Comparing the computed MD-based IR spectra with and without solvent, the additional bands at 800 -900 cm -1 occur also in the simulated spectrum of COF-1+solvent, albeit with much lower intensity than in experiment. Normal mode analysis of isolated molecules reveals that these bands belong to the C-O bending modes of 1,4-dioxane. The additional bands at 2850 -3050 cm -1 also appear in the computed spectrum, but, similarly as in experiment, with very low intensity. These bands correspond to the C-H stretching modes of 1,4-dioxane. The third small band at 1600 cm -1 is not present in our simulation. Vibrations of COF-1 or 1,4-dioxane are neither computationally or experimentally expected at this wavenumber. This band most likely originates either from the precursor 1,4-benzene diboronic acid (BDBA) (see Figure in the SI) or the second solvent used throughout the synthesis, namely mesitylene.