THE TRIMETHYLAMINE ADDUCT OF TRIMETHYLINDIUM IN THE VAPOR-PHASE - STRUCTURAL CHARACTERIZATION AND DISSOCIATION-ENERGIES FROM ELECTRON-DIFFRACTION AND INFRARED-SPECTROSCOPY

The electron diffraction structure of the trimethylamine adduct of tri methylindium is reported. As far as we know this is the first reported gas-phase study of an indium adduct. The molecule has C-3v symmetry a nd an indium-nitrogen bond length of 237.8(12) pm. The small changes i n the geometry of the trimethylindium moiety on coordination show that the adduct bond is weak. Similarly, the infrared spectra show only sm all shifts in frequencies for modes of the adduct compared to those of the acceptor and donor. The infrared spectrum of trimethylindium has been re-examined and reassignments made on the basis of 'hot' molecule s. The adduct is almost wholly associated in the vapour at room temper ature. Dissociation of the adduct at elevated temperatures has been st udied by electron diffraction and infrared spectroscopy. The former, m easured at constant pressure, has yielded an enthalpy of dissociation of 81 +/- 6 kJ mol(-1), the latter, measured at constant volume, gives an internal energy of dissociation of 84 +/- 2 kJ mol(-1). The differ ence between these two terms is discussed.