The vibrational spectra of phosphoric acid

It is in order to consider first the point symmetry of the phosphoric acid molecule, for, if we may consider the molecular symmetry (and, therefore, molecular vibrations) as a function of the symmetry of the surrounding crystal, our deliberations will be considerably simplified over the case in which one considers the symmetry of the unit cell as a whole. Considering the four different point symmetries available to a molecule of phosphoric acid, we have, as shown in Fig. 1, structures (l), (il), (ill), and (IV), corresponding to the point groups C3, C3, Cs, and C1, respectively. It is then clear that one might expect one or more of these to be present in the crystalline acid. If it should be possible to eliminate any number of these symmetries as possibilities,- then in conjunction with such data as might be available concerning the space symmetry, i. e., knowledge of the unit cell, it might be possible to draw some conclusions regarding the true point symmetry in the solid state with the use of the table of available site groups given by Halford(2)t Parenthetically, it may be mentioned that, due to the low scattering power of the hydrogen atom, x-ray diffraction techniques cannot always unequivocally determine structures involving such atoms. Finally, it may also be possible to make a reasonably complete band assignment. Towards this end, then, it was planned to investigate the solid acid, the deuterated solid acid, and various alkalai and alkaline earth salts of the acids in the solid state. This paper covers, in part, the work on the solid acids, and therefore deals with: 1. The study and interpretation of the infrared spectra of orthophosphoric and deuterated orthophosphoric acids at different temperatures. 2. The assignment of the observed frequencies of absorption to the normal modes of vibration. 3. The elimination of point symmetries that are incompatible with the obtained data. 4. Speculation regarding the point symmetry of the crystalline solid acid.