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Monochloracetic acid (CClH2–COOH) is a trimorphic substance. Only one of the three forms is stable, viz.: the α form; whereas the β and γ forms are metastable. The infrared spectrum of the forms α and β has been studied; that of the γ form has not been studied because of its extreme instability. It has been seen that the spectrum of the α form is different from that of the β form. The fundamental frequencies of monochloracetic acid have been assigned, bearing in mind the assignments made by the present authors in another paper for di- and trichloracetic acids. This assignment makes it possible to determine that the displacements which are observed when passing from form α to β chiefly affect the frequencies bound up with the carboxyl group and more especially those in the OH group. This has been confirmed by the preparation of the deuterated derivatives which likewise present themselves in α and β forms, the same relative differences in frequencies being observed. It is inferred from this study that the two crystalline forms differ in the structure of the intermolecular dimers (or polymers). The α form is the conventional dimer with two hydrogen bonds. In the β form this intermolecular bond is probably not the double hydrogen bond, since the OH groups show a greater mobility.

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