Time-resolved photodissociation of the molecular ions of propyl phenyl ethers

Article abstract of DOI:10.1021/ja00114a021

The fragmentation rate of n-propyl phenyl ether molecular ions has been measured by time-resolved photodissociation (TRPD) in the region of 2 eV internal energy, giving first-order rate constants on the order of 10⁴ s^-1. RRKM fitting suggests a dissociation energy barrier of 1.47 eV. Decomposition of the isopropyl isomer is at least 30 times faster in this energy region. Dissociative photoionization of isopropyl phenyl ether in a supersonic jet by laser resonance-enhanced multiphoton ionization (REMPI) shows that hydrogen scrambling is insignificant for this isomer in the 4.5 eV energy range, in contrast to the earlier finding of extensive hydrogen randomization under those conditions for the n-propyl isomer. PhOD^?+ is the only fragment detected in the TRPD or the REMPI of (CD₃)₂CHOPh. The REMPI results (guided by SCF calculations) also indicate that the two methyl groups are not equivalent in the most stable conformation of neutral isopropyl phenyl ether. A quantitative reaction coordinate is proposed for the n-propyl isomer dissociation, which postulates separate transition states for hydride migration and for proton transfer and an intermediate [iPr⁺ PhO^?] ion-neutral complex. The faster dissociation of the isopropyl isomer is not consistent with the energetics predicted by UHF calculations, and it seems likely that that dissociation proceeds via a different mechanism.