Theoretical study on intra-molecule interactions in TKX-50.

To fully and deeply understand the weak interactions in the gaseous structure of the TKX-50 molecule, two conformations I and II of the TKX-50 molecule confirmed in a crystal cell were optimized at the B3LYP/6-311g(d,p) level in the gas state, and the single point energy of the optimized structure was calculated at the M06-2X/ma-TZVPP level. Analyzing methods for weak interactions such as the interaction region indicator (IRI), topological basin analysis, and the extended transition state-natural orbitals for chemical valence (ETS-NOCV) theory with the help of Multiwfn code were employed to reveal the corresponding intramolecular weak interactions. The results showed that there were 5 kinds of intramolecular weak interaction in both conformations. They are two types of H bond, two types of intra-ring weak interaction, and one type of O-N bond within the molecular fragment containing the bis-tetrazole ring. The combined effect of all these weak interactions holds the bis-tetrazole ring of TKX-50 retaining an almost coplanar configuration. Meanwhile, the strength of these weak interactions is significantly different in conformation I and conformation II. The most obvious difference is that conformation II has a significant H transfer between intramolecular fragments due to the mirror rotation of almost 180° of cations (NHOH) perpendicular to the N-O bond axis thereof as compared to the reference conformation I. This conformational difference not only makes the weak interaction between the two conformations very different but also forms a quasi-covalent bond in conformation II with much larger bonding energy than other H bonds, thus resulting in conformation II having lower electron energy and more stable geometry. In addition, the order of breaking various H bonds in the combustion decomposition process of TKX-50 is deduced by comparing various H bonds.