Figure : Confusion matrix showing the performance of j-IR-vis in predicting functional groups from IR spectra. j-IR-vis was most successful at predicting aldehydes, amides, carboxylic acids, and nitro groups (Fig. ). In general, these functional groups exhibit more than one high-intensity stretching vibration in the diagnostic region (1500-4000 cm -1 ) of IR spectra. Aldehydes, amides, and carboxylic acid all show a strong signal for the C--O stretching vibration between 1640-1740 cm -1 and a second accompanying signal that allows that to be distinguished from other carbonyl-based groups: (1) aldehydes possess a C-H stretch (two signals between 2700-2900 cm -1 ); (2) primary and secondary amides a N-H stretch (one or two signals between 3100-3500 cm -1 ); and (3) carboxylic acids a O-H stretch (one signal between 2400-3400 cm -1 ). Nitro groups show a N-O stretching vibrations that produce two signals between 1475-1550 and 1290-1360 cm -1 .