The crystal structures of the low-temperature and high-pressure polymorphs of nitric acid

The bond valence model as a prospective approach: examination of the crystal structures of copper chalcogenides with Cu bond valence excess

Bond valence analysis has been applied to various copper chalcogenides with copper valence excess, i.e. where the formal valence of copper exceeds 1. This approach always reveals a copper bond valence excess relative to the unit value, correlated to an equivalent ligand bond valence deficit. In stoichiometric chalcogenides, this corresponds to one ligand electron in excess per formula unit relative to the valence equilibrium considering only Cu^I. This ligand electron in excess is 50/50 shared between all or part of the Cu-atom positions, and all or part of the ligand-atom positions. In Cu₃Se₂, only one of the two Cu positions is involved in this sharing. It would indicate a special type of multicentre bonding (`one-electron co-operative bonding'). Calculated and ideal structural formulae according to this bond valence distribution are presented. At the crystal structure scale, Cu-ligand bonds implying the single electron in excess form one-, two- or three-dimensional subnetworks. Bond valence distribution according to two two-dimensional subnets is detailed in covellite, CuS. This bond valence description is a formal crystal-chemical representation of the metallic conductivity of holes (mixing between Cu 3d bands and ligand p bands), according to published electronic band structures. Bond valence analysis is a useful and very simple prospective approach in the search for new compounds with targeted specific physical properties.

High-Pressure Polymorphism in Hydrogen-Bonded Crystals: A Concise Review

High-pressure polymorphism is a developing interdisciplinary field. Pressure up to 20 GPa is a powerful thermodynamic parameter for the study and fabrication of hydrogen-bonded polymorphic systems. This review describes how pressure can be used to explore polymorphism and surveys the reports on examples of compounds that our group has studied at high pressures. Such studies have provided insight into the nature of structure–property relationships, which will enable crystal engineering to design crystals with desired architectures through hydrogen-bonded networks. Experimental methods are also briefly surveyed, along with two methods that have proven to be very helpful in the analysis of high-pressure polymorphs, namely, the ab initio pseudopotential plane–wave density functional method and using Hirshfeld surfaces to construct a graphical overview of intermolecular interactions.

Crystal structure, thermal properties and detonation characterization of bis(5-amino-1,2,4-triazol-4-ium-3-yl)methane dinitrate

Bis(5-amino-1,2,4-triazol-4-ium-3-yl)methane dinitrate, BATZM·(NO₃)₂ or C₅H₁₀N₈²⁺·2NO₃^-, was synthesized and its crystal structure determined by single-crystal X-ray diffraction. It crystallizes in the space group Pbcn (orthorhombic) with Z = 4. BATZM·(NO₃)₂ is a V-shaped molecule where hydrogen bonds form a two-dimensional corrugated sheet with reasonable chemical geometry and no disorder. The specific molar heat capacity (C_p,m) of BATZM·(NO₃)₂ was determined using the continuous C_p mode of a microcalorimeter and theoretical calculations, and the C_p,m value is 366.14 J K^-1 mol^-1 at 298.15 K. The relative deviations between the theoretical and experimental values of C_p,m, H_T - H_298.15K and S_T - S_298.15K of BATZM·(NO₃)₂ are almost equivalent at each temperature. The detonation velocity (D) and detonation pressure (P) were estimated using the nitrogen equivalent equation according to the experimental density; BATZM·(NO₃)₂ has a higher detonation velocity (7927.47 ± 3.63 m s^-1) and detonation pressure (27.50 ± 0.03 GPa) than 2,4,6-trinitrotoluene (TNT). The above results for BATZM·(NO₃)₂ are compared with those of bis(5-amino-1,2,4-triazol-3-yl)methane (BATZM) and bis(5-amino-1,2,4-triazol-4-ium-3-yl)methane dihydrochloride (BATZM·Cl₂), and the effect of nitrate formation is discussed.

Structure and vibrational spectroscopy of methanesulfonic acid

In this work, we have used a combination of vibrational spectroscopy (infrared, Raman and inelastic neutron scattering) and periodic density functional theory to investigate the structure of methanesulfonic acid (MSA) in the liquid and solid states. The spectra clearly show that the hydrogen bonding is much stronger in the solid than the liquid state. The structure of MSA is not known; however, mineral acids typically adopt a chain structure in condensed phases. A periodic density functional theory (CASTEP) calculation based on the linear chain structure found in the closely related molecule trifluoromethanesulfonic acid gave good agreement between the observed and calculated spectra, particularly with regard to the methyl and sulfonate groups. The model accounts for the large widths of the asymmetric S-O stretch modes; however, the external mode region is not well described. Together, these observations suggest that the basic model of four molecules in the primitive unit cell, linked by hydrogen bonding into chains, is correct, but that MSA crystallizes in a different space group than that of trifluoromethanesulfonic acid.