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

Experimental and theoretical studies of gas-phase isomeric C4H5- anions have been performed. Deprotonation of 1,2-butadiene is expected to occur mainly at the 3-position, giving an anion which also is obtained after deprotonation of 2-butyne. The gas-phase acidities (ΔG°(acid)) of 1,2-butadiene and 2-butyne have been determined from the equilibria with methanol and pyridine and have led to ΔG°(acid) = 1574 ± 10 kJ mol-1 and ΔG°(acid) = 1596 ± 10 kJ mol-1, respectively. Deprotonated 1,2-butadiene and 2-butyne have been found to react similarly with N2O, neo-pentyl nitrite and methyl formate. These reactivity studies have indicated that the charge of the anion is delocalized over 2 carbon atoms. Ab initio calculations have been applied to isomeric C4H5- anions and have confirmed this hypothesis. The calculations have shown the existence of six stable 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 delocalized charge at carbon-1 and carbon-3.