Deprotonation of Isoxazole: A Photoelectron Imaging Study

We report a photoelectron imaging study of gas-phase deprotonation of isoxazole in which spectroscopic data are compared to the results of electronic structure calculations for the anion products corresponding to each of three possible deprotonation sites. The observed photoelectron spectra are assigned to a mixture of the anion isomers. Deprotonation at the most acidic (C5) and the least acidic (C4) positions yields the respective C5- and C4-isoxazolide anions, while the reaction at the intermediate-acidity C3 site leads to a cleavage of the O–N bond and an opening of the ring in the anion. Following photodetachment, the ground states of neutral C5- and C4-isoxazolyl are assigned to be σ radicals (X 2A′), while the ground-state neutral derived from the ring-open C3-anion is a π radical (X 2A″). The relative intensities of the spectral bands exhibit sensitivity to the ion source conditions, giving evidence of competing and varying contributions of the dominant C5 and C3, as well as possible C4, deprotonation pathways.