title: On the electronic structures and electron affinities of the m-benzoquinone (BQ) diradical and the o-, p-BQ molecules: a synergetic photoelectron spectroscopic and theoretical study.
abstract: Electron affinity (EA) is an important molecular property relevant to the electronic structure, chemical reactivity, and stability of a molecule. A detailed understanding of the electronic structures and EAs of benzoquinone (BQ) molecules can help rationalize their critical roles in a wide range of applications, from biological photosynthesis to energy conversion processes. In this Article, we report a systematic spectroscopic probe on the electronic structures and EAs of all three isomers-o-, m-, and p-BQ-employing photodetachment photoelectron spectroscopy (PES) and ab initio electronic structure calculations. The PES spectra of the three BQ(●-) radical anions were taken at several photon energies under low-temperature conditions. Similar spectral patterns were observed for both o- and p-BQ(●-), each revealing a broad ground-state feature and a large band gap followed by well-resolved excited states peaks. The EAs of o- and p-BQ were determined to be 1.90 and 1.85 eV with singlet-triplet band gaps of 1.68 and 2.32 eV, respectively. In contrast, the spectrum of m-BQ(●-) is distinctly different from its two congeners with no clear band gap and a much higher EA (2.89 eV). Accompanied theoretical study confirms the experimental EAs and band gaps. The calculations further unravel a triplet ground state for m-BQ in contrast to the singlet ground states for both o- and p-BQ. The diradical nature of m-BQ, which is consistent with its non-Kekulé structure, is primarily responsible for the observed high EA and helps explain its nonexistence in bulk materials.
authors: Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui, China.
authors: Qiang
authors: Q
authors: Fu
authors: Jinlong
authors: J
authors: Yang
authors: Xue-Bin
authors: XB
authors: Wang
mesh_terms: D016227:Benzoquinones / Q000737:chemistry*; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D056951:Photoelectron Spectroscopy; D011789:Quantum Theory
chemical_list: D016227:Benzoquinones; D005609:Free Radicals; C004532:quinone