Raman and IR signatures of [Mo$_3$ S$_4$ $^{4+}$ and [Mo$_3$ S$_13$ $^{2-}$ molybdenum sulphide molecular catalysts for solar hydrogen evolution

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.