Revisiting the Raman spectrum of diacetyl with jet-cooled experiments and ab initio molecular dynamics

A comprehensive combined experimental and theoretical study on the Raman spectra of diacetyl (2,3-butanedione) is presented here. Using a custom-built, high-sensitivity Raman setup, suitable for working with gas and liquid samples, as well as with supersonic gas jets, we revisit and refine the vibrational Raman signatures of this prototypical α -dicarbonyl. Intrinsic drawbacks such as high viscosity, low vapour pressure, and fluorescence were overcome, enabling us to unveil a new low-frequency Raman feature down to 93 cm − 1 and a resolved doublet near 1000 cm − 1 (merged in the liquid phase due to intrinsic broadening). For a more accurate description of fundamental bands, supersonic-jet spectra reduce thermal congestion and hot-band contributions, allowing improved determination of fundamental transitions ( ν = 0 → ν ′ = 1 ). The experimental signatures are confronted with state-of-the-art calculations, namely anharmonic perturbative methods and ab initio molecular dynamics. While static simulations predict a C 2 h minimum-energy structure, dynamical simulations account for finite-temperature structural fluctuations, resulting in a more realistic description and improved agreement for vibrational features, in particular Raman intensities. This work aims to highlight the value of dynamical approaches to compute and interpret Raman spectra of medium-size molecules.