Anion photoelectron spectroscopy of deprotonated indole and indoline

Anion photoelectron spectra of deprotonated indole have been obtained utilizing several photon energies. The slow electron velocity-map imaging spectrum of indolide allows for the determination of the electron affinity (EA) of indolyl, 2.4315 ± 0.0017 eV. The equilibrium geometry of indolide was shown to minimally distort upon photodetachment with only ring distortion vibrational modes of A′ symmetry becoming significantly excited. Photoelectron spectra of indolide accessing the electronic ground state of indolyl displayed a photon energy dependence due to electron autodetachment. Combining the EA of indolyl with the previous work studying the dissociation energy of H-indolyl allows for a new independent measure of ΔacidH0KoN–Hindole ≤ 348.7 kcal/mol, which improves the previous measurement of the gas phase acidity. The anion photoelectron spectrum of deprotonated indoline consisted of a featureless broad band extending from ∼1.3 eV to 1.7 eV electron binding energy. The congested nature of the spectrum is likely due to the presence of multiple isomers of deprotonated indoline, including ring-opened structures.