Knowledge Bank

“The interpretation of the vibrational spectra of simple carboxylic acids is greatly complicated by association of the molecules into stable dimers under most conditions. In the case of formic acid, however, the molecules are hydrogen bonded into infinite chains in the crystal giving relatively strong intermolecular forces instead of the weak interactions to be expected if dimers comprised the crystal units. The possibility thus exists of clarifying some of the controversial assignments of this compound by a study of the characteristic vibrational frequencies of the crystal. The Raman spectrum of large single crystals of formic acid has been obtained over a range of temperatures and many of the rather broad bands of the liquid have been observed to be split into several sharp components in the solid. As an aid to the interpretation, the spectrum of the monomeric gas at about $150\circ$ as well as the spectrum of the liquid at several temperatures has also been obtained. Assignments of the vibrational frequencies of the crystal have been made on the basis of a factor group analysis assuming $C^\{\backslash prime\}^\{\backslash prime\}\_\{2\{v\}\}$ symmetry for the unit cell. The results have been used in making an assignment of the vibrational frequencies of the monomer.”