Optical Characterization of Shock-Induced Chemistry in the Explosive Nitromethane using DFT and Time-Dependent DFT

Abstract

ABSTRACTA thorough understanding of chemistry in extreme environments is a major challenge in experimental as well as theoretical work. With continual improvements in ultrafast optical measurements and new methods for simulations of shock-induced chemistry for timescales approaching a nanosecond, the opportunity is beginning to exist to connect experiments with simulations on the same timescale. In the present work, we compute the optical properties of the energetic material nitromethane (CH3NO2) for the first 100 picoseconds behind the detonation shock front in a molecular dynamics simulation. We compute optical spectra using the Kubo-Greenwood approach with DFT Kohn-Sham electronic states and compare with spectra computed by linear-response time-dependent density functional theory (TDDFT). The latter typically yields more accurate spectra for molecular systems. At optical wavelengths, the TDDFT method offers a correction of up to 25% in the real part of conductivity relative to the Kubo-Greenwood calculation. We also study the effects of thermal electronic excitations on the calculated spectra, and find no discernible change at optical wavelengths. In all of our calculations, we observe a non-monotonic change over time in the entire spectrum of optical properties as decomposition products evolve. The most optically relevant decomposition products are found to be NO, CNO, CNOH, water, and larger transient molecules. In particular, the disappearance of transient NO and CNO molecules (about 90 picoseconds behind the shock front) is coincident with a substantial decrease in conductivity across the optical spectrum.