Symmetry matters: photodissociation dynamics of symmetrically versus asymmetrically substituted phenols

Abstract

We report a combined experimental (H (Rydberg) atom photofragment translational spectroscopy) and theoretical (ab initio electronic structure and vibronic coupling calculations) study of the effects of symmetry on the photodissociation dynamics of phenols. Ultraviolet photoexcitation to the bound S₁(¹ππ*) state of many phenols leads to some O–H bond fission by tunneling through the barrier under the conical intersection (CI) between the S₁ and dissociative S₂(¹πσ*) potential energy surfaces in the R_O–H stretch coordinate. Careful analysis of the total kinetic energy release spectra of the resulting products shows that the radicals formed following S₁ ← S₀ excitation of phenol and symmetrically substituted phenols like 4-fluorophenol all carry an odd number of quanta in vibrational mode ν_16a, whereas those deriving from asymmetrically substituted systems like 3-fluorophenol or 4-methoxyphenol do not. This contrasting behavior can be traced back to symmetry. Symmetrically substituted phenols exist in two equivalent rotamers, which interconvert by tunneling through the barrier to OH torsional motion. Their states are thus best considered in the non-rigid G₄ molecular symmetry group, wherein radiationless transfer from the S₁ to S₂ state requires a coupling mode of a₂ symmetry. Of the three a₂ symmetry parent modes, the out-of-plane ring puckering mode ν_16a shows much the largest interstate coupling constant in the vicinity of the S₁/S₂ CI. The nuclear motions associated with ν_16a are orthogonal to the dissociation coordinate, and are thus retained in the radical products. Introducing asymmetry (even a non-linear substituent in the 4-position) lifts the degeneracy of the rotamers, and lowers the molecular symmetry to C_s. Many more parent motions satisfy the reduced (a′′) symmetry requirement to enable S₁/S₂ coupling, the most effective of which is OH torsion. This motion ‘disappears’ on O–H bond fission; symmetry thus imposes no restriction to forming radical products with vibrational quantum number v = 0. The present work yields values for the O–H bond strengths in 3-FPhOH and 4-MeOPhOH, and recommends modest revisions to the previously reported O–H bond strengths in other asymmetrically substituted phenols like 3- and 2-methylphenol and 4-hydroxyindole.