The thermal reactions of 2-methylindene diluted in argon were studied behind reflected shock waves in a 2 in. i.d. pressurized driver single-pulse shock tube over the temperature range 1050-1300 K and overall densities of ∼3 × 10 -5 mol/cm 3 . A plethora of products resulting from decompositions, isomerizations, and ring expansion were found in the post shock samples. They were naphthalene as the product of five-membered ring expansion, 1-and 3-methylindene due to isomerizations, and CH 4 , C 2 H 4 , C 2 H 6 , C 2 H 2 , benzene, toluene, ethylbenzene, styrene, phenylacetylene, and indene as the result of fragmentation. Very minute yields of some other compounds were also observed. Except for the isomerizations that take place from the reactant as a starting material, the production of all the other products involve free radical reactions. The initiation of the free radical mechanisms in the decomposition of 2-methylindene takes place via ejection of hydrogen atoms from sp 3 carbons and dissociation of the methyl group attached to the ring. The H atoms and the methyl radicals initiate a free radical mechanism by abstraction of hydrogen atoms from sp 3 carbons and by dissociative recombination of H atom and removal of a methyl group from the ring. In addition to these dissociation reactions there are several breakdown processes that involve cleavage of the five-membered ring to produce both stable and unstable products. The ring expansion process that leads to the production of a high yield of naphthalene takes place only from a methylene indene radical intermediate rather than methylindene itself, whereas isomerizations take place from both the radical intermediates and the molecule. The total decomposition of 2-methylindene in terms of a first-order rate constant is given by: k total ) 10 13.69 exp (-59.4 × 10 3 /RT) s -1 . A reaction scheme containing 34 species and 71 elementary reactions was composed and computer simulation was performed over the temperature range 1050-1300 K at 25 K intervals. The agreement between the experimental results and the model prediction for most of the species is satisfactory

Isomerization and decomposition of 1,2-benzisoxazole were studied behind reflected shock waves in a pressurized driver, single-pulse shock tube. It isomerizes to o-hydroxybenzonitrile, and no fragmentation is observed up to a temperature where the isomerization is almost complete (approximately 1040 K at 2 ms reaction time). The isomerization experiments in this investigation covered the temperature range 900-1040 K. The lack of fragmentation is in complete contrast to the thermal behavior of isoxazole, where no isomerization was observed and the main decomposition products over the same temperature range were carbon monoxide and acetonitrile. In a series of experiments covering the temperature range 1190-1350 K, a plethora of fragmentation products appear in the post shock samples of 1,2-benzisoxazole. The product distribution is exactly the same regardless of whether the starting material is 1,2-benzisoxazole or o-hydroxybenzonitrile, indicating that over this temperature range the 1,2-benzisoxazole has completely isomerized to o-hydroxybenzonitrile prior to fragmentation. Two potential energy surfaces that lead to the isomerization were evaluated by quantum chemical calculations. One surface with one intermediate and two transition states has a high barrier and does not contribute to the process. The second surface is more complex. It has three intermediates and four transition states, but it has a lower overall barrier and yields the isomerization product o-hydroxybenzonitrile at a much higher rate. The unimolecular isomerization rate constants kinfinity at a number of temperatures in the range of 900-1040 K were calculated from the potential energy surface using transition-state theory and then expressed in an Arrhenius form. The value obtained is kfirst=4.15x10(14) exp(-51.7x10(3)/RT) s-1, where R is expressed in units of cal/(K mol). The calculated value is somewhat higher than the one obtained from the experimental results. When it is expressed in terms of energy difference it corresponds of ca. 2 kcal/mol.

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The thermal decomposition of benzoxazole diluted in argon was studied behind reflected shock waves in a 2 in. i.d. single-pulse shock tube over the temperature range 1000-1350 K and at overall densities of approximately 3 x 10(-5) mol/cm(3). Two major products, o-hydroxybenzonitrile at high concentration and cyclopentadiene carbonitrile (accompanied by carbon monoxide) at much lower concentration, and four minor fragmentation products resulting from the decomposition were found in the postshock samples. They were, in order of decreasing abundance, benzonitrile, acetylene, HCN, and CH=C-CN and comprised of only a few percent of the overall product distribution. Quantum chemical calculations were carried out to determine the sequence of the unimolecular reactions that led to the formation of o-hydroxybenzonitrile and cyclopentadiene carbonitrile, the major products of the thermal reactions of benzoxazole. A potential energy surface leading directly from benzoxazole to cyclopentadiene carbonitrile could not be found, and it was shown that the latter is formed from the product o-hydroxybenzonitrile. In order that cyclopentadiene carbonitrile be produced, CO elimination and ring contraction from a six- to a five-membered ring must take place. A surface where CO elimination occurs prior to ring contraction was found to have very high barriers compared to the ones where ring contraction occurs prior to CO elimination and was not considered in our discussion. Rates for all the steps on the various surfaces were evaluated, kinetic schemes containing these steps were constructed, and multiwell calculations were performed to evaluate the mole percent of the two major products as a function of temperature. The agreement between the experimental results and these calculations, as shown graphically, is very good.

The decomposition of 4-methylpyrimidine was studied behind reflected shock waves in a pressurized driver single-pulse shock tube over the temperature range 1160-1330 K at overall densities of ∼3 × 10 -5 mol/cm 3 . A plethora of decomposition products, both with and without nitrogen was found in the post-shock mixtures. They were HCN, CH 3 CN, C 2 H 3 CN, pyrimidine, C 2 H 5 CN, cis-and trans-CH 3 CHdCHCN, CH 2 dCHCH 2 CN, CHtC-CN, and C 2 N 2 , among the nitrogen containing products, and CH 4 , C 2 H 2 , CH 3 CtCH, C 2 H 6 , CH 2 d CdCH 2 , and C 2 H 4 as products without nitrogen. It is suggested that the decomposition of 4-methylpyrimidine has two major initiation steps. (1) An ejection of a hydrogen atom from the methyl group that is connected to the ring, C 4 and (2) split of the methyl group from the ring, C 4 H 3 N 2 -CH 3 f C 4 H 3 N 2 • + CH 3 • . The ejection of H atom from the ring is much slower and does not contribute much to the total rate. The H atoms and methyl radicals initiate a chain mechanism by abstraction of an H atom from the methyl group and by dissociative attachment of an H atom and removal of a methyl group from the ring. The pyrimidyl (C 4 H 3 N 2 • ) and methylene pyrimidine (C 4 H 3 N 2 -CH 2 • ) radicals decompose by ring cleavage followed by breakdown of the open-ring radical. The total decomposition of 4-methylpyrimidine in terms of a first-order rate constant is given by k total ) 10 15.37 exp(-83.5 × 10 3 /RT) s -1 . A reaction scheme containing 33 species and 91 elementary reactions was constructed to account for the observed product distribution. A discussion on the decomposition mechanism is presented.