Chemical properties of butadienyl anions in the gas-phase

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, ΔGacid0,a, 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. © 1995.