For the four modes highlighted in Figure , experimental frequencies of 5-FU adsorbed in H-FAU samples having Si/Al ratios of 2.5, 15, and 30 were reported by Datt et al. Since the results for all three samples were essentially identical, it is sufficient to discuss those obtained for the most Si-rich sample: Taking free 5-FU in Ar matrix as reference, these authors observed a slight red-shift of the ν(C2=O2) mode by -5 cm -1 and a more prominent red-shift of the ν(C4=O4) mode by -25 cm -1 . These changes are much smaller than those predicted by the DFT calculations for hydrogen-bonded 5-FU adsorbed in H-FAU_2H_1_3, as well as being qualitatively different from those computed for the protonated 5-FU case. Experimentally, a slight blue-shift of the ν(C=C) mode by +5 cm -1 was observed, whereas the ν(C5-F) mode was completely unaffected. Neither of these results agree with the computational predictions summarised above. These apparent discrepancies can be explained when taking into account that Datt et al. concluded that 5-FU adsorbed in their samples primarily interacted with Lewis acidic EFAL sites, not with framework protons. While a direct corroboration would require further calculations, the present results support the findings by Datt et al. that adsorption of 5-FU at Brønsted acidic framework protons was not the dominant process occurring in their samples.