Kinetics of the thermal isomerizations of gaseous vinylcyclopropane and vinylcyclobutane

Rate constants for the thermal isomerizations of vinylcyclopropane to cyclopentene have been measured over a wide temperature range, 577-1054 K, in a static reactor and a single-pulse shock tube; they are well represented by log(A, s(-1)) = 14.3 and E-a = 51.7 kcal/mol. This activation energy is higher than two previously reported values by some 2 kcal/mol; rate constants at high temperatures are about a factor of 2 larger than calculated from the Arrhenius parameters derived from the earlier low-temperature data. The thermal decomposition and isomerization reactions of vinylcyclobutane to give ethene plus 1,3-butadiene and cyclohexene have also been followed in shock-tube kinetic studies at 839-965 K. Combining the new rate constants with those from two lower-temperature studies gives the following: for the total consumption of vinylcyclobutane, log(A, s(-1)) = 14.5 and E-a = 49.3 kcal/mol; for production of ethene and butadiene, log(A, s(-1)) = 14.5 and E-a= 49.8 kcal/mol; and for isomerization to cyclohexene, log(A, s(-1)) = 13.4 and E-a = 47.5 kcal/mol. These values are close to previously reported Arrhenius parameters based on lower temperature static-reactor kinetic investigations. The diradical transition structure for the vinylcyclobutane to cyclohexene isomerization appears to be strain free, while the transition structure for the vinylcyclopropane to cyclopentene conversion retains some 4.6 +/- 0.9 kcal/mol of ring strain and torsional strain energy.