Position matters: competing O–H and N–H photodissociation pathways in hydroxy- and methoxy-substituted indoles

Abstract

H (Rydberg) atom photofragment translational spectroscopy (HRA-PTS) and complete active space with second order perturbation theory (CASPT2) methods have been used to explore the competing N–H and O–H bond dissociation pathways of 4- and 5-hydroxyindoles (HI) and methoxyindoles (MI). When 4-HI was excited to bound ¹L_b levels, (λ_phot ≤ 284.893 nm) O–H bond fission was demonstrated by assignment of the structure within the resulting total kinetic energy release (TKER) spectra. By analogy with phenol, dissociation was deduced to occur by H atom tunnelling under the barrier associated with the lower diabats of the ¹L_b/¹πσ*_(OH) conical intersection (CI). No evidence was found for a significant N–H bond dissociation yield at these or shorter excitation wavelengths (284.893 ≥ λ_phot ≥193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N–H bond fission is deduced to occur at λ_phot ≤ 271.104 nm, by direct excitation to the ¹πσ*_(NH) state. Analysis of the measured TKER spectra implies a mechanism wherein, as in pyrrole, the ¹πσ*_(NH) state gains oscillator strength by intensity borrowing from nearby bound states with higher oscillator strengths. HRA-PTS studies of 5-HI, in contrast, showed no evidence for O–H bond dissociation when excited on ¹L_b levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the ¹L_b/¹πσ* CI diabats in 5-HI is ∼60% greater than the corresponding area in 4-HI and O–H bond dissociation by tunnelling is thus much less probable. Only by reducing the wavelength to ≤ 255 nm were signs of N–H and/or O–H bond dissociation identified. By comparison with companion 5-MI studies, we deduce little O–H bond fission in 5-HI at λ_phot > 235 nm and that N–H bond fission is the dominant source of H atoms in the wavelength region 255 > λ_phot > 235 nm. The very different dissociation dynamics of 4- and 5-HI are traced to the position of the –OH substituent, and its effect on the overall electronic structure.