J. Phys. Chem. 1980, 84, 3573-3577 3573

Pressure-Temperature Dependence of the 0-6 Polymorph Interconverslon in Octahydro- 1,3,5,7-tetranitro- 1,3,5,7-tetrazocine

A. G. Landers and 1. B. Brill"

Depiirtment of Chemistry, University of Delaware, Newark, Delaware 1971 1 (Received: August 25, 1980)

The sensitivity of the (3 e 6 polymorph transition of octahydro-1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) to the combined effects of pressure and temperature has been determined in the solid phase by using Raman spectroscopy. The N2 gas pressure range was 0.1-138 MPa (0-2 X lo4 psig). In this pressure range the temperature for the j3 - 6 conversion increases from 176 to 210 "C. The line representing the conversion appears to have a slope break at 69 MPa (1 X lo4 psig). AH and A S for the conversion in the 0.1-69-MPa range are 10.1 kJ/mol and 2.34 J/K mol, respectively. In the 69-138 MPa range, AH is 17.7 kJ/mol and A S is 4.11 J/K mol. This transition is slightly dependent on the particle size for >63-pm HMX, but is significantly more sensitive to pressure and temperature in 3-pm HMX. The j3 - 6 and a - 6 phase transition temperatures at atmospheric pressure were redetermined by using slow heating and a new sample cell. By N2 gas heating, the (3 - 6 transition occurred at 165 "C and the a - 6 transition appeared at 193 "C.

Introduction The thermophysical properties of octahydro-1,3,5,7-

tetranitro-1,3,5,7-tetrazocine (HMX) at elevated temper- atures and pressures are of interest because of use of HMX as a monopropelland, material. The molecular structure of 0-HMX is shown in Figure 1. Our concern has been with the nature of the solid phase of HMX under extreme conditions. HMX if , known to exist in four polymorphs labeled p, a, 7, and 8, whose stability at room temperature decreases along this series.lY2 These forms have been compared graphi~alhy.~ The thermally induced inter- conversions among the polymorphs have been investigated a t atmospheric pressure both with1 and without2v4 the presence of solvent. In addition to these thermal trans- formations, we have found that y transforms to 0, and 6 transforms to p, at room temperature by application of p re s~ure .~ For instance, the thermal conversion of 0- - 6-HMX at equilibrium conditions and atmospheric pres- sure occurs at 158 "C.l At room temperature and by using a diamond anvil cell, the pressure for the reverse conver- sion of 6- - P-HMX could be identified only as "less than 0.5 kbar" (7000 psi).5 The results of the temperature- and pressure-induced transformations are consistent with one another. The less dense polymorphs become less stable a t high pressure because of their large molecular volumes, and more stable at high temperature because of their large entropy.

Pressures in the range of 21-48 MPa (3000-7000 psi) and high temperature exist in a rocket motor. No research has been reported on the combined effect of pressure and temperature on the phase transformations of HMX. This two-variable problem is of central interest because the relationship betweem pressure and temperature in the solid-phase transformations is not known. The conversion of 0- e 6-HMX is interesting because 0-HMX is the most stable form at room temperature while 6-HMX is the least stable. 6-HMX coulcl be present on the surface of crys- tallites as the propellant heats beyond the transition tem- perature. In light of the fact that the decomposition product distribution of the two forms appears to be dif- ferent,6 the p P 6 iriterconversion may play a role in combustion phenomena. However, because pressure and temperature are opposed forces in the p F? 6 intercon- version, it is not possible to say which polymorph is present a t a particular elevated temperature and pressure from the results available to dlate.

In the present work the p- P 6-HMX phase diagram is determined as a function of temperature in the pressure range of atmosphleric to 138 MPa (2 X lo4 psi). The effects of heating time and HMX particle size in this experiment were also determined. Thermodynamic parameters for the phase transition can be calculated from the results. Al- though a- and y-HMX might be involved in transforma- tions at the temperatures used, there was no evidence of their presence in these experiments. The transition ob- served was that between the most stable polymorph, 0- HMX, and the least stable form, 6-HMX.

The use of Raman spectroscopy to diagnose polymorph interconversions has been reinvestigated by using a newly designed sample cell. The results of slow heating are more reasonable than those found earlier for HMX4 and are now in qualitative agreement with those of Cady.2

Experimental Section Polymorph Preparation. The crude HMX was purified

to remove residual RDX (hexahydro-1,3,5-trinitro-s-tri- azine) by grinding the HMX to a fine powder, heating it in a vacuum oven to 150 "C for 24 h, and extracting the residue with The final product was obtained by evaporating the acetone on a Buchi Rotovaporator or slowly evaporating the solvent depending on the size of the crystals needed. The HMX was then washed with acetone, filtered, and dried in a vacuum desiccator over PZOb Four different crystal sizes were used in the study: HMX-C (coarse, 1.3 mm-300 pm), HMX-M (medium, 300-200 pm), and HMX-F (fine, <63 pm). These sizes were obtained by sieving the material through Fisher U S . standard sieves. HMX-E (extrafine, 3 pm) was used as received. Variations in treatment could have a small effect on the results due to slight differences in crystal strain and purity. In order to eliminate this factor, each complete phase diagram was determined on the same sample lot to ensure that the results were internally consistent. Several sample lots of different origin were tested, and the results were consistent to within 5%.

Instrumentatioii. All spectra were recorded with a Spex 1401 double-monochromator Raman spectrometer. A Spectra-Physics 146 4-W argon-ion laser was used for ex- citation. The laser was tuned to 488.0 nm with a power output of 0.5 W. The monochromator slit widths were set at 150 pm. The spectrometer utilized photon counting and was interfaced with a Nicolet 1180 data-acquisition system.

0022-3654/80/2084-3573$01.00/0 0 1980 American Chernical Society

Landers and Brill 3574 The Journal of Physical Chemistry, Vol. 84, No. 26, 1980

Figure 1. The molecular structure bf 0-HMX.

The spectra of the HMX polymorphs were recorded in the ring-stretching and -bending region (300-500 cm-l) since this region is diagnostic for each p~lymorph.~ Fluorescent decomposition products and surface deterio- ration lead to poor quality spectra at elevated temperature. As a result each spectrum was obtained by signal-averaging three scans. The total time required to record a spectrum was 15 min.

Pressure-Temperature Experiment. N2 gas was pres- surized from a 14-MPa (2000-psig) inlet pressure to a maximum of 138 MPa (2 X lo4 psig) by using a Haskel Engineering AG-152G single-acting single-stage gas booster (psi is a commonly employed pressure unit in propulsion technology and is used throughout the paper along the MPa). The pressure was monitored by an in-line Ashcroft Duragauge. The gauge was calibrated by using a Heise- Bourdon gauge placed in series. The sample cavity was a 10.5-cm long stainless-steel tube with an internal diam- eter of 1.6 mm such that a 1.5 X 100 mm glass capillary tube containing HMX would fit snugly and experience efficient heat transfer. The heating bath was an insulated glass battery jar filled with silicon oil and regulated by a Hallikainin Thermatrol temperature regulator to an ac- curacy of f0.5 "C. The temperature inside the glass ca- pillary was determined by placing a thermocouple in the capillary tube inside the stainless-steel cavity and im- mersing it in the controlled-temperature bath. The tem- perature in the capillary equaled that of the bath within 1 mifi of immersion.

The experiment was conducted by filling the capillary tube with 20-30 mm of HMX, placing the capillary tube in the sample cell, and pressurizing it to the desired pressure. The sample was then quickly immersed for 5, 10, or 30 min in the controlled-temperature bath. After the desired heating time, the sample was removed from the bath and depressurized, and the Raman spectrum recorded immediateIy. The pressure was increased in 500-psi increments as the phase transition was approached and 1000-2000-psi increments away from the transition.

The polymorph that exists at a particular temperature and pressure was found to remain for a period of up to 12 h because of the large hysteresis in the p 6 transfor- mation. I t is important to note that the pressure-tem- perature history of the sample affects the phase-transition temperature. We observe, for example, that it is not possible to cycle a sample of HMX up and down in tem- perature and pressure and obtain a reproducible phase- transition line. If a sample of HMX is heated and pres- surized to a condition where 0- and 6-HMX coexist, is

5 5 0 0 P S f G 187 %

5 0 0 0 PSI0 187%

4500 PSlG 187%

3 5 0 0 PSlG 187 OC

3 0 0 0 PSlG 187OC

0 PSI0 1 8 7 %

0 PSlG 2 5 %

r ' I - 1 - 1 ' I v i

CM -I

500 400 300

Flgure 2. Raman spectra of the ring torsion region of HMX as a function of pressure along the 187 O C isotherm and employing 5-min heating time. A 6- -* 0-HMX phase transition is occurring as the pressure is increased.

returned to atmospheric conditions until P-HMX alone is present, and then is reheated and repressurized to the original conditions, then only 6-HMX is present. These experiments show that 6-HMX is more readily obtained in samples which have already undergone a transformation at least once. For this reason a fresh sample of HMX was used for each heating run.

Slow-Heating Experiment. The method of measuring the temperature of HMX phase transitions by slow heating is basically that described el~ewhere.~J However, the sample cell was redesigned in order to eliminate several recently discovered problems. It was found that the ear- lier-used cell was not gastight after long periods a t high temperature. To prevent leaks, metal vacuum seals were used in place of Teflon tape to connect the cell to the heating line. This prevented shrinkage or slippage at the connection during the experiment. Gas leaks in the cell, which appeared to create a temperature gradient between the sample and the thermocouple, were thereby eliminated. Instead of external temperature monitoring, the temper- ature was internally monitored by placing a glass-encased J-type thermocouple inside the polycrystalline HMX. This method gives a true sample temperature and does not rely on the external gas temperature as indicative of the sample temperature.

Results and Discussion Criterion for the Phase Transition. The choice of the

observable5 to define the solid-phase transformation needs to be considered. Figure 2 shows the type of Raman data

p-6 Polymorph Interconversion in HMX

120

100

a o m a 2

60

40

20

180 190 200 2 1 0

T, ' C

Flgure 3. Two criteria for the ~t 6 phase transition. The solid line represents the disappearance of 6-HMX to form fl-HMX, and the dashed line represents the appearance of fl-HMX from 6-HMX. The data points on the solicl line were chosen in this study (coefficient of correlation is 0.997).

obtained in this study. The Raman spectrum at 25 "C and atmospheric pressure shows bands characteristic of p- HMX.4 At 187 "C and atmospheric pressure P-HMX has totally converted to 6-HMX after 5 min. When the pressure is increased to 21 MPa (3000 psig), the spectrum reveals that /3-HMX and 6-HMX are both present. As the pressure increases further, the 392-cm-l line characteristic of 6-HMX decreases in intensity and finally disappears a t 38 MPa (5500 psig). The phase transition at a given tem- perature was chosen as the pressure at which the 6-HMX polymorph disappeared. In Figure 2, for instance, the phase transition at 1187 "C occurs a t 38 MPa (5500 psig). When this criterion is used at atmospheric pressure, the transition occurs a t 175 "C in agreement with previous s t ~ d i e s . ~ In each experiment the sample was carefully moved in the laser beam with the monochromator set at 392 cm-l to ensure that the entire sample was homogeneous before each spectrum was run. From these experiments a plot of pressure VE,. temperature for the p e 6 transition can be made and ia shown in Figure 3.

The points on the solid line in Figure 3 represent the pressure at a given temperature where 6-MHX no longer exists. This criterion is experimentally convenient and reliable. Alternately, the points on the dashed line, where we found that P-MIIX could no longer exist, might have been used. These later points proved more difficult to extract because of fluoresence and poor spectral quality a t the temperature D f the experiment. Still a third crite- rion for the phase transition could be a line halfway be- tween the dashed line and the solid line in Figure 3. This line would be a semblance of equal distribution of p- and 6-MHX in the sample. However, the error in establishing the p - 6 conversioin line makes such a criterion no more accurate than the /3 - 6 conversion criterion itself. Moreover, the raw intensities of the Raman lines for p- and 6-HMX cannot be used here as an indication of the relative abundance of the two polymorph forms because the in- tensity of Raman scattering in solids is partly a function of crystallinity in the material. 0- and 6-HMX exhibit somewhat different scattering efficiencies probably because of this factor. Therefore, the solid line representing the disappearance of 6-HMX from the sample is the most reliable criterion for the phase transition.

It is important to note that the range of pressures and temperatures that define the limit of complete p- - 6- HMX and 6- -+ P-HMX conversion is small. It can be seen that the boundary lines are parallel to one another and consequently the slopes for all of the above criteria for the transition are the same.

The Journal of Physical Chemistry, Vol. 84, No.

16 1 7 2

12

6" I

0 6 a

4

0

26, 1980 3575

120

100

8 0 m n 2

60

40

20

170 180 190 200 210 T,'C

Flgure 4. The p- 6-HMX phase transition at two different heating times for 200-300-pm HMX: (0) 5 min; (A) 30 min. The coefficient of correlation for 5-min heating is 0.996.

Pressure and' Temperature Dependence of 0- FF 6- HMX. Two intersecting straight lines are obtained in Figure 3 for the 6 - p transition in 163-pm HMX over the pressure-temperature range studied. Consequently, two sets of thermodynamic parameters will be obtained. The enthalpy foir the transition can be calculated from the Clausius-Clapeyron equation (eq l), where AV is the

dP/dT = AH/(TAV) (1)

volume change during the 6- - P-HMX transition and T is the equilibrium temperature in K. T = 431 K according to the data of Teetsov and McCr0ne.l From the known densities of the two phases: AV = 8.66 cm3/mol. If one uses these values and dP/dT from Figure 3, AH = 10.1 kJ/mol for the p- a 6-HMX transformation in the 175-200 "C, 0.1-69-MPa (&lo4 psig) region. This value compares with an enthallpy value of 9.8 kJ/mol for the phase transformation calculated from DSC measurements a t 187 "C and atmosphleric pressure by Hall.lo A value of 9.40 kJ/mol was reported by Selig and attributed to unpub- lished data of C:rimmins.'l We regard the agreement between our enthalpy value and that of Hall as excellent given the differlence in the two experiments. We now know, however, that this value of 10.1 kJ/mol is valid for the /3 FF 6 transformation up to 69 MPa (1 X lo4 psi), a fact which could1 not be established from the DSC data.

The value of AH for the p P 6 transition in the 69- 138-MPa (1 X 1014-2 X lo4 psg) range can be calculated by using the slope of the line in Figure 3. AH = 17.7 kJ/mol in this range. It is typical of most compounds that AH for a solid-solid transformation increases with increasing temperature and pressure.12 No impurities, which might contribute to the1 slope break, could be detected by using mass spectrometry. It is possible that a mechanical property of the crystal is involved in the slope break.

Effect of Heating Time on the 0- FF 6-HMX Transition. The phase transition was found to be slightly dependent on the heating time, indicating that the e 6 transition is not truely at equilibrium in this experiment. This is in accordance with the hysteresis experiment described above. The equilibrium phase-transition temperature is believed to be 158 "C at atmospheric pressure,' whereas in the rapid-heating experiments here and before4 we find the transition in the neighborhood of 175 "C. Figure 4 shows the difference in the /3 P 6 transition for 5-min heating vs. 30-min heating. The longer heating time produces a slightly lower trisnsition temperature. However, there exists a limit to the time that HMX can be heated at these temperatures without producing fluorescent decomposition products which interfere with the Raman spectrum.

3576

TABLE I: the p- "t 8-HMX Phase Transition

The Journal of Physical Chemistty, Vol. 84, No. 26, 1980

Summary of Thermodynamic Parameters for

AH," AS," kJ/ J/(K

temp, "C press., MPa mol mol)

175-200 0.1-69 (0-1 X l o4 psig) 10.1 2.34 200-212 69-138 (1 X 104-2 X lo4 psig) 17.7 4.11 a Calculated for T = 431 K.

Landers and Brlil

Heating for 30 min above 200 "C results in spectra whose qualities are too poor to accurately identify the pressure where 6-HMX disappears.

It is important to note that the phase-transition lines as a function of heating time in Figure 4 are parallel to one another. (Several data points for 10-min heating time were also recorded, and they conform to this pattern). It is reasonable to assume that a very long heating time might result in equilibrium conditions and produce a transition line which is parallel to those shown in Figure 4. Since it is the slope of the line that is needed in eq 1, it seems valid to employ these "off-equilibrium" lines in Figures 3 and 4 to compute AH (vide supra). In turn, AS can be calculated by using the known equilibrium temperature for the 0- * 6-HMX transformation of 431 K.I Table I summarizes all of the thermodynamic parameters for the p FSC 6 transition from these data.

The velocity of the transition depends on the activation energy for the molecular movement in the solid. A large activation energy can make it particularly difficult to de- termine the precise equilibrium temperature of a solid- solid transition because the velocity of the 6 -+ p transition is slower near the equilibrium line that it is far to the right of the line. To the left of the line the transition at room temperature becomes slow enough that 6-HMX is meta- stable, and its spectrum can be readily recorded.

6-HMX Transition. The velocity of the solid-solid transformation can depend on the size of the crystals. Certain transformations are faster in the powder form while others are faster in the form of single crystals. The position of the line for the 6- -+ P-HMX transition was found to depend slightly on the size of the particles. Figure 5 shows that data for coarse (300 pm-1.3 mm), medium (200-300 pm), fine (63 pm), and extra fine (3 pm) sized crystals using a heating time of 5 min. The data show the tendency of the 0 -+ 6 transfor- mation to occur more readily in large crystals than in small crystals. In other words, in the neighborhood of the transformation, 6-HMX is the favored form in large crystals while 0-HMX is the favored form in small crystals. It is intriguing to note that 3-pm HMX failed to show the slope break in the pressure range studied. Attrition is used to prepare 3-pm HMX, and this may relieve crystal strains which could exist in larger-sized crystals. If the slope break is related to crystal strains, then the break might not be expected in the 3-pm sample.

Slow-Heating Experiments. The P -+ 6 and a - 6 phase-transition temperatures of HMX at atmospheric pressure and N2 gas heating were redetermined by using a newly designed sample cell. The temperatures previously obtained employing Nz gas heating were found to be The newly designed sample cell possesses glass-metal vacuum seals which do not leak at elevated temperatures. We discovered that the cell used in the earlier studies, while accurate during calibration runs and other short- duration experiments, developed leaks after the many hours of heating required for HMX phase-transition studies. These leaks affected the internal temperature. Using the new leak-tight cell, we found the phase transi-

Effect of Particle Size on the p-

"J

'2 8 j L I a 4 1 .*I 1:- , 8 , 1 ;I

. . O D

170 180 190 200 210 T, 'C

Flgure 5. The 0- P 6-HMX phase transition for 5-min heating times and four particle sizes: (0) HMX-coarse; (0) HMX-medium; (A) HMX-fine; (U) HMX-extra fine.

tions in slow heating to occur at 165 "C rather than 142 "C for the p- - 6-HMX conversion, and 193 "C rather than 149 OC for the a- - 6-HMX conversion. These values are higher than those reported by Teetsov and McCrone,l who used a solution-solid interface to achieve equilibrium. These new temperatures values are in agreement with temperature ranges obtained by Cady2 when no solvent was present. While higher than the equilibrium temper- atures, they are indicative of the phase transitions during reasonably slow heating (several degrees per minute) a t a gas-solid interface. They demonstrate the difficulty of reaching true thermodynamic equilibrium for solid-solid transitions involving HMX.

Conclusions The results obtained in the pressure-temperature

phase-transition study have a number of implications, many of which require further study. These studies are planned. First of all, because rocket-motor pressures are usually in the 3000-7000-psirange and temperature are much greater than those employed in these studies, it is safe to conclude that the 6-HMX polymorph is the stable form of HMX in rocket-motor conditions. Only the burning surface area and not an entire crystallite of HMX needs to be converted to the 6 form to function as the active species in the decomposition of HMX. Second, the thermodynamic parameters for the 0- -+ 6-HMX phase change can be determined quantitatively in the pressure domains studied here. It might be tempting to extrapolate these data beyond 138 MPa (2 X lo4 psi). However, if one notes the slope break at 69 MPa (1 X lo4 psi), dP/dT for the transition may increase, decrease, or remain the same above 2 X lo4 psi. This poses the provocative question of what the nature of HMX is at much higher pressures. Third, it is conceivable that the slope break in dP/dT which appears to be a function of particle size may play a role in combustion phenomena. If molecular events occur in the solid which create different burning forms, densities, and volumes, then different combus€ion rates and product distributions, fracturing, and stabilities might be antici- pated.

Acknowledgment, This research was sponsored by the Air Force Office of Scientific Research, Air Force Systems Command, USAF, under grant number AFOSR-76-3055.

References and Notes (1) Teetsov, A. S.; McCrone, W. C. Microsc. Cryst. Front 1965, 15,

(2) Cady, H. H.; Smith, L. C. Los Alamos, NM, May 3, 1962, Los Ahmos

(3) Brill, T. B.; Reese, C. 0. J. Phys. Chem. 1980, 84, 1376. (4) Goetz, F.; Brill, T. B. J. Phys. Chem. 1979, 83, 340.

13.

Scientific Laboratory, LAMS-2652.

J. Phys. Chem. 1980, 84, 3577-3581 3577

(9) Cady, H. H.; larson, A. C.; Cromer, D. 1. Acta Crystallogr. 1983, (5) Ooetz, F.; Brill, T. 5.; Ferraro, J. R. J. Phys. Chem. 1978, 82, 1912. (6) Goshgarkin, B. B. Oct 1978, Air Force Rocket Propulsion Laboratory,

(7) Goshgarkin, B. B., Air Force Rocket Propulsion Laboratory, personal

(8) Brill, T. EL; Goetz:, F. Prog. Astronaut. Aeronaut. 1978, 63, 1.

16, 617. AFRPL-TR-7 8-7 6.

communicatlon, 1978.

(10) Hall, P. G. Trans. Faraday SOC. 1971, 67, 556. (11) Selig, W. Exploslvstoffe 1989, 4 , 73. (12) Verma, A. R.; Krishna, P. "Polymorphism and Polytyplsm in Crystals";

Wiley: New 'York, 1966.

Scavenging of Electrons Prior to Thermalization in Ethanol

Duien Raiem" and Igor Dvornlk

"Ruder BoSkovi6" Instifute, 4 1000 Zagreb, Yugoslavla (Received: Aprll30, 1980; In Flnal Form: September 3, 1980)

The concept of epithermal electron scavenging, as known in the gas phase and in the case of high-mobility electron scavenging in nonpolar liquids, has been applied to the case of a high concentration of specific scavenger in a polar liquid. The conditions for epithermal electron scavenging exist in the system at an early stage of radiation action, in the time domain of the high-frequency dielectric constant. Radiolysis of ethanolic solutions of chlorobenzene has been studied, where the C1- ion yield represents the measure of dissociative attachment of bath solvated electrons and their precursors. The contribution of the former species can be suppressed by addition of suitable secondary scavengers. The relevance of the gas-phase electron scavenging to early event8 in the radiolysis of liquids and the specificity of electron scavenging by chlorobenzene are discussed.

Introduction In the course of electron energy loss in irradiated matter,

electrons falling into the subexcitation energy rage con- tinue to lose energy by scattering and by temporary neg- ative ion formation. Negative ion states can decay via several competing channels; the dissociation into an ionic and a neutral fragment is known as dissociative electron attachment. Halogen-containing hydrocarbons readily undergo dissociative electron attachment in the gas phaseel The energy requirement of this reaction, in the case of some compounds, allows the reaction to proceed only with sufficiently energetic electrons. By this mechanism the electron is converted1 into a halide ion and a corresponding hydrocarbon, thus providing a suitable means for moni- toring the interactions of subexcitation electrons by measuring the amount of the ensuing chemical change.

This approach has already been used by Khorana and Hamill,2 who studied the radiolysis of ethanolic solutions of alkyl halides in the presence of benzene and acid. They demonstrated the existence of two precursors of alkane from alkyl halide, acid being specific for one and benzene for the other. The latter precursor was identified as a dry electron, e-.

Events in the liquid phase are influenced by the prox- imity of solvent molecules, higher density causing faster degradation of electron kinetic energy. Thermalized electrons also have a finite probability of reacting with scavengers. The final chemical change on irradiating a liquid is the result of species reacting in early, as well as in subsequent, later stages of radiation action. The con- tribution of species other than subexcitation electrons must be eliminated by proper experimental design if only sub- excitation electrons are to be followed.

Chlorobenzene (PhCl) reacts efficiently with epithermal electrons in the gas phase via dissociative electron at- tachment. The cross section for the process peaks at an electron energy of 0.86 eVa3 On the other hand, chloro- benzene is only moderately reactive with solvated electrons (ke8- = 4.6 X 10s M-' t3-l in ethan01);~ therefore it is easy to find suitable secondary scavengers to suppress the

contribution of solvated electrons to the C1- ion yield. The concept of using ethanolic solutions of chloro-

benzene to demonstrate nonthermal (epithermal) electron reactions in condensed systems has been conceived in our previous work,&7 Those results were obtained with sys- tems containing oxygen. The present study was carried out in degassed systems which permit a quantitative dis- crimination between solvated electrons and their precur- sors. It is also the aim of the present work to discuss the relevance of gas-phase electron scavenging to the early events in the radiolysis of liquids and the specificity of electron scavenging by chlorobenzene. Experimental Section

Reagent-grade ethanol, supplied by Merck and used as received, and triply distilled water were mixed to prepare 96 vol % alcohol. All solutions of reagent-grade compo- nents were prepared by diluting to the desired volume with 96 vol % ethanol. Degassed samples containing 5 mL of solution in Pyrex vessels were irradiated by 6oCo y rays with doses between 1 X 1018 and 5 X l O l 8 eV mL-l. The concentration of radiolytically formed chloride ions was determined by m.ercurimetric titration with ethanolic so- lutions of Hg2+, using diphenylcarbazone as indicator. Results

At several constant concentrations of PhCl the concen- tration of secondary scavengers, [SI, was varied so that the total combined concentration did not exceed 1 M. Ra- diation chemical yields of C1- ions per 100 eV absorbed, G(Cl-), in the presence of perchloric acid (HC103, nitro- methane (MeNO:l), nitric acid (HNOJ, and nitrobenzene (PhN02) are shown in Figure 1, a-d, a t PhCl concentra- tions of 0.049,0.098,0.197, and 0.393 M, respectively. The competition between PhCl and a secondary scavenger S for precursors of C1- ions takes place in two concentra- tion-resolved regions. The fast initial decay of G(C1-) on addition of low concentrations of S is due to scavenging of solvated electrons. This is illustrated in Figure 1, a-d, by dashed lines which show the example calculated for S = PhN02.

0 1960 American Chemical Society