Hydrogen Bonding, Electron Transfer and Orbital Interactions between RDX and Al3O3-: Forces governing the initial encounter

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 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, Mulliken, 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(*) type) and the C-H···Al (2.836-2.871Å, Al(3p)-to-C-H(*) type) interactions are possible. Hirshfeld analysis predicts 6-25% transfer of the -1 charge to RDX while Hirshfeld-I, NPA, and Mulliken predict 7-22%, 2-7%, and 7-13% 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. Complexation is found to augment the resultant dipole moment from 2.7 (Al3O3- (rec)) and 8.6 (Al3O3- (kite)) to the 2.9-16.0 Debye range, signalling effects on the workfunction of the aluminum oxide surfaces upon binding.