Molecular Electrostatic Potential Approach to Determining the Steric Effect of Phosphine Ligands in Organometallic Chemistry

%VBur index and steric maps: from predictive catalysis to machine learning

Steric indices are parameters used in chemistry to describe the spatial arrangement of atoms or groups of atoms in molecules. They are important in determining the reactivity, stability, and physical properties of chemical compounds. One commonly used steric index is the steric hindrance, which refers to the obstruction or hindrance of movement in a molecule caused by bulky substituents or functional groups. Steric hindrance can affect the reactivity of a molecule by altering the accessibility of its reactive sites and influencing the geometry of its transition states. Notably, the Tolman cone angle and %VBur are prominent among these indices. Actually, steric effects can also be described using the concept of steric bulk, which refers to the space occupied by a molecule or functional group. Steric bulk can affect the solubility, melting point, boiling point, and viscosity of a substance. Even though electronic indices are more widely used, they have certain drawbacks that might shift preferences towards others. They present a higher computational cost, and often, the weight of electronics in correlation with chemical properties, e.g. binding energies, falls short in comparison to %VBur. However, it is worth noting that this may be because the steric index inherently captures part of the electronic content. Overall, steric indices play an important role in understanding the behaviour of chemical compounds and can be used to predict their reactivity, stability, and physical properties. Predictive chemistry is an approach to chemical research that uses computational methods to anticipate the properties and behaviour of these compounds and reactions, facilitating the design of new compounds and reactivities. Within this domain, predictive catalysis specifically targets the prediction of the performance and behaviour of catalysts. Ultimately, the goal is to identify new catalysts with optimal properties, leading to chemical processes that are both more efficient and sustainable. In this framework, %VBur can be a key metric for deepening our understanding of catalysis, emphasizing predictive catalysis and sustainability. Those latter concepts are needed to direct our efforts toward identifying the optimal catalyst for any reaction, minimizing waste, and reducing experimental efforts while maximizing the efficacy of the computational methods.

Comprehensive Structural and Electronic Properties of 2-Azaadamantane N-Oxyl Derivatives Correlated with Their Catalytic Ability

Herein, a comprehensive kinetic study is performed to compare the catalytic efficiency of 2-azaadamantane N-oxyl (AZADO) derivatives with that of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) used as radical catalysts in the aerobic oxidation of l-menthol. Furthermore, the correlation between the catalytic activity and structural/electronic parameters of AZADOs and TEMPO is elucidated. The reaction rate constants achieved with several AZADO derivatives exhibit moderate relationships with spectroscopic parameters, such as the hyperfine coupling constant of the N atom (AN) and NO stretching vibration frequency (νNO) observed in electron spin resonance and infrared spectra, respectively. The planarity C-(NO)-C angle (φ) at the N atom, determined by density functional theory (DFT) calculations, also strongly correlates with the AN and νNO. Moreover, the bond order of NO, which strongly depends on the structural and electronic properties of NO radicals, correlates with radical activity; thus, the radical activity can be predicted by DFT calculations, thereby accelerating the synthesis of new AZADO derivatives without requiring alcohol oxidation experiments.

Tris(aminomethyl)phosphines and Their Copper(I) (Pseudo)halide Complexes with Aromatic Diimines-A Critical Retrospection

Metal complexes feature a wide range of available geometries, diversified lability, controllable hydrolytic stability, and easily available rich redox activity. These characteristics, combined with the specific properties of coordinated organic molecules, result in many different mechanisms of biological action, making each of the myriads of the classes of metal coordination compounds unique. This focused review presents combined and systematized results of the studies of a group of copper(I) (pseudo)halide complexes with aromatic diimines and tris(aminomethyl)phosphines of a general formula [CuX(NN)PR₃], where X = I^- or NCS^-, NN = 2,2'-bipyridyl, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline or 2,2'-biquinoline, and PR₃ = air-stable tris(aminomethyl)phosphines. The structural and electronic properties of the phosphine ligands and luminescent complexes are discussed. The complexes with 2,9-dimethyl-1,10-phenanthroline, apart from being air- and water-stable, exhibit a very high in vitro antimicrobial activity against the Staphylococcus aureus and Candida albicans. Moreover, some of these complexes also show a strong in vitro antitumor activity against human ovarian carcinoma cell lines: MDAH 2774 and SCOV 3, CT26 (mouse colon carcinoma), and A549 (human lung adenocarcinoma) cell lines. The tested complexes are moderately able to induce DNA lesions through free radical processes, however the trends do not reflect observed differences in biological activity.

A Comprehensive Discovery Platform for Organophosphorus Ligands for Catalysis

The design of molecular catalysts typically involves reconciling multiple conflicting property requirements, largely relying on human intuition and local structural searches. However, the vast number of potential catalysts requires pruning of the candidate space by efficient property prediction with quantitative structure-property relationships. Data-driven workflows embedded in a library of potential catalysts can be used to build predictive models for catalyst performance and serve as a blueprint for novel catalyst designs. Herein we introduce kraken, a discovery platform covering monodentate organophosphorus(III) ligands providing comprehensive physicochemical descriptors based on representative conformer ensembles. Using quantum-mechanical methods, we calculated descriptors for 1558 ligands, including commercially available examples, and trained machine learning models to predict properties of over 300000 new ligands. We demonstrate the application of kraken to systematically explore the property space of organophosphorus ligands and how existing data sets in catalysis can be used to accelerate ligand selection during reaction optimization.

Tuning the donating strength of dye sensitizers using molecular electrostatic potential analysis

The significance of electron releasing groups at the donors of D–π–A systems for improving the donating strength and power conversion efficiency of photosensitizers.

Density functional theory study on the donating strength of donor systems in dye-sensitized solar cells

Donating strength of typically used donors in dye-sensitized solar cells.

Insight into 6-aminopenicillanic acid structure and study of the quantum mechanical calculations of the acid–base site on γ-Fe2O3@SiO2 core–shell nanocomposites and as efficient catalysts in multicomponent reactions

Magnetic 6-APA/γ-Fe₂O₃@Sio₂ nanocomposites have been developed by exploiting the potential of the acid–base bifunctional system to study the quantum mechanistic calculations.

Substituent Effect Transmission Power of Alkyl, Alkenyl, Alkynyl, Phenyl, Thiophenyl, and Polyacene Spacers

The transmission of substituent effect through a variety of spacers, that is to say, alkyl, alkenyl, alkynyl, phenyl, thiophenyl, and polyacene has been studied by modeling Y-G-X type molecular systems (Y: reaction center; G: spacer moiety; X: substituent) using B3LYP/6-31G(d,p) density functional theory calculations. The reaction center is always kept as a C=C double bond and the molecular electrostatic potential (MESP) minimum (V_min ) observed for this bond showed subtle variation with respect to the changes in the spacer unit and the nature of substituent. Strong linear correlations are observed between Hammett substituent constants (σ_I and σ_p ) and V_min , which recommend the aptness of V_min as an electronic descriptor to quantify the substituent effect. Since V_min offers an alternative measure of substituent effect, the correlation between V_min and σ_p has been used for assessing the transmission of substituent effect through a variety of spacer moieties. The highest transmission coefficient (γ) is always observed for smaller spacer length. Among all the spacers, alkenyl showed the highest and alkyl showed the lowest transmission power. The study recommends the use of short chains of C=C double, C≡C triple or a combination of both as spacers for the effective transmission of substituent effect to the reaction center.

Computational Ligand Descriptors for Catalyst Design

Ligands, especially phosphines and carbenes, can play a key role in modifying and controlling homogeneous organometallic catalysts, and they often provide a convenient approach to fine-tuning the performance of known catalysts. The measurable outcomes of such catalyst modifications (yields, rates, selectivity) can be set into context by establishing their relationship to steric and electronic descriptors of ligand properties, and such models can guide the discovery, optimization, and design of catalysts. In this review we present a survey of calculated ligand descriptors, with a particular focus on homogeneous organometallic catalysis. A range of different approaches to calculating steric and electronic parameters are set out and compared, and we have collected descriptors for a range of representative ligand sets, including 30 monodentate phosphorus(III) donor ligands, 23 bidentate P,P-donor ligands, and 30 carbenes, with a view to providing a useful resource for analysis to practitioners. In addition, several case studies of applications of such descriptors, covering both maps and models, have been reviewed, illustrating how descriptor-led studies of catalysis can inform experiments and highlighting good practice for model comparison and evaluation.

Role of sterics in phosphine-ligated gold clusters

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh₃) ligands on Au₈L₇²⁺ (L = PPh₃) gold clusters with three different tolyl ligands using electrospray ionization mass spectrometry to provide insight into how steric differences in the phosphines influence the extent of ligand exchange and the stability of the resulting mixed-phosphine clusters.

Quantification and classification of substituent effects in organic chemistry: a theoretical molecular electrostatic potential study

Substituent effects in organic chemistry are generally described in terms of experimentally derived Hammett parameters whereas a convenient theoretical tool to study these effects in π-conjugated molecular systems is molecular electrostatic potential (MESP) analysis. The present study shows that the difference between MESP at the nucleus of the para carbon of substituted benzene and a carbon atom in benzene, designated as ΔVC, is very useful to quantify and classify substituent effects. On the basis of positive and negative ΔVC values, a broad classification of around 381 substituents into electron withdrawing and donating categories is made. Each category is again sorted based on the magnitude of ΔVC into subcategories such as very strong, strong, medium, and weak electron donating/withdrawing. Furthermore, the data are used to show the transferability and additivity of substituent effects in π-conjugated organic molecules such as condensed aromatic, olefinic, acetylenic, and heterocyclic systems. The transferability properties hold good for ΔVC in all these molecular systems. The additive properties of substituent effects are strongly reflected on ΔVC and the predictive power of the data to assign the total substituent effects of multi-substituted systems is verified. The ΔVC data and the present classification of substituents are very useful to design π-conjugated organic molecular systems with desired electron rich/poor character.

The effect of σ/π, σ and π donors on the basicity of silylene superbases: a density functional theory study

In this study, we examined computationally that the combination of π-and σ-donor substituents results in higher basicity of silylene compounds compared to the usage of π-donor substituents alone.

Copper(i)-catalysed regio- and diastereoselective intramolecular alkylboration of terminal allenes via allylcopper(i) isomerization

We report the first copper(i)-catalysed intramolecular alkylboration of terminal allenes with an alkyl halide moiety.

The π–π stacking of tanshinone I and isotanshinone I with phenylalanine: The effects of isomerization, complexation and environment

The misfolding and aggregation of amyloid-[Formula: see text] (A[Formula: see text] peptides into amyloid fibrils is regarded as one of the possible causes of Alzheimer’s disease (AD). Aromatic interactions (i.e. [Formula: see text]–[Formula: see text] interactions) between tanshinone drugs extracted from Chinese herb Salvia Miltiorrhiza (SM) and aromatic residues of A[Formula: see text] peptides have been shown to prevent further growth of amyloid aggregates. In this work, the effects of isomerization, complexation and polarity of environment on the strength of [Formula: see text]–[Formula: see text] stacking interactions of tanshinone I (TS1) and isotanshinone I (IS1), as the two diterpenoid quinones in the SM herb, and their complexes with Mg[Formula: see text] cation, IS1-Mg[Formula: see text] and TS1-Mg[Formula: see text], with phenylalanine (PHE), as an aromatic amino acid in A[Formula: see text] structure have been investigated using the quantum mechanical calculations in the gas-phase, ether, acetone, DMSO, and water solvent. Molecular electrostatic potential (MEP), which are used to predict the nucleophile active sites, electron densities calculated at the bond critical points ([Formula: see text] and ring critical points ([Formula: see text] by the atoms in molecules (AIM) method and the donor–acceptor interaction energies ([Formula: see text] calculated using the natural bond orbital (NBO) method were used to investigate the interplay between [Formula: see text]–[Formula: see text] stacking and complexation. The results show that the IS1/TS1-Mg[Formula: see text] compounds have a more protective role compared to TS1/IS1-Mg[Formula: see text] compounds due to the stronger interaction with PHE of A[Formula: see text] in antiaggregation for further development of A[Formula: see text] inhibitors to prevent and disaggregate amyloid formation. Complexation with Mg[Formula: see text] increases the interaction diterpenoid drugs with PHE and makes notable changes in structural and electronic properties of diterpenoids. Also, the interactions between diterpenoid and PHE in less polar environment are more than other environments. Low-polarity environments have the best mimics of the A[Formula: see text] binding site.

Electrostatics for probing lone pairs and their interactions

The value of the molecular electrostatic potential minimum (V_min ) and its topographical features (position, as well as the eigenvalues and eigenvectors of the corresponding Hessian matrix) are recently proposed as the criteria for characterizing a lone pair (Kumar A. et al., J. Phys. Chem. 2014, A118, 526). This electrostatic characterization of lone pairs is examined for a large number of small molecules employing MP4/6-311++G(d,p)//MP2/6-311++G(d,p) theory. The eigenvector of the Hessian matrix corresponding to its largest eigenvalue (λ_max ), is found to be directed toward the lone pair-bearing-atom, with λ_max showing a strong linear correlation with V_min . Large magnitudes of V_min and λ_max indicate a charge-dense lone pair. The topographical features of V_min are seen to provide insights into the interactive behavior of the molecules with model electrophiles, viz. HF, CO₂ , and Li⁺ . In all the complexes of HF and majority of the other complexes, the interaction energy (E_int ) correlates well with the respective V_min, value, but for some deviations occurring due to other competing secondary interactions. The electrostatic interactions are found to be highly directional in nature as the orientation of interacting atom correlates strongly to the position of lone pair. In summary, the present study on a large number of test molecules shows that electrostatics is able to probe lone pairs in molecules and offers a simple interpretation of chemical reactivity. © 2017 Wiley Periodicals, Inc.

A theoretical study of π-stacking interactions in C-substituted tetrazoles

The π-stacking effects of benzene ring (Ben) with 1H- and 2H-tetrazole derivatives (1H-TZ-X and 2H-TZ-X) substituted at C5 (where X is Cl, COH, NO, NO2, CN, NH2, OH, OCH3, SH and H) has been investigated by the quantum mechanical calculations at the M06-2X/6-311++G** level. The results indicate the 1H-TZ-X||Ben complexes (|| donates π-stacking interaction) are more stable than 2H-TZ-X||Ben while in unstacked forms, 1H-TZ-X is less stable than 2H-TZ-X. All substituents enhance the π-stacking interaction relative to the unsubstituted ones and enhancement is higher for the electron-withdrawing substituents (EWSs). Also, investigation of the local and direct effect of substituents in stacking interaction showed that all substituents regardless of whether are electron donating or electron withdrawing have an additive effect in π-stacking interaction. Excellent correlations were found between the binding energies of the complexes and combination of substituent constant terms. The results showed that the electrostatic interaction alone is not responsible for stacking stabilization but charge penetration is important. Furthermore, analysis of aromaticity, AIM, ESP and NPA were investigated to obtain aromaticity index, non-bonding interactions, chemical reactivity and polarity (dipole moment), respectively.

Understanding ligand effects in gold clusters using mass spectrometry

This review summarizes recent research on the influence of phosphine ligands on the size, stability, and reactivity of gold clusters synthesized in solution. Sub-nanometer clusters exhibit size- and composition-dependent properties that are unique from those of larger nanoparticles. The highly tunable properties of clusters and their high surface-to-volume ratio make them promising candidates for a variety of technological applications. However, because "each-atom-counts" toward defining cluster properties it is critically important to develop robust synthesis methods to efficiently prepare clusters of predetermined size. For decades phosphines have been known to direct the size-selected synthesis of gold clusters. Despite the preparation of numerous species it is still not understood how different functional groups at phosphine centers affect the size and properties of gold clusters. Using electrospray ionization mass spectrometry (ESI-MS) it is possible to characterize the effect of ligand substitution on the distribution of clusters formed in solution at defined reaction conditions. In addition, ligand exchange reactions on preformed clusters may be monitored using ESI-MS. Collision induced dissociation (CID) may also be employed to obtain qualitative insight into the fragmentation of mixed ligand clusters and the relative binding energies of differently substituted phosphines. Quantitative ligand binding energies and cluster stability may be determined employing surface induced dissociation (SID) in a custom-built Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Rice-Ramsperger-Kassel-Marcus (RRKM) based modeling of the SID data allows dissociation energies and entropy values to be extracted. The charge reduction and reactivity of atomically precise gold clusters, including partially ligated species generated in the gas-phase by in source CID, on well-defined surfaces may be explored using ion soft landing (SL) in a custom-built instrument combined with in situ time of flight secondary ion mass spectrometry (TOF-SIMS). Jointly, this multipronged experimental approach allows characterization of the full spectrum of relevant phenomena including cluster synthesis, ligand exchange, thermochemistry, surface immobilization, and reactivity. The fundamental insights obtained from this work will facilitate the directed synthesis of gold clusters with predetermined size and properties for specific applications.

Designing metal hydride complexes for water splitting reactions: a molecular electrostatic potential approach

The hydridic character of octahedral metal hydride complexes of groups VI, VII and VIII has been systematically studied using molecular electrostatic potential (MESP) topography. The absolute minimum of MESP at the hydride ligand (Vmin) and the MESP value at the hydride nucleus (VH) are found to be very good measures of the hydridic character of the hydride ligand. The increasing/decreasing electron donating feature of the ligand environment is clearly reflected in the increasing/decreasing negative character of Vmin and VH. The formation of an outer sphere metal hydride-water complex showing the HH dihydrogen interaction is supported by the location and the value of Vmin near the hydride ligand. A higher negative MESP suggested lower activation energy for H2 elimination. Thus, MESP features provided a way to fine-tune the ligand environment of a metal-hydride complex to achieve high hydridicity for the hydride ligand. The applicability of an MESP based hydridic descriptor in designing water splitting reactions is tested for group VI metal hydride model complexes of tungsten.

Water-Soluble Phosphine Capable of Dissolving Elemental Gold: The Missing Link between 1,3,5-Triaza-7-phosphaadamantane (PTA) and Verkade's Ephemeral Ligand

We herein describe a tricyclic phosphine with previously unreported tris(homoadamantane) cage architecture. That water-soluble, air- and thermally stable ligand, 1,4,7-triaza-9-phosphatricyclo[5.3.2.1(4,9)]tridecane (hereinafter referred to as CAP) exhibits unusual chemical behavior toward gold and gold compounds: it readily reduces Au(III) to Au(0), promotes oxidative dissolution of nanocrystalline gold(0) with the formation of water-soluble trigonal CAP-Au(I) complexes, and displaces cyanide from [Au(CN)2](-) affording triangular [Au(CAP)3](+) cation. From the stereochemical point of view, CAP can be regarded as an intermediate between 1,3,5-triaza-7-phosphaadamantane (PTA) and very unstable aminophosphine synthesized by Verkade's group: hexahydro-2a,4a,6a-triaza-6b-phosphacyclopenta[cd]pentalene. The chemical properties of CAP are likely related to its anomalous stereoelectronic profile: combination of strong electron-donating power (Tolman's electronic parameter 2056.8 cm(-1)) with the low steric demand (cone angle of 109°). CAP can be considered as macrocyclic counterpart of PTA with the electron-donating power approaching that of strongest known phosphine electron donors such as P(t-Bu)3 and PCy3. Therefore, CAP as sterically undemanding and electron-rich ligand populates the empty field on the stereoelectronic map of phosphine ligands: the niche between the classic tertiary phosphines and the sterically undemanding aminophosphines.

Cationic gold clusters ligated with differently substituted phosphines: effect of substitution on ligand reactivity and binding

Loss of substituted phosphine ligands is strongly correlated with the electron donating ability of the phosphorous lone pair. The results indicate that the relative ligand binding energies increase in the order PMe₃ < PPhMe₂ < PPh₂Me < PPh₃ < PPh₂Cy < PPhCy₂ < PCy₃.

A theoretical investigation of substituent effects on the stability and reactivity of N-heterocyclic olefin carboxylates

Introduction of four phenyl groups at C-position and N-position not only favors decarboxylation but also ensures NHO as a strong nucleophile.

Molecular electrostatics for probing lone pair-π interactions

An electrostatics-based approach has been proposed for probing the weak interactions between lone pair containing molecules and π deficient molecular systems. For electron-rich molecules, the negative minima in molecular electrostatic potential (MESP) topography give the location of electron localization and the MESP value at the minimum (Vmin) quantifies the electron-rich character of that region. Interactive behavior of a lone pair bearing molecule with electron deficient π-systems, such as hexafluorobenzene, 1,3,5-trinitrobenzene, 2,4,6-trifluoro-1,3,5-triazine and 1,2,4,5-tetracyanobenzene explored within DFT brings out good correlation of the lone pair-π interaction energy (E(int)) with the Vmin value of the electron-rich system. Such interaction is found to be portrayed well with the Electrostatic Potential for Intermolecular Complexation (EPIC) model. On the basis of the precise location of MESP minimum, a prediction for the orientation of a lone pair bearing molecule with an electron deficient π-system is possible in the majority of the cases studied.

A Green Approach to the Production of Hybrid Diindolylmethane-Phenylboronic Acids via a 3MCR: Promising Antineoplasic Molecules

The current role of the “Green Chemistry Protocol” in multicomponent reactions is first highlighted. Then, the green approach to the production of three novel hybrid diindolylmethanes-phenylboronic acids via a 3MCR is discussed, which features the following: solventless conditions, the use of microwave irradiation to activate the reactions, the absence of catalyst, and an efficient atom economy. The products were achieved with moderate yields (41–61%) within a short time frame (5 min) and appropriately characterized by elemental analysis and spectroscopic methods (NMR:1H,13C,11B; MS: EI, CI, FAB+, HRMS). During the FAB+MS determinations, various artifacts that are associated with the boron atom via an interaction with thioglycerol from the matrix were noted in the corresponding FAB+mass spectra; in addition, the accurate mass determination of these adducts unequivocally confirmed the presence of the target molecules. Moreover, the activity of these target molecules was evaluated in the presence of six cancer cell lines (U251 = glia of the central nerve system, PC-3 = prostate, K562 = leukemia, HCT-15 = colon, MCF-7 = breast, and SKLU-1 = lung), which resulted in themeta-regioisomer being the most active. Finally, the products were also analyzed using computational chemistry in order to determine their most stable geometries and reactivities by computing the respective molecular electrostatic potentials.

Accurate prediction of cation-π interaction energy using substituent effects

Substituent effects on cation-π interactions have been quantified using a variety of Φ-X···M(+) complexes where Φ, X, and M(+) are the π-system, substituent, and cation, respectively. The cation-π interaction energy, E(M(+)), showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, ΔV(min) (the difference between the MESP minimum (V(min)) on the π-region of a substituted system and the corresponding unsubstituted system). This linear relationship is E(M(+)) = C(M(+))(ΔV(min)) + E(M(+))' where C(M(+)) is the reaction constant and E(M(+))' is the cation-π interaction energy of the unsubstituted complex. This relationship is similar to the Hammett equation and its first term yields the substituent contribution of the cation-π interaction energy. Further, a linear correlation between C(M(+))() and E(M(+))()' has been established, which facilitates the prediction of C(M(+)) for unknown cations. Thus, a prediction of E(M(+)) for any Φ-X···M(+) complex is achieved by knowing the values of E(M(+))' and ΔV(min). The generality of the equation is tested for a variety of cations (Li(+), Na(+), K(+), Mg(+), BeCl(+), MgCl(+), CaCl(+), TiCl(3)(+), CrCl(2)(+), NiCl(+), Cu(+), ZnCl(+), NH(4)(+), CH(3)NH(3)(+), N(CH(3))(4)(+), C(NH(2))(3)(+)), substituents (N(CH(3))(2), NH(2), OCH(3), CH(3), OH, H, SCH(3), SH, CCH, F, Cl, COOH, CHO, CF(3), CN, NO(2)), and a large number of π-systems. The tested systems also include multiple substituted π-systems, viz. ethylene, acetylene, hexa-1,3,5-triene, benzene, naphthalene, indole, pyrrole, phenylalanine, tryptophan, tyrosine, azulene, pyrene, [6]-cyclacene, and corannulene and found that E(M)(+) follows the additivity of substituent effects. Further, the substituent effects on cationic sandwich complexes of the type C(6)H(6)···M(+)···C(6)H(5)X have been assessed and found that E(M(+)) can be predicted with 97.7% accuracy using the values of E(M(+))' and ΔV(min). All the Φ-X···M(+) systems showed good agreement between the calculated and predicted E(M(+))() values, suggesting that the ΔV(min) approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.