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

1979; NRC Research Press; Volume: 57; Issue: 4 Linguagem: Inglês

10.1139/v79-068

1480-3291

Osvald Knop, Ian A. Oxton, Michael Falk,

Crystallization and Solubility Studies

The ν 1 and ν 4bc infrared absorptions of the NH 3 D + 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 ν 1 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 ν 1 in opposite directions. As a consequence, in a ν 4bc vs. ν 1 plot the halides are separated according to X, C.N., and certain other parameters. The effective radius of the ammonium ion in cubic (NH 4 ) 2 MX 6 is shown to increase with the strength of hydrogen bonding.The E a vs ν 1 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' ν 1 . 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), A 2 MX 6 .