These changes can explain the shift of a mode associated with ring deformations from 1383 cm -1 in free 5-FU to 1519 cm -1 . Altogether, the changes in the IR spectra are so pronounced that it should be straightforward to identify protonated 5-FU species in zeolite pores, even if only a fraction of the adsorbed molecules are protonated. For the 5-FU@H-FAU_2H_1_3 case,