Infrared Multiple-Photon Decomposition of Cyclopentene

Article abstract of DOI:10.1021/j150647a054

The infrared multiple-photon decomposition of cyclopentene yields cyclopentadiene as the major hydrocarbon product (>95percent).The other products are 1,4-pentadiene and cis- and trans-1,3-pentadiene, along with trace amounts of fragmentation products.The 1,3-pentadienes have not been previously reported in cyclopentene pyrolysis.Typical conversions obtained by focussing the laser beam into the sample ranged from less than 1percent to as much as 20percent per 1E3 pulses with estimated beam waist fluences of approximately 150 J/cm², indicating that cyclopentene excitation is strongly bottlenecked.No products were detected with collimated beams of 1 J/cm^-2 and after 1.7E4 pulses.Reaction product yields increase with increasing wavelength in the fundamental absorption band centered at 1048 cm^-1.Excitation originating in the Q branch of this transition results in very small yields.Total C₅ yields increase rapidly with increasing fluence at 1029 and 1033 cm^-1.A simple model of dissociation in which the focused beam geometry is a circular hyperboloid of revolution predicts fluence-yield behavior that is consistent with experimental observations.Conversion of cyclopentene decreases with increasing cyclopentene pressure between 0.01 and about 0.5 torr, and thereafter increases rapidly with increasing pressure.On the other hand, addition of N2, up to about 200 torr, causes total C₅ yields to decrease monotonically over the entire pressure range.A limited number of experiments were done with 1-methylcyclopentene.At the same wavelength (1019 cm^-1), pressure, fluence, and weak field absorbance, dehydrogenation of 1-methylcyclopentene gives yields that are as much as 20 times greater than yields from cyclopentene.