Near-UV photolysis of substituted phenols, I: 4-fluoro-, 4-chloro- and 4-bromophenol

Abstract

The experimental techniques of H (Rydberg) atom photofragment translational spectroscopy and resonance-enhanced multiphoton ionisation time-of-flight spectroscopy have been used to investigate the dynamics of H atom loss processes from gas phase 4-fluorophenol (4-FPhOH), 4-chlorophenol (4-ClPhOH) and 4-bromophenol (4-BrPhOH) molecules, following excitation at many wavelengths, λ_phot, in the range between their respective S₁–S₀ origins (284.768 nm, 287.265 nm and 287.409 nm) and 216 nm. Many of the Total Kinetic Energy Release (TKER) spectra obtained from photolysis of 4-FPhOH show structure, the analysis of which reveals striking parallels with that reported previously for photolysis of bare phenol (M. G. D. Nix, A. L. Devine, B. Cronin, R. N. Dixon and M. N. R. Ashfold, J. Chem. Phys., 2006, 125, 133318). The data demonstrates the importance of O–H bond fission, and that the resulting 4-FPhO co-fragments are formed in a select fraction of their available vibrational state density. All spectra recorded at λ_phot ≥ 238 nm show a feature centred at TKER ∼5500 cm⁻¹. These H atom fragments show no recoil anisotropy, and are rationalised in terms of initial S₁ ← S₀ (π* ← π) excitation and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S₀) state levels carrying sufficient O–H stretch vibrational energy to allow efficient transfer to (and round) the Conical Intersection (CI) between the S₀ and S₂(¹πσ*) Potential Energy Surfaces (PESs) at larger R_O–H, en route to H atoms and ground state 4-FPhO products. The vibrational energy disposal in the 4-FPhO products indicates that parent mode ν_16a promotes non-adiabatic coupling at the S₀/S₂ CI. Spectra recorded at λ_phot ≤ 238 nm reveal a faster (but still isotropic) distribution of recoiling H atoms, centred at TKER ∼12 000 cm⁻¹, attributable to H + 4-FPhO products formed when the optically excited ¹ππ* molecules couple directly with the ¹πσ* PES. Parent mode ν_16b is identified as the dominant coupling mode at the S₁(¹ππ*)/S₂(¹πσ*) CI, and the resulting 4-FPhO radical co-fragments display progressions in ν_18b (the C–O in-plane wagging mode) and ν_7a (an in-plane ring breathing mode involving significant C–O stretching motion). Analysis of all structured TKER spectra yields a C–F bond dissociation energy: D₀(H–OC₆H₄F) = 29 370 ± 50 cm⁻¹. The photodissociation of 4-ClPhOH shows many similarities, though the 4-ClPhO products formed together with faster H atoms at shorter wavelengths (λ_phot ≤ 238 nm, by coupling through the S₁/S₂ CI) show activity in an alternative ring breathing mode (ν_19a rather than ν_7a). Spectral analysis yields D₀(H–OC₆H₄Cl) = 29 520 ± 50 cm⁻¹. H atom formation via O–H bond fission is (at best) a very minor channel in the photolysis of 4-BrPhOH at all wavelengths investigated. Time-dependent density functional theory calculations suggest that this low H atom yield is because of competition from the alternative C–Br bond fission channel, and that the analogous C–Cl bond fission may be responsible for the weakness of the one photon-induced H atom signals observed when photolysing 4-ClPhOH at longer wavelengths.