Knowledge Bank

As part of a continuing study dealing with the interpretation of the effects of hydrogen bonding on the vibrational spectra of simple carboxylic acids, the polarized infrared spectrum of crystalline acrylic acid has been measured. Acrylic acid provides a particularly useful model compound for studying the power of this technique in understanding crystal spectra. Both the liquid phase infrared and X-ray diffraction results have been reported. The latter shows acrylic acid to be dimeric in the solid, the most common hydrogen bonded structure of carboxylic acids. In addition, the infrared and Raman spectra of liquid acrylic acid can best be interpreted on the basis of C dimeric symmetry. The results of the solid state infrared spectral studies will be discussed in terms of their consistency with the above observations. Particular attention will focus on whether the weak coupling of monomer units through the hydrogen bonds is sufficient to warrant interpretation of the experimental crystal spectrum on a dimeric basis or whether the hydrogen bonding acts as a perturbation of a monomer spectrum. In addition, solid state structural, implications from the polarized spectrum will be discussed in relation to the known crystal structure.