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

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.