Structures and Temperature Dependence of Hydrated Ion Clusters Probed by Infrared Photodissociation Spectroscopy Combined with a Temperature-Controlled Cryogenic Ion Trap Mass Spectrometer

Cryogenic ion trap mass spectrometry coupled with infrared photodissociation (IRPD) spectroscopy provides a powerful approach for probing the structures and conformational interconversion of gas-phase ions under well-defined thermal conditions. In this study, a home-built temperature-controlled cryogenic ion trap mass spectrometer coupled with messenger-tagging IRPD spectroscopy and a reflectron time-of-flight mass spectrometer (TOF MS) was reported. The instrument consists of a laser-ablation ion source, a temperature-controlled cryogenic Paul trap, mass-selection ion optics, a reflectron TOF MS for parent/fragment ion analysis, and a tunable infrared laser system. The temperature of the ion trap can be adjusted from 4 to 300 K, enabling precise control of the internal energy of trapped ions and the population of isomers. Collisions with buffer gas in the cryogenic environment efficiently cool flexible ions and stabilize weakly bound noncovalent complexes. In particular, the formation of messenger-tagged complexes becomes feasible. Particularly, the binding energies of messenger molecules are typically very low. Resonant absorption of a single infrared photon can induce efficient photodissociation. This process significantly enhances spectral sensitivity, effectively prevents band broadening from multiphoton effects, and improves both spectral resolution and signal-to-noise ratio by reducing saturation at high laser power. As a proof-of-concept application of the temperature dependence of ion structures, the hydrated gold cation cluster Au⁺(H₂O)₇ was investigated in the frequency region of OH stretching vibrational modes. Hydrated gold cation clusters Au⁺(H₂O)_n (n=1-23) were generated in the gas phase using a pulsed laser-ablation source. Au⁺(H₂O)₇ was selected the IRPD measurements at different trap temperatures. Previous theoretical studies predicted two low-lying isomers of Au⁺(H₂O)₇ with a small energy difference (≈3 kJ/mol). The global minimum isomer features a hydrogen-bonded ring motif formed by four water molecules, whereas the low-lying structure adopts an open-chain motif. At low temperatures, the IRPD spectrum of messenger-tagged complexes Au⁺(H₂O)₇(D₂) and infrared multiphotodissociation (IRMPD) spectra of Au⁺(H₂O)₇ at 4.2 K and 12 K agree well with the previously reported spectrum recorded in a supersonic jet. This confirmed that Au⁺(H₂O)₇ is dominated by the ring-containing global-minimum structure under cold temperatures. In contrast, the spectrum recorded near room temperature exhibits additional IRPD bands that are absent at low temperature. These new bands are assigned to the excitation of the hydrogen-bonded OH stretching modes associated with the higher-energy open-chain isomer. As the temperature increases, the water molecules in Au⁺(H₂O)₇ gradually convert from the low-temperature, ring-dominated structure to a slightly higher-energy open-chain configuration. Overall, this work demonstrated that temperature-controlled cryogenic ion trap mass spectrometry combined with IRPD spectroscopy can effectively reveal the microscopic structures of hydrated ions and their temperature-dependent conformational changes, providing a reliable experimental platform for studying the conformational dynamics of hydrogen-bonded clusters.