Nitro-sonium nitrate (NO+NO3 -) structure solution using in situ single-crystal X-ray diffraction in a diamond anvil cell

Abstract

At high pressures, autoionization - along with polymerization and metallization - is one of the responses of simple molecular systems to a rise in electron density. Nitro-sonium nitrate (NO⁺NO₃ ^-), known for this property, has attracted a large interest in recent decades and was reported to be synthesized at high pressure and high temperature from a variety of nitro-gen-oxygen precursors, such as N₂O₄, N₂O and N₂-O₂ mixtures. However, its structure has not been determined unambiguously. Here, we present the first structure solution and refinement for nitro-sonium nitrate on the basis of single-crystal X-ray diffraction at 7.0 and 37.0 GPa. The structure model (P2₁/m space group) contains the triple-bonded NO⁺ cation and the NO₃ ^- sp ²-trigonal planar anion. Remarkably, crystal-chemical considerations and accompanying density-functional-theory calculations show that the oxygen atom of the NO⁺ unit is positively charged - a rare occurrence when in the presence of a less-electronegative element.

Keywords: high-pressure single-crystal X-ray diffraction; nitro­sonium nitrate; positively charged oxygen atoms; structure refinement.

Figure 1

(a) Microphotographs of a N₂O sample at 15.8 GPa, before (top) and after (bottom) laser heating. The gold particles are encircled in white (top). The recrystallized matter in the area surrounding the laser-heated gold particles is where the chemical reaction occurred (bottom). (b) Raman spectra of NO⁺NO₃ ⁻ obtained upon the sample decompression from 55.0 down to 3.9 GPa. The Raman modes marked by asterisks belong to pure molecular nitro­gen (Bini et al., 2000 ▸). (c) The evolution of the Raman shift of the nitro­sonium nitrate Raman modes compared with those previously reported (Yoo et al., 2003; Kuznetsov et al., 2008 ▸).

Figure 2

The diffraction pattern (λ = 0.2891 Å) of nitro­sonium nitrate at 3.9 GPa following the sample laser heating at 37.0 GPa and subsequent decompression. The red dots, black line and green ticks represent the experimental data, the Le Bail fit and the positions of the diffraction lines of nitro­sonium nitrate, respectively. The diffraction lines of β-N₂ are not visible at this sample position.

Figure 3

(a) A monoclinic unit cell of NO⁺NO₃ ⁻ at 37 GPa. (b) A representation of the trigonal planar nitrate (top) and linear nitro­sonium (bottom) ions with bond lengths and bond angles indicated. (c)–(d) The environment of the O2 and N2 atoms forming the NO⁺ cation: both N2 and O2 atoms are fivefold coordinated by negatively charged oxygen atoms from the NO₃ ⁻ anions. The lengths are provided in Å.

Figure 4

(a) Pressure dependence of the unit-cell volume per formula unit of nitro­sonium nitrate. The volume of a 2:1 N₂–O₂ mixture (Olijnyk, 1990; Akahama et al., 1995; Gregoryanz et al., 2007 ▸) is significantly larger than that of NO⁺NO₃ ⁻ at corresponding pressures. The volume determined by Meng et al. (2006 ▸) (red dot) fits well the equation of state of nitro­sonium nitrate obtained in the current work. The data represented by the blue triangles were obtained by DFT calculations. (b) The lattice parameters of NO⁺NO₃ ⁻ as a function of pressure.