Unravelling molecular interactions in uracil clusters The C, N and O 1s XPS spectra of uracil and materials science. In a molecular cluster, the weak, non- clusters have been measured. A new covalent interactions modulate the core binding energy of nominally equivalent atoms. The measurement and interpretation of such bottom-up approach, which relies on fine variations in the XPS spectra provide the understanding computational simulations starting from the of the chemical equilibrium as well as stability and reactivity of crystallographic structure of uracil, has been supramolecular aggregates. Here, the XPS spectra of neutral clusters of uracil, one of the four nucleobases in the nucleic adopted to interpret the measured spectra. acid RNA, have been considered as case study of a bottom-up This approach sheds light on the different procedure to describe the interaction patterns within the cluster molecular interactions (H-bond, π-stacking, and to provide ab initio calculation of the binding energy, BE. dispersion interactions) at work in the RESULTS cluster and provides a good understanding The C, N and O XPS spectra of uracil clusters (Fig.1) have been of the observed chemical shifts in terms of measured at the PLEIADES beamline using a gas aggregation intramolecular and intermolecular screening source to produce neutral clusters. All spectra are broadened and occurring after the core-hole ionization. shifted by about 0.9 eV to lower BE with respect to those measured for the isolated uracil molecule, reported for comparison [1].To understand how the different molecular interactions affect these H-bonds and van der Waals interactions are ubiquitous in natureshifts, a bottom-up theoretical approach has been developed. and influence the structure, stability, dynamics, and function ofSeveral different cluster sizes (from dimer to dodecamer) have been molecules and materials throughout chemistry, biology, physicscut out from the uracil crystal structure [2] to sample all kinds of FIGURE 1 64