Asuka Fujii

A hemibond (two-center three-electron bond), formed by a charge resonance interaction, is a characteristic structural motif in radical cation clusters. Hemibonded clusters exhibit strong absorption in the UV–visible region, which is attributed to the charge resonance (CR) transition and serves as a marker band for hemibond formation. Despite its significance, how microsolvation affects the CR transition of the hemibonded ion core remains unclear. In this study, we performed electronic spectroscopy for (N2O)n+ (n = 2–6) clusters containing the hemibonded (N2O)2+ ion core to elucidate the microsolvation effect via clustering of N2O molecules. In the observed spectra, a charge transfer (CT) transition was detected in addition to the CR transition for n = 3. Our results reveal that the CR and CT transitions predominantly lead to the formation of N2O+ and (N2O)2+ fragments, respectively. As microsolvation proceeds, the CR band appears to exhibit a significant red shift. This is attributed to the contribution of the CT excitation to the N2O+ fragment channel via the thermal dissociation pathway.

We performed infrared photodissociation spectroscopy and theoretical computations on gas-phase protonated water–acetonitrile clusters, H+[(H2O)m(CH3CN)n] (m = 1, n = 1–3; m = 2, n = 1–6; and m = 3, n = 5), to determine their cluster structures and compared their infrared spectra with previously reported spectra of protonated water clusters in acetonitrile solutions. For the (m = 2, n = 4) cluster, which serves as a model for the protonated water dimer in an acetonitrile solution, the band center of the hydrogen-bonded OH stretch absorption was observed at a lower frequency than that in the solution spectra. This indicates that the formation of the second and subsequent solvation shells significantly weakens the hydrogen bonding between the protonated water dimer (ion core) and the first solvation shell in the solution phase. On the other hand, the spectrum of the (m = 3, n = 5) cluster, a model of the protonated water trimer with a completed first solvation shell, showed a much better agreement with that of the solution phase, suggesting a tendency where an increase in the protonated cluster size reduces the influence of the second solvation shell.

While O-H···O hydrogen (H) bonds have been vigorously investigated, gas-phase studies of S-H···S H bonds remain very limited, especially in cases where multiple H bonds form a network. In this study, we investigate S-H···S H bonds in hydrogen sulfide clusters, (H₂S)_n (n = 2-6), in the gas phase with size-selective infrared spectroscopy based on vacuum ultraviolet photoionization detection. For clusters with n ≥ 3, the H-bonded SH stretching frequency remains nearly constant irrespective of the cluster size, a trend that contrasts with previously reported observations from the Ar matrix isolation spectra. (H₂S)_n (n > 3) clusters prefer to form dense three-dimensional H-bond networks to enhance the contribution of dispersion forces. These behaviors of (H₂S)_n significantly differ from those of the OH H-bond networks in (H₂O)_n. This study reveals that S-H···S H-bond networks in (H₂S)_n are characterized by weak cooperativity and a substantial contribution from dispersion forces.

Electronic spectroscopy of (H₂O)_n⁺ (n = 2 and 3) radical cluster cations was investigated in the ultraviolet-visible region. For (H₂O)₂⁺, the charge resonance band attributable to the hemibonded isomer, an elusive species previously observed spectroscopically only in helium droplets, was identified in the gas phase. This observation establishes electronic spectroscopy as a sensitive probe of hemibonded structures in water radical cations.

Infrared spectroscopy confirms the formation of HOCO radicals in ionized H ₂ O–CO ₂ systems.

The features of the OH stretching overtone region in micro-solvated protonated methanol were explored using near-infrared spectroscopy and anharmonic vibrational simulations.

The hemibond, a nonclassical covalent interaction arising from charge-resonance between a radical and a neutral molecule, represents a distinctive bonding motif in open-shell systems. Its role has been widely discussed in radical reactions, radiation chemistry, and related biochemical processes. While hemibonds involving water molecules have garnered considerable interest, it remains unclear whether these interactions can persist under bulk solvation conditions. Here, we investigate hemibond formation in gas-phase [H₂O-X]⁺ clusters and examine the structural evolution upon microhydration. Infrared photodissociation spectroscopy of [H₂O-X]⁺-(H₂O)_n (X = O₂ and CS₂; n = 0-2) reveals that the hemibonded structure of [H₂O-X]⁺ persists during microhydration. These results elucidate the interplay between charge-resonance and charge-(induced) dipole interactions that govern hemibond stability and suggest that certain molecules may retain the ability to form stable hemibonds with water even in aqueous environments.

Hemibonded [H ₂ O–X] ⁺ (X = Kr and Xe) clusters respond differently to microhydration. The hemibond in [H ₂ O–Kr] ⁺ is disrupted, whereas that in [H ₂ O–Xe] ⁺ remains intact.

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-2 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.