The Use of the GSAM Approach for the Structural Investigation of Low-Lying Isomers of Molecular Clusters from Density-Functional-Theory-Based Potential Energy Surfaces: The Structures of Microhydrated Nucleic Acid Bases

Insight into Substrate Recognition by Urea-Based Helical Foldamer Catalysts Using a DFT Global Optimization Approach

Peptides and foldamers have recently gained increasing attention as chiral catalysts to achieve challenging (asymmetric) transformations. We previously reported that short helically folded aliphatic oligoureas in combination with achiral Brønsted bases are effective H-bonding catalysts for C-C bond-forming reactions─i.e., the conjugate addition of 1,3-dicarbonyl pronucleophiles to nitroalkenes─with high reactivity and selectivity and at remarkably low chiral catalyst/substrate molar ratios. This theoretical investigation at the density functional theory level of theory, aims to both analyze how the substrates of the reaction interact with the foldamer catalyst and rationalize a chain-length dependence effect on the catalytic properties. We confirm that the first two ureas are the only H-bond donors available to interact with external molecules. Moreover, each urea site interacts with one of the two reactants allowing a short distance between the two reacting carbons, thus facilitating the conjugated addition. Additionally, it was observed that the molecular recognition and catalyst-substrate interactions are mainly governed by electrostatic interactions but not orbital interactions (see from NBO if this is finally true). On these grounds, an electrostatic potential (ESP) analysis showed an important internal charge separation in the catalyst, the positive ESP region being concentrated around the first two ureas, with its area extending as the number of residues increases.

Fluoranthene dyes for the detection of water content in methanol

Three novel fluoranthene dyes were obtained by cycloaddition reactions using acrylonitrile and dialkyl acetylenedicarboxylates. Their fluorescence properties in different polar-organic solvents were investigated systematically. Meanwhile, spectral changes induced by the addition of water in methanol were observed, indicating that these fluoranthenes dyes can be efficiently used to detect the water content in methanol as probes. Significantly, the practical test measurements for the water contents in methanol illustrated the measured results with the three fluorescent probes were basically consistent with the water content added artificially. This demonstrated the potential of these fluoranthene dyes as probes in measuring the water content in methanol.

Ultrafast Excited-State Dynamics of Hydrogen-Bonded Cytosine Microsolvated Clusters with Protic and Aprotic Polar Solvents

Microsolvation effects on the ultrafast excited-state deactivation dynamics of cytosine (Cy) were studied in hydrogen-bonded Cy clusters with protic and aprotic solvents using mass-resolved femtosecond pump-probe ionization spectroscopy. Two protic solvents, water (H₂O) and methanol (MeOH), and one aprotic solvent, tetrahydrofuran (THF), were investigated, and transients of Cy·(H₂O)_1-6, Cy·(MeOH)_1-3, and Cy·THF microsolvated clusters produced in supersonic expansions were measured. With the aid of electronic structure calculations, we assigned the observed dynamics to the low-energy isomers of various Cy clusters and discussed the microsolvation effect on the excited-state deactivation dynamics. With the protic solvents only the microsolvated clusters of Cy keto tautomer were observed. The observed decay time constants of Cy·(H₂O) _n are 0.5 ps for n = 1 and ∼0.2-0.25 ps for n = 2-6. For Cy·(MeOH) _n clusters, the decay time constant for n = 1 cluster is similar to that of the Cy monohydrate, but for n = 2 and 3 the decays are about a factor of 2 slower than the corresponding microhydrates. With the aprotic solvent, THF, hydrogen-bonded complexes of both keto and enol tautomers are present in the beam. The keto-Cy·THF shows a decay similar to that of the keto-Cy monomer, whereas the enol-Cy·THF exhibits a 2-fold slower decay than the enol-Cy monomer, suggesting an increase in the barrier to excited-state deactivation upon binding of one THF molecule to the enol form of Cy.

Inner filter effect based selective detection of picric acid in aqueous solution using green luminescent copper nanoclusters

Designing superior probes to detect trace amounts of picric acid (PA) is of huge importance for homeland security and environmental protection.

Quantum chemical MP2 results on some hydrates of cytosine: binding sites, energies and the first hydration shell

A detailed quantum chemical investigation was undertaken to obtain the structure and energetics of cytosine hydrates Cyt·nH2O, with n = 1 to 7. The MP2(fc)/aug-cc-pVDZ level was used as the standard, with some DFT (B3LYP) and coupled cluster calculations, as well as calculations with the aug-cc-pVTZ basis set added for comparison. In a systematic search for microhydrated forms of cytosine, we have found that several structures have not yet been reported in the literature. The energies of different isomers, as well as binding energies are compared. When predicting the stability of a complex, we suggest using a scheme where the water molecules are extracted from a finite model of bulk water. Finally, based on energetic data, we suggest a rational definition of the first hydration shell; with this definition, it contains just six water molecules.

Interfacial chemical oxidative synthesis of multifunctional polyfluoranthene

Polyfluoranthene, synthesized by an interfacial cationic oxidative polymerization of fluoranthene, emits visible fluorescence that shows highly selective sensitivity to Fe³⁺.

A novel polyfluoranthene (PFA) exhibiting strong visual fluorescence emission, a highly amplified quenching effect, and widely controllable electrical conductivity is synthesized by the direct cationic oxidative polymerization of fluoranthene in a dynamic interface between n-hexane and nitromethane containing fluoranthene and FeCl₃, respectively. A full characterization of the molecular structure signifies that the PFAs have a degree of polymerization from 22–50 depending on the polymerization conditions. A polymerization mechanism at the interface of the hexane/nitromethane biphasic system is proposed. The conductivity of the PFA is tunable from 6.4 × 10^–6 to 0.074 S cm^–1 by doping with HCl or iodine. The conductivity can be significantly enhanced to 150 S cm^–1 by heat treatment at 1100 °C in argon. A PFA-based chemosensor shows a highly selective sensitivity for Fe³⁺ detection which is unaffected by other common metal ions. The detection of Fe³⁺ likely involves the synergistic effect of well-distributed π-conjugated electrons throughout the PFA helical chains that function as both the fluorophore and the receptor units.