alkaline-earth compounds.

A comprehensive computational exploration of plausible alternative mechanistic pathways for the intramolecular hydroamination (HA) of aminoalkenes by a recently reported class of kinetically stabilised iminoanilide alkaline-earth silylamido compounds [{N^N}Ae{N(SiMe3)2}⋅(thf)n] ({N^N} = iminoanilide; Ae = Ca, Sr, Ba) is presented. On the one hand, a proton-assisted concerted N-C/C-H bond-forming pathway to afford the cycloamine in a single step can be invoked and on the other hand, a stepwise σ-insertive pathway that involves a fast, reversible migratory olefin 1,2-insertion step linked to a less rapid, irreversible metal-C azacycle tether σ-bond aminolysis. Notably, these alternative mechanistic avenues are equally consistent with reported key experimental features. The present study, which employs a thoroughly benchmarked and reliable DFT methodology, supports the prevailing mechanism to be a stepwise σ-insertive pathway that sees an initial conversion of the {N^N}Ae silylamido into the catalytically competent {N^N}Ae amidoalkene compound and involves thereafter facile and reversible insertive N-C bond-forming ring closure, linked to irreversible intramolecular Ae-C tether σ-bond aminolysis at the transient {N^N}Ae alkyl intermediate. Turnover-limiting protonolysis accounts for the substantial primary kinetic isotope effect observed; its DFT-derived barrier satisfactorily matches the empirically determined Eyring parameter and predicts the decrease in rate observed across the series Ca>Sr>Ba correctly. Non-competitive kinetic demands militate against the operation of the concerted proton-assisted pathway, which describes N-C bond-forming ring closure triggered by concomitant amino proton delivery at the C=C linkage evolving through a multi-centre TS structure. Valuable insights into the catalytic structure-activity relationships are unveiled by a detailed comparison of [{N^N}Ae(NHR)] catalysts. Moreover, the intriguingly opposite trends in reactivity observed in intramolecular (Ca>Sr>Ba) and intermolecular (Ca<Sr<Ba) HA catalysis for the studied family of iminoanilide alkaline-earth amido catalysts are rationalised.

Ab initio study in the hydration process of metaphosphoric acid: the importance of the pnictogen interactions

A theoretical study of the hydration of metaphosphoric acid to yield phosphoric acid has been carried out by means of MP2/6-31+G(d,p) and MP2/aug-cc-pVTZ computational levels. Up to three explicit water molecules have been considered as well as the PCM solvation model to account for the effect of the bulk water. The reaction profile has been analyzed using the conceptual DFT methodology. The reactant structure is very dependent on the number of water molecules. The inclusion of more than one water molecule produces important cooperative effects and a shortening of the O···P pnictogen interaction besides the reaction barrier drops about 50 kJ mol−1. Reaction force at ξ 1 indicates the decreasing in the angular stress in the reaction site before reaching the TS as more explicit water molecules are taken into account. The analysis of the reaction electronic flux shows that for the three mechanisms studied, the principal reactive changes occur in the TS zone, while reactants and products remain in a zero-flux regime.

A comparative study between post-Hartree–Fock methods and density functional theory in closed-shell aurophilic attraction

The inter- and intramolecular aurophilic [ClAuPH3]2, [S(AuPH3)2] and [AuPH3]42+ interactions were studied using ab initio post-Hartree–Fock and DFT methodologies. The post-Hartree–Fock methods provide results closer to the experimental data than DFT-based methods. It is possible to highlight the results obtained by the SCS-MP2 and CCSD(T) methods. In the classic [ClAuPH3]2 dimer, the aurophilic interaction is driven by the induction and dispersion terms. When DFT is used, the best results of geometry and interaction energy are obtained with the PW91 level. We find -D3 Grimme correction, M06HF, M06L, M06 M062X, M052X, CAM-B3LYP and LC-ωPBE provided results of similar accuracy as MP2.

Photoabsorption and photodissociation studies of dimethyl sulphoxide (DMSO) in the 35,000–80,000 cm−1 region using synchrotron radiation

Photoabsorption and photodissociation studies of dimethyl sulphoxide and its deuterated isotopologue (DMSO-h6 and DMSO-d6) are performed using synchrotron radiation in the 35,000–80,000cm−1 region. In the photoabsorption spectrum, Rydberg series converging to the first three ionization potentials of DMSO at 9.1, 10.1 and 12.3eV corresponding to removal of an electron from the highest three occupied molecular orbitals (14a′, 7a″ and 13a′) are observed. Based on a quantum defect analysis, Rydberg series assignments are extended to higher members as compared to earlier works and a few ambiguities in earlier assignments are clarified. Analysis is aided by quantum chemical calculations using the DFT and TDDFT methodologies. Vibronic structures observed in the spectrum of DMSO-h6 in the regions 7.7–8.1eV and 8.1–8.8eV are attributed to the transitions 7a″→4p at 7.862eV and 14a′→6s/4d at 8.182eV, respectively. Photoabsorption spectra of DMSO-h6 and -d6 recorded using a broad band incident radiation show prominent peaks, which are identified and assigned to electronic and vibronic transitions of the SO radical. This provides a direct confirmation of the fact that DMSO preferentially dissociates into CH3 and SO upon UV–VUV excitation, as proposed in earlier photodissociation studies. An extended vibronic band system associated with the e1Π–X3Σ− transition of the SO radical is identified and assigned. The complete VUV photoabsorption spectrum of DMSO-d6 is also reported here for the first time.

Molecular engineering of donor–acceptor co-polymers for bulk heterojunction solar cells

We report a computational modeling, based on DFT and TD-DFT methodologies, on the structural, electronic, and optical properties of different donor–acceptor co-polymer system in bulk heterojunction solar cells. The donor moieties that were considered were the derivatives of thienocyclopentathiophene, fluorene, and thienobenzothiophene. We utilized for the acceptor groups the moieties of thieno[3,4-b]pyrrole-4,6-dione; thieno[3,4-b] thiophene-4,6-dione; tetrafluoro-1,3-dihydrothieno[3,4-c]thiophene and its non-fluorinated counterpart; various electron-donating substituents within the fused π-conjugated polymer system; and 3-fluoroselenophenothiophene. Among the donor–acceptor combination, the best in terms of molecular energy levels, energetic driving force, maximum absorption, calculated open-circuit voltage, reorganization energies, ionization potential, and electron affinity are poly[(bisthieno(bisthieno[3,2-b:2′,3′-d]thiophene)- alt-(5-alkyl-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione)] and poly[(4,8-dimethyl[1,3] oxazolo[5,4-f][1,3]benzoxazole)- alt-(thieno[3,4-d]pyrimidine)].

Evaluating the accuracy of density functional theory for calculating 1H and 13C NMR chemical shifts in drug molecules

The accuracy of different DFT methodologies for calculating 1H and 13C NMR chemical shifts in (R)-ispinesib, a complex drug molecule with multiple chemical groups and one stereocentre, has been evaluated. The accuracy of 6 basis sets and 16 different XC functionals was tested. In addition, we present a detailed study on the role of geometry optimisation (in gas and solution phase using a chloroform polarisable continuum model) on the accuracy of the calculated NMR spectra. NMR calculations using the double-ζ basis sets DGDZVP and 6-31++G(d,p) were found to be more accurate (and computationally more efficient) than those using larger triple-ζ basis sets. The O3LYP/DGDZVP methodology in solution phase using a geometry optimised at the same level of theory was found to be the most accurate method with mean absolute errors (MAEs) for 1H and 13C chemical shifts of 0.174ppm and 3.972ppm, respectively. Irrespective of the choice of XC or basis set used, complete geometry optimisation in either gas or solvent phase was found to be essential for attaining the highest accuracy in both 1H and 13C calculated chemical shifts. Finally, the role of molecular conformation was examined by calculating the Boltzmann-weighted 1H and 13C chemical shifts. Overall, we demonstrate that DFT shows exceptional promise for use in calculating the NMR chemical shifts in complex drug molecules. In the future, DFT calculations of NMR parameters are set to play an increasingly important role in drug discovery and chemical optimisation.

First principles account for large changes in electronic structure and bonding from LaCu to LaCuMg and LaCuMg4

Increasing amounts of magnesium in LaCu intermetallic are shown to bring drastic electronic, chemical and mechanical changes. Based on DFT methodologies this is quantified for the cohesive energies, relative charge transfers, bulk modules, electronic structures and bonding properties for the three compounds LaCu, LaCuMg and LaCuMg4. Particularly the cohesive energies increase with Mg amounts. This is illustrated by the increasing number of pair interactions: While in the binary the bonding is ensured by La–Cu, in the ternaries La–Cu, La–Mg and Cu–Mg interactions contribute to the bonding with the extra electrons brought by Mg occupying bonding states. Along the series, increasing magnesium contents lead to moderate increase of the compressibility.

Intermolecular hydroamination of vinylarenes by iminoanilide alkaline-earth catalysts: a computational scrutiny of mechanistic pathways.

A thorough computational exploration of the mechanistic intricacies of the intermolecular hydroamination (HA) of vinylarenes by a recently reported class of kinetically stabilised iminoanilide [{N^N}Ae{N(SiMe3)2}⋅(THF)n] alkaline-earth amido compounds (Ae = Ca, Sr, Ba) is presented. Two distinct mechanistic pathways for catalytic HA mediated by alkaline-earth and rare-earth compounds have emerged over the years that account equally well for the specific features of the process. On one hand, a concerted proton-assisted pathway to deliver the amine product in a single step can be invoked and, on the other, a stepwise σ-insertive pathway that comprises a rapid, reversible migratory olefin insertion step linked to a less facile, irreversible Ae-C alkyl bond aminolysis. The results of the study presented herein, which employed a heavily benchmarked and reliable DFT methodology, supports a stepwise σ-insertive pathway that involves fast and reversible migratory C=C bond insertion into the polar Ae-N pyrrolido σ bond. This proceeds with strict 2,1 regioselectivity via a highly polarised four-centre transition state (TS) structure, linked to irreversible intramolecular Ae-C bond aminolysis of the alkaline-earth alkyl intermediate as the energetically favourable mechanism. Turnover-limiting aminolysis is consistent with the significant KIE measured; the DFT-derived effective barrier matches the Eyring parameter empirically determined for the best-performing {N^N}Ba(NR2) catalyst gratifyingly well. It also predicts the observed trend in reactivity (Ca<Sr<Ba) correctly and the computationally estimated primary KIE is close to the observed values. Non-competitive kinetic demands militate against the operation of the alternative concerted proton-assisted pathway, which describes N-C bond formation triggered by concomitant amino proton transfer at the C=C linkage via a multi-centre TS structure. A detailed comparison of {N^N}Ae(NR2) catalysts revealed that the variation in the Ae-pyrrolido bond strength together with the degree of protection of the alkaline earth by a sterically encumbering iminoanilide ligand scaffold not only profoundly influences the performance in HA catalysis, but also the likelihood of traversing rival mechanistic pathways.

Consecutive losses of two benzyl radicals from the [M + Na]+ adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions

The successive loss of two benzyl radicals from the [M+Na]+ ions of the isomeric dihydroxybenzene dibenzyl ethers (2–4) and of the isomeric trihydroxybenzene tribenzyl ethers (5–7) under ESI/CID conditions has been studied by deuterium labelling, MSn experiments and DFT calculations. The fragmentation of the [M+Na]+ ions of 2 and 4 consists exclusively of the consecutive losses of two C7H7 (benzyl) radicals. This process is largely suppressed in the corresponding [M+Na]+ ions of the meta isomer 3 and also in those of the 1,3,5-analog, phloroglucinol tribenzyl ether (6), suggesting the facile formation of sodiated ortho- and para-quinone ions, [C6H4O2+Na]+, in the cases of 2 and 4, respectively. This finding is corroborated by a detailed investigation of the sodiated tribenzyl ethers of pyrogallol, [5+Na]+, and 1,2,4-trihydroxybenzene, [7+Na]+, and their isotopologs bearing differently labelled benzyl residues. Again, the successive loss of two C7H7 radicals is the only fragmentation channel for ions [7+Na]+ and strongly predominates for ions [5+Na]+, with the primary and secondary losses being highly regiospecific: ions [5+Na]+ lose the first benzyl residue almost exclusively from the central position (O-2) and only then a lateral C7H7 radical (from O-1 or O-3). Surprisingly, the very minor primary loss of a lateral benzyl group is followed by that of the other lateral one, suggesting a two-step isomerization process initiated by a 1,4-H shift. Ions [7+Na]+ lose the first benzyl radical almost exclusively from O-1, in very minor amounts from O-2 but not at all from O-4. The secondary loss of C7H7 subsequent to the major primary loss occurs from both O-2 and O-4 in similar amounts, reflecting the relative stabilities of the sodiated benzyloxy-substituted ortho- and para-benzoquinones. This and several other details of the energy profiles associated with the twofold benzyl loss from ions [5+Na]+ and [7+Na]+ were calculated by use of the DFT methodology and were found to agree very well with the regioselectivities observed. Finally, the pyrogallol-based ions [5+Na]+ were found to eliminate benzaldehyde in minor but significant relative amounts, which takes place with high regioselectivity from O-1 or O-3.

Cluster Approach To Model Titanium Dioxide as Isolated or Organic Dye Sensitized Nanoobjects

This paper proposes the cluster approach methodology to simulate electronic properties of semiconducting isolated nanocrystalline materials as well as functionalized by organic dye molecules. The proposed cluster approach considers the nanoobject construction with the crystal structure in the internal part while the surface is modified according to the environmental interaction. In this aim, the (TiO2)n clusters with n = 2-140, indoline dye molecule D102, and their hybrid composites were investigated. The electronic properties of (TiO2)n were computed thanks to different DFT potentials, considering the nanobject sizes evaluation, their environmental surface modification and saturation, and the interface effects occurring between the cluster and sensitizer. The studies prove that the electronic features of (TiO2)n nanoparticles with surface being altered by the external environment may be coherently computed using DFT methodology with LC-BLYP potential by modifying the long-range separation parameter μ. The values of μ depend on the composition of the investigated system, whereas the surface saturation of the studied clusters possessing suitable size did not have any critical impact on their electronic properties. It is shown that the developed methodology is also relevant to characterize the charge transfer involved in the hybrid forms associating dye molecules and (TiO2)n clusters. The mentioned process is crucial in the efficiency of photovoltaic devices based on the hybrid systems.

Benchmark Assessment of Density Functional Methods on Group II-VI MX (M = Zn, Cd; X = S, Se, Te) Quantum Dots.

In this work, we build a benchmark data set of geometrical parameters, vibrational normal modes, and low-lying excitation energies for MX quantum dots, with M = Cd, Zn, and X = S, Se, Te. The reference database has been constructed by ab initio resolution-of-identity second-order approximate coupled cluster RI-CC2/def2-TZVPP calculations on (MX)6 model molecules in the wurtzite structure. We have tested 26 exchange-correlation density functionals, ranging from local generalized gradient approximation (GGA) and hybrid GGA to meta-GGA, meta-hybrid, and long-range corrected. The best overall functional is the hybrid PBE0 that outperforms all other functionals, especially for excited state energies, which are of particular relevance for the systems studied here. Among the DFT methodologies with no Hartree-Fock exchange, the M06-L is the best one. Local GGA functionals usually provide satisfactory results for geometrical structures and vibrational frequencies but perform rather poorly for excitation energies. Regarding the CdSe cluster, we also present a test of several basis sets that include relativistic effects via effective core potentials (ECPs) or via the ZORA approximation. The best basis sets in terms of computational efficiency and accuracy are the SBKJC and def2-SV(P). The LANL2DZ basis set, commonly employed nowadays on these types of nanoclusters, performs very disappointingly. Finally, we also provide some suggestions on how to perform calculations on larger systems keeping a balance between computational load and accuracy.

Molecular Water Oxidation Mechanisms Followed by Transition Metals: State of the Art

One clean alternative to fossil fuels would be to split water using sunlight. However, to achieve this goal, researchers still need to fully understand and control several key chemical reactions. One of them is the catalytic oxidation of water to molecular oxygen, which also occurs at the oxygen evolving center of photosystem II in green plants and algae. Despite its importance for biology and renewable energy, the mechanism of this reaction is not fully understood. Transition metal water oxidation catalysts in homogeneous media offer a superb platform for researchers to investigate and extract the crucial information to describe the different steps involved in this complex reaction accurately. The mechanistic information extracted at a molecular level allows researchers to understand both the factors that govern this reaction and the ones that derail the system to cause decomposition. As a result, rugged and efficient water oxidation catalysts with potential technological applications can be developed. In this Account, we discuss the current mechanistic understanding of the water oxidation reaction catalyzed by transition metals in the homogeneous phase, based on work developed in our laboratories and complemented by research from other groups. Rather than reviewing all of the catalysts described to date, we focus systematically on the several key elements and their rationale from molecules studied in homogeneous media. We organize these catalysts based on how the crucial oxygen-oxygen bond step takes place, whether via a water nucleophilic attack or via the interaction of two M-O units, rather than based on the nuclearity of the water oxidation catalysts. Furthermore we have used DFT methodology to characterize key intermediates and transition states. The combination of both theory and experiments has allowed us to get a complete view of the water oxidation cycle for the different catalysts studied. Finally, we also describe the various deactivation pathways for these catalysts.