The photodissociation of oxetane at 193 nm as the reverse of the Paterno-Buchi reaction

We investigated the photodissociation of oxetane (1,3-trimethylene oxide) at 193.3 nm in a molecular-beam apparatus using photofragment-translational spectroscopy and selective photoionization. We measured time-of-flight (TOF) spectra and angular anisotropy parameters beta(t) as a function of flight time of products at m/z=26-30 u utilizing photoionization energies from 9.8 to 14.8 eV. The TOF distributions of the products alter greatly with the employed photon energy, whereas their beta(t) distributions are insensitive to the photon energy. Dissociation to H(2)CO+C(2)H(4) is the major channel in the title reaction. Three distinct dissociation paths with branching ratios 0.923:0.058:0.019 are responsible for the three features observed in the distribution of kinetic energy released in the channel H(2)CO+C(2)H(4). The observation of H(2) and H atoms, similar to 1% in branching, indicates that products H(2)CO and C(2)H(4) spontaneously decompose to only a small extent. Most HCO, C(2)H(3), and C(2)H(2) ions originate from dissociative photoionization of products H(2)CO and C(2)H(4). Except atomic H and H(2), the photoproducts have large angular anisotropies, beta >=-0.8, which reflects rapid dissociation of oxetane following optical excitation at 193.3 nm. The mechanisms of dissociation of oxetane are addressed. Our results confirm the quantum-chemical calculations of Palmer and provide profound insight into the Paterno-Buchi reaction.