The cage compound CL-20 (a.k.a., 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, HNIW, or 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.03,11.05,9]dodecane) is a well-studied high-energy-density material (HEDM). The high positive gas- ) and solid- ( ) phase heat of formation values for CL-20 conformers have often been attributed to the strain energy of this cage compound and, by implication, to the conventional ring strain energy (CRSE) inherent in isowurtzitane which may be viewed as a “parent compound” (although not the synthetic precursor) of CL-20. values and destabilization energies (DSEs), which include the contribution from CRSE, were determined by computation using a relatively new multilevel ab intio model chemistry. Compared to cubane, isowurtzitane does not have an exceptionally high CRSE. It is about the same as that of cyclopropane and cyclobutane. These investigations demonstrate that instead of the CRSE inherent in the isowurtzitane parent compound, the relatively high and DSE values of CL-20 conformers must be due, primarily, to torsional strain (Pitzer strain), transannular strain (Prelog strain), and van der Waals interactions that occur due to the presence of the six >N–NO2 substituents that replace the six methylene (–CH2–) groups in the isowurtzitane parent compound. These conclusions are even more pronounced when 2,4,6,8,10,12-hexaazaisowurtzitane is viewed as the “parent compound.” 1. Introduction The cage compound CL-20 (a.k.a., 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane, HNIW, or 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo [5.5.0.03,11.05,9]dodecane) is a high-energy-density material (HEDM) that has been developed and studied during the past several years (see Scheme 1). Scheme 1 This compound was first synthesized in 1987 by scientists at the China Lake Naval Weapons Center [1, 2]. There is substantial interest in the use of CL-20 as a high explosive and many of its properties (density = ~2？g？cm？3, detonation velocity = 9.4？mm？ms？1, oxygen balance = ？11, detonation pressure = 420？kbar, etc.) suggest that it is superior to RDX (1,3,5-trinitroperhydro-1,3,5-triazine) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), two high explosives that are widely used at the present time [3–6] (see Scheme 2). Scheme 2: Structures of RDX and HMX. In addition to its use as a high explosive compound, [3] CL-20 has potential use as a propellant [6]. CL-20 is often compared to octanitrocubane (ONC) which, like CL-20, is a polynitro cage compound. Although both have many properties
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