Theoretical and Experimental Vibrational and Nmr Studies of Α and Β-rdx

Cyclic nitramine hexahydro-1,3,5-trinitro-s-triazine (RDX), is an important energetic ingredient for propellants and explosives. To understand RDX explosive nature, the molecular structure needs to be examined in detail. Solid state RDX exist as three polymorphs: α, β and γ. The α-form is the stable polymorph at room temperature with Cs symmetry, β-polymorph has a molecular symmetry of C3v and γpolymorph may assume one of three orthorhombic structures: D2h, C2v or D2. Solid state Infrared and Raman spectra of α-RDX and C and N (ring) enriched isotopomers were recorded and fundamental frequencies were assigned using isotopic frequency shifts. Changes in vibrational signals associated with isotopic substitutions provide nearly unambiguous assignments of vibrational spectra of α-RDX, particularly where exact nature of vibrational modes had been either vague or contradictory. Assignments of the vibrational modes for α and β-RDX were made through comparisons between experimental and density functional calculations results. Calculated frequencies represent vibrational signatures for gas phase molecules. Hence, experimentally observed spectra for solid α and β-RDX may differ somewhat from calculated spectra for RDX AAE and AAA conformers. In the DFT calculation, B3LYP function tends to overestimate the spectral location of vibrational bands compared to experimentally observed values. The slight disagreement between theory and experiment could be a consequence of anharmonicity and of the