Photoelectron Spectroscopic Study of the Oxyallyl Diradical

Abstract

ABSTRACT: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 selfconsistent 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 theX 1 A 1 state are much broader than those of theã 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 ofb 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 theX 3 A 00 state of AC. The ground ( 2 A 00 ) and first excited ( 2 A 0 ) 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.

The synergy between qualitative theory, quantitative calculations, and direct experiments in understanding, calculating, and measuring the energy differences between the lowest singlet and triplet states of organic diradicals

“…90,91 In addition, the energy difference between the lowest energy peaks in the singlet and triplet vibrational progressions is 1.3 kcal/mol. This spacing, with the singlet peak at lower energy than the triplet, is, perhaps fortuitously, in essentially exact agreement with highest level calculations 94,95 of E ST for OXA.…”

“…13 contributes to the 1 A 1 wave function about the same amount that this type of structure contributes to the ground state of acetone. 92 Since the 3 B 2 state has a larger partial  bond to oxygen than the 1 B 2 state, it can be predicted that both 1 The best calculations performed to-date -(4/4) CASPT2 94 and EOM-SF-CCSD(dT), 95 both with an aug-cc-PVTZ basis set, predict a singlet ground state with E ST = -1.3 and -1.5 kcal/mol, respectively. Experimentally, OXA •-was generated by the reaction of acetone with O •-.…”

Investigation into 9(S)-HPODE-derived allene oxide to cyclopentenone cyclization mechanism via diradical oxyallyl intermediates

“…13 The singlet and triplet states of oxyallyl diradical have been observed directly by photoelectron spectroscopy and the singlet oxyallyl diradical was found to be 1.3 kcal/mol lower in energy than the triplet. 28, 32 Large basis set CASPT2 and EOM-SP-CCSD(dT) calculations agree with this experimental observation. 10, 28, 32 …”

Once upon Anion: A Tale of Photodetachment

“…This result demonstrated that transition-state features need not be minuscule contributors to a photoelectron spectrum but, in fact, can even be the dominant signature in a photoelectron spectrum (54)! More recently, we have observed the photoelectron spectrum of the oxyallyl anion, a diradical with a ground-state singlet state and the lowest triplet state only 55 meV higher in energy (55,56). High-level calculations suggest that the singlet state is not a minimum at the anion geometry but that it lies along the reaction pathway for ring closure to cyclopentanone.…”