Size-matched hydrogen bonded hydroxylammonium frameworks for regulation of energetic materials

Size matching molecular design utilizing host-guest chemistry is a general, promising strategy for seeking new functional materials. With the growing trend of multidisciplinary investigations, taming the metastable high-energy guest moiety in well-matched frameworks is a new pathway leading to innovative energetic materials. Presented is a selective encapsulation in hydrogen-bonded hydroxylammonium frameworks (HHF) by screening different sized nitrogen-rich azoles. The size-match between a sensitive high-energy guest and an HHF not only gives rise to higher energetic performance by dense packing, but also reinforces the layer-by-layer structure which can stabilize the resulting materials towards external mechanic stimuli. Preliminary assessment based on calculated detonation properties and mechanical sensitivity indicates that HHF competed well with the energetic performance and molecular stability (detonation velocity = 9286 m s−1, impact sensitivity = 50 J). This work highlights the size-matched phenomenon of HHF and may serve as an alternative strategy for exploring next generation advanced energetic materials.


IV. Computation and properties Packing Coefficient Calculations
Packing coefficients were calculated by using equation (1), where Vm and Vc are the molecular and crystal volumes, respectively. A volume enclosed through a surface with an assigned electronic density was considered as Vm. In this work, the electronic density was calculated at the theory level of M062x/6-311+G (d, p) and the density of 0.003 au was adopted for Vm calculations.

2D fingerprint plot for intermolecular contact
Hirshfeld surfaces in a crystal are constructed in terms of the electron distribution, calculated as the sum of spherical atom electron densities. 1,2 The normalized contact distance (dnorm) is determined by di and de, the distances from the surface to the nearest atom interior and exterior to the surface, respectively, and the van der Waals radii of the atoms. dnorm enables the identification of the regions of particular significance to intermolecular interactions. That is to say, a Hirshfeld surface is composed of lots of points, and each point parametrized as (di, de) can provide information about related contact distances from it. The smaller di + de implies closer atom−atom contact. Both di and de were constrained in a range of 0-3.0 Å. Mapping these (di, de) points and considering their relative frequencies, we can get a two-dimensional (2D) fingerprint plot. For any symmetrically dependent molecule in any crystal, the fingerprint is unique. This is the basis for identifying a crystal environment of a given molecule. The color mapping distinguishes the intensity of points, and the red and the blue represent the high and low intensities, respectively. Therefore, through the locations of (di, de) points and their relative frequencies discernible on the surface and the 2D fingerprint plot, we can ascertain the distances and intensities of these contacts. All the fingerprint plots were created using CrystalExplorer 17.5 3 in this work, and the surfaces were mapped over a dnorm range of -0.4 to 1.4 Å.

Methods for estimating the diameter of guest molecules
A method 7 for estimating the molecular kinetic diameter based on the iso-electronic density surfaces. The Gaussian 09 program (Revision D.01) was used to calculate the geometric optimization and frequency analyses of the guest structure, and the PBE0/def2-TZVP level is used. subsequently, the coordinates of the iso-electronic density surfaces vertices are calculated S 29 using Multiwfn, and finally the distance between the surface vertices is measured by VMD to derive the dimensions of the guest structure.

Supplementary Tab. 18. Enthalpies of the gas-phase species M (G2 method).
Based on the literature, the heat of sublimation is estimated with Trouton's rule. 8 The solid phase heat of formation of 1 was calculated with equation 1, in which T d represents the decomposition temperature. 8 (1) According to empirical formula (2) 9 the heat of sublimation can be computed from the temperature of melting (T represents either the melting point or the decomposition temperature when no melting occurs prior to decomposition). ∆Hsub = 188*(T+273.15)/1000 (2) The solid phase heats of formation of 1 and 4 were calculated with eq 3.
∆Hf, soliid = ∆Hf, gas-∆Hsub For energetic salts, the solid-phase heat of formation is calculated based on a Born-Haber energy cycle (Supplementary Fig. 36.). 9 The number is simplified by equation 4: where ρm [g cm −3 ] is the density of the salt, Mm is the chemical formula mass of the ionic material, and values for g and the coefficients γ (kJ mol −1 cm) and δ (kJ mol −1 ) are assigned literature values. 9 By using the measured room temperature densities, all the newly prepared compounds are listed in Supplementary Tab. 19.
Supplementary Tab. 19. The density, calculated lattice energy and calculated heat of formation.
[a] Density was calculated based on a single crystal at room temperature.
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Computational analysis of internal stress
Any external mechanical force acting on an energetic material can lead to a certain shape change to produce strain and store mechanical energy. 11 When this mechanical energy exceeds the limit of the energetic material, decomposition will be activated and trigger a series of explosions. 12 In order to explore the different performance of HHF-F and NF on safety issues, a force field was established for both unit cells and their abilities to handle external stimuli were probed. To simplify the simulation, only vertical compression and horizontal sliding of the crystal unit cell were investigated since all direction stimuli can be split into these two types. 13,14 Therefore, the deformation potential (P) can be expressed by the energy difference before and after deformation for energetic materials. For convenient comparison, the value is converted from mol units into volume units by dividing by the unit cell volume (eq 10).

P = (E after def -E before def )/V unit cell (10)
In this part of the calculations, single point energies were obtained from optimized structures using BLYP-D3 def2-SVP method 14

V Application expansion of size matching strategy
For N-rich azole anions, the combination of different skeletons and substituents can get various structures of different sizes. In general, for the monocyclic systems studied in this work, their dimensions are mostly in the range of 7-15 Å.
According to the source classification of acid hydrogen in N-rich azoles for energetic hydroxylammonium salts in the CSD database, the most abundant ones are nitroamino-and nitro-substituted heterocycles, respectively. The acidic hydrogen of HHF-F and HHF-T reported in this work is derived from nitroamine-based structures.
In order to expand the application of this strategy, we construct the nitroamine unit based on the monocyclic N-rich azoles as the skeleton and introduce the nitro group, amino group, alkyl group, amide group, and carboxylic acid in other positions to adjust the guest anion size. Thus, a candidate library of guest anions is established. According to the results of theoretical calculation, the diameter of the anionic guest GN1-4 is between 8-9 Å. Due to the large volume of the hydroxylamine structure itself, there is no S 41 structure whose predicted diameter is less than 8 Å. The GN-3 (8.8 Å) is the guest structure of HHF-F which is mainly discussed in this paper, and its predicted diameter is very close to the actual Hershfield surface diameter (8.5 Å) measured by the crystal structure.

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From the above supplementary figure 47 and 48, the guest molecules in HHF-N4 are successfully embedded in the hydrogen bond framework composed of ammonium hydroxide, displaying face-to-face stacking. Analysis of the above two compounds, HHF-N4 and HHF-N14 by two-dimensional fingerprints and Hirshfeld surfaces revealed that HHF-N4 (8.8 Å), which has guest anion sizes of 8-9 Å, also has more O...H interactions in the crystal structure than HHF-N14 (10.1 Å). This means that for the nitro group, more hydrogen bonds can be made to stabilize it. Supplementary Fig. 49. The 2D fingerprint plots in crystal stacking for HHF-N4 (a) and HHF-N14 (d); Pie graphs for HHF-N4 (b) and HHF-N14 (e) show the percentage contributions of the individual atomic contacts to the Hirshfeld surface, Hirshfeld surfaces (inside). Short contact for HHF-N4 (c) and HHF-N14 (f).
In this part, the Hirshfeld surface diameter of the guest anion is in good agreement with the predicted value, and their crystal packing mode is consistent with the conclusion that for a guest with size in the range of 8 to 9 Å, a more orderly crystal packing can be obtained. This indicates that the size-match strategy can be applied in this above series of N-rich azole anion guests. In addition, we extended our size-matching strategy to structure prediction of hydroxylamine hydrogen bond framework using a series of anions guest formed by N-rich heterocyclic azoles with acid hydrogen.
In addition, we extended our size-matching strategy to structure prediction of hydroxylamine hydrogen bond framework using a series of anions guest formed by N-rich heterocyclic azoles with acid hydrogen.
From the above predictions, some of the smaller sized structures (GH-2, 4, 21, 23, 27) give ionic diameters less than 8 when the structures do not contain nitramine groups. Also, the predicted diameters of the guest GH-5-12,19, 22, and 25 are in the range of 8-9 Å. The other guest structures have diameters greater than 9 Å. To verify the correlation between the predicted size and the actual crystal packing, we try to synthesis those hydroxylammonium salts via the following general steps： S 47 Supplementary Fig. 52. The general synthesis steps for HHF that N-rich heterocyclic azoles with acid hydrogen as the raw reagent.
In addition, we obtained ammonium salt crystals of GH-18 (ammonium 4-chloro-3,5dinitropyrazol-1-ide, crystal-N5) during the incubation of the hydroxymonium salt crystals of GH-18 ( Supplementary Fig. 53. g). During the preparation of such compounds, we observe that cyano-group turn into amino oximes ( Supplementary Fig. 53. d). This may be one of the reasons why there are few hydroxylammonium salts containing cyano-zoles in the current CSD database. Also some of the structures (Supplementary Fig. 53. a, b, h, i, j).) may lack sufficient acidity to obtain hydroxylammonium salt crystals, and the corresponding neutral compounds are directly precipitated from the system. The above-mentioned problems also reflect the difficulty of preparing energetic hydroxylammonium crystals.