Theoretical and experimental vibrational and NMR studies of α and β-RDX

Thumbnail Image
Infante Castillo, Ricardo
Embargoed Until
Hernández Rivera, Samuel P.
College of Arts and Sciences - Sciences
Department of Chemistry
Degree Level
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.

La nitramina cíclica hexahidro-1,3,5-trinitro-s-triazina (RDX) es un ingrediente energético importante en explosivos e impulsores. Para entender su naturaleza explosiva la estructura molecular debe examinarse detalladamente. RDX presenta tres polimorfos sólidos: α, β y γ. La forma α es el polimorfo estable a temperatura ambiente con simetría Cs; el polimorfo β tiene simetría C3v; el polimorfo γ puede asumir estructura ortorrómbicas: D2h, C2v o D2. Cambios en modos vibracionales asociados con sustituciones isotópicas permiten asignar espectros infrarrojo y Raman de α-RDX casi sin ambigüedad, especialmente donde la naturaleza exacta de los modos vibracionales ha sido confusa o contradictoria. Se midieron espectros Infrarrojo y Raman en estado sólido de αRDX e isotopómeros enriquecidos 13C y 15N (anillo) y se asignaron frecuencias fundamentales utilizando desplazamientos isotópicos. Las asignaciones de los modos vibracionales para α and β-RDX se realizaron por comparaciones de resultados experimentales y cálculos de densidad funcional. Las frecuencias calculadas representan señales vibracionales de moléculas en fase gaseosa. Espectros observados experimentalmente de sólidos α-RDX y β-RDX pueden diferir parcialmente de espectros calculados para confórmeros AAE y AAA. En cálculos de teoría de densidad funcional (DFT), la función B3LYP tiende a sobreestimar la localización de vibraciones fundamentales comparadas con mediciones experimentales. Desacuerdos entre teoría y experimentos pueden ser consecuencia de anharmonicidad y la tendencia general de métodos de química cuántica de sobreestimar constantes de fuerza en geometrías de equilibrio. La transición de fase sólido-sólido α-RDX→β-RDX se observó y documentó en tiempo real utilizando espectroscopia Raman. La conversión térmica ocurre a 204°C a presión atmosférica. El patrón vibracional de β-RDX sólido muestra que algunos modos presentes en la fase α-RDX, son vibraciones doblemente degeneradas, que aparecen como sencillas debido a aumento en simetría molecular. Se informaron cálculos desplazamientos químicos de resonancia magnética nuclear (RMN) para 13C y 15N en confórmeros de RDX y otras nitraminas cíclicas. Los datos comparados con valores experimentales en sólido y solución se analizaron con base a patrones y tendencias espectrales. Con el fin de establecer un protocolo consistente y conveniente para emplearse al confirmar espectros experimentales RMN de 13C y 15N de nitraminas, se utilizaron diferentes combinaciones de modelos y bases.
NMR studies,
Polymorphism of RDX
Infante Castillo, R. (2008). Theoretical and experimental vibrational and NMR studies of α and β-RDX [Thesis]. Retrieved from