Photodissociation dynamics of tetrahydrofuran at 193 nm

Abstract

Tetrahydrofuran (THF), a cyclic ether with the chemical formula C₄H₈O, can be considered the simplest analog of the deoxyribose backbone component of deoxyribonucleic acid (DNA). As such, it provides a useful model for probing the photochemistry of such biomolecular motifs. We present a velocity-map imaging study into the ultraviolet dissociation of THF at a wavelength of 193 nm. Excitation to the S₁ state occurs via a 3s ← n transition involving a lone-pair electron on the oxygen atom, and has been shown by other authors to result in rapid ring opening via cleavage of one of the C-O bonds to form a ring-opened C₄H₈O diradical, followed by C-C bond cleavage over a longer timescale to form either OCH₂ + C₃H₆ products (Channel 1a), HOCH₂ + C₂H₅ products (Channel 1b), or OCH₂CH₂ + C₂H₄ products (Channel 2). The C₂H₄O products formed via Channel 2 are unstable on the timescale of our experiment and dissociate further to form CH₃ and CHO. We also observe a number of minor products resulting from H or H₂ loss from the primary photofragments. The speed distributions observed for all photofragments are broad, indicating excitation of a range of rotational and vibrational states of the products. The angular distributions of the photofragments show an interesting speed dependence: the slowest products have almost isotropic angular distributions, but the magnitude of the recoil anisotropy increases monotonically with photofragment speed. The fastest products exhibit highly anisotropic angular distributions, with the recoil anisotropy parameter β approaching its limiting value of -1 (-0.75 for Channel 1 and -0.5 for Channel 2). This behaviour is attributed to the range of timescales over which the diradical intermediate dissociates into the observed photofragments. Rapid dissociation leads to fast photofragments which retain the correlation between the transition dipole moment for the S₁ ← S₀ excitation (which lies perpendicular to the ring) and the photofragment velocities (which lie predominantly in the plane of the ring). Slow dissociation results in a high degree of energy redistribution into internal modes, slower photofragments, and loss of correlation between the photofragment velocities and the transition dipole. The higher barrier associated with dissociation via Channel 2 suggests somewhat longer lifetimes for the diradical intermediate and is consistent with a corresponding reduction in the maximum observed value for β.