The effect of substituents on the stability and fragmentation of para benzoquinone molecular anion

Abstract

Quinones form a large group of organic compounds with various applications in industry, biological systems, and pharmacology. They can capture an electron with kinetic energy exceeding zero and keep it for a longer time. The principal structure responsible for electron capturing in quinones is the para-benzoquinone, PBQ. Several studies have been carried out on PBQ and its molecular anion, but less attention has been paid to its derivatives upon capturing an electron. As such, the focus of this study is to investigate the effects of substituents on the molecular anion properties of PBQ. These results will fill a void in available data and help in selecting new compounds as possible candidates for the synthesis of new electron acceptors. Both theoretical and experimental work were conducted to obtain the results. Electron donating substituents were found to decrease the stability of the molecular anion and led to the formation of more anion fragments under electron capture negative chemical ionisation mass spectrometry (ECNIMS) conditions while electron withdrawing substituents showed the opposite effect. The theoretical fragmentation schemes constructed for all dissociation channels in all molecules speculate that most of the fragment anions were formed due to loss of CO and/or C2H4 from the molecular anions, sometimes accompanied by migration of H-atom to a nearby or distant C-atom or elimination of H-atom/s. Electron donating derivatives retained several features of the spectra of PBQ itself. In addition to the pronounced molecular anion peaks and [M-CO]− , there were other significant peaks at m/z 41, 53 ,54, 55, 80 and 81 corresponding to the formation of [C2HO]− , [C3HO]− , [C3H2O]− , [C3H3O]− , [C5H4O/ C4O2] − and [C4HO2] − respectively with electron donating derivatives. Unlike electron donating derivatives, electron withdrawing derivative’s base peaks were due to the formation of Cl− or F− . The most common fragment anions apart from Cl− in chlorinated PBQs at m/z 41, 53 and 80 corresponds to [C2HO]− , [C3HO]− , and [C4O2] − , respectively. Density Functional Theory calculations on the electron affinities, reduction potentials and C=O vibrational frequencies, strongly supported the observations noticed in ECNI-MS. The destabilizing effect of electron donating groups decreased the electron affinity, reduction potential and C=O vibrational frequency of PBQ molecular anion while the stabilizing effect of electron withdrawing groups increased them. There was a linear correlation between the response of M− in ECNCI, electron affinities and reduction potentials.