STUDIES OF INTERMOLECULAR INTERACTIONS BY MATRIX-ISOLATION VIBRATIONAL SPECTROSCOPY - SELF-ASSOCIATION OF AMMONIA

Infrared and Raman spectra are reported for ammonia trapped in argon and nitrogen matrices over a range of concentrations. Infrared spectra are also reported for mixtures of ammonia and trimethylamine trapped in argon matrices. The results are discussed in relation to previous infrared studies. The experimental data demonstrate that the ammonia dimer contains one molecule acting as an electron acceptor and the other as an electron donor, i.e. it has an open chain structure of the type H3N⋯HNH2 (though not necessarily the classical linear structure). The hydrogen bonding interaction in the dimer is very weak, suggesting that the structure of the dimer in the matrix may be similar to that found in the gas phase. The data available for the ammonia trimer in matrices do not provide unequivocal evidence for either a cyclic or open chain structure; the symmetric cyclic structure appears to give the best fit to the observed spectra. © 1990.