Electronic spectra of (H2S)n+ (n = 2–6) and [(H2S)2–(H2O)m]+ (m = 1–2) in the gas phase

Abstract

A hemibond, a two-center, three-electron non-classical covalent bond, forms between a singly occupied and a fully-occupied non-bonding orbital. Hemibonded species exhibit a strong absorption in the near-ultraviolet (UV) to visible region, known as the charge resonance (CR) band, which serves as a well-established marker band for hemibond formation. Despite its significance, the influence of solvation effects on the CR band has not been fully understood due to the scarcity of direct comparison between condensed and gas phases. In this study, we investigated the CR band of hemibonded radical cation clusters, (H₂S)_n⁺ (n = 2–6) and [(H₂S)₂–(H₂O)_m]⁺ (m = 1–2), in the gas phase using mass-selected UV-vis photodissociation spectroscopy. For the (H₂S)_n⁺ clusters, the maximum absorption wavelength (λ_max) of the CR band displayed a blue-shift as the cluster size increased. The λ_max shift converged at n = 6, which corresponds to the completion of the first solvation shell around the hemibonded (H₂S)₂⁺ ion core. A comparison with the previously reported CR band in aqueous solution suggests that the first solvation shell plays a predominant role in influencing the electronic transition of the ion core. For the [(H₂S)₂–(H₂O)_m]⁺ clusters, the hemibonded ion core (H₂S)₂⁺ was disrupted upon the addition of two water molecules. Implications of the gas phase spectra for the previously observed spectrum in aqueous solution are discussed.