On contrary to the widely accepted conviction that the m/z 93 ion derived from phenol does not react with CO2, we demonstrate that it makes an adduct with CO2 to a small but demonstrable extent. For example, the product-ion mass spectrum recorded for the m/z 98 ion derived from [(2)H6]phenol showed a small peak at m/z 142 when CO2 was used as the collision gas. The formation of an m/z 137 adduct ion from the m/z 93 ion (generated either directly from phenol, or indirectly from salicylic acid by in-source decarboxylation) was demonstrated also by multiple-reaction-monitoring tandem mass spectrometric experiments. According to literature, the m/z 93 ion derived from salicylic acid does not undergo CO2 addition because it is deemed to exist only in the phenoxide form. This reaction has been previously proposed as a method for differentiating phenoxide ion from its isomeric hydroxyphenide ions. We propose that the m/z 93 ion, albeit small, exists also as the phenide form together with the predominant phenoxide ion.

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Abstract. The anilide anion (m/z 92) generated directly from aniline, or indirectly as a fragmentation product of deprotonated acetanilide, captures CO 2 readily to form the carbamate anion (m/z 136) in the collision cell, when CO 2 is used as the collision gas in a tandem-quadrupole mass spectrometer. The gas-phase affinity of the anilide ion to CO 2 is significantly higher than that of the phenoxide anion (m/z 93), which adds to CO 2 only very sluggishly. Our results suggest that the efficacy of CO 2 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 CO 2 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 CO 2 if the carboxyl group is esterified. Moreover, mixture of CO 2 and He as the collision gas was found to afford more efficient adduct formation than CO 2 alone, or as mixtures made with nitrogen or argon, because helium acts as an effective Bcooling^gas and reduces the internal energy of reactant ions.