2572 (10) (11) (12) (13) (14) (15) (16) (17) (18)
R. Shaw and F. E. Walker J. E. Dove and D. G. Jones, J . Chem. Phys., 55, 1531 (1971). I. Amdur, Adv. Chem. Phys., 10, 29 (1966). K. Takayanagi, Adv. At. Mol. Phys., 1, 149 (1965). F. H. Mies, J . Chem. Phys., 40, 523 (1964). R. E. Roberts, J . Chem. Phys., 49, 2880 (1968). R. L. McKenzie, J . Chem. Phys., 63, 1655 (1975). J. E. Dove and H. Teitelbaum, to be submitted for publication. H. 0. Pritchard, Chem. SOC., Spec. Rep., 1, 243-290 (1975). C.W. Gear, Commun. Am. Comput. Mach., 14, 176, 185 (1971).
(19) (a) F. H.Mies, J . Chem. Phys., 41, 903 (1964); (b) ibid., 42, 2709 (1965). (20) E. Kamaratos and H. 0. Pritchard, Can. J. Chem., 51, 1923 (1973). (21) (a) G. D. B. Strensen, J. Chem. Phys., 57, 5241 (1972); (b) G. D. Billing, Chem. Phys., 9, 359 (1975). (22) M. H. Alexander, Chem. Phys., 8, 86 (1975). (23) J. E. Dove and H. Teitelbaum, Chem. Phys., 6, 431 (1974). (24) J. H. Kiefer and R. W. Lutz, J . Chem. Phys., 44, 668 (1966). (25) N. C. Blais and D. G. Truhlar, J . Chem. Phys., 65, 5335 (1976).
Estimated Kinetics and Thermochemistry of Some Initial Unimolecular Reactions in the Thermal Decomposition of 1,3,5,7-Tetranitro-I ,3,5,7-tetraazacyclooctane in the Gas Phase Robert Shaw" Chemistry Department, Lockheed Missiles & Space Company, he., Lockheed Paio Alto Research Laboratory, Palo Alto, California 94304
and Franklln E. Walker Lawrence Livermore Laboratories, University of California, Livermore, California 94550 (Received May 27, 1977) Publication costs assisted by Lockheed Missiles & Space Company, Incorporated
A survey of the literature has shown that there are eight possible initial unimolecular steps in the thermal decomposition of HMX. Of the eight steps, Arrhenius parameters have been estimated for four of them: reaction, log As (where As = A/s-'), E/(kcal/mol); N-NO2 fission, 16.4, 46.2; homolytic C-N fission, 18, 60; five-center elimination of HONO, 10.8, 238; four-center elimination of HN02, 10.8, 238. The depolymerization of HMX to 4CH2NN02(a fifth possible step) was estimated to be 35.4 kcal/mol endothermic. The Arrhenius parameters are such that N-NO2 fission predominates at around 550 K. The estimated rate constant, lo-' s-l at 500 K, is one power of ten less than previously measured rate constants for the overall decomposition.
Introduction
4CH2NN02,5reaction 4; N-N02 fission,6-8 reaction 5 ;
The following initial steps in the thermal decomposition of HMX, 1,3,5,7-tetranitro-l,3,5,7-tetraazacyclooctane, have been postulated: transfer of an oxygen atom from a n -NOz group to a neighboring -CH2- group,lS2reaction 1; heterolytic C-N ~ l e a v a g ereaction ,~ 2; elimination of
/N\,,
T2 Y 02N-N\ CH2 N,
0
4
NP2
TH2 Y "\CH
02N
\
O ,
cp
\
(l)
C H N 2,C H ,2
- NI
I
I
NO2
NO2
02N
iH2 YrNo2 \" - I / 0-CH2
I \
0
CH
CH
2\N/CH2
N %CH2
I N02
(3)
CH2 ,N/CH2
I N02
The Journal of Physical Chemistry, Vol. 81, No. 25, 1977
"(32
i-"02 -
\
/ CHZ
CH2,
I N02
/HZ
OZN
I NO2
I N .
2
"pz
i"''\" Y rNo2 - 02"-1 -7
I
(2)
CH2NN02,4 reaction 3; concerted depolymerization to .
CHZ,"/CHZ
NO2
0 N-N
i
2
OZN
"p2
NO 7
y
rNo2 - -"\
/ CH2
rNo2iH'N\cH Y iH2 "?
2N ,C / H2
I
, "
homolytic C-N ~leavage,~,' reaction 6; and five-center
N\CH
N-NN(t)
\
\
Nf2 (-'
702
2H C,N,2H C
\
CH2
HUY
"\CH
rNo2
I
,NCN-742
02N-N
CH2N ,C / H2
02N-N
/CH2
rNo2 rNo2
N
"p?
Np?
02N-N\
CH2
,
/CHZ
i-"02
(6)
I N02
elimination of HON0,6,7-9reaction 7 . Recently Benson" has postulated that nitroalkanes may decompose by a four-center elimination of HN02, reaction 8. By analogy, HMX may also undergo four-center "02 elimination, reaction 9. It is not clear at present how to estimate the thermochemistry or the kinetics of the first three processes. However, we record here estimates of the kinetics of
2573
Thermal Decomposition of HMX
NO2
NO2
o\N/o
__1___1__ H-C-C-H
I
/
H
H
NO2
-
/O
O\ H-N
(8 1
H-C-C-H
I
H
I
H
NO2
N-N02 fission, homolytic C-N cleavage, and elimination of HONO or HNOz, and estimates of the thermochemistry of concerted depolymerization to 4-CHzNN02.
N-N02 Fission McGuire'l has pointed out that x-ray photoelectron spectra of dimethylnitramine and HMX have shown that the binding energies of the nitrogen atoms in the former are significantly different from those in the latter. McGuire'l therefore concludes that correlations drawn between dimethylnitramine decomposition routes and those projected for HMX are highly suspect. Nevertheless we have used dimethylnitramine as a model compound because we do not know how to correlate binding energies with bond strengths nor do we know of a model compound whose thermochemistry is more reliable. The model for N-N02 fission is reaction 10. For reAH: ,,,/(kcal/mol)
Ref
(CH,),NNO, = (CH,),N t NO, -0.3 38 7.9 12 13 14
(10)
action 1, AHoz98 = 46.2 kcal/mol, which may be taken as the activation energy for the forward reaction if that for the reverse reaction is assumed to be zero and if effects due to heat capacity and mole change are negle~ted.~ The bond strength of 46.2 kcal/mol is in excellent agreement with the values of 47 and 48 kcal/mol measured by Gowenlock, Jones, and Majer,1548 kcal/mol measured by Miller, Grigor, MUSSO, and Yount,16 and 47 kcal/mol reported by Tajima and others.17 The bond strength determined here is higher than the value of 41.2 kcal/mol determined by Korsunskii, Pepekin, Lebedev, and Apin.18 Of the 5 kcal/mol difference in bond strengths, 1.2 kcal/mol may be accounted for by the difference in the N-H bond strength in dimethylamine.13*18 The bond strength of 46.2 kcal/mol is considerably less than the value of 59 kcal/mol quoted by Mortimerlg and 55 kcal/ mol due to Cass, Fletcher, Mortimer, Quincey, and Springall." The latter value has been discounted by Korsunskii and co-workers18 as "an average of the bond energies and cannot be identified with the energy of the N-N bond in dimethylnitramine". The N-N02 bond dissociation energy of 46.2 kcal/mol is also much less than the N-N bond energy of 66 kcal/mol in 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) obtained by Orloff, Mullen, and RauchZ0from a correlation of
MO-calculated overlap populations and N-N bond strengths in N2, N2H2,and N2H4. We do not know why such a large difference exists. The Arrhenius A factor of 13 s-l was obtained from an average of the A factors for the decomposition of nitromethane and dinitrogen tetraoxide.21 That is why we estimate the Arrhenius parameters for reaction 10 to be log k s = 15.8 - 46.2/2.3RT [where ks = k/s-l, and where R = 1.987 cal/(mol K)]. These Arrhenius parameters are in poor agreement with those determined by Fluornoy22for reaction 10, namely, log ks = 20 - 53/2.3RT, although the rate constants at 465 K (the middle of Fluornoy's reciprocal temperature range) are in fair agreement, namely, log ks = -4.9 (Fluornoy) and log k s = -5.9 (this work). The estimated Arrhenius parameters, log ks = 15.8 46.2/2.3RT, are in fair agreement with those of Korsunskii and DubovitskiiZ3for the overall decomposition of dimethylnitramine, namely, log ks = 14.1 - 40.8/2.3 RT. In the middle of Korsunskii and Dubovitskii's temperature range, 493 K, their rate constant is log ks = -4.1, in reasonable agreement with log k s = -4.7 calculated from our estimated Arrhenius parameters a t the same temperature. Our Arrhenius parameters are consistent with a preliminary measurement of the rate of overall dimethylnitramine decomposition measured in a very pyrolysis apparatus by Barkerag HMX has four N-N02 bonds so we calculate that the rate constant for N-N02 fission is log ks = 16.4 - 46.2/ 2.3RT. Homolytic C-N Fission The C-N bond strength in CH3-NH2 is 85 kcal/m01.~~ Using isoelectronic analysis, Gowenlock, Jones, and Majer15 determined the C-N bond in CH3-NHCH3 to be 5 kcal/ mol weaker than that in CH3-NH2. Therefore we calculate the C-N bond strength in CH3-NHCH3 to be 80 kcal/mol. In HMX the C-N bond strength is weakened by ring strain, stabilization of the free electron on the nitrogen atom due to the NO2 group, and stabilization of the free electron on the carbon atom due to the N atom next to it. There are three approaches to estimating the ring strain (RS,HMX). The simplest6 is to take the average of the strains in eight-membered ring hydrocarbon^^^ giving RS,HMX = 11 kcal/mol. The second method is based on a comparison of the heats of formation of HMX and 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX). Using group notation AH,"(RDX) = 3[N-(NO2)(C),] + 3[C-(H),(N),I + RS,RDX = 45.8 k c a l / m 0 1 * ~ ~ ~ ~ (11) AH,"(HMX) = 4[N-(N02)(C),] + 4[C-(H),(N),I + RS,HMX = 59.8 k c a l / m 0 1 ~ ~ * * ~ (12) (4/3)AHf"(RDX) = 4[N-(NO,)(C),] + 4[C-(H),(N),] + (4/3)RS,RDX = 61.1 kcal/mol (13) (13) - (12) = (4/3)RS,RDX - RS,IIMX =
1.3 kcal/mol (14) RS,HMX = (4/3)RS,RDX - 1.3 kcal/mol (15) The ring strain in RDX is not known but it may be taken as equal to that in P 2 ,
The Journal of Physical Chemistry, Vol. 8 I, No. 25, 1977
R. Shaw and F. E. Walker
2574
which is 6.6 kcal/mol.28 That is RS,HMX = 8.6 - 1.3 = 7.3 kcal/mol. The third method uses group additivity directly. 4[N-(NB,)(C),] + 4[C-(H),(N),] RS,HMX =
+
69.8 kcal/rnol (16) Using the group valuesz9 N-(NOz)(C), = 17.8 and @-(H),(N), = -12.1 kcal/mol RS,HMX
=
59.8 - 71.2
+ 48.4 = 37 kcal/mol
It is not clear why this value is so different from the other two, but a review of the original literature29suggests that the group value of C-(H),(N), is in error. At any rate we reject, the value for RS,HMX obtained by the third method and take an average of the other two, namely, RS,HMX = 9 kcal/rnol. Levy30 has concluded that the substitution of a nitro group for a hydrogen atom in polynitromethanes will lower the activation energy for C-N02 fission by 5 or 6 kcal/mol. Ross31has found that substitution of a nitro group for a hydrogen atom on the 01 carbon atom will lower the activation energy for C-N fission by 11 kcal/mol. We will take the average value of 8 kcal/mol and assume6B7that the N-N02 stabilization is the same as the C-NO2 stabilization. Colussi and B e n s ~ nhave ~ ~ shown that the C-H bond strength in CH3NH2is 3 kcal/mol lower than the C-H bond strength in CH3CH3. We will take this as the stabilization of the -CH2- due to the adjacent N atom in the homolytic C-N cleavage of HMX. The C-N bond strength in 14MX is therefore Do(CH3-NHCH3) - RS,HMX NN02 stabilization - CH2N stabilization = 80 - 9 - 8 - 3 = 60 kcal/mol. Orloff, Mullen, and RauchZ0calculated the C-N bond energy in RDX to be the same as that in CH3NH2, namely, 85 kcal/mol, from a correlation of calculated overlap populations. We do not know why the difference between the thermochemical estimate for HMX and the MO calculation for RDX is as large as 25 kcal/mol. The intrinsic entropy of octene-1 is 22 cal/(mol K) more than that of cyclooctane.25 If this difference is taken to be the entropy of activation for ring opening, then the A factor may be estimated as 1Ol8 s-l. That is, for ring opening, log k s = 18 - 60/2.3RT. Five-Center Elimination of H O N O The rate constant for elimination of HONO from HMX may be estimated in two ways. The first way is by analogy with elimination of HONO from 2-nitropropane (CH,),CHNO, = CH,CHCH, + HONO
(17)
for which the rate constant isz1log hs = 11.3 - 40/2.3RT. Of the 40 kcal/mol for the activation energy, 20 kcal/mol is for the endothermicity, leaving 20 kcal/mol for the intrinsic activation energy.g Using dimethylnitramine as the model the thermochemistry of HONO elimination is A H ; ,,,/(kcal/mol) Re S
(CH,),NN02 = CH,NCH, -0.3 17.3 12 29
+
HONO (18) -18.3 14
Reaction 18 is 1-3 kcal/mol exothermic, so the activation energy has no endothermic contribution and is therefore 20 kcal/mol. Therefore the rate constant of reaction 18 is log hs = 11.3 - 20/2.3RT. However the measured rate constants for the pyrolysis of dimethylnitramine are log k s = 20 - 53/2.3RT (by FluornoyZ2)and log hs = 15.8 46.2/2.3RT by Korsunskii and D u b ~ v i t s k i i .A~t~473 K, Fluornoy’s rate constant is log k s = -4.6 in excellent saeement with Korsunskii and Dubovitskii’s rate constant The Journal of Physical Chemistry, Vol. 81, No. 25, 1977
of log hs = -4.8. At the same temperature, our estimated rate constant is about a factor of lo7 faster than the observed rate. Furthermore the observed productz2VrJis dimethylnitrosamine not N-methylmethylimine. It is not clear why our value is so far off but attention is drawn to Benson and O’Neal’s reservationsz1 about the mechanism of pyrolysis of the nitroalkanes. The second and preferred way of calculating approximately the rate constant for reaction 18 is to postulate that HONO elimination contributes no more than 10% to the rate of decomposition of dimethylnitramine at 473 K. Using Korsunskii and Dubovitskii’s rate constant a t 473 K gives log hs = -5.8 for reaction 18. If the Arrhenius A factor for HONO elimination is that recommended by Benson’O for nitroalkanes, namely, log As = 12, then E 2 38 kcal/mol for WON0 elimination from dimethylnitramine and, by analogy, from HMX. Barkerg has pointed out that O’Neal and Benson14 have calculated that cyclooctene is tighter than cyclooctane, and that the difference in entropy is 11.6 cal/(mol K). Barker has calculated that if this amount of entropy is lost when HMX goes to the transition state for HONO elimination, and if effects due reaction path degeneracy and loss of entropy when the NOz rotor is tied up then the A factor is 1010.8 s-l. That is, for HONO elimination, log k s I10.8 - 38/ 2.3RT. Four-Center Elimination of H N 0 2 BensonlO has estimated that for nitroalkanes the A factor for elimination of HNOz is similar to that for elimination of HONO. In principle is it possible to calculate the activation energy of the four-center elimination of HNOz using Benson and Haugen’s semiion pair technique.34 However we decided against it because of the uncertainty in estimating the necessary properties of HNOz and because no N-methylmethylimine was observed in the pyrolysis of dimethylnitramine. Again, as no N-methylmethylimine was reported in the pyrolysis of dimethylnitramine, the rate constant for HN02 cannot be greater than 10% of the observed rate constant, giving a lower limit of 38 kcal/mol for the activation energy. Therefore for HMX, we estimate the rate constant for HNOz elimination to be the same as that for HONO elimination, namely, log ks I10.8 - 38/ 2.3RT. Although four- or five-center elimination has not been observed for nitramines, Golden, Solly, Gac, and B e n ~ o n ~ ~ have observed four-center elimination of NH3 from CH3NHNH2, (log hs = 13.1 - 53.8/2.3RT). Depolymerization to 4-CH2NN02 It is not clear how the Arrhenius parameters for this reaction may be estimated but the thermochemistry is as follows. The heat of formation of methylhydrazine is 2.6 kcal/mol more than that for 1,l-diroethylhydra~ine.~~ Therefore, from the heat of formation of dimethylnitramine of -0.3 kcal/mol (ref 12), that for methylnitramine may be estimated as 2.3 kcal/mol. If the heat of hydrogenation is taken as 21.5 kcal/mol (ref 29 and 371, then = 23.8 kcal/mol. The heat of forAHfo298(CHzNN02) mation of solid HMX is 17.9 kcal/mol (ref 26) and its heat of sublimation is 41.9 kcal/mol (ref 27), giving a gas-phase heat of formation of 59.8 kcal/mol. In the gas phase, depolymerization of HMX to 4-CHzNN02 is thus 35.4 kcal/mol endothermic. Discussion In Figure 1, Robertson’s measured’ overall rate of de-
2575
Thermal Decomposition of HMX T/K 2000
1000
I
300
500 I
I
EXPERIMENTAL
10-
0 ROBERTSON(MELT)
I
1
ESTIMATED
-10
ELIMINATION OF HONOOR\ "02 LOG ks< 10.8-38/ ' \ -15 2 3 RT \ HOMOLYTIC C-N FISSION LOG ks=18-60/23RT -20, N-NOz FISSION LOG ks = 16 4-462/\ '\\ 23RT 1 2 3 1000 KIT Figure 1. Comparison of estimated and experimental rate constants for the decomposition of HMX [ k s = k / s - ' ] .
1:
,
,
I
composition in the melt and Barker's measured9 overall rate of decomposition in the gas phase are compared with the estimated rate constants for the elementary reactions, N-NOZ fission, homolytic C-N fission, and elimination of HONO or H N 0 2 . The results show that the fastest elementary reaction over the temperature range 333-2000 K is N-NOZ fission. In support of our conclusion that N-N02 fission is the fastest initial gas phase reaction, Miller and co-workers16 observed that the NOz peak had a relative intensity of 60 (NO = 100) in the vacuum pyrolysis of HMX a t 463 K. Furthermore Rauch and Fanelli38 observed NOz both visually and spectroscopically in the gas phase pyrolysis of RDX a t 480-500 K. However there is also some evidence of ring fission as Suryanarayana, Graybush, and Autera5 found that 99 % of the N 2 0 in the decomposition of l5NO2-1abeledHMX a t 503 K turned up as N15N0 (reactions 19 and 20). The NO NYZ I N '
02N
-
iH2
I
. CH2
\
?-NO2
--C
02N-N
?""'
NO2
\N - N O z
'CHZo
J 02N
N20
CHzO
jH2 -N,
Acknowledgment. For helpful support and discussions we thank W. Howard of Hercules, Inc., B. B. Goshgarian of the Air Force Rocket Propulsion Laboratory, D. M. Golden, J. R. Barker, D. S.Ross, and D. F. McMillan of SRI International, J. R. Keeffe of San Francisco State University, M. J. Kamlet of Naval Surface Weapons Center, White Oak, R. McGuire, G. R. Haugen, and E. James of Lawrence Livermore Laboratory, J. B. Levy of George Washington University, L. Phillips of ERDE, Waltham Abbey, H. E. O'Neal of San Diego State University, R. Miller of the Office of Naval Research, D. Laughran and R. N. Rogers of Los Alamos Scientific Laboratory, R. Fifer of the Ballistic Research Laboratory, Aberdeen Proving Ground, D. Garvin of the National Bureau of Standards, and D. M. Duran, G. A. Lo, E. L. Littauer, M. Furio, and C. Yasukawa of Lockheed's Palo Alto Research Laboratory.
References and Notes
NO2
.
HMX? Why does liquid RDX decompose 10 times faster than liquid HMX a t 543 K? Is the N-NO bond in dimethylnitrosamine stronger than the N-N02 bond in dimethylnitramine as proposed by Korsunskii and coworkersl8 but in opposition to Gowenlock and co-workis the main product in the e r ~ If? dimethylnitrosamine ~ ~ decomposition of dimethylnitramine, why is 1,3>5,7tetranitroso-1,3,5,7-tetrazacyclooctanenot the main product in the pyrolysis of HMX? Why do the Arrhenius parameters for the decomposition of nitramines vary from high values (e.g., log k s = 19.7 - 52.712.3RTfor HMX or log hs = 20 - 53/2.3RTfor dimethylnitramine) to low values (log k s = 10.5 - 30.712.3RT for N-methyl-Nchloromethylnitramine30)? As regards the concerted depolymerization of HMX to 4-CHzNNOZ (methylenenitrimine) the following quotation by Wright37may be relevant "There is no factual basis for the existence or intermediacy of methylenenitrimine" (in the synthesis of RDX). In conclusion, although many uncertainties remain concerning the mechanism of the decomposition of HMX, we believe that the present work provides useful guidelines in support of experiments in progress on the decomposition of HMX in solution.
N-N02
NO2
internal oxygen atom transfer and elimination of N 2 0 and CHzO may then continue. Furthermore, high molecular weight species have been detected in the thermal decomposition of HMX. Thus Goshgarian* found fragments up to mle = 222 and Miller and co-workers16 found fragments up to m l e = 148 (assigned to N(N02)CH2N(N02)CHZ). There is also evidence of HONO elimination. Rogers37 reports that HONO has been detected as a product of decomposition. Many questions remain unanswered. For example, what is the fate of the radical left when NOz is removed from
(1) A. J. B. Robertson, Trans. faraday Soc., 45, 85 (1949). (2) F. C. Rauch and A. J. Faneiii, J . Phys. Chem., 73, 1604 (1969). (3) K. P. McCarty, "HMX Propellant Combustion Studies", AFRPL-TR76-59, Air Force Propulsion Laboratory, Director of Science and Technology, Air Force Systems Command, Edwards, Calif. (4) B. B. Goshgarian, "JANNAF Workshop", Ballistics Research Laboratory, Aberdeen Proving Ground, Md., July 26 and 27, 1977. (5) B. Suryanarayana, R. J. Graybush, and J. R. Autera, Chem. I d . , 2177 (1967). (6) D. M. Golden, SRI International, private communication. (7) J. R. Keeffe, San Francisco State University,.private communication. (8) M. J. Kamiet, "JANNAF Workshop", Ballistics Research Laboratory, Aberdeen Proving Ground, Md., July 26 and 27, 1977. (9) J. R. Barker, SRI International, private communication. (10) S.W. Benson, "Thermochemical Kinetics", 2nd ed, Wiiey, New York, N.Y., 1976, p 115. (11) R. McGuire, Lawrence Livermore Laboratory, private communication. (12) R. C. Cass, S.E. Fletcher, C. T. Mortimer, P. G. Quincey, and H. D. Springall, J. Chem. Soc., 958 (1958). (13) D. M. Goklen, R. K. Solly, N. A. Gac, and S.W. Benson, J. Am. Chem. Soc., 94, 363 (1972). (14) "JANNAF Thermochemical Tables", Dow Chemical Co., Midland, Mich. (15) B. G. Goweniock, P. P. Jones, and J. R. Majer, Trans. Faraday SOC., 57, 23 (1961). (16) R. R. Miller, R. C. Musso, A. F. Grigor, and R. A. Yount, "Combustion Mechanism of Low Burning Rate Propellant", AFRPL-TR-69-130, Final Report, Contract FO4611-C-67-0049, May 1969. (17) Y. A. Tajima et al, "Research and Development of Burning Rate Catalysts for Nitramine-Based Solid Rocket Propellants" (C), Contract DA-30-069-AMC-122, National Lead Company, Final Report, Dec 1964. (18) B. L. Korsunskii, V. I . Pepekin, Yu. A. Lebederv, and A. Ya. Apin, Izv. Akad. Nauk SSR, Ser. Khim., 3, 525 (1967). (19) C. T. Mortimer, "Reaction Heats and Bond Strength", Pergamon Press, The Journal of Physical Chemistry, Vol. 8 1, No. 25, 1977
2576
R. G. Manning and J. W. Root
Eimsford, N.Y., 1962. (20) M. K. Orloff, P. A. Mulien, and F. R. Rauch, J . Phvs. Chem., 74 2189 (1970). (21) S.W. Benson and H. E. O'Neal, Nail. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 21 (1970). (22) J. M. Fluornoy, J . Chem. Phys., 36, 1106 (?962). (23) 6.L. Korsunskii and F. I.Dubovitskii, Dokl. Akad. Nauk. SSR, 155, 402 (1964). (24) S. W: Benson, "Thermochemical Kinetics", 2nd ed, Wiley, New York, N.Y., 1976, p 309. (25) S. W. Benson in ref 25, p 273. (26) D. R. Stull, E. F. Westrum, and G. C. Sinke, "The Chemical Thermodynamics of Organic Compounds", Wiley, New York, N.Y., 1969. (27) J. M. Rosen and C. Dickenson, J . Chem. Eng. Data, 14, 120 (1969). (28) S. W. Benson in ref 25, p 273. (29) S. W. Benson, F. R. Cruckshank, D. M. Golden, G. R. Haugen, H. E. O'Neal, A. S. Rodgers, R. Shaw, and R. Walsh, Chem. Rev., 69, 279 (1969). (30) J. B. Levy, "Second Progress Report and Final Report on Russian Literature Survey in Explosives Chemistry", Contract DAHC04-
(31) (32) (33) (34) (35) (36) (37) (38) (39)
72-A-0001, Naval Surface Weapons Center, White Oak, Silver Spring, Md., Sept 1976. D. S. Ross, unpublished work, reported by R. Shaw in Int. J . Chem. Kinet., 5, 261 (1973). A. J. Colussi and S. W. Benson, Inf. J , Chem. Kinet., 9, 307 (1977). L. Phillips, ERDE, Waltham Abbey, unpublished work, reported by J. R. Barker, SRI International private communication. S. W. Benson and G. R. Haugen, J . Am. Chem. SOC.,87, 4036 (1965). D. M. Goklen, R. K. Soliy, R. A. Gac, and S. W. Benson, Int. J. Chem. Kinet., 4, 433 (1972). J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds", Academic Press, New York, N.Y., 1970. R. Shaw, "Estimation of the Thermochemistry of Imidic Acid Derivatives" in "The Chemistry of Amidines and Imidates", S.Patai, Ed., Wiley, New York, N.Y., 1975. F. C. Rauch and A. J. Faneili, J . Phys. Chem., 73, 1604 (1969). G. F. Wright, "Methods of Formation of the Nitramine Group. Its Properties and Reactions" in "Chemistry of the Nitro and Nitoso Group", H. Feuer, Ed., Wiley, New York, N.Y., 1969.
Chemistry of Nuclear Recoil "F Atoms. 10. Studies of "F Caged Capture Processes in CH3CF3/H2Sand CH3CHF2/H2SLiquid Mixturest Ronald G. Manningf:and John W. Root" Department of Chemistry and Crocker Nuclear Laboratory, University of California, Davis, California 956 16 (Received April 26, 1977) Publication costs assisted by fhe U.S. Energy Research and Development Administration
Absolute nuclear recoil "F yields have been determined for liquid-phase fluoroethane/H2Smixtures at H2S concentrations ranging to 50 mol YO.The results have been interpreted using contrasting Miller-Dodson and steady-state nonthermal kinetic formulations. The Miller-Dodson collision densities do not change significantly with the addition of H2S. Nonthermal rate coefficients are closely comparable and independent of mixture composition for F-to-HF reactions with H2S,CH3CF3,and CH3CHF2,exceeding by roughly fivefold the composite values that include all fluoroethane organic reaction channels. Branching ratios for 18Fenergetic bimolecular vs. caging reactions are independent of H2Sconcentration. In contrast with the predictions of collision-theory cage-effect models, efficient F-to-HF reactions do not interfere with primary caged capture. Reaction probability functions for energetic F-to-HF processes in H2S and fluoroethanes are crudely similar and extend to center-of-mass energies of 25 eV or more.
Introduction High energy nuclear recoil 18Fresults have been reported for gaseous1,2and liquid3 fluoroethane reactant^.^ The liquid-phase studies were principally concerned with the detection and characterization of caging reactions. The first evidence for sigmoidal density-variation behavior was obtained by Richardson and Wolfgang in an I*Fstudy of CH~FI~ F -for-F
/-------+CHF', I ~ F +* CH,F-(
\-F%!cH,F~~F
+
F
+
H
in which the asterisk denotes translational excitation. Sigmoidal hot yield vs. density plots have also been obtained for other hot atom system^,^^^-^ and apparent substitution product yield increases with increasing density in the condensed phase have generally been ascribed to + Presented in p a r t at t h e Symposium on Reaction Mechanisms, Models and Computers, 173rd National Meeting of the American Chemical Societv. New Orleans, La.. 1977, Contributed Papers No. PHYS 255 and i56. Present Address: Stanford Research Institute, Menlo Park, Calif.
94025. The Journal of Physical Chemistry, Vol. 8 1, No. 25, 1977
caged-capture processes. The principal complication underlyisg this interpretation has to do with the possible role of unimolecular reactions, which exhibit a qualitatively similar density dependence. Substantial bona-fide unimolecular decomposition has been shown to accompany energetic F-for-F and F-for-H substitution processes in condensed-phase f l u o r ~ e t h a n e s ,and ~ ~ ~these ~ ~ , ~unimolecular mechanisms in themselves lead to sigmoidal density-variation behavior. In general, therefore, the information content of density-variation experiments is not adequate for the definitive detection or characterization of caging reactions. There have been many past attempts to characterize the essential dynamical features of the photochemical cage effect. These have been culminated and summarized in the molecular theoretical treatment of Bunker and Jac o b ~ o n . ~ Although -l~ the past two decades have witnessed the appearance of numerous experimental papers pertaining to caging processes in hot atom systems, even the systematics interpretation of these phenomena has remained c o n t r ~ v e r s i a l . ~In - ~ the present study we have sought to test the molecular theory" as well as other published interpretations of caging experiment^,^,^*^ and also to elucidate some specific dynamical characteristics
