Cycloaddition of Benzyne to Armchair Single-Walled Carbon Nanotubes: [2 + 2] or [4 + 2]?

Pyridine Functionalized Carbon Nanotubes: Unveiling the Role of External Pyridinic Nitrogen Sites for Oxygen Reduction Reaction

Pyridinic nitrogen has been recognized as the primary active site in nitrogen-doped carbon electrocatalysts for the oxygen reduction reaction (ORR), which is a critical process in many renewable energy devices. However, the preparation of nitrogen-doped carbon catalysts comprised of exclusively pyridinic nitrogen remains challenging, as well as understanding the precise ORR mechanisms on the catalyst. Herein, a novel process is developed using pyridyne reactive intermediates to functionalize carbon nanotubes (CNTs) exclusively with pyridine rings for ORR electrocatalysis. The relationship between the structure and ORR performance of the prepared materials is studied in combination with density functional theory calculations to probe the ORR mechanism on the catalyst. Pyridinic nitrogen can contribute to a more efficient 4-electron reaction pathway, while high level of pyridyne functionalization result in negative structural effects, such as poor electrical conductivity, reduced surface area, and small pore diameters, that suppressed the ORR performance. This study provides insights into pyridine-doped CNTs-functionalized for the first time via pyridyne intermediates-as applied in the ORR and is expected to serve as valuable inspiration in designing high-performance electrocatalysts for energy applications.

Mechanism and the Origins of Periselectivity in Cycloaddition Reactions of Benzyne with Dienes

Density functional theory calculations have been used to explore the reaction mechanism of (4 + 2) and (2 + 2) cycloadditions of benzyne with classical dienes. The results indicate the following: (1) (4 + 2) products arise via concerted pathways, (2) (2 + 2) products arise via stepwise pathways with diradical intermediates, and (3) these diradical intermediates are formed via isomerization of carbene intermediates. The origins of periselectivity in these reactions are analyzed using distortion/interaction analysis for the key steps, and they indicate that the tiny distortion in the very early [4 + 2] transition structure, coupled with an entropic favorability, controls selective (4 + 2) cycloaddition.

Switchable (2 + 2) and (4 + 2) Cycloadditions on Boron Nitride Nanotubes under Oriented External Electric Fields: A Mechanistic Study

The (2 + 2) and (4 + 2) cycloadditions are important approaches for the functional derivatizations of nanocarbon and hexagonal boron nitride (hBN) materials. However, as two competing reactions with similar reactivity, it is difficult to control the type of reactions and the corresponding adducts in practice. Here, we introduced a mechanistic study of the oriented external electric field (OEEF)-modulated cycloadditions of pristine and substituted benzynes on the zigzag boron nitride nanotubes. Owing to the distinct charge transfer directions between the competing (2 + 2) and (4 + 2) reactions and the resultant distinct responses of the barriers to the fields along the tube axis, we found that OEEF plays opposing catalytic roles in these two types of reactions and the effect of electric field as a catalyst or inhibitor can be easily reversed by flipping the field vector to achieve selective reactions and products at will.

Covalent Functionalization of Single-Walled Carbon Nanotubes by the Bingel Reaction for Building Charge-Transfer Complexes

Functionalization of nanotubes with donor and acceptor partners by the Bingel reaction leads to the formation of charge-transfer dyads, which can operate in organic photovoltaic devices. In this work, we theoretically examine the mechanism of the Bingel reaction for the (6,5)-chiral, (5,5)-armchair, and (9,0)-zigzag single-walled carbon nanotubes (SWCNTs), and demonstrate that the reaction is regioselective and takes place at the perpendicular position of (6,5)- and (5,5)-SWCNTs, and the oblique position of (9,0)-SWCNT. Further, we design computationally the donor-acceptor complexes based on (6,5)-SWCNT coupled with partners of different electronic nature. Analysis of their excited states reveals that efficient photoinduced charge transfer can be achieved in the complexes with π-extended analogue of tetrathiafulvalene (exTTF), zinc tetraphenylporphyrin (ZnTPP), and tetracyanoanthraquinodimethane (TCAQ). The solvent can significantly affect the population of the charge-separated states. Our calculations show that electron transfer (ET) occurs in the normal Marcus regime on a sub-nanosecond time scale in the complexes with exTTF and ZnTPP, and in the inverted Marcus regime on a picosecond time scale in the case of the TCAQ derivative. The ET rate is found to be not very sensitive to the degree of functionalization of the nanotube.

Microwave-induced covalent functionalization of few-layer graphene with arynes under solvent-free conditions

A non-conventional modification of exfoliated few-layer graphene (FLG) with different arynes under microwave (MW) irradiation and solvent-free conditions is reported. The described approach allows reaching fast, efficient and mild covalent functionalization of FLG.

Effects of Solvent and Side-Chain Length on the Cycloaddition of Cyclopentadiene to N-alkylmaleimides: A Dft Study

Quantum chemistry calculations have been performed to investigate the kinetics and mechanism of the cycloaddition reaction between cyclopentadiene and N-alkylmaleimides in the gas phase and different solvents. To investigate the effects of side-chain length on the cycloaddition reaction, the rate constants and kinetic parameters of the reaction between cyclopentadiene with N-methyl-, N-ethyl-, N-propyl- and N-butyl-maleimide were calculated. The results obtained indicate that the reaction in solvents is faster than in the gas phase. Moreover, the dipole moments of the transition states are larger than those of the reactants. Therefore, the reactions in the most polar solvent (water) are faster than in ethanol, n-hexane, 2,2,2-trifluoroethanol (TFE) and acetonitrile. Quantum mechanics-molecular mechanics (QM/MM) calculations on the reactions using the explicit solvent model for water and TFE indicate that hydrogen bond interactions of the solvents have a key role in the rate of the reaction and these are more important than the polarity of the solvent. Natural bond orbital analysis reveals that the charge transfer between the reactants in solvents is more than in the gas phase. Finally, HOMO–LUMO analysis indicates that solvents increase the reactivity of the reactants in comparison to the gas phase.

The Covalent Functionalization of Graphene on Substrates

The utilization of grown or deposited graphene on solid substrates offers key benefits for functionalization processes, but especially to attain structures with a high level of control for electronics and "smart" materials. In this review, we will initially focus on the nature and properties of graphene on substrates, based on the method of preparation. We will then analyze the most relevant literature on the functionalization of graphene on substrates. In particular, we will comparatively discuss radical reactions, cycloadditions, halogenations, hydrogenations, and oxidations. We will especially address the question of how the reactivity of graphene is affected by its morphology (i.e., number of layers, defects, substrate, curvature, etc.).