Dimethylalane, [Me2AlH](n), in the vapor phase and in hydrocarbon solution: Gas-phase electron diffraction, spectroscopic, colligative, and ab initiostudies

The nature of dimethylalane, [Me2AlH](n), in the vapor phase and in hydroca rbon solutions has been reinvestigated experimentally by gas electron diffr action (GED), IR and mass spectrometric, and vapor pressure depression meas urements, and also by quantum chemical calculations. The predominant vapor species at low pressures and temperatures near 470 K is the dimer (n = 2), but at temperatures near 330 K the vapor consists of a mixture of the dimer and a smaller proportion (ca. 30%) of the trimer (n 3). The results of the GED measurements have been analyzed, together with those of ab initio calc ulations, using the SARACEN method to determine the most reliable structure s of the doubly hydrogen-bridged dimer, Me2Al(mu-H)(2)AlMe2, and singly hyd rogen-bridged trimer, Me2Al(mu-H)Me2Al(mu-H)Me2Al(mu-H). Salient structural parameters in the r(a) structures were thus found to be (a) for the dimer, r(Al-H) 177.6(7), r(Al-C) 195.2(2), r(Al ... Al) 262.5(6), r(C-H) 111.4(2) pm; angle C-Al-C 117.7(4), angle Al-H-Al 95.3(6), angle Al-C-H 108.9(7)deg rees; and (b) for the trimer, r(Al-H) 171.1(23), r(Al-C) 195.8(3), r(Al ... Al) 315.3(8) pm; angle Al-H-Al 134.3(36), angle Al-C-H 110.1(31)degrees Ho wever, solution studies indicate the presence of an oligomer [Me2AlH](n) wi th n > 3, in addition to the dimer and, probably, the trimer.