Infante Castillo, Ricardo

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    Theoretical and experimental vibrational and NMR studies of α and β-RDX
    (2008) Infante Castillo, Ricardo; Hernández Rivera, Samuel P.; College of Arts and Sciences - Sciences; Castro Rosario, Miguel E.; Mina Camilde, Nairmen; Rivera Montalvo, Luis A.; Briano Peralta, Julio G.; Department of Chemistry; Mehta Chopra, Narinder K.
    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 γ- polymorph may assume one of three orthorhombic structures: D2h, C2v or D2. Solid state Infrared and Raman spectra of α-RDX and 13C and 15N (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 general tendency of quantum chemical methods to overestimate force constants at the equilibrium geometry. The elusive α→β solid-solid RDX phase transition has been documented in real time mode using Raman spectroscopy. The thermal conversion of α→β at atmospheric pressure occurred at 204°C. The vibrational pattern of solid β-RDX shows that several vibration signals present in α-RDX phase coalesce into single, doubly degenerate vibrations, possibly because of higher molecular symmetry. Calculated nuclear magnetic resonance (NMR) chemical shifts (13C and 15N) are reported for RDX conformers and others cyclic and acyclic nitramines. Data were compared with experimental solid and solution data, focusing on agreement of spectral patterns and trends. In order to establish a convenient and consistent protocol to be employed for confirming experimental 13C and 15N NMR spectra of nitramine compounds, different combinations of models and basis set were considered.