Semi-empirical calculations on the structure of the oxonium ion in various crystal sites and relative acidity scale

The structure of the H₃O⁺ ion embedded in a solid environment (H₃O⁺X^–, X^– = Cl^–, NO ₃ ^– , ClO ₄ ^– ) is studied using a modified version of CNDO/2. In this calculation the effect of the first shells of nearest neighbours is taken into account and the effects of second nearest neighbours are introduced by a simulation procedure. Electronic effects are also included. The ion structure is more planar in nitrate than in perchlorate environment and the hydrogen bonds are slightly bent. Trends in structural parameters are compared with chemical properties of the hydrogen bonds and parallels the Hammett acidity scale HNO₃ < HCl < HClO₄.