Hindawi Publishing Corporation Advances in Physical Chemistry Volume 2012, Article ID 175146, 7 pages doi:10.1155/2012/175146 Research Article A Theoretical Investigation of the Ring Strain Energy, Destabilization Energy, and Heat of Formation of CL-20 John A. Bumpus Department of Chemistry and Biochemistry, University of Northern Iowa, Cedar Falls, IA 50614, USA Correspondence should be addressed to John A. Bumpus, john.bumpus@uni.edu Received 10 April 2012; Accepted 13 August 2012 Academic Editor: Dennis Salahub Copyright © 2012 John A. Bumpus. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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[5.5.0.0 3,11 .0 5,9 ]dodecane) is a well-studied high-energy-density material (HEDM). The high positive gas- (Δ f H ◦ g ) and solid- (Δ f H ◦ s ) 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. Δ f H ◦ g 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 Δ f H ◦ g 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–NO 2 substituents that replace the six methylene (–CH 2 –) 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-hex- anitro-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. 0 3,11 .0 5,9 ]dodecane) is a high-energy-density material (HEDM) that has been developed and studied during the past several years (see 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, detona- tion 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). 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 that recommend them for use as high explosives [7–12], economic considerations appear to limit the large scale production and usefulness of ONC [11, 12]. On the other hand such problems for large- scale production of CL-20 appear to have been successfully addressed, although cost reduction is still an issue. CL- 20 is produced commercially by ATK-Thiokol Propulsion (Brigham City, UT, USA) [13]. The relatively high heat of combustion and heat of detonation values of many HEDMs are due, in part, to their high positive heat of formation values. Octanitrocubane (ONC) is a case in point (see Scheme 3). Estimates of gas-phase (Δ f H ◦ g ) and solid-phase (Δ f H ◦ s ) heat of formation values of this compound have been