Experimentally, gas-phase FT-IR results are available, but these experiments are conducted at high temperatures, which tend to break hydrogen bonds and thus eliminate the distinction between conformers. In contrast, matrix isolation spectroscopy is conducted at cryogenic temperatures within a frozen inert gas matrix. These experimental conditions stabilize different types of conformers, allowing them to be identified through the obtained IR spectra. It has been observed that, in general, the O-H stretching frequencies for Type I, Type II, and Type III amino acids are highly consistent when measured within the same matrix conditions. For instance, in argon matrices at approximately 12 K, the free O-H stretches for Type I and Type III amino acids appear near 3560 cm -1 , and the hydrogen-bonded O-H stretch for Type II appears near 3200 cm -1 . However, experimental studies on proline did not yield any identifiable signal near 3200 cm -1 , even though computationally, Type IIa proline is shown to be the lowest-energy conformer, and experimentally, two peaks occur in the C=O stretch region, which are believed to correspond to the Type Ia/IIIa and Type IIa conformers. This raises a perplexing question: where is the proline Type IIa hydrogen-bonded O-H vibrational signal in the matrix IR spectrum?