The linkage dendrogram provides visibility of the results of classification, because of which we employed the linkage function. Because this study used unsupervised learning, the AI classified IR spectra from a different perspective than a chemist. There are groups where the pairs at the ends of a branch are similar, but dissimilar when compared as a whole. In addition, some classification criteria were unclear and further improvements are required. However, the results showed that unsupervised ML, which uses images in their original form, is generally applicable in the field of chemistry. This study used the Tanimoto coefficient (TC) to objectively express molecular similarity . However, TC often results in a low degree of similarity, even when a chemist judges it to be high. For example, dodecane, dodecan-1-ol, and dodecan-1-amine hydrochlorides are similar because they are single-chain alkyl chains with functional groups attached at their ends. However, the average TC among them is 0.44, which is less than 0.8 (Tanimoto coefficients of 0.8 or higher are used to evaluate fingerprints) 19 . However, when using complex fingerprints such as extended connectivity fingerprints or Daylight fingerprint , the Tanimoto coefficient tends to be low. In this study, a value of 0.3 or higher is considered to correspond to a relatively high similarity 17d . Further, branches were considered to exhibit high similarity if the average TC was greater than 0.3. We also counted the number of combinations that were particularly similar with a score greater than 0.5.