The most diagnostic difference between the predicted IR spectra of the two conformers is the relative position of the carboxylic acid C=O stretching vibration (~1800 cm -1 ). The frequency shift is likely due to an n -π + interaction with the charged imidazole ring or an H-bonding interaction with the primary amine, which are not present in the lowest-energy conformer. These arguments suggest that compound H corresponds to ring-opened MDZ-N-glucuronide 1 but that we do not probe the predicted lowest-energy conformation. This also reveals limitations of reference-free IRIS identification: especially for high-MW compounds, spectral differences may become subtle and the number of low-energy conformers may become large. Nonetheless, we believe that even in such cases, an IRIS analysis can greatly restrict the number of isomers to be synthetically prepared as reference standards, thus still expediting the identification.