Isomerization and Decomposition of 3, 5-Dimethylisoxazole. Studies

Jul 1, 1995 - Decomposition of Anthranil. Single Pulse Shock-Tube Experiments, Potential Energy Surfaces and Multiwell Transition-State Calculations...
0 downloads 0 Views 1MB Size
Download PDF
J. Phys. Chem. 1995,99, 11436- 11446

11436

Isomerization and Decomposition of 3,5-Dimethylisoxazole. Studies with a Single Pulse Shock Tube Assa Lifshitz,* Dror Wohlfeiler; and Carmen Tamburu Department of Physical Chemistry, The Hebrew University, Jerusalem 91904, Israel Received: March 13, 1995; In Final Form: April 28, 1 9 9 9

The isomerization and decomposition of 3,5-dimethylisoxazole were studied behind reflected shocks in a pressurized driver single pulse shock tube over the temperature range 880- 1050 K and overall densities of -2.5 x mol/cm3. The main thermal reaction of 3,5-dimethylisoxazole is an isomerization to 2-methyl3-oxobutyronitrile (CH3COCH(CH3)CN). Acetonitrile, which is the main decomposition product in the system, is obtained from 3,5-dimethylisoxazole by a unimolecular process: 3,5-dimethylisoxazole CH3CN CH3CHC0. The unimolecular decomposition of the thermally excited isomer, (CH3COCH(CH3)CN)# CHzCHzCN CH3C0, is assumed to be the source for a large concentration of free radicals and for carbon monoxide. Thus, ethane, methane, and carbon monoxide are the major products among the species without nitrogen. It is believed that hydrogen cyanide, acetaldehyde, and 2-buten-3-one, which are found in the postshock mixtures, are obtained by a four-center unimolecular elimination from 2-methyl-3-oxobutyronitrile from both the thermally excited state (low temperatures) and a state of thermal equilibrium (high temperatures). The isomerization reaction, which involves cleavage of the N - 0 bond and migration of a methyl group from position 3 to 4 in the ring, is a first-order process with a rate constant given by kist = 10i5.90exp(-61.5 x 103/RT) s-I where R is expressed in units of cal/(K mol). The overall decomposition of 3,5-dimethylisoxazole e x p ( 4 9 . 5 x 103/RT)s-I. A reaction scheme in terms of a first-order rate constant is given by ktotal= 10'5.57 containing 26 species and 32 elementary reactions was constructed to describe the overall mechanism of 3,5-dimethylisoxazole decomposition and isomerization.

-

+

I. Introduction

-+

U

We have recently published two studies on the decomposition of isoxazole and 5-methylisoxazole behind reflected shocks in a single pulse shock tube.'V2 When subjected to shock heating, a plethora of decomposition products, with and without nitrogen, were analyzed in the postshock mixtures of these two compounds. On the basis of the experimental findings, we suggested reaction mechanisms, constructed kinetic schemes, and performed computer simulations with good agreements between the computed and experimental results. Ring cleavage always resulted in decomposition of the molecules into smaller fragments. We did not find any isomerization products in the postshock mixtures of both isoxazole and 5-methylisoxazole. Whereas all the decomposition channels in the two molecules were assumed to begin with a stretch in the weak N-0 bond in the ring, large differences in activation energies which resulted from differences in extent of stretching were observed. In isoxazole a minimal stretch was found to be enough to cause a migration of a H atom from position 5 to position 4 in the ring and to break the molecule into CO and CH3CN. In 5-methylisoxazole the N-0 bond had to be practically broken in order to allow migration of a methyl group from position 5 to position 4 and to break the molecule into CO and C2H5CN in the major reaction channel. Differences of some 25 kcallmol in activation energies together with several orders of magnitude variation in the preexponential factors were found. ~~

~

~~~

In partial fulfillment of the requirements for a Ph.D. Thesis submitted to the Senate of the Hebrew University by D.W. @Abstractpublished in Advance ACS Abstracts, June 15, 1995.

0022-365419512099-11436$09.00/0

3,5-Dimethylisoxazole is another member of the unstable isoxazole line of molecules. It is therefore expected that the decomposition of this molecule will also start by an extension of N-0 bond in the ring. It is of interest, therefore, to examine the thermal reactions of this molecule and to elucidate the effect of an additional methyl group on its kinetic stability and reaction channels. In this article we present a detailed investigation of the shock-initiated reactions of 3,5-dimethylisoxazole. As far as we are aware, the decomposition of 3,5-dimethylisoxazole has never been studied in the past either at low or at high temperatures.

11. Experimental Section 1. Apparatus. The thermal reactions of 3,5-dimethylisoxazole were studied behind reflected shocks in a pressurized driver, 5 2 mm i.d. single pulse shock tube. The tube and its mode of operation have been described in an earlier publication3 and will be given here very briefly. The 4 m long driven section was divided in the middle by a 52 mm i.d. ball valve. The driver had a variable length up to

0 1995 American Chemical Society

J. Phys. Chem., Vol. 99, No. 29, 1995 11437

Isomerization and Decomposition of 3,5-Dimethylisoxazole a maximum of 2.7 m and could be varied in small steps in order to obtain the most rapid cooling conditions. A 36 L dump tank was connected to the driven section at a 45" angle near the diaphragm holder in order to prevent reflection of transmitted shocks and to reduce the final pressure in the tube. The driven section was separated from the driver section by a "Mylar" polyester film of various thickness depending upon the desired shock strength. After pumping down the tube to approximately Torr, the reaction mixture was introduced into the section between the 52 mm i.d. ball valve and the end plate and pure argon into the section between the diaphragm and the valve, including the dump tank. Gas samples were collected from the tube in 150 cm3 glass bulbs through an outlet in the driven section near the end plate and analyzed using a Hewlett-Packard Model 5890A gas chromatograph with a flame ionization and nitrogen phosphor detectors. Reflected shock temperatures were calculated from the extent of decomposition of 2-chloropropane (C3H7Cl- C3H6 -k HC1) which was added in small quantities to the reaction mixture to serve as an internal standard. This decomposition is a firstorder unimolecular reaction4 with a rate constant given by kist exp(-50.9 x 103/RT)s-l. Reflected shock temperatures were calculated from the relation

-+-20

40

30

50

BO 70

90

time, min Figure 1. Gas chromatogram of a postshock mixture of 0.5% 3.5dimethylisoxazole in argon take on a NPD. The shock temperature is 996 K. The numbers by the chromatogram peaks (in parentheses)

indicate attentuation factors. (60)

Cti ,COC H(CH,)C N

I

%

.-c v)

40

C

2 where t is the reaction dwell time and decomposition defined as

x

is the extent of

20

.-C 0

2 = [C3H61J([C3H61f + [C,H,C1IJ

(11)

The remaining reflected shock parameters were calculated from the measured incident shock velocities. These were measured with two miniature high-frequency pressure transducers (Vibrometer Model 6QP500) placed 300 mm apart near the end plate of the driven section. The signals generated by the shock wave passing over the transducers were fed through a home-built piezoamplifier to a Nicolet Model 3091 digital oscilloscope. Time intervals between the two signals shown on the oscilloscope were obtained digitally with an accuracy of 2 ps (out of about 450),corresponding to approximately 15 K. A third transducer (P.C.B. Model ll3A26) placed in the center of the end plate provided measurements of the reaction dwell times (approximately 1.8 ms) with an accuracy of -5%. Cooling rates were approximately 5 x IO5 Ws. 2. Materials and Analysis. Reaction mixtures containing 0.5% 3,5-dimethylisoxazole and 0.1% 2-chloropropane in argon were prepared manometrically and stored at high pressure in 12 L glass bulbs. Both the bulbs and the line were pumped down to better than lop5 Torr before the preparation of the mixtures. 3,5-Dimethylisoxazole was obtained from Aldrich Chemical Co. and showed only one GC peak. The argon used was Matheson ultrahigh-purity grade, listed as 99.9995%, and the helium was Matheson pure grade, listed as 99.999%. All materials were used without further purification. Gas chromatographic analyses of the postshock mixtures were performed on two 2 m Porapak N columns with flame ionization and nitrogen phosphor detectors. Identification of reaction products was based on their GC retention times. The main reaction product was identified by its mass spectrum as an isomerization product of 3,5-dimethylisoxazoleusing a HewlettPackard Model 5970 mass selective detector. Its structure was determined by FTIR and NMR analyses which were performed on samples collected from several tests and combined together. It was identified as 2-methyl-3-oxobutyronitrile.NPD and FID

10

20

30

50

90

Retention time (min) Figure 2. Gas chromatogram of a postshock mixture of 0.5% 3,5dimethylisoxazole in argon take on a FID. The shock temperature is 996 K. The numbers by the chromatogram peaks (in parentheses) indicate attentuation factors.

chromatograms of 0.5% 3,5-dimethylisoxazole in argon shockheated to 996 K can be seen in Figures 1 and 2. They do not include carbon monoxide and hydrogen. Carbon monoxide, one of the decomposition products, was analyzed on a 2 m molecular sieve 5A column at 35 "C. It was reduced at 400 "C to methane prior to detection using a Chrompak methanyzer with a carrier composed of 50% hydrogen and 50% argon. These analyses gave the ratio [CO]/[C&]. From these ratios and the known methane concentration obtained in the Porapak N analyses, the concentration of CO could be calculated for each run. The ratio [CO]/[CH4] in a standard mixture of methane and carbon monoxide was determined periodically in order to verify complete conversion of the latter to methane in the methanyzer. The concentrations of the reaction products Cs(pr)i were calculated from their GC peak areas from the following relations? C,(pr)i = [A(pr,)JS(pr,)]{ C5(3,5-dimethylisoxazole),,/

A(3,5-dimethylisoxazole),) (III) C5(3,5-dimethylisoxazole), = (pl x %(3,5-dimethylisoxazole)~~/~~}/l~RT~ (IV)

A( 3,5-dimethylisoxazole), =

+

A(3,5-dimethylisoxazole),

x N ( p r , ) A(prJJS(prJ (V>

In these relations C5(3,5-dimethylisoxazole)~ is the concentration

11438 J. Phys. Chem., Vol. 99, No. 29, 1995

c

c

Lifshitz et al.

'0

a V

L

a n

E

1 0.1

0.01

.

,?7/

(7)

0 .001 ..

850

900

950

1000

1050

T (K) Figure 3. Product distribution in 3,5-dimethylisoxazole showing the nitrogen-containing species: (1) 3,5-dimethylisoxazole, (2) 2-methyl3-oxobutyronitrile, (3) acetonitrile, (4) acrylonitrile, (5) propylnitrile, (6) 2-methylpropylnitrile, (7) crotonitrile (cis and trans) and vinylacetonitrile, (8) hydrogen cyanide.

c.

C

a 2 a Q a

0

E

0.001

850

800

950

1000

1050

Figure 4. Product distribution in 3,5-dimethylisoxazole showing the oxygen-containing species (3,5-dimethylisoxazole is also shown for comparison): (1) 3,5-dimethylisoxazole, (2) carbon monoxide, (3) acetaldehyde, (4) 3-buten-2-one, (5) methyl ketene (assumed to be identical to acetonitrile, see text).

of 3,5-dimethylisoxazole behind the reflected shock prior to decomposition and A(3,5-dimethylisoxazole)o is the calculated GC peak area of 3,5-dimethylisoxazole prior to decomposition (eq V), where A(pri), is the peak area of a product i in the shocked sample, S(pri) is its sensitivity relative to 3S-dimethylisoxazole, and N(prJ is the number of its carbon atoms, Q ~ / Q I is the compression behind the reflected shock, and T I is room temperature. The sensitivities of the various products to the FID and NPD were determined relative to 3,5-dimethylisoxazolefrom standard mixtures. The areas under the GC peaks were integrated with a Spectra Physics Model SP4200 computing integrators and were transferred after each analysis to a PC for data reduction and graphical presentation.

850

900

850

1000

1050

Figure 5. Product distribution in 3,5-dimethylisoxazole showing the hydrocarbons (3,5-dimethylisoxazole is also shown for comparison): (1) 3,5-dimethylisoxazole, (2) methane, (3) ethylene, (4) ethane, ( 5 ) acetylene, (6) allene, (7) propyne, (8) butadiene.

obtained in the pyrolysis of isoxazole and 3-methylisoxazole, the main product in 3,5-dimethylisoxazole is a straight-chain isomerization product. The decomposition product of the highest abundance is acetonitrile, followed by carbon monoxide, methane, and ethane. The balance of oxygen vs carbon and that of nitrogen vs carbon among the decomposition products are shown in Figures 6 and 7, respectively. The concentrations of the oxygencontaining species are plotted in Figure 6 against one-fifth the sum of the concentrations of all the decomposition products (including the oxygen-containing species), each multiplied by the number of its carbon atoms. The concentration of methyl ketene was assumed equal to that of acetonitrile assuming that both molecules are formed in the same unimolecular process as will be shown later. One-fifth is the ratio of oxygen to carbon atoms in the reactant molecule. The 45" line in the figure represents a complete mass balance. As can be seen, within the limit of the experimental scatter, there is no major deviation from an oxygen-carbon balance over the temperature range of the investigation. Figure 7 shows a similar plot for the nitrogen-carbon balance. Figure 8 shows the rate constant for the overall reaction (isomerization and decompositions) of 3,5-dimethylisoxazole, calculated as a first-order rate constant from the relation

ktota,= -In{ [3,5-dimethylisoxazole]J

[3,5-dimethylisoxazole],)/t (VI) exp(-59.5 x The value obtained is given by ktotal = 1015.57 103/RT) s-' where R is expressed in units of cal/(K mol). Figure 9 shows an Arrhenius plot of the first-order rate constant of the isomerization reaction calculated from the

relation

111. Results Figures 3-5 show the distribution of reaction products of the nitrogen-containing species, oxygen-containing species, and the hydrocarbons. The lines are based on some 25 tests which were run with mixtures containing 0.5% 3,5-dimethylisoxazole in argon. Details of the experimental conditions and the distribution of reaction products are given in Tables 1-3. The percent of a given product in the total sample shown in the table corresponds to its mole fraction, irrespective of the number of carbon atoms it contains. In distinction from the results

k(isomerization) =

[CH,COCH(CH,)CN], [3,5-dimethylisoxazole], - [3,5-dimethylisoxazolel,k"a' (VII) The value obtained is kisome~zat,on - 1015.90exp(-61.5 x lo3/ RT) s-', Figures 10-13 show first-order rate constants for production of various reaction products plotted against reciprocal temperature. They were calculated from a relation similar to (VII),

J. Phys. Chem., Vol. 99, No. 29, 1995 11439

Isomerization and Decomposition of 3,5-Dimethylisoxazole

TABLE 1: Experimental Conditions and Product Distribution of Nitrogen-Containing Species T5,K 833 881 886 902 902 907 914 918 919 927 932 939 943 947 947 948 956 959 968 968 973 974 980 994 995 996 1000 1001 1010 1014 1036 1037 1040 1040 1052

t, ms 2.10 2.20 2.40 2.49 2.30 1.99 2.29 2.16 2.20 2.28 2.41 2.22 2.39 2.39 2.49 2.33 2.25 2.57 2.33 2.57 2.43 2.35 2.56 2.44 2.49 2.35 2.45 2.38 2.44 2.42 2.55 2.38 2.38 2.39 2.33

In units of

Ga 2.72 3.37 3.26 3.15 3.07 2.91 3.02 3.04 2.85 2.58 2.77 2.96 2.80 2.40 2.69 2.64 2.69 2.67 2.50 2.61 2.42 2.37 2.42 2.37 2.26 2.23 2.32 2.26 2.32 2.05 2.02 2.10 2.56 2.15 1.86

DMIb 99.79 98.56 97.42 97.03 97.31 95.39 94.87 93.03 93.17 87.61 90.62 83.48 85.74 82.11 83.92 82.49 77.59 76.21 68.16 68.69 53.63 57.13 53.66 39.69 41.16 43.60 40.19 35.22 30.61 23.17 10.89 10.16 8.19 9.77 3.80

isomer 0.071 0.796 0.741 1.562 1.695 2.125 2.881 3.613 3.565 5.035 5.665 9.091 7.670 8.527 8.905 9.151 11.94 13.61 18.12 17.08 16.93 19.18 18.59 25.19 25.74 25.28 28.79 27.95 28.89 31.55 29.32 28.15 29.82 30.06 18.38

CH3CN 0.011 0.234 0.488 0.419 0.365 0.855 0.602 1.052 1.055 2.532 1.263 2.458 2.188 2.998 2.384 2.755 3.433 3.569 4.699 4.528 8.745 7.142 7.998 9.954 9.069 8.747 9.121 11.88 12.67 12.83 14.20 18.42 14.07 13.13 12.99

2-mpn'

CzH3CN

HCN

0.0034 0.0031 0.0132 0.0075 0.0207 0.0178 0.0322 0.0223 0.0293 0.0402 0.0357 0.0845 0.0664 0.159 0.138 0.177 0.268 0.256 0.266 0.283 0.365 0.343 0.559 1.379 1.492 1.171 1.517 2.641

0.00096 0.00196 0.00196 0.00408 0.0056 0.0006 0.0076 0.0156 0.0196 0.0140 0.0364 0.0328 0.0448 0.0348 0.0524 0.0428 0.0428 0.111 0.0908 0.214 0.212 0.252 0.432 0.460 0.432 0.420 0.540 0.724 0.828 1.736 1.804 1.432 1.888 5.371

0.0014 0.0038 0.0020 0.0084 0.0053 0.0167 0.0153 0.0374 0.0219 0.0434 0.0422 0.0584 0.0479 0.0572 0.0590 0.0625 0.126 0.0985 0.203 0.182 0.21 1 0.272 0.315 0.264 0.282 0.347 0.436 0.469 0.694 0.577 0.448 0.606 0.601

CZHSCN

C3H5CNd

0.00013 0.0022 0.00059

0 . W 51

0.0013 0.0015 0.0040 0.0025 0.0060 0.0053 0.0083 0.0075 0.0080 0.0100 0.0097 0.0241 0.0179 0.0415 0.0353 0.0471 0.0665 0.0735 0.0623 0.0713 0.0877 0.117 0.125 0.228 0.197 0.154 0.215 0.451

0.0029 0.0012 0.0042 0.0033 0.0043 0.0056 0.0053 0.0069 0.0049 0.0048 0.0084 0.0166 0.0084 0.0217 0.0191 0.0176 0.0182 0.0198 0.0183 0.0204 0.0245 0.0253 0.0249 0.0249 0.0253 0.0054 0.0246 0.0975

moVcm3. 3,5-Dimethylisoxazole. 2-Methylpropylnitrile. cis-crotonitrile and trans-crotonitrile,vinylacetonitrile.

3 in the ring to that position. There are, however, marked differences in the preferred reaction paths that follow the N-0 kCH,CN = bond extensions in isoxazole and 5-methylisoxazole compared to 3,5-dimethylisoxazole. Following migrations of two hydro[CH,CNI, [3,5-dimethylisoxazole],- [3,5-dimethylisoxazole]t~totd gen atoms in isoxazole and one methyl group and one hydrogen atom in 5-methylisoxazole,the system is stabilized by elimina(VIII) tion of carbon monoxide and production of acetonitrile and propylnitrile, respectively. (See Introduction, reactions a and Values of E obtained from the slopes of the lines shown in b.) There are no products resulting from molecular rearrangeFigures 10- 13, and their corresponding preexponential factors ments. are summarized in Table 4. They were obtained from the low In 3,5-dimethylisoxazole the picture is different. The pretemperature-low conversion range in the figures before curferred path resulting from the N-0 bond extension is a vatures begin to occur. rearrangement to 2-methyl-3-oxobutyronitrile. This process The parameters for the decomposition products do not proceeds after a migration of one methyl group from position necessarily represent elementary unimolecular reactions. They 3 to position 4 in the ring has taken place. only provide rates of formation. Also, they do not imply that the decomposition reaction of 3,5-dimethylisoxazoleunder the conditions of the present experiment is unimolecular. As will be discussed later, the decomposition is composed of a large number of reactions involving free radicals. namely,

IV. Discussion 1. Isomerization. The main thermal reaction of 33dimethylisoxazole is an isomerization, contrary to decompositions that were found in isoxazoie and 5-methylisoxazole. The common feature to all the three molecules, however, is the mode of initiation of their main thermal reaction. In all the three molecules the reaction begins with an extension and/or cleavage of the weak N - 0 bond in the ring, a process that sensitizes the carbon atom in position 4. This sensitization makes position 4 in the ring a free radical in nature and allows migrations of hydrogen atoms and/or methyl groups from positions 5 and/or

The Arrhenius parameters for the isomerization process, namely, kiso = 10'5.90exp(-61.7 x 103/RT) s-l (Figure 9), clearly indicate that there is a substantial extension of the N - 0 bond in the ring. The activation energy is just 8-10 k c d m o l below the bond dissociation energy, which is estimated as 7073 kcdmol. This large extension makes the transition state a loose one, a fact that manifests itself by a large preexponential factor, namely, A = s-I. In view of the very large difference in stability of 33dimethylisoxazole and 2-methyl-3-oxobutyronitrile (AHf'298 of

Lifshitz et al.

11440 J. Phys. Chem., Vol. 99,No. 29, I995 TABLE 2: Experimental Conditions and Product Distribution of Oxygen-Containing Species T5, K

t,ms

C5"

833 881 886 902 902 907 914 918 919 927 932 939 943 947 947 948 956 959 968 968 973 974 980 994 995 996 1000 1001 1010 1014 1036 1037 1040 1040 1052

2.10 2.20 2.40 2.49 2.30 1.99 2.29 2.16 2.20 2.28 2.41 2.22 2.39 2.39 2.49 2.33 2.25 2.57 2.33 2.57 2.43 2.35 2.56 2.44 2.49 2.35 2.45 2.38 2.44 2.42 2.55 2.38 2.38 2.39 2.33

2.72 3.37 3.26 3.15 3.07 2.91 3.02 3.04 2.85 2.58 2.77 2.96 2.80 2.40 2.69 2.64 2.69 2.67 2.50 2.61 2.42 2.37 2.42 2.37 2.26 2.23 2.32 2.26 2.32 2.05 2.02 2.10 2.56 2.15 1.86

In units of of acetonitrile.

CH(CH3)COb 0.011 0.234 0.488 0.419 0.365 0.855 0.602 1.052 1.055 2.532 1.263 2.458 2.188 2.998 2.384 2.755 3.433 3.569 4.699 4.528 8.745 7.142 7.998 9.954 9.069 8.747 9.121 11.88 12.67 12.83 14.20 18.42 14.07 13.13 12.99

3-buten2-one CH3CHO 0.0822 0.0558 0.0776 0.481 0.269 0.0322 0.0787 0.186 0.607 0.621 0.0518 0.307 0.189 0.421 0.335 0.0367 0.353 0.0758 0.0944 0.657 0.150 0.606 0.141 0.0722 0.859 0.142 0.120 0.0779 0.637 0.144 0.760 0.0941 0.189 0.872 0.194 0.150 0.199 0.187 0.227 0.303 0.199 0.240 1.301 3.117 0.508 0.532 2.397 0.320 0.340 3.454 0.423 0.419 0.577 3.947 0.703 0.750 3.875 0.538 0.525 0.649 3.528 0.577 3.267 0.580 0.704 0.781 0.770 0.891 1.065 4.775 0.787 1.245 1.481 8.858 1.839 1.598 1.644 9.796 2.039 1.778 1.690 9.505 14.74 3.011 2.891 CO 0.0215

mol/cm3. Assuming equal to the concentration

3,5-dimethylisoxazole = -4.28 kcal/mo16 and AHf0298 of 2-methyl-3-oxobutyronitrile = -26.0 kcal/mo17), the latter is formed thermally excited with an excitation energy equal to Ea A H F 2 9 8 , where Ea is the activation energy of the isomerization

+

dimethylisoxazole is depleted, the decomposition from the state of thermal equilibrium determines the above-mentioned rate. Similar behavior where decompositions of thermally excited isomers played an important role in the overall kinetics was found in the isomerization-decomposition of oxirane,*methyloxirane? and 2,3-dimethylo~irane.'~In these systems too the isomerization products are considerably more stable than the oxirane and its derivatives. 2. Decompositions. a. 2-Methylpropylnitrile. A logical extension of reactions a and b (see Introduction) to 3 3 dimethylisoxazole would be migrations of one methyl group from position 3 and one from position 5 to position 4 in the ring to produce CO and of 2-methylpropylnitrile (c): CH.

Small quantities of the latter were indeed found in the postshock mixtures, but Figure 15 which compares production rates of the 2-methylpropylnitrile with and without toluene clearly shows that the production of this compound is depressed by the presence of toluene and thus involves free radicals. The outcome of these findings simply means that two simultaneous methyl group migrations to a single carbon atom (4) do not take place. The computer simulation shows that there are enough methyl and CHFHCN' radicals in the system to account for the observed concentration of 2-methylpropylnitrile: CH,'

+ CH3CHCN' - (CH,),CHCN

b. Acetonitrile. The concentration of acetonitrile is the highest among the decomposition products of 3,5-dimethylisoxazole. Cleavage of the ring with one methyl group migration from position 5 to position 4 in the ring can produce acetonitrile and methyl ketene:

-

process (61.5 kcal/mol). The thermochemistry of the 3,5dimethylisoxazole 2-methyl-3-oxobutyronitrileisomerization and decomposition is shown in Figure 14. The excitation energy of -83 kcdmol is sufficient, from a thermochemical viewpoint, to decompose and to undergo several elimination reactions: CH,COCH(CH,)CN H I . .. CH,CO-C--CN I: ? H,C+ H

iH : I CH,CO ;-C-CN f i I: H -CH,

-

CH,CO + CH,CHCN

-

CH,-CHCOCH,

+ HCN

CH,CHO + CH,-CHCN

As will be shown later, the decomposition of 2-methyl-3oxobutyronitrile, which is an important source of decomposition products, occurs from both a state of thermal excitation and a state of thermal equilibrium. At low temperatures the excited 2-methyl-3-oxobutyronitriledetermines the production rate of these products. At high temperatures, on the other hand, when the concentration of the latter becomes high and 3 3 -

In order to corroborate the assumption that the production of acetonitrile is indeed a unimolecular process, we have done the following two examinations, the results of which can be seen in Figures 16 and 17. Figure 16 shows GC peak ratios of acetonitrile and 3,5-dimethylisoxazole (in arbitrary units) in the presence and absence of toluene. In runs with toluene, a ratio of [toluene]d[3,5-dimethylisoxazole]oof approximately 10 was used. As can be seen, there is no influence of toluene on the production rate of acetonitrile. Figure 17 verifies the presence of methyl ketene in the postshock mixtures, a compound that is very hard to see in GC analyses. The figure shows two chromatograms (solid line and dashed line) of the same postshock mixture run on a GC with and without methyl alcohol. Methyl ketene tends to react with water absorbed in various locations on the way to the GC and produces propionic acid, which is also absorbed and hard to analyze. When the postshock mixture is collected in a bulb containing a small quantiy of methyl alcohol, methyl propionate is formed and can be, at least qualitatively, analyzed. The solid line in Figure 17 shows a chromatogram without methyl alcohol, and the broken line shows a chromatogram with methyl alcohol. As can be seen, two additional peaks appear. The big one is of methyl

J. Phys. Chem., Vol. 99, No. 29, 1995 11441

Isomerization and Decomposition of 3,5-Dimethylisoxazole

TABLE 3: Experimental Conditions and Product Distribution of Hydrocarbons Ts, K

t , ms

csa

833 881 886 902 902 907 914 918 919 927 932 939 943 947 947 948 956 959 968 968 973 974 980 994 995 996 1000 1001 1010 1014 1036 1037 1040 1040 1052

2.10 2.20 2.40 2.49 2.30 1.99 2.29 2.16 2.20 2.28 2.41 2.22 2.39 2.39 2.49 2.33 2.25 2.57 2.33 2.57 2.43 2.35 2.56 2.44 2.49 2.35 2.45 2.38 2.44 2.42 2.55 2.38 2.38 2.39 2.33

2.72 3.37 3.26 3.15 3.07 2.91 3.02 3.04 2.85 2.58 2.77 2.96 2.80 2.40 2.69 2.64 2.69 2.67 2.50 2.61 2.42 2.37 2.42 2.37 2.26 2.23 2.32 2.26 2.32 2.05 2.02 2.10 2.56 2.15 1.86

C2H6 0.0039 0.0500 0.144 0.0972 0.0776 0.214 0.155 0.250 0.299 0.784 0.325 0.750 0.646 1.158 0.714 0.841 1.162 1.161 1.698 1.592 3.735 2.937 3.429 4.462 4.410 4.001 3.601 4.989 5.839 5.492 8.125 8.512 8.232 8.287 9.182

CH4 0.0099 0.0438 0.115 0.158 0.114 0.185 0.207 0.178 0.216 0.398 0.216 0.461 0.403 0.575 0.446 0.503 0.635 0.878 1.109 0.984 2.192 1.814 2.125 2.910 2.816 2.505 2.344 3.358 3.945 3.288 4.770 5.318 5.516 5.151 6.142

c4H6

c 2 H 4

CzH2

0.0026 0.0081 0.0044 0.0063 0.0135 0.0128 0.0245 0.0254 0.0701 0.312 0.0799 0.065 1 0.113 0.0708 0.0872 0.131 0.131 0.219 0.198 0.473 0.401 0.498 0.722 0.724 0.675 0.609 0.850 1.071 1.063 2.054 2.248 2.266 2.45 1 4.684

0.0023 0.0017 0.0055 0.0034 0.0107 0.0072 0.0099 0.0073 0.0073 0.0163 0.0173 0.0256 0.0248 0.0491 0.0469 0.0559 0.0838 0.0889 0.0746 0.0613 0.115 0.142 0.156 0.321 0.380 0.331 0.449 1.167

ProPYne

allene

0.0033

0.0103 0.0057 0.0126 0.0083 0.0105 0.0209 0.0254 0.0454 0.03 19 0.0836 0.0754 0.0999 0.167 0.141 0.147 0.147 0.199 0.228 0.216 0.348 0.454 0.488 0.427 0.577

0.0026 0.0061 0.0049 0.0069 0.0055 0.0060 0.0095 0.0113 0.0139 0.0142 0.0323 0.0299 0.0314 0.0475 0.0470 0.0372 0.0381 0.0567 0.0660 0.150 0.126 0.125 0.128 0.132 0.217

0.0022 0.0039 0.0022 0.0037 0.0068 0.0087 0.0089 0.0077 0.0065 0.0120 0.0131 0.120 0.0245 0.0284 0.0338 0.0281 0.129

In units of 10-5 mol/cm3. nitrogen-carbon mass balance

10 :

1:

0.1 0.1

1

10

0.1

100

1

10

100

1/5 Z n,C, (arbit. units)

1 / 5 x t n,C, (arbit. units)

Figure 6. Oxygen-carbon mass balance among the decomposition

Figure 7. Nitrogen-carbon mass balance among the decomposition

products.

products.

propionate which is identified by its mass spectrum as shown in the figure, and the second (small) peak is of methyl acetate which was produced by a reaction of ketene with methyl alcohol. Although we could not determine the concentrationof methyl ketene quantitatively, its production in relatively large quantities provides a support for the assumption that acetonitrile and its counterpart methyl ketene are formed simultaneously in a unimolecular process. The rate constant obtained for this process, based on the production rate of acetonitrile and on the assumption that the latter does not seriously decompose" at these temperatures, is given by (Figure 10, Table 4) ~ H ~ = C N 10i5,60 exp(-62.1 x 103/RT)s-l. These Arrhenius parameters indicate that the N-0 bond extension in 3,5-dimethylisoxazole in the aforementioned path must be large before the molecule can be decomposed.

The isomerization reaction and the production of acetonitrile are the only unimolecular decompositions that exist in the decomposition of 3,5-dimethylisoxazole. In both there is single methyl group migration to position 4 in the ring following an almost complete rupture of the N - 0 bond in the ring. Both have high preexponential factors. All the other products that were identified are the result of free radical processes or direct reactions of 2-methyl-3-oxobutyronitrile. c. Other Decomposition Products. The relatively small concentration of hydrogen cyanide in the postshock mixtures and the inhibiting effect of toluene on the rate of its productions, as can be seen in Figure 18, indicates again that the production of hydrogen cyanide does not take place by a simple elimination from the ring. In addition to the four-center elimination from 2-methyl-3-oxobutyronitrileas suggested before, it can also be

Lifshitz et al.

11442 J. Phys. Chem., Vol. 99,No. 29, 1995

loool\

10. n v) w

1

Y 0.1

0

f

b

k - 3 . 7 2 ~ 10l6 exp(-59.5x1 03/RT) 3'

'

0.90

(CH,),CHCN I

" " " " " "

0.95

1.00

1.05

1.10

1.15

1/ Txl O3 (K.') Figure 8. First-order Arrhenius plot of the rate constant for the overall reaction (isomerization and decompositions) of 3,5-dimethylisoxazole.

r

loo

-

100.

,-

v) w

Y

0.1

I

2 '

0.90

0.95

1.00

1.05

1.10

1.15

0.01 0.90

0.95

1I T X o3 ~ (K,')

1.00

105

110

1.15

1 / Txl O3 (K.')

-

Figure 9. Arrhenius plot of the isomerization rate constant 3,5dimethylisoxazole 2-methyl-3-oxobutyronitrile. 1

1000 1

Figure 12. First-order Arrhenius plots for the production rates of 3-buten-2-one,allene, propyne, and butadiene. 1000

1

*

I

100 [ 10

-

A

v) v

l r

1 i

Y 0.1 i 0.01

0.1

a 0.01

f

'

0.001 0.90

[a t =

'

I

'

0.95

100

1.05

1.10

1.15

1/Tx103 (K.') Figure 10. First-order Arrhenius plots for the production rates of acetonitrile, acrylonitrile, propylnitrile, and methane.

obtained by hydrogen atom attack on acetonitrile'' which is a high concentration product: H'

t

+ CH3CN - HCN + CH,'

The decomposition of CH3CO' which is obtained from the decomposition of thermally excited 2-methyl-3-oxobutyronitrile is the main source of methyl radicals which lead in tum to the formation of methane, ethane, and other hydrocarbons. C 2 h CN', which is obtained in the same decomposition of 2-methyl3-oxobutyronitrile, is the main source for H atoms, acrylonitrile, propylnitrile, and other nitrogen-containing species. These processes are an integral part of the kinetic scheme. 3. Computer Modeling. On the basis of the mechanism described earlier, we constructed a reaction scheme consisting of 26 species and 32 elementary reactions. We did not make an attempt to model compounds that had very low concentrations

0.001 L 0.90

0.95

1.00

1.05

1.10

1.15

l / T x 1 0 3 (K") Figure 13. First-order Arrhenius plots for the production rates of

carbon monoxide, acetaldehyde, and hydrogen cyanide. (less than one-tenth of a percent at the highest temperature). The scheme is shown in Table 5. The first three columns in Table 5 give the three parameters A, n, and E for the forward rate constants, k, corresponding to the reactions as they are listed in the table. The rate constants are given as k = A P exp(-EIRT) in units of cm3, mol, s. Columns 4 and 5 give the values of the forward and reverse rate constants, at 900 K, as calculated from the rate parameters and the equilibrium constants of the reactions at the same temperature. In columns 6 and 7 the standard entropy and enthalpy change at 900 K for each reaction is given. Column 8 shows the source of the rate parameters used. The Arrhenius parameters for the majority of the reactions were either estimated by comparison with similar reactions for which the rate parameters are known or copied from similar reactions in the kinetic schemes of isoxazole or methylisoxazole

Isomerization and Decomposition of 3,5-Dimethylisoxazole

J. Phys. Chem., Vol. 99,No. 29, 1995 11443

TABLE 4: First-Order Arrhenius Parameters for Product Formation (880-1030 K) molecule total decomposition isomerization

log AIS-' 15.57 15.90 22.49 15.60 24.65 20.80 18.75 16.41 16.31 17.67 21.22 22.44 19.60 17.03 22.48 21.74 22.44

HCN acetonitrile acrylonitrile propylnitrile 2-methylpropylnitrile carbon monoxide methane ethane ethylene acetylene acetaldehyde 3-buten-2-one butadiene allene methylacetylene

E, kcdmol 59.5 61.7 103 62.1 110 95.9 83.8 67.9 68.2 73.5 93.0 103 86.0 74.1 102 104 103

I

without toluene

m-

0.05' 0.95

'

'

'

'

1 .oo

1.05

1.10

1 0 0 0 / T (K") Figure 16. Production rates of acetonitrile in the presence and absence of toluene. The latter has no effect on the rates, indicating that no

free radicals are involved in the process.

E

? -0

80

1 15 20 25 30 35 40 45 50 55 EO 65 70 75 80 85 90

m/z

1

1 CH,COCH(CH,)CNf

Figure 14. Thermochemistry of 3,5-dimethylisoxazole

3-oxobutyronitrile isomerization and decomposition.

.-M c

I

0.1 :

m-

\

- 2-methyl-

without toluene

a

-

20

15

2

35

30

Retention time (min)

Y

0

25

0.01

.

Figure 17. Gas chromatogram in the presence and absence of methyl

.-

E

P Y

0-

m

with toluene

\

m

I

\

alcohol. The appearance of a methyl propionate peak (its mass spectrum is also shown) indicates that methyl ketene is produced behind the shock (see text).

0

I

0.0003 0.95

'

'

'

1 .oo

I .05

1.10

l / T x l O ' (K.') Figure 15. Production rates of 2-methylpropylnitrile with and without

toluene. The latter decreases the production rate of 2-methylpropylnitrile, indicating that free radicals are involved in the process. decomposition.'.* Additional parameters were taken from various sources as listed in Table 5 . The thermodynamic properties of the species involved were taken from various literature The heat of formation of 3,5-dmisox(r), 36.6 k c d m o l at 298 K, was estimated on the basis of 93 k c d m o l bond strength for -CH2H. The heat of formation of CH3COC'(CH3)CN of 22.1 k c d mol at 298 K was estimated using the MST Structure and Properties program. Sensitivity analysis with respect to variations in the estimated AHf0 of the above-mentioned species showed that the results of the modeling were very insensitive to the estimated values.

without toluene

1:

0.1

.

0.01

with toluene 0.001 ' 0.85

" "

'

1 .oo

"

"

1.05

1.10

1 /TxlO' K-'

Figure 18. Production rates of hydrogen cyanide with and without toluene. The toluene decreases the production rate of hydrogen cyanide,

indicating that free radicals are involved in the process. Reactions 4-6 describe the decomposition and the four-center eliminations from the isomer in its thermal excited state, and reactions 9 and 10 describe the eliminations from a state of

Lifshitz et al.

11444 J. Phys. Chem., Vol. 99,No. 29, 1995 TABLE 5:

Reaction Scheme for the Isomerization and Decomposition of 3,5-Dimethylisoxazole (Values Are Given at 900 K)a

reaction 1. 3,Sdmisox CH3COCH(CH,)CN 2. 3,5-dmisox CH3CN + C2H4CO 3. 3,5-dmisox CH3CN C2H4 CO 4.3,5-dmisox CH3CHO C2H3CN 5.3,5-dmisox HCN CH~COCH=CHZ 6.3,5-dmisox CzH4CN CH3CO 7. 3,5-dmisox H 3,5-dmisox(r) H2 8. 3,5-dmisox CH3 3,5-dmisox(r) C h 9. CH3COCH(CH3)CN CH3CHO + C2H3CN 10. CH3COCH(CH3)CN HCN CH3COCH=CHz 11. 3,5-dmisox(r) CH3COCCH3CN 12. CH3COCCH3CN CH3CO C2H3CN 13. C2H4CN C2H3CN + H 14. C2H4CN C2H3 + HCN 15. CzHiCN H C2H4 HCN 16. C2H4CN H CzHsCN 17. CH3CO H CH3CHO 18. CH3CO C2H3 CH?COCH=CH2 19. CH3CO + Ar CH3 iCO Ar 20. CzH3CN C2H2 + HCN 21. CzH4CN CH3 (CH3)zCHCN 22. CH3 + CH3 CzH6 23. C2H3 + CzH3 C2& C2H2 24. CH3CN H CH3 HCN 25. CH3CN CH3 CH4 CHzCN 26. CH3CN H CH2CN H2 27. H CH2CN CH3CN 28. C2H3CN + H HCN + CzH3 29. CH3 CH2CN C2HsCN 30. CH3 CH3 CzHs H 31. C ~ H ~ - C Z % H 32. C2H3 -I-Ar C2H2 Ar H

--- ++ -- + + + + ---+ - + + -

+ +

-. -+ -

+ + +

+

+

+

+

+ +

A

-+

+

+ + + +

---+ + - + +

-+

+ +

+

A

n

E

7.94 x 1015 o 4.00 x 1015 o 1.00 x 10'4 o 5.00 x 1013 o 1.00 x 1014 o 1.21 1015 o 1.00 x 10'4 o LOO x 1013 o 4.50 x 10l2 0 5.00 x 10l2 0 7.94 x 1015 o 1.00 x 1014 o 1.00 x 1013 o LOO x 1013 o 8.00 x 1013 o 4.00 x 1013 o 2.00 x 10'3 o 2.00 x 1012 0 1.20 x 1015 o 1.78 x 10l2 0 5.44 x 10l2 -0.50 2.00 x 1013 o 1.20 x 1013 o 1.01 x 1013 o 2.00 x 10'2 0 2.61 10'3 o 4.86 x 10l2 0 1.01 1013 o 2.45 x lOI4 -0.50 2.80 1013 o 4.80 x lo8 1.19 3.00 1015 o

61.5 62.1 62.1 61.5 61.5 61.5 9.0 12.0 54.0 55.0

71.7 50.0 54.0 53.0

o o o o

12.5 68.0

o o o

7.0 9.0 8.0 0

7.0 0

13.5 37.2 32.0

kr

kr

9.26 3.30 8.25 x 5.83 x 1.17 x 1.41 6.53 x 1.22 x 3.48 x 2.21 x 3.09 x 7.23 x 7.73 x 1.35 8.00 x 4.00 x 2.00 x 2.00 x 1.11 x 5.48 x 1.81 x 2.00 1.20 x 2.02 1.31 x 2.98 x 4.86 x 2.02 x 8.17 x 1.44 x 1.45 x 5.01 x

10-1

10" 10'0

lo-' lo-' 10' lo-' 1013 1013 1013 10'2 1012

1011

1013 1013 1011 1Olo 10" 10l2 10" 10l2 1O-Io lo3 lo7

7.74 x 4.00 x 2.55 x 1.48 x 5.17 x 2.95 x 1.69 x 5.73 x 1.06 x 1.17 x 1.57 x 6.94 x 2.58 x 4.08 x 8.92 x 1.66 x 7.30 x 1.41 x 3.35 x 4.46 x 9.40 x 1.67 x 3.99 x 4.59 2.33 x 2.93 x 8.99 x 1.83 x 8.78 x 1.33 x 1.08 x 4.51 x

10-8 10-5

105 lo7

106 lo4 lo3

lo-" lo6 1Olo 109 10-2 10-7 10-7 10-7 1013 lo3 10-5 10-5 10-3

107 lo7 lo7 1O1O lOI4 10l2 10l6

AS,"

AH,"

11.10 42.52 74.71 45.94 42.71 49.75 6.49 -0.77 34.84 31.61 11.10 32.75 27.48 33.82 1.40 -31.25 -31.29 -40.86 31.42 32.70 -40.75 -40.71 -6.06 7.86 -2.29 4.77 -31.86 6.34 -37.92 -5.22 23.95 26.36

-23.28 -2.03 8.57 6.49 .46 46.63 -13.23 - 14.40 29.77 23.74 -28.28 29.94 48.02 49.43 -60.32 -92.01 -88.16 -95.60 14.33 41.96 -79.58 -91.04 -69.20 -7.93 -13.38 -12.21 -94.01 1.41 -83.96 11.63 38.04 40.55

source this invst. this invst. est est est est est est est est est est est est est est est est 18 acrilo est 15 16 est 1 1 1

est 1

17 15 18

AH: are expressed in units of kcaVmol and AS: in units of caV(K mol). Rate constants are expressed as k = AT" exp(-EIRT) in units of cm3,

s, mol, cal. 10

10

c

1 1

L

E

m

e

2 n m

-

01

fL

-m

0.1

E

E

Ool

0.01 0,001

I

0.001

900

950

1000

1050

0.0001

900

950

1000

1050

-f (K) Figure 19. Comparison between experimental and calculated mole percent of CHXN, C2H4, and C2H2.

Figure 20. Comparison between expenmental and calculated mole percent of CO, CZH~CN, and CZH~CN.

thermal equilibrium corresponding to the temperature Ts. The activation energies assigned for reactions 4-6 are the same as the one for the isomerization reaction (reaction 1). The preexponential factors Ad, As, and A6 were taken as relatively small fractions of Ai so as to fit best the experimental results. In Figures 19-22 we show comparisons between experimental and calculated mole percents of the isomerization and of the decomposition products. The plots are semilog plots so that comparisons at very low concentrations can be examined as well. The agreement for the products which are obtained by unimolecular reactions is very good. To a lesser extent is the agreement for the second generation of products. Of particular interest are the results of the calculations on the production of methyl vinyl ketone and acetaldehyde from 2-methyl-3-oxobutyronitrile as can be seen in Figures 23 and 24. The three lines on each figure correspond to (1) production

from the thermally excited isomer, ( 2 ) production from the isomer in a state of thermal equilibrium, and (3) the sum of (1) and (2). As can be seen, neither process 1 nor 2 can account alone for the production of these two species. Both processes are needed to account for their mole percent over the entire temperature range covered in this investigation. The contribution to the production of CH3CHO and CH3COCH=CH2 from the decomposition of the thermally equilibrated 2-methyl-3oxobutyronitrile is more pronounced at high temperatures since this decomposition has a high activation energy and since the concentration of the latter at a given reaction time is higher at higher temperatures. Table 6 shows the sensitivity spectrum of the reaction scheme. The sensitivity factor is defined in the tables as (A log CJA log k,) at t = 2 ms. SI,, = 1 means that a factor of 2 change in k, will cause a factor of 2 change in C,. SI,, was evaluated by changing k, by a factor of 2. Reactions that show no effect on

T (K)

J. Phys. Chem., Vol. 99, No. 29, 1995 11445

Isomerization and Decomposition of 3,5-Dimethylisoxazole

TABLE 6: Sensitivity Analysis at 900/1OOO Ka no.

isomer

1 1.0/0.57 2 -/-0.19 3 4 5 6

8 9 10 13 14 15 16 21 22 -0.34/-0.37 25 32 a

2,mpn

mvk

-1-0.54 -1-0.20

-/0.42 -/-0.17

C&

C2&

-1-0.58

-1-0.20 -/-0.18 1.0/0.77

C2H6 -1-0.34 -/-0.23

CzHz -1-0.14

CH3CHO HCN

CH3CN CzH3CN C2H5CN

0.10/0.75 -/0.33 -1-0.14 -1-0.16

-/-0.41 -/0.79

0.10/0.68 -/-0.16 0.80/0.31

0.97/0.51 0.64/0.39 -10.33 0.50/0.74

1.28/0.90 -0.98k0.35

-/-0.42 -/-0.19

0.96/0.45

0.99/0.75 1.60/ 1.27 -0.50/-0.1

CO

0.98/0.57 -/0.29

0.9810.78

1.25/0.68 -0.61/-

0.96/0.85

-/0.64

-/0.72 -/0.48

-/0.35 -/0.17 -/0.25 -/0.14

0.93/0.71

-/0.23 -/0.91

0.33

-/-0.30 0.3110.34 -/-0.17 OM/-0.20 0.82/0.55

-/-0.22 -0.33/-0.41

0.17/0.28 0.2010.36

0.33/0.16 0.27/-

(A log C/A log k ) at t = 2 ms. k is changed by a factor of 2. 2,mpn = 2-methylpropylnitrile. mvk = methyl vinyl ketone. 10

I

L

1

0.1

0.01

,' 7

isomer in

0.001

0.001

950

900

1000

900

1050

percent of C& and (CH3)zCHCN.

1050

T (K)

T (K)

Figure 21. Comparison between experimental and and calculated mole

1000

850

Figure 23. Experimental and calculated mole percent of CH3CHO.

The latter is obtained by a four-center elimination from 2-methyl-3oxobutyronitrile from both the thermally excited state (low temperatures) and thermal equilibrium (high temperatures).

1

lo

.

CH,COCH-CH,

e 0.01

p-

0.1

/'

t-A A

AA A

/Thermally excited Isomer

,

0.01

A

0.001'

Isomer In thermal equiiibrium

'.'"" 900

'

850

" ' " - ' '

1000

1050

T (K)

Experimental and calculated mole percent of CH2-CHCOCH3. The latter is obtained by a four-center elimination from 2-methyl-3-oxobutyronitrilefrom both the thermally excited state (low temperatures) and thermal equilibrium (high temperatures). Figure 24.

species which are formed by four-center elimination from the isomer are unaffected by k6. The other reactions in the scheme affect different products, which is self-evident from the reaction scheme.

V. Conclusions The thermal reactions of 3,5-dimethylisoxazole can be summarized in the following statements: 1. Contrary to isoxazole and 5-methylisoxazole, the main thermal reaction of 3,5-dimethylisoxazole is isomerization to

11446 J. Phys. Chem., Vol. 99, No. 29, 1995 2-methyl-3-oxobutyronitrile,which implies that migration of two methyl groups to one carbon atom (4) in the ring does not take place. 2. 2-Methyl-3-oxobutyronitrile is obtained with enough thermal energy to dissociate into free radicals and initiate chain reactions. 3. The unimolecular reactions of 3,5-dimethylisoxazolebegin with cleavage of the weak N - 0 bond in the ring which is characterized by large preexponential factors compatible with a biradical mechanism. 4. On the basis of a suggested mechanism, a reaction scheme containing 26 species and 32 elementary reactions successfully simulated production rates and distribution of reaction products.

Acknowledgment. This research was supported in part by a Grant from the G.I.F., the German-Israeli Foundation for Scientific Research and Development. The authors thank Professor P. Roth, who served as the cooperative investigator in this research, for his advice and encouragement. References and Notes (1) Lifshitz, A.; Wohlfeiler, D. J. Phys. Chem. 1992, 96, 4505. (2) Lifshitz, A,; Wohlfeiler, D. J . Phys. Chem. 1992, 96, 7367. (3) Lifshitz, A,; Tamburu, C.; Frank, P.; Just, Th. J . Phys. Chem. 1993, 97, 4085.

Lifshitz et al. (4)Tsang, W. In Shock Waves in Chemistry; Lifshitz, A., Ed.; Marcel Dekker: New York, 1991. (5) Lifshitz, A.; Bidani, M. J . Phys. Chem. 1989, 93, 1139. (6) Pedley, J. B.; Taylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds; Chapman and Hall: London, 1986. (7) Stein, S. E.; Rukkers, J. M.; Brown, R. L. NIST-Standard Reference Database 25. (8) Lifshitz, A.; Ben Hamou, H. J . Phys. Chem. 1983, 87, 1782. (9) Lifshitz, A.; Tamburu, C. J . Phys. Chem. 1994, 98, 1161. (10) Lifshitz, A.; Tamburu, C. J . Phys. Chem., submittedfor publication. (11) Lifshitz, A.; Moran, A,; Bidani, S. Int. J . Chem. Kinet. 1987, 19,

61. (12) Stull, D. R.; West”,

Jr., E. F.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds; John Wiley & Sons: New York,

1969. (13) Tsang, W.; Hampson, R. F. Phys. Chem. Ref. Data 1986,15, 1087. (14) Melius, K. BAC-MP4 Heats of Formation; Sandia National Laboratories: Livermore, CA, March 1993. (15) Westly, F.; Herron, J. T.; Cvetanovic, R. J.; Hampson, R. F.; Mallard, W. G. NIST-Chemical Kinetics Standard Reference Database 17, Ver. 5.0.

(16) Fahr, A.; Laufer, A. H. J . Phys. Chem. 1990, 94, 726. (17) Frank, P.; Braun-Unkhoff, M. Proc. 16th In?. Symp. Shock Tubes Waves 1987, 83. (18) Warnatz, I. In Combustion Chemistry; Gardiner, Jr., W. C., Ed.; Springer: New York, 1984; p 197. Jp9507307