Characterization of isomeric C4H5- anions in the gas phase; Theory and experiment

Experimental and theoretical studies of gas-phase isomeric C4H5- anions hav e been performed. Deprotonation. of 1,2-butadiene is expected to occur main ly at the 3-position, giving an anion which also is obtained after deproton ation of 2-butyne, The gas-phase acidities (Delta G degrees(acid)) of 1,2-b utadiene and 2-butyne have been determined from the equilibria with methano l and pyridine and have led to Delta G degrees(acid) = 1574 +/- 10 kJ mol(- 1) and Delta G degrees(acid) = 1596 +/- 10 kJ mol(-1), respectively. Deprot onated 1,2-butadiene and 2-butyne have been found to react similarly with N 2O, neopentyl nitrite and methyl formate, These reactivity studies have ind icated that the charge of the anion is delocalized over 2 carbon atoms. Ab initio calculations have been applied to isomeric C4H5- anions and have con firmed this hypothesis. The calculations have shown the existence of six st able C4H5- anions, The reaction between carbon-3 deprotonated 1,3-butadiene with methyl formate has shown that the 2-buta-1,3-dienyl anion has a deloc alized charge at carbon-1 and carbon-3, Copyright (C) 1999 John Whey & Sons , Ltd.