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

Physical Chemistry Chemical Physics Pub Date: 2011-07-13 DOI: 10.1039/C1CP21260E

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 1Lb levels, (λphot ≤ 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 1Lb/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 ≥ λphot ≥193.3 nm). Companion studies of 4-MI revealed different reaction dynamics. In this case, N–H bond fission is deduced to occur at λphot ≤ 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 1Lb levels. The present CASPT2 calculations assist in rationalizing this observation: the area underneath the 1Lb/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 λphot > 235 nm and that N–H bond fission is the dominant source of H atoms in the wavelength region 255 > λphot > 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.

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