Structures of 4-substituted thioanisole radical cations studied by time-resolved resonance Raman spectroscopy during pulse radiolysis and theoretical calculations

Abstract

To understand dimerization-reactivity, structures of 4-substituted thioanisole radical cations (ArSCH₃˙⁺) in aqueous solution were studied by the nanosecond time-resolved resonance Raman (ns-TR³) spectroscopy during pulse radiolysis and density functional theory (DFT) calculations. The downshift of the CC stretching mode of the benzene ring and the upshift of the C–S stretching mode upon oxidation were observed with aromatic sulfides such as 4-hydroxymethylthioanisole (MTPM), suggesting formation of a semi-quinoidal structure. For the 4-methoxythioanisole radical cation (MTA˙⁺), weak C_Ph–O stretching was observed additionally. Based on the DFT calculations, semi-quinoidal structures including an S atom and benzene ring were indicated for MTPM˙⁺ and so on, in which the positive charge and spin are localized on the S-atom and C₄-position, respectively. On the other hand, the quinoidal structure including the S and O atoms and benzene ring was indicated for MTA˙⁺. The results of DFT calculation agreed well with TR³ spectra of ArSCH₃˙⁺. The reactivities of ArSCH₃˙⁺ toward dimerization were discussed in terms of the conjugation between S non-bonding electrons and π-electrons of the aromatic system.