Gas-Phase Specific Reactivity of Isomeric 1,3-Benzodithiole Anions:  Tandem Mass Spectrometry and DFT Theoretical Studies

Abstract

1,3-Benzodithiole dideuterated at the dithioacetal carbon atom has been ionized in the chemical ionization source of a tandem triple quadrupole mass spectrometer under NICI condition. Two isomeric anions are formed by abstraction of an o-phenylic proton or through abstraction of a deuteron. The fragmentation patterns occurring under dissociative collisions differ and confirm the coexistence of two nonconverting carbanions. DFT calculations show that the dithioacetal anion is more stable than the phenylic. The nucleophilic reactivity toward CS 2 has been studied by collision-induced reactions in the collision cell of the tandem instrument. Both isomeric anion forms display characteristic fragmentations of the resulting [(M-H(D)) + CS 2 ] adduct anions, demonstrating a difference in reactivity: the dithioacetal anion reacts regioselectively at C and the o-phenylic at S. DFT calculations, performed with the aim of rationalizing this observed difference and understanding the formation of the diagnostic fragments, have been successful.

9 Organic gas phase ion chemistry

“…Gimbert, Arnaud and co-workers have examined the gas phase reactivity of two isomeric 1,3-benzodithiole anions, generated via deprotonation of 1,3-benzodithiole at either the dithioacetal carbon 55 or at the o-phenylic carbon 56 during negative ion chemical ionization (NICI), via experiment and theory. 183 Deuterium labelling at the dithioacetal position allowed these isomers to be distinguished. CID of these species resulted in distinctly different fragmentation products, confirming that the isomers do not interconvert, while ion-molecule reactions with CS 2 resulted in adduct formation for both species.…”