Grignard reagent formation via C-F bond activation: a centenary perspective

Examples of Grignard reagents obtained by C-F bond activation with magnesium kept appearing in the literature over the last century. Due to the high bond dissociation energy of the C-F...

Photodissociation of dicarbon: How nature breaks an unusual multiple bond

The dicarbon molecule (C2) is found in flames, comets, stars, and the diffuse interstellar medium. In comets, it is responsible for the green color of the coma, but it is not found in the tail. It has long been held to photodissociate in sunlight with a lifetime precluding observation in the tail, but the mechanism was not known. Here we directly observe photodissociation of C2. From the speed of the recoiling carbon atoms, a bond dissociation energy of 602.804(29) kJ·mol−1 is determined, with an uncertainty comparable to its more experimentally accessible N2 and O2 counterparts. The value is within 0.03 kJ·mol−1 of high-level quantum theory. This work shows that, to break the quadruple bond of C2 using sunlight, the molecule must absorb two photons and undergo two “forbidden” transitions.

The Influence of Titanium Dioxide on Silicate-Based Glasses: An Evaluation of the Mechanical and Radiation Shielding Properties

The mechanical and radiation shielding features were reported for a quaternary Na2O-CaO-SiO2-TiO2 glass system used in radiation protection. The fundamentals of the Makishima–Mazinize model were applied to evaluate the elastic moduli of the glass samples. The elastic moduli, dissociation energy, and packing density increased as TiO2 increased. The glasses’ dissociation energy increased from 62.82 to 65.33 kJ/cm3, while the packing factor slightly increased between 12.97 and 13.00 as the TiO2 content increased. The MCNP-5 code was used to evaluate the gamma-ray shielding properties. The best linear attenuation coefficient was achieved for glass samples with a TiO2 content of 9 mol%: the coefficient decreased from 5.20 to 0.14 cm−1 as the photon energy increased from 0.015 to 15 MeV.

Theoretical Insight into 20-Electron Transition-Metal Complexes (C5H5)2TM(E1E2)2 (TM = Cr, Mo, W; E1E2 = CO, N2, BF): Stabilities, Electronic Structures, and Bonding Nature

A systematic first-principles study is performed to investigate the 20-electron transition metal complexes (CH)TM(EE) (TM = Cr, Mo, W; EE = CO, N, BF). The bond dissociation energy (De) based on (CH)TM(EE) → (CH)TM(EE) + EE indicates much lower thermodynamic stability of (CH)TM(N) because of poor binding ability of N ligands. For the thermodynamic stable (CH)TM(EE) complexes (TM = Cr, Mo, W; EE = CO, BF), their 20-electron nature is derived from their occupied nonbonding molecular orbital mainly donated by ligands. Furthermore, charge transfer from TMs to the CH ligands is revealed by the atoms in molecules (AIM) theory, leading to the positive charges of the TM atoms. On the other hand, the nature of the TM-E bond has been thoroughly analyzed by the energy decomposition analysis (EDA) method. The absolute value of interaction energies (|ΔE|) between (CH)TM(EE) and EE has the same trend as the corresponding bond dissociation energy and Wiberg bond orders of TM-E bonds, following the order W > Mo > Cr with same ligands and BF > CO with same TM. Additionally, the largest contribution to the ΔE values is the repulsive term ΔE. Similar contributions from covalent and electrostatic terms to the TM-E bonds were found, which can be described as the classic dative bond with nearly same σ and π contributions. The stronger σ donations and π backdonations in (CH)TM(BF) than in (CH)TM(CO) indicate much more stability of (CH)TM(BF).