Abstract
Proton abstraction from 1,3‐butadiene by strong anionic bases in a Fourier‐transform ion‐cyclotron resonance mass spectrometer at relatively low pressure, yields an isomeric mixture of 1‐ and 2‐butadienyl anions. The gas‐phase acidity, Δ__G__°~acid~, of 1,3‐butadiene, associated with proton abstraction from the 2‐position, leading to 2‐butadienyl anions, is determined as 1627 ± 2 kJ/mol. 1‐Butadienyl anions, formed by proton abstraction from the 1‐position in 1,3‐butadiene, are found to be about 10 kJ/mol less stable than 2‐butadienyl anions. This stability difference is confirmed by high‐level ab‐initio calculations, which predict a planar, charge‐localized structure for the 1‐butadienyl anions, in which the butadiene π‐system is still intact. Additional stabilization of the 2‐butadienyl anion is realized by a 100° rotation of the central CC bond, which breaks up the butadiene π‐system in order to achieve allylic charge delocalization leading to a structure with two nearly orthogonal π‐systems. The isomeric 1‐ and 2‐butadienyl anions can be distinguished and identified conveniently and unambiguously by a probe reaction with nitrous oxide, which clearly reveals the distinct chemical nature of the two isomeric butadienyl anions.