30 Propellants and Explosives 4,30-34 (1979)

The Relationship of Impact Sensitivity with Structure of Organic High Explosives. 11. Polynitroaromatic Explosives

M. J. Kamlet and H. G. Adolph

Naval Surface Weapons Center, White Oak Laboratory, Silver Spring, Maryland 20910 (USA)

Die Beziehung zwischen Schlagempfindlichkeit und StruMur bei Organischen Sprengstoffen. 11. Polynitroaromatische Explosivstoffe

Die Schlagempfindlichkeit organischer Sprengstoffe hangt in er- ster Linie von ihrer Zersetzungsgeschwindigkeit in den unter dem Fallhammer erreichten Temperaturbereichen ab. Fur Verbindungs- klassen mit ahnlichem Zersetzungsmechanismus existiert offenbar eine statistisch gesicherte lineare Abhangigkeit (Empfindlichkeit/ Zusammensetzung) zwischen dem Logarithmus der ,,SO% Fallhohe“ und den OBI,,-Werten als Ma8 fur die Sauerstoffbilanz. AuRerdem wird gezeigt, daR polynitroaromatische Verbindungen mit CH-Grup- pen in a-Stellung zum aromatischen Ring schlagempfindlicher sind als Sprengstoffe ohne solche Gruppen. Diese Ergebnisse sind in Einklang mit der Tatsache, daR die bei der thermischen Zersetzung von TNT entstehenden Zersetzungsprodukte auf einen bevorzugten oxidativen inter- und intramolekularen Angriff auf die a-CH-Bin- dung hinweisen.

Relation entre sensibilite au choc et structure des explosifs organiques. 11. Explosifs arornatiques polynitres

La sensibilite au choc des explosifs organiques depend en premier lieu de leur vitesse de decomposition aux temperatures atteintes sous l’action du mouton de choc. Pour les familles de produits presentant le m&me genre de mecanisme de dtcomposition, il existe apparem- ment une relation lineaire (sensibilite/composition) statistiquement significative, entre le logarithme “hauteur de chute 50%” et les valeurs OB,,o qui indiquent le bilan en oxygkne. En outre on montre que les composes aromatiques polynitres, ayant des groupes CH en position a par rapport au noyau aromatique, sont plus sensibles au choc que les explosifs ne presentant pas de tels groupes. Ccs rtsultats concordent avec la constatation que les produits de la decomposition thermique de la tolite font apparaitre une action oxydante intermoleculaire et intramoliculaire preferentielle sur la liaison CH en position a.

Summary

Impact sensitivities of organic high explosives are primarily functions of the rates of the thermal decomposition processes taking place in the temperature regimes generated under the impact hammer. For classes of explosives with similar decomposition mechanisms, there appear to be statistically significant linear relationships (sensitivity/composition trends) between logarithmic 50% impact heights and values OBloo, a measure of oxidant balance. In addition, it is shown that polynitroaromatic explosives containing a C-H linkage alpha to the aromatic ring are more sensitive as a class than explosives lacking such a linkage. These results are con- sistent with the finding that products of thermal decomposition of TNT indicate a preferred site of inter- and intramolecular oxidative attack to be at the alpha C-H linkage.

1. Introduction

In Part I(’), we described the test used at the Naval Surface Weapons Center to measure impact sensitivities, and we reported sensitivitykhemical composition relationships for several classes of polynitroaliphatic explosives. In accordance with Wenograd’s suggestion that impact sensitivities of organic high explosives are governed by the thermal decomposition processes which take place at the widely varying temperatures generated under the impact hammer@), we demonstrated that, for families of high energy compounds with similar decomposition mechanisms, there appear to be rough linear relationships (sensitivity/composition trends) between logarithmic 50% impact heights and values of OBloo, a measure of oxidant balance. Among the polynitroaliphatic explosives, it was shown that at comparable oxidant balances compounds containing at least one N-nitro “trigger linkage” are more sensitive as a class than materials containing only C-nitro linkages.

Since the impact machine has neither the resolution nor the reproducibility required of a research tool, the work was done under the constraint that only limited reliability could be attributed to any individual impact result. For example, depending on operator technique, particle size distribution in the sample, atmospheric conditions (humidity, etc.), and multifold other causes, separate 25-shot determinations on twice recrystallized TNT have given 50% impact heights ranging from below 100 cm to above 250 cm.

For this reason, our initial assumption had been that any correlation of impact sensitivity with chemical structure or composition would necessarily depend on a large body of data determined for families of chemically related explosives. This did indeed prove to be the case. We found that individual out-of-line points tended to offset one another, that plots of the data showed statistically significant linear regression, and that the regression equations (sensitivitykomposition trends) were reasonably successful in predicting the impact behavior of yet-to-be-synthesized new explosives.

The OBIoo quantity chosen for comparison with log hSo% was a convenient compositional parameter which we defined as the number of equivalents of oxidant per hundred grams of explosive above the amount required to bum all hydrogen to water and all carbon to carbon monoxide. In calculating OBloo, an atom of oxygen represents two equivalents of oxidant, an atom of hydrogen one equivalent of reductant, and an atom of carbon two equivalents of reductant. Since carboxyl groups are considered to be “dead weight”, two equivalents of oxidant were substracted for each such group in the molecule. For C-H-N-0 explosives, the applicable equation is

100 (2 no - nH - 2 nc - 2 ncoo) Mol.Wt. OBI00 =

0340-7462/79/04024030$02.50/0 0 Verlag Chemie, GmbH, D-6940 Weinheim, 1979.

Propellants and Explosives 4,3&34 (1979)

where no, nH, and nc represent the number of atoms of the respective elements in the molecule, and ncOO is the number of carboxyl groups. For explosives balanced to the CO-level of oxygen balance, OBI,, = 0; at the C0,-level of oxygen balance, OB,,, = ca +2.5.

In the present paper we use exactly analogous procedures and reasoning to establish sensitivitykomposition relation- ships for nitroaromatic explosives. We are fortunate that the large body of impact information needed for these correlations had already been assembled at this laboratory during the period 195CL1960, and that a majority of the determinations had been carried out by the same competent operator.

2. Results

Impact sensitivities of thirty-eight polynitroaromatic and mixed nitroaromatic-nitroaliphatic explosives are listed in Table 1 together with molecular formulas, molecular weights, molar oxidant balances and values of OB,,,. These compounds have been divided into two classes, the basis for the division being the presence or absence of a hydrogen atom

Impact Sensitivity and Structure of Organic High Explosives 31

on a carbon alpha to the ring. The reason for this criterion of distinction will be discussed below.

Logarithmic 50% impact heights of these 38 compounds are plotted against values of OB,,, in Fig. 1. It is seen in the plot that, if all data points are considered equally, there is a general increase in impact height with decreasing OB,,,, but that the band within which all the data points fall is rather broad. Sensitivities vary from 13 cm to 24 cm at OB,,, = ca + 1, from 21 cm to 73 cm at OBI,, = ca 0, and from 52 cm to > 320 cm (the top of the scale) at OB,,, = ca -2. No quantitative relationship is evident.

If the distinction between classes is taken into account, however, some regularities begin to appear. Twelve of the thirteen compounds which have a hydrogen atom on an a-carbon show impact sensitivites toward the lower portion of the broad band; twenty-four of the twenty-five lacking this linkage lie in the upper portion. Indeed, if the categories are considered separately, most of the data points seem to show statistically significant clustering about two separate straight lines.

We have carried out least squares correlations of the log h,os results with corresponding values of OBloo, arbitrarily

Table 1. Impact Sensitivities and Oxidant Balances of Polynitroaromatic Explosives

No. Compound Molecular Formula Mol.Wt.

Compounds with no alpha C-Hlinkage

OBmoie OBioo h50% [cml

1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

2,3,4,5,6-pentanitroaniline 2,2,2-trinitroethyl-2,4,6-trinitrobenzoate 2,4,6- trinitroresorcinol 2,3,4,6- tetranitroaniline 2,2,2-trinitroethyl-3,5-dinitrosalicylate 2,2,2-trinitroethyl-3,5-dinitrobenzoate picric acid 2,4,6- trinitro-3-aminophenol 2,2’,4,4‘,6,6’- hexanitrobiphenyl 2,4,6-trinitrobenzoic acid 2,2-dinitropropyl-2,4,6-trinitrobenzoate 1,3,5- trini trobenzene 2,4,6- trinitrohenzonitrile picramide 1,6-dinitroresorcinoI 2,4-dinitroresorcinol 2,4,6-trinitroanisole 1,3-dimethoxy-2,4,6-trinitrobenzene 3-methoxy-2,4,6-trinitroaniline 1,3-diamino-2,4,6- trinitrobenzene 2,2‘,4,4’ ,6,6’ - hexanitrodiphenylamine 2,4,6- trinitrophloroglucinol 3- hydroxy-2,2‘,4,4’,6,6‘-hexanitrobiphenyl 3,3‘-dihydroxy-2,2’,4,4‘,6,6’- hexanitrobiphenyl 3,3‘-diamino-2,2’,4,4’,6,6’-hexanitrobiphenyl

Compounds with alpha C-H linkage 26. 1-(2,2,2-trinitroethy1)-2,4,6-trinitrobenzene 27. 1-(3,3,3-trinitropropy1)-2,4,6-trinitrohenzene 28. 1-(2,2,2-trinitroethyl)-2,4-dinitrobenzene 29. 2,4,6-trinitrobenzaldehyde 30. 2,2’,4,4‘,6,6’-hexanitrobibenzyl 31. TNT 32. 2,4,6-trinitrobenzyl alcohol 33. 2,4,6-trinitrobenzaldoxime 34. 2,4,6-trinitro-m-cresol 35. 3-methyl-2,2‘,4,4’,6,6’-hexanitrobiphenyl 36. 3,3’-dimethylL2,2’,4,4’,6,6‘-hexanitrobiphenyl 37. 3-methy1-2,2’,4,4’,6-pentanitrobiphenyl 38. 3,5-dimethyl-2,4,6-trinitrophenol

318 + 6 420 +4 245 + 1 273 f l 391 +1 375 -1 229 -1 244 -2 424 -4 25 7 -3 389 -5 213 -3 238 -4 228 -4 200 -4 200 -4 243 -5 273 -7 25 8 -6 24 3 -5 439 -5 26 1 + 3 440 -2 456 0 454 -6

376 +4 390 0 33 1 -1 24 1 -3 452 - 12 227 -7 243 -5 25 6 -4 24 3 -5 438 -8 45 2 - 12 393 - 13 257 -9

+ 1.88 + 0.95 +0.41 +0.37 + 0.26 - 0.28 -0.44 -0.81 -0.94 -1.12 - 1.28 - 1.46 - 1.68 - 1.75 - 2.00 -2.00 -2.06 -2.56 -2.32 -2.06 - 1.14 + 1.15 -0.45

0.00 - 1.32

+ 1.07 0.00

-0.30 - 1.24 -2.64 - 3.08 - 2.06 - 1.56 -2.06 - 1.81 -2.64 -3.30 -3.50

15 24 43 41 45 73 87

138 85

109 214 100 140 177

> 320 296 192 25 1

> 320 320 48 27 42 40

132

13 21 31 36

114 160 52 42

191 53

135 143 77

32 M. J. Kamlet and H. G. Adolph Propellants and Explosives 4, 30-34 (1979)

10 \ I I I I I L

-3 2 1 0 +1 +2

f24.11 0 ~ 1 0 0 - figure 1. Impact sensitivity of polynitroaromatics as a function of OBioo.

assigning 350 cm impact heights to the two explosives “shoot- ing” above the top of the 320 cm scale. For eleven explosives containing a hydrogen atom on a carbon affixed to a nitro- aromatic ring (compounds 34 and 38 excluded), the regres- sion equation describing the best straight line (sensitivity/ structure trend) is

log hsoqo = 1.33 - 0.26 OBloo (1)

with r (the correlation coefficient) = 0.97, and SD (the standard deviation) = 0.09 log unit. For twenty-four nitro- aromatic compounds lacking such an a-CH linkage (com- pound 21 excluded) the sensitivity/composition trend is best described by

log hs0% = 1.73 - 0.32 OBI00 (2)

with r = 0.96 and SD = 0.09 log unit. Since drop height settings on the ERL-NSWC impact machine are at 0.10 log unit intervals, the SD in both instances corresponds to about a one level uncertainty in impact sensitivities. Similar precision limits were observed earlier with the nitroaliphatic explosives(’).

3. Discussion

If, in the light of the inexactness of the impact machine, this apparent separation of the data points is to be ascribed to differences in chemical structure, a stronger case can be made if some independent or a priori basis for the separation of the compounds into two classes is first demonstrated. Toward this end, it is useful to repeat what we have pointed out earlier regarding the phenomena occurring under the impact hammer.

That the impact initiation of a high explosive is not a stable detonation is evident on comparing damage to the impact tools by primary and secondary explosives. The less energetic primaries, which on initiation go over rapidly to stable Chap- man-Jouguet detonations, do far more severe damage than the more powerful secondary explosives. In addition, while prod- ucts of C-J-detonations of C-H-N-0 explosives are primarily water, nitrogen, and the carbon analyses of products if impact explosions show relatively large amounts of nitrogen oxides and simple organic molecules like form- aldehyde(5). This indicates that impact explosions of secondary explosives are phenomena which more closely resemble relatively low temperature thermal decompositions than detonations. It further suggests that reaction kinetics, which do not influence the velocities of stable detonationd6), may play a significant role under the impact hammer.

Groups working with U b b e l ~ h d e ( ~ ) and Bowden(’) have devoted much attention to the mechanism of impact initiation. Bowden has suggested that initiations stem from hot spots in the explosive mass, generated by a number of possible routes including viscous heating, frictional heating, and adiabatic compression of entrapped gases. He has concluded that, to cause fires in PETN and NG, these hot spots must reach temperatures of at least 430 “C-500 “C.

A wide variety of physical and chemical properties may influence the birth of hot spots and their evolution into explosions. In addition to the reaction kinetics, these may include: (a) the heat evolved in the decomposition, (b) the heat capactiy, (c) thermal conductivity, (d) latent heats of fusion and evaporation of the explosive, (e) crystal hardness, (f) crystal shape, etc. In the case of liquids, dissolved gases, surface tension, and vapor pressure may also play a role.

By limiting the correlations to solid explosives and by proper design of the impact experiment, the contributions of certain of these properties to sensitivity may be minimized. Thus, resting the explosive samples on sandpaper (type 12 tools) lessens the effects of crystal hardness and crystal shape. The formation of hot spots is governed more by the sandpaper’s physical properties than by those of the explosives.

Other properties such as ( b d ) may differ from explosive to explosive, but if consideration is limited to organic com- pounds, the extent of variation is limited. It follows, there- fore, that while we may not ignore the part played by other properties, the most kkely correlations with impact sensitivity should depend on chemical reactivity and the rates and amounts of energy release.

Wenograd@) has convincingly corroborated this view by measuring ignition delays of thermal explosions in the 260 “ C to 1100 “ C range. He found that the temperature at which an explosion would occur within 250 ps (a time comparable to the interval under the impact hammer) varied greatly among explosives. He then demonstrated a monotonic pro- gression of the “critical temperatures” with measured impact sensitivities for a variety of explosives, suggesting that he had, in effect, simulated an impact experiment by a non- mechanical rate measurement.

With the above background, we may now consider whether the separate regression lines in Fig. 1 are simply an accidental consequence of the vagaries of the impact machine, o r whether they are the result of some more fundamental chemical difference between the two classes of compounds

Propellants and Explosives 4, 30-34 (1979) Impact Sensitivity and Structure of Organic High Explosives 33

being considered. Evidence in this regard is to be found in several studies of the thermal decomposition of polynitro- aromatic explosives.

Dacons and coworkers a t this Center") carried out partial thermal decompositions of T N T at 200°C. They reported that column chromatography of reaction mixtures indicated the presence of at least 20 and possible more than 50 discrete zones, each due to a t least one separate chemical entity. The number of zones suggested that T N T did not decompose by a single stepwise route, but rather that the detailed decom- position mechanism involved a number of simultaneous routes. Among the major decomposition products identified were 2,4,6-trinitrobenzaldehyde (I), 2,4,6-trinitrobenzyl alcohol (11), and 4,6-dinitroanthranil (111).

NO2 NO2 NO2

I I I I I I

Rogers("), carrying out TNT thermal decompositions at 233 "C-280 " C under a gas carrier stream to sweep out inter- mediate products, has reported similar results, also identifying trinitrobenzoic acid and trinitrobenzene (but not trinitro- benzaldehyde) among his products. These findings indicate that the thermal decomposition of T N T at 200 "C-280 " C occurs by both intra- and intermolecular reactions but that, whatever the route, a preferred point of initial attack is the methyl group. It is also likely that the attack occurs by abstrac- tion of a hydrogen atom. Shackelford and coworkers(") have offered further confirmation for such a mechanism, reporting a primary kinetic isotope effect in the thermal decomposition of TNT when the methyl group is deuterium labelled.

Additional striking evidence of the increased reactivity imparted by a C-H bond alpha to a polynitroaromatic ring has been obtained from thermal stability measurements. It has been mentioned that T N T self-ignites between 200 " C and 210 "C. 1,3,5-trinitrobenzene (TNB), on the other hand, is stable for extended periods at 300 "C without appreciable gas evolution o r evidence of degradation. Indeed,TNB is so stable a t 2 8 0 ° C that it has served as solvent in determining gas evolution by other dissolved thermally stable explosives(").

The significant difference between the two classes of explo- sives in Table 1 and Fig. 1 thus becomes apparent. A preferred site of initial attack, akin to that in TNT, is available for one group of compounds and not for the other. It follows that hot spot "critical temperatures" required to carry thermal decom- position reactions under the impact hammer over into thermal explosions should be lower for nitroaromatics containing the a-CH linkage, and they should be expected to "shoot" a t lower 50% impact heights than nitroaromatics lacking such a linkage.

For the former group of compounds, the sensitivityioxidant balance correlation is based on a similarity in decomposition mechanism or a t least position of initial attack for all members of the series. The latter group of compounds is of far more variegated structure, however, and from considerations of classical chemical reactivity, a number of widely differing decomposition mechanisms would be expected to apply. The use of a single sensitivityicomposition trend to describe the impact behavior of these explosives is simply a reflection of our lack of confidence in the capability of the impact machine to distinguish between them. One can, nevertheless use the

latter trend as a base with which the behavior of any sub-group can be compared.

Thus, for example, we had had some initial doubts as to whether a phenolic oxygen should be counted as equivalent to a nitro oxygen in calculating OBtoo. It seemed firmly established that phenolic oxygen does have some effect in increasing sensitivity. Compare, for example: 1,3,5-trinitro- benzene (100 cm), picric acid (87 cm), 2,4,6-trinitroresorcinol (43 cm), and 2,4,6-trinitrophloroglucinol (27 cm); hexanitro- biphenyl (85 cm), hydroxyhexanitrobiphenyl (42 cm), and dihydroxyhexanitrobiphenyl (40 cm); 2,2,2-trinitroethyl-3,5- dinitrobenzoate (73 cm) and trinitroethyl-3,5-dinitrosalicylate (45 cm); picramide (177 cm) and 3-amino-2,4,6-trinitro- phen_ol (138 cm). In these comparisons the average Alog h, , ,~/AOB,oo is -0.30, which is not far different from the -0.32 slope of the log h,,,% versus OBtoo plot. Evidently, then, present purposes are adequately served by counting a phenolic oxygen as equivalent to a nitro oxygen in determining OBloo. This result was admittedly surprising to us.

Sensitivities of compounds containing both polynitro- aromatic and polynitroaliphatic moieties are of some interest. In Part I of this series(') we reported the impact behavior of 3 3 polynitroaliphatic compounds containing nitro groups bonded only to carbon. For this group the applicable sensi- tivity/composition trend was described by the regression equation

log h,,,%, = 1.74 - 0.28 OBI,,. (3)

Since Eqs. (2) and (3) predict closely comparable sensitivities a t oxidant balances near zero, it is impossible to judge whether the esters 2,2,2-trinitroethyl-2,4,6-trinitrobenzoate, 2,2,2-trinitroethyl-3,5-dinitrosalicylate, 2,2,2-trinitroethyl- 3,5-dinitrobenzoate, and 2,2-dinitropropyl-2,4,6-trinitro- benzoate (compounds 2, 5, 6, and 11) should be considered with the applicable nitroaliphatic o r nitroaromatic series.

In the cases of compounds 26, 27, and 28, however, the appropriate equations predict widely differing sensitivities (Table 2). Here the results suggest that these compounds

Table 2. Impact Sensitivities of Polynitroalkyl Polynitroaromatic Explosives

No. Compound Pre- Pre- Measured dicted dicted Eq. (1) Eq. (3)

26. 1-(2,2,2-trinitroethyl)- 12cm 28cm 13cm

27. 1-(3,3,3-tnnitropropyl)- 22cm SScm 21 cm

28. 1-(2,2,2-trinitroethyl)- 26cm 66cm 31cm

2,4,6-trinitrobenzene

2,4,6-trinitrobenzene

2,4-dinitrobenzene

should be considered with the more sensitive class, and that their decomposition mechanisms under the conditions of the impact experiment resemble more closely those of T N T than of polynitroaliphatic explosives.

This illustrates a conclusion which we have found applicable in many aspects of new explosives synthesis, i.e., if a com- pound has structural features common to more than one sensi- tivitiy category, its impact behavior will conform most closely with the sensitivity/composition trend of the most sensitive (as was shown earlier with the polynitroaliphatic nitra- mines)(').

34 M. J. Kamlet and H. G. Adolph Propellants and Explosives 4, 3@34 (1979)

Of the 38 compounds considered, three stand out as anomalous. Hexanitrodiphenylamine (21) and 3,Sdimethyl- picric acid (38) are definitely more sensitive and 3-methyl- picric acid (34) is definitely less sensitive than the applicable equations would predict. We are satisfied that these 50% heights are not due to machine error, since replicate 25-shot determinations were carried out in these instances and the cited values are averages of the separate 25-shot tests.

These examples illustrate an important consequence of the correlation of molecular structure with sensitivity which we believe we have established. Abnormalities caused by unex- pectedly large secondary effects or, as we feel is here the case, by unexpected reactivity or lack of reactivity, are unmistakably thrust forward. Useful as is the ability to predict normal sensi- tivity behavior, the greatest promise of widening our under- standing of the reactivity of high energy compounds lies in the ability to isolate and study such exceptional behavior.

The extreme sensitivity of dimethylpicric acid may provide such an insight. Trinitrophloroglucinol and 1,3,5-triamino- 2,4,6-trinitrobenzene (h,(,%= >> 320 cm) are the only other hexasubstituted benzenes for which impact data are available, and in the latter compounds strong through-conjugation and hydrogen bonding forces favor coplanarity of the nitro groups. Spectral evidence is available(13) that, although the nitro groups in monomethylpicric acid are still essentially coplanar in solution, in dimethylpicric acid all three nitro groups have been forced out of planarity. If a non-coplanar (hence less con- jugated, less hydrogen bonded) nitro group is a better oxi- dizing agent toward methyl, or if a non-hydrogen bonded hydroxyl group is more easily oxidized than methyl, such sensitivity of dimethylpicric acid is readily rationalized. By an extension of this argument, if a through-conjugated, hydrogen bonded, coplanar nitro group is a poorer oxidizing agent toward methyl, the relative insensitivity of monomethylpicric acid may be rationalized.

We are hopeful that more detailed knowledge of the geometry of such structures from crystal structure determina- tions, as well as further studies of kinetics and mechanisms of thermal decomposition reactions, improved understanding of electronic interactions from absorption spectra studies, and other fundamental chemical and physical approaches may dovetail into a unified theory of structure, sensitivity, and thermal stability of organic high explosives. These important properties of high energy molecules would then become as predictable from a priori theory as are their detonation prop- erties.

4. Experimental

As carried out at the Naval Surface Weapons Center (NSWC), the technique used to determine impact sensitivities is a modification of that developed at the Explosives Research Laboratory (ERL), Bruceton, Pennsylvania, during World War 11. The salient features which distinguish the ERL-NSWC machine and method from other such tests are the particular tool design, the criterion of explosion, the use of flint paper to rest the sample on, and the statistical procedure used to conduct the exDeriment and reduce the data(14).

flint paper. The flat surface of the striker is rested on the sample through a closely fitted guide ring. The striker is then hit by a 2.50 kg weight dropped from heights which are varied according to the “up-down’’ experimental design described by Dixon and Massey(15). This design permits the calculation of a height from which drops will cause explosions 50% of the time. The impact sensitivities reported in Table 1 are 50% heights as generally determined from 25-shot sequences. A microphone-actuated noise-meter, set to record sounds louder than a preset intensity, decides whether a given shot has resulted in an explosion. The noisemeter is calibrated at that value which will cause TNT to have a 50% height of 160 crn.

Because the machine can distinguish between explosives with 50% impact heights of 15 cm and 20 cm about equally as well as between 150 cm and 200 cm, the logarithm of the 50% height is usually considered to be the best measure of relative impact sensitivity. 50% impact heights for some standard explosives not included in Table 1 are: Comp A-3, 60-70 cm; Comp-B, 60-70 cm; tetryl, 32-35 cm; RDX, 22-24 cm; HMX, 24-26 cm; BTNEU, 14-16 cm; PETN, 10-12 cm; BTNEN, 4-6 cm; lead azide, 2-4 cm.

5. References

(1) M. J. Kamlet, “The Relationship of Impact Sensitivity with Structure of Organic High Explosives. I. Polynitroaliphatic Ex- plosives”, Proceedings 6th Symposium (International) on Deto- nation, San Diego, California, Aug. 24-27, 1976, ONR Report ACR 221, p. 312.

(2) J. Wenograd, Trans. Faraday Soc. 57,1612 (1961). (3) M. J. Kamlet and J. E. Ablard, J . Chem. Phys. 48,36 (1968). (4) D. Ornellas, J . Phys. Chem. 72,2390 (1968). (5) A. J. B. Robertson and A. D. Yoffee, Nature (London) 161,806

(6) M. J. Kamlet and H. Hurwitz, J . Chem. Phys. 48,3685 (1968). (7) J. L. Copp, S. E. Napier, T. Nash, W. J. Powell, H. Skelley,

A. R. Ubbelohde and P. Woodward, Philos. Trans. Roy. SOC. London, Ser. A241, 197 (1948).

(8) F. P. Bowden and A. D. Yoffee, “Initiation and Growth of Explosions in Liquids and Solids”, Cambridge University Press, Cambridge 1952.

(9) J. C. Dacons, H. G. Adolph and M. J. Karnlet, J . Phys. Chem. 74,3035 (1970).

(1948).

(10) R. N. Rogers, Anal. Chem. 39,730 (1967). (11) S. A. Shackelford, J. W. Beckman and J. S. Wilkes, J. Org.

(12) J. M. Rosen and J. C. Dacons, Explosivstoffe 16, 250 (1968). (13) C. E. Moore and R. Peck, J . Org. Chem. 20,673 (1955). (14) E. H. Eyster and L. C. Smith, “Studies of the ERL Type 12

Dropweight Impact Machine at NOL”, NOLM-10003, Jan. 25, 1949; H. D. Mailory, Ed., “The Development of Impact Sensitivity Testing at the Explosives Research Laboratory, Bruceton, Penn- sylvania During the Years 1942-45”, NAVORD Report 4236, Mar. 16, 1956. The technique appears not to have been described in the general literature, but similar machines are described in “Military Explosives”, Department of the Army Technical Manual TM 9-1910, Washington D. C., April 1955.

(15) W. J. Dixon and F. J. Massey, “Introduction to Statistical Analy- sis”, McGraw-Hill Book Co., New York 1951, Chapter 19.

Chem. 42,4201 (1977).

The impact tools (type 12) consist of a 3.50 inch long striker and a 1.25 inch long anvil machined to 1.25 inch diameter from “Ketos” tool steel and hardened to Rockwell C-60-63. The contacting surfaces are polished. When, after successive polishing the length of the striker falls below 3.47 inches it is discarded. A conical heap, comprising about 0.035 g of the granular explosive is centered on the anvil on a piece of 5/0

Acknowledgements We are grateful to Drs. D. V. Sickman and C. Boyars for

useful discussions. The work was done under the Naval Surface Weapons Center’s Independent Research Program, Task IR-144.

(Received September 29, 1978)