Experimental and Theoretical Insights on the Structural, Electronic, and Magnetic Properties of the Quaternary Selenides EuPrCuSe3 and EuNdCuSe3.

Magnetic semiconductors EuPrCuSe3 and EuNdCuSe3 were obtained by using the halide flux method. Their crystal structures and magnetic properties were studied and discussed. Optical properties of the obtained selenides were studied by the means of diffuse reflectance spectroscopy, which revealed the values of 1.92/1.97 and 0.90/0.94 eV for the direct and indirect band gaps of Ln = Nd/Pr, respectively. The structural, electronic, and magnetic properties of the obtained compounds were additionally studied with spin-polarized density functional theory calculations, wherein both systems were found to be two new examples of semiconducting quaternary selenides with disperse conduction bands of Nd/Pr 5d character. The modeling showed that various magnetic orderings in the systems have subtle influences on the alignments/overlaps between the Se/Cu, Eu, and Pr/Nd bands, and that the spin-state energetics are very dependent upon the treatment of electron correlation, but a distinguishing feature in the case of ferromagnetic coupling is that the spin density on the Se atoms is maximized. Overall, the calculations are in good agreement with the experimental characterization of ferromagnetism in the bulk crystals, wherein the ferromagnetic transition occurs at temperatures of about 2.5 K for EuPrCuSe3 and about 3 K for EuNdCuSe3.

Genuine quadruple bonds between two main-group atoms. Chemical bonding in AeF- (Ae = Be-Ba) and isoelectronic EF (E = B-Tl) and the particular role of d orbitals in covalent interactions of heavier alkaline-earth atoms.

Quantum chemical calculations of anions AeF- (Ae = Be-Ba) and isoelectronic group-13 molecules EF (E = B-Tl) have been carried out using ab initio methods at the CCSD(T)/def2-TZVPP level and density functional theory employing BP86 various basis sets. The equilibrium distances, bond dissociation energies and vibrational frequencies are reported. The alkali earth fluoride anions AeF- exhibit strong bonds between the closed-shell species Ae and F- with bond dissociation energies between 68.8 kcal mol-1 for MgF- and 87.5 kcal mol-1 for BeF- and they show an unusual increasing trend MgF- < CaF- < SrF- < BaF-. This is in contrast to the isoelectronic group-13 fluorides EF where the BDE continuously decreases from BF to TlF. The calculated dipole moments of AeF- are very large between 5.97 D for BeF- and 1.78 D for BaF- with the negative end always at the Ae atom (Ae→F-). This is explained by the location of the electronic charge of the lone pair at Ae, which is rather distant from the nucleus. The analysis of the electronic structure of AeF- suggests significant charge donation Ae←F- into the vacant valence orbitals of Ae. A bonding analysis with the EDA-NOCV method suggests that the molecules are mainly covalently bonded. The strongest orbital interaction in the anions comes from the inductive polarization of the 2pσ electrons of F-, which leads to a hybridization of the (n)s and (n)pσ AOs at Ae. There are two degenerate π donor interactions Ae←F- in all anions AeF-, which provide 25-30% to the covalent bonding. There is another σ orbital interaction in the anions, which is very weak in BeF- and MgF-. In contrast, the second stabilizing σ orbital interaction in CaF-, SrF- and BaF- yields a strongly stabilizing σ orbital, because the Ae atoms use their (n - 1)dσ AOs for bonding. The energy lowering of the second σ interaction in the latter anions is even stronger than the π bonding. The EDA-NOCV results suggest that BeF- and MgF- have three strongly polarized bonds, whereas CaF-, SrF- and BaF- have four bonding orbitals. The quadruple bonds in the heavier alkaline earth species are made possible because they use s/d valence orbitals like transition metals for covalent bonding. The EDA-NOCV analysis of the group-13 fluorides EF gives a conventional picture with one very strong σ bond and two rather weak π interactions.

Nature of Beryllium, Magnesium, and Zinc Bonds in Carbene⋯MX2 (M = Be, Mg, Zn; X = H, Br) Dimers Revealed by the IQA, ETS-NOCV and LED Methods.

The nature of beryllium−, magnesium− and zinc−carbene bonds in the cyclopropenylidene⋯MX2 (M = Be, Mg, Zn; X = H, Br) and imidazol-2-ylidene⋯MBr2 dimers is investigated by the joint use of the topological QTAIM-based IQA decomposition scheme, the molecular orbital-based ETS-NOCV charge and energy decomposition method, and the LED energy decomposition approach based on the state-of-the-art DLPNO-CCSD(T) method. All these methods show that the C⋯M bond strengthens according to the following order: Zn < Mg << Be. Electrostatics is proved to be the dominant bond component, whereas the orbital component is far less important. It is shown that QTAIM/IQA underestimates electrostatic contribution for zinc bonds with respect to both ETS-NOCV and LED schemes. The σ carbene→MX2 donation appears to be much more important than the MX2→ carbene back-donation of π symmetry. The substitution of hydrogen atoms by bromine (X in MX2) strengthens the metal−carbene bond in all cases. The physical origin of rotational barriers has been unveiled by the ETS-NOCV approach.

Correction: Mahmoudi et al. Solvent-Induced Formation of Novel Ni(II) Complexes Derived from Bis-Thiosemicarbazone Ligand: An Insight from Experimental and Theoretical Investigations. Int. J. Mol. Sci. 2021, 22, 5337.

We received a complaint from the Université Catholique de Louvain [...].

Clarifying notes on the bonding analysis adopted by the energy decomposition analysis.

We discuss the fundamental aspects of the EDA-NOCV method and address some critical comments that have been made recently. The EDA-NOCV method unlike most other methods focuses on the process of bond formation between the interacting species and not just only on the analysis of the finally formed bond. This is demonstrated using LiF as an example. There is a difference between the interactions between the initial species which form the bond and are also the final product of bond cleavage, and the interactions between the fragments in the eventually formed molecule. The flexibility of the method allows the choice of the interacting fragments which helps to identify the charge and electron configuration of the fragments which describe the bond. This is very helpful in cases where the bond may be described with several Lewis structures. We reject the idea that it would be a disadvantage to have "bond path functions" as the energy components in the EDA, which actually indicate the variability of the method. The bonding analysis in a different sequence of the bond formation gives important results for the various questions that can be asked. This is demonstrated by using CH2, CO2 and the formation of a guanine quartet as examples. The fact that a bond is always defined by the bound molecule, the fragments, and their states is universal and deeply physical, as we show here again for various examples. The results of the EDA-NOCV method are in full accordance with the physical mechanism of the chemical bond as revealed by Ruedenberg.

Differentiation of isomeric metabolites of carbamazepine based on acid-base properties; Experimental vs theoretical approach.

Metabolism of carbamazepine is complex and leads to the three isomeric derivatives whose occurrence is dependent on the type of sample material. Their unambiguous differentiation is overall important. In this work, the qualitative analysis of 2-hydroxycarbamazepine, 3-hydroxycrbamazepine and carbamazepine-10,11-epoxide was attempted for the first time using capillary zone electrophoresis, based on the models linking electrophoretic mobility with pKa value determining the acidity of the hydroxyl groups. For this purpose, pKa values ​​were determined using electrophoretic and theoretical methods, and then the compliance of the obtained mobility models with the measured values ​​was analyzed. Despite the slight difference in acidity ​​(0.3-0.4 pH unit), the obtained results prove that the correct identification of the metabolites under consideration, and reliable prediction of the selectivity of their separation, are possible on the basis of experimentally determined pKa values, even with highly simplified methods assuming the lack of certain data. However, it is important to choose the optimal pH value, which should be close to pKa. On the other hand, worse results were obtained for the theoretically determined mobilities, which differed significantly from the experimental values. An attempt was also made to explain the acidity of hydroxycarbamazepines and the associated thermodynamic parameters - deprotonation enthalpy and entropy, with respect to their structure. The lack of intramolecular hydrogen bonds and the significant contribution of entropic effects stabilizing the protonated form seems to be significant. The higher pKa value for CBZ-2-OH probably results from the stronger effect of the energetically unfavorable organization of solvent dipoles due to ionization.

Solvent-Induced Formation of Novel Ni(II) Complexes Derived from Bis-Thiosemicarbazone Ligand: An Insight from Experimental and Theoretical Investigations.

In this work, we report solvent-induced complexation properties of a new N2S2 tetradentate bis-thiosemicarbazone ligand (H2LI), prepared by the condensation of 4-phenylthiosemicarbazide with bis-aldehyde, namely 2,2'-(ethane-1,2-diylbis(oxy)dibenzaldehyde, towards nickel(II). Using ethanol as a reaction medium allowed the isolation of a discrete mononuclear homoleptic complex [NiLI] (1), for which its crystal structure contains three independent molecules, namely 1-I, 1-II, and 1-III, in the asymmetric unit. The doubly deprotonated ligand LI in the structure of 1 is coordinated in a cis-manner through the azomethine nitrogen atoms and the thiocarbonyl sulfur atoms. The coordination geometry around metal centers in all the three crystallographically independent molecules of 1 is best described as the seesaw structure. Interestingly, using methanol as a reaction medium in the same synthesis allowed for the isolation of a discrete mononuclear homoleptic complex [Ni(LII)2] (2), where LII is a monodeprotonated ligand 2-(2-(2-(2-(dimethoxymethyl)phenoxy)ethoxy)benzylidene)-N-phenylhydrazine-1-carbothioamide (HLII). The ligand LII was formed in situ from the reaction of LI with methanol upon coordination to the metal center under synthetic conditions. In the structure of 2, two ligands LII are coordinated in a trans-manner through the azomethine nitrogen atom and the thiocarbonyl sulfur atom, also yielding a seesaw coordination geometry around the metal center. The charge and energy decomposition scheme ETS-NOCV allows for the conclusion that both structures are stabilized by a bunch of London dispersion-driven intermolecular interactions, including predominantly N-H∙∙∙S and N-H∙∙∙O hydrogen bonds in 1 and 2, respectively; they are further augmented by less typical C-H∙∙∙X (where X = S, N, O, π), CH∙∙∙HC, π∙∙∙π stacking and the most striking, attractive long-range intermolecular C-H∙∙∙Ni preagostic interactions. The latter are found to be determined by both stabilizing Coulomb forces and an exchange-correlation contribution as revealed by the IQA energy decomposition scheme. Interestingly, the analogous long-range C-H∙∙∙S interactions are characterized by a repulsive Coulomb contribution and the prevailing attractive exchange-correlation constituent. The electron density of the delocalized bonds (EDDB) method shows that the nickel(II) atom shares only ~0.8|e| due to the σ-conjugation with the adjacent in-plane atoms, demonstrating a very weak σ-metalloaromatic character.

Supramolecular structures of NiII and CuII with the sterically demanding Schiff base dyes driven by cooperative action of preagostic and other non-covalent interactions.

This work reports on synthesis and extensive experimental and theoretical investigations on photophysical, structural and thermal properties of the NiII and CuII discrete mononuclear homoleptic complexes [Ni(L I,II)2] and [Cu(L I,II)2] fabricated from the Schiff base dyes o-HOC6H4-CH=N-cyclo-C6H11 (HL I) and o-HOC10H6-CH=N-cyclo-C6H11 (HL II), containing the sterically crowding cyclo-hexyl units. The six-membered metallocycles adopt a clearly defined envelope conformation in [Ni(L II)2], while they are much more planar in the structures of [Ni(L I)2] and [Cu(L I,II)2]. It has been demonstrated by in-depth bonding analyses based on the ETS-NOCV and Interacting Quantum Atoms energy-decomposition schemes that application of the bulky substituents, containing several C-H groups, has led to the formation of a set of classical and unintuitive intra- and inter-molecular interactions. All together they are responsible for the high stability of [Ni(L I,II)2] and [Cu(L I,II)2]. More specifically, London dispersion dominated intramolecular C-H⋯O, C-H⋯N and C-H⋯H-C hydrogen bonds are recognized and, importantly, the attractive, chiefly the Coulomb driven, preagostic (not repulsive anagostic) C-H⋯Ni/Cu interactions have been discovered despite their relatively long distances (∼2.8-3.1 Å). All the complexes are further stabilized by the extremely efficient intermolecular C-H⋯π(benzene) and C-H⋯π(chelate) interactions, where both the charge-delocalization and London dispersion constituents appear to be crucial for the crystal packing of the obtained complexes. All the complexes were found to be photoluminescent in CH2Cl2, with [Cu(L II)2] exhibiting the most pronounced emission - the time-dependent density-functional-theory computations revealed that it is mostly caused by metal-to-ligand charge-transfer transitions.

Zeolites at the Molecular Level: What Can Be Learned from Molecular Modeling.

This review puts the development of molecular modeling methods in the context of their applications to zeolitic active sites. We attempt to highlight the utmost necessity of close cooperation between theory and experiment, resulting both in advances in computational methods and in progress in experimental techniques.

Lead(ii) coordination polymers driven by pyridine-hydrazine donors: from anion-guided self-assembly to structural features.

In this work, we report extensive experimental and theoretical investigations on a new series of PbII coordination polymers exhibiting extended supramolecular architectures, namely [Pb2(LI)(NCS)4]n (1), [Pb(HLII)I2]n (2), [Pb(LIII)I]n (3) and [Pb(HLIV)(NO3)2]n·nMeOH (4), which were self-assembled from different PbII salts and various pyridine-hydrazine based linkers, namely 1,2-bis(pyridin-3-ylmethylene)hydrazine (LI), (pyridin-4-ylmethylene)isonicotinohydrazide (HLII), 1-(pyridin-2-yl)ethylidenenicotinohydrazide (HLIII) and phenyl(pyridin-2-yl)methylenenicotinohydrazide (HLIV), respectively. It is recognized that the origin of self-assembling is fundamentally rooted in a dual donor (6s2/6p0 hybridized lone electron pair) and electrophilic behaviour of PbII. This allows production of extended topologies from a 1D polymeric chain in 4 through a 2D layer in 2 to the 3D frameworks in 1 and 3, predominantly due to the cooperative action of both covalent and non-covalent tetrel interactions of the overall type Pb-X (X = O, N, S, I). Counterintuitively, the latter, seemingly weak interactions, have appeared to be even stronger than the typical covalent bonds due to the presence of a bunch of supportive London dispersion dominated contacts: ππ, Lpπ, C-HO, C-HI, C-HH-C as well as more typical mainly electrostatically driven N-HO or N/O-HO hydrogen bonds. It is revealed that the constituting generally strong tetrel type Pb-X (X = O, N, S, I) bonds, though dominated by a classic Coulomb term, are therefore characterized by a very important London dispersion constituent, extremely strong relativistic effects and the two way dative-covalent Pb ↔ X electron charge delocalization contribution as revealed by the Extended Transition State Natural Orbital for Chemical Valence (ETS-NOCV) charge and energy decomposition scheme. It unravels that the pyridine-hydrazine linkers are also excellent London dispersion donors, and that together with the donor-acceptor properties of the heavy (relativistic) PbII atoms and nucleophilic counterions lead to extended self-assembling of 1-4.

Resonance Assisted Hydrogen Bonding Phenomenon Unveiled through Both Experiments and Theory: A New Family of Ethyl N-Salicylideneglycinate Dyes.

Extensive experimental and theoretical investigations are reported on the nature of resonance-assisted hydrogen bonding phenomenon (RAHB) and its influence on photophysical properties of the newly designed dyes differing in donor-acceptor properties, namely ethyl N-salicylideneglycinate (1), ethyl N-(5-methoxysalicylidene)glycinate (2), ethyl N-(5-bromosalicylidene)glycinate (3) and ethyl N-(5-nitrosalicylidene)glycinate (4). All compounds are thermochromic in the solid state and they contain a typical intramolecular O-H⋅⋅⋅N hydrogen bond formed between the hydroxyl hydrogen atom and the imine nitrogen atom, yielding the enol form in the solid state. It is unveiled, that the magnitude of RAHB effect fine tunes the strength of the O-H⋅⋅⋅N bonding and accordingly the relative populations of the enol, cis-keto and trans-keto forms leading to variation of the photophysical properties of 1-4. It is determined, that the electron-withdrawing NO2 in 4 amplifies the most RAHB effect causing the breaking of the O-H⋅⋅⋅N hydrogen bond and accordingly formation of the dominant cis-keto isomer in both the solid state and EtOH. To this end, the UV/Vis spectra of 1-3 in EtOH revealed the exclusive presence of the enol form, while the prevalent contribution of the cis-keto form was found for 4. Furthermore, only compound 4 is emissive in the solid state in ambient condition due to dual emission arising from the cis-keto* and trans-keto* forms, while 2 was found to be highly emissive in EtOH. It is revealed qualitatively and quantitatively, based on the ETS-NOCV charge and energy decomposition scheme and the EDDB population-based method, that RAHB is strongly a non-local phenomenon based on electrons pumping or sucking through both the π- and σ-channels, which accordingly exerts chemical bonding changes at both the phenyl ring and predominantly a distant O-H⋅⋅⋅N area.

Origin of Hydrocarbons Stability from a Computational Perspective: A Case Study of Ortho-Xylene Isomers.

It is shown herein that intuitive and text-book steric-clash based interpretation of the higher energy "in-in" xylene isomer (as arising solely from the repulsive CH⋅⋅⋅HC contact) with respect to the corresponding global-minimum "out-out" configuration (where the clashing C-H bonds are tilted out) is misleading. It is demonstrated that the two hydrogen atoms engaged in the CH⋅⋅⋅HC contact in "in-in" are involved in attractive interaction so they cannot explain the lower stability of this isomer. We have proven, based on the arsenal of modern bonding descriptors (EDDB, HOMA, NICS, FALDI, ETS-NOCV, DAFH, FAMSEC, IQA), that in order to understand the relative stability of "in-in" versus "out-out" xylenes isomers one must consider the changes in the electronic structure encompassing the entire molecules as arising from the cooperative action of hyperconjugation, aromaticity and unintuitive London dispersion plus charge delocalization based intra-molecular CH⋅⋅⋅HC interactions.

Substituent Effect on Conformational Preferences in Ground and Excited States of Selected Schiff Bases: An Insight from Theoretical Calculations.

2-Hydroxy aromatic Schiff bases that exhibit ESIPT (excited state intramolecular proton transfer) or TICT (twisted intramolecular charge transfer) photodeactivation pathways emerge as promising candidates for fluorescent sensors. In this computational work the influence of various substituents, differing in the electronic properties, on conformational preferences in the ground and S1 excited state for a series of 2-hydroxy-1-naphthylmethylene-hydrazine-based ligands is systematically studied using (TD)DFT calculations. In order to shed light on physical factors which might determine the obtained conformational preferences, extensive bonding analyses are performed. The results highlight the crucial role of a substituent's ability to form not only well-established intramolecular hydrogen bonds (e.g., O-H···N) but also unintuitive nonclassic weak interactions (e.g., C-H···O, C-H···N, and C-H···H-C) in the modulation of the equilibrium between naphthol-imine and keto-amine forms, and planar or twisted conformations, and, thus, in determination of photophysical properties of the considered bases.

Formation of active species from ruthenium alkylidene catalysts-an insight from computational perspective.

Ruthenium alkylidene complexes are commonly used as olefin metathesis catalysts. Initiation of the catalytic process requires formation of a 14-electron active ruthenium species via dissociation of a respective ligand. In the present work, this initiation step has been computationally studied for the Grubbs-type catalysts (H2IMes)(PCy3)(Cl)2Ru=CHPh, (H2IMes)(PCy3)(Cl)2Ru=CH-CH=CMe2 and (H2IMes)(3-Br-py)2(Cl)2Ru=CHPh, and the Hoveyda-Grubbs-type catalysts (H2IMes)(Cl)2Ru=CH(o-OiPrC6H4), (H2IMes)(Cl)2Ru=CH(5-NO2-2-OiPrC6H3), and (H2IMes)(Cl)2Ru=CH(2-OiPr-3-PhC6H3), using density functional theory (DFT). Additionally, the extended-transition-state combined with the natural orbitals for the chemical valence (ETS-NOCV) and the interacting quantum atoms (IQA) energy decomposition methods were applied. The computationally determined activity order within both families of the catalysts and the activation parameters are in agreement with reported experimental data. The significance of solvent simulation and the basis set superposition error (BSSE) correction is discussed. ETS-NOCV demonstrates that the bond between the dissociating ligand and the Ru-based fragment is largely ionic followed by the charge delocalizations: σ(Ru-P) and π(Ru-P) and the secondary CH…Cl, CH…π, and CH…HC interactions. In the case of transition state structures, the majority of stabilization stems from London dispersion forces exerted by the efficient CH…Cl, CH…π, and CH…HC interactions. Interestingly, the height of the electronic dissociation barriers is, however, directly connected with the prevalent (unfavourable) changes in the electrostatic and orbital interaction contributions despite the favourable relief in Pauli repulsion and geometry reorganization terms during the activation process. According to the IQA results, the isopropoxy group in the Hoveyda-Grubbs-type catalysts is an efficient donor of intra-molecular interactions which are important for the activity of these catalysts.

Kinetic and Potential Energy Contributions to a Chemical Bond from the Charge and Energy Decomposition Scheme of Extended Transition State Natural Orbitals for Chemical Valence.

This work provides novel physical insight into the nature of a chemical bond by exploring qualitative and quantitative relations between the natural orbitals for chemical valence (NOCV)-based deformation density bonding channels Δρ i ( i = σ, π, δ, etc.) and the corresponding kinetic Δ Ti and potential energy Δ Vi contributions within the charge and energy decomposition scheme ETS-NOCV implemented in the Kohn-Sham-based Amsterdam Density Functional (ADF) package. It is determined that interfragment dative and covalent-type electron charge reorganizations upon formation of a series of strong and weak bonds employing main-group elements are due to lowering of the negative kinetic energy contributions, as opposed to the intrafragment polarizations (e.g., hyperconjugations in ethane), which are, in contrary, driven by the potential energy (electrostatic) component. Complementary, formation of π-contributions in N2 is accompanied by lowering of both kinetic and potential energy constituents. Remarkably, well-known globally stabilizing back-donation (M → ligand, where M is a transition metal) and donation (ligand → M) processes, ubiquitous in organometallic species, have been discovered for the first time to be driven by the opposite Δ Ti/Δ Vi mechanisms, namely, the former contribution is associated with the negative kinetic term (which outweighs the positive potential energy), whereas the latter charge delocalization into electrophilic transition metals leads to an attractive electrostatic stabilization (and positive kinetic energy).

Aggregation-Induced Emission-Based Sensing Platform for Selective Detection of Zn2+ : Experimental and Theoretical Investigations.

Fluorescent chemosensors with aggregation induced emission enhancement (AIEE) emerge as promising tools in the field of sensing materials. Herein, we report the design, synthesis and applicability of a Schiff base chemosensor 1-(benzo[1,3]dioxol-4-ylmethylene-hydrazonomethyl)-naphthalen-2-ol (Hbdhn) of AIE characteristics that exhibits highly effective and selective response towards Zn2+ . The sensing effect of Hbdhn was evaluated by means of absorption/emission spectra and corresponding underlying photophysical mechanisms were proposed based on extensive quantum-chemical (TD)DFT calculations. The aggregated states in different DMSO/H2 O ratios and in a presence of Zn2+ were examined by fluorescence lifetime measurements, dynamic light scattering and scanning electron microscopy studies. The bioimaging abilities of Hbdhn were evaluated for Zn2+ in HepG2 cancer cells. The results demonstrate instant, stable in time and reproducible, colorimetric turn-on response with superb selectivity and sensitivity of Hbdhn towards Zn2+ , based on chelation enhanced fluorescence mechanism. AIEE improves further Hbdhn properties, leading to strong, long-lived fluorescence, with appearance of rod-like particles, in 90 % of water in DMSO and only 10 % of water in DMSO in the presence of Zn2+ . All these features combined with successful biomaging studies make Hbdhn one of the most promising candidate for practical applications among recently proposed related systems.

Structural versatility of the quasi-aromatic Möbius type zinc(ii)-pseudohalide complexes - experimental and theoretical investigations.

In this contribution we report for the first time fabrication, isolation, structural and theoretical characterization of the quasi-aromatic Möbius complexes [Zn(NCS)2LI] (1), [Zn2(µ1,1-N3)2(LI)2][ZnCl3(MeOH)]2·6MeOH (2) and [Zn(NCS)LII]2[Zn(NCS)4]·MeOH (3), constructed from 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane (LI) or benzilbis(acetylpyridin-2-yl)methylidenehydrazone (LII), respectively, and ZnCl2 mixed with NH4NCS or NaN3. Structures 1-3 are dictated by both the bulkiness of the organic ligand and the nature of the inorganic counter ion. As evidenced from single crystal X-ray diffraction data species 1 has a neutral discrete heteroleptic mononuclear structure, whereas, complexes 2 and 3 exhibit a salt-like structure. Each structure contains a ZnII atom chelated by one tetradentate twisted ligand LI creating the unusual Möbius type topology. Theoretical investigations based on the EDDB method allowed us to determine that it constitutes the quasi-aromatic Möbius motif where a metal only induces the π-delocalization solely within the ligand part: 2.44|e| in 3, 3.14|e| in 2 and 3.44|e| in 1. It is found, that the degree of quasi-aromatic π-delocalization in the case of zinc species is significantly weaker (by ∼50%) than the corresponding estimations for cadmium systems - it is associated with the Zn-N bonds being more polar than the related Cd-N connections. The ETS-NOCV showed, that the monomers in 1 are bonded primarily through London dispersion forces, whereas long-range electrostatic stabilization is crucial in 2 and 3. A number of non-covalent interactions are additionally identified in the lattices of 1-3.

Correction: Structural versatility of the quasi-aromatic Möbius type zinc(ii)-pseudohalide complexes - experimental and theoretical investigations.

[This corrects the article DOI: 10.1039/C9RA05276C.].

Photobleaching of pheomelanin increases its phototoxic potential: Physicochemical studies of synthetic pheomelanin subjected to aerobic photolysis.

Although melanin is a photoprotective pigment, its elevated photochemical reactivity could lead to various phototoxic processes. Photoreactivity of synthetic pheomelanin, derived from 5-S-cysteinyldopa (5SCD-M) and its photodegradation products obtained by subjecting the melanin to aerobic irradiation with UV-visible light, was examined employing an array of advanced physicochemical methods. Extensive photolysis of 5SCD-M was accompanied by partial bleaching of the melanin, modification of its paramagnetic properties, and significant increase in the ability to photogenerate singlet oxygen. The changes correlated with a substantial decrease in the melanin content of benzothiazine (BT) units and increase of modified benzothiazole (BZ) units. Synthetically prepared BZ exhibited higher efficiency to photogenerate singlet oxygen than the synthetic BT, and the free radical form of BZ, unlike that of BT, did not show measurable spin density on nitrogen atom, which was confirmed by quantum chemical calculations. Formation of modified BZ units in the photobleached 5SCD-M is responsible for the paramagnetic and photochemical changes of the melanin and its elevated phototoxic potential. Given a relatively constant pheomelanin-eumelanin ratio, such undesirable changes could occur in individual of all skin types.

Cyclodextrin-induced acidity modification of substituted cathinones studied by capillary electrophoresis supported by density functional theory calculations.

This paper shows that the acidity of substituted cathinones can change in a diversified and poorly predictable manner upon supramolecular interaction with cyclodextrins used as buffer additives in capillary electrophoresis. The direction and range of pKa shifts may be noticeably different for the particular cyclodextrins and cathinones, suggesting a strict correlation with structure. The most interesting results were observed for 2-hydroxyethyl-ß-cyclodextrin, which is capable for inducing the large negative and enantioselective apparent pKa shifts for α-pyrrolidinovalerophenone and methylenedioxypyrovalerone, even much above -1.0 pH unit. A thermodynamic analysis was performed, to identify the role of enthalpy and entropy in the formation and deprotonation of the respective diastereomeric complexes. The former process turned out to be driven by an energetically favorable increase in entropy, related probably to a hydrophobic effect. Deprotonation enthalpy in the complexed state, in turn, occurred to be more favorable than in the free molecule state, entailing the large drop in pKa after complexation. The DFT calculations allowed us to identify some structural effects that most likely contribute to these phenomena. At last, we have demonstrated that at low cyclodextrin concentration and pH ensuring partial ionization, pKa shifts contribute to chiral separation of the abovementioned cathinones. This analytically important effect may be helpful in anticipating the most efficient chiral separation mechanism of other systems.