Structural Principles of Polyhedral Allotropes of Phosphorus

We derive the structural principles of polyhedral allotropes of phosphorus, introducing three distinct families of black phosphorus nanostructures. The predicted tetrahedral, octahedral, and icosahedral phosphorus cages can also be considered as phosphorus fullerenes. Phosphorus cages up to P₈₈₈ are systematically investigated by quantum chemical methods, and their thermodynamic stabilities are compared with the experimentally known allotropic forms of phosphorus. The tetrahedral cages are thermodynamically favored over the octahedral and icosahedral structures, although large octahedral structures become nearly as stable as the tetrahedral ones. The stability trends of the studied polyhedral families can be rationalized on the basis of their structural characteristics. The phosphorus polyhedra can be further stabilized by fitting smaller structures inside larger ones, resulting in multilayered, bulk‐like cages. The synthesis of the predicted black phosphorus nanostructures is suggested to be viable from the thermodynamic point of view, and several approaches for their experimental preparation can be envisaged.     

ChemInform Abstract: The Chemistry of Phosphorus Suboxides.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

五员碳环、氮环和磷环比较的量子化学研究

用密度泛函(DFT)理论中的B3LYP和从头算(abinitio)理论中UHF,在6-31G基组水平上,对五员碳环、氮环和磷环进行几何优化计算,由所得结果讨论了分子的成键情况,对两类环(一类是C5H5、N5、P5;另一类是C5H5-、N5-和P5-)的相对稳定性分别作比较。结果表明:有机五员碳环比无机五员氮环、磷环稳定,而五员氮环与五员磷环相比,磷环更稳定。     

On the Origin of the Enhanced Acidity of Chalcocyclopentadienes (Cyclopentadiene Chalcogenols) in the Gas Phase

The intrinsic acidity of chalcocyclopentadienes (CpXH; X=O, S, Se, Te) is investigated by high‐level G3B3 and G2 ab initio as well as B3LYP DFT calculations, which show that, independent of the nature of the heteroatom, all chalcocyclopentadienes are stronger acids in the gas phase than cyclopentadiene. However the acidity does not increase regularly down the group, and the acidity enhancement for Te derivatives is five times larger than for O derivatives, but only twice that of S‐containing compounds. The most favorable deprotonation process corresponds to loss of the proton attached to the heteroatom, with the sole exception of the 5‐substituted 1,3‐cyclopentadienes, for which the O and S derivatives are predicted to behave as carbon acids. No matter the nature of the heteroatom, the 1‐substituted 1,3‐cyclopentadienes are the strongest acids. The intrinsic acidity of all isomers, namely, 1‐substituted, 2‐substituted, and 5‐substituted 1,3‐cyclopentadienes, increases with increasing aromaticity of the anion formed on deprotonation, and therefore the Te compound is the strongest acid for the three series. However, the intrinsic acidity of chalcocyclopentadienes is not dictated by aromaticity, so that, in general, the most stable deprotonated species do not coincide with the most aromatic ones.     

A theoretical study of the structure and bonding of UOX4 (X = F, Cl, Br, I) molecules: the importance of inverse trans influence.

During nitroxide-mediated polymerization (NMP) in the presence of a nitroxide R2(R1)NO*, the reversible formation of N-alkoxyamines [P-ON(R1)R2] reduces significantly the concentration of polymer radicals (P*) and their involvement in termination reactions. The control of the livingness and polydispersity of the resulting polymer depends strongly on the magnitude of the bond dissociation energy (BDE) of the C-ON(R1)R2 bond. In this study, theoretical BDEs of a large series of model N-alkoxyamines are calculated with the PM3 method. In order to provide a predictive tool, correlations between the calculated BDEs and the cleavage temperature (T(c)), and the dissociation rate constant (k(d)), of the N-alkoxyamines are established. The homolytic cleavage of the N-OC bond is also investigated at the B3P86/6-311++G(d,p)//B3LYP/6-31G(d), level. Furthermore, a natural bond orbital analysis is carried out for some N-alkoxyamines with a O-C-ON(R1)R2 fragment, and the strengthening of their C-ON(R1)R2 bond is interpreted in terms of stabilizing anomeric interactions.     

Novel Bonding Character of the Cyclic AlS2 and GaS2 Systems

The geometries, the harmonic vibrational frequencies and the bonding properties have been predicted for cyclic AlS2 and GaS₂ species at the density functional theory (DFT), MPn (n = 2, 3, 4), QCISD(T) and CCSD(T) levels with 6‐311 + G (2df) basis set. The novel bonding character was discussed.     

Theoretical insights into the mechanism for thiol/disulfide exchange

The mechanism for thiol/disulfide exchange has been studied with high-level theoretical calculations. Free energies, transition structures, charge densities, and solvent effects along the reaction pathway have been determined for the first time. Mechanistic results agree with experimental data, and support the idea that the thiolate is the reacting species and that the reaction indeed proceeds through an uncomplicated S(N)2 transition state. The transition structures have the charge density evenly concentrated in the attacking and leaving sulfur atoms. The charge densities allow us to rationalize the solvent effects. As transition structures have the charge density more widely distributed than reactants, hydrophobic environments catalyze the reaction. The effect can be so dramatic that disulfide exchange inside the active site of ribonucleotide reductase is estimated to be catalyzed 10(3) times faster than the reaction in water. It was also found that attack by thiol is much faster than previously assumed, if mediated through water chains. Although the present results, as well as experimental data, still suggest that thiolate is the main reaction species, water-mediated thiol attack is almost kinetically competitive, and can eventually become competitive under specific experimental conditions.     

Theoretical and photoluminescence studies on the d10-s2 AuI-TlI interaction in extended unsupported chains

The reactions of solutions of TlPF(6) and OPPh(3) in tetrahydrofuran or acetone with NBu(4)[AuR(2)] (R=C(6)Cl(5), C(6)F(5)) gave the new complexes [Au(C(6)Cl(5))(2)](2)[Tl(OPPh(3))][Tl(OPPh(3))(L)] (L=THF (1), acetone (2)) and the previously reported [Tl(OPPh(3))(2)][Au(C(6)F(5))(2)] (3). The crystal structures of complexes 1 and 2 display extended unsupported chains with short intermolecular interactions between alternating gold(I) and thallium(I) centres. Moreover, the Tl(I) centres show two different types of geometrical environments, such as pseudotetrahedral and distorted trigonal-bipyramidal, due to the presence of solvent molecules that act as ligands in the solid-state structure. Quasirelativistic and nonrelativistic ab initio calculations were performed to study the nature of the intermetallic Au(I)-Tl(I) interactions and are consistent with the presence of a high ionic contribution (80 %) and dispersion-type (van der Waals) interaction with a charge-transfer contribution (20 %) when relativistic effects are taken into account. All complexes are luminescent in the solid state at room temperature and at 77 K. Complexes 1 and 2 show site-selective excitation, probably due to the different environments around the Tl(I) centres. The DFT and time-dependent (TD)-DFT calculations are in agreement with the experimental excitation spectra for all complexes and confirm the site-selective excitation behaviour as a function of the Tl(I) geometrical environment.     

Weak Pnictogen Bond with Bismuth: Experimental Evidence Based on Bi−P Through-Space Coupling

To study pnictogen bonding involving bismuth, flexible accordion-like molecular complexes of the composition [P(C₆H₄-o-CH₂SCH₃)₃BiX₃], (X=Cl, Br, I) have been synthesised and characterised. The strength of the weak and mainly electrostatic interaction between the Bi and P centres strongly depends on the character of the halogen substituent on bismuth, which is confirmed by single-crystal X-ray diffraction analyses, DFT and ab initio computations. Significantly, ²⁰⁹Bi–³¹P through-space coupling (J=2560 Hz) is observed in solid-state ³¹P NMR spectra, which is so far unprecedented in the literature, delivering direct information on the magnitude of this pnictogen interaction.     

Assessing the Viability of the Methylsulfinyl Radical-Ozone Reaction

Although integral to remote marine atmospheric sulfur chemistry, the reaction between methylsulfinyl radical (CH₃SO) and ozone poses challenges to theoretical treatments. The lone theoretical study on this reaction reported an unphysically large barrier of 66 kcal mol⁻¹ for abstraction of an oxygen atom from O₃ by CH₃SO. Herein, we demonstrate that this result stems from improper use of MP2 with a single-reference, unrestricted Hartree-Fock (UHF) wavefunction. We characterized the potential energy surface using density functional theory (DFT), as well as multireference methodologies employing a complete active-space self-consistent field (CASSCF) reference. Our DFT PES shows, in contrast to previous work, that the reaction proceeds by forming an addition adduct [CH₃S(O₃)O] in a deep potential well of 37 kcal mol⁻¹. An O−O bond of this adduct dissociates via a flat, low barrier of 1 kcal mol⁻¹ to give CH₃SO₂+O₂. The multireference computations show that the initial addition of CH₃SO+O₃ is barrierless. These results provide a more physically intuitive and accurate picture of this reaction than the previous theoretical study. In addition, our results imply that the CH₃SO₂ formed in this reaction can readily decompose to give SO₂ as a major product, in alignment with the literature on CH₃SO reactions.     

Enthalpy and Entropy Barriers Explain the Effects of Topology on the Kinetics of Zeolite‐Catalyzed Reactions

The methylation of ethene, propene, and trans‐2‐butene on zeolites H‐ZSM‐58 (DDR), H‐ZSM‐22 (TON), and H‐ZSM‐5 (MFI) is studied to elucidate the particular influence of topology on the kinetics of zeolite‐catalyzed reactions. H‐ZSM‐58 and H‐ZSM‐22 are found to display overall lower methylation rates compared to H‐ZSM‐5 and also different trends in methylation rates with increasing alkene size. These variations may be rationalized based on a decomposition of the free‐energy barriers into enthalpic and entropic contributions, which reveals that the lower methylation rates on H‐ZSM‐58 and H‐ZSM‐22 have virtually opposite reasons. On H‐ZSM‐58, the lower methylation rates are caused by higher enthalpy barriers, owing to inefficient stabilization of the reaction intermediates in the large cage‐like pores. On the other hand, on H‐ZSM‐22, the methylation rates mostly suffer from higher entropy barriers, because excessive entropy losses are incurred inside the narrow‐channel structure. These results show that the kinetics of crucial elementary steps hinge on the balance between proper stabilization of the reaction intermediates inside the zeolite pores and the resulting entropy losses. These fundamental insights into their inner workings are indispensable for ultimately selecting or designing better zeolite catalysts.     

Light‐Induced Ambient Degradation of Few‐Layer Black Phosphorus: Mechanism and Protection

The environmental instability of single‐ or few‐layer black phosphorus (BP) has become a major hurdle for BP‐based devices. The degradation mechanism remains unclear and finding ways to protect BP from degradation is still highly challenging. Based on ab initio electronic structure calculations and molecular dynamics simulations, a three‐step picture on the ambient degradation of BP is provided: generation of superoxide under light, dissociation of the superoxide, and eventual breakdown under the action of water. The well‐matched band gap and band‐edge positions for the redox potential accelerates the degradation of thinner BP. Furthermore, it was found that the formation of P‐O‐P bonds can greatly stabilize the BP framework. A possible protection strategy using a fully oxidized BP layer as the native capping is thus proposed. Such a fully oxidization layer can resist corrosion from water and leave the BP underneath intact with simultaneous high hole mobility.     

Two‐Dimensional Phosphorus Oxides as Energy and Information Materials

Phosphorene is a rising star in electronics. Recently, 2D phosphorus oxides with higher stability have been synthesized. In this study, we theoretically explored the structures and properties of 2D phosphorus oxides. We found that the structural features of P_xO_y vary with the oxygen content. When the oxygen content is low, the most stable P_xO_y material can be obtained by the adsorption of O atoms on phosphorene. Otherwise, stable structures are no longer based on phosphorene and will contain P–O–P motifs. We found that P₄O₄ has a direct band gap (about 2.24 eV), good optical absorption, and high stability in water, so it may be suitable for photochemical water splitting. P₂O₃ adopts two possible stable ferroelectric structures (P₂O₃‐I and P₂O₃‐II) with electric polarization perpendicular and parallel to the lateral plane, respectively, as the lowest‐energy configurations, depending on the layer thickness. We propose that P₂O₃ could be used in novel nanoscale multiple‐state memory devices.     

Ab initio study of free-radical polymerizations: cost-effective methods to determine the reaction rates.

The addition of carbon-centered radicals to ethene, which are important in free-radical polymerization processes, are studied from a theoretical point of view. Experimental data for the rate constants are only available for the addition of methyl, ethyl, propyl and butyl radicals. The latter reactions are taken as model systems to derive a cost-effective method for the addition of alkyl radicals to ethene. The proposed model must be accurate and computationally feasible for additions in which larger radicals are involved. Accuracy is validated by direct comparison of theoretical and experimental rate constants in the temperature range from 300 to 600 K. A variety of electronic-structure methods were tested ranging from Hartree-Fock and post-Hartree-Fock methods to pure and hybrid density functional theory methods. Molecular partition functions were refined by treating large amplitude vibrations beyond the harmonic oscillator approximation.