Infrared spectra of the ammonium ion in crystals. Part VI. Hydrogen bonding in simple and complex ammonium halides

Abstract

The ν₁ and ν_4bc infrared absorptions of the NH₃D⁺ probe ion dispersed at low concentration in polycrystalline ammonium halides were used to investigate possible correlations between the frequencies of these absorptions and various structural and other parameters of the ammonium compounds. The most important among these parameters are the H … X distance, the acceptor strength of the halogen atom X for hydrogen bonding, and the coordination number C.N. and activation energy E_a for reorientation of the ammonium ion. It is found that the ν₁ and ν_4bc frequencies are affected by the acceptor strength and by the degree of 'compression' of the ammonium ion in the crystal, the volume effect. These two factors affect ν_4bc in the same direction but ν₁ in opposite directions. As a consequence, in a ν_4bc vs. ν₁ plot the halides are separated according to X, C.N., and certain other parameters. The effective radius of the ammonium ion in cubic (NH₄)₂MX₆ is shown to increase with the strength of hydrogen bonding.The E_a vs ν₁ plot contains two branches. The low-frequency branch is dominated principally by the strength of the hydrogen bonding and corresponds to C.N. 4 and 8 and to normal hydrogen bonds. The high-frequency branch is dominated principally by the volume effect and corresponds to C.N. 12 and to hydrogen bonds of highly dynamic character (fluxional hydrogen bonds). Ammonium ions with C.N. 6 may correspond to either branch, depending on the formal charge on X. Existence of the so-called symmetrically trifurcated hydrogen bond that has been proposed for the ammonium ion in certain coordinations is not supported by the present evidence.The problem of the fundamental vibrational frequencies of the 'free' ammonium ion is discussed and values are proposed for the 'limiting' ν₁. and ν_4bc frequencies in ammonium halides. Criteria of hydrogen bonding in ammonium halides are reviewed and comment is offered on symmetry aspects of crystallographic transformations in cubic hexahalometallates(IV), A₂MX₆.