| Abstract
| - Isomerization and decomposition of 1,2-benzisoxazole were studied behind reflected shock waves in apressurized driver, single-pulse shock tube. It isomerizes to o-hydroxybenzonitrile, and no fragmentation isobserved up to a temperature where the isomerization is almost complete (∼1040 K at 2 ms reaction time).The isomerization experiments in this investigation covered the temperature range 900−1040 K. The lack offragmentation is in complete contrast to the thermal behavior of isoxazole, where no isomerization was observedand 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 productsappear in the post shock samples of 1,2-benzisoxazole. The product distribution is exactly the same regardlessof whether the starting material is 1,2-benzisoxazole or o-hydroxybenzonitrile, indicating that over thistemperature range the 1,2-benzisoxazole has completely isomerized to o-hydroxybenzonitrile prior tofragmentation. Two potential energy surfaces that lead to the isomerization were evaluated by quantum chemicalcalculations. One surface with one intermediate and two transition states has a high barrier and does notcontribute to the process. The second surface is more complex. It has three intermediates and four transitionstates, but it has a lower overall barrier and yields the isomerization product o-hydroxybenzonitrile at a muchhigher rate. The unimolecular isomerization rate constants k∞ at a number of temperatures in the range of900−1040 K were calculated from the potential energy surface using transition-state theory and then expressedin an Arrhenius form. The value obtained is kfirst = 4.15 × 1014 exp(−51.7 × 103/RT) s-1, where R is expressedin units of cal/(K mol). The calculated value is somewhat higher than the one obtained from the experimentalresults. When it is expressed in terms of energy difference it corresponds of ca. 2 kcal/mol.
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