Hydrogen bond effects in multimode nuclear dynamics of acetic acid observed via resonant x-ray scattering

Viktoriia Savchenko1,2,3, Victor Ekholm4,5, Iulia Emilia Brumboiu1,6, Patrick Norman1, Annette Pietzsch7, Alexander Föhlisch7,8, Jan-Erik Rubensson4, Johan Gråsjö4,9, Olle Björneholm4, Conny Såthe5, Minjie Dong4, Thorsten Schmitt10, Daniel McNally10, Xingye Lu10, Pavel Krasnov2,3, Sergey P. Polyutov2,3, Faris Gel’mukhanov1,2,3, Michael Odelius11,a), and Victor Kimberg1,2,3,b)/ J. Chem. Phys. 154, 214304 (2021); / https://doi.org/10.1063/5.0049966

A theoretical and experimental study of the gas phase and liquid acetic acid based on resonant inelastic x-ray scattering (RIXS) spectroscopy is presented. We combine and compare different levels of theory for an isolated molecule for a comprehensive analysis, including electronic and vibrational degrees of freedom. The excitation energy scan over the oxygen K-edge absorption reveals nuclear dynamic effects in the core-excited and final electronic states. The theoretical simulations for the monomer and two different forms of the dimer are compared against high-resolution experimental data for pure liquid acetic acid. We show that the theoretical model based on a dimer describes the hydrogen bond formation in the liquid phase well and that this bond formation sufficiently alters the RIXS spectra, allowing us to trace these effects directly from the experiment. Multimode vibrational dynamics is accounted for in our simulations by using a hybrid time-dependent stationary approach for the quantum nuclear wave packet simulations, showing the important role it plays in RIXS.