Impact of C=C/B−N Replacement on the Diels–Alder Reactivity of Curved Polycyclic Aromatic Hydrocarbons

The influence of the replacement of C=C bonds by isoelectronic B−N moieties on the reactivity of π-curved polycyclic aromatic hydrocarbons has been computationally explored by means of density functional theory calculations. To this end, we selected the Diels–Alder cycloaddition reactions of the parent corannulene and its BN-doped counterparts with either cyclopentadiene or maleic anhydride. In addition, the analogous reactions involving larger buckybowls, such as BN-hemifullerene, BN-circumtrindene, and BN-fullerene, have been also considered. It has been found that whereas corannulene behaves as a dienophile, its BN counterpart better acts as a diene. In contrast, the larger BN-curved systems cannot be used as dienes in Diels–Alder reactions, but undergo facile (i.e., low barrier) cycloaddition reactions with cyclopentadiene. The observed trends in reactivity, which cannot be directly explained by using typical frontier molecular orbital arguments, are quantitatively described in detail by means of state-of-the-art computational methods, namely the activation strain model of reactivity combined with the energy decomposition analysis method. The results of our calculations highlight the crucial role of the curvature of the system on the reactivity and its influence on the strength of the orbital interactions between the deformed reactants during their transformations.     

“Dial-In” Emission from a Unique Flexible Material with Polarization Tuneable Spectral Intensity

The development of organic photoluminescent materials, which show promising roles as catalysts, sensors, organic light-emitting diodes, logic gates, etc., is a major demand and challenge for the global scientific community. In this context, a photoclick polymerization method is adopted for the growth of a unique photoluminescent three-dimensional (3D) polymer film, E, as a model system that shows emission tunability over the range 350–650 nm against the excitation range 295–425 nm. The DFT analysis of energy calculations and π-stacking supports the spectroscopic observations for the material exhibiting a broad range of emission owing to newly formed chromophoric units within the film. Full polarization spectroscopic Mueller matrix studies were employed to extract and quantify the molecular orientational order of both the ground (excitation) and excited (emission) state anisotropies through a set of newly defined parameters, namely the fluorescence diattenuation and fluorescence polarizance. The information contained in the recorded fluorescence Mueller matrix of the organic polymer material provided a useful way to control the spectral intensity of emission by using pre- and post-selection of polarization states. The observation was based on the assumption that the longer lifetime of the excited dipolar orientation is attributed to the compactness of the film.     

Side-on-End-on Coordination of Dinitrogen on a Polynuclear Vanadium Nitride Cluster Anion [V5N5]−

The side-on-end-on coordination of N₂ can be very important to activate and functionalize this very stable molecule. However, such coordination has rarely been reported. This study reports a gas-phase species (a polynuclear vanadium nitride cluster anion [V₅N₅]⁻) that can capture N₂ efficiently (12 %), and the quantum chemistry modelling suggests an unusual side-on-end-on coordination. The cluster anions were generated by laser ablation and the reaction with N₂ has been characterized by mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The back-donation interactions between the localized d–d bonding orbitals on the low-coordinated dual metal (V) sites and the antibonding π* orbitals of N₂ are the driving forces to adsorb N₂ with a high binding energy (about 2.0 eV).     

Synthesis of Pure Silica MWW Zeolite in Fluoride Medium by Using an Imidazolium-Based Long Dication

As the spacer length in 1,2-dimethylimidazolium-based dications increases beyond a specific point (six methylene units), they fail in structure-directing towards STW zeolites in any synthetic conditions. These dications can instead produce, under fluoride concentrated conditions, either *BEA [in the case of the eight-methylene-unit structure-directing agent (SDA)] or MWW (ten methylene units) zeolites. For any length of the dication, the default zeolite (MTW) is a relatively dense zeolite containing a unidimensional channel, whereas the zeolite demanding most specificity (STW, *BEA or MWW) is more porous, affording a larger concentration of the dication to be occluded. This work provides the first reported fluoride synthesis of pure silica MWW zeolites. Charge balance of the organic dications in this zeolite was achieved by combining “structural” silanolates, regular “connectivity defects” and occluded fluoride. Molecular mechanics calculations showed a perfect fit of the decamethylenebis(dimethylimidazolium) dication in the sinusoidal intralayer pore system of MWW. The calculations showed also that the dication is able to stabilize the interlayer space without disturbing the hydrogen-bonding system that holds the layers together in the as-made material. The ¹⁹F magic-angle spinning (MAS) NMR presented two distinct resonances at −71 and −83 ppm, which, on the basis of DFT calculations, we tentatively assigned to fluoride occluded in [4⁶6²] and [4¹5²6²] cages of the MWW structure, respectively. The same DFT study determines a different chemical shift of one methyl ¹³C nuclear magnetic resonance according to the imidazolium ring residing in the sinusoidal channels or in the large cup cavities, thus explaining an experimentally observed splitting of that resonance.     

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid-State NMR Spectroscopy,Polarized Neutron Diffraction,and First-Principle Calculations

The local magnetic structure in the [Fe^III(Tp)(CN)₃]⁻ building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions.     

A Tunable,Fullerene-Based Molecular Amplifier for Vibrational Circular Dichroism

Vibrational circular dichroism (VCD) studies are reported on a chiral compound in which a fullerene C₆₀ moiety is used as an electron acceptor and local VCD amplifier for an alanine-based peptide chain. Four redox states are investigated in this study, of which three are reduced species that possess low-lying electronic states as confirmed by UV/Vis spectroelectrochemistry. VCD measurements in combination with (TD)DFT calculations are used to investigate (i) how the low-lying electronic states of the reduced species modulate the amplification of VCD signals, (ii) how this amplification depends on the distance between oscillator and amplifier, and (iii) how the spatial extent of the amplifier influences amplification. These results pave the way for further development of tailored molecular VCD amplifiers.     

Statistics of reaction progress variable and mixture fraction gradients of a pulverised coal jet flame using Direct Numerical Simulation data

The statistical behaviour of the variances, covariance and gradients of the reaction progress variable (c), and the mixture fraction (ξ) have been analysed in a pulverised coal jet flame using a three-dimensional carrier phase direct numerical simulation (DNS) dataset. It has been observed that the Favre-probability density functions (PDFs) of c and ξ can be parametrised by the standard β function. Furthermore, the log-normal distribution has been found to accurately represent |?c| and |?ξ|. It is also found that ?c and ?ξ remain aligned throughout the flame brush. Finally the joint PDF of |?c| and |?ξ| has been compared with the product of the PDF of |?c| and PDF of |?ξ| extracted from carrier phase DNS, and it has been found that |?c| and |?ξ| are not statistically independent in the case investigated.The bivariate log-normal distributions with and without correlation have also been considered, and the former is found to be in better agreement with the carrier phase DNS data.     

Network topology of deeply supercooled water

Empirical potential structure refinements have been made to recent high-energy x-ray diffraction data, providing molecular models of deeply supercooled water. The average O-O coordination number is found to drop from 5.13 at 293?K to 4.85 at 244?K, within 3.5?Å. Triplet O-O-O bond angle distributions reveal a broad peak centred at 96.4° at 293?K which shifts to 100.0° at 244?K, indicative of the local geometry becoming increasingly tetrahedral with decreasing temperature. However, although the number of non-bonded interstitial molecules between the first and second shells is depleted upon cooling, the number of interstitial molecules forming triplets that are embedded within the hydrogen bonded tetrahedral network at θ_OOO?=?53°, remains constant. This is consistent with previous observations of an invariant O-O coordination number with temperature (4.24 out to 3.3?Å) and corresponds to non-bonded molecules positioned at close to half the ideal tetrahedral angle. Both -O-O-O- and hydrogen-bonded -O-H-O- ring length distributions show increases in 6 and 7-membered rings upon supercooling. This is concomitant with a shift and increase in intensity of peaks at r₄ ～8.7?Å and r₅ ～10.8?Å in the oxygen-oxygen pair distribution function, which in the models correspond to correlations between adjacent and next-nearest-neighbour hydrogen-bonded rings.     

First-Principles Investigation of β-FeOOH for Hydrogen Evolution: Identifying Reactive Sites and Boosting Surface Reactions

Akaganeite (β-FeOOH) is a widely investigated candidate for photo(electro)catalysis, such as water splitting. Nevertheless, insights into understanding the surface reaction between water and β-FeOOH, in particular, the hydrogen evolution reaction (HER), are still insufficient. Herein, a set of first-principles calculations on pristine β-FeOOH and halogen-substituted β-FeOOH are applied to evaluate the HER performance through the computational hydrogen electrode model. The results show that the HER on β-FeOOH tends to occur at Fe sites on the (010) surface, and palladium and nickel are found to serve as excellent co-catalysts to boost the HER process, due to the remarkably reduced free energy change of hydrogen adsorption upon loading on the surface of β-FeOOH, demonstrating great potential for efficient water splitting.     

Fullerene compounds with general formulae C60Ox (x = 1–3): Isomerism,stability and polarizability

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