Thermal Decomposition of 2- and 4-Iodobenzyl Iodide Yields Fulvenallene and Ethynylcyclopentadienes: A Joint Threshold Photoelectron and Matrix Isolation Spectroscopic Study.

The thermal decomposition of 2- and 4-iodobenzyl iodide at high temperatures was investigated by mass-selective threshold photoelectron spectroscopy (ms-TPES) in the gas phase, as well as by matrix isolation infrared spectroscopy in cryogenic matrices. Scission of the benzylic C-I bond in the precursors at 850 K affords 2- and 4-iodobenzyl radicals ( ortho - and para -IC 6 H 4 CH 2 • ), respectively, in high yields. The adiabatic ionization energies of ortho -IC 6 H 4 CH 2 • to the X̃ + ( 1 A') and ã + ( 3 A') cation states were determined to be 7.31 ± 0.01 and 8.78 ± 0.01 eV, whereas those of para -IC 6 H 4 CH 2 • were measured to be 7.17 ± 0.01 eV for X̃ + ( 1 A 1 ) and 8.98 ± 0.01 eV for ã + ( 3 A 1 ). Vibrational frequencies of the ring breathing mode were measured to be 560 ± 80 and 240 ± 80 cm -1 for the X̃ + ( 1 A') and ã + ( 3 A') cation states of ortho -IC 6 H 4 CH 2 • , respectively. At higher temperatures, subsequent aryl C-I cleavage takes place to form α,2- and α,4-didehydrotoluene diradicals, which rapidly undergo ring contraction to a stable product, fulvenallene. Nevertheless, the most intense vibrational bands of the elusive α,2- and α,4-didehydrotoluene diradicals were observed in the Ar matrices. In addition, high-energy and astrochemically relevant C 7 H 6 isomers 1-, 2-, and 5-ethynylcyclopentadiene are observed at even higher pyrolysis temperatures along with fulvenallene. Complementary quantum chemical computations on the C 7 H 6 potential energy surface predict a feasible reaction cascade at high temperatures from the diradicals to fulvenallene, supporting the experimental observations in both the gas phase and cryogenic matrices.