What makes an explosion happen?
Energy density and structure stability are of key concerns in designing and synthesizing energetic materials such as CNHC molecular crystals and the emerging cyclo-N5complexes for desired explosive functionalities with a mechanism being open for exploration. An extension of the recent progress in solvation [Sun, Unprecedented O::O compression and HH fragilization in Lewis solutions (perspective). PCCP 21, 2234-50 (2019)] suggests that a combination of the intermolecular hydrogen bond
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<p>Explosions can be categorized into two groups -constrained and spontaneous. The traditional CNHO molecular crystals such as TNT and TATB (1) and the emerging cyclo-N -5 complexes stabilized by acidic entrapment (2-5) undergo constrained explosion. One often expects high energy density yet low sensitivity to perturbation such as thermal excitation or shock wave impulse for practical applications. Once ignited, detonation takes place and the carrier erupts energy variously.</p><p>The manner, velocity, and amount of energy ejection is subject to its initial storage and the stability, which can be controlled by adjusting the relative number of protons and lone pairs carried by molecular motifs (1). Density functional theory computations (6) suggested that the nonlocal van der Waals interactions is dominant to the cohesive energies of the complex anisotropic molecular crystals, and the dispersion force play roles of even more important in stabilizing the structures (7).</p><p>However, comprehension of the intermolecular interaction and the manner of energy storage for the constrained explosion is still unclear.</p><p>Contrastingly, alkali metals (8) and molten alkali halides (9, 10) explode spontaneously when contacting to water -aquatic spontaneous explosion. When reacting with water at room temperature, Na and molten NaCl explode rigorously with production of H2, NaOH, shock waves and heat that ignites the H2 (8,10,11). In fact, bulk Na solvation proceeded in four stages in a few seconds (10): first, the Na is dissolved gradually and dyes the water into purple; secondly, the Na specimen releases gas; thirdly, the released gas burns; and finally explodes abruptly. The mixture of Na and K could form a liquid NaK alloy at room temperature. The NaK droplet shoots spikes explosively out when it enters the water, 0.2-0.4 ms immediately after the droplet contacting water (8,9), see photos in Figure 1a. This explosion process is too fast to be attributed to the heat repellant, instead, alkali metal atoms at the surface of a small cluster each lose its valence electron within picoseconds (8). These electrons dissolve in water and react in pairs to form molecular H2 and OHhydroxide. Likewise, solvation of or pouring the molten NaCl (Tm = 801 C) on ice causes massive explosion under the ambient condition. The explosion readily breaks a fish tank containing water and leaves a cave behind the ice as well. Figure 1 show photos captured from the aquatic explosion of NaK droplet in water and the molten NaCl poured in water and on ice (8,9).</p><p>However, replacing the NaK alloy with a drop of molten Al at ~1,000 °C in water, or pouring molten Cu at 1083C on ice, produces neither spark nor explosion, instead, displaying the Leidenfrost effect (12) and Rayleigh instability (13). The steam separates the liquid Al and water surface on a 0.1 s timescale. The molten Al just melts and spreads on ice. In contrast, solvation of molten Na2CO3 (Tm = 851 °C) and molten H3BO3 (Tm = 171 C) does not foster an explosion though the melting temperatures of the Na2CO3 and the H3BO3 are different from that of molten NaCl though the reactions also produce Na + and H + . Replacing H2O with liquid NH3 at −77.8 °C can form less-apparent transient spikes but does not foster an explosion (8).</p><p>*m, n, p, q are numbers of the specified intermolecular nonbonds.</p><p>Two steps are involved in the aquatic spontaneous explosion. Firstly, H2O dipoles tend to hydrate the Z alkali metals and the molten Zsalts as well to dissolve them into constituent ions, Z + and  -; the Z + replaces a H + of the H2O to form the basic ZOH and H2 gas for the Z metal, and H for the Z salt. Second, hydration of the ZOH base proceeds and turns the ZOH into the well-defined Z + and OHhydroxide (16). In basic solvation, each of the OHinteracts with its neighboring H2O to form an OH -4H2O motif and turns one O:H-O bond into the highly repulsive O::O super-HB. The second process repeats that of base solvation at a higher temperature.</p><p>The O::O compression is much stronger than the critical pressure 1.35 GPa for liquid water transiting into ice at the ambient temperature (19).  (c) the cyclo-N5 -: (3H3O + +2NH4 + ) complexes optimized from intensive quantum calculations (2,3).</p><p>The cyclo-N5ˉ can only be stabilized in acidic conditions of the excessive (H3O) + or (NH4) + (4,(20)(21)(22)(23)(24)(25)(26). For the TNT in Figure 2a, there are 10 H + and 2×(3+2×6) = 30 lone pairs ":" surrounding the motif. According to the nonbond counting rules (17), protons and lone pairs can interact with their same kinds of its neighboring motifs to form the X:H-Y, X::Y, or HH. There will be 20 HBs and 20 super-HBs per TNT motif with its neighbors. The combination of the X:H-Y tension and the X::Y compression stabilizes the crystals and stores energy into the covalent bonds, which ensures the constrained explosion of the TNT and other CNHO energetic crystals, as well.  Once the constraint X:H breaks, the system will collapse erupting energy. Short of X:H-Y tension endows the unconstrained explosion of alkali metals and molten alkali halides; absence of the X::Y compressions fosters no explosion of alkali metal in liquid NH3. One should be focused more on the stability and energy before calculating the impact of energy eruption. As the source of life, the electron lone pairs and protons and the forms of their interactions are the key to water, ice, aqueous solutions, energetic carrier for explosion, and beyond.</p><!><p>No conflicting interest is declared.</p>