The decomposition mechanism in shocked pentaerythritol tetranitrate (PETN) was examined using timeresolved emission spectroscopy. PETN single crystals were subjected to stepwise loading along [100] and [110] to peak stresses between 2 and 13 GPa. Due to concurrent changes in the optical transmission of PETN, emission spectra were analyzed using the absorption data acquired separately under the same loading conditions. Analyses of the corrected emission data revealed two bands in the spectra at ∼3.0 and ∼2.4 eV. Both bands were observed in every experiment regardless of stress or crystal orientation. However, their relative and absolute intensities, and temporal behavior revealed stress and orientation dependence. The emission was identified as chemiluminescence from the nitronium ion, NO 2 + , on the basis of its electronic structure and properties. NO 2 + electronic structure was analyzed using ab initio calculations, which showed transition energies matching those of the emitting intermediate observed experimentally. Several chemical pathways compatible with the formation of NO 2 + are considered and evaluated. Finally, a four-step chemical initiation mechanism in shocked crystalline PETN is proposed and discussed in detail.

Geometry optimizations and normal-mode analyses of the pentaerythritol tetranitrate (PETN) conformer belonging to the S 4 molecular point group and comprising the crystalline solid were performed using density functional theory methods (B3LYP and B3PW91). The basis sets used in this study were 6-31G(d) and 6-311+G(d,p). The structural results are in good agreement with experimental X-ray diffraction data. The predicted bond lengths and bond angles are accurate to within ∼2.5% and ∼1.2%, respectively. Raman and infrared spectra of crystalline PETN were measured and compared with the calculated spectra. The calculated and measured spectra agree very well in the spectral region below 1100 cm -1 ; the agreement is satisfactory for frequencies higher than 1100 cm -1 . On the basis of the calculations and analyses, normal mode assignments were made and mode symmetries determined.

A chemical mechanism to explain the observed anisotropy in the shock wave initiation of pentaerythritol tetranitrate (PETN) single crystals is proposed on the basis of semiempirical quantum chemical calculations. Building on the previously proposed model of steric hindrance to shear, the molecular mechanics of shear deformation at the lattice level is correlated with rotational conformations of PETN. The numerous stable conformations of PETN differ in symmetry and dipole moment values. The initial conformation belongs to the S 4 molecular point group and possesses no dipole moment. Because of shear deformations, the molecules change conformations. The [110] shocks result in sterically hindered shear and generate polar conformations. In contrast, the [100] shocks result in little or no polarization. Because the decomposition chemistry of PETN at 5−10 GPa is likely dominated by ionic reactions, local polarity of the lattice plays a crucial role in reactivity. The polar lattice stabilizes the transition state due to dipole−dipole interactions and, thus, facilitates the ionic dissociation. In contrast, the nonpolar lattice results in no stabilization and low reaction rates. Plausible ionic reactions are briefly discussed and experiments are suggested to verify the mechanism proposed.

The mechanism of amine sensitization of shocked nitromethane was investigated using time-resolved optical absorption spectroscopy in the visible. Neat nitromethane and mixtures of nitromethane with six different (primary, secondary, tertiary, and di-) amines were shocked to within 12−17 GPa peak pressure using step wave loading. Despite the small amine concentrations, profound differences were observed in the absorption spectra of neat and sensitized nitromethane. The changes in the absorption spectrum of reacting neat nitromethane consisted of irreversible broad-band (450−650 nm) loss of transmission through the sample after a short induction time. In contrast, a transient absorption peak at 525 nm developed in the spectra of the reacting mixtures. This feature did not depend on the particular amine used. We assign it to a transient intermediate formed in the shocked mixtures during the early stages of decomposition. On the basis of our analyses and the data available in the literature, we identify the intermediate as a radical anion of nitromethane, CH3NO2 •-. The implications of this on the mechanism of sensitization are discussed. Several possible radical anion mechanisms are considered and evaluated. The base catalysis by amines is favored as the most plausible mechanism of sensitization. This mechanism is discussed in detail.

Raman measurements and density functional theory (DFT) calculations were carried out to evaluate the propensity of pentaerythritol tetranitrate (PETN) to conformational changes. In the crystalline phase under ambient conditions, the PETN molecule has S 4 symmetry. Two instances of symmetry change were found and presented in this work. One involved compressing the PETN crystal statically to ≥5 GPa in a diamond anvil cell. The other was observed upon dissolving PETN in a polar solvent (acetone-d 6). DFT calculations indicate that changes in conformation/symmetry can be observed readily through changes in the vibrational spectrum. Several representative stable PETN conformations of various symmetries were found and examined. The calculated Raman spectra provide support for the conclusion that the S 4 symmetry, at ambient conditions, changes to C 2 under static high-pressure loading and to C 2 or C 1 in solution.