Time-Resolved Raman Measurements in Nitromethane Shocked to

Apr 1, 1994 - The Photochemistry of Crystalline Nitromethane under Static Pressure. Samuele Fanetti , Margherita Citroni , Naomi Falsini , and Roberto...
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J. Phys. Chem. 1994, 98, 4522-4529

Time-Resolved Raman Measurements in Nitromethane Shocked to 140 kbar G. I. Pangilinan' and Y. M. Gupta Shock Dynamics Center and Department of Physics, Washington State University, Pullman, Washington 99164-2814 Received: October 1 1 , 1993; In Final Form: December 22, 1993"

Time-resolved Raman spectra of nitromethane shocked to 140 kbar peak pressure using step wave loading have been obtained. The CN stretch (917 cm-l), CH3 stretch (2968 cm-l), and the NO2 stretch/CH3 bend (1400/ 1377 cm-1) vibrations are all observed to harden when peak pressure is attained in the liquid samples. The spectra obtained show no frequency softening up to 400 ns after peak pressure is reached in the material. Upon unloading from 140 kbar, the CN stretch vibration reverts back, showing no signs of an irreversible change. W e also observe that the vibrational frequencies increase nonlinearly with peak pressure, with the CH3stretching mode exhibiting the largest increase. The observed frequency hardening and broadening are suggestive of strong intermolecular interactions at these shock pressures. Implications of these observations with regard to attaining a precursor state for shock-induced chemical reactions are discussed.

I. Introduction A good understanding of shock-induced chemical reactions in condensed energetic materials is central to problems related to detonation, development of new energetic materials, and safe and efficient use of high explosives.' Despite the considerable use of high explosives, a detailed understanding of the chemistry and physics of the molecular processes involved in detonation phenomena remains a challenging problem.2 These limitations are widely recognized, and there has been an ongoing effort toward achieving such an ~nderstanding.~-l~ Progress in theoretical analysis has been significantly aided by new computational m e t h ~ d s . ~These J ~ analyses, however, are in need of experimental results to validate current hypotheses or promote new ones. It is not surprising that efforts to describe shock-induced chemical reactions in the condensed phase rely heavily on results from diverse phenomena such as gaseous reactions, photolysis, thermal decomposition, or pyrolysis. Results from these phenomena may not be directly applicable to shock experiments due to the uniqueness of shock loading conditions and the numerous chemical pathways that are possible. Therefore, a strong need exists for experimental results that describe molecular changes as energetic materials are shocked. Because of the strongly transient nature of the changes, the ability to monitor them with good time resolution is important. The lack of time-resolved measurements that probe microscopic properties of shocked energetic materials is not surprising because of the difficulty in performing such measurements. Materials of interest have to be subjected to high pressures and temperatures, and the probes need to have sufficient spatial and temporal resolution to record molecular changes that occur within a microsecond. The difficulties associated with the single event nature of such measurements in energetic materials cannot be overemphasized. In recent years, experimental techniques have been developed to permit time-resolved absorption,14 reflection,15J6 luminescence,17and Raman measurements18J9 in shocked nonenergetic materials to probechanges at themicroscopic level. Theobjective of the present work was to improve on the time-resolved Raman scattering method, previously developed, to examine the shock response of nitromethane to peak pressures approaching 140 kbar. Specifically, the C N stretch, the CH3 symmetric stretch, the NO2 symmetric stretch, and the CH3 symmetric bend of liquid nitromethane were monitored as peak pressures up to 140 kbar a

Abstract published in Advance ACS Abstracts, April 1, 1994.

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are reached, with 50-ns time resolution. Unloading experiments toverify irreversibility were also completed. These measurements are the first Raman measurements of shocked nitromethane above 100 kbar and also the first measurements on the CH3,and NO2 vibrations a t any shock pressure. On the basis of the behavior of these molecular vibrations obtained up to 400 ns after reaching peak pressure, we provide a microscopic picture of shocked nitromethane. Raman scattering is well suited to examine molecular changes since atomic vibrations are directly monitored. These measurements provide information on the frequency changes of molecular bonds, configurational changes as manifested by differing selection rules that occur at phase transitions, and electronic energy changes as seen in pressure-induced resonance.18-20Spontaneous Raman scattering measurements are, however, inherently difficult due to the weak signals that are generated.2' For example, timeresolved Raman scattering studies of chemical changes performed to study flash photolytic reaction require accumulating up to 10 000 events before acceptable signal-to-noise ratios are obtained;22the measurements described here had to be obtained with single shock events. Nitromethane was selected as the material to study because it serves as a prototypical energetic material.23 It is studied in liquid form so that complexities that may arise with solid samples such as crystallinity or presence of defects or dislocations do not arise. Aside from the number of vibrational studies discussed in detail in the next section, nitromethane's UV-vis absorption bandsz4are known, and its decomposition under various stimuli has been reported. C N scission is reported to be the first important step in UV p h o t o l y s i ~ ~or~ thermal *~6 decomposition,23 while with multiphoton dissociation,27 N O scission is believed to be an important process. The recent workz3by Constantinouon thermal decomposition of nitromethane is particularly noteworthy. Various hypotheses have been proposed to describe the first important chemical changes in shocked n i t r ~ m e t h a n e . ~ - l ~ . ~ ~ , ~ ~ Formation of aci-ion is proposed to be significant in the reaction in shocked nitromethane.'@-12 The formation of methyl nitrite is calculated28 to be a favorable process in the decomposition of shocked nitromethane. To explain the increase in shock sensitivity of nitromethane by amine addition, weakening of the C N bond preceded by hydrogen bonding has likewise been proposed.I3 Previous vibrational studies on nitromethane that are relevant to this work are briefly summarized in the next section. Section I11 describes the experimental methods used in our work and the results. A discussion of these results and comparison with earlier 0 1994 American Chemical Society

Time-Resolved Raman Measurements in Nitromethane

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work is presented in section IV, and the conclusions are presented in section V.

OMA

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SAPPHIRE

11. Summary of Previous Work

A number of studies on the vibrational spectra of nitromethane a t ambient temperature and pressure exist in the l i t e r a t ~ r e . ~ ~ - ~ ~ Symmetry assignments of the different Raman and far-infrared SAPPHIRE \WINDOW In the C, point mode assignments have been group, the 15 vibrational modes are divided into 5A1,4B1, 5B2, and 1A2 symmetries.” All the vibrational modes are Ramanactive, and the modes with the highest scattering efficiencies in liquid nitromethane are at 917 cm-1 (CN stretch), 2968 cm-l (CH3 stretch), and 1400 cm-1 (NO2 stretch).29.30 Because the C N bond is believed to play a role in the shock decomposition Figure 1. Schematicview of method used toobtain time-resolved Raman of 11itromethane,~J3it has been accorded special attention in scatteringspectra under shock loading as explained in the text: A, aperture; Raman studies a t ambient pressures. The C N bond Raman line L, lens; HF, holographic edge filter; MCP, mirochannel plate image shape has been studied, and it is explained in terms of a model intensifier; OMA, optical multichannel analyzer; PD, photodiode. that depends on thermal occupation of low-lying vibrational levels and 85 kbar, a t which reaction occurs. This finding is in and anharmonic coupling between vibrational modes.30 contradiction with the 68-kbar threshold proposed’* for the onset The importance of pressure in understanding shock-induced of a chemical reaction based on the disappearance of the C N chemical reactions in nitromethane has been recognized widely peak. To clarify the conflicting findings reported to date, several as evidenced by the number of vibrational studies in nitromethane points are noteworthy. First, analysis of shock experiments in a diamond anvil cell (DAC).12932-34 In Raman and far-infrared requires well-defined time and spatial resolution. The measureexperiments,32 the frequency shifts of the normal modes in ments in ref 38 consisted of a single measurement (per experiment) superpressed liquid nitromethane were obtained a t pressures up taken 4 ps after the shock wave had entered the sample. In to 17 kbar. The observed pressure-induced shifts (-8 cm-l for contrast, the CARS measurements37 were taken while the shock CH3 stretch, -7 cm-1 for NO2 stretch, and -2 cm-1 for CH3 wave was traversing approximately 3 mm of material with part bend a t 17 kbar) were explained by invoking a model that involved of the signal collected from the unshocked material. Second, a intermolecular interactions between two molecules. The pressure microscopic study of shocked nitromethane should include other range in this study was limited because nitromethane rapidly vibrations as well. Defining reaction onset or the precursor state crystallizes to a polycrystalline state a t about 20 kbar.34 requires information from other bonds of nitromethane. Finally, Vibrational studies above 17 kbar in the DAC on solid as discussed in more detail in section IV.D, we recommend caution nitromethane have also been reported. One study12 designed to in the assignment of a reaction onset from mode disappearance. infer isotopic mixing in CH3N02/CD3N02 mixtures yielded a Unloading experiments have to be performed to verify reversibility C N frequency shift of 13 cm-1 at 21 kbar. An earlier study33 of an observed spectral change before ascribing the change to a reported that all Raman frequencies of nitromethane increase chemical reaction. with pressure up to 117 kbar. Their conclusion33 that no precursor state to decomposition was attained in the pressure range studied 111. Experimental Method and Results is hard to establish since it is based solely on frequency positions that had large error bars (--f15 cm-l). No spectra are shown; A. Time-Resolved Raman Scattering Measurements. The line shape analysis is not provided, and the widths of the vibrational experimental configuration to obtain time-resolved Raman modes are not given. Furthermore, the NO2 symmetric stretch scattering data adapted from previous work18J9 is shown in Figure frequency position is missing in the report. 1. A flashlamp pumped dye laser (Candela SLL 5000) operating Thermal decomposition of nitromethane pressurized in a DAC a t 514 nm provided the incident beam. An aperture A was used to control the energy of the pulse that was focused to a 400-pm has also been examined. An early study35 reported that when pressurized to 50 kbar and heated to 150 OC, nitromethane would optical fiber (Mitsubishi ST-U400E-SY). The incident beam a t react producing a dark-colored compound. More the other end of the optical fiber was then focused with a 1:l infrared absorption studies show that both pressure (up to 71 imaging lens assembly onto the nitromethane sample sandwiched kbar) and temperature (up to 453 K) increase the rate of thermal between two sapphire crystals ( a or c cut, Union Carbide, CA) serving as optical windows. For these experiments, liquid decomposition, and the complex reaction mechanism may vary over large changes in pressure. Because of the differences between nitromethane (99+%, Aldrich) was used without further purishock wave and hydrostatic loading, shock-induced reaction may fication, and the sample thickness ranged from 140 to 307 pm. proceed quite differently from thermal decomposition observed Because of the 45O incident angle and astigmatism through the in the DAC. Results from shock experiments are thus necessary back sapphire window, the beam spot was about a millimeter in to understand shock-induced chemical reactions. diameter at the sample. The laser energy measured a t the output of the optical fiber was typically 120-140 mJ. By spreading the Under shock conditions, experimental results are minimal and laser energy over 3-ps duration and using the beam spot described conflicting and do not involve time-resolved measurements. Partly above, we achieved an acceptable signal collection without due to the hypothdsis proposing C N scission as the ratedetermining photochemical decomposition and/or stimulated Raman scatstep in the shock decomposition of nitromethane, only the C N tering. The laser intensity profile in time was also reproducible stretch has been probed up to now. In 1983, the C N stretch from shot to shot with at most 5% deviation. mode was monitored36for nitromethane single shocked, separately Backscattered Raman spectra were collected by another 1:1 to 50 and 85 kbar. A reaction onset was proposed to occur in the imaging lens system and sent through the collection optical fiber. 85 kbar experiment because no Raman mode was observed. The output of this fiber was focused to the input of a 0.6-m Coherent anti-Stokes Raman scattering (CARS) results3’ on spectrograph (dispersive stage of 1877 SPEX triplemate) through nitromethane indicated that decomposition, which would have a third 1:l imaging lens system. An aperture A between these decreased the C N peak intensity, had not occurred for nitwo lenses was adjusted to match thef-numbers of the optical tromethane single shocked to 76-kbar pressure. These two fiber and the spectrograph. The spectrograph is equipped with investigations seemingly indicate a pressure regime between 76

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The Journal of Physical Chemistry, Vol. 98, No. 17, 1994 NO2ICH3 (1400 cm"I1377 cm-')

CH3 (2968 cm")

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Figure 2. Five time-resolved Raman spectra of nitromethane at ambient pressure with 50-ns resolution. Shown are the CN stretch, CHs stretch, and NO1 stretch convoluted with the CHs bend vibrations.

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two gratings, which were interchangeably used to scan for either wide coverage (-2000 cm-I) or higher resolution ( 16 cm-1 band-pass) to quantify vibrational shifts with pressure. The output of the spectrometer was sent through a cylindrical lens to shorten the image height and a holographic filter (POC, CA) to filter out unwanted elastically scattered light. The transmitted beam was then collected by an electronic streak camera (Imacon 790) which dispersed the light in time, yielding at its output a two-dimensional image of light dispersed both in wavelength and in time. For these experiments, the streak rate of the camera and the image height formed by the cylindrical lens were chosen so that spectra with 50-ns resolution were obtained. The streakcamera output wassent througha proximity focused microchannel plate intensifier (ITT F4113) which gave an optical gain of about 5000. A lens system imaged the output from this intensifier to an array detector. For most of the experiments, an EG&G 1254 intensified vidicon detector was used. (In experiments 92052 and 92058, discussed in subsection C, an EG&G 1430P CCD detector was used.) Both detectors were controlled by an optical multichannel analyzer. The total time window for the experiments was limited by the detector array dimensions to 1.5 pts. The experiments required accurate synchronization of the laser pulse, the streak camera, the multichannel plate image intensifier, the vidicon detector, and the shock event. The streak camera was triggered so that the 1.5-ps time window encompassed the central portion of the 3-ps laser pulse. The vidicon detector and the intensifier were switched on at the same time as the streak camera. This synchronization was accomplished with preselected time delays from a delay generator (EG&G Model 9650) that was actuated by a pulse designed to trigger so that the streak camera captured impact in the first 250 ns of the 1.5-ps window. Photodiode records from the laser and from light reflected off a sample, a streak camera monitor pulse, and a pulse from the delay generator were recorded in a digital oscilloscope for diagnostic purposes. A typical result consisted of 30 tracks or spectra separated in time by 50 ns. Figure 2 shows five tracks with 50-11s resolution, obtained at ambient pressure, in which vibrational peaks corresponding to the C N stretch, CH3 stretch, and the NO2 stretch convoluted with a CH3 bend are seen. Weak structures that are visible are part of the collection noise. Moreover, a track-totrack variation in peak intensity, which can go as high as 20% of the signal, is seen in Figure 2. This variation is due to the cumulative response of the streak camera, microchannel plate intensifier, and detector. For each track, channel number was calibrated to cm-1 energy units. This was accomplished by taking five spectra of a known source such as the 5 14-nm argon ion line or the 546-nm mercury

Pangilinan and Gupta line for five different spectrometer settings. The peak of each spectrum corresponded to a specific channel number depending on the spectrometer setting. The spectrometer settings were chosen so that the peak positions of the five spectra covered about three-fourths of the total channel number. The five spectrometer settings and the corresponding channel number of the peaks in the spectra then yielded a channel number to energy conversion. For the experiments with higher resolution, this calibration yielded f cm-1 I3 cm-1 uncertainty in assigning peak positions. B. Pressure Loading and Unloading of Liquid Nitromethane. Shock waves which compress the sample weregenerated by impact between a sapphire impactor mounted on a projectile, and the sapphire front window of the sample cell. The projectile, 10.2 cm in diameter and weighing approximately 1 kg, was accelerated by a single stage gas gun'* to desired velocities. Two types of impact experiment^^^.*^ are briefly described here: step wave loading (SWL) and SWL followed by unloading. In SWL experiments, the back sapphire window facing the lens system was 25.4 mm in diameter and 12.5 mm thick, while the front sapphire window was 31.75 mm in diameter and 3.175 mm thick. When the sapphire disk (25.4-mm diameter and 12.5 mm thick) mounted on the projectile impacts the front window, a forward running shock wave traverses the front window and in about 280 ns reaches the front window sample interface. The shock wave subsequently reverberates between the sapphire windows until the sample reaches peak pressure, in about another 300-450 ns, depending on the sample thickness. Peak pressure is maintained in the samples for over 1 ps after impact. Release waves from the edges of the sapphire windows will reach the sample volume being studied beyond this time. Because the data are obtained only from the central 1 mm, the sample is in a state of uniaxial strain for approximately 1 ps. Unloading experiments differed from the SWL impact experiments in that thin sapphire crystals (either the impactor or the back window) were used to ensure arrival of unloading waves before the arrival of edge waves; loading and unloading were completed while the sample was in a state of uniaxial strain. In two experiments, the impactor was 3.175 mm thick instead of 12.5 mm. At impact, a backward moving shock wave traverses the impactor and hits a free surface, yielding a forward moving release wave that enters the sample at a later time. In another unloading experiment, a 12.5" impactor was used, but the back window was 1.58 mm thick. In this experiment, the release waves originated from the back window/free surface interface and reached the sample a t some later time. The pressure history in nitromethane was obtained by using wave propagation calculations and the equation of state of c-cut sapphire40 and n i t r ~ m e t h a n e .Parts ~ ~ a and b of Figure 3 show pressure vs time results of these calculations for S W L and unloading, respectively, for typical sample thicknesses and projectile velocities. t = 0 is the time at which the impactor hits the sapphire front window. About 280 ns later, the shock wave enters the sample. Pressure behind the shock front is then increased. Peak pressure is attained after about 300-450 ns rise time depending on the sample thickness, as a result of multiple reverberation of the shock, at the nitromethane/sapphire interface. For the unloading experiment shown in Figure 3b, a release wave from the thin back window entered the sample at about 670 ns after impact. In these measurements, data were analyzed at times when pressure reached at least 97% of the peak pressure. The accuracy of the calculated peak pressures depends only on the shock response of sapphire. Due to the isotropy of the elastic constants of sapphire,42 even with the use of a-cut sapphire material, the calculated pressure values in this work are very accurate. Temperature and volume calculations on the other hand can have bigger uncertainties.41 In these calculations the equation of state of nitromethane41 is based on the universal liquid Hugoniot

Time-Resolved Raman Measurements in Nitromethane

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(a) SW Loading ProJectile velocity

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Sample thickness = 267 p m

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(psec) Figure 3. Typical calculated pressure history in nitromethane with (a) step wave loading and (b) step wave loading following by unloading. The curves correspond to pressure history in shot numbers 92078 and 9303 1 listed in Table 1. TIME

equation43 with constants adjusted to match data from Lysne and Hardesty4 and Los Alamos.45 Specific heat was based on a single Einstein oscillator, and I?/ Vwas held constant. Thermal conduction through the sapphire windows can be ignored for times of interest.15 As will be discussed later, temperature and compression have important implications in describing shockinduced changes; only qualitative discussions regarding these parameters will be presented in this paper. C. Experimental Results. A total of 11 experiments were completed. The relevant parameters are summarized in Table 1. The impactor velocity and the sample thickness listed in the table are both measured quantities. The peak pressure, temperature, and volume compression (V/ Vo) a t peak pressure are calculated values as discussed in the previous subsection. In three experiments, spectral changes were obtained upon unloading. The experiments are also grouped according to spectral coverage. Because of the prevailing interest on the C N bond, five experiments examined this mode. Three experiments were completed on the CH3stretch. Only two experiments were performed on the NO;?/ CH3 bend because of the weak cross sections and the proximity of the two vibrational modes that cannot be resolved at shock pressures. An experiment in which all the modes are observed was also completed to show relative intensity changes in the vibrational modes. Due to different sample thicknesses and variations in incident laser energies for each of the experiments, time-resolved spectra are taken from the samples about 10 min prior to the impact experiment. These reference spectra serve two purposes: (a) the frequency shifts at a particular time can be obtained by comparing the experimental data and the reference data for the same track, and (b) theintensities for different experiments can be normalized. For the specific tracks where peak pressure is reached in the experiments, therecorrespond reference spectra that havedifferent

TABLE 1: Summary of Nitromethane Shock Experiments loading impactor sample expt no. type velocity (km/s) thickness (pm) l(91054) 2 (92004) 3 (92007) 4(92078) S(93031) 6(93013) 7(93025) a(92058) 9(92052) 10 (93035) 11 (92016)

unl

unl SWL SWL

unl SWL SWL SWL SWL SWL SWL

0.307 0.446 0.498 0.618 0.632 0.401 0.529 0.626 0.540 0.63 1 0.487

168 157 140 267 219 187 307 300 312 302 183

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Raman shift (cm-1) Figure 4. C N stretch vibration at (a) ambient pressure and at peak pressuresof(b)67,(c) 100,(d) 113,(e) 141,(f), 143,and(g) -1Okbar after unloading from 143 kbar. Frequency hardening and mode broadening are observed. There is no loss in scattering cross section at

shock pressures. Spectra from unloaded nitromethane confirm that no permanent changes have occurred in the C N stretch vibration. Data points are fitted with single Gaussians. No physical significanceis assigned to the noise in the shocked spectra. peak intensities, which can be normalized to one constant intensity by multiplying prefactors to references. If these prefactors are also multiplied to the corresponding data at peak pressures, the five experimental spectra would then be normalized to one reference intensity. Frequency hardening was observed at shock pressures in all of the experiments. Barring strong vibrational mode coupling, scission accompanied by displacive stretching of a specific bond would be manifested by mode softening. In the experiments performed up to peak pressures of 140kbar in the time window of the experiments, no mode softening was observed. Instead, mode hardening is observed a t peak pressures, and the frequencies remain constant at times after peak pressure is attained. Since the Raman modes do not exhibit time dependence a t constant pressures, Raman analysis is facilitated by averaging spectra taken a t the same pressure. Figures 4,5, and 6 show spectra averaged

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67 100 113 141 143 91 120 143 123 144 111

487 537 555 599 605 521 567 602 571 604 551

63 58 58 55 55 60 57 55 51 55 58

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vibrational modes examined CN

CN CN CN CN CH3 CH3 CH3 NOz NOz CN, CH3, NO2

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Figure 7. Time-resolved Raman spectra about the CN mode (Channel

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Raman shift (cml) Figure 5. NO2 stretch/CH3 bend vibrationsat (a) ambient pressure and (b) 123 and (c) 144 kbar. Hardening is observed. The two modes which are resolved at ambient pressures cannot be deconvoluted at shock pressures, and thus, data are fitted to single Gaussians.

175)with 50-11s resolution (shot number 92078). When the shock enters the liquid, a gradual shift to higher channel numbers (higher vibrational energies)corresponding to the shock-up process is seen. A slight increase in background and rms noise shows a seemingly disappearing vibration upon reaching peak pressure. Signal-to-noiseimprovementby averaging over six tracks yields no loss in scattering cross section as in Figure 4c. The asterisk is an instrument artifact.

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Figure 8. Comparison of Raman spectra at ambient pressure and at 109 kbar. The relativeenhancement of theNOzstretch/CH~bendvibrations compared with the CN stretch and CHI stretch vibrations is clear.

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Raman shift (cml) Figure 6. CH3 stretch vibration at (a) ambient pressure and (b) 91, (c) 120,and (d) 143kbar. Among theobservedvibrations,the CH3 exhibits the biggest frequency increaseand biggest width at shock pressures. Data are fitted with single Gaussians. from 150 to 300 ns at the indicated peak pressures for the CN, CH3, and N02/CH3 vibrations, respectively. The intensities in Figures 4-6 are normalized as discussed in the preceding paragraph. A reference spectrum a t ambient pressures is shown in Figures 4a, 5a, and 6a. I. The CN Stretch. Figure 4 shows the C N spectra at (a) ambient pressure and (b) 67, (c) 100, (d) 113, and (3) 141 kbar peak pressures. Parts f and g show spectra from an unloading experiment at a peak pressure of 143 and 10 kbar, respectively, corresponding to the state in Figure 3b, t 1 ps. There was an increase in root-mean-square (rms) noise with shock experiments as compared to the reference spectrum at ambient pressure. The curves were fitted to a single peak Gaussian which gave precise peak positions. A reduction in intensity with pressure was observed; however, a corresponding increase in width also occurred so no apparent change in scattering efficiency was noted. A slight increase in background, not seen in Figure 4,was also observed. This background increase and the rms noise in the spectra may interfere with the signal to indicate an apparent

--

signal loss. Figure 7 shows C N bond spectra taken a t 50-11stime intervals for the 141-kbar SWL experiment. The frequency shift with increasing pressure during reverberation are evident. Upon reaching peak pressure, the slight background increase and the increase in rms noise interfere with the signal. Signal averaging over six tracks at peak pressure, however, indicates that the scattering efficiency is maintained as seen in Figure 4e. Moreover, upon unloading from a peak pressure of 143 kbar (Figure 4 0 , the C N frequency, intensity, and width exhibited reversal. The rms noise persists in Figure 4g; nonetheless, loss in scattering efficiency was not observed. 2. The NO, Stretch-CHj Bend. The NO2 stretch/CH3 bend spectra are shown in Figure 5 at (a) ambient, (b) 123 kbar, and (c) 144 kbar pressures. Although the two modes are resolved at ambient conditions, they cannot be resolved at shock pressures. Hence, spectra were fitted to single Gaussians, and this introduced a larger uncertainty corresponding to the width of the Gaussians in the individual frequencies. There is no significant background increase in the NO1 stretch frequency, but an intensity increase is seen at shock pressures. This relative intensity enhancement is observed even at lower peak pressures. Figure 8 compares wide coverage spectra taken from ambient pressure and at 109 kbar. The relative intensity of the NO2 stretch/CH3 bend is seen to increase considerably in contrast to the intensities of the C N and CH3 stretch vibrations. Frequency hardening is also observed in these spectra. 3. The CH3 Stretch. Figure 6 shows the CH3 stretch mode at (a) ambient pressure and (b) 91, (c) 120, and (d) 143 kbar. Among the vibrational modes studied here, the CH3 stretch exhibited the largest frequency shifts and the highest broadening. No apparent time-dependent increase in background level was observed in the CH3 stretch region.

Time-Resolved Raman Measurements in Nitromethane

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IV. Discussion A. Frequency Shifts. A summary of the pressure-induced frequency shifts for C N stretch, CH3 stretch, and NO2 stretch/ CH3 bend from the present work is plotted in Figure 9. Curve fitting to single Gaussians gives very precise results; the error bars of f 3 cm-1 originate from fl-cm-1 uncertainties in the five calibration curves discussed in section 1II.A. Error bars are significantly larger for the NO2 stretch/CH~bend vibrations because the peaks cannot be deconvoluted at shock pressures, due to broadening in both peaks or anharmonic coupling between the two vibrations. Because of the uncertainty in calculating volumes, discussed in section III.B, mode Gruneisen parameters were not calculated. The frequency shifts seen in Figure 9 arequitelarge and suggest strong intermolecular interactions at shock pressures. The frequency shifts in the internal modes plotted here are more characteristic of shifts in lattice modes in molecular solids. For example, the N2 vibration at 2330 cm-l pressurized in the diamond anvil cell shows hardening of only about 20 cm-l at 100 kbar,46 and liquid N2 single shocked to 100 kbar shifts by 17 ~ m - l , 4 ~ while the CS2 breathing mode shifts by 6 cm-l when shocked to 100 kbar.18 In fact, the pressure-induced shifts in these internal modes of nitromethane are comparable to the shifts in lattice modes observed in benzenee4*These observations indicate that pressure, which selectively enhances effects of intermolecular interactions,20 strongly modifies the internal bonds in nitromethane. Strong intermolecular interactions may also explain why the CH3 stretch, which has the highest initial frequency, hardens most. In general, stiffer springs are expected to harden less with pressure,ZO and this is explained by the argument that strong bonds shorten only slightly thus stiffening is relatively slight. We thus expect that if only intramolecular forces are present, the CH3 stretching mode will harden least. The frequency shift observed with the CH3 stretch can be understood by invoking intermolecular forces involving the H atoms. In this simple spring model, the CH3 bond can be considered to have intramolecular and intermolecular contributions, SO that o (kintra + kinter)l/', where kintra and kintcrare spring constants and w is the vibrational frequency. At higher pressures, an increased contribution from kintcrwould yield a relatively large increase in w even if kintra increases only slightly. An interpolation through the data points marked in Figure 9 and the origin shows that, at high shock pressures, the CN and CH3 stretch frequencies change nonlinearly with pressure. This is expected of materials subjected to very high pressures.20 Although no mode softening was observed in this work, the curvature of the Raman modes may, at pressures higher than 140 kbar, exhibit a zero or negative frequency vs pressure slope,

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suggestive of a precursor for example to a phase transition as was observed in pressurized hydrogen.@ Finally, Figure 9 also shows the C N shifts measured by others. It appears that our data show higher shifts in contrast to earlier shock work. However, two major differences in our experiments and in the earlier work preclude any definitivecomparisons. First, the present work involved reverberation or SWL experiments while the earlier data were obtained from single-shock experiments. These loading conditions result in different temperature and volume compression values. It is not possible to account for thesevariations without a very reliable equation of state; such an equation of state is currently in de~elopment.~'Second, our results were obtained from time-resolved measurements of at most 307pm-thick samples within 400 ns after reaching peak pressures. In contrast, Renlund's measuremenW were made on millimeter thick samples and up to 4 ps after the shock entered the liquid. We emphasize that, in monitoring molecular changes that may have a time dependence, a microsecond may be long enough for these changes to occur. DAC datal2 on the CN stretching mode fits well with our data despite the difference in loading conditions. The NO2 stretch, CH3 stretch, and CH3 bend vibrational shifts 0btained3~at and below 17 kbar are not shown because these occupy a very small region of the pressure and frequency scales. Our data agree with these results in that the CHS stretch shifts most. We do not observea further splittingof theNO2 stretch/CH, bendvibrations at higher pressures as suggested by low-pressure DAC data. B. Broadening. Broadening, apparent in Figures 4-6, especially with the CH3 stretch mode is also an indication of strong intermolecular interactions at high pressures. The increase in width may arise from a distribution of bond strengths brought about by different local environments of the molecules in the scattering volume. This inhomogeneous broadening will have a strong effect if intermolecular interactions are strong. Moreover, the lifetime of vibrations can also decrease, thus causing mode broadening20 if more efficient coupling to new intermolecular decay channels is made possible at shock pressures. If the width is purely due to vibrational relaxation, to a first approximation (@At l / 2 , AE 80 cm-l), the lifetime of the CH3 vibration at 140 kbar has a lower limit of about 0.2 ps, well in the region of typical vibrational relaxation.50 Thermal effects may also contribute to broadening but cannot completely account for the observed widths. New peaks at frequencies close to the vibration modes will result in an apparent width increase if not resolved in the system collection. Vibration transitions from hot bands were reported in high-resolution (fwhm 0.7 cm-I) studies of the C N vibration in nitromethane isot0pomers3~and in studies of N2 single shocked to 340 kbar where temperatures are reported to reach 4400 K.47 At 300 K, the populations of molecules in the first excited vibrational states relative to the ground state, assuming Boltzmann distribution, are 0.012, 0.0012, and 6.4 X le7for the CN, NOz, and CH3 stretch vibrations, respectively. At 600 K, which is thecalculated temperature for the shocked nitromethane at 140 kbar, these values increase slightly to 0.1 1,0.034, and 0.0007. These values indicate that hot band contributions are small and would affect the C N vibration most due to its lowest frequency among the three stretch vibrations. Identifying hot band contributions in Figure 4 is not a profitable exercise at this point due to numerous approximations needed to specify anharmonic coupling constants. However, the fact that the width increased most for the CH3 stretch vibration and not the C N stretch vibration implies that hot band contributions are not sufficient to explain the observed broadening. C. Intensity Changes. An intensity enhancement with the NO2 stretch/CH3 bend vibrations can be seen in Figure 6 and also in the low dispersion spectrum in Figure 8 in which the intensities of the different modes can be compared. Selective

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4528 The Journal of Physical Chemistry, Vol. 98, No. 17, 19’94

Pangilinan and Gupta

to be discerned by our optical measurements. The broad widths enhancement of Raman modes has previously been explained by at shock pressure are consistent with an inhomogeneous resonance through electronic states of a specific ~ y m m e t r y . ~ ~ . ~measured ~ distribution of molecular states. It is very possible that some Even if the n l r * transition at 270 nma may red shift with pressure, molecules may have attained conditions for reactions to initiate. selective enhancement of the NOz/CH3 modes cannot be reconciled with the above model since all the stretch modes that The Raman measurements therefore probe molecules that are being compared have the same A1 symmetry and should thus approach states necessary for chemical reactions to start. This be enhanced equally. It remains to be seen whether pressure can is also reaffmed by the nonlinear frequency increase with pressure induce a more efficient light coupling to the nitro group of exhibited by the C N and CHSfrequencies. The curvature of the nitromethane and how this process can occur. vibrational frequency shifts with pressure shown in Figure 9 is consistent with approaching chemical changes. It is tempting to relate the enhancement at 1400 cm-1 to a band observed in the same frequency range measured in infrared We can only speculate on the molecular states attained in these absorbance obtained from reaction products in DAC work a t 20 experiments. The steep hardening of the stiffest vibration (CH3 kbar and 405 K.34 This, however, is most likely coincidental stretch) is inconsistent with compression solely of intramolecular since the reaction products in the DAC studies are end products springs.” In this spring model of bonds, this pressure-induced of the reaction, whereas, this investigation probes states prior to hardening can be understood if other bonds, e.g., intermolecular the onset of chemical reactions. forces involving the hydrogens in the liquid, are present. This D. SpectralBackground. As alluded to in section 11, the criteria intermolecularpotential possibly varies from molecule to molecule of assigning reaction onset from peak disappearances have to be and is partly responsible for the observed width of the CH3 done more carefully. Although we do not at this stage put meaning stretching vibration at shock pressures. Since the NO2 stretch/ to the absolute value of the background, this background and the CH3 bend vibrations does not broaden as much, the CH3 bending rms noise may increase to the same level as the C N peak and mode does not exhibit this inhomogeneous distribution as the artificially exhibit a disappearing C N vibration. Figure 7 shows CH3 stretching vibration. It is thus likely that at most one the C N vibration during shock-up to 141 kbar. Figure 4e shows hydrogen per molecule is involved with the intermolecular that, with improvement of the signal-to-noise ratio by time interaction so that the bending vibrations are not as affected as averaging, the C N vibration scattering efficiency actually remains the stretching vibration. the same even if it seems to disappear in Figure 7. This model is not inconsistent with bimolecular theories of It is clear that unloading experiments are necessary to test shock-induced reactions,3.5J4 nor C N scission preceded by irreversibility of changes before conclusions regarding onset of hydrogen bonding.13 We cannot at this point identify the exact reaction on the basis of modedisappearance can be drawn. Figure structure of this complex or complexes. It remains to be seen if 4g shows the C N vibration after it was unloaded from 143 kbar the observed enhanced Raman scattering efficiency of the NO2 (Figure 40. The peak position has not returned completely since stretch/CH3 bend vibrations can be explained from such a the pressure has not been completely unloaded in the time window complex. of the experiment. The width is broader than in spectrum a due to the pressure gradients in the sample at unloading and possible V. Concluding Remarks thermal effects as discussed earlier. One can conclude from the intensity that the C N vibration has not been irreversiblychanged The Raman modes of nitromethane shocked to peak pressures with 143 kbar loading. of up to 140 kbar by step wave loading have been monitored. Our E. Prelude to Shock-induced Reactions. The Raman spectra data indicate that no irreversible chemistry is observed in the (vibrational frequencies, widths, intensities, and background) experiments. These observations instead provide a picture of presented so far provide a miroscopic picture of nitromethane as nitromethane moelcules that may possibly undergo shock-induced it is shocked. We will now discuss the molecular states attained chemical reactions. The CN, CH3, and N02/CH3 modes are in these experiments and describe the roles they may play in seen to be sensitive to shock loading and thus make Raman shock-induced chemical reactions. It is important to remember scattering an excellent probe for monitoring microscopic changes at this point that pressure, volume, temperature, and time strongly induced by shock. The large frequency shifts of all the modes, affect chemical reactions and that these parameters are different and the broadening, are suggestive of strong intermolecular for different loading conditions in single shock, SWL, or DAC coupling under shock loading. The shifts of the CH3 and C N work. It should thus not be surprising that P and T reached in vibrations are nonlinear with pressure with the interesting result this work exceed P and T values necessary for thermal decomthat the CH3 stretch hardens most with pressure even if it is the position of nitromethane (405 K, 20 kbar)32.34or that 140 kbar stiffest vibration. An increase in concentration of a complex peak pressure also exceeds the Chapman-Jouget pressure (1 20 involving at most one hydrogen per molecule is speculated. The kbar) for commercial n i t r ~ m e t h a n e . ~The ~ molecular states C N stretch vibration exhibits reversibility after unloading from attained in this study are uniquely specific by SWL conditions 140 kbar, indicating that there is no permanent change on the and the experimental parameters. C N vibration up to the pressures examined. Finally, the NO2 stretch vibration convoluted with the CH3 bending mode shows The Raman measurements in this work show no evidence of a relative intensity enhancement at shock pressures. Reasons for chemical changes up to 140 kbar peak pressures. Within the this enhancement are currently being explored. 20% track to track variation of our instruments in measuring intensities, and barring resonance effects, no intensity loss is Understanding the shock-induced chemistry in nitromethane observed. No new modes (e.g., characteristic of aci-ionsl”l2 or requires advances in both experiment and theory. A coherent methyl nitrite28) were observed within the sensitivity of our picture of the microscopic changes in shocked nitromethane is instruments. Furthermore, no bonds were broken, nor was there far from complete. However, the present experiments,aside from evidence of mode softening (e.g., no C N scission as in photolysis demonstrating the sensitivity and importance of the Raman probe, and gaseous reactions,25.26 no N O scission as in multiphoton provide necessary data to pursue hypotheses on the microscopic di~sociation,3~ and no CH scission). There is no indication of events that occur at shock pressures. It is recognized that permanent chemical changes. temperature, volume compression, time of measurement, and degree of sensitivity, in addition to peak pressure, are important Reaction dynamics, however, do not preclude the possibility that certain molecules or group of molecules are undergoing factors in defining the state of shocked material. Toward this end, we are developing an equation of state for nitromethane.41 chemical changes during the time window of the experiments, Moreover, results from similar experiments on pure nitromethane without the reaction going to completion and at a rate too slow

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Time-Resolved Raman Measurements in Nitromethane shocked to higher peak pressures, amine-sensitizednitromethane, . . or nitromethane isotopes need to be obtained and compared with results from this work. We are currently pursuing measurements at higher shock pressures to determine a reaction threshold and what happens to the molecular bonds of nitromethane at this reaction onset. We are also studying amine-sensitized nitromethane to observe the effects of amine addition on thedifferent bonds at shock pressures in which chemical reaction has already started. Finally, isotopic studies are needed to isolate possible effects of anharmonic coupling between the vibrations in these studies. These results, together with advances in theory, will enable us to form a more coherent microscopic picture of shocked nitromethane and develop a better understanding of chemical reaction mechanisms under shock conditions.

Acknowledgment. Discussions with C. P. Constantinou and J. M. Winey are gratefully acknowledged. D. Savage and K. Zimmerman are thanked for their assistance in the experimental effort. Discussions with R. Engelke regarding experimental work on, and the handling of, nitromethane were most helpful. Professor G. E. Duvall is sincerely acknowledged for developing nitromethane's equation of state used in this work and for many stimulating discussions. This work was supported by ONR Grant NOOO14-90-5-1400, and the enthusiastic interest of Dr. R. S. Miller is acknowledged. References and Notes (1) See for example: Proceedings Ninth Symposium (International) Detonation. 1989. . .-..-....., . .. ..

(2) Davis, W. C. Sci. Am. 1987, 256, 106. (3) Odiot, S.; Blain, M.; Vauthier, E.; Fliszar, S. J. Mol. Struct. (THEOCHEM) 1993, 279. 233. . (4) Bardo, R. D. Shock Waues in Condensed Matter (1985);Gupta, Y . M., Ed.; Plenum: New York, 1986; p 843. ( 5 ) Bardo, R. D. Int. J. Quantum Chem.: Quantum Chem. Symp. 1986, 20, 455. (6) Bardo, R. D. Proc., Symp. Detonation, 7th 1981, 74. (7) Zerilli, F.J.; Toton, E. T. Phys. Rev. B 1984, 29, 5891. (8) Hardesty, D. R. Combust. Flame 1976, 27, 229. (9) Peyrard, M.; Odiot, S.; Lavenir, E.; Schnur, J. M. J . Appl. Phys. 1985, 57, 2626. (10) Engelke, R.; Earl, W. L.; Rohlfing, C. M. Int. J . Chem. Kinet. 1986, 18, 1205. (1 1) Engelke, R.; Earl, W. L.; Rohlfing, C. M. J. Chem. Phys. 1986,84, 142. (12) Engelke, R.; Schiferl, D.; Storm, C. B.; Earl, W. L. J. Phys. Chem. 1988,92,6815. (1 3) Cook, M. D.; Haskins, P. J.; Proc. 9th Symp. (Inr.)Detonation 1989, 1027. (14) Yoo, C. S.;Gupta, Y. M. J. Phys. Chem. 1990, 94, 2857. (15) Gustavsen, R.; Gupta, Y. M. J . Chem. Phys. 1991, 95, 451. (16) Gustavsen, R.; Gupta, Y. M. J . Appl. Phys. 1991, 69, 918. (17) Gupta, Y. M.; Shen, X . A. Appl. Phys. Lett. 1991, 58, 583. (18) Yoo, C. S.; Gupta, Y. M.; Horn, P. D. Chem. Phys. Lett. 1989,159, 178.

The Journal of Physical Chemistry, Vol. 98, No. 17, I994 4529 ~. (19) Gustavsen, R.; Gupta, Y. M. Submitted for publication in J. Appl. Phys. (20) Weinstein, B. A., Zallen, R. In Topics in Applied Physics; Cardona, M., Guntherodt, G., Eds.; Springer-Verlag: New York, 1984; Vol. 54, p 463. (21) Schmidt, S. C.; Moore, D. S.;Shaner, J. W. In Shock Waues in Condensed Matter (1983); Asay, J., Graham, R. A., Straub, G. K., Eds.; Elsevier Science Publishing: New York, 1984; p 293. (22) Tripathi, G. N. R. Time Resolved Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; John Wiley and Sons: New York, 1989. (23) Constantinou, C. P. The Nitromethane-Amine Interaction. Ph.D. Dissertation, Cambridge University, 1992. (24) Nagakura, S . Mol. Phys. 1960, 3, 152. (25) Haugen, G. R.;Steinmetz, L. L. Mol. Photochem. 1979, 9, 473. (26) Honda, K.; Mikuni, H.; Takahashi, M. Bull. Chem. Soc. Jpn. 1972, 45, 3534. (27) Wodtke, A. M.; Hintsa, E. J.; Lee,Y. T. J. Chem. Phys. 1986,84, 1044. (28) Dewar, M. J. S.;Ritchie, J. P. J. Org. Chem. 1985, 50, 1031. (29) Malewski, G.; Pfeiffer, M.; Reich, P. J. Mol. Srrucr. 1969, 3, 419. (30) Hill, J. R.; Moore, D. S.;Schmidt, S. C.; Storm, C. B. J. Phys. Chem. 1991, 95, 3039. (31) Verderame, F. D.; Lannon, J. A.; Harris, L. E.; Thomas, W. G.; Lucia, E. A. J. Chem. Phys. 1972, 56,2638. (32) Miller, P. J.; Block, S.;Piermarini, G. J. J . Phys. Chem. 1989, 93, 462. (33) Cromer, D. T.; Ryan, R. R.; Schiferl, D. J. Phys. Chem. 1985.89, 2315. (34) Piermarini, G. J.; Block, S.;Miller, P. J. J. Phys. Chem. 1989, 93, 457. (35) Brasch, J. W. J. Phys. Chem. 1980, 84, 2084. (36) Delpuech, A.; Menil, A. Shock Waves in Condensed Matter (1983); Asay, J. R.,Graham, R. A., Straub, G. K.. Eds.; Elsevier Science Publishing: New York, 1984; p 309. (37) Schmidt, S. C.; Moore, D. S.;Shaner, J. W.; Shampine, D. L.; Holt, W. T. Physica 1986, 13961408, 587. (38) Renlund, A. M.; Trott, W. M. In Shock Compression of Condensed Matter-1989; Schmidt, S . C., Johnson, J. N., Davison, J. W., Eds.; Elsevier Science Publishing; New York, 1990; p 875. (39) Fowles., G. R.; Duvall, G. E.; Asay, J.; Bellamy, P.;Feistmann, D.; Gray, T.; Michaels, T.; Mitchell, R. Rev. Sci. Insrrum. 1970, 41, 984. (40) Barker, L. M.; Hollenbach, R. E. J . Appl. Phys. 1970,4I, 4208. (41) Duval1,G. E. WashingtonStateUniversity,unpublishedresults,1993. (42) Graham, R. A.; Brooks, W. P. J. Phys. Chem. Solids 1971,32,2311. (43) Woolfolk, R. W.; Cowperthwaite, M.; Shaw, R. Thermochim. Acta. 1973, 5, 409. (44) Lysne, P. J.; Hardesty, D. R. J . Chem. Phys. 1973, 59, 6512. (45) Marsh, S. P., Ed. M L Shock Hugoniot Data; University of California Press: Berkeley, 1980; p 599. (46) Lesar, R.; Ekberg, S.A,; Jones, L. H.; Mills, R. L.; Schwalbe, L. A.; Schiferl, D. H. Solid Stare Commun. 1979, 32, 131. (47) Schmidt, S. C.; Moore, D. S.;Shaw, M. S.Phys. Rev. B 1987, 35, 493. (48) Ellenson, W .D.; Nicol, M. J. Chem. Phys. 1974, 61, 1380. (49) Sharma, S. K.; Mao. H. K.; Bell, P. M. Phys. Reu. Lett. 1980, 44, 886. (50) Fleming, G. R.; Wolynes, P. G. Phys. Today 1990,43, 24. (51) Compaan, A.; Cummins, H. Z. Phys. Rev. Lett. 1973,31,41. (52) Yu, P. Y.; Shen, Y. R. Phys. Rev. Lett. 1974, 32, 373. (53) Bdzil, J. B.; Engelke, R.; Christenson, D. A. J. Chem. Phys. 1981, 74, 5654.