New Mechanistic Insights into the Primary Thermolysis Reactions of 1,3,4,6-Tetranitrooctahydroimidazo-[4,5- d]imidazole (BCHMX) from Predictive Local Coupled Cluster Calculations

Igor N Melnikov; Alla N Pivkina; Vitaly G Kiselev

doi:10.1021/acs.jpca.3c06352

New Mechanistic Insights into the Primary Thermolysis Reactions of 1,3,4,6-Tetranitrooctahydroimidazo-[4,5- d]imidazole (BCHMX) from Predictive Local Coupled Cluster Calculations

J Phys Chem A. 2023 Dec 28;127(51):10860-10871. doi: 10.1021/acs.jpca.3c06352. Epub 2023 Dec 1.

Authors

Igor N Melnikov¹, Alla N Pivkina¹, Vitaly G Kiselev^{2

3}

Affiliations

¹ Semenov Federal Research Center for Chemical Physics RAS, 4 Kosygina Str., 119991 Moscow, Russia.
² Novosibirsk State University, 1 Pirogova Str., 630090 Novosibirsk, Russia.
³ Institute of Chemical Kinetics and Combustion SB RAS, 3 Institutskaya Str., 630090 Novosibirsk, Russia.

PMID: 38039193
DOI: 10.1021/acs.jpca.3c06352

Abstract

Theoretical studies of the decomposition mechanism of energetic materials quite often scrutinize only the primary thermolysis reactions. However, the secondary reactions are crucial, inter alia, for proper building of the combustion models and understanding the autocatalytic processes. In the present study, we applied predictive DLPNO-CCSD(T) calculations to elucidate the kinetics and decomposition mechanism of a novel promising energetic material, 1,3,4,6-tetranitrooctahydroimidazo [4,5-d] imidazole (BCHMX). We identified eight previously unknown BCHMX conformers, both cis and trans in accordance to the spatial position of the H atoms bonded to a carbon bridge. Among them, the relative enthalpies of cis isomers lie in the narrow range ∼10 kJ mol^-1 rendering them thermally accessible in the course of decomposition. The radical N-NO₂ bond cleavage via one of the novel conformers is the dominant primary decomposition channel of BCHMX with the kinetic parameters E_a = 168.4 kJ mol^-1 and log(A, s^-1) = 18.5. We also resolved several contradictory assumptions on the mechanism and key intermediates of BCHMX thermolysis. To get a deeper understanding of the decomposition mechanism, we examined a series of unimolecular and bimolecular secondary channels of BCHMX. Among the former reactions, the C-C bond unzipping followed by another radical elimination of a nitro group is the most energetically favorable pathway with an activation barrier ∼113 kJ mol^-1. However, contrary to the literature assumptions, the bimolecular H atom abstraction from a pristine BCHMX molecule by a primary nitramine radical product, not the nitro one, followed by another NO₂ radical elimination, is the most important bimolecular secondary thermolysis reaction of BCHMX at lower temperatures. The isokinetic temperature of the bimolecular and unimolecular secondary reactions is ∼620 K. Unimolecular reactions might be important in dilute solutions, where bimolecular reactions are suppressed. The secondary reactions considered in the present work might be pertinent in the case of related energetic nitramines (e.g., RDX, HMX, and CL-20).