Selective Chemical Upcycling of Mixed Plastics Guided by a Thermally Stable Organocatalyst.

Chemical recycling of plastic waste represents a greener alternative to landfill and incineration, and potentially offers a solution to the environmental consequences of increased plastic waste. Most plastics that are widely used today are designed for durability, hence currently available depolymerisation methods typically require harsh conditions and when applied to blended and mixed plastic feeds generate a mixture of products. Herein, we demonstrate that the energetic differences for the glycolysis of BPA-PC and PET in the presence of a protic ionic salt TBD:MSA catalyst enables the selective and sequential depolymerisation of these two commonly employed polymers. Employing the same procedure, functionalised cyclic carbonates can be obtained from both mixed plastic wastes and industrial polymer blend. This methodology demonstrates that the concept of catalytic depolymerisation offers great potential for selective polymer recycling and also presents plastic waste as a "greener" alternative feedstock for the synthesis of high added value molecules.

Diselenide Bonds as an Alternative to Outperform the Efficiency of Disulfides in Self-Healing Materials.

Self-healing materials are a very promising kind of materials due to their capacity to repair themselves. Among others, dichalcogenide-based materials are widely studied due to their dynamic covalent bond nature. Recently, the reaction mechanism occurring in these materials was characterized both theoretically and experimentally. In this vein, a theoretical protocol was established in order to predict further improvements. Among these improvements, the use of diselenides instead of disulfides appears to be one of the paths to enhance these properties. Nevertheless, the physicochemical aspects of these improvements are not completely clear. In this work, the self-healing properties of several disulfides, diselenides, and mixed S-Se materials have been considered by means of computational simulations. Among all the tested species, diphenyl diselenide based materials appear to be the most promising ones due to the decrease on the reaction barriers, instead of weaker diselenide bonds, as thought up to now. Moreover, the radical formation needed in this process would also be enhanced by the fact that these species are able to absorb visible light. In this manner, at room conditions, selenyl radicals would be formed by both thermal dissociation and photodissociation. This fact, together with the lower energetic barriers needed for the diselenide exchange, makes diphenyl diselenides ideal for self-healing materials.

Improvement of the electrochemical and singlet fission properties of anthraquinones by modification of the diradical character.

In this work, theoretical methods of quantum chemistry are employed to estimate the effects that the structural modification of 1,5- and 9,10-anthraquinone molecules might produce in their electronic structure, in the pursuit of a common strategy to improve the electrochemical and singlet fission features of conjugated quinones. The proposed modifications are the following: (i) substitution of the carbonyl oxygen atom, (ii) insertion of heteroatoms in the carbon backbone, and (iii) introduction of electron-withdrawing and electron-donating substituents in different positions of the rings. This work shows how specific modifications of the electronic structure can be used to tune the electrochemistry and photophysics of the quinones, improving their suitability to be singlet fission sensitizers and cathode materials. As it was previously known, intermediate diradical characters favor the accomplishment of the singlet fission process. This can be achieved for 1,5-anthraquinone by introduction of B and Si atoms and electron-donating groups in low spin density positions of the carbon backbone or N atoms in high spin density positions. The introduction of heteroatoms or substituents in 9,10-anthraquinone hardly facilitates the singlet fission process. Regarding the electrochemistry, it is observed that the reduction potentials greatly depend on the nature of the modification rather than on the diradical character.

Supramolecular-Enhanced Charge Transfer within Entangled Polyamide Chains as the Origin of the Universal Blue Fluorescence of Polymer Carbon Dots.

The emission of a bright blue fluorescence is a unique feature common to the vast variety of polymer carbon dots (CDs) prepared from carboxylic acid and amine precursors. However, the difficulty to assign a precise chemical structure to this class of CDs yet hampers the comprehension of their underlying luminescence principle. In this work, we show that highly blue fluorescent model types of CDs can be prepared from citric acid and ethylenediamine through low temperature synthesis routes. Facilitating controlled polycondensation processes, the CDs reveal sizes of 1-1.5 nm formed by a compact network of short polyamide chains of about 10 monomer units. Density functional theory calculations of these model CDs uncover the existence of a spatially separated highest occupied molecular orbital and a lowest unoccupied molecular orbital located at the amide and carboxylic groups, respectively. Photoinduced charge transfer between these groups thus constitutes the origin of the strong blue fluorescence emission. Hydrogen-bond-mediated supramolecular interactions between the polyamide chains enabling a rigid network structure further contribute to the enhancement of the radiative process. Moreover, the photoinduced charge transfer processes in the polyamide network structure easily explain the performance of CDs in applications as revealed in studies on metal ion sensing. These findings thus are of general importance to the further development of polymer CDs with tailored properties as well as for the design of technological applications.

Fullerene-Based Materials as Hole-Transporting/Electron-Blocking Layers: Applications in Perovskite Solar Cells.

Here we report for the first time an efficient fullerene-based compound, FU7, able to act as hole-transporting material (HTM) and electron blocking contact. It has been applied on perovskite solar cells (PSCs), obtaining 0.81 times the efficiency of PSCs with the standard HTM, spiro-OMeTAD, with the additional advantage that this performance is reached without any additive introduced in the HTM layer. Moreover, as a proof of concept, we have described for the first time efficient PSCs in which both selective contacts are fullerene derivatives, to obtain unprecedented "fullerene sandwich" PSCs.

Probing the structures and bonding of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging.

We report an investigation of a series of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging and ab initio calculations. Vibrationally resolved photoelectron spectra are obtained, allowing the electron affinities of Au-(C≡C) n -Au to be accurately measured as 1.651(1), 1.715(1), and 1.873(1) eV for n = 1-3, respectively. Both the Au-C symmetric stretching and a bending vibrational frequency are observed for each neutral auropolyyne. Theoretical calculations find that the ground state of Au2C2 - has a linear acetylenic Au-C≡C-Au- structure, whereas the asymmetric Au-Au-C≡C- structure is a low-lying isomer. However, for Au2C4 - and Au2C6 -, our calculations show that the asymmetric Au-Au-(C≡C) n - isomers are the global minima and the Au-(C≡C) n -Au- symmetric structures become low-lying isomers. All the asymmetric Au-Au-(C≡C) n - isomers are found computationally to have much higher electron binding energies and are not accessible at the detachment photon energies used in the current study. For neutral Au2C2n , the Au-(C≡C) n -Au auropolyyne structures are found to be the global minima for n = 1-3. The electronic structures and bonding for Au-(C≡C) n -Au (n = 1-3) are compared with the corresponding Au-(C≡C) n and Au-(C≡C) n -H species.

π⋠⋠⋠H+ ⋠⋠⋠π Hydrogen Bonds and Their Lithium and Gold Analogues: MP2 and CASPT2 Calculations.

Molecular systems in which two simple π-electron species, acetylene and ethylene, are linked by a cation located between them are analyzed in this study. In particular, the C2 H2 ⋅⋅⋅M+ ⋅⋅⋅C2 H2 , C2 H4 ⋅⋅⋅M+ ⋅⋅⋅C2 H2 , and C2 H4 ⋅⋅⋅M+ ⋅⋅⋅C2 H4 complexes (M+ =H+ , Li+ , Au+ ) are calculated with the use of MP2 and CASPT2 methods. The Quantum Theory of Atoms in Molecules (QTAIM), energy decomposition analysis (EDA), and Natural Bond Orbital (NBO) approaches are applied to deepen the understanding of the nature of M+ ⋅⋅⋅π interactions in these complexes. It is found that the interactions in gold and proton complexes are characterized by at least partial covalency, whereas interactions in lithium complexes are rather electrostatic in nature.

Theoretical design of conjugated diradicaloids as singlet fission sensitizers: quinones and methylene derivatives.

The electronic structures of 206 carbonyls and methylene derivatives based on conjugated cyclic hydrocarbons are computationally studied in this work using theoretical methods of quantum chemistry. The singlet open-shell nature of the ground state and its influence on the low-lying excited states is analyzed for 90 carbonyl (quinone, Q), 90 methylene (quinone dimethide, QDM) and 26 carbonyl-methylene (quinone methide, QM) mixed derivatives in the pursuit of new promising candidates for singlet fission sensitizers. Non-negligible diradical character is observed for most of the studied molecules, which is mainly determined by the nature and the relative position of the substituting groups in the bare rings. In general, the methylene group enhances to a greater extent the diradical character and the following trend is observed: y0(QDM) > y0(QM) > y0(Q). This feature leads to a decrease in the energy of the S0 → S1 and, especially, the S0 → T1 transitions, facilitating the accomplishment of the singlet fission energetic conditions: 2T1-S1 ≤ 0 (C1) and 2T1-T2 < 0 (C2). For all the methylene derivatives, these transitions have π → π* character, while some carbonyl-containing molecules, in particular those with low diradical character, show transitions with n → π* character, due to the presence of the lone pairs of the oxygen atom. From the total set of 206 molecules analyzed, 10 molecules with intermediate diradical character may be considered as potential candidates to undergo singlet fission efficiently.

The role of non-covalent interactions in the self-healing mechanism of disulfide-based polymers.

In this work, a theoretical protocol based on classical molecular dynamics has been defined, in order to study weak non-covalent interactions in diphenyl disulfide based compounds. This protocol is then used to study the influence of hydrogen bonds and π-π stacking in four selected cases, namely, monosubstituted and amine ortho trisubstituted urea and urethane-based diphenyl disulfides. In all cases, it has been observed that hydrogen bonds are much more relevant than π-π stacking, which has little influence. In addition, hydrogen bonds are the responsible to maintain the polymeric chains close, so that the disulfides may reach the reacting region, even in urethane-based materials, where the lower amount of hydrogen bonds formed make the chains more flexible and mobile. Combining the results obtained by classical molecular dynamics with those obtained earlier by means of quantum mechanics, we conclude that there are two main factors that are relevant to the self-healing properties of disulfide-based materials: firstly, the capacity to generate sulfenyl radicals by breaking the disulfide S-S bond and, secondly, the ability of these radicals to attack neighboring disulfides. The former is dominated by the bond dissociation energy of the S-S bond, while the latter is strongly influenced by two other factors. On the one hand, the hydrogen bonding interactions established between chains, and on the other, the energy barriers for the attack of sulfur radicals to neighbor disulfides. We have defined three new parameters to estimate the influence of these features, with the aim of predicting the self-healing capacity of disulfides and related materials, which will help experimentalists in the development of improved materials.

On the Termination Mechanism in the Radical Polymerization of Acrylates.

In a recent publication, Nakamura and co-workers studied the termination mechanism in the radical polymerization of acrylates. Contrary to conventional thinking, their conclusion is that termination is overwhelmingly by disproportionation. This finding impacts on a large body of the previous work in the polymerization of acrylic monomers which this work seeks to address. Analysis of the molecular weight distribution of acrylic polymers obtained under different polymerization conditions shows that termination by combination is the more probable mechanism for mutual termination of secondary radicals. It is proposed that in the experiments conducted by Nakamura and co-workers, backbiting plays a key role and their experimental data are reinterpreted, showing that they are more revealing with respect to the mode of termination of the midchain radical produced by backbiting, than to bimolecular termination of secondary radicals.

Innovative Poly(Ionic Liquid)s by the Polymerization of Deep Eutectic Monomers.

The incorporation of ionic liquid (IL) chemistry into functional polymers has extended the properties and applications of polyelectrolytes. However, ILs are expensive due to the presence of fluorinated anions or complicated synthetic steps which limit their technological viability. Here, we show a new family of poly(ionic liquid)s (PILs) which are based in cheap and renewable chemicals and involves facile synthetic approaches. Thus, deep eutectic monomers (DEMs) are prepared for the first time by using quaternary ammonium compounds and various hydrogen bond donors such as citric acid, terephthalic acid or an amidoxime. The deep eutectic formation is made through a simple mixing of the ingredients. Differential scanning calorimetry, nuclear magnetic resonance (NMR) and computational studies reveal the formation of the DEMs due to the ionic interactions. The resulting DEMs are liquid which facilitates their polymerization using mild photopolymerization or polycondensation strategies. Spectroscopic characterizations reveal the successful formation of the polymers. By this way, a new family of PILs can be synthesized which can be used for different applications. As an example, the polymers show promising performance as solid CO2 sorbents. Altogether the deep eutectic monomer route can lead to non-toxic, cheap and easy-to-prepare alternatives to current PILs for different applications.

Dihydrogen bond interactions as a result of H2 cleavage at Cu, Ag and Au centres.

A quantum chemical study of H2 activation at fluorides of coinage metals, MF (M = Cu, Ag and Au), and its splitting was performed. The following reaction path was analyzed: FMH2→ FHHM → HMFH, where both the molecular complexes and the corresponding transition states have been characterized at the CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVQZ level of theory. Further single-point CASSCF/CASPT2 calculations, including spin-orbit coupling effects, were also performed to analyze the role of non-dynamic correlation. The scalar relativistic effects are included via aug-cc-pVQZ-PP basis sets used for the metals. The dihydrogen-bonded copper (FHHCu) and silver (FHHAg) complexes are observed as a result of H2 cleavage, while the corresponding FHHAu gold complex is not found but the HAuHF arrangement is observed, instead. The energetic and geometrical parameters of the complexes have been analyzed and both the Quantum Theory of Atoms in Molecules approach and the Natural Bond Orbitals method were additionally applied to analyze the intermolecular interactions.

Design of new disulfide-based organic compounds for the improvement of self-healing materials.

Self-healing materials are a very promising kind of materials due to their capacity to repair themselves. Among others, diphenyl disulfide-based compounds (Ph2S2) appear to be among the best candidates to develop materials with optimum self-healing properties. However, few is known regarding both the reaction mechanism and the electronic structure that make possible such properties. In this vein, theoretical approaches are of great interest. In this work, we have carried out theoretical calculations on a wide set of different disulfide compounds, both aromatic and aliphatic, in order to elucidate the prevalent reaction mechanism and the necessary electronic conditions needed for improved self-healing properties. Two competitive mechanisms were considered, namely, the metathesis and the radical-mediated mechanism. According to our calculations, the radical-mediated mechanism is the responsible for this process. The formation of sulfenyl radicals strongly depends on the S-S bond strength, which can be modulated chemically by the use of proper derivatives. At this point, amino derivatives appear to be the most promising ones. In addition to the S-S bond strength, hydrogen bonding between disulfide chains seems to be relevant to favour the contact among disulfide units. This is crucial for the reaction to take place. The calculated hydrogen bonding energies are of the same order of magnitude as the S-S bond energies. Finally, reaction barriers have been analysed for some promising candidates. Two reaction mechanisms were compared, namely, the [2+2] metathesis reaction mechanism and the [2+1] radical-mediated mechanism. No computational evidence for the existence of any transition state for the metathesis mechanism was found, which indicates that the radical-mediated mechanism is the one responsible in the self-healing process of these materials. Interestingly, the calculated reaction barriers are around 10 kcal mol(-1) regardless the substituent employed. All these results suggest that the radical formation and the structural role of the hydrogen bonding prevale over kinetics. Having this in mind, as a conclusion, some new compounds are proposed for the design of future self-healing materials with improved features.

The Electronic Structure of the Al3(-) Anion: Is it Aromatic?

Multiconfigurational high-level electronic structure calculations show that the Al3(-) ring-like cluster anion has three close low-lying electronic states of different spin, all of them having strong multiconfigurational character. The aromaticity of the cluster has, therefore, been studied by means of total electron delocalization and normalized multicenter electron delocalization indices evaluated from the multiconfigurational wave functions of each state. The lowest-lying singlet and triplet states are found to be highly aromatic, whereas the next lowest-lying state, the quintet state, has much less, though non-negligible, aromatic character.

Performance of PNOF6 for Hydrogen Abstraction Reactions.

Radical formation through homolytic X-H bond cleavage in LiH, BH, CH4, NH3, H2O, and HF is investigated using natural orbital functional theory in its recent PNOF6 implementation, which includes interelectron-pair correlation, and the results are compared to those of the PNOF5 level of theory, CASSCF wave function methods, and experimental data. It is observed that PNOF6 is able to improve the estimation of the corresponding dissociation energies (De) with respect to PNOF5. When PNOF6 is combined with a better description of the electron pair, through the use of an extended number of coupled orbitals, we obtain further improvements of these quantities. The convergence of the corresponding De values with the number of coupled orbitals is also discussed, finding that a proper convergence of the results is attained with three orbitals. Next, we apply PNOF6 and its improved version PNOF6(3) to describe the thermodynamics of C-H homolytic bond cleavage for a data set of 20 organic molecules in which the C-H bond is broken in the context of different chemical environments. Finally, the radical stabilization energies obtained for such a general data set are compared with the experimental data, demonstrating that the inclusion of interelectron-pair correlation in natural orbital functional theory as in PNOF6 gives a resonable description of radical stability, especially as electron pair description is improved.

An integrated experimental and theoretical approach to probe Cr(vi) uptake using decorated halloysite nanotubes for efficient water treatment.

Halloysite nanotubes (HNTs) were surface functionalized using four distinct chemical moieties (amidoxime, hydrazone, ethylenediamine (EDA), and diethylenetriamine (DETA)), producing modified HNTs (H1-H4) capable of binding with Cr(vi) ions. Advanced techniques like FTIR, XRD, SEM, and EDX provided evidence of the successful functionalization of these HNTs. Notably, the functionalization occurred on the surface of HNTs, rather than within the interlayer or lumen. These decorated HNTs were effective in capturing Cr(vi) ions at optimized sorption parameters, with adsorption rates ranging between 58-94%, as confirmed by atomic absorption spectroscopy (AAS). The mechanism of adsorption was further scrutinized through the Freundlich and Langmuir isotherms. Langmuir isotherms revealed the nearest fit to the data suggesting the monolayer adsorption of Cr(vi) ions onto the nanotubes, indicating a favorable adsorption process. It was hypothesized that Cr(vi) ions are primarily attracted to the amine groups on the modified nanotubes. Quantum chemical calculations further revealed that HNTs functionalized with hydrazone structures (H2) demonstrated a higher affinity (interaction energy -26.33 kcal mol-1) for the Cr(vi) ions. This can be explained by the formation of stronger hydrogen bonds with the NH moieties of the hydrazone moiety, than those established by the OH of oxime (H1) and longer amine chains (H3 and H4), respectively. Overall, the findings suggest that these decorated HNTs could serve as an effective and cost-efficient solution for treating water pollution.

Tuning Reprocessing Temperature of Aliphatic Polyurethane Networks by Alkoxyamine Selection.

Recent studies have shown that the largest employed thermoset family, polyurethanes (PUs), has great potential to be reprocessed due to the dynamic behavior of carbamate linkage. However, it requires high temperatures, especially in the case of aliphatic PUs, which causes side reactions besides the desired exchange reaction. To facilitate the reprocessing of aliphatic PUs, in this work, we have explored the dynamic potential of alkoxyamine bonds in PU networks to facilitate the reprocessing under mild conditions considering their fast recombination ability. Taking advantage of the structural effect of the nitroxide and alkyl radicals on the dissociation energy, two different alkoxyamine-based diols have been designed and synthesized to generate PU networks. Our study shows that replacing 50 mol % of a nondynamic diol chain extender with these dynamic blocks boosts the relaxation times of the networks, enabling reprocessing at temperatures as low as 80 °C.

Quantum chemical study of the reactions between Pd+/Pt+ and H2O/H2S.

The study of the reactions of water and hydrogen sulfide with palladium and platinum cations has been completed in this work, in both low- and high-spin states. Our calculations predict that only the formation of platinum sulfide is exothermic (in both spin states), whereas for the remaining species the oxides and sulfides are found to be more reactive than their corresponding bare metal cations. An in-depth analysis of the reaction paths leading to metal oxide and sulfide species is given, including various minima, and several important transition states. All results have been compared with existing experimental and theoretical data, and earlier works covering the reaction of nickel cation with water and hydrogen sulfide to observe the trends for the group 10 transition metals.

Molecules with high bond orders and ultrashort bond lengths: CrU, MoU, and WU.

The structural and energetic parameters of MU heterobimetallic dimers (M = Cr, Mo, W) have been computed using the complete active space self-consistent-field method followed by second-order perturbation theory. Our results show that the effective bond order (EBO) of the MoU dimer (5.5) is higher than that for the tungsten dimer (5.2), known to date as the molecule with the highest EBO. These heterodimers present also ultrashort bond distances and remarkably large dissociation energies, which make these molecules suitable and interesting potential candidates in synthetic bimetallic organometallic chemistry.

Electronic spectroscopy and electronic structure of diatomic IrSi.

The optical spectrum of diatomic IrSi has been investigated for the first time, with transitions observed in the range from 17,178 to 23,858 cm(-1) (582-419 nm). A rich spectrum has been recorded, consisting of 14 electronic band systems and a number of unclassified bands. Thirty-one bands have been investigated with rotational resolution, allowing the ground state to be identified as X(2)Δ5∕2 arising from the 1σ(2)1π(4)2σ(2)1δ(3)3σ(2) configuration. The ground X(2)Δ5∕2 state is characterized by ΔG1∕2 = 533 cm(-1) and r0 = 2.0899(1) Å for the more abundant isotopic form, (193)Ir(28)Si (57.8%). The measured excited electronic states have equilibrium bond lengths ranging from 2.17 to 2.25 Å and vibrational frequencies ranging from 365 to 452 cm(-1). Ab initio calculations were also carried out on the molecule using the complete active space self-consistent field and multistate complete active space second-order perturbation theory methods, with relativistic and spin-orbit effects included through the restricted active space state-interaction with spin-orbit coupling method. The calculated ground state agrees with experiment, and a large number of excited states lying within 20,000 cm(-1) of the ground state are reported.