Photoelectron spectroscopic study of the oxyallyl diradical.

The photoelectron spectrum of the oxyallyl (OXA) radical anion has been measured. The radical anion has been generated in the reaction of the atomic oxygen radical anion (O(•-)) with acetone. Three low-lying electronic states of OXA have been observed in the spectrum. Electronic structure calculations have been performed for the triplet states ((3)B(2) and (3)B(1)) of OXA and the ground doublet state ((2)A(2)) of the radical anion using density functional theory (DFT). Spectral simulations have been carried out for the triplet states based on the results of the DFT calculations. The simulation identifies a vibrational progression of the CCC bending mode of the (3)B(2) state of OXA in the lower electron binding energy (eBE) portion of the spectrum. On top of the (3)B(2) feature, however, the experimental spectrum exhibits additional photoelectron peaks whose angular distribution is distinct from that for the vibronic peaks of the (3)B(2) state. Complete active space self-consistent field (CASSCF) method and second-order perturbation theory based on the CASSCF wave function (CASPT2) have been employed to study the lowest singlet state ((1)A(1)) of OXA. The simulation based on the results of these electronic structure calculations establishes that the overlapping peaks represent the vibrational ground level of the (1)A(1) state and its vibrational progression of the CO stretching mode. The (1)A(1) state is the lowest electronic state of OXA, and the electron affinity (EA) of OXA is 1.940 ± 0.010 eV. The (3)B(2) state is the first excited state with an electronic term energy of 55 ± 2 meV. The widths of the vibronic peaks of the X̃ (1)A(1) state are much broader than those of the ã (3)B(2) state, implying that the (1)A(1) state is indeed a transition state. The CASSCF and CASPT2 calculations suggest that the (1)A(1) state is at a potential maximum along the nuclear coordinate representing disrotatory motion of the two methylene groups, which leads to three-membered-ring formation, i.e., cyclopropanone. The simulation of b̃ (3)B(1) OXA reproduces the higher eBE portion of the spectrum very well. The term energy of the (3)B(1) state is 0.883 ± 0.012 eV. Photoelectron spectroscopic measurements have also been conducted for the other ion products of the O(•-) reaction with acetone. The photoelectron imaging spectrum of the acetylcarbene (AC) radical anion exhibits a broad, structureless feature, which is assigned to the X̃ (3)A'' state of AC. The ground ((2)A'') and first excited ((2)A') states of the 1-methylvinoxy (1-MVO) radical have been observed in the photoelectron spectrum of the 1-MVO ion, and their vibronic structure has been analyzed.