7767
J. Phys. Chem. 1994,98, 7767-7776
UV/Visible Absorption Spectra of Shocked Nitromethane and Nitromethane-Amine Mixtures up to a Pressure of 14 GPa C. P. Constantinou,' J. M. Whey, and Y. M. Gupta Shock Dynamics Center and Department of Physics, Washington State University, Pullman, Washington 99164-2814 Received: October 21, 1993; In Final Form: May 12, 1994'
Electronic and chemical changes in shock-compressed nitromethane and a mixture of nitromethane with ethylenediamine (0.1 wt %) have been examined using time-resolved optical absorption spectroscopy under stepwise loading to 14 GPa. Despite the small amine concentrations, profound differences are observed in the absorption spectra of pure and sensitized nitromethane. The changes in the absorption spectrum of pure nitromethane involve a maximum absorption edge shift of 20 nm and are completely reversible. In contrast, the mixture shows an irreversible time-dependent shift in the absorption edge of up to 90 nm toward longer wavelengths. A substantial component of the shift takes place after the sample has reached peak pressure. The time dependence and irreversibility of the absorption edge shift are caused by a chemical change. They confirm the sensitization of nitromethane in the presence of amines. The observed shift in the absorption edge is treated as a kinetic measurement. The derived kinetic parameters of the system are consistent with a previously suggested chemical mechanism for the sensitization of nitromethane by amines.
I. Introduction A good understanding of the mechanisms governing shockinduced chemical decomposition is central to scientific problems related with impact sensitivity and detonation in condensed energeticmaterials. Over the past twodecades, the energy release in impact experiments has been examined with a variety of timeresolved continuum measurements (pressure, particle velocity) and related analyses. Despite these developments, a detailed understandingof chemical decomposition at the atomic/molecular level remains an outstanding problem. An important goal of current research on energetic materials is to permit a direct examinationof the microscopic changes that accompany chemical reactions.' This information is needed to understand the interchange of energy between a shock wave and an energetic material. Well-characterized shock wave measurements have been continuously refined over the past 40 years and have been extensively used in the determination of high-pressure equations of state.2 In recent years, techniques have been developed in our laboratory that incorporate time-resolved spectroscopy into shock wave measurements in condensed material^.^ This provides us with the capability to probe the operative atomic/molecular mechanisms and to obtain information that complements the continuum studies; the latter contribute almost all of the past work.4 In this paper, we report on the application of time-resolved optical absorption spectroscopy to examine the shock-induced chemical decomposition in pure and sensitized nitromethane subjected to uniaxial strain compression under stepwise shock loading.5 Nitromethane was chosen for a number of reasons: it is liquid and, therefore, essentially homogeneous. Thus, many of the complexities associated with solid energetic materials are avoided. Furthermore, its chemical structure is relativelysimple, and it has the advantage of variable sensitivity in the presence of amines.6 Specifically, we present measurements on the shock-induced changes in the absorption spectra of nitromethane and a mixture of nitromethane with ethylenediamine (0.1 wt %). We examine the reversibility of these changes when the sample pressure is Abstract published in Advance ACS Abstracts, July 1, 1994.
relieved and their significance with respect to shock-induced chemical decomposition. Background information relevant to our results is provided in the next section. Section I11 summarizes the experimental methods, and the results are presented in section IV. Section V presents a discussion of our results and an analysis of the decomposition kinetics under shock conditions. The main findings are summarized in section VI. In the remainder of this paper we will make use of the abbreviations N M and N M EDA for nitromethane and nitromethane-ethylenediamine (0.1 wt %), respectively.
+
11. Background
The measurementsdescribed in this paper involve time-resolved changes in the near-UV and visible transmission spectra of nitromethane and N M + EDA, during dynamic compression of thesamples to pressuresup to 14GPa. TheUV/visibleabsorption spectrum of nitromethane consists of two broad bands. A strong band (log e = 4.2, where e is the molar extinction coefficient) occurs at a peak wavelength of 220 nm in the liquid7 and 198 nm in the gaseous phase* as compared to the calculated value of 200 nmO9A weaker band (log e = 0.2) appears centered around 275 nm. The two bands arise respectively from r2 to r3*and n, to 7r3* electronic transitions in nitromethane. In addition, another weaker transition, n, to q*, should occur at a shorter wavelength, probably in the vacuum ultraviolet. The lowest lying n, to a3* transition (centered around 275 nm) is the only one that could be probed in the experiments reported here. In the remainder of this section we briefly review previous work on nitromethane and nitromethane-amine mixtures that is relevant to this paper. We discuss, in sequence, work under ambient conditions, results at high hydrostatic pressures, and experiments on the behavior of the materials under shock compression. We end the section with a review of the evidence for the sensitization of nitromethane by amines. A. Ambient Experiments. Nitromethane is the simplest nitrocompound. Its decompositionin the gaseous phase is known to involve unimolecular breakage of the C-N bond.10 In the condensed phase, other mechanisms are more plausibly invoked; Constantinou7 has obtained experimental evidence that the thermal decomposition of liquid nitromethane follows a cubic
0022-3654/94/2098-7167%04.50/0 0 1994 American Chemical Society
7768 The Journal of Physical Chemistry, Vol. 98, No. 32, 1994 autocatalytic process that involves a nitromethane molecule reacting with a methyl radical and nitrogen dioxide. Constantinou and Chaudhri6 investigated the thermal decomposition of mixtures of nitromethane with five amines. They suggested a method for making accurate quantitative measurements on the sensitization, based on differential scanning calorimetry. From these measurements it was found that the reaction proceeds in two stages, the relative exothermicity of which depends on the amine concentration. The first stage in the reaction was suggested to involve the unimolecular decay of a charge-transfer complex formed between nitromethane and the amine. The UV/visible absorption spectra of nitromethane and mixtures of nitromethane with amines have been studied by Ungnade et al.,lI Constantinou et a1.,6 and cons tan ti no^.^ All of these studies found evidence for an electron donor-acceptor interaction. The implications of the UV/visible absorption spectra on the mechanism of sensitizationhave been discussed previously.6 B. HydrostaticHigh-PressureWork. In order to examine the effects of high pressure, nitromethane has been studied extensively in the diamond anvil cell.12 Although the measurements have demonstrated the invalidity of first-order unimolecular decay, the mechanism of decomposition under these conditions still remains an open issue.13 Static high-pressure experiments in a heated diamond anvil cell, in which nitromethane was decomposed in the solid phase, have provided evidence that the rateof reaction increases with pressure14 but again have not yet arrived at a detailed mechanism. Brasch15 studied the decomposition of nitromethane at static pressures (2-6 GPa) and elevated temperatures (373-433 IC) in the presence of an amine. It was found that the rate of decomposition under pressure was increased by small amounts of ethylenediamine. C. Shock Experiments. The shock response of liquid nitromethane has been a subject of investigations for many years. Continuum measurements have been made by Engelke and Bdzil16and Sheffield et al.17 The early studies on shockinitiation of nitromethane18 were interpreted in terms of the theory of thermal ignition.lg Despite large acknowledged uncertainties in measured shock pressures, assumed equations of state, and kinetic parameters, relatively good agreement was obtained between predicted and measured ignition times for shock pressures in excess of 8.0 GPa. HardestyZO was able to find further support for the thermal explosion model. Combining his own measurementsof the ignition time with those by Berke et al.21and assuming a first-order rate law, he was able to estimate kinetic parameters for the initiation reaction. The derived value for the activation energy suggested that radical reactions significantly influence the initiation of liquid nitromethane. However, the detailed chemical mechanism still remains unresolved. In the past, there have been attempts to use vibrational spectroscopy in the study of the shock initiation of nitromethane. These have concentrated on the C-N bond and have provided conflicting results. Renlund and Trott22measured the energy of the C-N stretching vibration up to 6.5 GPa under single-shock loading. They reported that at 6.8 GPa no Raman transitions were observed. They attributed this to a chemical reaction. Delpuech and Meni123also observed the disappearance of the Raman mode under single-shockconditionsat a stress somewhere between 5.0 and 8.5 GPa. Schmidt et al.24 have investigated the same mode using coherent anti-Stokes Raman scattering. They found no evidence of a chemical reaction behind single shocks of pressures up to 7.6 GPa. Walker has studied the shock initiation of mixtures of nitromethane with diethylenetriamine.25 The time delay for the appearance of a detonation front was found to decrease from 18
Constantinou et al. lmpactor
L~grfrom Xenon flashlamp
Detector
Sample \
Brass ,Cell
Optical
Figure 1. Experimental configuration for the light transmission experiments.
to
