Noncovalent Interactions and Electron Transfer between 1,3,5-Trinitro-1,3,5-triazinane and Al3O3−

As the first step in energy transfer from oxide passivated metal nanoparticles/surface defects to an energetic molecule for the controlled dissociation of the latter, the nature of the weak interactions between 1,3,5-trinitro-1,3,5-triazinane (RDX) and Al3O3− isomers are studied as a model system by DFT method with PBE0 functional and different basis sets (TZVP, Def2-TZVPP, TZ2P). Different electron density (NCI, IGMH, QTAIM, IQA, EDA-NOCV) and charge analysis methods (Hirshfeld, Hirshfeld-I, NPA) reveal that partial charge-transfer from Al3O3− to RDX via the C-H···O (1.851-2.481Å, O(2p)-to-C-H(σ*) type) and the C-H···Al (2.836-2.871Å, Al(3p)-to-C-H(σ*) type) interactions are possible. Hirshfeld analysis predicts 6-25% transfer of the -1 charge to RDX while Hirshfeld-I and NPA predict 7-22% and 2-7%, transfer, respectively. Al3O3− (HOMO)-RDX (LUMO+n, n=0-3) type interactions including the charge transfer via the (ON)O···Al (2.846-3.511Å) interactions are found to excite the Al3O3− (kite) anion. The binding energies are in the -10 to -31 kcal/mol range. Vertical detachment energies of the complexes are within 2.5-3.9 eV, higher than the bare Al3O3− isomers by 0.3-1.3 eV. Complexation weakens the RDX structure and reduces the C-H stretching frequencies by 10-350 cm−1. Modulations in the bond lengths in the Al3O3− isomers shift the frequencies in the 0-1300 cm−1 region. New intermolecular vibrations enrich the terahertz region of the IR spectra.