The geometric dependence of acidity: hexamethyldisilazide anion

Experimental data, and molecular orbital calculations at a variety of level s, provide support for the existence of hexamethyldisilazide anion with an sp-hybridized nitrogen when free of the counterion, but a more conventional sp(2) nitrogen when lithium is present. The cyclic compound 1,1,3,3-tetram ethyl-1,3-disila-2-azacyclopentane is experimentally 10.5 kcal/mol weaker a s an acid in the gas phase than the open chain equivalent, 1, 1, 1,3,3,3-he xamethyldisilazane, by ion cyclotron resonance spectrometry, The calculated energetics for each case agrees well with gas-phase and solution-phase exp erimental data. (C) 2001 Elsevier Science B.V.