Figure shows the calculated IR spectra for the wavenumber range from 1200 to 1800 cm -1 , emphasising the four most intense bands of free 5-FU (two C=O stretching modes, C5=C6 and C5-F stretching modes) as well as the corresponding bond distances. Intramolecular bond distances and, where assignable, frequencies of associated stretching modes are compiled in the ESI, Table . Relevant bending modes are also included in this table. Since some modes involve significant displacements of more than two or three atoms, it has to be stressed that this assignment is somewhat simplified and primarily intended to facilitate the following discussion. Table compares the calculated frequencies in the range of 1200 to 1800 cm -1 to experimental results obtained for 5-FU in an Ar matrix and to DFT-B3LYP calculations, both reported by Ivanov et al., and to experimental results obtained for solid 5-FU by Datt et al.