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Crystal Engineering Using the Unconventional Hydrogen Bond. Synthesis, Structure and Theoretical Investigation of Cyclotrigallazane

Campbell, J. P.; Hwang, J.-W.; Young, V. G., Jr.; Von Dreele, R. B.; Cramer, C. J.; Gladfelter, W. L.
J. Am. Chem. Soc. 1998, 120, 521.

Cyclotrigallazane, [H2GaNH2]3, was prepared by condensing liquid ammonia onto solid trimethylamine gallane, GaH3(NMe3), at -78oC and allowing the mixture to warm to room temperature and was characterized by IR, mass spectroscopy, elemental analysis, single crystal X-ray and neutron powder diffraction. Single-crystal X-ray diffraction at T = -167oC established that the (GaN)3 ring was in the chair conformation. Neutron powder diffraction data collected at 25oC on the fully deuterated analog were analyzed using Rietveld refinement to give an average bond distance for Ga-D of 1.56(3) angstroms, and N-D of 1.04(5) angstroms. The intermolecular interactions were dominated by four Ga-H - - - H-N unconventional hydrogen bonds per molecule that form a chain parallel to the crystallographic a axis. The crystallographically equivalent D - - - D bond lengths are 1.97 angstroms. Calculations revealed that in the gas phase, twist-boat conformations are preferred over chairs for cyclotrigallazane and the related boron and aluminum compounds by 0.9 to 2.6 kcal/mol at correlated levels of electronic structure theory. For cyclotriborazane and cyclotrigallazane, calculations suggest that each H - - - H hydrogen bond contributes about 3 kcal/mol to the binding energy (relative to chair monomer); this value is very slightly higher for cyclotrialumazane.

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