Isocyanocubanes


Of course, there are other high-energy groups than nitro-groups. Isocyano-groups are also highly energetic:
isocyano-group
The presence of two opposite formal charges (one a positive charge on an extremely electronegative nitrogen atom) connected by an already unstable triple bond makes for a very high-energy group. Adding isocyano-groups to cubane could be the method to creating HEDMs of epic proportions. It has already been observed that minute quantities of 1, 4-diisocyanocubane detonate violently when heated5. The best way of measuring the potential of a substance as an HEDM is by its enthalpy of formation - a high enthalpy of formation means the molecule contains a good deal of energy. Measuring the enthalpy of formation of an HEDM is never particularly safe, though - if the substance is explosive, attempts to measure the energy of its combustion (the usual first step towards finding the enthalpy of formation) can be deadly. For this reason, theoretical and semi-empirical methods are used to measure the enthalpies of formation of  the polyisocyanocubanes5.

The most accurate methods are ab initio MO methods, but these require vast amounts of computer power, and as such are not often practical. However, good  predictions of enthalpies of formation can be calculated using the  DFT-B3LYP method, but this cannot be used  for molecules that are too complex5. Another approach is to use another method, and then systematically correcting the results. All of these methods were used to calculate the enthalpies given below.

The main technique that these calculations were based on is that of the isodesmic reaction. An isodesmic reaction is one in which the number of each kind of formal bond is the same  in both the reactants and the products5. This usually involves splitting a large molecule up into two smaller ones, or a large molecule and lots of small molecules. The reaction scheme used in these calculations is:

C8H(NC)mH8-m + mCH4   into C8H8 + mCH3NC

The main restriction on using isodesmic reactions to calculate enthalpies is that if strained or conjugated systems are broken during the reaction, then the estimate of the enthalpy of formation will be wildly inaccurate. This is why the cubane skeleton remains intact on both sides of the reaction. The enthalpy of formation of all the other substances in the isodesmic reaction must be known as well - this is not a problem, as the enthalpies of formation of  methane, ethanenitrile, and cubane are all known.

The results are shown below5:
Compound
 Enthalpy of Formation/kJmol-1
Isocyanocubane
 816.93
1,2-diisocyanocubane
 1024.58
1,3-diisocyanocubane
 1021.23
1,4-diisocyanocubane
 1018.60
1,2,3-triisocyanocubane
 1239.81
1,2,4-triisocyanocubane
 1234.33
1,3,5-triisocyanocubane
 1234.26
1,2,3,4-tetraisocyanocubane
 1469.32
1,2,3,5-tetraisocyanocubane
 1464.75
1,2,3,6-tetraisocyanocubane
 1466.59
1,2,3,7-tetraisocyanecubane
 1467.41
1,2,5,6-tetraisocyanocubane
 1463.96
1,3,5,7-tetraisocyanocubane
 1461.91
1,2,3,4,5-pentaisocyanocubane
 1695.92
1,2,3,5,6-pentaisocyanocubane
 1693.51
1,2,3,5,7-pentaisocyanocubane
 1692.95
1,2,3,4,5,6-hexaisocyanocubane
 1927.23
1,2,3,4,5,7-hexaisocyanocubane
 1925.75
1,2,3,5,6,8-hexaisocyanocubane
 1924.98
Heptaisocyanocubane
 2161.03
Octaisocyanocubane
2402.21

It is fairly easy to see that the more isocyano-groups there are on the cubane shell, the higher its enthalpy of formation it is. Also, the closer the isocyano-groups are to each other, the higher its enthalpy of formation is. Certainly, the enthalpy of formation of octaisocyanocubane is immense, and, from this point at least, it seems as if octanitroisocyanocubane could well be an extremely potent HEDM.

However, there are several problems with this:
  1. Isocyano-groups are inherently unstable, and spontaneously convert into cyano-groups. The reaction is often slow, but it does occur at an observable and significant rate.  This means that isocyanocubanes will almost certainly not be storable for long periods. Depending on how quickly the isocyanocubanes become cyanocubanes, the isocyanocubanes might not be storable at all.
  2. These data say nothing about how fast the isocyanocubanes react. If they do not decompose quickly, they will be useless as propellants and explosives, although still usable as fuels. Conversely, if they are kinetically unstable at room temperature, they will be completely unusable as HEDMs.
  3. It is not known whether the isocyanocubanes are dense or not. Although cubanes are dense, as a rule, if polyisocyanocubanes are, for some reason, not dense, they will not be good explosives or propellants. Depending on just how dense they are, they might not be as efficient fuels per unit volume as other HEDMs that are used as fuels.
  4. These data do not say anything about the possibility of making the isocyanocubanes. If  polyisocyanocubanes with large numbers of  isocyano-groups cannot be made, then this work is meaningless. However, at least it is now known that research into this area is worthwhile...
  5. ...if the calculations of  the enthalpies of  formation are correct. It is entirely possible that they are not - they are only estimates. When the enthalpies of formation of the nitrocubanes were initially calculated, they were very inaccurate for the tetra-, penta-, hexa-, hepta-, and octanitrocubanes, because the people investigating them forgot to account for the increasing repulsion between  the nitro-groups - if something similar has happened with the isocyanocubanes, then this could very well turn out to be a dead end, research-wise.
  6. Isocyanocubanes, when they react, will not produce much gas. This means that the decomposition of isocyanocubane is unlikely to be as powerful as those of  nitro-explosives unless they are mixed with an oxygen-rich compound that will oxidise the cubane skeleton itself. This, of course, will lower the density of  the  substance, making it a worse explosive or propellant. It could still be an effective fuel.
  7. Cubane is still an extremely expensive compound, and multiply functionalised cubanes even more so. If cubane derivatives are ever going to have practical uses, then the cost of cubane needs to come down. As it is, even the powerhouse that is octaisocyanocubane is going to be prohibitively expensive.

Conclusion

Cubane-based HEDMs have great potential, especially for explosives and propellants. Amongst the nitrocubanes, TNC seems to be the best suited for use as a propellant, while HpNC has great potential as an explosive. Unless a denser ONC polymorph can be found, it is always going to be inferior to HpNC. The isocyanocubanes look promising, but much research needs to be done before it can be said whether they have any practical uses, They are certainly amongst the most energetic  CNH compounds known, if these predictions are correct.

However, for cubane-based HEDMs to be widely used, their cost needs to come down. Many of the featured substances can only be produced with  syntheses which have large numbers of steps, and all these start with an extremely expensive molecule - cubane. Until the cost of cubane is brought down, cubane-based HEDMs are always going to lose out to cheaper alternatives.

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