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

[Image: see text] 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.