The Lowest Singlet and Triplet States of the Oxyallyl Diradical

Abstract

Small S–T splitting: The photoelectron spectrum of the oxyallyl radical anion (see picture) reveals that the electronic ground state of oxyallyl is singlet, and the lowest triplet state is separated from the singlet state by only (55±2) meV in adiabatic energy. Oxyallyls are reactive intermediates, whose participation has been postulated in the ring opening of cyclopropanones1 and allene oxides,2 in the Favorskii rearrangement,3 and in a variety of synthetically useful transformations.4 The parent oxyallyl (OXA) can be viewed as a derivative of trimethylenemethane (TMM), in which one methylene group is replaced by an oxygen atom (Scheme 1). Trimethylenemethane (TMM) and oxyallyl (OXA). TMM has been the subject of many theoretical and experimental studies.5 ESR measurements by Dowd and co-workers showed that TMM has a triplet ground state.6 This fact is in accord with Hund’s rule, since triplet TMM has a degenerate pair of half-filled, nonbonding π molecular orbitals (MOs), ψ2 and ψ3, that are nondisjoint7 (i.e., they share common atoms, Figure 1). Lineberger and co-workers determined the energy difference between the triplet and the lowest, planar, singlet states to be (16.1±0.2) kcal mol−1 in photoelectron spectroscopic measurements.8 There is excellent agreement between this experimental investigation8 and prior ab initio calculations9 with respect to the singlet–triplet (S–T) energy separation. Schematic representation of the π MOs (top lobes only) of TMM (D3h symmetry) and OXA (C2v). In the OXA radical anion, MOs are doubly occupied up to 2b1, and a2 is singly occupied. The oxygen substitution in OXA strongly lifts the degeneracy of ψ2 and ψ3 in TMM, stabilizing the 2b1 MO relative to the a2 MO (Figure 1).10 The best ab initio calculations have predicted that the S–T splitting in OXA should be close to zero, with the singlet state possibly even slightly lower than the triplet state.11 The calculations also indicate that singlet OXA has a strong CO π bond;10 that is, the first of the two resonance structures, shown in Scheme 1, provides a much better description of the lowest singlet state of OXA than the second resonance structure, which is zwitterionic. In contrast to these theoretical efforts to understand the electronic structure, no direct experimental detection of OXA has been reported to date. Herein, we report the photoelectron spectrum of the OXA radical anion. The spectrum gives the relative energies of the lowest singlet and triplet states of OXA and provides information about important vibrations in each state. Photoelectron spectra of the OXA radical anion. Here, σ0 and β are the total cross section and the anisotropy parameter, respectively, and P2(cosθ) is the second Legendre polynomial. Depending on the MO from which detachment takes place, a unique angular distribution of photoelectrons is observed, and this angular dependence is characterized by the β parameter (−1≤β≤2). Peaks B, C, D, and E exhibit identical peak spacings of (405±10) cm−1. Peak A shows two features that distinguish it from these other four peaks. First, the β value for peak A is more negative than those for the other peaks. Its relative intensity diminishes to a greater extent than those of the other peaks at θ=0°. Second, peak A is much broader than peaks B–E. These observations lead us to believe that peak A and peaks B–E are associated with two different electronic states of OXA. B3LYP/6-311++G(d,p) calculations15 predict that the electron binding energy (eBE) of the OXA radical anion, relative to the triplet (3B2) state of OXA, is 1.979 eV, which matches quite well with the eBE of peak B. The C-C-C bond angles of 3B2 OXA and of the 2A2 ground state of the radical anion at their equilibrium geometries are calculated to be 121.9° and 114.4°, respectively. Thus, photodetachment to form 3B2 OXA is expected to activate the C-C-C bending motion. The corresponding normal mode of 3B2 OXA has a harmonic vibrational frequency of 408 cm−1 according to the DFT calculations. Therefore, it is concluded that peaks B–E represent a vibrational progression in the C-C-C bending mode of 3B2 OXA. The singlet (1A1) state of OXA cannot be adequately described by a single electronic configuration.10, 11 Therefore, (4,4)CASSCF and CASPT2 calculations15 were performed on this state, which we associate with peak A. The CASSCF/cc-pVTZ calculations find that the planar 1A1 state is a very shallow energy minimum, which vanishes when zero-point corrections are made to the energy of 1A1 OXA and to that of the transition state for its disrotatory ring closure to cyclopropanone. Upon inclusion of dynamic electron correlation at the CASPT2 level of theory, a scan along the b1 coordinate for the ring closure finds that formation of cyclopropanone from 1A1 OXA is barrierless. Thus, the broad width of peak A can be attributed to lifetime broadening arising from the transition-state nature of 1A1 OXA. The spectrum also exhibits a broad band at the base of peak E. This broad band is located at (1680±50) cm−1 relative to peak A, and another broad band can be seen at higher eBE, with the same spacing, relative to peak E. CASSCF calculations predict a shortening of the CO bond by 0.057 Å upon electron loss from the 2A2 state of the radical anion to form 1A1 OXA.16 Thus, it is reasonable to suppose a manifestation of a vibrational progression in CO stretching for the 1A1 state in the spectrum. It should be noted that acetone has a fundamental frequency of 1731 cm−1 for the CO stretching mode;17 hence, the observed peak spacing confirms the theoretical prediction of substantial CO double-bond character in 1A1 OXA.10 The observation of 1A1 OXA attests to the utility of negative ion photoelectron spectroscopy as a means of studying transition states.18 In summary, both the ground and first excited states of OXA have been observed in the photoelectron spectrum of the corresponding radical anion. The electron affinity of OXA is (1.942±0.010) eV. The ground state is 1A1, but the adiabatic energy of the 3B2 state is only (55±2) meV (1.3 kcal mol−1) higher than that of the singlet ground state. The spectrum indicates that the 1A1 state has a strong CO π bond and undergoes barrierless ring closure to form cyclopropanone. The results of previous electronic structure calculations10, 11 and those of the calculations conducted in the present study are in excellent agreement with the experimental findings reported herein. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.