Density functional calculations on dissociation reactions of radical anions of 5-fluorouracil derivatives

Fragmentation reactions upon electron attachment to 5-fluorouracil with CH2R substituents at N1 have been evaluated by means of density functional calculations. The present results show that electron attachment to R = F, HC=O or CN derivatives follows a stepwise pathway with radical anions as intermediates. For these compounds, the most stable species formed is the pi radical anion which bears an unpaired spin density at the C6=C5-C4=O pi-conjugated system of the uracil ring. Cleavage of the N1-CH2R or N1CH2-R bond of these intermediates proceeds through the mixing of the pi and sigma states by means of proper geometrical fluctuations along the reaction coordinate. No sigma radical anion could be characterised on any of these sigma basal potential surfaces. A noticeable decrease in the activation energy for the N1-CH2R bond dissociation was observed for R = H-C=O or CN. Therefore, such derivatives with unsaturated groups positioned vicinal to the N1-C1' bond are identified as targets for the development of novel radiation-activated antitumour drugs. On the other hand, the electron transfer to the compounds with R = Cl, Br is dissociative, i.e. it occurs without the mediation of radical anions. For compounds with R = halides or R = NO2, the fragmentation of the N1CH2-R bond is the preferred dissociation pathway.     

Exceptional gas-phase enantioselectivity of chiral tetramide macrocycles

Diastereomeric proton-bound complexes between some phenylalanine derivatives (A) and chiral tetramide macrocycles (M) exhibit an uncommon enantioselectivity when reacting with the enantiomers of 2-aminobutanes in the gas phase (B). The measured enantioselectivity depends mainly on two distinct factors: (i) the configuration of the A guest; and (ii) the structure and the relative stability of isomeric [MHA]+ complexes. No significant effects of the B configuration are observed. The diastereomeric [MHA]+ complexes with A = 1-naphthylalanine ethyl ester exhibit the largest enantioselectivity factor ever measured in the gas phase (khomo/khetero = 0.046). The origin of such an exceptional enantioselectivity is mainly attributed to the relative stability of the diastereomeric [MHA]+ complexes, as demonstrated by the comparison of the kinetic results with those from collision-induced dissociation of the trimeric [M2HA]+ adducts and with computational evidence.     

Superoxide Anion Disproportionation Induced by Li+ and H+: Pathways to 1O2 Release in Li−O2 Batteries

We explore the disproportionation reaction of superoxide anions in the presence of H⁺ and Li⁺ cations with high quality multiconfigurational ab-initio methods. This reaction is of paramount importance in Li−O₂ battery chemistry as it represents the source of a major degrading impurity, singlet molecular oxygen. For the first time, the thermodynamic and kinetic data of the reaction are drawn from an accurate theoretical model where the electronic structure of the reactant and products is treated at the necessary level of theory. Overall, the H⁺ catalyzed O₂⁻+O₂⁻ disproportionation follows a very efficient thermodynamic and kinetic reaction path leading to neutral ³O₂, ¹O₂ and peroxide anions. On the contrary, we have found that the Li⁺ catalysis promotes only the release of ³O₂ whereas the ¹O₂ formation is energetically unfeasible at room temperature.     

Visibility graph approach to the analysis of ocean tidal records

By using the recent method of the visibility graph, three time series of oceanic tide level in central Argentina were investigated. The degree distributions show a rich structure; in particular the maximum is due to the main periodic oscillations at 24 hours and 12 hours and higher harmonics. The degree distributions of the residuals (obtained removing from the original signals the cyclic components) suggest that the local effects, linked with the particular coastal conditions of the sites, are discernible for the degree k < 20, while the global effects, linked with linked with the more general and common atmospheric forcing and ocean current conditions, are visible for k > 100. Although a relationship between the spectral exponent α and the exponent of the degree distribution γ of tidal signals can be recognized, this cannot be simply stated due to the very rich and complex structure of time dynamics of tides. The present study, even if still preliminary, show the importance of the visibility graph method in investigating the complex time dynamics of observational and experimental signals.     

The dynamic chromatographic behavior of tri-o-thymotide on HPLC chiral stationary phases

The interconverting stereoisomers of tri-o-thymotide have been separated by HPLC on chiral stationary phases and the temperature dependence of the dynamic chromatographic patterns has been interpreted in terms of exchange between enantiomeric propeller and helical conformations. Computed low energy structures and CD spectra were used in absolute configuration assignment.     

Visibility graph analysis of wind speed records measured in central Argentina

Hourly means of wind speed time series recorded at two wind stations in central Argentina (one inland and the other coastal) are analyzed by means of the visibility graph method. The degree distribution of both series was calculated along with that of their shuffled, Gaussian- and uniform-distributed surrogates. The original series as well as their surrogates show an apparent exponential degree distribution. The v$v$–k$k$ (value–degree) plot of the original wind series indicates that the higher values of the series are not necessarily “hubs” for the series, while that of the surrogates are characterized by a reduced connectivity degree of the higher hubs.     

Cloaking apertureless near-field scanning optical microscopy tips

We numerically demonstrate that properly designed plasmonic covers can be used to enhance the performance of near-field scanning optical microscopy (NSOM) systems based on the employment of apertureless metallic tip probes. The covering material, exhibiting a near-zero value of the real permittivity at the working frequency, is designed in such a way to dramatically reduce the undesired scattering due to the strongly plasmonic behavior of the tip. Though the light scattering by the tip end is necessary for the correct operation of NSOMs, the additional scattering due to the whole probe affects the signal-to-noise ratio and thus the resolution of the acquired image. By covering the whole probe but not the very tip, we show that unwanted scattering can be effectively reduced. A realistic setup, working at mid-IR frequencies and employing silicon carbide covers, has been designed and simulated to confirm the effectiveness of the proposed approach.     

The "bridge" game: role of the fourth player in chiral recognition

A new team player: The "three-point interaction" model, which is usually employed to rationalize chiral recognition, does not account for the amazing enantioselectivity measured for the receptors of many proteic acceptors. Gas-phase experiments have indicated that at least a fourth "player" must be considered: the rigidity that a receptor opposes to distortions of its cavity resulting from noncovalent interactions with a chiral molecule (see picture).     

Chiral (cyclopentadienone)iron complexes with a stereogenic plane as pre-catalysts for the asymmetric hydrogenation of polar double bonds

In this paper, we describe a small library of easy-to-prepare chiral (cyclopentadienone)iron pre-catalysts for enantioselective CO and CN hydrogenations. Starting from readily accessible achiral materials, six chiral (cyclopentadienone)iron complexes (1a-f) possessing a stereogenic plane were synthesized in racemic form. Based on the screening of pre-catalysts (±)-1a-f in the hydrogenation of ketones and ketimines, we selected two complexes (1a and 1d) for resolution by semipreparative enantioselective HPLC. The absolute configuration of the separated enantiomers of 1a and 1d was assigned by XRD analysis (1a) and by comparison between experimental and DFT-calculated ECD and ORD spectra (1d). The enantiopure pre-catalysts (S)-1a and (R)-1d were tested in the asymmetric hydrogenation of several ketones and ketimines and showed good activity and modest enantioselectivity, the e.e. values ranging from very low to moderate (54%).