Formation of Carbamate Anions by the Gas-phase Reaction of Anilide Ions with CO₂

The anilide anion (m/z 92) generated directly from aniline, or indirectly as a fragmentation product of deprotonated acetanilide, captures CO₂ readily to form the carbamate anion (m/z 136) in the collision cell, when CO₂ is used as the collision gas in a tandem-quadrupole mass spectrometer. The gas-phase affinity of the anilide ion to CO₂ is significantly higher than that of the phenoxide anion (m/z 93), which adds to CO₂ only very sluggishly. Our results suggest that the efficacy of CO₂ capture depends on the natural charge density on the nitrogen atom, and relative nucleophilicity of the anilide anion. Generally, conjugate bases generated from aniline derivatives with proton affinities (PA) less than 350 kcal/mol do not tend to add CO₂ to form gaseous carbamate ions. For example, the anion generated from p-methoxyaniline (PA = 367 kcal/mol) reacts significantly faster than that obtained from p-nitroaniline (PA = 343 kcal/mol). Although deprotonated p-aminobenzoic acid adds very poorly because the negative charge is now located primarily on the carboxylate group, it reacts more efficiently with CO₂ if the carboxyl group is esterified. Moreover, mixture of CO₂ and He as the collision gas was found to afford more efficient adduct formation than CO₂ alone, or as mixtures made with nitrogen or argon, because helium acts as an effective “cooling” gas and reduces the internal energy of reactant ions. [Figure not available: see fulltext.]