| Time-resolved absorption and Raman spectroscopies were used to probe molecular-level changes in nitromethane (NM) subjected to stepwise loading. A complete equation of state for unreacted NM was also developed, enabling temperature calculations for the shocked liquid.;For peak pressures up to 140 kbar, no sign of chemical reaction was observed with either absorption or Raman spectroscopy. In the absorption experiments, for peak pressures above 170 kbar, extensive reaction was indicated by an irreversible redshift of the absorption band edge followed by a loss of transmission through the sample. Modest increases in the initial sample temperature produced large decreases in the reaction induction time. The induction time-shock temperature data, analyzed using thermal explosion theory, exhibited no pressure dependence within the pressure range examined here. Comparison with other work suggests that induction times measured using different experimental techniques are not necessarily equivalent.;In the Raman experiments, reaction was indicated at 160 kbar by a rapidly-rising luminescent background after peak pressure was reached. The lower threshold for reaction, relative to the absorption experiments, and measurable spectral changes during the induction time for the absorption experiments, suggest that the Raman experiments were a more sensitive probe of reaction. There was no measurable extent of reaction at this pressure. However, spectral changes originating from the bulk NM were observed. After peak pressure was reached, large-scale progressive broadening was observed in the CH;The decomposition reaction was assumed to initiate from the proposed precursor state. Given the evidence for extensive intermolecular interactions and an associative precursor state, a bimolecular reaction mechanism was suggested. In contrast, prior work on amine-sensitized NM has suggested unimolecular CN bond scission as an important early reaction step. |
