The infra-red absorption bands associated with the COOH and COOD groups in dimeric carboxylic acids. I. The region from 1500 to 500 cm-1

The infra-red spectra of a considerable number of carboxylic acids and their COOD derivatives have been investigated between 1500 and 500 cm-1, as dimeric units in the liquid or crystalline solid states. Under these conditions the COOH group is shown usually to give rise to strong absorption bands in the regions 1420 ±20, 1300 ± 15 and 935 ± 15 cm-1. The first two of these are found to correspond to closely coupled OH deformation and C—O stretching vibrations occurring in the plane of the (COOH)2 dimeric ring; the latter is caused by the out-of-plane OH deformation vibration. COOD groups have absorption bands in the ranges 1350 ±50, 1050 ± 10 and 675 ±25 cm-1, which can be assigned respectively to the C—O stretching mode and the in-plane and out-of-plane OD deformation vibrations. Less constant absorption bands of the COOH group between 700 and 575 cm-1 are attributed to O—C = O skeletal deformation vibrations, and the corresponding bands are found at slightly lower frequencies in the spectra of the COOD derivatives. The results of the infra-red investigation are compared with the Raman spectra of such acids and with related infra-red and Raman frequencies of other molecules. Infra-red spectra of some equimolecuiar mixtures of acids with water have also been studied.