Is the Excited‐State H‐atom Transfer in Hypericin Concerted?¶

The excited-state intramolecular H-atom transfer of hypericin (Hyp) was investigated as a function of pH in monodispersed reverse micelles formed by sodium bis(2-ethylhexyl)sulfosuccinate/heptane/water and in complexes with Tb3+ under conditions in which one of the two carbonyl groups of Hyp is incapable of accepting a hydrogen atom. The results of pump-probe transient absorption experiments provide no evidence for a concerted H-atom transfer mechanism.     

Does Excited‐State Proton‐Transfer Reaction Contribute to the Emission Behaviour of 4‐Aminophthalimide in Aqueous Media?

4‐Aminophthalimide (AP) is an extensively used molecule both for fundamental studies and applications primarily due to its highly solvent‐sensitive fluorescence properties. The fluorescence spectrum of AP in aqueous media was recently shown to be dependent on the excitation wavelength. A time‐dependent blue shift of its emission spectrum is also reported. On the basis of these findings, the excited‐state solvent‐mediated proton‐transfer reaction of the molecule, which was proposed once but discarded at a later stage, is reintroduced. We report on the fluorescence behaviour of AP and its imide‐H protected derivative, N‐BuAP, to prove that a solvent‐assisted excited‐state keto–enol transformation does not contribute to the steady‐state and time‐resolved emission behaviour of AP in aqueous media. Our results also reveal that the fluorescence of AP in aqueous media arises from two distinct hydrogen‐bonded species. The deuterium isotope effect on the fluorescence quantum yield and lifetime of AP, which was thought to be a reflection of the excited‐state proton‐transfer reaction in the system, is explained by considering the difference in the influence of H₂O and D₂O on the nonradiative rates and ground‐state exchange of the proton with the solvent.     

6‐Chloroisocytosine and 5‐bromo‐6‐methylisocytosine: again,one or two tautomers present in the same crystal?

It is well known that pyrimidin‐4‐one derivatives are able to adopt either the 1H‐ or the 3H‐tautomeric form in (co)crystals, depending on the coformer. As part of ongoing research to investigate the preferred hydrogen‐bonding patterns of active pharmaceutical ingredients and their model systems, 2‐amino‐6‐chloropyrimidin‐4‐one and 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4‐one have been cocrystallized with several coformers and with each other. Since Cl and Br atoms both have versatile possibilities to interact with the coformers, such as via hydrogen or halogen bonds, their behaviour within the crystal packing was also of interest. The experiments yielded five crystal structures, namely 2‐aminopyridin‐1‐ium 2‐amino‐6‐chloro‐4‐oxo‐4H‐pyrimidin‐3‐ide–2‐amino‐6‐chloropyrimidin‐4(3H)‐one (1/3), C₅H₇N₂⁺·C₄H₃ClN₃O⁻·3C₄H₄ClN₃O, (Ia), 2‐aminopyridin‐1‐ium 2‐amino‐6‐chloro‐4‐oxo‐4H‐pyrimidin‐3‐ide–2‐amino‐6‐chloropyrimidin‐4(3H)‐one–2‐aminopyridine (2/10/1), 2C₅H₇N₂⁺·2C₄H₃ClN₃O⁻·10C₄H₄ClN₃O·C₅H₆N₂, (Ib), the solvent‐free cocrystal 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one (1/1), C₅H₆BrN₃O·C₅H₆BrN₃O, (II), the solvate 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one–N‐methylpyrrolidin‐2‐one (1/1/1), C₅H₆BrN₃O·C₅H₆BrN₃O·C₅H₉NO, (III), and the partial cocrystal 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one–2‐amino‐6‐chloropyrimidin‐4(3H)‐one (0.635/1/0.365), C₅H₆BrN₃O·C₅H₆BrN₃O·C₄H₄ClN₃O, (IV). All five structures show R₂²(8) hydrogen‐bond‐based patterns, either by synthon 2 or by synthon 3, which are related to the Watson–Crick base pairs.     

Structure and Bonding in First‐Row Transition‐Metal Dicarbides: Are They Related to the Stability of Met‐cars?

First-row transition-metal dicarbides MC(2) (M=Sc-Zn) have been investigated by using quantum-mechanical techniques. The competition between cyclic and linear isomers in these systems has been studied and the bonding scheme for these compounds is discussed through topological analysis of electron density. All of the systems have been found to prefer a C(2v)-symmetric arrangement, although for ZnC(2) the energy difference between this and the linear isomer is rather small. In most cases the C(2v)-symmetric structure corresponds to a T-shaped structure, with the exceptions of TiC(2), CoC(2), and NiC(2) which have been shown to be true rings. A detailed analysis of the variation of the energy of the system with geometry has been carried out. An analysis of the bonding, taking into account the main interactions between the valence orbitals of both fragments, the M atom and the C(2) molecule, has allowed the main features of these compounds to be interpreted. A clear correlation between the dissociation energies of the first-row transition-metal dicarbides and the bonding energies of the corresponding met-cars was observed.     

Does the Surface Matter? Hydrogen‐Bonded Chain Formation of an Oxalic Amide Derivative in a Two‐ and Three‐Dimensional Environment

We report on a multi‐technique investigation of the supramolecular organisation of N,N‐diphenyl oxalic amide under differently dimensioned environments, namely three‐dimensional (3D) in the bulk crystal, and in two dimensions on the Ag(111) surface as well as on the reconstructed Au(111) surface. With the help of X‐ray structure analysis and scanning tunneling microscopy (STM) we find that the molecules organize in hydrogen‐bonded chains with the bonding motif qualitatively changed by the surface confinement. In two dimensions, the chains exhibit enantiomorphic order even though they consist of a racemic mixture of chiral entities. By a combination of the STM data with near‐edge X‐ray absorption fine‐structure spectroscopy, we show that the conformation of the molecule adapts such that the local registry of the functional group with the substrate is optimized while avoiding steric hindrance of the phenyl groups. In the low coverage case, the length of the chains is limited by the Au(111) reconstruction lines restricting the molecules into fcc stacked areas. A kinetic Monte Carlo simulated annealing is used to explain the selective assembly in the fcc stacked regions.     

One‐ or Two‐Electron Transfer? The Ambiguous Nature of the Discharge Products in Sodium–Oxygen Batteries

Rechargeable lithium–oxygen and sodium–oxygen cells have been considered as challenging concepts for next‐generation batteries, both scientifically and technologically. Whereas in the case of non‐aqueous Li/O₂ batteries, the occurring cell reaction has been unequivocally determined (Li₂O₂ formation), the situation is much less clear in the case of non‐aqueous Na/O₂ cells. Two discharge products, with almost equal free enthalpies of formation but different numbers of transferred electrons and completely different kinetics, appear to compete, namely NaO₂ and Na₂O₂. Cells forming either the superoxide or the peroxide have been reported, but it is unclear how the cell reaction can be influenced for selective one‐ or two‐electron transfer to occur. In this Minireview, we summarize available data, discuss important control parameters, and offer perspectives for further research. Water and proton sources appear to play major roles.     

Is the placental proteome impaired in well‐controlled gestational diabetes?

In pregnancy complicated by gestational diabetes mellitus (GDM), the human placenta shows several pathological functional and structural changes, but the extent to which maternal glycemic control contributes to placental abnormalities remains unclear. The aim of this study was to profile and compare the proteome of placentas from healthy pregnant women and those with GDM, to investigate the placenta‐specific protein composition and possible changes of its function in presence of GDM. Quantitative proteomic analysis, based on LC‐MS^E approach, revealed that higher (approximately 15% increase) levels of galectin 1 and collagen alpha‐1 XIV chain (although the difference regarding the latter was at the limit of significance) were present in GDM samples, while heat shock 70 kDa protein 1A/1B was less abundant in GDM placental tissue. These data seem to indicate that GDM, when well controlled, did not markedly affect the placental proteome.     

Rydberg or Valence? The Long‐Standing Question in the UV Absorption Spectrum of 1,1′‐Bicyclohexylidene

The electronic excited states of the olefin 1,1′‐bicylohexylidene (BCH) are investigated using multiconfigurational complete active space self‐consistent‐field second order perturbation theory in its multi‐state version (MS‐CASPT2). Our calculations undoubtedly show that the bulk of the intensity of the two unusually intense bands of the UV absorption of BCH measured with maxima at 5.95 eV and 6.82 eV in the vapor phase are due to a single ππ* valence excitation. Sharp peaks reported in the vicinity of the low‐energy feature in the gas phase correspond to the beginning of the π3s_R Rydberg series. By locating the origin of the ππ* band at 5.63 eV, the intensity and broadening of the observed bands and their presence in solid phase is explained as the vibrational structure of the valence ππ* transition, which underlies the Rydberg manifold as a quasi‐continuum.     

Sterically Active Electron Pairs in Lead Sulfide? An Investigation of the Electronic and Vibrational Properties of PbS in the Transition Region Between the Rock Salt and the α‐GeTe‐Type Modifications

Recently, we have investigated the energy landscape of PbS for many different pressures on the ab initio level by using Hartree–Fock and density functional theory to globally search for possible thermodynamically stable and metastable structures. The perhaps most fascinating observation was that besides the experimentally known modification exhibiting the rock salt structure a second minimum exists close‐by on the landscape showing the low‐temperature α‐GeTe‐type structure. In the present study, we investigate the possible reasons for the existence of this metastable modification; in particular we address the question, whether the α‐GeTe‐type modification might be stabilized (and conversely the rock salt modification destabilized) by steric effects of the non‐bonding electron pair.     

Targeting RNA G‐Quadruplex in SARS‐CoV‐2: A Promising Therapeutic Target for COVID‐19?

The COVID‐19 pandemic caused by SARS‐CoV‐2 has become a global threat. Understanding the underlying mechanisms and developing innovative treatments are extremely urgent. G‐quadruplexes (G4s) are important noncanonical nucleic acid structures with distinct biofunctions. Four putative G4‐forming sequences (PQSs) in the SARS‐CoV‐2 genome were studied. One of them (RG‐1), which locates in the coding sequence region of SARS‐CoV‐2 nucleocapsid phosphoprotein (N), has been verified to form a stable RNA G4 structure in live cells. G4‐specific compounds, such as PDP (pyridostatin derivative), can stabilize RG‐1 G4 and significantly reduce the protein levels of SARS‐CoV‐2 N by inhibiting its translation both in vitro and in vivo. This result is the first evidence that PQSs in SARS‐CoV‐2 can form G4 structures in live cells, and that their biofunctions can be regulated by a G4‐specific stabilizer. This finding will provide new insights into developing novel antiviral drugs against COVID‐19.     

Electron ionization mass spectra of 8‐aryl‐3,4‐dioxo‐2H,8H‐6,7‐dihydroimidazo[2,1‐c][1,2,4]triazines. Do they exhibit tautomerism in the gas phase?

The electron ionization mass spectra of the title compounds (1: a R = H, b 2-CH(3), c 4-CH(3), d 2,3-diMe, e 2-OCH(3), f 4-OCH(3), g 2-Cl, h 3-Cl, i 4-Cl, j 3,4-diCl) were recorded at 70 eV to determine the effects of substituents and the possible keto-enol tautomerism. The compounds showed several common fragment ions but also fragment ions which divided them into three classes, namely 1a-1d (parent compound and Me-substituted derivatives), 1e and 1f (MeO-substituted derivatives), and 1g-1j (Cl-substituted derivatives). The presence of the HOCN(+.) ion as well as the exponential dependence of its total ion current in the case of p- and also 3-Cl-substituted compounds (1a, c, f, h-j) on the Hammett sigma constants and the loss of CHO or one or two HOCN moieties can be somewhat easier explained by the presence of the enol form but as a whole the results support the predominance of the keto form, in parallel to the situation in solution.     

Is the HO4− Anion a Key Species in the Aqueous‐Phase Decomposition of Ozone?

The role of the HO₄^? anion in atmospheric chemistry and biology is a matter of debate, because it can be formed from, or be in equilibrium with, key species such as O₃ + HO^? or HO₂ + O₂^?. The determination of the stability of HO₄^? in water therefore has the greatest relevance for better understanding the mechanism associated with oxidative cascades in aqueous solution. However, experiments are difficult to perform because of the short‐lived character of this species, and in this work we have employed DFT, CCSD(T) complete basis set (CBS), MRCI/aug‐cc‐pVTZ, and combined quantum mechanics/molecular mechanics (QM/MM) calculations to investigate this topic. We show that the HO₄^? anion has a planar structure in the gas phase, with a very large HOO? OO bond length (1.823 Å). In contrast, HO₄^? adopts a nonplanar configuration in aqueous solution, with huge geometrical changes (up to 0.232 Å for the HOO? OO bond length) with a very small energy cost. The formation of the HO₄^? anion is predicted to be endergonic by 5.53±1.44 and 2.14±0.37 kcal mol^?1 with respect to the O₃ + HO^? and HO₂ + O₂^? channels, respectively. Moreover, the combination of theoretical calculations with experimental free energies of solvation has allowed us to obtain accurate free energies for the main reactions involved in the aqueous decomposition of ozone. Thus, the oxygen transfer reaction (O₃ + OH^? → HO₂ + O₂^?) is endergonic by 3.39±1.80 kcal mol^?1, the electron transfer process (O₃ + O₂^? → O₃^? + O₂) is exergonic by 31.53±1.05 kcal mol^?1, supporting the chain‐carrier role of the superoxide ion, and the reaction O₃ + HO₂^? → OH + O₂^? + O₂ is exergonic by 12.78±1.15 kcal mol^?1, which is consistent with the fact that the addition of small amounts of HO₂^? (through H₂O₂) accelerates ozone decomposition in water. The combination of our results with previously reported thermokinetic data provides some insights into the potentially important role of the HO₄^? anion as a key reaction intermediate.     

Power‐to‐Syngas: An Enabling Technology for the Transition of the Energy System?

Power‐to‐X concepts promise a reduction of greenhouse gas emissions simultaneously guaranteeing a safe energy supply even at high share of renewable power generation, thus becoming a cornerstone of a sustainable energy system. Power‐to‐syngas, that is, the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to syngas for the production of synfuels and high‐value chemicals, offers an efficient technology to couple different energy‐intense sectors, such as “traffic and transportation” and “chemical industry”. Syngas produced by co‐electrolysis can thus be regarded as a key‐enabling step for a transition of the energy system, which offers additionally features of CO₂‐valorization and closed carbon cycles. Here, we discuss advantages and current limitations of low‐ and high‐temperature co‐electrolysis. Advances in both fundamental understanding of the basic reaction schemes and stable high‐performance materials are essential to further promote co‐electrolysis.