CRYSTAL ENGINEERING USING THE UNCONVENTIONAL HYDROGEN-BOND - SYNTHESIS, STRUCTURE, AND THEORETICAL INVESTIGATION OF CYCLOTRIGALLAZANE

Cyclotrigallazane, [H2GaNH2](3), was prepared by condensing liquid amm onia onto solid trimethylamine gallane, GaH3(NMe3), at -78 degrees C a nd allowing the mixture to warm to room temperature and was characteri zed by IR, mass spectroscopy, elemental analysis, single-crystal X-ray , and neutron powder diffraction. Single-crystal X-ray diffraction at T = -167 degrees C established that the (GaN)(3) ring was in the chair conformation. Neutron powder diffraction data collected at 25 degrees C on the fully deuterated analogue were analyzed with Rietveld refine ment to give an average bond distance for Ga-D of 1.56(3) Angstrom and a N-D of 1.04(5) Angstrom. The intermolecular interactions were domin ated by four Ga-H ... H-N unconventional hydrogen bonds per molecule t hat form a chain parallel to the crystallographic a axis. The crystall ographically equivalent D ... D bond lengths are 1.97 Angstrom. Calcul ations revealed that in the gas phase, twist-boat conformations are pr eferred over chairs for cyclotrigallazane and the related boron and al uminum compounds by 0.9 to 2.6 kcal/mol at correlated levels of electr onic structure theory. For cyclotriborazane and cyclotrigallazane, cal culations suggest that each H ... H hydrogen bond contributes about 3 kcal/mol to the binding energy (relative to the chair monomer); this v alue is very slightly higher for cyclotrialumazane.