Raman and IR signatures of [Mo ₃ S ₄ ] ⁴⁺ and [Mo ₃ S ₁₃ ] ²⁻ molybdenum sulphide molecular catalysts for solar hydrogen evolution

Abstract Molybdenum sulphide clusters [Mo ₃ S ₄ ] ⁴⁺ and [Mo ₃ S ₁₃ ] ²⁻ are key molecular models for Mo–S–based catalysts and promising candidates for energy‐conversion applications. While their catalytic chemistry has been widely explored, there is a notable gap in the investigation of their vibrational properties. In this work, we provide reference Raman spectra of both clusters, with a complete analysis of their vibrational modes from experimental and theoretical perspectives. High-quality crystalline samples were synthesised and characterised by scanning electron microscopy and energy-dispersive x -ray spectroscopy. Raman measurements were performed using 488 and 532 nm excitation, and detailed Lorentzian fitting enabled identification of all density functional theory (DFT)-predicted Raman-active modes. For [Mo ₃ S ₄ ] ⁴⁺ , prominent bands near 200, 350, and 450 cm ⁻¹ are assigned to Mo–S–Mo bending, Mo–S stretching, and terminal S vibrations, respectively. For [Mo ₃ S ₁₃ ] ²⁻ , the most intense peaks occur at 285, 329, 360, 453, and 518 cm ⁻¹ , corresponding to mixed Mo–S vibrations and the strong symmetric stretching of terminal S–S ligands. Complementary DFT-calculated infrared (IR) spectra show that [Mo ₃ S ₁₃ ] ²⁻ exhibits a vibrational profile closely matching its Raman activity, whereas [Mo ₃ S ₄ ] ⁴⁺ displays additional intense IR bands at ∼230, 270, and 320 cm ⁻¹ , which are weak or absent in the Raman spectra. Finally, Raman spectroscopy proved more sensitive than x -ray diffraction for identifying these materials when supported on other types of materials such as Sb ₂ Se ₃ . This work offers a detailed vibrational reference for [Mo ₃ S ₄ ] ⁴⁺ and [Mo ₃ S ₁₃ ] ²⁻ , establishing Raman and IR spectroscopy as robust techniques for characterising Mo–S molecular clusters in both fundamental and applied research.