On the instability of iodides of heavy main group atoms in their higher oxidation state

The inert pair effect-the tendency of the s orbital of heavy atoms to stay unreactive, is a consequence of the relativistic contraction of the s orbitals. While the manifestations of this on the reactivity depend on the nature of the substituents, this aspect is often overlooked. Divalent Pb prefers inorganic substituents, whereas tetravalent Pb prefers organic substituents. Among the inorganic substituents, again there are specific preferences-tetravalent Pb prefers F and Cl more than Br and I. It is as though the relativistic contraction of the s orbital of Pb is more significant with Br and I substituents than with Cl, F, and alkyl substituents. Herein, we address this problem using the molecular orbital approach and support it with quasi-relativistic density functional computations. We explain why typical hypervalent systems, like 12-X-6, and 10-X-5 (X is a heavy atom, the number preceding X is the number of valence electrons surrounding X, and the number after X is the coordination number) with less electronegative substituents carrying a lone pair (such as iodine), and Lewis octet molecules like PbI₄ are unstable, but their dianions (14-X-6, 12-X-5, PbI₄^2-) are not. For heavy atoms, the relativistic contraction of the s orbital renders the antibonding combination of s with ligand orbitals (σ₁*) very low-lying, making it a good acceptor of electrons. Thus, compounds where σ₁* is empty are kinetically unstable when an electron donor with appropriate energy (such as the lone pair on iodine or bromine) is present in the vicinity. Donor-acceptor interaction between σ₁* and the lone pair on I or Br (F and Cl lone pairs are energetically far away from σ₁*) is responsible for the instability of such compounds. The kinetic stability of tetraalkyl lead compounds is due to the absence of lone pairs on the alkyl substituents. This work illustrates the key factor responsible for the instability of heavy element iodides by taking into consideration the covalent nature of the bonds, while the existing explanations assume a purely ionic bonding, which is an oversimplification.

Gas-Phase Fluorination of g-C3N4 for Enhanced Photocatalytic Hydrogen Evolution

Graphitic carbon nitride (g-C₃N₄) has attracted much attention because of its potential for application in solar energy conservation. However, the photocatalytic activity of g-C₃N₄ is limited by the rapidly photogenerated carrier recombination and insufficient solar adsorption. Herein, fluorinated g-C₃N₄ (F-g-CN) nanosheets are synthesized through the reaction with F₂/N₂ mixed gas directly. The structural characterizations and theoretical calculations reveal that fluorination introduces N vacancy defects, structural distortion and covalent C-F bonds in the interstitial space simultaneously, which lead to mesopore formation, vacancy generation and electronic structure modification. Therefore, the photocatalytic activity of F-g-CN for H₂ evolution under visible irradiation is 11.6 times higher than that of pristine g-C₃N₄ because of the enlarged specific area, enhanced light harvesting and accelerated photogenerated charge separation after fluorination. These results show that direct treatment with F₂ gas is a feasible and promising strategy for modulating the texture and configuration of g-C₃N₄-based semiconductors to drastically enhance the photocatalytic H₂ evolution process.

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

The preparation and structure of Ge3F8 - a new mixed-valence fluoride of germanium, a convenient source of GeF2

The new binary mixed-valence fluoride of germanium, Ge3F8, has been obtained by heating GeF4 with powdered Ge in an autoclave (390 K/4 bar/48 h). The structure contains pyramidal Ge(II)F3 and octahedral Ge(IV)F6 units, linked by fluoride bridges. The new compound is the missing member of the series (GeF2)n·GeF4 (n = 2, 4, or 6). Sublimation of (GeF2)n·GeF4in vacuo provides a convenient source of GeF2 in ca. 30% overall yield.

The origin of diffuse scattering in crystalline carbon tetraiodide

Total scattering neutron powder diffraction measurements were performed on the tetragonal phase (a=6.4202(5) Å, c=9.5762(12) Å) of CI4. The experiments were followed by reverse Monte Carlo (for powder diffraction) modelling. Detailed analyses of the resulting particle configurations revealed that the observed diffuse scattering originates from the libration of the molecules. By examining the partial radial distribution functions a distinct carbon-iodine peak at 4.5 Å is found, which appears as a consequence of corner-to-face mutual alignment of two molecules. The occurrence of edge-to-edge alignments is also significant within the first carbon-carbon coordination shell.

Predicted crystal structures of tetramethylsilane and tetramethylgermane and an experimental low-temperature structure of tetramethylsilane

No crystal structure at ambient pressure is known for tetramethylsilane, Si(CH3)4, which is used as a standard in NMR spectroscopy. Possible crystal structures were predicted by global lattice-energy minimizations using force-field methods. The lowest-energy structure corresponds to the high-pressure room-temperature phase (Pa\overline{3}, Z = 8). Low-temperature crystallization at 100 K resulted in a single crystal, and its crystal structure has been determined. The structure corresponds to the predicted structure with the second lowest energy rank. In X-ray powder analyses this is the only observed phase between 80 and 159 K. For tetramethylgermane, Ge(CH_3)_4, no experimental crystal structure is known. Global lattice-energy minimizations resulted in 47 possible crystal structures within an energy range of 5 kJ mol−1. The lowest-energy structure was found in Pa\overline{3}, Z = 8.