Lone-pair interactions and photodissociation of compressed nitrogen trifluoride

Polymerization of Nitrogen in Nitrogen-Fluorine Compounds under Pressure

A wide range of polynitrogen species have attracted much attention because of their potential applications as high-energy-density materials. Until now, predicted polynitrogen was found to be negatively charged, with charge transfer from introduced atoms to nitrogen in nitrogen-bearing compounds. Using an evolutionary algorithm combined with first-principles calculations, stoichiometries and structures in nitrogen-fluorine compounds at pressures ranging from 0 to 200 GPa are investigated. In addition to two fluorine-rich compounds NF3 and NF5, two other compounds, NF and N6F, emerge with increasing pressure. N6F, as a nitrogen-rich compound, will become stable at pressures greater than 180 GPa with a positively charged nitrogen network. Above 120 GPa, the NF compound with polymeric zigzag nitrogen chains is discovered, and it is quenchable to the ambient conditions, acquiring the highest energy density of 5.38 kJ/g among reported binary covalent nitrogen compounds. These newly predicted N-F compounds are useful in understanding the chemistry of polynitrogen.

Phase Stability of Chloroform and Dichloromethane at High Pressure

Chloroform (CHCl3) and dichloromethane (CH2Cl2) are model systems for the study of intermolecular interactions, such as hydrogen bonds and halogen–halogen interactions. Here we report a joint computational (density-functional perturbation theory (DFPT) modelling) and experimental (Raman scattering) study on the behaviour of the crystals of these compounds up to a pressure of 32 GPa. Comparing the experimental information on the Raman band positions and intensities with the results of calculations enabled us to characterize the pressure-induced evolution of the crystal structure of both compounds. We find that the previously proposed P63 phase of CHCl3 is in fact a metastable structure, and that up to 32 GPa the ambient-pressure Pnma structure is the ground state polymorph of this compound. For CH2Cl2 we confirm the stability of the ambient-pressure Pbcn structure up to 32 GPa. We show that the high-pressure evolution of the crystal geometry of CHCl3 in the Pnma structure is a result of the subtle balance between dipole–dipole interactions, hydrogen bonds and Cl···Cl contacts. For CH2Cl2 (Pbcn structure) the dipole–dipole interactions and hydrogen bonds are the main factors influencing the pressure-induced changes in the geometry.

Crystal Structures of α- and β-Nitrogen Trifluoride

The crystal structures of α-NF₃ and β-NF₃ are reported for the first time. As shown by powder neutron diffraction, the low-temperature α-NF₃ crystallizes in the orthorhombic space group Pnma ( oP16) with lattice parameters a = 6.71457(13) Å, b = 7.30913(14) Å, c = 4.55189(8) Å, V = 223.396(7) ų, and Z = 4 at T = 6 K. The intramolecular atom distances in α-NF₃ are 1.3639(16) and 1.3677(11) Å for N-F, and 2.1216(16) and 2.120(2) Å for F···F. The F-N-F bond angles are 101.92(7)° and 101.63(10)°. All data are in excellent agreement with quantum-chemical predictions and previously reported experimentally obtained gas-phase data. The high-temperature β-NF₃ is a plastic crystal, space group P4₂/ mnm ( tP120), with the lattice parameters a = 15.334(6) Å, c = 7.820(3) Å, V = 1838.6(12) ų, and Z = 30 at T = 60 K. Its crystal structure is closely related to that of the Frank-Kasper sigma phase.

Hexacoordinated nitrogen(V) stabilized by high pressure

In all of its known connections nitrogen retains a valence shell electron count of eight therefore satisfying the golden rule of chemistry - the octet rule. Despite the diversity of nitrogen chemistry (with oxidation states ranging from + 5 to −3), and despite numerous efforts, compounds containing nitrogen with a higher electron count (hypervalent nitrogen) remain elusive and are yet to be synthesized. One possible route leading to nitrogen’s hypervalency is the formation of a chemical moiety containing pentavalent nitrogen atoms coordinated by more than four substituents. Here, we present theoretical evidence that a salt containing hexacoordinated nitrogen(V), in the form of an NF₆⁻ anion, could be synthesized at a modest pressure of 40 GPa (=400 kbar) via spontaneous oxidation of NF₃ by F₂. Our results indicate that the synthesis of a new class of compounds containing hypervalent nitrogen is within reach of current high-pressure experimental techniques.

High-pressure stabilization of argon fluorides

On account of the rapid development of noble gas chemistry in the past half-century both xenon and krypton compounds can now be isolated in macroscopic quantities. The same does not hold true for the next lighter group 18 element, argon, which forms only isolated molecules stable solely in low temperature matrices or supersonic jet streams. Here we present theoretical investigations into a new high-pressure reaction pathway, which enables synthesis of argon fluorides in bulk and at room temperature. Our hybrid DFT calculations (employing the HSE06 functional) indicate that above 60 GPa ArF2-containing molecular crystals can be obtained by a reaction between argon and molecular fluorine.