Ultrafast Chemical Reactions in Shocked Nitromethane Probed with

Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 United States. J. Phys. Chem. A , 2014, 118 (14), pp 2559–2567. DOI: 10.1021...
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Ultrafast Chemical Reactions in Shocked Nitromethane Probed with Dynamic Ellipsometry and Transient Absorption Spectroscopy Kathryn E. Brown,* Shawn D. McGrane, Cynthia A. Bolme, and David S. Moore Shock and Detonation Physics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 United States S Supporting Information *

ABSTRACT: Initiation of the shock driven chemical reactions and detonation of nitromethane (NM) can be sensitized by the addition of a weak base; however, the chemical mechanism by which sensitization occurs remains unclear. We investigated the shock driven chemical reaction in NM and in NM sensitized with diethylenetriamine (DETA), using a sustained 300 ps shock driven by a chirped Ti:sapphire laser. We measured the solutions’ visible transient absorption spectra and measured interface particle and shock velocities of the nitromethane solutions using ultrafast dynamic ellipsometry. We found there to be a volumeincreasing reaction that takes place around interface particle velocity up = 2.4 km/s and up = 2.2 km/s for neat NM and NM with 5% DETA, respectively. The rate at which transient absorption increases is similar in all mixtures, but with decreasing induction times for solutions with increasing DETA concentrations. This result supports the hypothesis that the chemical reaction mechanisms for shocked NM and NM with DETA are the same. Data from shocked NM are compared to literature experimental and theoretical data. static pressure30 also shows an increase in aci ion concentration. The presence of the aci ion in NM and its increase with base addition, UV irradiation, or high pressure have not been disputed; however, its role in NM initiation and the amine sensitization mechanism has.25,26,31,32 There are two general possibilities as to the role of the aci ion in the amine sensitization in NM. Either an increased presence of the aci ion causes sensitization or its presence is coincidental to sensitization. There have been experimental findings to support both cases. Cook and Haskins26 and Constantinou and co-workers25,31,32 have suggested a sensitization mechanism in which an added base forms a charge transfer complex with the NM, weakening and ultimately causing the scission of the C−N bond. In this hypothesis, there is an increased presence of the aci ion, but the aci ion is not involved in the decomposition of NM. More prevalent in the literature, however, is the hypothesis that the increased presence of the aci ion is the mechanism for amine sensitization in NM. As noted above, the aci ion has been shown to be the rate-determining species in liquid NM detonation.23 In this causal hypothesis, the addition of a base results in a higher concentration of this reactive ion and thus will yield a higher reaction rate. Gruzdkov and Gupta performed emission, fluorescence,33 and absorption experiments34 on amine-sensitized NM. Their results did not support C−N scission, but rather the formation of a radical anion through the aci ion. However, they acknowledged the possibility of there being two separate mechanisms for neat

I. INTRODUCTION Ultrafast experiments can access chemical initiation information of shocked materials that is unattainable in traditional plateimpact experiments.1−5 Likewise, theory and simulations of shocked materials tend to be on the ultrafast time scale,6−10 allowing for direct comparison of theory to experiment. Recently, Armstrong et al.3 published a paper on shockinduced, ultrafast chemical reactions in hydrogen peroxide, where they were able to successfully compare their data to a calculated product Hugoniot.11 The chemical decomposition mechanism of shocked nitromethane (NM) has been a subject of debate over the last several decades. NM is an insensitive explosive liquid, the smallest and simplest of the nitro-organic molecular explosives. The shock to detonation transition,12−16 as well as the equation of state for shocked NM,17−20 has been addressed by several groups. However, for as widely and extensively as shocked NM has been studied, kinetic and chemical models remain incomplete. Nitromethane, CH3NO2, is a weak acid in equilibrium with its “aci” isomer, CH2NO2H, and the aci conjugate base, the nitronate anion, or “aci ion”, CH2NO2−.21 Engelke et al. were the first to propose22 and show evidence via 13C nuclear magnetic resonance measurements that the aci ion is the ratecontrolling species of liquid nitromethane detonation.23 It is well established that the addition of a basic amine sensitizes NM.22,24−28 As there is a very small amount of the aci ion in neat NM (one estimate gives 5 aci ions per 1 million nitromethane molecules at 0 °C),21 addition of a base, including diethylenetriamine (DETA), shifts its acid−base equilibrium and produces more aci ion. NM that has been sensitized by ultraviolet (UV) irradiation29 or subjected to high © 2014 American Chemical Society

Received: December 23, 2013 Revised: March 12, 2014 Published: March 14, 2014 2559

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Figure 1. Experimental schematic for shock generation, UDE, and transient absorption spectroscopy. Adapted and reproduced with permission from ref 36. Copyright 2007, AIP Publishing LLC. http://dx.doi.org/10.1063/1.2767376.

measurements utilized a portion of the beam that was spectrally clipped and chirped (∼300 ps) and the remainder of the beam was compressed to 2.5 km/s.

450 nm. Transient absorption spectroscopy data for NM+5% DETA under reactive and unreactive conditions are shown in Figure 5. The data for Figure 5a correlate to UDE us−up points that are in the unreactive regime of NM+5% DETA, up < 2.0 km/s. The data for Figure 5b corresponds to UDE data that is 2.0 km/s ≤up < 2.5 km/s, and the data for Figure 5c correspond to UDE data that is up ≥ 2.5 km/s. The data for both panels a and b of Figure 5 are considered to be in the reactive regime of NM+5% DETA.

Figure 6. Interface particle and shock velocity data points for NM including (cyan diamonds) plate-impact points from refs 47 and 48 and UDE points from NM in the (red circles) unreactive, (blue squares) “cusp”, and (green triangles) reactive regimes. The black solid line is the Winey et al. parametrized ULH for NM.20 The dotted red line is the Hugoniot for NM products.18

ULH-form equation given by Winey et al.,20 hereafter referred to as the “W-ULH”, to be Δus = 0.24 km/s. The data17 used by Winey et al. to calculate the W-ULH are for up < 1.5 km/s; the data we used to calculate standard deviation were between up = 1.3 km/s (our lowest point) and up = 2.0 km/s, as our data in that region have consistently low scatter (approximately 3%). This is up to approximately the same up range covered in the literature by traditional plate impact experiments (cyan diamonds), compiled in refs 47 and 48. Our data lie on average Δus = 0.2 km/s above the W-ULH and traditional shock data. A slightly high us for a given up could be due to hydrogen bonding with NM,49 which may affect compression on our time scale. Up to up < 2.0 km/s, 88% of our data points

IV. DISCUSSION It is apparent from both the UDE and transient absorption data that NM, with and without DETA, reacts under our highest up shock conditions. There is a volume-increasing reaction that occurs around interface up = 2.4 km/s for neat NM and NM +1% DETA solutions, and up = 2.2 km/s for NM+3% and 5% DETA. This result is consistent with sensitization of the NM with DETA. A. Nitromethane UDE. Detail of our NM UDE data and comparison to literature are shown in Figure 6. We calculated the standard deviation of our UDE data with respect to the 2562

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shocked temperature at 19 GPa to be approximately 1800 K, which is comparable to the calculated von Neumann spike temperature of 1872 K at a detonation pressure of 21 GPa.18 For a similar initial shock temperature, Hervouët et al.10 predicted that the concentration of NM molecules would diminish slowly over the course of 300 ps. Their calculated temperature required to diminish the concentration of NM in less than 50 ps was 3000 K. Pellouchoud and Reed calculated that under similar shock velocity to our experiment, the time required to diminish NM molecules was on the order of 20 ps.6 C. Shocked Refractive Index Calculations. Our UDE measurements of NM also yielded shocked refractive index information. The initial, unshocked refractive index was 1.39.39,53 Shocked NM under our reactive conditions was found to have a maximum refractive index of ∼1.7 at 800 nm, consistent with previous findings.54 Pellouchoud and Reed6 calculated the real part of the shocked refractive index to be >2 at 800 nm. Their calculated imaginary part of the shocked refraction index, k, was determined to have an initial increase to ∼0.6 at 400 nm and ∼0.4 at 800 nm, followed by a decrease by roughly a factor of 2 at 400 nm and by a factor of approximately 10 at 800 nm over 100 ps. Our UDE analysis can include fits for k. A value of k > 0.1 is inconsistent with our data. Using our transient absorption data and relations for the absorption coefficient, we can estimate k:

lie within one standard deviation (red circles) of the W-ULH, and 12% lie between one and two standard deviations. The region up to up = 2.0 km/s and higher up points that are within one standard deviation of the W-ULH are in the unreactive region of NM. Between up = 2.0 km/s and 2.4 km/s, the scatter in our data greatly increases, with 41% within one standard deviation of the W-ULH, 35% between one and two, 18% between two and three, and 6% between three and four. This region has been denoted the “cusp” region. Data in this region and in the unreactive region that are between one and four standard deviations from the W-ULH are denoted by blue squares. In the cusp region, our assumption for a steady state, single wave structure fails, as that is where the reaction rate is comparable to the time scale of our experiments. The points in the cusp region were fit in the same manner as the unreactive points, but it must be stated that these are not Hugoniot points. The third region (green triangles) corresponds to reacted or partially reacted NM. It has been defined as starting at the first point that lies outside of four standard deviations from the WULH; in the case of pure NM, that point is at up = 2.4 km/s and us = 6.9 km/s. This is in comparison to the detonation conditions calculated by Menikoff and Shaw to be up = 3.0 km/ s and us = 6.25 km/s.18 In this region, so long as NM is reacting faster than the time scale of our measurement, our assumption of constant velocity is likely close to accurate and the data can be fit reasonably well with a single wave. The NM detonation products Hugoniot calculated by Menikoff and Shaw is also shown in Figure 6 (red dashed line).18,50 We consider the points denoted by green triangles to be more closely related to the products Hugoniot than the W-ULH. There are no plateimpact us−up data available for overdriven NM, making our data the first of their kind. Our data points lie remarkably close to the expected products curve. This concurrence indicates that a substantial amount of the shock-induced decomposition of NM takes place within our time scale, likely in tens of picoseconds. This rapid formation of volume-expanding products is unexpected, given that VISAR data from detonation experiments indicate a decay over several tens of nanoseconds.12,51,52 It is worth noting again that our measurement of up is the particle velocity of the Al−NM interface. In a volume-increasing reaction such as the one of NM, there is an expansion wave that effectively slows that interface velocity, effectively depressing our interface up in the reactive region from the true NM up. Reactive wave modeling is needed to quantify the up depression. There is some overlap between the data points for reactive and unreactive NM. This overlap could be due to varying experimental conditions, including variations in the Al or sapphire substrate or shot-to-shot laser energy fluctuations. B. Shock Pressure and Temperature. Using the Rankine−Hugoniot relation,

P = ρ0 usu p

I = I 0 e −αx

k=

αλ 4π

(4)

(5)

where I and I0 are the intensities before and during the shock (eq 1), α is the absorption coefficient, x is the path length of the shocked region, and λ is the wavelength. The length of the shocked region was estimated as the shock velocity multiplied by each 50 ps time step. As we cannot access either 400 or 800 nm in our absorption experiments, we used 410 and 750 nm. Values of k were consistent between 100 and 300 ps. At 410 nm, we found k to be ∼0.004, 2 orders of magnitude smaller than predicted at 400 nm. At 750 nm, we found k to be ∼0.001, over 1 order of magnitude smaller than predicted at 800 nm. D. Transient Absorption. The transient absorption spectra from DETA were compared to the data previously obtained from cyclohexane,1,37 a molecule that has been shown to be unreactive under our conditions.1 The DETA spectra did not significantly differ from the cyclohexane spectra. Both the absence of transient absorption and UDE data taken for DETA (Figure 3) suggest that DETA alone is unreactive, but that the addition of DETA to NM causes an increase in reactivity. There is a slight increase in the apparent transient absorption of DETA. A similar phenomenon has been shown previously with other unreactive materials including cyclohexane, and we have attributed it to time-dependent changes to the Al reflective surface, including an increase in scattering due to roughening of the shocked Al.1 Figure 7 shows the average absorption from ten separate shock events over all wavelengths between 410 and 750 nm of shocked NM (a) and NM+5% DETA (b) under three different shock conditions. The data for neat NM (a) can be related to the regions discussed in section IVA and shown in Figure 6. The red circles correspond to data taken under unreactive conditions, the blue squares from the “cusp” region, and the green triangles from the reactive region. For NM+5% DETA (b), the conditions are the same as in Figure 5, where the red circles correspond to transient absorption data taken

(3)

where ρ0 is the density of NM under ambient conditions and ρ0 = 1.14 g/mL,41 we can calculate the highest shock pressures we obtained under nonreactive conditions. The cusp region begins after P = 12 GPa, and the highest nonreactive pressure in the cusp region is P = 17 GPa. The Rankine−Hugoniot relations are only valid for nonreactive materials, but if we use eq 3 to estimate the pressure of our first reactive point, we get P = 19 GPa. Using the equation of state relating pressure and temperature calculated by Winey et al.,20 we can estimate our 2563

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up, reactive region begins to increase after about 100 ps, and at the highest up, the absorption begins to increase almost immediately. The difference in rate of change in the concentration of the absorbing species, i.e., optical density with time (OD/ps) between the lower up and highest up (Figure 7b, blue squares and green triangles) in the reactive region, is apparent, indicating an overall faster reaction rate for higher up. In Figure 7c, the average absorption over 410−750 nm for NM (purple circles), NM+1% DETA (left-pointing green triangles), NM+3% DETA (orange squares), and NM+5% DETA (magenta triangles) under the highest up condition is shown. The data are offset incrementally by OD = 0.05 with increasing concentration of DETA. The rate of change in the concentration of the absorbing species was determined for each data set starting at the time that absorption begins to increase. For neat NM, between 200 and 300 ps, the slope is 0.6 ± 0.06 OD/ns; for NM+1% DETA, 0.5 ± 0.05 OD/ns between 200 and 300 ps. For NM+3% DETA, the slope is 0.8 ± 0.07 OD/ns between 150 and 300 ps; for NM+5% DETA, 0.7 ± 0.07 OD/ ns between 50 and 300 ps. Within two standard deviations, the rates of absorption increase for neat NM, NM+1% DETA, NM +3% DETA, and NM+5% DETA are similar. This phenomenon is indicative of a similar reaction rate between the four solutions, but having different induction times. Previous studies have reported that NM sensitization occurs from additions of less than 1% DETA. For example, Engelke22 reported that the failure diameter of NM decreases by 30% with the addition of 0.01% DETA and by 70% with 2.5% DETA (wt.). Later, Engelke et al. reported that the addition of 0.25% DETA decreased the chemical reaction zone of NM by ∼25%.28 Walker24 examined the effect of DETA on detonation delay time of NM. The bulk of the sensitization occurred with the addition of 1% DETA, causing the detonation delay time to decrease by 88%. Addition of 5% DETA further sensitized NM, decreasing the delay time by 97% over neat NM. We saw only a slight increase in transient absorption magnitude with the addition of 3% DETA, but a significant increase in transient absorption magnitude and decreased onset time delay with 5% DETA. A higher concentration of DETA might be required to see stronger evidence of NM sensitization on our sub-ns time scale. The identification of the absorbing chemical species is unclear. It may be final reaction products (including solid carbon) and/or reaction intermediates. The UV absorption of the aci anion peaks at 233 nm.31,55 As noted by Constantinou et al.,31 the mixture of DETA and NM is yellow, despite both liquids being colorless. The visible absorption spectrum of a NM+5% DETA solution in a 10 mm cuvette is shown in Figure 8. In our 6−12 μm-thick samples, this absorption around 480 nm is negligible. In shock ring up transient absorption experiments performed by Gruzdkov and Gupta34 on NM sensitized with a different amine, ethylenediamine (EDA), a reactive intermediate was determined to have an absorption peak around 525 nm that developed prior to a broadband absorption. They identified the 525 nm absorbing species in that case to be a radical anion, CH3NO2•−. Our transient absorption in every case smoothly increased toward the UV and was quite broad, spanning our entire range. As the spectral lineouts from neat NM and NM+5% DETA (Figure 9) only appear to differ in magnitude and the rate at which the absorption increases, not in spectral shape, we feel that this is

Figure 7. Average transient absorption from 410 to 750 nm at 0−300 ps after shock arrival. (a) NM: red circles, blue squares, and green triangles correspond to data taken in the unreactive, cusp, and reactive regimes, respectively. (b) NM+5% DETA: red circles, blue squares, and green triangles correspond to data taken in the unreactive regime, the reactive regime at 2.0 km/s ≤ up < 2.5 km/s, and the reactive regime at up > 2.5 km/s, respectively. (c) Transient absorption under reactive conditions for (purple circles) NM, (left-pointing green triangles) NM+1% DETA, (orange squares) NM+3% DETA, and (magenta triangles) NM+5% DETA.

under unreactive conditions, where up < 2.0 km/s; the blue squares and green triangles are from data taken under reactive conditions, 2.0 km/s ≤ up < 2.5 km/s and up > 2.5 km/s, respectively. Each data set in Figure 7 is offset by increments of optical density (OD) = 0.05 for clarity. Error bars are the standard deviation from the average of 10 spectra at each time and shock drive energy. The absorption caused by the timedependent changes in the Al reflective surface in each case (assumed to be the same as in DETA) was subtracted; absorption spectra in Figures 4, 5, and 9 were not adjusted. The shot to shot variation in the transient absorption measurements of DETA was similar to the shot to shot variations in the reactive materials. In the case of NM (Figure 7a), the data in neither the unreactive nor cusp regions (red circles, blue squares, respectively) have any discernible increase to their transient absorption over time. We attribute the nonzero value in this case to be a product of variation in the substrate and laser energy and profile between DETA and NM measurements. This interpretation is consistent with the UDE data in the unreactive up regimes; there is no apparent reaction occurring. Our transient absorption measurements do not appear to be sensitive to the reactions that are beginning to occur in the cusp region. However, data corresponding to high up in the reactive region show that there is a definite increase in transient absorption starting after around 200 ps. In the case of NM with 5% DETA, there is likewise no absorption for the lowest up, unreactive case. The transient absorption in the lower 2564

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spectroscopy. Our UDE-measured Hugoniot data agree with literature values, and both UDE and transient absorption data indicate that there is a volume-increasing chemical reaction that takes place above interface up = 2.4 km/s for neat NM and interface up = 2.2 km/s for NM+5% DETA. The similarity of the spectral evolution of the transient absorption between neat NM and sensitized NM supports the hypothesis that the decomposition mechanism in neat NM is the same as the decomposition mechanism in sensitized NM. The reaction rates of the formation of an absorbing species in shocked neat and sensitized NM were similar, but increased addition of DETA reduced the induction time, indicating that DETA may be a catalyst for the first reactive step in NM decomposition. Chemical-specific techniques, including ultrafast vibrational spectroscopy (i.e., coherent anti-Stokes Raman scattering) will be valuable in assessing the nature of the reactive and absorbing chemical species.

Figure 8. Visible absorption spectrum of NM+5% DETA under ambient conditions.

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ASSOCIATED CONTENT

S Supporting Information *

All of the tabular UDE data points in Tables S1 and S2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*K. E. Brown: phone, (505) 695-8666; e-mail, kebrown@lanl. gov.

Figure 9. Spectral lineouts of transient absorption under reactive conditions for (a) NM and (b) NM+5% DETA. Bottom to top: 0, 50, 100, 150, 200, 250, 300 ps shock-probe delay.

Notes

The authors declare no competing financial interest.

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consistent with the hypothesis that the mechanism for NM +DETA decomposition follows the same pathways as the decomposition of neat NM, rather than there being a different reaction pathway for sensitized and neat NM. Our absorption data and the literature data do not rule out either the aci ion or the radical anion as the absorbing species. The apparent structure in the spectra (Figure 9) is not resolved well enough to provide any conclusive species identification. Our experiment did not access wavelengths below 400 nm, and as Gruzdkov and Gupta used multistep shocks, their temperatures were lower than for our single shocks. In addition to impacting the rate of reaction, increased temperature allows for thermal population of higher vibrational states, broadening and red shifting the absorption spectrum. Recently, Pellouchoud and Reed6 published a paper in which they used time-dependent density functional theory (TDDFT) to simulate optical properties of shocked NM over the first 100 ps. Their simulations predict that the absorption of NM initially transiently increases, but then decreases by over 50%. It would be interesting to see if our shocked NM acts similarly. However, our current transient absorption spectroscopy time step and noise do not allow for this level of specificity. Consider shocked NM whose shock-probe delay was 300 ps. The probe looks at the integral of the entire thickness of NM, including material at the shock entrance that has been compressed for 300 ps, and material at the shock exit that is at that moment being compressed.

ACKNOWLEDGMENTS Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. The authors thank Dana Dattelbaum and Ralph Menikoff for useful discussions. The authors gratefully acknowledge the support of this study by Rick Martineau through Science Campaign 2: HE Science.

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REFERENCES

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V. CONCLUSION We characterized NM and NM+DETA shocked by a temporally shaped 300 ps pulse from a Ti:sapphire laser using ultrafast dynamic ellipsometry and transient absorption 2565

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